Comparison of North America Lithospheric Thickness from Seismic Tomography & Thermo-Dynamic Models

To better understand the longevity of continental lithosphere and the origin of its strength, it is necessary to understand how seismic observations of lithosphere structure are related to the thermal and mechanical structure of the lithosphere. In addition, while the strength of tectonic plates is commonly compared in terms of the effective elastic thickness, it is not clear what portion of the lithosphere contributes to the elastic thickness. We have compared predicted lithosphere thickness for North America from the surface wave tomography model NA04 [1] with the thermal lithosphere thickness predicted by converting the seismic velocity structure to a temperature structure using the predicted seismic velocities for a pyrolitic mantle composition [2] corrected for attenuation [3], and the mechanical lithosphere thickness resulting from instantaneous dynamic flow models with either a composite (Newtonian, non-Newtonian & plastic yielding) or Newtonian-only viscosity structure [4]. We find that the predicted thermal lithosphere thickness (depth to 900C), which is consistent with observed heat-flow, is 100-125 km in cratonic regions, but less than 75 km in the Basin & Range Province (BRP). The mechanical thickness (depth to the maximum strain-rate gradient) is consistently deeper in cratonic regions (175-200 km), but similar to the thermal thickness in the BPR. However, if the mechanical thickness is defined in terms of a strain-rate cut-off for deformation at time-scales longer than 1 billion years, then predicted lithosphere thickness is only 25-50 km in the BPR. We find that these estimates of lithosphere thickness are not strongly dependent on the assumed yield stress of cold lithosphere because the base of the mechanical lithosphere is deforming viscously. However, models with a composite viscosity structure predict 20% thicker lithosphere in the cratonic regions compared to Newtonian viscosity models, consistent with the expectation that mantle flow is less coupled to the lithosphere because strain-rate weakening reduces viscosity. Estimates of mechanical lithosphere thickness are up to a factor of two larger than estimates of elastic thickness [5]. References: [1] S. van der Lee & A. Frederiksen, in AGU Monograph on Seismic Earth: Array and Broadband Seismograms, 2005. [2] W. Xu, C. Lithgow-Bertelloni, L. Stixrude, & J. Ritsema, E&PSL, 275:70-79, 2008. [3] S. Goes & S. van der Lee, JGR, 107(B3), 2002. [4] M. I. Billen & G. Hirth, G-cubed, 8(Q08012), 2007. [5] J. F. Kirby & C. J. Swain, JGR, 114(B08401):1-36, 2009.