Woodlark Rift Research Articles

Lu–Hf garnet geochronology applied to plate boundary zones: Insights from the (U)HP terrane exhumed within the Woodlark Rift

High-pressure and ultra high-pressure (U)HP metamorphic rocks occur in many of the world's major orogenic belts, suggesting that subduction of continental lithosphere is a geologically important process. Despite the widespread occurrence of these rocks, relatively little is known about the timescales associated with (U)HP metamorphism. This is because most (U)HP terranes are tectonically overprinted and juxtaposed against rocks with a different history. An exception to this are the Late Miocene (U)HP metamorphic rocks found in active metamorphic core complexes (MCC) in the Woodlark Rift of southeastern Papua New Guinea. This region provides a rare opportunity to study the garnet Lu–Hf isotopic record of (U)HP metamorphism in a terrane that is not tectonically overprinted. In order to constrain the timing of garnet growth relative to the history of (U)HP metamorphism and the evolution of the Woodlark Rift, Lu–Hf ages were determined, in conjunction with measurements of Lu and major element zoning, for garnets from three metamorphic rocks. Garnets from the three samples yielded different ages that, instead of recording the spatial and temporal evolution associated with a single metamorphic event, provide information on the timing of three separate plate boundary events. The youngest Lu–Hf age determined was 7.1 ± 0.7 Ma for garnets in a Late Miocene coesite eclogite. The age is interpreted to record the time when a garnet-bearing partial melt of the mantle crystallized within subducted continental lithosphere at (U)HP conditions. The young Lu–Hf age from the coesite eclogite is in contrast to a 68 ± 3.6 Ma Lu–Hf age obtained on large (1–2 cm) garnet porphyroblasts, from within the Pleistocene amphibolite facies shear zone carapace bounding exposures of (U)HP rocks in the D'Entrecasteaux Islands. This older age records the growth of garnet in response to continental subduction and ophiolite obduction in the region north and east of Australia during late Mesozoic/early Cenozoic time. In order to relate the (U)HP metamorphic rocks to the larger-scale tectonic evolution of the Woodlark Rift, a third Lu–Hf age of 11.2 ± 2.1 Ma was determined for dynamically recrystallized garnet from the southeastern margin of the rift. While the tectonic implications of the Lu–Hf age from the southeastern-rifted margin are not fully known, this result suggests that the eastern and western parts of the Woodlark Rift might have a different metamorphic history. The garnet Lu–Hf results demonstrate that prior to rifting, the Woodlark Rift underwent a complex history of metamorphism involving garnet growth and recrystallization. The ability of the Lu–Hf system to date Late Miocene garnet growth, and at the same time preserve Latest Cretaceous garnets within Pleistocene shear zones in the Woodlark Rift demonstrate that Lu–Hf garnet geochronology is an essential component of investigations where the primary objective is documenting and understanding the full range of plate boundary processes recorded in metamorphic minerals.

Diapiric exhumation of Earth's youngest (UHP) eclogites in the gneiss domes of the D'Entrecasteaux Islands, Papua New Guinea

The Woodlark Rift in Papua New Guinea hosts the world's youngest (2–8 Ma) eclogite-facies rocks and extensional deformation has played a key role in exhuming these (U)HP rocks at rates of > 20 mm/yr. During the Eocene Papuan arc-continent collision Australian Plate-derived continental rocks were subducted to (U)HP depths. There they remained for up to 30 m.y. until the Pliocene when asthenospheric circulation ahead of the west-propagating Woodlark spreading ridge introduced heat and fluids. This caused rocks to break away from the paleosubduction channel, recrystallize in the eclogite facies, and rise as Rayleigh–Taylor instabilities. The diapirs ascended adiabatically undergoing partial melting, which lowered their viscosity and increased buoyancy. (U)HP crust ponded near the Moho at ~ 2–4 Ma, thickening the crust to ~ 40 km (11 kb). Domal uplifts emerged above sea level, and these are still underlain by an unusually thick crust (> 26 km) for a rift that has stretched by factor of ~ 3 since 6 Ma. After ponding, they acquired a flat-lying foliation during amphibolite-facies retrogression. Vertical shortening accompanied the gravitationally driven outflow of ponded lower crust. The weak material was extended parallel to the rift margin, thinning ductilely by < 1/3. The flow was dominated by pure shear ( W k ~ 0.2), and was mechanically decoupled from – and orthogonal to – plate motion in the rift. Top-E shear fabrics suggest that this flow was westward, perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the Woodlark spreading ridge. At < 2 Ma, the gneisses were upwardly juxtaposed against an ophiolitic upper plate to form nearly symmetric gneiss domes that cooled at > 100 °C per m.y. and were mechanically incorporated into the rift's upper crust. Final exposure was by normal faulting and minor erosion. Such exhumation may also apply to other (U)HP terranes where less evidence for Moho ponding is preserved.

