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 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 In the Peruvian sub-Andean foreland basin system, the construction of serial balanced cross-sections from a good set of structural data and an extensive knowledge of the stratigraphy and geodynamic evolution allow a more refined definition of the unexplored plays, as subthrusts, duplexes, or pre-Andean structures. Sequential restorations are proposed by coupling thermochronologic analyses with growth strata studies. The results show significant north–south variations in geometry, timing and rates of deformation, and foreland sedimentation. These latitudinal variations are not only related to the pre-Andean basins’ inheritance but also to the interactions between thrusts propagation, erosion, and sedimentation. Thermochronologic ages correspond to the most recent thrust-related uplifts and are supplemented by the study of stratigraphic foreland basin records that can bring to light oldest tectonic events. North of the Peruvian sub-Andean zone, thrusts propagation is controlled by thick-skinned and thin-skinned salt tectonics. Northern thick-skinned tectonics has westward vergence and is inherited from a Middle Permian fold-and-thrust belt. To the south, thrusts deformation is largely controlled by the geometry of the preserved Paleozoic sedimentary wedge and becomes progressively thin skinned. Total sub-Andean shortening varies between 70 km (43 mi) in the north and 47 km (29 mi) in the south. Sub-Andean deformation started in the Late Cretaceous. After a period of quiescence during the middle Eocene, it reactivated and is still active. Three stages of sub-Andean deformation are clearly identified and help to define the preservation time in the suggested petroleum plays.

ABSTRACT The Camisea multi-trillion cubic feet (tcf) gas and condensate fields are located at the southern edge of the Ucayali Basin of southeastern Peru. The Ordovician to Neogene sedimentary succession was deformed by late Miocene to Present Day contraction related to the Peruvian flat-slab subduction regime. This produced thin-skinned, north-northeast-vergent thrust-fault-related folds that form the traps of the Camisea fields. The architecture of the frontal thin-skinned thrust system is characterized by a faulted detachment fold system at Cashiriari and a gently dipping north-northeast-vergent thrust ramp system and associated kink-band hanging-wall anticlines and back-thrusts at San Martin. At San Martin, these form brittle thrust wedge systems that terminate in triangle zones in the Paleogene–Neogene strata of the foreland basin at the leading edge of the fold-and-thrust belt. The basal detachment of the thin-skinned system is located at the top of the Ordovician–Silurian synrift sequence and at the base of the Devono–Mississippian postrift units. Steep Ordovician–Silurian extensional faults offset the basement and form half-graben structures that influence the topography of the postrift strata and the basal detachment geometry. The Cashiriari Anticline is modeled as gentle inversion fault-propagation fold at the early stages of the Andean deformation and then was amplified forming a detachment fold during the late Miocene to Present Day phase of strong contraction. Small displacement limb-break thrusts displace the Cashiriari fold limbs. In contrast, the San Martin fault-fold system is modeled as a simple shear fault-bend fold that forms a wedge thrust and a triangle zone. The San Martin folds are hanging-wall kink-band-style fault-bend systems where the positions of the underlying thrust ramps were controlled by the basement fault systems and the topography of the postrift units. The hinterland of the Camisea frontal thin-skinned fold-and-thrust belt is interpreted to be a system of large inverted basement fault blocks that were uplifted and exhumed as the Andean deformation moved outboard from the hinterland to the foreland and transferred displacement onto the thin-skinned sedimentary wedge at the edge of the basin. This study shows how the underlying basement fault architectures and rift basin geometries can control the styles of the thin-skinned Andean deformation in the sub-Andean system.

