Suturing and extensional reactivation in the Grenville orogen, Canada

Jay P.Busch* Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109 Klaus Mezger Max-Planck Institut für Chemie, D-55020, Mainz, Germany Ben A. van der Pluijm Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109

ABSTRACT Sutures are zones of weakness within orogenic belts that have the potential to become reacti- vated during orogenic evolution. The Robertson Lake shear zone marks a major tectonic bound- ary in the southeastern Grenville orogen of Canada that has been intermittently active for at least 130 m.y. The shear zone played a major role in the compressional stage of the orogenic cycle as well as during postorogenic collapse. The zone separates the Elzevir terrane to the west and the Fron- tenac terrane to the east. Sphene ages (U-Pb) indicate that these two terranes have distinct tectonothermal histories and that the shear zone represents a “cryptic suture.” In its current state, the shear zone is a low angle (30¡ESE dip) plastic to brittle extensional shear zone that separates the Mazinaw (footwall) and Sharbot Lake (hanging wall) domains. Integration of structural, meta- morphic, and chronologic data leads to a model that describes the complete evolution of this fun- damental tectonic boundary that evolved from an early compressional zone (ca. 1030 Ma) to a late extensional zone (until at least 900 Ma).

INTRODUCTION similarities to modern orogenic belts, such as the sion is a significant process in the tectonic evolu- During the contractional stages of orogenesis, Alpine-Himalayan chain. It is now also estab- tion of the Grenville orogen (van der Pluijm and strain tends to be focused along shear zones that lished that synorogenic and postorogenic exten- Carlson, 1989; Mezger et al., 1991; Cosca et al., are characterized by gently dipping thrust faults and/or steeply dipping strike-slip faults. Shear zones are regions of material weakness and it is likely that they are preferentially (re)activated during later stages. Such multiply active shear zones are known from shallow-crustal rocks in modern orogenic belts, but similar behavior in Figure 1. Map showing U-Pb cooling and metamorphic the deeper part of an orogen requires study of ages, regional structures, and deeply eroded, ancient mountain belts, which tectonic boundaries in south- will also provide complementary insights into the east Ontario, Grenville orogen. dynamics of orogenic evolution. Ages are from: c, Corfu and In this paper we provide information on the evo- Easton (1995); m, Mezger et al. (1993); and c+, Corfu et al. lution of a shear zone in a deeply eroded part of the (1995). Abbreviations: M, Mazi- Grenville orogen of Ontario, Canada. This well ex- naw domain; SL, Sharbot Lake posed and completely evolved mountain belt al- domain; F, Frontenac domain; lows direct access to rocks deformed and meta- MSZ, Mooroton shear zone; RLSZ, Robertson Lake shear morphosed at middle to lower crustal levels during zone; SLSZ, Sharbot Lake orogenesis. Therefore, the Grenville orogen pro- shear zone. Stippled pattern in- vides the opportunity to gain insight into deep dicates Paleozoic sedimentary crustal tectonic processes with the ultimate goal of rocks. Boundary with question defining the complete history of a major orogenic mark is proposed boundary of Easton (1992). Curvature of cycle. The potential also exists to use the Grenville structures in Mazinaw domain orogen as an analogue for deep crustal tectonic is interpreted to be product of processes in modern orogenic belts, and this has emplacement of Sharbot Lake been explored successfully in some cases (e.g., domain via sinistral transpres- sion. Inset shows Elzevir ter- Windley, 1986; Hanmer, 1988; Mezger et al., rane (ET) and Frontenac ter- 1993; van der Pluijm et al., 1994). The Grenville rane (FT) on either side of orogen is traditionally interpreted as a product of Robertson Lake shear zone ductile thrusting and crustal imbrication during and study area relative to convergence (e.g., Davidson, 1984; Hanmer, Gneiss belt (GB), Metasedi- mentary belt (MB), and Gran- 1988; Hildebrand and Easton, 1995) and has clear ulite terrane (GT).

*Present address: Exxon Production Research Com- pany, Houston, Texas 77252.

Geology; June 1997; v. 25; no. 6; p. 507Ð510; 3 figures; 1 table. 507 Figure 2. A: Thermobarometric data vs. distance from Robertson Lake shear zone showing pronounced baric field gradients. Dotted lines are error esti- mates. B: Generalized cooling curves for Sharbot Lake and Mazinaw domains illustrating relative crustal levels and juxtaposition of rocks with different cool- ing ages (both modified from Busch et al., 1996a).

