A BALANCING ACT OF CRUST CREATION AND DESTRUCTION ALONG THE WESTERN MEXICAN CONVERGENT MARGIN

Convergent margins are the manufacturers of continents, but they are also the main drivers of continental demise via sediment subduction and tectonic erosion. Balancing the competition between crust production and destruction is therefore key to our understanding of the origin and evolution of continents. Using a comprehensive geochemical and volumetric database of arc volcanism and forearc lithologies, we provide the first attempt to balance the volumes of crust being consumed, reworked and produced along the western Mexican margin over the past million years.

The Mexican subduction zone represents a typical erosive margin where large volumes of tectonically eroded forearc lithologies have dominated the crustal input to the subduction channel since the late Miocene. Although the Quaternary continental arc is compositionally heterogeneous, mantle-derived high Mg# andesites erupting from large stratovolcanoes represent roughly 80% of the estimated extrusive volume (~900 km3). In order to fully capture the compositional heterogeneity of the forearc lithologies, and further test their potential contribution to arc volcanoes, we employed a high-density sampling strategy of river-mouth fluvial sediments draining to the coastal plain, complemented with off-shore piston-core hemipelagic sediments and strategically located bedrock samples. We observed that stratovolcanoes in the region show the same isotopic variation found in sediments and bedrocks located at equivalent distances along the trench, strongly suggesting that reworking of subducted forearc lithologies in fact plays an important role in arc petrogenesis. We further estimate that at least 25% of the magma mass of andesites must come from reworking of subducted forearc lithologies, with almost an equal proportion derived from melts of the subducted oceanic crust. Furthermore, while the total volume of subducted continental material (~9,650 km3) over the past million years appears to be effectively counterbalanced by contemporaneous magmatism (considering a typical intrusive/extrusive ratio of ~10; Lipman and Backman, 2015, GES), the amount of crustal reworking calculated for this particular arc section would be no less than 20%, about 2.5 times higher than previous estimates for the Mexican arc (8%; Clift et al., 2009, ESR).