Late mesozoic-cenozoic evolution of the Northwestern Mexico magmatic arc zone

Abstract

Late Mesozoic - Cenozoic radiometric data on igneous rocks from the extensive magmatic province of northwestern Mexico are used to investiga te on the evolution of the continental margin. Results show that magmatic activity changed systematically with time from about 140 my B.P. to the present. Between about 140 and 60 my B.P. the trench-arc gap increased from about 100 to 400 km with a velocity varying from 1 to 12 cm/yr. The area covered by magmatic products also increased up to ~540 km, which, combined with the trench-arc gap changes, resulted in a ~1 200 km wide spatial magmatic are. Magmas were dominantly calc-alkaline and tectonism was predominantly compresiona!. Between about 60 and 35 - 30 my B.P., magmatic activity developed at points farthest from the trench, giving high-K calc-alkalic and alkaline rock suites. Tectonism changed from mainly compresional (Laramide orogeny, ~80-45 my B.P.) to mainly extensional (mid-Tertiary orogeny, ~35 - 15 my B.P. Finally, between about 35 - 30 and 10 - O my B.P., magmatic activity was rapidly displaced towards the trench and tectonism was mainly extensional. Bimodal volcanism developed including the emplacement of a thick ignimbritic sequence, and apparently continued long after cesation of active plate subduction. Major plate reorganization occurred during this interval, associated with the encounter of segments of the spreading center and trench, which resulted in replacement of plate subduction by transform motion. Major recent events include the development of the Gulf of California, about 15 my ago, with active spreading occurring mainly during the last 4.5 my. The changes observed in the trench-arc gap and in the width of the zone of magma tic manifestations are interpreted in terms of a model of variable subduction zone dips and variable depth and range of magma generation. Major factors involved were the change of age of the plate being subducted (progressively younger), rate of plate convergence, absolute motion of upper plate, sediment accretion, obduction, subduction of oceanic plateaus and tectonic extension. Relative importance of these factors, however, remains to be quantified.