1. Tectonic Evolution of the Northern New England Fold Belt: The Permian-Triassic Hunter-Bowen event.
  2. R J Holcombe; C J Stephens1, C R Fielding1; D Gust; T A Little1,; R Sliwa1,; J. Kassan1,, J McPhie; A Ewart1

    1. Abstract
    2. The New England Fold Belt in Queensland is a complex arrangement of terranes with boundaries dominated by structures that were active during the contractional Hunter-Bowen Orogeny (event). This event extended for around 35 m.y. from Late Permian (~265 Ma) to late Middle Triassic (~230 Ma) time. The present NNW-trending structural grain of the fold belt is largely due to this deformation, but most faults have been reactivated during post-Late Triassic faulting. The northern New England Fold Belt (NNEFB) can be subdivided into: 1) a northern region (Connors Arch and lateral structures) within which deformation is characterised by open folds and variable, but generally minor, thrusting, 2) a central region of thin-skinned, fold-thrust deformation with cross-orogen tear faults, and within which strain is strongly partitioned and there is variable cleavage development (Gogango Overfolded Zone and more eastern terranes), and 3) a southern region of thick-skinned deformation within which basement appears to have been involved in deformation. Within the central region, the Marlborough Block is a composite out-of sequence thrust nappe terrane of ophiolitic components, low and high grade metasedimentary rocks, and metagranitoid, juxtaposed, at deeper structural levels, by early thrusting, and finally thrust at least 80km west over the fold-thrust belt. Various elements of the fold belt, including the Yarrol/Calliope terrane and Gympie Block, may be allochthonous elements transported for distances of tens to hundreds of kilometres within the fold belt. Calc-alkaline magmatism in the NNEFB during the Early and Middle Triassic may have been in response to the initiation, or onshore migration, of a magmatic arc, and its termination coincided with the last phase of contraction. The tectonic regime became extensional by the Late Triassic with widespread granite intrusion and development of silicic volcanic complexes and of localised extensional sedimentary basins.

      Pulses of contractional deformation in the fold belt are recorded by the distribution and nature of sediment in the foreland Bowen Basin to the west. Thrusting is indicated by the presence of coarse clastic wedges shed into the basin for greater distances at successively higher stratigraphic levels, reflecting the advancement of the thrust front. The final contractional event appears to have re-initiated at the eastern margin of the fold belt, rather than step westward from the previous thrust front, and to have been more intense than previous pulses. Bowen Basin sedimentation closed at about 232 Ma following accumulation of the most voluminous of the clastic wedges. Fluid flow associated with shear and dilational structures during this final contractional event is thought to be responsible for gold mineralisation within the fold belt, and for widespread diagenetic mineralisation within sediments of the Bowen Basin.

    3. Introduction
    4. The present geometry of folded terranes in the New England Fold Belt (NEFB) in coastal Queensland (Fig. 1) is largely controlled by structures produced during the Permo-Triassic Hunter-Bowen event yet its nature and timing have been poorly defined. In a companion paper (Holcombe et al., this volume), we discuss the constraints on the transition from active accretion in mid-Carboniferous times to widespread extension through the Late Carboniferous and Early Permian. This transition is interpreted in terms of eastward retreat of the convergent slab, and migration of the volcanic arc offshore. This paper presents a synthesis of our current understanding of the Late Permian to Late Triassic tectonic evolution of this region (see Fig. 2 of Holcombe et al., this volume). We will argue that this ~35 my period records the westward migration of a continental magmatic arc during a period of crustal contraction, and subsequent transition to an extensional (and ultimately intra-plate) setting.

      All of the terranes of the northern New England Fold Belt (NNEFB) are affected by Permo-Triassic thrusts and folds, although the greatest intensity of deformation occurs within the Gogango Overfolded Zone (GOZ: Fig. 1) and in the old accretionary terranes east of the Yarrol Fault. Folds associated with thrusts in the GOZ are characterised by pervasive slaty cleavage, in contrast to the much more localised cleavage development north and south of this area. It is in the GOZ that the magnitude and style of this contractional event can be most clearly characterised, but Late Permian thrusts have now been mapped in the North D’Aguilar Block (NDB) in southern Queensland (Little, 1992, Sliwa, 1994, Donchak et al., 1995), although with eastward vergence.

      Explanations for Late Permian-Triassic sedimentation patterns in the Bowen Basin have evolved toward thrust-loaded foreland basin models (e.g., Flood, 1983; Murray, 1990; Baker et al., 1993), replacing earlier extensional or wrench-related models (e.g., Evans & Roberts, 1980; Korsch and others, 1988). We now regard the Bowen Basin as preserving in its sedimentological signature a large component of the post-Carboniferous evolutionary history of the adjacent NEFB. In particular, thick wedges of coarse clastic material along the eastern margin of the basin indicate episodic influx from the active fold belt, and reflect the pulsed nature of the contractional event.

      The Late Permian to Late Triassic represents a major period of magmatism in the NEFB, although magmatic compositions appear to change through time, and the igneous rocks of this interval can be separated into relatively discrete suites (Gust et al., 1993). Early Triassic volcanic and plutonic rocks with calc-alkaline geochemical characteristics occur within the broad contractional cycle, while Late Triassic silicic caldera-related volcanics and granite plutons overprint the fold-thrust belt structures.

      The data presented here are derived from a number of research projects in southern and central coastal Queensland, as well as drawing on several Bowen Basin studies. In particular, the deformation history is based on ongoing study areas in the Fitzroy region around Rockhampton and Marlborough in coastal central Queensland (Figs.1,2), and in southern Queensland (Figs.1,3). Our understanding of the magmatic history is based on data gathered throughout the NNEFB during our studies, and from the Connors Arch region (Dear, 1989, 1994; Allen et al., 1994, and pers.comm., 1996).

    5. Fold-thrust Deformation
      1. Marlborough-Fitzroy area
      2. Two fault styles dominate the Fitzroy region: Late Permian-Triassic thrusts and associated tear faults; and Cretaceous-Tertiary normal faults (Fig. 2). Significant post-Triassic strike-slip movement has also occurred on some of the faults in the region (e.g., the Broad Sound Fault). Many of the Cretaceous-Tertiary normal faults exploit the pre-existing thrust-tear fault architecture of the region to such an extent that almost all faults, of any age, have a Cretaceous-Tertiary brittle overprint. Many of the thrusts in Figure 2 are continuous with a normal fault that forms the boundary to an adjacent Cretaceous or Tertiary basin. Although there are undoubtedly elements of crustal fracturing in the region that controlled basin development during Early Permian extension (Hammond, 1987; Fielding et al., this volume), we have not positively identified any regional fault patterns within the NNEFB that can be associated directly with this event.

        Contractional structures developed during the Permo-Triassic thrust event (thrusts, folds, cleavage) are pervasively developed for a distance of at least 100 km inland from the coast and continue with varying intensity into the Bowen Basin. Regional deformation related to this event is strongly heterogeneous. The most deformed terranes in the eastern part of the belt include remnants of the Devonian-Carboniferous accretionary terranes in the Coastal Block and parts of the Calliope terrane (Mt Holly Beds). Further west, the GOZ describes an arcuate shape in plan (Fig. 2) that includes a ~20 kilometre-wide, west-verging thrust-fold belt along the trend of the Connors-Auburn Arch (Fig. 1). Much of the intervening Yarrol Basin units and Calliope terrane are significantly less deformed, at least at outcrop scale.

        Several styles of contractional deformation are preserved across the region. In the GOZ, the most common style of deformation is multiple imbricate thrusts of small throw (few tens to few hundreds of metres) associated with well-developed mesoscopic folding and cleavage indicating a distributed ductile response. Broad areas of steep to overturned dips suggest that thrust propagation folds are common. Fergusson et al. (1990, 1993) described similar pervasive folding and cleavage in the accretionary rocks of the Coastal Block. The intervening terranes are characterised by fewer thrust faults that have larger throw, accompanied by fault bend folds but little cleavage.

