Constraining timing of Alpine Fault gouge formation at Gaunt Creek and Waikukupa River, South Island, New Zealand.

Abstract:

Neotectonic brittle faults are commonly associated with near-surface deformation and fluid transport. Fault slip and fluid-rock interactions both contribute to generation of fault gouge composed of rock fragments and authigenic clay minerals. Numerous studies have demonstrated it is possible to determine the absolute timing of brittle faulting by isotopic (K-Ar) dating of authigenic illite clay minerals. Precise size separation combined with mineral characterization (SEM, TEM, XRD, LPS etc.) yields reliable results. The Alpine Fault in the South Island of New Zealand, marks the Australian-Pacific plate boundary. An amphibolite-facies mid-crustal ductile shear zone (mylonite series rocks) in the Pacific Plate hanging wall is exhumed along a current brittle fault marked by cataclasite and fault gouge. Preliminary investigations of two <2 micron illite gouge separates from fault gouge samples collected from surface exposures of the Alpine Fault at Gaunt Creek and Waikukupa River yield K-Ar ages of resp. 4.1 ± 0.4 and 1.9 ± 0.2 Ma, corresponding to the late Pliocene. The high error is caused by low radiogenic 40Ar content of c. 12 to 15%. The K-Ar illite ages are consistent with well-defined field constraints and within error similar to c. 1 to 2.5 Ma 40Ar/39Ar ages for micas from hanging wall metapelites and amphibolites and to published K-Ar mica and near-zero apatite fission track ages. The overlapping illite and mica ages indicate that hanging wall rocks were rapidly exhumed and cooled c. 1 – 4 Ma ago, and that exhumation was coeval with extensive hydration in the brittle part of the Alpine Fault resulting in illite neoformation. The ages of fault gouge illite provide absolute time constraints on the youngest, retrograde, neotectonic movements on this part of the Australian-Pacific plate boundary.