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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.

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison to backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inver

The Ordovician Macquarie Arc is most widely exposed in the Lachlan Fold Belt of central New South Wales. Complex relationships between the arc and the Ordovician turbidite mega-fan are partly explained by anticlockwise rotation of the arc during the Ordovician. Thus, initially two lobes of the mega-fan formed to the north and south of the east-west trending arc, using present-day coordinates. The arc consists of the western Goonumbla-Trangie Volcanic Belt, replacing the inappropriate term Junee-Narromine Volcanic Belt, and an eastern composite of the Molong, Rockley-Gulgong and Kiandra Volcanic Belts. These two major segments of the arc are separated by Ordovician quartz turbidites of the Kirribilli Formation and it is probable that the arc has been duplicated by a sinistral strike-slip fault. Eastonian palaeogeographic reconstruction of the eastern segment of the arc highlights a prominent limestone platform in the western Molong Volcanic Belt that grades eastwards into a realm of mainly deep-marine sedimentation and volcanic activity. By analogy with Guam in the western Pacific Ocean, the limestone platform is equated to a frontal arc ridge. This implies that the associated subduction zone was along the western side of the arc and not to the east, as in previous reconstructions. A wide zone of deformed Ordovician quartz turbidites, making up the Girilambone and Wagga-Omeo Zones west of the Macquarie Arc, is interpreted as a subduction complex that formed rapidly in the Late Ordovician. Flipping of the subduction zone was a relatively long event, inferred to have occurred during the latest Ordovician to early Silurian Benambran Orogeny. This was driven by collision of the subduction complex with northern continuations of the Stawell and Bendigo Zones, with a new west-dipping subduction zone forming to the east.

Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since similar to 0.78-0.436 Ma and accommodated similar to 13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out-of-sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well-drained, sand-rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

Development of the East Gondwana passive margin and when it occurred are constrained by the composition of low grade mafic schists and U-Pb ages of detrital zircons in psammitic schists from the Bathampton Metamorphics in the Anakie Inlier of central Queensland. These rocks show considerable variation in light lithophile elements due to post-magmatic processes. They have flat heavy rare earth element patterns, low-TiO2 (<2 wt%) contents and their immobile element Ti, V, Y, La, Nb, Th and Zr values, indicate that they have an NMORB- like magmatic affinity. However, they differ from N-MORB in that they show light rare earth depleted patterns and lower incompatible trace element contents. Their relative low abundance and association with metasediments suggest they formed in a magma-poor rifted margin setting. They are associated with psammitic rocks with detrital zircon ages indicating probable deposition in the late Neoproterozoic at ca 600 Ma. A magma-poor rifted margin in northeastern Australia differs from the volcanic passive setting that occurred in southeastern Australia at this time. These findings support development of the East Gondwana margin at 600 Ma that may have been related to rifting of a microcontinent off East Gondwana well after the breakup of Rodinia at ca 750 Ma.

Gravel beds drilled during Integrated Ocean Drilling Program Expedition 316 at Site C0007 in the eastern Nankai Trough, ~100km offshore from the Kii Peninsula of Honshu, southwest Japan, have been investigated using 26 samples from two horizons (Cores 316-C0007C/C-17H and 316-C0007D-12R). The gravel from Core 316-C0007c-17H is from a 1.7m thick layer, whereas the gravel from Core 316-C0007D-12R is from a layer 4.55m thick. The upper parts of both layers are normally graded. The gravel is clast-supported, polymictic, of granule to pebble size, moderately to poorly sorted, and with subrounded to angular fragments. Clast types are similar from both cores with abundant fine-grained sedimentary clasts that are well cemented, and many clasts contain small quartz veins indicative of a source with a very low metamorphic grade. The main sedimentary clast types are chert of various colors (black, gray, light yellow-brown, clear and rare red and purple), siltstone, and sandstone. Other clast types are vein quartz and igneous clasts of mainly intermediate volcanic composition, minor plutonic rock fragments, and minor metamorphic rock fragments. Intraformational soft mud fragments occur in most samples.

The southeastern Georgetown Inlier (Greenvale Province) consists of Early Palaeozoic metamorphic rocks in fault contact along the Lynd Mylonite Zone with the Palaeoproterozoic to Mesoproterozoic craton of northeastern Australia. It has a central assemblage of metamorphosed silicic volcanic and sedimentary rocks considered equivalent to the Late Cambrian to Early Ordovician Seventy Mile Range Group that developed in an extensional backarc in the Charters Towers Province to the southeast. In the western part of the Greenvale Province the Oasis Metamorphics has a U-Pb zircon SHRIMP metamorphic age of 476 ± 5 Ma and is intruded by the granodioritic Lynwater Complex with U-Pb zircon ages of 486 ± 5 Ma and 477 ± 6 Ma. These ages are consistent with these rocks forming basement and intrusive equivalents to the extensional volcanic basin. Existing geochronological constraints on the Halls Reward domain, located at the eastern margin of the province, are consistent with it being basement to the extensional basin. Several domains are recognised in the Greenvale Province with either dominantly steep or low to moderate dips of the main foliation and each experienced multiple deformation with locally up to four overprinting structural phases. Steepening of foliation in several of the domains is attributed to contractional deformation in the Early Silurian that is inferred to have overprinted low-angle foliation developed during extensional tectonics in the backarc setting. Contractional deformation related to the Early Silurian Benambran Orogeny is considered responsible for multiple deformation in the Greenvale Province and reactivation of domain-bounding faults.

