The Walls Boundary Fault zone and the Møre Trøndelag fault complex : a case study of two reactivated fault zones
It is commonly observed that ancient faults or shear zones can become reactivated again and again, either within the same or even superimposed tectonic episodes, yet millions of years apart. Rocks of the continental crust show such effects particularly well, owing to their longevity, because through their buoyancy, continental rocks resist recycling back into the Earth's mantle over long time-scales. The Møre Trøndelag Fault Complex (MTFC), Central Norway and the Walls Boundary Fault (WBF), Shetland, were studied to elucidate the kinematic, geometric and textural evolution, in order to assess fault linkages, fault-rock preservation styles and the controlling factors on fault reactivation The WBF is a crustal-scale, reactivated fault that separates distinctively different basement terranes; the Caledonian front to the west from Dal radian type rocks to the east. The WBF initiated as a late-Caledonian sinistral strike-slip fault (c.l00-200km offset) associated with the development of mylonites and cataclasites. Dextral strike-slip reactivation (c.65km) in the Permo-Carboniferous related to inversion of the Orcadian Basin and led to the development of cataclasite and fault gouge assemblages. Later dip-slip and finally sinistral strike-slip (c.l5km. Tertiary?) reactivation were localised within earlier formed fault gouges. The ENE-WSW-trending MTFC in Central Norway is a 10-20 km wide, steeply dipping zone of fault-related deformation. The MTFC has a prolonged and heterogeneous kinematic history. The complex comprises two major fault strands: the Hitra-Snasa Fault (HSF) and the Verran Fault (VF). These two faults seem to have broadly initiated as part of a single system of sinistral shear zones during Early Devonian times (409+12 Ma). Sinistral transtensional reactivation (dated as Permo-Carboniferous; 291 + 14 Ma) of the ENE-WSW-trending HSF and VF led to the development of cataclasites and pseudotachylites together with the formation of N-S-trending faults leading to the present-day brittle fault geometry of the MTFC. Several later phases of reactivation were focused along the VF and N-S linking structures during the Mesozoic probably related to Mid- Late Jurassic/Early Cretaceous rifting and Late Cretaceous / Early Tertiary opening of the North Atlantic. Based on apparent offshore trends, it has been suggested that the MTFC and the WBF may have been linked at some stage during their evolution and subsequent reactivation. This is consistent with the present study, as early Devonian movements along both the WBF and the MTFC are sinistral. Differences in the magnitude, dynamics and senses of displacement in the Permo-Carboniferous, however, seem to militate against linkage of these faults in the late Palaeozoic. There is no compelling evidence for direct Mesozoic or Tertiary linkage, although both structures were reactivated to some extent during these times. It seems that the formation and reactivation of the WBF and MTFC were associated with broadly similar regional tectonic processes and therefore, to some extent, share similar kinematics. Although both the MTFC and the WBF show clear proof of repeated reactivated, superficially similar geometries or alignments should not be used as a basis for correlating structures, in the absence of direct kinematic evidence. Displacements along the MTFC and the WBFZ are repeatedly localised along the earlier formed fault rocks, suggesting that these fault rocks are intrinsically weak compared to the surrounding rocks. A complex interaction exists between the geometrical properties of the fault network and fault-zone weakening mechanisms operative within fault rocks around the level of the frictional-viscous transition. Together these factors control fault reactivation in the long term. In the case of reactivated, sub-vertical, strike-slip fault zones the preservation and exhumation of these fault rocks both depend on the architecture and magnitude of later reactivations.