Abstract The genetic analysis of fold and thrust belts is facilitated by tracking the evolution of their organic endowment (petroleum tectonics). Petroleum tectonic analysis of convergent orogenic systems provides an audit of the processes that control the deformation and kinematics of orogenic belts. The distribution and deformation paths of the organic endowment intervals are key factors in determining the petroleum system evolution of fold and thrust belts. This comparison of orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper, mechanical stratigraphy and inherited architecture on the creation, preservation and destruction of petroleum accumulations. The Zagros, Pyrenees, Sevier and Beni Sub-Andean convergent systems share key characteristics of fold and thrust belts, with major differences in scale, degree of incorporation of organic endowment in evolution of the fold and thrust belt and its foreland, and preservation of fold and thrust belt wedge-top deposits. The Zagros is an orogen dominated by flexural processes that is a perfect storm for hydrocarbon generation and preservation. Its multiple stacked sources ensure continuous hydrocarbon generation while stacked detachments foster a low taper and thick wedge-top basins. The Pyrenees is also a flexurally dominated orogen, but the early consumption of its source rocks led to minimal survival of hydrocarbon accumulations during exhumation in a long lasting, high-taper orogenic wedge. The Sevier was initially a flexural orogen that was later dominated by dynamic uplift of the fold and thrust belt and distal foreland subsidence with foreland deformation. The consumption of its pre-orogenic sources during the early low-taper phase indicates a probable robust petroleum system at that time. However, the late high-taper phase exhumed and destroyed much of the early petroleum system. The addition of syntectonic foreland sources to be matured by both local and dynamic subsidence created an additional later set of petroleum systems. Post-orogenic events have left only remnants of world-class petroleum systems. The Beni segment of the Sub-Andean Orogen is a flexural system with probable dynamic overprints. Its most robust petroleum system probably occurred during its early low-taper flexural phase, with dynamic subsidence enhancement. Its late high-taper phase with possible dynamic uplift shuts down and stresses the petroleum systems. Comparison of these orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper kinematics, mechanical stratigraphy, distribution of source rocks relative to shortening and inherited architecture on the creation, preservation and destruction of petroleum accumulations in fold and thrust belts.

Abstract The margins to evolving orogenic belts experience near layer-parallel contraction that can evolve into fold–thrust belts. Developing cross-section-scale understanding of these systems necessitates structural interpretation. However, over the past several decades a false distinction has arisen between some forms of so-called fault-related folding and buckle folding. We investigate the origins of this confusion and seek to develop unified approaches for interpreting fold–thrust belts that incorporate deformation arising both from the amplification of buckling instabilities and from localized shear failures (thrust faults). Discussions are illustrated using short case studies from the Bolivian Subandean chain (Incahuasi anticline), the Canadian Cordillera (Livingstone anticlinorium) and Subalpine chains of France and Switzerland. Only fault–bend folding is purely fault-related and other forms, such as fault-propagation and detachment folds, all involve components of buckling. Better integration of understanding of buckling processes, the geometries and structural evolutions that they generate may help to understand how deformation is distributed within fold–thrust belts. It may also reduce the current biases engendered by adopting a narrow range of idealized geometries when constructing cross-sections and evaluating structural evolution in these systems.

Abstract Fruta del Norte is a completely concealed and extremely well-preserved, Late Jurassic epithermal gold-silver deposit of both low- and intermediate-sulfidation type, which is located in the remote Subandean mountain ranges of southeastern Ecuador. Currently defined indicated resources are 23.8 million metric tons (Mt) averaging 9.61 g/t Au and the total endowment is 9.48 Moz Au. The deposit, notable for the widespread occurrence of visible gold and bonanza grades, will be bulk mined underground. Fruta del Norte was discovered in 2006 during greenfield exploration and systematic drill testing of a conceptual geologic model, which predicted that auriferous veins would occur in andesitic volcanic rocks inferred to underlie a zone of arsenic- and antimony-anomalous silicification in fluvial conglomerate. The host andesitic volcanic rocks, crosscutting feldspar porphyry, and associated phreatic breccia are part of a roof pendant in the Zamora batholith. Together, they are products of a continental-margin volcanoplutonic arc of Middle to Late Jurassic age. The deposit lies beneath the northern extremity of the ~16-km-long, Suárez pull-apart basin where it is localized by steep, second-order faults within the regionally extensive Las Peñas strike-slip fault zone. The pull-apart basin was progressively filled by fluvial conglomerate, dacitic ignimbrite, finer grained siliciclastic sedimentary rocks, and, finally, andesite flows. The Fruta del Norte deposit comprises a 1.3-km-long and up to >300-m-wide vein stockwork associated with quartz-illite-pyrite alteration. The deposit comprises two principal vein types, one in the south dominated by quartz, manganoan carbonates, and abundant base metal sulfides and the other in the north dominated by manganese- and base metal-poor quartz, chalcedony, and calcite. Adularia is a minor gangue mineral in both. Both vein types are abruptly transitional upward and westward to a third important ore type characterized by intense silicification and chalcedony veining, with disseminated and veinlet marcasite (± pyrite). An extensive silica sinter horizon directly overlies the andesitic rocks and/or occurs as interbeds in the lowermost 20 m of the conglomerate and, consequently, is in unusual proximity to the underlying gold-silver orebody. Much of the conglomerate lacks silicification except for a narrow, steeply inclined zone exposed above the deposit, which led to its discovery.

