SUPPOSITIONS ABOUT SHALLOW EXPRESSION OF DEEP SEATED IGNEOUS PROCESSES. Carrillo M.M. Instituto de Geologia, U.N.A.M. C. U., México D.F. Mexico. e-mail: mcm@servidor.unam.mx

A general comparison of two collided areas in Central Mexico suggests the existence of a Cordilleran mobile belt or megasuture bounded to the north-east by complementary continental subduction. Three rock assemblages separated by two major angular unconformities characterize the megasuture: a) a Late Jurassic pelagic and epimetamorphic volcanosedimentary broken sequence which record two main compressional deformations, Nevadan? and Laramide; b) a Laramide structured Cretaceous marine sedimentary deposits and, c) a Cenozoic silicic volcanic rocks and felsic intrusions of the Sierra Madre Occidental showing strike slip and normal faults which deform the Mesozoic rocks also. Toward the northeast crops out the at least 4 000 m thick marine sedimentary rocks of the marginal Sierra Madre Oriental fold/thrust belt: which include in their southwestern border Late Jurassic to Early Cretaceous coarse siliciclast and alkaline volcanoclastic rocks probably deposited during high angle normal faulting. The style of deformation is mostly thin skinned, whose subducted crystalline basement deduced from palinspastic reconstruction should intercept the deeply and high angle detached faults which channelized magma of the S.M.Oc. This fact presume sinking of the sialic plate to depths enough to cause early andesitic to rhyolitic vulcanism and late basaltic extrusion as faults deepened. Further data agree with this scheme is provided by intermediate to felsic shallow intrusions during Early Eocene synchronic to principal late Laramide movements; Oligocene felsic stocks and Neogene tabular intrusions also striking mostly along or across the Laramide trend, whereas Quaternary simaic intrusions were emplazed during extension. The megasuture and the S.M.Or. passive margin contact seems to be the kilometric length and about 20 m thick the El Fronton fault zone and subsidiary faults.