Adelaide Rift Research Articles

An 40Ar– 39Ar investigation of the Mertz Glacier area (George V Land, Antarctica): Implications for the Ross Orogen–East Antarctic Craton relationship and Gondwana reconstructions

George V Land (Antarctica) includes the boundary between Late Archean–Paleoproterozoic metamorphic terrains of the East Antarctic craton and the intrusive and metasedimentary rocks of the Early Paleozoic Ross–Delamerian Orogen. This therefore represents a key region for understanding the tectono-metamorphic evolution of the East Antarctic Craton and the Ross Orogen and for defining their structural relationship in East Antarctica, with potential implications for Gondwana reconstructions. In the East Antarctic Craton the outcrops closest to the Ross orogenic belt form the Mertz Shear Zone, a prominent ductile shear zone up to 5 km wide. Its deformation fabric includes a series of progressive, overprinting shear structures developed under different metamorphic conditions: from an early medium-P granulite-facies metamorphism, through amphibolite-facies to late greenschist-facies conditions. 40Ar– 39Ar laserprobe data on biotite in mylonitic rocks from the Mertz Shear Zone indicate that the minimum age for ductile deformation under greenschist-facies conditions is 1502 ± 9 Ma and reveal no evidence of reactivation processes linked to the Ross Orogeny. 40Ar– 39Ar laserprobe data on amphibole, although plagued by excess argon, suggest the presence of a ∼1.7 Ga old phase of regional-scale retrogression under amphibolite-facies conditions. Results support the correlation between the East Antarctic Craton in the Mertz Glacier area and the Sleaford Complex of the Gawler Craton in southern Australia, and suggest that the Mertz Shear Zone may be considered a correlative of the Kalinjala Shear Zone. An erratic immature metasandstone collected east of Ninnis Glacier (∼180 km east of the Mertz Glacier) and petrographically similar to metasedimentary rocks enclosed as xenoliths in Cambro–Ordovician granites cropping out along the western side of Ninnis Glacier, yielded detrital white-mica 40Ar– 39Ar ages from ∼530 to 640 Ma and a minimum age of 518 ± 5 Ma. This pattern compares remarkably well with those previously obtained for the Kanmantoo Group from the Adelaide Rift Complex of southern Australia, thereby suggesting that the segment of the Ross Orogen exposed east of the Mertz Glacier may represent a continuation of the eastern part of the Delamerian Orogen.

Re-Os geochronology of postglacial black shales in Australia: Constraints on the timing of “Sturtian” glaciation

New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits in southern and central Australia, respectively, challenge the prevailing consensus of three Neoproterozoic glaciations. The end of Sturtian glaciation in the Adelaide Rift Complex is constrained by a Re-Os date of 643.0 ± 2.4 Ma from the overlying Tindelpina Shale Member (basal Tapley Hill Formation). A Re-Os date of 657.2 ± 5.4 Ma for the basal Aralka Formation constrains the age of the underlying Areyonga glacial deposits in the Amadeus Basin. The Re-Os ages show that the Sturtian and Areyonga glacial deposits are younger than other radiometrically dated (ca. 685–750 Ma) Neoproterozoic glacial intervals previously regarded as possible correlatives. Thus, the “Sturtian” ice age was markedly diachronous, and/or there was more than one “Sturtian”-type glaciation. Some Neoproterozoic glacial deposits may represent the products of regional and diachronous glaciation associated with protracted breakup of the supercontinent Rodinia rather than “snowball” or “slushball” Earth ice ages.

