Combining Multi-Scale Analyses to Unravel the Tectonic History of Shear Zones and Geodynamic Events

Shear zones are volumes of rocks characterized by intense ductile deformation compared to the surrounding walls. They are some of the main tectonic structures accommodating stress in the lithosphere, and they exist at all observation scales and can form at nearly all structural levels. In everyday use, shear zones are known to be linked to seismic risk and fluid circulations leading to ore deposit distribution; but from a structural geology point of view, shear zones are unevaluable structures to decipher local and global tectonics, the rheological behaviour of the earth's crust and mantle, and for shaping tectonic models.

Studying the complex strain geometry and the (often long-lasting) deformation history of shear zones is challenging for geologists as at a single observation scale it is not possible to obtain all the necessary information to understand the deformation (i)geometry, (ii)conditions, and (iii)timing. For this reason, the study of such structures requires a multidisciplinary and multiscale approach to be carried out. Detailed field observations and structural measurements are crucial, as they provide the context for all subsequent microstructural analyses. Field observations deriving from geological mapping and structural analysis must be integrated with quantitative analysis from microtectonics, petrology and geochronology. From a structural geology perspective, the variation (and reduction) of the observation scale requires the integration of progressively more sophisticated analytical approaches which include field measurements, petrography, textural analysis, and crystallographic orientation analysis. The combination of multiple observations at the microscale allows us to obtain important information on the flow geometry (such as the kinematic vorticity number), which is essential for understanding the tectonic significance of the obtained results. Hereafter, two cases of study are presented illustrating the development and application of methods for the study of shear zones.

The first case study illustrates how the combination of multiple observations at the microscale was used to produce a new method to quantify the kinematic vorticity number in shear zones. This new method, named C’-c, combined the orientation of the C’ shear bands with the distribution of the quartz c-axes to estimate the degree of non-coaxial deformation component in a shear zone. This method was first tested on well-known shear zones in Himalaya and Sardinia and then used to investigate the kinematic vorticity of the Abloviak shear zone (Torngat orogen, northern Quebec) and the Ben Hope Thrust (Scottish Highlands).

The advantages of an integrated cross-disciplinary and multi-scale approach for studying shear zones are illustrated by the second case study, that of the tectonic evolution of the Intra Tantato shear zone, a large-scale structure with incredibly well-preserved lower crustal deformation. In this example, observations from geological mapping, structural analysis at all scales, and quartz Crystallographic Preferred Orientation are combined with metamorphic petrology and geochronology on monazite and garnet. The obtained results helped to: (i) identify and describe the main deformation mechanisms responsible for the rheological behaviour of the felsic portion of the lower continental crust; (ii) understand the tectonic significance of the Intra Tantato shear zone as a complex structural lineament active through multiple orogenic events.

Zoom link: https://ubc.zoom.us/j/61227908464?pwd=eE9IaGhPZFF6OW5OSWROL3U4cmJYQT09(link is external)

Meeting ID: 612 2790 8464

Passcode: 009405

Please note that after the seminar, the candidate will give a chalk-talk regarding their research plan over the next five years at ESB 5104 @2:30 pm, you are welcome to join as well!