Field and petrographic analysis of mylonitic fabrics : implications for tectonic corrugation development, Tanque Verde Ridge, Arizona, USA

Tanque Verde Ridge (TVR) in Saguaro National Park, Arizona, USA, represents a large-scale lineation-parallel ridge (tectonic corrugation) within the Santa Catalina-Rincon Core Complex. Field and petrographic features record a history of lineation-parallel extension and boudinage occurring across a ductile to brittle deformational continuum during core complex exhumation. Lineation-perpendicular ductile to brittle features suggest the importance of extension orthogonal to lineations during exhumation and need to be factored into the debate on tectonic corrugation evolution. TVR is a foliation-defined antiformal flexure, concordantly bounded at the toe by a brittle detachment fault system. The footwall mylonite is strongly foliated (suggestive of flattening, rather than constriction) and lineated (WSW-trending). Ductile, micro- (thin section) to macro- (hundreds of meters) scale features are oriented both lineation-parallel (e.g., S-C' fabrics, asymmetric porphyroclasts, shear zones) and perpendicular (e.g., shear zones, asymmetric quartz fabrics). These features record dominantly WSW, and secondarily NNW and SSE, directed non-coaxial shear, respectively. Across the corrugation, foliation flexures exhibit both lineation-parallel and perpendicular fold axes. Distended necks of micro- to meso-scale boudins contain NNW-SSE striking zones of intense brittle fracturing, brecciation and cataclasis. The ductile, mylonitic foliation is passively affected across these boudin neck zones as foliation dip varies from sub-horizontal to >45°. This suggests a temporal link between mylonite formation, extension-related deformation and the development of the corrugated surface. Oriented thin sections and field observations from across TVR and across outcrop-scale corrugation-like features constrain the conditions during mylonite development to lower-amphibolite facies and the spatial relationship of micro- to meso-structural features across the corrugation. On the basis of field and petrographic evidence, a new model for the evolution of tectonic corrugations is proposed, whereby original anastamozing shear zones within the deep-seated, ductile shear zone are amplified in response to bidirectional extension (including boudinage, and spatially related brittle fracturing). Shear zone modification is progressive and is synchronous with non-coaxial simple shear. Overall flattening of the shear zone is predicted in response to this extension. Notably, mylonitic fabrics at TVR do not record a history of large-scale constrictional deformation. Tectonic corrugations can initiate at depth as a result of original shear zone morphologies, be modified by boudinage during bidirectional extension, and be translated up-dip during progressive core complex exhumation. Corrugations are manifestations of deep-seated, multi-scale processes that influence shear zone dynamics during large-scale crustal extension.