40Ar– 39Ar dating of detrital muscovite in provenance investigations: a case study from the Adelaide Rift Complex, South Australia

Detrital zircon ages are commonly used to investigate sediment provenance and supply routes. Here, we explore the advantages of employing multiple, complimentary techniques via a case study of the Neoproterozoic and Cambrian of the Adelaide Rift Complex, South Australia. Detrital muscovite Ar–Ar ages are presented from stratigraphic units, or equivalents, that have previously been the subject of U–Pb detrital zircon dating, and, in some cases, whole-rock Sm–Nd isotope studies. The zircon age ranges and whole-rock Sm–Nd isotope data suggest that early Neoproterozoic sediments from near the base of the Adelaide Rift Complex comprise a mixture of detritus derived from the adjacent Gawler Craton (Palaeoproterozoic to earliest Mesoproterozoic) and overlying Gairdner flood basalts. In contrast, detrital muscovites from this level have a broad scatter of Mesoproterozoic infrared (IR) laser total fusion Ar–Ar ages, while UV laser traverses indicate that the age spread reflects partial resetting by multiple heating events, rather than a mixture of sources. Younger Neoproterozoic sediments document replacement of the Gawler Craton by the more distant Musgrave and/or Albany–Fraser Orogens as the main provenance. The Cambrian Kanmantoo Group marks an abrupt change in depositional style and a new sediment source. The Kanmantoo Group have older Nd model ages than underlying strata, yet are dominated by near to deposition-aged (∼500–650 Ma) detrital zircons and muscovites, suggesting rapid cooling and exhumation of a tectonically active provenance region. Although this source remains uncertain, evidence points towards the distant Pan-African orogenic belts. Deposition in the Adelaide Rift Complex was terminated in the late Early Cambrian by the Delamerian Orogeny, and the results of previous detrital mineral dating studies from the Lachlan Fold Belt to the east are consistent with at least partial derivation of these sediments from reworked upper Adelaide Rift Complex (Kanmantoo Group), rather than a continuation of sediment supply from the Kanmantoo Group sediment source. More broadly, the data suggest a close link between basin formation and orogen exhumation, and we also speculate that mantle plumes have played a significant role in crustal evolution at this Palaeo-Pacific margin of Gondwanaland, challenging the notion that subduction zones are the principle sites of crustal growth and sediment provenance.

History of Neoproterozoic rift basins in South China: implications for Rodinia break-up

Abstract We present here a first-order analysis of the Neoproterozoic continental rift basins in South China, focusing on the north-south trending Kangdian basin in the western part, and the Yangtze-side of the northeasterly-trending Nanhua basin. The Yangtze-side of the Nanhua basin is subdivided into the Hunan-Guangxi, the Jiangnan Ridge, and the northern Zhejiang sub-basins. The Neoproterozoic successions in these basins consist of four major sequence-sets, representing four phases of rifting. The first phase occurred at ca. 820 Ma after the 830–820 Ma bimodal magmatism and rapid crustal unroofing, and is represented by the Shiqiaopu Formation in the Hunan-Guangxi sub-basin and the Luojiamen Formation in the northern Zhejiang sub-basin. The second phase at ca. 800 Ma is represented by the Hongchicun Formation in the northern Zhejiang sub-basin. The third, a major rift phase, occurred at ca. 780–750 Ma, represented by the Sanmenjie Formation in the Hunan-Guangxi sub-basin, the Luokedong and Jingtan formations in the Jiangnan Ridge sub-basin, and the Shangshu Formation in the northern Zhejiang sub-basin. The final phase recorded the rift–drift transition during the Nantuo glaciation, estimated at ca. 750–690 Ma, with no volcanism. This was followed by a sag phase to the remainder of the Neoproterozoic, which saw the entire Yangtze Block covered with either platform carbonate deposits or basinal carbonaceous and cherty deposits. The basin history in South China prior to ca. 600 Ma can be well correlated with that in the Adelaide rift system in southeastern Australia, suggesting that South China could have been adjacent to eastern Australia in the supercontinent Rodinia, but broke away from Australia by the end of the continental rifting at ca. 750–690 Ma.

