Deformational History of Manhattan Rocks and Its Relationship with the State of In-situ Stress in New York City, New York

SHAH, Ajitkumar N., MTA-New York City Transit, 2 Broadway, 7th Floor, New York, N.Y. 10004, , CHANG, C.C., Parsons Brinkerhoff, 2 Broadway, 5th Floor, New York, N.Y. 10004, and KIM, K., Nu-Terra, LLC, 350 West, 31 street, Suite 805, New York, N.Y.10001.

Abstract

Since the late 19th Century, engineers and geologists have recognized the substantial

In-situ stress within the Manhattan rocks of New York City. Kemp (1895) reported stressed rocks during the construction of Ravenswood gas tunnel between Manhattan and Queens. The New York City Transit engineers have observed the rock movements during the construction of 63rd street subway line in 1976.

Two distinct lithotectonic units have been recognized in 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 NE/SW trending structural feature in the area. The rocks occurring west of Cameron thrust are known as Manhattan Formation and to the east are known as Hartland Formation (Fig. 1, Shah and others, 1998). 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 geological history of the area has its deeply rooted origin to the tectonics since Precambrian times. Gates (2000) has given an excellent account through his tectonic models since 1.1 billion years, i.e. after completion of Grenville Orogeny. The rocks of Grenville super continent form the basement for the New York State region. The collapse of Grenville mountains and the rifting later on gave the passage to dense volcanic rocks to form the new crust i.e. Proto North America. The remaining rift basin continued to widen to form a new ocean, i.e. Iapetus (Proto Atlantic). During this time an Island Arc with deep sea flysch deposits and differentiated volcanic and magmatic rocks (Hartland Formation) developed in the Iapetus ocean. The Proto North American continent during its collision course in the ESE/SE direction along with the vast sedimentary continental shelf deposits (miogeosynclinal) known as Manhattan Formation collided with the WNW/NW moving Island Arc along with Hartland Formation rocks. The collision area is marked by Cameron thrust fault zone (Fig.2).

During the process of deposition, movement and collision, the sediments and rocks of Proto North American plate and Island Arc were deformed. The deformational history is imprinted in the multiple stages (at least five) in these rocks (Fig. 3-7). Since Precambrian times the stresses have been operative in the WNW-NW/ESE-SE direction due to the closing of the Iapetus (Proto Atlantic) ocean. These forces have been resolved time to time with the successive deformational events. The major event took place during Cameron thrusting, which is almost perpendicular to the main operative forces causing a major strain relief in NE/SW direction.

The Manhattan Formation rocks are subjected to stresses resulting not only from the overlying strata but also from the locked-in stresses due to the deformational stresses of tectonic origin. The excavation through these rocks disrupts the stress field and a new set of stresses is induced within them. The magnitudes and directions of these in-situ stresses are essential for the design and construction of underground structures.

The state of in-situ stresses from the analytical data taken from number of places in Manhattan confirms the maximum horizontal stress between the WNW-ESE and NW-SE direction. The stresses caused by strain relief due to the Cameron thrusting, i.e., in the NE-SW direction are also recorded in the vicinity of the thrust zone (Fig. 8, After Zoback and others, 1985).

References

  1. Gates, A.E. (2000), “The plate tectonic history of New York State”, In Geology of New York, Isachsen, Landing, Lauber, Richard, Rogers (Editors) Second Edition, New York State Museum, pp.11-20.
  1. Kemp, J. F. (1895), “The geological section of the East River, at Seventieth Street, New York.” Transactions of New York Academy of Sciences, Vol. XIV, pp. 273-276
  1. Shah A.N., Joe Wang and Naresh Samtani (1998), “Geological hazards in the consideration of design and construction activities of the New York City area”, Environmental and Engineering Geosciences, Vol. IV, No. 4, 1998, pp. 524-533.
  1. Zoback, M.D., R.N. Anderson and D. Moos, (1985), “In-situ stress measurements in a 1 km deep well near the Ramapo Fault zone”, EOS Transactions, 66, p 363.

Figure 1, Generalized geological map of New York City (Shah and others, 1998)

Figure 2, Taconic Orogeny–Collision of Island Arc and Proto North America(Gates,2000)

Figure 3, Deformational Model D-1

Figure 4, Deformational Model D-2

Figure 5, Deformational Model D-3

Figure 6, Deformational Model D-4

Figure 7, Deformational Model D-5

Figure 8, After Zoback and others, 1985

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