Kinematic analyses of discfolds in Devonian Millboro Formation in the Central-Southern Appalachian junction zone

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Virginia Polytechnic Institute and State University

A method of kinematic analysis has been applied to discfolds in the Devonian Millboro Formation in the junction zone and adjacent, differently trending segments of the Central and Southern Appalachian Valley and Ridge Province. Types and kinematic signatures of shearing movements that produced discfolds at twenty localities have been recognized. Discfolds are a type of flexural fold produced by planar shearing movements during northwestern translation and deformation of the Paleozoic cover, and their analyses reflect the orientations of the large-scale movements that produced the major folds and faults. Most analyses indicate northeast-striking, southeast-dipping slip planes, northwest-trending slip lines, and overall vergence of the shear zones to the northwest. Discfolds on northwest limbs of major anticlines were produced by shearing along late-stage, southeast-dipping reverse faults, and those on the southeast limbs by shearing along southeast-dipping, bedding-parallel reverse faults and/or by large-scale flexural flow. Analyses of discfolds in the junction zone indicate movement of material out of the zone toward both the northeast and southwest.

Visual and statistical comparisons between analyses of the central and southern segments show contrasts in the orientations of the slip planes and slip lines. Thus, the transport directions of the two segments converged toward the northwest, and are consistent with those that must have operated earlier in the formation of the major folds and faults. This conclusion coupled with the apparently similar timing of the discfolds indicates that there was, at most, only slight northwestern rotation of the southern segment in relation to the central.