Exhumation of a collisional orogen: A perspective from the North American Grenville Province
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1 Geological Society of America Memoir Exhumation of a collisional orogen: A perspective from the North American Grenville Province Margaret M. Streepey* Department of Geological Sciences, Florida State University, Tallahassee, Florida , USA Carolina Lithgow-Bertelloni Ben A. van der Pluijm Eric J. Essene Department of Geological Science, University of Michigan, Ann Arbor, Michigan , USA Jerry F. Magloughlin Department of Earth Resources, Colorado State University, Fort Collins, Colorado , USA ABSTRACT Combined structural and geochronologic research in the southernmost portion of the contiguous Grenville Province of North America (Ontario and New York State) show protracted periods of extension after the last episode of contraction. The Grenville Province in this area is characterized by synorogenic extension at ca Ma, supported by U-Pb data on titanites and 40 Ar- 39 Ar data on hornblendes, followed by regional extension occurring along crustal-scale shear zones between 945 and 780 Ma, as recorded by 40 Ar- 39 Ar analysis of hornblende, biotite, and K-feldspar. By ca. 780 Ma the southern portion of the Grenville Province, from Ontario to the Adirondack Highlands, underwent uplift as a uniform block. Tectonic hypotheses have invoked various driving mechanisms to explain the transition from compression to extension; however, such explanations are thus far geodynamically unconstrained. Numerical models indicate that mechanisms such as gravitational collapse and mantle delamination act over timescales that cannot explain a protracted 300 m.y. extensional history that is contemporaneous with ongoing uplift of the Grenville Province. Rather, the presence of a plume upwelling underneath the Laurentian margin, combined with changes in regional stress directions, permitted the observed uplift and extension in the Grenville Province during this time. The uplift history, while on a slightly different timescale from those of most plume models, is similar to that seen in models of uplift and extension caused by the interaction of a plume with the base of the lithosphere. Some of the protracted extension likely reflects the contribution of farfield effects, possibly caused by tectonic activity in other cratons within the Rodinian supercontinent, effectively changing the stress distributions in the Grenville Province of northeastern North America. Keywords: Grenville, Rodinia, extension * streepey@quartz.gly.fsu.edu. Streepey, M.M., Lithgow-Bertelloni, C., van der Pluijm, B.A., Essene, E.J., and Magloughlin, J.F., 2004, Exhumation of a collisional orogen: A perspective from the North American Grenville Province, in Tollo, R.P., Corriveau, L., McLelland, J., and Bartholomew, M.J., eds., Proterozoic tectonic evolution of the Grenville orogen in North America: Boulder, Colorado, Geological Society of America Memoir 197, p For permission to copy, contact editing@geosociety.org Geological Society of America. 391
2 392 M.M. Streepey et al. INTRODUCTION Central to many questions in structural geology and tectonics regarding the evolution of orogens is how crust overthickened by continental collisions is modified and stabilized after an orogenic event. To understand how the crust evolves after orogenesis, it is necessary to study ancient mountain belts, the deep cores of which are exposed at the surface today in high-grade metamorphic terranes. Because results of studies of the temporal evolution of such areas give insight into the time and rates involved in crustal stabilization, these results can be used both to study the general problem of crustal stabilization and to predict the deep behavior of young orogenic belts. The Grenville Province in northeastern North America is an outstanding, well-studied example of an exposed, deeply eroded, ancient mountain system. The province is affected by a ca to 1.3 billion-year-old set of orogenic events, seen in cratonic blocks worldwide, and culminating in the formation of the supercontinent Rodinia (Hoffman, 1991; Dalziel, 1997). One of the best continuous exposures of Grenville-aged rocks is in northeastern North America between Labrador, Canada, and New York state, where Grenville deformation is thought to have occurred in an arc-continent collision at ca Ga and a continent-continent collision at ca Ga (Moore and Thompson, 1980; Easton, 1992; Rivers, 1997; Davidson, 1998; Carr et al., 2000; McLelland et al., 2001). The terrane is characterized by slices of crust that are separated by ductile shear zones in which more of the deformation is concentrated, some of which record normal motion overprinting an earlier contractional history. The colliding craton causing continentcontinent collision in this segment of Rodinia is not known, as the proposed collision with Amazonia has recently been questioned by paleomagnetic evidence (Tohver et al., 2002). Early rifting attempts are recorded in some of the blocks of Rodinia (Li et al., 1999; Karlstrom et al., 2000; Dalziel and Soper, 2001; Tack et al., 2001; Timmins et al., 2001). Most major rifting events involving the eastern Laurentian margin (present-day coordinates) appear to have