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Geothermal anomalies in the crust and upper mantle along Southern Rocky Mountain transitions

Marshall Reiter^,1

¹ New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA

Heat-flow, temperature gradient, and gravity data are analyzed along five profiles crossing from the Colorado Plateau or Great Plains to the high topographic areas of the Southern Rocky Mountains. These profiles cross large, negative, Bouguer gravity anomalies associated with the high topography in the region, and are within the area studied by two recent seismic investigations. The heatflow and gravity profiles typically cross from maximum to minimum values in the region, allowing an FFT (fast Fourier transform) algorithm to be applied to the data so that high frequencies can be filtered. The resulting smoothed data can be fitted by the first few terms of a sine series to estimate mid-depths of crustal and upper mantle step sources, although anomalies could also be caused by different source geometries at much shallower or much deeper depths. The analysis provides a depth comparison for thermal- and gravity-anomaly sources modeled with similar geometries, and a comparison of thermal source depths with seismic-velocity anomalies.

There is considerable similarity along the profiles between the gravity and heat-flow data, and anomaly sources appear to be both in the crust and upper mantle. There often appears to be spatial coincidence between thermal- and gravity-anomaly sources and seismic-velocity anomalies proposed in the upper mantle, implying thermal influence to these gravity and seismic anomalies. Thermal anomalies proposed to have sources in the crust in some areas are consistent with gravity and shear-wave anomalies and support studies in those areas, suggesting high crustal radiogenic concentrations in relatively less dense, granitic crustal rocks.

Key Words: Geothermal anomalies • crust and upper mantle • Southern Rocky Mountain transitions • FFT filtering • anomaly half-width and source depth

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