CONSTRAINTS ON TIMING AND MAGNITUDE OF UPLIFT FOR THE WESTERN MARGIN OF THE SOUTHERN SAN LUIS BASIN OF THE RIO GRANDE RIFT, NORTHERN TUSAS MOUNTAINS, COLORADO AND NEW MEXICO

The northern Tusas Mountains occupy the western margin of the southern San Luis Basin segment of the Rio Grande rift and are characterized by down-to-west faulting accommodating an east-dipping dip-slope surface documented to have developed after 25.5 Ma, and possibly as late at 20 Ma, based on new geologic mapping, geochronology, and geochemistry. Hinsdale Formation basaltic lava flows in the northern Tusas Mountains form two compositionally and spatially distinct groups; tholeiitic basalts form a northern lobe and mildly alkaline basalts form a southern lobe. Regionally, Hinsdale basalts are dominantly mildly alkaline compositions, whereas tholeiitic compositions are rare. The tholeiitic lavas are compositionally indistinguishable from lavas in the southernmost San Luis Hills to the northeast, a known source area for Hinsdale lavas. Westernmost distal tholeiitic lavas at an elevation of nearly 3200 m are 50 km from their likely source in the San Luis Hills. To achieve such a distance, even for low viscosity lavas, a southwest-sloping and channelized surface is necessary, which indicates the San Luis Hills were high relative to the Tusas Mountains at the time of emplacement at 25.5 Ma. A southwest-directed drainage system is supported by the presence of rounded low-grade turquoise-bearing clasts in sediments below the distal Hinsdale lava that indicate material was being transported from a source in the San Luis Hills (the only regional source for turquoise). North of the tholeiitic Hinsdale lavas, below lavas from the Pliocene volcano at Los Mogotes, a sequence of 20.2 to 20.9 Ma Hinsdale lavas are interbedded with Los Pinos Formation sediments stratigraphically above 26.3 to 26.6 Ma Hinsdale lavas. Both horizons of Hinsdale lavas in this area dip basinward at approximately the same angle (5-7 degrees) as the tholeiitic lavas to the south, suggesting faulting affecting both areas did not begin until after the ~20 Ma lavas were emplaced. Since initiation of rift-related faulting, down-to-west faults and east-dipping half-grabens have accommodated no less than 700 m of down-to-east displacement between the Tusas Mountains and the down-dropped San Luis Hills, although closer to 1 km offset is observed between the lowest Hinsdale lavas and those at high elevations.