Geological Society of America Memoir 197 2004

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 32306-4100, USA Carolina Lithgow-Bertelloni Ben A. van der Pluijm Eric J. Essene Department of Geological Science, University of Michigan, Ann Arbor, Michigan 48109-1063, USA Jerry F. Magloughlin Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523-1482, 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. 1040 Ma, supported by U-Pb data on titanites and 40Ar-39Ar data on hornblendes, followed by regional extension occurring along crustal-scale shear zones between 945 and 780 Ma, as recorded by 40Ar-39Ar analysis of hornblende, biotite, and K-feldspar. By ca. 780 Ma the southern portion of the Grenville Province, from Ontario to the Adiron- dack 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 man- tle delamination act over timescales that cannot explain a protracted 300 m.y. exten- sional history that is contemporaneous with ongoing uplift of the Grenville Province. Rather, the presence of a plume upwelling underneath the Laurentian margin, com- bined 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 far- field 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

*E-mail: [email protected].

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. 391–410. For permission to copy, contact [email protected]. © 2004 Geological Society of America. 391 392 M.M. Streepey et al.

INTRODUCTION extension in the Laurentian part of the Grenville orogen is the primary focus of this contribution. Central to many questions in structural geology and tecton- ics regarding the evolution of orogens is how crust overthick- Geologic Setting ened by continental collisions is modified and stabilized after an orogenic event. To understand how the crust evolves after oro- One of the continuous exposures of rocks that shows genesis, it is necessary to study ancient mountain belts, the deep Grenville-aged deformation occurs in North America. The east- cores of which are exposed at the surface today in high-grade ern edge of the belt abuts the edge of the Appalachian thrust front metamorphic terranes. Because results of studies of the tempo- and is bounded to the west by the Archean Superior Province and ral evolution of such areas give insight into the time and rates other Archean and Proterozoic provinces. Because of the later- involved in crustal stabilization, these results can be used both ally continuous nature of this belt, it offers an excellent opportu- to study the general problem of crustal stabilization and to pre- nity to study lithotectonic relationships in the orogen. dict the deep behavior of young orogenic belts. The Grenville Province is composed of lithotectonically The Grenville Province in northeastern North America is distinct blocks representing the autochthonous terrains of the an outstanding, well-studied example of an exposed, deeply Laurentian craton as well as allochthonous blocks accreted to eroded, ancient mountain system. The province is affected by a the Laurentian margin during Grenville orogenesis (Easton, ca. 1.0- to 1.3 billion-year-old set of orogenic events, seen in 1992; Rivers, 1997; Davidson, 1998; Hanmer et al., 2000; Fig. cratonic blocks worldwide, and culminating in the formation of 1, inset). These blocks are separated by major crustal-scale shear the supercontinent Rodinia (Hoffman, 1991; Dalziel, 1997). zones and contain distinct, smaller domains that are also sepa- One of the best continuous exposures of Grenville-aged rocks rated by major ductile shear zones (e.g., Davidson, 1984; Eas- is in northeastern North America between Labrador, Canada, ton, 1992). A significant amount of strain recorded by these and New York state, where Grenville deformation is thought to rocks is concentrated into these zones of deformation, which have occurred in an arc-continent collision at ca. 1.3–1.2 Ga provide the key to unraveling the tectonic history of the region. and a continent-continent collision at ca. 1.1–1.05 Ga (Moore In many cases, these shear zones appear to be multiply active, and Thompson, 1980; Easton, 1992; Rivers, 1997; Davidson, with the latest episode of deformation recording extension, or 1998; Carr et al., 2000; McLelland et al., 2001). The terrane is appearing to record extensional activity, synchronous to characterized by slices of crust that are separated by ductile Grenville-aged contractional pulses (Mezger et al., 1991b; Cul- shear zones in which more of the deformation is concentrated, shaw et al., 1994; Busch et al., 1997; Martignole and Reynolds, some of which record normal motion overprinting an earlier 1997; Ketchum et al., 1998; Streepey et al., 2001). The current contractional history. The colliding craton causing continent- structural expression of the region is of an extensional terrain, continent collision in this segment of Rodinia is not known, as and the challenge then lies in determining both the magnitude, the proposed collision with Amazonia has recently been ques- timing, and origin of extension as well as the earlier, contrac- tioned by paleomagnetic evidence (Tohver et al., 2002). Early tional history of the area. rifting attempts are recorded in some of the blocks of Rodinia In this paper, we focus on the eastern Metasedimentary Belt (Li et al., 1999; Karlstrom et al., 2000; Dalziel and Soper, 2001; of the Grenville Province and its boundary with the adjacent Tack et al., 2001; Timmins et al., 2001). Most major rifting Granulite Terrane (Fig. 1). This area spans southeastern Ontario events involving the eastern Laurentian margin (present-day and northwestern New York state. The Metasedimentary Belt is coordinates) appear to have occurred in the late Neoprotero- one of three major crustal slices that comprise the Grenville zoic. Rifting in this region resulted in the opening of the Iape- Province in this region (Fig. 1). It lies between the Gneiss Belt tus Ocean, which has been dated in the north at ca. 600 Ma and the Granulite Terrane and contains variably metamorphosed (Torsvik et al., 1996; Svenningsen, 2001) and in the south at (greenschist to granulite-facies) metasediments, metagranitoids, 570–550 Ma (Torsvik et al., 1996). However, with documented and metavolcanic rocks (Easton, 1992). pulses of rifting having occurred in Baltica, Congo, China, and The Metasedimentary Belt contains several small shear the southwestern United States from ca. 900 Ma to ca. 700 Ma, zones that juxtapose lithologically and geochronologically dis- any extensional activity in the eastern Laurentian block, pres- tinct domains. These shear zones within the Metasedimentary ent-day northeastern North America, during this period may Belt dip to the southeast, and the two major boundaries, the Ban- reflect initial stages of Rodinia’s breakup (Li et al., 1999; Tan- croft shear zone and the Robertson Lake shear zone, show late ner and Bluck, 1999; Streepey et al., 2000; Dalziel and Soper, extensional motion. The Carthage-Colton shear zone is located 2001; Timmins et al., 2001). Well-exposed Grenville structures at the eastern edge of the Metasedimentary Belt, and separates in North America provide strong constraints on the nature of it from the Granulite Terrane of the Adirondack Highlands. This extensional activity in the area and also, when compared to shear zone also shows a late extensional history, but dips shal- extensional activity in other Rodinian blocks that occurred dur- lowly to the northwest, creating a grabenlike geometry between ing roughly the same period, on the processes that control the the Robertson Lake shear zone and the Carthage-Colton shear breakup of supercontinents. The driving mechanism(s) for zone (Fig. 1). Exhumation of a collisional orogen 393

Figure 1. Generalized map of the Meta- sedimentary 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 able, the geochronologic results are limited or incomplete and require a multidisciplinary approach, with emphasis placed on cannot provide a full, detailed cooling history of the rocks. the field relationships, peak metamorphic pressures and tem- In order to develop a more geodynamically complete pic- peratures, and the corresponding geochronologic data that con- ture of the exhumation history of the Grenville Province, we strain the cooling and exhumation history. Because most of the have developed a two-dimensional numerical model of a slice of rocks have experienced more than one phase of deformation and crust representing this region. The structures assigned to the metamorphism, structural relationships in the field can be com- model are taken directly from field studies in the region, and the plex, and field analysis alone is not enough to completely con- rheologies are assigned based on existing literature (Ranalli, strain the significance of these boundaries. 1995). The numerical models explore possible driving mecha- This study presents a synthesis of geochronologic informa- nisms for the observed phenomenon of extension in this oro- tion combined with structural analysis and thermobarometric genic belt. From geochronologic and structural information, the data to describe the kinematics of the uplift or exhumation his- timescales involved in the transition from compression to exten- tory of a segment of the Grenville Province in northeastern sion have been evaluated and have placed constraints on the North America. A summary of ages is given, adding to regional amount of displacement across shear zones. Although how this compilations (Cosca et al., 1991, 1992, 1995; Mezger et al., orogenic belt extends following collision is known, why it 1991a, 1992, 1993; van der Pluijm et al., 1994). In addition, new extends is less evident. It remains uncertain whether extension 40Ar-39Ar ages from amphiboles in the Adirondack Lowlands can be attributed to a single mechanism, such as gravitational and the Adirondack Highlands are presented and further con- collapse, or whether it requires a combination of mechanisms, strain the geologic history of the area. such as mantle delamination in addition to changes in far-field Whereas the combination of structural, petrologic, and stresses. Whereas numerical models cannot provide constraints geochronologic information is critical to constructing a kine- that uniquely solve this problem, they give insights as to whether matic model of the evolution of the region, it does not give a geo- or not proposed mechanisms can act in a way that fits field dynamic picture of the development of the late stages of observations over the period of time dictated by geochronologic modification and stabilization of overthickened crust. This constraints. information allows us to develop reasonable geologic hypothe- ses about timing of late, postorogenic extension and the nature GEOCHRONOLOGIC SUMMARY of motion between blocks of crust, but it does not explain the physical processes behind the evolution. In addition, geochrono- In studies of ancient metamorphic terranes, motion along logic data are restricted to lithologies and assemblages that con- ductile shear zones can often be delineated with a combination tain minerals with the appropriate elements for radiogenic of ages that yield information on the timing of latest metamor- dating. In areas where the appropriate assemblages are not avail- phism and ages that record the cooling or exhumation history of 394 M.M. Streepey et al. the terrane (e.g., van der Pluijm et al., 1994). In such studies it Figure 2, A and B, shows temperature and pressure maps of is critical to constrain the pressure-temperature (P-T) conditions the Metasedimentary Belt from Streepey et al. (1997; thermo- of metamorphism in order to determine whether geochronologic barometric data from references therein). Metamorphism in the ages are ages of cooling from a peak metamorphic event or area reached upper-amphibolite to granulite-facies metamor- growth ages. Minerals yield cooling ages if the peak conditions phism. Maximum temperatures in the study area from just west of metamorphism are higher than the closure temperatures of the of the Robertson Lake shear zone to just east of the Carthage- minerals and growth ages if the minerals can be shown to have Colton shear zone ranged from 600 to 650 °C in and around the grown during metamorphism but at conditions below their clo- Robertson Lake shear zone and increased to the east to 700–750 sure temperatures. Therefore, in order to best interpret geo- °C in and around the Carthage-Colton shear zone. Pressures chronologic data in the eastern portion of the Metasedimentary were 600 to 800 MPa over the region. Belt, it was necessary to initially assess the metamorphic con- In order to best interpret radiometric ages from polymeta- ditions of the terrane. morphic terranes, it is essential not only to have quantitative