1992, 1995; Culshaw et al., 1994; Busch and van REGIONAL GEOLOGY inaw domain has been associated with internal der Pluijm, 1996) and of orogenic belts in general The Robertson Lake shear zone lies within the imbrication of the domain during compression (e.g., Dewey, 1988). Metasedimentary Belt of the Grenville orogen from 1000 to 1050 Ma (Corfu and Easton, 1995). The present configuration of portions of the and separates the Sharbot Lake and Mazinaw do- Abundant fold closures and the regional trend of Grenville orogen is a product of extensional proc- mains (Fig. 1). The Metasedimentary Belt has axial surfaces and thrust faults are curved near esses, so that relating the shallowly dipping tec- been divided into several lithotectonic domains on the shear zone (Fig. 1). tonite fabrics to synorogenic thrusting may not the basis of lithologic, chronologic, and geophy- The Sharbot Lake domain is the westernmost always be warranted (e.g., White et al., 1994). sical data (e.g., Easton, 1992). Throughout this domain of the Frontenac terrane and is domi- Moreover, the significance of shear zones is often paper the term domain is used to imply a rela- nated by mafic and intermediate metaigneous obscured due to the high-grade metamorphism tively homogeneous volume of rock bounded by rock and marble. The age of metamorphism in and polyphase deformation typically associated a structural discontinuity, and the term terrane is the Sharbot Lake domain has been constrained with deeply eroded orogenic belts, and overprint- used when it can be demonstrated that the volume by U-Pb dates from sphenes and zircons as from ing extensional deformation commonly leaves of rock is tectonically distinct from neighboring 1140 to 1170 Ma (Mezger et al., 1993; Corfu et little evidence for the earlier history of the shear rocks. Easton (1988) first recognized the regional al., 1995; this study). Hildebrand and Easton zones. Thus, a multidisciplinary approach is re- extent of the Robertson Lake shear zone and the (1995) proposed that marbles of the Sharbot quired to unravel the complete orogenic history contrast in metamorphic facies and rock types on Lake domain represent platform carbonates that of high grade metamorphic terranes (see also van either side of the zone. The zone has been defined have been overridden by a northwest-directed der Pluijm et al., 1994). as an east-southeast-dipping, crystal-plastic mylo- thrust sheet that may extend across the Grenville Whereas most of the Grenville orogen of On- nite zone with locally intense cataclastic deforma- orogen of Ontario into the Gneiss Belt (Fig. 1). In tario has been mapped in detail and numerous tion (Easton, 1988; Busch and van der Pluijm, this interpretation metamorphism is associated petrologic and geochemical studies have been 1996). Normal shear-sense indicators within the with magmatism in the hanging wall and em- completed, the nature of amalgamation and com- zone have been reported for both brittle and crys- placement of a hot thrust sheet (magmatic arc) plete orogenic history are still poorly defined. In tal plastic structures (Busch and van der Pluijm, over platform carbonates at 1161 Ma. Many fact, in many regions the significance of shear 1996 and references therein). Quantitative ther- rocks retain U-Pb and 40Ar-39Ar crystallization zones and the identification of terrane boundaries mobarometric data indicate that displacement ages of 1170 to 1224 Ma (Corfu et al., 1995; are still under debate. Tectonic models developed along the Robertson Lake shear zone has pro- Busch et al., 1996b; A. Davidson, 1995 personal using the chronologic and petrologic approach duced discontinuities in metamorphic field gradi- communication), indicating that metamorphism generally lack a kinematic-geometric framework ents (Busch et al., 1996a), and 40Ar-39Ar cooling did not reset portions of the Sharbot Lake domain for evaluating the mode of terrane amalgamation ages are diachronous across the zone, which has and the Sharbot Lake domain did not undergo the along individual terrane boundaries. This study been interpreted to be a result of late extensional 1030 Ma metamorphic event documented in the focuses on the eastern region of the Grenville displacement that lasted until at least 900 Ma Mazinaw domain. On the basis of lithologic and orogen near the Robertson Lake shear zone, (Busch et al., 1996b) (Fig. 2). geochemical data, Easton (1992) suggested that which separates two domains in southeastern On- The Mazinaw domain is the easternmost do- the southeastern Sharbot Lake domain is geneti- tario (Fig. 1). This region has received consider- main in the Elzevir terrane and is dominated by cally related to the Mazinaw domain (Fig. 1). able attention in recent years and abundant struc- deformed plutonic rocks, clastic metasedimen- tural, petrologic, and chronologic data now exist tary rocks (quartzo-feldspathic schist, gneiss and ROBERTSON LAKE SUTURE ZONE (Easton, 1988; Cosca et al., 1992; Mezger et al., mica schist), and metavolcanic rocks (amphibole The Mazinaw domain is generally character- 1993; Corfu and Easton, 1995; Corfu et al., 1995; schist) that probably represent a Middle Protero- ized by amphibolite facies metamorphism and the Busch and van der Pluijm, 1996; Busch et al., zoic continental margin (Moore and Thompson, Sharbot Lake domain is characterized by upper 1996a, 1996b), which provides a unique opportu- 1980). As many as three metamorphic events amphibolite to granulite facies metamorphism nity to develop a complete tectonic-structural have affected the Mazinaw domain, resulting in a (e.g., Carmichael et al., 1978). New U-Pb ages of model of crustal shear zone evolution and in- complex tectono-metamorphic history (Easton, sphene extracted from marble in the Sharbot Lake sights into the dynamic processes acting in the 1992; Mezger et al., 1993; Corfu and Easton, domain were obtained to constrain the age of deep sections of orogenic belts. 1995). The youngest metamorphism in the Maz- metamorphism (ca. 1140Ð1160 Ma, Table 1; see

