BASEMENT FAULT MAPS OF ALABAMA

Two maps of basement faults in Alabama were compiled to provide a basis for understanding the origin of earthquakes in Alabama and for predicting areas in the state most susceptible to earthquake damage.

Precambrian Laurentian basement rocks underlie the Paleozoic rocks of the Black Warrior basin and the Appalachian orogen in Alabama. The Suwannee-Wiggins suture marks the outboard limit of the Laurentian continental crust. Structural elements in the Precambrian basement rocks north of the Suwannee-Wiggins suture in Alabama include the Grenville front, the New York-Alabama lineament, the Paradox Transcontinental fault zone (Alabama-Oklahoma transform fault), a northwest-striking basement fault system in the southern Black Warrior basin, sediment-filled grabens of the Black Warrior basin, the Birmingham basement fault system, transverse zones and faults in the Appalachian orogen, the Clingman-Ocoee lineament system, the Central Piedmont suture (Towaliga-Bartletts Ferry faults), and the Altamaha magnetic anomaly.

Pre-Mesozoic (Paleozoic) basement rocks beneath the sediments of the Gulf Coastal Plain of Alabama are composed of three lithotectonic elements: Ancestral North America, the Suwannee terrane, and the Wiggins terrane. In the early Mesozoic, rifting associated with the formation of the Gulf of Mexico and the Atlantic Ocean resulted in numerous northwest- and northeast-trending normal faults and formed northeast-trending horsts and grabens. The largest graben system, the South Georgia basin, is located in part in southeast Alabama.

The northwest-trending Alabama-Oklahoma fault system formed the southern edge of the North American plate during the Jurassic. Two spatially related, genetically distinct fault systems are recognized: (1) an early Mesozoic system that developed during initial rifting of the Pangean supercontinent and rims the Gulf of Mexico, displacing pre-Mesozoic rocks; and (2) a regional peripheral fault system that strikes subparallel to the updip limit of Middle Jurassic salt and developed during the Late Jurassic to Miocene. Normal and strike-slip faults are known to occur in basement below Mobile Bay but are poorly understood.