Continental breakup and UHP rock exhumation in action: GPS results from the Woodlark Rift, Papua New Guinea

Abstract

We show results from a network of campaign Global Positioning System (GPS) sites in the Woodlark Rift, southeastern Papua New Guinea, in a transition from seafloor spreading to continental rifting. GPS velocities indicate anticlockwise rotation (at 2–2.7°/Myr, relative to Australia) of crustal blocks north of the rift, producing 10–15 mm/yr of extension in the continental rift, increasing to 20–40 mm/yr of seafloor spreading at the Woodlark Spreading Center. Extension in the continental rift is distributed among multiple structures. These data demonstrate that low‐angle normal faults in the continents, such as the Mai'iu Fault, can slip at high rates nearing 10 mm/yr. Extensional deformation observed in the D'Entrecasteaux Islands, the site of the world's only actively exhuming Ultra‐High Pressure (UHP) rock terrane, supports the idea that extensional processes play a critical role in UHP rock exhumation. GPS data do not require significant interseismic coupling on faults in the region, suggesting that much of the deformation may be aseismic. Westward transfer of deformation from the Woodlark Spreading Center to the main plate boundary fault in the continental rift (the Mai'iu fault) is accommodated by clockwise rotation of a tectonic block beneath Goodenough Bay, and by dextral strike slip on transfer faults within (and surrounding) Normanby Island. Contemporary extension rates in the Woodlark Spreading Center are 30–50% slower than those from seafloor spreading‐derived magnetic anomalies. The 0.5 Ma to present seafloor spreading estimates for the Woodlark Basin may be overestimated, and a reevaluation of these data in the context of the GPS rates is warranted.

Southeast Papuan crustal tectonics: Imaging extension and buoyancy of an active rift

“…The simplest interpretation is that deformation is continuous along an interconnected set of currently active normal faults and transfer structures making up the D'Entrecasteaux Fault Zone. Seismicity also reveals fault zones crossing Normanby Island and beneath the eastern Papuan Peninsula, consistent with some fraction of plate motion occurring south of the MCCs as indicated by GPS data [ Wallace et al , ]. Seismicity on the south side of Fergusson lies along a coastline that shows geomorphic evidence for the present activity [ Miller et al , ].…”

“…Sparse subcrustal seismicity is observed northwest of Goodenough Island reaching 120 km depth, but none of the published focal mechanisms show thrust faulting (Figure ), and the scattered hypocenters on the Trobriand Platform do not lie consistently at plate interface depths. Geodetic data also suggest that the Trobriand Trough is currently inactive or extensional [ Wallace et al , , ], so we infer that any convergence here happened before 0.52 Ma or earlier.…”

“…We interpret the Ward Hunt Strait Fault Zone as a similar transfer zone linking extension between the Mai'iu Fault and the D'Entrecasteaux Fault Zone. Documentation of this transfer zone resolves a paradox in regional kinematics: although the D'Entecasteaux Fault Zone shows much evidence for rapid modern motion [ Abers , ; Baldwin et al , ; Miller et al , ; Wallace et al , ], the Kiribisi Basin immediately to the west is virtually undeformed and shows only a couple kilometers of extension [ Fitz and Mann , ]. The existence of a transfer structure allows large extension across Goodenough Island without disrupting adjacent basin strata.…”

Earthquake supercycles as part of a spectrum of normal fault slip styles

“…Normal stress and a ‐ b distributions are described in section 2.3.2, and we vary the plate velocity V p to simulate different rates of extension on normal faults. We use rates of V p = 3, 6, 9 mm/yr which correspond to modern extension rates in the Wasatch Fault zone [ Friedrich et al ., ], the central East African Rift [ Stamps et al ., ], and the Mai'iu Fault zone in SE Papua New Guinea [ Wallace et al ., ], respectively.…”

Anisotropy beneath a highly extended continental rift

“…Seismic tomography shows an abrupt transition from slow continental crust to fast oceanic crust to the immediate east of the DI [ Ferris et al ., ]. GPS data show that the current relative motion between the Trobriand Block and AUS is −2.74°/Myr about a pole at (9.43°S, 147.5°E) [ Wallace et al ., ]. The difference in poles results in a ∼30% discrepancy between long‐term geologic rates of divergence (∼30 mm/yr) and geodetic rates (∼15–20 mm/yr) across the DI [ Wallace et al ., ] possibly due to a tectonic rearrangement at ∼0.52 Ma [ Taylor et al ., ].…”