Deformation Processes in Fault Rocks

Deformation Processes in Fault Rocks

Textural and microstructural evidence for semi-brittle flow in natural fault rocks with varied mica contents (1)
S.R. White
Fig. 1 (a) Location of Siberia Fault Zone in South Island, New Zealand. (b) Structural map of SFZ and related Cenozoic structures.

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.
Fig. 2 (a) Field expression of SFZ; height of exposure c. 100 m. Sp is country rock schistose foliation. (b) Sketch from photograph showing example of interleaved foliated fault rocks. (c) Tightly folded and intercalated green (gn) and grey (gy) foliated cataclasite.
Fig. 4 Hand sample and thin section photographs of foliated cataclasite. (a) Partially aligned subangular quartz clasts in mica-rich matrix. (b) Banded and foliated cataclasite with mica-poor (light grey) and mica-rich (dark grey) domains. (c) Microscopic fabric of domain with up to 60% mica. (d) Contact between quartzofelspathic domain (top) and mica-rich domain (centre and lower right). (e) Quartzofeldspathic(qf)-rich domain. (f) Very well foliated cataclasite with greater than 90% phyllosilicates. 
Fig. 8 Plane polarised light ultrathin section (=0.015 mm) views of muscovite in foliated cataclasite. (a)Overview of the matrix texture. (b) Detail from (a) showing undulose extinction textures. (c) Kinked muscovite with subgrains, parting along cleavage.  
(d) Single muscovite flake reduced to a mosaic of subgrains.
Fig. 9 SFZ fault rocks placed in a conceptual model for fault zones (based on Sibson 1983, Scholz 1988, Shimamoto 1989). The rocks probably formed in, or close to, the brittle-ductile transition for grain-scale deformation, corresponding to the field of semi-brittle rheology, and the interval of maximum shear strength for continental rocks.

(1) published in International Journal of Earth Sciences (Geol Rundsch), 90: 14-27 (2001)
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