## 6 Conclusions  
This study provides a comprehensive synthesis of the frictional behavior of granitic faults, integrating key insights from theoretical frameworks and experimental observations. We first reviewed the classical rate-and-state friction (RSF) law, establishing its foundational role in modeling fault dynamics. Through a systematic comparison of dry and hydrothermal experiments on granitic fault gouges, we highlight distinct frictional responses (velocity-stepping, constant-velocity, and slide-hold-slide tests) under varying conditions. Our analysis emphasizes the critical roles of temperature, normal stress, pore fluid pressure, and slip velocity in modulating friction strength and sliding stability, revealing how these parameters interact to govern fault behavior across different regimes. Notably, we demonstrate that microstructural evolution--such as grain size reduction, pressure solution, and fluid-assisted healing--acts as a key mechanism driving transitions between velocity-weakening and velocity-strengthening regimes, with direct implications for earthquake nucleation and arrest. The key conclusions drawn are as follows:  
1. Granitic faults in the crystalline upper-to-middle continental crust represent key structures controlling the seismogenic behavior and long-term deformation of the continental lithosphere. Their frictional properties are thus central to understanding earthquake initiation and rupture dynamics.
2. Laboratory friction experiments, including velocity-stepping (VS), constant-velocity (CV), and slide-hold-slide (SHS) protocols, consistently demonstrate that granitic gouges exhibit measurable and reproducible frictional behavior. Notably, the VS tests reveal frictional stability regimes (velocity-strengthening or -weakening), while SHS tests provide quantitative estimates of fault healing and restrengthening during interseismic periods.
3. Microstructural observations indicate that shear deformation induces localized shear, significant grain size reduction, pressure solution creep, and mineralogical alteration within the fault gouge. These microphysical processes are sensitive to temperature, normal stress, pore fluid pressure, and strain rate and play a pivotal role in modulating the rheology and frictional response of the fault zone.  
In the continental crust, the interplay between microstructural evolution and frictional properties suggests that nucleation is favored in zones where brittle-frictional weakening mechanisms dominate but are bounded by ductile, velocity-strengthening regions that arrest rupture.  
Methodology, H.L.; Resources, S.L. and W.D.; Data curation, H.L., S.L., and W.D.; Formal analysis, H.L. and W.D.; Writing original draft, H.L., S.L., and W.D. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key Research and Development Program of China (Grant 2023 YFC3012001) and the National Nonprofit Fundamental Research Grant of China, Institute of Geology, China Earthquake Administration (Grant JB-24-31).  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
We gratefully acknowledge [PERSON] for his constructive reviews, which significantly improved the clarity of the manuscript.  
The authors declare no conflicts of interest.