Precambrian extension in the Llano Uplift, Texas

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

OSTI ID:6003457

Carter, K E ^[1]; Reese, J; Helper, M A ^[2]

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1. Baylor Univ., Waco, TX (United States). Dept. of Geology
2. Univ. of Texas, Austin (United States). Dept. of Geological Sciences

Grenville-age, post-collisional extension in the southeastern Llano Uplift is recorded by extensional differentiation crenulation cleavages, boudinaged and folded veins, quartz fibers, folds and associated axial planar foliations. These structures overprint three (D[sub 1], D[sub 2], and D[sub 3]) distinct deformation phases linked to crustal shortening, and preserve evidence of an evolution from NE-directed noncoaxial shortening through N-S extension. A WNW-trending extensional crenulation cleavage (S[sub ext]), with top-down-to-the-south shear-sense, crenulates the regionally dominant S[sub 2] foliation. In some domains a similarly trending extensional cleavage crenulates S[sub 3], but shows top-down-to-the-NE motion. Folds associated with S[sub ext] are boudinaged, whereas veins at steeper angles are folded. Quartz fibers in en echelon veins are oriented normal to S[sub ext] surfaces and are parallel to the extension direction of boudins. These structures are consistent with top-down-to-the-south motion that would occur under a N-S extension. A crosscutting extensional crenulation cleavage trends NW and shows top-down-to-the-SW motion. Folds associated with this cleavage trend NNW and fold pre-existing structures. Extensional fabrics are overprinted by cordierite and statically recrystallized biotite associated with later granite plutonism. The geometry, shear-sense, and relative age of all late ductile structures are consistent with N-S extension and may reflect collapse of Llano crust following earlier contractional phases of the Grenville orogeny.

• OSTI ID: 6003457
• Report Number(s): CONF-9303212--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:1
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
DEFORMATION
DEVELOPED COUNTRIES
GEOLOGIC AGES
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
NORTH AMERICA
OROGENESIS
PLATE TECTONICS
PRECAMBRIAN ERA
RECRYSTALLIZATION
TECTONICS
TEXAS
USA