Last Edited: 29/01/97 12:46 Printed: 03/04/96 8:33
Tectonic Evolution of the Northern New England Fold Belt: The Permian-Triassic Hunter-Bowen event.
R J Holcombe1; C J Stephens, C R Fielding11; D Gust2; T A Little1,; R Sliwa1,; J. Kassan1,, J McPhie3; A Ewart
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.
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).