Kinematics of deformation at a thrust fault ramp (?) from syntectonic fibers in pressure shadows

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Abstract
Strains thought to be associated with movement of a thrust sheet over a ramp are recorded by cleavages and syntectonic fibers within a thrust slice of the Hamburg sequence north of Reading, Pennsylvania. The three cleavages and bedding are coaxial with the regional east-northeast strike, and three related sets of fibers are found in pressure shadows on microscopic, near-spherical pyrite framboids. Viewed east-northeast at XZ sections, the oldest fiber set in the pressure shadows begins with a short segment at ∼40° to the slaty cleavage (S1), but smoothly reorients clockwise to parallelism with the gently dipping S1. A crenulation cleavage (S2) crudely fans a broad antiform in S0 and S1, but consistent north-northwest-side-up offsets across cleavage planes throughout the fold, and localized development of an extremely stylolitic character to S2 in the core of the fold, suggest that this cleavage preceded folding. Fibers of the first set curve sharply but continuously into fibers of the second set, which are parallel to S2. A third cleavage (S3) crenulates S2 and exhibits consistent top-toward-south-southeast offsets. Short fibers parallel to S3 are found on framboids in zones between S2 lamellae, but within the S2 lamellae, long fibers in cylindrical pressure shadows together with other features indicate a volume gain, subhorizontal extension during F3 development. The progression through these three deformation phases fits well with the strains expected during the movement of a thrust sheet: (1) on a flat; (2) through a closing bend onto a ramp; and (3) through an opening bend onto a higher flat.

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