LAYER PARALLEL SHORTENING IN A LEADING EDGE FOLD USING DEFORMED MUDCRACK POLYGONS IN THE CENTRAL APPALACHIANS, MD: TRANSITION BETWEEN THE THRUST BELT AND FORELAND REGION

The Wills Mountain anticline is a large-scale, northwestward-vergent leading edge fold of the central Appalachian fold and thrust belt. This fold involves Silurian and Devonian strata and developed during the Alleghenian orogeny above a blind westward-vergent thrust and duplex system that involved Cambro-Ordovician rocks. Nearly vertical Silurian strata in the frontal limb of the Wills Mountain anticline are well exposed in a 0.5-km long outcrop in western Maryland near the town of Pinto. Unique bedding plane exposures of the Silurian Wills Creek Formation in this outcrop reveal deformed mudcrack polygons. The lengths and rakes of long and short axes were measured in 120 deformed polygons. The average ratio between the long and short axes is 1.70, representing 41% layer parallel shortening. Center-to-center analysis of the deformed polygons (n=348) produced an axial ratio of 1.57, corresponding to layer parallel shortening (LPS) of 36%. Restoring the rakes of the short axes of the polygons indicates a northwest-southeast direction for this episode of LPS. To ensure feasibility of a center-to-center analysis of deformed mudcrack polygons, an analysis of modern mudcracks was performed yielding an axial ratio of 1, showing that undeformed mudcrack polygons have an anticlustered distribution of centers.

Previous total shortening estimates through this portion of the Appalachians range from 25-40%, based upon layer parallel features, cleavage, and solution loss, while measurements at the Pinto outcrop show approximately 36-41% shortening in layer parallel features alone. This amount of LPS is relatively high compared to the amounts of LPS reported for the external parts of other thrust belts (e.g., Sevier, Pyrenees). Published values of Alleghenian LPS in the Appalachian foreland (e.g. - plateau of northwestern Pennsylvania) range from 9% to 23% and in the hinterland (e.g. - the Blue Ridge of western Virginia) range from 55% to 68%. Our estimates of LPS are intermediate to these end members and reveal a systematic trend of decreasing LPS from the hinterland, through the fold and thrust belt, and into the foreland.