Abstract The continental lithosphere of southwestern Gondwana, comprising the southern part of South America and southern Africa, was largely assembled before the end of the Proterozoic. Geologic studies indicate that the basement anisotropy that controlled the development of Phanerozoic basins was established by Neoproterozoic–Early Cambrian tectonism. This tectonism reactivated the older terrane boundaries or cut across them. The backbone linking the system of Pan-African and Brasiliano basins was a system of northeast-trending structures. There were four areas of pronounced Neoproterozoic–Early Cambrian basin subsidence in the study area: the Chiquitanas trough in Bolivia, the Puncoviscana basin in Argentina, the Dom Feliciano-Ribeira basins in Brazil, and the Damara-Nama basin complex of southern Africa.

Abstract More than 4500 wildcat wells were drilled from 1980 to 1990 in South America. Approximately 355 of these resulted in hydrocarbon discoveries. An estimated 12% of the discoveries contain reserves greater than 100 MMBO. Several of the larger finds (>500 MMBO), such as Cusiana (Colombia), Funial-Musipan (Venezuela), Caño Limón (Colombia), and Marlim (Brazil), are important among the giant fields found worldwide since 1980. Most of the larger discoveries were made by national oil companies in Venezuela, Mexico, and Brazil. The probability of finding large oil fields (>500 MMBO) is greatest in the Campos, Llanos, Reforma-Campeche, Maracaibo, and Maturin basins. Smaller, but still significant fields (50-250 MMBO) may still be found in the Neuquén, San Jorge, Austral, Tarija, Marañon-Napo-Putumayo, Magdalena, and Tampico-Misantla basins. More than 170 BBO of proven reserves have been found in the highest potential Latin American basins. Undiscovered oil resources of 40-80 BBO are estimated to remain in the same group based on historical field size data and current geologic knowledge. Frontier and emerging basins may also contain significant resources, but limited data makes it difficult to estimate their undiscovered potential.

Abstract From the Cambrian to Late Jurassic, the basins and arches of southern South America were oriented approximately north-south. Subsequently, southwest-northeast trending stresses related to the breakup of Gondwana and the opening of the South Atlantic imposed new structural alignments, the effects of which were widespread. These two tectonic regimes encompass six stages of basin formation: Cambrian-Middle Devonian terrigenous clastics, carbonates, and intrusives occurred along the western edge of the Brazilian, Puna, and Pampas shield areas. Carboniferous–Late Jurassic sedimentation in the intracratonic rifts was mainly of continental origin. Marine clastics accumulated in a foreland basin in front of a volcanic arc on the western edge of the continent. This period ended with the Late Jurassic breakup of Gondwana and extrusive magmatism. Late Jurassic extension marked by widespread marine flooding affected vast regions of the continent. Clastics, evaporites, and acidic volcanics constitute the “Andean foreland” succession which was associated with a volcanic arc system. In Patagonia, acidic volcanics covered the North Patagonia massif and Deseado craton and earlier basins. After breakup, sedimentary prisms formed along the Atlantic margin on the western margin. Thick sequences of shales, limestones, evaporites, and pyroclastics were associated with middle–Late Cretaceous volcanic arc and back-arc settings. New acidic intrusions and the Andean batholith are also dated to this period. The Late Cretaceous–early Tertiary (Laramide) was marked by development of the Andean fold and thrust belt and final emplacement of the Andean batholith. A flexural foreland basin formed in front of the Andes. Passive margin sedimentation dominated the eastern margin. The Tertiary was a time of Andean mountain building and passive margin subsidence. The thrust belt supplied thick sedimentary fills to the foreland basin. Shallow transgressions covered much of Patagonia and the Pampa plains. This tectonic evolution is expressed in a complex array of composite basins. These tectonic, structural, and depositional patterns were also responsible for a suite of petroleum systems, many of them commercially significant.

