Using USArray to Determine Lithospheric Thickness in the Central Andes From Teleseismic Depth-Phase Precursors

The Central Andean margin is a natural laboratory for understanding orogenic processes associated with plateau uplift and subduction of oceanic lithosphere beneath continental margins. Here we analyze seismograms of intermediate to deep Andean earthquakes recorded at USArray stations in a search for underside reflections from lithospheric discontinuities. The large number of USArray stations allows the use of cross-correlation and slant-stacking techniques to identify arrivals that are likely associated with the Moho, the lithosphere-asthenosphere boundary (LAB) and other deep interfaces in the central Andes. Our results suggest the MOHO beneath Puna-Altiplano varies in thickness from 50 to 80 km, consistent with estimates from other techniques. We also observe, for the first time, precursors that seem to represent reflections from the underside of the lithospheric-asthenosphere boundary. Importantly, this technique utilizes higher frequencies than other LAB-imaging methods such as S-wave receiver functions. Through using these higher frequencies we imply that this transition may be much sharper than previously documented. Earthquakes in the northeastern Puna (23-24S; 67-66.5W) indicate LAB precursors at depths of 135-150 km whereas further northeast under the Cordillera Oriental (22S-20S; 66-65W), the LAB deepens to 175-190 km. A relatively abrupt transition occurs between these two locales. At the southern end of the Subandean fold-thrust belt near 22.5S at 66W the LAB deepens markedly from 150 to 175km. To the east in the foreland, a north-south trending profile at 65W shows a deep, stable lithospheric thickness of ~200 km from 28S to 24S. Further south above the Pampean flatslab between 69W and 67W at 31S, we also observe the LAB. Southeast of Pie de Palo (68W) lithospheric thickness is measured to be 80 km. The LAB is not seen beneath Pie de Palo and reappears to the east in the Sierras Pampeanas at 67W at a depth of 100 km.