DEFORMATIONAL HISTORY OF THE MANHATTAN ROCKS AND ITS RELATIONSHIP WITH THE STATE OF IN-SITU STRESS IN THE NEW YORK CITY AREA, NEW YORK

Since the late 19th Century, engineers and geologists have recognized the substantial in situ stress within the Manhattan rocks of New York City. Two distinct lithotectonic units have been recognized in the New York City area, viz., Manhattan Formation (Manhattan Schists, Inwood dolomite and calcareous schists and Fordham gneiss) and Hartland Formation (gneiss, granite gneiss, granodiorite-gneiss, granodiorites, diorites. Amphibolites and schists). These two units are separated by Cameron thrust fault, a major structural feature in the area. The rocks occurring west of Cameron thrust fault are known as Manhattan Formation and to the east are known as Hartland Formation. Both these lithotectonic units have undergone polyphase deformations (at least five) and metamorphism which have resulted in complex fold and fracture patterns. Two major fault patterns, which control the tectonics of this area, have been observed, i.e. NE-SW (caused by Cameron thrust fault) and WNW-ESE (caused by Manhattanville-125th Street fault). The collision took place between the continent carrying the Manhattan Formation, i.e. ancestral North American Plate and the ancestral African Plate carrying Island arc meta-sediments and metavolcanics of Hartland Formation at the location of present Cameron thrust fault zone. The deformational sequence of the Manhattan Formation rocks since the deformational activity in the mio-geosynclinal environment since Early Pre-Cambrian through the tectonic collision and the final structural activity (Middle Devonian) have been analyzed. The state of in-situ stress from the analytical data confirms the maximum horizontal stress between the WNW-ESE and NW-SE direction.