A shear zone origin for Alleghanian (Permian) multiplekanat.jsc.vsc.edu/gey3120/Goldstein1994.pdf · A shear zone origin for Alleghanian (Permian) multiple deformation in eastern

TECTONICS, VOL. 13, NO. 1, PAGES 62-77, FEBRUARY 1994

A shear zone origin for Alleghanian (Permian) multiple deformation in eastern Massachusetts

Arthur G. Goldstein

Department of Geology, Colgate University, Hamilton, New York

Abstract. The area around Worcester, Massachusetts, has been used to determine which deformational and metamorphic features are due to the Alleghanian (Permian) orogeny and which are pre-Alleghanian. Isolated, fault-bounded inliers of Carboniferous rocks display evidence of a single metamorphism and the formation of two prominent cleavages. The first cleavage formed synchronously with metamorphism. Pre-Carboniferous metasedimentary rocks have been affected by two metamorphisms and contain three prominent cleavages. The initial cleavage formed during the first metamorphism and the second cleavage formed during the second, retrogressive metamorphism. Thus the second two cleavages and the second metamorphism in pre-Carboniferous rocks are interpreted as Alleghanian. Normal displacements on two distinct faults are bracketed by the formation of the first and second Alleghanian cleavages. The initial faulting, along the newly defined Wachusett mylonite zone (WMZ), formed a wide zone of ductile mylonites which dips moderately to the northwest and contains elongation lineations which trend northwest. The second faulting, along the Clinton-Newbury fault (CNF) occurred along a steeply inclined plane which cuts the WMZ mylonites and contains a thin zone of phyllonites and mylonites which have elongation lineations which trend west. Alleghanian cleavages and metamorphism are confined to a mappable zone which is approximately 15 km wide where well defined. This zone is interpreted as a ductile shear zone which moved twice, forming the first and second Alleghanian cleavages. Early-formed, pre- Alleghanian metamorphic minerals are retrograded to hydrous phyllosilicates, and new hydrous minerals formed in the shear zone, indicating that it was a pathway for fluid flow. The initial motion was left- lateral with a thrust component. The second cleavage formed during top-to-the-northwest normal displacements. Thus the history of Alleghanian tectonism in this area began with sinistral faulting and then included three

Copyright 1994 by the American Goophysical Union.

Paper number 93TC02522. 0278-7407/94/93TC02522510.00

displacements along normal faults or shear zones. This suggests that the strain history included two distinct episodes, with the second experiencing slightly different strain orientations at different times, resulting in first WMZ normal motion, then CNF normal motion and finally normal displacements related to the second Alleghanian cleavages. This history agrees well with other work on the nature of the Alleghanian orogeny in the northeast United States.

INTRODUCTION

One of the more pressing problems for the study of multiply deformed rocks in the interior of mountain belts is the absolute timing of the formation of different fabric elements (cleavages, !ineations). A particularly clear case is the uncertainty of Alleghanian (Permian) deformational features in the multiply deformed and polymetamorphosed rocks of the northern Appalachians. Carboniferous rocks are present only locally in southern New England, in the Narragansett basin of Rhode Island and Massachusetts, in small basins in northern Rhode Island and eastern Massachusetts and several small inliers north of

Worcester, Massachusetts (Figure 1). The southern Narragansett Basin is multiply deformed and metamorphosed to upper amphibolite facies. However, the intensity of deformation and metamorphism within the basin decreases rapidly northward [Dallmeyer, 1982; Murray et al, 1979; Skehan et al, 1986]. The Harvard conglomerate [Thompson and Robinson, 1976] and rocks of the "Worcester Coal Mine" (Figure 2) are both believed to be Carboniferous, have been metamorphosed to middle greenschist facies (chloritoid) and contain a bedding-parallel foliation and a later crenulation cleavage [Thompson and Robinson, 1976; Hepburn, 1978]. However, in the granitic gneisses which surround the Narragansett basin and the polydeformed metasedimentary and metaigneous rocks which form the basement for carboniferous

rocks near Worcester, Massachusetts, it has not been possible to unequivocally distinguish Alleghanian fabrics from older deformational effects, except in one small area [Dreier, 1984]. Thus our

Goldstein: Alleghanian Orogeny in Eastern Massachusetts

N.H.

MASS.

FITCHBURG PLUTON

0

CONN.

10 20 Mi.

I I ? •0 •0 30Km.

Fig. 1. Generalized tectonic map of southeastern New England showing area studied. CNF, Clinton-Newbury fault; BBF, Bloody Bluff fault; LCF, Lake Char fault; HHF, Honey Hill fault; WF, Willimantic fault.

understanding of the Alleghanian orogeny in the northern Appalachians is defined almost exclusively by the geology of the southern Narragansett basin [Mosher, 1983] and the Norfolk basin, the small appendage which projects from the northwest Narragansett basin [Cazier, 1987].

There are several important tectonic problems which require a more in-depth understanding of the latest Paleozoic orogeny in southern New England. The Alleghanian orogeny in the central and southern Appalachians was a major event, marking the final closure of Iapetus and collision of eastern North America and northwestern Africa [e.g., Hatcher et al., 1989]. Most workers agree that the Iapetus ocean had closed in the northern Appalachians and Caledonides by Siluro-Devonian time [e.g., Rodgers, 1988] and that the nature of late Paleozoic deformations in the northern Appalachians will help us to better understand the nature of the collision to the south. For that matter, unless late Paleozoic effects can be separated from earlier fabrics, the exact nature of the Acadian (Siluro- Devonian) and perhaps the Taconian (Ordovician) orogenies cannot be accurately defined in southeastern New England. Finally, Goldstein [1982a, b; 1989], Goldstein and Owens [1985] and Getty and Gromet [1992] have described low-angle normal faults which underlie

much of southeastern New England (Figure 1). Goldstein [1989] and Getty and Gromet [1992] have proposed that those faults are Alleghanian, and Goldstein [1989] has suggested that they are associated with a releasing bend in a major orogen-parallel system of dextral strike-slip faults. It has not been possible to correlate this normal faulting with deformational events in the Narragansett basin. Can these low-angle normal faults be placed in an Alleghanian deformational sequence and can the origin proposed by Goldstein [1989] be confirmed? This is especially important because low-angle normal faults are becoming widely recognized in mountain belts worldwide [e.g., Dewey, 1988] and their origin(s) are a subject of considerable debate. It is important to evaluate the origin of the low-angle normal faults in the northern Appalachians in light of models for the origin of other such faults and to evaluate their significance in the Alleghanian orogeny.

