HR: 0830h
AN: V21D-0556    [PDF]
TI: Pre-Obduction, Syn-Magmatic Extensional Deformation and Unroofing of a Fore-Arc Ophiolite, the Thetford-Mines Complex of Southern Quebec.
AU: Schroetter, J
EM: jschroet@nrcan.gc.ca
AF: INRS Eau-Terre-Environnement, 880 Chemin Ste-Foy, Quebec, PQ G1V 4C7 Canada
AU: Page, P
EM: ppage@nrcan.gc.ca
AF: INRS Eau-Terre-Environnement, 880 Chemin Ste-Foy, Quebec, PQ G1V 4C7 Canada
AU: * Bedard, J H
EM: jbedard@nrcan.gc.ca
AF: Geological Survey of Canada, 880 Chemin Ste-Foy, Quebec, PQ G1V 4C7 Canada
AU: Tremblay, A
EM: tremblay.a@uqam.ca
AF: Univ. Queb. Montreal, 201, President Kennedy local PK 6151, Montreal, PQ H3C 3P8 Canada
AB: The Ordovician Thetford Mines Ophiolite Complex (TMOC) is a peri-continental supra-subduction zone fore-arc terrane obducted onto the Laurentian margin during the Taconic Orogeny. Stratigraphic correlations suggest that the Mont-Orford and Asbestos Ophiolites are correlative, which implies obduction of a 100 km long oceanic slab. The TMOC is affected by syn-obduction (syn-emplacement) deformation, and two post-obduction events: (i) Silurian backthrusting and normal faulting, and (ii) Acadian folding and reverse faulting. The post-obduction deformation tilted the southern part of the TMOC to the vertical, exposing from base to top: cumulate Dunitic, Pyroxenitic and Gabbroic Zones, a hypabyssal unit (either sheeted dykes or a subvolcanic breccia), and an ophiolitic extrusive/sedimentary sequence, upon which were deposited (unconformably) a forearc basin. Our mapping has revealed the presence of numerous pre-obduction faults, spaced c.1 km apart on average. In the plutonic part of the crust, the faults are manifested as sheared or mylonitic dunites and syn-magmatic breccias, and may correspond to along-strike breaks in lithology. Fault breccias are cut by undeformed, 10-m scale, websteritic to lherzolitic intrusions, demonstrating the pre- to syn-magmatic nature of the faulting, and suggesting a role in transfer of melt to the surface. Assuming that rhythmic cumulate layering was originally paleo-horizontal, then kinematic analysis implies that these were originally normal faults separating a series of tilted (30-90 degrees) blocks. Swarms of dykes are oriented parallel to the major faults and locally constitute a sheeted complex, locally removed by syn-volcanic unroofing and erosion. In the upper part of the crust, the faults correspond to marked lateral changes in the thickness and facies assemblages seen in supracrustal rocks, are locally marked by prominent subvolcanic breccias, and have upwardly decreasing throws, which together suggest that they are growth faults. In most places, the base of the exposed volcano-sedimentary sequence is a major erosional surface, which can penetrate down to the Pyroxenitic Zone. The evidence for coeval extension and magmatism, and the presence of a sheeted dyke complex, imply that the TMOC formed by seafloor spreading. The dominance of a boninitic signature in cumulate and volcanics rocks suggests that spreading occurred in a subduction zone environment, possibly in a fore-arc setting.
DE: 7220 Oceanic crust
DE: 8015 Local crustal structure
DE: 8159 Rheology--crust and lithosphere
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting