Steven D. Boger

Early Paleozoic tectonism within the East Antarctic craton: The final suture between east and west Gondwana?

New U-Pb SHRIMP ages from East Antarctica point to the existence of a laterally continuous orogenic belt that bisects the East Antarctic craton. This orogenic belt juxtaposes Archean crust to the south and east against Neoproterozoic metamorphic rocks to the north and west. It defines the margin of a separate lithospheric block that consists of a large section of East Antarctica and India that did not form part of east Gondwana or Rodinia as they are currently reconstructed. Instead, this Indo-Antarctic continent accreted with west Gondwana along the Mozambique suture shortly before collision and suturing along a second “Pan-African” suture now cropping out in the southern Prince Charles Mountains and Prydz Bay regions of Antarctica. This scenario is consistent with (1) the abrupt termination of ca. 990–900 Ma tectonism recognized in the northern Prince Charles Mountains–Rayner Complex–Eastern Ghats against Paleozoic orogenic belts, (2) the lack of terranes of equivalent age found elsewhere in either Antarctica or other previously adjacent continents, and (3) the distinct detrital-zircon populations obtained from either side of this proposed suture.

Terminal suturing of Gondwana and the onset of the Ross-Delamerian Orogeny : the cause and effect of an Early Cambrian reconfiguration of plate motions

Abstract A review of U–Pb age data from intrusive and volcanic rocks for Antarctica and Australia has highlighted the remarkable synchronicity in the shift from passive margin or continental arc tectonism, to convergent orogenesis along the Pacific margin of Gondwana. This event, the Ross–Delamerian Orogeny, marks a distinct and rapid shift in the style of deformation, rate of uplift, volume of magmatism and syn-orogenic sedimentation along the entire 4000+ km length of the orogen. The Ross–Delamerian Orogeny is constrained to have begun at 515±5 Ma, an interval that also encompasses the youngest collisional events that lead to the suturing of Gondwana. We propose that these geographically separate events are intimately linked and represent the cause and effect of a major tectonic reorganisation in the motion of the Earth’s plates. We suggest the mechanism controlling this Early to Middle Cambrian plate reorganisation was the rapid reduction in relative motion, driven by continental collision, between the final two components of Gondwana. Due to the need to maintain a zero sum for all plate motions, the change in the rate of convergence at this boundary resulted in compensatory changes elsewhere in the global plate circuit. One such change was an increase in the rate and perhaps direction of convergence along the Pacific margin of Gondwana.

Neoproterozoic deformation in the Radok Lake region of the northern Prince Charles Mountains, east Antarctica; evidence for a single protracted orogenic event

Abstract Ion microprobe dating of structurally constrained felsic intrusives indicate that the rocks of the northern Prince Charles Mountains (nPCMs) were deformed during a single, long-lived Neoproterozoic tectonic event. Deformation evolved through four progressively more discrete phases in response to continuous north–south directed compression. In the study area (Radok Lake), voluminous granite intrusion occurred at ∼990 Ma, contemporaneous with regionally extensive magmatism, peak metamorphism, and sub-horizontal shearing and recumbent folding. Subsequent upright folding and shear zone development occurred at ∼940 Ma, while new zircon growth at ∼900 Ma constrains a final phase of deformation that was accommodated along low-angle mylonites and pseudotachylites. This final period of deformation was responsible for the allochthonous emplacement of granulites over mid-amphibolite facies rocks in the nPCMs. The age constraints placed on the timing of deformation by this study preclude the high-grade reworking of the nPCMs as is postulated in some of the recent literature. Furthermore, 990–900 Ma orogenesis in the nPCMs is at least 50 Myr younger than that recognised in other previously correlated Grenville aged orogenic belts found in Australia, east Africa and other parts of the Antarctic. This distinct age difference implies that these belts are probably not correlatable, as has been previously suggested in reconstructions of the supercontinent Rodinia.

