Paleozoic crustal blocks of onshore and offshore central Argentina:New pieces of the southwestern Gondwana collage and their role in the accretion of Patagonia and the evolution of Mesozoic south Atlantic sedimentary basins
Francisco Pángaro a ,*,Víctor A.Ramos b
a Petrobras de Argentina S.A.,Exploration and Production,Argentina b
IDEAN,Universidad de Buenos Aires,CONICET,Argentina
a r t i c l e i n f o
Article history:
Received 21October 2011Received in revised form 9March 2012
Accepted 29May 2012
Available online 15June 2012Keywords:
Ventania-Cape Fold Belt Patagonia
Tectonic evolution Río de la Plata Craton Colorado Basin Colorado Syntaxis
a b s t r a c t
The southwestern Gondwana basement block con ?guration in the central Argentinean offshore area was analyzed using gravimetric,magnetic and seismic data and existing onshore tectonic models.The resultant maps,the distribution of the Mesozoic and Cenozoic basins and Paleozoic structural features were used to validate the interpretations and to produce a new regional tectonic model.Pre-Carboniferous southwestern Gondwana of South America was interpreted as an open margin formed from east to west by the Dom Feliciano Belt,the Río de la Plata Craton,the Pampean Belt and the Pampia and Cuyania terranes.The collision of the Patagonia allochthonous terrane during the Late Paleozoic resulted in the development of the Ventania-Cape Fold Belt,which was mapped for the ?rst time off the Argentinean coast out to 600km from the shore.A strong change in the orientation of the Fold Belt is referred to as the Colorado Syntaxis,a mirror image of the Cape Syntaxis in South Africa.This change re ?ects the buttressing effect of the cratonic blocks that hamper the northward propagation of syn-collisional deformation and resulted in a 180-km shift of the orogenic front.The Mesozoic basins and the basement block distribution were analyzed.The Pampean Belt,a deformed area produced by the Pampia accretion to the cratonic area,is the locus to two episutural basins,the General Levalle and Macachín basins.The Salado Basin was interpreted as an episutural basin controlled by a 2.1e 2.0Ga suture within the Rio de la Plata Craton.The Colorado Basin is composed of four segmented depocenters that re ?ect different emplacement controls:the location of the western Colorado Basin was controlled by the Upper Paleozoic orogen;the distributions of the central and eastern Colorado depocenters,orthogonal to the continental boundary,were also strongly in ?uenced by the Upper Paleozoic structures and were offset by lineaments that re ?ect Dom Feliciano fabric;the Colorado Basin external depocenter that parallels the continental margin was also controlled by these lineaments.We interpret a time gap of some 50Ma between the beginning of the evolution of the margin-orthogonal depocenters and the Atlantic breakup.
ó2012Elsevier Ltd.All rights reserved.
1.Introduction
The Argentinean continental shelf between 35 and 45 S has a width between 300and 600km between the shore and the continent-ocean boundary de ?ned by the G Anomaly (LeBrecque and Rabinowitz,1977).The amount of data on the pre-Mesozoic geology of this area,which could provide constraints for the pre-breakup of the southwestern Gondwana reconstruction,is
limited.Only a few exploratory wells from the oil industry have provided fragmentary information of the Upper Paleozoic units that underlie the Mesozoic rift and drift sedimentary sequences,and data from only four of the offshore wells have been associated with an uncharacterized igneous-metamorphic basement,two near the Buenos Aires southern shoreline and two in the Salado Basin (Tavella and Wright,1996;Fryklund et al.,1996)(Fig.1).Although thousands of kilometers of 2D seismic lines provide reasonable images of the Paleozoic units,no detailed interpreta-tions were available at the time of this study.Efforts have been made to assess the crustal structure and terrane con ?guration of the area using indirect data such as aero-magnetometric surveys (Ghidella et al.,1995;Max et al.,1999),gravimetric modeling (Kostadinoff,1984;Introcaso,2003;Franke et al.,2006)and deep
*Corresponding author.Present address:Av.República de Chile 330,20031-170Río de Janeiro,Brazil.Tel.:t552168759508;fax:t552121442457.
E-mail addresses:francisco.pangaro@https://www.doczj.com/doc/2911934622.html,.br ,fpangaro@https://www.doczj.com/doc/2911934622.html, (F.
Pángaro).
Contents lists available at SciVerse ScienceDirect
Marine and Petroleum Geology
journal ho mep age:www.elsevier.co m/lo cate/marp
etgeo
0264-8172/$e see front matter ó2012Elsevier Ltd.All rights reserved.https://www.doczj.com/doc/2911934622.html,/10.1016/j.marpetgeo.2012.05.010
Marine and Petroleum Geology 37(2012)162e 183
Figure 1.A e Generalized geologic map of the pre-Mesozoic units of Argentina and Uruguay and offshore control points.Terrane boundaries were determined according to Ghidella et al.(1995),Max et al.(1999),Chernicoff and Zapettini (2002,2004),Basei et al.(2008),Ramos (2008).Age of the Agua Blanca granite was determined according to Thover et al.(2012).The offshore well information on Paleozoic units was modi ?ed from Fryklund et al.(1996)and Tavella and Wright (1996).PA:Piedra Alta terrane.PDE:Punta del Este terrane.NP:Nico Pérez terrane.COB:continent-ocean boundary.B e Available database for the interpretation of the Paleozoic units.1e Wells with well-logs,cores and cuttings description and biostratigraphic data.2e Well-logs and cores and cuttings descriptions.3e Only lithologic descriptions available.4e Only well-logs available.5.e Interpretations based on published data,see text for references.6e No data available.7e No pre-Mesozoic units drilled.8e Wide angle seismic.9e Conventional seismic.10e Seismic lines of Figures 4,8,9and 12.11e Meters drilled through pre-Mesozoic units.12e Correlation cross-section of ?gure 1-C.C e Cross section displaying the Paleozoic e Mesozoic unconformity (U2)and the main ?ooding surfaces picked for regional seismic interpretation of the Upper Paleozoic units (C1and C2).1e Sandstones.2e Shales.3e Alternated siltstones and ?ne sandstones.4e Diamictites.GR e Gamma ray log.DT e Sonic velocity log.The column to the right displays a partial reconstructed Upper Paleozoic stratigraphic column based on biostratigraphic data and regional seismic interpretation.The seismic image of the major surfaces interpreted is shown in Figure 4.
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183163
re?ection and refraction seismic(Franke et al.,2006,2010; Schnabel et al.,2008),but this information has not been suf?ciently integrated with onshore data and is mainly focused on character-izing the Gondwana breakup mechanisms,with the exception of the conclusions on basement fabrics presented by Max et al.(1999). Recent publications by Pángaro et al.(2011),Pángaro and Ramos (2011)and Domínguez et al.(2011)provide new insights into the regional structural features of the Argentinean continental shelf by integrating different sources of data at the regional and semi-regional scales.
The aim of this research was to?ll a310,000sq.km void in the pre-breakup SW Gondwana reconstruction by integrating re?ec-tion seismic interpretations,satellite gravimetric data,well data, previously interpreted aero-magnetic data,and existing tectonic models based on the surface geology of Argentina,Uruguay and South Africa.Our interpretations are based on thousands of km of 2D seismic data of variable quality,some25wells for which a variable amount of data was available(Fig.1b and c),and surface geology from onshore Argentina.Integrating this new interpreta-tion with previous onshore models led to an improved terrane con?guration of the Argentinean continental shelf and provided new piercing points for the pre-134Ma SW Gondwana recon-structions.The Ventania-Cape Fold Belt was mapped for the?rst time through the entire width of the Argentinean continental shelf. Its in?uence on tectonic evolution was analyzed in an integrated manner.New and well-documented hypotheses for the opening mechanisms and timing of the Colorado and Salado rift basins are presented.These models and the basement block arrangement led to important conclusions about the pre-Mesozoic con?guration of SW Gondwana.
2.Regional setting
2.1.Regional overview
The regional structural con?guration of central Argentina is dominated by the long lived Río de la Plata Craton,a2.1Ga stable cratonic block extending from the Piedra Alta Terrane of west central Uruguay(Dalla Salda et al.,1988;Pankhurst et al.,2003; Kr?ner and Cordani,2003;Cingolani,2011)(Fig.1)southward to the Colorado Basin.This craton is bounded to the east by the Dom Feliciano Belt(Fragoso Cesar,1980),which is composed of terranes that were accreted between the Neoproterozoic and Early Cambrian periods;this is well documented in outcrops throughout southern Brazil and Uruguay(Basei et al.,2008).The western margin of proto-Gondwana evolved through the collision of a series of terranes accreted from Cambrian to Devonian times.These include Pampia,accreted at the Precambrian e Cambrian boundary (Ramos,1988);Cuyania,accreted during the Ordovician(Astini et al.,1996);and Chilenia,accreted during the Devonian(Ramos, 1999).The con?guration of the southern margin of the Upper Paleozoic proto-Gondwana is still debated.Several authors have interpreted the existence of an open margin to the south that was closed by the collision of the Patagonia terrane during Carbonif-erous to Permian times(Ramos,1984,2008;Rapalini,2005;Milani and De Wit,2008;López de Lucchi et al.,2010),whereas others have reported an already welded Gondwana encompassing all of Patagonia as an autochthonous terrane(Gregori et al.,2008).de Almeida et al.(2000)on their analysis of the proposed“South American platform”also suggest that the Patagonian block would be exotic to it based on the relative young thermal age of its basement.
The main Phanerozoic structural features of eastern central Argentina and the Argentinean continental shelf can be separated by age into two groups,the?rst encompassing the Upper Paleozoic features and the second encompassing the Jurassic(?)to Creta-ceous features,which are related to the rifting and later breakup of the Gondwana.The Paleozoic structural features include the Clar-omecóBasin,a10km thick Paleozoic basin,and the Ventania Fold Belt(Fig.2).The former is interpreted as containing two sequences: the lower Paleozoic platform deposits of southwestern Gondwana, with sediment sourced from the north,and the Carboniferous to Permian southerly sourced deposits,which have been interpreted as part of the in?ll of a foreland basin formed during the Patagonia accretion(Ramos,1984,2008;López Gamundi et al.,1995;Lesta and Sylwan,2005).The Sierra de la Ventana Fold Belt (Harrington,1970;Tomezzoli and Vilas,1999;Tomezzoli and Cristallini,2004)is an approximately NW trending structural feature extending onshore for120km and with a minimum extension of680km into the continental shelf.Even though this feature has been interpreted offshore by re?ection and refraction seismic methods(Franke et al.,2006),this is the?rst time it has been documented up to the eastern boundary of the South Amer-ican Plate with detailed mappings of its distribution.To the SW of the Ventania Fold Belt,a potentially250km wide deformation area is interpreted as the inner part of an Upper Paleozoic orogen that crops out in the Cerro de los Viejos(Fig.1),where mylonitic rocks indicate late Paleozoic ductile fabric with a northwest-trending foliation and S e C structures that indicate top-to-northeast ver-gence(Ramos,2008and references therein).The offshore exten-sion of this orogen has not been unequivocally documented due to a lack of suf?cient data,but gravimetric and magnetic fabrics,and paleogeomorphological features of the pre-Mesozoic unconformity strongly suggest continuity in the northeastern portion of Patago-nia,as discussed subsequently.