It will be shown in this paper that the Alleghanian deformational events in eastern Massachusetts are

complex, involving the formation of two prominent cleavages, one of which is associated with large-scale folding, and two distinct episodes of faulting. Further, it will be shown that the cleavages, folds and the associated metamorphic hydration and

64 Goldstein: Alleghanian Orogeny in Eastern Massachusetts

PH •

! BOUNDARY 'OF ß STERLING SHEAR ZONE

i ,/, )11: b

"' / // •1 I i '/I // /

/I I/ / ;i • I

• /

.

I

WCM

/ /

BH

Fig. 2. Tectonic map of the Worcester, Massachusetts, area shown in Figure 1. Rocks of the Merrimack trough are unpatterned except in the Wachusett Mylonite Zone where they are given a pattern indicating mylonitization. Carboniferous rocks are shown with a vertical rule pattern in three locations: PH, Pin Hill; BH, Bare Hill; WCM, Worcester Coal Mine. Roman numerals refer to structural domains discussed in the text and shown in

Figure 5.

retrogression are confined to a 15-km-wide zone with sharp boundaries. The most likely origin for the cleavages is as strain accommodation features within a wide ductile shear zone, referred to as the Sterling Shear Zone (SSZ), which has experienced complex multiple motions.

STRUCTURAL GEOLOGY OF EASTERN

MASSACHUSETTS

The area around Worcester, Massachusetts (Figure 2), is ideal for unraveling the nature of the

Alleghanian orogeny in pre-Carboniferous rocks. Several small inliers of Carboniferous rocks occur

surrounded by igneous and metasedimentary rocks of the Merrimack trough and the metasedimentary and metavolcanic rocks of the Nashoba terrane. The Carboniferous rocks can be subdivided into two

sequences with slightly different lithologies. The inliers of Harvard conglomerate at Harvard (Pin Hill) and Bolton (Bare Hill), Massachusetts, contain, in decreasing abundance, gray, siliceous phyllite and matrix supported metaconglomerate with clasts consisting principally of quartzite and

Goldstein: Alleghanian Orogeny in Eastern Massachusetts 65

metasandstone. The lithologies exposed at the well- known Worcester coal mine and several other small

inliers nearby [Grew, 1973] contain black carbonaceous phyllite, metaconglomerate and metaanthracite. The age of the deposits at the Worcester coal mine is constrained by the presence of plant fossils to be middle Pennsylvanian [Westphalian [Grew et al., 1970; Lyons and Darrah, 1979]), essentially the same age as those of the Narragansett and Norfolk basins. Fossils have not been found in the rocks of the Harvard and Bolton

deposits, but abundant carbonaceous detritus strongly suggests an age similar to that of the Worcester coal mine deposits.

The basement for the Carboniferous

metasedimentary rocks is composed of a sequence of phyllites, quartzites and calcareous quartzites of uncertain age [Peck, 1976] intruded by granitic stocks and plutons. Lyons et al. [1982] argue that these rocks, which lie in what they term the Merrimack trough, are Precambrian and are distinct from the Paleozoic metasedimentary rocks of the Kearsarge-Central Maine Synclinorium to the east (formerly referred to as the Merrimack synclinorium). The two sequences are separated, for most of their outcrop length, by granitic rocks of the Fitchburg pluton (402 + 11 to 372 + 7 Ma, Rb-Sr and 390 +15, Pb-Pb [Zartman and Naylot, 1984]). The Merrimack trough is bounded on the east by the Clinton-Newbury fault which separates it from the rocks of the Nashoba terrane, which are composed of metavolcanic and metasedimentary rocks, intruded by numerous felsic plutons, which cannot

be positively correlated with rock sequences in any other part of the Appalachians and which have been metamorphosed to upper amphibolite facies [Hepburn et al, 1987b].

Faults

The Clinton-Newbury fault (CNF) is a prominent west to northwest dipping fault which separates the Nashoba terrane from lower metamorphic grade rocks of the Merrimack trough to the west. It has been described as a west-over-east thrust fault

[Skehan, 1968]. New mapping (Figure 2), combined with orientations of mineral elongation lineations in mylonites and other fault rocks (Figure 3) and analysis of kinematic indicators in oriented thin sections (Figure 4) allows this initial description to be modified. The CNF exists in two separate fault segments (Figure 2 and Hepburn and Munn [1984]). Each segment is marked by a reasonably thin zone of mylonitization (approximately 100-200 m wide) with elongation lineations which trend E-W (Figure 3). No geological units can be found that are displaced across the CNF, and thus determination of the sense of displacement must be based on less direct observations. Immediately adjacent to the CNF along much of its length in the footwall is a rusty- weathering schist (the Tadmuc Brook Schist of Bell and Alvord [1976]. Within approximately 100 m of the fault this schist has been mylonitized producing a highly sheared phyllonite. The phyllonite is cut by numerous, closely spaced shear zones each with normal displacement (Figure 4). Such structures

ß POLE T

O MINERAL ELONGATION LINEATION

Fig. 3. Lower hemisphere, equal-area stereographic plots of structural data for the Wachusett Mylonite Zone (WMZ) and the Clinton-Newbury Fault (CNF). Mineral elongation lineations are shown as open circles and poles to mylonitic and phyllonitic foliation are contoured for the WMZ (contours 2, 4, 6, 8, 10, 12% per 1% area; 46 data points).