Pan-African intraplate deformation in the northern Prince Charles Mountains, east Antarctica

New structural and metamorphic data coupled with U–Pb SHRIMP zircon and Rb–Sr step-leach biotite ages help constrain a period of Early Palaeozoic (Pan-African) deformation recognised in the northern Prince Charles Mountains, east Antarctica. This period of deformation is accommodated along discrete northeast trending mylonites that preserve up-dip reverse kinematics with dominantly southeast over northwest vergence. Ambient P–T conditions of 524±20°C and 7.6±4 kbar accompanied deformation. This phase of deformation post-dated the intrusion of planar felsic dykes that yield ages of c. 550 Ma and pre-dated Rb–Sr biotite ages of c. 475 Ma that record cooling of the terrane below c. 300°C. These mylonites are identical in age to continental collisional events recognised in the southern Prince Charles Mountains and Prydz Bay, which lie to the south and east of the northern Prince Charles Mountains, and similar in age to orogenesis recognised to the west in Lutzow-Holm Bay. These belts represent sutures between the component lithospheric blocks of east and west Gondwana. The northern Prince Charles Mountains lie between these sutures. Consequently, the mylonites we report here are interpreted to have formed in an intraplate setting and developed in response to stresses applied along the plated margins as a consequence of continental collision during the amalgamation of Gondwana.

Hot rocks in a cold place: high sub-glacial heat flow in East Antarctica

Numerical models are the primary predictive tools for understanding the dynamic behaviour of the Antarctic ice sheet. However, a key boundary parameter, sub-glacial heat flow, remains poorly constrained. We show that variations in abundance and distribution of heat-producing elements within the Antarctic continental crust result in greater and more variable regional sub-glacial heat flows than currently assumed in ice modelling studies. Such elevated heat flows would have a fundamental effect on ice sheet behaviour and highlight that geological controls on heat flow must be considered to obtain more accurate and refined predictions of ice mass balance and sea-level change. Supplementary materials: Heat flow and heat production definitions, heat production data, and details of the 2D model of the gross geometry of the continental lithosphere for a section through Prydz Bay are available at www.geolsoc.org.uk/SUP18690.

Brittle faulting in the Prince Charles Mountains, East Antarctica: Cretaceous transtensional tectonics related to the break-up of Gondwana

Abstract The Lambert Glacier–Amery Ice Shelf occupies a narrow NNE–SSW-orientated fault-bound depression referred to as the Lambert Graben. Deep faults associated with this structure are recognised geophysically, and are interpreted to extend at least 700 km inland from the Antarctic coast. Kinematic and palaeostress data from quartz- and calcite-bearing faults, inferred to represent the surface expression of these deeper structures, suggest that a single faulting event occurred in response to NW–SE-directed extension, oblique to the axis of the graben. The bulk of the movement along these faults was dextral strike slip, accommodating components of both normal and reverse offset. In the northern Prince Charles Mountains, these faults disrupt the Permo-Triassic Amery Group and juxtapose it against Proterozoic basement. Equivalent strike-slip faults in the southern Prince Charles Mountains produce dextrally offset tectonic boundaries and metamorphic isogrades across the Lambert Glacier. The similarity in orientation between the palaeostress field calculated for these faults and the Cretaceous divergence vector between India and Antarctica strongly supports the inference that faulting was of Cretaceous age, and related to the break-up of Gondwana.

Detrital zircon U–Pb and 40Ar/39Ar hornblende ages from the Aileu Complex, Timor-Leste: provenance and metamorphic cooling history

Geochronological data from the Aileu Complex provide new constraints on the development of the Banda Arc–continent collision. Detrital zircons of the Aileu Complex have major U–Pb age modes at 270–440 Ma, 860–1240 Ma and 1460–1870 Ma, most compatible with a sediment source located in SE Asia as part of the now fragmented Sula Spur. 40Ar/39Ar cooling ages of hornblende demonstrate an extended cooling history across the complex, with the eastern parts cooling through hornblende closure temperature by 10 Ma and central parts by 6 Ma, consistent with a variable exhumation history. The onset of cooling by 10 Ma implies that metamorphism was probably coeval with initiation of the Banda Arc. We propose that the Aileu Complex cooling ages record deformation related to fragmentation of the Sula Spur and early development of the Banda Arc, rather than collision between the Australian continent and the Banda Arc. Supplementary material: Detrital zircon U–Pb analytical results, hornblende argon isotope geochronology sample details, hornblende 40Ar/39Ar step heating analytical results and hornblende electron microprobe analytical results are available at www.geolsoc.org.uk/SUP18702.

