ABSTRACT The last stratigraphic and structural assessments of the Ene Basin (Peru, Block 108) defined a prospective petroleum system. Over the past 50 years, this basin has been studied by a number of oil companies and scientists. Surface geology and ~750 km (~460 mi) of 2-D seismic sections have provided most of the information, since no exploratory well has been drilled to date. The presence of well-known source and reservoir rocks, several hydrocarbon manifestations at the surface, and large anticlines define two main plays, currently making the Ene Basin the frontier basin with greatest exploration potential in Peru. The Ene Basin is part of the Peruvian sub-Andean system developed ~300 km (185 mi) east from the Pacific trench, cratonward of the Eastern Andes. As shown by their common stratigraphy, the evolution of this intermontane basin was related to the southern Ucayali and Camisea Basins until they were separated by the uplift of the Otishi and Shira basement blocks during the Andean orogeny. The Ene Basin is divided into two main structural domains based on their different mechanical stratigraphy that imprinted contrasting structural styles: (1) the northwestern domain displays marked stratigraphic similarities with the northerly Pachitea Sub-Basin, being characterized by a thick Mesozoic succession, salt domes, and a deformation style related to the inversion of the southeastern rim of the Triassic–Jurassic Pucará extensional basin; (2) the southeastern domain is affected by thin-skinned structural deformation and exhibits a similar stratigraphy than the easterly Ucayali Basin, characterized by thin to absent pre-Cretaceous Mesozoic units and variable Cretaceous–Paleozoic unconformable relationships. Contrasting structural styles and the uneven distribution of shortening are related to differing degrees of interaction between the two main structural domains and the surrounding basement blocks.

Abstract Forearc basins are areas underexplored by academic or industrial research. They are not considered as potential areas by the industry because only three giant oil fields have been formed in this type of basin. One of them is located in the northern Peruvian forearc system: the Talara Basin (1.7 billion bbl to date). The aim of this work is to provide insights for the oil industry about the tectonic style and the tectonic-sedimentation relationships within this basin to decipher its petroleum potential, using surface and subsurface data (seismic lines, well-logs). From a tectonic point of view, we demonstrate that the basin is not a pull-apart basin, but an accretionary prism built by deep-seated north-verging thrusts reworked by shallow gravitational normal faults. A tectonic model is developed, which involves the northwestward propagation of thrust culminations accompanied by the development of southeast- facing normal faults since Eocene times. This tectonic model renews our perception about the entire Peruvian forearc and suggests that anticline traps, which have never been explored, should become a new target for a future hydrocarbon exploration.

ABSTRACT This study is focused on the Salaverry Basin, in the forearc of the Peruvian Andes, which contains three oil seeps sampled and analyzed, suggesting at least one active petroleum system within the basin. The development of the basin is controlled by the interaction between the Nazca-Farallon and South American plates, together with Nazca ridge subduction. Seismic stratigraphic analysis calibrated with available well and core data was carried out to understand the spatial-temporal evolution of Cenozoic depositional systems in the basin. As a result, the Cenozoic succession is best subdivided into five second-order sequences (2S1–2S5) composed of nine third-order sequences (3S1–3S9). Subsequently, 3-D seismic geomorphologic analysis afforded images of depositional elements from mapped seismic sequences. Further analysis of these images enhanced interpretation of depositional systems and lithological predictions. Hence, this study adds new potential reservoir rocks into the 2S1, 2S2, 3S2, 3S3, 3S5, and 3S6 sequences and seal rocks into the 3S1, 3S4, 3S5, and 3S7–3S9 sequences, for the petroleum systems previously defined in the basin. These new petroleum system elements are related to the potential plays proposed in this study, which in turn, through a follow-up assessment, could become very attractive exploratory prospects located in the central portion of the Salaverry Basin, shallow water offshore Peru (west South America).