A 78˚W 77˚W Temperatures (˚C) >750

700-750

650-700 . MBBZ 600-650 RLSZ 550-600 BSZ 500-550 SLSZ 45˚N <500

Figure 2. Regional thermobarometric gradients in the Metasedimentary Belt. (A) Contoured temperatures. The lowest MSZ temperatures are less than 500 °C in the Elzevir terrane, and the highest temper- N CCSZ atures are granulite facies (700 °C and 0 30 km above) in the Gneiss Belt, the Frontenac terrane, and the Adirondack Highlands. (B) Contoured pressures. Pressures gen- erally follow the pattern of temperatures B Pressures (MPa) and indicate regional metamorphism in 78˚W 77˚W the region. Pressures over most of the > 800 area are 700–800 MPa. BSZ—Bancroft shear zone; CCSZ—Carthage-Colton shear zone; MBBZ—Metasedimentary 700-800 Belt Boundary Zone; RLSZ—Robert- MBBZ son Lake shear zone; SLSZ—Sharbot 600-700 Lake shear zone.

BSZ SLSZ < 600 45˚N

RLSZ

MS

N CCSZ

0 30 km Exhumation of a collisional orogen 395 assessments of P-T conditions but also to have accurate closure low crustal levels (Mezger et al., 1993; Streepey et al., 2000, temperatures for minerals used in analysis and some constraints 2001). However, some investigators have proposed that at least on the diffusion mechanisms active in isotopic resetting. For this portions of the Adirondack Lowlands may have been deformed study we consider volume diffusion in grains to be the primary during the Ottawan orogeny (Wasteneys et al., 1999), and more mechanism of isotopic resetting. In addition, published and detailed isotopic work may be useful in resolving the extent and widely used closure temperatures for the minerals titanite, horn- nature of Ottawan deformation in the Lowlands. East of the blende, biotite, and K-feldspar are considered appropriate for this Carthage-Colton shear zone, peak metamorphism in the Adiron- study of a slowly cooled, regionally metamorphosed terrane dack Highlands occurred during the Ottawan orogeny, although (titanite: 600–700 °C, Mezger et al., 1991a, Scott and St-Onge, there is some evidence that this terrane was also deformed dur- 1995; hornblende: 480–500 °C, McDougall and Harrison, 1999; ing the Elzevirian orogeny (McLelland et al., 1988; McLelland biotite: 300 °C, McDougall and Harrison, 1999; K-feldspar: 150 and Chiarenzelli, 1989; Kusky and Lowring, 2001). In this to 300 °C, Zeitler, 1987, McDougall and Harrison, 1999, Lovera scenario, the Frontenac terrane, bounded by the east-dipping et al., 1991). Because peak temperatures of regional metamor- Robertson Lake shear zone and the west-dipping Carthage- phism are close to or generally exceed the closure temperature of Colton shear zone, is a block of crust that has been largely titanite in the U-Pb system, the U-Pb ages of titanite constrain protected from the Ottawan pulse of orogenesis, while the either the timing of latest metamorphism or cooling ages very Adirondack Highlands and the Elzevir terrane recorded meta- close to the timing of peak metamorphism in this area. The 40Ar- morphic ages during this period. 39Ar ages of hornblende, biotite, and K-feldspar, which have The cooling history of the terranes immediately adjacent to lower closure temperatures than titanite, constrain most of the these two shear zones is discussed in order to evaluate their sig- cooling history of the study area and are considered to have closed nificance as postorogenic extensional structures. The ages com- to the K-Ar system at some time after peak metamorphism. piled in this study include ages determined from regional U-Pb The U-Pb ages from zircon, garnet, monazite, and titanite and 40Ar-39Ar geochronologic data (Busch and van der Pluijm, have been determined for the Metasedimentary Belt in numer- 1996; Busch et al., 1996b, 1997; Streepey et al., 2000, 2001, ous studies (Mezger et al., 1993; Corfu and Easton, 1995, 1997; 2002, and new results) to provide a complete cooling history for Perhsson et al., 1996; Wasteneys et al., 1999; Corriveau and van the eastern half of the Metasedimentary Belt. Sample locations Breemen, 2000; McLelland et al., 2001). We provide a brief and their corresponding ages are given in Table 1. The U-Pb ages summary of the metamorphic ages of the Metasedimentary Belt; of titanite give the timing of early deformation, thought to have the reader is referred to reviews by Rivers (1997), Carr et al. some transpressive component along both the east-dipping (2000), Hanmer et al. (2000), and McLelland et al. (2001) for Robertson Lake shear zone and the west-dipping Carthage- detailed descriptions of the early metamorphic history of the Colton shear zone. The 40Ar-39Ar ages of hornblendes, biotites, Metasedimentary Belt. The Metasedimentary Belt yields infor- and K-feldspars, combined with structural analysis of shear mation on two major periods of “Grenville-aged” orogenesis that zones, record the timing of later extensional motion along both represent an arc accretion event from ca. 1300–1190 Ma (the shear zones. New 40Ar-39Ar ages of hornblendes further detail Elzevirian orogeny), culminating in a continent-continent colli- the cooling history of the crust adjacent to the Carthage-Colton sion at ca. 1080–1020 Ma (the Ottawan orogeny). These events shear zone in northwest New York state. represent two episodes of contraction, possibly separated by extensional events, which are recorded by magmatic activity and Robertson Lake Shear Zone emplacement of large anorthosite complexes during these peri- ods (McLelland et al., 1988; Davidson, 1995). The Robertson The Robertson Lake shear zone is a multiply active, east- Lake shear zone and the Carthage-Colton shear zone separate dipping shear zone separating the eastern Elzevir terrane (Maz- rocks showing a marked discontinuity in metamorphic age, and inaw domain) and the western Frontenac terrane (Sharbot Lake so are fundamental boundaries separating blocks with distinct domain) within the Metasedimentary Belt (Fig. 1; Easton, metamorphic histories. From the Metasedimentary Belt bound- 1988). The latest episode of motion recorded along this zone is ary thrust zone east to the Robertson Lake shear zone (encom- down-to-the-east, as shown by shear-sense indicators including passing the Bancroft and Elzevir terranes), metamorphic S-C, C–C′ fabrics, with these crystal plastic structures crosscut minerals record geochronological evidence of metamorphism by brittle fabrics delineating an uplift history during deforma- during the latest contractional event during the Ottawan orogeny. tion (Busch and van der Pluijm, 1996). Its early history records The Frontenac terrane (including the Adirondack Low- a transpressive event at ca. 1030 Ma, indicating imbrication via lands) from east of the Robertson Lake shear zone to the sinistral transpression of the Mazinaw and the Sharbot Lake Carthage-Colton shear zone shows evidence of metamorphism domains (Busch et al., 1997). 40Ar-39Ar cooling ages of horn- related to the earlier Elzevirian orogeny. It does not, however, blendes and micas in the Robertson Lake shear zone show a appear to record metamorphic ages related to the Ottawan marked difference across the zone. Combined with the structural orogeny, indicating that this terrane was either laterally sepa- information and the nature of the offset, extensional motion had rated from the Elzevir terrane during this period or was at shal- to have occurred to juxtapose the crustal blocks on either side of 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 4971225 372600 RVL20A 1205 Ma n.d. Sharbot Lake 4973050 367025 CDN58 947 Ma n.d. Elzevir/Mazinaw 4968970 356050 MTG72 952 Ma n.d. Elzevir/Mazinaw 4957060 355300

From Busch and van der Pluijm (1996) Northing Easting RVL118B n.d. 969 Ma Sharbot Lake 4971225 372600 LVT130B n.d. 901 Ma Elzevir/Mazinaw 4990750 361920