Pb isotopic domains from the Indian Ocean sector of Antarctica: implications for past Antarctica–India connections

Abstract New feldspar lead isotope compositions of crystalline rocks from the Indian Ocean sector of East Antarctica, in conjunction with the review of data from elsewhere within the continent and from continents formerly adjacent within Gondwana, refine boundaries and evolutionary histories of terranes previously inferred from geological mapping and complementary isotope studies. Coastal Archaean Vestfold and Napier complexes have overlapping compositions and had Pb isotopes homogenized at 2.5 Ga sourced from or within already fractionated protoliths with high and variable U–Pb. Identical compositions from the Dharwar Craton of India support a correlation with these Antarctic terranes. The Proterozoic–Palaeozoic Rayner Complex and Prydz Belt yield more radiogenic compositions and are broadly similar and strongly suggest these units correlate with parts of the Eastern Ghats Belt of India. A strikingly different signature is evident from the inboard Ruker Complex, which yielded unradiogenic compositions. This complex is unlike any unit within India or Australia, suggesting that these rocks represent exposures of an Antarctic (Crohn) Craton. Compositions from the enigmatic Rauer Terrane are consistent with a shared early history with the Ruker Complex but with a different post-Archaean evolution. Supplementary material: Feldspar LA-ICP-MS Pb isotope data are available at www.geolsoc.org.uk/SUP18622

Boron- and phosphate-rich rocks in the Larsemann Hills, Prydz Bay, East Antarctica: tectonic implications

Abstract Granulite-facies paragneisses enriched in boron and phosphorus are exposed over c. 15×5 km2 in the Larsemann Hills, Antarctica. The most widespread are biotite gneisses containing centimetre-sized prismatine crystals, but tourmaline metaquartzite and borosilicate gneisses are richest in B (676–19 700 µg/g or 0.22–6.34 wt%; B2O3). Chondrite-normalized rare-earth element (REE) patterns give two groups: (1) LaN>150, Eu*/Eu<0.4, which comprises most apatite-bearing metaquartzite and metapelite, tourmaline metaquartzite, and Fe-rich rocks (up to 2.3 wt%; P2O5); (2) LaN<150, Eu*/Eu > 0.4, which comprises most borosilicate and sodic leucogneisses (2.5–7.4wt%; Na2O). Enrichment in boron and phosphorus is attributed to premetamorphic hydrothermal alteration, either in a rifted, most likely marine basin or in a mud volcanic system located inboard of a c. 1000 Ma continental arc that was active along the leading edge of the Indo-Antarctic craton. This margin developed before collision with the Australo-Antarctic craton (c. 530 Ma) merged these rocks into Gondwana and sutured them into their present position in Antarctica. Rocks lithologically similar to those in the Larsemann Hills include prismatine-bearing granulites in the Windmill Islands, Wilkes Land, and tourmaline–quartz rocks, sodic gneisses and apatitic iron formation in the Willyama Supergroup, Broken Hill, Australia.

Provenance of the Upper Cretaceous to Lower Tertiary Sedimentary Relicts in the Renbu Mélange Zone, within the Indus-Yarlung Suture Zone

The Upper Cretaceous to Lower Tertiary sediments in the Indus-Yarlung suture zone provide critical records on the history of accretion along the southern margin of Asia prior to, and during, the India-Asia collision. In this article, we report field and petrographic observations, in situ detrital zircon U-Pb ages, Lu-Hf isotopic analyses, and Cr-spinel electron microprobe data from the Upper Cretaceous to Lower Tertiary sedimentary rocks of the Renbu mélange zone in east Xigaze, southern Tibet. The Renbu mélange zone consists of two serpentinite mélange subzones separated by a mud-matrix mélange. Using similarities found in the compositions of detrital Cr-spinels and detrital zircon U-Pb ages, we propose to correlate the northern Renbu mélange subzone with the upper part of the Xigaze forearc basin. Detrital zircons from sandstones in the southern Renbu mélange subzone indicate an influx of Cretaceous–early Cenozoic zircon grains with juvenile Hf isotopic compositions, suggesting a provenance from the Lhasa terrane, especially the Gangdese arc. Compared to the mélange in the western Xigaze, our new results show that much younger sediments were deposited on the top of the accretionary wedge, with the youngest U-Pb age cluster including six single grains in the range of 64–53 Ma and a large peak at 67 Ma. Our results support the idea that the foreland basin developed along the Indus-Yarlung suture during the India-Asia collision.