Depth and strength of the lithosphere-asthenosphere transition across the Southwestern United States
We detect and analyze conversions of shear to compressional waves across the southwestern United States in order to study variations in lithospheric structure across tectonic and magmatic terranes. Our results are published and discussed in detail in a recent EPSL publication (http://dx.doi.org/10.1016/j.epsl.2013.11.026), whose abstract summarizes our findings:
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 carton -- 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.
The depth and strength of the lithosphere-asthenosphere boundary determined from bootstrap-tested Sp common conversion point stacks, along with estimated uncertainties, can be downloaded here: LekicFischerEPSL_southwestUS_LABdepthandstrength. For a detailed discussion of the bootstrap procedure, see the LekicFischer2013_EPSL_Supplement for this publication, as well as that accompanying Lekic, French, & Fischer, 2011.