South Bismarck Plate Research Articles

Tectonic and magmatic evolution of the Bismarck Sea, Papua New Guinea; review and new synthesis

The Bismarck Sea, located north of Papua New Guinea and just south of the equator, formed during the final stages of a long, complex geological development of the Melanesian Borderland, which resulted from Cenozoic convergence between the Australian and Pacific-Caroline Plates and the opening of back-arc basins. The sea, which straddles two oppositely facing trenches, the inactive Manus trench and the active New Britain trench, covers two basins, the New Guinea Basin to the west and the Manus Basin to the east. These basins are separated by the shallow Willaumez-Manus Rise, trending roughly from WNW to ESE. The origin of these major structural units and their relationship with the present-day zone of major seismicity along the Bismarck Sea Seismic Lineation remains unclear. A detailed examination of geophysical and geochemical data, combined with geologic and geodetic information from surrounding regions, attests that the Bismarck Sea went through some rather unusual events of back-arc development beginning in the middle Pliocene. Around 3.5 Ma, the northern tip of New Guinea came into contact with the Finisterre-Huon Range. The event triggered a back-arc opening that eventually divided the Bismarck seafloor into North and South Bismarck Plates. The rapid opening also caused an upwelling of anomalously hot upper mantle beneath the Bismarck Sea. A large volcanic outflow during this period may have contributed to the formation of the rise west of Manus Island. Both the North and South Bismarck Plates interacted with surrounding plates, and small and large changes may have occurred throughout their history. A sudden shift in local plate motion, perhaps caused by locking such as plate collision, may have caused the overlying lithosphere to decouple from the mantle upwelling. The Willaumez-Manus Rise was probably created under such circumstances when a large volume of rising magma leaked out along a strike-slip plate boundary linking the spreading centers in the New Guinea and Manus basins. An alternative view is that the Willaumez-Manus Rise represents a magmatically robust segment of a single spreading axis that linked the two basins. The further docking of Finisterre-Huon Range with the New Guinea Highlands allowed the South Bismarck Plate to open faster toward the east in the Manus Basin and eventually to jump to the present-day Bismarck Sea Seismic Lineation. The Willaumez Transform was initiated by this more recent plate-boundary reorganization. Seafloor spreading commenced in the Manus Spreading Center <0.78 Ma, and the overall pattern suggests that rifting in the Bismarck Sea is propagating eastwards in the Southeastern Rift and westwards in the Western Spreading Ridges. One feature that distinguishes the Bismarck Sea from most other recently opened back-arc spreading systems in the western Pacific, except for the northern North Fiji and Lau basins, is the anomalously large distance (more than 400 km) between the arc and the spreading axis. This reflects the complex, combined geodynamic processes in this region, i.e., arc-continent collision, mantle upwelling, and rapid back-arc opening specific to this region. Central Bismarck Sea lavas are also unusual as they contain chemical components characteristic of the lower mantle. This may reflect proximity to the Melanesian Borderland, long a site of major subduction, and therefore of upper mantle enrichment by subduction-derived components, compared to other spreading systems.

Tectonic interpretation of aftershock relocations in eastern Papua New Guinea using teleseismic data and the arrival pattern method

SUMMARY The arrival pattern (AP) method and teleseismic data have been used to relocate aftershocks following three major (Mw >= 7.8) interplate earthquakes that occurred in eastern Papua New Guinea in 2000 November. The first event was an Mw= 8.0 strike-slip earthquake that ruptured the Weitin fault over a length of ∼100 km. No aftershocks were located in the upper 15 km, suggesting that the accumulated strain was fully released in this event. The two subsequent Mw= 7.8 thrust events occurred on the New Britain trench. Wadati–Benioff zones dipping ∼20° define the locations of the thrust events at 152°E and 153°E on fault planes with strikes of 75° and 65°, respectively. The plot of aftershocks defines a complex seismic zone beneath New Ireland where events have occurred in the overriding Pacific and South Bismarck plates above the deeper zone of subduction of the Solomon Sea Plate. These events cluster in a broad zone to the northeast of the Weitin fault rather than defining a linear feature that might have been expected following a near-vertical major strike-slip event.

