C2: GEOLOGY OF THE NORTHERN TACONIC ALLOCHTHON - STRAIN VARIATION IN THRUST SHEETS, BRITTLE FAULTS, AND POSTRIFT DIKE EMPLACEMENT
Jean Crespi
Jennifer Cooper Boemmels
Jessica Robinson
Geosciences
University of Connecticut
Storrs, CT 06269
INTRODUCTION
This field trip has three purposes: (1) to present a synthesis of structural and strain data for the Taconic slate belt, which lies in the northern part of the Giddings Brook thrust sheet in the Taconic allochthon, (2) to present an analysis of fault-slip data for post-cleavage faults in the region, and (3) to present the results of work on mafic dikes
in the Taconic lobe of the New England–Québec igneous province. The stops have been selected to illustrate the along-strike variation in structure and strain, to show representative post-cleavage faults, and to include dikes in a variety of orientations. The data presented at the stops cover a region that extends for about 60 km along strike in the Giddings Brook thrust sheet, from Hubbardton, Vermont, in the north to Salem, New York, in the south (Figs. 1 and 2).
Emplacement of the Taconic allochthon took place during the Early to Late Ordovician Taconic orogeny. In the northeastern United States, the Taconic orogeny has traditionally been interpreted as resulting from the collision of a west-facing volcanic arc with the eastern margin of Laurentia (Bird and Dewey, 1970; Rowley and Kidd, 1981; Stanley and Ratcliffe, 1985). In this interpretation, emplacement of the Taconic allochthon occurred in a proforeland setting. New geochronological data, however, indicate that the orogeny involved several phases (Karabinos et al., 1998, 2017; Macdonald et al., 2014, 2017). During the first phase, a west-facing volcanic arc built on peri-Gondwanan basement collided with a fragment of a hyperextended eastern Laurentian margin. During the second phase, this composite ribbon terrane collided with Laurentia proper. Slab break-off and a reversal of subduction polarity resulted in the establishment of an east-facing volcanic arc. During the third and final phase, the Taconic allochthon was emplaced. In this revised interpretation, emplacement of the Taconic allochthon occurred in a retroarc setting.
The eastern North American margin developed after Paleozoic contractional tectonics culminated in the formation of Pangea. Early Mesozoic rifting of Pangea was accompanied by the development of a system of rift basins of Late Triassic and Late Triassic–Early Jurassic age and by massive outpourings of basalt of the ∼201 Ma central Atlantic magmatic province (CAMP). The extension direction during rifting was generally northwest–southeast (Withjack et al., 2012). In the northeastern United States, CAMP dikes trend northeast–southwest, consistent with the extension direction for rifting. Postrift magmatic bodies include the plutons and sheet intrusions of the Early Cretaceous New England–Québec (NEQ) igneous province (McHone, 1984; McHone and Butler, 1984). The origin of these intrusives has long been debated. They may be related to passage of the Great Meteor hotspot (Morgan, 1972; Sleep, 1990), edge-driven convection (Matton and Jébrak, 2009), opening of the North Atlantic Ocean (McHone and Butler, 1984), delamination of over-thickened crust, or some combination of these mechanisms.

C2: GEOLOGY OF THE NORTHERN TACONIC ALLOCHTHON - STRAIN VARIATION IN THRUST SHEETS, BRITTLE FAULTS, AND POSTRIFT DIKE EMPLACEMENT

Parking for Stop 1

STOP 1. High Strain Subdomain (Poultney/Deep Kill Formation)
This exposure of the Lower to Middle Ordovician Poultney/Deep Kill Formation was created in the mid 1990s when County Road 18A was constructed to improve road traffic safety. The strata lie on the overturned limb of a regional-scale syncline and in the immediate footwall of the out-of-sequence Middle Granville thrust. Our analysis of the orientations of slaty cleavage and the stretching lineation and of syntectonic fibers places the exposure in the high strain subdomain on the eastern edge of the central domain.
