par Christophe Clarens DOCTEUR PROCESSUS ET CONTRÔLES ...

Title : Processes and controls of the Miocene at actual sedimentation in an extensional basin: Example of theComondu formation and the Alfonso – El Carrizal basin (B.C.S., Mexico).Abstract :Geodynamic evolution of the continental margin of California involved the formation of divergentsedimentary basins and is recorded by various markers, whom we wanted to study through the example of theAlfonso – El Carrizal transtensional basin, created during the opening of the Gulf of California (10-3,5 My).This analysis made it possible to differentiate the three main phases, from the evolution of this margin and moreprecisely in Baja California. It thus enables us to determine the origin and the history of the basin and tounderstand the filling processes during the evolution of the margin.The filling sedimentological study of the basin was carried out for each three phase: subduction,transformation and opening of the Gulf and enabled us to differentiate four sequences. Thus, Alfonso - ElCarrizal basin has a substrate resulting from an intense volcanic activity related to the phase of subduction whichaffects the margin of Baja California. The S1 sequence translates a phase of subsidence into response to thissubduction and is made up of deposits of pro-delta. It marks the beginning of configuration of the margin of BajaCalifornia in the future volcanic arc, accompanied by a gradual rising of the continental edge. The phase offilling of a basin of fore-arc in extension, is recorded by the deposit of the sequence S2 (middle to upperMiocene), composed of volcanoclastic and volcanic deposits resulting from andesitic volcanicity due to thesubduction of the Pacifique plate under the North American plate. This S2 separate sequence of the S1 sequenceby unconformity (D0), also recorded by the beginning of NNW transformation of the horsts and the grabens inthe La Paz area. The study of the magnetic fabrics by AMS method, applied to volcanic debris avalanche (VDA)deposits and of rhyolitic deposits it possible to highlight the existence of a volcanic source, located near the areaof La Paz at the origin of such deposits. A new transtension Pliocene phase allowed the opening of Gulf ofCalifornia and the formation and the development of Alfonso - El Carrizal basin along the major faults of La Pazand El Carrizal which marked the final stage of the rifting, represented by the appearance of the transformantfaults and the formation of the oceanic crust in the Gulf of California, after approximately 3,5 My. The latter isrecorded by the deposits of deltaic facies of the S3 sequence. A great period of erosion separates the S3 and S4sequences corresponding to the post-rift limit D2 which corresponds to the phase of tilting and rotation of theblocks formed after NNW-SSE fracturing. This new provision involves the migration of the deposits and theinstallation of the S4 sequence in NW part of Alfonso - El Carrizal basin. It is made up of marine sedimentsforming marine terraces along the peninsular coast. This Pleistocene post-rift transgressive sequence wasinterpreted like a marine high level procession forming part of the phase of opening, which continues actually inthe divergent margin of the Gulf of California.Keywords: fore-arc basin – subduction – gulf of California – ignimbrite – volcanic debris avalanche – rift.