THE DYNAMIC HISTORY OF 220 MILLION YEARS OF SUBDUCTION BELOW MEXICO: A CORRELATION BETWEEN SLAB GEOMETRY AND OVERRIDING PLATE DEFORMATION BASED ON GEOLOGY, PALEOMAGNETISM, AND SEISMIC TOMOGRAPHY

The North American Cordillera has been shaped by a history of accretion of arcs and other buoyant crustal fragments to the western margin of the North American Plate. Accretion of these terranes resulted from a complex tectonic history including episodes of both intra-oceanic subduction within the Panthalassa/Pacific Ocean, and continental margin subduction. Western Mexico, at the southern end of the Cordillera, contains a Late Cretaceous-present day continental margin arc, as well as older Mesozoic arc and SSZ ophiolite terranes. Interpretations of the origin of these older subduction-related rock assemblages vary from far-travelled exotic intra-oceanic island arc character to autochthonous extended continental margin origin. Here, we present new paleomagnetic data from four localities: (1) the Norian SSZ Vizcaíno Peninsula Ophiolite; (2) its Lower Jurassic sedimentary cover; and (3 and 4) Aptian sediments derived from the Guerrero arc. Additionally, we analyze seismic tomographic images of the positive wave speed anomaly below Mexico, stretching from the uppermost mantle at the Middle America trench in the west, to the lowermost mantle below the Atlantic in the east. The length and continuity of this slab indicates long-lived, uninterrupted eastward subduction of the attached Cocos Plate and its predecessor, the Farallon Plate. The paleomagnetic data show that the Mexican Mesozoic ophiolite and arc terranes have a paleolatitudinal plate motion history equal to that of the continent. This suggests that these rock assemblages were part of the overriding plate and were only separated from the North American continent by temporary fore- or backarc spreading. The entire Triassic-present day Mexican subduction record can therefore be linked to the Cocos slab. We correlate episodes of overriding plate shortening and extension to flat and steep segments of the slab. By integrating paleomagnetic, geological, and tomographic evidence, we provide a model that reconciles absolute North American plate motion and the deformation history of Mexico since Late Triassic time with modern mantle structure. We conclude that phases of fore- and backarc extension within the continental margin and subsequent closure of these basins led to the growth of the Mexican segment of the North American continent without addition of any far-travelled exotic terranes.