Lithoprobe line 55: integration of out-of-plane seismic results with surface structure, metamorphism, and geochronology, and the tectonic evolution of the eastern Grenville Province

Lithoprobe line 55, in the Grenville Province of eastern Quebec, provides u nusually good control on the three-dimensional (3-D) geometry and structura l relationships among the major lithological units there. Archean basement underlies the exposed Proterozoic rocks, along the entire seismic line, and there is a lateral ramp in this basement immediately behind a lobate stack of thrust slices of high-pressure metamorphic rocks comprising the Manicou agan Imbricate Zone (MIZ). Integration of the 3-D geometry with P-T and geo chronological data allows derivation of a tectonic model for the region. Th e MIZ was buried to depths > 60 km at 1050 Ma. Preservation of its high-pre ssure assemblages, and the absence of metamorphism at 990 Ma, which is char acteristic of lower pressure metamorphic rocks that tectonically overlie th em, indicates the MIZ rocks were rapidly unroofed, early in the tectonic hi story. There were two discrete pulses of crustal thickening during the Gren villian Orogeny in this region. The first, involving imbrication of Labrado rian and Pinwarian rocks that comprised part of southeast Laurentia, culmin ated in the Ottawan pulse at ca. 1050 Ma, and produced the high-pressure me tamorphism of the MIZ. Its effects were rapidly reversed, with extrusion of the MIZ rocks to shallow crustal levels at ca. 1020 Ma. The crust was agai n thickened, with the Moho subsiding to depths > 60 km, in the Rigolet puls e at ca. 990 Ma. The site of extrusion of the MIZ was probably controlled b y the subsurface lateral ramp. High geothermal gradients indicate that extr usion may have been aided by lithospheric delamination in the crustal-thick ening zone.