Receiver function imaging of lithospheric structure and the onset of melting beneath the Galápagos Archipelago

- We image the crust and mantle beneath Galápagos with receiver functions and the SIGNET array.
- A Moho phase at 11 km and a deeper velocity increase near 37 km depth are imaged.
- The lithosphere-asthenosphere boundary is imaged at 66-82 km, deeper than thermal models (∼50 km).
- The onset of melting is imaged at 125-145 km, deepest by the hotspot and 2 regions near the ridge.
- A compositional lithosphere and multiple plume diversions explain the observations.

The Galápagos Archipelago represents an opportunity to investigate the properties of young oceanic lithosphere, the effects of a hotspot anomaly on lithospheric thickness, and melting dynamics in a hotspot-ridge interaction. Here we use data recorded by the SIGNET array and permanent station PAYG on the Islands Santa Cruz and Isabela, respectively. We used P-to-S (Ps) and S-to-P (Sp) receiver functions to constrain crust and mantle structure. A simultaneous deconvolution method was used to constrain 1-D structure and also for the modeling of robust features. A migrated extended multitaper method was used to investigate 3-D structural variations. Ps images a velocity increase with depth at 11±7km, probably the base of the pre-plume crust, or old Moho. Sp imaging and modeling images a second, deeper velocity increase at 37±7km depth. A velocity decrease with depth is imaged on average at 75±12km likely associated with the lithosphere-asthenosphere boundary. This discontinuity is imaged deeper, 82 km, in the southwest and shallower, 66 km, in the northeast near the spreading ridge. Although the trend is consistent with lithospheric thickening with age, the thickness is much larger than predicted by conductive cooling models of 0-10 My oceanic lithosphere. We infer a compositional contribution to velocity variations. Finally, a velocity increase with depth is imaged at ∼125 to 145±15km depth that is likely associated with the onset of melting. The discontinuity is imaged deeper in 3 sectors of the Galápagos platform-ridge region, all coincident with the slowest surface wave shear velocity anomalies in the upper 100 km. One is located in the southwest in a hypothesized plume location. The other two are to the northwest and northeast, possibly illuminating multiple plume diversions related to complex plume-ridge interactions.