GPS and seismological constraints on active tectonics and arc‐continent collision in Papua New Guinea: Implications for mechanics of microplate rotations in a plate boundary zone

The island of New Guinea is located within the deforming zone between the Pacific and Australian plates that converge obliquely at ∼110 mm/yr. New Guinea has been fragmented into a complex array of microplates, some of which rotate rapidly about nearby vertical axes. We present velocities from a network of 38 Global Positioning System (GPS) sites spanning much of the nation of Papua New Guinea (PNG). The GPS‐derived velocities are used to explain the kinematics of major tectonic blocks in the region and the nature of strain accumulation on major faults in PNG. We simultaneously invert GPS velocities, earthquake slip vectors on faults, and transform orientations in the Woodlark Basin for the poles of rotation of the tectonic blocks and the degree of elastic strain accumulation on faults in the region. The data are best explained by six distinct tectonic blocks: the Australian, Pacific, South Bismarck, North Bismarck, and Woodlark plates and a previously unrecognized New Guinea Highlands Block. Significant portions of the Ramu‐Markham Fault appear to be locked, which has implications for seismic hazard determination in the Markham Valley region. We also propose that rapid clockwise rotation of the South Bismarck plate is controlled by edge forces initiated by the collision between the Finisterre arc and the New Guinea Highlands.

Coralgal composition of drowned carbonate platforms in the Huon Gulf, Papua New Guinea; implications for lowstand reef development and drowning

Collision between the South Bismarck plate and the northern edge of the Australian plate has produced an actively subsiding foreland basin in the western Huon Gulf. A series of drowned carbonate platforms and pinnacles are preserved on this margin due to a combination of this rapid subsidence and eustatic sea-level changes over the last 450 ka. We analyzed sedimentary and coralgal data from the platforms to better understand lowstand reef development and drowning in the Huon Gulf. The recovered limestones are divided into five main sedimentary facies: coral reef, coralline–foraminiferal nodule, coralline–foraminiferal crust, Halimeda, and planktonic foraminiferal limestones. Based on a comparison with modern analogues in the Indo-Pacific and elsewhere, we identified coral reef, deep fore-reef slope, deeper fore-reef slope, and pelagic/hemipelagic paleoenvironmental settings. An analysis of facies relationships and their paleoenvironmental meanings revealed lowstand corals reefs preserved at the top of the platforms that grew within ∼10 m of sea level. Two different coral assemblages were identified within this facies: (1) a shallow, high energy reef community characteristic of windward margins and limited to the deep platforms (1947, 2121, 2393 m), and (2) another shallow community but indicative of more moderate lower energy reef conditions and limited to the middle (1113, 1217, 1612 m) and shallow platforms (823 m). The change from high to lower energy reef growth conditions suggests that oceanographic/climatic conditions in the Huon Gulf have changed substantially through time, primarily through the closure of the Gulf as a result of tectonic rotation and uplift of the Huon Peninsula over the last 450 ka. Despite major environmental perturbations (i.e. relative sea-level and temperature changes) the platforms and the shallow water coral reefs exposed at the top have been able to re-establish themselves time and time again over the last 450 ka. We also identified two different incipient drowning scenarios influenced by the rate of relative sea-level rise. More rapid drowning in the middle and deep platforms produced a thin veneer of coralline–foraminiferal nodule and Halimeda limestones over the shallow coral reef material while the slower drowning experienced by the shallowest platforms allowed thick coralline–foraminiferal crust limestones to develop. We recognize three main stages of platform development: (1) initiation and growth characterized by shallow coral reef growth as the platforms grew close to sea level during the lowstands, (2) incipient drowning marked by a shift to coralline–foraminiferal nodule, crust and Halimeda limestones as the platforms began to drown during rapid eustatic sea-level rise and continued subsidence, and (3) the complete drowning of the platforms characterized by platform ‘turn off’, increased bioerosion, Fe–Mn precipitation and pelagic/hemipelagic sedimentation as the platform surfaces finally drop below the photic zone.

Resolving slip vector azimuths and plate motion along the southern boundary of the South Bismarck Plate, Papua New Guinea

The interaction of the Australian, South Bismarck and Solomon Sea Plates in Papua New Guinea is the source of frequent earthquakes that occur as a result of subduction and arc continent collision. Previous investigators have drawn attention to a discontinuity in the horizontal azimuth of slip vectors along the southern boundary of the South Bismarck Plate, with those to the west of 148°E being systematically rotated 20ndash;30° clockwise compared to those located east of 148°E. This has led to the suggestion that relative motion may be occurring between the Huon Peninsula and New Britain or that more than two plates are acting south of the South Bismarck Plate. Global positioning system (GPS) measurements since 1991 indicate that there is no internal deformation occurring within the South Bismark Plate and that at least two distinct plates are in contact with the southern edge of the South Bismarck Plate. We show from a study of a recent earthquake dataset that the change in slip vector azimuth can be modelled by the interaction of the overriding South Bismarck Plate with the underthrusting Australian and Solomon Sea Plates, consistent with the GPS observations, while maintaining the South Bismarck Plate as a rigid entity. We found that a transition zone exists between 147°E and 148°E where the underlying plate changes from the Australian Plate to the Solomon Sea Plate. There are insufficient data at present to indicate whether or not a third plate, the Woodlark Plate, is also interacting directly with the South Bismarck Plate in this transition zone. Slip vector azimuths were used to estimate an Euler pole (6.74°S, 144.64°E), which describes the relative motion of the South Bismarck and Solomon Sea Plates along the New Britain Trench.

