Archive for the category: Tectonic and Paleogeographic evolution of Mesozoic Eastern Australia

Wainman, Dr Carmine¹, McCabe, Prof Peter¹, Reynolds, Dr Peter¹

¹Australian School of Petroleum and Energy Resources, University of Adelaide, Adelaide, Australia

Volcanogenic rocks are important components of the Late Triassic to Early Cretaceous infill of the Great Australian Superbasin, including the widespread deposition of air-fall volcanic ash (tuff) preserved in the Jurassic Walloon Coal Measures (WCM) of the Surat Basin. With the paucity of known igneous bodies in eastern Australia, these tuff beds provide important clues on the tectono-magmatic environment of eastern Gondwana during the Mesozoic. To better understand the source and character of the volcanic province, age-constrained tuffs (168 to 148 Ma) were analysed in detail. Bed thickness, petrography (supported by XRD data), zircon crystal size and their geochemistry were documented. New datasets reveal these buff-coloured tuffs, mostly preserved within coal seams, are between 0.01 and 2 m thick with sharp lower and upper contacts. They are dominated by splintery, angular quartz clasts (approx. 10–100 μm in diameter) supported in an amorphous, white-buff coloured matrix consisting of clay minerals (predominantly smectite). The same beds are poorly sorted, lightly compacted and lack any structure. Tuff isopach maps from the WCM show elongate lobes that thin from current day northeast to southwest (5 m to <1 m). Dated zircon crystals average 170 μm in size, are euhedral to tabular in shape, and have moderate U values (100 to 1000 pm) and elevated Y values (>500 pm). Integrating these datasets demonstrate that these tuffs were (1) produced from volcanoes fed by intermediate to felsic magmas, (2) that the prevailing paleowind direction was from east-southeast to west-northwest and (3) sourced from volcanoes approximately 280 to 1000 km away which erupted with a volcanic explosivity index (VEI) of approximately 8. We infer that these tuffs originated from a long-lived (late Palaeozoic to Cretaceous) continental arc related to the westward subduction of the paleo-Pacific oceanic crust beneath eastern Australia. These tuffs were most likely derived from the Whitsunday Igneous Association as supported from similar studies from Early Cretaceous strata of the Eromanga Basin. These previous findings will help better constrain the timing of when eastern Gondwana transitioned from a convergent to a divergent margin, and define future targets for ocean drilling to locate the parent igneous bodies in the Tasman Sea.

Biography

I am a Visiting Research Fellow at the University of Adelaide. I completed my PhD at the same university and received an MSci in Geology from the University of Southampton. My research focuses on Permian and Jurassic coal-bearing strata in Australia, and the Mesozoic evolution of Australia’s southern margin.

Spandler, Carl¹, Henderson Bob², Foley, Elliot², Roberts, Eric², Kemp, Tony³

¹The University of Adelaide, Adelaide SA 5005, ²James Cook University, Townsville, QLD 4811, ³The University of Western Australia, Perth, WA 6907

The Phanerozoic tectonic setting of eastern Australia involved two separate regimes. The older setting was a Cambrian to Triassic active convergent margin as registered by a succession of orogenic systems collectively grouped as the Tasmanides. The younger setting was a passive margin, driven by plate divergence that initiated in the Cretaceous, and continues to characterize eastern Australia. The tectonic setting of eastern Australia during the gap between these contrasting tectonic regimes (approx. 130 m.y. from the late Triassic to Cretaceous) has been poorly documented and remains open to question. While extensive continental detritus of this age is preserved in the Great Artesian Basin, recognized igneous activity is restricted to the Whitsunday Igneous Province that formed from 132 Ma to 95 Ma.

Here we investigate a suite of igneous rocks/units that includes the Grahams Creek Formation (>0.25 M km³) and a series of small plutons and volcanic units that are exposed in the region between the Sunshine Coast and Maryborough in SE Queensland. The plutonic rocks range from I–type, hornblende-rich gabbros and diorites, to granodiorites, and quartz syenites, while the Grahams Creek Formation consists of a thick sequence (up to 1200 metres) of volcanic to volcaniclastic rocks of basaltic to dacitic composition. Both plutonic and volcanic components have distinctive subduction-related trace element compositions, including relative depletion in Ti, Nb and Ta, and enrichment in Pb, Sr, K, Rb, Th, U, Ba and Cs. These compositions are typical of hydrous magmatic rocks from continental arc settings. Uranium-Pb dating of magmatic zircons from these samples returned ages between 145 and 140 Ma; an age range that is distinctly older that the Whitsunday Igneous Province. The initial Hf and O isotope composition of these zircons (εHf = +8 to +12.5; δ¹⁸O = +5.7 to +6.5) is consistent with a juvenile mantle source for these magmas.

