Balcones Fault

The Balcones Fault or Balcones Fault Zone is a tensional structural system in the U.S. state of Texas that runs approximately from the southwest part of the state near Del Rio to the north central region near Dallas along Interstate 35. The Balcones Fault zone is made up of many smaller features, including normal faults, grabens, and horsts. One of the obvious features is the Mount Bonnell Fault.

The location of the fault zone may be related to the Ouachita Mountains, formed 300 million years ago during a continental collision. Although long since eroded away in Texas, the roots of these ancient mountains still exist, buried beneath thousands of feet of sediment. These buried Ouachita Mountains may still be an area of weakness that becomes a preferred site for faulting when stress exists in the Earth's crust.

The Balcones Fault has remained inactive for nearly 15 million years, with the last activity being during the Miocene epoch. This activity was related to subsidence of the Texas Coastal Plain, most likely from the large amount of sediment deposited on it by Texas rivers. The Balcones Fault is in one of the lowest-risk zones for earthquakes in the United States.

The surface expression of the fault is the Balcones Escarpment, which forms the eastern boundary of the Texas Hill Country and the western boundary of the Texas Coastal Plain and consists of cliffs and cliff-like structures. Subterranean features such as Wonder Cave and numerous other smaller caves are found along the fault zone.

...
Wikipedia