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2010
Interpreting New England subduction complex
rocks using deep-sea drilling results from the
Nankai Trough (offshore southwest Japan)
Christopher L. Fergusson
University of Wollongong, [email protected]
Publication Details
This conference paper was originally published as Fergusson CL, Interpreting New England subduction complex rocks using deep-sea
drilling results from the Nankai Trough (offshore southwest Japan), Proceedings of the New England Orogen conference, Armidale,
New South Wales, Australia, November 2010.
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[email protected]
Interpreting New England subduction complex rocks using deep-sea
drilling results from the Nankai Trough (offshore southwest Japan)
Abstract
Two distinct structural styles are considered likely to have affected parts of the subduction complex of the
New England Orogen. Imbricate thrusting, as occurs by duplexing at a ramp along the décollement at the base
of the subduction complex, accounts for the structure of units such as the Gundahl Complex (also the
Wandilla Formation of central Queensland and the Sandon beds of the Armidale district). These units have
relatively straight-forward imbricate structure although in the case of the Gundahl Complex the thickness of
units is condensed and reflects factors controlling deposition on the ocean plate and in the trench. In
comparison units such as the Coramba beds contain excessively thick sections of turbidite wedge deposits that
only seem explicable by complicated internal imbricate thrust repetition perhaps requiring earlier thrusts
being cut by out-of-sequence thrusts. Much more age control than presently exists is required to improve our
understanding of the sedimentary and structural processes that have developed these units. Deep-sea drilling
and study of modern subduction zones, such as the Nankai Trough, provides an essential starting point for the
interpretation of ancient subduction complexes.
Keywords
mélange, offscraping, subcretion, subduction, tectonic erosion, turbidites, GeoQUEST
Disciplines
Life Sciences | Physical Sciences and Mathematics | Social and Behavioral Sciences
Publication Details
This conference paper was originally published as Fergusson CL, Interpreting New England subduction
complex rocks using deep-sea drilling results from the Nankai Trough (offshore southwest Japan),
Proceedings of the New England Orogen conference, Armidale, New South Wales, Australia, November 2010.
This conference paper is available at Research Online: http://ro.uow.edu.au/scipapers/978
Interpreting New England subduction complex rocks
using deep-sea drilling results from the Nankai Trough
(offshore southwest Japan)
Chris L. Fergusson
School of Earth & Environmental Sciences, University of Wollongong, Australia
Keywords: mélange, offscraping, subcretion, subduction, tectonic erosion, turbidites.
Introduction
Interpretations of the New England Orogen for the Late Devonian to Carboniferous have a forearc
system with from west to east a volcanic chain, forearc basin and subduction complex (Murray et al., 1987;
Leitch et al., 2003; Korsch et al., 2009). Units of the inferred subduction complex include widespread,
largely coherently deformed turbidite successions such as the Shoalwater Formation of central Queensland
and the Coffs Harbour association of northeastern New South Wales. These units lack widespread age
control and therefore their structure remains incompletely resolved. Imbricate thrust repetition is considered
likely as expected in subduction complexes but cannot be convincingly demonstrated due to the lack of well
defined stratigraphy. The Shoalwater Formation, a unit that extents along strike over 400 km, is dominated
by quartz-rich sandstones (Leitch et al., 2003) and regarded by some geologists as anomalous in a subduction
complex setting. Other units in the inferred subduction complex consist of lithic sandstone, mudstone, tuff,
chert, altered basalts and include disrupted rocks (mélanges). The diversity within these units illustrates more
convincingly internal thrust imbrication in comparison to the turbidite units. Coherent turbidite units and the
more diverse disrupted units are typical of subduction complexes such as the Shimanto Belt of southwest
Japan (Taira et al., 1988). The subduction-related setting of the New England Orogen for the Late
Carboniferous is also indicated by blueschists of the North D’Aguilar Block north of Brisbane (Little et al.,
1992).
Interpretation of subduction complex terranes is facilitated by comparison with modern systems that
have been investigated by ocean drilling and seismic investigations such as in the Nankai Trough where the
Philippine Sea plate is being subducted under southwest Japan. This review aims to present an updated
interpretation of depositional environments and structure of subduction complex units of the Coffs Harbour
Block utilising insights from the Nankai Trough.
