TECTONIC STABILITY OF THE SAN-LUIS HILLS, NORTHERN RIO-GRANDE RIFT, COLORADO - EVIDENCE FROM PALEOMAGNETIC MEASUREMENTS

The San Luis Basin is the largest of four tilted, fault bounded Neogen e basins of the northern Rio Grande rift in New Mexico and Colorado. I t formed during the second of two episodes of extension beginning in m iddle Oligocene and extending to the present. The San Luis Hills are a n intrarift horst of Oligocene intermediate and basalt rocks preserved in the middle of the San Luis Basin. Previous paleomagnetic and struc tural studies in the Espanola Basin to the south indicate counterclock wise rotation of that region during the most recent extension. Paleoma gnetic samples from the two exposed formations in the San Luis Hills y ield a mean direction of inclination = 55.6 degrees and declination = 347.2 degrees (alpha(95) = 6.8 degrees, N=23) that is coincident with the expected Oligocene direction for North America. These rocks show n o indication of rotation or severe tilting during rift extension.