MOHO TEMPERATURE AND COMPOSITIONAL CONTROLS ON LITHOSPHERIC BENDING STRENGTH IN THE WESTERN UNITED STATES

We use measurements of mantle P-wave velocity from the Moho refracted phase, Pn [Buehler and Shearer, 2012, 2014], and mineral physics [Schutt and Lesher, 2006, 2010] to estimate temperature in the uppermost few km of the western U.S. mantle. Relative to other approaches to modeling the deep geotherm, or mapping surface wave velocities to temperatures, Pn requires fewer assumptions and provides a less uncertain temperature within a tightly-constrained depth. Moho temperatures are lowest in the high-plains region of Wyoming and western Kansas/Nebraska, while highest temperatures are observed under recent (<10 Ma) volcanic provinces where they generally exceed 850C. Moho temperatures east of the Laramide deformation front are also quite hot—~850C, but crustal thicknesses here are 10-20 km thicker than in the Basin and Range, so these temperatures are not surprising.

Using a range of estimates of crustal heat production values, surface heatflow measurements are extrapolated to depth under the assumption of steady-state conduction, and with the constraint that Pn velocity observations are fit. Preliminary results are very encouraging, and also provide an indication of where Pn velocities are modulated by composition rather than temperature or where the assumption of steady-state heat flow is invalid.

These geotherms are used to predict lithospheric bending strength parameterized as effective elastic thickness, Te, for various assumed rheologies. The model predictions are compared to measurements [Lowry & Pérez-Gussinyé, 2011], to show that a weak, hydrous rheology and/or hydrous partial melt is required to fit observations in the westernmost U.S. The hydrous rheology zone significantly overlaps with the part of North America formed from accreted terranes over the last 300 M.y. To the east of this region, a dryer and stronger rheology is needed to fit the Te observations.