Nankai Trough
Considerable deep-sea drilling has been undertaken in the Nankai Trough and is on-going with
expeditions planned using the R/V Chikyu that has the capability to drill to significant depths within the
hanging wall of the subduction zone using riser technology. This costly exercise is driven by the need to
better understand and eventually predict earthquakes, a topic of obvious interest to the Japanese people.
Here, I briefly summarise the results of deep-sea drilling and seismic surveys in the Nankai Trough based on
published reports (Shipboard Scientific Party, 2001; Moore et al., 2005; Kimura et al., 2009; Tobin et al.,
2009) that are available online on web sites of the Ocean Drilling Program (http://www-odp.tamu.edu/) and
the Integrated Ocean Drilling Program (http://www.iodp.org/).
The Philippine Sea plate is presently subducting to the northwest under southwest Japan, part of the
Eurasian plate, at a rate of ca 40 mm/a (4 km/Ma). The Philippine Sea plate in this region consists of the
Shikoku Basin that formed by backarc sea-floor spreading behind the Izu island arc up to ca 15 Ma. The
extinct ridge is associated with the Kinan seamounts. The Shikoku Basin has basaltic basement overlain by a
thick sequence of mainly mud containing dispersed ash layers. In the deeper parts of the basin occurs a
turbidite succession with quartz-rich and sedimenticlastic sands. At the Nankai Trough the Philippine Sea
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plate bends down to the northwest, but given the young age of the ocean floor (ca 30–15 Ma) and the thick
sedimentary succession, the topographic depression is less than 5000 m deep and therefore is not a deep-sea
trench. In the Nankai Trough, the Shikoku Basin is overlain by a wedge of trench-fill sediment consisting of
turbidites derived from the eastern end of the Nankai Trough at the collision zone of the Honshu Arc and the
Izu island arc. These sediments are rich in volcaniclastic material and were derived from the collision zone
and active volcanoes such as Mt Fuji, i.e. they are not derived from an active arc associated with the Nankai
Trough subduction zone. A major submarine channel meanders along the deepest part of the Nankai Trough
seaward of the deformation front.
The Nankai Trough Accretionary Prism is forming by offscraping at a deformation front at the base
of the inner slope of the Nankai Trough and has built outwards from the mid Tertiary Shimanto Belt exposed
on the oceanward side of Shikoku Island (Taira et al., 1988). A major topographic depression occurs on the
inner slope of the Nankai Trough offshore from Muroto Peninsula on Shikoku. This depression formed by
subduction of a major seamount, similar to those of the present-day Kinan seamounts, causing tectonic
erosion of the overlying prism and collapse of the inner trench slope. A transect of deep-sea drilling sites is
located on the southern side of the depression (Fig. 1) and they demonstrate that rapid frontal accretion has
occurred over the last 1–2 Ma with the deformation front advancing 35–40 km. At Sites 808 and 1174, late
Pleistocene outer trench wedge and axial channel sands of the trench wedge are being accreted. Frontal
accretion has caused the formation of a fold-thrust belt with imbricate thrust faults and fault-bend folds. Thus
seamount subduction has been followed by rapid recovery as the wedge attempts to re-establish a normal
taper angle. Structural complexity only occurs in Site 1178 where thrust imbrication is indicated by
repetition of trench wedge facies and nannofossil biostratigraphy.
Figure 1. Summary stratigraphic columns based on deep-sea drilling results of the Ocean Drilling Program
Legs 131, 190 and 196 for the Muroto Transect. Site 808 has had the column corrected for repetition along
the frontal thrust. Ages based on shipboard nannofossil identifications. See Shipboard Scientific Party (2001,
figure F6) for location of drill sites (http://www-odp.tamu.edu/publications/190_IR/chap_01/c1_f6.htm).