        The largest of these fault-bend folds is the Craigilee Anticline which appears to be a breached fault propagation fold (Fig. 4). The main floor thrust to this anticline (Rookwood Thrust, Fig. 2) is a major shallow crustal discontinuity. Although time-equivalent Permian units are incorporated in both footwall and hangingwall, the thrust marks the eastern boundary of the well-cleaved rocks that define the GOZ. Rocks of the Yarrol and Calliope terranes (Fig.1) occur only within the hangingwall of the thrust, whereas rocks of the Permo-Carboniferous Connors-Auburn Arch occur only within the footwall. Early Permian marine units (e.g. Rookwood Volcanics, Berserker Beds - see Holcombe et al., this volume) apparently only occur to the east of the Rookwood Thrust.

        The Marlborough Block is an enigmatic, thin (<2 km), composite terrane that was transported westward along a very low angle, out-of-sequence thrust (the Marlborough Thrust, Fig. 2) over the earlier thrust packages (Fig. 4). The basal thrust is a brittle structure with little ductile deformation, even within metres of the contact. In contrast, the Marlborough Block internally is an amalgam of numerous fault-bounded packages of greenschist to amphibolite facies rocks with kilometre-scale, ductile response in the rocks adjacent to most faults. Some of these faults and associated shear zones have clear thrust geometry and kinematics whereas others are more ambiguous in either dip or sense-of-shear. These latter structures tend to occur near foliated S-type granitoids, and may be either thrusts that were rotated during translation of the block, or remnants of extensional faults that developed during emplacement of the granitoids, as have been observed in the NDB in southern Queensland (Holcombe et al., this volume). The internal thrusts of the Marlborough Block are interpreted to have developed during thrusting in a deeper part of the thrust belt, and the package translated to its present, structurally high level position along the younger Marlborough Thrust. In this model the Marlborough Thrust must represent an upper flat on a system that ramps down to the east. We interpret 40Ar/39Ar cooling ages of 242.9±0.4 Ma and 248.8±0.5 Ma on biotite from foliated granodiorite in the Marlborough block (Holcombe et al., this volume) to reflect exhumation and cooling following the deeper level, more ductile, thrusting event.

        Deformation is also strongly partitioned along the fold belt, with tear faults separating compartments with fold-thrust packages of differing geometry (e.g., in the area shown in Fig. 2). The northern termination of the GOZ fold-thrust belt is a tear fault system (Holcombe et al., 1995) equivalent to the Stanage Fault Zone of Henderson et al. (1993). The system is a complex zone of linked faults that also separates the allochthonous and para-allochthonous fold-thrust belt and Marlborough Block, to the south, from a gently folded, autochthonous terrane to the north. Fault styles developed within the zone include pure thrusts, strike-slip and oblique-slip vertical faults, and oblique-slip thrusts. The Stanage Fault Zone thus appears to have been a major tear fault system to both the nappe emplacement of the Marlborough Block, and to the in-sequence thrust belt in the footwall.

        The orientation of the overall thrust convergence vector appears to be SW-directed, as indicated by the consistent orientation of tear faults. Strong strain partitioning in the fold-thrust belt makes both the construction of balanced cross-sections and precise estimation of the overall crustal shortening difficult. Fergusson (1991) estimated 60% shortening within the GOZ, an estimate consistent with the intensity of cleavage, and an overall upper crustal shortening of 50-90 km across the GOZ and Bowen Basin Folded Zone (Fig. 2). By matching the easternmost exposures of rocks that we equate with Connors/Camboon Volcanics (Holcombe et al., this volume, and Fig. 1), north and south of the Stanage Fault Zone, we estimate thrust-tear fault contraction of >30 km along the Stanage Fault Zone, consistent with a 30 km translation proposed by Leitch et al. (1994). By correlating deformed syntectonic granite terranes in the Marlborough Block (Holcombe et al., this volume) with the Broome Head Metamorphics on the coast (Morand, 1993) we estimate ~50 km translation on the out-of-sequence Marlborough Block nappe system, in broad agreement with early estimates by Murray (1974).

      3. North D’Aguilar Block
      4. The effects of the Permo-Triassic contractional deformation are subdued in the southeast Queensland section of the NEFB, relative to the Fitzroy region. The earliest recognisable thrust structure is the west-dipping Claddagh Thrust (Little, 1992) in the northern NDB. The thrust juxtaposes amphibolite facies rocks over low grade (anchizonal) rocks within the older accretionary complex. The thrust can be traced for over fifty kilometres to the south where it splits into a system of imbricates (Sliwa, 1994) that are responsible for the >5 km thickness of the Jimna Phyllite (Fig. 3). Other east verging thrust imbricates have now been mapped on the eastern margin of the NDB (Donchak et al., 1995).

        The age of movement on the Claddagh Thrust and its imbricates is poorly constrained between Early Permian and Early Triassic. Both the ~305 Ma Claddagh Granite (Little et al., 1995) and the ?Early Permian Marumba Beds (Fig. 3) are allochthonous within this thrust system. The upper limit for thrusting is the age of the Early and Middle Triassic Toogoolawah Group of the Esk Trough which unconformably overlies these early thrusts. Although poorly dated at this stage, a group of ~241 Ma K/Ar whole rock and amphibole ages (240.9±11 Ma, hornblende; 241.9±8 Ma, 236±7, whole rock; Irwin, 1973; Kerr, 1974) ages from the Neara Volcanics is currently used to constrain the age of this group. The Monsildale Granodiorite is a multiphased body that intrudes subvertical Marumba Beds a few kilometres from where it is unconformably overlain by very gently dipping Toogoolawah Group rocks (Bryden Beds). Although it is not known whether the pluton stitches the thrusts at depth, K/Ar hornblende ages of 247.5±3 Ma (Sliwa, 1994) and 240.1±3 Ma (Kwiecien,1996) on the older phase of the granodiorite provides a minimum age on the thrusting and tilting, assuming that the granodiorite itself has not been involved in the tilting.

        40Ar/39Ar ages on white mica from the Mt Mee area in the southern NDB more tightly constrain the lower limit of thrust-related contraction in this area (Holcombe and Little, 1994). Whereas the polymetamorphic rocks and syntectonic granitoids in the northern NDB were exhumed through the blocking temperature for argon diffusion in these minerals (~350°C) during regional extension at about 296 Ma (Little et al., 1995), the structurally deeper epidote-blueschist rocks at Mt Mee remained below this blocking temperature until exhumed rapidly (~0.3 km/m.y.) at ~260 Ma (261.6±0.6 Ma - 257.9±0.8 Ma, Holcombe and Little, 1994). At Mt Mee, a regional upright antiform is associated with third generation axial plane fabrics which were initiated under greenschist facies conditions associated with the growth of coarse albite porphyroblasts. (Holcombe and Little, 1994). These metamorphic fabrics, and associated mesoscopic folding, become very much more intense adjacent to the North Pine Fault which is the western boundary of this epidote-blueschist facies terrane. Although this fault is now defined by its post-Late Triassic movement, a precursor fault associated with contractional deformation clearly must have been active prior to the rocks passing through the ~350°C isotherm at ~260 Ma. The rapid exhumation at ~260 Ma is distinctly younger in age than the regional extension that initiated Bowen Basin sedimentation and is marginally younger than the initiation of thrust-loading sedimentation in the Basin. Thus we would interpret the event creating the conditions for rapid exhumation at Mt Mee as being initiation of the regional thrust-fold contraction and the North Pine Fault as a likely early thrust.

        Other ambiguous, open, upright folds (up to several kilometres wavelength) overprint earlier metamorphic fabrics throughout the North and South D’Aguilar Blocks (e.g., Holcombe, 1977), and are overprinted by contact metamorphism adjacent to the ~230-220 Ma suite of granitoids. In the central part of the NDB one such set of large flexures fold the imbricate thrusts of the Jimna Phyllite (Fig. 3; Sliwa, 1994) and are likely related to the youngest contractional event described below.

