Study on finite strain and its relationship with magnetic fabric along Ailao Shan‐Red River shear zone

Abstract

The finite strain along Ailao Shan‐Red River shear zone has been studied. The axial ratios of strain markers and shear strains vary greatly, arising principally from deformation intensity and from contrasting competencies. Ellipticity of chloritic amygdales in basalt ranges from 6.79 to 10.47 (corresponding shear strain γxz = 2.22–2.93), and ellipticity of garnets from 1.56 to 2.95 (γxz = 0.45–1.14) in core gneiss. The S‐C angle of the sheared rocks varies from 2° to 28° (γsc = 1.35–28.60), mostly in a range 2° to 7°. Optical observation, thermal demagnetization, and hysteresis curves were conducted to determine the nature of minerals and the contribution of paramagnetic and ferromagnetic minerals to magnetic susceptibility. The result shows that ferromagnetic minerals and hornblende dominate the magnetic fabric of samples with high to very high susceptibility, and play an important role in the magnetic fabric of samples with low to intermediate susceptibility. The ferromagnetic minerals are mainly pseudo‐single‐domain and multidomain Ti‐poor magnetite. Most samples have their magnetic lineation lying between the C and S surfaces. In the remaining samples, deviations from this common alignment principally resulted from the magnetic minerals being deflected by porphyroclasts, mineral cleavage, and cracks. The axial ratio correlation is very poor between the magnetic and actual or imaginary strain ellipsoid calculated from the S‐C fabric. This poor correlation suggests that the shape of the finite strain ellipsoid cannot be evaluated using the magnetic fabric when the latter is a combined result from multiple minerals, and especially when the shape effect of multidomain titanomagnetite is important.