Balcones Fault
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The Balcones Fault is a zone of normal faulting in Texas (USA) that runs approximately from the southwest part of the state to the north central region along Interstate 35. Like most fault zones, the Balcones Fault zone is made up of many small, mostly unnamed faults. One of the more well-known faults within the zone is the Mount Bonnell Fault, which runs through central and west Austin.
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 zone was most recently active about 15 million years ago 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 zone is not active today, and 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.
Many cities are located along this fault zone, and that is not a coincidence. Frequently, springs such as San Marcos Springs, Barton Springs, San Pedro Springs, and Comal Springs are found in the fault zone and provide a source of fresh water and an obvious place for human settlement.
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