      5. Esk Trough

      The Esk Trough is a narrow, NNW-trending belt of Early Triassic rocks, flanked by fault-bounded slivers of Permian rocks. The belt overlies and includes several regional lineaments in the NNEFB and also potentially overlies the Early Carboniferous cratonic margin, separating accretionary complex rocks to the east from forearc basin rocks to the west. The structure of the belt is generally regarded as a graben, or half-graben, with its present margins approximating original rift margins. Seismic profiling across the southern extension of the belt clearly defines an asymmetric structure with steeply faulted eastern margin but no basal structures suggestive of an extensional origin (Korsch et al., 1989). The predominant rock types within the Esk Trough are a terrestrial sequence of andesitic, mainly volcaniclastic, rocks (Neara Volcanics), a package of interbedded clastic sedimentary rocks (Bryden Formation) that at least locally underlie these volcanic rocks, and a sandstone-dominated alluvial and lacustrine sequence (Esk Formation) overlying the volcanic strata.

      The present margins of the Esk Trough are sharply delineated faults that are characterised by Late Triassic or younger movement such that the original geometry of any trough is uncertain. In the central area of the Esk Trough we have observed local unconformable contacts with underlying sequences on both the eastern and western margin of the belt. At the western margin subvertical Esk Formation units overlie a similarly steeply dipping ,slightly metamorphosed, pillow basaltic unit of unknown age. At the eastern margin of the belt in this area the shallowly dipping Bryden Formation is in strong angular unconformity with underlying ?Early Permian rocks, and further north gently tilted units of the Neara Volcanics onlap the basement rocks of the North D’Aguilar Block. Thus, the present western margin reflects strong post-depositional fold and fault structures but the current eastern margin appears to be broadly depositional and only moderately modified by the late strike-slip faulting. Remnants of the Neara Volcanics that overlie the basement rocks are mainly volcanic-dominated, whereas thick beds of coarse volcaniclastic conglomerate ("boulder beds") characterise deposits within the axis of the belt. These data suggest that the Esk Trough was at least a depocentre, if not a fault-bounded basin, during the Early and Middle Triassic. Remnants of Late Permian marine sedimentary and volcanic sequences are preserved ad development. Ongoing studies indicate that the dominant palaeoflow direction in the Esk Formation was southward and westward. There is, to date, no sedimentological evidence that the present fault margins confined either the coarse conglomeratic facies or the overlying Esk Formation.

      Structures and stratigraphic relationships associated with the Esk Trough constrain elements of the Triassic part of the Hunter-Bowen event. The Neara Volcanics lie unconformably over thrusts along the western margin of the North D’Aguilar Block, providing a minimum age on the initiation of thrusting in this area. The sequences within the Esk Trough are also folded into >1 kilometre wavelength, low to moderate amplitude folds and are unconformably overlain by flat-lying 228.4±0.6 Ma volcanics of a later extensional magmatic phase (see below). This folding, constrained within the interval 241-228 Ma, appears to be the last Triassic contractional deformation in the area, and is interpreted as part of the final deformation associated with the Hunter-Bowen event. Fold axial traces generally lie parallel to the axis of the Esk Trough belt except near the fault margins where locally axial traces trend west to west-northwest in a sense that is consistent with a component of dextral wrenching parallel to the trough. West-trending axial traces also occur in fault slices of adjacent Late Permian fault blocks (Northbrook Beds; Cressbrook Creek Group) that lie outside the belt. Deformation intensity in this folding event is low and and the folds lack axial plane cleavage.

      Unlike that in the Fitzroy region, Permian and Triassic thrusting in southern Queensland appears to be thick-skinned and involve basement rocks, rather than thin-skinned. Except for the metamorphic rocks of the Mt Mee area and within the Gympie Block, cleavage is rarely associated with folds related to the Permo-Triassic event in southern Queensland. Thrust vergence in southern Queensland is eastward, in contrast to the consistent westward vergence in central Queensland.

    6. Thrust loading signatures in Bowen Basin sequences
    7. Development of the Bowen Basin began with extensional sub-basins in the Early Permian (Phase 1 of Fielding et al., 1995) that was followed by a period of thermal sag in the latest Early Permian to early Late Permian (Phase 2). A major change in the petrology and depositional environment of basinal sediments in the Late Permian (Baker et al., 1993) was manifested in the introduction of first-cycle, volcanic lithic detritus shed from the east. The basin developed a marked cross-sectional asymmetry typical of loaded foreland basins (Busby & Ingersoll, 1995). Sediments shed westward across the GOZ joined major south-flowing, axial drainage systems (Fielding et al., 1995, this volume). Depositional environments were initially marine, but rapidly became coastal plain to alluvial plain systems as the basin was oversupplied with coarse sediment. This change is interpreted as a response to the onset of thrust loading of the eastern Bowen Basin, and accompanied the resurgence of volcanism to the east (Phase 3, see below).

      Carboniferous calc-alkaline volcanics within the Connors Arch (Fig. 2) are widely interpreted as describing the position of a magmatic arc along the eastern margin of the Bowen Basin (Day et al., 1978). Fergusson (1991), in contrast, showed the Connors Arch as a structural high produced during Permo-Triassic thrusting. The Connors Arch west of Marlborough is an open, antiformal structure with shallow (<30°) limb dips, complicated locally (particularly on the eastern limb) by steep faults. A complete Permian succession is preserved on both limbs of the antiform and around the southern limit of the Arch (Malone et al., 1969). Continental and shallow marine Permian sediments are transgressive across Connors Volcanics basement with only local angular discordance. West-directed palaeoflow directions occur in Phase 2 and Phase 3 sequences both east and west of the Arch, and no basin margin facies transitions are evident close to the Arch. While Carboniferous volcanics along the arch may have been exposed during formation of the Early Permian extensional sub-basins, these had no topographic expression by the middle Permian transgression. We suggest, on the basis of the characteristics of Triassic sediments within the Bowen Basin, that a topographic high did not form until the latest Middle Triassic as the thrust front migrated westward across the basin.

      The sedimentological transition from thermal sag to thrust loading is exposed in the Moah Creek Beds along the Fitzroy River, west of Rockhampton (Fielding et al., this volume, Fig. 4; and in press) where a monotonous sequence of thinly interbedded marine siltstone and fine sandstone passes abruptly upward into disorganised conglomerate and diamictite enclosed within thinly-bedded strata similar to the thick interval exposed below. Slump folds encased in a mud-breccia matrix are interpreted as the products of mainly debris and turbidity flow processes. The lower part of the exposed sequence is typical of the Phase 2 sag sequences and the first evidence of instability is a 5 metres thick, foundered horizon consisting of detached and transported masses of sandstone occurring within the siltstone package about 50 metres below the main transition to conglomerate-rich units. The section is interpreted as having accumulated in an unstable submarine environment ahead of an approaching subsurface thrust front with the foundered horizon representing the earliest indication of the approaching front. Similar packages of coarse grained mass flow deposits have been recognised at this stratigraphic position over distances of at least 350 km along the eastern Bowen Basin margin (Fielding et al., in press).

      Within the eastern Bowen Basin, a number of distinct wedges of coarse clastic sediments occur within the Late Permian to Early Triassic succession (Fig 5; Kassan, 1994; Fielding et al., 1995). These wedges comprise conglomerates and breccias of metasedimentary, volcanic and intrusive, and intraformational lithologies. Clast composition and palaeocurrent data indicate derivation from a highland to the east, that was associated with active volcanism. Successive wedges appear to have penetrated further west into the basin (Fig. 5). We interpret them as arising from pulses of thrusting in a rising hinterland to the east. In the case of the initial pulses recorded in the Late Permian sequences (such as the Moah Creek beds described above) this active volcanic hinterland was to the east of the current coastline. Elliott (1993) and Korsch and Totterdell (1995) have documented two discrete periods of thrust deformation within the Triassic Bowen Basin fill from seismic data. Each thrusting episode followed a period of westward propagation of a coarse clastic wedge, the later (late Middle Triassic) event terminating sediment accumulation across the entire basin.

