ALBERT

Description: The Bolla Bollana Formation is an exceptionally thick ( ca 1500 m), rift-related sedimentary succession cropping out in the northern Flinders Ranges, South Australia, which was deposited during the Sturtian (mid Cryogenian) glaciation. Lithofacies analysis reveals three distinct facies associations which chart changing depositional styles on an ice-sourced subaqueous fan system. The diamictite facies association is dominant, and comprises both massive and stratified varieties with a range of clast compositions and textures, arranged into thick beds (1 to 20 m), representing stacked, ice-proximal glaciogenic debris flow deposits. A channel belt facies association, most commonly consisting of normally-graded conglomerates and sandstones, displays scour and fill structure of ca 10 m width and 1 to 3 m depth: these strata are interpreted as channelised turbidites. Rare mud-filled channels in this facies association bear glacially striated lonestones. Finally, a sheet heterolithics facies association contains a range of conglomerates through sandstones to silty shales arranged into clear, normally graded cycles from the lamina to bed scale. These record a variety of non-channelised turbidites, probably occupying distal and/or interchannel locations on the subaqueous fan. Coarsening and thickening-up cycles, capped by dolomicrites or mudstones, are indicative of lobe build out and abandonment, potentially as a result of ice lobe advance and stagnation. Dropstones, recognised by downwarped and punctured laminae beneath pebbles to boulders in shale, or in delicate climbing ripple cross-laminated siltstones, are clearly indicative of ice rafting. The co-occurrence of ice-rafted debris and striated lonestones strongly support a glaciogenic sediment source for the diamictites. Comparison to Pleistocene analogues enables an interpretation as a trough mouth fan, most probably deposited leeward of a palaeo-ice stream. Beyond emphasising the highly dynamic nature of Sturtian ice sheets, these interpretations testify to the oldest trough-mouth fan recorded to date. This article is protected by copyright. All rights reserved.

Description: In polyphase tectonic zones, integrating a study of fault and fracture with calcite twin analysis can determine the evolving paleo-stress magnitudes and principle stress directions that affected the area. This paper presents the results of the analyses of fractures, striated faults and calcite twins collected within the Khao Khwang Fold-Thrust Belt (KKFTB) in central Thailand (SE Asia). Here we attempt to reconstruct the orientation of the principal stresses that developed during the tectonic evolution of this highly deformed, polyphase orogen. Tectonic data were collected in the Permian carbonates of the Khao Khad Formation of the Saraburi Group, and five successive tectonic stages are determined that are interpreted to have developed before, during, and after, the Triassic Indosinian Orogeny. The first three stages pre-date the main deformation event: the first stage is interpreted as a pre-Indosinian N-S extensional stage, the second stage described a N-S strike-slip and compressional regime, largely perpendicular to the fold axes of the main structures, while the third stage is associated with an E-W compressional strike-slip phase. A further two stages took place after, or during, the main folding event and correspond to N-S compression and to an E-W composite strike-slip/contractional stage, the latter which is interpreted to represent Cenozoic deformation related to the India-Asia collision.

Description: Gondwana amalgamated along a suite of Himalayan-scale collisional orogens, the roots of which lace the continents of Africa, South America and Antarctica. The Southern Granulite Terrane of India is a generally well-exposed, exhumed, Gondwana-forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran-Cambrian (ca. 600 – 500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat Cauvery Shear System – PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high-grade shear zones within the PCSS system show that the Moyar-Salem-Attur Shear Zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike-slip motion, while the Cauvery Shear Zone (CSZ) to the south of the PCSS system can be loosely constrained to ca. 740 – 550 Ma and is associated with dip-slip dextral transpression and north-side-up motion. To the south of the proposed suture zone (the Cauvery Shear Zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D 1 – D 4 ), some of which are likely to be contemporaneous. The timing of HP-UHT metamorphism and deformation (D 1 –D 3 ) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to ca. 550 – 500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery Shear Zone during the Ediacaran/Cambrian.

Description: Abstract Madagascar is a key area for unraveling the geodynamic evolution of the transition between the Rodinia and Gondwana supercontinents as it contains several suites of c. 850–700 Ma magmatic rocks that have been postulated to correlate with other Rodinian terranes. The Bemarivo Domain of northern Madagascar contains the youngest of these units that date to c. 750–700 Ma. We present zircon Hf and O isotope data to understand northern Madagascar's place in the Neoproterozoic plate tectonic reconfiguration. We demonstrate that the northern component of the Bemarivo Domain is distinct from the southern part of the Bemarivo Domain and have therefore assigned new names—the Bobakindro Terrane and Marojejy Terrane, respectively. Magmatic rocks of the Marojejy Terrane and Anaboriana Belt are characterized by evolved εHf(t) signatures and a range of δ18O values, similar to the Imorona‐Itsindro Suite of central Madagascar. These magmatic suites likely formed together in the same long‐lived volcanic arc. In contrast, the Bobakindro Terrane contains juvenile εHf(t) and mantle‐like δ18O values, with no probable link to the rest of Madagascar. We propose that the Bobakindro Terrane formed in a juvenile arc system that included the Seychelles, the Malani Igneous Suite of northwest India, Oman, and the Yangtze Belt of south China, which at the time were all outboard from continental India and south China. The final assembly of northern Madagascar and amalgamation of the Bobakindro Terrane and Marojejy Terrane occurred along the Antsaba subduction zone, with collision occurring at c. 540 Ma.

