New Mexico Geological Society Annual Spring Meeting — Abstracts

Teleseismic studies indicate that the upper mantle beneath the Colorado Rocky Mountains has low seismic velocity and is at a temperature consistent with the presence of a small percentage of partial melt. The lowest mantle velocity feature in the region, the Aspen Anomaly, coincides with the intersection of the NE-trending Proterozoic Colorado mineral belt and the NNWtrending extension of the Rio Grande rift. This anomaly is similar in spatial scale to low-velocity anomalies in the Yellowstone and Rio Grande rift regions, and is characterized by very sharp velocity transitions. Its interpreted association with both a dipping Proterozoic paleosuture zone in the lithosphere and with the Cenozoic San Juan volcanic field and Rio Grande rift systems suggests feedbacks between Cenozoic asthenospheric small-scale convection and ancient lithospheric compositional and rheologic heterogeneities.

The geometry and tectonic history of the Aspen Anomaly are being evaluated through an integrated experiment involving: 1) passive IRIS PASSCAL-supported imaging (~70 stations), with data collection planned for 2008-2009; 2) geologic and thermochronologic studies of the uplift history of the highest elevation region of the Colorado Rockies; and 3) studies of mantle to surface interconnections via mantle degassing, hydrochemistry, and neotectonics. The provocative time-space correlations between Cenozoic rock uplift and denudation patterns, magmatism, modern hydrothermal systems, and the modern day mantle anomaly indicate that the Aspen Anomaly may have been an active tectonic feature of the southern Rockies throughout the Cenozoic. The highest peaks of the Colorado Rockies are located above, and major drainage is radial away, the mantle anomaly suggesting broad mantle-driven epeirogenic surface uplift. Quaternary faults in Colorado follow both rift- related NNW trends and NE trends suggesting that epeirogenic uplift may be expressed in the crust as block movements in a segmented lithosphere above the buoyant mantle domain. Mantle degassing (CO₂ and ³He) and high heat flow through hot springs and CO₂ springs indicate continued mantle devolatilization. The emerging data and the ongoing experiment offer rich potential for understanding interconnections and feedbacks between the mantle and the near-surface systems.