HORST BLOCKS AND REGIONAL ANTITHETIC FAULTS WITHIN THE BALCONES FAULT ZONE, SOUTH-CENTRAL TEXAS

The Balcones fault zone is a set of mainly Neogene normal faults that are observed displacing Cretaceous strata at the surface. Seismic and subsurface mapping show that the principal faults displace the pre-Cretaceous unconformity and root in Ouachita-facies thrust sheets.

Across the San Marcos Arch, a series of regional antithetic faults are developed 30 km downdip of the Balcones. This series, the Luling fault zone, also roots in Ouachita rocks. To the southwest, only one Luling fault is present in Bexar County; none are documented farther southwest.

However, southwest of the Arch, several horst blocks and horst-like folds bounded by regional antithetic faults occur within the Balcones system itself. These include the Alamo Heights horst and the Culebra anticline in Bexar County, and the Frio Hill horst in Uvalde County. No similar horsts have been mapped northeast of Bexar County. What is the significance of this change in structural style?

A section along the San Marcos Arch shows a crudely symmetric graben structure, reminiscent of Cloos’ models for detachment at the head of a flat decollement. This suggests that Balcones and Luling faults may result from extension along Ouachita thrust planes or a mid-crustal ductile front. Alternatively, the system may be seen as keystone normal faults due to downward flexuring of the crust.

What happens to the southwest? The appearance of horst blocks may represent a component of left-lateral transtension as the Balcones system turns gradually westward. Or, the decollement surface may shallow southwestward allowing Luling-type antithetics into the Balcones system.

In the same area, post-Ouachita but pre-Cretaceous (possibly Pennsylvanian or Permian) ‘redbed’ sediments are reported from several deep wells. These have been inferred to represent post-thrusting graben fill. Similar sediments have not been reported to the northeast. Late Paleozoic graben faulting may have been reactivated and be responsible for the anomalous fault patterns.

Horsts in the south-central Texas Balcones localize spring outflows from the confined Edwards Aquifer. The block-bounding faults confine flowpaths in the aquifer, and may be partly responsible for the anomalous downdip edge of fresh water in Uvalde County.