occurred in the late Neoproterozoic. Rifting in this region resulted in the opening of the Iapetus Ocean, which has been dated in the north at ca. 600 Ma (Torsvik et al., 1996; Svenningsen, 2001) and in the south at Ma (Torsvik et al., 1996). However, with documented pulses of rifting having occurred in Baltica, Congo, China, and the southwestern United States from ca. 900 Ma to ca. 700 Ma, any extensional activity in the eastern Laurentian block, present-day northeastern North America, during this period may reflect initial stages of Rodinia s breakup (Li et al., 1999; Tanner and Bluck, 1999; Streepey et al., 2000; Dalziel and Soper, 2001; Timmins et al., 2001). Well-exposed Grenville structures in North America provide strong constraints on the nature of extensional activity in the area and also, when compared to extensional activity in other Rodinian blocks that occurred during roughly the same period, on the processes that control the breakup of supercontinents. The driving mechanism(s) for extension in the Laurentian part of the Grenville orogen is the primary focus of this contribution. Geologic Setting One of the continuous exposures of rocks that shows Grenville-aged deformation occurs in North America. The eastern edge of the belt abuts the edge of the Appalachian thrust front and is bounded to the west by the Archean Superior Province and other Archean and Proterozoic provinces. Because of the laterally continuous nature of this belt, it offers an excellent opportunity to study lithotectonic relationships in the orogen. The Grenville Province is composed of lithotectonically distinct blocks representing the autochthonous terrains of the Laurentian craton as well as allochthonous blocks accreted to the Laurentian margin during Grenville orogenesis (Easton, 1992; Rivers, 1997; Davidson, 1998; Hanmer et al., 2000; Fig. 1, inset). These blocks are separated by major crustal-scale shear zones and contain distinct, smaller domains that are also separated by major ductile shear zones (e.g., Davidson, 1984; Easton, 1992). A significant amount of strain recorded by these rocks is concentrated into these zones of deformation, which provide the key to unraveling the tectonic history of the region. In many cases, these shear zones appear to be multiply active, with the latest episode of deformation recording extension, or appearing to record extensional activity, synchronous to Grenville-aged contractional pulses (Mezger et al., 1991b; Culshaw et al., 1994; Busch et al., 1997; Martignole and Reynolds, 1997; Ketchum et al., 1998; Streepey et al., 2001). The current structural expression of the region is of an extensional terrain, and the challenge then lies in determining both the magnitude, timing, and origin of extension as well as the earlier, contractional history of the area. In this paper, we focus on the eastern Metasedimentary Belt of the Grenville Province and its boundary with the adjacent Granulite Terrane (Fig. 1). This area spans southeastern Ontario and northwestern New York state. The Metasedimentary Belt is one of three major crustal slices that comprise the Grenville Province in this region (Fig. 1). It lies between the Gneiss Belt and the Granulite Terrane and contains variably metamorphosed (greenschist to granulite-facies) metasediments, metagranitoids, and metavolcanic rocks (Easton, 1992). The Metasedimentary Belt contains several small shear zones that juxtapose lithologically and geochronologically distinct domains. These shear zones within the Metasedimentary Belt dip to the southeast, and the two major boundaries, the Bancroft shear zone and the Robertson Lake shear zone, show late extensional motion. The Carthage-Colton shear zone is located at the eastern edge of the Metasedimentary Belt, and separates it from the Granulite Terrane of the Adirondack Highlands. This shear zone also shows a late extensional history, but dips shallowly to the northwest, creating a grabenlike geometry between the Robertson Lake shear zone and the Carthage-Colton shear zone (Fig. 1).
3 Exhumation of a collisional orogen 393 Figure 1. Generalized map of the Metasedimentary Belt (MB) of the Grenville Province (Ontario and New York). The map shows the Metasedimentary Belt in between the Gneiss Belt (GB) and the Granulite Terrane (GT). The Bancroft shear zone (BSZ), Robertson Lake shear zone (RLSZ), and Carthage-Colton shear zone (CCSZ) are shown in their most current expression as normal faults. Other shear zones shown are the Metasedimentary Belt Boundary Zone (MBBZ) and the Sharbot Lake shear zone. The inset map shows the Grenville Province of northeastern North America with the Grenville Front tectonic zone (GFTZ) as it abuts the Archean Superior Province. Other abbreviations: MT Morin terrane; LSZ Labelle shear zone. After Streepey et al. (2001). Studies of the deformation histories of these shear zones require a multidisciplinary approach, with emphasis placed on the field relationships, peak metamorphic pressures and temperatures, and the corresponding geochronologic data that constrain the cooling and exhumation history. Because most of the rocks have experienced more than one phase of deformation and metamorphism, structural relationships in the field can be complex, and field analysis