From Streepey et al. (2000) and unpublished data A102 990 Ma n.d. Lowlands Popple Hill Formation, near Russell. A112 1055 Ma 940 Ma Lowlands Near Gouverneur, Hwy 58/812, N Jct. of Poplar Hill Rd. A114 990 Ma n.d. Lowlands Along Grass River, Hwy 17, W side of road, 1.5 km NW of Russell. A117 967 Ma n.d. Highlands 0.5 mi. E of Rt. 27 and Brouser’s Corner. A124 1043 Ma n.d. Lowlands Emoryville Rd., ca. 0.5 mi. NW of power plant, ca 0.25 mi. W of turnoff to Talcville. A125 1066 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. A128 1020 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. A129 1079 Ma 924 Ma Lowlands S. of Trout Lake, Rt. 19 to Edwards, ca. due W of westernmost extreme of Cedar Lake. A133 975 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. A134 972 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. A135 1165 Ma n.d. Highlands Just N of Sterling Rd., near power line, about 150 from junction with Joe Indian Rd. A136 943 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. A137 1005 Ma n.d. Highlands Rt. 56, about 3 mi. S of Stark. W side of hwy, roadcut of amphibolite migmatites. A138 983 Ma n.d. Highlands Small outcrop on E side of Hwy 56, about 1 mi. S of 137, about 4 mi. S of Stark. A140 947 Ma n.d. Highlands Hwy 3, just E of Pitcairn. Exposures on embankment N of the road, at crest of hill. A142 953 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). A145 935 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. SE596-49 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. CN596-56 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. PP596-60 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-1 1000 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. DH98-1 998 Ma n.d. Within CCSZ North 44 deg., 22 min. West 75 deg., 10 min. RWH-1 973 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. CR3-87 941 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. DH2-8 1012 Ma n.d. Within CCSZ Directly across Rt. 87 from CR3-87. CR6-A4 1005 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. Exhumation of a collisional orogen 397

the shear zone (Busch et al., 1996b; Fig. 3). The timing of the 900 Ma biotite ages in the Mazinaw domain (Busch and van der transition from compression to extension can be somewhat con- Pluijm, 1996). At the time of biotite closure, rocks were at fairly strained by the age of the latest transpressive event in the region shallow crustal depths of ∼10 to 12 km assuming an average at ca. 1030 Ma (Busch et al., 1997), but cannot be directly deter- geothermal gradient. Because of the offsets in cooling ages of mined. The termination of extension across the Robertson Lake the rocks that are presently exposed at the surface crustal level, shear zone cannot be constrained from 40Ar-39Ar analyses of it is clear that extensional activity did not terminate across the hornblende and biotite, as both show differences of ca. 70–100 Robertson Lake shear zone until sometime after 900 Ma. Ma across the zone. Results from the analysis of K-feldspar in The 40Ar-39Ar ages of K-feldspars give some information the region suggest that the entire block was uplifting uniformly on the termination of extension along the Robertson Lake shear by 780 Ma, suggesting termination of extension between 900 zone, but do not completely constrain it. Unlike hornblendes or and 780 Ma (Streepey et al., 2002). micas, which are considered to have one diffusion domain and Ages taken from Busch and van der Pluijm (1996), Busch therefore a single closure temperature, K-feldspars are thought et al. (1996b, 1997), and Streepey et al. (2002) are compiled in to have multiple diffusion domains and therefore multiple clo- Figure 3. These data constrain the cooling history of the terrane sure temperatures (Lovera et al., 1991). Analysis of K-feldspars from immediately after orogenesis in the Robertson Lake shear gives, instead of a single age, a temperature-time path for the zone area to the time at which the terrane was uplifting as a uni- grain. Thermal modeling of K-feldspar spectra from the Mazi- form block. In addition, the geometry of normal fault motion naw and the Sharbot Lake domains show that the two domains and the amount of displacement along the shear zone are evident were juxtaposed by at least 780 Ma, meaning that the termina- (Busch et al., 1996a). The U-Pb ages from titanites document tion of extension across the Robertson Lake shear zone must the difference in metamorphic ages between the Mazinaw and have occurred between 900 Ma and 780 Ma (Streepey et al., the Sharbot Lake domains. As the closure temperature of titan- 2002). Thus, postorogenic extension across the Robertson Lake ite is ca. 600–700 °C (Mezger et al., 1991a; Scott and St-Onge, shear zone terminated between 140 and 260 m.y. after the final 1995), titanite ages give the timing of metamorphism or a cool- expression of contractional tectonics in the area at ca. 1040 Ma. ing age that is very close to the age of metamorphism in the amphibolite to granulite-facies rocks of the Elzevir and the CARTHAGE-COLTON SHEAR ZONE Frontenac terranes. Metamorphism in the Sharbot Lake domain (the hangingwall block of the Robertson Lake shear zone) The Carthage-Colton shear zone separates the eastern Fron- occurred at ca. 1140 to 1170 Ma (Mezger et al., 1993; Corfu and tenac terrane (Adirondack Lowlands) from the Granulite Ter- Easton, 1995; Busch et al., 1997). These ages are generally sim- rane (Adirondack Highlands; Fig. 1). It is ∼150 km east of ilar to those across the entire width of the Frontenac terrane the Robertson Lake shear zone and dips to the west, toward the (Mezger et al., 1993). However, the youngest metamorphic ages Robertson Lake shear zone. With this geometry, the Frontenac in the Mazinaw domain (the footwall block of the Robertson terrane is a grabenlike block bounded by two shear zones dip- Lake shear zone) range from ca. 1010 Ma to 1050 Ma (Mezger ping toward one another. Upper amphibolite-facies marbles et al., 1993; Busch et al., 1997), showing a ca. 100-m.y. differ- and other metasediments dominate the Adirondack Lowlands ence in the timing of the latest metamorphic event across this lithologies, whereas the Adirondack Highlands are comprised boundary. Transpressional activity is interpreted to have caused predominantly of granulite-facies metaigneous assemblages. juxtaposition through imbrication of the two terranes at ca. 1030 The Carthage-Colton shear zone crops out as a zone of intense Ma (Busch et al., 1997). deformation between the two terranes, although the exact loca- The 40Ar-39Ar cooling ages of hornblendes, micas, and K- tion of the boundary has been debated (Geraghty et al., 1981). feldspars constrain the post-titanite cooling and unroofing his- The Carthage-Colton shear zone had a long history of activ- tory of the blocks of crust adjacent to the Robertson Lake shear ity over the duration of the Grenville orogenic cycle. The zone and constrain the timing and nature of postorogenic exten- Adirondack Lowlands and Highlands both appear to have been sion along this zone. Hornblende ages for rocks in the vicinity affected by the ca. 1190 Ma arc-continent collision at the end of of the Robertson Lake shear zone are shown in Figure 3. The the Elzevirian orogeny (Mezger et al., 1991a, 1992; Wasteneys offset in ages shown by the titanite U-Pb geochronology is also et al., 1999; Kusky and Lowring, 2001). However, only the shown by the cooling ages of hornblende, with hornblende ages Adirondack Highlands appear to have been pervasively meta- of ca. 1010 Ma in the Sharbot Lake domain (hangingwall), morphosed by the granulite-facies Ottawan orogeny, which has which are at least 60 m.y. older than rocks immediately across been dated at 1090–1040 Ma (McLelland et al., 1996). The the Robertson Lake shear zone in the Mazinaw domain, where entire Frontenac terrane, from east of the Robertson Lake shear hornblende ages are ca. 950 Ma (Busch and van der Pluijm, zone to just west of the Carthage-Colton shear zone, appears to 1996; Busch et al., 1996a). Biotite, which closes to the K-Ar sys- have escaped widespread thermal metamorphism and resetting tem at ca. 300 °C, continues to show an offset across the Robert- of isotopic ages from this pervasive deformational event, either son Lake shear zone, with biotites in the Sharbot Lake domain by being at shallower crustal levels during that period or by recording ages of 970 Ma that are at least 70 m.y. older than the being laterally separated. Figure 3. Maps showing hornblende, biotite, and K- feldspar 40 Ar-39 Ar 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 defor- mation 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—Meta- sedimentary Belt Boundary Zone; MSZ—Moorton shear zone; MT—Morin terrane; RW—field outcrop name; SLSZ—Sharbot Lake shear zone. Exhumation of a collisional orogen 399

