Progressive localization of deformation during exhumation of a major strike-slip shear zone: Norumbega fault zone, south-central Maine, USA

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Tectonophysics 273 (1997) 185-201

Progressive localization of deformation during exhumation of a major strike-slip shear zone: Norumbega fault zone, south-central Maine, USA David E West, Jr. a,*, Mary S. Hubbard b a Department of Geology, Earlham College, Richmond, IN 47374, USA b Department of Geology, Kansas State University, Manhattan, KS 66506, USA Received 29 February 1996; accepted 12 December 1996

Abstract

Detailed structural studies along with 4°Ar/39Ar thermochronology provide important constraints on the temporal, spatial and structural evolution of a major orogen-parallel strike-slip shear zone, the Norumbega fault zone in Maine. Detailed structural analysis in south-central Maine reveals two very different styles of dextral noncoaxial deformation, a wide zone (>25 km) of heterogeneously distributed ductile shear structures, and a relatively narrow zone (~1 kin) of lower greenschist facies high-strain mylonitization. 4°Ar/a9Ar thermochronology indicates that these two zones of deformation developed during a prolonged period of regional exhumation following Middle Devonian amphibolite facies metamorphism. The wide zone of dextral shear is interpreted to reflect a major episode of moderate temperature (post-amphibolite facies metamorphism but pre-regional cooling below 320°C) Late Devonian to Early Carboniferous transcurrent tectonism. The narrow zone of dextral shear represents both a younger (latest Carboniferous) and significantly cooler (<320°C) deformational event occurring at much higher structural levels. A general model of increasingly narrow, but more highly focused noncoaxial deformation during progressive regional exhumation is supported by the observations.

Keywords: structural geology; strike-slip faults; northern Appalachians; 4°Ar/39Ar ages

1. Introduction

Over the course of the last two decades considerable advances have been made towards recognizing and understanding the tectonic significance of largescale transcurrent shear zones. While detailed kinematic analysis has provided a wealth of information on the structural evolution and geometries of these shear zones, our understanding of the temporal evo*Corresponding author. Tel.: +1 (317) 983-1231; Fax: +1 (317) 983-1497; E-mail: [email protected]

lution of such structures remains rather incomplete. Although field studies coupled with thermobarometry (e.g., Hanmer, 1988) have suggested that during exhumation progressively younger mylonite belts become narrower, the details of the absolute timing of such processes are lacking. In particular, our understanding of the longevity of major shear zones, as well as the structural and spatial evolution of these zones with time is clearly inadequate (Means, 1995). In this paper we characterize a well exposed portion of a major, long-lived orogen-parallel shear zone in the northern Appalachian orogen, the Norumbega fault

0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S 0 0 4 0 - 1 9 5 1 ( 9 6 ) 0 0 3 0 6 - X

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Fig. 1. Generalized geologic map of coastal northern New England and adjacent New Brunswick, modified from Osberg et al. (1985) and McLeod et al. (1994). CB = Casco Bay Group and correlatives. K G = Mt. Katahdin Granite, M = Massebesic Gneiss Complex, M R = Miramichi Anticlinorium, SB = Sebago Batholith. N.B, = New Brunswick, N H . = New Hampshire.

zone in Maine (Fig. 1). In particular, with the aid of 4°Ar/39Ar thermochronology, the emphasis is placed on how the width, structural style, and conditions of displacement varied with time along this structure. Numerous high-angle faults and shear zones with a history of strike-slip motion have been identified along the length of the Appalachians (e.g., Gates et al., 1986), although the tectonic significance of such structures remains controversial. These structures are often cited as major crustal sutures or terrane boundaries, although in many cases detailed studies are lacking. One such structure is the Norumbega fault zone. Despite a general lack of detailed study prior to 1990, Keppie (1985) shows the Norumbega fault zone to be a major left-lateral boundary between exotic terranes. In contrast to this, Berry and Osberg (1989), in a tectonic synthesis of eastern Maine barely mention the Norumbega fault zone, apparently considering it to be a relatively late, tectonically insignificant structure. Thus an additional goal of this work will be to evaluate the tectonic significance of

the Norumbega fault zone in northern Appalachian orogenesis.

