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Small-Scale Mantle Heterogeneity and Dynamics beneath the
Colorado Rocky Mountains Revealed by CREST
Jonathan MacCarthy (New Mexico Institute of Mining and Technology), Rick Aster (New Mexico Institute of Mining and
Technology), Ken Dueker (University of Wyoming), Steven Hansen (University of Wyoming), Karl Karlstrom (University
of New Mexico)
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.
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.
Figure 1 – (A) Depth slices through ∆Vp model at 90 km, showing outlines of
the Colorado Plateau and southern Rocky Mountains. Cross-section lines 1-6
are shown in (B).
Figure 2 – (A) Depth slices through ∆Vs model at 90km, showing outlines of the
Colorado Plateau and southern Rocky Mountains. Cross-section lines 1-6 are
shown in (B).
2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-209