    8. Early and Middle Triassic calc-alkaline suites
    9. About half of the exposed granitoid ages in the NNEFB have K/Ar ages between 230-250 Ma (Gust et al., 1993), within the later part of the broad Hunter-Bowen thrust event. The granitoids are widely distributed throughout the fold belt south of Broad Sound and east of the GOZ and are predominantly intermediate in composition. The plutonic rocks are coeval with terrestrial volcanism that is overwhelmingly andesitic in composition.

      Volcanic rocks of this age are poorly represented north of the Stanage Fault Zone at Broad Sound, although Triassic radiometric ages are recorded from intrusives and minor volcanic rocks in the Whitsunday region (Ewart et al., 1992; Parianos, 1993; Allen et al., pers.comm.,1996) and from mafic dykes and minor granitoid intrusions within the Urannah Complex (Allen et al., 1994; and pers.comm., 1996). Andesitic volcanic and volcaniclastic sequences south of Broad Sound unconformably overlie basement of Devonian-Carboniferous to Late Permian age.

    10. Late Triassic extensional basins and magmatic suites
      1. High level granites and volcanics
      2. In contrast to the intermediate-dominated composition of Early and Middle Triassic granitoids of southern Queensland, the Late Triassic (~230 Ma to ~220 Ma) is characterised by intrusions of predominantly silicic granite composition associated with the development of volcanic complexes of rhyolite and minor mafic lava and ignimbrite (Stephens et al., 1993). One large-scale caldera associated with ignimbrite development has been identified within these units (Stephens, 1992) and the restricted distribution of many of these sequences suggests that other calderas may be present. Ignimbrites correlated with this event are unconformable on Early to Middle Triassic andesite in the northern Esk Trough.

        In southern Queensland flat-lying andesite giving an 40Ar/39Ar cooling age of 228.4±0.6 Ma on amphibole unconformably overlies folded Early to Middle Triassic rocks (Little et al., 1993). Elsewhere, hornblende in the same sequence yielded a concordant 40Ar/39Ar total fusion age of 232±4 Ma (Murray, 1992, pers.comm.) and ~225 Ma K/Ar ages have been derived from similar andesitic and rhyolitic flows and dykes that overlie basement rocks in the NDB (Table 1: Holcombe et al., this volume; Roberts, 1992; Sliwa, 1994;).

        The Station Creek Adamellite, which intrudes rocks of the NDB, provides important constraints on the timing of events in the area as it also intrudes the thrust sheets within the NDB and may be comagmatic with the overlying volcanics. We have recently obtained 40Ar/39Ar step heating plateau ages of 232.8±0.4 Ma and ~236 Ma on biotite from this body, although the data have not yet been completely interpreted.

        These Late Triassic volcanics and plutons are largely undeformed, although the Station Creek Adamellite on the northeastern flank of the NDB is locally faulted and ductilely sheared (Edgar, 1992; J.Tang, personal communication), and there is local brittle-ductile deformation in the North Arm Volcanics, to the east of the NDB.

      3. Extensional basins

      The change in character of magmatism to silicic volcanic structures was accompanied by development of small to moderate-sized north-northwest elongate basins with coarse-grained, grossly fining upward, alluvial fills, thick localized coal bodies, and evidence of synchronous bimodal volcanism. Basins of this nature include the Ipswich (Falkner, 1986; Staines et al., 1995) and Tarong Basins (Pegrem, 1995) and part of the Callide Coal Measures (Biggs et al., 1995). The Tarong Basin has a half-graben geometry (Williams, 1993), as do a number of less well-described coeval basins in the subsurface (eg., Wiltshire, 1982). Although the cross-sectional geometry of the other major basins is not well constrained, our work (CRF) suggests that these basins are probably similar in geometry. Bimodal volcanism is most evident within the Ipswich Basin where basalt flows and a significant felsic pyroclastic unit (Brisbane/Hector Tuffs) lie at, or near, the base of the sequence. All three of the basins studied thus far contain abundant thin felsic tuffaceous horizons within coal-dominated sequences.

      Models proposed for the origin of these basins are diverse ( e.g., Murphy et al., 1976; Falkner, 1986; Korsch et al., 1989; Pegrem, 1995) but on the basis of the asymmetric cross-sectional basin geometry and regional association with extensive coeval bimodal volcanic sequences, we regard the basins as characteristic of an extensional tectonic environment.

    11. Younger basins and structures
    12. The post-Triassic evolution of the region is marked by development of extensive, Jurassic-Cretaceous basins (eg., Surat; Clarence-Moreton; Maryborough Basins) across the Late Triassic extensional basins. One of these basins that impacts on the interpretation of structures seen within the fold belt is the Maryborough Basin which, unlike others of its age, is gently to moderately folded. This deformation, which is constrained stratigraphically as mid-Cretaceous or younger, is the only recognised post-Triassic contractional episode in the region. It is the most likely event in which much of the late regional faulting within the NNEFB formed. Effects of this deformation include small displacement reverse faults that cut the Mesozoic basin rocks throughout southern Queensland, and presumed coeval faults of similar geometry and kinematics within the fold belt.

      Major faults, commonly with ~10 km sinistral strike-slip, broadly define the structural grain of southern Queensland. Almost all of the older terranes are bounded by these younger faults which locally displace plutons of the ~230-220 Ma magmatic suite. Examples of these faults exploiting the older fault architecture occur in the NDB, where Early Permian syn-metamorphic deformational structures intensify toward the North Pine Fault. This fault, however, appears to have sinistrally offset several Late Triassic plutons and a Late Triassic volcanic formation by ~8 km. The North Pine Fault is continuous with the composite Perry Lineament to the north, where a Late Triassic (~221 Ma) volcanic complex shows a sinistral strike displacement of about 9 km (Stephens, 1992).

      Similar sinistral strike-slip faults occur in the central Queensland part of the NNEFB (e.g., the Broadsound Fault with ~20 km strike separation: Fig. 2; offshore in the Whitsunday region: Ewart et al., 1992) but fault patterns in the northern region are dominated by steep normal faults that bound the numerous Cretaceous and Tertiary Basins (e.g., the Cretaceous Styx Basin, and the Tertiary Duaringa Basins).

    13. Discussion
    14. The major contractional period defined here as the Hunter-Bowen event lasted for about 35 my, from ~265 to ~230 Ma. Stratigraphic evidence from the foreland basin fill suggests that this event was strongly pulsed, and that successive thrust pulses penetrated further into the basin. The final contractional event appears to have re-initiated at the eastern margin of the fold belt, rather than step westward from the previous thrust front, and to have been more intense than previous pulses. The presence of a broadly synchronous magmatic event within the NNEFB suggests that arc magmatism was superimposed on the actively rising mountain belt.

      In the Fitzroy region the commencement of thrust contraction is constrained as Late Permian (~265 Ma). The oldest sedimentary unit in this region that was derived from the approaching thrust front (and subsequently involved in the thrusting) lies within the Late Permian Moah Creek Beds, Barfield Formation, and the equivalent Boomer Formation. The maximum age on thrusting is thus constrained to Kazanian on biostratigraphic evidence (Fielding et al., this volume). Episodic deformation in the Fitzroy area is indicated by the out-of-sequence thin-skinned emplacement of the Marlborough thrust nappe. This thrust overrides earlier thrusts that involve latest Permian rocks (Dinner Creek Conglomerate), and the only other constraint on the timing of this thrust are 242.9±0.4 Ma and 248.8±0.9 Ma 40Ar/39Ar cooling ages on biotite in sheared and foliated metagranites that we surmise were related to a deeper-seated, earlier, thrust environment. The presence and magnitude of the nappe indicates a significant renewal of a contractional deformation from the east after this time.