McLelland et al. (1996) proposed that the Carthage-Colton was uplifting as a uniform block by ca. 780 Ma (Heizler and shear zone was the locus for extensional collapse of the Ottawan Harrison, 1998; Streepey et al., 2002). orogen at ca. 1050 Ma, which resulted in exhumation of the high-grade core of the Adirondack Highlands, presumably while NUMERICAL MODELING the orogen was still under a contractional stress field and as a direct result of the orogenic event, either through gravitational Geochronology paired with structural geology shows that collapse or through mantle delamination beneath the orogen. extensional motion took place along a large segment of the Streepey et al. (2001) suggested that the Carthage-Colton shear Metasedimentary Belt well after the Grenville contractional zone was involved in transpressive deformation similar to that orogeny, during a period that was considered to be relatively qui- documented across the Robertson Lake shear zone at ca. 1040 escent. Application of 40Ar-39Ar and U-Pb geochronology Ma. It is clear that the Carthage-Colton shear zone was active details the kinematics and timing of this transition and the during or immediately after the latest episode of Grenville con- amount and nature of extensional deformation that occurred traction. In addition, cooling ages from hornblendes and biotites after it, but gives few constraints on the mechanisms that pro- show that the Carthage-Colton shear zone was reactivated in an duced a regional extensional event during this period. extensional regime similar to that observed along the Robertson Extension occurred at least 100 m.y. after the latest con- Lake shear zone. tractional event in the Grenville Province of New York and We present sixteen new 40Ar-39Ar hornblende analyses southeastern Ontario. Given this timescale, it is difficult to make from the University of Michigan Radiogenic Isotope Laboratory a causal link between orogenesis and extension. Whereas sev- combined with published 40Ar-39Ar hornblende and biotite ages eral attempts have been made to create a Himalayan analog to near and along the Carthage-Colton shear zone (Streepey et al., the Grenville Province (e.g., Windley, 1986), a component of 2000, 2001). Standard operating procedures for the collection of major extension in the latter is clearly postorogenic, so the hornblende and biotite analyses in this laboratory are described timescales for the two are different. In Tibet, for example, grav- in detail in Streepey et al. (2000). Our results are shown in Fig- itational potential energy due to the elevated topography of the ure 3, and isotopic data are presented in Table 2.1 Plateaus were plateau played a major role in driving collapse (e.g., Shen et al., defined as occurring where 50% or more of the total 39Ar was 2001). However, in the Grenville Province the duration and ter- released in three or more consecutive steps and where the ages mination of this extensional event were so much later than com- of the steps overlapped at the 2σ level of error. pression that gravitational collapse became an ineffective The hornblende ages in the Adirondack Lowlands of the driving mechanism. Earlier synorogenic extensional events (as Frontenac terrane show that these locations within this slice of proposed by McLelland et al., 2001, and references therein) at crust reached 500 °C at ca. 1036 Ma (Fig. 3; Table 2). Horn- ca. 1050 Ma were different in nature and almost certainly can be blende ages across the Carthage-Colton shear zone are ∼947–983 ascribed to processes such as gravitational collapse, mantle Ma. The offset in ages indicates active movement along the delamination, or some combination of these driving mecha- Carthage-Colton shear zone after hornblendes closed to the K- nisms. The exhumation of the Grenville orogen was not solely Ar system, or after 950 Ma. The nature of the offset, combined the result of erosional and isostatic processes, but was due at with the regional fabrics, indicates that this motion must have least in part to active extension along shear zones, which led to been extensional (Heyn, 1990; Streepey et al., 2000). Some uplift of the region. Geochronologic data have allowed con- hornblendes in the Dana Hill metagabbro tightly cluster at 945 struction of a kinematic model of denudation and unroofing of Ma. Though there is some textural complexity in these samples, midcrustal levels of the orogenic belt. When uplift is discussed with young ages coming from a variety of veins and other tex- in the context of this paper, we refer to the exhumation of the tures, these ages fit well within the regional framework of exten- core of the Grenville Province and do not constrain a measure sion (bracketed by regional hornblende and biotite 40Ar-39Ar of the paleotopographic surface. ages) and indicate that this was likely a time of deformation A model investigation has been made of the evolution of a along the Carthage-Colton shear zone (Streepey et al., 2001). block of overthickened crust, with three common driving mech- Biotite 40Ar-39Ar ages are ca. 900–930 Ma on both sides of the anisms proposed to explain postorogenic extension. We have shear zone, indicating that this block of crust was uniformly evaluated the driving forces necessary to generate uplift and uplifting by this time (Streepey et al., 2000). K-feldspar ages are extension that match the kinematics of deformation in the similar to those found in the Robertson Lake shear zone area, Grenville Province as documented through field and laboratory further supporting the idea that the entire Metasedimentary Belt 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 1GSA Data Repository item 2004059, Appendix, hornblende spectra, is avail- along the Robertson Lake shear zone versus the Carthage- able on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA, [email protected], or at www.geosociety.org/pubs/ Colton shear zone detailed in the geochronology (see earlier and ft2004.htm. Fig. 3). TABLE 2. HORNBLENDE ARGON DATA 39 39 40 39 37 39 36 39 40 39 40 σ Power Ar frac Ar mol Ar/ Ar Ar/ Ar Ar/ Ar Ar*/ ArK % Ar atmos Ca/K Age (Ma) 1 error (Ma) A112-1 400 0.17977 1.33E-15 82.67166 3.62469 0.01034 79.6151 3.69723 6.65081 1044 2 401 0.12911 9.53E-16 82.13706 3.8956 0.00050 81.9893 0.17990 7.14789 1068 2 402 0.04571 3.37E-16 80.99802 3.88507 –0.00025 81.0706 –0.08961 7.12857 1059 3 420 0.12908 9.52E-16 80.80290 3.97513 –0.00010 80.8318 –0.03576 7.29382 1056 2 440 0.04952 3.65E-16 79.80750 3.82902 –0.00050 79.9564 –0.18657 7.02572 1048 3 460 0.06496 4.79E-16 80.32021 3.94636 –0.00069 80.5247 –0.25459 7.24103 1053 3 480 0.01616 1.19E-16 77.82671 3.64937 –0.00247 78.5576 –0.93913 6.69609 1034 9 560 0.08421 6.21E-16 79.86897 3.63473 0.00025 79.7954 0.09212 6.66923 1046 3 640 0.10076 7.43E-16 80.25100 4.00081 0.00004 80.2387 0.01532 7.34094 1050 3 720 0.03772 2.78E-16 79.24898 3.52251 0.00293 78.3825 1.09337 6.46332 1032 6 800 0.03863 2.85E-16 81.53749 4.13691 0.00093 81.2615 0.33849 7.59066 1061 5 880 0.01055 7.78E-17 80.54386 4.17951 0.00409 79.3344 1.50162 7.66883 1041 14 1000 0.06427 4.74E-16 80.99230 3.91662 0.00074 80.7727 0.27114 7.18646 1056 3 1040 0.02004 1.48E-16 78.72645 3.90584 –0.00173 79.2368 –0.64826 7.16668 1040 8 4800 0.02954 2.18E-16 80.08239 3.91963 –0.00173 80.5944 –0.63935 7.19198 1054 6 J value 9.84667E-03 ± 1.38181E-05 Total 39 K vol = 2.12529E-10 CCNTP/G Total gas age = 1051.97 ± 1.39196 Ma

A114-2 360 0.00871 6.42E-17 437.13157 4.65082 1.19085 85.2341 80.5015 8.53361 1091 56 400 0.00357 2.63E-17 84.83118 2.64341 0.06967 64.2452 24.2670 4.85029 877 71 440 0.01623 1.20E-16 78.79309 2.49256 0.05687 61.9881 21.3280 4.57350 853 17 480 0.03422 2.53E-16 75.25700 2.55393 0.00739 73.0722 2.90312 4.68611 970 8 520 0.07799 5.75E-16 76.43780 2.69612 0.00273 75.6316 1.05471 4.94701 996 4 560 0.10642 7.85E-16 76.70364 2.71598 0.00102 76.4032 0.39169 4.98345 1004 3 600 0.19840 1.46E-15 76.44511 2.70967 0.00100 76.1492 0.38709 4.97187 1001 2 640 0.06504 4.80E-16 75.72533 2.70733 0.00017 75.6755 0.06580 4.96758 997 4 680 0.05337 3.94E-16 74.36980 2.69303 0.00144 73.9429 0.57402 4.94134 979 4 720 0.14419 1.06E-15 75.23356 2.76182 0.00045 75.1018 0.17513 5.06756 991 3 760 0.00258 1.90E-17 74.30312 2.82691 0.00236 73.6062 0.93794 5.18699 976 57 840 0.20909 1.54E-15 75.25175 2.73198 0.00072 75.0377 0.28445 5.01281 990 2 880 0.04428 3.27E-16 75.13989 2.7655 0.00005 75.1238 0.02142 5.07431 991 6 2000 0.03130 2.31E-16 77.06652 3.00146 0.00076 76.8428 0.29030 5.50727 1008 6 5000 0.00462 3.41E-17 80.45262 3.77874 0.03093 71.3128 11.3605 6.93347 952 40 J value 9.74593E-03 ± 1.69588E-05 Total 39 K vol = 1.50293E-10 CCNTP/G Total gas age = 992.403 ± 1.77968 Ma

A117-1R 400 0.06297 4.65E-16 113.32355 5.04689 0.04675 99.5093 12.1901 9.26035 1226 3 440 0.13070 9.64E-16 74.12110 4.74351 0.00057 73.9514 0.22895 8.70369 982 3 480 0.17212 1.27E-15 74.65890 4.74913 0.00060 74.4807 0.23869 8.7140 987 2 520 0.10834 7.99E-16 73.44085 4.73651 0.00155 72.9816 0.62534 8.69084 972 3 560 0.08989 6.63E-16 71.93891 4.7711 0.00197 71.3563 0.80987 8.75431 955 4 600 0.05587 4.12E-16 71.29738 4.84565 0.00070 71.0905 0.29017 8.89110 952 5 640 0.04701 3.47E-16 70.75044 4.83448 0.00259 69.9852 1.08161 8.87061 941 4 680 0.02548 1.88E-16 70.30706 4.75113 0.00502 68.8222 2.11197 8.71767 929 4 760 0.08086 5.97E-16 74.37964 4.80444 0.00205 73.7752 0.81265 8.81549 980 3 880 0.04753 3.51E-16 72.89371 4.88858 0.00407 71.6920 1.64858 8.96987 959 6 1000 0.13615 1.00E-15 73.20374 5.31663 0.00025 73.1300 0.10074 9.75528 973 2 2000 0.04257 3.14E-16 73.61771 5.22413 0.00348 72.5902 1.39574 9.58556 968 4 4800 0.00053 3.93E-18 98.69721 5.98711 0.08950 72.2505 26.7958 10.98552 964 390 J value 9.78019E-03 ± 1.65238E-05 Total 39 K vol = 1.44533E-10 CCNTP/G Total gas age = 987.869 ± 1.61561 Ma

