## 1 Introduction  
Seismic fault mechanics have traditionally focused on the brittle upper crust of tectonically active regions. In classical models, deformation below the brittle-ductile transition (BDT) is thought to be governed by crystal plasticity, while earthquakes are attributed to rapid brittle failure. Consequently, seismic activity was not expected to occur in the deeper, ductile portions of the crust. However, recent observations challenge this assumption. Seismic nucleation has been documented in strong lower crustal rocks within the continental lithosphere, particularly beneath the BDT [1]. In the upper crust, deformation primarily occurs via brittle processes such as frictional sliding and faulting, whereas dislocation creep dominates in the middle and lower crust under thermally activated conditions [2].  
During the seismic cycle, the deformation mechanisms of fault rocks are significantly influenced by temperature, strain rate, and pore fluid pressure [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. The BDT is thus recognized as a critical zone for controlling the depth extent of seismicity in the continental crust [15, 16], with its behavior highly sensitive to temperature, strain rate, and pore fluid pressure [11, 12, 13, 14, 17]. Importantly, deformation mechanisms within this transition zone vary across different seismic phases--includingoseismic loading, post-seismic relaxation, and interseismic creep--due to variations in strain rate and fluid conditions [11, 12, 13, 14, 18]. During interseismic periods, low strain rates promote ductile flow, while higher strain rates during coseismic and post-seismic phases favor brittle failure.  
To fully understand the transient creep behavior and strain rate sensitivity of rocks in the BDT during the seismic cycle, it is essential to investigate the microstructures and deformation mechanisms preserved in deep fault rocks [19]. Key evidence for such processes comes from pseudotachyltyte and mlonite, which together record the complex interplay of brittle and ductile deformation in the lower crust [20]. Mylonites form under ductile conditions in shear zones, while pseudotachylytes typically originate from high-speed coseismic slip, producing solidified frictional melt along fault surfaces [21, 22, 23, 24, 25].  
Traditionally, pseudotachyltyte has been considered a product of dry, high-velocity frictional melting [23, 26]. However, new studies suggest that pseudotachylyte may also form during slower processes such as ductile creep, microseismic rupture, and slow slip events [1, 19, 27, 28]. Furthermore, pseudotachyltyte formation has been observed in fluid-rich fault zones [29] and may be preceded by intense fragmentation and grain size reduction [30]. The preservation of pseudotachylyte is also problematic due to its susceptibility to post-seismic alteration and weathering during exhunation [26, 31, 32]. Nevertheless, pseudotachyltyte has now been documented at depths of 22-50 km [33, 34, 35], indicating that earthquake processes can extend well below the traditionally defined BDT. Microscale amorphization structures observed in both natural fault gouges and experimental granite deformation further suggest that intense shear, rather than melting alone, can produce pseudotachylyte-like textures [36, 37, 38, 39].  
The Anning River fault zone provides an ideal natural laboratory for such research. We aim to investigate the chemical composition, microstructures, and deformation mechanisms of pseudotachylytes, cataclasites, and mylonites from the brittle-ductile transition zone in this study. Based on the deformation and petrogenetic origin of pseudotachylyte, a deformation model was established that captures the behavior of fault rocks during coseismic, post-seismic, and interseismic periods in the brittle ductile transition zone, thereby contributing to a more comprehensive understanding of seismic fault dynamics.