Geophysical expression of buried range-front embayment structure: Great Sand Dunes National Park, Rio Grande rift, Colorado

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Abstract

Great Sand Dunes National Park and Preserve (GRSA, Colorado) liesalong the eastern margin of the San Luis Basin and the tectonically activeSangre de Cristo fault system that are part of the northern Rio Grande rift.GRSA lies within a prominent embayment in the range front where two separatesections of the Sangre de Cristo fault system intersect. Fault scarpsare observed along both intersecting fault zones within older basin-fill alluvium,but have been obscured by the actively migrating dunefield. The dunesand is also strongly magnetic, locally limiting the usefulness of aeromagneticmethods for mapping concealed structure. This study uses airbornegeophysical methods, primarily airborne gravity gradient data, along withconstraints from geologic mapping and limited subsurface data and groundwatermodeling, to interpret the subsurface basin geometry and range-frontstructure of the embayment. Using forward modeling of the gravity gradientdata and locations of faults inferred from gravity gradient and aeromagneticlineaments, several previously unrecognized tectonic elementsare interpreted adjacent to the range front. Some of the largest rift-relatedfault offsets are demonstrated to be basinward of the normal fault zonesmapped at the surface along the range front of the Sangre de Cristo Mountains,along faults concealed under the dunefield and subparallel to the twofault sections. A fault-bounded structural bench, likely composed of Proterozoicrocks, underlies most of the high dunefield at depths of 500 m to 1 km.The bench is truncated on its southwest margin by a northwest-trending,southwest-dipping normal fault. A northeast-trending, northwest-dippingnormal fault with ~600 m of estimated relief lies under the southern marginof the dunefield and bounds a structurally higher bench of Proterozoicrocks concealed at <400 m depth near the range front. The northwest- andnortheast-trending geophysical lineaments generally correspond well withthe trends of faults mapped at the surface, and with both pre- and syn-riftstructures in the Sangre de Cristo Mountains. Aeromagnetic anomaliesare explained by variations in the magnetization of pre-rift rocks, and thestrongly magnetic dune sand.

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Drenth, B. J., Grauch, V. J. S., Ruleman, C. A., & Schenk, J. A. (2017). Geophysical expression of buried range-front embayment structure: Great Sand Dunes National Park, Rio Grande rift, Colorado. Geosphere, 13(3), 974–990. https://doi.org/10.1130/GES01439.1

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