GSA GEOPHYSICS DIVISION GEORGE P. WOOLLARD LECTURE: CONTRIBUTIONS OF MODERN SEISMIC IMAGING TO UNDERSTANDING THE FORMATION OF THE ANDES
The South American subduction zone has two regions of flat slab subduction in Peru, and central Argentina separated by a segment of “normal” subduction and an active magmatic arc. In the depth range of 80-120 km, the Argentina flat slab has high rates of seismicity while the Peru flat slab has much less seismicity suggesting a possible difference in hydration between the two regions. Both these flat slab segments show indications of strong coupling to the over-riding plate and associated slab tears. In Argentina, the slab tear is down-dip of the flat slab and mantle anisotropy (as measured by shear-wave splitting) suggests mantle flow from below the slab through the large tear at ~200-350 km depth. The Peru flat slab also has several tears both parallel and perpendicular to the slab strike. In both flat slabs the upper plate deformation extends ~100-200 km further inboard of where the slab begins to re-subduct.
The thick crust (up to ~75 km) of the central Andes has strong positive radial anisotropy in the mid- to lower crust that we interpret as the result of mineral alignment due to ductile crustal deformation and flow. The active arc and backarc of the Puna Plateau in southern Bolivia and northern Argentina show evidence of MASH zones at the crust-mantle transition and large mid-crustal low-velocity bodies. These large low-velocity bodies represent a partially molten mid-crust where magma can further evolve to higher silica concentrations before erupting. Associated with the largest of these low-velocity bodies, we observe very strong positive radial anisotropy interpreted as a horizontally layered magmatic storage system. These results place new constraints on the relative roles of magmatic addition and crustal shortening in the formation of the Ande