ANALYSIS OF REGIONAL FRACTURE TRENDS IN THE WESTERN HUDSON HIGHLANDS, NEW YORK

In conjunction with bedrock mapping for the StateMap program in NY State, fracture maps (including fracture attitude and fracture density for multiple sets) were produced for the Monroe and Sloatsburg 7.5' quads., located in the Hudson Highlands. This area of the Hudson Highlands is mostly underlain by Grenville metamorphic and igneous rocks. Fracture orientations are highly consistent across the entire Grenville massif of the western Hudson Highlands and some fracture sets correlate with mapped faults and topographic lineaments. Minor fractures that are parallel to foliation occur in all bedrock units. The general strike of these fractures is NE-SW, and the dip is variable depending on the attitude of the local foliation. Although most topographic lineaments are parallel to, and can be correlated with bedrock units, numerous short(1-5 km long) topographic lineaments trend NW-WNW across the regional bedrock structure. These lineaments are most pronounced west of the Ramapo Fault in the Sterling Forest. Steeply dipping to subvertical fractures that strike NW-SE can be found at almost every bedrock exposure, and the fracture density ranges from 3-6 per meter (counted perpendicular to the fracture strike). A second set of subvertical fractures strikes ENE-WSW. This fracture set appears to be more localized, but occurs in bedrock that is near long (>10 km) topographic lineaments that also transect the regional structural grain of the Hudson Highlands.

Some of these WNW fractures are associated with dextral/ north-side-up near vertical faults with the same trend. These faults have up to 2 km offset and produce strong topographic expression. Fresh talus slopes at their bases in this highly glaciated area and recent earthquake activity suggest that they may be relatively young. Their apparent association with Mesozoic features also suggests that they may be related to stresses produced during the opening of the Atlantic Ocean. Similarly oriented dextral strike-slip faults farther south in NJ have documented mid-late Cretaceous activity. These extensive WNW fracture & fault systems appear to be the youngest structural features in the region and may reflect current stress fields.