Abstract
1 We map the thermal state of the North American mantle between
depths of 50 and 250 km by inverting P and S velocities of three
recent seismic tomographic models. In the well-resolved regions,
temperatures derived from P velocities agree with those derived from
S velocities within the estimated uncertainties, and generally, the
seismic temperatures are in agreement with those inferred from
surface heat flow. Adiabatic mantle temperatures are found as shallow
as 50 km under most of the Basin and Range. Warm, subsolidus mantle
and known crustal structure can account for the high average
elevation and large-scale variations in topography of western North
America. In the cratonic mantle beneath the stable eastern part of
North America, temperatures at 50-100 km are on average 500 degreesC
cooler than under the tectonic western part of the continent and
adiabatic mantle temperatures are not reached until 200-250 km depth.
To balance the effect on topography of the thermally implied density
increase for the North American craton, we infer a compositionally
induced density decrease equivalent to a 1% depletion in iron over a
depth interval of 50-250 km. In regions where T-P differs
significantly from T-S we drop our assumption that variations in
seismic velocity are only due to thermal structure. A discrepancy
between T-P and T-S between 50 and 150 km depth under the Cascades
and the Gulf of California can be accounted for by the presence of 1
to 2 vol % of fluids and/or melt. Another such discrepancy beneath
Wyoming remains enigmatic.
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