Thermal history of the Eastern Socorro Basin, Socorro County, New Mexico, based on apatite fission-track thermochronology
Shari A. Kelley, Charles E. Chapin, Steven M. Cather, and Mark Person


Apatite fission-track (AFT) analysis of clasts from the north-trending outcrop belt of the Eocene Baca Formation east of the Rio Grande near Socorro, New Mexico, reveals an intriguing north to south middle Cenozoic thermal pattern. Clasts from the northern end of the outcrop belt in Palo Duro Canyon yield AFT ages of 45 to 57 Ma, ages that are equivalent to or older than the stratigraphic age of the unit. The mean track lengths measured in these clasts are 13.7 to 13.8 μm. The AFT ages decrease to 30 to 33 Ma a short distance (4 km) south of Palo Duro Canyon. Further south along the outcrop belt, the AFT ages are 12 to 18 Ma and mean track lengths are long (13.8 to 14.0 μm) in exposures east and southeast of Socorro. AFT ages for Baca Formation increase eastward into the northern Jornada del Muerto, where the ages are 22 to 29 Ma and the mean track lengths are short (11 to 12 μm), indicative of partial resetting. To the south, outcrops of Baca Formation along U.S. Highway 380 east of the village of San Antonio yield 31 to 46 Ma AFT ages, similar to the ages found at the north end of the outcrop belt. The AFT ages decrease to 22 to 39 Ma at the southernmost exposure of the unit, which is located about five kilometers to the southwest of the highway outcrops. AFT ages from Proterozoic basement outcrops on the eastern margin of the Socorro basin also decrease from north to south toward Socorro. AFT ages >45 Ma for Abo and Proterozoic clasts in the Baca Formation provide constraints on the denudation history of the Laramide Sierra uplift. The partially reset Oligocene apparent AFT ages and associated reduced mean track lengths and totally reset Miocene AFT ages east of Socorro define a middle Cenozoic thermal anomaly on the eastern margin of the Socorro Basin that has a N-S extent of ~20 km.

Two thermal sources may have heated the Baca Formation to temperatures above 110°C during middle Cenozoic time. First, the eastern margin of the Socorro caldera lies ~10 km to the west of the outcrops with late Miocene cooling ages. Heat from the caldera, which had to be distributed convectively rather than conductively, reset or partially reset the fission-track clock at 32 Ma. Temperatures remained higher than 110°C until middle Miocene time as a consequence of continued volcanic activity in the Mogollon-Datil volcanic field to the southeast and burial of the area by at least 1 km of tuffs, lava flows, and volcaniclastic sediments. The area then cooled rapidly during uplift of the Loma de las Cañas starting ~18 Ma. Second, the outcrops with late Miocene cooling ages lie near the Socorro accommodation zone. A middle Miocene paleohydrothermal system with recharge in volcanic highlands southwest of Socorro, eastward flow through fractured rocks along the accommodation zone, and discharge in the eastern Socorro Basin may have locally increased the geothermal gradient. This hydrothermal regime was disrupted during late Miocene time by rift-related faulting and volcanism. The middle Cenozoic thermal anomaly preserved in apatite fission-track data from the Eocene Baca Formation in the eastern Socorro Basin likely formed as a result of elevated heat flow due to both volcanism and hydrothermal activity.


  1. Kelley, Shari A.; Chapin, Charles E.; Cather, Steven M.; Person, Mark, 2009, Thermal history of the Eastern Socorro Basin, Socorro County, New Mexico, based on apatite fission-track thermochronology, in: Geology of the Chupadera Mesa, Lueth, Virgil W.; Lucas, Spencer G.; Chamberlin, Richard M., New Mexico Geological Society, Guidebook, 60th Field Conference, pp. 347-358.

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