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A re-evaluation of arc-continent collision and along-arc variation in the Bismarck Sea region, Papua New Guinea

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The Bismarck Sea region of Papua New Guinea is marked by recent arc–continent collision giving rise to a highly dynamic tectonic environment, characterised by complex plate interactions that are yet to be fully understood. We present a new crustal and upper mantle crustal architecture model for northeastern Papua New Guinea and western New Britain that reveals complex tectonic geometries of overprinting slab subduction and partial continental subduction, resulting in a unique setting in which to investigate along-arc magmatic variation. Earthquake hypocentre databases are combined with detailed topography and seafloor structure together with geology and regional-scale gravity to unravel the sub-surface structure of northeastern Papua New Guinea. These data are used in conjunction with an updated 3-D slab map of the region to propose a new interpretation of the area whereby Australian continental crust extends as an underthrust block beneath the accreted Finisterre Terrane. The subducting continental crust combined with slab stagnation has resulted in a complex pattern of arc-related geochemical signatures from east to west along the Bismarck arc. In the east, where the Solomon Sea plate is subducting beneath New Britain, the sedimentary component is low, whereas in the west, the arc volcanics exhibit a greater sedimentary component, consistent with subduction of Australian crustal sediments. As a result, a new plate reconstruction is provided for the region together with a forward-looking reconstruction of the Papuan peninsula, the Solomon Sea plate and New Britain that illustrates that the same process will likely be repeated in some 5–10 m.y.
Topography, bathymetry and major tectonic elements of the study area. (a) Major tectonic boundaries of Papua New Guinea and the western Solomon Islands; CP, Caroline plate; MB, Manus Basin; NBP, North Bismarck plate; NBT, New Britain trench; NGT, New Guinea trench; NST, North Solomon trench; PFTB, Papuan Fold and Thrust Belt; PT, Pocklington trough; RMF, Ramu-Markham Fault; SBP, South Bismarck plate; SCT, San Cristobal trench; SS, Solomon Sea plate; TT, Trobriand trough; WB, Woodlark Basin; WMT, West Melanesian trench. Study area is indicated by rectangle labelled Figure 1b; the other inset rectangle highlights location for subsequent figures. Present day GPS motions of plates are indicated relative to the Australian plate (from Tregoning et al. 1998, 1999; Tregoning 2002; Wallace et al. 2004). (b) Detailed topography, bathymetry and structural elements significant to the South Bismarck region (terms not in common use are referenced); AFB, Aure Fold Belt (Davies 2012); AT, Adelbert Terrane (e.g. Wallace et al. 2004); BFZ, Bundi Fault Zone (Abbott 1995); BSSL, Bismarck Sea Seismic Lineation; CG, Cape Gloucester; FT, Finisterre Terrane; GF, Gogol Fault (Abbott 1995); GP, Gazelle Peninsula; HP, Huon Peninsula; MB, Manus Basin; NB, New Britain; NI, New Ireland; OSF, Owen Stanley Fault; RMF, Ramu-Markham Fault; SS, Solomon Sea; WMR, Willaumez-Manus Rise (Johnson et al. 1979); WT, Wonga Thrust (Abbott et al. 1994); minor strike-slip faults are shown adjacent to Huon Peninsula (Abers & McCaffrey 1994) and in east New Britain, the Gazelle Peninsula (e.g. Madsen & Lindley 1994). Circles indicate centres of Quaternary volcanism of the Bismarck arc. Filled triangles indicate active thrusting or subduction, empty triangles indicate extinct or negligible thrusting or subduction.
… 
Gravity and seismicity correlation for the southwest Bismarck – Huon Peninsula region. All earthquakes illustrated are from the NEIC earthquake database (1990 – 2010) and projected on the Bismarck Sea seafloor gravity anomaly map and topography (a, c); and associated earthquake density distribution maps (b, d). (a, b) Gravity and seismic correlation between 40 and 100 km depth; the top surface of the slab map above 100 km depth is shown as a white shaded area (a) and area outlined in black (b). A large area of anomalously high earthquake density trending east-southeast – west-northwest and outlined by the red dashed line does not show a relationship with the defined windows for slab-related seismicity (a, b); furthermore, this anomalous seismic region correlates well with the negative (low; purple) gravity anomaly to the north of- and beneath the Huon Peninsula. We make note of the two highest density earthquake clusters; the largest in extent of the two lies at the southeast tip of the Huon Peninsula, this is a region of overlap between both the slab map and anomalous seismicity/gravity correlation implying multiple earthquake sources superimposed; the second is located at the approximate centre of the Finisterre Terrane and holds the greatest observed density . (c, d) Gravity and seismicity correlation between 100 and 300 km depth; the top surface of the slab below 100 km to termination is shown according to the previous description; as this surface represents the top of the slab, where the top surface is above 100 km depth, earthquakes occurring below this depth and within the slab are not atypical. Seismicity in this depth range correlates well with the outlined slab map and show little relationship with gravity anomaly trends. A zone of intense, high density seismicity is present north of the Huon Peninsula, this has previously been referred to as the ‘ Finisterre Nest ’ (Abers & Roecker et al. 1991).
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TAJE_A_824505.3d (TAJE) 09-08-2013 11:44
A re-evaluation of arc–continent collision and along-arc
variation in the Bismarck Sea region, Papua New Guinea
R. J. HOLM*AND S. W. RICHARDS
School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland 4811, Australia.
The Bismarck Sea region of Papua New Guinea is marked by recent arc–continent collision giving rise to
a highly dynamic tectonic environment, characterised by complex plate interactions that are yet to be
fully understood. We present a new crustal and upper mantle crustal architecture model for northeast-
ern Papua New Guinea and western New Britain that reveals complex tectonic geometries of overprint-
ing slab subduction and partial continental subduction, resulting in a unique setting in which to
investigate along-arc magmatic variation. Earthquake hypocentre databases are combined with
detailed topography and seafloor structure together with geology and regional-scale gravity to unravel
the sub-surface structure of northeastern Papua New Guinea. These data are used in conjunction with
an updated 3-D slab map of the region to propose a new interpretation of the area whereby Australian
continental crust extends as an underthrust block beneath the accreted Finisterre Terrane. The subduct-
ing continental crust combined with slab stagnation has resulted in a complex pattern of arc-related
geochemical signatures from east to west along the Bismarck arc. In the east, where the Solomon Sea
plate is subducting beneath New Britain, the sedimentary component is low, whereas in the west, the
arc volcanics exhibit a greater sedimentary component, consistent with subduction of Australian crustal
sediments. As a result, a new plate reconstruction is provided for the region together with a forward-
looking reconstruction of the Papuan peninsula, the Solomon Sea plate and New Britain that illustrates
that the same process will likely be repeated in some 5–10 m.y.
KEY WORDS: Papua New Guinea, Bismarck arc, arc–continent collision, gravity, seismicity, arc
geochemistry.
INTRODUCTION
The boundary between the northern Australian plate
and the Pacific plate, which includes the Bismarck Sea
region of Papua New Guinea (Figure 1), comprises some
of the youngest and most active tectonic elements of the
southwest Pacific (e.g. Taylor 1979; Abbott 1995; Martinez
& Taylor 1996; Weiler & Coe 2000). Northward motion of
the Australian plate has led to a scenario where both
continental and oceanic crust is interacting along the
northern plate boundary. The complexities of present-
day crustal and mantle geometries have emerged from
new information and a reinterpretation of the mecha-
nisms leading to the tectonic amalgamation of the area
is required. Here we focus on just the latest 4 Ma or so in
northeastern Papua New Guinea where in this short
time arccontinent collision has consumed tectonic
plates and uplifted mountain ranges to more than
4000 m, neighboured by contemporary island arc mag-
matism, culminating in highly dynamic and striking
geological landscapes. Numerous workers have sought to
explain these processes of, for example, arc volcanism in
the western Bismarck Sea, the source of earthquakes, or
the timing and nature of arccontinent collision; how-
ever, these models lack an overarching geological model
with cross-disciplinary foundations that can account for
all geological phenomena.
We present a compilation and reinterpretation of
an extensive catalogue of previous data, including
topography/bathymetry, earthquake hypocentres,
regional-scale gravity, geology and geochemistry, and
models depicting the complex tectonic history of Papua
New Guinea and the southwest Pacific. From this we re-
evaluate and address gaps in our knowledge of the pres-
ent-day 3-D tectonic setting of northeast Papua New
Guinea and the Bismarck Sea. Using a new and robust
regional tectonic model, we assess the role of recent arc
continent collision in construction of the present-day
tectonic puzzle that is Papua New Guinea.