The recognition of this new suite of magmatic rocks, together with new chemical analyses of mafic rocks from the Whitsunday Igneous Province and detrital zircon records of quartzo-feldspathic sedimentary sequences of the Great Artesian Basin (see Foley et al. 2020, this session), allow a re-evaluation of the tectonic setting of eastern Australia across the Mesozoic. We propose that the plate convergence regime along eastern Australia that formed the New England Orogen persisted across the Triassic, Jurassic and into the Cretaceous, with the newly-recognised arc rocks representing the youngest episode of continental arc magmatism recorded on the Australian continent.  Eastwards rollback of the slab in the Early Cretaceous led to continental extension and opening of a continental back-arc (analogous to the present-day Okinawa Trough) that formed the Whitsunday Igneous Province. Continued slab rollback and extension of the overlying plate in the late Cretaceous and Cenozoic lead to rifting and fragmentation of the eastern continental margin to form the current configuration of the eastern Australia-Zealandia, where large tracts on thinned continental crust remain submerged.

Biography

Carl Spandler is a petrologist/geochemist with interest in broad aspects of geology, particularly crustal growth processes, plate tectonics, mantle evolution and critical minerals ore systems.

Henderson, Robert ¹, Foley, Elliot¹, Roberts, Eric¹

¹James Cook University, Townsville, QLD 4811

Jurassic fluviatile infill of the Nambour Basin consists largely of quartzose sandstone of the Myrtle Creek and Landsborough formations and succeeding heterolithic sandstone, shale and coals of the Tiaro Coal Measures. Detrital zircon age spectra for representative sandstone samples constrain the ages and sediment sources of these units. Maximum depositional ages from zircon, broadly consistent with published biostratigraphic age determinations, assign this succession as Early to Middle Jurassic (195-163 Ma). With a coastal location in southern Queensland, these units represent the easternmost record of Jurassic sedimentation for Australia.

Zircon age spectra from Early Jurassic samples of the Myrtle Creek Formation are dominated by a 650 – 500 Ma (Pacific-Gondwana) age cluster, with a Grenville age cluster (1300-950) also showing prominence. Detrital zircon of these ages is characteristic of Cambro-Ordovician metasediments of the Tasmanides as widely represented in southeastern Australia. Rocks of this Tasmanide assemblage stood as epeirogenic Jurassic upland, shedding sediment northwards and eastwards across southeast Queensland. Transport vectors obtained for the Myrtle Creek Formation fluviatile sandstone horizons support this conclusion.   Devonian – Triassic ages of detrital and igneous zircon characteristic of the New England Orogen, which abuts the Nambour Basin and forms a broad crustal tract to its west, are sparingly represented in these samples.

Relief across the orogen in southeastern Australia, as generated by the Permo-Triassic (260-230 Ma) Hunter Bowen Orogeny, had therefore been reduced towards base level, with little contribution to ongoing erosion and sediment production, by the Early Jurassic (~190 Ma). By implication, the base level surface forming the floor to the Great Artesian Basin, marking a unconformity of remarkable extent, continued eastwards as a surface of low relief across much of the New England Orogen.  Detrital zircon from sandstone samples of the Middle Jurassic Tiaro Coal Measures indicate a continuing provenance contribution from Cambro-Ordovician Tasmanide metasediments but also from a more pronounced New England Orogen source, suggesting some physiographic rejuvenation of this crustal sector subsequent to the Early Jurassic.

Jurassic aged zircon is scarce in most samples. However, it is well represented in a sample from the Myrtle Creek Formation and dominates the detrital zircon age spectrum of an arkosic sample from the Tiaro Coal Measures. As no source terrain for Jurassic zircon is known for the crustal fabric of eastern Australia, it must have been derived from igneous assemblages on continental crust to the east, now represented by submerged northern Zealandia, which rifted from the Australian continent during Late Cretaceous – Paleocene. An eastern source is supported by westerly paleocurrent directions measured for the sandstone intervals from which these samples were obtained.

