Curved fold-and-thrust accretion during the extrusion of a synorogenic viscous allochthonous sheet: The Estepa Range (External Zones, Western Betic Cordillera, Spain)

摘要

New structural, borehole, and time domain electromagnetic data allow us to reconstruct the 3-D geometry of the Estepa range (External Zones of the Western Betic Cordillera), which constitutes an isolated curved fold-and-thrust range made up of Jurassic shallow marine carbonates and Cretaceous to Paleocene marls and marly limestones. Contraction and rising load linked to the accretion of the Subbetic nappe enhanced the expulsion of Triassic evaporites and overpressured clays during the early and middle Miocene, finally forming part of a highly deformed melange unit. This plastic extrusion favored the disconnection of External and Middle Subbetic blocks, including the Sierra de Estepa Range. There, deformation is characterized by northwest-vergent curved folds-and-thrusts associated with NE-SW frontal ramps and NW-SE lateral transfer faults that segmented the folds active between 18 and 12 Ma. At a local scale, curvature was achieved under a nearly uniform NW/WNW transport direction suggesting a primary arc type. Friction variation across the heterogeneous detached plastic basal Triassic unit mainly controlled the curvature and segmentation of the fold systems. Transport direction is consistent with previously published slip data from the Internal Subbetic and the Campo de Gibraltar units. After the Late Serravallian, accretion slowed down and a shallow marine unit deposited unconformably placed over the main thrust and folds. The sequence of deformation and emplacement of these ranges is interpreted as a continuous process linked to the early Miocene E continental collision of the Iberian margin thin continental crust and subduction of the Flysch trough oceanic crust beneath the Alboran continental domain.