      The initiation of thrusting is less constrained in southern Queensland, but there is a clear indication of two phases of contractional deformation separated by an interval of calc-alkaline magmatism. On the western margin of the NDB, thrusts that carry the Late Carboniferous Claddagh Granite and the ?Early Permian Marumba Beds are unconformably overlain by the Early-Middle Triassic (~241 Ma) volcanic sequence of the Esk Trough. The youngest age for these thrusts is thus ~241 Ma, but the oldest age is poorly constrained. Nonetheless, the white mica 40Ar/39Ar ages from the Mt Mee area indicates rapid exhumation of the metamorphic basement rocks at ~260 Ma, an event that may relate to the commencement of Hunter-Bowen contractional deformation in this area, and an age that is consistent with initiation of this event elsewhere.

      Termination of the thrust and folding events in southern Queensland is constrained to the interval 241-228 Ma. The folded Early and Middle Triassic volcanic succession within the Esk Trough is unconformably overlain by ~228 Ma flat-lying intermediate volcanics but related rocks in the region have a range of K/Ar ages from ~225 to 235 Ma. (Table1: Holcombe et al., this volume) The new ~235 Ma 40Ar/39Ar ages we have obtained on the Station Creek Adamellite (see above), might provide an even tighter constraint on the age of the terminal Hunter-Bowen folding in this area.

      Analysis of radiometric dates (Gust et al., 1993) and local detailed mapping (Stephens, 1992) clearly distinguishes the presence of Early and Late Triassic volcanic rocks of contrasting composition and style. All of these Triassic volcanics were grouped during the initial 1:250,000 scale mapping of the NNEFB, and only recently has the presence of two compositionally distinct events been reflected in the stratigraphic nomenclature (Cranfield, 1994).

      Early and Middle Triassic magmatism has not been systematically studied at this time, but data on granitoids and volcanics (unpublished theses at UQ and QUT) and limited isotopic data from volcanics in the Esk Trough (Ewart et al., 1992) show not only the calc-alkaline character of this event but the overwhelmingly intermediate composition of the rocks. These data are consistent with a period of continental margin arc-related magmatism during the Early and Middle Triassic. Such an interpretation is supported by the observation that the Late Permian-Early Triassic sediments derived from the approaching thrust-front to the east contain first-cycle volcanic detritus. Holcombe et al. and Fielding et al. (this volume) emphasise that there is no evidence in the Early to mid-Permian rocks of the NNEFB for the presence of an arc-related volcanic terrane. In tectonic terms, the Permo-Triassic magmatism thus requires the initiation of subduction below the region, or the migration of the arc onshore from a position somewhere to the to east, during the Hunter-Bowen contraction event.

      Stephens (1992) and Stephens et al. (1993) interpreted Late Triassic silicic volcanism in terms of an extensional tectonic environment. Criteria cited included the discrete, caldera-forming nature of the volcanism, characteristic of continental extensional environments, the bimodal, silicic-dominated composition of the volcanics, and the regional silicic granite-dominated composition of coeval intrusives. These data suggest that the relatively rapid re-establishment of voluminous arc magmatism within the NEFB during the Permo-Triassic, clearly associated with a broader cycle of tectonic contraction, was replaced during the Late Triassic by an extensional environment and crustal melting of the recently- arc-impregnated crust. The latest Triassic is further characterised by localised, discrete caldera development and emplacement of granite with mild A-type geochemical affinities (Stephens, 1992; Gust et al., 1993), supporting the concept that the region underwent a transition from convergence to extension that continued into the latest Triassic.

      The position and nature of any arc that operated after the Middle Triassic, or of any Permo-Triassic subduction complex, is uncertain. However, a possible mechanism for the transition from presumed subduction to extension during this time may lie in one of our suggested interpretations of the Late Carboniferous-Early Permian evolution of the NNEFB (Holcombe et al., this volume). We suggest one possible scenario is that the subducting slab again underwent roll-back during the late Middle Triassic driving extension and resulting in re-establishment of the volcanic arc some distance to the present east of the NEFB (and the present coastline). Voluminous first-cycle volcaniclastic debris and numerous tuffs within the Surat Basin (Exon, 1976) suggest that volcanism sourced from an unidentified terrane continued through the Jurassic leading up to the major Early Cretaceous breakup-related magmatism (Ewart et al., 1992).

      1. Metallogenic aspects
      2. Two major styles of mineralisation are associated with the broad Hunter-Bowen event in the NNEFB. Porphyry-style mineralisation is commonly developed in association with the Permo-Triassic calc-alkaline intrusives of the fold belt (Horton, 1978). Of somewhat more enigmatic origin is the occurrence of epithermal gold mineralisation, associated with quartz-rich alteration systems, and locally with carbonate veining, that consistently gives ~235 to 245 Ma K/Ar alteration ages. Examples include the major deposits of Cracow and Gympie, plus the smaller deposits at Manumbar in the NDB, Mt Mackenzie in the southern Connors Arch, and perhaps at Mt Wickham in the northern Connors Arch. We also include mineralisation at Rannes, within the GOZ, within this association on the basis of alteration style and structural association. In some instances, such as at Gympie and Rannes, mineralization is within thrust-related sheared or cleaved rocks. In others, such as Cracow, Mt Mackenzie and Mt Wickham, mineralisation is more typical high level, mesothermal to epithermal in nature.

        A line of significant gold deposits occurs along the eastern margin of the Bowen Basin, including Cracow, Rannes, Mt Mackenzie and Mt Wickham. Of these occurrences, only Rannes occurs within strongly thrust-deformed rocks. At Rannes, well-developed silicic alteration systems associated with gold and minor base metal mineralisation occur in locally-sheared Camboon Volcanics. Mineralisation appears to occur both on thrusts, and in zones that cut across the thrust trend at a high angle. The other deposits comprise more classical alteration systems of similar high T, low P grade, but also overprint the Late Carboniferous-Early Permian volcanic succession.

        Further east at Gympie, strain is strongly partitioned in the volcaniclastic sandstones of the Rammutt Formation. Where cleavage is developed, it is a strong pressure solution fabric and is accompanied by a marked stretching lineation defined both by the shape of pressure-solved clasts and, more particularly, by mica beards and fringes developed on clasts. A characteristic feature of these rocks is the development of a network of fine extensional veins perpendicular to the stretching lineation, and infilled with fibrous quartz (and minor carbonate) that are parallel to this lineation. Similar veins sets occur perpendicular to the stretching lineation in the gold-bearing black slates and are known locally as the "Gympie vein set". These veins (and the associated mineralisation) are thus syntectonic with the cleavage-forming deformation, and alteration associated with these veins has given a K/Ar age of ~235 Ma (Cuneen, 1994).

        Gold mineralisation at Manumbar in the NDB occurs in carbonate-quartz veins within rocks equated to the Early Triassic Neara Volcanics. Mining is currently occurring within a single, major vein, but numerous en echelon swarms of fibrous extensional veins occur in the field. There is no other obvious deformation apart from the brittle-ductile deformation associated with the vein swarm, and alteration associated with mineralisation has yielded a ~235 Ma age (M. Garman, personal communication).

        In all cases, the deposits are localised within volcanic or volcaniclastic rocks, and within areas that are characterised by late (i.e., post-Permian) Hunter-Bowen structures. The line of deposits along the eastern margin of the Bowen Basin occur along the western limb of the structural arch that defines the Basin margin, and which we regard as forming during the Middle Triassic. The timing of mineralisation clearly just precedes the late, major contractional pulse that closed the basin, and is broadly coeval with K/Ar ages on mineralogically-pure cleat-filling illite within the Late Permian coal measures (Fig. 6). We believe that this late pulse of the Hunter-Bowen event not only was responsible for the development of the structural Connors-Auburn Arch, but also promulgated a major fluid flux within structures deforming the NNEFB and through sediments and structures within the eastern Bowen Basin (Faraj et al., 1996). The meteoric composition of the mineralising fluids at Cracow (Golding et al., 1987) is interpreted to reflect the nature of fluids generated during this event.

