ABSTRACT
Studies of foliation and associated microstructures are presented
from a variety of rock types and localities in the metamorphic
terrains of the northeastern United States.
At Islesboro, Maine cleavage is well developed in lower
greenschist grade siltstones and interbedded pelites of lower
Paleozoic age. Cleavage in the siltstone units consists of two
types of discrete mica films: short film segments and lengthened
mica film. Thick secondary mica-rich layers are also exhibited in
some siltstones. The short film segments appear to be basic
elements through which cleavage development progresses by a
linkage process that is poorly understood. The linkage of these
segments results in the production of lengthened mica films.
Further development of the larger mica films may result in the
production of the thicker mica-rich secondary layers. Detailed
statistical and microstructural studies of these mica films
suggests that fracturing, solution transfer, mica and opaque
mineral accumulation, and mica crystallization are important
mechanisms involved in mica film formation.
At New Paltz, New York, lower greenschist facies slates contain a
number of soft-sediment clastic dikes. The author's work suggests
that these clastic dikes are pre-cleavage structures which have
been redistributed during folding. Folded clastic dikes contain a
cleavage as an axial surface foliation, and this cleavage is
continuous with the slaty cleavage in the surrounding pelite.
Bedding folds and dike folds have a common axial surface. Previous
workers have suggested that clastic dikes might be made parallel
to slaty cleavage by rotation during deformation instead of by
injection during dewatering. The evidence presented here supports
this view and countermands the hypothesis of tectonic dewatering
as a mechanism for the production of slaty cleavage on a regional
scale (Maxwell, 1962).
In the Ludlow, Vermont vicinity, abundant tabular garnets and
biotite "cross-micas" occur in epidote-amphibolite grade
metasediments. Evidence is presented here that conclusively
demonstrates that shear displacement of originally equant grains
parallel to schistosity is responsible for the tabular grain
shape. An additional example of sheared garnets is presented from
granulite facies gneisses of the Overlook, New York vicinity in
the Adirondacks. The cross-micas in these rocks have incurred
shear displacements along (001) surfaces and the overall shape
changes due to deformation closely approximate a simple shear
model operating on a grain scale. Shear strain in each cross-mica
is determined and plotted on a map of the rock surface being
investigated. The distribution illustrates that higher shear
strains are found in cross-micas that are close to the translation
surfaces along which garnet porphyroblasts have been sliced. The
study illustrated here could be extended to other types of
cross-mica and may prove to be a useful method for the
investigation of strain in foliated rocks.
The final section of this thesis presents the author’s work
concerning the problem of transposition in deformed metamorphic
rocks. Much confusion presently exists in the application of the
term transposition, primarily because of an erroneous translation
of Sander’s (1911) original description of this structure. In an
attempt to clarify this situation a new translation of Sander is
presented along with a number of examples from the Central Vermont
metasedimentary sequence.
Gregg, W.J., 1979. The development of foliations in low, medium
and high grade Metamorphic Tectonites. Unpublished PhD
dissertation, State University of New York at Albany. 230pp., +x;
2 folded plates (maps)
University at Albany Science Library call number: SCIENCE
MIC Film QE 475 A2 G74X
Copies of this PhD dissertation can be ordered
from Proquest UMI
Front matter (title,
table of contents, abstract, acknowledgements) - 0.3MB pdf
file
Photo pages in dissertation
(greyscale
and colour photos with captions): - 22.6MB pdf file
Plate 1 - uncoloured coastal outcrop structural map (scale ~1:530; mapped at 1:240) pdf file 2.3 MB
Macroscopic
structural
setting
of
cleavage at Turtle Head, Isleboro, Maine
Plate 2 - uncoloured outcrop map (scale
1:6,000) pdf file 1.3 MB
Geological
outcrop
map
of the Overlook Pegmatite and vicinity, Town of Corinth, NY
Fig. 3.19 - map
of a cleavage mica film in thin section S-29a (scale 700:1)
pdf file 1.4 MB
Return to PhD dissertations completed
in the Geological Sciences Program, University at Albany