Exploration Tectonics research

Earth Sciences 
The University of Queensland

New England Fold Belt Projects

The following are Industry and ARC-funded projects that have concentrated on the geological and tectonic evolution of the northern New England Fold Belt in Queensland, Australia. They are projects of ExTec (and previous similar groupings - now under the umbrella of the Research Exploration Initiative UQ.

The NEFB (see map) is an Early Devonian to Late Triassic convergent margin orogen that has undergone repeated cycles of crustal contraction and extension as the arc has attempted to move from a continental to an oceanic setting. Together, the projects have provided a much better understanding of the terranes and timing of events in this orogen and this is summarised in our evolving Time-Space plot for the Devonian-Triassic and for the Phanerozoic of Queensland in general and in our publications.

2004

The Esk Basin, northern New England Fold Belt

C R Fielding; R J Holcombe; Lorraine Campbell (PhD)

The following is a synopsis of Lorraine Campbell's PhD thesis (awarded March 2005). [The full thesis abstract can be downloaded from here].

The Esk Trough in southeast Queensland lies within the northern part of the New England Fold Belt, a Palaeozoic to early Mesozoic accretionary/orogenic complex that parallels part of the eastern margin of the Australian plate. The geological history of the Esk Trough was largely unknown and it had been presumed that it was a discrete basin, initiated and filled as a graben/rift valley during the Triassic. The purpose of the study was to determine the origin of the basin, describe its geological evolution and account for its present position within the northern New England Fold Belt. The Esk Trough is filled with Early–Middle Triassic continental sedimentary and intermediate volcanic rocks of the three units of the Toogoolawah Group. Structural cross-sections compiled from field data in this study have resulted in a redefinition of the stratigraphic order of the Toogoolawah Group with the middle unit having been reassigned as the youngest unit and the remaining two units having been correlated. Incorporated within the project area are Permian sedimentary and volcanic sequences that lie adjacent to the Esk Trough on its eastern, southeastern and southwestern boundaries, the relationship of which to each other and to the Esk Trough sequence was unknown. Of these Permian rocks, the two most southern sequences are correlated and concluded to be basal and genetically part of the same basin as the Triassic Toogoolawah Group. The remaining Permian unit is revealed as having been a component of a thrusted highland that was the major provenance for the sediments of the Toogoolawah Group. The evolutionary history of the Esk Basin (new name) is redefined as consisting of an Early Permian phase of extension, a mid-Permian phase of passive thermal subsidence and a latest Permian–Early Triassic phase of foreland loading. This parallels the tectonic evolution of the Bowen Basin in central Queensland and this, plus similarities in rock units between the two basins, leads to the conclusion that the Esk Basin developed in a depocentre on the southeastern margin of the larger Bowen Basin and was likely contiguous with it. Evidence is also presented for the existence of a continental volcanic-arc active during the Early–Middle Triassic in southeast Queensland, the arc having been active during an hiatus in deformation in this region. The developmental history of the Esk Basin incorporates the period of the Hunter-Bowen Orogeny within eastern Australia. This study of the Esk Basin has confirmed that, within southeast Queensland, thrusting of the Hunter-Bowen Orogeny had produced an exposed fold-thrust highland by the Early Triassic and had resulted in arc magmatism migrating westwards onto the continent, and that the terminal thrusting of this orogenic event occurred prior to the end of the Middle Triassic.

Aims:

2000-2002

The role of extension at long-lived convergent continental margins: the northern New England Fold Belt

R J Holcombe; C R Fielding; Scott Bryan (Research Associate)
[Download conference poster on structure and timing of the Marlborough Block (3700Kb)]
Aims:
1) To investigate the Devonian - Carboniferous structural, magmatic and depositional record of the “Andean” margin of eastern Australia (northern NEFB), and address the following questions:
· Why has extensional tectonics dominated at a long-lived (>200 m.y.) convergent margin setting?
· Why have back-arc basins (e.g., Drummond and Bowen basins) formed so far behind the interpreted volcanic arc?
· Can any magmatic arcs, in fact, be defined?
2) To assess the role of vague cross-orogen lineaments such as the ‘Darling River Lineament in the NEFB in partitioning and focussing deformation or extension along the NEFB. Along-orogen variations in the modern Andes are related to cross-orogen lineaments (probably representing lithospheric scale boundaries), which have partitioned these variations in depositional, magmatic and structural style. An assessment of how much of the present terrane architecture is an artefact of the Permo-Triassic Hunter Bowen Orogeny is also overdue.

