ABSTRACT
Continuous outcrop mapping (1:240) along coastal exposures in southern
Maine has led to the recognition of a complex brittle structural history
that established the pre-existing structural grain for, and culminated
in, the intrusive development of several Mesozoic dike swarms. The structural
grain within the Rye Formation of the southern part of the field area consists
of a dominant NE-trending near-vertical gneissic (mylonitized) foliation
on which is superimposed the intense brittle shear fracturing of the pseudotachylyte-bearing
Fort Foster Brittle Zone. This brittle shear fracturing was responsible
for the later localization of explosive igneous breccias and felsic melts
above the largely unexposed Gerrish Island Igneous Complex. The later emplacement
of the dominant N25E-trending dike swarm, possibly Late Jurassic to Early
Cretaceous in age, was primarily independent of the pre-existing structural
grain and directly related to a system of late rusty-weathering open brittle
fractures. The structural grain within the Kittery Formation of the northern
part of the field area consists of a prominent N60E-trending vertical planar
bedding-anisotropy as limbs of Acadian fold structures and a second N45E-trending
vertical planar cleavage anisotropy related to a late Z-shaped asymmetric
fold and dextral shear zone development. These bedding-cleavage anisotropies
are responsible for the structural localization of a prominent N60-45E
trending dike swarm, possibly Early Triassic to Early Jurassic in age,
associated with the alkaline syenite complex at Agamenticus. The bedding-anisotropy
is found to play an important role in determing the character of strain
accommodation between en echelon dike terminations. A younger NNW-NNE trending
secondary dike swarm is interpreted as a termination structure for the
prominent N25E-trending dike swarm exposed farther south at Gerrish Island.
The emplacement of this N25E-trending dike swarm and the development of
the related late brittle fracture system involves a N65W-S65E, Late Jurassic-Early
Cretaceous, crustal extension and its interaction with the prominent N60E-trending
vertical bedding-anisotropy and the large rigid cylindrical Agamenticus
intrusion. Finally, a regional synthesis of Mesozoic structural developments
in eastern North America results in a model for a complex decoupling history
during central Atlantic rifting. This model incorporates the varied interaction
between dextral shear, extension, sinistral shear and final crustal separation
along a wide, arcuate, near-pole, small-circle transform system and the
linear belt of pre-existing Appalachian structural grain. The N60-45E and
N25E trending dike swarms studied in southern Maine would most likely be
related to the Triassic-Jurassic extensional phase and final Jurassic-Cretaceous
crustal separation phase, respectively, within the proposed model.
Swanson, M.T., 1982. The structure and tectonics of Mesozoic dike swarms
in eastern New England. Unpublished PhD dissertation, State University
of New York at Albany. 278pp., +xxiii; 10 folded plates (maps)
University at Albany Science Library call number: SCIENCE MIC
Film QE 40 Z899 1982 S92
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