A NEW APPROACH FOR KINEMATIC VORTICITY QUANTIFICATION USING QUARTZ CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS AND C’ SHEAR BANDS

The proportion of pure and simple shear recorded in deformed rocks is a key characteristic necessary to solve important geological problems such as quantifying transport parallel stretch in shear zones and estimating the angle of convergence between tectonic units or continents. In terms of instantaneous strain, this variable reflects the relative orientation of the flow apophyses A₁ and A₂.

The sectional kinematic vorticity number (Wn), is the approximation of this parameter in deformed rocks. Several methods for estimating Wn exist that use the relative orientations of structures such as foliations, veins, shear bands, and crystallographic preferred orientation (CPO) as proxies for the orientation of A₁ and A₂. Because estimations typically require two or more of these structures, their occurrence restricts the use of these methods to the rocks where appropriate structures are well represented. The development of new techniques, therefore, using new or different combinations of structures is necessary to enable vorticity investigation across a wider range of rocks.

In this study, we present a new Wn estimation method based on the orientations of quartz CPOs and C’ shear bands. This method is based on the measurement of β and θ. Where β is the angle between the main foliation and A₂ (approximated by the plane normal to the central sector of single and Type 1 crossed girdle quartz c-axis distributions), and θ represents the angle between C’ planes (nucleating on the A₁-A₂ bisector plane) and the foliation. Using these two angles it is possible quantifying the sectional vorticity number using the following formula: Wn = cos[2(θ-β)].

Data obtained by applying the β/θ method to previously examined rocks from the Main Central Thrust and South Tibetan Detachment System of the Himalaya are consistent with vorticity data quantified by other methods. In addition, we present new original β/θ data from the Ben Hope thrust in the northern Scottish Highlands on different lithotypes. These results show that the β/θ method can be reliably used to estimate the Wn in the presence of quartz CPO and C’ shear bands, which increases the potential candidates for kinematic vorticity analysis in natural shear zones.