In the eastern Nankai Trough, a three dimensional seismic survey has documented the structure of
the Nankai Trough Accretionary Prism and shown that it is a well developed fold-thrust belt with structural
complexity developed at the frontal thrust confirmed by drilling at Sites C0006 and C0007 (Fig. 2). Thrust
imbrication of a relatively thin, (?)early Pleistocene trench wedge turbidite succession is documented at both
sites. It is thought that this structural complexity is related to the recent subduction of a small seamount
Screaton et al., 2009). Thrust imbrication is indicated by three lines of evidence: (1) nannofossil ages
although difficulties arise with reworking, (2) repetition of facies, particularly the mud-rich outer turbidite
wedge at Site C0006 and the repetition of the channel sand and gravel at Site C0007, and (3) logged fault
zones in the cores. Repetition of sand and gravel channel facies at Site C0007 has also been suggested to
reflect facies advance and retreat although I consider this unlikely. These sites illustrate a style of structural
imbrication that has resulted in internal thickening by fault repetition of similar facies.
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Figure 2. Left: simplified stratigraphic column for Site C0006. Right: simplified stratigraphic column for Site
C0007. Both sites are in the eastern Nankai Trough off the Kii Peninsula of Honshu. Columns are redrawn
from Expedition 316 Proceedings (Kimura et al., 2009). See Tobin et al. (2009) for location map (figure F1).
Coffs Harbour Block
Following from initial reconnaissance work by numerous geologists, investigations by R. J. Korsch
has provided the published framework for much of the geology of the coastal and southern Coffs Harbour
Block, whereas I have provided information on the central Coffs Harbour Block west of Grafton. Space
restrictions allow only a few of these publications to be cited here.
Several major units are mapped in the Coffs Harbour Block (Fig. 3) (Korsch, 1978a, b; Fergusson,
1984). These are folded into a large-scale synform, which is the southern closure of the Texas–Coffs
Harbour Megafold (Murray et al., 1987). Internally these units have generally steep dips with stratigraphic
younging indicators mainly to the north, although reversals occur in some areas (Fergusson, 1982). From
north to south the main units are: the Gundahl Complex, Cunglebung Creek beds, Coramba beds, Brooklana
Beds and Moombil beds.
The Gundahl Complex is a unit of lithic sandstone, mudstone, tuff, chert and altered basalt; it has
widespread zones of tectonic mélange including abundant evidence for stratal disruption of soft sediments
and the development of high fluid pressures. Within parts of the unit internal imbrication of trench floor
sequences is apparent (Fig. 4). The Gundahl Complex has a lithological content and structural association
characteristic of many exposed ancient subduction complexes such as the Shimanto Belt and the Franciscan
Complex of California. Trench-floor slices of the Gundahl Complex shown in Fig. 4 are thin with reduced
thicknesses of all comparable facies to those in the Nankai Trough such as at Sites 808, 1174 and 1173 (Fig.
1). The implication is that the Gundahl Complex formed either at a sediment-starved margin or subduction of
a very young plate occurred and thus only thin pelagic and trench turbidite units were formed. Accretion of
the Gundahl Complex was probably at depth beneath the subduction complex at steps in the décollement (i.e.
subcretion at ramps). Mélanges in the Gundahl Complex probably formed either from thickening of fault
zones between imbricate slices or were emplaced as mud-rich diapirs along fault zones (Moore and Byrnes,
1987; Pickering et al., 1988).
The Cunglebung Creek beds are dominated by mudstone and probably represent accretion of deepmarine basinal facies equivalent to the Shikoku Basin deposits. The Coramba beds are dominated by thinbedded and thick-bedded turbidites with horizons containing abundant thick layers of massive to graded
sandstone and rarely conglomerate (Fig. 5). The sandstones are dominated by dacitic volcanic detritus with
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some feldspar-rich horizons (Korsch, 1978b). From comparison with the Nankai Trough, the depositional
environment for the Coramba beds is interpreted as a trench wedge with the mudstone-rich parts representing
deposition in the outer trench wedge and the sandstone units having formed in an axial channel in the inner
trench wedge (cf. column in Fig. 5 with Sites 808, 1173 and 1174, Fig. 1). These trench wedge deposits are
considered most likely to have been added to the subduction complex by frontal accretion. The great
thickness of these facies in the Coramba beds is at variance with those encountered in even the thick trench
wedge deposits of the western Nankai Trough sites. The eastern Nankai Trough frontal thrust sites show
internal imbrication of the type that may well be widespread in the Coramba beds and account for the
apparently exaggerated thicknesses of the trench wedge deposits in this unit.