Rotations in the actively colliding Finisterre Arc Terrane: paleomagnetic constraints on Plio-Pleistocene evolution of the South Bismarck microplate, northeastern Papua New Guinea

We report paleomagnetic results from 12 Plio-Pleistocene localities in the actively colliding Finisterre Arc Terrane of northeastern Papua New Guinea (PNG). Calcareous, hemipelagic cover rocks possess a stable, syn-collisional remagnetization indicating a clockwise rotation of the colliding terrane through about 40° in post-Miocene time. A decrease in paleomagnetic declination anomalies as a function of along-strike distance in the Finisterre Arc Terrane, analyzed by our preferred model of a linear remagnetization and a migrating Euler pole, suggests an average rotation rate of 8° Ma −1, in good agreement with the instantaneous rate from global positioning system geodesy. Thus, we propose that this rotation results from a coherent, rigid-body rotation of the Finisterre Terrane rather than from sequential docking of independently colliding blocks of the terrane. Moreover, we conclude that these paleomagnetic declinations result mainly from South Bismarck Plate motion, and not decoupled rotation of the crustal terrane independent of the underlying lithosphere. We examine models of a syn-collisional remagnetization with both fixed and migrating Euler poles of South Bismarck/Australia plate relative motion, and suggest that the Euler pole describing South Bismarck Plate motion has migrated southwestward to its present location on the collision suture in response to the propagating collision. This plate kinematic model agrees with the variability in depth of the seismogenic slab beneath the collision zone. Our best-fit model of pole migration describes South Bismarck/Australia relative motion producing a highly oblique collision in its early stages, with the Finisterre Arc Terrane converging along a left-lateral Ramu-Markham suture, gradually changing to the nearly orthogonal convergence observed today.

Combining AIRSAR data of Huon peninsula and Hawaii MR1 data of Huon Gulf, Papua New Guinea, for studies of tectonic and sedimentary processes

Two swaths of airborne synthetic aperture radar (AIRSAR) imagery, acquired over the southern Huon Peninsula, Papua New Guinea, have been digitally merged with aerial photogrammetric topography and with Hawaii MR] acoustic side‐scan imagery and bathymetry of the adjacent seabed of Huon Gulf. This example of land‐sea integration provides a means of examining relationships between tectonic lineaments and sediment transport pathways on land and on the subjacent seabed. Imaging methods and geometries dictate both the regions covered and the information content of the data. The principal limitations in merging the two data sets arise because the coastal and inner shelf zone, in water depths less than 500 m, is inaccessible to the imaging techniques that were used. Indirect methods allow some inferences to be made about the geomorphology of this zone, using surface sound channel data and sea surface imagery. The area examined is part of an active tectonic collision zone in which the northern margin of the Australian plate is being subducted beneath the South Bismarck plate. The edge of the South Bismarck plate has been uplifted 4,000 m since the late Pliocene, forming the Finisterre Range on Huon Peninsula. The record of relative emergence and submergence is seen in subaerially exposed Quaternary marine sediments and in drowned carbonate reefs. The merged radar and sonar imagery clearly indicate rapid erosion and deformation, both on land and under water. The imagery shows details of sediment transport and depositional systems, which are generating large terrestrial fluvial/alluvial plains, coastal fan deltas, and submarine channel deposystems. Fault structures and outcrops of bedrock units are clearly delineated both on land and on the seabed. These observations help explain patterns of onshore and submarine outcrops in terms of interactions between modern offshore sedimentation patterns and progressive deformation.

