Deformation mechanisms, Rheology and Tectonics 2011 Abstract Volume

Testing transpression models in brittle shear zones compared with ductile deformation. A standardized procedure with contrasting approaches

Manuel Díaz-Azpiroz1, Leticia Barcos1, Carlos Fernández2, Dyanna Czeck3

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

Comparison between complex transpression models and data from natural shear zones proves to be difficult, especially because multiple variables can combine in different ways to produce comparable results. To avoid this ambiguity, Fernández et al. (this volume) propose a standardized and objective procedure to constrain three main transpression parameters: obliquity (φ), kinematic vorticity number (Wk) and extrusion obliquity (υ). This protocol was fi rst defined to be applied to ductile shear zones where fi nite strain markers are common. However, transpression has been identified also in relation to upper crustal deformation zones where strain is often highly partitioned into several structural domains and discrete structures (folds and faults). Therefore, the protocol is adapted to these brittle shear zones, comprising six successive steps.

Step 1: In ductile shear zones, transpression obliquity (φ) is deduced from the location of the simple shear component, as the intersection between the shear zone boundary (SZB) and the vorticity normal section (VNS). The section of maximum fabric asymmetry recognised in the field represents a good proxy for the VNS. In brittle shear zones, the simple shear direction can be directly obtained from slickenlines in representative discrete faults. Steps 2-3: Comparison between the orientations of the finite strain ellipsoids (FSE) deduced from ductile shear zones and that resulting from the model is rather simple using lineations and/or the orientation of the X-axis of the FSE. This task is quite more difficult in brittle deformation, where a single FSE imposed on a shear zone produces discrete structures that accommodate different components of the bulk strain. Fault population data, fold analysis and, if it is an active zone, earthquake focal solutions can be combined to obtain the FSE for each set of structures, which must be further evaluated to estimate the bulk FSE. Steps 4-5: Ellipticity (Rs), orientation (angle θ) of the strain ellipse at the VNS and the shape of the FSE obtained by the model are compared with similar data deduced for the shear zone to determine a range of Wk values. These data can be determined in ductile shear zones if reliable strain markers are present. In brittle shear zones, the shape of the FSE can be approximated via balanced cross-sections, by the shortening across the shear zone (Z-axis), the upward extrusion (X-axis) and the lateral extension (Y-axis). In steeply dipping shear zones, the maximum horizontal stretching axis would correspond to the Y-axis, and it can be deduced from the orientation of fold-axes and/or the maximum horizontal extension accommodated by normal faults. Step 6: This step applies only for recent or active oblique convergent tectonic limits (as many brittle transpression zones are) where information about plate velocities is normally available. The geometric relationship between plate velocity vectors and the orientation of the deformed zone permits to further constrain φ and Wk. Currently, this protocol is being applied to a brittle transpressional shear zone developed in relation to a recess zone within the fold-and-thrust belt of the Betic Cordillera (southern Spain).

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