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, ajshah@nyct.com, 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
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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).
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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.
2. 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
3. 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.
4. 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)
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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
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Figure 5, Deformational Model D-3
Figure 6, Deformational Model D-4
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Figure 7, Deformational Model D-5
Figure 8, After Zoback and others, 1985
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