Search for: All records

The structure of the lithosphere-asthenosphere boundary (LAB) beneath oceanic plates is key to understanding how plates interact with the underlying mantle. Prior contradictory geophysical observations have been used to argue for a thin, melt-rich boundary that decouples the plate from the rest of the mantle, or for a much broader anisotropic and thermally controlled boundary that indicates significant coupling with the rest of the mantle. The predictions of models based on these interpretations can be tested most easily in a subduction zone setting where the steady increase in pressure at the base of the subducting plate’s LAB will have differing effects on melt and anisotropy. Melt remains stable within the mantle to ~150-250 km (for carbonate melt) or to ~330 km (for silicate melt), while anisotropy induced by different processes should have no significant change until ~250 km to ~440 km depth. We calculate P-to-S receiver functions (PRFs) using varying frequency bands at broadband seismic stations with >4 years of data from the Servicio Geológico Colombiano’s Red Sismológica Nacional de Colombia to investigate the characteristics of the LAB of the subducting Nazca oceanic plate from the coast to the Andean foreland (corresponding to slab LAB depths of ~50 km to >400 km). The use of PRFs permits identification and analysis of anisotropy across the boundary while calculation at a range of frequency bands permits tuning of the PRFs to differing spatial scales to determine the size and abruptness of the boundary. We find that the P-to-S converted phase of the subducted Nazca plate’s LAB is detectable 4-5 seconds after the converted phase of the plate’s Moho to at least ~150 km depth. Assuming the slab has an average Vp/Vs of 1.75 to 1.78 and Vp of 8.2 km/s (+2.5% dVp), this corresponds to a plate thickness of ~50 km, matching the expected thickness given the Nazca plate’s age in the region (~10-20 Myrs). We find that the Nazca plate’s LAB is most consistently detectable in the <0.24 Hz band and largely undetectable in the <2.4 Hz band, indicating the LAB is gradational and between 10 and 30 km in thickness. Amplitude variations and complexities in the LAB converted phases further indicate that the boundary marks a change in anisotropy most consistent with the LAB representing a sheared zone between the plate and underlying mantle.