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.