NEW CONSTRAINTS ON RIFT-ASSOCIATED FAULTING IN SUNSHINE VALLEY, NORTHERN NEW MEXICO

Sunshine Valley is an area of late-Tertiary to recent extension along the Rio Grande rift zone near Questa, New Mexico. Gravity and aeromagnetic maps of Sunshine Valley reveal N-S trending gradients that are interpreted as faults at depth associated with rifting. However, some distinctive features on the high-resolution aeromagnetic map, likely associated with faults, do not correspond to anomalies on the existing gravity map. To address this discrepancy, we collected over 100 gravity points spaced 300-500 meters apart on E-W trending lines that cross the N-S trending features identified on the aeromagnetic map. The data outline the basin within Sunshine Valley as a narrow gravity low centered close to the western front of the Sangre de Cristos Mountains. The mountains and the plateau on the western side of the Rio Grande gorge are gravity highs. The basin and mountains are separated by N-S trending steep gradients of about 8-15 mGal that are consistent with a normal faulting interpretation, corroborating our interpretation of the aeromagnetic map. However, the location, shape and extent of the faulting revealed by our data are different from prior maps. The gradients are asymmetric, with the steepest gradient on the east side of Sunshine Valley, consistent with one or two major normal faults. The gradient extending across the Rio Grande gorge on the western side of the valley is gradual, likely representing a series of en echelon normal faults that each offset basement rocks only a small amount. Near Ute Mountain, the gradient on the west side of the valley splits into two distinct gradients, one of which corresponds with both a lineament in the aeromagnetic data and a topographic escarpment. There is also a good correspondence between many of the other individual faults revealed by the geophysics and topographic escarpments and fault scarps identified by geologic mapping. The rift geometry and subsurface geology shown by our geophysical modeling can be compared to other parts of the Rio Grande rift, as well as other rift zones in order to better comprehend rift formation and fault interaction.