Abstract Calibrated geohistory analyses of single data points and cross sections are used to construct tectonic subsidence curves. There are a limited number of distinct curve types, of which seven end-members are discussed. These curve types and their shapes are genetically related to plate tectonic processes. One regional backstripped cross section across South America is discussed. The tectonic subsidence curves are used as calibration for a paleogeographic analysis of Gondwana in the Phanerozoic. It is suggested that the southern part of South America and parts of Antarctica are composed of terranes that were formerly located west of the present position of the Arequipa massif in Chile and Peru. These displaced terranes formed the western edge of Gondwana from the Cambrian-Ordovician to the end of the Devonian. During these periods, they were involved in intracratonic rifting. During the time of the basal Carboniferous unconformity, the terranes were translated southward to approximately their present positions. Seven reconstructions are presented that are representative of the major episodes in the development of Gondwana.

Abstract Palinspastic paleogeographic maps of western and northern South America, including the entire 8500-km “Andean system” from Trinidad to Cape Horn, are presented for nine Mesozoic-Cenozoic time intervals. The maps show (1) the spatial record of formational lithostratigraphic units; (2) continental, shallow marine, and deeper marine paleoenvironments and the location of active magmatic arcs through time; (3) progressive structural and tectonic development; (4) relative motions of adjacent plates affecting the Andes; and (5) paleolatitude. Phases and causes of geologic development are summarized from the maps and other information. Depositional systems are related to tectonic evolution, with implications drawn for hydrocarbon systems and history. It is shown that tectonic, depositional, and hydrocarbon histories are closely interrelated, having occurred in fairly discrete pulses through time, each with its own significance to hydrocarbon potential.

Abstract A major characteristic of the oil industry is that geologic data tend to be kept within each company. In South America, where state-owned companies and exploration monopolies predominate, there are many datasets and many different interpretations. These “virgin” data constitute a valuable basis for assembling a correlation study across the continent. The authors of this paper are making probably the first attempt to put together extensive, previously unavailable information and present it to the geologic community. The geology of Gondwana is strikingly different from the geology of Laurasia. Gondwana basins tend to be dominated by siliciclastics, whereas northern hemisphere basins are rich in limestones. This largely reflects the cold climate that predominated in Gondwana during most of the Paleozoic. Thick diamictites and sandstones deposited during glaciation in the Carboniferous–Permian are widespread in the Gondwana of South America. These sandstones are the principal reservoirs in the Bolivian and northwestern Argentinian gas fields. Gas and condensate in these fields are sourced from the underlying Devonian black shales. A regional unconformity on top of the Devonian shales played an important role in oil migration into overlying beds in Bolivia and Argentina. Structural traps created during the Hercynian orogeny, and later during the Andean orogeny, were also important for hydrocarbon accumulation near the Cordillera de Los Andes. A huge area east of the Andes is still a frontier area. This includes the Paraná and Chaco-Paraná basins in Brazil, Paraguay, Uruguay, and Argentina, covering an area larger than 1.7 million km 2 . There are fewer than 200 exploration wells drilled in these basins.

Abstract Although glaciated basins are usually associated with nonproductive, poorly sorted strata, hydrocarbons occur in several late Paleozoic glaciated basins of central and southern South America. In Bolivia, the Chaco-Tarija basin has commercial production from more than 30 fields in glacially influenced submarine channel systems (Palmar, Santa Cruz, and Bermejo fields) that accounts for about 60% of current national reserves. Correlative deposits in Argentina host the Campo Duran and Madrejones oil fields. In Brazil, the Paraná basin has significant but as yet subcommercial gas shows in thick marine turbidite sandstones of the glacially influenced Itararé Group. The Chaco-Paraná basin of Argentina is one of the largest onshore targets for exploration in South America, but it is virtually untested. Glacially influenced foreland basins of Argentina (Tepuel and Paganzo-Maliman) contain complex glacigenic stratigraphies of interbedded tillites and poorly prospective sandstones. In contrast, the glacially influenced marine infills of intracratonic basins in Brazil (Paraná), Bolivia, and Argentina (Chaco-Tarija and Chaco-Paraná) contain thick sequences of pebbly mudstones and regionally extensive reservoir quality sandstones. The key to the occurrence of good reservoirs and associated trapping mechanisms in these intracratonic basins is the interplay of sediment supply, regional tectonics, and relative sea level changes. Glacial scouring of extensive cratons by ice sheets resulted in the delivery of huge volumes of glaciofluvial sand to deltas. Structural control of drainage patterns on the craton by basement lineaments resulted in persistent sediment sources and depocenters. Frequent earthquake activity along reactivated basement lineaments resulted in downslope mass flow of deltaic sediments and the deposition of thick, amalgamated sand turbidites (reservoirs). Pebbly mudstone seals most likely record higher relative sea levels, resulting from basin subsidence, and deposition from suspended sediment plumes and icebergs. Source rocks are provided by Devonian and Permian shales. This model may be applicable to other parts of Gondwana that contain thick, prospective sandstones in glacially influenced intracratonic basins.