PNG Transect 1998–2001

We report paleomagnetic results from 12 Plio-Pleistocene localities in the actively colliding Finisterre Arc Terrane of northeastern Papua New Guinea (PNG). Calcareous, hemipelagic cover rocks possess a stable, syn-collisional remagnetization indicating a clockwise rotation of the colliding terrane through about 40° in post-Miocene time. A decrease in paleomagnetic declination anomalies as a function of along-strike distance in the Finisterre Arc Terrane, analyzed by our preferred model of a linear remagnetization and a migrating Euler pole, suggests an average rotation rate of 8° Ma−1, in good agreement with the instantaneous rate from global positioning system geodesy. Thus, we propose that this rotation results from a coherent, rigid-body rotation of the Finisterre Terrane rather than from sequential docking of independently colliding blocks of the terrane. Moreover, we conclude that these paleomagnetic declinations result mainly from South Bismarck Plate motion, and not decoupled rotation of the crustal terrane independent of the underlying lithosphere. We examine models of a syn-collisional remagnetization with both fixed and migrating Euler poles of South Bismarck/Australia plate relative motion, and suggest that the Euler pole describing South Bismarck Plate motion has migrated southwestward to its present location on the collision suture in response to the propagating collision. This plate kinematic model agrees with the variability in depth of the seismogenic slab beneath the collision zone. Our best-fit model of pole migration describes South Bismarck/Australia relative motion producing a highly oblique collision in its early stages, with the Finisterre Arc Terrane converging along a left-lateral Ramu-Markham suture, gradually changing to the nearly orthogonal convergence observed today.

Large‐scale structures on Gazelle Peninsula, New Britain: Implications for the evolution of the New Britain arc

Geological mapping of fault systems on the Gazelle Peninsula, eastern New Britain arc, combined with a reinterpretation of existing sea floor data indicate that faults previously thought to be a possible location of the boundary between the North and South Bismarck Plates, do not appear to be directly related to the plate boundary spreading centres and transform faults in the 3.5 Ma Manus Basin. Structure on the Gazelle Peninsula is dominated by the Mediva Fault (new name) and the Wide Bay Fault System, both north‐northwest trending, deep‐seated features. The Mediva Fault, an element of the Baining Mountain Horst and Graben Zone, is an extensional structure which has focused Middle Miocene intrusive activity, controlled Mio‐Pliocene sedimentation in the central Gazelle Peninsula, and displaced Quaternary volcanic deposits. The Wide Bay Fault System has been active since at least the Late Oligocene. One hundred kilometres of sinistral strike‐slip motion is likely on this fault since at least the late Middl...

The Papua New Guinea Satellite Crustal Motion Surveys

Abstract Satellite Doppler and GPS measurements have been used to determine plate tectonic motion in Papua New Guinea (PNG). A network, which straddles most of the plate boundaries of the region, was first observed with Doppler receivers in 1981 and many of these sites were recently reobserved with GPS in three successive field campaigns carried out in 1990, 1991 and 1992. The measurements indicate that the deformation of the crust in PNG, between the Australian and Pacific plates, is characterized by the rapid motion (> 10 cm/yr) of two small plates, a Solomon Sea/Woodlark plate and the South Bismarck plate, and that the majority of movement occurs along the Bismarck Sea seismic lineation as strikeslip motion, across the New Britain Trench as convergence and across the Woodlark Basin spreading system as extension. Current rates of motion generally agree with those obtained from geology.

Quaternary uplift rates at a plate boundary, Lae urban area, Papua New Guinea

Uplift along the southern margin of the South Bismarck plate in Papua New Guinea is a result of plate convergence, terrane docking and triple-junction migration. Lithofacies data combined with paleontological and 14C dating yield uplift rates for the Markham Valley north flank and the Lae urban area in the range 2.0–7.6 m/10 3 yr. These rates are established for the past 21,000 years and have probably obtained for 10 6 yr. Depositional surfaces > 8100 years old in the Lae urban area are tilted. The uplift may be occurring by steady creep or in episodic increments of > 1 m with recurrence intervals of the order of 10 2 yr, as seems to be the case for the northern Huon Peninsula terraces. If episodic uplift is occurring, earthquakes of magnitude > 7.0 can be anticipated accompanied by slump-induced tsunamis.

Suturing history of an allochthonous terrane at a modern plate boundary traced by flysch-to-molasse facies transitions

The southern boundary of the South Bismarck plate in Papua New Guinea changes character from west to east as a consequence of oblique collision and suturing of the allochthonous Finisterre Terrane with the margin of the Australian continent and plate. Plate rearrangements are occurring, and a triple junction is migrating eastwards along the plate boundary. Characterization of modern sedimentary facies leads to the following correlations between facies and tectonic settings: Tectonic setting Sector of plate boundary Facies terrane already docked Markham Valley fluvial molasse terrane now docking Huon Gulf marine molasse terrane yet to dock western New Britain Trench conglomeratic flysch These relationships provide the basis for a tectonic-depositional model showing the distribution and relationships between facies, in time-distance space, along the plate boundary. Facies transitions are time-transgressive and their trajectories provide a measure of the rate of triple-junction migration. Rapid uplift of the southern margin of the South Bismarck plate has exposed older sediments related to this history of terrane suturing. Their facies characterization provides support for the model, but age control is insufficient for conclusive demonstration of its validity.

