Structural styles of the Camisea fold-and-thrust belt, southeast Peru

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 thinskinnedfold-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.