Characterization of the Oceanic Lithosphere-Asthenosphere Boundary in the Colombian Subduction Zone through Receiver Function Analysis

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).

The goal of this project is to test two competing models of the boundary separating the Earth’s stiff outer layer (lithosphere) from the underlying, weaker layer (asthenosphere). One model proposes that this boundary is a narrow channel that decouples the lithosphere and asthenosphere. The other model proposes that this boundary forms a broad band of deformation that couples the lithosphere and asthenosphere.

Each model has important implications for our understanding of plate tectonics—the process which drives the Earth’s long-term geologic evolution—and its relation to the Earth’s interior. Each model carries predictions about the boundary between the lithosphere and asthenosphere. These predictions can be tested using receiver functions.

In collaboration with the Universidad Nacional de Colombia, this project uses recordings from seismic stations of the Colombian National Seismic Network located above the Colombian subduction zone. In this region, the characteristics of Nazca oceanic plate’s lower boundary can be examined over a range of depths to determine which of the models is most accurate.

Understanding which model is most accurate will benefit society by helping to constrain the force balance acting on tectonic plates, especially around subduction zones where potentially destructive earthquakes driven by these forces occur. This project further benefits society by deepening international collaboration between institutions in the US and Colombia as well as furthering STEM education by providing training in seismology to a postdoctoral fellow.

Observations of the oceanic lithosphere-asthenosphere boundary (LAB) support two categories of contradictory models for coupling between the lithosphere and underlying asthenosphere, and correspondingly contradictory implications for plate tectonics. One category interprets the boundary to be a narrow, melt-rich channel that decouples lithospheric plates from the asthenosphere.

Plate motion is thus driven largely by slab pull. The other category interprets the boundary as a broad zone which couples plates to the asthenosphere. Plate motion is thus strongly influenced by asthenospheric flow induced drag.

These models predict differing structures and localizations of shear along the LAB. Channel models predict two closely spaced, highly sheared boundaries while coupled models predict either a single boundary with little shear or a single highly sheared boundary. The predicted features respond differently to increasing pressure due to subduction.

Channels narrow with depth and coupled structures persist to greater, differing depths, so observing a plate as it subducts offers an ideal natural experiment to test these models. Seismic waves with different frequency contents interact with these structures such that high frequencies can better image channels while low frequencies can better image coupled structures.

This project uses P-to-S-wave receiver function analysis of radial and transverse components recorded at broadband seismic stations to examine structural characteristics and shear localization along the boundary. The complete characterization of structure and anisotropy caused by shear requires long-term observations at seismic stations from the coast to the back arc at a range of frequency bands.

This project uses data from 2010 to the present from 26 stations in the Colombian National Seismic Network to investigate and characterize the subducting Nazca oceanic plate’s LAB from depths of <100 km at the coastline to >400 km in the back arc. These observations will provide constraints on the depth, thickness, and anisotropic properties (if present) of the boundary and use these to test the proposed models.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.