Abstract: Dynamic shear coalescence of rock discontinuities dominates the dynamic failure process of rock masses in landslides and other geohazards. However, the influence of mineral heterogeneity on the dynamic shear behavior of non-persistent discontinuities remains poorly understood. This study built a grain-based model(GBM)in UDEC to account for mineral heterogeneity, and performed numerical direct shear tests on granite rock bridges at various shear rates. We investigated how structural heterogeneity and material heterogeneity affect the shear rate dependence of rock bridges and the associated microcracking mechanisms. Three structural heterogeneity levels(So=1.09, 1.16, 1.87)and three minerals(quartz, feldspar, hornblende)were used, with single-mineral models as control groups. Results show that: (1)Shear strength of rock bridges rises with increasing shear rate, and the rate effect becomes stronger as structural heterogeneity increases. (2)Higher shear rates reduce wing-crack angles and concentrate microcracks in the rock bridge zone. (3)The ratio of transgranular to intergranular cracks increases with shear rate, which explains the rate-dependent strength at the microscale. (4)Under conditions of high structural heterogeneity(So=1.87), material heterogeneity further intensified stress concentration, significantly increased the number of transgranular cracks, and enhanced the rate dependence of shear strength. This study reveals the controlling role of transgranular-intergranular crack evolution in the dynamic shear behavior of rock bridges, providing a theoretical basis for understanding the dynamic shear instability of discontinuities in crystalline rocks and the initiation of dynamic instability in landslides containing non-persistent discontinuities.