HR: 0800h
AN: T51A-0441    [Abstracts]
TI: Anomalous Cleavage-Bedding relation: flexure folding of non-parallel planes in NW India
AU: * Bhattacharyya, T
EM: bhattacharyya@quartz.gly.fsu.edu
AF: Tapas Bhattacharyya, Department of Geological Sciences. Florida State University. 108 Carraway Building., Tallahassee, Fl 32306 United States
AB: Failure to put a relative time stamp on cleavages or comparable metamorphic bandings in multiply deformed terrains may lead to apparently irresolvable geometric situations. Secondary metamorphic banding within upright non-plunging folds in impure marble of Proterozoic Delhi Supergroup shows deviations from the time-tested cleavage-bedding acute angle relations in locating fold closures. In adjacent beds from the same limb of macroscopic folds the cleavage-bedding relations indicate opposing directions of closures, suggesting that each neighboring pairs of beds belong to two different limbs of isoclinal folds. The confusion reaches its highest where within the same bed, in the same limb of a macroscopic fold, the secondary bandings show contrasting cleavage-bedding acute angle relation (1) at the same bedding surface, and (2) at the two different bounding surfaces of the bed. A model involving change in orientation and length of early metamorphic banding in the limbs of late flexure folds on beds explains the apparent anomaly. This change is a function of the pre-flexure angle between metamorphic banding and the bedding. The sense of shear being different in the two limbs of a flexure, the metamorphic banding would develop contrasting geometries in the two limbs. As the magnitude of shear changes continuously along a limb, the new geometry of early secondary banding will also vary continuously in the same limb. The model suggests that even in isoclinal folds the examination of cleavage-bedding relation in only one limb may not always lead to a correct correlation of the metamorphic banding as the axial planar structure of the fold. In areas with strongly cross-laminated rocks flexure folding of beds would lead to selective folding of cross laminations within individual beds that would resemble, and can be mistaken as, soft-sediment deformation folds. Keywords: Metamorphic banding, Cleavage, Acute angle relation, Fold closure, Superposed Deformation, Flexure, Soft sediment deformation.
DE: 8005 Folds and folding
DE: 8020 Mechanics
DE: 8025 Mesoscopic fabrics
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting