The goal of this project is to understand the solid earth response to the loss of ice mass in the Southern Patagonia Icefield (SPI) since the Little Ice Age, which ended about 1850.   This requires an understanding of the structure of the crust and upper mantle beneath the SPI, since this controls the elastic response and viscous flow in the solid earth that accompanies ice mass loss.  We constrained these parameters by installing and operating a network of 27 broadband seismographs throughout much of southern Chilean and Argentinian Patagonia from November 2018 to February 2021. The results of the seismic analysis shows low upper mantle seismic velocities and very thin rigid lithosphere that coincides with a gap in the subducting plate beneath Patagonia.  The gap in subduction has enabled hotter, less viscous mantle material to flow underneath the region.  The low viscosity mantle allows rapid response to the loss of ice in recent decades, leading to extremely rapid uplift of the SPI region, as observed in recent geodetic studies.   Using the seismic structure, we derive an estimate of the three-dimensional variation of mantle viscosity beneath Southern Patagonia (figure 1). This estimate is being used by other investigators to reconcile geodetic uplift measurements with estimates of ice mass loss. 

                  The directional dependence of seismic velocity, termed seismic anisotropy, allows us to study the crystalline fabric of the mantle that results from deformation due to mantle flow at depth.  We use S-wave measurements of seismic anisotropy beneath the seismic stations to infer the directions of mantle flow.   We find that mantle material flows eastward in a warm, low viscosity shallow mantle channel, through the gap in subduction, from below the Chile Rise spreading center under the South American continent (figure 2).  This mantle flow thermally and mechanically erodes the lithosphere and feeds the region of hot mantle beneath the SPI.   

                  The deployment of 27 seismic stations also allows us to study the local seismicity and infer the stress distribution in a region undergoing rapid deglaciation.  Most of the local seismicity is within the upper 25 km of the earth and located near the SPI.  Earthquakes to the east of the SPI show compressional faulting, whereas earthquakes beneath the SPI and to the west show a complex pattern, with mostly strike-slip and normal faulting.  Very few earthquakes occur near the subduction megathrust plate boundary to the west.   We attribute these observations to stress fields induced by the rapid deglaciation and uplift of the SPI region.  In addition, we interpret the lack of seismicity in the subduction megathrust as indicating that the plate boundary may be locked, with stresses building up to a major earthquake in the future.

Last Modified: 03/23/2024
Modified by: Douglas A Wiens