“BOTTOM UP” SUBDUCTION GEOMETRY INITIATION OF EXTREME LOCALIZED EXHUMATION AT OROGENY SYNTAXES

The redistribution of mass on the Earth’s surface by surface processes is hypothesized to provide a coupling between tectonics and climate. Prominent examples of these interactions have been suggested in the last decade for the Himalaya, Andes, and St Elias (Alaska) orogens. A key question that emerges from previous work is what are the relative roles and contributions of tectonics and erosion to producing the thermochronometer record of mountain building we observe today? Studies of upper plate deformation in subduction collisions are almost always investigated through the use of 2D cross sections or cylindrical bending of flat sheets, an approach which omits the 3D geometry imposed by instabilities in bending curved shells. While this reduction of geometric complexity may be reasonable for most of the extent of subduction zones, geometric effects become large in orogen syntaxes, where geometric stiffening resists further bending in the dip direction, forming a rigid, shallowly dipping indenter. The initiation of rapid and localized deformation at the ends of subduction arcs (syntaxes) has long been attributed to anomalously high erosion rates at Nanga Parbat and Namche Barwa in the Himalaya and at Mt. St. Elias in Alaska. However, an erosion-dominated mechanism ignores the 3D geometry of subducting curved shells at syntaxes. Here we present an alternative explanation for rapid exhumation at syntaxes based on the 3D thermo-mechanical evolution of collisions with geometrically stiffened syntaxial indenters in subducting slabs. Comparison of model predictions with existing thermochronometer data reproduces the defining characteristics of syntaxial mountains, and offers an explanation for their spatial correlation with arc termini. These results demonstrate a ‘bottom up’ tectonic rather than ‘top down’ erosional initiation of rapid syntaxis exhumation and the importance of 3D subduction geometry.