alone is not enough to completely constrain the significance of these boundaries. This study presents a synthesis of geochronologic information combined with structural analysis and thermobarometric data to describe the kinematics of the uplift or exhumation history of a segment of the Grenville Province in northeastern North America. A summary of ages is given, adding to regional compilations (Cosca et al., 1991, 1992, 1995; Mezger et al., 1991a, 1992, 1993; van der Pluijm et al., 1994). In addition, new 40 Ar- 39 Ar ages from amphiboles in the Adirondack Lowlands and the Adirondack Highlands are presented and further constrain the geologic history of the area. Whereas the combination of structural, petrologic, and geochronologic information is critical to constructing a kinematic model of the evolution of the region, it does not give a geodynamic picture of the development of the late stages of modification and stabilization of overthickened crust. This information allows us to develop reasonable geologic hypotheses about timing of late, postorogenic extension and the nature of motion between blocks of crust, but it does not explain the physical processes behind the evolution. In addition, geochronologic data are restricted to lithologies and assemblages that contain minerals with the appropriate elements for radiogenic dating. In areas where the appropriate assemblages are not available, the geochronologic results are limited or incomplete and cannot provide a full, detailed cooling history of the rocks. In order to develop a more geodynamically complete picture of the exhumation history of the Grenville Province, we have developed a two-dimensional numerical model of a slice of crust representing this region. The structures assigned to the model are taken directly from field studies in the region, and the rheologies are assigned based on existing literature (Ranalli, 1995). The numerical models explore possible driving mechanisms for the observed phenomenon of extension in this orogenic belt. From geochronologic and structural information, the timescales involved in the transition from compression to extension have been evaluated and have placed constraints on the amount of displacement across shear zones. Although how this orogenic belt extends following collision is known, why it extends is less evident. It remains uncertain whether extension can be attributed to a single mechanism, such as gravitational collapse, or whether it requires a combination of mechanisms, such as mantle delamination in addition to changes in far-field stresses. Whereas numerical models cannot provide constraints that uniquely solve this problem, they give insights as to whether or not proposed mechanisms can act in a way that fits field observations over the period of time dictated by geochronologic constraints. GEOCHRONOLOGIC SUMMARY In studies of ancient metamorphic terranes, motion along ductile shear zones can often be delineated with a combination of ages that yield information on the timing of latest metamorphism and ages that record the cooling or exhumation history of
4 394 M.M. Streepey et al. the terrane (e.g., van der Pluijm et al., 1994). In such studies it is critical to constrain the pressure-temperature (P-T) conditions of metamorphism in order to determine whether geochronologic ages are ages of cooling from a peak metamorphic event or growth ages. Minerals yield cooling ages if the peak conditions of metamorphism are higher than the closure temperatures of the minerals and growth ages if the minerals can be shown to have grown during metamorphism but at conditions below their closure temperatures. Therefore, in order to best interpret geochronologic data in the eastern portion of the Metasedimentary Belt, it was necessary to initially assess the metamorphic conditions of the terrane. Figure 2, A and B, shows temperature and pressure maps of the Metasedimentary Belt from Streepey et al. (1997; thermobarometric data from references therein). Metamorphism in the area reached upper-amphibolite to granulite-facies metamorphism. Maximum temperatures in the study area from just west of the Robertson Lake shear zone to just east of the Carthage- Colton shear zone ranged from 600 to 650 C in and around the Robertson Lake shear zone and increased to the east to C in and around the Carthage-Colton shear zone. Pressures were 600 to 800 MPa over the region. In order to best interpret radiometric ages from polymetamorphic terranes, it is essential not only to have quantitative A 78 W 77 W Temperatures ( C) > N MBBZ BSZ RLSZ SLSZ <500 B N 0 30 km MBBZ 78 W BSZ MSZ 77 W SLSZ CCSZ Pressures (MPa) > Figure 2. Regional thermobarometric gradients in the Metasedimentary Belt. (A) Contoured temperatures. The lowest temperatures are less than 500 C in the Elzevir terrane, and the highest temperatures are granulite facies (700 C and above) in the Gneiss Belt, the Frontenac terrane, and the Adirondack Highlands. (B) Contoured pressures. Pressures generally follow the pattern of temperatures and indicate regional metamorphism in the region. Pressures over most of the area are MPa. BSZ Bancroft shear zone; CCSZ Carthage-Colton shear zone; MBBZ Metasedimentary Belt Boundary Zone; RLSZ Robertson Lake shear zone; SLSZ Sharbot Lake shear zone. 45 N RLSZ < 600 MS N CCSZ 0 30 km
