SUBDUCTION AND ASSOCIATED MAGMATISM. CENTRAL MEXICO

We present a reconnaisance geologic map in Central Mexico of a Cordilleran megasuture bounded eastward by the Sierra Madre Oriental foreland fold/thrust belt which is floored by a gneissic basement. The megasuture records a dominant compressional regimen of deformation signed by two major angular unconformities of: a) an epimetamorphic pelagic volcano-sedimentary Jurassic sequence which has been intensely thrusted and polyfolded and uplifted before Lower Cretaceous rocks, b) Laramide deformed Cretaceous marine shallow deposits and, c) Tertiary andesites and mainly rhyolite sheets of the Sierra Madre Occidental. The SMOr includes synextensional Jurassic-Cretaceous marine rocks and some basaltic flows, compressed by the Laramide orogenesis and carried up late synkinematic granitoid intrusions and ore deposits. To the south the SMOr supports with angular unconformity synvolcanic to postvolcanic extensional faulting of the active Mexican Volcanic Arc of basaltic composition. Here, the Aljibes half graben master fault is listric and is structuraly connected to the Laramide El Doctor thrust fault suggesting extension in the upper part and compression in the lower part. A balanced cross section shows the SMOr to be thin skinned and shortened about 40 % of a 130 km length strip. Ampferer subduction should dispose deeply the basement due to MVA connected detached faults channelising magma, whose tendency from Oligo-Miocene rhyolites of the SMOc. to late Miocene-Quaternary basaltic rocks of the MVA. indicate deepening of faulting. Earlier subduction is indicated by the Jurassic broken formation which suppose an accretionary prism formed by offscraped rocks from Wadatti-Benioff subducted lithospheric plate inclined to the west.