2. Geologic setting The study area is located in the northern Appalachian orogenic belt along the southeastern margin of the central Maine terrane (Zen, 1989) within the Merrimack-Harpswell terrane of Maine (Rankin, 1994). The geologic history of this region is complex, with the rocks multiply deformed, metamorphosed and intruded during several orogenic events related to Appalachian mountain-building processes. The general geologic relationships of the study area are shown in Fig. 2. The stratified rocks in this region can be divided into several lithostratigraphic belts based on their internal stratigraphy (Berry and Osberg, 1989). Boundaries between these belts in most cases correspond to relatively high-angle faults and shear zones. It should be noted, however, that multiple episodes of severe deformation have made

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Undivided Central Maine Seq. ~

St. Croix Belt

Bucksport/Appleton Ridge Fins.

~

Cape Elizabeth Fro. & Upper Casco Bay

~

F-B Sequence and Cushing Fro.

N

Undivided Plutonic Rocks i mm mm n

0

187

T

mm mm

Kilometers

10

Fig. 2. Generalizedgeologicmap of the study area, modifiedfrom Pankiwskyj(1976) and Osberg et al. (1985). TM = Three Mile Pond pluton, SG = Lake St. GeorgeGranitic Gneiss, LS = Lincoln Sill, W = WaldoboroPlutonic Complex,HPf = HackmatackPond fault, SPf = SennebecPond thrust fault (southeast dipping).

the original stratigraphic positions of the lithologic units very difficult to determine. Compositional layering, although present in nearly all outcrops, most likely reflects transposition of original bedding in response to deformation and recrystallization. The lithotectonic belts identified in the study area are listed and briefly described below in order of their exposure from northwest to southeast. 2.1. Lithotectonic belts

The central Maine terrane (Fig. 2), exposed to the northwest of the Hackmatack Pond fault (Panki-

wskyj, 1996) contains rocks of the central Maine sequence. This sequence consists of a very thick Upper Ordovician (?)-Early Devonian assemblage of metamorphosed wacke, shale, and minor limestone (Osberg, 1988). The Cushing Formation and Falmouth-Brunswick sequence, exposed immediately southeast of the central Maine sequence, are lithologically variable with the dominant lithology being layered quartzofeldspathic gneisses. These rocks most likely represent an Ordovician metavolcanic and volcanogenic metasedimentary section (Osberg et al., 1995). The Cape Elizabeth Formation and associated upper portions of the Casco Bay Group con-

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sist primarily of an Ordovician-aged metasedimentary assemblage with minor metavolcanics (Pankiwskyj, 1976; Hussey, 1985; Osberg et al., 1995). The Appleton Ridge and Bucksport Formations of Late Ordovician to Early Silurian age are metamorphosed pelitic and calc-silicate rocks, respectively. The Sennebec Pond fault in the extreme southeastern portion of the study area appears to mark a major tectonostratigraphic boundary in the region. Berry and Osberg (1989) have noted that fossils in rocks east of this structure preserve a non-North American fauna, whereas those to the west have North American fossil assemblages. East of the Sennebec Pond fault, rocks of the St. Croix Belt (Ludman, 1987) consist largely of Late Cambrian to Early Ordovician clastic metasedimentary rocks. 2.2. Metamorphism The study area is located along the northeastern termination of high-grade metamorphism in New England (Thompson and Norton, 1968). In Maine, this termination consists of four northeast-trending lobes (Guidotti, 1989), and the study area is located within the easternmost high-grade metamorphic lobe. All the stratified rocks shown in Fig. 2 have been metamorphosed to at least middle amphibolite facies conditions, and in many places rocks have mineral assemblages characteristic of sillimanite ÷ K-feldspar-grade metamorphism. Rocks of the central Maine sequence preserve evidence for at least three low-pressure Devonian metamorphic events (Novak and Holdaway, 1981; Osberg, 1988). In the adjacent rocks of the Cushing Formation to the east, evidence of an even earlier Barrovian metamorphic event also is found (Pankiwskyj, 1976). East of the Sennebec Pond fault there is evidence for at least two amphibolite facies low-pressure metamorphic events and later, perhaps regionally extensive retrograde events (Berry, 1987; Guidotti, 1989). West et al. (1995) have demonstrated that the amphibolite facies metamorphic events east of the Sennebec Pond fault are Silurian in age. 2.3. Intrusive rocks Texturally and compositionally diverse intrusive rocks are found in the study area. Although not

subdivided in Fig. 2, these intrusive rocks can be divided into three groups based on compositions and apparent relationships to deformational events: (1) strongly foliated gneissic granitoids; (2) porphyritic mafic syenite of the Lincoln Sill; and (3) weakly to nonfoliated granitoids. In addition to these larger intrusive bodies, pegmatites (generally not mappable at the scale of 1 : 24,000) are ubiquitous in the region and are typically peraluminous, containing coarsegrained muscovite, tourmaline and garnet as accessory minerals. The majority of these pegmatites have been affected by dextral shear deformation and are most likely Devonian in age.

