Orthogonal folding of extensional detachments:: Structure and origin of the Sierra Nevada elongated dome (Betics, SE Spain) -: art. no. 1012

[1] A close relationship of the kinematics and timing between low-angle extensional faulting and upright folding is established for the Miocene detachment systems in the hinterland of the Betics in southeastern Spain. Folding accompanied tectonic denudation developing elongated domes with fold axes both parallel and perpendicular to the direction of extension. The geometry, kinematics, and tectonic evolution of two major sequentially developed extensional fault systems have been characterized in several E-W elongated mountain ranges of the central Betics on the basis of new cartographic and structural data and a comprehensive revision of other available geological and geophysical observations. The extensional systems have an average WSW direction of extension and led to exhumation of the two lower metamorphic complexes of the Betics, the Nevado-Filabride and the Alpujarride, during the middle and upper Miocene (Serravallian to Tortonian). The extended domain contains a core complex, with distal and proximal antiformal hinges separated by around 60 km and with a fold amplitude of similar to6 km (measured parallel to the direction of extension). The total amount of extension across the core complex is about 109-116 km, corresponding to a stretching factor (beta) of 3.5-3.9, estimated using the distance between fold axial surfaces and a geometrical model accomplishing footwall deformation during tectonic denudation via subvertical simple shear. The elongated domal and basinal geometry of the detachments and their respective footwalls is due to the interference between two sets of orthogonal large-scale open folds, trending N-S and E-W. Longitudinal, N-S trending folds are interpreted as isostatic folds, i.e., folds that developed in response to differential unloading of the extensional detachment footwalls inducing ductile flow in the middle crust. These folds formed in a rolling-hinge anticline in which rotation migrates westward through the footwall as it is progressively unroofed. In contrast, E-W trending folds, being subparallel to the direction of extension, have a contractional origin and progressively affect to the west the isostatically readjusted segments of the detachments once they are inactive. Orthogonal folding occurred since the Serravallian to lower Pliocene. Extension is still active from the rolling-hinge anticline in Sierra Nevada to the west. The middle Miocene to Pliocene tectonic evolution of the Sierra Nevada elongated dome at the core of the Betic hinterland is another example of the coexistence of extension and contraction during continued overall convergence and mountain-building.