BASIN TO MOUNTAINS: CARBONIFEROUS INVERSION OF THE OUACHITA TROUGH, ARKANSAS AND OKLAHOMA USA

The Ouachita Mountains in Oklahoma and Arkansas expose late Paleozoic, lower greenschist facies metasedimentary rocks that represent the structurally inverted remnants of a major Paleozoic deep-water sedimentary basin. This basin initiated as a sediment-starved passive margin, and evolved into a rapidly deepening remnant ocean basin with subsequent deposition of siliciclastic sediments. Basin evolution was driven by the “soft” collision between Laurentia and a volcanic arc complex or microcontinent outboard of the leading edge of the South American segment of Gondwana. The relative timing of sediment deposition is known from stratigraphic/paleontological studies, and a small number of intercalated volcanic ash beds. However, modern thermochronologic data, capable of constraining the timing and style of basin inversion and orogenic evolution, are lacking.

Zircon fission track and U-Th/He are useful thermochronometers in the more thermally mature parts of the orogen, recording the time rocks cooled through ~ 265°C to ~165°C, respectively. Published apatite and titanite fission track ages, along with bulk-mica and whole-rock ⁴⁰Ar/³⁹Ar ages, indicate greenschist rocks of the eastern metamorphic core have reset thermochronometers because of intrusive bodies emplaced during a Cretaceous event. There are no reset (Cretaceous) cooling ages in the western, less thermally overprinted, parts of the orogen.

New zircon fission track ages range from 307 ± 19 Ma to 333.4 ± 39 Ma within and along the flanks of the Benton uplift. These ages indicate progressive cooling to the south and west along the axis of the uplift and are consistent with vitrinite reflectance data that indicate a continuous trend in thermal maturation from the Benton uplift to the Broken Bow uplift. In Oklahoma, a single new zircon fission track analysis from the northern part of the Broken Bow uplift yielded 295.3 ± 25 Ma. Nearby rocks yield U-Th/He ages in the range 235.7 ± 19 Ma to 181.6 ± 14.5 Ma. Assuming a geothermal gradient of 20°C/km, the exhumation averaged ~50 m/My over this time interval. A new U-Th/He age from a sample within the Arkoma Basin is similar to U-Th/He dates of the Broken Bow uplift. This is consistent with derivation of Arkoma Basin sediments from the syn-tectonic rocks of the Ouachitas without heating above the U-Th/He reset temperature.