The Jurassic(?)to Tertiary sedimentary basins developed throughout the Río de la Plata Craton and the Dom Feliciano Belt and can be grouped into three categories.The?rst category consists of a series of episutural basins,as described by Bally et al.(1981), related to the suture between the Río de la Plata Craton-Dom Feliciano Belt block and the Pampia and Patagonia terranes.These include the General Levalle,Macachín,and western Colorado Basins(Fig.2).The second category is composed of a group of basins and sub-basins orthogonal to the continental margin.These include the Colorado Basin’s central and eastern depocenters,the Salado Basin,and the Punta del Este Basin.The third group consists of the basins parallel to the continental margin and closely related to the?nal breakup of Gondwana.Included in this group is the external depocenter of the Colorado Basin.In contrast,only two depocenters in the postulated Patagonia terrane,the Rawson and Valdés basins,developed parallel to the main structural trend of the contact between Patagonia and the Río de la Plata Craton-Dom Feliciano Belt block.These constitute minor NNW oriented depo-centers interpreted to be the result of extensional reactivation of Upper Paleozoic compressive features.
2.2.Regional Paleozoic stratigraphy
The Paleozoic stratigraphy of southwestern Gondwana in central Argentina is summarized in Figure3,which shows a wide-spread unconformity that separates Cambrian to Devonian deposits from Carboniferous to Permian deposits.The older ones are composed mainly of sequences of quartz-rich platform deposits of Cambrian to Devonian age materials of the Curamalal and Ventana Groups(Harrington,1980)and correlative units.These sequences overlie the metamorphic,igneous and sedimentary Proterozoic to Cambrian rocks of the Río de la Plata Craton,with sediment input from the northeast,as based on the paleocurrents measured by Reinoso(1968)and Andreis and Cladera(1992)and recently con?rmed by detrital zircon analyses by Uriz et al.(2011).These
F.Pángaro,V.A.Ramos/Marine and Petroleum Geology37(2012)162e183 164
units correlate with the Cape Supergroup of South Africa,as recognized by Keidel (1916)and du Toit (1927),and are interpreted as forming a set of passive margin sequences that ?anked the SW Gondwana after a Cambrian rifting event (Rapela et al.,2003).Lying over these above an unconformity are the Upper Carboniferous to Permian sedimentary rocks of the PillahuincóGroup,an approxi-mately 3.5km thick package of mainly southerly sourced sediments that re ?ect a drastic change in the regional structural con ?guration of southwestern Gondwana and a marked input from volcanic rich sources (Andreis y Cladera,1992;López Gamundi,2006).2.3.Regional offshore Paleozoic stratigraphic constraints
In the offshore portion of the study area,the pre-Mesozoic rocks are mainly preserved in strongly rotated half-grabens formed during the Jurassic (?)to Cretaceous syn-rift phase of the pre-breakup and breakup rifting events (Fig.4).Through detailed mapping and correlation with exploratory wells for which biostratigraphic control is available (Archangelsky,1996;Balarino,2009),the sedimentary column preserved in these half-grabens was reconstructed down to the Upper Carboniferous glacial deposits,which correlate with the Sauce Grande Formation of the Sierra de la Ventana area for which Di Pasquo et al.(2008)docu-mented a Pennsylvanian e Cisuralian age,and with the Dwyka Group in South Africa dated as Upper Pennsylvanian (Stollhoffen et al.,2008)to Sakmarian (Bangert et al.,1999).Offshore,the only age constraint for these units comes from biostratigraphic analyses of Permian assemblages in sediments overlying the Pennsylvanian to Cisuralian diamictites in the exploratory wells Puelche x-1(Archangelsky,1996),Cruz del Sur x-1,Estrella x-1(Balarino,2009),and Gaviotín x-1(Veroslavsky et al.,2003).The Upper Paleozoic units are mappable in re ?ection seismic data thanks to two regional ?ooding surfaces characterized by strong acoustic impedance contrast (Figs.1and 4)that allow recognition of the interpreted portion of the Upper Paleozoic package.The lower one corresponds to the post glacial transgression above the Sauce Grande Fm.e Dwyka Gr.diamictites,drilled in the Puelche x-1well (Fig.1)and dated 288.0?3and 289.6?3.8Ma in the western Cape region of South Africa (Bangert et al.,1999).The upper one corresponds to a basin-wide transgression dated as Sakmarian to Arstinskian in the Cruz del Sur x-1well (Balarino,2009)where a package of ?ne to medium sandstones is overlain by a thick interval of black shales (Fig.1)that were correlated with the Whitehill Fm.of South Africa and the IratíFm.of the ParanáBasin of Brazil.The age of this regional ?ooding event was dated in both Brazil and South Africa around 278Ma (Ventura Santos et al.,2006;Werner,2007).Upper Paleozoic sedimentary rocks usually overlie a re ?ection-free seismic package that could represent an igneous or metamorphic basement or poorly imaged Lower Paleozoic units that correlate with the onshore passive margin deposits of the Curamalal and Ventana groups (Ordovician to Devonian).These Lower Paleozoic units were interpreted in seismic lines in the central Colorado Basin,where several kilometers of strongly folded strata underlie a package correlated to the Upper Paleozoic units.One tying point to characterize the Lower Paleozoic units might be the Pejerrey x-1well (Fig.1)that drilled through 80m of quartz rich hyaline and hard sandstones that provided no fossiliferous remains during biostratigraphic analyses;these sandstones bear a re ?ection
free
Figure 2.Phanerozoic structural features and isopach map of the Mesozoic plus Cenozoic sedimentary rocks.SLuc:Santa Lucía Basin.Mer:Merín Basin.GLev:General Levalle Basin.PdE:Punta del Este Basin.Mac:Macachín Basin.WCol:western Colorado https://www.doczj.com/doc/2911934622.html,ol:central Colorado Basin.ECol:eastern Colorado Basin.ExtCol:external Colorado Basin.Ventania FB:Ventania Fold Belt.COB:continent-ocean boundary.The onshore sedimentary thickness is based on Webster et al.(2004),Veroslavsky et al.(2004),Chebli et al.(1999)and Zambrano (1974).The thicknesses of the Rawson and Valdés basins are from Marinelli and Franzin (1996).
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183165
seismic image and are seen to underlie the tabular and high amplitude re ?ections that characterize the Upper Paleozoic dia-mictites.To date,the only certain reference to these units in the subsurface is from the onshore ClaromecóBasin,where Lesta and Sylwan (2005)observed a sedimentary pile in excess of 10km of Paleozoic units encompassing Cambrian to Upper Permian mate-rials.The seismic line provided in Figure 4shows re ?ective inter-faces some 500ms beneath the base of the Carboniferous diamictites that might represent the Lower Paleozoic units.3.Gravimetric data preparation and processing
Based on available satellite gravity data (Sandwell and Smith,1997),a regional onshore -offshore Bouguer anomaly map was constructed to guide the interpretation of major structural features over an area of 1,200,000sq.km,with a special emphasis on the offshore area where integrated structural data for the pre-Cretaceous was unavailable (Fig.5).Several pre-processing steps were taken to minimize spurious effects,especially those in the extended shallow continental shelf where satellite gravimetric data are sensitive to errors.Calibrating the resultant maps with struc-tural interpretations of a dense 2D seismic offshore coverage and the known onshore geology (Fig.1)enabled a reasonable level of accuracy for the maps used for regional and semi-regional interpretations.
The main objective of the gravimetric data processing step was to determine gravity contrasts underneath the Paleozoic e Meso-zoic boundary and time equivalent surfaces.For this purpose,the density differences in the Mesozoic to Upper Paleozoic sediment transition were obtained from exploratory wells in the Colorado and Salado basins and from the structural high between them.The ?rst step of pre-processing was to determine the grid cell size considering two factors:the size of the Jurassic to Cretaceous rift related structures,which are typically tens of kilometers long half-grabens,and the smallest feature radius to be detected without introducing excessive noise due to lack of resolution and the mentioned pitfalls of public domain satellite gravimetric data.The 3-by-3-km grid used during processing proved to be suf ?ciently accurate to interpret mid-scale structural features such as rotated half-grabens,with lateral density contrasts as low as 0.1g/cm 3(Fig.4).
Limitations related to public domain satellite data are at a maximum in two offshore areas:the slope between the lower platform and the abyssal plain due to steep slopes and
deep
Figure 3.Pre-Mesozoic stratigraphy of southwestern Gondwana and northern Patagonia.See Figure 1for the location of the schematic stratigraphic columns.The onshore stratigraphy is from Harrington (1980),Rapela et al.(2003),Uriz et al.(2011),Cingolani (2011),Oyhant?abal et al.(2011).1.Folded strata.2.Carbonate rocks.3.Granitic rocks.4.Diamictites.5.Clastic rocks.6.Fine grained clastic rocks.7.Metamorphic rocks.The offshore Upper Paleozoic ages are based on biostratigraphy,see text for references.The offshore Ordovician to Devonian rocks were interpreted using seismic data and no biostratigraphic well control.
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183
166
canyons and the shelf in water depths shallower than 100m.Because the latter feature encompasses most of the analyzed area,the possible errors were carefully https://www.doczj.com/doc/2911934622.html,paring the free air gravity and bathymetry data suggested that the error in the bathymetry data is not always in phase with gravimetric anomalies;thus,a smoother bathymetric map of the continental shelf was constructed for use in the removal of the mass defect caused by the water column.The error introduced by steep canyons was not ef ?ciently removed,and the area was discarded in the gravimetric interpretation.In the ?rst step of processing,a Bouguer anomaly map was calculated by removing the offshore water column and the onshore topography effect;the latter was performed using a density of 2.63g/cm 3,equivalent to that for the upper Paleozoic sedimentary rocks beneath the Mesozoic sedimentary column in the Colorado and Salado basins.The main goal of this density selection was to aid the delineation of possible Paleozoic basins and basement highs in the areas of poor seismic coverage or quality.