Abstract Despite the presence of Late Devonian world-class source rock and several noncommercial discoveries, the northern Bolivian Beni Basin and its associated fold-and-thrust belt is one of the most underexplored sub-Andean basins in South America. To address the petroleum potential of the region, we carried out an integrated study of the tectono-stratigraphic settings of the northern Bolivian sub-Andes. We provide two balanced cross-sections, five stratigraphic correlations, and nine distribution maps of key Paleozoic to Cenozoic stratigraphic units. The revised distribution maps show that the Devonian and Carboniferous–Permian source-rocks are present over most of the sub-Andean fold-and-thrust belt (SAZ), whereas the Silurian and Late Cretaceous source rocks are limited to the southern or northern segments of the SAZ. On the other hand, potential Jurassic, Late Cretaceous, and Late Paleogene to Neogene sandstone reservoirs show global widespread distribution. Tentative sequential restorations of the structural cross-sections are provided and suggest a main Miocene shortening period followed by a Pliocene-Pleistocene period of vertical uplift of the whole SAZ. Based on both the stratigraphic and structural review of the study area, the potential plays of the northern Bolivian sub-Andes are discussed. The main exploration risks are related to the timing and presence of the interpreted structures in depth.

ABSTRACT The Madre de Dios Basin, located in the northern sub-Andean zone of Bolivia is an underexplored sub-Andean basin. A complete stratigraphic revision, including biostratigraphy, core description, and sismo-stratigraphy, has been carried out; it suggests some changes in the historical sedimentary models and allows the identification of several reservoir and seal pairs. This revision not only integrates the results of previous studies but also provides new and original interpretations of the existing data set. The geochemical study indicates the existence of an Upper Devonian world-class source rock, in which, the Frasnian interval is characterized with a type I-II kerogen and a source potential index (SPI) higher than 6 ton/m 2 ; the Famennian interval has a type II kerogen and its SPI reaches 3 t/m 2 . The Carboniferous and Permian formations have levels with notable content of organic matter but do not classify as source rock in this area because of their low SPI. To evaluate the hydrocarbon potential of the basin, a 3-D dynamic model has been built. The thermal calibration of the temperature and maturity data is only possible taking into account an increase of the heat flow during Triassic–Jurassic time. As a consequence, 90% of the hydrocarbons are expelled before Cretaceous times by the identified kitchen in the center of the basin. The remaining 10% were expelled between the Oligocene and present time. Considering a petroleum system yield of 1%, the yet to find (mean) of the studied area is evaluated at 7 Gbbl of oil equivalent. The main challenge of the basin remains in finding traps.

ABSTRACT The southern sub-Andean fold-thrust belt of Bolivia and northwestern Argentina is constructed from a ~10-km (~6-mi) thick stratigraphic pile of post-Ordovician to Neogene deposits that have been shortened above a detachment located in Silurian to Upper Ordovician horizons. Hydrocarbon accumulations in this fold-thrust belt include a variety of plays, with reservoirs ranging in age from Devonian to Neogene. Giant gas fields, however, are restricted to deep structures involving Devonian reservoirs. Exploration for this play relies on structural models, as seismic imaging is challenged by geological and topographical conditions. Duplex systems seem to be the dominant thrust system type, including passive, active, and composite roof-thrust geometries. Pure structural wedges are common either in structural plunges or early stage structures. The main controlling factor for the development of duplexes and structural wedges is the presence of two major detachments, the basal detachment, mainly located along the Silurian Kirusillas Formation and the Devonian Los Monos Formation. Once duplex horses, or wedges, start developing in the lower structural level, the overpressured Los Monos Formation is passively uplifted and lithostatic pressure decreases. As this happens, the overpressure increases significantly, triggering pseudoplastic deformation within the Los Monos Formation, which results in the classic complexities of the southern sub-Andean belt. Regional variations in the Silurian–Devonian stratigraphic package seem to be an important control on modes of deformation. Overall structural complexities in the lower structural level increase toward the most distal parts of the Silurian-Devonian basin, with additional detachments developed in the Icla Formation, and a marked decrease in the thickness and mechanical strength of the quartzite packages of the Devonian section. This results in complex and unpredictable trap geometries and a more challenging exploration for the deep plays.

