TEMPORAL AND SPATIAL CHANGES IN SHORTENING DIRECTION DURING PROGRESSIVE DEFORMATION OF THE CENTRAL APPALACHIAN FOLD-AND-THRUST BELT: EVIDENCE FROM JOINTS AND VEINS

In a deforming fold-and-thrust belt, structures do not form instantaneously across the belt, but instead are related to the passage of a deformation front, which starts in the hinterland and progresses toward the foreland. Therefore, structures in one part of the belt are not necessarily time-correlative with those in another part. Changes in the regional stress field during the development of the orogen will result in changes the orientation of the structures being formed progressively toward the foreland. In addition, structures associated with later deformation events (and different stress fields) may overprint earlier-formed structures.

The Late Paleozoic central Appalachian fold-and-thrust belt in West Virginia and northern Virginia is rich in syntectonic extensional joints and veins (mineral filled fractures) that, along with other mesoscale structures, record the shortening history for the orogen. Joint and vein orientations, cross cutting relationships, and vein mineral paragenesis are used to constrain the timing of structure formation.

The earliest joints are pre-folding ENE-striking and NNW-striking sets that contain multiple stages of mineralization. They are common in the Pendleton Co., WV area, but are rare to the north. They are interpreted to be related to an early NNW-directed Alleghanian shortening event, possibly the formation of the Southern Appalachians. These structures are followed by very common pre-to syn-folding NE-striking and NW-striking sets that formed with the forelandward growth of folds associated with the development of the underlying Cambro-Ordovician duplex. These sets contain multiple stages of mineralization. The NW set exhibits a progressive swing in orientation to a more westerly strike from east to west across the Valley & Ridge, and reflects a change in shortening direction over time.

Syn- to post-folding, rarely mineralized WNW-striking joints are found primarily in the Wills Mt. and Allegheny Front area, and are interpreted to be associated with the emplacement of the Wills Mt. duplex. Finally, post-folding, rarely mineralized EW-striking joints and veins overprint pre-existing structures throughout the Valley & Ridge, and are interpreted to be related to a late Alleghanian E-W shortening event associated with out-of-sequence thrusting in the Blue Ridge.