Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 84

model that explained many of the geological characteristics of the area (including faults
created in an extensional regime and carbonatite intrusions). Since that pioneering work,
the normal faults of the St. Lawrence paleorift system have been described and mapped
at the surface as well as in the subsurface (Du Berger et al ., 1991 ; Castonguay et al .,
2010 ; Tremblay and Roden-Tice, 2011 ) . The SLRS is a half-graben created during the
late Precambrian opening of the proto-Atlantic (Iapetus) Ocean (Kumarapeli, 1985 ) . Many
of the faults trend NE-SW and mark the boundary between the Grenville Province of
the Canadian Shield to the northwest and the St. Lawrence Lowlands to the southeast.
The grabens of Ottawa-Bonnechere and Saguenay River intersect the SLRS and are both
interpreted as Iapetan failed arms (aulacogens; Kumarapeli, 1985 ; Tremblay and Roden-
Tice, 2011 ) . Faults related to the Ottawa-Bonnechere and Saguenay grabens trend mostly
WNW-ESE.
The SLRS faults have been studied in the Montreal region (Rocher et al ., 2003 ) aswellas
inCharlevoix, Quebec City, and in the St. Lawrence estuary ( Figure 4.2 ) . SLRS faults consist
of cohesive cataclastic rocks, with some major fault zones being marked by 10-20-meter-
thick fault breccias, ultracataclasite, and foliated fault gouge (Tremblay and Roden-Tice,
2011 ) . The rift system is a crustal-scale fault zone where reactivation occurred along Late
Precambrian to early Paleozoic faults attributed to Iapetus rifting (Sanford, 1993 ) . Studies of
the age of formation and reactivation of these faults revealed various periods of reactivation
(Rocher et al ., 2003 ; Faure et al ., 2006 ) . Following the formation of the Appalachian orogen
(started in the Ordovician) when these faults were under a compressive stress environment,
rifting of the Atlantic Ocean-Labrador Sea during Mesozoic times possibly reactivated
them (e.g., Kumarapeli and Saull, 1966 ; Carignan et al ., 1997 ) . Apatite fission-track age
discontinuities between the two sides of these faults are interpreted as the result of normal
faulting at c . 200 Ma (Jurassic) followed by tectonic inversion about 150 Ma ago (Tremblay
and Roden-Tice, 2011 ) . The latter study provides support for Atlantic-related, extensional
and compressive deformation within the interior of the Canadian Shield, more than 500 km
west of the axis of the Mesozoic rift basins. The faults were most recently active during
the Late-Triassic-Jurassic period, which corresponds to the creation of the current Atlantic
Ocean with the separation of North America and Africa. The emplacement of igneous rocks
of the Monteregian Hills occurred approximately at the same time, i.e., in early Cretaceous
times (McHone and Butler, 1984 ; Foland et al., 1986 ; Pe-Piper and Jansa, 1987 ) . In the
Montreal region, the E-W and SE-NW extensional tectonics are documented by brittle
normal faults.
Beneath the Paleozoic and Appalachian cover, seismic reflection profiles in the
St. Lawrence Lowlands have shown the upper crustal morphology of the faulted blocks
(Theriault et al ., 2005 ; Castonguay et al ., 2010 ) . SLRS faults exist at depth beneath
the Appalachians, and their positions are revealed by their magnetic and gravity signa-
tures (Charlevoix: Lamontagne, 1999 ; Lower St. Lawrence: Lamontagne et al ., 2003 ;
St. Lawrence Lowlands: Lamontagne et al ., 2012 ) . Although we know the positions of
some of these rift faults beneath the sedimentary cover, our knowledge is far from com-
plete. Based on geophysical information, Wheeler ( 1995 , 1996) defined the northwestern