TERTIARY UPLIFT AND DEPOSITIONAL HISTORY OF THE BRAZOS UPLIFT IN NORTHERN TUSAS MOUNTAINS, NEW MEXICO AND COLORADO

The northern Tusas Mountains of northern New Mexico and southern Colorado represent the western margin of the San Luis Basin of the Rio Grande rift. The San Luis Basin is generally considered a half graben with an intervening intra-rift horst with greatest subsidence along the Sangre de Cristo fault zone on the east margin. Little attention has been paid to development of the west margin other than to classify faulting as down-to-west faults accommodating tilting of the half graben. Recent geologic mapping, geochronology, and gravity modeling allows for a more complete time-integrated understanding of this area.

Uplift of the northern Tusas Mountains during the time of the Laramide orogeny stripped the area of Mesozoic rocks and developed deeply incised valleys in Precambrian rocks. The southeastern extent of the San Juan volcanic field activity overlapped and filled the extensive paleotopography with intermediate composition volcanic rocks overlain by ignimbrite outflow from caldera-forming eruptions at the Platoro-Summitville caldera complex. After cessation of volcanic activity around 28.5 Ma, material shed from the volcanic highlands of the San Juan volcanic field to the north and the San Luis Hills to the northeast was deposited in a roughly north-south trending depositional basin as the Los Pinos Formation. The axis of this deposition is interpreted to be coincident with Broke Off Mountain where Hinsdale basalt flows cap more than 350 meters of exposed Los Pinos sediments. The 25.5-Ma Hinsdale basalt likely originated from near the San Luis Hills to the northeast and extends west to elevations nearly equivalent to the highest elevations along the Brazos uplift. Westward thinning of the Los Pinos Formation under the Hinsdale lava shows the onlap onto older volcanic and Precambrian rocks and marks the edge of the depositional basin at this time. The presence of northeast-derived Hinsdale basalt at high elevations indicates the Brazos uplift was still a relative low at 25.5 Ma. After this time down-to-west faulting accommodated vertical movement of the Brazos uplift with additional down-to-west faulting and east tilting distributed basinward. Subsidence of more than 850 meters exists between the uplifted western flank in the Tusas Mountains and Hinsdale basalt below the valley floor as interpreted from well logs.