3. Structural geology The main goal of this study was to locate and describe structures in south-central Maine related to noncoaxial deformation. This endeavor was made difficult by the fact that nearly all the rocks in this region preserve evidence of several different episodes of deformation. Although the noncoaxial structures represent the latest deformational phases in the region, the orientations of pre-existing structures are very important in the development of structures related to superimposed deformational events. Because of this structural complexity, all structural fabrics observed at each outcrop were described and their orientations noted where possible. Temporal relationships between the different structures were established using basic cross-cutting relationships. In addition, the relationships between these structures and the metamorphism was established using microstructural analysis. Table 1 shows the relative timing of the major tectonic features in the area deduced from field and petrofabric relationships. ]"able I Relative timing of tectonic features Development of compositional layering Episode of intrusion Development of isoclinal folds and axial planar foliation Latest amphibolite facies metamorphism (coarsening of foliation) Episode of intrusion Development of asymmetric Z-folds, shear bands, and asymmetric boudinage (wide zone of dextral deformation) High-strain mylonitization (Sandhill Corner fault) Brittle faulting and jointing, pseudotachylyte generation

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189

Fig. 3. Fabric elementsfrom south-centralMaine. Lowerhemisphereequal-areaprojections.(a) Poles to compositionallayering/foliation outside the high-strain zone. (b) Mineral lineations outside the high-strain zone. (c) Poles to myloniticfoliationin the high-strain zone. (d) Mineral lineationsin the high-strain zone.

3.1. General f e a t u r e s

Nearly all outcrops of stratified rock in the study area are characterized by regular and systematic compositional layering. In these outcrops, the orientation of compositional layering and the dominant foliation (generally defined by coarse-grained micaceous minerals) are very consistent and essentially parallel. In light of this observation and knowledge that the region has been polydeformed and extensively recrystallized, this layering is best referred to simply as compositional layering rather than bedding. The orientation of the compositional layering and foliation is remarkably consistent over the region, striking approximately N30-50°E and dipping

steeply, generally greater than 60° (Fig. 3a). Several different types of lineations are observed in the field area, including intersection lineations and minor fold axes. Of most interest, however, are well developed quartz rods, common in the Cape Elizabeth Formation, and the parallel alignment of elongate mineral grains such as hornblende and sillimanite. The mineral lineations trend northeast-southwest with plunges generally less than 25 ° (Fig. 3b). These lineations are interpreted to represent the general direction of transport during the noncoaxial deformational events discussed below. Generally upright northeast-trending isoclinal folds are ubiquitous in the region. Foliation and compositional layering are essentially parallel to the axial

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surfaces of these folds. While these folds have been traditionally ascribed to largely pure shear compressional events associated with the Devonian Acadian Orogeny (e.g., Osberg, 1988), more recent interpretations suggest they may related to the dextral noncoaxial deformational events described below (e.g., Hubbard, 1992: Swanson, 1992). In addition to these isoclinal folds, asymmetric folds with northto slightly northeast-trending axial surfaces are also found throughout the study area. These structures fold compositional layering, foliation, and the axial surfaces of the earlier isoclinal folds, and are interpreted to be related to the dextral shear deformation discussed in detail below. The noncoaxial deformational features in this region are here divided into two different types based upon the style of deformation and the spatial distribution of these structures: (1) a wide zone (>25 km) of heterogeneously distributed structures which affects nearly the entire area shown in Fig. 2; and (2) a relatively narrow zone ( ~ 1 km) of high-strain mylonitization (labeled as such in Fig. 2). These are here referred to as the wide strain zone and the narrow high-strain zone, respectively. The narrow high-strain zone was originally mapped by Pankiwskyj (1976) as the Sandhill Comer fault and it is what has been traditionally referred to as the Norumbega fault zone in this region (e.g., Stewart and Wones, t974; Osberg et al., 1985). More recent work (e.g., Hubbard et al., 1995), however, suggests that both the wide and narrow strain zones should be included in discussions of the Norumbega fault zone. In this paper we suggest that all the northeast-trending dextral noncoaxial features found in this region are related to one evolving structure.