508 GEOLOGY, June 1997 Mezger et al., 1993, for analytical techniques). domain (footwall) show little variation, indicating Metamorphic temperatures in the region are rela- uniform unroofing of the footwall since 940 Ma tively high (650Ð750 ¡C; Busch et al., 1996a) and (Cosca et al., 1991, 1992; Busch et al., 1996b). thus are equal to or slightly higher than the closure Phlogopite, muscovite, and biotite cooling ages temperature for Pb in sphene (650Ð700 ¡C, Mez- are 924 to 890 Ma (Busch et al., 1996b). Cooling ger et al., 1993; Scott and St-Onge, 1995), so that curves constructed for the Mazinaw domain show these ages reflect cooling immediately after peak a period of relatively rapid cooling (~5 ¡C/m.y.) metamorphism. Regardless of the precise closure from 940 to 890 Ma (Fig. 2). The cooling history temperature for Pb in sphene, the ages document of Sharbot Lake domain is characterized by a sim- Figure 3. Model for terrane emplacement and that the Mazinaw domain has a metamorphic his- ilar cooling rate (~5 ¡C/m.y.), but from 1010 to subsequent extensional reactivation along tory that is temporally different from the Sharbot 970 Ma. Hornblende and biotite cooling ages in Robertson Lake zone. A: Transpressional em- Lake domain. When combined with the contrast in the Sharbot Lake domain are 70 to 130 m.y. older placement (sinistral) of Sharbot Lake domain. Mazinaw domain is undergoing metamorphism rock types between domains, the Robertson Lake than in the Mazinaw domain, indicating that the and internal imbrication, and Sharbot Lake do- shear zone is interpreted as a cryptic suture, as Sharbot Lake domain was at shallow crustal lev- main is at shallow crustal levels. Shaded re- originally hypothesized by Mezger et al. (1993) on els as early as 1029 Ma. Juxtaposition of rocks gions are rocks currently exposed in Mazinaw the basis of only one U-Pb sphene age (1152 Ma) with different biotite cooling ages (ca. 900 Ma vs. and Sharbot Lake domains. B: By 940 Ma, Maz- inaw domain had reached 500 ¡C and was be- from the Sharbot Lake domain. Sphene ages from ca. 1030 Ma) indicates that normal faulting along ing uniformly unroofed. Displacement contin- both sides of a proposed boundary in the southern the Robertson Lake shear zone must have been ued until at least 900 Ma, on the basis of offset Sharbot Lake domain (Easton, 1992) indicate that active until at least 900 Ma. Moreover, because biotite cooling ages. Depths are calculated this boundary does not separate rocks with tempo- hornblende cooling ages are uniform in the Maz- from thermochronologic data using ~25 ¡C/km rally distinct metamorphic histories (Fig. 1). Thus, inaw domain, thermochronologic data indicate and from barometric data using crustal density of 2700 kg/m3 and errors from metamorphic despite the lithologic and geochemical similarity that rotation of the footwall during extension must barometry. Dotted lines are displacement direc- between the Mazinaw domain and the southwest- have ceased by ca. 940 Ma. tions along the Robertson Lake zone and ern Sharbot Lake domain, there is no evidence for dashed lines show degree of unroofing. a common tectono-metamorphic history. These MODEL OF TERRANE EMPLACEMENT new ages are similar to metamorphic ages from the On the basis of metamorphic, chronologic, and Frontenac domain and Adirondack Lowlands do- structural data, it is possible to construct a model near the shear