Open Collections

Graziani, Riccardo

University of British Columbia

Despite the major role played by shear zones in large-scale deformation accommodation processes across structural levels, the present record of such structures from the lower crust is limited because of sparse exposure of such rocks on Earth. Significant gaps, therefore, remain in our knowledge about the deformation processes and behavior of lower crustal shear zones during collision. This contribution aims to fill that gap by studying the dynamics of the continental lower crust at both the micro and regional scale and by developing new methods for studying the tectonic evolution of lower crustal terranes. Using the tilt angle (β) of a quartz crystallographic preferred orientation (CPO) and the orientation of the C′ shear bands (θ) it was possible to develop a new technique for quantifying the sectional vorticity number (Wₙ) in medium to high-metamorphic grade deformation environments typical of deep crustal levels. This vorticity number estimation technique, named the C′-c method, can be applied using the following formula: Wₙ = Cos[2(θ-β)]. Microstructural and quartz CPO analyses of the lower crustal Intra Tantato shear zone (ITSZ; East Athabasca Mylonite Triangle of northern Saskatchewan, Canada) reveals that specimens with few hydrated mineral phases deformed mainly by dislocation creep and dynamic recrystallization. In contrast, in specimens rich in hydrated mineral phases, deformation was dominated by pressure solution and (re)precipitation processes that enhanced phase mixing and promoted grain boundary sliding. These results indicate a strong mineralogical and fluid control on deformation and potential strain softening processes in the lower crust. Finally, quartz CPO analysis combined with garnet Lu/Hf and Monazite U/Pb geochronology were used to reconstruct the history of the ITSZ as a major thrust structure that was active at 1904 ± 19 Ma and resulted in nappe stacking within the continental lower crust. Microstructural and geochronological data also outline normal-sense reactivation of the ITSZ at 1860 ± 30 Ma during the exhumation of the East Athabasca Mylonite Triangle.

Text

2021-08-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/79541

Earth and Environmental Sciences

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/