5 Exhumation of a collisional orogen 395 assessments of P-T conditions but also to have accurate closure temperatures for minerals used in analysis and some constraints on the diffusion mechanisms active in isotopic resetting. For this study we consider volume diffusion in grains to be the primary mechanism of isotopic resetting. In addition, published and widely used closure temperatures for the minerals titanite, hornblende, biotite, and K-feldspar are considered appropriate for this study of a slowly cooled, regionally metamorphosed terrane (titanite: C, Mezger et al., 1991a, Scott and St-Onge, 1995; hornblende: C, McDougall and Harrison, 1999; biotite: 300 C, McDougall and Harrison, 1999; K-feldspar: 150 to 300 C, Zeitler, 1987, McDougall and Harrison, 1999, Lovera et al., 1991). Because peak temperatures of regional metamorphism are close to or generally exceed the closure temperature of titanite in the U-Pb system, the U-Pb ages of titanite constrain either the timing of latest metamorphism or cooling ages very close to the timing of peak metamorphism in this area. The 40 Ar- 39 Ar ages of hornblende, biotite, and K-feldspar, which have lower closure temperatures than titanite, constrain most of the cooling history of the study area and are considered to have closed to the K-Ar system at some time after peak metamorphism. The U-Pb ages from zircon, garnet, monazite, and titanite have been determined for the Metasedimentary Belt in numerous studies (Mezger et al., 1993; Corfu and Easton, 1995, 1997; Perhsson et al., 1996; Wasteneys et al., 1999; Corriveau and van Breemen, 2000; McLelland et al., 2001). We provide a brief summary of the metamorphic ages of the Metasedimentary Belt; the reader is referred to reviews by Rivers (1997), Carr et al. (2000), Hanmer et al. (2000), and McLelland et al. (2001) for detailed descriptions of the early metamorphic history of the Metasedimentary Belt. The Metasedimentary Belt yields information on two major periods of Grenville-aged orogenesis that represent an arc accretion event from ca Ma (the Elzevirian orogeny), culminating in a continent-continent collision at ca Ma (the Ottawan orogeny). These events represent two episodes of contraction, possibly separated by extensional events, which are recorded by magmatic activity and emplacement of large anorthosite complexes during these periods (McLelland et al., 1988; Davidson, 1995). The Robertson Lake shear zone and the Carthage-Colton shear zone separate rocks showing a marked discontinuity in metamorphic age, and so are fundamental boundaries separating blocks with distinct metamorphic histories. From the Metasedimentary Belt boundary thrust zone east to the Robertson Lake shear zone (encompassing the Bancroft and Elzevir terranes), metamorphic minerals record geochronological evidence of metamorphism during the latest contractional event during the Ottawan orogeny. The Frontenac terrane (including the Adirondack Lowlands) from east of the Robertson Lake shear zone to the Carthage-Colton shear zone shows evidence of metamorphism related to the earlier Elzevirian orogeny. It does not, however, appear to record metamorphic ages related to the Ottawan orogeny, indicating that this terrane was either laterally separated from the Elzevir terrane during this period or was at shallow crustal levels (Mezger et al., 1993; Streepey et al., 2000, 2001). However, some investigators have proposed that at least portions of the Adirondack Lowlands may have been deformed during the Ottawan orogeny (Wasteneys et al., 1999), and more detailed isotopic work may be useful in resolving the extent and nature of Ottawan deformation in the Lowlands. East of the Carthage-Colton shear zone, peak metamorphism in the Adirondack Highlands occurred during the Ottawan orogeny, although there is some evidence that this terrane was also deformed during the Elzevirian orogeny (McLelland et al., 1988; McLelland and Chiarenzelli, 1989; Kusky and Lowring, 2001). In this scenario, the Frontenac terrane, bounded by the east-dipping Robertson Lake shear zone and the west-dipping Carthage- Colton shear zone, is a block of crust that has been largely protected from the Ottawan pulse of orogenesis, while the Adirondack Highlands and the Elzevir terrane recorded metamorphic ages during this period. The cooling history of the terranes immediately adjacent to these two shear zones is discussed in order to evaluate their significance as postorogenic extensional structures. The ages compiled in this study include ages determined from regional U-Pb and 40 Ar- 39 Ar geochronologic data (Busch and van der Pluijm, 1996; Busch et al., 1996b, 1997; Streepey et al., 2000, 2001, 2002, and new results) to provide a complete cooling history for the eastern half of the Metasedimentary Belt. Sample locations and their corresponding ages are given in Table 1. The U-Pb ages of titanite give the timing of early deformation, thought to have some transpressive component along both the east-dipping Robertson Lake shear zone and the west-dipping Carthage- Colton shear zone. The 40 Ar- 39 Ar ages of hornblendes, biotites, and K-feldspars, combined with structural analysis of shear zones, record the timing of