Seismic structure and the extent of the slab window beneath the Northern and Southern Patagonia Icefields

Abstract:

Subduction of the Chile Ridge has formed a slab window beneath Patagonia, but the spatial extent of the region perturbed by the slab window is poorly constrained. The geometry of the slab window has important implications for glacial isostatic adjustment across the Northern and Southern Patagonia Icefields, as geodynamic processes associated with the slab window are expected to dramatically thin the lithosphere and lower the viscosity of the upper mantle, enhancing the solid earth response to glacial unloading.

Plate-kinematic reconstructions suggest that the mantle anomaly due to the slab window may be broad, as the northern boundary of the slab window has moved northward from Tierra del Fuego to the present-day latitude of the Nazca-Antarctic-South America triple junction (46S) over the past 16 Ma (Breitsprecher and Thorkelson 2009). Previous regional seismic studies show a slow mantle anomaly associated with the triple junction (Russo et al. 2010), but limited data coverage precluded mapping the southern extent of the window.

We present a new seismic velocity model for Patagonia based on data from 28 passive seismic stations deployed as part of the GUANACO experiment during 2018, along with previous temporary stations and permanent stations of the Chile seismograph network. We jointly invert Rayleigh wave dispersion curves from ambient noise cross-correlation and teleseismic earthquakes with P receiver functions to obtain a regional isotropic velocity model for the crust and the upper mantle down to 200 km. Prominent features of the model include a slow anomaly at upper crustal depths associated with the sediment-filled Magallanes basin in the Southeast, and a slow anomaly in the uppermost mantle associated with the slab window in the North. The slowest part of the anomaly attributed to the slab window occurs north of 49S. This indicates that the Northern Patagonia Icefield, and perhaps the northernmost part of the Southern Patagonia Icefield, are underlain by significantly thinner lithosphere and lower upper mantle viscosity than regions farther south.