3.2. Wide zone of dextral shear deformation Structures consistent with ductile dextral shear deformation are heterogeneously distributed over a region at least 25 km wide (perpendicular to strike). These structural features at the outcrop scale include asymmetric boudinage, shear bands and asymmetric folds. The southeastern extent of these shear structures roughly coincides with the location of the Sennebec Pond fault (Fig. 2) in this region. The northwestern extent of this deformation is unknown, although it clearly extends west of the Hackmatack

Pond boundary into the central Maine sequence. It should be noted that Osberg (1968, 1988) describes structures consistent with dextral shear deformation well west of the present study area. The structures associated with the wide zone of dextral shear deformation are well developed at the outcrop scale. North- to slightly northeast-trending asymmetric folds all have the same clockwise sense of rotation (Fig. 4a). This sense of asymmetry is consistent with dextral noncoaxial deformation. Although the axes of these folds are variable in orientation, most plunge rather steeply to the north or northeast. Asymmetric boudinage is also commonly associated with the wide zone of shear deformation (Fig. 4b). This structure typically occurs where large theology contrasts exist within a given outcrop, such as amphibolite layers or pegmatites in a dominantly schistose rock. This boudinage occurs at all scales, from thin section to possibly map-scale boudinage of entire formations. The sense of rotation and displacement associated with this boudinage is again, in most cases, consistent with dextral shear. Swanson (1992), along-strike to the southeast of the present study area, has provided an in-depth study of the asymmetric boudinage development associated with the Norumbega fault zone. Probably the most common features suggestive of noncoaxial deformation in this region are shear bands (Fig. 4c), typically found in the pelitic lithologies. These shear bands are evenly spaced (1- to 5-cm intervals) across an outcrop and deflect foliation at a fairly low angle (30 ° or less). The sense of drag associated with the shear bands is also consistent with a dextral sense of shear. The wide zone of dextral shear deformation is characterized by considerable variations in strain across strike. Many outcrops show considerable evidence of intense, relatively high-temperature strain (evidence of both quartz and feldspar recrystallization), while some outcrops (a minority) show little evidence for significant noncoaxial deformation.

3.3. Narrow high-strain zone All of the deformational, metamorphic and intrusive features described above are cut by a northeaststriking zone of high-strain mylonitization (shown as the 'high-strain zone' in Fig. 2). This zone was originally mapped by Pankiwskyj (1976) as the Sandhill

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191

Fig. 4. Structural features associated with the wide zone of dextral shear deformation. (a) Asymmetric Z-folds in the Casco Bay Group (compass points north). (b) Asymmetric boudinage in the Casco Bay Group (compass points north).

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Fig. 4. (continued). (c) Oriented hand sample (northeast to the right) illustrating the post-metamorphic aspect of the wide zone of dextral shear deformation. Note the sheared and boudinaged staurolite porphyroblasts. Width of photo is approximately 7 cm.

Comer fault, and has been correlated with the regionally extensive Norumbega fault zone (Osberg et al., 1985). Detailed mapping has shown that the Sandhill Comer fault is a ductile shear zone approximately 1 km wide. This shear zone contains several high-strain zones of intense mylonitization. These high-strain zones are up to 100 m wide and are characterized by near-vertical mylonites and ultramylonites striking N35-45°E (Fig. 3c). Variations in strain also exist within these high-strain zones, even on the scale of a given thin section. The narrow high-strain zone in this region is intraformational, with rocks of the Cape Elizabeth Formation present on both sides (Fig. 2). Mineral lineations of a different type than are found in the high-strain zone. Here, large muscovite porphyroclasts (up to 2 cm) are smeared out along mylonitic foliation planes creating a well defined mineral lineation. These mineral lineations plunge shallowly to the northeast and southwest (Fig. 3d). Meso-scale kinematic indicators in the form of shear bands, asymmetric boudinage and asymmetric folds

are present in areas of lower strain within the highstrain zone. These all indicate a dextral sense of shear. Many of the mylonitic rocks associated with the high-strain zone, however, are very fine-grained and require the use of microstructural kinematic analysis to determine the movement sense (e.g., Fig. 5a). Observations of thin sections cut perpendicular to the mylonitic foliation and parallel to the mineral elongation direction reveal shear bands, muscovite fish, rotated porphyroclasts, broken porphyroclasts, asymmetric microfolds, and both grain shape and lattice preferred orientation in quartz (Fig. 5b). Feldspar porphyroclasts in these high-strain mylonites are cracked and broken suggesting relatively low temperatures of deformation (<450°C, FitzGerald and Stiinitz, 1993). Ductile deformation in quartz-rich layers, however, indicate temperatures above 300 + 50°C (Tullis and Yund, 1977). The extremely fine-grained and incompletely recrystallized matrix material surrounding the porphyroclasts is consistent with lower greenschist facies conditions during this deformational episode.

