Lithospheric Structure of the Pampean Flat Slab Region From Double-Difference Tomography

Date: 4/25/2019

Time: 02:45 PM

Room: Cascade II

We obtain earthquake locations and a 3D velocity model of the flat slab subduction zone in west-central Argentina using a regional-scale double-difference tomography algorithm with earthquake data recorded by two temporal broadband networks. In this region, the flat subduction of the Nazca Plate including the Juan Fernandez Ridge is spatially correlated with a volcanic gap and the Sierras Pampeanas in the overriding South American Plate. Our results show the subducting Nazca Plate as a continuous band of increased P-wave velocities coinciding with the Wadati-Benioff Zone. In the overriding South American Plate the lithosphere mantle appears to be heterogeneous but mostly characterized by a Vp/Vs of 1.75-1.77, which is consistent with depleted peridotites. Two Vp/Vs anomalies deviate from this mantle with lower (1.70-1.73) and higher (1.78-1.82) Vp/Vs, which are interpreted as localized dry and hydrated regions, respectively. The lower Vp/Vs is consistent with an enrichment of 40-80 % of orthopyroxene and the higher Vp/Vs with up to 5% mantle hydration. The size, orientation, and location of these anomalies suggest the progressive eastward dehydration of the subducting slab and the presence of an east-dipping large-scale lithospheric suture, which is interpreted as evidence of an ancient subduction zone and also as a weak zone that facilitates the hydration of the upper plate. Finally, the relocated slab earthquakes illuminate the slab geometry and suggest that at depths of ~100 km the flat slab segment is 240-km wide and has a slight westward dip (2º) before it resumes its descent into the mantle. We support the interpretation that the hydrated and moderately overthickened oceanic crust of the Juan Fernandez Ridge may be an important factor that contributes to the flat slab geometry, but the observation of a wider flat slab segment than the width of the ridge offshore (~100 km) also implies that there might be additional contributing factors for the flattening.

Presenting Author: Lepolt Linkimer

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