Slaty cleavage strikes north-northeast and dips moderately east-southeast, and the stretching lineation plunges approximately down the dip of slaty cleavage. Syntectonic fibers associated with slaty cleavage development are curved in XZ sections consistent with top-to-west-northwest non-coaxial flow. The fibers yield stretch values for the X direction of about 2.2–2.3. They also record a small amount of extension in the Y direction. The central domain, in contrast, is characterized by north-northwest strikes for slaty cleavage, a moderately raking stretching lineation, and lower stretch values for the X direction.
Many mesoscale folds are present in the exposure (Fig. 3). They are unusual because the hinge lines plunge moderately to steeply southeast. Hinge lines of folds in the Taconic slate belt are typically subhorizontal to gently plunging, and, in the central domain, the plunge direction is south-southeast. The bedding-cleavage intersection lineation also plunges to the southeast although not as steeply as the fold hinge lines. This difference is likely because the mesoscale folds are mostly in the middle of the exposure and the bedding-cleavage intersection lineation was measured at the ends of the exposure where bedding is planar. The folds in the exposure are also asymmetrical such that they have an S shape when viewed down plunge.
The fold geometry can be explained by initial development of parasitic folds and subsequent rotation of the hinge lines of the parasitic folds toward the stretching lineation. The S-shaped fold asymmetry is consistent with an origin as parasitic folds on the overturned limb of a west-vergent, regional-scale fold. In addition, the orientation of the hinge lines is consistent with an initial gentle south-southeast plunge and counterclockwise rotation toward the downdip stretching lineation during top-to-west-northwest non-coaxial flow. This interpretation implies that this area was a long-lived region of high strain, the folds starting to rotate during folding and continuing to rotate during slaty cleavage development. The Middle Granville thrust may be a late-stage expression of this high strain zone.
Mesoscale, post-cleavage faults in the strata typically contain laminated veins with well-developed steps. Some of the faults have formed along folded surfaces, a good example lying on the northeast side of County Road 18A near the middle of the exposure (Fig. 4). These faults are normal faults on the long limbs of the folds and reverse faults on the short limbs of the folds. Slip was subparallel to the hinge lines. In the example in Figure 4, the outer part of the fold has moved up relative to the inner part of the fold, and the striae show a systematic variation in orientation around the fold such that they are closer to exactly parallel to the hinge line in the hinge.

STOP 2a. Northern Domain (Mettawee Slate/Middle Granville Slate)
Cedar Point quarry exposes the hinge zone of a north-northeast-trending, regional-scale syncline. The folded strata belong to the Lower Cambrian Mettawee Slate/Middle Granville Slate, a purple and green slate that contains reduction spots useful for strain analysis. The quarry lies in the northern domain where slaty cleavage strikes north-northeast and dips gently east-southeast and where the stretching lineation plunges approximately down the dip of slaty cleavage. Here, slaty cleavage is overprinted by a crenulation cleavage, which becomes the dominant cleavage to the east of the approximate longitude of Lake Bomoseen and Lake St. Catherine. Although this crenulation cleavage is locally present in the Taconic slate belt, it does not appear to affect the orientation of slaty cleavage. In the northern domain, where the crenulation cleavage is more common than in the central and southern domains, the mean orientation of slaty cleavage for sites without a crenulation cleavage is essentially the same as the mean orientation of slaty cleavage for sites with a weakly to moderately developed crenulation cleavage.
At Stop 2a, the concave side of a folded bedding surface is visible through the graffiti. The intersection of the axial planar slaty cleavage with bedding indicates the hinge line of the syncline is gently plunging, which is typical of folds in the Taconic slate belt. At Stop 2b, the convex side of a folded bedding surface and the profile plane of the syncline are visible. The tight to isoclinal nature of the syncline is typical of folds in the Taconic slate belt.