D.P. West, Jr., M.S. HubbardlTectonophysics 273 (1997) 185-201

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Fig. 5. Structural features associated with the narrow zone of high-strain dextral shear deformation. (a) Outcrop photograph of delta feldspar porphyroclast consistent with a dextral sense of shear (horizontal outcrop, northeast to the left). (b) Oriented hand sample from the high-strain zone (northeast to the right). Light layers represent deformed pegmatites.

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Fig. 5 (continued). (c) Oriented photomicrograph showing muscovite and plagioclase porphyroclasts ('fish') set in an extremely fine-grained recrystallized matrix of phyllosilicates and quartz (northeast to the right). Width of photo equals 2.5 cm.

4. Timing of dextral shear deformation 4.1. Narrow high-strain zone

The large muscovite porphyroclasts found in the high-strain mylonite zone described above provide an excellent opportunity to test the effects of mylonitization on argon isotopic systems in muscovite. West and Lux (1993) have presented the results of detailed 4°Ar]a9Arstudies on muscovite porphyroclasts from these mylonites. Only a brief summary of this work will be presented here. The spatial distribution of 4°Ar/a9Ar muscovite plateau ages from outside the high-strain zone in this region is very systematic, decreasing from approximately 360 Ma in the northeast to 320 Ma in the southwest (Fig. 6a). These ages date the time of cooling through muscovite closure temperatures (~320°C, Snee et al., 1988) following Devonian high-grade metamorphism. The systematic age distribution over a large area, combined with the relatively undisturbed age spectra demonstrate that the

region has been cooler than --~320°C since the end of Mississippian time. The systematic southwesterly decrease in muscovite ages shown in Fig. 6a is very similar to the regional pattern of mica ages observed by other workers (Dallmeyer, 1989; DeYoreo et al., 1989; Lux and West, 1993) in the high-grade metamorphic terrane of west-central Maine. DeYoreo et al. (1989) attributed this pattern to differential unroofing, where areas to the southwest were more deeply buried following Devonian tectonism and thus took longer to uplift through isotherms corresponding to mineral closure temperatures. A similar interpretation of slow, differential unroofing during the Late Devonian-Early Carboniferous is favored for this region following Early to Middle Devonian high-grade metamorphism. In strong contrast to the release spectra from the unmylonitized samples, all the age spectra for muscovite from the high-strain mylonites are strongly discordant (e.g., Fig. 6b). They are characterized by young ages at low extraction temperatures, which systematically increase to ages that equal the plateau

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(a) 0 360

Kilometers

10

(b)

320300~" 280 -

r~ < 260

l~90-1051am(tg =32.8Ma)

[ ~ 60-90~ra(tg- 323Ma) , , , 100 0

%39Ar

100

Fig. 6. (a) Distribution of 4°Ar/39Ar mineral ages from south-central Maine. Uncertainties in ages are about +1%. h = hornblende age, m = muscovite age. Map symbols are the same as in Fig. 2. (b) Comparison of muscovite age spectra from the three finest grain size fractions from mylonitic sample Raz-43. The lowest extraction temperature increment has been omitted for clarity. (c) Line tracing of (a) showing the age convergence at lowest and highest extraction temperatures. The maximum age of 345 Ma equals the plateau age of the nearby nonmylonitized sample, and the minimum age of approximately 290 Ma is interpreted to be the best estimate of the timing of mylonitization in the narrow high-strain zone.

ages for muscovite collected outside the high-strain zones. Detailed petrographic observations suggest that these systematic discordances reflect a mixing of argon components from older, relict, muscovite porphyroclasts and fine-grained white mica aggregates that recrystallized during mylonitic deformation. 4°mr/39Ar total gas ages of five different grain size fractions separated from the same mylonite sample (3 are shown in Fig. 6b) become progressively younger with decreasing grain size, indicating a larger component of the recrystallized grains in the finer grain size fractions. Although the three finest grain size fractions give different total

gas ages and do not overlap in age for most of their release spectra, their initial increments do coincide at approximately 290 Ma. This observation indicates a minimal older age contribution from the relict porphyroclasts in the initial increments and suggests that the 290 Ma age provides a good estimate for the time of mineral growth associated with mylonitic deformation in the high-strain zone. The distribution of 4°Ar/39Ar muscovite ages from outside the high-strain zone shown in Fig. 6a indicates that ambient temperatures in this region have been below approximately 320°C since about 320 million years ago. Because the zone of high-strain

