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

Abstract

Great Sand Dunes National Park and Preserve (GRSA, Colorado) lies along the eastern margin of the San Luis Basin and the tectonically active Sangre 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 separate sections of the Sangre de Cristo fault system intersect. Fault scarps are observed along both intersecting fault zones within older basin-fill alluvium, but have been obscured by the actively migrating dunefield. The dune sand is also strongly magnetic, locally limiting the usefulness of aeromagnetic methods for mapping concealed structure. This study uses airborne geophysical methods, primarily airborne gravity gradient data, along with constraints from geologic mapping and limited subsurface data and groundwater modeling, to interpret the subsurface basin geometry and range-front structure of the embayment. Using forward modeling of the gravity gradient data and locations of faults inferred from gravity gradient and aeromagnetic lineaments, several previously unrecognized tectonic elements are interpreted adjacent to the range front. Some of the largest rift-related fault offsets are demonstrated to be basinward of the normal fault zones mapped at the surface along the range front of the Sangre de Cristo Mountains, along faults concealed under the dunefield and subparallel to the two fault sections. A fault-bounded structural bench, likely composed of Proterozoic rocks, 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-dipping normal fault with ~600 m of estimated relief lies under the southern margin of the dunefield and bounds a structurally higher bench of Proterozoic rocks concealed at <400 m depth near the range front. The northwest-and northeast-trending geophysical lineaments generally correspond well with the trends of faults mapped at the surface, and with both pre-and syn-rift structures in the Sangre de Cristo Mountains. Aeromagnetic anomalies are explained by variations in the magnetization of pre-rift rocks, and the strongly magnetic dune sand.

Chronostratigraphic Studies of the Ootun Area Revealing the Late Holocene Plume Volcanism of the Oldoinyo Lengai, Ngorongoro, Tanzania

“…However, there are a few examples of magnetic ash dune fields which are similar to those found in the Ngorongoro area. The Great Sand Dunes National Park which is found in Rio Grande rift, Colorado, USA, is one of them [45].…”

Plants and Ethnobotany of the San Luis Valley

“…A northern section extends from near Poncha Pass to the southern side of the Blanca Massif, where the range abruptly swings eastward. Geophysical and drill-hole data indicate that faults concealed beneath the northern valley floor that subparallel the northern fault section also accommodate some of the half-graben's structural relief in this area (Watkins 1996;Grauch et al 2012Drenth et al 2017). The central fault section starts near Highway 160 and ends at about the state line.…”

“…Elevation differences between Proterozoic rocks exposed at the crest of the Sangres and the top of Proterozoic rocks in the Baca Graben indicate that nearly 8,000 m of vertical slip has occurred on faults that bound the east side of the Baca Graben during the history of the rift. Part of this displacement is accommodated by the northern Sangre de Cristo fault, while sub-parallel faults concealed beneath the valley floor also accommodated some of it (Watkins 1996;Drenth et al 2017). Other parts of the northern sub-basin have significantly less fill.…”