TheBalcones (/bælˈkoʊnɛz/) Fault orBalcones Fault Zone is an area of largelynormal faulting[1] in theU.S. state ofTexas that runs roughly from the southwest part of the state nearDel Rio to the north-central region nearDallas[2] alongInterstate 35. Derived from the Spanish word in architecture for “balcony,” or “vantage point” in geology and nature, the Balcones feature is said to have been named bySpanish explorerBernardo de Miranda in 1756, part ofEl Camino Real, or the "King’s Highway" that skirted the fault line.[3] The Balcones Fault zone is made up of many smaller features, includingnormal faults,grabens, andhorsts.[4] One of the obvious features is theMount Bonnell Fault.[5]
The location of the fault zone may be related to theOuachita Mountains, formed 300 million years ago during a continental collision. Although long since worn away in Texas, the roots of these ancient mountains still exist, buried beneath thousands of feet ofsediment. These buried Ouachita Mountains[6] may still be an area of weakness that becomes a preferred site for faulting when stress exists in the Earth'scrust.
The Balcones Fault has remained inactive for nearly 15 million years, with the last activity being during theNeogene period. This activity was related tosubsidence of theTexas 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 forearthquakes in the United States.[7]
The surface expression of the fault is the Balcones Escarpment,[8] which forms the eastern boundary of theTexas Hill Country and the western boundary of the Texas Coastal Plain, and consists of cliffs and cliff-like structures. Subterranean features such asWonder Cave and numerous other smaller caves are found along the fault zone.
Many springs are located along this fault zone. Springs such asSan Pedro Springs,Comal Springs,San Marcos Springs,Barton Springs, andSalado Springs are found in the fault zone and provide a source of fresh water and a place for human settlement.
The Balcones Fault Zone[9] is a demarcation line for certain ecological systems and for species distributions, e.g., the California fan palm (Washingtonia filifera) is the only species ofpalm tree native to the continental United States west of the Balcones Fault.[10]
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