Seismicity in an active rift exposing ultra-high pressure metamorphic rocks: D'Entrecasteaux Islands, Papua New Guinea

Continental crust has rifted in the Woodlark Rift, Papua New Guinea for the last 6Ma, with sea floor spreading where extension is largest and farthest from the Euler pole. The D'Entrecasteaux Islands (DIs) lie immediately west of the present-day seafloor spreading centers. Metamorphic core complexes (MCCs) dominate these islands, some of which contain the youngest known ultra-high pressure (UHP) metamorphic rocks. These have been exhumed at <20 mm/year since 5-6Ma from >90 km depth, coeval with continental rifting. Sea floor magnetic anomalies and GPS data indicate 20-30 mm/yr extension should be accommodated across the rift near the DIs; however, it has been debated whether that takes place along the MCC-bounding faults, or parallel structures bounding the Papua Peninsula. To address these questions, we report on regional seismicity from the 2010-2011 CDPapua passive seismic experiment, which comprised an array of 31 on-land and 8 ocean-bottom seismographs over a 200km x 300km region centered on the DIs. We have used P- and S- wave arrivals to detect and locate local earthquakes. Most detected earthquakes lie outside the array, at very active subduction zones 300-600 km distant, but still an average of 3.3 local earthquakes within the array are observed to occur each day. Unexpectedly, we located a cluster of intermediate depth earthquakes, ranging from 70-110 km depth. This deep activity represents the first evidence of sub-crustal seismicity in this region and comes from the depths at which UHP rocks originate, ~100 km along strike from where they are exposed. The seismicity lies toward the Euler pole from the DIs, where extension is less, so potentially represents a not-yet-exhumed source region of UHP rocks in a less-extended region. Most crustal seismicity lies proximate to the D'Entrecasteaux Islands and is concentrated on the dome-bounding faults, revealing that these structures are actively accommodating extension and that MCC exhumation persists. Just SW of the westernmost DI, a dense cluster of seismicity down to 30 km is observed on what may be a transfer structure connecting seismicity along the DIs with deformation on mainland Papua. Further to the south, we observe a more diffuse cluster of events at lower-crustal depths, 20-40 km, beneath the easternmost Papuan Peninsula. While geologic evidence suggests that a major normal fault follows the north coast of the Papua Peninsula, we see little microseismic activity on this structure. As more of this dataset is analyzed, the observed seismicity will provide a clearer understanding of continental rifting in the Woodlark Basin and may allow us to understand the nature and history of subduction leading to UHP metamorphism.