Geology & Geophysics publications

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2.5-D modelling of seismic wave propagation: Boundary condition, stability criterion and efficiency
(1998) Cao, S.; Greenhalgh, Stewart A.; School of Earth and Environmental Sciences : Geology and Geophysics
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3-D, 40Ar-39Ar geochronology in the Paranà continental flood basalt province
(1996) Stewart, Kathy; Turner, Simon; Kelley, Simon P.; Hawkesworth, Chris; Kirstein, Linda; Mantovani, Marta
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3D geological modeling of the Trujillo block: insights for crustal escape models of the Venezuelan Andes
(Elsevier, 2012) Dhont, D.; Monod, B.; Hervouët, Y.; Backé, G.; Klarica, S.; Choy, J.
The Venezuelan Andes form a N50°E-trending mountain belt extending from the Colombian border in the SW to the Caribbean Sea in the NE. The belt began to rise since the Middle Miocene in response to the E-W collision between the Maracaibo block to the NW and the Guyana shield belonging to South America to the SE. This oblique collision led to strain partitioning with (1) shortening along opposite-vergent thrust fronts, (2) right-lateral slip along the Boconó fault crossing the belt more or less along-strike and (3) crustal escape of the Trujillo block moving towards the NE in between the Boconó fault and the N-S-striking left-lateral Valera fault. The geology of the Venezuelan Andes is well described at the surface, but its structure at depth remains hypothetic. We investigated the deep geometry of the Mérida Andes by a 3D model newly developed from geological and geophysical data. The 3D fault model is restricted to the crust and is mainly based on the surface data of outcropping fault traces. The final model reveals the orogenic float concept where the mountain belt is decoupled from its underlying lithosphere over a horizontal décollement located either at the upper/lower crust boundary. The reconstruction of the Boconó and Valera faults results in a 3D shape of the Trujillo block, which floats over a mid-crustal décollement horizon emerging at the Boconó-Valera triple junction. Motion of the Trujillo block is accompanied by a widespread extension towards the NE accommodated by normal faults with listric geometries such as for the Motatan, Momboy and Tuñame faults. Extension is explained by the gravitational spreading of the upper crust during the escape process. © 2012 Elsevier Ltd.
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3D meteoroid trajectories
(Elsevier, 2019) Sansom, E.; Jansen-Sturgeon, T.; Rutten, M.G.; Devillepoix, H.; Bland, P.; Howie, R.; Cox, M.; Towner, M.; Cupák, M.; Hartig, B.
Meteoroid modelling of fireball data typically uses a one dimensional model along a straight line triangulated trajectory. The assumption of a straight line trajectory has been considered an acceptable simplification for fireballs, but it has not been rigorously tested. The unique capability of the Desert Fireball Network (DFN) to triangulate discrete observation times gives the opportunity to investigate the deviation of a meteoroid’s position to different model fits. Here we assess the viability of a straight line assumption for fireball data in two meteorite-dropping test cases observed by the Desert Fireball Network (DFN) in Australia – one over 21 s (DN151212_03), one under 5 seconds (DN160410_03). We show that a straight line is not valid for these two meteorite dropping events and propose a three dimensional particle filter to model meteoroid positions without any straight line constraints. The single body equations in three dimensions, along with the luminosity equation, are applied to the particle filter methodology described by Sansom et al. (2017). Modelling fireball camera network data in three dimensions has not previously been attempted. This allows the raw astrometric, line-of-sight observations to be incorporated directly. In analysing these two DFN events, the triangulated positions based on a straight line assumption result in the modelled meteoroid positions diverging up to 3.09 km from the calculated observed point (for DN151212_03). Even for the more typical fireball event, DN160410_03, we see a divergence of up to 360 m. As DFN observations are typically precise to  < 100 m, it is apparent that the assumption of a straight line is an oversimplification that will affect orbit calculations and meteorite search regions for a significant fraction of events. Previous article in issue
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3D seismic analysis investigating the relationship between stratigraphic architecture and structural activity in the intra-cratonic Cooper and Eromanga basins, Australia
(Elsevier, 2018) Kulikowski, D.; Amrouch, K.
