Fault rocks from the Siberia Fault Zone (SFZ) in southern New Zealand are derived from schists with varied mica contents. Regional evidence indicates that the rocks are exhumed from depths of 8-10 km and temperatures of 200-250ºC. Foliated cataclasites in a zone 5-40 m thick are accompanied by interlaced pseudotachylyte, and are cut through by a late-stage master fault and zones of random fabric cataclasite.
Textures and microstructures in the foliated cataclasite reflect contemporaneous or cyclic operation of cataclastic, crystal-plastic and solution transfer deformation mechanisms, partitioned differently between different phases.
The deformation regime is interpreted as a form of semi-brittle flow, facilitated by crystal-plastic deformation of phyllosilicate phases in a relatively weak interconnected matrix. Quartz and feldspar are deformed mainly by cataclasis. The presence of pseudotachylyte indicates the fault zone was seismically active, and non-localised semi-brittle flow was punctuated by high strain-rate earthquake events.
Late-stage formation of a discrete master fault probably reflects a change from semi-brittle flow to brittle faulting. The presently exposed level of the fault zone is thought to represent a section of the mid-crustal brittle-ductile transition in the seismogenic zone. Thus, this study provides a tangible natural example of theoretically and experimentally predicted fault rocks.