Crustal thickness in the central Andes from teleseismically recorded depth phase precursors

Abstract

SUMMARY Although crustal thickness is a critical constraint for geodynamic models of Andean orogenesis, relatively few measurements exist from local seismic studies. In this study, we use reflections from the underside of the Moho as illuminated by intermediate depth earthquakes to provide new estimates of crustal thickness for the central Andes (16–34 ◦ S). These reflected signals (pmP) are identified as precursors to the depth phase pP when recorded at teleseismic distances (35–85 ◦ ). Although relatively small in amplitude, the pmP phase is often clear even on single seismograms. Less obvious pmP phases were enhanced by stacking traces from arrays. This method was most effective for events of M w > 6 and depth > 100 km. Crustal thickness determined in this study ranges from 59 to 70 km in southern Peru, from 49 to 80 km across the Puna-Altiplano and from 50 to 60 km above the Pampean flat slab. The lack of such phases for certain events may be evidence of heterogeneity at the Moho, and some precursors appear to correspond to intracrustal (magma?) and lithospheric mantle discontinuities. The results obtained using the pmP technique combined with those from other studies show the crustal thicknesses under the Central Andes are more variable than expected from purely isostatic considerations. We infer that one or a combination of geodynamic processes involving lower crustal flow, lower crustal delamination, and mantle–crustal lithospheric coupling can explain the highly variable topography on the Moho beneath the Central Andes.