A124-1R 360 0.03019 2.23E-16 125.33636 4.21346 0.04743 111.3220 11.1814 7.73112 1338 5 400 0.17033 1.26E-15 80.50973 3.98635 0.00127 80.1340 0.46669 7.31440 1051 2 401 0.10968 8.09E-16 80.73303 3.89882 0.00148 80.2960 0.54133 7.15380 1053 3 402 0.04250 3.14E-16 81.25107 3.85999 0.00243 80.5319 0.88512 7.08255 1055 5 420 0.01263 9.32E-17 79.31704 3.95845 0.01442 75.0558 5.37241 7.26321 1000 14 460 0.21815 1.61E-15 81.20917 3.91409 0.00086 80.9539 0.31434 7.18182 1060 2 461 0.08099 5.98E-16 81.15436 3.89173 0.00176 80.6357 0.63911 7.14079 1056 3 480 0.10678 7.88E-16 81.52008 3.8357 0.00041 81.3986 0.14902 7.03798 1064 2 520 0.04500 3.32E-16 80.23726 3.91163 0.00240 79.5267 0.88558 7.17730 1045 4 560 0.02183 1.61E-16 79.20467 4.03858 0.00249 78.4681 0.92995 7.41024 1035 8 600 0.00773 5.70E-17 77.39229 4.28706 0.00899 74.7364 3.43173 7.86617 997 22 640 0.01163 8.58E-17 78.55942 4.56397 0.00192 77.9916 0.72279 8.37426 1030 14 720 0.03076 2.27E-16 79.95281 4.34586 –0.00010 79.9817 –0.03614 7.97406 1050 6 800 0.02660 1.96E-16 80.78376 4.17887 0.00348 79.7551 1.27335 7.66765 1048 8 1200 0.05685 4.19E-16 81.00487 4.0396 0.00053 80.8471 0.19477 7.41211 1058 4 4000 0.02837 2.09E-16 81.85268 4.09907 –0.00085 82.1048 –0.30802 7.52123 1071 6 J value 9.87089E-03 ± 1.20594E-05 Total 39 K vol = 2.58136E-10 CCNTP/G Total gas age = 1063.43 ± 1.33794 Ma

A125-1 400 0.25390 1.87E-15 84.45171 3.90485 0.00647 82.5402 2.26343 7.16486 1056 2 401 0.07938 5.86E-16 84.04833 3.75689 0.00251 83.3075 0.88143 6.89338 1063 4 440 0.13038 9.62E-16 83.99765 3.72575 0.00136 83.5946 0.47983 6.83624 1066 3 480 0.10856 8.01E-16 83.76073 3.70631 0.00069 83.5557 0.24478 6.80057 1065 3 520 0.22332 1.65E-15 84.00499 3.8024 0.00102 83.7027 0.35984 6.97688 1067 2 560 0.08583 6.33E-16 85.40903 3.84262 0.00098 85.1192 0.33935 7.05068 1080 4 600 0.00564 4.16E-17 77.55969 4.87365 0.00266 76.7725 1.01495 8.94248 999 34 680 0.00783 5.77E-17 84.75936 4.43573 –0.01303 88.6097 –4.54267 8.13895 1113 29 820 0.01984 1.46E-16 83.30916 4.05837 –0.00825 85.7468 –2.92602 7.44655 1086 14 1000 0.01869 1.38E-16 83.44706 4.02577 –0.00840 85.9295 –2.97487 7.38673 1088 14 4800 0.06665 4.92E-16 83.66405 4.09005 0.00077 83.4373 0.27103 7.50468 1064 4 J value 9.63302E-03 ± 1.87146E-05 Total 39 K vol= 1.26569E-10 CCNTP/G Total gas age = 1065.08 ± 1.92921 Ma

A128-1 360 0.02003 1.48E-16 103.37114 0.86044 0.05595 86.8393 15.9927 1.57879 1114 5 380 0.00976 7.20E-17 45.93718 0.55937 0.00118 45.5872 0.76186 1.02636 668 14 420 0.00899 6.63E-17 85.67338 1.20975 0.00381 84.5478 1.3138 2.21972 1092 13

(continued) TABLE 2. Continued 39 39 40 39 37 39 36 39 40 39 40 σ Power Ar frac Ar mol Ar/ Ar Ar/ Ar Ar/ Ar Ar*/ ArK % Ar atmos Ca/K Age (Ma) 1 error (Ma) 460 0.01472 1.09E-16 95.15639 2.10311 0.00229 94.4805 0.71029 3.85892 1186 11 500 0.06145 4.53E-16 83.47260 4.65351 0.00067 83.2755 0.23613 8.53855 1080 2 520 0.16436 1.21E-15 78.39011 4.93948 0.00060 78.2127 0.22632 9.06327 1029 1 530 0.06040 4.46E-16 78.09354 4.64889 0.00062 77.9101 0.23490 8.53007 1026 2 540 0.02521 1.86E-16 71.34198 3.67143 0.00056 71.1764 0.23209 6.73657 957 5 560 0.04059 2.99E-16 75.79147 4.50351 0.00002 75.7861 0.00709 8.26332 1005 3 620 0.10396 7.67E-16 76.91635 4.80667 0.00048 76.7755 0.18312 8.81958 1015 2 660 0.11611 8.57E-16 78.49321 4.95174 0.00055 78.3297 0.20831 9.08576 1031 2 720 0.11533 8.51E-16 79.95906 5.11697 0.00065 79.7680 0.23895 9.38894 1045 2 780 0.08676 6.40E-16 80.74414 4.89144 0.00065 80.5511 0.23908 8.97512 1053 2 1000 0.06995 5.16E-16 80.56078 4.96834 0.00099 80.2677 0.36380 9.11622 1050 2 4000 0.10239 7.55E-16 66.01429 3.44906 0.00134 65.6196 0.59788 6.32855 899 1 J value 9.83709E-03 ± 1.44148E-05 Total 39 K vol = 2.63518E-10 CCNTP/G Ttotal gas age = 1022.11 ± 1.28867 Ma

A129-1 360 0.02336 1.72E-16 97.70120 6.7107 0.05614 81.1131 16.9784 12.31321 1044 11 500 0.56485 4.17E-15 84.98144 6.63161 0.00053 84.8238 0.18550 12.16809 1079 2 540 0.09031 6.66E-16 84.37694 6.5040 –0.00120 84.7317 –0.42045 11.93394 1079 4 580 0.10879 8.03E-16 85.28425 6.53173 0.00166 84.7932 0.57578 11.98483 1079 3 620 0.02093 1.54E-16 85.02355 6.49323 0.00091 84.7555 0.31527 11.91418 1079 13 1000 0.12513 9.23E-16 82.43000 6.78991 0.00165 81.9417 0.59238 12.45855 1052 3 2000 0.06351 4.69E-16 84.95944 7.47248 0.00029 84.8723 0.10257 13.71097 1080 4 4800 0.00313 2.31E-17 79.77944 6.68513 –0.00745 81.9811 –2.75968 12.26629 1052 88 J value 9.65675E-03 ± 1.79665E-05 Total 39 K Vol= 1.17160E-10 CCNTP/G Total gas age = 1075.01 ± 2.10352 Ma

