The Taconic Allochthon is an elongate belt of Cambro-Ordovician, argillaceous sediments with lesser occurrences of calcareous and siliceous lithologies. These lithologically distinctive strata lie tectonically juxtaposed over a coeval medial-Ordovician carbonate shelf sequence. This geometry resulted from an arc-continent collision in which a westward-migrating arc (Ammonoosuc Volcanics) collided with an eastern North America passive margin and the accumulated accretionary prism of the forearc region was thrust onto the passive continental margin.
The present thesis examined the structural sequence at the north end of the Allochthon and the continuity of this sequence into the adjacent Parautochthonous Shelf strata. Structurally, the study area is quite complex and at least five phases of deformation can be distinguished.
In the study area, two "slaty cleavages" were found. The earliest (S2), newly discovered during this research, strikes E/W and lies axial planar to the isoclinal, recumbent Ganson Hill Syncline. This is in turn transected by the NE-trending, "regional" slaty cleavage (S3) which is well known from the western Taconics. This "later" cleavage is that slaty cleavage dominant in outcrop and shows a similar orientation for both the Allochthon and Parautochthon. The "slaty" cleavage in the Shelf Sequence, however, has not been modified by later folding as has the Allochthon fabric.
In the Allochthon, the crenulation cleavage (S5) is best developed near thrust zones although a weak crenulation lineation is present in nearly every outcrop in the study area. The NE-trending crenulation folds range from open to fairly tight, the latter being most common near thrusts. They are ubiquitously south-plunging with few exceptions.
The late crenulation cleavage is nearly absent within the Parautochthon. At only three places a weak crenulation cleavage was found within the carbonates. Coupled with the observations from within the Allochthon, this suggests that many of the late crenulation-age imbricate thrusts within the Allochthon sole or flatten to the Basal Thrust (Giddings Brook Fault) of the Allochthon.
An anomalous crenulation cleavage fabric is typically found in fault zones which developed synchronously with the crenulation F5 folding. This anomalous discrete crenulation cleavage (named here S4) post-dates the slaty cleavages (S2 and S3) and is generally transected by the NE-striking crenulation cleavage (S5), The anomalous E/W striking fault zone cleavage (S4) has an orientation normal to the fault plane or shear surface and the intersection lineation lies parallel to the transport direction as defined by chloritic and quartzose slickensides.
The magnitudes of strain associated with the Ganson Hill "early" slaty cleavage (S2), the "regional" slaty cleavage (S3) and the crenulation cleavage (S5), were determined for a small number of localities using buckled quartz veins, reduction spots, and a combination of the t’ alpha method and buckled veins, respectively. The "early" slaty cleavage (S2) shows a 74% shortening, the "late" slaty cleavage (S3) shows 68-72% shortening and the crenulation strain (S5) shows a variable shortening magnitude of 27-45% normal to the respective cleavages.
Microstructurally, the rocks of the Allochthon study area show all gradations of cleavage morphology due to varying proportions of silica and pelitic material. Slaty cleavage commonly appears as a differentiated layering or an anastamosing network of aligned phyllosilicates. The crenulation cleavages show a diversity of morphologies ranging from a discrete to a zonal fabric. New mica growth is typically seen in the cleavage domains. Microstructure of the Parautochthon is monotonous and cleavage is defined by stylolitic opaque seams and a weak grain shape foliation. In these rocks, mechanical twinning is present but evidence of recrystallization/recovery textures is common. Fault rocks from the Allochthon show microstructures indicative of complex growth patterns during deformation of quartz fibers and chloritic gouge material, in an environment dominated by high shear strains. Many specimens allow determination of shear sense by the geometry of cleavage rotation and the geometry of shear surfaces, analogous to those found in ductile shear zones
Hoak, T.E., 1987. Structural Analysis Across the Northeast
Boundary of the Taconic Allochthon, West-Central Vermont.
Unpublished MSc. thesis, State University of New York at Albany.
236pp +xv; 2 folded plates (maps).
University at Albany Science Library call number: SCIENCE Oversize (*) QE 40 Z899 1987 H63
thesis (scanned text with colour/greyscale pictures) - 16.1MB pdf file
Plate 1 - Geology of the northeast boundary of the Taconic Allochthon
(coloured outcrop map, scale 1: 10,000) - 10MB pdf file
Plate 2 - Structure sections
(uncoloured geological cross-sections) - 3MB pdf file
Return to MS Theses completed in the Geological Sciences Program, University at Albany