Results to date:

• Recognition of mafic to silicic magmatic cycles in the Devonian-Carboniferous evolution of the Yarrol stratigraphic system. Late Devonian mafic volcanics pass upward into a silicic volcanogenic system around the Devonian-Carboniferous boundary before the first oolitic limestones appear in the Early Carboniferous.
• The Campwyn Volcanics sequence to the north is very similar to the Yarrol sequence with a similar mafic to silicic transition at approximately the same stratigraphic level. (In contradiction to a previous major study of this terrane, we have found that a consistent stratigraphy can be defined with a more defined facies architecture than previously described).
• Geochemical results confirm that the mafic magmatism in the Yarrol Basin had an arc to transitional back-arc signature, in contradiction to the forearc model for this basin. Geochemical results are not yet available for the Campwyn Volcanics. Our palaeocurrent data in the mafic/silicic sequences in both the Yarrol and Campwyn areas support the back-arc scenario by indicating a (proximal?) volcanic source to the east rather than to the west. (Our data for the Campwyn volcanics directly contradicts the dataset from the previous study].
• Based on five Time-Space transects, incorporating over 1000 old and new age dates and other sub-project observations, we now recognize up to four discrete crustal deformation episodes within the Hunter-Bowen event within a single transect. Like the Andean system, however, these episodes do not necessarily correspond across transect boundaries suggesting that crustal contraction varies temporally along the orogen and is partitioned across major crustal structures.
• Although most pulses would have formed fold-thrust mountain systems, the major mountain-forming/plateau forming event in the Hunter-Bowen orogeny occurs in the Late Permian at 253Ma during emplacement of the Marlborough duplex (see  below). This is the major crustal loading event during the orogeny and its timing corresponds to the initiation of widespread terrestrial sand deposition into the loaded Bowen Basin.
• In contrast to the sequences exposed in the main NEFB, we have identified a marked pre-Permian east-verging thrust sequence within seismic sections in the Denison Trough at the southwestern margin of the Bowen Basin. This is probably a mid-Carboniferous event – perhaps the distal signature of the Alice Springs Orogeny.

1998 -99

Cross-Orogen Structures and Along-Orogen Tectonic Transitions, Northern New England Fold Belt

R J Holcombe; C R Fielding; (UQ External Support Enabling Grant - original ARC application with D.Gust, QUT)
This has been combined with:
Structural, stratigraphic, and volcanic framework of the Middle-Late Devonian (~380Ma) terranes in the Northern New England Fold Belt, Monto area
R J Holcombe; C R Fielding, & Scott Bryan (North Ltd funded project)
This has now evolved into the previously described project
 

1996-1998

Geochemistry, petrology, and tectonic significance of the Marlborough ophiolite, northern New England Orogen

Y Niu; R J Holcombe ; Michael Bruce (PhD student)

[Major results so far show:

• that the ophiolitic component of the Marlborough Block has a MORB signature with an Neoproterozoic-Early Cambrian (~580 Ma) model age
• that the ophiolitic rocks are intruded by ~380 Ma dykes with an oceanic arc-like signature, and by Permo-Triassic dykes with a continental arc signature;
• that the Permo-Triassic granitoids intruding the Block have an arc-signature.
• that the ultramafic rocks exposed on South Percy Island have a suprasubduction signature and are unrelated to the Marlborough Block ultramafics. They are likely either part of the ophiolitic suite that is intercalated into the Carboniferous accretionary complex (similar to the North D'Aguilar suite) or they are arc equivalents of the Early Permian back-arc basins (such as the Rookwood Volcanics).

