Small-Scale Mantle Heterogeneity and Dynamics beneath the Colorado Rocky Mountains Revealed by CREST

Small-Scale Mantle Heterogeneity and Dynamics beneath the Colorado Rocky Mountains Revealed by CREST, Figure 2 Figure 2
(A) Depth slices through ΔVs model at 90km, showing outlines of the Colorado Plateau and southern Rocky Mountains. Cross-section lines 1-6 are shown in (B).
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Recent crustal thickness and shear wave velocity estimates from the CREST (Colorado Rockies Experiment and Seismic Transects, NSF Continental Dynamics) experiment indicate that the highest elevations of the Colorado Rocky Mountains (> 2.5 km) are not primarily supported in the crust. Mantle buoyancy and dynamics are therefore of fundamental importance. We present results of teleseismic body wave tomography of the upper mantle beneath western Colorado interpreted in consort with ongoing CREST results. Using a network of over 160 CREST and USArray stations with a minimum spacing of ~24 km, we invert approximately 20,000 P arrivals and nearly 10,000 S arrivals for regularized 3-D models of upper mantle Vp and Vs structure. We find Vp perturbations relative to AK135 of >6% and Vs variations of >8%, with structure being largely confined to the upper 300 km of the mantle. The previously noted "Aspen Anomaly" of low mantle velocities in this region is revealed to be fragmented, with the lowest Vp and Vs velocities beneath the San Juan mountains being clearly distinct from low velocities associated with the northern Rio Grande Rift. The San Juan anomaly probably represents thermal and/or chemical heterogeneity in the uppermost mantle related to voluminous Cenozoic magmatism and possible subsequent piecewise lithospheric dripping and/or crustal delamination. CREST thermochronology and Colorado River incision constraints additionally point to significant Neogene uplift and denudation in the southwestern and central Colorado Rockies associated with mantle forcing. A northeast-southeast grain in shallow Vs domains parallel to the Colorado Mineral Belt may be influenced by Proterozoic accretionary lithospheric architecture. We find that the low velocity anomalies beneath southwest Colorado in particular may provide significant support for enigmatic high elevations. These high-resolution tomography results CREST illuminate a remarkably high-degree of spatial heterogeneity in the region of transition between tectonic and stable North America that may reflect vigorous small-scale convection and related processes linking the lithosphere and mantle transition zones, with characteristic length scales of just 10s of km.
</p><p>Acknowledgements: The CREST project is supported by the National Science Foundation Continental Dynamics Program, award #0607837, with instrument and field support from the IRIS PASSCAL Instrument Center.</p>

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