In the second step,a 5th-order Bouguer anomaly polynomial residue map was created to remove the strong in ?uence of
crustal
Figure 4.Time-migrated seismic line and Bouguer anomaly pro ?le in milligals for the northern ?ank of the eastern Colorado Basin showing strongly rotated half-grabens on which upper Paleozoic units are preserved.The pre-Upper Carboniferous appears as a nearly re ?ection-free package in the central portion;however,some re ?ections are visible at 3s depth in the southern portion of the line.The small paleotopographic feature in the Paleozoic e Mesozoic unconformity in the central portion is typical of remnant highs caused by differential erosion of Upper Carboniferous diamictites that are hundreds of meters thick.The C1?ooding surface that corresponds to the widespread Cisuralian post-glacial transgression is interpreted at a high amplitude re ?ection that is the result of a strong change in acoustic wave propagation velocity;this is evident at the Pu.x-1well in Figure 1.The C2?ooding surface is interpreted at the top of a relatively highly re ?ective package of parallel re ?ections that were drilled in the CDS.x-1well (Fig.1).The top of the Upper Permian is inferred based on the onlap relationship of syn-rift deposits above it.See Figure 2for location.
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183167
thinning associated with the continental to oceanic crust transition and any regional anomalous effect.Possible sources for long wavelength gravimetric anomalies could be mantle variations and dynamic topography effects;however the length of the features interpreted in the 5th-order polynomial residue is typically 100e 150km while the regional effect of the mentioned sources would bear a much longer wavelength,in the order of several hundreds to thousands of kilometers.Proof of this are the
results
Figure 5.a e Bouguer anomaly map based on satellite gravity data.A grid size of 3?3km was used during preprocessing.The density used for the Bouguer correction was 2.63gr/cubic cm.b e Bouguer anomaly ?fth-order polynomial residue.Values in milligals.SLuc:Santa Lucía Basin.Mer:Merín Basin.GLev:General Levalle Basin.PdE:Punta del Este Basin.Mac:Macachín Basin.WCol:western Colorado https://www.doczj.com/doc/2911934622.html,ol:central Colorado Basin.ECol:eastern Colorado Basin.ExtCol:external Colorado Basin.CmLow:Claromecógravity low.COB:continent-ocean boundary.
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183
168
obtained by Crovetto et al.(2007)who analyzed the geoid’s undulation and its very limited impact on the gravimetric anomaly of the Salado Basin,and the scale of dynamic topography anomalies of South America presented by Shephard et al.(in press)who show that the effect of the latter is despicable at our scale of analysis.As a result,the reasonably detailed Bouguer anomaly map and its5th-order residue enabled interpretations of the main structural features at semi regional scale(Fig.5a and b).In the third step, gravimetric back stripping was performed based on the lines proposed by Hammer(1963)to remove the effect of the known Mesozoic sedimentary basins.The result was a Bouguer anomaly map from which the mass defects due to density differences between Mesozoic and Paleozoic sediments were removed.This map reveals mass and density excesses underneath the Jurassic to Tertiary sediments(Fig.6a and b).The existence of volcanic rocks in the rift axis,known at the Salado and Merín basins and interpreted in the Colorado Basin’s eastern depocenter,would enhance the mass excess created by crustal thinning and later cooling of litho-spheric mantle material at the base of the lower crust.
4.Gravity data interpretation
The interpretation of the gravity-derived maps was conducted by integrating previous studies(Kostadinoff,1984,2007;Introcaso and Ramos,1984;Max et al.,1999;Introcaso,2003;Crovetto et al., 2007;álvarez,2007;Gregori et al.,2009;Domínguez et al.,2011; Pángaro et al.,2011)with newly constructed gravity,structure and sedimentary thickness maps(Fig.2).The Bouguer anomaly residue enables the interpretation of gravimetric fabric and the segregation of several domains characterized by differential orientations of local highs and lows of tens to hundreds of kilometers of extension. Major highs that occasionally intersect the background texture at an angle are evident in areas such as the Colorado and Salado basins (Fig.7).
4.1.The Río de la Plata Craton
This craton is mainly characterized by a brittle NE-oriented fabric,the dominant feature observed in the gravimetric data coinciding with the orientation of the main fault direction mapped in the Tandil area of w2.1Ga Paleoproterozoic basement rock outcrops(Cingolani,2011).However,the main structural trend within the craton,which is not visible in the gravimetric maps, corresponds to a N70 W orientation schistosity and penetrative ductile deformation of2e2.1Ga,which predates the aforemen-tioned faults.The gravimetric fabric is uniform from the ClaromecóBasin area in the south through400km to the north to the Salado Basin gravimetric anomaly,a curved,roughly ESE-oriented high generated by crustal thinning(Introcaso and Ramos,1984;Crovetto et al.,2007).This feature is interpreted as being controlled by the fabric resulting from a Paleoproterozoic collision between the Tandilia terrane to the south and the Buenos Aires terrane to the north(Ramos,1999).
North of the Salado Basin,the craton’s gravimetric fabric adopts an arcuated pattern that parallels the Salado high and represents the Paleoproterozoic structural grain of the Buenos Aires terrane. The northern boundary of Tandilia that contacts the Salado Basin has been interpreted as a suture based on the ma?c and ultrama?c Belt of El Cortijo,an island arc system that was trapped between the Buenos Aires and the Tandilia terranes(Teruggi et al.,1988; Cingolani,2011).
This characteristic extends northward into Uruguay,where Oyhant?abal et al.(2011)described a good?t between gravimetric data and terrane boundaries that coincide in cases with ultrabasic-basic belts interpreted as sutures between the Piedra Alta and Buenos Aires terranes.The west-northwest-trending San José-Montevideo ma?c and ultrama?c belts represent this suture,which extends into the Martín García Island(Dalla Salda et al.,1988;Dalla Salda,1999;Cingolani,2011).
The arcuated fabrics are interrupted by a NE-oriented series of anomalies extending from the Salado Basin to the southern Bra-zilian coast;these include the Mesozoic Santa Lucía and Merín basins of Uruguay(Veroslavsky et al.,2004)and the recently proposed Quilmes trough in Argentina(Rossello et al.,2011).The interpretation and distribution of Bouguer anomalies off the Rio de la Plata estuary show that the arcuated fabrics extend east up to an area characterized by NE lineaments,suggesting the continuity of the craton(Fig.7).
The craton’s western and southwestern boundaries are marked by a series of gravity highs and lows at an angle with its internal fabric that de?ne an area where crustal thinning-related maxima evolved and led to the formation of the episutural basins of General Levalle,Macachín and the eastern Colorado depocenter(Figs.2,6 and7).These areas of boundary parallel gravimetric anomalies are interpreted to represent the Pampean Belt and the Upper Paleozoic orogen,both of which are related to terrane collisions. The eastern boundary of the Río de la Plata Craton gravimetric domain lies almost entirely offshore.Its interpretation in Uruguay and southern Brazil is based on existing onshore data(Oyhant?abal et al.,2011;Basei et al.,2008)for an area in which the gravimetric fabric of the stable craton meets the NNE-oriented fabric of the Dom Feliciano Belt and less clear fabric of the Nico Pérez terrane.In the south,the Punta Mogotes well in Argentina yielded metapelites with a metamorphic age of740e840Ma(Rapela et al.,2011)that are interpreted as part of the Dom Feliciano Belt;Pángaro et al. (2011)proposed that this boundary was further offshore,but the recently published data by Rapela et al.(2011)has de?ned the craton’s eastern boundary.This boundary can be interpreted southward to the northern?ank of the Colorado Basin,where Pángaro et al.(2011)identi?ed two NE-oriented,NW-dipping deeply rooted shear zones mapped all the way to the Moho in wide angle re?ection seismic lines.These shear zones are interpreted as offsetting the Colorado Basin depocenters,and the eastern one is the focus of emplacement of a Late Cretaceous volcanic?eld composed of hundreds of presumably monogenetic volcanoes,with a K e Ar age of70?3Ma according to dating in the Puelche x-1well (Linares,1978).The emplacement of the Upper Cretaceous volcanic rocks is interpreted as being controlled by the reactivation of a major structural feature that might represent a Dom Feliciano Belt inherited weakness zone.
Two outstanding features within the Río de la Plata Craton are the ESE-oriented high amplitude e high wavelength gravity low of the ClaromecóBasin(Ramos and Kostadinoff,2005)and the high amplitude maximum of the Salado Basin(Introcaso and Ramos, 1984).The former is interpreted to re?ect the presence of a thick Paleozoic succession encompassing the Cambrian to the Upper Permian,whereas the latter is interpreted to re?ect a non-compensated crustal thinning associated with the Salado rift basin,which evolved between the Jurassic and the Tertiary (Introcaso and Ramos,1984;Crovetto et al.,2007).
4.2.The Dom Feliciano Belt
The second gravimetric domain is the offshore extension of the Dom Feliciano Belt,which is interpreted from Uruguay where it crops out(Oyhant?abal et al.,2011)into the Colorado Basin in a series of NE-oriented lineaments that locally controlled the emplacement of crustal thickness anomalies related to the early rifting and breakup.The Colorado Basin external depocenter,which is parallel to the continental boundary,and to the vast gravity low
F.Pángaro,V.A.Ramos/Marine and Petroleum Geology37(2012)162e183169
in the east,which re ?ects the extension of the outer high of the Colorado Basin (Figs.5b and 6b )are examples of this.These features and the areas between them are,in general,characterized by NE-oriented gravimetric fabric.One exception is the area to the east of the Claromecógravity low,where fabric adopts a boundary parallel fashion (Figs.5b and 7).
The presence of nearly perpendicular high amplitude Bouguer anomalies in several areas of the Belt re ?ects the loci of
crustal
Figure 6.a e Compensated Bouguer anomaly map constructed by removing mass defects due to the difference between Mesozoic and Cenozoic sedimentary rock densities and the density chosen for the Bouguer anomaly correction (see text for explanation).b e Fifth-order polynomial residue of the compensated Bouguer map.Values in milligals.SLuc:Santa Lucía Basin.Mer:Merín Basin.GLev:General Levalle Basin.PdE:Punta del Este Basin.Mac:Macachín Basin.Wcol:western Colorado https://www.doczj.com/doc/2911934622.html,ol:central Colorado Basin.ECol:eastern Colorado Basin.ExtCol:external Colorado Basin.CmLow:Claromecógravity low.COB:continent-ocean boundary.
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183
170
thickness anomalies developed during rifting as described by Pángaro et al.(2011).These include the Colorado Basin central and eastern depocenters,the Punta del Este Basin and a small depo-center to the east of the Salado Basin,all of which stand out in the compensated Bouguer anomaly (Fig.6).
