THERMOCHRONOLOGY OF ACTIVE PLATE BOUNDARIES

Plate boundaries are transient features that record the 3-D interplay between continental and oceanic lithosphere, and mantle asthenosphere over geologic time. Thermochronology provides a set of tools to unravel the thermal and tectonic evolution. An integrated approach combining high (e.g., U/Pb, ⁴⁰Ar/ ³⁹Ar) to low (e.g., fission track, and (U-Th/He)) temperature thermochronologic (T-t), petrologic (P-T), and structural (D) data is often required to fully document protolith ages, and to constrain the timing of metamorphism, magmatism, and exhumation of rocks deformed within active plate boundaries.

Two examples from the obliquely convergent Australia (AUS) - Pacific (PAC) plate boundary zone are used to illustrate the application of thermochronology to assess plate boundary processes. In the South Island of New Zealand oblique AUS-PAC continent-continent convergence led to formation of the Southern Alps. Thermochronologic studies of the hanging wall (PAC plate) east of the Alpine Fault reveal an asymmetric zone of lower crustal rocks exhumed since ~6 Ma at rates of ~6-9 mm/yr. In contrast, thermochronology of the footwall (AUS plate) provides a record of the earlier (since 23–25 Ma) development of the plate boundary not preserved elsewhere in the Southern Alps. The data set records the thermal response to the AUS-PAC transition from a strike-slip to obliquely convergent plate boundary. Eastern Papua New Guinea also evolved within the AUS–PAC plate boundary zone. There, the Woodlark Basin forms the plate boundary between AUS and the Woodlark (WLK) microplate, and at its western end is undergoing the transition from distributed rifting to seafloor spreading. P-T-t-D studies of the lower plates of the D'Entrecasteaux Islands metamorphic core complexes, located west of the active seafloor spreading rift tip, document variably retrogressed 8-2 Ma eclogites exhumed at plate tectonic rates (>1 cm/yr). The rapid transition from subduction to rifting within this HP/UHP terrane is revealed by the P-T-t-D data set. Thermochronologic studies of active plate boundaries permits assessment of geologic and tectonic processes whereas in studies of Paleozoic and older plate boundary zones analytical errors associated with isotopic ages may potentially hinder recognition of rapid plate boundary events.