THRUST FAULTS AND FOLDS INTERPRETED AS POST-GLACIAL POP-UPS IN DEVONIAN STRATA OF LAKESHORE BLUFFS ALONG LAKE ERIE, WESTERN NEW YORK STATE, USA

More than a dozen small thrust faults and folds occur along lakeshore bluffs between Ripley Beach and Eighteen Mile Creek, NY. These contractional structures include tent-shaped buckle folds with irregular, open axial fractures similar to pop-up folds observed elsewhere in quarries and post-glacial terrains; stacked thrust faults and associated folds with opposing vergences; and a bedding-parallel fault with gouge and upward splays. Some of these structures were described in classic publications by Hall (1843), Gilbert (1892), and Grabau (1898). Several structures crop out at publicly-accessible beaches such as Ripley Beach, NY. There a compressional fold displays increasing amplitude and breadth upward, enabled by multiple detachments. The basal thrust fault in the lower part of the bluff includes gouge below the axis of a compact fold with round hinge. Toward the top of the lakeshore bluff the fold has broader, planar limbs with angular hinge; open space between detachment and overlying folded beds there indicates very shallow and recent (post-glacial) deformation. Devonian bedrock on the western New York lakeshore is overlain by little till and proglacial sediment overburden.

Horizontal compression at Ripley Beach was nearly N-S. Sixty-one km to the northeast the azimuth of horizontal compression indicated by folds and faults rotates clockwise from NE-SW at Point Breeze Beach to NW-SE near the mouth of Eighteen Mile Creek 13.5 km farther northeast. Variations in apparent SHmax can reflect different timing of postglacial uplift, local stress variations due to fault stress release, or local stress fields that resulted from reduced overburden load. For example in northwestern PA and western NYS, pop-up structures with highly variable orientations occur in Devonian shale bedrock along streams that were incised into the landscape. In Lake Ontario, Jacobi et al. (2007) found that the longest, oldest post-glacial pop-ups on the lakefloor strike WNW, parallel to the trend of early glacial rebound isobases of Lake Iroquois. Recent modeling (Chester et al., 2024) confirms that early horizontal compression locally develops normal to the peripheral bulge of the retreating glacial front. Younger, shorter Lake Ontario pop-ups have a variety of strikes, some of which appear to follow older fracture and fault trends.