Mid-Neoproterozoic (∼830–750 Ma) isotope stratigraphy of Australia and global correlation

Carbon and strontium isotope studies of numerous drillcore and outcrop sections within individual Australian Neoproterozoic basins, in a tectonic and sedimentologic framework, have allowed the construction of isotopic records for the interval 830–750 Ma. This, combined with the discovery of a restricted stratigraphic range for the acritarch Cerebrosphaera buickii in mid-Neoproterozoic strata of Australia, makes possible correlation with successions in Canada, Namibia and Spitsbergen. That part of the mid-Neoproterozoic succession in Spitsbergen which contains C. buickii was previously considered younger than the Sturtian glacial epoch, but this study has indicated that the Akademikerbreen Group is older than that. A correlation scheme is proposed along five tie lines, and is based on a correlation between Australia and Canada at ∼830 Ma, and Australia, Spitsbergen and Namibia at ∼760 Ma. The lowest ever recorded seawater 87Sr/ 86Sr ratios in the upper Shaler Supergroup of Canada (0.70561–0.70670) compare with ratios in evaporites of the ∼830 Ma Bitter Springs Formation, Amadeus Basin, Australia (0.70568–0.70720). Comparable δ 13C carb records support a correlation. At about 760 Ma, a correlation is made between the heaviest mid-Neoproterozoic δ 13C carb values of 7.2‰ (upper Burra Group, Adelaide Rift Complex) and 8.1‰ (Kanpa Formation, western Officer Basin) in Australia, 8.5‰ in Spitsbergen (Backlundtoppen Formation, Akademikerbreen Group), and 8.5‰ in Namibia (Ombombo Subgroup, Congo Craton). The stratigraphic range of C. buickii in Australia and Spitsbergen supports this correlation. A compilation of δ 13C carb and 87Sr/ 86Sr records between about 830 and 750 Ma from Australia, Canada, Namibia and Spitsbergen reveals numerous negative excursions, which are not associated with preserved glacial deposits. It is necessary, therefore, to consider additional models to the ‘Snowball Earth’ to explain the large and rapid carbon isotopic excursions.

Isotope stratigraphy of the Ediacarian (Neoproterozoic III) of the Adelaide Rift Complex, Australia, and the overprint of water column stratification

This study establishes a high resolution δ 13C-chemostratigraphic framework for the type Ediacarian (terminal Proterozoic) section in the Adelaide Rift Complex, a framework that contributes to the emerging global chronostratigraphic synthesis of a period that saw momentous changes in the Earth's surface environment. Only 15 m of the 3.5 km thick type section consists of shallow-water carbonates of the kind that have formed the basis of most previous studies of Neoproterozoic chemostratigraphy. However, the organic (δ 13C org) record reveals a surprising richness of information and shows excellent utility in correlation between sections in the Adelaide Rift Complex. Lateral variations in δ 13C org profiles correspond to expected diachronous patterns of basin fill. With the addition of 87Sr/ 86Sr data, correlation can also be made to the Officer and Amadeus Basins, consistent with known lithostratigraphic and sequence-stratigraphic constraints. Sublittoral micritic carbonates of the upper Nuccaleena Formation (‘cap dolostone’) and lower and middle Wonoka Formation show strong basinwide 13C depletion (δ 13C carb=−3 to −10‰) despite compelling evidence for preservation of depositional δ 13C carb (such as good textural preservation, evidence for sea floor lithification, and in the Wonoka Formation, pristine 87Sr/ 86Sr and low Mn/Sr). Associated with the depleted carbonates are rare subaqueous evaporite, relatively heavy δ 13C org and anomalously 13C-depleted organic carbon of benthic microbial mat origin, and a common cause — salinity stratification of the water column — is postulated. The indicated palaeogeographic setting — an evaporative Mediterranean-like basin tenuously connected to the world ocean — is entirely consistent with previous suggestions of one or more Messinian-type desiccation events to explain very deep (1 km) incised valleys or canyons in the Adelaide Rift Complex and Officer Basin, at horizons within the Wonoka Formation and correlates. Positive excursions in δ 13C org during basin stratification may in part be due to 12C depletion of surface waters by photosynthetic carbon fixation. Despite the proposed semi-isolation of Australian Ediacarian basins from the world ocean, major δ 13C excursions — which can to some extent be differentiated by the monotonic rise in Ediacarian marine 87Sr/ 86Sr — appear to be correlatable to the upper Windermere Supergroup, northwest Canada.