Late Miocene coesite-eclogite exhumed in the Woodlark Rift

Late Miocene-Pliocene eclogites were exhumed in the Woodlark Rift of eastern Papua New Guinea, an actively extending region west of the Woodlark Basin seafl oor spreading center. We report the discovery of coesite in late Miocene eclogite from the lower plate of one of the D'Entrecasteaux Islands metamorphic core complexes within the Woodlark Rift. Zircon crys- tallization temperatures (650-675 °C) and 238 U/ 206 Pb age (ca. 8 Ma), and rutile thermometry (695-743 °C) combined with garnet-pyroxene thermometry (600-760 °C) and garnet-pyroxene- phengite barometry (18-27 kbar), indicate that the coesite-eclogite was exhumed from mantle depths (≥90 km) to the Earth's surface at plate tectonic rates (cm yr -1 ). This late Miocene coesite- eclogite is the youngest exhumed ultrahigh-pressure (UHP) rock on Earth, and its preservation ahead of the westward-propagating seafl oor spreading center forces reevaluation of models for UHP exhumation, as well as the geologic and tectonic evolution of the Woodlark Rift.

Can microplate rotation drive subduction inversion

We propose a model for the exhumation of Late Miocene coesiteeclogite in the Woodlark Rift of Papua New Guinea. Reorganization within the obliquely convergent Australian–Pacifi c plate boundary zone led to formation of the Woodlark microplate. Counterclockwise rotation of the microplate relative to the Australian plate resulted in extensional reactivation of a subduction thrust (subduction inversion) and the exhumation of high- and ultrahigh-pressure (HP-UHP) rocks within the Australian–Woodlark plate boundary zone. The model invokes plate tectonic processes to drive rapid exhumation and predicts spatial and temporal patterns of exhumation to assess its applica bility to HP-UHP terranes worldwide.

Continental rifting and metamorphic core complex formation ahead of the Woodlark spreading ridge, D'Entrecasteaux Islands, Papua New Guinea

We evaluate the role of a metamorphic core complex (MCC) on Normanby Island in the Woodlark rift. Located &lt;30 km from an active mid‐ocean ridge (MOR), a &gt;1 km thickness of blueschist‐derived mylonites formed in a midcrustal shear zone during the Pliocene at ∼400–500°C. This top‐to‐the‐north zone appears to have reactivated the gently dipping base of the Papuan ophiolite (Papuan Ultramafic Body, PUB), and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the MCC and rapid subsidence of the Woodlark Rise. Mylonites in the MCC's lower plate have been exhumed along a detachment as a result of &gt;50 km of slip at rates of &gt;12 mm/yr. The inactive, back‐tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the Plio‐Pleistocene plate motion. A second SE vergent detachment has been established on the opposite flank of this rolling‐hinge style MCC, probably since &lt;0.5 Ma. Centimeters per year slip rates on these two faults can account for most of the Pleistocene plate motion in this eastern sector of the Woodlark rift, and confirm the important role of MCCs in exhuming very young HP rocks in this rift. Paleopiezometry of mylonites using recrystallized quartz grain size indicates flow stresses of ∼30 MPa before the rocks were overprinted by extension fractures. These results imply high pore fluid pressures (λ &gt; 0.8) at depth, and provide a sufficient mechanism for activating low‐angle normal faults in the rift. MCC inception was not localized to the tip of the Woodlark MOR. Instead, extreme crustal thinning near the MCC preconditioned later continental breakup. The lower crust appears to be weak, thickening beneath unloaded footwalls to uplift MCCs above sea level, and flowing laterally to even out regional crustal thickness contrasts on a 1–6 m.y. timescale. Deep‐seated transforms separate rheologically distinct domains in which extension has been localized along the weak PUB to cause MCC formation, vs. those in which slip is distributed across an imbricate zone of more uniform strength normal faults. The Trobriand fault connects in the eastern Woodlark rift to the Owen Stanley fault in the Papuan Ranges, which is probably moving at nearly the full plate velocity.

Crustal structure across the transition from rifting to spreading: the Woodlark rift system of Papua New Guinea

