Deformation mechanisms, Rheology and Tectonics 2011 Abstract Volume

Protocol for testing the model of oblique transpression with oblique extrusion in

ductile shear zones

Carlos Fernández1, Manuel Díaz-Azpiroz2, Dyanna Czeck3

1 Departamento de Geodinámica y Paleontología, Universidad de Huelva, Spain 2 Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Spain 3 Department of Geosciences, University of Wisconsin, USA fcarlos@uhu.es

Recognizing triclinic deformation paths in ductile shear zones is a difficult task. Objective criteria are needed fi rst to identify such types of complex deformation regimes in nature, and second to constrain the values of the controlling parameters, mainly the obliquity angle (φ), the kinematic vorticity number (Wk), and the extrusion obliquity angle (υ). In this work, a standardized procedure has been designed that exploits the method outlined by Czeck and Hudleston (2003), completed to take into account the complexities added by a localized nonvertical extrusion direction according to the theoretical model of Fernández and Díaz-Azpiroz (2009). The protocol includes considering information in fi ve steps, which must be sequentially applied to a given natural case. Step 1: orientation of the vorticity-normal section, as afi rst approximation it can be considered as the plane with the maximum fabric asymmetry, thereby providing the range of possible φ values. Step 2: Orientation of the mineral and stretching lineations. Step 3: Orientation of the X-axis of the measured finite strain ellipsoid. Information from steps 2 and 3 are compared with the λ1 patterns deduced from the theoretical model to obtain a range of Wk values. Step 4: Ellipticity (Rs) and orientation (angleθ) of the strain ellipse at the vorticity-normal section that is compared with theoretical Rs vs. θ curves to determine again a range of Wk values. The results of steps 2 to 4 are checked together and only those compatible combinations of φ , Wk and υ values are considered. Often, more than one such combination yields compatible solutions. Step 5: To further constrain the result, the measured finite strain ellipsoid is plotted on a Flinn diagram against the theoretical shapes derived from the distinct combinations obtained from steps 2 to 4.

The procedure has been successfully applied to a few examples of natural ductile shear zones with differing amounts of available information. Cases with complete information –kinematic data, fabric orientation, determination of fi nite strain ellipsoid-like the Wabigoon-Quetico boundary, allow an acceptable testing of the model, yielding valuable and constrained knowledge on the vorticity and obliquity of the shear zone. In shear zones with more limited data –kinematic data, fabric orientation-as is the Southern Iberian shear zone, only two or three of the described steps can be applied, and broad ranges of Wk and υ values are obtained. Even in this last case, the protocol guarantees an objective procedure to identify the triclinic nature of the shear zone, providing some helpful estimates of vorticity and obliquity.

REFERENCES Czeck, D., Hudleston, P.J. (2003). Testing models for obliquely plunging lineations in transpression: A natural example and theoretical

discussion. Journal of Structural Geology 25, 959-982. Fernández, C., Díaz Azpiroz, M. (2009). Triclinic transpression zones with inclined extrusion. Journal of Structural Geology 31,

1255-1269.





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