Figure 3. Geology of the central and southern Coffs Harbour Block in the southeastern New England
Orogen. Map extracted from the Geoscience Australia digital surface geological map of Australia at
1:1,000,000 scale. Abbreviations: Bb = Brooklana beds, Bucc = Buccarumbi (see Fig. 5 for detail of inset),
Cb = Coramba beds, CCb = Cunglebung Creek beds, GC = Gundahl Complex, Mb = Moombil beds, NRB =
Nymboida River bend (see Fig. 4 for detail of inset). Stars show location of age control, Tourn = Tournaisian
radiolarians (Aitchison, 1988), 320 Ma – refers to two samples from coastal headlands at Woolgoolga with
SHRIMP U-Pb ages on zircon and Ar-Ar ages on hornblende (Korsch et al., 2009).
The Brooklana beds are a unit of mudstone and less abundant lithic sandstone but also include chert
and altered basalts although not as common as in the Gundahl Complex. The Moombil beds are another
mudstone-dominant unit like the Cunglebung Creek beds. As for the other units imbrication would account
for the great thicknesses of these units.
Conclusions
Two distinct structural styles are considered likely to have affected parts of the subduction complex
of the New England Orogen. Imbricate thrusting, as occurs by duplexing at a ramp along the décollement at
the base of the subduction complex, accounts for the structure of units such as the Gundahl Complex (also
the Wandilla Formation of central Queensland and the Sandon beds of the Armidale district). These units
have relatively straight-forward imbricate structure although in the case of the Gundahl Complex the
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thickness of units is condensed and reflects factors controlling deposition on the ocean plate and in the
trench. In comparison units such as the Coramba beds contain excessively thick sections of turbidite wedge
deposits that only seem explicable by complicated internal imbricate thrust repetition perhaps requiring
earlier thrusts being cut by out-of-sequence thrusts. Much more age control than presently exists is required
to improve our understanding of the sedimentary and structural processes that have developed these units.
Deep-sea drilling and study of modern subduction zones, such as the Nankai Trough, provides an essential
starting point for the interpretation of ancient subduction complexes.
Figure 4. Simplified geological interpretative map of the Gundahl Complex of the Nymboida River bend
area showing imbricate repetition of the trench succession. Arrows indicate direction of younging in the
ocean floor and trench wedge fault slices. Stratigraphic column shows detail of the trench floor stratigraphy.
Redrawn from Fergusson (1985, figures 4 and 5).
Figure 5. Geological map of the Buccarumbi area in the central Coffs Harbour Block 35 km west-southwest
of Grafton and representative stratigraphic column. Map has been redrawn and modified from Fergusson
(1982, figure 7) and shows subdivision of the Coffs Harbour sequence into an outer trench wedge unit
dominated by thin-bedded turbidites and an axial channel of the trench wedge consisting of thick-bedded
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turbidites and thick intervals of massive lithic sandstone. Column is redrawn and modified from Fergusson
(1985, figure 12).
References
Aitchison, J. C., 1988. Late Paleozoic radiolarian ages from the Gwydir terrane, New England Orogen,
eastern Australia. Geology, 16, 793-795.
Fergusson, C. L., 1982. Structure of the Late Palaeozoic Coffs Harbour Beds, northeastern New South Wales.
Journal of the Geological Society of Australia, 29, 25-40.
Fergusson, C. L., 1984. Tectonostratigraphy of a Palaeozoic subduction complex in the central Coffs Harbour
Block of northeastern New South Wales. Australian Journal of Earth Sciences, 31, 217-236.
Fergusson, C. L., 1985. Trench-floor sedimentary sequences in a Palaeozoic subduction complex, eastern
Australia. Sedimentary Geology, 42, 181-200.
Kimura, G., Screaton, E. J., Curewitz, D., and the Expedition 316 Scientists, 2009. Expedition 316 summary.