ABSTRACT The sub-Andean zone of southern Bolivia is a typical thin-skinned fold-and-thrust belt with remarkable regularity in the geometry and spacing of the structures. This is a typical feature of fold-and-thrust belts where the basement is not involved in the deformation. However, when the structural geometry and evolution are analyzed in detail, many deviations from such regularity are evidenced. This paper has studied the processes that might have affected the development of some of the structures along the southern sub-Andean zone. Special attention is given to the La Vertiente structure, an elongated anticline with low relief developed at the latitude of the Pilcomayo River. According to the interpretation of growth strata based on 2-D seismic lines, the beginning of the deformation for this structure has been dated by several authors at around 6 Ma. Restoration of structural cross-sections suggests that the La Vertiente structure was originated with a spacing of 65 km (40 mi), which represents two to three times the maximum spacing that characterizes the rest of the structures of the southern sub-Andean zone. With the aim of explaining this “anomaly,” different factors that could lead to an increase in the maximum spacing of this order have been analyzed. We present a model where increase erosion capacity of the Pilcomayo River, as a result of the capture of a large drainage network area that was previously part of the Parapetí River, would have generated a dramatic decrease in the effective basal friction coefficient of the Silurian shales of the Kirusillas Formation. This change, in turn, would be responsible for the transient increase in spacing between structures in the La Vertiente structure.

ABSTRACT The sub-Andean foreland basin of southern Bolivia chronicles erosional unroofing of the central Andean fold-thrust belt during Cenozoic shortening. Analyses of five stratigraphic sections document regional paleosol development in forebulge to distal foredeep depozones, followed by proximal accumulation of a greater than 4 km (2.5 mi) thick upward coarsening and thickening succession of mixed fluvial to megafan deposits. New timing constraints from zircon U-Pb ages for sandstones and interbedded volcanic horizons indicate Oligocene to late Miocene facies migration and eastward progradation from growing structures and point sources of sediment. Detrital zircon U-Pb results, sandstone/conglomerate compositions, and paleocurrent data reveal subarkosic and sublitharenitic sand derived from Andean sources to the west, with (1) contributions from upper Paleozoic strata (420–570 Ma zircon age components) of the inter-Andean Zone; (2) variable input from the Eastern Cordillera, including lower Paleozoic strata (650–800 Ma zircon ages), Mesozoic strata (66–360 Ma zircon ages), and recycled Paleogene basin fill; and (3) later arrival of a cosmopolitan age assemblage from upper Paleozoic to Cenozoic strata of the incipient sub-Andean Zone. Eastward advance of the fold-thrust belt corresponds to an increase in sediment accumulation from 3 m (10 ft)/m.y. to 90–2000 m (295–6562 ft)/m.y. The integrated results suggest a progressive early–middle Miocene transition from foredeep to wedge-top deposition within the eastern inter-Andean Zone to western sub-Andean Zone, with a generally post–12 Ma age for most sub-Andean structures. We propose that either (1) pre-Cenozoic stratigraphic and structural heterogeneities promoted nonsystematic activation and shortening advance, with an unsteady eastward migration of flexural foreland subsidence, or (2) the fold-thrust belt and foreland basin advanced systematically eastward, with a marked southward reduction in the magnitude of shortening along the inter-Andean-sub-Andean Zone. Surface uplift associated with advancing deformation guided the creation of topographic barriers in the fold-thrust belt and proximal foreland, inducing variable unroofing patterns, drainage network evolution, and deposition of fluvial megafans.

ABSTRACT In the southern sub-Andean region of Bolivia, most of the structurally controlled hydrocarbon traps are related to the development of the Andean fold-and-thrust belt. The present study analyzed the structure and the hydrocarbon accumulations at the south sub-Andean “wedge-top.” The structural decoupling in the allochthonous section is characterized by two vertical overlapping structural levels, and the general geometry is defined as a trailing imbricate fan system. Backthrusts, lateral ramps, and tear faults are associated with the shallow structural level. They can act either as barriers or as secondary migration pathways for hydrocarbon accumulations. Related traps show three-and four-way dip closure, but only the latter were productive. The Curiche and Tajibo gas fields are two examples that produce from Cenozoic units in this structural domain. The deeper structural level is defined by thrust sheets with no forelimb development, generating three-way dip closure traps. The Tacobo gas field was the first Huamampampa Formation discovery of this level in the wedge-top. Surface geology, 2-D/3-D seismic, exploratory wells, and petroleum system modeling have been integrated to analyze the development of the wedge-top structures. The timing of generation, migration, and remigration of hydrocarbons was also analyzed. The petroleum system model shows that most of the source rocks reached the critical generation moment prior to the Andean orogeny. The generated hydrocarbon contributed to the development of detachments and thrust that were conduits of migration from the Neogene to present.