The eastern boundary of the Dom Feliciano Belt,the G Anomaly (LeBrecque and Rabinowitz,1977),is coincident with a maximum of the Bouguer anomaly residue and re ?ects the transitional continental crust.To the west,the boundary in Uruguay consists of the suture between the Belt and Río de la Plata Craton (Oyhant?abal et al.,2011),and in the offshore portion of the Río de la Plata estuary,it consists of the boundary between the aforementioned arcuated fabrics of the craton and the clear NE fabric interpreted as the southern extension of the Punta del Este terrane (Fig.7).A good piercing point to the south is the Punta Mogotes well,where metamorphism ages re ?ect a Dom Feliciano Belt af ?nity (Rapela et al.,2011).In the south,the limit is interpreted to be in the eastern boundary of the Claromecógravity low and extending further south into the Colorado Basin,where it offsets the central and western depocenters (Pángaro et al.,2011).The southern boundary of the Belt with the Patagonia terrane is marked by an abrupt change of gravimetric fabric from a NE to a NNW orienta-tion,which de ?nes a sharp boundary.4.3.Patagonia
The northeastern portion of the Patagonia allochthonous terrane (Ramos,1984,2008;Rapalini,2005;Milani and De Wit,2008;López de Lucchi et al.,2010;Rapalini et al.,2010)is charac-terized in most of the area analyzed by a marked NNW trend of
gravity Bouguer anomalies that are consistent with the orientation of the Rawson and Valdés sedimentary basins offshore (Fig.7).The entire area of the Patagonia terrane lacks Bouguer anomalies with high amplitudes such as those present in the Dom Feliciano Belt and the Río de la Plata Craton (Figs.5a and 6a ).These two char-acteristics clearly segregate this area,especially in the offshore region,where the northeastern boundary is a sharp line of SSE orientation that correlates with the boundary proposed by Ghidella et al.(1995)and Max et al.(1999)via the interpretation of airborne regional magnetic ?eld data.This SSE-oriented boundary changes to a WNW orientation as it reaches the western end of the Dom Feliciano Belt in the vicinity of the Colorado Basin western depo-center.To the west of this point,the boundary is not recognizable through gravity data interpretation for 150km.Further west,the high amplitude Rio Colorado gravimetric high marks the north-ernmost extent of Patagonia.The NNW-oriented fabric is inter-preted to correlate with an area of thick-skinned deformation that constitutes the offshore extent of the inner part of the Upper Paleozoic orogen,which resulted from the Patagonia collision and crops out into the Cerro Los Viejos and North Patagonian Massif areas (Rapalini et al.,2010and references therein)(Fig.7).4.4.Pampia,the Pampean Belt and the Upper Paleozoic orogen Along the western margin of the Río de la Plata Craton between 33
and 38 S,a 100e 150km wide area of parallel gravity anomalies de ?nes a region interpreted as a deformation Belt associated with the accretion of the Pampia terrane to the Río de la Plata Craton during Neoproterozoic to Cambrian times (Ramos,1988;Thover et al.,2012).This area,referred to as the Pampean Belt,was
the Figure 7.Interpretation of the terrane boundaries of onshore and offshore central Argentina based on the Bouguer anomaly maps and onshore published data (see text for references).The depocenters of the Mesozoic and Cenozoic sedimentary basins are superimposed.The terrane boundaries were extrapolated from controlled onshore locations and based on the interpreted gravimetric fabrics.Note the sharp offset of the Colorado Basin depocenters and their location on the Dom Feliciano Belt and the interpreted Permian orogen.The Yaminué,Cerro de los Viejos and Sierra de la Ventana localities are highlighted to show the potential width of the Permian orogen;the NNW fabrics that allow its interpretation extend between 38 and 43 S over both Patagonia and the area between Pampia and the Río de la Plata Craton.
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183171
loci for the development of the General Levalle and Macachín basins,which experienced minor crustal thinning compared to those that developed over the Río de la Plata Craton and the Dom Feliciano Belt.Farther west,the Pampia terrane shows NNW-oriented fabric that reaches as far as39 S(Fig.7)immediately to the north of the E e W-oriented Rio Negro gravimetric high.Here, the southwestern tip is interpreted to be in contact with the Patagonia terrane and might have represented the open southern continental margin of the Gondwana supercontinent prior to the postulated Carboniferous to Permian collision event(Ramos,1984, 2008;Rapalini,2005;Milani and De Wit,2008;López de Lucchi et al.,2010;Rapalini et al.,2010).The boundary between the Pampia and the Río de la Plata Craton in the southern portion is based on the recognition of the Pampean magmatic arc in isolated and minor outcrops identi?ed in the Pampa plains and dated by Chernicoff et al.(2012).
South of37 S,the area of NW-oriented fabric broadens to a width of250km between the Sierra de la Ventana and the Cerro de los Viejos locality;the former is considered to be the orogenic front of the Upper Paleozoic deformation event related to the Patagonia collision,and the latter is recognized because its ductile deformation and metamorphism age dated as late Paleozoic (Ramos,2008and references therein)and is interpreted to be part of the syncollisional orogen.
With the information available,the boundary between the Pampean Belt and the Permian orogen cannot be identi?ed because the data are isolated(Fig.1).However,the gravimetric data allow us to infer the existence of a wide area of syncollisional northeast-verging deformation extending for more than250km between Cerro Los Viejos and Sierra de la Ventana.Offshore,south of40 S, the northeastern Patagonia terrane is characterized by the afore-mentioned NNW fabric,which is interpreted by correlation with available seismic data as the prolongation of the Permian orogen’s hinterland.This piece of information was integrated with the Fold Belt extension offshore and newly mapped features to provide new possible constraints for assessing the scope of the late Paleozoic deformation.In the North Patagonian Massif,where regional gravimetric interpretation yielded poor results(Fig.7),Rapalini (2005)and Rapalini et al.(2010)provide a detailed analysis of Carboniferous and Permian granitoids in the area of Yaminué,and interpret that they were emplaced during a major NNE e SSW thrusting event with a SSW vergence as early recognized by von Gosen(2003).Rapalini et al.(2010)interpret the North Patago-nian Massif to be part of the hinterland of the orogen associated to the closure of the Colorado Ocean due to the frontal collision of Patagonia.The result would be an orogen with a total width exceeding500km in the study area.
4.5.Jurassic e Cretaceous crustal thickness anomalies
The Bouguer anomaly map,compensated by removing post-Permian sediment mass defects(Fig.6),enables an interpretation of three regional basement features:a linear WNW high corre-sponding to the Salado Basin,a series of highs of an approximate NNW orientation in the area of the western boundary of the Río de la Plata Craton ending in the Colorado Basin’s western depocenter, and the Colorado Basin’s eastern and central depocenters,which have a NW orientation and an offset of150km between their axes.
The Salado gravity high has been thoroughly described in the literature as being caused by an uncompensated crustal thinning (Introcaso and Ramos,1984).Recently,Crovetto et al.(2007) included dynamic topography as a contributor to the magnitude of the anomaly but concluded that crustal thinning was the main source for the high Bouguer anomaly values.The eastern Colorado gravimetric high has been correlated to crustal thinning(Introcaso,2003),whereas some authors refer to high density bodies to account for the anomalies in the central and eastern portions of the Colorado Basin(Franke et al.,2006).Recently,Pángaro and Ramos (2011)analyzed the Colorado Basin depocenters and identi?ed the existence of four crustal thinning related anomalies that are relatively independent from one another.This re?ects a segmenta-tion of the basin that directly correlates with the distribution of the Mesozoic sediments.The one-to-one correlation of the highs with crustal thinning was veri?ed by direct mapping of the re?ection Moho in depth-converted deep imaging seismic data as that pre-sented in Figure8;the Moho hinge interpretation is in agreement with previous wide angle re?ection seismic interpretations by Franke et al.(2002).In our interpretation the re?ection Moho was picked at the base of a2e5km thick high amplitude set of re?ec-tions following the criteria described by Cook et al.(2010)and references therein.The NNE-oriented highs in the Pampean Belt correlate with the series of episutural basins,documented through potential?eld methods and drilling in the General Levalle Basin (Webster et al.,2004).By correlation with the offshore cases and seismic interpretation of the offshore portion of the Salado Basin, the crustal thinning during pre-breakup and breakup can be interpreted as the source of the main gravimetric residual highs observed in the compensated Bouguer anomaly map.
4.6.Regional integration of gravimetric data interpretation
Through the integration of the Bouguer anomaly map inter-pretation,gravimetric back-stripping and seismic-based Mesozoic thickness maps,several valuable regional observations that strengthen the interpretation of basement block distribution have been made.First,the locations of at least three Mesozoic episutural basins in the Pampean Belt and the Permian orogen in a NNE-oriented stripe have been identi?ed(Fig.7).These basins are restricted to the suture areas resulting from the attachment of Pampia to the Río de la Plata Craton and the later accretion of Patagonia to southwestern Gondwana.Another valuable piece of information is the sharp offset of the Colorado Basin depocenters, which are accompanied by an equal offsetting of the Bouguer anomalies(Figs.6b and7),and the fact that the western boundary of the Central Colorado depocenter coincides with the boundary of the Dom Feliciano Belt.Another Bouguer anomaly that shows a strong in?uence of previous regional structural features is the Salado Basin,which is mainly restricted to a Proterozoic suture within the cratonic area and shows a rapid decrease in amplitude as it reaches the Dom Feliciano Belt.
The Merino e Santa Lucia e Quilmes lineament(Rossello et al., 2011)is not a clear feature in the compensated Bouguer anomaly map,but it is traceable in the Bouguer residue map.It shows no correlation with regional fabrics and cuts through the Dom Felic-iano Belt and the Río de la Plata Craton indistinctively.Finally,the continental border parallel anomalies that are restricted to an area of100e150km landward from the G anomaly suggest a strong in?uence of the Dom Feliciano Belt fabric on their location.
https://www.doczj.com/doc/2911934622.html,parison of gravity data interpretation with the EMAG2 magnetic anomaly map
In order to support the gravity based interpretations the results were contrasted with a regional scale magnetic anomaly mapping based on the2arc min resolution earth magnetic anomaly map provided by Maus et al.(2009).This dataset covers the Argentinean continental shelf entirely and roughly an80%of the onshore study area.When compared to Max et al.(1999)magnetic anomaly maps of the Argentinean continental shelf,acquired with a20?20km grid and a nominal altitude of300m,the EMAG2proves being very
F.Pángaro,V.A.Ramos/Marine and Petroleum Geology37(2012)162e183 172
precise and adequate for regional structural interpretation.Following the lines proposed by Ross et al.(1994)regional magnetic anomaly interpretation allows the discretization of basement domains characterized by the intensity and wavelength of the anomalies and by the resultant magnetic fabrics and their relation with adjacent domains.A general rule of thumb provided by these authors suggests that aeromagnetic anomalies are sourced mainly by basement features,and that sedimentary basins produce a damping effect re ?ecting the subtraction of high frequencies so basement features beneath thick sedimentary covers would not have a strong related magnetic anomaly.