Abstract This study of the Chaco basin is based on field studies of outcrops and on exploration data. The Chaco basin covers 246,725 km 2 of western Paraguay and consists of several depocenters or subbasins, each with a unique tectonostratigraphic record. In the northwest, the Curupaity and Carandaity subbasins contain a well-developed Paleozoic succession. In contrast, Mesozoic subsidence was marked in the southern Pirity and Pilar subbasins and in the shallow Bahia Negra platform and San Pedro low to the east. These depocenters are separated by structural highs. Uppermost Proterozoic-Recent sedimentary sequences are present in the Chaco basin. The subsidence history of the Chaco basin is recorded in four major unconformity-bounded sequences. Northwest- and northeast-oriented structural lineaments of Eocambrian Brasiliano origin controlled the patterns of subsidence. Mesozoic extensional tectonics related to the opening of the South Atlantic reorganized the structural pattern of the Chaco basin; this episode is expressed in a system of half-grabens. Cenozoic Andean orogenesis imposed the final structural readjustment and established the Chaco area as a modem foreland basin. Upper Devonian marine shales and Upper Cretaceous shales and carbonates are the primary source rocks for hydrocarbons. The principal reservoir zones are Carboniferous channel sandstones in the Curupaity and Carandaity subbasins and Stratigraphie and structurally controlled sandstone reservoirs of Mesozoic age in the Pirity subbasin.

Abstract The geologic evolution of Bolivia and the central Andean system during the past 500 m.y. was largely controlled by the geodynamics of the South American margin of western Gondwana. The Phanerozoic strata were deposited in mainly marine environments until the Early Triassic, after which continental environments predominated. However, there were six restricted marine transgressions in the Late Cretaceous-Danian and one in the late Miocene. The Late Cambrian–Early Ordovician margin was initially a passive margin. It became an active one during a Middle Ordovician compressional episode and was controlled by large-scale transtensional or transpressional conditions from the Late Ordovician to the Triassic. The Late Ordovician–Mississippian evolution was characterized by vigorous subsidence of the marine foreland, which was filled with thick, shallowing-upward sequences showing northeastward onlaps. Ashgill and latest Devonian–Mississippian glaciomarine and fluctuating sea level processes are recorded in the succession. Shallow marine carbonates, marls, and sandstones, as well as some evaporites and eolianites, were deposited during Pennsylvanian–Early Triassic time. After Middle Triassic rifting was aborted, the Bolivian basin behaved in a cratonic way until it was caught up in the Andean system due to the onset of transtension along the margin in the Late Jurassic. It became part of the Andean foreland domain in early Senonian time. Andean thrust deformation propagated into Bolivia from the west in the late Oligocene and progressed eastward through Neogene time. Organic-rich units correlate with Paleozoic highstand deposits and younger transgressions. Generation, migration, and trapping of hydrocarbons depended mainly on Cenozoic sedimentary burial and tectonic loading and hence on propagation of Andean deformation.

