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The Ordovician Thetford Mines Ophiolite Complex (TMOC) is an oceanic terrane accreted to the Laurentian margin during the Taconic Orogeny and is affected by syn-obduction (syn-emplacement) deformation and two post-obduction events (Silurian backthrusting and normal faulting, and Acadian folding and reverse faulting). The southern part of the TMOC was tilted to the vertical during post-obduction deformation and preserves a nearly complete cross-section through the crust. From base to top we distinguish cumulate Dunitic, Pyroxenitic and Gabbroic Zones, a hypabyssal unit (either sheeted dykes or a subvolcanic breccia facies), and an ophiolitic extrusive-sedimentary sequence, upon which were deposited sedimentary rocks constituting the base of a piggy-back basin. Our mapping has revealed the presence of subvertically dipping, north-south-to 20~ faults, spaced c. 1 km apart on average. The faults are manifested as sheared or mylonitic dunites and synmagrnatic breccias, and correspond to breaks in lithology. The fault breccias are cut by undeformed websteritic to peridotitic intrusions, demonstrating the pre-to synmagmatic nature of the faulting. Assuming that rhythmic cumulate bedding was originally palaeo-horizontal, kinematic analysis indicates that these are normal faults separating a series of tilted blocks. In the upper part of the crust, the north-south-striking fault blocks contain north-south-striking dykes that locally constitute a sheeted complex. 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 upward decreasing throws suggesting that they are growth faults. The base of the volcano-sedimentary sequence is a major erosional surface in places, which can penetrate down to the Dunitic Zone. The evidence for coeval extension and magmatism, and the discovery of a locally well-developed sheeted dyke complex, suggest that the TMOC formed by sea-floor spreading. The dominance of a boninitic signature in cumulate and volcanic rocks suggests that spreading occurred in a subduction zone environment, possibly in a forearc setting.

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[1] The Thetford Mines Ophiolite Complex (TMOC) preserves a complete ophiolitic sequence, and occupies the hanging wall of a major SE dipping normal fault, the Saint-Joseph fault. Preobduction, synobduction, and postobduction structures can be recognized in the TMOC. NS trending, preobduction, paleonormal faults are parallel to ultramafic minor intrusions, and to sheeted dykes, recording extension related to seafloor-spreading in a pericontinental suprasubduction zone basin. WNW trending synobduction, synmetamorphic fabrics are found toward the base of the TMOC and in the underlying continental margin rocks, but are absent in the upper part of the TMOC and overlying sedimentary rocks. These Ordovician (Taconian) structures record the development of a dynamothermal aureole immediately below the mantle/margin contact, and emplacement of the young ophiolite onto the continental margin. Postobduction structures include Late Silurian/Early Devonian, SE verging backthrusts and back folds that inverted the TMOC; and Middle Devonian (Acadian) NW verging folds and reverse faults. The tectonic history established for the TMOC is consistent with that of the adjacent Laurentian margin, and can be applied to the southern Québec ophiolitic belt as a whole. The structural synthesis of the ophiolitic belt, complemented with new observations and our compilation of stratigraphical, geochemical, geochronological, and petrological data, suggests that the southern Québec ophiolites may represent the remnants of the obduction of a single large slab of suprasubduction oceanic lithosphere extending for over a 100 km of strike length.

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