Pressure-temperature-time-deformation paths of former mid-crustal rocks, northern Monashee Complex of the southeastern Canadian Cordillera: A model of synconvergent exhumation by sequential ductile extrusion.

摘要

It is generally accepted that the high-grade rocks exposed in the southeastern Canadian Cordillera were exhumed from the middle crust by large-magnitude extension in the Paleocene-Eocene following gravitational collapse of a thickened crustal welt and/or change in far-field stress. Some models further proposed that gneiss domes of the Monashee Complex, the deepest structural level of the southeastern Canadian Cordillera, were produced by extension-assisted diapirism. However, a small number of studies also proposed that part of the exhumation occurred during convergence. This thesis aims at gaining a better understanding of the processes that led to the exhumation of former mid-crustal rocks to the surface of the Earth.;The focus of the second part shifts toward upper structural levels, and various exhumation processes are tested for the Lower Selkirk allochthon (LSa). LSa is a 10--15 km thick panel of migmatitic rocks that underlies the older lower-grade rocks of the Intermontane Belt and overlies the younger rocks of the Monashee Complex. Structural, metamorphic and geochronologic data are combined to construct a detailed Pressure-Temperature-time-deformation path (P-T-t-d) for the thrust shear zone at the base of the LSa. This path indicates that the last increments of thrust shear strain took place between ∼62 and 57 Ma, and were associated with near-isothermal decompression. A compilation of previously published data further suggests that normal shearing along the shear zone that defines the roof of the LSa also occurred in the Paleocene. Combined with several depth-time paths that point to synchronous burial of the underlying Monashee Complex, this coeval motion of two oppositely-verging shear zones bounding a migmatitic panel suggests that the LSa was exhumed to upper crustal level primarily by Paleocene syn-convergent ductile extrusion, rather than large-magnitude extension.;The four P-T-t paths constructed in the last part indicate that rocks of the Monashee cover sequence, which are now sandwiched in between the LSa above and the Monashee Complex basement below, were exhumed from a depth of ∼40 km to less than ∼14 km depth by ductile extrusion after ductile extrusion of the LSa. Fold nappes developed during extrusion, most likely induced by internal density contrast within the extruding channel. A first-order force balance analysis indicates that underthrusting of a felsic basement beneath a denser orogenic wedge should produce a lateral pressure gradient large enough to drive low-viscosity rocks, such as migmatites, up a 30° ramp.;Consequently, results of this thesis strongly argue against large-magnitude extension for exhumation of high-grade rocks of the southeastern Canadian Cordillera at the latitude of the Monashee Complex. Extension probably played a role but only for the last stages of exhumation, once the rocks were already exhumed to upper crustal levels.;The first part of the thesis focuses on the core zone of the northern dome of the Monashee Complex, Frenchman Cap. After confirming previous studies in that all gray granite dykes in basement gneiss of the dome are part of a Paleoproterozoic suite, these dykes are used as time markers to demonstrate that only the upper ∼1.5 km of the exposed 5--6 km basement section has a penetrative foliation developed during the Cordilleran orogeny. Below, strain patterns identified as Cordilleran are limited to open folds and meter-scale shear zones. Furthermore, phase equilibria forward modeling points to a maximum burial depth of ∼20 km for the deepest exposed structural level of the dome. Data presented in this first part suggest that a downward-migrating base of convergence was frozen-in in the Eocene, and preclude diapirism as a viable doming mechanism for Frenchman Cap dome (FCD).