Abstract Devonian-Permian data of western Bolivia and adjacent regions are used to construct a paleogeography of the central Andes. Four phases characterize the sedimentation history. (1) Shallow marine clastic deposition occurred through the Devonian (Lochkovian-Frasnian), with an increase in sedimentation in Emsian-Eifelian time. Lithofacies distribution and sediment thicknesses indicate primarily a western source. Endemic, high-latitude (>55° S) fauna with several megafaunal originations in Bolivia also contain organisms characteristic of North Africa and northeastern United States (Middle Devonian megafaunas and Late Devonian palynomorphs). (2) Latest Devonian-Early Carboniferous (Famennian-Viséan) sedimentation is characterized by glaciomarine and fan-deltaic sedimentation. Clasts are derived from underlying sedimentary units and andesitic, granitic, and tuffaceous rocks. (3) A middle Carboniferous (Serpukhovian-Bashkirian) hiatus in sedimentation occurred, its age and duration varying across the region. (4) Söiciclastic and carbonate deposition occurred in Late Carboniferous-middle Permian time (Moscovian?-Artinskian). The clastics were derived from a western source. Carbonate rocks (Copacabana Formation) were deposited in situ, in low latitudes (< 25° S lat). Devonian sedimentation is inferred to have occurred on continental crust in a foreland setting, with a western magmatic arc. Restoration of the San Nicolas batholiths (U-Pb zircon, 425 and 394-388 Ma) relative to the Devonian basins suggests that they may have constituted the magmatic arc. Intra-arc basins may have existed near present-day coastal Peru. Following the middle Carboniferous hiatus, sedimentation continued in a back-arc region, although differentiation of distinct Carboniferous and Permian basins, the intrusion of plutons inboard of and within the basin along strike, and extensional faulting in the Late Permian indicate major changes in the tectonic setting, possibly including a reorientation of the subducting slab to a low angle. Uncertainties in the tectonic setting interpretation are introduced by the incomplete Stratigraphie record, which is obscured in the Altiplano and Cordillera Occidental, and by the undefined history of plate boundary interactions, such as possible postdepositional strike-slip motion and tectonic erosion along the plate margin.

Abstract Northwestern Argentina has undergone a complex tectonosedimentary history. From Silurian through Late Jurassic time, sedimentary sequences were deposited in intracontinental basins. The beginning of this stage is linked to a first-order orogenic event, the Ocloyic phase. Above the Ocloyic unconformity, 6 km of sedimentary rocks were deposited during the Silurian-Jurassic interval. This record consists of two major unconformity-bounded sequences separated by the Lower Carboniferous Chanic unconformity. The first sequence, spanning the Silurian-Devonian, contains predominantly marine facies arranged in cycles of different hierarchies that were deposited in an epeiric flexural basin related to the Ocloyic orogen. The less intense Chanic orogeny changed the subsidence style and combined flexural and thermal causes. The low average sea level and variable climatic conditions during the late Paleozoic and early Mesozoic controlled the environmental characteristics of this second sequence, which is represented by predominantly continental facies of glacial and arid origin. This stage of intracontinental evolution ended with rupture and fragmentation of the basins in response to Cretaceous rifting, which marked the Gondwana break-up. Several of the units deposited during the studied interval produce oil or gas that were generated in the Upper Devonian marine shales. The complex burial history led to the variable thermal maturity of these source rocks. One of the most important control factors of commercial accumulations here is source rock availability because the Devonian rocks were affected by erosive cycles of different ages.