• Goldstein: Allegh•ni•n Orogeny in E•s•ern M•ss•chuse•s

have been called extensional crenulation cleavages by Platt and Vissers [1979] and normal-slip crenulations by Dennis and Secor [1987]. These microshear zones shows normal (west-down) displacement, indicating that at least the last phase of movement on the Clinton-Newbury fault was normal rather than thrust (west-up) as proposed by Skehan [1968]. A normal sense of displacement is also more in keeping with the rather steep dip (60 ø- 80 ø to the west) of the fault, the occurrence of much higher metamorphic facies in the footwall of the fault (Nashoba terrane) than in the hanging wall (Merrimack trough) and the preservation of isolated inliers of Carboniferous rocks exclusively in the hanging wall of the fault (Figures I and 2).

Between the two segments of the CNF is a very wide mylonite zone herein named the Wachusett mylonite zone (WMZ), after Lake Wachusett on whose eastern shores can be found the best

exposures of the mylonites (Figure 2). Mylonites of the WMZ had protoliths which were mostly from the Nashoba terrane (mafic schists and granites) but the contact between rocks of the Nashoba block and

Merrimack trough is preserved within the WMZ and a small panel of mylonitized Merrimack rocks is also present. Mineral elongation lineations in the WMZ trend generally N30øW (Figure 3). Kinematic analysis of mylonites from the WMZ reveals normal (west-side-down) motion. The most common kinematic indicators seen in oriented thin section are

asymmetric recrystallization tails on feldspar augen (sigma grains) and asymmetric muscovite augen (Figure 4). Also observed, but less common, are asymmetric quartz grain shapes, c and s fabrics and asymmetric microfolds in the mylonitic foliation.

The WMZ is most probably distinct from the CNF for the following reasons: (1) The orientation of

lineations within WMZ mylonites is different from that in the CNF fault rocks, although some overlap is present between the two populations; (2) The width of the CNF zone is quite narrow compared to the width of the zone of mylonitization of the WMZ; (3) The mylonitic foliations within the WMZ are truncated by the CNF. The orientations of WMZ foliations tend to be more easterly than CNF foliations, although considerable overlap exists. Where the WMZ is in contact with the CNF, one does not see a smooth curvature of WMZ foliations

into the more northeasterly CNF orientations. Rather, there is a reasonably abrupt change in orientation.

Cleavages, Folds and Metamorphisms

Detailed structural analysis of rocks of the Merrimack trough reveals a complex set of superposed fabrics which define distinct deformational events. Dt is characterized by a ubiquitous bedding parallel foliation (SoS0. Dz produced macroscale folds overturned to the east and a penetrative, subvertical axial planar crenulation cleavage (Sz). Dz fold axes are subhorizontal and are parallel to intersection lineations (Lz). D3 is evidenced by a locally well-developed subhorizontal crenulation cleavage (S3). The latest fabric identified (S4) is a vertical crenulation cleavage which is only present locally, is intensely developed where found and occurs in zones which vary from centimeters to tens of centimeters in width. Because

this cleavage is so locally developed, it is not considered further in this study.

Changes in the orientations of fabric elements can be observed between the eastern portion of the Merrimack trough and the western portion (Figure

Fig. 4. Photomicrographs of microstructures in rocks of the Merrimack trough and from ß the WMZ. Figures 4a through 4d are from the WMZ, thin sections were cut parallel to lineation and perpendicular to foliation. The down-plunge direction of the lineation (NW) is to the right of all photographs (i.e., near vertical, northwest striking sections viewed looking to the southwest). Mylonitic foliation is horizontal. (a) Large (approximately 1.5 mm) muscovite augen showing asymmetry with respect to mylonitic foliation but also shearing of augen along top of grain. Width of photograph is 3.2 mm, plane polarized light. (b) Asymmetric recrystallization tails (sigma grain) on plagioclase augen. Tails are composed of fine grained recrystallized plagioclase and muscovite and the grain is rimmed with muscovite. Photograph is 1.5 mm across, plane polarized light. (c) Asymmetric intrafolial folds in quartz, biotite, plagioclase, muscovite mylonite. Note that folds deform the mylonitic foliation but are clearly intrafolial. Photograph is 6.5 mm across. (d) Asymmetric recrystallization tails (sigma grain) on K-feldspar augen. Photograph is 3.2 mm across, plane polarized light. (e) Chlorite (Mz) overgrowing garnet (M0 in Worcester phyllite. Sz is subhorizontal, Photograph is 1.5 mm across. Plane polarized light. (f) Pseudomorph after staurolite (Sp) and andalusite (A), which is largely retrogressed to white mica (W). Photograph is 1.4 cm across. Plane polarized light. (g) Chlorite overgrowing S: crenulation cleavage. Photograph is 0.75 mm across, plane polarized light. (h) Chlorite deformed by Sz crenulation cleavage. Note that large chlorites are reoriented by Sz and that some chlorites appear to be growing parallel to Sz.


