HISTORY OF MUD DIAPIRISM AND TRIGGER MECHANISMS IN THE WESTERN ALBORAN SEA

The distribution and morphology of the mud diapirs in the Western Albo ran Basin were studied using multichannel and high resolution seismic profiles (airgun). The diapirism in the Western Alboran Basin forms di apiric ridges and mud volcanoes. Three types of contact relationships between the diapirs and the sedimentary cover have been identified: (1 ) inverse-piercing contacts; (2) normal-fault contacts; and (3) subver tical contacts. These geometric relationships and the study of the sed imentary cover composed of six seismic units (Lower Miocene to Quatern ary) allow us to establish the timing and the geodynamic framework und er which the diapirism has evolved. The diapir distribution was contro lled through the geodynamic evolution of the Alboran Basin related to extensive, compressive and strike-slip processes. Four phases of diapi rism are proposed in order to explain the evolution of diapirism: reac tive, active, passive and collapse phases. We establish an evolutionar y model of diapirism from the Langhian to the present. Diapirism start ed in the Langhian-lower Serravallian controlled by extensional proces ses that allowed a reactive phase to develop. Upper Serravallian-lower Tortonian diapirism was characterized mainly by active diapirism indu ced by compressional and strike-slip processes. During the upper Torto nian-lower Messinian passive diapirism was predominant until the upper Messinian, when an active diapiric phase developed under a transtensi ve tectonic setting. Extensional processes continued acting during the Pliocene and Lower Quaternary in the Western Alboran Sea, inducing re active diapirism though it was punctuated by several active phases. Th e Upper Quaternary was characterized by a generalised collapse phase l inked to extensional processes that developed in many mud ridges, alth ough mud volcanism and active diapirism was also favoured by different ial loading produced by a contouritic drift.