Abstract Several superimposed tectonic stages distinguished by varying structural styles are recognized in the Andes of northern Argentina (22-28° S lat). The oldest structures occur in the Precambrian crystalline basement. This basement forms the central core of the region and is made up of several multiply deformed belts. These belts were intruded by several generations of granitoids and were amalgamated during the Panamerican orogeny (Late Brazilian orogeny, 700-600 Ma). A westward-vergent foldbelt containing Ordovician marine sediments shows eastward-dipping axial plane cleavage. It lies along the western border of the crystalline core and acts as host rock to pretectonic intrusives. Development of the folding is assigned to the Late Ordovician Ocloyic orogeny. The sub-Andean ranges and Puna Silurian-Devonian successions were folded during the Late Devonian-Early Carboniferous at the beginning of the Gondwanan cycle (Chanic orogeny). This tectonic cycle is represented in several late Paleozoic basins that surround the study area. The inversion of those basins probably took place during the middle Permian San Rafael orogeny. The Andean cycle commenced with the opening of rift troughs filled with thick continental deposits during Early Cretaceous-Eocene time. The inversion of these troughs began with the late Eocene Inca movements, but was completed during the Miocene Quechua and Pliocene-Pleistocene Diaguita orogenies. From late Oligocene time onward, continental basins developed, and an extensive Miocene-Pleistocene volcanic arc originated on the western flank of the study area. These Cretaceous-Cenozoic basins were inverted by the Diaguita orogeny. Andean tectonics caused clearly differentiated morphostructural units. The most westerly of these is the Puna Plateau, characterized by Precambrian basement and Paleozoic rocks sheets that were thrust over Tertiary continental successions. East of the Puna, the Eastern Cordillera represents a tectonic stack of Precambrian basement and Paleozoic rock sheets thrust eastward over the sub-Andean ranges. This latter belt forms the outermost unit, made up of large faulted anticlines. South of 27° S lat, a change occurs in the architecture of the Andean foreland. The sub-Andean ranges and the Eastern Cordillera are replaced by faulted blocks of Precambrian crystalline basement and Paleozoic granitic intrusions, which form the Pampean ranges. This paper summarizes the evolution of the oil-bearing basins of northern Argentina.

Abstract The Carboniferous-Permian Paganzo succession straddles the Pampeanas, Precordillera, and Chilenia terranes. Late Devonian-Early Carboniferous diastrophism of the Chanic event separated very different early and late Paleozoic histories of basin formation. The Paganzo basin was initiated in the Visean by reactivation of old terrane boundaries. The early Paganzo consisted of a suite of discrete fault-controlled depocenters interpreted as transtensional pull-apart basins linked to right-lateral displacement along major crustal faults. Younger phases of basin formation were characterized by amalgamation of these various depocenters into a single broad basin. The Paganzo succession is divided into four supersequences by major hiatuses. These are the Guandacol, Tupe, and lower and upper Patqula-De la Cuesta supersequences. Each is constructed by stacked unconformity- bounded depositional sequences. These four supersequences record the various stages of basin evolution. The Guandacol sediments were deposited in isolated basins. Fieldwork shows a pattern of rapid subsidence and stacking of coarse alluvial facies along basin-bounding faults. The characteristics of the finer grained strata indicate a periglacial influence. The overlying Tupe supersequence suggests a gradual cessation of fault activity as the various depocenters were yoked together. Tupe stratigraphy onlaps the Guandacol-Tupe unconformity and buries some of the previous interbasin highs. In Patqula-De la Cuesta time, the Paganzo basin had widened to its maximum extent. Significant transgressions are recorded in Tupe (Westphalian–Stephanian) and Patqula-De la Cuesta (Artinskian and Kazanian) stratigraphy. Extensive geochemical studies show that Patquia source rocks are oil prone. Although indications are that the Paganzo basin is prospective, it remains largely untested. Regional studies show that the strike-slip faults that controlled Carboniferous basin development in northwestern Argentina diverge northward where they become involved in the Chaco salient of the Bolivian Andes. The Tupambi-Tarija and Escarpment sequences of Bolivia are broadly contemporaneous with the Guandacol and Tupe stratigraphy of the Paganzo basin. They share similar depositional characteristics typical of rapidly subsiding transtensional basins, including stacked alluvial facies, thick debris flow diamictites, massive soft sediment deformation, and dewatering structures. The Escarpment Formation represents an expansion of the earlier Tupambi-Tarija depocenters and contains an anastomosing drainage system.