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 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 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 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.

ABSTRACT The tectonic evolution of the Marañón Basin and its related basins, the Huallaga and Santiago Basins, in northern Peru, spans more than 250 m.y. of Mesozoic–Cenozoic subsidence. Basin evolution began with an initial rifting in the Late Permian–Early Triassic. This period of extension was accommodated by inherited structural inhomogeneities and a southwest-oriented extension, which dissected the Paleozoic sequences into a series of roughly northwest-southeast-trending grabens and half grabens filled with volcanic and continental-derived sediments. Fault-controlled subsidence was followed by regional postrift subsidence, and a thick section of Triassic to Jurassic marine to transitional sediments was deposited over the preexisting extensional features. These included one of the potential source rocks for the western part of the basin (the Aramachay Formation). The Late Jurassic to Early Cretaceous Jurua orogeny and later peneplanation produced a major regional unconformity. Subsequent Cretaceous sedimentation, mostly controlled by eustatic processes and related regressive–transgressive cycles, is composed of a thick section of continental to proximal and shallow marine deposits, comprising the main reservoirs and the main source rock for the basin, responsible for most of the oil discoveries in the northeastern region. Several compressional and structural inversion episodes, related with the subduction of the Nazca plate in Late Cretaceous and that culminated in the Miocene, have modified the basin and isolated the Huallaga and Santiago subbasins to the west, both structured by complex thrust systems. Cenozoic deposits constitute the foreland basin system infill and contain more than 4000 m (13,120 ft) of mostly fluvial and deltaic deposits with minor marine incursions. In the sub-Andean zone they constitute the “molasses” from the rising Andean cordillera to the west. This history of tectonic evolution is reflected in a complex structural framework in the western part and a well-developed foreland system to the east with a broad topographic high, known as the Iquitos Arch, which corresponds to the present forebulge.

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 Santiago Basin of the northern Peruvian sub-Andes is a structurally complex region related to a combination of thin- and thick-skinned deformation and the impact of salt tectonics during Andean deformation. Oil shows in this basin are very common, and even though the first exploration campaigns started in the 1940s, no commercially exploitable hydrocarbons have been discovered yet. We present three basin-scale structural transects and refined structural interpretations, based on vintage 2-D seismic data and well tops that help elucidate the relationships between thin-skinned and deep-seated, thick-skinned structures. Two dip sections were kinematically restored to the top of the Yahuarango formation, one of the youngest pre-Andean units. We calculated the depth to the intra-basement detachment to be approximately 20 km (12 mi), a value that correlates with other thick-skinned detachments and earthquake hypocenters from the region. We recognized a varied inventory of salt-related structures, which we interpret to be part of the approximately 800-km (500-mi)-long Peruvian Salt Belt. The onset of salt movement occurred soon after salt deposition, likely through sediment loading. Our data suggest that Miocene-Pliocene basin deformation starting at 5.3 Ma has been sustained until the present-day. Shortening ranges from 7.31 km (4.54 mi) to 7.56 km (4.70 mi) (5.9% and 6.1%, respectively), corresponding to Miocene-Pliocene deformation rates of 1.3–1.4 mm/yr. These values are significantly lower than those of adjacent regions in the sub-Andes. This may be related to the combined effects of pressure solution, strain accommodation, or deflection by crustal-scale faults farther west.