Partial Report for Gulf-margin normal faults, Texas (Class B) No. 924
Compiled in cooperation with the Texas Bureau of Economic Geology
citation for this record: Wheeler, R.L., compiler, 1999, Fault number 924, Gulf-margin normal faults, Texas, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/hazards/qfaults, accessed 05/24/2013 09:53 PM.
|Synopsis||A belt of mostly seaward-facing normal faults borders the northern Gulf of Mexico in westernmost Florida, southwestern Alabama, southern Mississippi, all of Louisiana and southernmost Arkansas, and eastern and southern Texas (Ewing and Lopez, 1991 #2032). For the purposes of his compilation, the Gulf Coast faults are divided in four large groups because they number in the hundreds. To reflect regional differences in the characteristics of the faults, those in Florida and Alabama are evaluated together in a single group, as are those in Mississippi, those in Louisiana and Arkansas, and those in Texas (described here).
Because numerous individual faults are combined into a single group for this compilation, it is not possible to provide to provide digital information about the azimuth, length, and dip of each individual fault.
The gulf-margin normal faults in Texas are assigned as Class B structures because their low seismicity and because they may be decoupled from underlying crust, making it unclear if they can generate significant seismic ruptures that could cause damaging ground motion.|
|County(s) and State(s)||TEXAS |
|Physiographic province(s)||COASTAL PLAIN |
|Reliability of location||Poor|
Compiled at 1:2,500,000 scale.
|Geologic setting||A belt of mostly seaward-facing normal faults borders the northern Gulf of Mexico. These gulf-margin faults face southwest in westernmost Florida, southwestern Alabama, and southern Mississippi; south in Louisiana and southernmost Arkansas; and southeast in eastern and southern Texas (Ewing and Lopez, 1991 #2032). In early to middle Mesozoic time, the opening of the Gulf of Mexico formed a south-facing, rifted, passive margin at the southern edge of North America (DuBar and others, 1991 #2010; Salvador, 1991 #2019; Salvador, 1991 #2020). Subsequently, the rifted margin was buried beneath the thick, Middle Jurassic, Louann Salt and an overlying, carbonate and clastic, marine sequence that continues to accumulate today. This post-rift sequence thickens seaward (Salvador, 1991 #2020). It is at least 2 km thick everywhere in the belt of gulf-margin normal faults. At the coastline, the sequence is at least 10 km thick west of the Mississippi River and at least 5 km thick farther to the east. Thicknesses exceed 12 km under coastal Texas and southern Louisiana and perhaps 16 km offshore Louisiana. |
Rapid deposition and the resulting enormous thickness of the post-rift sediments caused them to collapse and spread seaward. Salt flowed southward and pierced upward, and the overlying sediments extended on listric, normal, growth faults that flatten downward into detachments in the salt and in overpressured shales (Ewing, 1991 #1994; Nelson, 1991 #1995). These listric normal faults, their splays, and their antithetic and transfer faults make up the belt of gulf-margin normal faults described here.
Regional fluctuations in the overall deposition rate divide the belt of gulf-margin faults into two parts with different main ages of faulting and different degrees of Quaternary faulting. (1) The Interior zone of Ewing (1991 #1994) includes the entire belt except southern Louisiana, coastal Texas, and their offshore extensions. Triassic-Jurassic rifting and sedimentation, including deposition of the Louann Salt, led to Mesozoic growth faulting and salt tectonism. A line of large grabens approximates the landward limit of Jurassic salt, and Cenozoic faulting is sparse in the Interior zone (Ewing, 1991 #1994; Salvador, 1991 #2019; Ewing and Lopez, 1991 #2032). The San Marcos arch plunges southeastward from the Llano uplift of central Texas toward the coast. On the arch, the landward limit of the Louann Salt is embayed southeastward to within 60 km of the coast (Ewing, 1990 #3659; Ewing and Lopez, 1991 #2032). Accordingly, on the arch the landward edge of the Interior zone extends southeastward from the line of large grabens just mentioned to the Karnes fault zone (Ewing, 1991 #1994, his Figure 1 and p. 35). Approximately 100 km west of the Karnes fault zone at the western end of the Charlotte-Jourdantown fault zone, the landward limit of the belt of normal faults steps gulfward approximately 40 km to exclude most of the Rio Grande embayment (Ewing, 1990 #3659; Ewing and Lopez, 1991 #2032). The embayment contains some thin salt but lacks the large, southeast-facing, normal faults that indicate gulfward gravitational collapse (Ewing, 1991 #1994, p. 33). (2) The Coastal zone of Ewing (1991 #1994) covers southern Louisiana, coastal Texas, and their offshore extensions, and is separated from the Interior zone by the Early Cretaceous shelf edge (Ewing, 1991 #1994; Ewing and Lopez, 1991 #2032). Sawyer and others (1991 #3685) summarized total tectonic subsidence (TTS) analyses to infer that the Early Cretaceous shelf edge formed above a crustal boundary that was inherited from the crustal thinning accompanying Triassic-Jurassic rifting. The boundary separates less thinned continental crust on the north from more thinned continental crust on the south. The TTS results indicate that the boundary approximately overlies crust that was thinned to half its original thickness before deposition of the post-rift sequence (Sawyer and others, 1991 #3685, figures 5 and 6). After formation of the Early Cretaceous shelf edge, Late Cretaceous and especially Cenozoic clastic sediments prograded southward, and their load led to abundant Cenozoic and continuing growth faulting and salt tectonism (for example DuBar and others, 1991 #2010, p. 584-585; Salvador, 1991 #2019). The post-rift sequence as a whole is at least 9-11 km thick throughout the Coastal zone (Salvador, 1991 #2020). In addition to causing seaward gravitational collapse of the thick post-rift sequence, the crustal load from rapid Quaternary sedimentation may also aid Quaternary normal faulting and reactivate Tertiary faults of the Coastal zone by imposing extensional bending stresses on the post-rift sequence; older extensional stresses imposed by the Mesozoic sediment load have had time to relax (Nunn, 1985 #2215).