Similar criteria was followed in the pioneering work by Ghidella et al.(1995)and Max et al.(1999)who recognized the existence of three main basement domains in the Argentinean shelf between 47 and 35 of south latitude:the “Patagonia Platform domain ”to the south,the “La Plata domain ”to the north and the “Marginal sub-domain of the La Plata domain ”between them.In their inter-pretation Max et al.(1999)de ?ne the latter as an area tectonically reworked during the accretion of Patagonia.
Our regional interpretation of the EMAG2data (Fig.9)allowed the segregation of four magnetic domains that are in phase with the gravimetric interpretation.The Dom Feliciano Belt is characterized by NNW trending anomalies that were interpreted by Max et al.(1999)as “Precambrian structural grain ”in their La Plata domain;in our interpretation this domain re ?ects the Dom Feliciano fabric
also visible in the Bouguer anomaly maps.The Río de la Plata Craton is characterized by a NE fabric intersected by less de ?ned WNW anomalies,especially west of Punta Mogotes;the former correlates with the main fault direction mapped in outcrops (Cingolani,2011)and the latter re ?ects the Precambrian penetrative ductile defor-mation mentioned in 4.1.Further interpretations within the Rio de la Plata Craton are restrained by data quantity and quality of the EMAG2database.The southwestern portion of the study area is characterized by a zone of rounded anomalies without any preferred orientation and a moderate amplitude;these give way to the NE to a series of well de ?ned NW oriented anomalies.When analyzed in the context of the proposed major tectonic boundaries this third domain is interpreted to stand for the Patagonia terrane and the Permian orogen mapped in the contact area between Patagonia and the Río de la Plata Craton-Dom Feliciano Belt block.This interpretation of the NW magnetic fabric and their correlation with a Permian orogen was ?rst proposed by Max et al.(1999);the continuity of the NW oriented anomalies into the onshore area linking the offshore proposed orogen with the Cerro de los Viejos locality strengthens the proposal of an extended Permian orogen.6.Formation mechanism of the Colorado Basin
The Colorado Basin,now interpreted as being formed by four clearly discernible depocenters,has been debated for
decades,
Figure 8.Structural cross section of the eastern Colorado depocenter.a e Bouguer anomaly values in milligals obtained from satellite gravity data processing.Note the good ?t of the low-frequency variations with the re ?ection Moho depth variations on b.The high-frequency oscillations re ?ect the contrast between Upper Paleozoic sediments and underlying rocks,especially in the major downthrown blocks.b e Depth-converted seismic lines of the eastern depocenter of the Colorado Basin,2times vertical exaggeration.The southern third of the section is a depth converted version of wide angle-deep imaging line BGR 98e 13.Red arrows indicate the base of the re ?ection Moho.Note the good correlation of this feature with both sedimentary thickness and gravity anomalies.A slight pushed-down effect is interpreted below the main depocenter as a depth migration error consequence.See Figures.1and 2for location.c e Structural cross section,two times vertical exaggeration.Note the south dipping master shear and the prominent step some 70km south of the northern tip of the section.The interpretation of the Upper Paleozoic section was possible as a result of the restoration of the two mega-rollovers resulting from the fault geometry.In the main depocenter,the Paleozoic units are not interpreted due to poor seismic images,but this is area is interpreted as the potential location for folded strata,which are indicated by the dashed arrow.d e Restored cross section,no vertical exaggeration.The minimum extension was estimated to 45km.Two pinpoints were located north and south of the depocenter.The difference between the southern ?ank restoration and the reference level can be attributed to regional tilting and thermal subsidence.Note the approximately 6Km eroded pre-Mesozoic thickness in the northern ?ank.Based on previous work by Pángaro and Ramos (2011).(For interpretation of the references to colour in this ?gure legend,the reader is referred to the web version of this article.)
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183173
Figure 9.a e Magnetic anomaly map based on EMAG2data,see text for references.b e Regional interpretation of magnetic anomalies.Note thewell de ?ned NNE magnetic fabrics of the Dom Feliciano Belt and the NW to NNW fabrics of the Patagonia allochthonous terrane.The Río de la Plata Craton displays two populations of anomalies oriented NE and ESE.The major tectonic boundaries interpreted in the Bouguer anomaly maps were overlain to contrast the results of both methods.Note the striking coincidence of the NW oriented fabrics of the Permian orogen and the main domain boundaries de ?ned by Max et al.(1999)(doted lines).In their interpretation the “Marginal sub-domain of the La Plata domain ”represents the deformation Belt associated to the collision of Patagonia.1e Major tectonic boundaries interpreted.2e Positive magnetic anomaly.3e Negative magnetic anomaly.4e Main sedimentary basins.5e Magnetic domain boundaries by Max et al.(1999).Domains interpreted by Max et al.(1999):I e “Patagonia Platform domain ”.II e “Marginal sub-domain of the La Plata domain ”.III e “La Plata domain ”.RDPC:Río de la Plata Craton.DFB:Dom Feliciano belt.PO:Permian https://www.doczj.com/doc/2911934622.html,ol:central Colorado Basin.ECol:eastern Colorado Basin.ExtCol:external Colorado Basin.CMco:ClaromecóBasin.
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183
174
especially due to its location in an area lacking data on the pre-Mesozoic units and in the area where the boundary between the Patagonia allochthonous terrane and SW Gondwana was formerly interpreted(Ramos,1984).Previous models have postulated a combined extension and strike slip deformation(Nürnberg and Müller,1991),with a horizontal slip of some20e30km.A similar model was later adopted by Fryklund et al.(1996)and Franke et al. (2006).The latter author proposed an aborted rift basin that evolved prior to the Atlantic opening by either cutting through or interfering with Paleozoic structures.As part of a regional struc-tural analysis of the Argentinean continental shelf,Ramos(1996) analyzed gravimetric data and,through that analysis,interpreted the existence of a south-dipping master shear that might coincide with the suture between the Patagonia allochthonous terrane and the Río de la Plata Craton.
Recently,Pángaro and Ramos(2011)used depth-converted deep imaging seismic data to assess the formation mechanism of the Colorado Basin’s eastern depocenter.The seismic line,structural model and gravimetric pro?le shown in Figure8enable a straight-forward interpretation of the re?ection Moho at depths of30km and a bulge of4km,coinciding geographically with the maximum thickness of Mesozoic sediments and the maximum amplitude in both raw and compensated Bouguer anomaly maps.A south-dipping master detachment shows a prominent ramp underneath the northern limit of the main depocenter and can be interpreted via structural restorations(Pángaro and Ramos,2011).North of this ramp,a150km wide area of strongly rotated fault blocks helps to de?ne an asymmetric rift basin with a clear axial thermal subsi-dence depocenter and a vast ramp of mechanical subsidence due to block rotation.The minimum extension was estimated through restoration via inclined shear method to be at least45km,equaling a20%stretching factor.A regional6km doming and denudation process was documented by carefully characterizing the Paleozoic sediments and constructing tectonic subsidence curves in the northern and southern?anks of the depocenter(Pángaro and Ramos,2011).This stretching factor was calculated by restoring only Colorado Basin-related faults and not including any Atlantic opening-related stretching.The denudation event is interpreted on a regional scope because it has been documented throughout the Colorado Basin.
Applying concepts for the interpretation of volcanic facies on seismic data(Pángaro et al.,2006),the eastern depocenter’s deepest portion was interpreted as a pile of more than1km of volcanic rich strati?ed deposits and prominent volcanic edi?ces that reach a height of1.5km were detected.These are interpreted to represent the typical bimodal syn-rift volcanism with initial basaltic lavas and a later more acidic activity capable of forming high relief stratovolcanoes.The areal extent of the volcanic facies coincides with that of the axial crustal thickness anomaly.In contrast in the central depocenter,even though a less developed crustal thickness anomaly is interpreted through seismic and gravimetric data,no volcanic rocks have been identi?ed to date; this might be in part due to a smaller number of deep imaging seismic lines being available or simply to the absence of volcanism in this portion of the basin.
In all the interpretations carried on so far allow stating with a great degree of con?dence that the Colorado Basin’s eastern and central depocenters evolved as strongly asymmetric rift basins with a south-dipping master detachment,a minimum extension ranging between45and40km,and a straight forward relationship between the thickness of the sedimentary pile and crustal thick-ness anomalies.No evidences of strike slip deformation were detected in the rifting process of any of the Colorado Basin’s depocenters;in particular,the eastern depocenter shows a wide area of rotated blocks showing axis parallel faults.Towards the continental margin,the breakup rifting event adds complexity to the fault array but still no strike slip deformation is envisaged as an important process during the formation of the pre breakup aborted rift basins.
The off-setting of the depocenters is interpreted to be controlled by Dom Feliciano Belt structural features because the areas of offset coincide with northeast-oriented,west-dipping sets of re?ections identi?ed via seismic interpretation from the top of the basement some5km deep,up to tens of kilometers deep(Figs.2,6b and7). These might represent?rst-order structural elements associated with the amalgamation of crustal blocks during the belt’s formation.
Structural control of preexisting features on the distribution of Colorado Basin depocenters are at this point undeniable;their strongly asymmetric geometry with south-dipping master detachment,their offsetting through discrete structural elements, and the possible relationship of the western depocenter with the suture area between Patagonia and the Río de la Plata Craton all lead to this interpretation.The missing piece of information to date has been the nature and origin of the structural features controlling their distribution and loci of emplacement.
7.The Ventania-Cape Fold Belt and the Permian orogen
To determine the main control in the evolution of the Colorado Basin,a new element needs to be incorporated into the regional collage of basement blocks,structural features and sutures:the offshore extension of the Ventania-Cape Fold Belt.In general,its existence has previously been postulated in regional models with neither direct nor indirect data support(Urien and Zambrano, 1996)and with a somewhat vague trace;others proposed an eastward decrease in the intensity of deformation and the absence of the Fold Belt offshore,based only on the absence of evidence of its existence(Juan et al.,1996).Fryklund et al.(1996),in their analysis of the Colorado Basin’s southern?ank,mention the lack of adequate data available to characterize the pre-rift geology and to identify folded https://www.doczj.com/doc/2911934622.html,ter,Franke et al.(2006)mention the offshore extension of this feature through the interpretation of a deep imaging seismic line across the Colorado Basin depocenter without providing further interpretations.
To assessing this problem,with the presumption that the Ven-tania Fold Belt should extend offshore in the Argentinean conti-nental shelf,a detailed mapping of the pre-Mesozoic units was carried out where seismic quality allowed.It must be borne in mind that a serious obstacle to this task is the fact that most of the available seismic data was acquired using arrays shorter than4km; this resulted in a serious limitation in the imaging of beds dipping more than13 at depth greater than3km;these beds should not be imaged due to the geometric limitations of the marine seismic acquisition method.
