C6: ACADIAN EXTENSION AROUND THE CHESTER DOME, VERMONT
Paul Karabinos
Department of Geosciences
Williams College
Williamstown, MA 01267
INTRODUCTION
Low-angle normal faults are important both in regions undergoing crustal extension and in mountain belts dominated by crustal shortening. Extension and shortening may be coeval in mountain belts, as has been proposed for the high Himalayas (Hodges et al., 1992) and Tibetan plateau (Molnar and Tapponnier, 1978), or extension may postdate shortening and thickening of continental crust, as has been suggested for the Basin and Range province (Coney, 1979; Sonder et al., 1987). Extension was also an important process in the evolution of ancient mountain chains (e.g. the Alps, Selverstone et al., 1984; Selverstone, 1985).
Although extension appears to be a fundamental process during orogenesis, major Paleozoic normal faults in the Appalachians have not been widely recognized. A notable exception is the Lake Char-Honey Hill fault in southeastern New England, which has been interpreted as an Alleghenian low-angle normal fault (Goldstein, 1989; Getty and Gromet, 1992). Before Goldstein's (1989) breakthrough work showed that the sense of displacement was normal on this fault, it was universally regarded as a thrust. Astonishingly little evidence exists for normal faulting during the Acadian orogeny in the New England Appalachians. Castonguay et al. (2001) presented thermochronological evidence for Silurian to Early Devonian exhumation and extension along the St. Joseph fault in Quebec, and Spear (1992) suggested that the Monroe Line along the New Hampshire-Vermont border might be a normal fault based on contrasting styles of metamorphism of rocks across the contact.
Karabinos (1999, 2000) proposed that faults previously interpreted as Taconic thrusts around the Chester dome in southeastern Vermont (Thompson et al., 1990; Ratcliffe et al., 1997) are really Acadian normal faults. This field trip presents evidence for an extensional shear zone around the Chester dome and explores the tectonic implications of Early Devonian crustal extension. If normal faulting occurred in western New England, an entirely new aspect of the Acadian orogeny will have to be taken into account in tectonic reconstructions, the interpretation of geologic maps, and models of ore genesis within the belt.

C6: ACADIAN EXTENSION AROUND THE CHESTER DOME, VERMONT

STOP 1. BASEMENT-COVER CONTACT, NW MARGIN OF CHESTER DOME.
Gneiss on E side of road, opposite pullout, belongs to the Middle Proterozoic Mount Holly Complex (i.e. basement) and is composed of quartz, plagioclase, biotite, epidote, ± muscovite, ± microcline. It contains abundant highly-deformed pegmatites that distinguish it from plagioclase schist and gneiss of the Late Proterozoic Hoosac Formation. Walk N along road. The first outcrop on the W side of the road belongs to the cover sequence. Before looking at it, head into the woods on the E side of the road to see more of the Mount Holly complex. The contact appears to be buried in the small drainage on the E side of the road. The basement gneisses are highly sheared and the mylonitic fabric dips steeply to the W. Outcrops up the hill contain large-scale C-S fabric and rotated boudins that indicate a tops-to-the-west sense of shear.
The first outcrop on the W side of the road is quartz, muscovite, biotite, chlorite, ± plagioclase, ± garnet schist. Many layers contain graphite. Further N are exposures of quartz-rich gneiss with a distinctive pinstripe texture. I think these rocks belong to the Cram Hill and Moretown Members of the Missisquoi Formation of Doll et al. (1961). Reverse direction and drive N on Route 103.

STOP 2. CAVENDISH GORGE, AGE OF THE CAVENDISH Fms.
Walk from gate up the dirt road to the power station. One of the tonalite samples dated by Ratcliffe et al. (1997) was collected from the outcrop next to the power station. Ratcliffe et al. (1997) argued that the 1.4 Ga tonalite intruded the metasedimentary units of the Cavendish Formation here in the gorge, the type locality of the Cavendish Schist of Richardson (1929), making them older than the tonalite and part of the Mount Holly Complex. I think that the contact between the tonalite and the marble, calc-silicate, quartzite, and schist is tectonic in origin. I collected a sample of quartzite from the gorge
about 50 m upstream from the power station and separated detrital zircons. Single grain Pb-evaporation and SHRIMP ages of 25 grains are all younger than 1.4 Ga (Karabinos et al., 1999), consistent with a tectonic contact between the tonalite and the metasedimentary rocks, but inconsistent with the intrusive contact interpretation. Furthermore, the plagioclase schist here in the gorge does not contain abundant highly-deformed pegmatite like the gneiss we saw in the basement at Stop 1. I correlate the plagioclase-rich schist with the Hoosac Formation and the marble, calc-silicate, and quartzite with the Tyson Formation.