The forearc of Central Chile (33°–34°S) is formed by three N-S–trending morphostructural units, including, from west to east, the Coastal Cordillera, the Central Depression, and the Principal Cordillera. The Cenozoic sedimentary rocks that could represent the erosional material generated throughout the morphotectonic evolution of these units accumulated in the marine Navidad Basin. The age of the marine deposits is controversial, as foraminifer biostratigraphy indicates that marine deposition started during the late Miocene, whereas 87 Sr/ 86 Sr data indicate that deposition started during the early Miocene. We carried out single heavy mineral microprobe analysis and standard heavy mineral analysis of these deposits in order to qualitatively identify the geological units subjected to erosion in the central Chilean forearc during Cenozoic times. Our analysis focused mainly on unweathered and unaltered detrital garnet, pyroxene, and amphibole. The textural characteristics and geochemical signature of these minerals were used to determine their original rock type; their magmatic affinity, in the case of pyroxenes of volcanic origin; and their metamorphic grade, in the case of amphiboles of metamorphic origin. We have also compared the composition of detrital garnet, pyroxene, and amphibole with preexisting chemical data of these minerals in the possible source rocks, which, along with the analysis of the detrital heavy mineral suite in each sample, allows us to determine the specific geological unit from which they were generated. Three erosional-depositional stages are recorded by our analysis. Whereas the chemistry of pyroxene and amphibole characterized volcanic-subvolcanic sources within the present-day Central Depression for the first stage, the Central Depression and the Principal Cordillera for the second stage, and the Principal Cordillera for the third stage; the composition of garnet is indicative of metamorphic and plutonic sources within the Coastal Cordillera during all three stages. If marine deposition inside the Navidad Basin started during the early Miocene, the provenance results would record erosion and deposition contemporary with volcanic activity. On the other hand, if marine deposition started during the late Miocene, the provenance results show a retrograde erosive response to landscape for a regional uplift event proposed for that period in the study area. Also, assuming that provenance results are directly related to the action of faults, our data indicate that the main relief-generating fault during the early stages of Andean uplift corresponds to the Los Ángeles–Infiernillo Fault, rather than the San Ramón Fault, as stated by the proposed morphotectonic models for the study area. In addition, the ubiquitous provenance from the Coastal Cordillera is more likely to represent the erosion of nearshore basement rocks affected by faulting along the eastern border of the Navidad Basin, rather than uplift and erosion of the Coastal Cordillera, as previously considered. Single-mineral geochemical analysis of detrital pyroxene and amphibole can be used in other sedimentary basins related to arc-magmatic systems with short transport distances, like the ones in the western Andean border, where these minerals tend to be largely unweathered. In particular, our work represents an advance in this field, as the chemistry of detrital amphibole has not been used before to discriminate source rocks presenting different geochemical signatures.

Integrated magmatic, structural, and geophysical data provide a basis for modeling the Neogene lithospheric evolution of the high Central Andean Puna-Altiplano Plateau. Reconstruction of three transects south of the Bolivian orocline in the Altiplano and Puna Plateau shows processes in common, including subduction characterized by relatively shallow and changing slab dips, crustal shortening, delamination of thickened lower crust and lithosphere, crustal melting, eruption of giant ignimbrites, and deep crustal flow. Temporal similarities in events in the three transects can be correlated with changes in the rate of westward drift of South America and slab rollback. Temporal differences between the three transects can be attributed to variations in Nazca plate geometry in response to southward subduction of the aseismic Juan Fernandez Ridge. Subduction of the north-south arm of the ridge can explain an Oligocene flat slab under the Altiplano, and subduction of a northeast arm of the ridge can explain a long period of relatively shallow subduction characterized by local steepening and shallowing. Major episodes of ignimbrite eruption and delamination have occurred over steepening subduction zones as the ridge has passed to the south. Late Miocene to Holocene delamination of dense lithosphere is corroborated by published seismic images. The southern Altiplano transect (17°S–21°S) is notable for high, structurally complex Western and Eastern Cordilleras flanking the Altiplano Basin, the eastern border of which is marked by late Miocene ignimbrites. The broad Subandean fold-and-thrust belt lies to the east. The Neogene evolution can be modeled by steepening of a shallowly subducting plate, leading to mantle and crustal melting that produced widespread volcanism including large ignimbrites. Major uplift of the plateau at 10–6.7 Ma was dominantly a response to crustal thickening related to Subandean shortening and peak lower-crustal flow into the Altiplano from the bordering cordilleras as the ignimbrites erupted, and partly a response to delamination along the eastern Altiplano border. A smaller ignimbrite volume than in the northern Puna suggests the Altiplano lithosphere never reached as high a degree of melting as to the south. An Oligocene flat-slab stage can explain extensive Oligocene deformation of the high plateau region. The northern Puna transect at ~21°S–24°S is notable for voluminous ignimbrites (>8000 km 3 ) and a narrower Subandean fold-and-thrust belt that gives way southward to a thick-skinned thrust belt. The evolution can be modeled by an early Miocene amagmatic flat slab that underwent steepening after 16 Ma, which led to mantle melting that culminated in widespread ignimbrite eruptions beginning at 10 Ma, peaking in the backarc at ca. 8.5–6 Ma, restricted to the near arc by 4.5 Ma, and ending by 3 Ma. The formation of eclogitic residual crust caused periodic lower-crustal and lithospheric mantle delamination. Late Miocene uplift was largely due to crustal thickening in response to crustal shortening, magmatic addition, and delamination. Crustal flow played only a minor role. The high degree of mantle and crustal melting can be explained as a response to steepening of the early Miocene flat slab. The southern Puna transect at ~24°S–~28°S is notable for eastward frontal arc migration at 8–3 Ma, intraplateau basins bounded by high ranges, long-lived Miocene stratovolcanic-dome complexes, voluminous 6–2 Ma ignimbrites, 7–0 Ma backarc mafic flows, and the latest Miocene uplift of the reverse-faulted Sierras Pampeanas ranges to the east. Its evolution can be modeled by a moderately shallow slab producing widespread volcanism with subsequent steepening by 6 Ma, leading to delamination of dense lithosphere culminating in the eruption of the voluminous Cerro Galan ignimbrite at 2 Ma.