This research uses four three-dimensional (3D) seismic surveys located in Australia's largest onshore hydrocarbon province, the intra-cratonic Cooper and Eromanga basins, to present an approach that extracts important structural and stratigraphic information from geophysical data that can then be used to define the tectonostratigraphic evolution of subsurface provinces. The methodology consists of: (1) analysing isopach maps; (2) cross-section interpretation of stratigraphic features, erosional surfaces, and faults; and (3) constraining the evolution of fault activity. Most faults within this province are basement-involved with high dip angles. The primary fault set is NE-SW striking, with secondary sets striking N-S, E-W and NW-SE. These high angle faults most likely developed as normal faults before being reactivated by five of the six major tectonic events. Field scale NW-SE strike-slip faults are prolific and can often be overlooked due to the low seismic resolution. A close relationship between on-lapping features and present-day structural highs was found during each of the major structural events, particularly within hydrocarbon-rich Permian stratigraphy, inferring that present-day structures were present throughout basin development and intermittently reactivated. Significant stratal-package thinning, and a high presence of on-lapping features, were associated with regional basement-involved faults, particularly along the Gidgealpa-Merrimelia-Innamincka and Murteree-Nappacoongee ridges. Initial structural trap development occurred during the early Permian, but was most significant during the Late Triassic. Hydrocarbon accumulations were unaffected by structural growth after the critical moment in the petroleum system (90 Ma), as the final period of fault activity was during the Late Cretaceous. This research constrains the tectonostratigraphic evolution of the intra-cratonic Cooper and Eromanga basins, while detailing an approach that extracts and analyses important structural and stratigraphic information from geophysical data, where outcrop is not accessible.
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50 years and worlds apart: rethinking the Holocene occupation of Cloggs Cave (East Gippsland, SE Australia) five decades after its initial archaeological excavation and in light of GunaiKurnai world views
(Taylor & Francis (Routledge), 2021) David, B.; Fresløv, J.; Mullett, R.; Delannoy, J.J.; McDowell, M.; Urwin, C.; Mialanes, J.; Petchey, F.; Wood, R.; Russell, L.; Arnold, L.J.; Stephenson, B.; Fullagar, R.; Crouch, J.; Ash, J.; Berthet, J.; Wong, V.N.L.; Green, H.
In this paper we report on new research at the iconic archaeological site of Cloggs Cave (GunaiKurnai Country), in the southern foothills of SE Australia’s Great Dividing Range. Detailed chronometric dating, combined with high-resolution 3D mapping, geomorphological studies and archaeological excavations, now allow a dense sequence of Late Holocene ash layers and their contents to be correlated with GunaiKurnai ethnography and current knowledge. These results suggest a critical re-interpretation of what the Old People were, and were not, doing in Cloggs Cave during the Late Holocene. Instead of a lack of Late Holocene cave occupation, as previously thought through the conceptual lens of ‘habitat and economy’, Cloggs Cave is now understood to have been actively used for special, magical purposes. Configured by local GunaiKurnai cosmology, cave landscapes (including Cloggs Cave's) were populated not only by food species animals, but also by ‘supernatural’ Beings and forces whose presence helped inform occupational patterns. The profound differences between the old and new archaeological interpretations of Cloggs Cave, separated by five decades of developing archaeological thought and technical advances, draw attention to archaeological meaning-making and highlight the significance of data capture and the pre-conceptions that shape the production of archaeological stories and identities of place.
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50 years of different landscape management influencing retention of metals in soils
(Elsevier, 2012) Chrastný, V.; Komárek, M.; Procházka, J.; Pechar, L.; Vaněk, A.; Penížek, V.; Farkaš, J.
Abstract not available
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A 3D lithospheric electrical resistivity model of the Gawler Craton, Southern Australia
(Inst Mining Metallurgy, 2007) Maier, R.; Heinson, G.; Thiel, S.; Selway, K.; Gill, R.; Scroggs, M.