To cope with the limited resolution of most of the available seismic information,our?rst task was to map the areas with an unmistakable absence of folded strata.This was possible through the interpretation of the Upper Carboniferous and Permian strata in rotated normal fault downthrown blocks as those of the seismic line presented in Figures4and8;the seismic-to-rock correlation was possible through a detailed analysis of four exploration wells drilling a total of5.5km of Upper Paleozoic rocks;two of this wells are presented in Figure1.It was found that there are no Upper Paleozoic folded strata either north or south of the eastern Colorado depocenter(Fig.8)or south of the central depocenter.Seismic data underneath the western depocenter is generally of very poor quality and does not allow the characterization of the pre-Mesozoic strata,with the exception of a few locations where folded strata were detected under the Paleozoic e Mesozoic unconformity.The
F.Pángaro,V.A.Ramos/Marine and Petroleum Geology37(2012)162e183175
detailed mapping of this unconformity in the western depocenter area led to the identi?cation of palaeo-topographic features that were interpreted as buried hills with a NW orientation,similar to that of the extrapolated Ventania Fold Belt fold’s axis(Fig.10).
This piece of data,in conjunction with the last onshore piercing point that lies in the southern tip of the Sierra de la Ventana in the onshore of province of Buenos Aires,led to the interpretation that the distribution of the Ventania Fold Belt offshore covers a roughly ESE-oriented area with a width of100e150km(Fig.11).It can be seen in detail from west to east that the Ventania Fold Belt’s prolongation maintains a SE azimuth from the last piercing point onshore up to the similarly oriented Colorado Basin’s western depocenter;at that point,roughly60 W40 S,the deformed area shows a45-degree rotation to an east-west orientation for230km, becoming parallel to the Colorado Basin central depocenter.It continues eastward,and as it reaches the eastern depocenter,the orientation of the potentially deformed strip changes to an ESE orientation,similar to that of the depocenter’s axis for some 200km,until it reaches the South American plate boundary.
With this potential distribution of the folded strata in mind,and with the aid of seismic lines with better resolution acquired with longer arrays,the interpretation of the offshore prolongation of the Ventania Fold Belt was straightforward in several localities, allowing the con?rmation of what was inferred from the indirect data(Fig.12).Work to characterize the Fold Belt’s structural style is still in progress,but to date,several structures with a vertical relief in excess of2km,reaching as much as5km and involving the pre-Carboniferous strata were identi?ed.
One dif?culty to interpret the Ventania Fold Belt in seismic lines is the lack of suf?cient wells with enough information to allow a con?dent well to seismic correlation.To interpret the seismic line displayed in Fig.12the sedimentary column of the area was put together using data from the Puelche x-1well that drilled1605m of Upper Paleozoic strata some70km south of the seismic line,the Cruz del Sur x-1well,which is150km to the ESE and drilled 1490m,and several localities in which the non-folded Upper Paleozoic rocks preserve an almost complete original thickness. With this data integrated,a circa5km stratigraphic column encompassing the Upper Paleozoic rocks was constructed;this column is characterized,as is that of Fig.4,by three highly re?ective stratigraphic surfaces representing the top of the Upper Carbonif-erous to Lower Permian diamictites(?ooding surface C1of Figs.1 and4),the top of a regressive sandstone package(?ooding surface C2),and towards the top a series of high amplitude re?ections not drilled in any well offshore,that are interpreted via seismic analysis and correlation with outcrops of Sierra de la Ventana as continental deposits of Upper Permian age.Underneath the Upper Paleozoic rocks,an approximately6.6km thick package of sediments is interpreted;these probably correspond to the Lower and Middle Paleozoic sandstone bearing units of the Cura-malal and Ventana Group that outcrop in Sierra de la Ventana (Fig.3).The base of these is interpreted in a highly re?ective event that might represent Neoproterozoic to Cambrian rocks as those present in the Tandil area;the high amplitude re?ections could be due to highly variable acoustic impedance resulting from the interlayering of Cambrian limestones and clastic rocks.Another interpretation could be that these re?ections correspond to the top of the crystalline basement.Near30km deep,a set of re?ections is interpreted as the re?ection Moho.Another dif?culty interpreting this line was the fact of it being a strike line to the Ventania Fold Belt;this may result in information coming from the sides of the seismic line due to the structural complexity of the Fold Belt.
Regardless of all the mentioned limitations,it was possible to interpret a sedimentary pile in excess of14km deeply affected by faulting and folding.Moreover,the thickness measured for the Upper Paleozoic units based on the identi?cation of the three high impedance layers mentioned is consistent with the extrapolated thickness estimated in neighboring localities adding con?dence to the identi?cation and mapping of major stratigraphic surfaces.The areas left in blank in Figure12are almost devoid of any re?ection rendering them uninterpretable;however these could correspond either to basement rocks or to strongly folded strata of Lower to Middle Paleozoic that is characterized for its poor seismic image.
A set of apparently west dipping deeply rooted thrust faults is interpreted,the effect of these faults on the sedimentary cover is a strong folding accompanied by minor thrusting,The half-graben in the eastern portion of the line is interpreted as a possible extensional reactivation of a thrust fault.
On a regional level,combining the fold belt’s distribution with the basement blocks map,valuable observations regarding possible regional structural controls can be made.First,the Fold Belt displays an abrupt change in geographic orientation in the same area where the eastern boundary of the Río de la Plata Craton is interpreted to be located;this could well be the mirror image of the Cape Syntaxis(Johnston,2000and references therein)in South Africa where the Cape Fold Belt turns almost90 from an EW to a NNW orientation as it leaves the Kalahari Craton to enter the Gariep Belt,the African corresponding orogen of the Dom
Feliciano
Figure10.Time seismic line south of the western Colorado Basin depocenter.Prominent buried hills are visible underneath Mesozoic strata.The passive onlap?ll allows the interpretation of preexisting features not related to the Jurassic-Cretaceous rifting event.The central feature is more than750m high,as determined by a seismic wave propagation velocity of3400m/sec for the Mesozoic sediments.The orientation of these highs is displayed in Figure10and is mainly NW.See Figures1and11for location.
F.Pángaro,V.A.Ramos/Marine and Petroleum Geology37(2012)162e183
176
Belt as described by Kr?ner and Cordani (2003)and references therein.Second,the width of the Fold Belt diminishes abruptly from more than 150km in the Permian orogen area south of the craton to less than 100km in the Dom Feliciano Belt,and it adopts an orientation parallel to the Colorado Basin ’s central and eastern depocenters (Fig.11).
Taking into consideration the onshore control of the Permian deformation (that is,the Cerro de los Viejos locality with its top to the northeast foliation and the orogenic front at Sierra de la Ventana),gravity data were analyzed in conjunction with one available seismic line cutting through the NW-oriented gravimetric anomalies of the northeastern Patagonia and the adjacent Dom Feliciano Belt.This resulted in the identi ?cation of clear compressive structures with an apparent northeast transport direction,which combined with the gravimetric fabrics,is consistent with the top to the northeast foli-ation of Cerro de los Viejos.On the other hand,the southwest verging structural deformation described in the North Patagonian Massif (von Gosen,2003;Rapalini,2005;Rapalini et al.,2010)could not be interpreted offshore due to lack of seismic data and its presence is only inferred through the extrapolation of onshore data and the interpretation of the gravimetric fabrics (Fig.11),8.Regional integration
Combining all new and previously published data,a regional tectonic framework of offshore and onshore western Gondwana and its impact on the evolution of the study area during the Upper Paleozoic and Mesozoic times is envisaged (Fig.13).Using onshore outcrop-controlled locations,coastal borehole data and gravity,magnetic and seismic data,the Dom Feliciano Belt,Río de la Plata Craton,Pampia and the potential extension of the Permian orogen were mapped into the offshore areas.Considering the hypotheses regarding the allochthonous (Ramos,1984,2008)or para-autochthonous origin of Patagonia (Rapalini,2005,Rapalini et al.,2010;López de Lucchi et al.,2010),the pre-Carboniferous SW open margin of Gondwana prior to the collision was composed of four blocks in onshore Argentina and the Argentinean continental shelf:the Cuyania and Pampia terranes,with the attached Pampean Belt comprising Pampean and cratonic fragments,the southern extent of the Río de la Plata Craton and the Dom Feliciano Belt.The margin had an EW orientation in the western portion of the area analyzed and a NW orientation in the Río de la Plata Craton and the Dom Feliciano southern boundaries (Fig.13a).
The Ventania Fold Belt and its prolongation in the Cape Fold Belt are considered to be the northernmost extent of the Carboniferous to Permian syncollisional deformation related to the accretion of Patagonia,and pre-existing structural features exerted a great deal of control on them.The most noticeable aspect of these features is that the deformation was unable to penetrate deeply into the cratonic area;this lack of penetration is in contrast to the north-wards extent of deformation in the Dom Feliciano Belt,resulting in the 45 change in orientation of the Fold Belt,referred to here as the Colorado Syntaxis (Fig.13b).Its counterpart in South Africa,the Cape Syntaxis,displays a similar control because the deformation did not penetrate deeply into the Kalahari Craton (de Beer,1995).Considering that the main structural trends in the Dom
Feliciano
Figure 11.The Ventania Fold Belt distribution both onshore and offshore,terrane boundaries,surface geology and Mesozoic plus Cenozoic isopach maps.The area highlighted pale purple represents folded strata that were directly interpreted using seismic or outcrop techniques,while the area in red shows the zones were it was inferred from gravimetric maps or poor seismic image areas.Note the 45 of variation in orientation in coincidence with the central Colorado depocenter ’s western limit and the Dom Feliciano Belt boundary.Note the wide area of undeformed Upper Paleozoic units south of the Ventania Fold Belt.Data on onshore tectonic transport direction is based on Rapalini et al.(2010)and references therein.(For interpretation of the references to colour in this ?gure legend,the reader is referred to the web version of this article.)
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183177
Belt are NE-oriented and are parallel to the continental margin,the orthogonal orientation of the Fold Belt is likely the result of rheo-logical controls rather than the in ?uence of pre-existing structures.Segmentation of the Colorado Basin ’s depocenters,which are interpreted to re ?ect offsetting in the compressive structures by Dom Feliciano Belt fabric,could be the consequence of the belt ’s control in the compressive deformation (Fig.13c).
It is then postulated that onshore and close to the coast,the deformation extended as far north as the Rio de la Plata Craton allowed due to the buttressing effect of the cratonic block;eastward and farther from the coast,in the absence of the cratonic block,it extended into the Dom Feliciano Belt and was propagated more than 100km northwards,resulting in the Colorado Syntaxis (Fig.13b).
