Scales of Deformation and Strain Localization in the Lower Crust, Arunta Block, Australia

The Arunta block, central Australia, is a well-known exposure of lower crustal material exposed at the earth's surface. These lower crustal granulites preserve deformation fabrics developed at lower crustal conditions (7-9 kb, 700-900\deg), and are largely unmodified by the younger orogenic events that exhumed it. Our work, to date, concentrates on the mafic granulites of the Mt Hay region. This contribution focuses on two aspects of our geological investigations, resulting from detailed (1:500 and 1:12,000) geological field studies: 1) The scales of observed lithological heterogeneity; and 2) The recognition of strain localization in the lower crust. Lithological heterogeneity occurs over several orders of magnitude from 0.01 to 1000 m at Capricorn ridge. At the 100 m scale, the most relevant scale for seismic work, heterogeneity is defined by major lithological changes: anorthositic granulite, pyroxene granulite, pyroxene-plagioclase granulite, and "transitional" zones. Contacts between layers are abrupt, occurring over less than 1 m across strike. The contact zones are semi planar in map view (~20\deg variation in strike). These lithologic domains extend parallel to strike for several km. Direct observation cannot be made at scales larger than 100 m because of the lack of sufficient outcrop. As a sidenote, no lattice preferred orientation of quartz or feldspar occurs. Demonstrable localization (shear zones) at lower crustal conditions occurs on the 100 m scale on Capricorn Ridge, with respect to the less deformed main section of Mt. Hay. Localization is characterized as cm- to m-scale segregations of different lithologies, and rapid changes in lithologies. In general, strain seems to localize preferentially in the quartzofeldspathic domains in this setting.