P- and S-Wave Receiver Function Images of Crustal Imbrication beneath the Cheyenne Belt in Southeast Wyoming

Abstract

We extend the receiver function deconvolution methodology of Bostock (2004) to S-wave receiver functions and develop a method of source function spec- trum estimation to constrain the crustal structure across the Archean-Proterozoic Cheyenne belt suture in southeast Wyoming using data from a dense deployment of seismic stations. S-wave receiver functions are particularly useful because free- surface reverberations do not contaminated the direct Sdp arrivals, and the S-wave receiver function image is able to validate our P-wave receiver function image. P- and S-wave receiver function images and a teleseismic P-wave tomogram find a structure consistent with the imbrication of Proterozoic lower crust across the Chey- enne belt. Both P and S-wave receiver function images delineate a double Moho north of the Cheyenne belt: the Archean Moho is imaged at 41-43 km depth with a deeper velocity step at 60-62 km depth. South of the Cheyenne belt, the P-wave receiver function image finds the Proterozoic Moho dipping ∼7° northwest consistent with observed back-azimuth dependent Pms amplitudes. Given the lateral continuity with the northwest dipping Proterozoic Moho, the deeper velocity step of the double Moho is interpreted as the imbricated Proterozoic Moho. Modeling of P-wave re- ceiver function amplitudes suggests a 6:4-7:4 km=sec velocity step across the shal- lower Archean Moho and a 7:4-7:9 km=sec velocity step across the deeper imbricated Proterozoic Moho. We speculate that imbrication of the Proterozoic lower crust was contemporaneous with the 1.76 Ga uplift and deformation of the 50 km-wide Palmer Canyon block immediately north of the Cheyenne belt exposed in the Laramie Mountains.