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5). The Wekepeke fault [Peck, 1976] forms a boundary between two domains, I and II (Figure 2), across which all fabric elements except S3 are rotated approximately 45 ø clockwise about a near vertical axis. Thus in Domain I SoS, forms a girdle which strikes east-west, S2 strikes north-south and L2 (fold axes and intersection lineations) trends due north (Figure 5). In Domain II, however, SoS, forms a girdle which strikes northwest, Sz strikes northeast and Lz trends southwest (Figure 5). The exception to this rotation occurs for S3, which maintains a constant orientation within both domains (Figure 5). Further to the west, all traces of fabrics related to Dz and D• are missing. Within this westernmost area, Domain III, SoS, strikes northwesterly and dips moderately towards the southwest. A mineral lineation, defined primarily by biotite but rarely by andalusite, trends northwest with a shallow plunge. The transition from Domain II to Domain III is well

exposed in roadcuts along I-190 and the Dz and D• fabrics both disappear across a zone approximately 500 m in width [Goldstein, 1992]. Outcrops within the transition area contain mylonite zones several tens of centimeters thick. The mylonites strike northeasterly and dip approximately 50 ø to the northwest, slightly oblique to S2. Lineations in these mylonites rake between 40 ø and 50 ø to the northeast. Subhorizontal crenulation lineations on the mylonitic foliations are due to the postmylonitic superposition

of S•. Kinematic indicators are rare in these biotitic metaquartzites and give conflicting shear senses which cannot be interpreted without ambiguity. Sz curves smoothly into parallelism with the mylonites.

Two distinct metamorphisms can be identified in rocks of the Merrimack.Trough (Figure 4). The earliest, M,, formed garnet, staurolite and andalusite in pelitic lithologies. These minerals, along with other metamorphic minerals form S,, indicating that they formed during D,. S, is neither contained as inclusions in nor wrapped around any porphyroblasts. Within Domains I and II, where the Sz and S• cleavages are present, M, metamorphic minerals are retrograded and new metamorphic minerals have formed. In pelitic lithologies, pseudomorphs of staurolite are present as patches of schist with the outlines of crystal faces (Figure 4F). Andalusite crystals are present as crystal-shaped patches of fine-grained white mica some of which contain remnants of andalusite in their center.

Garnets have rims of coarse chlorite (Figure 4E) and new chlorite crystals have grown in the matrix (Figure 4G). The chlorite growth spanned the formation of Sz; some chlorite crystals have been rotated into the crenulation cleavage whereas others crosscut it.

In contrast to the pre-Carboniferous rocks described above, the Carboniferous rocks at the three small localities in eastern Massachusetts (Pin

III

I

Fig. 5. Lower hemisphere, equal-area stereographic plots of structural data from three domains in the study area. All contours are in intervals of 3 sigma using the Kamb method. SoS, are poles to bedding and pervasive bedding parallel foliation. L, are biotite and, locally, andalusite mineral alignment lineations. Sz are poles to crenulation cleavage axial planar to D2 folds. Lz are intersection lineations and fold axes of Dz folds. S3 are poles to subhorizontal crenulation cleavage.


Hill, Bare Hill and the Worcester Coal Mine, Figure 2) contain only two cleavages: a pervasive bedding- parallel foliation (S0 and a crenulation cleavage (S:). Orientations of fabrics in the Carboniferous rocks do

not correlate with those in their basement. SoS• in Pennsylvanian rocks is highly variable, commonly striking northeast and dipping to the northwest moderately to steeply. However, northwest strikes are also common as are east-west orientations. No

pattern has been detected in the orientations and no fold hinges have been observed. Rather, the variation in orientations seems to reflect different

orientations in different fault blocks. S2 is less variable in orientation, tending to strike northeast and dip steeply northwest. A single metamorphism has affected these rocks, producing porphyroblasts of chloritoid and a bedding-parallel metamorphic foliation (S0 defined principally by muscovite. Chloritoid growth spanned the formation of S•; some porphyroblasts are aligned with the foliation whereas others cut across this fabric. The second

cleavage, S2, is clearly later than chloritoid growth.

Carboniferous rocks and their pre-Carboniferous basement. For example, at Bare Hill in Bolton, Massachusetts (Figure 2), SoS• in Carboniferous rocks strikes approximately E-W and dips 50ø-90 ø N and S2 strikes consistently N30øE and dips 40ø-70 ø SE (Figure 6). In pre-Carboniferous rocks within approximately 300 m of outcrops of Carboniferous rocks S2 (postulated as equivalent to S• in Carboniferous rocks) is oriented N55øE 75NW and S3 (postulated as equivalent to S2 in Carboniferous rocks) is oriented N70øW 15øNE (Figure 6). The orientation of cleavages in the pre-Carboniferous rocks are close to those observed throughout Domain I whereas the orientations in Carboniferous rocks are

clearly different. It is proposed that, because the Carboniferous rocks are bounded on at least two

sides by faults, the fabrics in the Carboniferous rocks have been rotated after their formation. The

other two outcrop areas of Carboniferous rocks in

71ø37'30 ,,

TIMING OF DEFORMATIONS AND

METAMORPHISMS: A COMPLEX

ALLEGHANIAN EVENT

The following observations allow the timing of faulting, folding, cleavage formation and metamorphisms to be accurately defined:

1. Pre-Carboniferous rocks contain three

prominent cleavages whereas Carboniferous rocks contain only two.

2. Pre-Carboniferous rocks experienced two metamorphic events whereas Carboniferous rocks experienced only one.

3. Mylonites of the WMZ and the CNF contain a subhorizontal crenulation cleavage, S3 (S3 in pre- Carboniferous rocks) but never contain S:, despite their proximity to rocks which were affected by the D2 event. Further, SoS• and S: are rotated across the Wekepeke fault whereas Sa is not.

These observations lead to the conclusions that the

Alleghanian (Permian) orogeny in eastern Massachusetts consisted of the following sequence of structural events: (1) Formation of S: and associated folds in pre-Carboniferous rocks; accompanied by M: metamorphism (equivalent to S• and M• in carboniferous rocks); (2) normal displacements on the WMZ; direction of motion N30øW; (3) normal displacements on the CNF; direction of motion N90øW; accompanied by displacement along the Wekepeke fault; (4) formation of Sa.

