TECTONIC AND CLIMATIC CONTROLS ON AN INTRA-CONTINENTAL RIFT FLUVIAL SYSTEM, THE TAOS PLATEAU-SAN LUIS VALLEY LINKAGE, NORTHERN RIO GRANDE RIFT, NORTH AMERICA

The headward migration and fluvial pattern of the Rio Grande north of Taos, NM, is directly controlled by coupled climatic and tectonic activity. The pre-middle Pleistocene headwaters of the Rio Grande were constrained to the Red River-Cabresto Creek drainage network, south of the Red River fault zone. Pre-middle Pleistocene tectonic activity along the eastern margin of Cerro de Los Taoses created a western structural and topographic boundary to the low-energy fluvial system. The Red River fault zone was the northern structural and topographic barrier to the fluvial system and has no apparent post-middle Pleistocene displacement. North of the Red River fault zone, two separate closed basins existed: Sunshine Valley and the northern San Luis basins. Within the Taos Plateau and Sunshine Valley regions, three pre-deep-canyon-incision fluvial deposits are present, prograding and aggrading across the present position of the Rio Grande gorge. In the San Luis Valley, north of the San Luis Hills, Lake Alamosa existed until middle Pleistocene time, approximately 440 ka; the draining of this lake incorporated approximately 25,000 km² of watershed and induced deep canyon incision to the south. Sequential climatic fluctuations based on the marine oxygen isotope record correlate the three pre-canyon-cutting deposits to glacial stages 16, 14, and 12. Prior to canyon incision at Arroyo Hondo, faulting along the Dunn fault, a strand of the larger Gorge fault zone, pulled the river system to the east and constricted the lateral extent of the floodplain. Subsequent canyon incision utilized a pre-existing rhombohedral structural fabric within the Servilleta Basalt, creating a rectilinear drainage pattern. A subsidence in post-middle Pleistocene tectonic activity along the Southern Sangre de Cristo fault zone allowed deep dissection of the Taos Plateau.