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mylonitization developed some 30 million years subsequent to this regional cooling below muscovite blocking temperatures, ambient temperatures must have been below 320°C during this high-strain mylonitization. This thermal constraint is consistent with the lower greenschist facies estimate based on textural evidence. Finally, a general lack of significant offset in regional Early Carboniferous mineral age patterns across the high-strain zone (Fig. 6a) suggests that displacement along this zone was probably less than 30 km (West and Lux, 1993). 4.2. Wide zone

All the structures associated with the wide zone of dextral shear deformation (asymmetric folding, boudinage and shear banding) overprint an earlier episode of isoclinal folding and strong foliation development which has been dated at Early Devonian in the central Maine sequence (Osberg et al., 1995). In south-central Maine the only significant geologic feature younger than the wide zone of dextral shear deformation appears to be the high-strain mylonites, which have been dated at approximately 290 Ma. The wide zone of dextral shear deformation is thus constrained to be post-Early Devonian and pre-latest Carboniferous, a time span of nearly 100 Ma. Unfortunately, direct dating of these shear features is very difficult because little neomineralization appears to be associated with this deformation. The age of these features must therefore be estimated using indirect methods, such as relationships to metamorphic events of known age. In south-central Maine, the timing of the last high-grade metamorphic event is well constrained by 4°mr/39Arhornblende ages to be Middle to Late Devonian (~380-370 Ma; West et al., 1988, 1993, 1995). Throughout the study area, elongated metamorphic minerals such as hornblende and coarsegrained sillimanite have a strong preferred orientation (see Fig. 3b), typically well aligned within foliation planes. This strongly suggests that the noncoaxial deformation was syn- to post-metamorphic. In addition, porphyroblasts such as garnet, andalusite and staurolite which grew during the last amphibolite facies metamorphism are commonly deformed by structures related to the shear deformation (e.g., Fig. 4c). These relationships suggest that the major-

ity of the shear deformation in south-central Maine was post-metamorphic or post-Middle Devonian. It should be noted that in some portions of the study area, porphyroblasts are essentially undeformed and appear to have grown under relatively static conditions. These are areas of low strain, however, and further reflect the inhomogeneous distribution of these shear features in south-central Maine. It should also be noted that curved inclusion trails in porphyroblasts consistent with dextral shear have been observed locally (e.g., Hubbard et al., 1991, and in this field area). These features should, however, be interpreted with caution (Bell et al., 1992; Passchier et al., 1992), and a more rigorous study of these textures is required before their significance and relationship to metamorphic events of known age are completely understood. In summary, 4°Ar/39Ar ages for muscovite from the study area range from about 360 to 320 Ma (Fig. 6a) and indicate the time of post-metamorphic cooling below about 320°C. Although the relationship between the shear deformation and cooling through mica closure temperatures cannot be ascertained directly, some information can be gained on the timing of this deformation. Nearly all of the structures associated with the wide zone of noncoaxial deformation formed in a ductile environment. Unfortunately, it is difficult to quantify the temperatures at which these structures formed. Variations in strain rate, lithologic differences, and fluid interaction all play an important, but at present, unquantifiable role in the development of ductile fabric. Despite these uncertainties, it is believed that most of the shear structures that characterize the wide strain zone developed at temperatures above 320°C or above muscovite closure temperatures. If this is the case, much of the deformation associated with the wide zone of dextral shear occurred prior to 320 Ma, or prior to regional cooling below muscovite closure temperatures. 5. Discussion

The 4°Ar/39Ar studies discussed above strongly suggest that the age of the narrow zone of high-strain mylonitization is approximately 290 Ma. The timing of noncoaxial deformation associated with the wide strain zone is more difficult to determine. Based on

