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Recent Aeromagnetic and Gravity Studies in the San Luis Valley, South-Central Colorado

Drenth, Benjamin 1; Grauch, V. J.1
1 U.S. Geological Survey, Denver, CO.

In the past decade, advancements in aeromagnetic and gravity data acquisition and interpretation have significantly enhanced the utility of these methods for studies of sedimentary basins. In particular, modern aeromagnetic surveys can be used to map intrabasin faults and reveal a more comprehensive view of fault patterns than can be observed from the surface alone. Modern gravity interpretation methods allow the user to account for variable densities and develop models of basin geometry that are more geologically realistic. Combining intrabasin fault patterns from aeromagnetic data with 3D gravity models of basin geometry provides insights into and constraints on structural framework of basins and improves applied studies that rely on this knowledge.

The San Luis Valley of Colorado is a natural laboratory where aeromagnetic and gravity methods have critical importance to studies of ground-water, petroleum, and geothermal resources. The valley is coincident with one of the basins that make up the Rio Grande rift, and includes thick accumulations of rift sediments, ground-water resources, syn-rift volcanic flows, and active faults. Aeromagnetic and gravity studies over the valley began in 2003, in order to facilitate 3D mapping in the context of these geologic features. The geometry of the basin is estimated from 3D gravity inversion and allows delineation of basin segments. In particular, a poorly understood region of the valley between Alamosa and the New Mexico border is found to contain three sub-basins filled with at least 1 km of rift sediments, separated by two intrabasin structural highs. Rift sediments reach their greatest thicknesses near the Sangre de Cristo fault, and more recent stages of rifting appear to be localized on the eastern portion of the valley. Aeromagnetic data confirm a previously inferred northeast-trending fault on the north side of the San Luis Hills that forms a major subsurface structure across most of the basin. North-south-trending gradients on the aeromagnetic map also indicate many north-south faults that cut both volcanic rocks and rift sediments. Aeromagnetic data recently collected over Great Sand Dunes National Park and south to Blanca Peak suggest that both basement-related and intrabasin faults generally parallel the mountain front. Longer-wavelength anomalies appear to map basement structural benches.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009