ABSTRACT The Argentinean and Bolivian sub-Andean fold-and-thrust belt is located in the orogenic front between 18–23°S and 64–63°W and immediately to the east of the Puna-Altiplano system and Eastern Cordillera. This fold-and-thrust belt is characterized by thin-skinned structures detached in Lower Paleozoic sequences. The compressional stage that affected this part of the orogen began in late Miocene and is ongoing. Examining the morphology of the folds and thrusts by combining digital elevation models, geological maps, and field observations allows us to present tectonic and geomorphological data on lateral fold growth and spatially linked shortening along the San Antonio range. The crest of this fold chain shows multiple culminations, slightly right-stepping, that merge to form the full San Antonio structural trend. Forked drainage patterns and the presence of wind gaps suggest recent fold amplification and lateral growth from these culminations. Linear and stepped linkage geometries are found between single structures. A northward increase in surface elevation and fluvial erosion follows the south to north increase in fold amplitude, aspect ratio, and change in structural style from folding to thrusting. The observed along-strike alternation of topographic highs and lows correlates well with the locations of subsurface hydrocarbon traps and saddle spill points indicating that the shallow and deep deformation patterns are coupled as a single structural feature.

ABSTRACT Tight fractured sandstones of the Devonian Huamampampa Formation are associated with large gas discoveries in the sub-Andean fold-and-thrust belt of southern Bolivia. A LIDAR-based fracture characterization of the Abra del Condor backlimb anticline, a structural-stratigraphic analog, is used as the basis for a fracture stratigraphy determination. Fracture characterization using LIDAR is integrated with outcrop scanlines and is framed by stratigraphy and structural positions within this thrust-related anticline. SEFL software was used to process LIDAR data, dividing the outcrop by orientations. A workflow to extract modeled fracture planes and their associated orientations, lengths, and heights results in five fracture sets, partially validated by fracture outcrop scanlines. Multiple virtual scanlines are used to measure fracture intensity, identify fracture stratigraphic units, and define fracture-associated parameters of abundance and size distribution. Our LIDAR-based fracture characterization indicates a distribution of fracture intensities according to their structural position, decreasing from the hinge to the backlimb. From the five fracture sets identified, one set of orthogonal fractures dominates. Moreover, most of the fractures are contained or bounded within their fracture stratigraphic units and calculated fracture spacing ratio and the fracture space index show a nonexistent relation between fracture spacing and the fracture stratigraphic unit thickness.

ABSTRACT The sub-Andean system through Argentina and Bolivia is composed of a fold-and-thrust belt developed from 9 Ma until today, as a result of an east-northeast-verging compressive stress field. Depending on the area evaluated, thin- or thick-skinned deformation characterizes the structural style throughout this orogenic system. The differences in structural styles depend on variables such as the sedimentary column involved, internal facies and thickness changes, detachment level features, climatic influence, and the presence of inherited extensional and compressional structures. The existing balanced structural cross-sections sometimes present difficulties for solving the rate and chronology of the deformation. The aim of this chapter is to present suitable new deformation models integrating distinct kinematic characteristics and to analyze the variables involved in the southern sub-Andean thin-skinned fold-and-thrust belt. The structural framework proposed for the southern sub-Andean system in Bolivia and northwestern Argentina is based in the identification of four rheological levels. Levels 1 and 3, with a shale-dominated composition (Kirusillas and Los Monos Formations, respectively), are deformed as a weak isotropic material and can be simulated using Trishear kinematic modeling. On the other hand, rheological levels 2 and 4 (Tarabuco–Santa Rosa–Icla–Huamampampa Formations and Carboniferous–Cenozoic interval, respectively), with a sand/shale alternating composition, are structured as a strongly heterogeneous interval responding to the compressive stress field with parallel folding. A simple shear kinematic model could be used to simulate this deformation. This behavior has been tested as a feasible model for the deep structure in significant oil/gas fields in Argentina and Bolivia.

