The North China Craton contains one of the longest, most complex records of magmatism, sedimentation, and deformation on Earth, with deformation spanning the interval from the Early Archaean (3.8 Ga) to the present. The Early to Middle Archaean record preserves remnants of generally gneissic meta-igneous and metasedimentary rock terranes bounded by anastomosing shear zones.

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S U M M A R YVelocities of shear waves with different polarizations, as well as compressional-wave velocities, have been measured using a pulse transmission method in a series of physical models. The observations were made in a composite material filled with rubber inclusions, of variable aspect ratio, simulating cracks filled with a 'weak' material. The inclusions were oriented in a plane but distributed randomly in position. At a crack density of 5 per cent the experimentally measured velocities of shear and compressional waves are compared with results predicted by a theoretical model of Hudson (1981). Good agreement between the Hudson theory and the experiment is obtained for shear waves at all aspect ratios, but the agreement between theory and experiment for compressional waves is observed only for an aspect ration of 0.012. For dispersion analysis, velocities were obtained at different frequencies using a multiple-filtering technique. Results of dispersion analysis indicate that the maximum velocity dispersion for both shear waves (Sl,S2) occurs at a 0" angle of incidence and the minumum velocity dispersion occurs at a 90" angle of incidence. The variation of anisotropy as a function of aspect ratio and crack density can be related to Thomsen's (1986) anisotropy parameter y = (V,?/V,: -1)/2. This variation suggests that the crack density is more important than the effect of the aspect ratio. Finally, we show that the ratio VJV, is anisotropic. The present paper is a continuation of the elastic wave propagation discussed by Ass'ad et u1. 1993.

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