The global importance of arccontinent collision has
been addressed by several authors related to the Africa
Europe collision (e.g. Rosenbaum et al. 2002; Kley & Voigt
2008) or the IndiaAsia collision (e.g. Hendrix et al. 1994;
Sobel & Dumitru 1997; Najman et al. 2010); however, the
southwest Pacific offers the unique opportunity to
observe the process in action. Furthermore, recognition
of the subtle processes and mechanisms of collision that
are not apparent at the surface, such as crustal under-
thrusting and associated arc magmatism, will contribute
*Corresponding author: rob.holm@my.jcu.edu.au
Ó2013 Geological Society of Australia
Australian Journal of Earth Sciences (2013)
60, 605–619, http://dx.doi.org/10.1080/08120099.2013.824505
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Figure 1 Topography, bathymetry and major tectonic elements of the study area. (a) Major tectonic boundaries of Papua New
Guinea and the western Solomon Islands; CP, Caroline plate; MB, Manus Basin; NBP, North Bismarck plate; NBT, New Britain
trench; NGT, New Guinea trench; NST, North Solomon trench; PFTB, Papuan Fold and Thrust Belt; PT, Pocklington trough;
RMF, Ramu-Markham Fault; SBP, South Bismarck plate; SCT, San Cristobal trench; SS, Solomon Sea plate; TT, Trobriand
trough; WB, Woodlark Basin; WMT, West Melanesian trench. Study area is indicated by rectangle labelled Figure 1b; the other
inset rectangle highlights location for subsequent figures. Present day GPS motions of plates are indicated relative to the
Australian plate (from Tregoning et al. 1998,1999; Tregoning 2002; Wallace et al. 2004). (b) Detailed topography, bathymetry
and structural elements significant to the South Bismarck region (terms not in common use are referenced); AFB, Aure Fold
Belt (Davies 2012); AT, Adelbert Terrane (e.g. Wallace et al. 2004); BFZ, Bundi Fault Zone (Abbott 1995); BSSL, Bismarck Sea
Seismic Lineation; CG, Cape Gloucester; FT, Finisterre Terrane; GF, Gogol Fault (Abbott 1995); GP, Gazelle Peninsula; HP,
Huon Peninsula; MB, Manus Basin; NB, New Britain; NI, New Ireland; OSF, Owen Stanley Fault; RMF, Ramu-Markham Fault;
SS, Solomon Sea; WMR, Willaumez-Manus Rise (Johnson et al. 1979); WT, Wonga Thrust (Abbott et al. 1994); minor strike-slip
faults are shown adjacent to Huon Peninsula (Abers & McCaffrey 1994) and in east New Britain, the Gazelle Peninsula
(e.g. Madsen & Lindley 1994). Circles indicate centres of Quaternary volcanism of the Bismarck arc. Filled triangles indicate
active thrusting or subduction, empty triangles indicate extinct or negligible thrusting or subduction.
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to our understanding of collision events and terrane
accretion at ancient convergent margins.
TECTONIC SETTING
Papua New Guinea and much of the southwest Pacific
occupy a zone of oblique convergence between the Aus-
tralian and Pacific plates (Figure 1). The tectonic history
of the region is significantly more complex than other
arcs owing to the number of recognised small plates
within the region. This scenario arises from the posi-
tioning of Papua New Guinea within a regional-scale col-
lision zone between the Australian continental crust in
the south (Abbott 1995; Hall 2002; Davies 2012) and the
Ontong Java Plateau in the northeast (Petterson et al.
1999; Hall 2002; Mann & Taira 2004). The relative direc-
tion of plate convergence has resulted in development of
oblique spreading centres and the formation of numer-
ous micro-plates and associated plate boundaries. The
principal tectonic elements comprising this complex
zone are shown on Figure 1, but emphasis is placed on
the Australian plate, the Finisterre Terrane (described
in detail below), New Britain and the North and South
Bismarck plates.
Previous research has suggested that from the Upper
Oligocene to the latest Neogene, northern Papua New
Guinea is marked by a series of arccontinent collisions.
The youngest and most significant of these collisions
resulted in the accretion of the Adelbert and Finisterre
Terranes, the latter of which forms a prominent topo-
graphic high known as the Finisterre Range (Figure 1;
Abbott et al. 1994; Abbott 1995). Abbott et al. (1994) stud-
ied clastic sequences on the southern flanks of the Fini-
sterre Range and concluded that the collision must have
initiated at ca 3.73.0 Ma. The Adelbert and Finisterre
Terranes are largely composed of Paleogene through to
earliest Neogene volcanic arc rocks overlain by Miocene
to PlioPleistocene limestone (Jaques & Robinson 1977;
Weiler & Coe 2000). Collision of these terranes with
Papua New Guinea is interpreted to have resulted from
the closure of the Solomon Sea at the New Britain trench
owing to subduction-driven convergence between the
Australian and South Bismarck plates (e.g. Abbott 1995;
Hill & Raza 1999; Weiler & Coe 2000). Oblique collision
started in the west and propagated southeastwards, pro-
ducing progressive thrusting and uplift of the north
coast Adelbert and Finisterre Ranges (Johnson & Jaques
1980; Abbott 1995; Weiler & Coe 2000).
At present, the ongoing convergence between the Fin-
isterre Terrane and the Australian plate is accommo-
dated by activity along the Ramu-Markham Thrust Fault
(e.g. Cooper & Taylor 1987; Abbott et al. 1994; Pegler et al.
1995). All previous studies regarding this episode of arc
continent collision have focused on the Finisterre Ter-
rane, the uplifted and exposed upper plate. The outstand-
ing topography of the Finisterre Range, however, only
arises as the Finisterre Terrane is thrust over the for-
mer northern coastward margin of Papua New Guinea.
This concept, and the nature or expanse of the now
underthrust Papua New Guinea margin has only been
suggested in passing by previous studies, but should be
regarded as an important, although missing piece of
Papua New Guinea. This statement is particularly signif-
icant given the prominence of major suture zones and
structures converging with the Ramu-Markham Fault
and underthrust beneath the Finisterre Terrane, for
example the Owen Stanley Fault and the Aure Fold Belt
(Figure 1).
The recent collision of the Adelbert and Finisterre
Terranes is reflected in the regional tectonics of the Bis-
marck Sea. The inferred timing of plate coupling at
3.7 Ma (Abbott et al. 1994) is coincident with the earliest
breakup and opening of the New Britain back-arc,
whichinturncreatedtwonewmicro-plates,theNorth
and South Bismarck plates (Taylor 1979). The South Bis-
marck plate is currently rotating clockwise at a rate of
8/Ma relative to Australia (Tregoning et al. 1999;
Wei le r & Coe 2000; Wallace et al. 2004) while the west-
northwest motion of the North Bismarck plate is simi-
lar to the Pacific plate (Tregoning et al. 1998;Wallace
et al. 2004) suggesting almost complete coupling
betweentheNorthBismarckandPacificplates
(Figure 1). In the eastern Bismarck Sea, the East Manus
spreading centre separates the North and South Bis-
marck Sea plates (Martinez & Taylor 1996). However, in
the wester n Bismarck Sea, the boundary becomes the
Bismarck Sea seismic lineation, defined primarily by
earthquake epicentre locations and characterised by
left-lateral transform faults and associated step-over
rifts (Denham 1969;Taylor1979). Thus, the Manus Basin
accommodates the majority of extension and rotation
in the eastern part of the Bismarck Sea while in the
west, the Bismarck Sea seismic lineation becomes a dis-
crete, eastwest oriented strike-slip plate boundary
(Figure 1; Cooper & Taylor 1987;Llaneset al. 2009).
North-dipping subduction of the Solomon Sea plate
beneath New Britain, in addition to the convergence
responsible for terrane accretion, has resulted in the for-
mation of the active Bismarck volcanic arc. The arc
occupies the northern part of the island of New Britain
and extends to the west where it is present as a series of
volcanic islands off the northwest coast of New Britain
and northeast coast of the Papua New Guinea mainland
where it forms the West Bismarck Arc (Figure 1). The
composition of the volcanics centred on and around the
island of New Britain range from basalt to rhyolite with
typical low-K, island arc tholeiite signatures (Jakes &
Gill 1970). The compositions differ markedly in the West
Bismarck arc with predominantly a medium-K charac-
ter (Woodhead et al. 2010). Along-arc variation, recently
been investigated by Woodhead et al. (2010), is discussed
below.
DATA AND DATA-ANALYSIS TECHNIQUES
Earthquakes
The interpretations and reconstructions presented
below have been resolved using a variety of datasets
combined into a single 3-D tectonic map of the region
using the software GOCAD. Seismic data provide a use-
ful indicator for active tectonic structures such as faults
and subducting slabs (Figure 2). We utilise a combina-
tion of earthquake records including the EHB hypo-
centre catalogue (Engdahl et al. 1998; Engdahl 2006) and
Bismarck Sea collision tectonics 607
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Figure 2 Seismicity and major structure of northwest Papua New Guinea. Earthquakes are derived from the NEIC earthquake
database (19902010) for the 0100 km depth bin. (a) Seismicity, structure and geology of the southwest Bismarck SeaHuon
Peninsula region. Structures and labels follow Figure 1b; see text for discussion of geology. (b) Earthquake density distribu-
tion map for the same region. Earthquake densities are contoured from high density (white) to low density (blue). We note
there are high-density earthquake clusters adjacent to the point where the Bundi Fault Zone and Owen Stanley Fault intersect
and under-thrust the Ramu-Markham Fault.
