Airborne SAR images provided essential clues to the tectonic setting of (1) the MbLg 6.5 Saguenay earthquake of 25 November 1988, (2) the Charlevoix-Kamouraska seismic source zone, and (3) some of the low *eve* seismic activity in the Eastern seismic background zone of Canada. The event occurred in the southeastern part of the Canadian Shield in an area where the boundary between the Saguenay graben and the Jacques Cartier horst is not well defined. These two tectonic blocks are both associated with the Iapetan St-Lawrence rift. These blocks exhibit several important structural breaks and distinct domains defined by the lineament orientations, densities, and habits. Outcrop observations confirm that several lineament sets correspond to Precambrian ductile shear zones reactivated as brittle faults during the Phanerozoic. In addition, the northeast and southwest limits of recent seismic activity in the Charlevoix-Kamouraska zone correspond to major elements of the fracture pattern identified on the SAR images. These fractures appear to be related to the interaction of the Charlevoix astrobleme with the tectonic features of the area. 20 refs.
Paleomagnetic results are reported for six Archean volcanics and six Proterozoic diabase dike sites in the La Grande Riviere basin using AF and thermal demagnetization. The area is now flooded so that future work is not possible. The limited geological and statistical data base makes interpretation of the results necessarily tentative. The dikes yield an A component pole at 84°W, 74°N from magnetization residing in higher stability domains, which likely reflects emplacement either at ~2150 or ~1825 or ~1650 Ma ago, and a B component pole at 33°W, 4°S residing in lower stability domains, which likely reflects metamorphism during the Hudsonian Orogeny at ~1725 Ma, or, less likely, during Grenvillian uplift at ~975 Ma. The volcanics appear to yield a prefolding C component pole at 28°W, 49°S that dates extrusion at ~2900 Ma and a postfolding D component pole at 306°W, 37°N that dates metamorphism by the Kenoran Orogeny at ~2600 Ma.
The Saguenay region is divided into three lithostructural units, namely (1) a gneiss complex; (2) batholithic anorthosite and mangerite complexes; and (3) late- to post-kinematic plutons. Three age groups of rocks in the gneiss complex are defined by increasing complexity of structure and mobilizates. Rocks of age group I (paragneiss, granite I) show extremely complex structure and contain three generations of mobilizates. Structures in rocks of age group II (granite II, amphibolite II dykes) are less complex and two generations of mobilizates are present. Rocks of age group III (granite III, amphibolite III dykes) contain one foliation and one generation of mobilizates. Metamorphosed mafic dykes are stratigraphic markers permitting correlation of age groups over vast tracts of terrain.Well-preserved igneous structures (cumulate layering) and textures (cumulate textures, ophitic textures) prove that anorthosite and mangerite crystallized from magmas. Anorthosite has been granulated and foliated, metamorphosed at high pressure with formation of orthopyroxene–spinel coronas by reaction of olivine and plagioclase, and intruded by three successive generations of mafic dykes (diorite, leucotroctolite, amphibolite IIIa).Mineral associations in the gneiss complex suggest metamorphism during slow cooling at about 5 kbar (500 MPa) pressure. Successive mineral associations in anorthosite and its cross-cutting dykes suggest a history of decreasing pressure (from >7–9 kbar (700–900 MPa)) and temperature (from 1200–1400 °C). Thus the anorthosite was emplaced at its present level by diapirism. Structures in the gneiss complex and anorthosite are consistent with the model of diapiric rise of anorthosite and mangerite plutons, but suggest presence of a generation of orogenic structures prior to diapirism.
We describe a paleomagnetic study of Mid-Proterozoic sandstone of the Sakami Formation. The thermal demagnetization is more effective than by alternating field; this indicates that part of the magnetic memory is located in hematite. The mean direction of magnetization after thermal and alternating field treatments is 031, −25 α 95 = 7.1°, K = 73 and the corresponding paleopole position is 103°W, 18°S (d m = 11°, d p = 8°). These results are important because a part of this geological formation will be covered with water after completion of the development works of La Grande Rivière. [Journal Translation]
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