STOP 3. Basement-Cover Contact, NE Margin Of Chester Dome.
Outcrop immediately E of pullout is quartz-rich, biotite, chlorite, ± plagioclase schist with graphite in some layers. Farther E are more outcrops of this lithology and some layers contain highly fractured garnet crystals partially altered to chlorite along the cracks. These rocks are part of the Silurian to Devonian sequence, probably the Northfield or Waits River Formation. As we will see, the intense zone of shearing along the northeastern margin of the Chester dome extends all the way from Middle Proterozoic basement through the pre-Taconic cover sequence and into the Silurian units.
Walk back down the road to the W. Outcrop beneath roots of fallen tree on S side of road is similar to the first outcrop and contains a fine-grained, unmetamorphosed mafic dike, probably related to Mount Ascutney, a Cretaceous intrusion. Outcrop along road is spotty but it is much better in the woods on both sides of the road. Below the schist of the Northfield Formation are highly deformed felsic and mafic gneisses that contain a mylonitic fabric. It is difficult to assign these rocks to a specific unit with certainty, because they are highly deformed and out of context, but they look more like Barnard Gneiss to me than anything else. This is the only rock type belonging to the Late Proterozoic to Ordovician sequence that I have found along this traverse.
Only 0.3 mile W of the pullout is an outcrop of highly sheared augen gneiss. This rock was mapped as Bull Hill Gneiss by Doll et al. (1961) but not all layers contain microcline, which is typical of Bull Hill Gneiss. In thin section it is clear that the eyes in some layers are composed of plagioclase aggregates. Doll et al. (1961) showed Bull Hill Gneiss directly below Hoosac Formation around much of the Chester dome. It is common to find Bull Hill Gneiss at the basement-cover contact in the southern part of the dome, but other feldspar-rich gneisses belonging to the Mount Holly Complex are more typically found at the contact in the northern part of the dome.

Outcrop on corner contains unmetamorpbosed dike.

STOP 4. GNEISSES IN MOUNT HOLLY COMPLEX WITH ACADIAN DIKES.
The tonalitic gneiss seen here is a common lithology in the Mount Holly Complex of the Chester dome. Also, the deformation fabric in this outcrop is typical of that found in the Mount Holly Complex rocks within the core of the dome, away from the basement-cover contact. Late cross-cutting felsic dikes, like those seen here, are found in many outcrops in the dome below the mylonite zone. Ratcliffe et al. (2001) reported a zircon SHRIMP age from a granitic dike near Gassetts of 392 ± 6 Ma.

Road parallels the W flank of the dome
The narrow valley that the road follows S of Grafton parallels the W flank of the dome near the contact between the Hoosac Formation and the Bull Hill Augen Gneiss.

STOP 5. BASEMENT-COVER CONTACT, SE MARGIN OF DOME.
Walk back about 20 m to W to first outcrop; look for signs of blasting dating from construction of the old railway bed parallel to the West River. Rock is quartz, microcline, plagioclase, biotite, epidote gneiss belonging to the Mount Holly Complex. This does not look like typical Bull Hill Gneiss to me, although that is what is shown below the Hoosac Formation by Doll et al. (1961). Walk E along road past pullout. There are some outcrops of plagioclase schist that look like Hoosac Formation. Continue E along road to where the river is just S of the road. River outcrop of mafic schist with some pelitic schist layers. The pelitic schist contains some large garnet crystals that were incompletely altered to chlorite. The extent of retrogression increases significantly to the E along this traverse. This outcrop and others further E contain cleavage surfaces with sprays of biotite after amphibole. There are some outcrops along the road to the E of quartz-rich schist with distinctive pinstripe texture. I would assign the mafic and pelitic schist found in the river outcrops to the Moretown Formation. I see no rocks here that I would assign to the Pinney Hollow, Ottauquechee, or Stowe Formations.