A133-1 400 0.10466 7.72E-16 78.38483 3.18793 0.00641 76.4909 2.41619 5.84941 1001 2 440 0.28683 2.12E-15 73.06987 3.12962 0.00103 72.7641 0.41847 5.74242 963 1 441 0.08772 6.47E-16 73.68601 3.14167 0.00127 73.3109 0.50907 5.76453 968 3 460 0.02532 1.87E-16 73.45835 3.18647 0.00229 72.7827 0.91977 5.84673 963 8 500 0.04963 3.66E-16 74.45830 3.22276 0.00102 74.1561 0.40586 5.91332 977 6 540 0.11704 8.64E-16 74.81581 3.20948 0.00134 74.4197 0.52945 5.88895 980 3 580 0.04971 3.67E-16 74.77719 3.13684 0.00234 74.0855 0.9250 5.75567 976 5 620 0.01130 8.34E-17 71.75809 3.13448 0.01051 68.6532 4.32688 5.75134 920 18 700 0.12656 9.34E-16 73.38544 3.21055 0.00100 73.0899 0.40273 5.89092 966 2 780 0.04513 3.33E-16 74.21991 3.26527 0.00039 74.1052 0.15456 5.99132 976 4 880 0.04618 3.41E-16 74.49200 3.20771 0.00037 74.3841 0.14485 5.88571 979 4 1000 0.03645 2.69E-16 73.88380 3.81057 –0.00254 74.6357 –1.01768 6.99187 982 6 1200 0.00925 6.83E-17 74.92031 3.98344 0.00117 74.5737 0.46264 7.30906 981 29 5000 0.00423 3.12E-17 74.50266 3.69954 –0.01149 77.8975 –4.55667 6.78815 1015 33 J value 9.69066E-03 ± 1.73722E-05 Total 39 K vol = 1.89216E-10 CCNTP/G Total gas age = 972.995 ± 1.63359 Ma A134-1 360 0.05806 4.28E-16 88.65932 3.11553 0.02434 81.4682 8.11096 5.71657 1061 3 380 0.08990 6.63E-16 74.67086 2.89451 0.00067 74.4739 0.26378 5.31103 991 2 400 0.13506 9.97E-16 72.87802 2.93911 0.00036 72.7716 0.14602 5.39286 973 2 420 0.13788 1.02E-15 72.07033 2.91703 –0.00002 72.0766 –0.00871 5.35235 966 2 430 0.07872 5.81E-16 72.99311 2.94646 0.00062 72.8086 0.25278 5.40635 973 2 440 0.06587 4.86E-16 72.61572 2.93576 0.00075 72.3934 0.30617 5.38672 969 3 460 0.08062 5.95E-16 72.72679 2.93459 0.00137 72.3216 0.55714 5.38457 968 2 480 0.05728 4.23E-16 72.60468 2.93469 0.00191 72.0408 0.77664 5.38475 966 4 500 0.06910 5.10E-16 73.46718 2.96128 0.00186 72.9177 0.74792 5.43354 975 2 520 0.03623 2.67E-16 72.74153 2.89763 0.00251 72.0009 1.01816 5.31675 965 4 560 0.02905 2.14E-16 72.46758 2.99739 0.00192 71.8994 0.78405 5.49980 964 5 600 0.01910 1.41E-16 70.95202 3.05735 0.00210 70.3310 0.87527 5.60982 948 6 680 0.03939 2.91E-16 72.01860 3.25967 0.00347 70.9928 1.42436 5.98105 955 3 760 0.03514 2.59E-16 71.75096 3.20298 0.00185 71.2048 0.76118 5.87703 957 4 840 0.01835 1.35E-16 72.69965 3.60576 0.00535 71.1194 2.17367 6.61607 956 6 4000 0.05026 3.71E-16 72.23752 4.28732 0.00242 71.5225 0.98982 7.86664 960 4 J value 9.82456E-03 ± 1.50837E-05 Total 39 K vol = 3.12587E-10 CCNTP/G Total gas age = 974.680 ± 1.34455 Ma

A135-1 360 0.00480 3.54E-17 189.06855 3.53975 0.38769 74.5070 60.5926 6.49495 996 29 420 0.03215 2.37E-16 97.21990 2.83047 0.00822 94.7920 2.49733 5.19352 1193 6 460 0.10836 8.00E-16 94.16484 2.76105 0.00110 93.8398 0.34519 5.06615 1184 2 520 0.28549 2.11E-15 92.27292 2.80599 0.00096 91.9885 0.30824 5.14861 1167 1 540 0.17551 1.30E-15 91.59555 2.83532 0.00085 91.3458 0.27267 5.20242 1161 2 560 0.07382 5.45E-16 93.82666 2.84512 0.00231 93.1434 0.72821 5.22040 1178 3 620 0.14952 1.10E-15 91.60731 2.84698 0.00091 91.3373 0.29475 5.22382 1161 2 660 0.06962 5.14E-16 91.92043 2.86217 –0.00076 92.1449 –0.24420 5.25169 1168 3 720 0.03631 2.68E-16 91.53615 2.89577 –0.00059 91.7105 –0.19047 5.31334 1164 6 840 0.04446 3.28E-16 92.45542 2.94274 0.00034 92.3555 0.10808 5.39952 1170 5 4000 0.01996 1.47E-16 92.06961 3.09668 0.00578 90.3630 1.85361 5.68198 1152 9 J value 9.88469E-03 ± 1.11152E-05 Total 39 K vol= 1.54303E-10 CCNTP/G Total gas age = 1167.36 ± 1.24451 Ma

A136-1 360 0.0290 2.14E-16 107.51473 3.13684 0.05980 89.8435 16.4361 5.75567 1149 8 400 0.08390 6.19E-16 70.25276 3.13401 0.00122 69.8912 0.51466 5.75048 950 4 420 0.16589 1.22E-15 69.97159 3.07525 0.00036 69.8666 0.15005 5.64266 950 2 440 0.2397 1.77E-15 69.01690 3.14081 0.00033 68.9185 0.14258 5.76295 939 1 450 0.07365 5.43E-16 68.46276 3.10302 0.00155 68.0044 0.66950 5.69361 930 5 480 0.04626 3.41E-16 68.16077 3.18853 0.00122 67.8009 0.52798 5.85051 928 5 520 0.08498 6.27E-16 67.39505 3.13191 0.00063 67.2088 0.27636 5.74662 921 2 560 0.02044 1.51E-16 67.85553 3.2227 0.01155 64.4438 5.02793 5.91321 891 20 580 0.01323 9.76E-17 67.22698 3.08304 –0.00313 68.1531 –1.3776 5.65695 931 15 640 0.03406 2.51E-16 70.51416 3.1958 0.00225 69.8502 0.94159 5.86385 949 7 800 0.12837 9.47E-16 68.48829 3.14145 0.00091 68.2200 0.39174 5.76413 932 2 880 0.02252 1.66E-16 67.33619 3.24219 0.00315 66.4059 1.38156 5.94897 913 9

(continued) TABLE 2. Continued 39 39 40 39 37 39 36 39 40 39 40 σ Power Ar frac Ar mol Ar/ Ar Ar/ Ar Ar/ Ar Ar*/ ArK % Ar atmos Ca/K Age (Ma) 1 error (Ma) 4000 0.0580 4.28E-16 69.01521 3.22167 0.00061 68.8346 0.2617 5.91132 939 4 J value 9.91445E-03 ± 1.20341E-05 Total 39 K vol = 2.21028E-10 CCNTP/G Total gas age = 943.299 ± 1.37251 Ma

A137-2 360 0.07434 5.49E-16 79.54528 2.6555 0.00766 77.2828 2.84427 4.87248 1010 3 380 0.18242 1.35E-15 76.64371 2.56293 0.00054 76.4855 0.20642 4.70262 1002 2 400 0.29765 2.20E-15 77.58981 2.57025 0.00031 77.4988 0.11730 4.71606 1012 6 401 0.02462 1.82E-16 76.83948 2.49381 0.00419 75.6012 1.61151 4.57580 993 7 440 0.15378 1.13E-15 76.76473 2.56553 0.00078 76.5340 0.30057 4.70739 1002 1 460 0.11356 8.38E-16 76.79623 2.55097 0.00117 76.4516 0.44876 4.68068 1002 3 480 0.03633 2.68E-16 77.37406 2.59891 0.00191 76.8109 0.72784 4.76864 1005 3 500 0.01628 1.20E-16 76.30115 2.4661 0.00684 74.2795 2.64957 4.52495 980 9 560 0.02836 2.09E-16 76.61391 2.58639 0.00493 75.1571 1.90149 4.74567 989 6 800 0.05065 3.74E-16 76.83979 2.70719 0.00161 76.3642 0.61893 4.96732 1001 4 4000 0.02203 1.63E-16 77.06461 3.48322 0.00603 75.2818 2.3134 6.39123 990 7 J value 9.70862E-03 ± 1.72082E-05 Total 39 K vol = 2.77185E-10 CCNTP/G Total gasage= 1004.41 ± 2.31283 Ma

A138-1 360 0.00341 2.51E-17 80.22103 3.86201 0.07427 58.2728 27.3597 7.08626 812 38 400 0.01336 9.86E-17 75.38258 2.98881 0.00238 74.6796 0.93255 5.48406 987 11 420 0.13042 9.62E-16 74.15896 2.96001 0.00045 74.0267 0.17835 5.43121 980 2 440 0.10992 8.11E-16 74.45032 2.96696 0.00062 74.2657 0.24798 5.44396 983 2 460 0.10226 7.55E-16 74.44556 2.98933 –0.00039 74.5612 –0.15534 5.48501 986 2 480 0.08788 6.48E-16 74.58835 3.00933 0.00059 74.4137 0.23416 5.52171 984 3 500 0.11664 8.61E-16 74.50427 2.99731 0.00092 74.2331 0.36397 5.49965 983 2 520 0.07957 5.87E-16 74.44747 2.97613 –0.00003 74.4556 –0.01093 5.46079 985 3 560 0.20707 1.53E-15 74.33688 3.00375 0.00039 74.2214 0.15535 5.51147 982 1 580 0.02001 1.48E-16 73.73513 2.98427 –0.00007 73.7562 –0.02858 5.47572 978 7 640 0.08036 5.93E-16 74.68295 2.98614 0.00071 74.4746 0.27898 5.47916 985 1 4000 0.04910 3.62E-16 74.24016 3.0346 0.00125 73.8720 0.49591 5.56807 979 4 J value 9.75238E-03 ± 1.69034E-05 Total 39 K vol = 2.45945E-10 CCNTP/G Total gas age = 982.380 ± 1.49725 Ma