608 R. J. Holm and S. W. Richards
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the USGS National Earthquake Information Center
(NEIC) database for the period between 1973 and 2010
(Figure 2). In addition, Centroid-Moment-Tensor (CMT)
earthquake solutions were derived from the Harvard
Global CMT database (19762010) and plotted within
ArcScene in ArcGIS using the USGS 3D Visualisations of
Earthquake Focal Mechanisms extension. The NEIC
earthquake database and CMT database include all
earthquakes with moment magnitude values (Mw)
greater than Mw 4.5; the EHB database utilises earth-
quakes greater than Mw 4.3.
The earthquake hypocentre data were scrutinised
using a variety of software and techniques. Initially, the
data were imported into the 4DEarth model (GOCAD)
where slab surface models were derived. Details of the
slab model are presented below. In order to estimate
earthquake abundance distributions and clustering (Fig-
ure 2), a simple gridding function was used to derive a
map highlighting the number of earthquakes within
grid cells with dimensions of 0.04 0.04latitude and lon-
gitude. This grid cell size allowed the minimum number
of cells to be chosen without biasing towards the genera-
tion of many separate but isolated clusters or points. In
addition, the use of equi-dimensional cell size removed
any directional bias; therefore, the trends observed in
the density distribution maps represent a true cluster
orientation. The results are presented as a series of den-
sity distribution maps which are plotted for depth bins
of 040 km, 40100 km and 100300 km (depths of 020,
2040, 4060, 6080, 80100, 100120 and 120140 km are
contained within the Supplementary Papers).
Seafloor gravity and topography
Seafloor gravity data for the Bismarck Sea (Figure 3) are
sourced from the Australian Bureau of Mineral Resour-
ces (1970). The gravity model provided has been cor-
rected for a uniform ocean thickness.
Topography and bathymetry data derived from the
National Oceanic and Atmospheric Administration
ETOPO1 1-minute global relief model (Amante & Eakins
2009) provide an additional framework for correlation
and interpretation.
Geochemical data
Woodhead et al. (1998,2010) created an extensive geo-
chemical dataset for the New Britain and West Bismarck
arcs based on new and existing geochemical data from
New Britain from Johnson & Chappell (1979) and
Woodhead & Johnson (1993). The majority of these geo-
chemical data were produced for major and trace ele-
ments by X-ray fluorescence (XRF) and limited use of
spark-source mass spectrography (SSMS; Johnson &
Chappell 1979; Woodhead & Johnson 1993; and references
therein). Woodhead & Johnson (1993) and Woodhead
et al. (1998,2010) used inductively coupled plasma mass
spectrometry (ICPMS) and Pb, Sr, Nd and Hf isotope
analyses to develop the best compilation of data for east-
ern Papua New Guinea. Woodhead et al. (2010) investi-
gated along-arc geochemical changes in Bismarck arc,
and we re-evaluate these data in the context of the new
tectonic model presented here.
Figure 3 Seafloor free-air gravity anomaly map for the southwest BismarckHuon Peninsula region (Data from Australian
Bureau of Mineral Resources 1970).
Bismarck Sea collision tectonics 609
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INTERPRETATION AND RESULTS
Subduction zone and slab architecture
Previous work in the region focused on establishing an
accepted plate boundary model using information such
as seismicity and instantaneous GPS motions (e.g.
Denham 1969; Johnson & Molnar 1972; Ripper 1982;
Abers & Roecker 1991; Pegler et al. 1995; Wallace et al.
2004). It is widely accepted that multiple subduction
zones have existed since the beginning of crustal amal-
gamation of Papua New Guinea, however the details per-
taining to the geometry and type of crust subducting
along the northeastern Papua New Guinea coast and
western New Britain are unresolved, despite its infancy.
Construction of new 3-D subducted slab models, up to
600 km depth in the mantle, build on earlier work by
OKane (2008). Earthquake hypocentre data (Engdahl
et al. 1998; Engdahl 2006) are primarily used to generate
the 3-D models of subducted slab with all earthquakes
below 100 km assumed to occur within the subducting
plate (Isacks et al. 1968). The method for constructing
slabs follows that outlined in Richards et al. (2007,2011).
The Global CMT database is examined in 3-D to assist in
interpreting the geometry of the slab. The final inter-
preted slab geometry of the composite Australian plate
(Solomon Sea plate, Woodlark Basin and Australian plate)
subducted at the New Britain and San Cristobal trenches,
and termed the Solomon slab, is presented in Figure 4.
Miller et al. (2006) used a similar method of analysing
slab geometries in conjunction with earthquake failure
solutions beneath the southern Mariana Arc.
Overall, the Solomon slab exhibits a moderate dip
between the surface and 100 km depth; below this depth,
the slab is steeply dipping. West of the New Britain
trenchTrobriand trough triple-junction, the Solomon
slab currently resides at a depth of 100 km, and remains
close to this depth until it terminates in the west beneath
central Papua New Guinea. Furthermore, a north-
dipping slab component is modelled in the west which
extends to 250 km depth below west New Britain and
continues to the west at shallower depths (Figure 4), con-
sistent with findings from Johnson & Molnar (1972), John-
son & Jaques (1980), and Abers & Roecker (1991).A
restricted south-dipping slab component is also imaged
but this is limited to the region adjacent to the Huon Pen-
insula, accounting for observations made by Ripper
(1982), Cooper & Taylor (1987),Pegleret al. (1995),and
Woodhead et al. (2010). The lack of a definitive modern
seismic or tomographic signature for either an extensive
slab at depth or plate interface seismicity at the trench
(Hall 2002; Hall & Spakman 2002) suggests that there is
very little evidence for substantial southward subduction
at the Trobriand trough (Johnson & Molnar 1972; Johnson
&Jaques1980;Abers&Roecker1991) in agreement with
the slab map present here. A small tear in the slab is
interpreted below the eastern margin of the Huon Penin-
sula; this fundamentally separates the wester n slab
domain from the remaining Solomon slab in the east.
Adjacent to east New Britain and New Ireland, the
curvature of the trench and subducted slab, and associ-
ated subduction of an originally flat oceanic crustal
sheet have resulted in the development of a vertical tear
in the slab (Figure 4); in line with findings suggested by
Cooper & Taylor (1989). At present, the tip of the tear ter-
minates beneath southern New Ireland and exhibits a
western and an eastern flank propagating beneath the
TabarLihirTangaFeni arc; the western flank propa-
gated beneath Lihir (where the slab lies some 550 km
below owing to the steep dip). The tear here is significant
because it provides a window where the asthenosphere
can penetrate from the rear of the slab to the front. To
Figure 4 3-D model of the Solomon slab comprising the sub-
ducted Solomon Sea plate, and associated crust of the Wood-
lark Basin and Australian plate subducted at the New
Britain and San Cristobal trenches. Depth is in kilometres;
the top surface of the slab is contoured at 20 km intervals
from the Earths surface (black) to termination of slab-
related seismicity at approximately 550 km depth (light
brown). Red line indicates the locations of the Ramu-Mark-
ham Fault (RMF)New Britain trench (NBT)San Cristobal
trench (SCT); other major structures are removed for clar-
ity; NB, New Britain; NI, New Ireland; SI, Solomon Islands;
SS, Solomon Sea; TLTF, TabarLihirTangaFeni arc. See
text for details.
610 R. J. Holm and S. W. Richards
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the east, the Solomon slab is dipping beneath the Solo-
mon Islands and reaches a maximum interpreted depth
of 500 km. The subducted slab here exhibits less struc-
turethan the slab to the west; however, research focused
on mapping the subducted extent of the Woodlark Basin
rift is ongoing.
Gravity and seismicity correlation
At the relatively shallow sub-crustal depths (in the order
of 50100 km), upper crustal features can mask seismic
tomography and earthquake distribution. We instead
utilise regional seafloor gravity free-air anomaly data
obtained during the 1970 Hamme Cruise (Figure 3;
Australian Bureau of Mineral Resources 1970) to help
interpret crustal boundaries. In particular, we focus on
the region adjacent to the Huon Peninsula and north
coast of Papua New Guinea (Figure 1). Figure 5 presents
the gravity data together with seismicity and the
interpreted slab model. A large gravity-low anomaly is
observed trending sub-parallel to the northern coast of
Papua New Guinea defined by negative gravity values
(Figure 3). The gravity low is particularly intense to the
north of the Huon Peninsula (Davies et al. 1987; Honza
et al. 1987). This anomalous gravity-low also corresponds
with the location of intense seismicity beneath the Huon
Peninsula at depths of between 0 and 100 km (Figure 5).
Gravity and seismicity anomalies of the two datasets cor-
relate extremely well suggesting a relationship between
the two, to a depth of up to 100 km. This level of seismic-
ity has been attributed to the presence of Australian lith-
osphere at up to 100 km depth (Pegler et al. 1995;
Woodhead et al. 2010); however, this has only been
explored in 2-D sections adjacent to the eastern Huon
Peninsula without consideration given to the 3-D extent
of the seismicity.
