SEQUENTIAL EXTRUSIVE FLOW ABOVE A BASEMENT RAMP DOCUMENTED FROM DETAILED P-T-T-D PATHS, MONASHEE COMPLEX, SE CANADIAN CORDILLERA

The Monashee Complex (MC) of southeastern Canadian Cordillera is formed by two domal culminations; the northern Frenchman Cap and the southern Thor-Odin domes. Gneisses of North-American basement that have not been involved in the Cordilleran Orogeny core Frenchman Cap dome. An apparent coherent folded sequence of migmatitic metasediments and infolded basement slices crops out above the basement. The Sybley Creek Syncline (SCS), the structurally highest regional-scale fold of the MC, is located in the footwall of the Monashee décollement, which is a thrust-sense ductile shear zone forming the base of the Sil-Kfs-grade Selkirk Allochton panel.

Phase diagram sections (pseudosections) were modelled with Perple_X'06 for different structural levels of the west flank of Frenchman Cap dome and combined to existing and newly obtained geochronological data to derive detailed P-T-t-d paths. Four structural levels, that are now juxtaposed, were identified based on their contrasted P-T-t-d paths. At the upper level, Selkrik Allochton rocks experienced protracted flow in the middle crust throughout the Cretaceous and were exhumed to upper-crustal conditions between 70-60 Ma. By contrast, rocks from the upper limb of the SCS, that are now located immediately below the Selkirk Allochton, were at >35 km/800^oC at ca 59 Ma and were rapidly exhumed to <15 km/550oC by 55 Ma. SCS likely formed during exhumation of its upper limb because its lower limb did not reach the St-out isograd until 59 Ma, records 750^oC/31 km between 59-53 Ma and was exhumed to the And field by 53 Ma. Similarly, the lower part of MC cover sequence and upper basement stayed within the Sil-Kfs field until ca 50 Ma before being rapidly exhumed. A novel model of sequential buoyancy-driven extrusion of partially molten crust above an underthrusted basement ramp is proposed. This extrusion was triggered by the density (viscosity) drop predicted by Perple_X'06 to occur across the Ms-out isograd. High-grade rocks of SE-Canadian Cordillera thus records a complex and episodic exhumation history varying both in time and space. The exhumation process was controlled in part by density gradient internally created during metamorphism, but also by rheological contrast between fertile partially molten metapelites and refractory orthogneis.