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Younger basins and structures

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).

    1. Discussion

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

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.

      1. 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:

  1. both the Calliope and Yarrol terranes may be very thin (<2 km);

  2. since both the hangingwall and footwall of the Rookwood Thrust contain Bowen Basin sequences, the thrust displacement of the allochthonous terranes would not be expected to greatly exceed a few tens of kilometres;

  3. the Tungamull and Parkhurst Faults are part of a single imbricate thrust system (albeit reactivated during Cretaceous-Tertiary normal faulting) carrying allochthonous terranes that include both the accretionary rocks and elements of the Yarrol terrane.

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?

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.

      1. 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.

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