This project centers on two questions: 1. What props up the Rocky Mountains of Colorado and New Mexico ? is the surface elevation high because of extra-thick, buoyant crust underneath, or is the crust no thicker than in the surrounding areas and it is instead hotter than usual mantle sitting underneath that supports the unusually high elevations? To answer this question, we must map the crust-mantle boundary under the Rockies. Conventional methods to do this have so far left a fair amount of ambiguity regarding crustal thickness. We propose to solve this by investigating question 2: Can we use a recently developed method that has been successfully applied to shallow geophysical problems, such as hydrological imaging, to find the much deeper crust-mantle boundary?
We will use already collected seismic data sets and apply new analysis techniques to provide constraints on the dynamics of a tectonically enigmatic area. The Southern Rocky Mountain region has recorded a complex history of continental assembly, mountain building far from plate boundaries, and incipient continental rifting. Because the mechanism and timing of uplift of this large area are still under debate, the region has been the subject of intensive study with active and passive source seismic profiles and networks. A recurrent theme from these studies is that the high surface elevations are not mirrored by a thick crustal root, however the most recent crustal thickness estimates still differ by ~10 km. This difference has significant implications for mechanisms to explain the support of high topography in the region, specifically whether the surface elevation is isostatically balanced within the crust or support for the high elevations has to come from deeper in the Earth. Our project is an interdisciplinary collaboration to apply techniques developed by the team for shallow high-frequency seismic applications to lithospheric scale seismology. Specifically, we plan to use diffracted seismic waves, from earthquakes and from ambient seismic noise, to image the Moho (the seismic velocity contrast that can be identified with the crust-mantle boundary). Detection of Moho diffractions is to date limited to earthquakes and explosion sources. Our approach is to perform interstation crosscorrelations of previously recorded explosions and earthquakes and of ocean-generated ambient seismic noise to simulate a signal source at each station location. This technique would allow detection of Moho diffractions and therefore Moho mapping in areas where conventional techniques are not applicable.
