TECTONICS OF THE LANTERN HILL FAULT,
SOUTHEASTERN CONNECTICUT: EVIDENCE FOR THE
EMBRYONIC OPENING OF THE ATLANTIC OCEAN
ALTAMURA, R.J.1, Department of Geosciences, The
Pennsylvania State University, University Park, PA
16802
The Lantern Hill fault (LHF) is a 10-mile long, N-S trending
hypersilicified breccia zone that transgresses meta-igneous and meta-
sedimentary units of the Proterozoic Z Avalonian terrane in
southeastern Connecticut. The fault zone is massively silicified along
its trace, especially at its northern end, at the intersection with the
Honey Hill fault, the terrane boundary between Avalonian and Gander
terranes.
An analysis of stress and motion-sense indicators as recorded
by faults and quartz-filled fractures indicates an extensional tectonic
regime with a subvertical sigma 1 and sigma 3 = approximately
00/105. Analysis of bore-hole data of the U.S. Silica Company
suggests that the main silcified zone strikes NNE and dips 50o west
and is comprised of steeply-dipping one-meter thick parallel quartz-
filled fractures. These quartz veins strike 15o clockwise to the LHF
and represent evidence for oblique slip along a pre-existing regional
fault. Main-stage fracturing and mineralization in the LHF occurred
during the Middle Triassic (Altamura and Lux, 1994). Deposition of
redbeds exposed 30 miles to the west in the Hartford basin began 20
Ma later during the Late Triassic. The tectonic setting and regional
stress field from which this failed-rift basin (de Boer and Clifford,
1988) was the same as that from which formed the LHF.
Hypersilicified breccia zones similar to the LH occur in the footwall
of the eastern border fault (EBF) of the Hartford basin and elsewhere
in southern New England and may be correlative. In the proposed
model, regional WNW-ESE extension was accompanied by
hypersilicifiaction and brecciation during the embryonic opening of
the Atlantic Ocean during the Middle Triassic (approximately 238
Ma: Anisian) - some 20 Ma prior to deposition of graben-fill
sediments and tholeiitic basalts in the rifts. Whereas establishment of
the Hartford rift marked the end of significant extensional tectonics
along the LHF, the establishment of the Mid-Atlantic Ocean spreading
center during the Jurassic marked an end to extensional tectonics
along the EBF. At this point the regional stress configuration was
dominated by approximately E-W compression due to ridge push.
Altamura, R.J. and Lux, D.R., 1994, 40Ar/39Ar and K-Ar ages for
muscovite from a giant quartz lode and alaskite host rocks,
Avalonian terrane, southern New England: Geological Society
of America Abstracts with Programs, 26, 7, p. 529.
1Present Address: Dept. of Geology & Planetary Sciences, University
of Pittsburg, Johnstown, PA 15904
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