The architecture of long-lived fault zones: insights from microstructure and quartz lattice-preferred orientations in mylonites of the Median Tectonic Line, SW Japan

Abstract

We combine field mapping with quartz microstructure and lattice preferred orientations (LPO) to constrain the mechanisms and spatio-temporal distribution of deformation surrounding the Median Tectonic Line (MTL), SW Japan. In the study area, the MTL occurs either as a narrow gouge zone or as a sharp contact between hangingwall quartzofeldspathic mylonites to the north and footwall pelitic schists to the south. Along the northern margin of the MTL, there exists a broad zone of mylonitic rocks, overprinted by cataclastic deformation and a damage zone associated with brittle deformation. The mylonitic shear zone is dominated by coarse-grained protomylonite up to~100 m from the MTL, where fine-grained ultramylonite becomes dominant. We observe a systematic variation in quartz LPO with distance from the MTL. In protomylonites, quartz LPOs are dominantly Y-maxima patterns, recording dislocation creep by prism<a> slip at~500°C. Closer to the MTL, we observe Rand Z-maxima, and single and crossed girdles, reflecting dislocation creep accommodated by mixed rhomb<a> and basal<a> slip, likely under cooler conditions (~300°C-400°C). Some ultramylonite samples yield weak to random LPOs, interpreted to result from the influx of fluid into the shear zone, which promoted deformation by grainsize-sensitive creep. Following cooling and uplift, deformation became brittle, resulting in the development of a narrow cataclasite zone. The cataclasite was weakened through the development of a phyllosilicate foliation. However, healing of fractures strengthened the cataclasites, resulting in the development of anastomosing cataclasite bands within the protomylonite.

Poly-cataclasites: Implications to the Seismic Cycle

“…The present study focuses mainly on the region seen on the map in Fig 1. This region has relatively simple deformational history and was thought be a good location to study the early stages of the MTL 2 as overprinting of later brittle stage (post-Ichinokawa phase) is less significant eg 29,30 .…”

“…The Ryoke granitoids in the field region was previously describe to have originated from two main protolith namely granite and tonalite with geological distribution as seen in Fig 1 29,30 . Numerous brittlely deformed domains was observed in this region.…”

“…The microstructures for the interseismic period on the other hand is usually described as the sealing of fractures through the precipitation of minerals and compaction 25,26 . This paper attempts to related the seismic cycle phases to natural samples that had typically undergone continuous polyphase deformation throughout its history, 15,27 also known as polycataclasites 28 . Investigation was conducted on the exhumed fault rocks of the Ryoke Granitoids in Tsukide, Mie Prefecture, Japan that has undergone direct influence from the activity of the MTL from the Kashio until the Ichinokawa phase 29,30 . Detailed descriptions of the poly-cataclasites' microstructural and mineralogical heterogeneity allowed us to unravel its development with respect to the seismic cycle phases 31 .…”

Fluid‐Assisted Strain Localization in Quartz at the Brittle/Ductile Transition

“…Propagation of shear bands outside of the coarse and stiff quart porphyroclasts might have been promoted by temperature decrease. This might be a local effect in the Calamita Schists, as several papers documented that in retrograde shear zones, domains that deformed by basal <a> and rhomb <a> slip tend to be relatively narrow compared to the domains, which registered higher-temperature prism <a> deformation, as a result of strain localization during decreasing temperature (see, e.g., Shigematsu & Yamagishi, 2002;Shigematsu et al, 2009;Czertowicz et al, 2019). This might be a local effect in the Calamita Schists, as several papers documented that in retrograde shear zones, domains that deformed by basal <a> and rhomb <a> slip tend to be relatively narrow compared to the domains, which registered higher-temperature prism <a> deformation, as a result of strain localization during decreasing temperature (see, e.g., Shigematsu & Yamagishi, 2002;Shigematsu et al, 2009;Czertowicz et al, 2019).…”

“…The presence of strong Y-max textures possibly hindered dislocation creep by basal <a> slip, which should be energetically favored at low-temperature conditions (geometric hardening; Poirier, 1980;Toy et al, 2008), forcing early strain localization in phyllosilicate-rich layers and shear bands. This might be a local effect in the Calamita Schists, as several papers documented that in retrograde shear zones, domains that deformed by basal <a> and rhomb <a> slip tend to be relatively narrow compared to the domains, which registered higher-temperature prism <a> deformation, as a result of strain localization during decreasing temperature (see, e.g., Shigematsu & Yamagishi, 2002;Shigematsu et al, 2009;Czertowicz et al, 2019). Therefore, domains that deformed by basal <a> slip might be present in the Calamita Schists and possibly have registered a different evolution at low metamorphic-grade conditions.…”