A SYN- TO POST-TECTONIC RECORD OF COOLING FROM THERMOCHRONOLOGY OF THE LATE PALEOZOIC OUACHITA OROGEN IN WESTERN ARKANSAS

The Ouachita fold and thrust belt and its linked Arkoma foreland basin formed during late Paleozoic closure between Gondwana and Laurentia. Although the orogenic system has been long studied, the timing of thrusting, uplift, erosion, and sedimentation are incompletely known. Here, we summarize thermochronology data that record syn- to post-tectonic cooling across the orogen in western Arkansas. Vitrinite reflectance and illite crystallinity indicate the heating of pre-tectonic Ordovician through Mississippian deep-water strata to as high as 300˚ C. These strata are exposed in the Benton uplift in the central part of the orogen. Zircon fission-track (ZFT) dates from Benton uplift strata indicate an average Late Mississippian cooling (~340 Ma; Johnson, 2011). Syn-tectonic Upper Mississippian Stanley Shale and Lower Pennsylvanian Jackfork Sandstone strata that flank the Benton uplift yield average Late Pennsylvanian ZFT ages (~303 Ma, 6 sites). A Middle Pennsylvanian Atoka Formation site from the distal foreland of northern Arkansas (never deeply buried) yielded an old, dispersed ZFT date (515 ± 55 Ma), indicating only partial resetting of its detrital signal. In contrast, ZFT dates from Atoka Formation in frontal northern and southern thrust sheets within the orogen yield ZFT ages (~319 Ma, 6 sites) that overlap with the depositional age (< 316 Ma), indicating full resetting during rapid syn-tectonic burial (> 7 km) and exhumation. Zircon (U/Th)/He thermochronology dates from 8 sites in the frontal thrust sheets of the Ouachita orogen are latest Pennsylvanian to Triassic in age, ranging from 299 Ma to 215 Ma. Grain ages within these individual sites exhibit high dispersion and eU-age correlations consistent with slow cooling linked to post-orogenic exhumation. Time-temperature modeling supports rapid Pennsylvanian burial and subsequent slower Permian-Triassic exhumation of the syn-tectonic strata within the Ouachita orogen.

Johnson, H.E., 2011, Texas A&M MS thesis