      3. Possible allochthoneity of the Yarrol/Calliope terranes

The thrust geometries of the GOZ shown in the cross-sections of Figure 5 are regarded as reasonable extrapolation of the available surface data. Strong strain partitioning and disruption by Cretaceous and Tertiary faults, however, makes confident interpretations of these sections to depth difficult. One source of variation in interpretation based on extrapolation of these sections, however, is that placed on the geometry of the Berserker Block. We have noted that basement to the footwall rocks of the Rookwood Thrust is consistently Connors Volcanics or equivalents, whereas basement to the hangingwall rocks is consistently rocks of the Yarrol/Calliope terranes. If the Berserker Block is stratigraphically equivalent to the Connors Volcanics (sensu lato), as suggested in Holcombe et al. (this volume), then the simplest geometry that satisfies this structural and stratigraphic interpretation is that the Berserker Block (Fig. 2) is a window through the Rookwood Thrust system. The implication of this interpretation is that the hangingwall, with its Siluro-Devonian basement, is very thin-skinned and must have a displacement of several tens of kilometres. The ramifications of such a model are that:

While these interpretations are highly speculative, they do bear upon matters such as the position of the continental margin and accretionary complex during the Early Carboniferous and older convergent tectonic events.

      1. Gympie block: how allochthonous is it?
      2. We see a paradox in the histories of different age packages of cleavaged rocks of the NDB. The cleaved Early Permian Cambroon beds on the eastern margin of the NDB are in fault contact to the west with the ?Early Carboniferous Booloumba beds of the accretionary terrane (Fig. 3) and structural relationships suggest considerable translation on this fault (tentatively correlated with an early phase of movement on the Bracalba Fault; Sliwa, 1994). The folded chert-argillite sequences of the Booloumba beds contain a single upright cleavage characteristic of the anchizonal (upper plate) accretionary rocks that we interpret as being developed during the mid-Carboniferous accretion. There are, however, no overprinting cleavage fabrics in the older rocks that correspond to the cleavage present in the adjacent Early Permian rocks. That the cleavage forming event in the Early Permian rocks was not sufficiently intense to be transmitted into the basement rocks is considered unlikely, given the polydeformational fabrics in the Permian rocks at some localities. More likely, the fault separating the two units has considerable displacement and was active after cleavage formation in the Early Permian rocks, but before the boundary was intruded by Late Triassic (~220 Ma) granite. The amount of any such displacement is unknown but must be sufficient to have juxtaposed rocks of entirely different deformational responses to the same event.

        Terranes east of the Cambroon Beds that may have been included in such a displacement include remnants of the accretionary rocks and the Early Permian-Early Triassic units of the Gympie Basin. The concentration of post-Early Permian brittle and ductile deformation along the eastern margin of the NDB, the probability of significant translation on the Bracalba(?) Fault, and the presence of Middle Triassic cleavage-forming deformation within the Gympie Basin, suggests that the units of the Gympie Basin also have been translated, to some degree, into its present location. Holcombe et al., (this volume) note the similarity of the Early Permian sediments and volcanics in the Gympie Block with other extensional marine basin sediments in the most eastern parts of the NNEFB. In contrast with the much less cleaved rocks in the adjoining blocks in southern Queensland, the style of deformation in the Gympie Block with its widespread cleavage, and variable cleavage intensity and orientations typical of thrust terranes, is similar to that in the rocks of the fold-thrust belt that we have studied in the Fitzroy area to the north. We suggest that the Gympie Block may be an element of a more northerly terrane of the NNEFB that has been displaced south by dextral strike-slip motion during the later part of the Hunter-Bowen event. We would speculate that it initiated as one of the suite of Early Permian marine extensional basins that formed within the old accretionary terranes along the NEFB, thus accounting for its present location to the east of the southern accretionary exposures.

        The well-cleaved ?Early Triassic Kin Kin Phyllite is the youngest rock unit in the Gympie Block and thus any major strike-slip displacement must postdate that time and yet be completed by the end of Hunter-Bowen contraction at ~230Ma. Typical strike-slip fault displacement rates on major faults in California are within the range of of 1 to 10mm/yr, increasing to 25-35mm/yr for the San Andreas plate margin system (Petersen and Wesnousky, 1994). Hence a moderately fast movement rate of 10mm/yr would produce 100km of dispacement over 10 my. We have noted that the ultimate Hunter-Bowen contractional event was more intense than previous pulses, and that it produced out-of-sequence thrust nappe structures at the eastern margin of the fold belt. It is this event that would be the most likely driving force for any displacement of the Gympie Block, although movement rates would have to be 10-20mm/yr.

      3. The NEFB "double orocline" and dextral wrenching

A major factor in the consideration of possible displaced terranes in the NNEFB has been the problem of explaining the major double oroclinal flexure in northern NSW and southern Queensland. Murray et al. (1987) developed a model for the formation of the oroclinal flexure invoking large scale dextral displacement of terranes in the eastern NEFB on a transform fault during the Late Carboniferous. This model, and subsequent variations (e.g., Fergusson et al., 1993) postulate a large displacement (~500 km) dextral strike-slip fault in the NNEFB that accommodates the oroclinal bending to the south. A major problem with this model has been the lack of documented dextral strike-slip structures of that age in the NNEFB, although any such structure could well be masked by the later contractional deformation.

We would suggest that the most likely deformation event with the required geometry to produce the dextral oroclinal flexure would be during the Hunter-Bowen event. In the NNEFB, the original meridional structural grain that was imparted by the Early Carboniferous accretionary events was overprinted by a NNW-trending grain transverse to WSW-verging thrusts during the Hunter-Bowen event. A WSW contractional vector would provide an ideal structural environment for dextral slip on the pre-existing structural grain.

    1. Acknowledgements
    2. This paper incorporates the results of several independent ARC-funded projects (#A38830041; #A39130279; #A39331366; # A39232338, #A3931187) over the past ten years. We particularly acknowledge the financial and logistical support of Queensland Metals Corporation (and in particular Ian Howard-Smith, David Horton, and Darcy Milburn) without which the unifying work in the Fitzroy region would never have evolved. We are indebted to the many students who have contributed as part of the various projects. In particular, aspects of the PhD work of Joe Tang, Terry Harbort, Lorraine Campbell, and Basim Faraj, the MSc work of Bill Kwiecien and Jim Hanson, and the Honours work of Terry Harbort, Vanessa Muscio, Murray Patterson, Bernadette Williams, Eris O’Brien, Sean Joyce, Craig Roberts, Jason Moultrie, and Steve Downey have added to our understanding of the region. We must also acknowledge the discussions, sometimes spirited, over the years with Cec Murray, Russell Korsch, Chris Fergusson, John Draper, and others that have refined our ideas. In a review paper like this it is likely that we have missed proper acknowledgement of some sources. Constructive reviews by Vince Morand, Peter Cawood, and Cec Murray have considerably strengthened the paper.

    3. References
    4. Allen, C.M., Chappell, B.W., Wooden, J.L. and Williams, I.S. 1994. Ages, compositions and sources of the Urannah batholith. In R.A. Henderson and B. K. Davis (eds.) Extended conference abstracts : new developments in geology and metallogeny: Northern Tasman orogenic zone, James Cook University, Townsville, Economic Geology Research Unit Contribution 50, 107-108.

      Baker, J.C., Fielding, C.R., de Caritat, P. and Wilkinson, M.M. 1993. Permian evolution of sandstone composition in a complex back-arc extensional to foreland basin: the Bowen Basin, eastern Australia. Journal of Sedimentary Petrology 63, 881-893.

      Biggs, M., Burgess, A.W. and Patrick, R.B. 1995. Callide Basin. In Ward, C.R., Harrington, H.J., Mallett, C.W. and Beeston, J.W. eds., Geology of Australian Coal Basins, Geological Society of Australia Coal Geology Group Special Publication 1, 471-488.

      Bryan, W.H., 1925. Earth movements in Queensland. Proceedings of the Royal Society of Queensland, 37, 3-82.