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metamorphic textures, knowledge of the age of this metamorphism, as well as the time of cooling below muscovite closure temperatures in this region, it is believed that much of this deformation occurred in Middle Devonian-Early Carboniferous time. Here we consider two possibilities that we feel best explain the differences in timing and structural style between the two zones of dextral shear deformation. (1) The two zones represent two fundamentally different and unrelated structures that formed at two different times, one simply overprinting the other. (2) The two zones are related, and they reflect an extended period of right-lateral shear strain operating during a prolonged episode of regional uplift and exhumation. We believe the latter of these two possibilities best explains our observations and provide the following lines of evidence to support this interpretation. First, there is a conspicuous lack of evidence for any other style of deformation occurring during the long time interval between the two episodes of dextral shear deformation. In other words, the only type of deformation recorded in these rocks from Middle Devonian to nearly Early Permian time is that of dextral shear deformation. While other studies in the Appalachians have made the case for overprinting, unrelated episodes of dextral shear (e.g., Secor et al., 1986), they invariably involve intervening deformational events. In this region there is no evidence of intervening deformational episodes. Secondly, the remarkable parallelism of structures associated with the wide and high-strain zones suggest no appreciable change in the orientation of shear stresses in this region during the time interval between these two deformational events. Finally, despite this general parallelism of structures, the style of deformation, as well as the spatial distribution of structures observed in the two zones of dextral shear, suggest different thermal conditions during deformation, although differences in strain rate and fluid content could explain these differences as well. 5.1. Shear zone evolution through time

The very systematic distribution of Early Carboniferous 4°Ar/a9Ar muscovite ages in the study area (Fig. 6a) indicate that this was a time of slow but steady regional exhumation following Middle

197

Devonian amphibolite facies metamorphism. The wide zone of ductile dextral shear deformation reflects relatively deep, high-temperature shearing that likely initiated either during or immediately after peak-metamorphic conditions. This style of deformation likely continued through the Late Devonian and Early Carboniferous, a time of regional cooling between hornblende and muscovite blocking temperatures (between -~500 ° and 3200C). Although structures associated with this deformational event are distributed over an across-strike width of greater than 25 km, it seems unlikely that this whole zone was being actively sheared at any one time. Studies in other deeply exhumed large-scale ductile shear zones (e.g., Sibson, 1986; Mandl, 1987; Hanmer, 1988) suggest that laterally migrating zones of higher strain are more likely, with the width of these zones varying as metamorphic conditions and strain rates change. Thus the strain inhomogeneities observed across this wide zone of dextral shear deformation likely reflect a prolonged period of lateral shear zone migration within this area. Field relationships, as well as the 4°mr/39Arthermochronology, clearly indicate that the narrow zone of high-strain mylonitization post-dates the structures found in the wide zone of dextral deformation. This narrow zone of lower greenschist facies shearing represents a later episode of dextral shear deformation superimposed on the earlier zone. Fabrics within these mylonites as well as the 4°mr/39Arthermochronology indicate that this narrow zone likely developed at significantly cooler temperatures associated with higher structural levels. Because the structures associated with the wide and narrow zones formed during the progressive unroofing of the region, they provide excellent records of how the width of the shear zone as well as the style of deformation changed dramatically with both time and temperature. Fig. 7 is an attempt at a four-dimensional diagram showing how dextral shear deformation varied with time, temperature, depth, and present surface expression in south-central Maine. The diagram is essentially a modified version of Sibson's (Sibson, 1977, 1986) classic model of a downwardly widening crustal profile through a crustal-scale ductile shear zone. With time the present erosional surface (the top of the diagram) was exhumed through progressively shallower

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Narrow high-strain zonc

.275 ,300 T i

T

.325 m

m

.350

P e

r

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(Ma)

.375

t

u r e

oc

Fig. 7. A four-dimensional diagram showing how dextral shear deformation varied with time, temperature, depth, and present surface expression in south-central Maine. The diagram is essentially a modified version of Sibson's (Sibson, 1977, 1986) classic model of a downwardly widening crustal profile through a crustal-scale ductile shear zone. Northeast is to the upper right of the diagram.

crustal levels and cooler temperatures. During this time span dextral shear strain was becoming progressively more focused into a narrower corridor. The structures exposed at the present erosional surface thus represent an accumulation of nearly 100 million years of dextral shear strain in south-central Maine.

5.2. Tectonic significance of the Norumbega fault zone

The wide zone of dextral shear structures described here in south-central Maine is interpreted to reflect a major episode of Middle Devonian to Early Carboniferous moderate-temperature regional dextral shear deformation. The eastern limit of this shear deformation in south-central Maine appears to approximately coincide with the Sennebec Pond fault (Fig. 2), as rocks within the St. Croix Belt show little evidence of this penetrative style of deformation. It should be noted, however, that nonpenetrative structures consistent with a dextral sense of shear have

been described well east of the study area along the coast of Maine (e.g., Engelder, 1989). The western limit of this dextral shear deformation is not known, but it apparently extends well out into the central Maine sequence (see F3 deformation of Osberg, 1988; Solar, 1996). The wide zone of noncoaxial deformation described here is also found along much of the previously mapped length of the Norumbega fault zone, both north and south of the present study area (Swanson, 1992; Ludman, 1995; Hubbard et al., 1995). How this episode of regional shear deformation fits in with the overall tectonic evolution of the northern Appalachians is an important question. The timing of this deformation (Middle Devonian to Early Carboniferous) in south-central Maine is consistent with the timing of the later stages of the Devonian Acadian Orogeny (Osberg et al., 1989). In southern Maine, however, Swanson (1992) provided no constraints on the timing of dextral transpression, but suggested that it was 'Late Acadian-Alleghanian'.