ABSTRACT The south sub-Andean zone of Bolivia is characterized by east-verging thrusts, detaching within Silurian and Devonian series, and associated to a series of north–south to north-northeast–south-southwest regional anticlines and synclines holding large gas accumulations. This chapter is a case history of the multi-TCF (trillion cubic feet) Incahuasi discovery and how an integrated, multidisciplinary approach allows a progressive improvement in the understanding of the trap geometry, reducing the uncertainty during exploration and field appraisal. While-drilling structural interpretation and biostratigraphic analyses suggested that the discovery well, prognosed mainly by surface geology and targeting the top of the Huamampampa Formation as a bright discontinuous seismic reflector on a 2-D seismic test line, was drilling the overturned limb of the surface anticline instead of penetrating the core of the structure. The top reservoir was found deeper by a side track well. The appraisal wells all contributed to progressively constrain the complex geometry of the trap while leaving several possible geometric interpretations of the reservoir structure. The Huamampampa Formation is repeated and overturned by a series of thrusts that increase the internal complexity of the field. These results also confirmed that the shales of the Icla Formation behave as an effective detachment below the reservoir, contributing to the structural complexity of the field. A calibrated/constrained inversion of 3-D magnetotelluric acquisition performed over the structure shows the near-top Huamampampa Formation resistivity anomaly plunging to the north, which helps to identify the possible structural spills. In map view, the variation of orientation provides information on local bends of the target and hence is an important element to be considered for optimizing future appraisal wells. The Incahuasi discovery provided the opportunity to test the validity and efficiency of many geological and geophysical tools while noting their limitations in a context where only a few hundred meters of horizontal error can lead to missing the reservoir and trap.

ABSTRACT The quartz-rich sandstones of the Upper Cretaceous Vivian Formation constitute the most important reservoirs in the prolific Marañón foreland basin of northern Peru. The Vivian sandstones are largely fluvial in the northeast and transition to a marine shoreface and open shelf in the west. The formation consists of two sand-rich units (Lower and Upper Vivian) with better reservoir characteristics in the Lower Vivian. Porosity versus depth analysis for the dominantly fluvial Vivian sandstones shows a simple trend of decreasing porosity with depth, with facies-dependent variations for wave-reworked facies. This simple linear trend (correlation coefficient R2 > 0.77) indicates that overburden stress is the dominant factor that determines the porosity reduction. Further west, in the wedge-top Santiago Basin, the Vivian sandstones exhibit anomalously low porosities at relatively shallow depths, a distinct diversion from the regional trend. The depth difference between these low porosities and the porosities of the regional trend was used as a proxy for the uplift that the basin has suffered during the Andean deformation in the latest Miocene. The resulting values (up to approximately 2600 m [8530 ft]) are consistent with uplift estimates derived from other methods and suggest that parts of the foredeep have been partially uplifted.

ABSTRACT The analysis of the pre-Andean history of the Central Andes shows a complex tectonic evolution. The basement of the Andean continental margin was formed by the accretion of Precambrian blocks during the formation of Rodinia in late Mesoproterozoic times. There are two magmatic arcs of Grenvillian age, one developed on the margin of the craton, known as the Sunsas belt, and another on the accreted terranes. The suture between these blocks with the Amazonian craton has been continuously reactivated by tectonic and magmatic processes. The terranes of Paracas and Arequipa, both of Grenvillian age, have a contrasting Paleozoic evolution. The Arequipa terrane amalgamated to the craton by the end of the Mesoproterozoic, and during the Paleozoic its suture acted as a crustal weakness zone. This zone concentrated the extension and the formation of a large platform in the retro-arc basin, where the Eopaleozoic sediments accumulated. The Famatinian magmatic arc of Ordovician age (475–460 Ma) is preserved in this segment along the continental margin. The Eopaleozoic extension that affected the Paracas terrane reopened the old suture and formed oceanic crust between Amazonia and Paracas. The subduction of this oceanic crust developed a magmatic arc over the cratonic margin, which is preserved in the Eastern Cordillera of Peru as orthogneisses associated with metamorphic rocks of Famatinian age. There are ophiolitic assemblages, paired metamorphic belts, and intense deformation associated with the Paracas collision (~460 Ma)against the Amazonian craton. In northern Eastern Cordillera of Peru the late Paleozoic orogen has within-plate granitic belts and was far away from the active margin. The orogen was deformed and uplifted in two phases (336–285 Ma and 280–235 Ma) known as the early and late Gondwanide orogenies. They are preserved as medium grade metamorphic belts developed along the Paracas segment. Further south along the Arequipa segment in southern Peru and Bolivia, the late Paleozoic–Triassic rocks are represented by granites and acidic volcanic rocks, which are not metamorphosed and are associated with sedimentary rocks. Relics of a magmatic arc are exposed as tonalites and metamorphic rocks (~260 Ma) along the northern continental margin of Peru and in the near offshore platform. The extensional regime that dominated most of the Mesozoic developed rift basins in the hanging-wall of the terrane sutures, which controlled the structural highs and basin margins. The Peruvian Late Cretaceous orogeny produced the emplacement of the Coastal batholith, the beginning of deformation along the coast, and the first foreland basins. The giant Ayabacas submarine syn-tectonic collapse is also controlled by previous sutures. The Cenozoic Andean evolution was dominated by a wave of shallowing of the subducted slab, the migration of the magmatism to the foreland, the steepening of the oceanic plate, and the consequent “inner arc” magmatism. The “inner arc” plutonic and volcanic rocks are the expression of deep crustal melts, associated with crustal delamination and lithospheric mantle removal. The flattening of the oceanic slab is related to ablative subduction and shortening in the Altiplano and Eastern Cordillera. The steepening is associated with rapid removal of mantle lithosphere and crustal delamination, expressed at surface by the “inner arc” magmatism. The suture crustal weakness zones between different terranes partially controlled the location of the delaminated blocks and the “inner arc” magmatism. Both processes triggered the lower crust ductile shortening and subsequent upper crustal brittle development of the sub-Andean fold-and-thrust belt.

