POLYPHASE DEFORMATION OF DIAPIRIC AREAS IN MODELS AND IN THE EASTERN PREBETICS (SPAIN)

Abstract

Fingerlike bodies of evaporite rocks have been observed in many regions affected by multiple tectonic phases, such as the Atlas, the Pyrenees, or the Zagros Mountains, where they have been interpreted as fault-plane diapiric injections or collapse-related salt welds. In this article, we suggest a new interpretation of these structures as squeezed diapirs based on detailed structural and sedimentological field data of the Bicorb-Quesa diapir (eastern Prebetics) and offer five analog models, which simulate the polyphase deformation of the eastern Prebetics. In this area, diapirs formed from the Oligocene to Langhian during an extensional phase related to the opening of the Valencia Trough. These diapirs were later affected by a Serravallian contractional phase, which inverted the preexisting grabens and created new folds and thrusts. The pre-existing diapirs were necked and/or squeezed, forming secondary welds with a fingerlike geometry that isolated the diapir bulbs from their source layer. Extrusion of diapiric material was also accelerated during this phase, but no new diapirs formed. Finally, the area was again affected by an extensional phase during the Tortonian, which reactivated the normal faults and created a new set of diapirs. The new diapirs formed where the overburden was thinner, that is, at the toe of the major reactivated faults. Commonly, these faults coincide with the bounding faults of the major grabens formed during the first extensional phase, and therefore, the new diapirs grow close to the location of the squeezed diapirs. The models also show that the faults created during the initial extension prevailed as the main focus for deformation during the polyphase history. Deformation in the overburden and the viscous layer was mainly accommodated along the major grabens formed during the first extensional stage. During shortening, the initial major grabens deformed as complex anticlines, and during the subsequent extensional phase, most deformation occurred by the collapse of these anticlines along preexisting faults, fault welds, and the flank of the squeezed diapirs. The source layer is compartmentalized, accumulating and withdrawing material in the same locations (the initial grabens and horst). As a result, the soure layer is easily depleted beneath the initial horst, forming primary welds.