LATE CENOZOIC BRITTLE-DUCTILE DEFORMATION AND FLUID FLOW RECORDED IN A FOSSIL BRITTLE-DUCTILE TRANSITION ZONE, SOUTHERN ALPS, NEW ZEALAND

A late Cenozoic brittle-ductile transition zone (BDTZ), ~1 km thick, is exposed in biotite-zone rocks in the central Southern Alps, South Island, New Zealand. The exhumed BDTZ lies in a rapidly exhumed, east-tilted section of rocks ~7 km structurally above the Alpine fault, a major dextral-reverse structure between the Pacific and Australian plates.

The BDTZ contains veins and discrete vertical brittle-ductile shear zones that overprint all other metamorphic fabrics in the Alpine Schist. The dextral-oblique shears strike subparallel to the Alpine fault, and are remarkably systematic in their spacing (average 0.59 ± 0.03 m), kinematics (NW-up throw, antithetic to the Alpine fault) and slip magnitude (average 15.2 ± 0.71 m). Deformed quartz veins show more ductile deformation than their quartzofeldspathic host. We infer that the shears were sequentially activated, in escalator-like fashion, to accommodate tilting of the delaminated Pacific Plate rocks onto the SE-dipping Alpine fault ramp. The width of ductile deformation adjacent to each shear steadily increases structurally downwards through the BDTZ, and the shears coalesce downward into a wide zone of pervasively back-sheared garnet-zone rocks. A high velocity geophysical anomaly is coincidental with this ramping of Pacific Plate rocks up onto the Alpine fault and may represent enhanced rates of metamorphic fluid release.

The shears are lined with lineated qtz-cal-chl veins that were emplaced syntectonically with slip, recording presence of fluids. Preliminary results indicate that the vein-precipitating fluids are saline (2.3 wt % NaCl) and are of metamorphic origin. Ongoing fluid inclusion work, oxygen isotope thermometry, and electron microprobe analyses aim to establish the temperature and pressure conditions under which the shear-infilling veins formed. This will also yield a depth estimate of the BDTZ under the Southern Alps at present, as well as evaluate shifts in fluid composition through the BDTZ and its potential influence on brittle-ductile behaviour.