later extensional motion along both shear zones. New 40 Ar- 39 Ar ages of hornblendes further detail the cooling history of the crust adjacent to the Carthage-Colton shear zone in northwest New York state. Robertson Lake Shear Zone The Robertson Lake shear zone is a multiply active, eastdipping shear zone separating the eastern Elzevir terrane (Mazinaw domain) and the western Frontenac terrane (Sharbot Lake domain) within the Metasedimentary Belt (Fig. 1; Easton, 1988). The latest episode of motion recorded along this zone is down-to-the-east, as shown by shear-sense indicators including S-C, C C fabrics, with these crystal plastic structures crosscut by brittle fabrics delineating an uplift history during deformation (Busch and van der Pluijm, 1996). Its early history records a transpressive event at ca Ma, indicating imbrication via sinistral transpression of the Mazinaw and the Sharbot Lake domains (Busch et al., 1997). 40 Ar- 39 Ar cooling ages of hornblendes and micas in the Robertson Lake shear zone show a marked difference across the zone. Combined with the structural information and the nature of the offset, extensional motion had to have occurred to juxtapose the crustal blocks on either side of
6 TABLE 1. SAMPLE LOCATIONS Hornblende Biotite Sample age age Domain Location UTM Coordinates UTM Coordinates From Busch et al. (1996) Northing Easting RVL118A 1011 Ma n.d. Sharbot Lake RVL20A 1205 Ma n.d. Sharbot Lake CDN Ma n.d. Elzevir/Mazinaw MTG Ma n.d. Elzevir/Mazinaw From Busch and van der Pluijm (1996) Northing Easting RVL118B n.d. 969 Ma Sharbot Lake LVT130B n.d. 901 Ma Elzevir/Mazinaw From Streepey et al. (2000) and unpublished data A Ma n.d. Lowlands Popple Hill Formation, near Russell. A Ma 940 Ma Lowlands Near Gouverneur, Hwy 58/812, N Jct. of Poplar Hill Rd. A Ma n.d. Lowlands Along Grass River, Hwy 17, W side of road, 1.5 km NW of Russell. A Ma n.d. Highlands 0.5 mi. E of Rt. 27 and Brouser s Corner. A Ma n.d. Lowlands Emoryville Rd., ca. 0.5 mi. NW of power plant, ca 0.25 mi. W of turnoff to Talcville. A Ma n.d. Lowlands Emoryville Rd., E of Hailsboro, small outcrop north of road, NW of house, 1.8 mi. E of bridge, 2.1 mi. from Island Branch Rd. A Ma 964 Ma Lowlands Trout Lake Rd., N of Edwards, along W side of Trout Lake, S side, W of 2 islands, W side of road. Cut about 40 m long. A Ma 924 Ma Lowlands S. of Trout Lake, Rt. 19 to Edwards, ca. due W of westernmost extreme of Cedar Lake. A Ma n.d. Highlands Crest of White s Hill, White Hill Rd., SE of Parishville about 3 mi. at abandoned lookout. 150 on small trail to the W. A Ma n.d. Highlands Very low outcrops along both sides of the highway. S side of White Hill, about 1 mi. S of lookout. Outcrop on White Hill Rd., within amphibolite. A Ma n.d. Highlands Just N of Sterling Rd., near power line, about 150 from junction with Joe Indian Rd. A Ma 937 Ma Highlands Small outcrop on Joe Indian Rd. at the crest of a small hill. Located at the N70 W, 60 N symbol on Leonard and Buddington s map. A Ma n.d. Highlands Rt. 56, about 3 mi. S of Stark. W side of hwy, roadcut of amphibolite migmatites. A Ma n.d. Highlands Small outcrop on E side of Hwy 56, about 1 mi. S of 137, about 4 mi. S of Stark. A Ma n.d. Highlands Hwy 3, just E of Pitcairn. Exposures on embankment N of the road, at crest of hill. A Ma 919 Ma Highlands Small outcrop on the E side of Mud Lake Rd. at the junction of Mud Lake and Briggs Rd. (SE corner of intersection). A Ma n.d. Highlands About 2 mi. E of Texas on Texas Rd., 6 mi. from Hwy 812. HM95 n.d. 915 Ma Lowlands NW of Russell on Rt. 17 in Devil s Elbow, 2 mi. S of Hermon. Sample from E side of outcrop. SE n.d. 895 Ma Highlands On Rt. 87, N of Dana Hill Rd., 1.9 mi. S of Whipporwill Corners. Outcrop on NW side of road. LB596-31b n.d. 925 Ma Lowlands 4.5 mi. from intersection of Hwy 3 and Hwy 812, on 812 S of Balmat. Sample from E side of outcrop. LB93 n.d. 904 Ma Lowlands Outcrop just N of LB596-31b. CN n.d. 924 Ma Highlands 0.2 mi. S of junction of Hanson Rd. and Orebed Rd. on Orebed Rd., near Colton. Outcrop on W side of road. PP n.d. 899 Ma Lowlands 1 mi. SW of Pierrepont on Rt. 2. Outcrop on E side of road. From Streepey et al. (2001) Latitude Longitude RWS Ma n.d. Within CCSZ North 44 deg., 22 min. West 75 deg., 10 min. RWS-3c 999 Ma n.d. Within CCSZ North 44 deg., 22 min. West 75 deg., 10 min. EA1 981 Ma n.d. Within CCSZ North 44 deg., 22 min. West 75 deg., 10 min. DH Ma n.d. Within CCSZ North 44 deg., 22 min. West 75 deg., 10 min. RWH Ma n.d. Within CCSZ North 44 deg., 22 min. West 75 deg., 10 min. CR7-DH1 989 Ma n.d. Within CCSZ Dana Hill outcrop: On N side of Dana Hill Rd. CR1-DH1 980 Ma n.d. Within CCSZ Dana Hill outcrop: On N side of Dana Hill Rd. CR Ma n.d. Within CCSZ 1.8 km N of intersection between Dana Hill Rd. and Rt. 87, on Rt. 87. CR2-A2 948 Ma n.d. Within CCSZ Directly across Rt. 87 from CR3-87. DH Ma n.d. Within CCSZ Directly across Rt. 87 from CR3-87. CR6-A Ma n.d. Within CCSZ Directly across Rt. 87 from CR3-87. CR12-A4 998 Ma n.d. Within CCSZ Directly across Rt. 87 from CR3-87. CR9-A4 944 Ma n.d. Within CCSZ Directly across Rt. 87 from CR3-87. H87-5b 1019 Ma n.d. Within CCSZ Directly across Rt. 87 from CR3-87. n.d. no data; CCSZ Carthage-Colton shear zone; UTM Universal Transverse Mercator.