Seismicity maps of the New Guinea-Solomon Islands region

Using the data of all tectonic earthquakes with M ≧ = 7.0, which occurred between 1900.01.01. and 1972.12.31., between 130° and 163°E and 0° and 12°S, maps for the distribution of tectonic flux and epicenters were constructed. These were compared with the tectonic map of the area investigated (Weber Deep, New Guinea, Bismarck Sea, New Ireland, New Britain, Solomon Sea, Bougainville Island, Solomon Islands). It was found that the so-called active centers of the tectonic flux are arranged either exactly on, or very near the plate boundaries established by other authors. The map of epicentral distribution shows a less clear correlation of this kind. The pattern of distribution of shallow ( h = 0–99 km) shocks is rather diffuse on New Guinea itself, namely, but less diffuse over oceanic areas where many of the epicenters are to be found near or just on the suggested boundaries of lithospheric plates. The intermediate and deep shocks ( h ≧ = 100 km ) form three, well-separated regions. The first is related to the Weber Deep, the second to the westernmost edge of the south Bismarck plate and that of the Solomon plate, respectively; finally the third one to the easternmost corner of the two Bismarck plates and, respectively, the northeastern edge of the Solomon plate. The data of altogether 152 shocks were applied and this number appeared to be small for the construction of Benioff zones. Due to the research carried out by other authors, we know, however, that Benioff zones do exist in some parts of the area investigated. The deepest shock in the magnitude-range of 7.0–8.6 originated at 430 km beneath the surface. The distribution of shocks down to this depth by and large corresponds to the pattern which is often experienced at and underneath typical island arcs, with an outstanding maximum between 0 and 99 km. This statement is valid for the number and summarized energy of the earthquakes alike. The distribution of volcanoes is very particular and strange as many of them are situated either exactly on or very near the plate boundaries. In general, that is in the case of less-complicated island arcs, such as — say — the Indonesian Arc, the volcanoes are usually at a distance of about 280–300 km or more from the axis of the oceanic trench which is the nearest plate boundary measured from them. The area investigated in this paper is really one of the most complicated regions of the world.

Part 6. Present plate boundary seismic, volcanic and kinematic processes: Seismicity and earthquake focal mechanisms in the New Guinea Solomon Islands region

There is general agreement between plate tectonic theory and seismicity for the New Guinea Solomon Islands region. Two minor plates, the Solomon Sea and South Bismarck plates, are sandwiched within the collision zone between the continental fronts of the Pacific and India plates, and derive their relative motions from the collision. The azimuth of collision is approximately east?northeast.

Focal mechanisms and plate tectonics of the southwest Pacific

Ninety-six new focal mechanisms were determined for earthquakes on the belt of seismic activity separating the Pacific and Australian plates. The direction of convergence of these plates varies from NE-SW to E-W. The Australian plate underthrusts the Pacific plate to the ENE under the Solomon and New Hebrides islands and overthrusts the Pacific to the east along the Tonga-Kermadec arc and the North Island of New Zealand. The data for the Macquarie ridge concur with the idea that the pole of rotation for the Pacific and Australian plates is nearby and to the east of this feature. The data also suggest a NNE-SSW convergence of the Pacific and Australian plates in northwestern New Guinea. The relative motions of the plates near the Bismarck Archipelago are complex because of the presence of at least three additional small plates. The south Bismarck plate, the best defined, underlies the southern part of the Bismarck Sea. It is bounded on the north by an E-W belt of seismicity at about 3°S defining a left-lateral strike-slip fault. The New Britain arc forms the southern boundary, where the Solomon Sea floor underthrusts the south Bismarck plate to the NNW. There is some evidence for SW convergence of the south Bismarck and Australian plates in northeastern New Guinea. Small plates, less well-defined seismically, are also proposed under the northern part of the Bismarck Sea and under the Solomon Sea. The plate underlying the Solomon Sea floor is bounded by the Solomon and New Britain arcs and by eastern New Guinea. The southern boundary is not sharply defined by seismic data. The Solomon Sea plate is moving approximately NW with respect to the Australian plate and underthrusting the Pacific plate to the NE along the Solomon arc. The consistent pattern of relative motions of these three small plates allows quantitative estimates of relative rates of motion between them. These data demonstrate that plate tectonics is applicable even for regions with dimensions of only a few hundred kilometers. Geologic data from New Guinea are used to speculate about earlier plate motions in that area.