An additional component of the anomalous gravity
low is present to the southeast of our interpreted upper
Figure 5 Gravity and seismicity correlation for the southwest BismarckHuon Peninsula region. All earthquakes illustrated
are from the NEIC earthquake database (19902010) and projected on the Bismarck Sea seafloor gravity anomaly map and
topography (a, c); and associated earthquake density distribution maps (b, d). (a, b) Gravity and seismic correlation between
40 and 100 km depth; the top surface of the slab map above 100 km depth is shown as a white shaded area (a) and area outlined
in black (b). A large area of anomalously high earthquake density trending east-southeastwest-northwest and outlined by
the red dashed line does not show a relationship with the defined windows for slab-related seismicity (a, b); furthermore, this
anomalous seismic region correlates well with the negative (low; purple) gravity anomaly to the north of- and beneath the
Huon Peninsula. We make note of the two highest density earthquake clusters; the largest in extent of the two lies at the south-
east tip of the Huon Peninsula, this is a region of overlap between both the slab map and anomalous seismicity/gravity corre-
lation implying multiple earthquake sources superimposed; the second is located at the approximate centre of the Finisterre
Terrane and holds the greatest observed density. (c, d) Gravity and seismicity correlation between 100 and 300 km depth; the
top surface of the slab below 100 km to termination is shown according to the previous description; as this surface represents
the top of the slab, where the top surface is above 100 km depth, earthquakes occurring below this depth and within the slab
are not atypical. Seismicity in this depth range correlates well with the outlined slab map and show little relationship with
gravity anomaly trends. A zone of intense, high density seismicity is present north of the Huon Peninsula, this has previously
been referred to as the Finisterre Nest(Abers & Roecker et al. 1991).
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mantle crustal anomaly. This is not as seismically active
as the remainder of the anomalous zone and falls within
the normal bounds of slab-related seismicity for the
region. This anomaly is also consistent with elevated
bathymetry at the surface, and can therefore be attrib-
uted to thickened crust between the Ramu-Markham
FaultNew Britain trench and the Wonga Thrust.
Previous interpretations of geochemistry
Woodhead et al. (2010) concluded that the West Bismarck
and New Britain arcs are both typicalsubduction-
related volcanic arcs, and although contiguous along
strike, exhibit very different geochemical characteris-
tics. The arc was divided into two parts, the West Bis-
marck arc and New Britain arc with the line separating
the two drawn between the western-most volcanoes of
New Britain (Cape Gloucester, Langila, Aimaga, Tangi
and Gloucester) and the remainder of New Britain in the
east (Woodhead et al. 2010). In the most general terms,
the distinction between New Britain and West Bismarck
arcs equates to a tholeiiticcalc-alkaline transition
(Jakes & Gill 1970; Woodhead et al. 1998,2010), and may
reflect underlying differences in the nature and composi-
tion of the mantle wedge or subducting plate, or the pro-
cesses of mass transfer between the two, or alternatively
is a consequence of collisional processes during
accretion of the Adelbert and Finisterre Terranes
(Woodhead et al. 2010).
Along-arc geochemical trends of elements, ratios, and
isotopic variation utilising a compilation of data from
both Woodhead et al. (1998,2010) are shown in Figure 6.
Further characteristics of this arc will be discussed
below. Woodhead et al. (2010) identified important differ-
ences between the geochemistry of the two arcs. Arc lavas
from the New Britain volcanic front are derived from a
mantle source highly depleted in many incompatible
trace elements (Woodhead et al. 1998), while the least
evolved West Bismarck arc lavas generally have higher
HFSE contents than the New Britain volcanic front.
Extreme element depletion in the New Britain lavas is
typically attributed to prior melt extraction in the back-
arc Manus Basin, however, the same process does not
operate to the same extent on the mantle source of the
West Bismarck lavas (Woodhead et al. 1993,2010). The
Sm/La ratios (Figure 6), which are higher in New
Britain, suggest a depleted mantle source when compared
with the West Bismarck arc lavas. Furthermore, the
decrease in the Sm/La ratio to the west together with a
decrease in the size of volcanic edifices and eruption rate
(Johnson 1977) suggests the degree of mantle melting falls
dramatically from east to west (Woodhead et al. 2010).
In addition, Woodhead et al. (2010) finds Th/La ratios
in the West Bismarck arc are lower than that of bulk
Figure 6 Along-arc geochemical variation in selected major and trace elements, trace element ratios, and isotopic ratios for
the West Bismark and New Britain arcs (data from Woodhead et al. 1998,2010). See text for further discussion.
612 R. J. Holm and S. W. Richards
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continental crust and the average arc(Plank 2005) sug-
gesting that a sedimentary component is apparent in the
West Bismarck arc lavas. Woodhead et al. (2010) noted
that prior to collision with the South Bismarck plate, the
Australian plate likely carried high Th/La sediments
derived from mainland Papua New Guinea. This is sup-
ported by the similar Th/La ratios to average sediments
from the Solomon Sea (Woodhead et al. 1998), which also
contain a substantial volcaniclastic input derived from
the Papua New Guinea Highlands (Crook 1987). More-
over, Pb-isotopic compositions of the West Bismarck
lavas, which contain relatively radiogenic Pb compared
with Manus Basin MORB (Figure 6), suggest a strong
crustalsignature is evident, again similar to Pb compo-
sitions found in the Solomon Sea sediments. Hafnium
and Nd isotope ratios show opposite but similar trends
exhibiting the highest ratio values in the New Britain
arc and decreasing in the West Bismarck arc; Woodhead
et al. (2010) interpreted this as a response to arcconti-
nent collision where increased proximity to a crustal
source dramatically increased the proportion of conti-
nent-derived detritus delivered to the subducting slab.
These geochemical observations are important and dif-
ferentiate the New Britain arc magmas, predominantly
mantle derived but with a very small sedimentary com-
ponent, from the West Bismarck arc where the sedimen-
tary component is interpreted to be much greater.
DISCUSSION
Crustal architecture
The seismological activity or inactivity of major struc-
tures and plate boundaries should be apparent over a
regional-scale, even taking into account the relatively
short geological window of earthquake recording. The
principal cause of seismicity in the Bismarck Sea region
is subduction at the New Britain trench; this has long
been recognised and accepted as the origin for shallow
through to deep earthquakes. On the same regional-
scale, additional earthquake trends related to major
structure and plate boundary activity are those of the
Bismarck Sea seismic lineation and New Guinea trench
in the north and northwest, and the Papuan Fold and
Thrust Belt to the southwest (Figures 1,2). These major
tectonic structures have long been defined and are well
understood. However, shallow to intermediate depth
seismicity of the northeast Papua New Guinea mainland
is characterised by a somewhat chaotic distribution of
earthquakes (Figure 2). While much of the shallow seis-
micity has previously been correlated with upper crustal
structure (Cooper & Taylor 1987; Abers & McCaffrey
1994; Stevens et al. 1998), it is evident that a significant
proportion cannot be clearly related to any recognised
structural control (Figures 2,5).
As presented in this study, the anomalous seismicity
is focused beneath the Finisterre Terrane and immedi-
ate adjacent areas (Figures 2,5). This zone is defined by
an uncharacteristically high abundance of earthquakes
compared with typical backgroundseismicity
(Figure 5), and defines a zone extending from the surface
to approximately 100 km depth beneath northeast Papua
New Guinea. Further more, this feature correlates with
a negative gravity anomaly that cannot be easily related
to any near surface geological phenomena (Figure 3).
Similar gravity lows observed adjacent to trenches are
commonly interpreted as subducted low-density crust
(Morales et al. 1999; Mishra et al. 2000), or alternatively,
as crustal thickening and stacking of low-density crust
during orogenesis (Stern 1995; Casas et al. 1997). Both
scenarios are typical of convergent margin settings
much like the recent history of the Bismarck Sea region.
We propose the anomalous gravity-low in combination
with anomalous seismicity is the expression of a previ-
ously undefined crustal block underthrust beneath the
Adelbert and Finisterre Terranes during collision with
the Papua New Guinea margin. In addition, this crustal
block is interpreted to be continental crust that is the
underthrust and subducted leading edge of Papua New
Guinea (Figure 7). This is significant in that the nature
of the continental margin has not previously been con-
sidered in the context of the northern Papua New
Guinea accreted terranes and holds implications for the
dynamics of terrane collision processes. The extent of
the underthrust margin is further supported by CMT sol-
utions. These are illustrated in Figure 7 for between 40
and 90 km depth and highlight a regional compressional
stress field orientated WNWESE, consistent with the
trend of the underthrust margin; this regional stress dis-
tribution has previously been recognised in Australian
lithosphere by Woodhead et al. (2010). At the eastern
boundary of the underthrust margin, we see evidence
for more complex deformation occurring through trans-
lational source mechanisms and an additional dilational
regime, rotated into a generally northeastsouthwest
orientation (Figure 7).