The extension of the Upper Paleozoic orogen is still a matter of research,but the extrapolation of the onshore data to offshore features and the interpretation of gravimetric and magnetic fabrics along with available seismic lines led to the mapping of a 300km wide orogen that expands up to 600km as it evolves into the Dom Feliciano Belt (Fig.13b).However,in the widest area of the orogen,south of the Colorado Basin ’s central and eastern depocenters,where the Colorado Syntaxis developed,a gap in the Upper Paleozoic compressive deformation was interpreted from seismic lines.South of both the central and western Colorado Basin depo-centers,non folded strata were interpreted;a good example of this is the southern portion of the seismic line presented in Figure 8.The extent of the non-deformed Paleozoic strata area is approximately 200km;therefore,the orogen in this area would be formed by two separate deformation belts totaling 600km,300km of which correspond to the southwestern inner portion,200km to the non deformed area and only 100e 150km of which correspond to the northeastern orogenic front,namely the offshore continuation of the Ventania Fold Belt.Therefore we envisage for the Colorado Syntaxis area and eastwards a Jura-type Fold Belt with a non folded area between the orogenic front and the orogen ’s core similar to that described by Escher et al.(1997)in the Alps.This system might have evolved between the Upper Carboniferous and Upper Permian,since the major thrusting event in the Yaminué
complex
Figure 12.Depth-converted wide angle seismic line BGR 98-1C indicating the offshore extension of the Ventania Fold Belt.The data quality was good enough to interpret at least three major faults involving the basement and deforming a sedimentary pile in excess of 14km.Even though the line is not perpendicular to the tectonic transport direction,the structures still show a height of more than 5km.The bottom section shows the same seismic line with no vertical exaggeration.The small insert to the right of the interpreted section shows the cumulative thickness of sedimentary rocks in meters interpreted above the interpreted Upper Cambrian to Lower Ordovician unconformity.The thickness of the sedimentary pile was estimated in the central portion of the section to be more than 14.4km.The bottom cross-section has no vertical exaggeration.1e Tertiary and Quaternary rocks.2e Upper Cretaceous.3e Upper Paleozoic.4e Lower to Middle Paleozoic.5e Proterozoic plus crystalline basement or crystalline basement.6e Interpreted fault.7e Inferred fault.8e C2?ooding surface (see text for details).9e C1?ooding surface.10e Base of Carboniferous diamictites.11e Top Cambrian or top of the crystalline basement.12e Re ?ection Moho.
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183
178
was dated Upper Carboniferous (Rapalini et al.,2010)and the Ventania Fold Belt deformation involves Permian rocks.
Mesozoic extensional events were also strongly in ?uenced by the Proterozoic and Upper Paleozoic structural features,which controlled the loci of the crustal thinning during initial rifting and the preferred area of breakup (Fig.13c).Two non-juxtaposed rifting events are interpreted to have taken place:one such event with an onset during Lower to Middle Jurassic and associated to the widespread extension that accompanied the Karoo e Ferrar LIP at c.183Ma (Encarnación et al.,1996)that resulted in the formation of the Colorado and Salado basins;and a later event leading to ?nal breakup and opening of the Atlantic related to the Etendeka LIP at c.130Ma (Peate,1997).This interpretation envisages a time gap of some 50Ma between the rifting at Colorado and Salado basins and the ?nal opening of the South Atlantic.
The ?rst rifting event resulted mainly in continental-border orthogonal depocenters and was controlled by the location of the Ventania Fold Belt,the Permian orogen and the Pampean Belt in the case of the Colorado Basin,and by the Paleoproterozoic suture between the Buenos Aires and Tandilia terranes in the case of the Salado Basin.Therefore we interpret these basins as the result of tensional reactivation of Upper Paleozoic features in the case of Colorado Basin as previously suggested by Ramos (1996),and Proterozoic ones in the case of Salado Basin.Tensional and transtensional reactivation of Paleozoic to Lower Triassic compressive features has been widely interpreted and accepted as forming mechanism of South African basins,both intermontaine and offshore (Tankard et al.,2009;Stankiewicz et al.,2008;Lindeque et al.,2011).The rifting event that led to the breakup was entirely restricted to the Dom Feliciano Belt,and only in the northern portion of the study area did it extend into the Río de la Plata Craton via the formation of the Merín-Santa Lucia-Quilmes lineament.This observation is supported by the age of the Laguna Merín volcanic and intrusive rocks,which were dated younger than 133Ma,corresponding to the Parana-Etendeka volcanism (Cernuschi Rodilosso,2011).
Why did not the crustal thinning propagate into the Río de la Plata Craton?Although the control of the Ventania Fold Belt was documented,there is another factor to be taken into account when analyzing the distribution of the crustal thickness anomalies illus-trated by the Bouguer anomaly maps:the impossibility that the thermal anomaly affected the roots of the thick cratonic ’s more stable nuclei,which is the Tandilia terrane.In a regional analysis of the Santos,Campos and Spirito Santo basins in offshore
Brazil,
Figure 13.Interpretation of major basemen blocks and structural features of southwestern Gondwana during the Upper Paleozoic and Cretaceous.a e Con ?guration prior to Patagonia collision.The Pampean Belt resulting from the accretion of Pampia to the Río de la Plata Craton extends south to the Gondwana open margin.b e Upper Paleozoic,collision of Patagonia allochthonous terrane and development of the Permian orogen affecting the Pampean Belt,the Río de la Plata Craton,the Dom Feliciano Belt and Patagonia.The tectonic transport direction offshore is interpreted to be perpendicular to the fold axis and to gravimetric highs associated with structures detected to the seismic south of the Colorado Basin;onshore tectonic transport direction after Rapalini et al.(2010)and references therein.Note the trajectory of the orogenic front and the Colorado Syntaxis controlled by the buttressing effect of the Río de la Plata Craton.c e Location of Jurassic (?)to Cretaceous crustal thickness anomalies interpreted by integrating geologic,seismic and gravity data.Note the strong control of pre-existing features on their distribution and extent.The cratonic block is only affected where the 2.1Ga suture is interpreted (see text for further details).The remaining anomalies are restricted mainly to the Pampean Belt,the Permian orogen core and the Permian orogenic front.d e Jurassic (?)to Cretaceous sedimentary basins.The northeastern area of the Patagonia terrane is the locus for small basins not related to crustal thickness anomalies.This could re ?ect the reactivation of Upper Paleozoic orogen features during extension.Note the development of the 70?3Ma volcanic ?eld above the lineament off-setting the eastern and central Colorado depocenters.All information was carefully georeferenced.Projection is UTM 21S,datum WGS 1984.
F.Pángaro,V.A.Ramos /Marine and Petroleum Geology 37(2012)162e 183179
Zalán et al.(2011)postulate,based on their interpretation of deep imaging seismic lines,that the propagation of thermal anomalies and the related crustal thinning into cratonic nuclei is hampered by the existence thick and stable crust.Based on similar criteria,the Colorado Basin’s eastern depocenter was restrained from propa-gating westwards by the presence of the Río de la Plata Craton, which acted as a buttressing block.As a consequence,a shift in the location of the crustal thickness anomaly is generated,probably through a pre-existing Dom Feliciano weakness zone(Fig.13c and
d)that controlled the Upper Paleozoic deformation as well.
9.Integration into the southwestern Gondwana context
All of the data that were gathered and interpreted were entered into a geo-referenced database to allow pre-breakup reconstruc-tions with publicly available software(https://www.doczj.com/doc/2911934622.html,)(Fig.14). To avoid noise,South African and South American plates were restored as rigid blocks to134Ma.The result is a pre-breakup restoration that bears an incipient opening south of the latitude of Cape Town.The only constraint for the restoration was the matching of the G anomaly in the study area;this led to an overlap in the north that is beyond the scope of this analysis.For quality control,the results were contrasted with those of Moulin et al. (2010),with satisfactory results;further reconstructions that consider intraplate deformations are in progress.
The shape and symmetry of the Cape and Colorado syntaxes are both controlled by the geographical extension of the Paleo to Mesoproterozoic Río de la Plata and Kalahari cratonic blocks.The northerly offset of the Upper Paleozoic orogenic front,the Cape-Ventania Fold Belt on both sides of the Atlantic,is strikingly consistent.
The Bouguer anomaly and residual Bouguer anomaly maps of South Africa show a great deal of similarities with their South American counterpart.South of the Cape Fold Belt,in the South African offshore,the gravimetric fabrics and the sedimentary basins are WNW-oriented.On the opposite side,the Patagonia terrane displays a NNW-oriented fabric and similarly oriented small basins controlled mainly by mechanical subsidence,such as the Rawson and Valdés.The intermontaine Upper Jurassic to Cretaceous sedi-mentary basins of southern South Africa and correlative basins off its southern coast have been interpreted as extensional reactivation of Upper Paleozoic to Lower Triassic compressive features(Paton et al.,2006;Broad et al.,2006;Tinker et al.,2008;Stankiewicz et al.,2008);in this context the trend of gravity anomalies inter-preted in the offshore portion of the Patagonia terrane and that correlate with the orientation of relatively small sedimentary basins could represent an analog to the well documented exten-sional reactivation process in South Africa.Therefore the defor-mation Belt related to the accretion of a continental block during Upper Paleozoic times is interpreted through the southern South African shelf,the Argentinean continental shelf south of the Colo-rado Basin,and into the onshore in the area between Patagonia and the Río de la Plata Craton,where the Upper Paleozoic compressive deformation is constrained by metamorphic age(Tickyj et al.,1997; Rapalini,2005;Rapalini et al.,2010).The angular separation of25 between northeastern Patagonia and the South African southern continental shelf gravimetric fabrics might be the consequence of the incipient opening at the time of the restoration or of intraplate deformation.When analyzed along with the G anomaly on both sides,these fabrics should provide a good piercing point for future work on intraplate deformation quanti?cation during breakup.
The buttressing effect of the Río de la Plata and Kalahari cratonic blocks deeply controlled the northerly expansion of the Permian deformation related to the accretion of Patagonia.The non-deformed area between the frontal Fold Belt and the core of the orogen has not yet been analyzed in detail,but a preliminary interpretation is the development of an extended deep detachment transferring deformation300km northwards.This detachment might not have developed in the cratonic blocks due to rheological constraints associated with different depths of brittle e ductile transitions with respect to the Dom Feliciano Belt and the corre-sponding Gariep Belt.Therefore we envisage a Jura-style defor-mation front for the offshore Ventania Fold Belt east of the Colorado Syntaxis.This interpretation is supported by similar
conclusions
Figure14.Paleogeographic restoration at134Ma.Note the development of the Colorado and Cape syntaxes and their outstanding symmetry.Their existence is interpreted to be a consequence of the cratonic blocks’buttressing effect.The Permian orogen,as interpreted in northeastern Patagonia,has its African counterpart in the Saldana Belt.The gap between the main orogenic area and the offshore Ventania Fold Belt is interpreted as a consequence of the development of a regional detachment in the Dom Feliciano Belt.Damara Belt,Gariep Belt and Kalahari Craton boundaries after Gray et al.(2008).SB e Saldania Belt.
