Deformation mechanism transitions during the seismic cycle recorded by quartz CPO in fault-related silica layers

Silica layers composed of quartz grains typically a few micrometers or smaller are texturally distinct from typical quartz veins and occur as μm- to mm-thick layers along fault slip zones. The ultrafine quartz within these layers exhibits uniform interference colors in optical microscopy. Such features are commonly interpreted as indicating crystallographic preferred orientation (CPO), a fabric typically associated with ductile deformation but developed within brittle fault zones. Despite their widespread occurrence in faults developed in various rock types, the deformation mechanisms and ultrafine quartz CPO-forming mechanisms through the seismic cycle remain poorly understood.

In this study, we analyze the microstructures of silica layers observed in three upper crustal faults in Korea developed in sedimentary rocks, granite, and rhyolite within the Cretaceous Gyeongsang Basin, where average burial depths reach ~6 km. All observed natural silica layers are composed of fine-grained quartz (<2 μm. These layers display uniform interference colors in optical microscopy, while EBSD analyses reveal clustering of quartz c-axes. However, some faults are characterized by densely packed comminuted grains with nanopores, whereas others display polygonal quartz grains together with nanopores and illite aligned parallel to the fault plane, as well as shape-preferred orientation of quartz and adjacent calcite grains. These observations suggest that ultrafine quartz within silica layers may have experienced diffusion-related processes in the presence of fluids.

To investigate whether diffusional processes active after fault slip and cataclasis affect CPO development, we conducted hydrothermal rotary shear experiments on single-crystal quartz gouge (<63 μm) under identical P-T-fluid conditions (600°C, effective normal stress of 120 MPa, pore fluid pressure of 80 MPa) using three different velocity histories: (1) fast slip alone (V=300 μm/s), (2) fast slip followed by slow slip (V=0.1 μm/s), (3) fast slip followed by hydrothermal holding without further shear (22 h). The fast slip produces intense comminution within slip localized zones without the development of CPO-like features. In contrast, both the subsequent slow slip and hydrothermal holding result in the development of CPO-like features at the optical scale within the grain-size-reduced zones, accompanied by surface indentations on larger quartz grains and linear aggregates of euhedral ultrafine quartz. However, TEM observations reveal that ultrafine quartz grains within these zones display random crystallographic orientations, with no evidence for a preferred orientation.

Integrating natural and experimental observations, we interpret silica layers to form through a two-stage process: intense grain-size reduction by comminution during seismic slip, followed by fluid-assisted, time-dependent reorientation of ultrafine quartz during post-seismic or interseismic periods. Silica layers characterized by CPO-like features at the optical scale therefore record transitions in deformation mechanisms during the seismic cycle and provide key geological constraints for understanding slip behavior, mechanical properties, and the role of fluids in upper crustal faults. Further investigation is required to clarify the relationship between these optical features and crystallographic orientations at the nanoscale.