RECEIVER FUNCTION INVESTIGATION OF THE CRUST AND UPPER MANTLE OF THE ROCKY MOUNTAIN FRONT

The Rocky Mountain front is a region of varied geological terrains representing nearly the full range of Earth’s history. This includes the currently and recently tectonically active provinces: Rio Grande Rift, Colorado Plateau and Rocky Mountains and differently aged Precambrian terrains such as: the Archaean Wyoming Province (2.4-3.9 Ga), Trans-Hudson Orogen, Yavapai Province (1.74-1.78 Ga), Yavipai-Mazatzal transition zone, Mazatzal Province (1.65-1.7 Ga) and Grenville Province (1.1 Ga). As a result, it is ideal for investigating the relationship between tectonic features and lithospheric development and stabilization.

The EarthScope transportable array creates a 70 km grid of seismic stations. This provided two years (each) of seismic data that we processed into receiver functions for single station analysis of Vp/Vs ratios and depths to various crust and upper mantle boundaries. The Moho depths are greatest along the transition from the Rocky Mountains to the Great Plains and beneath the Colorado Plateau. The depths to the Hales Discontinuity (80 km) does not correlate with currently active tectonic features, except it appears to be related to the Rio Grande rift but is more shallow and offset slightly to the east. Lithosphere-Asthenosphere boundary (LAB) depths are generally deeper to the south than to the north. The depth of the boundary between the deep and shallow LAB coincide, roughly, with the boundary between the Yavipai-Mazatzal transition zone and Mazatzal Province. Average Vp/Vs ratios, from the surface to each of the three boundaries, were determined by stacking receiver functions with differing Vp/Vs ratios and selecting the values which produced the most coherent frequency and alignment of the Ps phase and various reverberations. The average Vp/Vs ratio from the surface to the Moho shows no correlation to geologic provinces. By stripping the Moho Vp/Vs ratios from those derived from the Hales discontinuity, we estimate average Vp/Vs ratios for the interval between the Moho and Hales discontinuities for each station. This Moho-to-Hales interval Vp/Vs ratio map has lower values to the south of the boundary between the Yavipai-Mazatzal transition zone and the Mazatzal Province. The interval Vp/Vs values between the Hales and LAB show no clear pattern with regard to the geologic provinces.