TECTONIC EVOLUTION IN THE WEST-CENTRAL PART OF THE SAN LUIS BASIN, NORTHERN RIO GRANDE RIFT, COLORADO AND NEW MEXICO

We constrain tectonism in the west-central part of the San Luis Basin in the northern Rio Grande rift by conducting Oligocene to Miocene paleogeographic reconstructions. Reconstructions are based on the distribution, stratigraphic relationships, and ⁴⁰Ar/³⁹Ar geochronology of intercalated volcaniclastic and volcanic deposits along the eastern margin of the southeast San Juan Mountains and the northern Tusas Mountains. Eruption of the Chiquito Peak Tuff from the Platoro caldera at about 29 Ma provides a maximum age for overlying volcaniclastic deposits of the Los Pinos Formation. Distribution of the tuff records a south-southeast-sloping paleosurface from the southeast San Juan Mountains into the northern Tusas Mountains where stream flow was directed to the southeast by a topographic barrier. After 29 Ma, syndepositional faulting along the margin of the southeast San Juan Mountains is indicated by stratal thickening across a faulted monocline. Inverse gravity modeling suggests 200-800 m of Los Pinos deposits may be preserved beneath Pliocene lava flows in a north-south-trending graben east of the faulted monocline. On the eastern margin of the graben, a west-sloping surface and associated streamflow into the graben were established prior to being overridden by lava flows from the San Luis Hills at 26.5 Ma. Graben-axial streams are inferred to have flowed south into the northern Tusas Mountains, where positive topography diverted flow to the southeast. However, by 25.5 Ma, deposition overtopped the topographic barrier and streams flowed southwest across the Tusas Mountains. Hinsdale Formation lava flows originating in the San Luis Hills flowed over 50 km southwest across the Tusas Mountains, which is consistent with imbricate clast orientations below the lava flows that indicate stream flow to the west and southwest. Eastward tilting along the western margin of the San Luis Basin in response to activation of the incipient Sangre de Cristo fault zone occurred sometime after the emplacement of 20.5 Ma lava flows. Up to 2 km of vertical displacement is calculated between the Tusas Mountains and the San Luis Hills, which is consistent with Miocene and younger displacement established along the Sangre de Cristo fault zone at the same latitude.