LONG-LIVED SEISMICITY IN THE OZARKS SUGGESTS BASEMENT FAULT REACTIVATION DURING MARGIN TECTONISM

In southwestern Missouri, Ordovician and Mississippian strata contain evidence for Paleozoic deformation on the Springfield Plateau. Soft sediment deformation features, including clastic dikes and a sill that are contained within Ordovician strata, and are co-located near mapped faults that offset overlying Mississippian units. Clastic dikes require unlithified sand to be mobilized shortly after deposition, thus should be similar age to the stratigraphic units that surround them. Clastic dikes are in Ordovician strata, therefore do not temporally coincide with nearby faults that offset Mississippian rocks. We present structural data, including, outcrop scale surveys, joint and clastic dike orientation data, and regional gravity maps to test hypotheses for the genesis of injectites in the Ozarks. Given the proximity to mappable faults, we interpret clastic dikes to be seismic in origin. If injectites are Ordovician in age, they are unlikely to be related to ongoing tectonism along the Laurentian margin, because the Springfield Plateau is roughly 700 km from the Taconic orogeny and injectites are oriented N-S and E-W which does correlate with Taconic structural trends. We propose that clastic dikes are related to long lived seismicity generated from reactivated deep basement faults, that accommodated displacement during the Ordovician and Mississippian.