Abstract
Abstract. The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose, which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities, and is free from multiples, unlike P receiver functions. Only two major discontinuities are observed in the entire area down to about 300 km depth. These are the crust-mantle boundary (Moho) and a negative boundary, which we correlate with the lithosphere-asthenosphere boundary (LAB), since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926). Our S receiver function LAB is at a depth of 70–80 km in large parts of westernmost North America. East of the Rocky Mountains, its depth is generally between 90 and 110 km. Regions with LAB depths down to about 140 km occur in a stretch from northern Texas, over the Colorado Plateau to the Columbia basalts. These observations agree well with tomography results in the westernmost USA and on the east coast. However, in the central cratonic part of the USA, the tomography LAB is near 200 km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100 km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) and Lekic and Romanowicz (2011) as a recently discovered mid-lithospheric discontinuity (MLD). A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al., 2007).
Highlights
The radial structure of the Earth’s interior is basically determined from seismology
Gutenberg (1926) suggested that a seismic low velocity zone in the upper mantle, which he had deduced from P phase observations, could possess reduced viscosity and permit continents to move laterally
The surface wave lithosphere-asthenosphere boundary (LAB) is near 200 km depth, and S receiver functions observe a negative discontinuity near 100 km depth
Summary
Mechie1 1Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany 2National Geophysical Research Institute NGRI (CSIR), Hyderabad, India 3Freie Universitat Berlin, Fachbereich Geowissenschaften, Berlin, Germany. Received: 30 November 2011 – Published in Solid Earth Discuss.: 6 January 2012 Revised: 17 April 2012 – Accepted: 30 April 2012 – Published: 24 May 2012
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