If we infer the underthrust Papua New Guinea mar-
gin is similar in extent to the margin prior to terrane
accretion, we can begin to add detail to this crustal block
and place it in the context of the surrounding structure
and geology expressed at the surface. Within the seismic
signature of the underthrust block (Figures 2,5) there
are earthquake clusters typified by an increase in the
density of seismic activity that are confined to the north
side of the Ramu-Markham FaultNew Britain trench
plate boundary. The regional context of these clusters,
and likewise the source regions, has not previously been
investigated. It is clear from Figure 2 that these clusters
are proximal to major structures; the Bundi Fault Zone,
Aure Fold Belt, and Owen Stanley Fault, where the struc-
tures are currently being underthrust beneath the Fini-
sterre Terrane at the Ramu-Markham Fault. We
interpret this seismicity as possible reactivation of the
former structural suture zones during their passage
beneath the Finisterre Terrane. It is reasonable to
assume this structure will continue to depth in the
downgoing crustal block, and likewise can be defined by
a similar earthquake cluster at greater depths as
observed between approximately 20 and 60 km
(Figures 2,5). Furthermore, we propose that based on
this premise, and similarities in geological context, that
is, ophiolite belts of similar interpreted age adjacent to,
and exhibiting analogous structural and tectonic rela-
tionships to major suture zones, that the Bundi Fault
Zone and Owen Stanley Fault can be correlated as the
same structural discontinuity (Figure 7). This provides
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a new context for major structures within Papua New
Guinea as regionally significant suture zones rather
than discrete and unrelated geological phenomena.
Along-arc geochemical variation
Given the constraint provided by the new crustal archi-
tecture model presented here, we can begin to address
the implications of these findings and re-evaluate cur-
rent geological models of northeastern Papua New
Guinea. Most significantly, we can reinterpret the geo-
chemical signatures of the currently active New Britain
and West Bismarck arcs within a robust tectonic frame-
work. It becomes clear from the Solomon slab map that
the New Britain arc is related to the Solomon slab adja-
cent to the island of New Britain (Figure 7). However, in
the western Bismarck Sea, the north-dipping limb of the
subducted slab is located to the south of the West Bis-
marck arc and beneath the accreted Finisterre and Adel-
bert Terranes and the Papua New Guinea mainland
(Figure 7), spatially removed from the active West Bis-
marck arc. This observation immediately brings into
question the relationship between the West Bismarck
arc and the subducted Solomon slab proposed in previ-
ous studies. Instead, we suggest that the source of fluids
and potential crustal melts is readily available in the
form of the underthrust edge of Papua New Guinea crust
(Figure 7).
The extent of the underthrust margin at depth is out-
lined by both gravity and seismic signatures, consistent
with the interpretations of Davies et al. (1987), Honza
et al. (1987) and Woodhead et al. (2010). At a depth of
approximately 100 km in the mantle, the underthrust
continental margin is likely to be undergoing dewater-
ing processes and contributing fluids to the mantle
wedge. Given these fluids are derived from a continental-
crustal source rather than oceanic crust, this accounts
for the high sediment-signature input into the magmas
and comparatively reduced slab influence addressed
above, in contrast to the New Britain arc in the east.
This concept is further supported by the frontal edge of
the underthrust margin correlating spatially with the
overlying arc (Figure 7). Therefore, we define the New
Britain and West Bismarck arc as two distinct entities
with different source regions and different geochemical
affinities. These two arcs are the expression of either
slab-derived fluids (New Britain arc) or continental
crust-derived fluids (West Bismarck arc). There is, how-
ever, the added complication that the two arcs form a sin-
gle, more or less morphologically continuous volcanic
arc. Therefore, a zone must be present where fluids
derived from both subducted slab and underthrust conti-
nental crust are mixing and both contribute to arc mag-
matism (Figure 7). This model is consistent with an
observed continuum in the geochemical signatures
between the two arcs, transitioning from a slab
Figure 7 Interpretation of present-
day tectonic plate configuration
and magmatic arc distribution in
northeastern Papua New Guinea.
Gravity anomaly map is provided
as a base map. Bold black outlines
illustrate the extent of the under-
thrust continental crust, formerly
the leading edge of the Papua New
Guinea mainland; and the associ-
ated correlation of the Bundi Fault
Zone and Owen Stanley Fault in the
under-thrust crust. CMT solutions
are shown for the under-thrust
margin between 40 and 90 km
depth. Below the under-thrust mar-
gin, the distribution of subducted
oceanic crust of the Solomon slab is
shown for comparison, and is con-
toured at 40 and 100 km until termi-
nation to the slab. The Bismarck
arcis divided into the West Bis-
marck arc, New Britain arc, and
mixing zone between the two; these
are derived from continental crust,
oceanic slab, and a combination of
the two respectively. Cross-sections
through the plate arrangement are
provided to illustrate the 3-D frame-
work of the new plate arrangement
and context of corresponding fluid
sources of the equivalent magmatic
arc. See text for discussion.
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signature-dominated melt in the east through to the
crustal signature-dominated melt in the west (Figure 6).
This influence of slab-derived fluids is apparent up to
approximately 146.5E at which point the slab becomes
removed to the south beneath the underthrust continen-
tal margin and effectively blocked from any further con-
tribution of fluids in the arc building process (Figure 7).
This explains the delay in the apparent peak crustal sig-
natures in arc geochemistry up to this point as observed
by Woodhead et al. (2010).
It is also worth noting that further to the northwest
along the trend of the West Bismarck arc and towards
the Bismarck Sea seismic lineation, we see a geochemi-
cal trend that is consistent with a return to more mantle-
like signatures. We suggest the Bismarck Sea seismic lin-
eation behaves as a leaky transform in line with findings
from Llanes et al. (2009) resulting in variable continen-
tal-derived fluid contribution to arc magmatism.
Geodynamic evolution of the Bismarck Sea
Given the regional significance of the findings outlined
in this study, we propose a new plate tectonic reconstruc-
tion for the Bismarck Sea region that builds on previous
reconstructions of the region (e.g. Abbott 1995; Hill &
Raza 1999; Weiler & Coe 2000; Hall 2002) and incorporates
the new tectonic elements introduced in this paper.
Reconstructions of the Australian plate include all previ-
ously accreted terranes. GPS measurements of current,
geologically instantaneous plate motions (Tregoning
et al. 1998,1999; Tregoning 2002; Wallace et al. 2004) form
the basis for this new Bismarck Sea reconstruction
while the published timing for events and sea floor mag-
netic anomalies from Taylor (1979), Goodliffe et al.
(1997), Taylor et al. (1999) and Gaina & M
uller (2007) are
used to infer the direction and rate of seafloor spreading.
Paleomagnetic rotation data for the Finisterre Terrane
from Weiler & Coe (2000) is used to further constrain
reconstructions.
New reconstructions highlighting the significance of
the leading Australian continental margin are shown in
Figure 8. Between 3 and 4 Ma the Australian plate col-
lided with the Adelbert-Finisterre Terrane, closing the
western New Britain trench and forming the Ramu-
Markham Fault (Abbott et al. 1994; Abbott 1995). This is
coincident with decoupling and the initial formation of
the North Bismarck plate, South Bismarck plate and
associated Bismarck Sea seismic lineation (Martinez &
Figure 8 Tectonic reconstruction of the Bismarck Sea region. The inferred Papua New Guinea northeastern continental mar-
gin is shown in grey, and highlights the continuation of the BundiOwen Stanley suture and the seaward continental shelf; yel-
low dotted line represents the Finisterre Volcanics of the Adelbert and Finisterre Ranges. Magnetic isochrons and spreading
centres are included for the Woodlark Basin (Taylor et al. 1999), Solomon Sea (Gaina & M
uller 2007) and Manus Basin (Taylor
1979). Filled triangles and open triangles indicate normal and slow or extinct subduction respectively. AFT, Adelbert-
Finisterre Terrane; SBP, South Bismarck Plate; NBP, North Bismarck Plate; AP, Adelbert microplate; MSC, Manus Spreading
Centre; RMFZ, Ramu-Markham Fault Zone; OJP, Ontong Java Plateau. Reconstructions are presented in a fixed hot spot refer-
ence frame.
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Taylor 1996). Continued advance of the Australian plate
impinging on the western South Bismarck plate resulted
in the onset of clockwise rotation of the South Bismarck
plate (e.g. Weiler & Coe 2000). Furthermore, the South
Bismarck plate became decoupled from the North Bis-
marck plate and the subducting Solomon Sea plate,
allowing the South Bismarck plate to rotate freely about
a pole southeast of the Finisterre Terrane (Tregoning
et al. 1999). Rotation of the South Bismarck plate resulted
in retreat of the New Britain trench in the east and rift-
ing and sea-floor spreading at the Manus spreading cen-
tre from 3 Ma (Taylor 1979; Martinez & Taylor 1996).