These conclusions are based, in part, on the correlation of the two cleavages in Carboniferous rocks with the second two cleavages in pre- Carboniferous rocks. As noted above, the orientations of these fabrics are not the same in

400 m.

C.I. --- 50 ft.

Fig. 6. Map of the Bare Hill locality near Bolton, Massachusetts (based on new mapping and on Zen [1983]). Orientations of cleavages and bedding believed to be correlative are noted. Ph, Harvard conglomerate; SOvh, Vaughn Hills formation; SZtb, Tadmuc Brook Schist. SoS• in the Harvard conglomerate is believed to be contemporaneous with S2 in the Vaughn Hills formation as is S2 in the conglomerate and S3 in the Vaughn Hills formation.


eastern Massachusetts are similarly fault bounded [Grew, 1973; Thompson and Robinson, 1976; Zen, 1983] accounting for the wide divergence of orientations of fabrics in these rocks. It is significant to note that SoS• and S,• are nearly orthogonal to each other in Carboniferous rocks as are S,• and S3 in pre-Carboniferous rocks. Finally, the relationship between metamorphism and the formation of S• in Carboniferous rocks and S,• in pre-Carboniferous rocks is the same, suggesting strongly that the two cleavage-forming events are coincident. Thus for this area in eastern Massachusetts it can be

documented with a high degree of assurance that the Alleghanian orogeny was a fairly complex event involving tight folding, metamorphism, two distinct episodes of ductile normal faulting and formation of crenulation cleavages. It is unlikely that any of the features ascribed to the Alleghanian orogeny are Triassic or later considering the unmetamorphosed, unfolded and uncleaved nature of Triassic rocks in

the Connecticut Valley basin to the west of this area and in the small Middleton basin, east of this area.

One important question regarding timing of deformations is the relative timing of S,• and the thin mylonite zones at the western margin of the zone of Alleghanian cleavages. As noted above, both S,• and the mylonites predate the formation of S3 and curves smoothly into the thin mylonite zones. These observations could be interpreted as indicating that the formation of S2 predates the formation of the mylonites or that the two events are synchronous. In either case, both events must be Alleghanian.

INTERPRETATION AND SPECULATION: A

SHEAR ZONE MODEL FOR ALLEGHANIAN DEFORMATION

One of the more significant observations noted above is that the Alleghanian fabrics are confined to a zone between 7 and 13 km wide, where well defined. The western limit of this zone is marked

by the abrupt loss of S,• and S3 and the presence of thin (approximately 5 cm thick) mylonite layers, but the eastern boundary is more difficult to define. East of the CNF and WMZ the Nashoba zone

contains many sets of superposed cleavages and foliations as well as numerous mylonites and shear zones. Unfortunately, it is not possible to unequivocally define any of these as Alleghanian. •Ar/39Ar cooling ages from rocks in the Nashoba terrane record cooling during the Carboniferous (325 - 351 Ma hornblende; 308 Ma biotite; 299 Ma Rb/Sr biotite [Hepburn et al., 1987a], suggesting that no thermal events affected that terrane during the latest Paleozoic. Thus the tentative eastern boundary of the zone of Alleghanian deformation is taken as the CNF and, locally, the WMZ.

There are two possible interpretations of the

observations detailed above. One is that the

deformations associated with S: and S3 are dominantly pure shear phenomena, and that the boundaries of the zone of Alleghanian deformation and metamorphism are cleavage fronts. The second interpretation is that the deformations are related to simple shear and that the boundaries of the zone are shear zone boundaries. It is not possible, with the data available, to unequivocally distinguish between the two models. However, the shear zone model is more likely because the well-defined boundaries are suggestive of a shear zone, because thin mylonites are found exclusively at the western boundary of the zone of Alleghanian cleavages, because there is an association between S: and S3 and major terrane- bounding faults and mylonites (WMZ and CNF) and because S: and S3 are spatially related, despite having formed at different times. In addition, pure shear within a well-defined zone causes strain

compatibility problems, unless considerable volumes of rock are lost. Thus for shortening to have occurred within the zone of Alleghanian cleavages by dominantly pure shear processes, considerable uplift would have had to occur within a narrow zone, an unlikely event. Thus the interpretation proposed in this paper is that S: and S3 developed as strain accommodation features in a wide zone of

ductile shear which was active during the Alleghanian orogeny. The shear zone will be referred to as the Sterling shear zone (SSZ) for the town of Sterling, Massachusetts, which is located within that zone. It represents not only a zone within which fabrics formed but also a zone of fluid

flow. Within the SSZ early metamorphic minerals (garnet, staurolite, andalusite) are retrograded to hydrous minerals (chlorite, biotite and white mica), and new hydrous minerals have formed, indicating enhanced fluid availability during M2 as compared to Mz.

If the shear zone model is correct, the geometry of the crenulation cleavages in that zone may be used to determine the kinematics of shear zone dis-

placement. Several models have been proposed for the formation of crenulation cleavages in shear zones. Platt and Vissers [1980] discussed the development of extensional crenulation cleavages (ECC) in shear zones, a phenomenon which they attribute to a lower ductility parallel to a planar fabric than across it. The lower ductility allows the formation of asymmetric shear bands, which can be used as kinematic indicators [Simpson and Schmid, 1983]. Similarly, Dennis and Secor [1987] showed that two sets of crenulations can form in shear zones

where planar anisotropic fabrics are oblique to shear zone boundaries. Their normal slip crenulations (similar to ECC) and reverse slip crenulations serve to maintain the width of the shear zone as slip on planar anisotropies occurs. The geometry of these asymmetric crenulations is controlled by the orientation of the anisotropy with respect to the

72 Goldstein: Allegh•ni•n Orogeny in Esstern M•ss•chusetts

shear zone boundary and the sense of shear across the shear zone. Berthe et al. [1979], Lister and Snoke [1985] and many others have discussed the formation of S and C fabrics in mylonites, dual sets of planar fabrics developed primarily in highly sheared isotropic rocks. S planes are defined by a shape fabric of many minerals in the rock, are penetrative and are believed to track the orientation of the finite

strain ellipsoid in the shear zone. C planes are shear bands which form parallel to the shear zone boundaries [Simpson and Schmid, 1983]. None of these models of fabric development apply to the SSZ, however, because S2 is a zonal crenulation cleavage, not a shear band cleavage.

