A Symmetry-based Method for Kinematic Analysis of Large-slip Brittle Fault Zones

We apply a symmetry-based, statistical method for kine­ matic analysis of mesoscale fault-zone structures to resolve the trans­ port directions for the Rosario fault zone and Lopez StructuralCom­ plex in the San Juan Islands. Both moderately dipping fault zones accommodated large amounts of Late Cretaceous slip ( > 30 km), but the transport direction of nappes overlying these fault zones is controver­ sial. Individual asymmetric folds and Riedel composite structures consisting of arrays of fault-related planar shear fractures and folia­ tions are represented by an internal rotation axis, which describes the sense of shear and orientation of each structure. Synoptic stereo­ graphic analyses show that the internal rotation axes lie in a plane parallel to the fault zone but are widely dispersed within it. The fault-parallel distribution of internal rotation axes has a Gaussian form, centered around a mean direction. We propose that the mean of the distribution reflects a determinis­ tic process, the monoclinic symmetry of progressive deformation in an idealized, non-coaxial shear zone, and that stochastic and antecedent processes result in random variations about the mean. Therefore, the circular mean of the distribution is used to define a synoptic internal rotation axis, which according to the model should he normal to the mirror plane of symmetry for the deformation and hence normal to the average slip vector for the fault zone. The symmetric Gaussian disper­ sions of internal rotation axes in the Lopez and Rosario fault zones are hypothesized to have resulted from processes such as local perturba­ tions in the orientation of layering with respect to the slip direction or shear plane or rotations about fault-normal axes. Sfip vectors derived from synoptic analyses of mesoscale folds are top-to-the-south for the Lopez fault zone and top-to-the-southwest for the Rosario zone. Riedel stru' ctures in the Rosario fault zone formed within two conjugate sets of localized shear zones; slip on one set was top-to-the-northwest and on the other, top-to-the-southeast. The inter­ nal rotation axes for the Riedel structures have a bimodal distribution and inferred orthorhombic symmetry indicating bulk northeast­ southwest shortening normal to the Rosario zone. The overall deforma­ tion in the zone is interpreted as a general non-coaxial shear with monoclinic symmetry. * Department of Geological Sciences, U niversity of Washington, Seattle, Washington 98 1 95. ·r, ** Department of Geology and Geophysics, Yale University, New Haven, Connecticut Ob.JI I.