1995-1997-2001

Structure and metallogenesis of the Fitzroy region (partly QMC-funded)

R J Holcombe; C R Fielding; T Harbort (PhD student)
[See here for most recent abstract]
Results so far have concentrated on the Marlborough block and these show:

• The Marlborough Block contains three separate components, a mafic/ultramafic ophiolitic component, a variably deformed S-type granite suite and associated amphibolite facies metamorphics, and accretionary rocks similar to the adjaent Wandilla and Shoalwater terranes;
• The Block as a whole is a thin, out-of-sequence thrust sheet that overlies and truncates the regional fold-thrust belt. The basal detachment fault is brittle and subhorizontal to gently undulating.
• Internally, strongly ductile amphibolite-facies shear zones separate tilted sheets of metagranite/metasedimentary schist from an ophiolitic suite. A further set of greenschist facies ductile shear zones separate this higher grade sandwich against lower grade metasedimentary rocks of the accretionary complex.
• A brittle fault/cataclastic zone separates two such stacked sequences and together with the basal deatchment, define a two-horse duplex of a previously stacked ductile thrust sequence assembled at mid-crustal levels.
• The Marlborough Block duplex was emplaced at 253Ma based on Ar-Ar dating of granitoids below the sheet and intruding the sheet.

1994-1995

Marlborough-Fitzroy Queensland Metals Corp project (QMC-funded)

C J Stephens; R J Holcombe; C R Fielding (Applied Regional Exploration Geology and Tectonics Group)
[Project aimed at providing a geological framework for regional exploration by QMC in the Marlborough-Rockhampton region [see map]. The project arose out of a new base-metal discovery (Develin Creek) in previously unprospective Permian volcanics, and conflict in the surrounding four 1:250000 geological maps as to the stratigraphic and structural framework. Major conclusions arising from this project (other than establishing the local stratigraphic framework are:

• that the northern part of the New New England Fold Belt is dominated by a Late Permian to Middle Triassic shallow fold and thrust belt [see sections], and that this fault architecture has been thoroughly exploited by extensive Cretaceous and Tertiary normal faulting [see map];
• that the fold-thrust belt includes rocks of the Bowen Basin, a set of Early Permian extension subbasins that were transformed into a deep foreland-loaded basin during the Late Permian-Middle Triassic thrusting events[see map];
• that the location of the previously postulated Connors Arc is a structural culmination and not the original locus of an arc;
• that a previously postulated deep marine forearc basin (Grantleigh Trough) does not exist.]

1994-1996

Late Palaeozoic and Early Mesozoic evolution of the New England Orogen in southeast Queensland

D A Gust (QUT); R J Holcombe; C R Fielding; Joe Tang (PhD, QUT); Lorraine Campbell (PhD)
[This project is a continuation of the previous with emphasis on the Triassic history. In particular, we are concentrating on the Esk Trough, an early Triassic basin filled with andesitic volcaniclastics and volcanics; on coeval granitoids; and on the Late Triassic granitoids and volcanics that mark the beginning of protracted extension in eastern Australia].
See here for most recent abstract on the Esk "Trough"

1993-1995

Permian volcanics of southeastern Queensland

A Ewart;C R Fielding; C J Stephens; J McPhie (Tas); Amanda Jones (PhD)
[This project is a continuation of the previous with emphasis on the Triassic history. In particular, we are concentrating on the Esk Trough, an early Triassic basin filled with andesitic volcaniclastics and volcanics; on coeval granitoids; and on the Late Triassic granitoids and volcanics that mark the beginning of protracted extension in eastern Australia].

1992

Late Palaeozoic Tectonics of the New England Orogen in Southeast Queensland

R J Holcombe; C R Fielding; S K Dobos
[Continuation of the previous project with emphasis on Early Permian extensional basins superimposed on the old accretionary rocks, and on the timing and structure of the Late Permian thrusting that marks the beginning of the Hunter-Bowen orogeny].

1989-1991

Tectonic transect and terrane analysis across the Yarraman, Esk, and D'Aguilar blocks, Southeast Queensland

R J Holcombe; C R Fielding; S K Dobos T A Little (post-doctoral research associate); R Sliwa (Ph D)
[The project ended up concentrating on the structure of the North D'Aguilar Block accretionary rocks and their subduction and exhumation history. The block includes epidote-blueschist facies ophiolitic rocks that have undergone a thermal greenschist facies overprint as they were exhumed in a core-complex structure in the Late Carboniferous]. Deep level and high level components of the accretionary complex are now juxtaposed across a low angle detachment fault that was also synchronous with intrusion of syntectonic S-type granitoids].

 

Department of Earth Sciences

The University of Queensland
Queensland, 4072, Australia

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Web page created by Rod Holcombe6 April 2004