Date

2015

Document Type

Thesis

Degree

Master of Science

Department

Earth and Environmental Sciences

First Adviser

Meltzer, Anne S.

Other advisers/committee members

Stachnik, Josh C.; Zeitler, Peter K.

Abstract

Origin and support of high topography in an intracontinental setting is not fully understood. The Hangay Dome in central Mongolia spans an area of ~200,000 km2 and reaches elevations of ~4,000 m. It has a complex accretionary history associated with the Central Asian Orogenic Belt and is bound to the north, south, and west by active strike-slip faults. The extent to which the accretionary history or present day deformation contributes to current topography remain open questions. Geodynamic models that have been proposed to account for current topography include far-field effects of Pacific Plate subduction or the India-Asia collision, rifting stemming from the Lake Baikal region, mantle-plume activity, upwelling of the asthenospheric mantle, lithospheric delamination, and/or the underplating of magmatic rocks at the base of the crust. In order to determine which whether upper mantle structure might contribute to the origin of high topography in the Hangay, two years of teleseismic P and S body wave data are inverted for 3D velocity variations in Vp and Vs in the upper mantle beneath the Hangay. Velocity perturbations range between +/-3% for the P wave model and +/-7% for the S wave model. Changes in velocity are a function of temperature, density, composition, and presence of melt or fluid. Thermal anomalies are the primary causes for velocity perturbations in the upper mantle. The Hangay is underlain by non-uniform low velocity zones that correlate well with areas of the Hangay that have experienced volcanism in the past ~30 Ma. High velocity zones are located off the edges of the dome to the west, east, and south. One low velocity anomaly in particular, is located near the headwaters of the Orkhon River beneath a region that had experienced magmatism ~15-20 million years ago. Interestingly, this region sits between two areas ~50 km away on either side that have experienced magmatism in the last 3 million years. This low velocity anomaly has a ∆Vp of -4% and ∆Vs of -9%. This high perturbation may imply a compositional difference or presence of fluid/melt. Analysis of the results from this study, combined with results from previous studies, is consistent with asthenospheric upwelling that could be caused by small-scale edge-driven convection or delamination of the lower lithosphere.

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