Geometry and kinematics of the Fuegian thrust-fold belt, southernmost Andes

Abstract

Published balanced structural interpretations of the Fuegian thrust-fold belt (southernmost Andes) fail to address aspects such as basement shortening in the thrust-fold belt or its structural connection with the central belt of the orogen. We tackle this deficiency by constructing four serial balanced cross sections based on surface geologic information, 2-D reflection seismic lines, and well logs, which depict the first-order structural geometry and kinematics from the thrust front to the internal thrust-fold belt. The interpretation reveals three main tectonostratigraphic packages separated by major detachments: basement (Paleozoic-Upper Jurassic), Cretaceous, and Paleogene, all variably involved in thrusting along strike. Minimum shortening in the Paleogene cover ranges between 16% and 43%, and significant style changes are observed across and along the thrust wedge. We show that basement thrust sheets have a major role in the construction of the thrust-fold belt; some of them were emplaced below the thrust front during late Oligocene or early Miocene times. The kinematic evolution of the thrust-fold belt indicates that the active detachments depended on the existence of high pore pressure to propagate forward. Our model shows that this pore pressure buildup was controlled by overburden, mean rock impermeability, and strain rate; the variable interaction among these factors controlled the propagation and style of the thrust wedge.