Evidence of micro-continent entrainment during crustal accretion

Peer reviewedPublisher PD

Provenance of rifted continental crust at the nexus of East Gondwana breakup

The Bruce Rise, a prominent bathymetric feature offshore the Bunger Hills (East Antarctica), has basement of unknown crustal affinity and age. In East Gondwana, the Bruce Rise is reconstructed near the Naturaliste Plateau (offshore SW Australia) and microcontinents now submerged in the eastern Indian Ocean. New zircon U-Pb-Hf data from two c. 1150 Ma granite samples dredged from the eastern escarpment of the Bruce Rise demonstrate that these rocks are dominated by xenocrystic cargo. Mesoproterozoic xenocrystic cores show textural evidence of melt-mediated coupled dissolution-precipitation to form rim domains with apparent ages that skew towards c. 1150 Ma. The zircon U-Pb-Hf signatures from the xenocrysts in the Bruce Rise granites, and from c. 1230 to 1180 Ma felsic-intermediate granites and orthogneisses from the conjugate Naturaliste Plateau basement, suggest late Mesoproterozoic magmatism occurred at a transition in the regional tectonic architecture between a reworked Archean cratonic margin and Proterozoic juvenile crust. On the basis of plate reconstructions, exhumation and thinning of the basement to the Bruce Rise/Naturaliste Plateau occurred predominantly during rifting of India (prior to c. 120 Ma). Minor further thinning likely occurred leading up to the onset of seafloor spreading between Australia/Naturaliste Plateau and Antarctica/Bruce Rise (from c. 90 to 84 Ma)

Linking mainland Australia and Tasmania using ambient seismic noise tomography: Implications for the tectonic evolution of the east Gondwana margin

For nearly half a century, a number of conflicting tectonic models have been postulated to explain the enigmatic geological relationship between Tasmania and Victoria, with a view to unifying our understanding of the evolution of the eastern margin of Gondwana in Australia. In this study, ambient noise data from an array of 24 broadband seismometers is used to produce a high-resolution 3-D crustal shear wave velocity model of Bass Strait, the key to understanding the missing link. We apply a novel transdimensional and hierarchical Bayesian inversion approach to construct group velocity maps in the period range of 2-30. s, and subsequently invert group velocity dispersion for 3-D shear wave velocity structure. This allows us to image, for the first time, the entire crust beneath Bass Strait in high detail and elucidate the geometry and position of key crustal features with corroboration from complementary datasets. The three sedimentary basins related to the failed rifting event associated with the Australia-Antarctica breakup, in particular Bass Basin, clearly emerge from the tomographic solution model. A key feature of the 3-D shear wavespeed model is a distinct mid-lower crustal NW-SE high velocity zone which extends from northwestern Tasmania to south-central Victoria, confirming a Proterozoic geological connection. We also image three north-south high velocity belts that appear to span Bass Strait, with some interruption from velocity variations possibly related to more recent tectonic events. These belts are consistent with recent gravity and magnetic maps, and may indicate the presence of an exotic Precambrian terrane (the Selwyn Block). The model also images the crustal velocity structure of the southern Stawell and Bendigo Zones, and their internal large-scale multi-layer characters, a legacy of their Early Paleozoic intra-oceanic origins. Another high velocity anomaly imaged in the mid-lower crust is an east-west lineament beneath the northern part of Bass Strait, which may be an intrusive feature associated with the failed rift

Pedo-geophysics teaching and research in the Adelaide hills

Please see page 6 of PDF for this item.Graham Heinson, Nick Direen, Mark Thomas, Andrew Baker, Rob Fitzpatrick, Patrick James, Brendan Coleman, Matthew Hutchens, Hashim Carey and the 3rd year Mineral and Environmental Geophysics Clas

A crustal‐scale view at rift localization along the fossil Adriatic margin of the Alpine Tethys preserved in NW Italy.

International audienceFossil rifted margins, whereby originally extended continental crust is subsequently stacked in orogenic belts, provide the opportunity to track rift-related tectonics across different crustal levels. In this study, the tectonothermal evolution of the fossil Adriatic continental margin, sampled in the Italian Southern Alps, is investigated combining new (U-Th)/He zircon (ZHe) thermochronology from upper crustal rocks with existing data from the originally underlying lower crust, to shed light on the processes responsible for rift localization in the Alpine Tethys system. The Adriatic microplate records a protracted rift evolution, whereby distributed upper crustal stretching at 245-190 Ma was followed by rift localization along its future western edge, culminating in mantle exhumation at 165-160 Ma. A progressive westward younging of ZHe ages, from 280-240 Ma in the Lombardian Basin to 215-200 Ma near the Sostegno and Fenera Basins, indicates that anomalously high thermal gradients were established in the Late Triassic in the area where rifting later localized. The inferred episodic heating was contemporaneous with protracted fluid flow, minor magmatism, and ductile shearing within the originally underlying lower crust. Subsequent normal faulting was initiated post-185Ma, as constrained by exhumation-related ZHe ages in detrital zircons from a syntectonic sandstone. The spatial distribution of the detected heating-cooling cycle suggests that rift localization along the western edge of the Adriatic Plate was probably favored by a crustal-scale thermal anomaly, established at 215-210 Ma, followed by thermal decay by 200-190 Ma. Subsequent crust-wide extension, starting at 185-180 Ma, led to excision of continental crust and mantle exhumation