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Malo and Bourque (1993) describe an episode of significant dextral shear deformation in the Gasp6 Peninsula of Quebec which is of Middle Devonian age. Additionally, de Roo and van Staal (1994) describe dextral shear deformation of Devonian age in central New Brunswick. Although these widely separated areas of dextral shear deformation appear to be of roughly the same age, their relationships to each other are uncertain. Irregardless, ductile dextral shear deformation of Middle Devonian to Early Carboniferous age seems to be present in many parts of the northern Appalachians and likely reflects a major episode of dextral transpression at this time. The high-strain zone in south-central Maine represents an episode of latest Carboniferous dextral strike-slip faulting. Based on the lack of offset in regional muscovite-age patterns across this high-strain zone, displacements were probably less than 30 km (West and Lux, 1993). Although the timing and amount of displacement associated with this highstrain zone is consistent with estimates provided in other areas (e.g., Wones and Thompson, 1979), its relationship to other high-strain zones along the Norumbega fault zone is uncertain. The timing of this high-strain mylonitization in south-central Maine approximately coincides with timing of terrane accretion and deformation in parts of southeastern New England (see Hatcher, 1989). In addition, the Cobequid-Chedabucto fault in Nova Scotia (the Meguma and Avalon terrane suture) was an active dextral strike-slip fault at this time (Mawer and White, 1987). Thus, the high-strain zone in south-central Maine may reflect strains associated with terrane accretions in other parts of the northern Appalachians.

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to NE-SW penetrative sub-horizontal mineral lineations in the region. Metamorphic textures indicate that this deformation was largely post-metamorphic (post-Middle Devonian) but ductile fabrics suggest that this deformation occurred prior to regional cooling below muscovite blocking temperatures (Middle Carboniferous). The narrow zone of high-strain mylonitization, located within the wide zone, is what has been traditionally referred to as the Nommbega fault zone in this region (e.g., Osberg et al., 1985). Kinematic analysis in this zone indicates that movement was also dextral strike-slip, essentially parallel to the direction of movement found in the wide zone. Systematically discordant 4°Ar/39Ar release spectra from muscovite from this zone indicate that this high-strain mylonitization occurred in latest Carboniferous time. The distribution of 4°mr/39Armineral ages in this region indicates a prolonged period of relatively slow exhumation following Middle Devonian amphibolite facies metamorphism. Dextral shearing associated with the wide zone of deformation is believed to have commenced during the early stages of regional cooling immediately following peak-metamorphic conditions. The age of the narrow zone of high-strain mylonitization is nearly 100 million years younger than the age of this metamorphism. Both mineral textures and the 4°mr/39Arthermochronology suggest that this high-strain mylonitization occurred at lower greenschist facies conditions (<320°C). A general model of increasingly narrow, but more highly focused noncoaxial deformation during progressive regional exhumation is supported by the observations.

Acknowledgements 6. Conclusions Detailed structural analysis along a portion of the Norumbega fault zone in south-central Maine reveals two very different styles of dextral noncoaxial deformation, a wide zone (>25 km) of heterogeneously distributed shear structures, and a relatively narrow zone ('-~1 km) of intense high-strain mylonitization. The wide zone is characterized by asymmetric folds, asymmetric boudinage, and shear bands that are all consistent with dextral noncoaxial deformation. These structures are interpreted to reflect a major episode of dextral shear approximately parallel

This research was funded in part by National Science Foundation Grant EAR-9218833. We wish to thank Spike Berry, Chuck Guidotti, Art Hussey, Allan Ludman, Dan Lux, Phil Osberg, Dave Stewart, Gary Solar, and Mark Swanson for many stimulating discussions on the significance of the Norumbega fault zone in Maine. We would also like to thank Jim Hibbard, Terry Engelder, and an anonymous reviewer for their critical reviews that helped to clarify many aspects of this paper.

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