7 Exhumation of a collisional orogen 397 the shear zone (Busch et al., 1996b; Fig. 3). The timing of the transition from compression to extension can be somewhat constrained by the age of the latest transpressive event in the region at ca Ma (Busch et al., 1997), but cannot be directly determined. The termination of extension across the Robertson Lake shear zone cannot be constrained from 40 Ar- 39 Ar analyses of hornblende and biotite, as both show differences of ca Ma across the zone. Results from the analysis of K-feldspar in the region suggest that the entire block was uplifting uniformly by 780 Ma, suggesting termination of extension between 900 and 780 Ma (Streepey et al., 2002). Ages taken from Busch and van der Pluijm (1996), Busch et al. (1996b, 1997), and Streepey et al. (2002) are compiled in Figure 3. These data constrain the cooling history of the terrane from immediately after orogenesis in the Robertson Lake shear zone area to the time at which the terrane was uplifting as a uniform block. In addition, the geometry of normal fault motion and the amount of displacement along the shear zone are evident (Busch et al., 1996a). The U-Pb ages from titanites document the difference in metamorphic ages between the Mazinaw and the Sharbot Lake domains. As the closure temperature of titanite is ca C (Mezger et al., 1991a; Scott and St-Onge, 1995), titanite ages give the timing of metamorphism or a cooling age that is very close to the age of metamorphism in the amphibolite to granulite-facies rocks of the Elzevir and the Frontenac terranes. Metamorphism in the Sharbot Lake domain (the hangingwall block of the Robertson Lake shear zone) occurred at ca to 1170 Ma (Mezger et al., 1993; Corfu and Easton, 1995; Busch et al., 1997). These ages are generally similar to those across the entire width of the Frontenac terrane (Mezger et al., 1993). However, the youngest metamorphic ages in the Mazinaw domain (the footwall block of the Robertson Lake shear zone) range from ca Ma to 1050 Ma (Mezger et al., 1993; Busch et al., 1997), showing a ca. 100-m.y. difference in the timing of the latest metamorphic event across this boundary. Transpressional activity is interpreted to have caused juxtaposition through imbrication of the two terranes at ca Ma (Busch et al., 1997). The 40 Ar- 39 Ar cooling ages of hornblendes, micas, and K- feldspars constrain the post-titanite cooling and unroofing history of the blocks of crust adjacent to the Robertson Lake shear zone and constrain the timing and nature of postorogenic extension along this zone. Hornblende ages for rocks in the vicinity of the Robertson Lake shear zone are shown in Figure 3. The offset in ages shown by the titanite U-Pb geochronology is also shown by the cooling ages of hornblende, with hornblende ages of ca Ma in the Sharbot Lake domain (hangingwall), which are at least 60 m.y. older than rocks immediately across the Robertson Lake shear zone in the Mazinaw domain, where hornblende ages are ca. 950 Ma (Busch and van der Pluijm, 1996; Busch et al., 1996a). Biotite, which closes to the K-Ar system at ca. 300 C, continues to show an offset across the Robertson Lake shear zone, with biotites in the Sharbot Lake domain recording ages of 970 Ma that are at least 70 m.y. older than the 900 Ma biotite ages in the Mazinaw domain (Busch and van der Pluijm, 1996). At the time of biotite closure, rocks were at fairly shallow crustal depths of 10 to 12 km assuming an average geothermal gradient. Because of the offsets in cooling ages of the rocks that are presently exposed at the surface crustal level, it is clear that extensional activity did not terminate across the Robertson Lake shear zone until sometime after 900 Ma. The 40 Ar- 39 Ar ages of K-feldspars give some information on the termination of extension along the Robertson Lake shear zone, but do not completely constrain it. Unlike hornblendes or micas, which are considered to have one diffusion domain and therefore a single closure temperature, K-feldspars are thought to have multiple diffusion domains and therefore multiple closure temperatures (Lovera et al., 1991). Analysis of K-feldspars gives, instead of a single age, a temperature-time path for the grain. Thermal modeling of K-feldspar spectra from the Mazinaw and the Sharbot Lake domains show that the two domains were juxtaposed by at least 780 Ma, meaning that the termination of extension across the Robertson Lake shear zone must have occurred between 900 Ma and 780 Ma (Streepey et al., 2002). Thus, postorogenic extension across the Robertson Lake shear zone terminated between 140 and 260 m.y. after the final expression of contractional tectonics in the area at ca Ma. CARTHAGE-COLTON SHEAR ZONE The Carthage-Colton shear zone separates the eastern Frontenac terrane (Adirondack Lowlands) from the Granulite Terrane (Adirondack Highlands; Fig. 1). It is 150 km east of the Robertson Lake shear zone and dips to the west, toward