Fragmentation of the South Bismarck plate in response
to collision with the thickened crust of the BundiOwen
Stanley suture zone formed the independent Adelbert
microplate. Right lateral offset of the OligoceneLower
Miocene Finisterre Volcanics in the Adelbert and Fini-
sterre Terranes is interpreted to have resulted from
crustal displacement along the Adelbert microplate
South Bismarck plate boundary. Continued north-
northeast motion of the Australian plate combined with
clockwise rotation of the South Bismarck plate resulted
in ongoing subduction of the Solomon Sea plate and ini-
tiation of rifting adjacent to the Manus spreading centre
with opening the Manus microplate from ca 1Ma
(Martinez & Taylor 1996).
The concept of a separate Adelbert microplate,
although presented here for the first time in a regional
context, is not a new idea and initially arose from geolog-
ical observations of the volcanic island-arc terranes in
the Adelbert and Finisterre Ranges (e.g. Jaques & Robin-
son 1977; Abbott et al. 1994; Wallace et al. 2004). A physi-
cal boundary between the Adelbert and Finisterre
Ranges was interpreted in early mapping of cross fault-
ing in the Finisterre Ranges (Jaques & Robinson 1977;
Abbott et al. 1994), however, dextral translational faults
defined by Abers & McCaffrey (1994) (Figure 1) have
been adopted as the Adelbert-Finisterre boundary in
this study. The most apparent contrast between the Adel-
bert microplate and South Bismarck plate is a dramatic
difference in elevation (Figure 1; Abbott 1995). Abbott
(1995) interpreted that the Gowop Limestone cap both
the Adelbert and Finisterre Ranges and that the contrast
in elevation of the Adelbert Range is due to less total
uplift of the Adelbert block, and cannot be attributed to
erosion in the Adelbert Range. This contrast is likely to
be the result of differential plate motion with northward
motion and clockwise rotation of the Adelbert micro-
plate relative to the South Bismarck plate resulting in a
lower rate of convergence with the Australian plate.
Such differential motion also explains the observed off-
set of the Finisterre Volcanics common to both ranges
(Figure 8; Jaques & Robinson 1977; Abbott et al. 1994).
Moreover, the nature of the continental crust under-
thrust beneath the Adelbert and Finisterre Terranes
likely holds implications for the degree of uplift. Recon-
structions show the Finisterre Terrane was forced over,
and currently overlies the thickened crust of the Bundi
Owen Stanley suture zone, in contrast to the marginal
continental crust beneath the Adelbert Terrane, thus
resulting in reduced total uplift compared with the Fini-
sterre Ranges. This is a new approach to explain differ-
ential uplift of the Adelbert and Finisterre Terranes
where the nature and geometry of the prior continental
margin, in combination with collision obliquity, controls
accretion dynamics.
It is also apparent from previous reconstructions and
those presented here that arccontinent collision in
northeast Papua New Guinea was a consequence of obli-
que collision between the larger Australian and Pacific
plates. This process zippedshut the intervening
Solomon Sea between the Papua New Guinea mainland
and the outboard Adelbert and Finisterre Terranes
(Figure 8;Abbott1995;Hill&Raza1999). However, the zip-
ping process continues to the present-day along this mar-
gin, therefore, collision is considered time transgressive
and will continue to migrate eastwards. Figure 9 presents
forward-looking tectonic reconstructions where contin-
ued subduction of the Solomon Sea plate at the New
Britain trench will ultimately consume the Solomon Sea
leading to arccontinent collision between the Papuan
Peninsula (Australian plate) and New Britain. We esti-
mate it will take approximately 5 m.y. to consume the
Solomon Sea at present plate motion rates. As in the pre-
vious instance of the Finisterre Terrane, we predict slab
pull forces acting on the subducted Solomon Sea slab cou-
pled to the Australian plate at the point of collision will
lead to drawdown and underthrusting of the leading edge
of continental crust. This process initiates a plate configu-
ration where New Britain is allowed to overthrust the
continental margin. Throughout this process we predict
the apparent continual outboard migration of the associ-
ated subduction-derived magmatic arc as the accreting
plate overrides the loci of slab dewatering (Figure 9).
The tectonic process of arccontinent accretion is not
unique to the tectonic evolution of Papua New Guinea.
In the geological record, there are many recognised arc
continent collision episodes (e.g. Teng 1990; Rosenbaum
et al. 2002; Whattam 2009; Najman et al. 2010), and simi-
larly occurrences of continental crust entering a trench
and failing to subduct (e.g. New Caledonia; Aitchison
et al. 1995; Rawling & Lister 2002; Spandler et al. 2005).
These collision events are typically distinguished in
field studies by the presence of ultra-high-pressure meta-
morphic terranes and/or obduction of ophiolite sequen-
ces, however, these observed rock types are not present
en masse in northeast Papua New Guinea, and it seems
apparent that the processes of terrane accretion and par-
tial subduction of continental crust remain active. In the
distant future, we can reasonably expect some exhuma-
tion of subducted continental crust will occur in line
with findings in ancient arccontinent collision events.
Recognising this process in the recent geological past,
and in the near future for New Britain, provides valu-
able insight into the dynamics and tectonic settings that
give rise to such geological phenomena. Furthermore,
the role of subducted continental crust as a potential
source of fluids in magmatic arc generation, in this case
the West Bismarck arc, is a relatively new avenue of geo-
logical research and will only occur under exceptional
tectonic circumstances. Nevertheless, recognition of the
expression of continental crust-derived arcs at the sur-
face and an understanding of the tectonic events leading
up to the generation of such apparent geological anoma-
lies will be invaluable in unravelling complex tectonic
regions globally.
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CONCLUSIONS
We re-evaluate the tectonics of northeastern Papua New
Guinea and the South Bismarck Sea region, and present
a new and robust model for the 3-D architecture of the
crust and upper mantle in this region. The new tectonic
plate model accounts for all the observed geological phe-
nomena including geology, geophysics, seismicity, arc
geochemistry, topography, bathymetry, landscape mor-
phology, and a newly developed slab model of crust sub-
ducted at the New Britain trench, while also drawing
from studies and concepts introduced by numerous
workers over decades of research. Compilation and re-
evaluation of these data reveal a previously unrecog-
nised crustal block at depth beneath the Finisterre Ter-
rane on the northern Papua New Guinea coast, defined
by anomalous seismicity and a correlative negative grav-
ity anomaly. We interpret this as the leading edge of Aus-
tralian continental crust, which has become partially
subducted beneath the Adelbert and Finisterre Terranes
during arccontinent collision and terrane accretion.
This continental crust extends to a depth of approxi-
mately 100 km in the mantle where it is currently under-
going dewatering reactions and contributing fluids to
the upper plate resulting in formation of the West
Bismarck arc adjacent to the northeastern Papua New
Guinea coastline. The West Bismarck arc is a distinct
arc, separate from the slab-derived New Britain arc to
the east, however, a mixing zone exists between the two
where fluids derived from both normal subduction pro-
cesses and also partially subducted continental crust are
contributing to arc magmatism. In light of these findings
we present a new tectonic reconstruction for the recent
development of the Bismarck Sea in conjunction with a
forward-looking reconstruction to highlight the role of
marginal continental crust in the dynamics of arcconti-
nent collision and accretion. Furthermore, we stress the
importance of the recognition of continental crust and
marginal provinces in collisional tectonic settings and
their potential to contribute in arc building processes.
ACKNOWLEDGEMENTS
We would like to thank Nautilus Minerals for providing
data from the Hamme Cruise. We also thank Carl Spandler,
Thorsten Becker, Patrice Rey, Gideon Rosenbaum and
Robert Hall and an anonymous reviewer for helpful
criticisms and suggestions.
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Received 7 June 2012; accepted 8 July 2013
SUPPLEMENTARY PAPER
Earthquake density distribution maps for depth bins of
020, 2040, 4060, 6080, 80100, 100120 and 120140 km.
Earthquake densities are contoured from high density
(white) to low density (blue).
Bismarck Sea collision tectonics 619
Downloaded by [JAMES COOK UNIVERSITY], [R. J. Holm] at 15:30 20 August 2013
... The Manus Basin is an active backarc basin associated with the New Britain Trench . The north and northeastern margins of the Manus Basin are bounded by a partly active volcanic arc, interpreted to have formed above the westdipping subduction of the Pacific Plate (Figure 1; Connelly, 1976;Baldwin et al., 2012;Holm and Richards, 2013) and extends from Manus Island in the north to New Ireland in the east (Lee and Ruellan, 2006). An active, subduction-related volcanic arc bounds the southern basin margin, but the volcanism here is caused by subduction along the New Britain Trench. ...
... This is reflected in the variety of interpretations presented in past studies, including a backarc rift in old arc crust (Monecke et al., 2014); an ancient forearc crust stretched between two bounding faults in a pull-apart basin (Binns and Scott, 1993;Martinez and Taylor, 1996;Sinton et al., 2003); a transitional island arc and backarc rift Hannington et al., 2011); and a backarc spreading rift (Beaulieu et al., 2015). Microplate formation, rotation, and adjacent subduction, characterized by slab rollback of a steeply dipping slab and the formation of a slab tear add to the complexity of tectonic models of the region (Wallace et al., 2005;Baldwin et al., 2012;Holm and Richards, 2013). ...
