--> Abstract: The Ubiquity of Wrench Faults in the USA, William McBee, Jr, #90097 (2009)

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The Ubiquity of Wrench Faults in the USA

William McBee, Jr.1

1Consultant

In mid-Proterozoic (1.6-1.0 Ga), Laurentia (ancient North America) was subjected to external transpressional forces that resulted in the emplacement of four NW-SE-trending, sinistral, major shear fractures and several minor ones. Some split the crust resulting in the injection of mid-Proterozoic granite and diorite dikes, dated 1.6-1.0 Ga. From that time to mid-Permian, transpressional forces produced left-lateral, oblique, displacements of 10-80 mi. These movements qualify the major fractures as megashears, i.e. with offsets >6 mi, quasi-vertical planes, etc. Each of the four major breaks appears to transect the entire craton. They are listed below from south to north:

Texas Megashear: South Texas to offshore southern California. Active mid-Proterozoic-Holocene. Offset several hundred mi. Oklahoma Megashear: Northeast Texas to central Utah. Active mid-Proterozoic-mid- Permian. Offset ~50 mi. Central Plains Megashear: Southeast Missouri-to northwestern Montana. Probably active mid-Cambrian to late Paleozoic. Offset ~50 mi.

Belle Plaines Megashear: Virginia-to Montana. Probably active mid-Cambrian to late Paleozoic. Offset ~80 mi. Other known wrench faults are:

The right-lateral Nemaha Fault in Kansas and Oklahoma. Known activity from mid-Ordovician to mid-Permian. Offset <10mi? The left-lateral Matador-Red River fault in north Texas. Only known activity mid-Pennsylvanian. Displacement <10 mi?. The left-lateral Lewis and Clark ‘lineament’ in Montana. Only known activity from mid-Protozoic to mid-Paleozoic. Displacement 110 mi. Apparent extension of the Belle Plaine Megashear.

The left-lateral Keokuk and Whitetail fault zones. Eastern Oklahoma. Only known activity mid-Pennsylvanian. Displacement undetermined.

Collateral faulting and folding resulting from wrench-fault movements are responsible for the creation of many prolific oil and gas structures associated with source beds.

 

 

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