COLORADO ROCKIES EXPERIMENT AND SEISMIC TRANSECTS (CREST): TIME-SPACE PATTERNS OF CENOZOIC UPLIFT-MAGMATISM AND THEIR CORRESPONDENCE TO THE ASPEN ANOMALY

The mantle beneath the Colorado Rocky Mountains has an overall low velocity (<4.2 km/s for surface waves) that is consistent with a few percent partial melt in the mantle. At a finer scale, body wave tomographic images show that large velocity variations ("reddite versus blueite") occur across very sharp domain boundaries, and there is a surprisingly heterogeneous >100 km scale mantle velocity structure (and Moho topography). One of the most significant features of the mantle velocity field is an enigmatic zone we refer to as the Aspen anomaly. Although incompletely imaged, this anomaly seems to align with both a northern extension of the Rio Grande rift and with the Proterozoic-age NE-trending Colorado mineral belt. There are provocative time-space correlations between Cenozoic rock uplift and denudation patterns, magmatism, and the modern day mantle anomaly that suggest it is a presently active tectonic feature that may reflect Cenozoic small scale asthenospheric convection. Yet an ancient influence on this anomaly is supported by dipping velocity boundaries, dipping anisotropy (for example the Paleoproterozoic subducted slab beneath the Cheyenne belt), the depth extent of mantle anomalies to depths of >200 km, and by receiver function images that suggests horizontal layering within the lithosphere to > 200 km depth. Two end-member models need testing, with the expectation that components of each may be correct. 1) The Aspen anomaly and San Juan volcanic field may be genetically aligned with the Proterozoic Colorado mineral belt and hence attributable to the long-lived compositional effects of a dipping paleosuture in the lithosphere. 2) Alternatively, the plume-like form and coincidence with a northern extension of the Rio Grande rift system may represent Cenozoic and ongoing asthenospheric upwelling. 3) A merged hypothesis is that the Aspen anomaly represents a "smart plume" where active asthenospheric upwelling is focused by ancient compositional pre-conditioning. Active tectonic expressions above the Aspen anomaly may include broad mantle-driven epeirogenic surface uplift, mantle degassing through hot springs and CO2 springs, and localized Quaternary faulting of a previously segmented lithosphere.