Extremely fractured quartz within Milun fault zone: implications for pulverization

Fault rocks are influenced by physical conditions such as frictional properties, temperature, effective normal stress, and differential stress. Their formation is examined with respect to energy distribution in fault zones, fault slip velocity, and etc. In this study, the fault-zone materials were retrieved from Hole-A of MiDAS project and were examined at 491.3 m in which the extremely fractured quartz was found in the vicinity of upper boundary of the active Milun Fault zone. The quartz was analyzed using optical microscopy, X-ray powder diffraction (XRD), and synchrotron XRD and Laue diffraction to understand their microstructures and potential deformation mechanisms. Microstructural observations showed angular, extremely fractured quartz grains with intragranular fracturing and no significant shear strain. XRD analyses showed a notable rightward peak shift in the 491.3 m quartz compared to quartz from other depths (389.1 m and 505.45 m), suggesting compressive stress-induced strain. Synchrotron-based XRD confirmed the absence of amorphous phases, indicating the quartz experienced rapid brittle deformation rather than prolonged shear. Laue diffraction demonstrated significant lattice distortion and high residual stress within the quartz, further supporting a mechanical origin. On ther other hand, triaxial compression tests on synthetic quartz were conducted to simulate deformation under semi-static deformation conditions. These tests revealed that strain localization are inconsistent with observations from the extremely fractured quartz. Based on these findings, thermal fracturing and comminution due to shear deformation were excluded as primary mechanisms. Instead, the results suggest that the pulverized quartz formed under extreme high strain rates likely associated with seismic rupture dynamics. This study provides a comprehensive microstructural characterization of pulverization, advancing our understanding of fault zone processes during earthquakes.