Description: The Ediacaran Jibalah Group comprises volcano-sedimentary successions that filled small fault-bound basins along the NW-SE trending Najd fault system in the eastern Arabian-Nubian Shield. Like several other Jibalah basins, the Antaq basin contains exquisitely preserved sedimentary structures and felsic tuffs, and hence is an excellent candidate for calibrating late Ediacaran Earth history. Shallow marine strata from the upper Jibalah Group (Muraykhah Formation) contain a diversity of load structures and intimately related textured organic (microbial) surfaces, along with a fragment of a structure closely resembling an Ediacaran frond fossil and a possible specimen of Aspidella . Interspersed carbonate beds through the Muraykhah Formation record a positive δ 13 C shift from –6 to 0‰. U-Pb zircon geochronology indicates a maximum depositional age of ~570 Ma for the upper Jibalah Group, consistent with previous age estimates. Although this age overlaps with that of the upper Huqf Supergroup in nearby Oman, these sequences were deposited in contrasting tectonic settings on opposite sides of the final suture of the East African Orogen. This article is protected by copyright. All rights reserved.

Description: Understanding the evolution of the northern Paraguay Belt, Brazil, is critical in two current controversies: (i) the number, timing and significance of Ediacaran glaciations; and (ii) the timing of amalgamation of South American Gondwana. The Neoproterozoic Alto Paraguay Group forms much of the northern Paraguay Belt. The Serra Azul Formation, within this Group, contains unequivocal evidence for a glacial influence on sedimentation, including multi-directional striations on sandstone clasts and striated, polished and bullet-shaped mudstone clasts. However, the age of the Serra Azul Formation is not well-constrained. The northern Paraguay Belt also formed after the traditionally accepted time for amalgamation of South American Gondwana. If the orogen represents closure of an ocean, then this traditional view is incorrect. A significant number of single grain 40 Ar/ 39 Ar detrital muscovite cooling ages ( ca 120) from the Alto Paraguay Group are presented. The three youngest grains from the Serra Azul Formation yield a weighted mean age of 640 ± 15 My providing a robust maximum depositional age for this formation. This age, when considered with other data, suggest that the Serra Azul Formation developed in a mid-Ediacaran glaciation consistent with that expressed in the Gaskiers Formation of Newfoundland, Canada. Cryogenian 40 Ar/ 39 Ar detrital muscovite ages from the Alto Paraguay Group are hard to reconcile with the known geology of Amazonia and are interpreted as being sourced from the evolving orogen to the east – from an arc terrane, possibly the Goiás-Paranapanema Massif. Detrital muscovites in the upper part of the Alto Paraguay Group are as young as 544 ± 7 My, consistent with mounting evidence that indicate a Cambrian age for orogenesis within the Paraguay Belt during the final amalgamation of Gondwana. This article suggests that the data best support a model where ocean closure in the region continued until Ediacaran/Cambrian times, with final ocean closure represented by orogenesis in the Paraguay–Araguaia orogen. This article is protected by copyright. All rights reserved.

Description: Multi-method thermochronology along the Vakhsh-Surkhob fault zone reveals the thermotectonic history of the South Tian Shan–Pamirs boundary. Apatite U/Pb analyses yield a consistent age of 251 ± 2 Ma, corresponding to cooling below ~550–350°C, related to the final closure of the Palaeo-Asian Ocean and contemporaneous magmatism in the South Tian Shan. Zircon (U–Th–Sm)/He ages constrain cooling below ~180°C to the end of the Triassic (~200 Ma), likely related either to deformation induced by the Qiangtang collision or to the closure of the Rushan Ocean. Apatite fission track thermochronology reveals two low-temperature (〈120°C) thermal events at ~25 Ma and ~10 Ma, which may be correlated with tectonic activity at the distant southern Eurasian margin. The late Miocene cooling is confirmed by apatite (U–Th–Sm)/He data and marks the onset of mountain building within the South Tian Shan that is ongoing today. This article is protected by copyright. All rights reserved.

Description: Abstract The Tianshan and Altai mountain belts dominate the present‐day topography of Central Asia, and are major constituents of the Central Asian Orogenic Belt. These mountain belts have been extensively studied using low‐temperature thermochronology to understand their uplift and exhumation history. In comparison, few studies have focussed on the spatial extent and timing of intracontinental deformation in the intervening ranges, such as the West Junggar Mountains. Apatite fission track (AFT) and apatite U–Pb dating of igneous samples from the West Junggar Mountains reveals AFT central ages ranging from latest Carboniferous to Middle Jurassic, and mean confined track lengths between 14.1 and 12.0 μm, which are negatively correlating to AFT central ages. Apatite U–Pb dating produced mid‐Carboniferous–early Permian ages within uncertainty of zircon U–Pb crystallization ages, indicating cooling to below ~450–550 °C immediately following emplacement. Thermal history modelling of our AFT data combined with published (U–Th)/He data predicts rapid cooling to 〈60 °C during the early–middle Permian (285–260 Ma), followed by slower cooling rates during the Mesozoic–Cenozoic. The rapid cooling event coincides with (1) the timing of major basin‐ward thrusting in the Junggar Basin adjacent to the study area and (2) exhumation in the Chinese Altai to the north. In contrast to previous studies, our results imply that the West Junggar Mountains do not record significant exhumation during the Mesozoic–Cenozoic. This suggests that Mesozoic deformation of the Central Asian Orogenic Belt was less pervasive and widespread than previously assumed.