Monday, April 30, 2012

Geological setting of the Saint Lawrence valley

The Saint Lawrence valley region is located in an intra-plate tectonic setting since about 200 MYBP but has been the locus of several major tectonic events in the last approximately 1 GYBP

The earliest tectonic phase is the Grenville Oregon (Figure 1) from about 1100 to about 900 MYBP, which is associated with the accretion of allochthonous terranes to the south-east margin of the Laurentia (North America) craton (e.g., Karlstrom et al. 2001). In the late Proterozoic (700–600 MYBP), the region was affected by the rifting and opening of the Iapetus (proto-Atlantic) Ocean (e.g., Kumarapeli 1985). This major episode of extension corresponds to the formation of large-scale systems of normal faults along the rifted margin and associated aulacogens across most of eastern North America. Major structures of the Iapetan system (Figure 1) include the Saint Lawrence, Ottawa, and Saguenay grabens in Canada and the Reelfoot Rift, Rough Creek grabens, and southern Oklahoma aulacogen in the centre of the US (Wheeler 1995). The closing of the Iapetus Ocean corresponds to different development phases of the Appalachian Oregon during the mid to late Paleozoic, possibly as late as Permian (about 250 MYBP) (Williams 1979; Faure et al. 1996). The Appalachian nappes were thrust over the North America craton as far west as the Saint Lawrence valley (Figure 1). During the Jurassic rifting and opening of the North Atlantic ocean, the reactivation of Iapetan normal faults marks the last phase of significant tectonic activity in the Saint Lawrence area (Lemieux et al. 2003).

Geological setting of the Saint Lawrence valley

Figure 1. Geological setting of the Saint Lawrence valley (modified from Mazzotti et al. 2005)

The Saint Lawrence valley is characterized by large eastward dipping normal faults with up to a few kilometres of motion documented in the Precambrian basement (Kumarapeli 1985; Tremblay et al. 2003). The normal fault system is capped by westward verging thrust faults and nappes of the Appalachian Oregon. This Paleozoic cover is only a few kilometres thick in most of the Saint Lawrence region. A meteorite impact (about 350 MYBP) in the southern part of the Charlevoix seismic region contributed additional complexity by creating an approximately 60 km diameter system of concentric faults and fractures (Lemieux et al. 2003).

Faure, S., Tremblay, A. and Angelier, J., 1996. Alleghanian paleostress reconstruction in the northern Appalachians: Intraplate deformation between Laurentia and Gondwana. Geological Society of America Bulletin, 108(11), p.1467.

Karlstrom, K.E. et al., 2001. Long-lived (1.8–1.0 Ga) convergent orogen in southern Laurentia, its extensions to Australia and Baltica, and implications for refining Rodinia. Precambrian Research, 111(1-4), pp.5-30.

Kumarapeli, P.S., 1985. Vestiges of Iapetan rifting in the craton west of the northern Appalachians. Geoscience Canada, 12(2), pp.54-59.

Lemieux, Y., Tremblay, A. and Lavoie, D., 2003. Structural analysis of supracrustal faults in the Charlevoix area, Quebec: relation to impact cratering and the St-Laurent fault system. Canadian Journal of Earth Sciences, 40(2), pp.221-235.

Mazzotti, S. et al., 2005. GPS crustal strain, postglacial rebound, and seismic hazard in eastern North America: The Saint Lawrence valley example. Journal of Geophysical Research, 110(B11), p.B11301.

Tremblay, A., Long, B. and Massé, M., 2003. Supracrustal faults of the St. Lawrence rift system, Quebec: kinematics and geometry as revealed by field mapping and marine seismic reflection data. Tectonophysics, 369(3-4), pp.231-252.

Wheeler, R.L., 1995. Earthquakes and the cratonward limit of Iapetan faulting in eastern North America. Geology, 23(2), p.105.

Williams, H., 1979. Appalachian Orogen in Canada. Canadian Journal of Earth Sciences, 16(3), pp.792-807.

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