THRUST CONTROL ON THERMAL MATURITY OF THE FRONTAL OUACHITA MOUNTAINS, CENTRAL ARKANSAS, USA

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Abstract
Three types of anomalies in thermal maturity exist within the frontal thrust‐belt of the Ouachita Mountains, Arkansas, USA. The anomalies were documented by combining structural mapping with measurements of vitrinite reflectance; all samples were obtained from surface exposures. Maturity‐inversion occurs along the Y‐City Fault, where Jackfork and Johns Valley strata (Lower Pennsylvanian) have been thrust over less‐mature sequences of the lower Atoka Formation (Middle Pennsylvanian). An apparent shear‐heating aureole is also associated with this important thrust, although the spatial extent of the aureole is limited. When allowances are made for both structural and stratigraphic position, we find that internal gradients in vitrinite reflectance within individual thrust‐sheets are abnormally low. We attribute these gradients to tectonic burial. Weak remnants of the primary stratigraphic gradients are retained in most instances, so organic metamorphic reactions evidently failed to equilibrate during thrust‐loading; rapid unroofing of the thrust‐belt may have begun even as some faults remained active. Similar patterns of maturation should be expected within other thrust‐belts where peak burial and heating events were synchronous with deformation. Under these circumstances, subsurface projections of maturation trends should be keyed to structural (rather than stratigraphic) position.