TRANSITION FROM STRAIN PARTITIONING TO GENERAL SHEAR IN ULTRAMYLONITES BELOW THE BRITTLE-DUCTILE TRANSITION

Partitioning of oblique kinematics into orthogonal strike-slip and dip-slip components is well documented for shallow structural levels in transpressional and transtensional structures, but at depths below the brittle-ductile transition, strain partitioning is not energetically favored. High strain zones that form at upper greenschist and higher metamorphic grades in transpression zones commonly exhibit general shear characteristics rather than superposed simple shear fabrics. Recognition and interpretation of general shear in very high strain and high strain rate rocks depends upon correct identification and interpretation of both the extension direction and the transport direction, which may be difficult to determine in thick ultramylonite zones. An example is illustrated from a transpressional terrane boundary in central Argentina where high strain rate ultramylonite fabrics that formed over a depth range from ca. 8km (minimum) to 22 km (maximum)are exposed. Foliations and lineations are rare, especially in exceptionally thick ultramylonite zones, but where present indicate down-dip extension. Evidence for substantial strike-slip movement is deduced from relatively small vertical displacements on long (>250km) and locally thick (up to 16km) high strain zones as well as by disparate metamorphic histories of adjacent terranes. Evidence for dip-slip movement is confined to shear bands in rocks that deformed near the brittle-ductile transition and thin, highly sheared quartz veins with grain shape preferred orientation in amphibolite-grade ultramylonites. Rotational porphyroclast systems visible on sections cut parallel to the down-dip lineation include delta- and theta-types, but the majority are either symmetrical or indicate a very low vorticity number, despite the regional indications for strike-slip displacement. Our data support the hypothesis that in low vorticity number, ductile transpressional zones, the maximum rotational component of strain may be at a very high angle to the stretching lineation.