... The term 'lineament' is used here to describe linear features that may have either structural or tectonic origins including faults and fractures, or represent linear geomorphological features such as ridge crests and volcanic chains/ridges (e.g., Peacock et al., 2016). Finally, the seafloor domain classification informed the generation of the morphotectonic map (see Morphotectonic Map of the East Manus Basin) through interpretation and integration of the final BTM model and DEMs with recent regional tectonic reconstructions (e.g., Martinez and Taylor, 1996;Wallace et al., 2004;Wallace et al., 2009;Holm and Richards, 2013;Holm et al., 2016). ...
Article
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Backarc basin systems are important sites of extension leading to crustal rupture where basin development typically occurs in rifting phases (or stages) with the final successful stages identified by the formation of spreading ridges and new oceanic crust. The East Manus Basin is a young (<1 Ma), active, rapidly rifting backarc basin in a complex tectonic setting at the confluence of the oblique convergence of the Australian and Pacific plates. Here we undertake the first comprehensive spatial-temporal morphotectonic description and interpretation of the East Manus Basin including a link to the timing of, and tectonic controls on, the formation of seafloor massive sulfide mineralization. Key seafloor datasets used in the morphotectonic analysis include multi-resolution multibeam echosounder seafloor data and derivatives. Morphotectonic analysis of these data defines three evolutionary phases for the East Manus Basin. Each phase is distinguished by a variation in seafloor characteristics, volcano morphology and structural features: Phase 1 is a period of incipient extension of existing arc crust with intermediate to silicic volcanism; Phase 2 evolves to crustal rifting with effusive, flat top volcanoes with fissures; and Phase 3 is a nascent organized half-graben system with axial volcanism and seafloor spreading. The morphotectonic analysis, combined with available age constraints, shows that crustal rupture can occur rapidly (within ∼1 Myr) in backarc basins but that the different rift phases can become abandoned and preserved on the seafloor as the locus of extension and magmatism migrates to focus on the ultimate zone(s) of crustal rupture. Consequently, the spatial-temporal occurrence of significant Cu-rich seafloor massive sulfide mineralization can be constrained to the transition from Phase 1 to Phase 2 within the East Manus Basin. Mineralizing hydrothermal systems have utilized interconnected structural zones developed during these phases. This research improves our understanding of the early evolution of modern backarc systems, including the association between basin evolution and spatial-temporal formation of seafloor massive sulfide deposits, and provides key morphotectonic relationships that can be used to help interpret the evolution of paleo/fossilized backarc basins found in fold belts and accreted terrains around the world.
... The tectonic setting of the region is complex, dominated by oblique northeast-southwest plate convergence (Fig. 1A). Manam is located in the segment of the Bismarck volcanic arc where arc-continent collision took place in the late Miocene to Pliocene during closure of the Solomon Sea (43)(44)(45)(46). This suturing of arc and continent destroyed the submarine trench and is consistent with a hanging slab that has been detected by seismic tomography at about 100-km depth below the north coast ranges (47,48). ...
... Considering modern geophysical reconstructions of the rather unique tectonic regime in which Manam is situated (Fig. 1A), the observed discordance with global geochemical trends is perhaps not that unexpected. Following arc-continent collision and partial obduction of the Adelbert-Finisterre Terrane over the leading edge of the Australian continental crust, the Western Bismarck arc is no longer a site of active subduction (43)(44)(45)(46) (Fig. 1A). Regional convergence is now accommodated along the Ramu-Markham fault zone leaving a hanging remnant slab beneath the southern portion of the arc, the margins of which are outlined by distinct gravity and seismic signatures (46,47). ...
... Following arc-continent collision and partial obduction of the Adelbert-Finisterre Terrane over the leading edge of the Australian continental crust, the Western Bismarck arc is no longer a site of active subduction (43)(44)(45)(46) (Fig. 1A). Regional convergence is now accommodated along the Ramu-Markham fault zone leaving a hanging remnant slab beneath the southern portion of the arc, the margins of which are outlined by distinct gravity and seismic signatures (46,47). Any carbonate sediments entering the Western Bismarck trench during closure of the Solomon Sea are likely long melted and replaced by carbon-poor sediment addition following terrane accretion, potentially sourced from the underthrust continental crust (45,46,70). ...
Article
Full-text available
Volcanic emissions are a critical pathway in Earth's carbon cycle. Here, we show that aerial measurements of volcanic gases using unoccupied aerial systems (UAS) transform our ability to measure and monitor plumes remotely and to constrain global volatile fluxes from volcanoes. Combining multi-scale measurements from ground-based remote sensing, long-range aerial sampling, and satellites, we present comprehensive gas fluxes - 3760 ± [600, 310] tons day−1 CO2 and 5150 ± [730, 340] tons day−1 SO2 - for a strong yet previously uncharac-terized volcanic emitter: Manam, Papua New Guinea. The CO2/T ratio of 1.07 ± 0.06 suggests a modest slab sediment contribution to the sub-arc mantle. We find that aerial strategies reduce uncertainties associated with ground-based remote sensing of SO2 flux and enable near-real-time measurements of plume chemistry and carbon isotope composition. Our data emphasize the need to account for time averaging of temporal variability in volcanic gas emissions in global flux estimates.
... Many examples of former back-arc basins ending as propagators into the continental margins have been later undergoing shortening, leading ultimately to the formation of spectacular mountain range . For example, the closure of a V-shaped margin at the Solomon Sea has been described (Cooper and Brian, 1989;Abbott et al., 1994;Holm and Richards, 2013). The Pyrenées in Europe also result from the closure of the narrowest part of propagating basin. ...
Thesis
This study focuses on the tectonic configuration of a rare example of the subduction-related opening on the downgoing plate of a subduction zone. The birth, the stages, and the cessation, of activity of the South China Sea (SCS) basin in Cenozoic seem to be related with the decay of the Proto-South China Sea (PSCS), by subduction in Borneo and Palawan. However, the mechanisms are still poorly constrained in terms of leading parameter and fundamental mechanism. Being narrow, the SCS margin offers the advantage of easily feasible correlation of sedimentary infill, structural styles and magmatic activities. The comprehensive and regional tectono-stratigraphic studies on subsurface seismic data and field observation could allow me to understand the genetic relation between the ocean opening and the adjacent subduction zone.The SCS continental crust stretching and thinning by ubiquitous large extensional detachment faults were expressed in a diachronous rifting and breakup, initiated in the east while rifting continued in the west until the propagator’s arrival. To illustrate the pre-breakup geometries of the southwestern SCS margins, we restore two conjugate sections near the first oceanic magnetic anomaly. The COT configuration, not only illustrates local asymmetrical hyper-extension, but also appears in map view to have a rhomb-shape controlled by N-S abrupt segments and E-W hyper-extended ones. The spatial variation of the crustal structures records an initial N-S extension simultaneous with the first phase of seafloor spreading in the eastern SCS, and a significant change of extensional direction shortly later (circa 23Ma) to NW-SE. During the post-spreading period, in the ocean floor, few key seismic images show that the local uplift in diameter of few kilometers goes with important magma production, interestingly within an episodic subsidence period in regional scale. This large global subsidence is anomalous compared to the theoretical thermal subsidence of post-rifting basins. Meanwhile, collision initiated since Late Oligocene in the PSCS, and mélange and shale-prone delta deposits characterize the shallow part of the accretionary wedge. This unit was occasionally brought to the surface and the overlying strata were deformed as circular synclines and anticlines. I, therefore, suspect that the mechanism of exposing the mélange involved a diapiric process that pushed this low viscosity material upward. Accordingly, the entire process illustrates a tectonic evolution involving shale tectonics from the end of the collision to the post-collision setting of northern Borneo, delineating an age of termination of orogeny when the seafloor spreading also ceased at ca. 16 Ma.The evolution of the rifting, spreading and cessation of rifting in the South China Sea seems to be controlled by the effect of subduction in the PSCS margin. The closure of the PSCS undergo a longer period of shortening in front of Palawan and Sabah where the oceanic domain is wide – none or very limited continental crust - whereas a limited shortening to the tip of the basin in the Sarawak region. The lithospheric heterogeneity of lower plate (thick continental crust in the south and thin one to oceanic in the north) induced a counterclockwise rotation off the subduction zone, and simultaneously, the contiguous SCS margin that change the extensional direction from N-S to NW-SE by 23Ma. The slab ultimately broke off in the Late Miocene to induce large plutons in the PSCS margin, and dramatic subsidence as well as local magmatic intrusions in the SCS during the post-spreading period. This suggests a slab detachment which provoked deep mantle upwelling to the surface. This dual tectonic evolution on two opposite margins therefore illustrates how far-field subduction process may impact the regional tectonics.