The Woodlark rift system is one of the few places where active ocean basin formation can be studied. Within this rift system, continental extension rates are some of the fastest on the planet, and extension progresses eastwards to full seafloor spreading. We use results from a recent passive seismic experiment to address the role of magmatism prior to the onset of seafloor spreading. We invert local earthquake P and S traveltimes for 3-D structure around and ahead of the active spreading tip. From the local earthquake tomography we observe three main structures. Seismic velocities in the crust show a sharp contrast between regions that resemble continental crust ahead of the spreading tip and oceanic crust to the east. In the continental portion, P velocities increase from 5.6 km s−1 to 6.9 km s−1 between 10 and 25 km depth, indicating a bulk felsic to intermediate composition, similar to other continental regions. Beneath the seismic Moho (23–28 km depth, as defined by receiver functions) a 10–15-km-thick gradient zone exist locally with velocities from 7.0 to 7.9 km s−1, which may reflect an underplated mafic, granulite facies lowermost crust, or perhaps magmatic intrusion into the lithospheric mantle. In contrast, farther east and closer to the active spreading tip, velocities rapidly increase from 6.5 to 7.2 km s−1 between 8 and 18 km depth. These fast crustal velocities appear in a narrow zone roughly 60 km wide and indicate a mafic crust, similar to oceanic crust. Our velocity results suggest that the transition from diffuse continental rifting to localized seafloor spreading likely occurs across a narrow zone. Magmatism prior to the onset of seafloor spreading may not play a significant role in altering crust until the onset of seafloor spreading, except through underplating at the base of the crust.

Mantle compensation of active metamorphic core complexes at Woodlark rift in Papua New Guinea.

In many highly extended rifts on the Earth, tectonic removal of the upper crust exhumes mid-crustal rocks, producing metamorphic core complexes. These structures allow the upper continental crust to accommodate tens of kilometres of extension, but it is not clear how the lower crust and underlying mantle respond. Also, despite removal of the upper crust, such core complexes remain both topographically high and in isostatic equilibrium. Because many core complexes in the western United States are underlain by a flat Moho discontinuity, it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating. These processes should decouple upper-crust extension from that in the mantle. In contrast, here we present seismic observations of metamorphic core complexes of the western Woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension. These core complexes are actively being exhumed at a rate of 5-10 km Myr(-1), and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density, as indicated by low seismic velocities. We conclude that, at least in this case, the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon, but must involve mantle extension.

Evolution of the Miocene-Recent Woodlark Rift Basin, SW Pacific, inferred from sediments drilled during Ocean Drilling Program Leg 180

Abstract The results of drilling during Ocean Drilling Program Leg 180 provide insights into fundamental processes of continental break-up, because rifting can be related to westward propagation of a spreading centre into continental crust. A generally north-south transect of holes was drilled across the Woodlark Rift on the uplifted northern rift margin on the Moresby Seamount (Sites 1114 and 1116), on the hanging wall of the low-angle (25–30°) extensional Moresby Detachment Fault (Sites 1108, 1110–1113 and 1117) and across the downflexed northern rift margin (Sites 1118, 1109 and 1115). The results, when placed in the regional tectonic context, document a history of Palaeogene ophiolite emplacement, followed by Miocene arc-related sedimentation. Regional uplift and emergence of the forearc area took place in Late Miocene time. Submergence to form the Woodlark Rift began in latest Miocene time, marked by widespread marine transgression and shallow-water deposition, accompanied by input of air-fall tephra and volcaniclastic sediments. During Pliocene time, deposition within the rift basin was dominated by deep-water turbidites, including high-density turbidites in the south. Strong extension along the north-dipping Moresby Detachment Fault was active during Pleistocene time, associated with uplift of the Moresby Seamount and shedding of fault-derived talus, mainly of meta-ophiolitic origin. During Pleistocene time, a carbonate platform was constructed to the NW, trapping clastic sediment and resulting in a switch to slower, more pelagic and hemipelagic deposition within the Woodlark Rift Basin. The marked change in rift basin configuration during Pleistocene time may relate to westward propagation of the Woodlark oceanic spreading centre at c. 2 Ma.

Evidence for seismogenic normal faults at shallow dips in continental rifts

Abstract Several recent observations indicate that normal faulting earthquakes occasionally occur on faults dipping &lt;35°, dips often considered shallow. Most of these occur in the Woodlark and Aegean rifts. These two rifts are found to generate significantly more earthquakes than others and are the most rapidly extending, and so display the widest variety of fault behaviour. Even within the Woodlark Rift system extension rates vary along strike, with the shallowest-dipping faults confined to the most rapidly rifting segment. Here, several events (M w 6.0–6.8) feature nodal planes dipping 23–35°. These planes are subparallel to shear zones bounding nearby metamorphic core complexes, including one imaged to 8–9 km depth by seismic reflection profiling. In the western Gulf of Corinth at least one large event (M w 6.4) occurred on a fault dipping c. 33°. Similarly to the Woodlark example, this rift segment exhibits a high opening rate (10–20 mm a −1 ). Several other cases elsewhere, based on older historical data, microseismicity, or geological inference suggest seismic slip at similar or shallower dips. However, no documented large earthquake exhibits seismic slip on subhorizontal surfaces (dip &lt;10–15°). Stress rotation may explain the 23–35° dips, but thus far no realistic mechanism has been found. More likely, these faults represent surfaces somewhat weaker than surrounding rock, through some combination of modest cohesion of the surrounding rock and slightly lower frictional coefficients on the fault. Such weakening may be a consequence of high slip rates, which rapidly generate large offsets, and of mature fault systems.