Mosher and Berryhill [1991] and Burks [1985] discussed a model of crenulation formation in shear

zones quite different from those discussed above. In their model, based on observations in the Narragansett basin of Rhode Island, zonal crenulation cleavages form when the preshearing schistosity is at high angles to the shear zone boundaries and contains the kinematic "a" direction.

Crenulations form initially at approximately 45 ø to the shear zone boundary and rotate as shear continues. At some point, crenulations can no longer efficiently accommodate shortening and a new set of crenulations form, at the same initial angle as the first set. Both sets rotate as shearing continues until a third set forms. As many as four sets of superposed crenulations have been recognized in rocks of the Narragansett basin [Mosher and Berryhill, 1991 ] and their consistent overprinting and relative age relationships have been used as a shear sense indicator in that area.

Not all crenulations formed in noncoaxial strain

environments develop multiple superposed sets. Gray and Durney [1979] describe crenulation cleavages formed in rocks at the base of the Morcles nappe in the Swiss alps which experienced crenulation cleavage formation during a period of noncoaxial strain. They conclude that the crenulation cleavage followed the orientation of the XY section of the bulk strain ellipse, departing by only 4 ø from that orientation at very high strains. Similarly, Gray and Willman [ 1991 ] interpret complex fault motion histories for the Ballarat slate belt from the orientations of overprinted crenulation cleavages, with a single cleavage forming during a single episode of motion. New cleavages only formed when the direction and sense of fault motion

changed. Thus it appears that at least two modes of crenulation cleavage are possible in regions experiencing noncoaxial strain; (1) early formed crenulations "lock up" after some increment of strain and new cleavages form, with the sense of overprinting yielding information about shear sense, or (2) early formed crenulation cleavage accommodates noncoaxial strain throughout the deformation without the formation of new cleavages and, perhaps, tracks the orientation of the XY plane

of the finite strain ellipsoid. In such a model, the sense of obliquity between the cleavage and the shear zone boundary yields information about the sense of shear zone displacement.

If the SSZ exists and S2 and S3 are related to motion on that zone, they must have formed in a manner described by model 2 above, because superposed multiple sets of crenulation cleavages have not been observed. Thus a history of motion on the SSZ can be deduced from the orientations of

fabrics within the zone and some knowledge of the orientation of the shear zone boundary. ^ thorough description of displacement within the SSZ must include the orientation of the shear zone and both

the direction and sense of displacement. The SSZ has been mapped by defining the distribution of S2, S3 and related folds (Figure 2). The shear zone strikes N to NNE but there are scant data which bear

on its dip. The mylonites found along its western border strike N30øE, approximately parallel to the mapped trace of the SSZ western boundary, and dip 50øNW. The CNF, assumed to be the eastern border of the SSZ, also dips to the west or northwest, suggesting that the SSZ is a west or northwest dipping feature. The only independent evidence which bears on the direction of motion of the SSZ during D 2 are the lineations on the mylonites along the western boundary, which rake between 40 ø and 50 ø NE, suggesting nearly equal amounts of dip-slip and strike-slip displacement. ^ sinistral component of displacement is indicated by the angular relationships between mylonite zones and S2 at the SSZ western margin. The mylonites strike N30øE and S2 strikes N50ø-55øE, an angular relationship consistent with a sinistral component of displacement. Further, SoS• is folded tightly within the SSZ requiring that it lay within the shortening field of strain for SSZ displacement. For SoS• to lie in that orientation, shearing must have had a sinistral component. Dextral displacement would have caused SoS• to swing into parallelism with the SSZ boundary with S2 overprinting but without any folding of SoS•. Thus the most likely displacement for S2 related displacement on the SSZ is sinistral with an equal component of thrust displacement, required by the orientation of lineations within mylonites. It is not possible to accurately determine the amount of motion which occurred during this episode, but the tight folding and the near parallelism between the shear zone boundary and S2 suggests very high strains, such as would be generated by several tens of kilometers of displacement on a 15-kin-wide shear zone (shear strain =2- 4).

The next episode of motion associated with the SSZ produced mylonites within the Wachusett Mylonite Zone (WMZ). This interpretation is based on the observation that WMZ mylonites overprint S2 in rocks of the Merrimack trough. As noted above, these mylonites occur along the boundary between


rocks of the Nashoba terrane and the Merrimack trough and resulted from normal motion in a direction approximately N30ø-40øW. It is difficult to evaluate the amount of motion associated with this phase of faulting. However, deformation was sufficient to form a mylonite zone approximately 2 km thick. It is interesting to speculate on the tectonic associations of the WMZ. Mylonites of the Lake Char-Honey Hill-Willimantic fault system are very similar to those of the WMZ in several ways. Both the orientation of lineations and the sense of shear are the same. Both mylonitization events occurred during the Alleghanian orogeny. Finally, both mylonite zones are very thick and are localized at the contact between "basement N and Ncover, • although the basement and cover for each are different. The Lake Char system is localized along the contact between latest Precambrian granitic rocks and overlying metasedimentary and metavolcanic rocks. The WMZ, however, is localized between a very high grade metasedimentary/metavolcanic and metaplutonic basement (the Nashoba zone) and a lower metamorphic grade metasedimentary cover. Mylonites of the Lake Char system can be traced along the basement-cover contact from Connecticut into southern Massachusetts, where they are associated with the southern portion of the Bloody Bluff fault (Burlington mylonite zone of Castle et al. [1976]), but die out along that contact towards the northeast [Goldstein, 1989]. It is possible that this late Paleozoic normal motion climbed from one

