PRE-OBDUCTION STRUCTURAL AND MAGMATIC EVOLUTION OF THE THETFORD MINES OPHIOLITE COMPLEX: A RECORD OF FOREARC EXTENSION AND SEAFLOOR SPREADING

The Ordovician Thetford Mines Ophiolitic Complex (TMOC) is an oceanic terrane accreted to the Laurentian margin during the Taconic Orogeny, and is affected by syn-obduction 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 cross-section through the crust. From base to top we distinguish cumulate Dunitic, Pyroxenitic and Gabbroic Zones, a hypabyssal unit (either sheeted dykes or 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 reveals the presence of sub-vertically dipping, N-S to NNE-striking faults, spaced ~1 km apart on average. The faults are manifested as sheared or mylonitic dunites and syn-magmatic breccias, and correspond to lithologic breaks. The fault breccias are cut by undeformed websteritic to peridotitic intrusions, demonstrating the pre- to syn-magmatic nature of the faulting. Assuming that rhythmic cumulate bedding was originally paleo-horizontal, then kinematic analysis indicates that these are normal faults separating a series of tilted blocks. In the upper crust, N-S-striking faults are associated with N-S-striking dykes, locally constituting a sheeted dyke complex. The faults correspond to lateral changes in thickness and facies of supracrustal rocks, are locally marked by prominent subvolcanic breccias, and have upwardly decreasing throws, suggesting that they are growth faults. The base of the volcano-sedimentary sequence is an 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 seafloor spreading. The dominance of a boninitic signature in cumulates and lavas suggests that spreading occurred in a subduction zone environment, possibly in a forearc.