THE ROLE OF MANTLE DYNAMICS IN SHAPING LANDSCAPES OF THE WESTERN GREAT PLAINS: AN ANALYSIS OF POST-MIOCENE PATTERNS OF EROSION AND TILTING IN NE NEW MEXICO AND SE COLORADO

The western Great Plains have seen a dramatic change since the late Miocene from an environment of widespread aggradation associated with Ogallala Group deposition to one of widespread erosion that continues to the present. The causal mechanisms for the onset of erosion, as well as its temporal and spatial variability, remain the subject of an enthusiastic debate between the relative roles of climatically-modulated geomorphic parameters and tectonic rock uplift as drivers of long-term fluvial incision. We attempt to distinguish between these drivers with topographic analyses and ⁴⁰Ar/³⁹Ar evidence on the western Great Plains of NE New Mexico and SE Colorado where late Cenozoic volcanism of the Jemez lineament and post-Miocene incision have combined to create modern landscapes characterized by deep (≤400 m) bedrock canyons and (inverted) high (≤500 m) basalt-capped mesas. Data sets and surface features include: (1) NE-trends in volcanic and erosional topography, (2) stream profile (k_sn) analysis which identifies anomalously steep (k_sn >80) river gradients within a NE-trending zone of broad (50-100 km) convexities in stream profiles; (3) differential rates of landscape denudation (measured from ⁴⁰Ar/³⁹Ar-dated paleosurfaces to local modern base level) which increase away from a NE-trending hinge-line of low denudation; and (4) a tilted 3.6 Ma paleosurface indicating tectonic tilting of 24 millidegrees. These time/space patterns are not well explained by bedrock variability or climatic forcings, but are instead spatially related to the NE-trending Jemez lineament and its associated low-velocity mantle anomaly. Given this connection, we suggest dynamic, mantle-driven uplift may have caused a NE-trending flexural bulge that has influenced the magnitude and pattern of erosion in this area since the end of the Miocene. At longer timescales, apatite fission-track ages from the Plains have revealed a tilted, middle Cenozoic 110 ºC isotherm that relates to an earlier episode of long-wavelength flexure oriented along a N-S axis parallel to the Rocky Mountain front. Our interpretation is that post-5 Ma, mantle-driven rock uplift along the NE-trending Jemez zone is overprinted on this dominant N-S grain and has caused drainage reorganization via capture of E-flowing paleostreams by the S-flowing Canadian River.