zone. During transpression, imbri- mains to the southeast (Mezger et al., 1993), and for the evolution of the Robertson Lake shear zone cation of the Mazinaw domain was accompanied therefore, the Sharbot Lake, Frontenac, and Adi- and adjacent terranes. The Mazinaw domain was by sinistral displacement along the Robertson rondack lowlands domains are considered to rep- undergoing high-grade metamorphism at 1000 Lake suture zone, which produced the curved map resent a coherent tectonic element (called the to 1050 Ma, whereas the western Sharbot Lake pattern of regional structures in the Mazinaw do- Frontenac terrane) at least since 1160 Ma. domain was at shallow crustal levels (300 ¡C) as main (Fig. 1). The period of rapid cooling of the early as 1029 Ma (cooling age of biotite), and the Sharbot Lake domain (1010Ð970 Ma) is consistent EXTENSIONAL REACTIVATION eastern Sharbot Lake domain reached 500 ¡C at with enhanced unroofing of the hanging wall in Peak metamorphic pressures increase in the 1010 Ma. Thus, the Sharbot Lake domain was the waning stages of transpression. The transition Mazinaw domain (footwall) toward the Robertson cooling at shallow crustal levels while the Mazi- to extensional tectonics in the region took place Lake shear zone from 550 MPa to 770Ð890 MPa, naw domain was still undergoing metamorphism. during the interval between peak metamorphism whereas metamorphic pressures decrease across The Sharbot Lake domain was juxtaposed with (at 1030 Ma) and 940 Ma, which reactivated the the zone to 500 MPa in the hanging wall and in- the Mazinaw domain during the 1030 Ma meta- Robertson Lake suture zone. Isostatically induced crease away from the zone to 900 MPa (Fig. 2). morphic event in the Mazinaw domain (Fig. 3A). flexural rotations were complete by 940 Ma and Uniform metamorphic ages but varied metamor- Rocks now exposed in the Sharbot Lake domain were followed by a period of rapid cooling of the phic pressures within each domain indicate that were at shallow crustal levels and thus did not un- footwall (940Ð890 Ma) that coincided with final rocks now exposed at the surface were originally dergo this metamorphic event. The pre-extension, phases of normal faulting and the transition from at varied depths. These observations are resolved high-dip angle of the Robertson Lake shear zone crystal-plastic to brittle deformation along the by a model of isostatically induced flexural rota- associated with terrane accretion suggests that Robertson Lake shear zone (Fig. 3B). tions that accompanied extension and produced pure dip-slip thrusting was not operative, because gradients in metamorphic pressure adjacent to the such movement typically occurs on faults with a DISCUSSION shear zone. The magnitude of rotation inferred shallow dip. Rather, imbrication and metamor- The early significance of the Robertson Lake from barometric data constrains the pre-extension phism in the Mazinaw domain (ca. 1030 Ma) re- shear zone for the history of the Grenville orogen dip angle of the terrane boundary (“suture”) to val- sulted from transpressional convergence, which can be appreciated only after removal of the effects ues of 65¡ to 90¡ (Busch et al., 1996a). offers an explanation for both the metamorphic of extension. The timing and conditions of meta- Hornblende cooling ages across the Mazinaw history and the curvature of regional structures morphism in each domain are important param-

