Long-term evolution of subduction zones and the development of wide magmatic ares

Abstract

Late Mesozoic-Cenozoic igneous rocks cover a large arca in southwestem United States and northem Mexico, which extends over several provinces with contrasting structural and petrotectonic characteristics and lithospheric structure. Chemical and petrographic data show magmatic are affinities, particularly for the calc-alkaline suites, supporting a gem'tic link with the plate convergence process between the North American and the Farallon and Kula plates. However, the wicith of the magmatic are (in excess of 1100 km) and the trench-arc gap (estimated from paleogeographic reconstructions) are in marked contrast when compared to the ranges observed in contemporary subduction zone-magmatic are systems. Geochronological and stratigraphic studies have documented apparent east-west migration pattems of activity, which support that the magmatic province is the result of a long-term evolution of the convergent continental margin. The wide magmatic province defined by the space-time pattem of geochronological data is referred to as a spatial magmatic are. Several models with changing geometrical, kinematic and dynamic relationships are discussed: (1) variable subduction dipconstant depth and range of melting (magma generation zone); (2) variable dip-variable depth and range of melting; (3) low-angle subduction and variable trench-arc gap (lateral migration of trench due to variable dip in the shallow zone and sediment accretion); (4) extensional tectonism; and (5) intra-arc and back-arc extension. The space-time patterns show that the spatial magmatic are was displaced away from the trench up to 450 km bctween 120 Myr to 55 Myr, and then back towards the trench between 30 Myr to 20-15 Myr. The width of the spatial magmatic are increased up to 550 km from 120 Myr to 70 Myr and then remained fairly constant up to 20 Myr. Igneous rocks at the easternmost end of the magmatic are occur in the Gulf alkaline province, whose geochemical and petrographic characteristics show a transition from subduction related to intraplate extension, in agreement with a model of low-angle subduction and lateral migration of activity during the Tertiary.