Early Alleghanian oblique dextral extension and magmatism along the Modoc fault zone, eastern Appalachian Piedmont, SC-GA

Conference · Mon Mar 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:6111937

Sacks, P E ^[1]; Secor, Jr, D T ^[2]; Maher, Jr, H D ^[3]; Wright, J ^[4]; Dallmeyer, R D ^[5]

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1. Western Michigan Univ., Kalamazoo, MI (United States). Geology
2. Univ. of South Carolina, Columbia, SC (United States). Geological Sciences
3. Univ. of Nebraska, Omaha, NE (United States). Geography and Geology
4. Rice Univ., Houston, TX (United States). Geology and Geophysics
5. Univ. of Georgia, Athens, GA (United States). Geology

The Modoc fault zone is a prominent zone of simple shear that has been mapped for 250 km from near Columbia, SC to the Ocmulgee River, in central GA. The steeply northwest-dipping fault zone is up to 5 km wide and contains variably mylonitic paragneiss and synkinematic sheets of mylonitic granite. Rotated tension gashes, reverse-slip-slip-crenulations, and asymmetric porphyroclasts in the fault zone are interpreted to indicate oblique dextral and normal movement. U/Pb zircon ages of 315--300 Ma yielded by some of these granite sheets are interpreted to date the time of movement on the Modoc fault zone, relatively early during the Alleghanian orogeny (ca 330--265Ma). Concurrent with movement along the Modoc fault zone, granite bodies (dated at 320--300 Ma) were intruded into both the hangingwall and the footwall sides of the fault. Cooling ages of ca 308 Ma (U/Pb monazite) and ca 305--288 Ma (40Ar/39Ar hornblende) from footwall rocks near the Savannah River indicate rapid cooling from temperatures above 700 starting with movement along the Modoc fault zone. Published geobarometry results suggest that footwall rocks were uplifted from depths of ca 29km and juxtaposed next to hangingwall rocks at depths of ca 11km by movement along the Modoc fault zone. Taken together, the crustal omission, uplift and rapid cooling of the footwall blocks, and the oblique normal sense of shear indicate at least a component of crustal extension along the Modoc fault zone. Intrusion of granite into and adjacent to the fault indicates magmatism accompanied movement on the fault at ca 315--300 Ma. Regardless of tectonic mechanism, extension associated with either crustal delamination or dextral transcurrent motion of accreted terranes, it is clear that crustal extension and magmatism was important during early phases of the Alleghanian orogeny in this part of the orogen, and it may have also been important elsewhere.

• OSTI ID: 6111937
• Report Number(s): CONF-9304188--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:4
• Country of Publication: United States
• Language: English

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