Geodynamics of the Patagonian Slab Window constrained by Shear Wave
Splitting and Seismic Imaging
Abstract
Patagonia is one of the key places to study the interaction of plate
tectonics and mantle flow patterns with geological processes. This part
of the continent is shaped by the northward migration of the Chile
Triple Junction, currently marked by subduction of the Chilean spreading
ridge at latitude 46oS, opening a slab window beneath Southern
Patagonia. The idea of slab window was hypothesized to explain the
volcanic gap between north Patagonia and the southern part of the
peninsula. The analysis of volcanic rock composition shows the
transition between a domain with the signature of slab melt
(metasomatized MORB) and a domain with no slab signature (OIB source
mantle). Along the Pacific coast, other slab windows were suggested in
Central America, California and North Cordillera. The analysis of
uplifted terranes and seismic imaging tried to constrain the geometry of
these slab windows and map the mantle flow pattern that controls the
present-day surface expression (topography, volcanism distribution).
From a limited seismic coverage, early studies mapped the Patagonian
slab window from body wave tomography and shear wave splitting. The
recent deployment of a temporary seismic array from 2018 to 2021 and the
Chilean seismic networks fills the data gap between the seismically
active northern part of Patagonia and the more poorly studied southern
part. This presentation will show the results of our recent seismic
studies in Patagonia and help constrain the geodynamical processes
associated with the slab window. From the analysis of SKS and similar
core phases, we determine the pattern of azimuthal seismic anisotropy
resulting from the mantle flow pattern beneath South America. Fast
splitting directions are generally NE-SW throughout most of Southern
Patagonia, similar to the pattern of large-scale azimuthal seismic
anisotropy from global and regional surface wave models. However,
between 46oS - 48oS, we observe large splitting values and an E-W
direction showing the effect of the slab edge. This is consistent with
models of rapid upper mantle flow from the Pacific around the southern
edge of the Nazca slab. Seismic imaging using receiver functions and
Rayleigh waves from earthquakes and ambient noise show very low upper
mantle velocities and an absence of mantle lithosphere in this region,
suggesting the lithosphere has been thermally eroded by the dynamics of
the slab window. We will also show and discuss preliminary results of a
body wave tomographic analysis of the same seismic station dataset.