LITHOLOGIC HETEROGENEITY AND STRUCTURAL ORDERING IN SHEAR ZONES: POSSIBLE IMPLICATIONS FOR SEISMIC ANISOTROPY IN THE LOWER CRUST

As a general rule, internal organization of structural elements will increase with increasing strain in metamorphic tectonites. This ordering fundamentally reflects variations in the mechanical behavior of lithologic domains in polymineralic rocks. These physical domains will deform at different rates, resulting in localization of strain in weaker materials and along the boundaries of lithologic domains. This is true even where the rock has not deformed by dislocation creep. In such cases, the rock will not exhibit a strong crystallographic preferred orientation, but may show preferred alignment of other structural elements.

We illustrate this point by comparing the geometry and internal structure of moderate versus high strain domains produced by deformation in the lower crust (7-9 kb, 700-900°C). Detailed (1:500 and 1:12,000) geological field studies in the Arunta Block, central Australia, document lithologic heterogeneity over several orders of magnitude, from 0.01 to 1000 m, in the Mt. Hay area. Microstructural analysis shows no evidence for dislocation creep. Mt. Hay ridge is a >5 km thick zone of moderate strain, in which primary structures are locally preserved, and primary compositional domains have irregular shapes and thicknesses on the order of 10-100 meters. The adjacent, >4 km thick, high strain zone (Capricorn ridge) is characterized by the transposition of igneous intrusive bodies into parallelism with mesoscale compositional layers and lenses as small as 0.01 m thick, and the shape preferred orientation of mineral grains, which together define foliation. Strain is highest along major lithologic boundaries. Overall, the boundaries of lithologic domains from the 0.01 to 1000 m scales, as well as the dominant grain boundaries, become more planar and parallel with increasing strain, and the spacing of compositional bands decreases. We suggest that these attributes of the higher strain zones may constructively enhance the larger scale lithologic contrasts, producing a significantly greater seismic anisotropy, and perhaps reflectivity, than adjacent domains with similar bulk composition but less ordered internal structure.