      Busby, C. and Ingersoll, R. (eds.) 1995. Tectonics of Sedimentary Basins, Blackwell, Oxford.

      Cranfield L.C., 1994. 1:250 000 Geology Series Explanatory Notes, Maryborough Queensland:SG56-6. Queensland Department of Minerals and Energy, 120p.

      Cuneen, R. 1994. New Insights into the Gympie Goldfield. Queensland Exploration Potential 1994, Symposium Abstracts, Queensland Department of Minerals and Energy, Brisbane, 26.

      Day, R.W., Murray, C.G. and Whitaker, W.G. 1978. The eastern part of the Tasman Orogenic Zone. In Scheibner, E. ed., The Phanerozoic structure of Australia and variations in tectonic style, Tectonophysics 48, 327-364.

      Donchak, P.J.T., Little, T.A., Sliwa, R. and Holcombe, R.J. 1995. Geology of metamorphic units of the North D’Aguilar Block - Goomeri, Nannago and Nambour 1:100 000 sheet areas. Queensland Geological Record, 1995/7, Queensland Department of Minerals and Energy 88p.

      Edgar, S.A., 1992. Geology and geochemistry of the Woolooga area, southeast Queensland. B.Sc. (Honours) Thesis, University of Queensland (unpublished).

      Elliott, L.G. 1993. Post-Carboniferous tectonic evolution of eastern Australia. Australian Petroleum Exploration Association Journal 33, 215-236.

      Ewart, A., Schön, R.W. and Chappell, B.W. 1992. The Cretaceous volcanic-plutonic province of the central Queensland (Australia) coast - a rift-related 'calc-alkaline' province. Transactions of the Royal Society of Edinburgh 83, 327-346.

      Exon, N.F. 1976. Geology of the Surat Basin in Queensland. Bureau of Mineral Resources, Geology and Geophysics, Bulletin 166.

      Falkner, A.J. 1986. Sedimentology of the Blackstone Formation, Ipswich Coal Measures, southeast Queensland. PhD Thesis, University of Queensland (unpublished).

      Faraj, B.S., Fielding, C.R. and Mackinnon, I.D.R. 1996. Cleat mineralisation of the Upper Permian Baralaba/Rangal Coal Measures, Bowen Basin, Australia. In Gayer, R. and Harris, I. eds. Coal bed Methane and Coal Geology. Geological Society of London Special Publication 97, 151-164.

      Fergusson C.L. 1991. Thin-skinned thrusting in the northern New England Orogen, central Queensland, Australia. Tectonics 10, 797-806.

      Fergusson C.L., Henderson R.A. and Leitch E.C. 1990. Structural history and tectonics of the Palaeozoic Shoalwater and Wandilla terranes, northern New England Orogen, Queensland. Australian Journal of Earth Sciences 37, 387-400.

      Fergusson, C.L., Henderson, R.A., Leitch, E.C. and Ishiga, H., 1993. Lithology and structure of the Wandilla terrane, Gladstone-Yeppoon district, central Queensland, and an overview of the Palaeozoic subduction complex of the New England Fold Belt. Australian Journal of Earth Sciences, 40: 403-414.

      Fergusson C.L., Henderson R.A. and Leitch, E.C. 1994. Tectonics of the New England Fold Belt in the Rockhampton Gladstone region, central Queensland. In: Holcombe, R.J., Stephens, C.J. and Fielding, C.R. (eds.), 1994 Field Conference Guidebook: Capricorn Region, central coastal Queensland, Geological Society of Australia, Queensland Division, Brisbane, 1-16.

      Fielding, C.R., Stephens, C.J., Kassan, J. and Holcombe, R.J., 1995. Revised palaeogeographic maps for the Bowen Basin, Central Queensland. In Follington, I.L., Beeston J.W. and Hamilton L.H. eds., Bowen Basin Symposium 1995, Proceedings, Geological Society of Australia, Brisbane, 7-15.

      Fielding, C.R., Stephens, C.J., and Holcombe, R.J., (in press). Submarine mass-wasting deposits as an indicator of the onset of foreland thrust loading: Late Permian Bowen Basin, Queensland, Australia. Terra Nova.

      Fielding, C.R., Stephens, C.J. and Holcombe, R.J. submitted. Permian stratigraphy and palaeogeography of the northern New England Fold Belt in coastal central Queensland. Australian Journal of Earth Sciences (this volume).

      Flood, P.G. 1983. Tectonic setting and development of the bowen Basin. In Waterhouse, J.B. ed., 1983 Field Conference - Permian areas, Biloela, Moura, Cracow, Geological Society of Australia Queensland Division, Brisbane, 7-21.

      Golding, S.D., Wilson, A.F., Scott, M., Anderson, P.K. Waring, C.L., Flitcroft, M. and Rypkema, H.A. 1987. Isotopic evidence for the diverse origins of gold mineralisation in Queensland. In Herbert, H.K. ed. Gold in Queensland. Papers of the Department of Geology, University of Queensland 12, 65-83.

      Gust, D.A., Stephens, C.J. and Grenfell, A.T. 1993. Granitoids of the northern NEO: their distribution in time and space and their tectonic implications. In Aitchison, J.C. and Flood, P.G. eds. New England Orogen, Eastern Australia, NEO '93 Conference Proceedings, University of New England, 565-572

      Hammond, R. 1987. The Bowen Basin, Queensland, Australia: an upper crustal extension model for its early history. Bureau of Mineral Resources, Geology and Geophysics, Record 1987/51, 131-142.

      Henderson R.A., Fergusson C.L., Leitch E.C., Morand V.J., Reinhardt J.J. and Carr P.F. 1993. Tectonics of the northern New England Fold Belt. In Flood P.G. and Aitchison J.C., eds New England Orogen, eastern Australia. Department of Geology and Geophysics, University of New England, Armidale, 505-515.

      Holcombe, R.J., 1977. Structure and tectonic history of the Brisbane Metamorphics in the Brisbane area. Journal of the Geological Society of Australia 24, 475-490.

      Holcombe, R.J.and Little, T.A. 1994. Blueschists of the North D'Aguilar Block: Structural development of the Rocksberg Greenstone and associated units near Mt Mee, southeast Queensland. Australian Journal of Earth Sciences, 41, 115-130.

      Holcombe, R.J., Fielding, C.R., and Stephens,C.J. 1995. Tear fault termination of the fold-thrust belt in the Northern New England Orogen. Geological Society of Australia Abstracts 40, 72.

      Holcombe, R.J., Stephens, C.J., Fielding, C.R., Gust, D.A., Little, T.A. Sliwa, R., McPhie, J. and Ewart, A. submitted. Tectonic Evolution of the Northern New England Fold Belt: Carboniferous to Early Permian transition from accretion to extension. Australian Journal of Earth Sciences (this volume).

      Horton, D.J. 1978. Porphyry-type copper-molybdenum mineralization belts in Eastern Queensland. Economic Geology 73, 904-921.

      Irwin. M.J. 1973. The igneous and sedimentary geology of the Kinbombi area, southeast Queensland. BSc Honours Thesis, University of Queensland (unpublished).

      Jones, P.J. (compiler). 1996. AGSO Phanerozoic Timescale 1995. Oxford University Press, Melbourne, 32p.

      Kassan, J. 1993. Basin analysis of the Triassic section in the Bowen Basin, Queensland, PhD Thesis, University of Queensland (unpublished).

      Kerr, I.D. 1976. Aspects of the geology and geochemistry of the Neara Volcanics and the Kilkivan Mercury Deposits, southeast Queensland. BSc Honours Thesis, The University of Queensland (unpublished).

      Korsch, R.J. and Totterdell, J.M. 1995. Structural events and deformational styles in the Bowen Basin. In Follington, I.L., Beeston, J.W. and Hamilton, L.H. eds., Bowen Basin Symposium 1995 Proceedings, Geological Society of Australia Coal Geology Group, Brisbane, 27-35.

