SYN-CLEAVAGE TWINNING IN CALCITE IN FOLDED CLASTIC DIKES, DELAWARE WATER GAP, NEW JERSEY

Ordovician Martinsburg turbidites preserve a classic Taconic orogen axial planar cleavage (N40°E, SE dips) in regional thrust-related folds (N40°E, shallow plunges). Within the turbidites there are a series of sub-vertical layers of calcareous siltstone (clastic dikes of Maxwell, 1962) which are folded (N55°E, shallow plunges, horizontal axial plane) disharmonically and have a discordant bedding-cleavage relationship with the surrounding slates. Cleavage planes in the siltsones are more widely spaced and contain horizontal, but locally sheared, calcite veins. One siltstone fold was sampled, producing 5 parallel slabs and 2 thin sections per slab. Mechanical twins in calcite were measured (n=211) using the Groshong technique, recording a vein-parallel, sub-horizontal shortening strain (158°, 4°), vertical extension and a shortening strain magnitude of -7.5%. Differential stress was calculated at -383.2 bars. A minor strain overprint is preserved (14% NEVs) and records a sub-horizontal shortening (36°, 6°). The twinning strain is consistent with SE-NW thrust shortening, and sub-horizontal shortening normal to evolving axial planar cleavage planes; locally, the sub-horizontal shortening is layer-normal and parallel to the horizontal cleavage (calcite veins) planes.