[1] Anelastic loss of seismic wave energy, or seismic attenuation (1/Q), provides a proxy for temperature under certain conditions. The Q structure of the upper mantle beneath central Alaska is imaged here at high resolution, an active subduction zone where arc volcanism is absent, to investigate mantle thermal structure. The recent Broadband Experiment Across the Alaska Range (BEAAR) provides the first dense broadband seismic coverage of this region. The spectra of P and SH waves for regional earthquakes are inverted for path averaged attenuation operators between 0.5 and 20 Hz, along with earthquake source parameters. These measurements fit waveforms significantly better when the frequency dependence of Q is taken into account, and in the mantle, frequency dependence lies close to laboratory values. Inverting these measurements for spatial variations in Q reveals a highly attenuating wedge, with Q < 150 for S waves at 1 Hz, and a low-attenuation slab, with Q > 500, assuming frequency dependence. Comparison with P results shows that attenuation in bulk modulus is negligible within the low-Q wedge, as expected for thermally activated attenuation mechanisms. Bulk attenuation is significant in the overlying crust and subducting plate, indicating that Q must be controlled by other processes. The shallowest part of the wedge shows little attenuation, as expected for a cold viscous nose that is not involved in wedge corner flow. Overall, the spatial pattern of Q beneath Alaska is qualitatively similar to other subduction zones, although the highest wedge attenuation is about a factor of 2 lower. The Q values imply that temperatures exceed 1200°C in the wedge, on the basis of recent laboratory-based calibrations for dry peridotite. These temperatures are 100-150°C colder than we infer beneath Japan or the Andes, possibly explaining the absence of arc volcanism in central Alaska.

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that signifi cant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6-10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500-1000 m may account for ~25%-50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

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Diffusive and ballistic Rayleigh wave dispersion data from three PASSCAL seismic deployments are combined with crustal thickness constraints from receiver function analysis to produce a high‐resolution shear velocity image of the Yellowstone hot spot track crust and uppermost mantle. This synoptic image shows the following crustal features: the eastern Snake River Plain (ESRP) high‐velocity midcrustal layer, low‐velocity lower crust beneath the 4.0–6.6 Ma Heise caldera field, high‐velocity lower crust beneath the <2.1 Ma Yellowstone Calderas, and low‐velocity upper crustal volume beneath the <2.1 Ma Yellowstone caldera fields. The low‐velocity lower crust beneath the 4.0–6.6 Ma Heise caldera field is found to extend outward 50–80 km from the ESRP margins, consistent with outflow of the magmatically heated and thickened ESRP lower crust. In addition, the lack of 10 km of crustal thickening of the ESRP crust, associated with the estimated 10 km of magmatic thickening, requires that the ESRP lower crust has flowed outward in a complex fashion governed by preexisting lower crustal strength heterogeneity. Within the northern Wyoming province, the so‐called 7.x km/s lower crustal magmatic layer is found to extend westward to the N‐S oriented pre‐Cambrian rift margin. The high‐velocity, hence high‐density, 7.x layer beneath the <2.1 Ma Yellowstone caldera fields has apparently inhibited heating of the subcaldera lower crust and instead magmatic heat and fluids are exchanged with the country rock above 13 km depth. The narrow 80 km diameter plume imaged by body wave tomograms, after being sheared to horizontal by plate drift, is manifest as a very low velocity (3.9 km/s) layer that is only about 110 km wide. The ESRP mantle lithosphere has been thinned to about 28 km thickness by the plume's transport of heat and magma upward, lateral advection of the lower lithosphere by plume shear, and ongoing lithospheric dilatation.