The late Neoproterozoic Grassy Group of King Island, Tasmania: correlation and palaeogeographic significance

There is a very strong lithostratigraphic and δ 13C-chemostratigraphic resemblance between the well-exposed diamictite–cap dolostone–shale succession on eastern King Island, and the Marinoan glacials and lowermost Wilpena Group of the Adelaide Rift Complex. The strength of the resemblance suggests original lateral continuity of sedimentation between King Island and the Adelaide Rift Complex together with the contiguous central Australian Neoproterozoic epicratonic basins. On King Island, unlike mainland Australia, clastic sedimentation is interrupted by a thick succession of mafic, rift-related basalts and picrites at the level of the lower Brachina Formation equivalent. The evidence is consistent with King Island — and possibly the rest of western Tasmania — lying close to the Australian craton in Marinoan times, and rifting shortly after ca. 600 Ma.

Mid-Neoproterozoic biostratigraphy and isotope stratigraphy in Australia

Palynomorph and stromatolite assemblages from mid-Neoproterozoic strata of Australia appear highly significant when collated with results obtained from isotope stratigraphy, and allow correlation across the Centralian Superbasin and Adelaide Rift Complex. The majority of palynomorphs from the lower mid-Neoproterozoic (∼830 Ma) have limited biostratigraphic value, although a few apparently have restricted ranges. Carbonates at this stratigraphic level contain the distinctive stromatolite Acaciella australica and other taxa of the Acaciella australica Stromatolite Assemblage. These carbonates have δ 13C carb values of between −4 and 0‰, and associated anhydrite has 87Sr/ 86Sr ratios of 0.7069. These depleted δ 13C carb values are immediately preceded by values of 6.1‰ in the western Officer and Amadeus Basins of the Centralian Superbasin. Similar C and Sr isotopic features are seen in the similarly aged upper Shaler Supergroup of Canada. Upper mid-Neoproterozoic (∼760 Ma) carbonates contain the distinctive stromatolite Baicalia burra and other stromatolites of the Baicalia burra Stromatolite Assemblage. Siliciclastic horizons in the same interval contain the acritarch Cerebrosphaera buickii, and have δ 13C carb values of between 0 and 8‰. The highest mid-Neoproterozoic δ 13C carb values of between 6.7 and 8.1‰ occur within the stratigraphic range of C. buickii in Australia. In the western Officer Basin and Adelaide Rift Complex the mid-Neoproterozoic δ 13C carb peaks are preceded by values of ca. 0‰. The apparently restricted range of C. buickii in Australia suggests the need for reinterpretation of the stratigraphic position of its occurrence in the Svanbergfjellet Formation, Akademikerbreen Group, Spitsbergen, which was previously considered a post-Sturtian-glacial deposit.

Evolution of late proterozoic to early paleozoic Adelaide foldbelt, Australia: Comparisons with postpermian rifts and passive margins

A tectonic and sedimentary facies model is proposed to explain progressive evolution of the late Proterozoic to early Paleozoic Adelaide Rift (Geosyncline) of southern Australia. Tectonic and stratigraphic similarities are noted between the Adelaide Rift and many post Permian rifts and passive continental margins. Also the time span of the pre oceaniccrust accretion stage of the rifting process may be of the same order of magnitude, both in the Adelaide Rift and in post-Permian passive margins. These observations suggest that the underlying cause of the rifting process and the resultant crustal response have not changed significantly since late Precambrian times. More specifically the so-called “breakup unconformity”, observed in stratigraphic sequences beneath many present day passive continental margins, has been shown by various authors to correlate in time with earliest oceanic crust accretion, and it often separates underlying non-marine or paralic from fully marine shelf strata. In the Adelaide Rift, the unconformable Precambrian—Cambrian boundary is proposed as the analogue of this breakup unconformity, thereby explaining the apparently sudden influx of largely marine metazoans in Cambrian strata immediately above this unconformity.

Fractures and movement in the Adelaide rift zone

Fractures and movement in the Adelaide rift zone