Abstract The southern Altiplano rift basin forms part of the Cretaceous rift system that extends from Peru to northwestern Argentina. Seismic and potential field data suggest that basin formation was controlled by a preexisting structural grain consisting of northwest-southeast and northeast-southwest tectonic lineaments within a Precambrian-Paleozoic basement. Variable extension was achieved through transfer fault zones that coincide with northwest-southeast trending lineaments. Rift subsidence can be divided into an early rift (Berriasian- Cenomanian) and a late rift episode (Turonian-Campanian) with a total accumulation of up to 1000 m of sandstones and shales. A period of tectonic quiescence followed that resulted in a regional sag basin characterized by thin-bedded, calcareous lacustrine deposits. This sequence includes the oil-prone shales and reservoir carbonates of the El Molino Formation, which has been correlated with the hydrocarbon-producing Yacoraite Formation in northwestern Argentina. At the beginning of the Tertiary, rift-related subsidence had ceased and was gradually replaced by subsidence resulting from Andean compression. An Eocene unconformity marks the subtle change to this new episode of basin formation. This was followed by a major Oligocene unconformity that characterizes the onset of increased subsidence rates related to the emplacement of large thrust sheets of the Eastern Cordillera. More than 4 km of synorogenic sediments accumulated in the adjacent Altiplano foredeep. Inversion of the Cretaceous rift structures took place during this compressional phase. In the southern Altiplano, this inversion resulted in hydrocarbon traps similar to those in the rift basins of northwestern Argentina.

Abstract The foothills of the central Andes of northwestern Argentina hinder the interpretation of the complex structural rift system developed during late Mesozoic extension. Andean compressive deformation inverted the Salta rift system, resulting in a series of complex structures with trends oblique to the main Andes. The Lomas de Olmedo basin, a failed branch of the rift system located east of the Andean orogenic front, was selected to undertake deep reprocessing of the available industrial seismic lines. A 150-km-long seismic section of the basin, recorded with Vibroseis and dynamite sources, was reprocessed. Extended correlation applied to the Vibroseis seismic data yielded reliable results down to 9 sec two-way travel time. Acoustic horizons identified within this interval include the deepest synrift deposits in the axial part of the basin and a deep oblique discontinuity in the crust. On this basis, a complete cross section of the basin was made. This study documents the asymmetry of the rift, with a prominent zone of thermal uplift in the northern edge. Truncation of the Paleozoic beds and identification of a deep oblique discontinuity at 7–8 sec (18–21 km deep) suggest that a northward-dipping detachment controlled the asymmetry of the system. The rift structure is mildly modified by folding related to Cenozoic tectonic inversion in the southern sector of the basin. This inversion was controlled mainly by strike-slip displacements along the previous normal faults.

Abstract TWo north-trending, west-verging, fault-bounded Neogene basement uplift systems (Sierras Chicas of Cordoba and Serramas Occidentales of San Luis) of the Sierras Pampeanas of central Argentina are inverted Early Cretaceous rifts. Their geometry and position 2000 km from the Atlantic continental margin and the geometry of Neogene inversion is dependent on the earlier fabric of the basement rocks. The trends of reactivated faults in the rifts are consistent with an Early Cretaceous extension direction orthogonal to the Atlantic spreading center. The principal north-northwest rift trends were produced by dextral-oblique rifting along previous basement sutures, and isolated depocenters may have formed as transtensional pull-apart basins. The Sierras Chicas are the easternmost of the Pampean uplifts. They were uplifted along the eastward-dipping Punilla thrust fault zone. Three Cretaceous depocenters containing two depositional megasequences and volcanics are preserved as remnants of a larger basin. The sediments were deposited in restricted half-grabens dominated by alluvial fans and playa lakes. Paleocurrent analyses indicate that the Punilla fault was a normal fault during deposition. Neogene inversion of normal fault trends thrusted proximal fanglomerates over their former source terrain. Cretaceous rocks on the hanging wall of the Punilla fault zone were folded into a west-verging monocline in the Sierra de Pajarillo area. The steep limb of the monocline is underlain by a fault-bounded wedge of cataclastically deformed basement rocks. The Serramas Occidentales of San Luis are similar to the Sierras Chicas of Cordoba. Depositional environments are similar, and fault-bounded depocenters can be identified within the larger Cretaceous San Luis basin. The Cretaceous normal faults follow basement fabric. Neogene inversion of the Serramas Occidentales produced short-cut faults and back-thrusts, a vertical thrust-bounded Cretaceous section (at Sierra Quijadas), and a dramatic change of trend (north-northwest to northeast) in the basement thrust faults (Sierra del Gigante).

