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 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.