Field-based constraints on the origin of the Benton Uplift, Ouachita Mountains .

摘要

The Ouachita Mountains are an east-west trending fold-and-thrust belt exposed in west central Arkansas and southeastern Oklahoma. The Mississippian-Pennsylvanian Ouachita orogeny occurred when the Sabine arc terrane collided during south-facing subduction, during closure of the Cambrian passive margin of the North American plate. Deep-water Paleozoic strata were incorporated into an accretionary wedge as the Sabine terrane approached the North American plate. Folds and thrust faults in the Ouachita Mountains generally verge north due to tectonic transport of an accretionary wedge from south to north. Structures in the Benton and Broken Bow Uplifts (located in the core of the wedge) locally appear antithetic to this deformation, i.e., a sub-set of data from both uplifts consists of south-vergent thrust faults and asymmetric to overturned folds. Interpretations for the formation of south-vergent structures includes the subsequent overturning of thrusts and other initially north-vergent structures (i.e. Blythe, 1988; Nielsen, 1989; Arbenz, 2008) as well as antivergent folding of originally north-vergent folds during thrust fault reactivation (i.e. Babaei, 1990). Field data from the southwestern Benton Uplift in west-central Arkansas are most consistent with south-directed backthrusting, as indicated by asymmetric z-folds defined by the Arkansas Novaculite and thrust-duplication of north-dipping Paleozoic strata. Out-of-sequence backthrusting is attributed to internal wedge thickening as explained by the critical taper model (Davis et al., 1983; Dahlen, 1990). The Ouachita Mountains Paleozoic strata are cut by steeply-dipping conjugate joints that strike NNW and NNE. The conjugate joint set is inferred to represent north-south shortening parallel to the maximum compressive paleostress direction, and east-west extension parallel to the Ouachita orogenic front (and the minimum compressive paleostress direction). The joints are interpreted as a record of orogen-parallel extension. In the context of "critical taper" mechanics, this extension would be driven by late-stage sub-vertical thinning of the Ouachita accretionary wedge, i.e., due to a transient sub-vertical reorientation of maximum compressive paleostress. Data from this study, however, provide preliminary evidence for non-plane strain lateral spreading of the wedge, i.e. sub-horizontal orogeny-parallel extension, following emplacement onto North American continental crust.