GRAVITY CONSTRAINTS ON FAULTING, VOLCANISM AND BASIN GEOMETRY IN THE RIO GRANDE RIFT, NEAR TAOS, NM

From 2014-2019 the Florida State University field camp and students conducted a series of gravity and magnetic experiments to examine various parts of the Rio Grande rift and associated structures near Taos, NM. Approximately 400 new gravity points were measured, coupled with 278 recalibrated points from past astronaut training exercises. Structures examined range from: the Questa caldera, to the rift bounding Sangre de Cristo fault, to horst blocks near Tres Piedras, the Rio Hondo pluton, the San Luis valley itself and most recently, the transition from the Sangre de Cristo to the Embudo faults in the Ranchos de Taos area. In addition, off axis rift-related magmatic structures in the Raton basin, such as the Spanish Peaks dikes and intrusions have also been studied.

North and south of the Questa region, the gravity structure of the San Luis Valley is dominated by the approx. -20 mgal anomaly (-275 to -255 mgal Complete Bouguer Anomaly (CBA)) of the Taos graben with the steepest gravity gradient located at the rift bounding Sangre de Cristo fault. Near Questa, the Bouguer anomaly of the Taos graben is lessened to –10 mgal due to the Late Oligocene volcanic and plutonic rocks of the Questa caldera. Within the Sangre de Cristo range, the Questa caldera can be identified by a -20 mgal Bouguer anomaly due to the density contrast with Proterozoic arc rocks (diorite and amphibolite). In addition, in the Taos ski valley area, the Miocene Rio Hondo pluton exhibits a -10 mgal Bouguer anomaly with respect to the Proterozoic basement.

The largest gravity contrasts in the Taos area occur at the southern end of the San Louis Valley along the Embudo fault, which has been interpreted as a transform like structure within the Rio Grande rift. In this area, there is a -40 mgal Bouguer anomaly from the center of the Taos graben (-275 mgal CBA) across the Embudo fault and into the southern Taos / Picuris range (-235 mgal CBA). The Taos graben is filled with poorly consolidated alluvial fan sediments, rift-related basalt flows, and Santa Fe Group sedimentary rocks, whereas across the Embudo fault, Proterozoic metamorphic and plutonic rocks are exposed to the southwest and to the northeast are Paleozoic sedimentary rocks. Overall, these investigations have refined the gravity structure of the Taos region and provided students with real world geophysical experience.