A140-1 360 0.04811 3.55E-16 64.05867 1.55391 0.00826 61.6178 3.81037 2.85121 856 4 400 0.15487 1.14E-15 70.53351 2.27729 0.00118 70.1852 0.49382 4.17851 949 1 420 0.07203 5.31E-16 70.73917 2.29034 0.00040 70.6197 0.16889 4.20246 953 2 440 0.12733 9.40E-16 70.67584 2.34235 0.00044 70.5457 0.18414 4.29789 952 3 460 0.05076 3.75E-16 70.53867 2.28231 –0.00001 70.5418 –0.00444 4.18772 952 3 500 0.03876 2.86E-16 70.67347 2.29519 0.00156 70.2130 0.65155 4.21136 949 6 540 0.10871 8.02E-16 72.45699 2.38892 0.00278 71.6342 1.13555 4.38334 964 2 600 0.13072 9.65E-16 71.07979 2.39188 0.00051 70.9285 0.21285 4.38877 956 3 640 0.09072 6.69E-16 71.10540 2.37214 0.00069 70.9027 0.28507 4.35255 956 3 720 0.07920 5.84E-16 70.81559 2.3225 0.00092 70.5424 0.38577 4.26147 952 3 1000 0.06688 4.93E-16 68.26967 2.13221 0.00043 68.1432 0.18525 3.91231 927 3 4000 0.03192 2.35E-16 66.52505 1.88886 0.00066 66.3294 0.29411 3.46580 908 6 J value 9.85806E-03 ± 1.30213E-05 Total 39 K vol = 2.00423E-10 CCNTP/G Total gas age = 946.275 ± 1.27061 Ma

A142-1 340 0.10645 7.85E-16 86.98428 2.63419 0.03012 78.0844 10.2316 4.83338 1023 2 360 0.00028 2.07E-18 83.87368 –1.95789 0.29163 –2.30401 102.7470 –3.59246 –41 717 440 0.00059 4.38E-18 87.43051 3.62755 –0.09082 114.2680 –30.6958 6.65606 1352 158 460 0.00183 1.35E-17 80.86233 3.13292 0.03457 70.6461 12.6341 5.74848 947 65 500 0.00211 1.55E-17 76.33558 2.88432 0.01508 71.8787 5.83853 5.29233 960 65 600 0.00280 2.07E-17 75.58747 3.20468 0.04974 60.8882 19.4467 5.88015 842 33 660 0.15283 1.13E-15 73.79989 2.59028 0.00183 73.2585 0.73359 4.75281 974 1 680 0.02791 2.06E-16 71.04472 2.62043 0.00324 70.0865 1.34876 4.80813 941 4 700 0.02705 2.00E-16 72.02760 2.60012 0.00257 71.2679 1.05474 4.77086 953 6 720 0.11147 8.22E-16 71.32644 2.56527 0.00112 70.9957 0.46370 4.70692 950 2 740 0.04120 3.04E-16 71.01069 2.5882 0.00227 70.3400 0.94449 4.74899 944 3 800 0.04011 2.96E-16 71.33549 2.55555 0.00076 71.1102 0.31581 4.68908 952 3 880 0.06211 4.58E-16 71.96983 2.61321 0.00125 71.6005 0.51317 4.79488 957 3 920 0.06160 4.55E-16 71.66352 2.55238 0.00195 71.0886 0.80225 4.68327 951 3 1000 0.08337 6.15E-16 70.81080 2.57033 0.00128 70.4329 0.53367 4.71620 945 2 1080 0.17788 1.31E-15 70.92116 2.57601 0.00037 70.8111 0.15518 4.72662 948 1 1160 0.08034 5.93E-16 70.87883 2.58045 0.00103 70.5750 0.42866 4.73477 946 2 4000 0.02009 1.48E-16 70.34298 2.60776 0.00529 68.7793 2.22294 4.78488 927 7 J value 9.76808E-03 ± 1.67233E-05 Total 39 K vol = 2.63802E-10 CCNTP/G Total gas age = 959.956 ± 1.43517 Ma

A145-1 360 0.00415 3.06E-17 394.93547 2.5978 –0.00141 395.3520 –0.10547 4.76661 2873 27 420 0.03778 2.79E-16 69.46958 2.57393 –0.00355 70.5180 –1.50918 4.72281 955 7 480 0.16300 1.20E-15 68.41997 2.65508 –0.00040 68.5377 –0.17207 4.87171 935 2 490 0.18985 1.40E-15 68.42456 2.65659 –0.00051 68.5766 –0.22220 4.87448 935 1 500 0.01403 1.04E-16 68.09059 2.5561 –0.00679 70.0984 –2.94874 4.69009 951 24 580 0.07373 5.44E-16 68.25978 2.6678 –0.00280 69.0863 –1.21084 4.89505 940 3 640 0.09774 7.21E-16 68.43730 2.63232 0.00040 68.3192 0.17257 4.82994 932 1 680 0.04941 3.65E-16 68.32619 2.64628 0.00041 68.2053 0.17693 4.85556 931 2 720 0.07108 5.24E-16 67.62825 2.60158 0.00048 67.4864 0.20974 4.77354 923 2 920 0.05431 4.01E-16 67.94276 2.63883 0.00075 67.7222 0.32463 4.84189 926 2 1000 0.17403 1.28E-15 68.09987 2.78113 0.00040 67.9816 0.17368 5.10299 929 1 4000 0.07089 5.23E-16 67.82803 2.77444 –0.00023 67.8951 –0.09889 5.09072 928 4 J value 9.90225E-03 ± 1.07859E-05 Total 39 K vol = 3.21601E-10 CCNTP/G Total gas age = 947.267 ± 1.11592 Ma 406 M.M. Streepey et al.

Model Setup shear zones and is subject to different types of stresses acting in the presence of gravity, such as symmetric extensional stresses Postorogenic extension is a feature commonly observed in that stretch the block and vertical stresses that simulate the pres- mountain belts, regardless of the age of the orogen (Dewey, ence of an upwelling or downwelling mantle. The latter is an 1988). Physical mechanisms proposed to explain this phenom- approximation of the dynamical effects of mantle convection, enon include (a) gravitational collapse; (b) changes in the stress particularly of the deformation of the lithosphere subject to such regime, perhaps induced by changes in plate motion that cause normal stresses. We have not included isostatic effects on topog- either reduced rates of convergence or a transition to extensional raphy that would have been derived from the density contrast stresses; (c) mantle-induced uplift and extension, including the between the upwelling or downwelling and the normal mantle. effects of mantle plumes or mantle delamination and slab break- We refer to them as bottom heating sources for convenience. off (e.g., Mareschal, 1994; Burg and Ford, 1997). Gravitational We have solved the equations of motion for an incompress- collapse acts on all mountain belts during orogenesis and tends ible elastic-plastic material via the finite element method, using to occur shortly afterward, but the timescales and maximum the general-purpose finite element code ABAQUS (version 6.2; amount of extension produced by gravitational collapse suggest Hibbit et al., 2000). We considered two-dimensional models of that it is unlikely to have been the primary driving force behind an overthickened block of crust with a geometry similar to that the observed extension. of the Grenville Province in this region at the peak of orogenesis. Changes in the stress regime acting in the region, referred The dimensions of the block measure 300 km in length, with a to in this paper as far-field stresses, are likely to have acted over depth of 60 km. In this block we site weak zones in positions large areas beyond the core of the orogen and were therefore that are determined by the present-day surface positions of the homogeneous across the relatively narrow region between the Robertson Lake shear zone and the Carthage-Colton shear zone two shear zones. In other words, while far-field stresses could (Fig. 4). have acted over long periods of time and led to the required We have chosen a simple elastic-plastic rheology, with val- amount of extension and exhumation, they cannot explain the ues determined experimentally or taken from the literature (e.g., asymmetric uplift observed in the Grenville Province. Ranalli, 1995). For this study we did not address heterogeneities Slab break-off and mantle delamination are often invoked in rheology as a function of depth (or temperature), but main- as mechanisms acting in mountain belts (Sacks and Secor, 1990; tained a homogenous crustal material outside the weak zones Platt and England, 1994). Both are suggested as inducing uplift (the parameters defining crust and weak zones are given in Table and exhumation of orogens by replacing cold mantle with hot- 3). Boundary conditions were no-slip in the vertical direction ter asthenospheric mantle. However, both require an initial and free-slip in the horizontal direction and at the surface. Rota- period of subsidence while the slab is in the process of breaking tion in and out of the plane of the page was not allowed. The off (and even thereafter), or as the mantle delaminates. This is block was subjected to extensional side loads and vertical trac- the required dynamic response of the crust to sinking material. tions that simulated the presence of a plume or a delaminating Both mechanisms should also lead initially and locally to com- slab. The time evolution from the latest Grenville compression pression in the region immediately above the delamination. Our to the final periods of extension when the crust must have been structural, geochronological, and kinematic constraints on the uplifting as a uniform block was 300 m.y. After a 300-m.y. time Grenville suggest this was not the case there. Even if the cold evolution, the predicted stresses and displacements were com- and hot mantle had been as much as a thousand degrees apart in pared to field observations. temperature, this would have led to only 3% of the difference in Our block of crustal material was subjected to a two-sided density, and hence would have had only an isostatic effect on the extensional load of 100 MPa. The bottom of the block was sub- order of 2 km, which is less than the observed amount of uplift jected to a concentrated vertical stress ranging from 10 to 50 seen in the Grenville orogen. Furthermore, isostatic uplift acts MPa, acting upward to simulate a plume or downward to simu- over short timescales and cannot alone explain the protracted late a delaminating mantle. A gravitational body force was period of exhumation. applied over the block for the duration of the analysis. The A mantle plume will initially generate uplift, via dynamic model was then allowed to evolve over a 300-m.y. period. topography, on the order of 1–2 km (Lithgow-Bertelloni and Sil- ver, 1998; Panasyuk and Hager, 2000). It can also lead to exten- Results sion, as it induces stresses on the overlying lithosphere as well as adding excess gravitational potential energy due to the added Models using only far-field stresses simulated by applying topography. However, the uplift caused by the plume will cease a two-sided 100 MPa load to the block show the required once the head reaches the bottom of the lithosphere. amounts of extension but do not explain the temporal differ- Given that each individual mechanisms seems unable to ences between zones or the uplift of the block. Models that explain all geological observations, our study has focused on the include only plume stresses explain the uplift of the block but impact of a combination of mechanisms on the deformation and do not generate local stresses around weak zones that are high uplift of an overthickened crustal block. The block contains enough to induce significant amounts of normal faulting. The Exhumation of a collisional orogen 407