contact to another, structurally higher contact. Motion on the CNF must have postdated motion

on the WMZ, as detailed above. However, both the CNF and the WMZ are normal faults so that the transition from one to the other did not result from a

profound change in the regional strain field. Rather, they record a change in the direction of crustal elongation. There are no fabrics present within the SSZ which can be correlated with normal

motion on the CNF or WMZ but their proximity to the zone of Alleghanian cleavage development (SSZ) suggests that the motions which occurred on these faults is related to both earlier (D2) and later (D3) motions on the SSZ. It is not known why CNF and WMZ motions were concentrated along the eastern margin of the SSZ, whereas earlier (D2) and later (D3) motions were distributed across the entire zone. Faster strain rates may be responsible, with slower rates causing distributed motion and faster rates causing more isolated displacements.

Both CNF and WMZ mylonites and phyllonites locally contain S3 so that the formation of that crenulation cleavage must have occurred after motion on both faults. Additional evidence also

suggests that D3 occurred after motion on the CNF. The orientations of SoS• and S2 within the block bounded by the Wekepeke fault and the CNF - WMZ (domain I of Figures 2, 5, and 7) are rotated

D 2 EARLY ALLEGHANIAN SINISTRAL MOTION

/

/ /

N30W EXTENSION WACHUSETT MYLOiq!TE ZONE

E-W EXTENSION

CLINTON-NEWBURY FAULT WEKEPEKE FAULT

D• DISTRIBUTED NW-DOWN MOTION

Fig. 7. Sequence of structural events in the SSZ.


approximately 45 ø counterclockwise with respect to the region to the west of the Wekepeke fault (domain II) whereas the orientations of S3 remain constant across the area (Figure 5). The westerly trend of elongation lineations within the CNF and the normal sense of motion require a sinistral component of motion on that fault. This strike-slip component could have had the effect of rotating a block in the hanging wall in a counterclockwise sense. Thus the block bounded by the Wekepeke fault and CNF- WMZ is interpreted as having rotated during motion on the CNF. The presence of a low-dipping crenulation cleavage within a shear zone suggests a normal component of motion because such motion within a west-dipping shear zone would result in a state of strain in which the plane of maximum finite shortening would dip shallowly to the west, as long as shear zone motion was not large. The orientation of S3, striking northeast and dipping northwest, suggests normal displacement in a northwest direction.

TECTONIC SIGNIFICANCE OF ALLEGHANIAN DISPLACEMENTS

The following discussion explores the possible correlations of deformational events in the study area and other areas affected by the Alleghanian orogeny. The preceding discussion can be summarized as comprising two aspects: (1) data which bear on the nature and timing of structural events in eastern Massachusetts and (2) an interpretation of those events as associated with a wide zone of ductile shear. The nature and timing of Alleghanian deformations and metamorphisms in eastern Massachusetts has been detailed above and is

well constrained. The interpretation of the shear zone origin is more speculative but is consistent with the data. The history of displacements within the SSZ can be used to speculate on the nature of the Alleghanian orogeny in southeastern New England. For completeness, both the shear zone model and a pure shear model will be discussed, although the shear zone model is more likely correct.

The results presented above show that the Alleghanian orogeny in eastern Massachusetts consisted of four separable fabric-forming events. Assuming a shear zone model, the first event would have been sinistral ductile shearing with a thrust component and would have resulted from shortening in an approximately north-south direction. The last three events are associated with

normal displacements and indicate crustal extension first in a northwest direction (WMZ), then in an east-west direction (CNF) and finally in a northwest direction once more (S3). A profound change in regional strain is recorded in the change from sinistral strike-slip ductile shearing, with a thrust component, to normal displacements. Goldstein

[1989] suggested that Alleghanian normal faulting in southeastern New England on the Lake Char and related faults was associated with a releasing bend in a late Alleghanian dextral strike-slip fault system. Although this model cannot be corroborated by this study, it is never-the-less consistent with the results of this study. Getty and Gromet [1992] suggested that Alleghanian normal displacements in southeastern New England are the reflection of gravitational collapse. At this time, it is not possible to differentiate between this model and that

proposed by Goldstein [1989]. Goldstein's [1989] model of normal motions related to dextral faulting is more in keeping with what is known about Alleghanian tectonics and the lack of evidence of late Paleozoic topography which would have resulted in gravitational collapse [Zen, 1991 ]. Both models are consistent with the high grade of metamorphism in the southern Narragansett Basin, as that feature would lie in the footwall of the normal faults and

experience uplift, much like the footwalls of the normal faults in metamorphic core complexes.

There is excellent agreement with other information about the Alleghanian orogeny in southeastern New England and the shear zone model discussed above. Mosher and Berryhill [ 1991 ] showed that following east-west shortening, the rocks of the Narragansett basin were subjected first to sinistral ductile faulting along north-northeast striking zones and then by dextral displacements along east-northeast striking zones. It is appealing to speculate that the sinistral shearing (D2) followed by dextral-related normal faulting (WMZ, CNF, D3) observed in the Worcester, Massachusetts, area is correlative with these two phases of deformation recorded slightly to the south. If this correlation is correct, it suggests that Alleghanian deformations experienced by rocks of the Narragansett basin are reflective of strain experienced by a large area of southeastern New England. The possibility of regional sinistral faulting associated with the terminal Paleozoic orogeny has not been considered by most workers. This study suggests that it should be, but that evidence for it may be recorded in fabrics not traditionally related to ductile shearing. Evidence for other Alleghanian ductile shear zones in pre-Carboniferous rocks could be the presence of crenulation cleavages and retrograde metamorphism confined to reasonably narrow zones.

