1.1 MA MELT PULSES IN PSEUDOTACHYLYTE FROM THE ALPINE FAULT, NZ

The Alpine Fault is a ~650 km long structure through the continental crust of the South Island, NZ. With an apparent dextral displacement of at least 480 km, movement along the fault was accompanied by a component of thrusting that exhumed pseudotachylyte-bearing amphibolite facies rocks. Motion along the central part of the Alpine Fault has involved oblique-slip displacement, with minimum slip rates over the past 14 ka exceeding 18 mm/year and hangingwall uplift of ~10 mm/year. Pseudotachylyte veins along the central portion of the fault have been attributed to coseismic frictional melting at depth. The studied pseudotachylyte vein contains abundant biotite as the only K-bearing mineral and shows a symmetrically layered zonation, originally interpreted as a quenching texture. It is particularly suitable for study in lacking any mineralogical signs of low-temperature alteration. Based on detailed high-resolution transmission electron microscopy (HRTEM), melted wall-rock biotite is preserved in the marginal zones, whereas neocrystallized microlitic biotite characterizes both margin and center of the vein. Syn-flow crystallization of biotite, strain features and erosion at crystal-glass contacts indicate that cyclic heating, melting and crystallization occurred during a large earthquake episode lasting several tens of seconds, which reflects the propagation properties of frictional faulting. Electron microscopy also permits reliable dating of the vein. Whereas 40Ar/39Ar total gas ages as old as ~10 Ma are observed in wall rock samples, ages from the vein center give a 1.1(+/- .02) Ma age for this event. Taking ~1.1 Ma for the age of coseismic melting and cooling of the vein allows us to predict the depth at which frictional melting occurred. The rate of uplift and exhumation along the central region of the Alpine Fault is ~10 mm/yr over the past few million years. This leads to a formation depth of ~11 km, which coincides with the base of today’s seismogenic zone in the area. Using modeled isotherms indicates regional temperatures at this depth of 250-300 °C that agree well with biotite passing through its closure temperature following frictional heating. These depths are beneath the levels of hydrothermal circulation of heated meteoric waters, which caused extensive alteration to clay minerals in shallower portions (upper 6 km) of the fault.