Textural and microstructural evidence for semi-brittle flow in natural fault rocks with varied mica contents

Abstract

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±250 C. 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 cataclasites 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 was seismically active, and non-localised semi-brittle flow was episodically 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 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.

Surprisingly young Rb/Sr ages from the Simav extensional detachment fault zone, northern Menderes Massif, Turkey

“…We therefore consider these rocks as low-grade mylonites and/or foliated cataclasites (Fig. 5a), similar to those described in the literature (Chester et al, 1985;Lin, 1997Lin, , 1999Lin, , 2001White, 2001;Edwards and Ratschbacher, 2005;Lin et al, 2005). We prefer not to choose between low-grade mylonites and foliated cataclasite terminology because 'there is a gradual transition between cataclasites and low-grade mylonites' (quoted from Trouw et al, 2010).…”

“…Our observations are consistent with the literature where the low-grade mylonites/foliated cataclasites are commonly interpreted to reflect mixed deformation mechanisms with contemporaneous operation of cataclastic flow (cf. Tulis and Yund, 1987;Menéndez et al, 1996), semi-brittle flow (Hirth and Tullis, 1994;Kohlstedt et al, 1995) and solution transfer deformation mechanisms (White, 2001). The deformation regime in the detachment fault zone is thought to be fluid-assisted semi-brittle flow, facilitated by crystal-plastic deformation mechanisms (cf.…”

Heterogeneous strength and fault zone complexity of carbonate-bearing thrusts with possible implications for seismicity

“…Wojtal and Mitra, 1986;Prior and Behrmann, 1990;Willemse et al, 1997;Lacroix et al, 2011;Gratier et al, 2013). Therefore, we think that our integrated analysis has broad applicability to faults hosted in carbonatic multilayers worldwide.…”

Microfabric diversity and grain shape analysis of fault rocks from the selected areas of the Western Tatra Mountains

“…In the DB de for ma tion com plexes dy namic rec rystallisation by bulg ing (BLG; Passchier and Trouw, 2005) is ac tive, form ing fine-grained quartz ag gre gates. Tem per a tures dur ing these pro cesses were less than 400°C (Hirth and Tullis, 1992;White, 2001).…”