STOP 2b. Northern Domain (Mettawee Slate/Middle Granville Slate)
Reduction spots are common in the slabs in the slate pile at Stop 2b. Two types of reduction spots are present: those with irregular shapes and those that are elliptical. The latter have been measured and reported on by Wood (1974), Hoak (1992), and Goldstein et al. (1995). The reduction spots here and elsewhere in the Taconic slate belt plot in the flattening field of a Flinn plot, and Goldstein et al. (1995) interpreted their horizontal distribution on a
Flinn plot as indicating the slates underwent a minimum of 55% volume loss during slaty cleavage development. Our strain data for the northern domain from strain fringes around pyrite framboids in black slates are not consistent with the reduction spot data. The strain fringes do not show fiber growth in the Y direction, and so we infer plane strain deformation for slaty cleavage development. In addition, the strain fringe data yield internally consistent data when combined with the orientation of slaty cleavage if constant volume is assumed. The 2.28 mean stretch value for the X direction obtained from strain fringes for the northern domain gives a strain ratio for the XZ plane of 5.2 for constant volume and a strain ratio for the XZ plane of 10.4 for 50% volume loss. These values can be plotted on an RXZ versus θ’ graph for simultaneous simple shear and volume change (Fossen and Tikoff, 1993) using the 28° mean dip of slaty cleavage in the northern domain and an appropriate dip for the Giddings Brook thrust sheet. We use a thrust sheet dip of 5° east, giving a θ’ value of 23°. A value of 5.2 for RXZ results in the northern domain plotting close to the curve for simple shear, which is by definition a constant volume deformation. A value of 10.4 for RXZ results in the northern domain plotting in the volume gain field, which is at odds with the 50% volume loss assumption used to obtain RXZ.

STOP 3a. Bomoseen Dike at Glen Lake
Glen Lake Trail (visit vtstateparks.com/assets/pdf/bomoseentrailmap.pdf for a copy of the trail map) is well marked with blue trail blazes, and the walk to Stop 3a is relatively easy with little elevation change, the occasional fallen tree, and some irregular terrain where the trail passes over exposed tree roots. About 7 minutes (∼0.25 miles) from the trailhead, the trail splits shortly after passing a foundation. Take the right fork, which passes on the east side of a cellar hole. The left fork follows the water’s edge and is not the main trail. About 10 minutes (∼0.35 miles) from the trailhead, the trail climbs up a small ridge. A hemlock tree with a girth of about 2 m and a blue trail blaze stands at the top of the ridge. The south-southwest side of the Bomoseen dike is exposed in the vicinity of this tree. Continue to follow the trail as it descends into a small valley to the north of the ridge. The valley is before the trail bends downhill to the left. Walk off trail just upslope from the valley floor to where a recently fallen tree has exposed the north-northeast side of the Bomoseen dike. Return to the trail and follow it downhill to the right. Rubble next to the trail of the dike and wall rock marks the dike-wall rock contact on the north-northeast side of the dike. Follow the trail up- hill back to the valley floor and walk off trail to the east-southeast toward Moscow Rd. Several exposures of the dike and of the wall rock to the south-southwest of the dike are present in this area. Return to the trail and follow it back to the trailhead.
The exposure of the Bomoseen dike at Stop 3a consists of several small and discontinuous outcrops of the dike between the eastern shore of Glen Lake and Moscow Road. Outcrops of the wall rock (West Castleton Formation on the Bedrock Geologic Map of Vermont (Ratcliffe et al., 2011) and Poultney and Hatch Hill formations on Rowley’s map of the Lake Bomoseen area (Rowley, 1983, Plate 1)) are also present.
The Bomoseen dike was named by McHone and McHone (1993) for an exposure of the dike on VT-30 along the eastern shore of Lake Bomoseen. The dike appears on Dale’s map of the Taconic slate belt (Fig. 5) (Dale, 1899) and is reproduced on Fowler’s map of the Castleton area (Fowler, 1950, Plate II) and on the Bedrock Geologic Map of Vermont (Ratcliffe et al., 2011). It trends west-northwest–east-southeast and is relatively wide and long compared to other NEQ dikes in the Taconic lobe.
