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
A. Hynes et al., 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, CAN J EARTH, 37(2), 2000, pp. 341-358
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.