U–Pb detrital zircon ages in variably metamorphosed, dominantly fine-grained clastic successions are used in northeastern Australia to identify two major successions along the East Gondwana margin. The older succession is of probable Late Neoproterozoic age and is considered part of a passive margin associated with rifting at c. 600 Ma. Most detrital zircons have ages in the range 1000–1300 Ma and were probably derived from an extension of a Late Mesoproterozoic (1050–1200 Ma) orogenic belt from the central Australian Musgrave Complex located 1500 km to the west. No evidence has been found for 600–800 Ma rifting of a Rodinian supercontinent and therefore it is suggested that breakup must have occurred well outboard of the present Early Palaeozoic East Gondwana margin. The younger succession is of Early Palaeozoic age and contains the distinctive 500–600 Ma detrital zircon signature that is widespread in East Gondwana in addition to some samples with ages in the range 460–510 Ma consistent with local igneous sources. The younger succession is related to the active margin of Gondwana that developed on the former passive margin in a back-arc setting, and the source of 510–600 Ma zircons is considered to be a composite of rift-related and back-arc volcanic sources.

The Ross-Delamerian orogenic belt formed along the early Paleozoic active Pacific margin of the newly merged Gondwana supercontinent. In its northern-most segment in the Townsville region of northeastern Australia, we have identified a short contractional phase of the Delamerian orogeny in the Argentine Metamorphics postdating formation of a mafic breccia with a U-Pb zircon age of 500 ± 4 Ma. Contraction was followed by widespread inferred extensional deformation with formation of flat-lying foliation, domal features, and amphibolite grade and greenschist retrograde metamorphism all synchronous with latest Cambrian to Early Ordovician extensional backarc volcanism, sedimentation and intrusions. One of these intrusions gives a U-Pb zircon age of 480 ± 4 Ma. Foliation related to the extensional deformation is cross-cut by a late granodiorite dyke with a U-Pb zircon age of 461 ± 4 Ma. Late east-west contractional deformation affected the higher grade part of the assemblage. In contrast to the Ross-Delamerian orogenic belt in the Transantarctic Mountains and southeastern Australia, the orogenic belt in northeastern Australia was affected by a short episode of contraction at ∼495 Ma followed by long-lived backarc extension from ∼490 Ma to 460 Ma with subsequent contractional deformation.

The Early Palaeozoic Cape River Metamorphics consist mainly of psammitic gneiss and schist and occur as an extensive linear belt at the western margin of the Charters Towers Province 200 km southwest of Townsville in the northern Tasmanides. A prominent foliation (S2) is the main structure in the belt and is associated with tight to isoclinal folds, subparallel mineral and intersection lineations, and boudinaged pods of vein quartz and pegmatite. In the southwest, the main foliation is a crenulation cleavage (S2) related to D2 deformation. It overprints steeply dipping foliation (S1) formed in a D1 deformation but no associated folds have been found. Gently plunging, upright, open folds (D3 deformation) with axial planar S3 crenulation cleavage have affected the main foliation (S2). These deformations were associated with upper greenschist to lower amphibolite facies metamorphism. Amphibolite-grade orthogneiss containing S2 and S3, deformed granite and migmatite of the Fat Hen Creek Complex occurs in the northeast. In the southwest, the main foliation (S2) is folded around a map-scale, gently plunging synclinorium indicating that S2 formed with a subhorizontal orientation. In metamorphic rocks, the origin of widespread, intense subhorizontal foliation, usually associated with recumbent folds, has been considered problematic and in many cases is attributed to crustal extension. We relate the origin of D2 structures to subvertical shortening (i.e. extension) resulting in orientations that are strikingly divergent to those of upright D1 and D3 structures that were induced by compression. The proposed extensional event is poorly constrained in timing but it affected much of the Fat Hen Creek Complex, the oldest known phase of which is 493 Ma, and occurred prior to 40Ar/39Ar cooling ages at 423 – 409 Ma that also post-dated the D3 deformation.

