Shear wave velocities in the Pampean flat-slab region from Rayleigh wave tomography: Implications for slab and upper mantle hydration

Abstract

The Pampean flat-slab region, located in central Argentina and Chile between 29° and 34° S, is considered a modern analogue for Laramide flat-slab subduction within western North America. Regionally, flat-slab subduction is characterized by the Nazca slab descending to ~100 km depth, flattening out for ~300 km laterally before resuming a more ?normal? angle of subduction. The onset of flat-slab subduction is associated with the inboard migration of deformation from the high Andes into the Precordillera and Sierras Pampeanas, as well as the eastward migration and eventual cessation of arc related volcanism. Flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, suggesting a relationship between the two. We use ambient-noise tomography and ballistic surface waves to calculate a regional 3D shear velocity model that encompasses both flat-slab subduction and normal-angle subduction located immediately to the south. Within the crust we find that shear wave velocity variations are largely related to changes in lithology, with basins and bedrock exposures clearly defined as low- and high-velocity regions, respectively. In the south, where normal-angle subduction is occurring, a low-velocity feature is observed in the upper mantle beneath the active arc, consistent with the presence of partial melt. We argue that subduction related hydration plays a significant role in controlling shear wave velocities within the upper mantle. In the normal-angle subduction zone in the southern part of the study area, the slab is visible as a high-velocity body with a low-velocity mantle wedge above it, extending eastward from the active arc. These low velocities are likely due to hot asthenosphere emplaced as corner flow.Where flat-slab subduction is occurring, slab velocities increase to the east while velocities in the overlying lithosphere decrease, consistent with the slab dewatering and hydrating the overlying mantle. As the flat slab steepens and assumes a normal angle of subduction, we observe a dipping low velocity layer above it, consistent with cooled asthenosphere or hydrated lithospheric mantle sandwiched between the subducting slab and high velocity lithosphere of the Rio de la Plata craton. Shear velocities suggest that the slab is more hydrated in the flat-slab region than to the south and that the Rio de la Plata cratonic lithosphere may be inhibiting corner flow in this area. The hydration of the downgoing slab may be contributing to the excess buoyancy of the down going oceanic lithosphere.