McHone and McHone (1993) reported a width of 12 m for the Bomoseen dike at the VT-30 exposure (the entire width of the dike is no longer preserved). Our observations of the dike at the Glen Lake exposure give a comparable width. The location of the dike-wall rock contact on the north-northeast side of the dike can be located fairly closely on the trail by the change from dike rubble to slate rubble. A sample we collected from the southern side of the exposure of the dike closest to Moscow Road contains the dike-wall rock contact on the south-southwest side of the dike.
The Bomoseen dike is shown as extending continuously for nearly 5 km on Dale’s map of the Taconic slate belt (Dale, 1899). In addition to the Glen Lake and VT-30 locations, the Bomoseen dike is well exposed along North Road.
Of interest at Stop 3a is the well-developed pencil cleavage in the wall rock adjacent to the dike-wall rock contact on the north-northeast side of the dike. This pencil cleavage has been formed by the intersection of the well-developed slaty cleavage with closely spaced dike-parallel fractures.

STOP 3b. Scotch Hill Syncline
The exposure of the Scotch Hill syncline at Stop 3b (Fig. 6) is one of the best-known exposures in the Taconic slate belt. It lies in the northern domain. Slaty cleavage forms a divergent cleavage fan with the overall orientation of slaty cleavage similar to the mean orientation of slaty cleavage in the northern domain. Because sampling is prohibited at the exposure, the orientation of the stretching lineation was not determined, and the characteristics of strain fringes around subspherical core objects were not assessed.
The folded strata, which belong to the Poultney/Deep Kill Formation, contain two sets of veins, one that predates folding and one that formed at the same time as folding. A subhorizontal, north-northeast-trending fold axis for the Scotch Hill syncline is shown by both the pole to the best-fit plane to the poles to bedding and by the bedding-cleavage intersection lineation at the exposure (Figs. 7A and 8A). The poles to slaty cleavage lie on the best-fit plane to the poles to bedding (Fig. 7A). The prefolding veins are similarly oriented throughout the fold, and their poles do not plot on the best-fit plane to the poles to bedding (Fig. 7B). The synfolding veins lie in a range of orientations, and their poles plot on the best-fit plane to the poles to bedding (Fig. 7B).
The prefolding veins are long, thin, and planar (Fig. 8B–D). They are commonly boudined (Fig. 8B), and they are crosscut by the synfolding veins (Fig. 8C, D). The synfolding veins have a less uniform morphology but are commonly short, thick, and sigmoidal (Fig. 8C–F). They are either boudined or folded depending on the angle between the vein and slaty cleavage. Synfolding veins that are especially thick, however, do not appear deformed. Although the prefolding veins appear approximately parallel to slaty cleavage on the profile plane of the fold, their non-coaxiality with the fold can be seen on west-northwest-facing bedding surfaces on the subvertical limb of the fold. The trace of cleavage on the bedding surfaces is subhorizontal whereas the trace of the prefolding veins plunges to the south. The coaxiality of the synfolding veins with the fold can also be seen on these surfaces. Both the trace of cleavage and the trace of the synfolding veins are subhorizontal (Fig. 8A).
The asymmetry of the synfolding veins is consistent with flexural folding. On the gently dipping limb of the fold, the synfolding veins are commonly irregularly shaped, but a large vein has a classic sigmoidal shape, the S shape when viewed to the north-northeast indicating thrust sense of shear (Fig. 8E). On the subvertical limb of the fold, the synfolding veins are commonly sigmoidal, the Z shape when viewed to the north-northeast indicating westside- up sense of shear (Fig. 8F). These opposite senses of shear are as predicted for flexural folding with a pinned fold hinge. The mystery of the prefolding veins having similar orientations on the two limbs of the fold despite being prefolding is solved when shearing related to flexural folding is applied to their orientation. The change in orientation of the prefolding veins from limb rotation was essentially offset by the change in orientation from flexural folding- related shearing within the bedding layers.

STOP 4. Central Domain (Indian River–Pawlet/Austin Glen Formations).
The bedrock geology along Tanner Hill Road was mapped by Jacobi (1977) and is described in Rowley et al. (1979) and Landing (2002). The road transects the Tanner Hill syncline, and, although many of the exposures are small, the stratigraphic succession is readily observed and contacts between units are easily located. Exposures on the west limb of the syncline are somewhat inaccessible because of a new maple sugaring operation.
