Balcones Fault

{{short description|Fault zone in Texas, United States}}

{{about|the fault zone|the Dixieland band|Balcones Fault (band)}}

File:Mount Bonnell 1889.jpg]]

File:Balcones Fault Trend.png

File:Balcones, and the Mexia-Talco-Luling Fault Trends.png of the Balcones, and the Mexia-Talco-Luling Fault Trends, where black lines are faults, the blue shaded area is the Claiborne Group, yellow is the Jackson Group, and tan is the Wilcox Group]]

The Balcones{{Pronunciation needed|date=November 2024}} Fault or Balcones Fault Zone is an area of largely normal faulting{{cite web | title=Major Faults of the Edwards Aquifer | publisher= Edwards Aquifer |url=http://www.edwardsaquifer.net/faults.html | access-date=11 May 2010| archive-url= https://web.archive.org/web/20100413142453/http://www.edwardsaquifer.net/faults.html| archive-date= 13 April 2010 | url-status= live}} Edwards Aquifer in the U.S. state of Texas that runs roughly from the southwest part of the state near Del Rio to the north-central region near Dallas{{cite web|last1=Laubach|first1=Stephen E.|title=Tectonic Map of Texas|url=http://www.beg.utexas.edu/UTopia/images/pagesizemaps/tectonic.pdf|publisher=Bureau of Economic Geology, The University of Texas at Austin|date=1997|access-date=30 July 2015}} along Interstate 35. The Balcones Fault zone is made up of many smaller features, including normal faults, grabens, and horsts.{{cite web |url=http://www.lib.utexas.edu/geo/balcones_escarpment/pages71-76.html |title=Structural Style in an En Echelon Fault System, Balcones Fault Zone, Central Texas: Geomorphologic and Hydrologic Implications |last=Grimshaw |first=Thomas W. |author2=Charles Woodruff, Jr. |publisher=The University of Texas |year=1986 |access-date=2008-10-27 |archive-url=https://web.archive.org/web/20081107202028/http://www.lib.utexas.edu/geo/balcones_escarpment/pages71-76.html |archive-date=2008-11-07 |url-status=dead }} One of the obvious features is the Mount Bonnell Fault.{{cite encyclopedia |url=http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-5235917-1/txu-oclc-5235917-1-b134a.html |encyclopedia=First Annual Report of the Geological Survey of Texas, 1889 |editor=E. T. Dumble |title=A brief description of the Cretaceous rocks of Texas and their economic uses |first=Robert T. |last=Hill |author-link=Robert T. Hill |publisher=State Printing Office |location=Austin |year=1891 |page=134 |access-date=2008-10-27 |archive-url=https://web.archive.org/web/20100621053628/http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-5235917-1/txu-oclc-5235917-1-b134a.html |archive-date=2010-06-21 |url-status=dead }}

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 worn away in Texas, the roots of these ancient mountains still exist, buried beneath thousands of feet of sediment. These buried Ouachita Mountains{{cite web | title=Ouachita Mountains | publisher=J.S. Aber | url=http://academic.emporia.edu/aberjame/struc_geo/ouachita/ouachita.htm | access-date=11 May 2010 | archive-url=https://web.archive.org/web/20100728125755/http://academic.emporia.edu/aberjame/struc_geo/ouachita/ouachita.htm | archive-date=2010-07-28 | url-status=dead }} J.S. Aber 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 Neogene period. 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.{{cite web |url=https://earthquake.usgs.gov/hazmaps/products_data/2002/2002April03/US/USpga500v4.gif |title=Peak Acceleration (%g) with 10% Probability of Exceedance in 50 Years| publisher=United States Geological Survey |format=GIF |date=October 2002 |archive-url=https://web.archive.org/web/20070627054315/http://earthquake.usgs.gov/research/hazmaps/products_data/2002/2002April03/US/USpga500v4.gif |archive-date=2007-06-27}}

The surface expression of the fault is the Balcones Escarpment,{{Handbook of Texas | name=Balcones Escarpment| id=rxb01| retrieved=30 July 2015}} Texas State Historical Association 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.

Many cities are located along this fault zone. Springs such as San Pedro Springs, Comal Springs, San Marcos Springs, Barton Springs, and Salado Springs are found in the fault zone and provide a source of fresh water and a place for human settlement.

The Balcones Fault Zone{{cite web | title=Balcones Fault Zone | publisher= UT Austin |url=http://www.beg.utexas.edu/UTopia/centtex/centtex_what.html | access-date=11 May 2010}} UT Austin is a demarcation line for certain ecological systems and for species distributions, e.g., the California fan palm (Washingtonia filifera) is the only species of palm tree native to the continental United States west of the Balcones Fault.{{cite web|url=http://globaltwitcher.auderis.se/artspec_information.asp?thingid=90942 |first=C. Michael |last=Hogan |title=California Fan Palm: Washingtonia filifera |publisher=GlobalTwitcher.com |date=2009-01-05 |access-date=2009-07-13 |url-status=dead |archive-url=https://web.archive.org/web/20090930184850/http://globaltwitcher.auderis.se/artspec_information.asp?thingid=90942 |archive-date=2009-09-30 }}