An investigation of the pampean flat slab and its role in andean tectonics through the analysis of enhanced earthquake catalogues

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Maharaj, Ariane
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The Andean margin of South America is an archetypal example of a convergent tectonic environment that has played a fundamental role in the creation, evolution, and consumption of both continental and oceanic lithosphere. An enigmatic aspect of this margin is the existence of several regions of flat slab subduction, the causes and consequences of which remain poorly understood despite years of intensive investigation. The research discussed in this dissertation focuses on the Pampean flat slab beneath central Chile and Argentina, partly in response to the discovery of an unusual pattern of P wave arrival times from earthquakes beneath Argentina recorded in Chile, but also because recently developed techniques in automated phase onset estimation allow us to take significantly greater advantage of a substantial archive of seismic data that has been collected in this region over the past several decades. After constructing enhanced catalogues of seismic activity and analyzing them with various techniques of seismic tomography, the improved images of the elastic wavespeed structure of this region allow insights into the state of the subducted Nazca plate, the effects of subducting anomalous bathymetry such as ridges and seamounts shortening on the interaction between the Nazca plate and the overriding South American plate, and the consequences of lithospheric shortening of the South American plate on the geometry of the Nazca plate. We find that dense clusters of seismicity associated with the Pampean flat slab are most likely a consequence of the subduction of ridges and seamounts, and that the extensive alteration and devolatilization of the oceanic lithosphere by these features leads both to an increased buoyancy and to the release of volatiles that hydrate the South American mantle and fracture the lower crust. The existence of these features far from the trench lends support to the contention that subduction erosion is more episodic than continuous. We also find 2 low velocity anomalies, particularly in Vp, within and below the subducting slab. The western low velocity anomaly correlates with the Juan Fernandez Ridge and is postulated to be due to an increase in silica content or the presence of supercritical fluids. The eastern low velocity anomaly seems to be a result of rising hot asthenosphere. Furthermore, it appears that the gradient in wavespeeds that generated the pattern of anomalous P wave residuals that originally motivated this research may be due more to the low wavespeeds in overthickened crust beneath the high Andes grading into the higher wavespeeds of the Nazca plate, rather than a pronounced lithospheric root beneath a normal crustal root. At the same time, much of the original signal remains, which suggests that the ultimate cause of the higher apparent wavespeeds may be due as much to anisotropy as opposed to lateral heterogeneity.
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Rensselaer Polytechnic Institute, Troy, NY
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