GEOLOGY, June 1997 509 eters for assessing whether a tectonic boundary is tension are a product of the latter. Plate tectonic ince, Ontario: Canadian Journal of Earth Sci- a terrane or subdomain boundary, but definition of reconstructions for this time period (900 Ma) are ences, v. 31, p. 160Ð175. the geometric evolution of the boundary is re- not well defined and the temporally nearest rift- Davidson, A., 1984, Tectonic boundaries within the Grenville Province of the Canadian shield: Jour- quired to evaluate the nature of terrane emplace- ing phase along the Laurentian margin is 740 Ma nal of Geodynamics, v. 1, p. 433Ð444. ment. The proposed model resolves the diachron- (Su et al., 1994). Perhaps the late, postorogenic Davidson,A., and Carmichael, D. M., 1997,An 1161 Ma ous metamorphism of domains in the region and extension evident along the Robertson Lake suture in the Frontenac terrane, Ontario segment of accounts for the geometry of footwall structures shear zone is a yet unrecognized phase of the Grenville orogen: Comment, v. 25, p. 88Ð89. Dewey, J. F., 1988, Extensional collapse of orogens: and geometry prior to extension along the zone. (aborted?) continental breakup. To test this hy- Tectonics, v. 7, p. 1123Ð1139. From our chronologic data and analysis of pre- pothesis one should look for contemporaneous Easton, R. M., 1988, The Robertson Lake mylonite extensional geometry, it follows that the Robertson normal faulting in cratons that were juxtaposed zone—A major tectonic boundary in the Central Lake shear zone is a fundamental tectonic bound- with Laurentia at that time. Metasedimentary Belt, eastern Ontario. Geologi- ary (a deeply eroded plate boundary or “cryptic su- cal Association of CanadaÐMineralogical Associ- ation of CanadaÐCanadian Society of Petroleum ture”) in the Grenville orogen, which separates the ACKNOWLEDGMENTS Supported by grants from the National Science Geologists joint meeting Program with Abstracts, Elzevir terrane (Bancoft, Elzevir, and Mazinaw Foundation (EAR 93-05736 and EAR 96-27911), the v. 13, p. A34. domains) from the Frontenac terrane (Sharbot Scott Turner Fund (University of Michigan), and the Easton, R. M., 1992, Part 2, The Grenville Province and Lake and Frontenac domains, and Adirondack Geological Society of America (4875-92 and 5122- the Proterozoic history of central and southern Ontario, in Thurston, P. C., et al., eds., Geology of Lowlands). 93). We thank R. M. Easton for introducing us to the Robertson Lake shear zone, his insights into the re- Ontario: Ontario Geological Survey Special Vol- The new chronologic data and structural model gional geology, and generous field assistance. Com- ume 4, p. 714Ð904. provide insights into the significance of crustal- ments by journal reviewers J. McLelland and L. Hanmer, S., 1988, Ductile thrusting at mid-crustal lev- scale shear zones, fault reactivation, and tectonic Ratschbacher helped focus the manuscript. els, southwestern Grenville Province: Canadian boundaries in the Grenville orogen. The proposed Journal of Earth Sciences, v. 25, p. 1049Ð1059. Hildebrand, R. S., and Easton, R. M., 1995, An 1161 Ma thrust contact above marbles of the Sharbot Lake REFERENCES CITED Busch, J. P., and van der Pluijm, B. 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B., 1978, Isograds around the Hastings metamorphic Pb closure temperature in titanite based on rocks sional terrane emplacement along the zone, this ‘low’(field trip guidebook): Geological Society of from the Ungava orogen, Canada: Implications major crustal discontinuity was reactivated by ex- AmericaÐGeological Association of CanadaÐMin- for U-Pb geochronology and P-T-t path determi- tension resulting in movement along this fault that eralogical Association of Canada, p. 325Ð346. nations: Geology, v. 23, p. 1123Ð1126. Su, Q., Goldberg, S. A., and Fullagar, P. D., 1994, Pre- lasted at least 130 m.y. Corfu, F., and Easton, R. 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J., Mezger, K., and van der western Grenville and western Himalaya, in tings of orogenic extension. Is this extension Pluijm, B. A., 1995, Constraints on the duration Moore, J. M., et al., eds., The Grenville Province: Geological Association of Canada Special Paper related to gravitational instability and orogenic of tectonic processes: Protracted extension and deep-crustal rotation in the Grenville orogen: Ge- 31, p. 341Ð348. collapse (e.g., Dewey, 1988), or is the extension ology, v. 23, p. 361Ð364. related to plate tectonic reorganization and conti- Manuscript received November 26, 1996 Culshaw, N. G., Ketchum, J. W. F., Wodicka, N., and Revised manuscript received February 28, 1997 nental breakup (i.e., postorogenic)? On the basis Wallace, P., 1994, Deep crustal extension follow- Manuscript accepted March 4, 1997 of the lag time between contraction and cessation ing thrusting in the southwestern Grenville Prov- of extension it is likely that the latest stages of ex-

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