In: Kinoshita, M., Tobin, H., and the Expedition 314/315/316 Scientists, Proc. IODP, 314/315/316:
Washington, DC (Integrated Ocean Drilling Program Management International, Inc.).
doi:10.2204/iodp.proc.314315316.131.2009
Korsch, R. J., 1978a. Stratigraphic and igneous units in the Rockvale-Coffs Harbour region, northern New
South Wales. Journal and Proceedings of the Royal Society of New South Wales, 111, 13-17.
Korsch, R. J., 1978b. Petrographic variations within thick turbidite sequences: an example from the late
Palaeozoic of eastern Australia. Sedimentology, 25, 247-265.
Korsch, R. J., Adams, C. J., Black, L. P., Foster, D. A., Fraser, G. L., Murray, C. G., Foudoulis, C., and Griffin,
W. L., 2009. Geochronology and provenance of the Late Paleozoic accretionary wedge and Gympie
Terrane, New England Orogen, eastern Australia. Australian Journal of Earth Sciences, 56, 655-685.
Leitch, E. C., Fergusson, C. L., and Henderson, R. A., 2003. Arc to craton provenance switching in a Late
Palaeozoic subduction complex, Wandilla and Shoalwater terranes, New England Fold Belt, eastern
Australia. Australian Journal of Earth Sciences, 50, 919-929.
Little, T. A., Holcombe, R. J., Gibson, G. M., Offler, R. O., Gans, P. B., and McWilliams, M. O., 1992.
Exhumation of late Paleozoic blueschists in Queensland, Australia, by extensional faulting. Geology, 20,
231-234.
Moore, J. C., and Byrne T., 1987. Thickening of fault zones: a mechanism of melange formation in accreting
sediments. Geology, 15, 1040-1043.
Moore, G. F., Mikada, H., Moore, J. C., Becker, K., and Taira, A., 2005. Legs 190 and 196 synthesis:
deformation and fluid flow processes in the Nankai Trough accretionary prism. In: Mikada, H., Moore,
G.F., Taira, A., Becker, K., Moore, J. C., & Klaus, A. (Eds.), Proc. ODP, Sci. Results, 190/196: College
Station, TX (Ocean Drilling Program), 1-26. doi:10.2973/odp.proc.sr.190196.201.2005
Murray, C. G., Fergusson, C. L., Flood, P. G., Whitaker, W. G., and Korsch, R. J., 1987. Plate tectonic model
for the Carboniferous evolution of the New England Fold Belt. Australian Journal of Earth Sciences, 34,
213-236.
Pickering, K. T., Agar, S. M., and Ogawa, Y., 1988. Genesis and deformation of mud injections containing
chaotic basalt-limestone-chert associations: examples from the southwest Japan forearc. Geology, 16, 881885.
Screaton, E., Kimura, G., Curewitz, D., and others, 2009. Interactions between deformation and
fluids in the frontal thrust region of the NanTroSEIZE transect offshore the Kii Peninsula, Japan:
Results from IODP Expedition 316 Sites C0006 and C0007. Geochemistry, Geophysics,
Geosystems, 2, doi:10.1029/2009GC002713. http://gcubed.org/
Shipboard Scientific Party, 2001. Leg 190 summary. In: Moore, G. F., Taira, A., Klaus, A., et al., Proc.
ODP, Init. Repts., 190: College Station, TX (Ocean Drilling Program), 1-87.
doi:10.2973/odp.proc.ir.190.101.2001
Taira, A., Katto, J., Tashiro, M., Okamura, M., and Kodama, K., 1988. The Shimato Belt in Shikoku, Japan—
evolution of Cretaceous to Miocene accretionary prism. Modern Geology, 12, 5-46.
Tobin, H., Kinoshita, M., Ashi, J., Lallemant, S., Kimura, G., Screaton, E. J., Moe, K. T., Masago, H.,
Curewitz, D., and the Expedition 314/315/316 Scientists, 2009. NanTroSEIZE Stage 1 expeditions:
introduction and synthesis of key results. In: Kinoshita, M., Tobin, H., & the Expedition 314/315/316
Scientists, Proc. IODP, 314/315/316: Washington, DC (Integrated Ocean Drilling Program Management
International, Inc.). doi:10.2204/iodp.proc.314315316.101.2009
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