The small quarry immediately east of the parking area for this stop is in the Middle Ordovician Indian River Formation, a red slate interpreted as the oldest synorogenic unit in the Giddings Brook thrust sheet (Landing, 2002, 2012). Macdonald et al. (2017) dated a sample from a half-meter thick silicified layer of green slate in this quarry using LA-ICPMS and CA-IDTIMS. Zircon grains interpreted to be of primary volcanic origin and in the youngest population obtained by LA-ICPMS analysis yielded a CA-IDTIMS age of 466.06 ± 0.21 Ma.
Stop 4 lies in the central domain between the Taconic Frontal thrust and the Lee Road fault. The syncline extends for the length of the Taconic slate belt, the axial trace trending north-northeast–south-southwest in the northern and southern domains and north–south in the central domain. Folds between the Lee Road fault and Middle Granville thrust in the central domain have axial traces that trend north-northwest–south-southeast. The anomalous orientation of the fold axial traces is interpreted to be a result of the central domain having undergone inclined transpression (Crespi et al., 2010). Consistent with such a three-dimensional deformation, the stretching lineation is not downdip, and the strain fringes record extension in the Y direction. Moreover, in addition to the top-to-west-northwest sense of shear given by fibers in strain fringes in the XZ plane throughout the Taconic slate belt, fibers in strain fringes in the central domain are locally curved in the XY plane, the sense of curvature indicating sinistral shear.
Mesoscale, post cleavage normal faults are present in the exposures along Tanner Hill Road, most commonly in the Indian River Formation. Some of the faults contain laminated quartz and chlorite veins, but most lack fault-zone vein material, most likely because the faults are weathered. Faults with thick panels of laminated quartz and chlorite veins and well-developed steps can be found in out-of-place blocks in the quarries on either side of Tanner Hill Road. Although some faults are parallel to slaty cleavage, most strike parallel to but dip more steeply than cleavage (Fig. 9). The angle between the faults and slaty cleavage is on average about 10°–15°.

STOP 5. Southern Domain (Hatch Hill Formation) and Bardin Dike.
This exposure of the Lower Cambrian to Lower Ordovician Hatch Hill Formation consists of interbedded black slate and quartz arenites. Subvertical bedding is visible in the small roadcut on the north side of the large bend in Bardin Road, and subhorizontal bedding is visible in the small roadcut on the south side of the large bend in Bardin Road. Subvertical to steeply east-dipping and subhorizontal bedding is visible in the outcrops in the woods immediately west of Bardin Road. The exposure lies on the upright limb of a regional-scale syncline on Shaw’s map of portions of the Granville, Hartford, Hebron, and Pawlet townships (Shaw, 1984) and on the overturned limb of a regional- scale syncline on the Bedrock Geologic Map of Vermont (Ratcliffe et al., 2011). The bedding-cleavage relations at the exposure indicate a north-northeast-trending anticlinal hinge between the area with subvertical to steeply east-dipping bedding and the area with subhorizontal bedding. This anticline is parasitic to the regional-scale fold.
The characteristics of Stop 5 are typical of the southern domain. Slaty cleavage strikes north-northeast and dips about 30° east. The stretching lineation is about down the dip of slaty cleavage. Syntectonic fibers associated with slaty cleavage development are generally straight in XZ sections. Where they exhibit subtle curvature, the sense of curvature is consistent with top-to-west-northwest non-coaxial flow. The fibers yield a mean stretch value for the X direction of 1.6 and a mean stretch value for the Y direction of 1.1. The main differences between the southern and northern domains are that the fibers in the southern domain are not as curved as they are in the northern domain and that the stretch values for the X direction in the southern domain are not as high as they are in the northern domain. These differences are consistent with the southern domain lying at a higher structural level, i.e., farther from the base of the Giddings Brook thrust sheet, than the northern domain.
