WHAT DO EXPOSED FAULTS IN THE LLANO INLIER HAVE TO DO WITH INDUCED EARTHQUAKES IN NORTH CENTRAL TEXAS?

Earthquake swarms (Azle/Reno, DFW Airport, Cleburne, Irving and Venus) in the Fort Worth Basin associated with the Barnett Shale play are located along faults that cut the crystalline basement and the overlying Paleozoic rocks. These types of faults are exposed at the surface in the Llano inlier consisting of Precambrian basement rocks overlain nonconformably by Paleozoic sediments which are disconformably overlain by Cretaceous sediments. The Marble Falls 1:24000 sheet (Barnes, 1982) also provides constraints on the nature of faults that cross-cut basement and Paleozoic rocks but not the overlying Cretaceous sediments. The Paleozoic tectonism consists of numerous normal faults that exhibit maximum displacements (100 meter scale) that juxtapose Pennsylvanian carbonates (e.g. near Marble Falls against Precambrian Town Mountain Granite). At the Hoover Point roadcut on FM 1431, cm to meter scale normal faults crosscut glauconitic cross-bedded sandstones. These faults are shear-failures with dips >70 degrees. The faults are coated with red clay, most likely derived from the overlying karst in Ellenburger Group. Meter scale faults have spacing at the decimeter scale whereas cm scale faults have meter scale spacing. The hill at Hoover Point is actually a graben that has experienced topographic inversion. A second fault, mapped over 16 km on the Marble Falls sheet, that bounds the northwest side city of Marble Falls preserves of remnant of upturned Pennsylvanian beds on the hanging wall of a normal fault whose footwall is Precambrian Town Mountain Granite. The km scale spacing of these 100 meter scale displacement normal faults is recorded on the large scale geologic map of Texas which clearly shows that none of the faults cross cut the Cretaceous unconformity. The fault on the northwest side of the town of Marble Falls, is a topographically inverted lineament governed by the steeply dipping beds truncated by the footwall crystalline rocks. This topographic inversion may predate the Cretaceous unconformity or it may be a recent geomorphic feature, either way the Paleozoic faults have not been active since at least the Cretaceous, indicating that recent earthquake activity is due to reactivation of Paleozoic faults stimulated by changes in effective stress due to changes in hydrostatic pressure resulting from oil and gas activities.