the Robertson Lake shear zone. With this geometry, the Frontenac terrane is a grabenlike block bounded by two shear zones dipping toward one another. Upper amphibolite-facies marbles and other metasediments dominate the Adirondack Lowlands lithologies, whereas the Adirondack Highlands are comprised predominantly of granulite-facies metaigneous assemblages. The Carthage-Colton shear zone crops out as a zone of intense deformation between the two terranes, although the exact location of the boundary has been debated (Geraghty et al., 1981). The Carthage-Colton shear zone had a long history of activity over the duration of the Grenville orogenic cycle. The Adirondack Lowlands and Highlands both appear to have been affected by the ca Ma arc-continent collision at the end of the Elzevirian orogeny (Mezger et al., 1991a, 1992; Wasteneys et al., 1999; Kusky and Lowring, 2001). However, only the Adirondack Highlands appear to have been pervasively metamorphosed by the granulite-facies Ottawan orogeny, which has been dated at Ma (McLelland et al., 1996). The entire Frontenac terrane, from east of the Robertson Lake shear zone to just west of the Carthage-Colton shear zone, appears to have escaped widespread thermal metamorphism and resetting of isotopic ages from this pervasive deformational event, either by being at shallower crustal levels during that period or by being laterally separated.
8 Figure 3. Maps showing hornblende, biotite, and Kfeldspar 40Ar-39Ar ages across the Robertson Lake shear zone (RLSZ) and the Carthage-Colton shear zone (CCSZ). Across the RLSZ, the ages are from Busch et al. (1996), Busch and van der Pluijm (1996), and Streepey et al. (2002). Hornblende ages are italicized. Across the CCSZ, ages are from Streepey et al. (2000, 2001, 2002) and new results. Pairs separated by a comma indicate hornblende and biotite ages from the same sample. The age distributions across the RLSZ indicate motion along the shear zone after ca. 900 Ma and before ca. 780 Ma. The age distributions across the CCSZ indicate deformation between ca. 950 and 930 Ma, most likely at ca. 945 Ma. DH-1 field outcrop name; GB Gneiss Belt; GFTZ Grenville Front tectonic zone; GT Granulite Terrane; MB Metasedimentary Belt; MBBZ Metasedimentary Belt Boundary Zone; MSZ Moorton shear zone; MT Morin terrane; RW field outcrop name; SLSZ Sharbot Lake shear zone.
9 Exhumation of a collisional orogen 399 McLelland et al. (1996) proposed that the Carthage-Colton shear zone was the locus for extensional collapse of the Ottawan orogen at ca Ma, which resulted in exhumation of the high-grade core of the Adirondack Highlands, presumably while the orogen was still under a contractional stress field and as a direct result of the orogenic event, either through gravitational collapse or through mantle delamination beneath the orogen. Streepey et al. (2001) suggested that the Carthage-Colton shear zone was involved in transpressive deformation similar to that documented across the Robertson Lake shear zone at ca Ma. It is clear that the Carthage-Colton shear zone was active during or immediately after the latest episode of Grenville contraction. In addition, cooling ages from hornblendes and biotites show that the Carthage-Colton shear zone was reactivated in an extensional regime similar to that observed along the Robertson Lake shear zone. We present sixteen new 40 Ar- 39 Ar hornblende analyses from the University of Michigan Radiogenic Isotope Laboratory combined with published 40 Ar- 39 Ar hornblende and biotite ages near and along the Carthage-Colton shear zone (Streepey et al., 2000, 2001). Standard operating procedures for the collection of hornblende and biotite analyses in this laboratory are described in detail in Streepey et al. (2000). Our results are shown in Figure 3, and isotopic data are presented in Table 2. 1 Plateaus were defined as occurring where 50% or more of the total 39 Ar was released in three or more consecutive steps and where the ages of the steps overlapped at the 2σ level of error. The hornblende ages in the Adirondack Lowlands of the Frontenac terrane show that these locations within this slice of crust reached 500 C at ca Ma (Fig. 3; Table 2). Hornblende ages across the Carthage-Colton shear zone are Ma. The offset in ages indicates active movement along the Carthage-Colton shear zone after hornblendes closed to the K- Ar system, or after 950 Ma. The nature of the offset, combined with the regional fabrics, indicates that this motion must have been extensional (Heyn, 1990; Streepey et al., 2000). Some hornblendes in the Dana Hill metagabbro tightly cluster at 945 Ma. Though there is some textural complexity in these samples, with young ages coming from a variety of veins and other textures, these ages fit well within the regional framework of extension (bracketed by regional hornblende and biotite 40 Ar- 39 Ar ages) and indicate that this was likely a time of deformation along the