The Early Palaeozoic Cape River Metamorphics consist mainly of psammitic gneiss and schist and occur as an extensive linear belt at the western margin of the Charters Towers Province 200 km southwest of Townsville in the northern Tasmanides. A prominent foliation (S2) is the main structure in the belt and is associated with tight to isoclinal folds, subparallel mineral and intersection lineations, and boudinaged pods of vein quartz and pegmatite. In the southwest, the main foliation is a crenulation cleavage (S2) related to D2 deformation. It overprints steeply dipping foliation (S1) formed in a D1 deformation but no associated folds have been found. Gently plunging, upright, open folds (D3 deformation) with axial planar S3 crenulation cleavage have affected the main foliation (S2). These deformations were associated with upper greenschist to lower amphibolite facies metamorphism. Amphibolite-grade orthogneiss containing S2 and S3, deformed granite and migmatite of the Fat Hen Creek Complex occurs in the northeast. In the southwest, the main foliation (S2) is folded around a map-scale, gently plunging synclinorium indicating that S2 formed with a subhorizontal orientation. In metamorphic rocks, the origin of widespread, intense subhorizontal foliation, usually associated with recumbent folds, has been considered problematic and in many cases is attributed to crustal extension. We relate the origin of D2 structures to subvertical shortening (i.e. extension) resulting in orientations that are strikingly divergent to those of upright D1 and D3 structures that were induced by compression. The proposed extensional event is poorly constrained in timing but it affected much of the Fat Hen Creek Complex, the oldest known phase of which is 493 Ma, and occurred prior to 40Ar/39Ar cooling ages at 423 – 409 Ma that also post-dated the D3 deformation.

Leg 190 of the Ocean Drilling Program yielded discoveries about the early stages of tectonic and sedimentary evolution of a trench-slope basin in the Nankai subduction zone of southwest Japan. Lithofacies character, biostratigraphy, and seismic-reflection data show that the basin's architecture was constructed during the early Quaternary by frontal offscraping of coarse-grained trench-wedge deposits. Clast types in muddy gravel beds indicate that one of the trench's polymictic sources was enriched in low-grade metasedimentary rocks. Outcrops of the Shimanto Belt on the island of Shikoku contain comparable lithologic assemblages, so we suggest that some of the turbidity currents and debris flows were funneled from that source through a transverse canyon-channel system. Offscraped trench deposits are mildly deformed and nearly flat lying beneath the slope basin. Bedding within the basin laps onto a hanging-wall anticline that formed above a major out-of-sequence thrust fault. Rapid uplift brought the substrate above the calcite compensation depth soon after the basin was created. The sediment delivery system probably was rerouted during subduction of the Kinan seamounts, thereby isolating the juvenile basin from coarse sediment influx. As a consequence, the upper 200 meters of basin fill consist of nannofossil-rich hemipelagic mud with sparse beds of volcanic ash and thin silty turbidites. Intervals of stratal distruption are also common, and the soft-sediment folding resulted from north- to northeast-directed gravitational failure. The Nankai accretionary prism has grown 40 km in width during the past 1 My, and the slope basin is already filled to its sill point on the seaward side. The stratigraphy displays an upward fining and thinning trend, in contrast to the upward coarsening and thickening mega-sequences depicted by some conceptual models for the slope basins.

The Nankai Trough accretionary prism is considered an “end‐member” prism accreting a coarse terrigenous sediment section in a setting with structural simplicity, unparalleled resolution by seismic and other geophysical techniques, and large historic earthquakes. It therefore has been the focus of Ocean Drilling Program (ODP) drilling to address several unresolved questions concerning accretionary processes and prism evolution. At six sites cored along two transects across the Nankai Trough accretionary prism during ODP Leg 190, lithostratigraphy and sediment diagenesis vary markedly. For the first time, reference sites at the seaward ends of the two transects defined the stratigraphic framework of the accreting/subducting Shikoku Basin sedimentary section. A thick section of Miocene turbidites and smectite‐rich mudstone is present within the subducting section at the Ashizuri site. The turbidites and mudstones are absent in the correlative section at the Muroto site; variations in lithology, mineralogy, and hydrologic properties of the incoming sediments probably contribute to the difference in prism wedge taper between the two transects, while possibly controlling the seismic character of the active plate boundary. The décollement in both transects is localized within a common stratigraphic unit (∼5.9–7 Ma) within the lower Shikoku Basin facies. The décollement is also a major boundary for both physical and mechanical properties. A broad low‐chloride pore water anomaly in the lower Shikoku Basin unit, first identified at Site 808, progressively decreases in magnitude from prism to basin along the Muroto Transect. Physical properties relationships, evidence for mineralogic changes in the sediments, and pore fluid chemistry suggest that the chloride anomaly results primarily from in situ diagenetic reactions in the sediments, possibly augmented by flow of freshened fluid from depth. New constraints on stratigraphy and age of units along more landward parts of the Mur