F.Pángaro,V.A.Ramos/Marine and Petroleum Geology37(2012)162e183
180
provided by Lindeque et al.(2011)who,based on wide angle seismic,propose that the Cape Fold Belt e Karoo Basin may represent a thin skinned Jura-type Fold Belt.
The impact of the Proterozoic and Paleozoic inheritance on the Mesozoic extension was signi?cant.The General Levalle and Mac-achín episutural basins formed in the Pampean Belt and in the transition to the northernmost branch of the Upper Paleozoic orogen,while the western Colorado Basin formed in the Upper Paleozoic orogen.The Salado Basin formed in a2.1Ga suture.The Colorado central and eastern depocenters developed orthogonal to the Dom Feliciano fabric and were deeply controlled by the Permian orogenic front.The external Colorado depocenter evolved parallel to the continental margin and was likely controlled by Dom Feliciano inheritance.
10.Concluding remarks
A revised basement block distribution for onshore and offshore central Argentina was developed,and the offshore prolongation of the Upper Paleozoic orogen associated with the collision of Pata-gonia was mapped for the?rst time.
The southwestern Gondwana margin of Argentina prior to the Patagonia collision is envisaged as open and formed by the Dom Feliciano Belt,the Río de la Plata Craton,the Pampean Belt and the Pampia and Cuyania terranes.This con?guration resulted in an inhomogeneous collisional front,and the contrasting rheology between the cratonic block and the Dom Feliciano Belt led to the formation of the Colorado Syntaxis,a150km indentation of the orogenic front that mirrors the Cape Syntaxis in South Africa. Prominent structures were mapped in the offshore portion of the Ventania Fold Belt and interpreted to represent the orogenic front of a hundreds of kilometers wide orogen,the relief of which is visible in seismic lines underneath the Mesozoic Paleozoic unconformity.
The Mesozoic basins indicate structural control on their emplacement,re?ecting a strong Proterozoic and Paleozoic inher-itance.In particular,the Colorado Basin is presented as a segmented,aborted rift dominated by simple shear along south dipping discontinuities and prominent axial crustal thickness anomalies.These discontinuities are interpreted as major compressive fault zones formed during the Upper Paleozoic colli-sional event.Through the correlation with well documented examples of South Africa,the Rawson and Valdés basins might also be interpreted as the result of the extensional reactivation of Upper Paleozoic compressive features in the core of the orogen.Contrary to previous models,strike slip deformation is not considered to be an important basin-forming mechanism.Further,a strong diachronic evolution is interpreted between the Colorado and Salado basins and the Atlantic breakup,the former being correlated to a Karoo LIP age equivalent rifting resulting in a circa50Ma time gap between them.
Acknowledgments
Petrobras de Argentina S.A.is acknowledged for supporting the research project that prompted the development of most of the concepts in this article.Dr.Pablo Pazos is acknowledged for the revision of early versions of this manuscript.This work was greatly improved thanks to the observations made by two anonymous reviewers.
References
álvarez,G.T.,2007.Extensión noroccidental de la cuenca paleozoica de Claromecó, provincia de Buenos Aires.Revista de la Asociación Geológica Argentina62(1), 86e91.Andreis,R.R.,Cladera,G.,https://www.doczj.com/doc/2911934622.html,s epiclastitas pérmicas de la cuenca Sauce Grande (Sierras Australes,Buenos Aires,Argentina),Parte II:Emplazamiento tectónico de lasáreas de aporte.In:4 Reunión Argentina de Sedimentología,vol.1.Actas, https://www.doczj.com/doc/2911934622.html, Plata.
Archangelsky,S.,1996.Capítulo4.Palinoestratigrafía de la Plataforma Continental.
In:Ramos,V.A.,Turic,M.A.(Eds.),13 Congreso Geológico Argentino y3 Congreso Exploración de Hidrocarburos(Buenos Aires),Geología y Recursos Naturales de la Plataforma Continental Argentina,Relatorio.Asociación Geo-lógica Argentina and Instituto Argentino de Petróleo,Buenos Aires,pp.67e72. Astini,R.A.,Ramos,V.A.,Benedetto,J.L.,Vaccari,N.E.,Ca?as,F.L.,https://www.doczj.com/doc/2911934622.html, Pre-codillera:un terreno exótico a Gondwana.In:Congreso Geológico Argentino, No.13y Congreso de Exploración de Hidrocarburos,No.3,Vol. 5.Actas, pp.293e324.Buenos Aires.
Balarino M.L.,2009.Palinoestratigrafía del Paleozoico Superior de la Cuenca Colo-rado,República Argentina y su correlación conáreas relacionadas.PhD thesis.
Directors:Gutiérrez P.R.and Ballent,S.Universidad nacional de La Plata.Fac-ultad de Ciencias Naturales y Museo,2009.Thesis N 1017.Abstract.
Bally,A.W.,Bernoulli,D.,Davis,G.A.,Montadert,L.,1981.Listric normal faults.In: Oceanologica Acta Supplement,vol.4,pp.87e102.París.
Bangert,B.,Stollhoffen,H.,Lorenz,V.,Armstrong,R.,1999.The geochronology and signi?cance of ash fall tuffs in the Glaciogenic Carboniferous e Permian Dwyka Group of Namibia and South Africa.Journal of African Earth Sciences29,33e50. Basei,M.A.S.,Frimmel,H.E.,Nutman, A.P.,Preciozzi, F.,2008.West Gondwana amalgamation based on detrital zircon ages from Neoproterozoic Ribeira and Dom Feliciano belts of South America and comparison with coeval sequences from SW Africa.In:Geological Society,London,Special Publications,vol.294, pp.239e256.
Broad,D.S.,Jungslager,E.H.A.,McLachlan,I.R.,Roux,J.,2006.Offshore Mesozoic basins.In:Johnson,M.R.,Anhaeusser,C.R.,Thomas,R.J.(Eds.),The Geology of South Africa.Geological Society of South Africa and Council for Geoscience, pp.553e571.
Cernuschi Rodilosso, F.,2011.Geology of the Cretaceous Lascano-East Intrusive Complex:Magmatic Evolution and Mineralization Potential of the Merín Basin, Uruguay.Department of Geosciences,Oregon State University,Wilkinson Hall 104,Corvallis,Oregon,USA.Masters thesis,Oregon State University.
Chebli,G.A.,Mozetic,M.E.,Rossello,E.A.,Buhler,M.,1999.Cuencas sedimentarias de la Llanura Chacopampeana.In:Caminos,R.(Ed.),1999.Geología Argentina,vol.
29.SEGEMAR,Anales,Buenos Aires,Argentina,pp.627e644.
Chernicoff,J.,Zapettini,O.E.,2002.Delimitación de los terrenos tectonoestrati-grá?cos de la región centro-austral Argentina:evidencias aeromagnéticas.
Revista geológica de Chile.dec.2003,vol.30,no.2,p.299e316.ISSN0716e0208. Chernicoff,J.,Zapettini,O.E.,2004.Geophysical evidence for terrane boundaries in South-Central Argentina.Gondwana Research7(4),1105e1117.
Chernicoff, C.J.,Zappettini, E.O.,Santos,J.O.S.,Godeas,M.C.,Belousova, E., McNaughton,N.J.,2012.Identi?cation and isotopic studies of early Cambrian magmatism(El Carancho Igneous Complex)at the boundary between Pampia terrane and the Río de la Plata craton,La Pampa province,Argentina.Gondwana Research21(2e3),378e393.
Cingolani,C.A.,2011.The Tandilia system of Argentina as a southern extension of the Rio de la Plata cratón:an overview.International Journal of Earth Sciences (Geol Rundsch)100,221e242.
Cook,F.A.,White,D.J.,Jones,A.G.,Eaton,D.W.S.,Hall,J.,Clowes,R.M.,2010.How the crust meets the mantle:lithoprobe perspectives on the Mohorovicic disconti-nuity and crust e mantle transition.Canadian Journal of Earth Sciences47, 315e351.https://www.doczj.com/doc/2911934622.html,/10.1139/E09-076.
Crovetto,C.B.,Novara,I.L.,Introcaso,A.,2007.A stretching model to explain the Salado Basin(Argentina).Boletín del Instituto de?siografía y geología77(1e2), 2007.
Dalla Salda,L.,Bossi,J.,Cingolani,C.,1988.The Rio de la Plata cratonic región of southwestern Gondwana.Episodes11(4),263e269.
Dalla Salda,L.,1999.Cratón del Río de la Plata,1.Basamento granítico-metamór?co de Tandilia y Martín García.In:Caminos,R.(Ed.),1999.Geología Argentina,vol.
29(4).Instituto de Geología y Recursos Minerales,Anales,pp.97e106.
de Almeida, F.F.M.,de Brito Neves, B.B.,Carneiro,C.D.R.,2000.The origin and evolution of the South American Platform.Earth Sciences Reviews50,77e111. de Beer,C.H.,1995.Fold interference from simultaneous shortening in different directions:the Cape Fold Belt syntaxis.Journal of African Earth Sciences21, 157e169.
Di Pasquo,M.,Martínez,M.A.,Freije,H.,2008.Primer registro palinológico de la Formación Sauce Grande(Pennsylvaniano-Cisuraliano)en las Sierras Australes, provincia de Buenos Aires,Argentina.Ameghiniana45(1),69e81.
Domínguez,R.F.,Marchal,D.,Sigismondi,M.,Espejón,C.A.,Vallejo,E.,2011.Car-acterización de dominios estructurales e in?uencia de estructuras preexistentes en hemigrábenes de rift en el sector centro-norte de la plataforma continental Argentina.In:XVIII Congreso Geológico Argentino,Neuquén.Abstracts on CD, pp.1415e1416.
du Toit,A.,1927.A Geological Comparison of South America with South Africa.In: With a Paleontological Contribution by F.Cowper Reed,vol.381.Carnegie Institut of Washington,Publication,Washington,158p.
Encarnación,J.,Fleming,T.H.,Elliot,D.H.,Eales,H.V.,1996.Synchronous Emplace-ment of Ferrar and Karoo Dolerites and the Early Breakup of Gondwana: Geology,vol.24.https://www.doczj.com/doc/2911934622.html,/10.1130/0091-7613(1996)024<0535:SEO-FAK>2.3.CO;2,pp.535e538.
Escher,A.,Hunziker,J.C.,Marthaler,M.,Masson,H.,Sartori,M.,Steck,A.,1997.
Geologic framework and structural evolution of the western Swiss-Italian Alps.
F.Pángaro,V.A.Ramos/Marine and Petroleum Geology37(2012)162e183181