Biography

Researcher with long term interests in the Tasmanide orogenic system, especially the Mossman and New England Orogens. Research interests also in Paleozoic and Mesozoic sedimentary basins located in Queensland inclusive of stratigraphy, sedimentology, paleontology and tectonic setting with contemporary utilisation of detrital zircon LAICPMS geochronolgy in basin studies

Foley, Elliot¹, Henderson, Robert¹, Roberts, Eric¹, Kemp, Tony², Spandler, Carl³

¹James Cook University, Townsville, QLD 4811, ²The University of Western Australia, Perth, WA 6009, ³The University of Adelaide, Adelaide SA 5005

The tectonic setting of the east Gondwana margin during the Jurassic and Early Cretaceous is an enduring geological unknown. Whereas Paleozoic to early Mesozoic (~520 to 220 Ma) accretionary orogenic domains in eastern Australia are considered an exemplary record of convergent margin processes, the Late Triassic to Cretaceous represents an enigmatic gap in this record due to the paucity of exposed igneous and metamorphic rocks. This latter witnessed the deposition of vast quantities (>1.5 x 10⁶ km³) of sediment into the Great Australian Superbasin, including Jurassic silicic tuff horizons and a substantial Cretaceous component identified as volcanogenic.

The nature of magmatism that provided this volcanogenic material is debated, with two principal hypotheses posited. One suggests a continental magmatic arc enduring from the Carboniferous to mid-Cretaceous. The second model favours intraplate, rift-related magmatism unrelated to subduction, exemplified by the early-mid Cretaceous Whitsunday Igneous Province, a silicic large igneous province (SLIP) generated in the prelude to rupture of east Gondwana in the Late Cretaceous. Resolution of this question has been hampered by the sparse Jurassic-Cretaceous igneous record for eastern Australia. To overcome this deficiency, we investigated detrital zircon from the Great Australian Superbasin as a proxy record for subjacent igneous activity, employing U-Pb geochronology and Hf isotopic analysis to evaluate Mesozoic magmatism and clarify this enigmatic episode of east Gondwana crustal evolution.

Detrital zircon ages indicate that magmatism along the east Gondwana margin continued into the mid Cretaceous, with short-lived (~10 Myr) pulses of Mesozoic magmatic activity indicated by peaks at ~160, ~140, and ~100 Ma. A trend of increasing igneous activity, from the Jurassic towards eventual Late Cretaceous continental rupture of east Gondwana, as predicted by the SLIP hypothesis, is not supported by the detrital zircon record. Hf isotopic analysis of dated zircons shows a strongly positive εHf signature (+8 to +12) throughout the Mesozoic to ~95 Ma indicative of juvenile sources for the original igneous parent rocks. Similar positive εHf signatures are characteristic of Permian – Triassic granitic rocks of the New England Orogen for which a continental magmatic arc setting has been long accepted.

A potential Australian igneous source for Cretaceous zircon, the Whitsunday Igneous Province, is of limited aerial extent and a Jurassic source is unknown. Northern Zealandia, now submerged, formed the eastern borderland of east Gondwana prior to the Late Cretaceous, and must have been the main locus of Jurassic and Cretaceous magmatism.

Biography

Elliot Foley is a PhD Candidate at James Cook University and a member of the multidisciplinary Jurassic Arc Research Group.  His research focuses on the sedimentology, stratigraphy, provenance and hydrocarbon potential of basin fill in the northern sector of the Jurassic-Cretaceous Great Australian Superbasin.

Campbell, Matthew J ¹, Rosenbaum, Gideon ¹, Allen, Charlotte M. ², Spandler, Carl ³

¹School of Earth and Environmental Sciences, The University of Queensland, Brisbane, QLD, Australia,  ²Institute for Future Environments, Queensland University of Technology, Brisbane, QLD, Australia, ³Geosciences, James Cook University, Townsville, QLD, Australia.