Abstract Most of the hydrocarbon reserves in the Cuyo basin are contained in 15 oil fields that are mainly structurally controlled. They include several pools in the 40 million m3 range. Tectonic analysis based on seismic data tied to well control suggests that most closures relate to folds and reverse faults that are genetically associated with an earlier extensional fault system that developed during the early Mesozoic collapse of a late Paleozoic orogenic belt. Reconstruction of the source rock (Middle Triassic Cacheuta Formation) and reservoir paleogeography (Triassic-Tertiary Potrerillos, Rio Blanco, Barrancas, and Papagayos formations) indicates that synsedimentary extension and differential subsidence were key factors that induced an irregular distribution of organic-rich strata and porosity development. Cenozoic contraction linked to Andean orogenesis inverted the Triassic half-grabens and created structural closures. This resulted in local reservoir enhancement and access to effective charge after late Cenozoic regional migration. Prospective closures consist of elongate, irregularly spaced to en echelon anticlines and plunging noses. Axial surfaces display eastward or westward vergence, and shallow folds are replaced at depth by faulted structures (e.g., Tupungato, Barrancas, La Ventana, and Río Tunuyán fields). The roots of the structural highs involve Stratigraphie depocenters and high-angle faults that show normal separation at depth and reverse separation at intermediate levels (e.g., Vizcacheras field). The amount of inversion decreases from west to east, and in the most deformed areas, the cores of the folds were penetrated by faulting and popped up structures shaped as bivergent thrust wedges bounded by master faults and converging back-thrusts.

Abstract The Malargüe fold and thrust belt formed by Mesozoic rift inversion during Tertiary compressional orogeny. Mesozoic extension created the Neuquén basin in west-central Argentina and controlled most structural styles and the geometry of the fold and thrust belt, which is characterized by basement-cored oppositely verging structures. Subsidence curves and palinspastically restored isopach maps of Mesozoic sedimentary fill describe a complex pattern of asymmetric half-grabens bounded by major faults of opposite polarity and accommodation zones related to a rift phase during Late Triassic-Early Jurassic time, as well as Middle Jurassic-Early Cretaceous postrift regional subsidence. Balanced cross sections show the relationship between preexisting extensional fabrics and contractional basement-involved thrusts and back-thrust structures that generated the half-graben inversion. Overpressured shales and three evaporite levels favored formation of duplexes, triangle zones, and detachment of the cover as a result of basement-involved shortening.

Abstract The tectonic evolution of the Neuquén basin spans about 220 m.y. of Mesozoic-Cenozoic subsidence. Initial rifting in the Triassic was driven by extensional collapse of the Permian-Triassic orogen. This period of extension was accommodated by inherited structural inhomogeneities and a southwest-oriented extensional stress field. From the Aalenian onward, fault-controlled subsidence was replaced by regional subsidence. Several episodes of structural inversion modified the shape of the depocenter and rejuvenated fringing sedimentary source areas. The most significant inversion occurred in the late Oxfordian-earliest Kimmeridgian when the Dorsal de Huincul was formed. This Late Jurassic diastrophism marks a fundamental reorganization of extensional stress fields related to fragmentation of southwestern Gondwana and the Atlantic opening. Late Jurassic-Cretaceous extension was northwest directed. This history of tectonic evolution is reflected in a complex structural framework, at least two major hydrocarbon source rock intervals, and numerous reservoir zones.