Date

5-1-2019

Document Type

Thesis

Degree

Master of Science

Department

Earth and Environmental Sciences

First Adviser

Anne S. Meltzer

Abstract

Mongolia has a complex tectonic history. The lithosphere was formed from multiple plate collisions in the Neoproterozoic - Early Paleozoic associated with the Central Asian Orogenic Belt. The region has since been modified by Mesozoic rifting, Cenozoic magmatism, and major strike-slip faulting along terrane boundaries and sutures. To gain a deeper understanding of modern deformation within the Hangay Dome in Central Mongolia, two years of seismicity, recorded by a dense array of 72 broadband seismic stations, was used to assess the distribution of seismicity and determine crustal velocity structure. Results from a joint tomographic inversion for earthquake location and 3D velocity structure show a relatively uniform crust. P-wave velocities in the upper 10 km of the crust range from 5.5-6.1 km/s, from ~6.2-6.6 km/s in the mid-crust, and spans velocities of ~6.7-7.3 km/s in the lower-most crust. Seismicity in the Hangay Dome aligns with faults along terrane boundaries, indicating that terrane sutures are zones of crustal weakness. Events relocated in the inversion outline the Bulnay and Bogd faults, where historic Mw > 8 earthquakes have occurred. Considerable seismicity is observed on the South Hangay – Bayanhongor Fault System, including a ML 5.2 earthquake and aftershock sequence. Seismicity is also observed along the Egiin Davaa and Mogod Faults. Seismicity in the Hangay Dome is generally confined to the upper 30-35 km, suggesting a rheological transition from brittle to ductile at this depth. Source characterization of seismicity along the South Hangay segment of the South Hangay – Bayanhongor Fault system is left-lateral strike slip, along with oblique normal faulting due to releasing bends in the arcuate fault system. The Bayanhongor Fault segment mainly exhibits reverse slip; however, there are some focal mechanisms which suggest oblique strike-slip motion. This suggests that the Bayanhongor Fault is acting as a restraining bend along the larger strike slip fault system. The primarily left-lateral strike slip motion along the South Hangay – Bayanhongor Fault System is consistent with the escape tectonics regime influencing Central and Eastern Mongolia. These results indicate that the modern tectonic regime is reactivating ancient terrane sutures in the Hangay Dome, and structural inheritance appears to have a large control on the distribution of seismicity in Mongolia.

Available for download on Friday, February 26, 2021

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