Deformation behavior of perthitic feldspar under brittle–ductile transition conditions: effects of pre-existing lamellar fabric

 Large-magnitude earthquakes in the continental crust predominantly occur near the brittle–ductile transition zone, where the deformation behavior of rocks plays an important role in earthquake nucleation and energy release.Rocks deforming under high-temperature and high-pressure conditions within the brittle–ductile transition zone may exhibit mechanical responses controlled not only by temperature and stress level, but also by pre-existing microstructural features; in particular, perthitic feldspar, a widespread feldspar solid solution in the crust, commonly contains exsolution-related lamellar structures that may introduce orientation-dependent deformation behavior.Despite its common occurrence in mid-crustal rocks, the influence of pre-existing lamellar fabric orientation on the deformation behavior of perthitic feldspar, especially under brittle–ductile transition conditions, remains poorly constrained by experiments.Based on this background, we conducted high-temperature and high-pressure deformation experiments using a Griggs-type solid-medium apparatus to systematically investigate the deformation behavior of perthitic feldspar with different pre-existing lamellar fabric orientations.Samples were prepared with lamellar orientations at angles of 0°, 45°, and 90° relative to the maximum principal stress, and deformed at a confining pressure of 1 GPa, over a temperature range of 600–1050 °C, at strain rates ranging from 5 × 10⁻⁵ to 2 × 10⁻⁶ s⁻¹. Microstructures of the samples before and after deformation were characterized using scanning electron microscopy and electron backscatter diffraction, and the mechanical responses and microstructural features were compared among samples with different fabric orientations.The mechanical results show significant differences in peak strength among the three lamellar fabric orientations, with sample strength decreasing in the order of 45°, 0°, and 90° at the same temperature.All samples entered a plastic deformation regime above 800 °C (σ_d<Pc).Microstructural observations reveal that at low temperatures (<900 °C), pervasive brittle cracks crosscut both feldspar phases and are accompanied by localized ductile shear zones; at intermediate temperatures (900–950 °C), cracks are mainly confined within albite grains and are commonly oriented perpendicular to grain boundaries; at high temperatures (>950 °C), samples exhibit bulk plastic flow with a marked reduction in cracking.Notably, samples with a 45° lamellar orientation experienced pronounced bulk fragmentation at 1000 °C and 1050 °C.EBSD results show that K-feldspar does not develop significant changes in crystallographic preferred orientation during deformation, whereas albite exhibits progressively heterogeneous orientation patterns with increasing temperature, consistent with plastic deformation associated with subgrain rotation recrystallization.Together, the mechanical and microstructural results demonstrate that pre-existing lamellar fabric orientation exerts a significant influence on the deformation behavior of perthitic feldspar under brittle–ductile transition conditions, providing experimental constraints on strength anisotropy in feldspar-rich rocks.