Kinematic Constraints within a Deep Crustal, Granulite Facies, Normal Shear Zone: Mt. Hay Block, Central Australia

The Mt. Hay block (Arunta inlier, central Australia) contains two granulite facies (~700-800°C) structural domains deformed at ~1780 and ~1700 Ma. The older, N-side-up, domain contains an S₁ foliation defining a sheath-like fold inclined to the NE, and a NE-dipping, roughly axial-planar, S₂ foliation. Tectonite fabric ranges from L>>S (lineation much stronger than foliation) to L=S (Staffier, 2007).

The >5 km thick, cross-cutting Capricorn ridge shear zone (CRSZ) records S-side-up shear. Here, S₁ and S₂ foliations are transposed into a steeply SW-dipping S₃ foliation with lineations plunging steeply SSE. Parallelism of grain shape fabrics and lithologic boundaries, and locally sheath folds, all indicate high strain.

Fabric mapping within the CRSZ delineates strain gradients recording part of the strain path. Mesoscale structures such as boudinage and conjugate extensional shear bands record a component of stretching perpendicular to the lineation. Strain markers such as mineral grains, feldspar porphyroclasts and polycrystalline feldspar “blebs” define plane to oblate fabric ellipses (SPO and ELLIPSOID programs; Launeau and Robin, 2005). Together, these observations reflect the conversion of an L-dominated, but structurally heterogeneous, older fabric into an S-dominated fabric during a deformation combining flattening with shear.

By removing rotation due to uplift, the CRSZ restores to a moderately dipping (30-50°N), N-side-down normal shear zone, whose strike was subparallel to the inferred Proterozoic plate boundary. The restored lineation is clockwise from the normal to the plate boundary, suggesting oblique divergence. If the restoration is correct, the CRSZ would have been oblique to shallow-crustal faults expected to form in oblique divergence. This geometry, and the evidence for a flattening deformation, illustrate the challenge in relating smaller-scale and crustal-scale kinematics, and in formulating models for whole-crust deformation using deep crustal exposures whose shallower correlative structures have been removed.