INITIAL STAGES OF RIFTING IN THE NORTHERN RIO GRANDE RIFT: EVIDENCE FROM 40AR/39AR GEOCHRONOLOGY FROM THE SOUTHERN SANGRE DE CRISTO MOUNTAINS

The Rio Grande rift at the latitude of the Colorado-New Mexico border is a down-to-the-east half-graben bordered by high-angle range front faults along the Sangre de Cristo Mountains. Pre- and syn-rift volcanic rocks and basin-fill sedimentary rocks are preserved along the uplifted flanks of the Culebra Range at elevations as much as 1050 m above the valley floor.

We present new field observations and 40Ar/39Ar data for a sequence of middle to late Cenozoic rocks preserved in fault blocks along the headwaters of the Costilla River. This sequence, approximately 35-12 m.y. old, is about 500 m thick and includes both pre- and syn-rift units. At the base, the Vallejo Formation (VF) rests unconformably on Proterozoic basement rocks. Above the VF are welded tuff boulders and rare laharic conglomerate, possibly related to volcanism in the San Juan volcanic field 100 km to the west. These rocks are overlain by 25 m.y. old ash flow tuffs and welded tuffs of the Questa Caldera.

Deposition of this flow sequence was followed by block faulting and erosion of the bounding Proterozoic rocks that comprise most clasts in gravels of the Santa Fe Formation (SFF). Basaltic andesite lavas (15 Ma) flowed over a broad erosional surface cut on Proterozoic rocks and the graben-fill gravels of the SFF. During post-15 m.y. extensional deformation, these flows were tilted east about 20-25°. The final volcanic episode occurred at about 12 m.y., producing a small cinder cone and related lava flows.

Voluminous eruptions of tholeitic basalt of the northern Toas Plateau volcanic field followed after a hiatus of approximately 7 m.y. A basalt in this field was dated at 4.5 Ma dips 35° north and documents faulting at, and subsequent to, 4.5 m.y. Within the San Luis Valley, the local rate of uplift is about 37 m/m.y. and the rate of displacement of the preserved section is as high as 114 m/m.y.