Contrasting lithospheric signatures across the western United States revealed by Sp receiver functions

Abstract

To explore how lithospheric structure varies between tectonic and magmatic terranes of the western United States, we use observations of shear-to-compressional wave conversions across lithospheric velocity interfaces. With a newly developed automatic method designed to take advantage of the EarthScope Transportable Array and other broadband seismic arrays deployed across the region, and an optimized deconvolution technique based on the extended-time multi-taper method, we map variations in the depth to the seismic Moho, lithosphere–asthenosphere boundary (LAB) and mid-lithospheric discontinuities (MLDs). Beneath areas that have undergone substantial extension in the Neogene—including the eastern Great Basin, the southern Basin and Range, and the southern Rio Grande Rift—we detect strong Sp conversions across a relatively shallow (60–80 km) seismically-defined LAB. We detect Sp conversions across an 80–90 km depth discontinuity beneath the High Rockies, which we interpret as the seismically-defined LAB. Beneath areas that have remained relatively undeformed—including the Wyoming craton and the Great Plains craton—we detect Sp conversions in the 90–140 km depth range that are substantially weaker and more distributed in depth than those beneath less stable regions. Beneath the Colorado Plateau, Sp CCP stacks show that lithospheric thinning has accompanied magmatic encroachment and the LAB beneath the margins of the Plateau is indistinguishable from the LAB of the surrounding extensional provinces. In contrast, beneath central and northern Colorado Plateau, weaker Sp conversions are observed in the 90–140 km depth range. The observed variations in lithospheric structure across structural blocks with different magmatic and deformational histories imply that: (1) both mechanical thinning and magmatic and thermal erosion have likely contributed to the thin lithosphere observed in the Basin and Range and southern Rio Grande Rift; (2) at the margins of the Colorado Plateau and beneath the High Rockies, where extension has been relatively minor, strong and shallow seismic LABs indicate magmatic alteration of the lithosphere; (3) MLDs seen in comparatively thick, stable lithosphere produce weaker Sp conversions and are often more distributed in depth than seismically-observed LABs in regions of thin lithosphere; (4) MLDs and weak, deep phases consistent with a seismic LAB are sometimes observed together.