SCALE DEPENDENT INFLUENCE OF PRE-EXISTING SHEAR ZONES ON BRITTLE FAULT GEOMETRY, ARCHITECTURE AND FLUID FLOW STRUCTURE DURING RIFTING

Pre-existing structures are invariably present when continental rifting initiates and are commonly thought to exert strong controls on the geometry and kinematics of rift systems. Here, we present field and microstructural observations from faults in northeast Brazil that demonstrate the influence of pre-existing shear zones on subsequent brittle fault development is scale dependent. At the regional scale, syn-rift Cretaceous faults trend parallel to subvertical, crustal-scale Brasiliano (c. 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust, indicated by shear-wave splitting, controlled the overall orientation and kinematics of the rift faults. However, at scales up to hundreds of meters, mylonitic foliations have little influence on fault architectures. Faults crosscut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. Changes in fault rock mechanical properties were the most important control on the development of deformation features and coupled fluid flow structures through time. Initially, flowing structures localized upon subtle ductile heterogeneities. Following brittle fault initiation, both the fault core and damage zone acted as conduits for fluid flow. In the later stages of faulting, pseudotachylyte welding created a strong, low-permeability fault core and fully annealed damage zone. During exhumation, the welded fault zone acted as a mechanical heterogeneity that caused long, open fractures to propagate perpendicular to the fault. These open fractures form a modern hydraulically conductive zone surrounding the fault that is illuminated by earthquakes triggered by water level fluctuations in a surface water reservoir. The conductive zone extends 100s of meters from the edge of the annealed damage zone, and is far wider than would be predicted using traditional fault scaling relationships. Our results show that although faulting may be strongly influenced by the presence of a pre-existing shear zone at the regional scale, at the field scale, complex coupled hydraulic and mechanical interactions can exert a far greater influence the spatial and temporal evolution of structures.