      Korsch, R.J., Harrington, H.J., Wake-Dyster, K.D., O'Brien, P.E., and Finlayson, D.M. 1988. Sedimentary basins peripheral to the New England Orogen: their contribution to understanding New England tectonics. In Kleeman, J.D. ed., New England Orogen, Tectonics and Metallogenesis, University of New England, Armidale, 134-140.

      Korsch, R.J., O'Brien, P.E., Sexton, M.J., Wake-Dyster, K.D. and Wells, A.T., 1989. Development of Mesozoic transtensional basins in easternmost Australia. Australian Journal of Earth Sciences 36, 13-28.

      Kweicien, W. 1996. Geology, geochemistry, and petrogenesis of the Monsildale Granodiorite. Unpub. MSc thesis, Queensland University of Technology.

      Leitch, E.C. 1975. Plate tectonic interpretation of the Palaeozoic history of the New England Fold Belt. Geological Society of America Bulletin 86, 141-144.

      Leitch E.C., Fergusson C.L. and Henderson R.A. 1994. Ophiolitic and metamorphic rocks in the Percy Isles and the Shoalwater Bay region, New England Fold Belt, central Queensland. Australian Journal of Earth Sciences, 41, 571-579.

      Little,T.A. 1992. Geology of a northern part of the North D'Aguilar block, southeast Queensland. Qld Dept of Resource Industries Record, 1992/22, 28p, 1:25000 map.

      Little,T.A.; Holcombe,R.J.; and Sliwa,R. 1993. Structural evidence for extensional exhumation of blueschist-bearing serpentinite-matrix melange, New England Orogen, southeast Queensland, Australia. Tectonics 12, 536-549.

      Little,T.A.; McWilliams,M.O. and Holcombe,R.J. 1995. 40Ar/39Ar thermochronology of epidote-blueschists from the North D'Aguilar Block, Queensland, Australia: Timing and kinematics of subduction complex unroofing. American Geological Society Bulletin 107, 520-535

      Morand V.J. 1993. The Broome Head Metamorphics: high grade metamorphism in the northern New England Fold Orogen. In Flood P.G. and Aitchison J.C., eds. New England Orogen, eastern Australia. Department of Geology and Geophysics, University of New England, Armidale, 591-598.

      Murphy, P.R., Schwarzbock, H., Cranfield, L.C., Withnall, I. and Murray, C.G. 1976. Geology of the Gympie 1:250,000 Sheet Area. Geological Survey of Queensland Report 96.

      Murray, C.G. 1990. Tectonic evolution and metallogenesis of the Bowen Basin. In Bowen Basin Symposium 1990 Proceedings, Geological Society of Australia (Queensland Division), 201-212.

      Murray, C.G. 1974. Alpine-type ultramafics in the northern part of the Tasman Geosyncline - Possible remnants of Palaeozoic ocean floor. In Denmead A.K., Tweedale G.W. and Wilson A.F. eds The Tasman Geosyncline - A Symposium. Geological Society of Australia, Queensland Division, Brisbane. 161-181.

      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.

      Parianos, J., 1993. Carboniferous to Tertiary geology of the Airlie Block, northeast Queensland. M.Sc. thesis, The University of Queensland (unpublished).

      Pegrem, B.J. 1995. Tarong Basin. In Ward, C.R., Harrington, H.J., Mallett, C.W. and Beeston, J.W. eds., Geology of Australian Coal Basins, Geological Society of Australia Coal Geology Group Special Publication 1, 465-470.

      Petersen, M.D. and Wesnousky, S.G., 1994. Fault slip rates and earthquake histories for active faults in southern California. Bulletin of the Seismological Society of America, 84, 1608-1649.

      Roberts, C.A., 1992. The geology of the Fat Hen Creek area, Kilkivan, Southeast Queensland. B.Sc. (Honours) Thesis, University of Queensland (unpublished).

      Sliwa, R. 1994. Regional structural geology of the central North D’Aguilar Block, southeastern Queensland. PhD thesis, University of Queensland (unpublished).

      Sliwa,R.; Holcombe,R.J.; Fielding, C.R.; Little,T.A., Bryan, S.E.; Fifoot,A. 1993. Early Permian marine fault basins formed during exhumation of the New England Orogen subduction complex in southeastern Queensland.In: Flood, P.G. and Aitchison, J.C., eds. New England Orogen, eastern Australia., Dept of Geology and Geophysics, University of New England, Armidale, 557-564.

      Staines, H.R.E., Falkner, A.J. and Thornton, M.P. 1995. Ipswich Coalfield. In Ward, C.R., Harrington, H.J., Mallett, C.W. and Beeston, J.W. eds., Geology of Australian Coal Basins, Geological Society of Australia Coal Geology Group Special Publication 1, 455-464.

      Stephens, C.J., 1992. The Mungore Cauldron and Gayndah Centre Late Triassic large-scale silicic volcanism in the New England Fold Belt near Gayndah, southeast Queensland. PhD Thesis, University of Queensland (unpublished).

      Stephens, C.J., Schön, R.W.S. and Ewart, A. 1993. Mesozoic crustal extension in the northern NEO: Geochemical and isotopic evidence from large scale silicic magmatism. In Aitchison, J.C. and Flood, P.G. eds. New England Orogen, eastern Australia NEO '93 Conference Proceedings, University of New England, 637-642.

      Waterhouse,J.B. and Sivell,W.J. 1987. Trans-Tasman relationships between the Permian-Triassic rocks of Gympie, southeast Queensland, and those of New Caledonia and New Zealand. In: Murray, C.G and Waterhouse, J.B. (eds), 1987 Field Conference Guide, Gympie District, Geological Society of Australia, Queensland Division, 48-59.

      Williams, B.M. 1993. The Geology of the Meandu Coal Deposit and the Central Tarong Basin, southeast Queensland. Bsc. (Honours) Thesis, University of Queensland (unpublished).

      Wiltshire, M.J. 1982. Late Triassic and Early Jurassic sedimentation in the Great Artesian Basin. In Moore, P.S. and Mount, T.J. eds., Eromanga Basin Symposium, Summary Papers, Geological Society of Australia and Petroleum Exploration Society of Australia, Adelaide, 59-67.

       

    5. List of Figures

Figure 1

Location map of major tectonic entities and study areas in the northern New England Fold Belt.

Figure 2

Structural framework of the Fitzroy region showing units and features discussed in the text. Dotted lines labelled i, ii, and iii are the cross-section lines AB, CD, and EF respectively of Figure 4.

Figure 3

Structural framework of the North D’Aguilar Block in southern Queensland showing units and features discussed in the text.

Figure 4

Cross-sections across the Gogango Overfolded Zone in the Fitzroy region. The Rookwood Thrust is a major thrust that carries Yarrol Block basement over Connors Arch basement in the footwall. Early Permian marine basins occur predominantly to the east (in the hangingwall) of this thrust. The Marlborough Thrust (section AB) is an out-of-sequence thin thrust nappe that overrides the earlier thrusts. The large antiformal structure in CD and EF is the Craigilee Anticline which is a breached fault propagation fold from a thrust that splays off the Rookwood Thrust. The steep faults are Cretaceous and Tertiary normal faults. The locations of section lines are shown on Figure 2. Both vertical and horizontal scale bars are in 1 km increments.

Figure 5

Time-stratigraphic distribution of coarse clastic sediment wedges shed into the Bowen Basin from the east, showing the extent and episodic nature of coarse siliciclastic sediment derived from the approaching thrust front to the east. Bar marked Au deposits indicates age range for K/Ar ages from structurally-focussed gold mineralisation in the NNEFB. Small bars indicate specific K/Ar ages from illite in the Bowen Basin (Faraj et al., 1996), interpreted as fluid flux event(s). Three periods of thrust deformation are shown; the earliest is constrained mainly by 40Ar/39Ar dating in southeast Queensland, whereas the second and third are recognised from seismic records across the Bowen Basin (Korsch and Totterdall, 1995).