Structure of the subduction transition region from seismic array data in southern Peru

S U M M A R Y Data from three seismic arrays installed in southern Peruwere analysed using receiver functions from P, PP and PKP wave phases, in order to image the subducted Nazca slab. The arrays cover the transition region from flat slab subduction in central Peru to normal subduction with an angle of about 30◦ further south. A previous study used data from the first array in the normal subduction region to image the Moho depth and slab, and showed the existence of a mid-crustal structure at 40 km depth that is suggested to be a possible underthrusting of the Brazilian shield. Here, we discuss new observations from the other two arrays that span the transition between the two subduction regimes and the flat subduction region. The results provide an image of the flattened slab from the coast to approximately 300 km inland and also across the transition region from flat to 30◦ subduction, which appears to be a bend rather than a tear in the slab. In the flat slab region, the slab is well defined near the coast and flattens out at 100 km depth beneath the Altiplano. The slab appears to start flattening some 400 km in advance of the subduction of the Nazca Ridge and the flattening is maintained for 1300 km after its passage. The Moho begins at a depth of around 30 km near the coast and has a maximum depth of 75 km beneath the Altiplano, consistent with the results of the other arrays. Both arrays also show a positive impedance mid-crustal structure at 40 km depth, which if explainable by underthrusting of the Brazilian shield, would add further support to the observations from the normal subduction region and show the northward and westward extent of the signal. The underthrusting hypothesis would explain the missing crust from the shortening budget needed to support the Altiplano. The Vp/Vs ratios for both arrays exhibit average values between 1.73 and 1.75 indicating a lesser likelihood that there is a high degree of partial melting or magma bodies at depth in this region. The receiver function results provide new imaging of the flat slab and transition from normal to flat slab subduction which allows for comparison of different subduction regimes.