... Hence, our findings suggest that oxidized, Sr-rich and Cu-rich forearc mantle sources might be viable candidates to source porphyry-type magmatism if there is sufficient heat to cause the melting of the forearc mantle (e.g., the occurence of a slab tear associated with ridge subduction). Evidence in support for the above model can be found in Pliocene to Pleistocene porphyry-epithermal Cu-Au deposits in Papua New Guinea, which share the following common features: (a) these ore deposits are located in the New Ireland forearc mantle wedge (McInnes et al., 2001); (b) the forearc mantle is metal-rich and oxidized (McInnes et al., 1999(McInnes et al., , 2001; (c) a tear has been identified in the underlying slab (Holm et al., 2019;Holm & Richards, 2013). Thus, like the Vanuatu Cu-rich Forearc samples, the strong correlation between the location of porphyry-epithermal systems above the forearc mantle and a slab tear suggest that the conjunction of these two factors would likely favor the generation of porphyry-epithermal Cu-Au systems. ...
Article
Full-text available
The effects of buoyant ridge subduction have been researched for decades. However, it remains unclear how this process influences magma chemistry. Here we use a compilation of geochemical data, well‐established geochemical proxies (i.e., Ba/Nb, Th/Nb) and mantle redox modeling (i.e., V/Sc, Cu) to propose that the subduction of a ridge underneath the central portion of the Vanuatu arc causes shallow‐angle subduction and the development of a slab tear. We suggest that the shallow slab pinches out the asthenospheric mantle and bulldozes ancient metasomatized lithospheric mantle from the forearc toward the main‐arc. Slab‐fluxed melting of this bulldozed material could account for the along‐arc ⁸⁷Sr/⁸⁶Sr‐¹⁴³Nd/¹⁴⁴Nd variations of the Vanuatu magmas. The influx of hot sub‐slab material into the Vanuatu arc mantle wedge through a slab tear produces magmatism within the forearc. Modeling of V/Sc and Cu systematics suggest that the mantle source of the forearc magmas has a higher fO2 and Cu content than the source of the main‐arc and rear‐arc samples. The main‐arc and rear‐arc mantles were metasomatized by both slab‐derived fluids and melts. Whereas release of high Cu‐SO2 slab‐derived fluids caused oxidation and Cu enrichment of the forearc mantle. These systematics indicate a decrease in the fO2 of slab fluxes with increasing depth‐to‐slab and distance‐from‐trench. Our findings highlight the role of ridge subduction in controlling the along‐arc and across‐arc variations in the chemistry of Vanuatu arc magmas. This updated geodynamic model, based on geochemistry, is consistent with recent geophysical constraints and 3D numerical modeling.
... This 1000 km-long volcanic arc extends onto the larger island of New Britain to the east, and arc volcanism in this setting is associated with the northward subduction of the Solomon microplate and of a relict slab further west, where the arc has collided with the New Guinea continental margin (Baldwin et al., 2012;Honza et al., 1989;Johnson et al., 1987;Taylor, 1979). This tectonically complex zone of microplates lies in a region of oblique convergence between the Pacific and Australian plates (Baldwin et al., 2012;Holm and Richards, 2013;Woodhead et al., 2010). The eastern and western ends of the Bismarck arc are cut by the Bismarck Sea Seismic Lineation, a seismically active series of left-lateral transform faults and spreading segments separating the South Bismarck plate and the North Bismarck plate (Baldwin et al., 2012;Taylor, 1979; Fig. 1). ...
Article
Volcanic island sector-collapses have produced some of the most voluminous mass movements on Earth and have the potential to trigger devastating tsunamis. In the marine environment, landslide deposits offshore the flanks of volcanic islands often consist of a mixture of volcanic material and incorporated seafloor sediments. The interaction of the initial volcanic failure and the substrate can be highly complex and have an impact on both the total landslide deposit volume and its emplacement velocity, which are important parameters during tsunami generation and need to be correctly assessed in numerical landslide-tsunami simulations. Here, we present a 2D seismic analysis of two previously unknown, overlapping volcanic landslide deposits north-west of the island of Sakar (Papua New Guinea) in the Bismarck Sea. The deposits are separated by a package of well-stratified sediment. Despite both originating from the same source, with the same broad movement direction, and having similar deposit volumes (~15.5–26 km³), the interaction of these landslides with the seafloor is markedly different. High-resolution seismic reflection data show that the lower, older deposit comprises a proximal, chaotic, volcanic debris avalanche component and a distal, frontally confined component of deformed pre-existing well-bedded seafloor-sediment. We infer that deformation of the seafloor-sediment unit was caused by interaction of the initial volcanic debris avalanche with the substrate. The deformed sediment unit shows various compressional structures, including thrusting and folding, over a downslope distance of more than 20 km, generating >27% of shortening over a 5 km distance at the deposit's toe. The volume of the deformed sediments is almost the same as the driving debris avalanche deposit. In contrast, the upper, younger landslide deposit does not show evidence for substrate incorporation or deformation. Instead, the landslide is a structurally simpler deposit, formed by a debris avalanche that spread freely along the contemporaneous seafloor (i.e., the top boundary of the intervening sediment unit that now separates this younger landslide from the older deposit). Our observations show that the physical characteristics of the substrate on which a landslide is emplaced control the amount of seafloor incorporation, the potential for secondary seafloor failure, and the total landslide runout far more than the nature of the original slide material or other characteristics of the source region. Our results indicate the importance of accounting for substrate interaction when evaluating submarine landslide deposits, which is often only evident from internal imaging rather than surface morphological features. If substrate incorporation or deformation is extensive, then treating landslide deposits as a single entity substantially overestimates the volume of the initial failure, which is much more important for tsunami generation than secondary sediment failure.
... In this case, we have included one transect just south of where the putative interaction between these slabs is located, but the complexity of the region may not be captured in our 2D models. Further to the west, the subducting slab along the Solomon trench beneath the Bismark Sea is heavily contorted and likely torn in the lower part of the upper mantle (Holm & Richards, 2013) which adds three-dimensional (3D) complexity that is well beyond the scope of this study. In some cases, such as the westernmost end of the Sumatra arc, the slab subducts normally in the shallow upper mantle, but is heavily deformed in the lower upper mantle and transition zone (Pesicek et al., 2008). ...
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RECEIVED MARCH 17, 2003; ACCEPTED DECEMBER 1, 2004 Arc magmas and the continental crust share many chemical features, but a major question remains as to whether these features are created by subduction or are recycled from subducting sediment. This ques-tion is explored here using Th/La, which is low in oceanic basalts (<0Á2), elevated in the continents (>0Á25) and varies in arc basalts and marine sediments (0Á09–0Á34). Volcanic arcs form linear mixing arrays between mantle and sediment in plots of Th/La vs Sm/La. The mantle end-member for different arcs varies between highly depleted and enriched compositions. The sedimentary end-member is typically the same as local trench sediment. Thus, arc magmas inherit their Th/La from subducting sediment and high Th/La is not newly created during subduction (or by intraplate, adakite or Archaean magmatism). Instead, there is a large fractiona-tion in Th/La within the continental crust, caused by the preferen-tial partitioning of La over Th in mafic and accessory minerals. These observations suggest a mechanism of 'fractionation & foun-dering', whereby continents differentiate into a granitic upper crust and restite-cumulate lower crust, which periodically founders into the mantle. The bulk continental crust can reach its current elevated Th/La if arc crust differentiates and loses 25–60% of its mafic residues to foundering.
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Northern New Guinea has been regarded as a region where the polarity of an island arc reversed following collision with the Australian continent in the Tertiary. However, the evidence for this reversal is not compelling. Because present-day volcanism off the north coast of mainland Papua New Guinea is associated with a steeply northward-dipping Benioff zone (almost vertical) and late Cainozoic volcanoes in the central highlands to the south cannot be related to any present-day Benioff zone, a more acceptable interpretation is that, following collision, the northward-dipping slab beneath the arc became suspended nearly vertically. The active marginal basin lying to the north of the arc is unlikely to be subducted southwards beneath the mainland, because the lithosphere beneath marginal basins appears to be neither thick nor cold enough for the initiation of subduction. Polarity reversal, therefore, may not be the inevitable consequence of continent—arc collisions. Instead, the downgoing slab may steepen, equilibrate with the surrounding mantle and lose its identity. Continuing convergence may be taken up at other plate boundaries and the accreted arc may never again become active.
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A brief description is given of the main structural units in the New Guinea-Solomon Islands region. It is shown how the distribution of earthquakes is related to these features. A new trend, named the Bismarck Sea seismic lineation, that consists of a line of shallow earthquakes across the Bismarck Sea, is revealed. This trend does not correspond to any known surface feature. Cross sections both parallel and perpendicular to the strike of the New Britain arc and the Solomon chain are presented. They show that the north New Guinea region of uplift is on a different trend to the island arc structure of New Britain. The zone of earthquakes associated with the arc dips to the north, whereas that associated with the zone of uplift dips to the south. This study has improved the definition of the seismic zones in the New Guinea-Solomon Islands region, and most of the features can be explained in terms of sea-floor spreading.