STRESS, FLUID PRESSURE CYCLING, AND TRANSIENT DEEP EMBRITTLEMENT BENEATH THE SOUTHERN ALPS, NEW ZEALAND: INSIGHT FROM A PALEO-BRITTLE-DUCTILE TRANSITION ZONE

Oblique convergence between the Australian and Pacific plates along the oblique-slip Alpine Fault has resulted in the rapid uplift and exhumation of an east-tilted section of Pacific Plate crust from a depth of ~25-30 km, forming the Southern Alps. In the central part of the Southern Alps, this crustal section includes superb exposures of a paleo-brittle-ductile transition zone (BDTZ) located in biotite zone rocks at ~7 km structurally above the SE-dipping Alpine Fault. The ~1 km thick BDTZ is subparallel to the Alpine Fault, which dips ~40° SE, and contains vertical brittle-ductile shears that overprint all other metamorphic fabrics in the Alpine Schist. The shears are infilled by ductilely deformed hybrid shear-extensional quartz-calcite veins. Stable isotope thermometry of these veins indicate they were emplaced at temperatures of 450 ± 50°C, as is consistent with new ⁴⁰Ar/³⁹Ar data on two samples of white mica in the host schist. Cooling ages of ~3-4 Ma require late Cenozoic exhumation of the BDTZ rocks from >400 ± 50°C (inferred closure temperature). Based on the Alpine Fault’s slip-rate of ~10 mm/yr, the ages suggest original depths of ~ 22 km, and an unperturbed geothermal gradient there of 18-22°/km.

We infer these dextral-oblique shears were sequentially activated, like the steps of an escalator, to accommodate tilting of the delaminated Pacific Plate rocks onto the Alpine Fault ramp. The shears are brittle faults where they cut quartzofeldspathic schist, but are ductile shear zones where they intersect pre-existing quartz veins >2 cm thick. Based on frictional failure criteria we infer that fluid pressures close to lithostatic (l=~0.95) and high strain rates allowed transient embrittlement of the lower crust. Simultaneously, the thick quartz veins underwent dislocation creep at differential stress values of ~100 MPa. Using quartz flow laws, this infers high strain rates (1 x 10^-9 - 10^-10) during quartz deformation. Fluid inclusions trapped in the infilling veins suggest a post-failure fluid pressure of ~310 MPa (l=~ 0.6), suggesting a large and transient drop in fluid pressure that extended well below the base of the seismogenic zone. These data provide new insight into deformation and crustal rheology of up-ramped rocks subject to transient conditions of high strain rate and deep embrittlement.