An igneous dike, which we have named the Bardin dike, lies in the hinge zone of the anticline indicated by the bedding-cleavage relations at the exposure (Fig. 10). This dike together with many of the other dikes/dike segments in the southern part of the Taconic lobe was mapped in 1895 by Florence Bascom, a pioneer in the advancement of women in geology. Bascom’s work contributed to Dale’s efforts to map the Taconic slate belt, and the dikes that she mapped appear on Dale’s map of the Taconic slate belt (Dale, 1899). The Bardin dike is about 2 m wide. Its trend is subparallel to the axial traces of the regional-scale folds in the southern domain and to the axial trace of the local anticline that it has intruded. Like the Bomoseen dike, a well-developed pencil cleavage is present in the wall rock adjacent to the dike. It can be observed on both sides of the dike and has been formed by the intersection of the well-developed slaty cleavage with closely spaced dike-parallel fractures. Bascom’s mapping shows the Bardin dike as one of a series of discontinuous dike segments that extends for about 12 km from Belcher, New York, to South Granville, New York (Fig. 10).

STOP 6. Central Domain/Southern Domain Boundary (Mount Merino Formation) and Granville Dike.
This exposure of the Middle to Upper Ordovician Mount Merino Formation changes from interbedded black slate and chert in the northwestern part of the exposure to mainly chert in the southeastern part of the exposure. Slaty cleavage, which is well developed in the northwestern part of the exposure, dips moderately east, and bedding is subvertical. The exposure lies on the upright limb of a regional-scale anticline on Kidd’s map of the Lee Road area (Kidd, 1976)/Fisher’s map of the Glens Falls–Whitehall region (Fisher, 1985) and on the Bedrock Geologic Map of Vermont (Ratcliffe et al., 2011). The bedding-cleavage relations here, however, indicate the exposure is on an overturned limb. This implies the strata are on the limb of a fold that is parasitic to the regional-scale fold.Stop 6 lies near the boundary between the central and southern domains where the trend of the fold axial traces changes from north-northwest–south-southeast in the central domain to north-northeast–south-southwest in the southern domain. Slaty cleavage strikes slightly east of north and dips moderately east. The stretching lineation is about down the dip of slaty cleavage. We have not analyzed syntectonic fibers at this location.
A narrow igneous dike lies near the southeastern end of the exposure. This dike does not appear on Dale’s map of the Taconic slate belt (Dale, 1899). It is shown, however, on Kidd’s map of the Lee Road area (Kidd, 1976) and is reproduced on Fisher’s map of the Glens Falls–Whitehall region (Fisher, 1985). We have named this dike the Granville dike. It is about 25 cm wide and has been mapped for a distance of about 0.4 km.
Like the Bardin dike, the Granville dike is one of a number of northeast-trending dikes/dike segments in the southern part of the Taconic lobe. These dikes have the same general trend in the southern domain, where the structural grain trends north-northeast–south-southwest, and in the central domain, where the structural grain trends north-northwest–south-southeast. Thus, while the dikes generally follow the structural grain in the southern domain, they cut across it in the central domain. The northeast-trending Granville dike cuts across the structural grain, but this is not immediately apparent at Stop 6 because both the dike and bedding are subvertical and because the orientation of bedding is difficult to see in three dimensions in the chert layers next to the dike. About 2 m to the northwest of the dike, bedding strikes about 035–040 (Fig. 11). Because the dike is slightly magnetic and somewhat curved, we used the orientation of dike-parallel fractures in the vicinity of the dike as a proxy for the orientation of the dike. These fractures strike about 060–075 (Fig. 11), which is oblique to bedding and similar to the 055–060 trend of the dike shown on Kidd’s map of the Lee Road area (Kidd, 1976). The oblique relation between the Granville dike and the structural grain is readily seen on Kidd’s map of the Lee Road area (Kidd, 1976)/Fisher’s map of the Glens Falls–Whitehall region (Fisher, 1985) where the dike is shown cutting across the contact between the Indian River and Mount Merino formations (Fig. 11).
Several brittle faults are present in this exposure. They are either oblique to both bedding and slaty cleavage or parallel to bedding.