PhD Project in Tectonics at UBC Okanagan - September 2017 or January 2018

The past 15 years have seen major advancements in our knowledge of how large orogenic systems evolve. Much of this progress has come from testing geologic predictions made by thermo-mechanical models, through the construction of detailed P-T-t-D paths across the now exhumed exhumed midcrustal terranes. This type of work in the Himalaya has outlined a series of cryptic faults at various locations along the length of the orogen. The occurrence of such structures indicates a more complex history for the exhumed Himalayan mid-crust with significantly more internal horizontal shortening and vertical thickening than previously recognized. These recognized ‘tectonometamorphic discontinuities’ in the Himalaya are only recent discoveries and as such little data exist on their geologic characteristics and their overall role in the kinematic evolution of the orogen. Potentially similar structures identified in the Canadian Cordillera and in Northern Saskatchewan may act as proxies to those in the Himalaya, providing opportunities to better understand their role in how convergence is accommodated in collisional orogenic systems.

The East Athabasca mylonite zone of northern Saskatchewan comprises more than 20,000 km2 of late Archean to Paleoproterozoic lower continental crust. It can be separated internally into three domains with broadly similar pressure-temperature-time (P-T-t) paths that include late Archean granulite facies metamorphism and deformation ca. 2.60-2.55 Ga and a subsequent granulite facies event at 1.9 Ga. These include the Chipman (or Southeast) domain in the east, separated from the Lower Deck (or Northwest domain) to the northwest by the 1.88 Ga Cora Lake shear zone, and the Upper Deck (or Southern domain), which occurs to the south of the Lower Deck. The interface between the Upper and Lower decks is marked by a fabric transition from heterogeneous mylonites below (in the Lower Deck) to homogeneously developed mylonite above (in the Upper Deck). Higher P recorded in the Upper Deck has been interpreted to indicate that it was thrust over the Lower Deck, however, the contact has a general lack of macroscopic shear sense indicators. The rare outcrop-scale shear sense indicators that have been reported generally record dextral shear indicating an overall top-down-to-the southwest, or extensional, shear sense along the boundary contact. New quartz c-axis data from 11 specimens collected across the Upper Deck/Lower Deck transition outline a strain gradient across the boundary consistent with its interpretation as a shear zone. The c-axis fabrics record sinistral shear across the structure with deformation temperatures of ~575 - 700 (± 50) ˚C, which is compatible with thrusting of the Upper Deck over the Lower Deck post-peak metamorphism. Moreover, deformation temperatures in the Upper Deck are within error of those suggested for eclogite facies metamorphism at 1.9 Ga indicating they may be genetically related.

This part of the project will continue this initial microstructural work and expand it to include at least two more transects of the Lower Deck/Upper Deck boundary to determine if similar characteristics are observed. This will involve a 6 week field season in N. Saskatchewan in the Summer of 2018. In addition, the student will take advantage of EBSD to identify misorientations in zircon and/or monazite. Misoriented domains will then be targeted using LA-ICP-MS U(Th)-Pb geochronology to attempt to constrain the timing of (re)crystallization and thereby directly date deformation.

The second part of this project will examine the Monashee décollement in the Canadian Cordillera. The Monashee décollement is a midcrustal structure exposed in the Thor-Odin and Frenchman Cap domes of the Monashee complex in the southern Canadian Cordillera. Its occurrence is contested, with some researchers arguing that it does not exist, while disagreement about its kinematics exists amongst those that do recognize it. Interestingly, the timing of metamorphism across the disputed structure shows a pattern similar to that observed across discontinuities in the Himalaya; decreasing metamorphic ages and increasing pressure down structural section. This indicates that the Monashee décollement may reflect comparable convergence accommodation processes. The Monashee complex provides excellent exposures to investigate this structure and previous studies have demonstrated that the rocks in the region contain the assemblages required to carry-out the analyses necessary to understand this enigmatic, yet potentially important, fault.

This PhD project will be conducted under the supervision of Dr. Kyle Larson at the University of British Columbia, Okanagan in Kelowna, BC with collaborations with Félix Gervais (École Polytechnique, Montréal) and John Cottle (University of California, Santa Barbara). UBC Okanagan hosts a series of analytical tools including an Automated Fabric Analyser G60, a LA-ICP-MS, a Cameca SX5FE electron probe microanalyser, and a Tescan SEM with EBSD. Please contact Dr. Larson (kyle.larson@ubc.ca or 1-250-807-8564) with expressions of interest or any questions you might have ASAP. Start-dates for the projects can be Fall 2017 or Winter 2018.