Paleozoic-Mesozoic supra-subduction units, which originally formed along the paleo-Pacific margin of east Gondwana, are now preserved in eastern Australia, Antarctica and Zealandia. Previous works have characterized the temporal and geochemical history of magmatism within this broad accretionary orogenic system but the Zealandia continent remains a problematic piece of the Gondwana puzzle due to (1) 94% of the continent being submerged beneath the southwest Pacific Ocean and (2) a number of major phases of deformation that culminated in the oceanward dispersal of continental fragments of Zealandia. Here we reconstruct the Paleozoic and Mesozoic evolution of the active continental margin of Zealandia (eastern Gondwana), using a combination of detrital zircon geochronology, trace-element geochemistry, and Hf isotope data from several Paleozoic-Mesozoic terranes in New Zealand and New Caledonia. We find that zircon grains dated 360–160 Ma from New Zealand are characterized by εHf_i (+15 to +2) and trace-element compositions typical of predominantly juvenile magmatic sources. In contrast, the εHf_i (+15 to –5) and trace-element compositions of detrital zircon grains dated 245-140 Ma from New Caledonia reflect a mix of juvenile and evolved crustal sources. Secular trends in trace-element and Hf isotope compositions of zircon grains suggest that magmatism and continental crustal growth in Zealandia during the Devonian–Cretaceous were controlled by switches from trench advance to trench retreat. Orogenesis and crustal growth were controlled by a long-lived westward-dipping subduction system, which during the Permian–Triassic, was intermittently affected by distinct phases of arc accretion (e.g., of the Brook Street intra-oceanic arc) and orogenesis (e.g., driven by trench advance). These phases of orogenesis coincided with the Gondwanide Orogen (265–230 Ma), which might have been controlled by a plate-scale reorganization event following the final assembly of Pangea supercontinent.

Biography

Recent PhD graduate in geology from the University of Queensland. My PhD was focused on the Paleozoic to Mesozoic tectonic history and crustal architecture of the southwest Pacific region (Zealandia).

Betts, Peter¹, Moresi, Louis², Whittaker, Joanne³, Miller, Meghan²

¹School of Earth Atmosphere and Environment, Monash University, Melbourne, Australia, ²Research School of Earth Sciences, Australian National University, Canberra, Australia, ³Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia

Congested subduction occurs when buoyant lithosphere within the downgoing plate interacts with the with the convergent margin trench and prevents the slab from subducting in the region of collision. This can trigger several dynamic behaviours in the slab including slab tearing beneath the congestion and roll back of the trench away from the collision zone.  Congested subduction also drives contemporaneous shortening and extension along different parts of the overriding plate, and has the potential to dismember and segment subducting slabs.

During the Cretaceous collision of the Hikurangi Plateau with the east Gondwana margin triggered a major re-organisation of the margin culminating in the opening of the Tasman Sea and separation of Zealandia from Gondwana.  Tectonic models for this plateau accretion have proposed collision between the plateau and Campbell Plateau (South Island of New Zealand) at ca. 100 Ma, which triggered the opening of the Tasman Sea at ca 84 Ma.   These models are appealing because they explain the present-day relationship between the Hikurangi Plateau and the New Zealand along the Pacific-Australia plate margin. 

In this abstract, we provide an alternative model for the collision of the Hikurangi Plateau and Gondwana that is informed by numerical modelling of congested subduction.  We present this alternative tectonic model as a series of G-plates reconstructions.  Our new model requires collision of the Hikurangi Plateau with the Gondwana along the North Island of New Zealand, rather than the South Island.  Suturing of the Hikurangi Plateau with the North Island resulted in crustal shortening in front of the collision zone and triggered collapse of the Gondwana margin to the south and north.  Initiation of asymmetric trench roll back to the south is evidenced by the ubiquitous extension in the overriding plate, affecting the Campbell Plateau and Chatham rise.  Asymmetric opening of Tasman Sea and opening of the Bounty Trough at 84 Ma.  Counter clockwise rotation of the Chatham and Campbell plateaus segmented the slab along the Gondwana margin.  Soft collision of southern Zealandia (Chatham and Campbell Plateau) along the southern margin of the Hikurangi Plateau occurred at ca 70 Ma and stable subduction was established outboard of this accreted margin.  In this model, the Alpine fault initiated as a sinistral fault at the transition between trench advance in the North Island and trench roll-back to the south. 

Biography

Peter Betts has a diverse portfolio of research that addresses tectonic problems through Earth history.  He is currently working on the Tectonic process related to congested subduction, the opening of new oceans, and the role of inherited structures on continental reactivation.