ABSTRACT Stratigraphic, geochemical, and biomarker data from the Huallaga Basin suggest that organic carbon-rich shales and limestones of the Upper Triassic to Lower Jurassic Aramachay Formation of the Pucará Group, previously identified as potential hydrocarbon source rocks in Peruvian sub-Andean basins, were deposited under low oxygen or anoxic conditions within a semirestricted basin. Rock-Eval and total organic carbon (TOC) data from surface and subsurface locations show that although most Aramachay Formation shale and limestone outcrop samples have relatively high organic carbon content, the unit has little remaining genetic potential; T max data indicate that the thermal maturity of nearly all outcrop samples ranges from wet to dry gas. Visual kerogen analyses show that type II amorphous kerogen is the dominant type in the Aramachay Formation. Cretaceous rocks within the Huallaga Basin are dominated by type II/III and type III kerogen and generally lack sufficient TOC to be effective source rocks for oil. Geochemical and biomarker data indicate that rock extracts and seep oils were derived from mixed shale and carbonate source facies dominated by marine algal and bacterial organic matter and are similar to “Jurassic” oils described from the Marañon and northwestern Ucayali Basins. Hydrocarbon generation and expulsion models suggest that the generation and expulsion of oil from the Aramachay Formation (likely the middle Aramachay Formation) began from west to east in the Huallaga Basin, starting in the now-exhumed western part of the basin during the Early Cretaceous, extending through the middle Oligocene in the central part of the basin and into the Present in the eastern part of the basin. Estimates of vitrinite reflectance (R o ) based on biomarker data indicate that Marañon Basin oils derived from the Aramachay Formation were likely generated during the peak oil phase of generation; oils in the northwestern Ucayali Basin were generated during the late oil phase of generation. Petroleum extracts from outcrop samples in the northern part of the basin and oils from seeps along the southeastern frontal thrust of the basin indicate a late oil level of thermal exposure. Migration of oils into the Marañon and northwestern Ucayali Basins likely occurred prior to the early Pliocene, when formation of the Andean frontal thrust cut off migration routes from the Huallaga Basin.

ABSTRACT This chapter focuses on the role of basement fabrics and inverted extensional faults that strongly affect the frontal zones of the fold-and-thrust faults of sub-Andean basins in Peru and Bolivia. This review examines the relationships of hinterland deformation in the basement with the Present Day topography from the Andean plateau to the sub-Andean foreland basin. Preexisting, steep basement–involved extensional faults that were inverted in the last phase of Andean deformation (~10 Ma to the Present Day) produced basement-cored uplifts that transferred thick-skinned shortening eastward onto the thin-skinned thrust fault and fold systems detached above the basement. Regional cross sections are reviewed and revised in the light of analysis of seismic data as well as mechanically feasible models of the hinterland to foreland transfer of displacement. Steep inverted faults with dominantly high vertical uplift in the hinterland exhume the older stratal packages together with crystalline basement, and these units provide the source for the largely Neogene to Holocene syn-tectonic foreland basins in front of the advancing thrust wedge of the sub-Andean system in Peru and Bolivia.