Figure 4. (A) Setup of the finite element model. Dimensions are given, as are boundary conditions and loads during analysis. (B) Contour map of resulting stress distributions. Highest stresses are seen around shear zones, with lowest stresses occurring within the block bounded by shear zones. Model parame- ters are given in Table 3. CCSZ—Carthage- Colton shear zone; RLSZ—Robertson Lake shear zone.

same behavior is observed if delamination is simulated. How- for 250 m.y. and plume stresses that induced the required ever, plume and delamination stresses are different in one impor- amount of asymmetric uplift observed in the region. The tilt pro- tant respect. To have generated the asymmetric uplift observed duced in the Frontenac or Adirondack Lowlands regions in the region, delamination must have acted locally on the west- matches the tilt inferred by Dahl et al. (2001) from regional ern rather than the eastern edge (by the Robertson Lake shear hornblende ages. zone) of the region, since their dynamic effect would have pulled In this combined model, the results show a 300-m.y. period down the crust. Any uplift generated by this mechanism would of uplift and extension, with the greatest uplift occurring have required that the mantle previously attached to the crust be underneath the Adirondack region, matching observed geo- replaced by hotter asthenospheric mantle and that the delami- chronologic results that show extensional motion along the nating mantle sink far enough into the mantle to no longer exert Carthage-Colton shear zone due to rapid uplift of the Adiron- stress on the lithosphere. The isostatic uplift from hotter mantle dack Highlands. Plots of contoured deviatoric stresses (Fig. 4, would have acted on timescales comparable to those of post- B) show high concentrations of stresses beneath the Carthage- glacial rebound. Models that include only gravitational energy Colton shear zone. The surface around the Robertson Lake shear imposed on the block of crust with sited weak zones do not show zone and the Carthage-Colton shear zone regions dropped to the appropriate amount of extension. However, the total magni- very low stresses as the weak zones activated as faults. All strain tude of gravitationally driven extension should have changed was confined to the weak zones, indicating that they became the with the inclusion of paleotopography. The timescales over loci of permanent deformation in the region. As expected, shear which it would have acted, however, would have remained the stresses were highest in the regions immediately adjacent to same. We suggest that the most likely physical scenario requires faults. the presence of far-field stresses that induced periodic extension DISCUSSION

TABLE 3. MODEL PARAMETERS Two-dimensional finite-element models that simulate the Crust Weak zone Grenville Province at the peak of orogenesis may be used to Young’s modulus (GPa) 80 50 evaluate the temporal and spatial characteristics of uplift and Poisson’s ratio 0.25 0.25 extension in the region. Postorogenic extension along the Density (kg/m3) 2650 2650 Carthage-Colton shear zone occurred at ca. 945 Ma, according Yield stress (MPa) 150 15 to the argon ages of hornblende within the Carthage-Colton 408 M.M. Streepey et al. shear zone (Streepey et al., 2001). The time of extension across thermal instabilities under the supercontinent can lead to the Robertson Lake shear zone is less precisely known, but is regional uplift and extension. bracketed between 900 Ma and 780 Ma, as documented by 40Ar- Results elsewhere in Rodinia indicate that extension is wide- 39Ar analyses of biotite and K-feldspar in the hangingwall and spread. Bottom-heated sources commonly generate large amounts footwall blocks of the Robertson Lake shear zone. Figure 5 shows of magma, creating mafic igneous provinces or massive dike the combined kinematic and dynamic model of the postorogenic swarms. Whereas dike swarms in the Adirondack region are extensional history of the Metasedimentary Belt. Sometime after much younger, on the order of 580 Ma (Geraghty et al., 1979), 1045 and before 945 Ma, the entire region underwent a transition abundant dike swarms at ca. 780 Ma occur in the western United from a contractional regime to an extensional environment, caus- States (Park et al., 1995). Extensional activity in the Baltica ing normal motion along the Robertson Lake shear zone and the craton has been suggested at ca. 870 Ma (Dalziel and Soper, Carthage-Colton shear zone. Extension terminated along the 2001). In addition, ages of extension have been suggested at Carthage-Colton shear zone at ca. 945 Ma, as shown by argon ca. 920 Ma along the western edge of the Congo craton (Tack ages of hornblende that are offset across the zone and biotite ages et al., 2001). Mafic dike swarms in the South China block that are not offset along the Carthage-Colton shear zone (Fig. 3), have been dated at ca. 820 Ma. Collectively these results support but did not cease along the Robertson Lake shear zone until some- the idea of widespread extensional activity in the Late Mesopro- time after 900 Ma. Since extension occurred so much later than terozoic or Early Neoproterozoic, before the onset of large-scale contraction, uplift of the crustal blocks that comprise this portion rifting of the Rodinian supercontinent along this side of the of the Grenville cannot be explained by protracted orogenesis. Laurentian margin. It is possible that other evidence of magma- This differs significantly from the case in southern Asia, which tism in the North American Grenville Province during this has often been proposed as an analogue to the Grenville Province. period has been obscured by Phanerozoic tectonic activity in the In the Himalayan region, India is still underthrusting Asia, lead- Appalachian orogen. ing to continued uplift of the Tibetan plateau even while the Finally, bottom heating alone does not result in the pattern plateau is actively extending, probably due to gravitational insta- of geology seen at the surface today. It would have been neces- bilities or flow in the lower crust (e.g., Shen et al., 2001). sary to apply a force to the block, approximately equal to a plate- One viable mechanism for generating uplift while also driving force, to generate both the amounts of uplift and the causing active extension is heating from below. This heat source amounts of extensional displacement documented in the field can take several forms, including the formation of a plume, man- (Fig. 4). Therefore, we surmise that an initial instability in the tle delamination, or slab break-off, all responsible for replace- mantle triggered an adjustment of the plates while Rodinia ment of crust by hot asthenospheric mantle. In addition, it has remained a coherent supercontinent. This adjustment, or change long been postulated that the existence of a supercontinent cre- in far-field stress, would have triggered initial stages of breakup ates a blanket over the mantle (e.g., Gurnis, 1988). With heat of the supercontinent, generating failed rifts or weakening the produced in the mantle unable to escape via spreading centers, crust, in advance of later breakup.

Figure 5. Kinematic and dynamic model of regional extension in the Grenville Province of southeastern Ontario and northwestern New York. Between 1040 and 945 Ma, the entire region shifted from a compressional stress regime to an extensional regime. Extension along the Carthage-Colton shear zone (CCSZ) ended at 945 Ma, while extension across the Robertson Lake shear zone (RLSZ) did not terminate until after 900 Ma. Exhumation of a collisional orogen 409

ACKNOWLEDGMENTS Dalziel, I.W.D., 1997, Overview: Neoproterozoic-Paleozoic geography and tec- tonics: Review, hypothesis, environmental speculations: Geological Soci- This research was funded by the David and Lucille Packard ety of America Bulletin, v. 109, p. 16–42. Foundation (CLB), National Science Foundation (NSF) grant Dalziel, I.W.D., and Soper, N.J., 2001, Neoproterozoic extension on the Scot- tish promontory of Laurentia: Paleogeographic and tectonic implications: EAR-9980551 (CLB), NSF grant EAR-9627911 (BvdP), an Journal of Geology, v. 109, p. 299–317. NSF postdoctoral fellowship (JFM), the Geological Society of Davidson, A., 1984, Tectonic boundaries within the Grenville Province of the America (MMS), and the University of Michigan F. Scott Turner Canadian shield: Journal of Geodynamics, v. 1, p. 433–444. Foundation (MMS). We thank Chris Hall and Marcus Johnson Davidson, A., 1995, A review of the Grenville orogen in its North American type for their generous assistance in the Radiogenic Isotope Geo- area: Journal of Australian Geology and Geophysics, v. 16, p. 3–24. Davidson, A., 1998, An overview of Grenville Province geology, Canadian chemistry Laboratory at the University of Michigan. Klaus Shield, in Lucas, S.B., and St-Onge, M.R., eds., Geology of the Precam- Mezger and Larry Ruff are thanked for helpful discussions. brian Superior Province and Precambrian fossils in North America: Boul- der, Colorado, Geological Society of America, Geology of North America REFERENCES CITED Series (DNAG), v. 7, p. 207–270. Dewey, J.F., 1988, The extensional collapse of orogens: Tectonics, v. 7, p. 1123– Burg, J.-P., and Ford, M., 1997, Orogeny through time: An overview, in Burg, 1139. 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