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Abstract: Apatite Fission-Track Thermochronology Mapping in the Southern Rocky Mountains-Rio Grande Rift Province

Shari A. Kelley, Charles E. Chapin

Approximately 250 apatite fission-track (AFT) cooling ages have been obtained on rocks of Proterozoic to Eocene depositional or intrusive ages in New Mexico and Colorado. AFT ages record the time at which these rocks cooled through the 140°-60°C temperature range. Track-length data permits modelling of cooling histories, including cooling rates, depth of burial, magnitude of denudation, and intrusive/hydrothermal effects. Results indicate that AFT thermochronology is a valuable adjunct to regional tectonic analysis and can yield data of interest to hydrocarbon exploration. Some examples are: 1) Paleozoic AFT ages on Proterozoic rocks in the Front Range and northern Wet Mountains indicate that these rocks have not been buried deeper than 3-4 km since the late Pal ozoic; 2) 473-100 m.y. old AFT ages on both flanks of the Front Range contrast with 67-50 m.y. old AFT ages and uplifted partial annealing zones in the interior of the range and indicate that during Laramide deformation the flanks were thrust laterally above low angle faults while the interior was strongly uplifted; 3) lack of late Cenozoic AFT ages on the Front Range-High Plains escarpment can be used to infer erosional exhumation of Laramide structure rather than late Cenozoic normal faulting; 4) 30-5 m.y. old AFT ages of Proterozoic rocks on escarpments bordering Rio Grande rift basins, compared to 65-30 m.y. old AFT ages at high elevations constrain the time of rifting and rates of denudation; 5) significant differences in AFT aged across transverse faults within ranges reveals the s gmented nature of uplifts; and 6) unexpected early Miocene AFT ages extending from near the Colorado-New Mexico border southward along the east flank of the Sangre de Cristo Range through the Pedernal Hills and into the northern Jornado del Muerto Basin suggests a combination of high heat flow and erosional stripping of 1.5-2 km of Cenozoic sediments. Relatively high levels of thermal maturity observed in the Raton, Las Vegas, Tucumcari, and Estancia basins may be related to this belt of Miocene cooling ages.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada