Investigating Cordilleran Crustal Structure Through Use of Local Crustal Earthquakes and Teleseismic Receiver Functions

Abstract

Local earthquake locations and teleseismic receiver functions can provide insight into the structure of orogenic crust. This dissertation summarizes three studies that focus on using locations of local earthquakes, determination of crustal velocity models, and calculation of teleseismic receiver functions to better understand parts of two Cordilleran orogenic systems. In the southern Sierra Nevada of North America, we used broadband seismic data from a temporary network to locate 131 local earthquakes, ranging in depth from 3.1 to 47.1 km, which occurred between June 2005 and May 2006. This catalog of earthquakes improves on those reported by regional catalogs for this period, locating more events and more accurately determining their depth. These small earthquakes cluster into two distinct groups and occur above a Moho that shows a sharp, high-amplitude character at < 40 km depth to the northeast but deepens and becomes more diffuse and low-amplitude to the southwest. These earthquakes provide information on where the localized foundering of lithosphere and upwelling of asthenosphere may be occurring in the southern Sierra Nevada. In the second study, we analyzed local earthquakes from a 2-year broadband seismic deployment in the north-central and central Andes (between latitudes ~10°-20°S) and found the crustal events were constrained mainly to the upper ~10 km. Based on the shallow depths of the crustal earthquakes we interpret the mid and lower crust to be ductile, a rheology consistent with the Andean Low Velocity Zone located at a depth of ~15-25 km that is identified across the Central Andean Plateau. For our third study we used receiver functions to provide a more detailed map of crustal thickness than previously available for the central Andes in northwest Bolivia and southern Peru. Overall, the high elevations have thick crust, with crustal thicknesses of >70 km beneath the Altiplano and Western Cordillera, and lower elevations have thinner ~40 km thick crust beneath the active deformation front. However, there are two isolated areas of thinner than expected crust beneath the >4 km high peaks of the Cordillera Real that we suggest are due to climate induced exhumation. We also identified mid-crustal conversions in the receiver function stacks that correspond with the basal décollement in the Subandean fold and thrust belt and the top of a low velocity zone corresponding to the previously mentioned Andean Low Velocity Zone.