Carthage-Colton shear zone (Streepey et al., 2001). Biotite 40 Ar- 39 Ar ages are ca Ma on both sides of the shear zone, indicating that this block of crust was uniformly uplifting by this time (Streepey et al., 2000). K-feldspar ages are similar to those found in the Robertson Lake shear zone area, further supporting the idea that the entire Metasedimentary Belt 1 GSA Data Repository item , Appendix, hornblende spectra, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO , USA, editing@geosociety.org, or at ft2004.htm. was uplifting as a uniform block by ca. 780 Ma (Heizler and Harrison, 1998; Streepey et al., 2002). NUMERICAL MODELING Geochronology paired with structural geology shows that extensional motion took place along a large segment of the Metasedimentary Belt well after the Grenville contractional orogeny, during a period that was considered to be relatively quiescent. Application of 40 Ar- 39 Ar and U-Pb geochronology details the kinematics and timing of this transition and the amount and nature of extensional deformation that occurred after it, but gives few constraints on the mechanisms that produced a regional extensional event during this period. Extension occurred at least 100 m.y. after the latest contractional event in the Grenville Province of New York and southeastern Ontario. Given this timescale, it is difficult to make a causal link between orogenesis and extension. Whereas several attempts have been made to create a Himalayan analog to the Grenville Province (e.g., Windley, 1986), a component of major extension in the latter is clearly postorogenic, so the timescales for the two are different. In Tibet, for example, gravitational potential energy due to the elevated topography of the plateau played a major role in driving collapse (e.g., Shen et al., 2001). However, in the Grenville Province the duration and termination of this extensional event were so much later than compression that gravitational collapse became an ineffective driving mechanism. Earlier synorogenic extensional events (as proposed by McLelland et al., 2001, and references therein) at ca Ma were different in nature and almost certainly can be ascribed to processes such as gravitational collapse, mantle delamination, or some combination of these driving mechanisms. The exhumation of the Grenville orogen was not solely the result of erosional and isostatic processes, but was due at least in part to active extension along shear zones, which led to uplift of the region. Geochronologic data have allowed construction of a kinematic model of denudation and unroofing of midcrustal levels of the orogenic belt. When uplift is discussed in the context of this paper, we refer to the exhumation of the core of the Grenville Province and do not constrain a measure of the paleotopographic surface. A model investigation has been made of the evolution of a block of overthickened crust, with three common driving mechanisms proposed to explain postorogenic extension. We have evaluated the driving forces necessary to generate uplift and extension that match the kinematics of deformation in the Grenville Province as documented through field and laboratory studies. Two critical field observations in this area that must be explained are the continuous regional uplift during the time of extension and the time lag in termination of extensional motion along the Robertson Lake shear zone versus the Carthage- Colton shear zone detailed in the geochronology (see earlier and Fig. 3).
10 TABLE 2. HORNBLENDE ARGON DATA Power 39 Ar frac 39 Ar mol 40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar 40 Ar*/ 39 ArK % 40 Ar atmos Ca/K Age (Ma) 1σ error (Ma) A E E E E E E E E E E E E E E E J value E-03 ± E-05 Total 39 K vol = E-10 CCNTP/G Total gas age = ± Ma A E E E E E E E E E E E E E E E J value E-03 ± E-05 Total 39 K vol = E-10 CCNTP/G Total gas age = ± Ma A117-1R E E E E E E E
11 E E E E E E J value E-03 ± E-05 Total 39 K vol = E-10 CCNTP/G Total gas age = ± Ma A124-1R E E E E E E E E E E E E E E E E J value E-03 ± E-05 Total 39 K vol = E-10 CCNTP/G Total gas age = ± Ma A E E E E E E E E E E E J value E-03 ± E-05 Total 39 K vol= E-10 CCNTP/G Total gas age = ± Ma A E E E (continued)
12 TABLE 2. Continued Power 39 Ar frac 39 Ar mol 40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar 40 Ar*/ 39 ArK % 40 Ar atmos Ca/K Age (Ma) 1σ error (Ma) E E E E E E E E E E E E J value E-03 ± E-05 Total 39 K vol = E-10 CCNTP/G Ttotal gas age = ± Ma A E E E E E E E E J value E-03 ± E-05 Total 39 K Vol= E-10 CCNTP/G Total gas age = ± Ma A E E E E E E E E E E E E E E J value E-03 ± E-05 Total 39 K vol = E-10 CCNTP/G Total gas age = ± Ma
13 A E E E E E E E E E E E E E E E E J value E-03 ± E-05 Total 39 K vol = E-10 CCNTP/G Total gas age = ± Ma A E E E E E E E E E E E J value E-03 ± E-05 Total 39 K vol= E-10 CCNTP/G Total gas age = ± Ma A E E E E E E E E E E E E (continued)
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