Covered bridge on left, outcrop of quartz-rich gneisses on right.
These rocks could belong to the lower part of the cover sequence (Rosenfeld, 1988) or to the basement. Approximately 0.3 mi. back to the southeast, near the sharp bend in the road, is sheared Bull Hill augen gneiss on the north side of Route 30. The Bull Hill Gneiss intruded the Mount Holly Complex at approximately 960 Ma (Karabinos and Aleinikoff, 1990), after the Grenville orogeny. Therefore, the deformation fabric in this unit is from the Taconic and Acadian orogenies.

STOP 6. TOWNSHEND DAM SPILLWAY, MYLONITE ZONE
SW MARGIN OF CHESTERDOME.
Walk around gate and down dirt road past the equipment sheds for Townshend Dam. Just before the road crosses a small and unappealing stream, walk through the reeds to the large outcrop to your right. Here is highly sheared plagioclase schist and gneiss of the Hoosac Formation. Some dark biotite-rich layers are present. Note the extreme isoclinal folds and boudinage in this outcrop.
If it is not too wet, it is possible to walk up the spillway and see rocks on both sides of the cut. Another approach is to walk back to the road, continue across the bridge about 100 m, and head into the pine trees on the right. Look for the end of the chain link fence and then walk around and past it to the N, toward the edge of the spillway using extreme caution - there are some steep cliffs with a lot of vertical relief. From the top of the spillway it is possible to look to the other side and appreciate the extreme deformation of these rocks. There are abundant folds, boudinage, and truncated layers. The rocks on this side of the spillway are made up of interlayered mafic and pelitic schist, with sprays of biotite after amphibole, typical of the Moretown Formation. Between the rocks of the Hoosac Formation just seen and the Moretown Formation, there are some layers of garnet-rich pelitic schist that could be part of the Pinney Hollow Formation. They contain quartz, plagioclase, muscovite, biotite, chlorite, and garnet up to 3 cm in diameter. Some of the garnets have quartz inclusion trails that indicate a counter-clockwise sense of rotation when viewed to the N. This is consistent with other kinematic indicators that show tops to the W. An interesting feature of this section is that the degree of retrogressive alteration of garnet to chlorite is highly variable. Furthermore, some layers contain graphite, usually as inclusions in plagioclase and garnet. I believe that the graphite was precipitated from infiltrating fluids during metamorphism and that the variable retrogression reflects irregular fluid flow, probably during the normal-sense shearing that preceded the doming phase of deformation.
This is the location of garnet dated by Christensen et al. (1989) using 87Sr/86Sr variations; they found that the garnet grew at approximately 380 Ma over a 10 m.y. interval. Thompson et al. (1993) assigned rocks in the spillway to the Pinney Hollow and Ottauquechee Formations. If this assignment is correct, these units are very thin compared to their outcrop widths on the E flank of the Green Mountain massif.

STOP 7. TOWNSHEND DAM ROADCUT
MORE OF THE MYLONITE ZONE, SW MARGIN OF CHESTERDOME.
Please be very careful looking at this roadcut. It is a dangerous curve and cars drive by very fast. Stay off the pavement!
The most prominent feature of this long roadcut is the strong mylonitic fabric parallel to the compositional layering. There are abundant folds, boudinage, and rotated garnets. However, what impresses me is the extent of disrupted layering throughout the outcrop. The pelitic layers contain quartz. plagioclase, muscovite, biotite, chlorite, and garnet. The mafic layers contain quartz, plagioclase, hornblende, muscovite, biotite, garnet, and epidote.
Rosenfeld et al. (1988) called these rocks Ottauquechee Formation. I would prefer to assign them to the Moretown Formation. The rocks on the W side of the roadcut show the distinctive pinstripe texture of the Moretown Formation. Rocks in the middle and eastern end of the roadcut are admittedly more difficult to assign, but look like Moretown Formation to me based on their lithology. Whatever units these intermediate rocks belong to, there is not much distance between rocks belonging to the Hoosac Formation to the E and rocks that are clearly part of the Moretown Formation to the W. Compared with the typical sequence seen on the E flank of the Green Mountain massif, only several kilometers to the W, the section between the Hoosac and Moretown Formations here is highly attenuated. Outcrops W of here along Route 30 are mapped as Moretown Formation and show the distinctive pinstripe texture. They do not, however, have the strong mylonitic fabric seen in this roadcut.