Epicenter maps show only sparse, low-magnitude seismicity within the fault belt (Engdahl, 1988 #1959; Stover and Coffman, 1993 #1986). The only damaging earthquakes reported through 1989 in this huge tract of land are four MMI VI earthquakes in westernmost Florida (1780), southern Louisiana (1930), and eastern Texas (1891, 1932) (Stover and Coffman, 1993 #1986). This level of seismicity is even less than that of sparsely seismic North and South Dakota, which together cover approximately the same area as the belt of gulf-margin faults and which had seven earthquakes of MMI VI since 1909 (Stover and Coffman, 1993 #1986). Furthermore, some of the sparse seismicity in the normal-fault belt may be artificially induced. Earthquakes of m bLg 3.4 and 3.9 and M of 4.0 and 4.7 in southeastern Texas and M 4.9 in southwestern Alabama may have been induced by extraction of oil and gas or injection of fluids for secondary recovery (Pennington and others, 1986 #1876; Chang and others, 1998 #1806; Gomberg and others, 1998 #1828; Gomberg and Wolf, 1999 #3440). Therefore, the natural seismicity rate in the normal-fault belt might be even less than the recent historical record would indicate.
The post-rift sequence and its belt of gulf-margin normal faults may be mechanically decoupled from the underlying crust. The stress field is extensional throughout the post-rift sequence in both the Interior and Coastal zones of the normal-fault belt, as determined mostly from drill-hole data that demonstrate fault slips and well-bore breakouts (Zoback and Zoback, 1991 #2006). The orientations of Shmin are radial to the Gulf of Mexico, in contrast to the east-northeast trends of SHmax that characterize most of North America east of the Rocky Mountains; the stress field in the crust beneath the thick post-rift sequence is unknown (Zoback and Zoback, 1991 #2006). Consistent with the stress field in the post-rift sequence, the normal-faulting focal mechanism of the 1997, M 4.9 earthquake in southwestern Alabama indicated south-southwest extension (Chang and others, 1998 #1806). The presence of the normal faults throughout the post-rift sequence from westernmost Florida to southern Texas (Ewing and Lopez, 1991 #2032) demonstrates that the sequence is sliding and extending seaward on detachments in weak salt and overpressured shales.
In summary, the belt of gulf-margin normal faults from Florida through Texas has strikingly low historical seismicity; the stress field and seismogenic potential of the underlying crust are unknown; and, therefore, the ability of the fault belt to generate significant seismic ruptures that could cause damaging ground motion is unclear. Accordingly, the fault belt is assigned to class B.
|Sense of movement||Normal|
|Dip Direction||SE; NW|
|Geomorphic expression||Scarps and drainage, topographic, and tonal lineaments (DuBar and others, 1991 #2010), particularly near the coast.|
|Age of faulted surficial deposits||Cretaceous to Holocene (Winker, 1990 #3698; Barnes, 1992 #3703).|
|Most recent prehistoric deformation||Latest Quaternary (<15 ka)|
|Slip-rate category||Less than 0.2 mm/yr|
|Date and Compiler(s)||1999|
Russell L. Wheeler, U.S. Geological Survey
|References||#3703 Barnes, V.E., supervisor, 1992, Geologic map of Texas: Bureau of Economic Geology, The University of Texas at Austin, 4 sheets, scale 1:500,000.|
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