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Exhumation of a Collisional Orogen: a Perspective from the North American Grenville Province

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Exhumation of a Collisional Orogen: a Perspective from the North American Grenville Province 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
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