|shkolakz.ru|| 1 ... 2 3 4 5 6
Younger basins and structuresSimilar 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).
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. 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. 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).
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).
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.
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.
<< предыдущая страница следующая страница >>