The model discussed above is also in keeping with the conclusions of Sacks and Secor [1990] who postulated a kinematic model for late Paleozoic continental collision in which Pennsylvanian sinistral faulting along northeast directions was followed by WNW and then E-W convergence. The E-W convergence would have been associated with the pan- Appalachian dextral faults believed to be related to normal faults in southeastern New England (Lake Char, Honey Hill, Willimantic, CNF, WMZ) as suggested by Goldstein [1989] and discussed above.


The results of this study are also in keeping with the results of Geiser and Engelder [1983] who showed that rocks of the Appalachian foreland in New York and Pennsylvania contain evidence for two distinct pulses of Alleghanian deformation. The first pulse, the Lackawana phase, produced structures which are compatible in orientation with sinistral faulting along northeast directions in New England. The second pulse of Alleghanian deformation, the Main phase, would have produced dextral faulting along northeast strikes and would have generated normal displacements in dextral releasing bends. It is significant that the results of this study and those of Geiser and Engelder [19113] indicate two distinct pulses of Alleghanian deformation. Geiser and Engelder's [19113] Lackawana phase can be considered correlative with D•_ of this study and the Main phase can be considered correlative with motion on the WMZ and CNF and D3. The shear zone model discussed above is also consistent with

the widespread occurrence of post-Acadian-pre- Triassic sinistral and dextral strike-slip faults in Maine, the Maritime provinces of Canada and Newfoundland [Hatcher, 19118]. Also, as noted above, most workers agree that the Iapetos ocean had closed in the northern Appalachians and Caledonides by the end of the Devonian. Thus, deformation associated with strike-slip faulting and shearing is a likely mode of deformation for Permian events.

The occurrence of Alleghanian sinistral faults in southeastern New England, unrecognized in the central and southern Appalachians, may lend credence to the continental escape model of LeFort [19118] and Vauchez et al. [19117]. This model considers the Reguibat uplift of northwest Africa to have acted as a rigid indenter during Alleghanian collision of North America and Africa. The model

has been difficult to accept, because of a lack of Alleghanian sinistral faults in the northern Appalachians, although numerous dextral faults exist in the central and southern Appalachians. Hatcher et al. [1989] show that central and southern Appalachian dextral faulting would have been contemporaneous with sinistral faulting in the Narragansett basin, lending additional credence to the continental escape model.

If the shear zone model is not correct, we must postulate a different correlation between eastern Massachusetts Alleghanian events and those noted elsewhere. In this case, the earliest Alleghanian event in eastern Massachusetts, D2, would not represent sinistral shearing, but rather northwest- southeast shortening. Such an event would most likely correlate with D•and D2 in the Narragansett basin, evidenced by isoclinal folding and metamorphism. Sinistral ductile faulting in the Narragansett basin would not be represented in eastern Massachusetts and dextral faulting in the basin would be equivalent to WMZ, CNF and D3 in eastern Massachusetts.

CONCLUSIONS

1. The effects of the Alleghanian orogeny in eastern Massachusetts can be accurately defined for the area around Worcester, Massachusetts. These are (1) tight folding and formation of near vertical axial plane crenulation cleavage (S2), (2) ductile normal faulting along the boundary of the Nashoba terrane and the Merrimack trough (Wachusett Mylonite Zone), (3) normal faulting along the Clinton- Newbury fault, (4) formation of folds and axial plane crenulation cleavage (S3) with shallow northwestward dips, and (5) formation of locally developed vertical crenulation cleavage with a WNW strike. This study shows that a larger area of southeastern New England experienced complex Alleghanian deformation and metamorphisms than previously thought.

2. S2 and S3 are developed within a zone which has a well-defined western boundary, interpreted to be a shear zone (the SSZ). The occurrence of S2 and S3 with retrogression of early-formed high-grade metamorphic minerals suggests that the SSZ was also a zone along which fluids were migrating. The shear zone interpretation is supported by the association with major faults on the east side of the area and by the presence of thin mylonite zones exclusively at the western boundary of the zone of multiple cleavage development.

3. Faults described previously as thrusts (Clinton- Newbury fault and related structures, Skehan [1968]) have been reexamined in this study and have been found to have normal displacements. The older fault, the Wachusett Mylonite Zone, is considered to be correlative with the Lake Char fault, not in its structural position but in the geometry and kinematics of its displacement. The Clinton- Newbury fault is slightly younger than the W•MZ, has a distinctly different direction of displacement and has a steeper dip than the WMZ.

4. Comparison with the work of others, notably Mosher [1983], Sacks and Secor [1990] and Geiser and Engelder [1983] suggests a coherent picture of the Alleghanian orogeny in southeastern New England. Early Alleghanian structures were driven by a generally north-directed shortening and the formation of sinistral faults and shear zones. Later

pulses of the Alleghanian orogeny can mostly be related to dextral faulting which is, later than most of the west-directed thrusting and folding in the southern and central Appalachians [Sacks and Secor, 1990]. Early isoclinal folding in the Narragansett Basin has not been unequivocally correlated with events which affected eastern Massachusetts.

Acknowledgements. This work was supported by grants from the National Science Foundation (EAR 87-08244) and the Colgate University Research Council. Field assistance by Chris Vyhnal, John Hopper and John Ix is gratefully acknowledged as


are many discussions with Chris Hepburn, Dave Ashenden, Rachel Burks, Mike Wells, and Mike Williams. Reviews by Sharon Mosher and Jim Skehan improved the paper immensely.

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(Received August 19, 1991; revised August 23, 1993; accepted September 9, 1993.)

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