A three-dimensional lithospheric electrical resistivity model of the Archaean-Proterozoic Gawler Craton in southern Australia has been developed, to define the tectonic framework of the craton and identify craton margins under regolith cover. Knowledge of cratonic margins is important as upwards of 60% of known mineral wealth is located in palaeoconvergent zones between ancient cratons. The research was conducted in three phases. First, all previous magnetotelluric (MT) and geomagnetic depth sounding (GDS) data were compiled to establish a regional scale induction database. Almost 400 observation sites were found in the literature and from personal communication, collected over the last 30 years by various research groups. Of these, most measurements were GDS only, but recent 2D MT sites along major profiles provide further depth constraints. Second, eight long period MT sites were collected from central areas of the craton to provide a window into the deeper lithosphere and asthenosphere. Data were collected over a number of weeks to establish high quality MT responses in the bandwidth 10-10⁴ s. One-dimensional modelling indicated that the lithosphere was resistive (>100 Ω m), with a more conductive midmantle at 70 km and a decrease in resistivity at the transition zone. These MIT responses agree with the long period (10⁴-10⁷ s) continental lithosphere induction responses of Olsen (N. Olsen: 'Long-period (30 days-1 year) electromagnetic sounding and the electrical conductivity of the lower mantle beneath Europe', Geophys. J. Int., 1999, 138, 179-187). Finally, a 3D resistivity model of the lithosphere was compiled from 2D inversions of MT profiles, with constraints from lithosphere asthenosphere soundings, and regional scale GDS data. The Gawler Craton is shown to have a resistive core of dimension 500 km, surrounded by more conductive terrains. The core is largely, but not exclusively, Archaean crust, surrounded by conductive Proterozoic crust. The edge of the core on the eastern side of the Gawler Craton correlates with the location of several iron oxide-copper-gold deposits, including the world class Olympic Dam deposit, potentially making mapping of large scale resistivity structures a useful tool for regional scale exploration. © 2007 Institute of Materials, Minerals and Mining and The AusIMM.
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A chemostratigraphic overview of the late Cryogenian interglacial sequence in the Adelaide Fold-Thrust Belt, South Australia
(Elsevier Science BV, 2001) McKirdy, D.; Burgess, J.; Lemon, N.; Yu, X.; Cooper, A.; Gostin, V.; Jenkins, R.; Both, R.
Within the Neoproterozoic sequence of the Adelaide Fold-Thrust Belt, South Australia, two of the most severe ice ages in Earth history (Sturtian and Marinoan) are recorded in the glacigenic rocks which mark the base and top of the 4.5 km-thick Umberatana Group, the focus of the present chemostratigraphic study. Rock units from three drillholes and ten measured outcrop sections, located in the southern and central Flinders Ranges and on the Stuart Shelf, were sampled for isotopic analysis of their co-existing calcite (δ¹³Ccal), dolomite (δ¹³Cdol) and kerogen (δ¹³Corg). Strontium and sulphur isotopic analyses were also run on selected samples. The sample sites are in tectonic domains where regional metamorphism has been insufficient to significantly alter the isotopic composition of the kerogen. The resulting data, when integrated with the lithostratigraphy of the Umberatana Group, yield one of the most complete records of secular variation in the C-isotope signature of the late Cryogenian ocean between ∼ 750 and 680 Ma. Its composite δ¹³Ccarb curve begins with a sharp positive excursion (-3 to + 1.5‰ in the dolomitic Tindelpina Shale which caps Sturtian glacial diamictites (Wilyerpa Formation). Then follows a steady climb through the shoaling upward Tapley Hill Formation reaching +4‰ in the Brighton Limestone and culminating in a plateau of δ¹³C enrichment within the platformal carbonates of the Etina Formation (+ 8 to + 10‰). The upper part of the temporal trend shows a decline to + 6 to + 7‰ in the stromatolite reef facies of the Enorama Shale; and a dramatic drop to -8 to -9.5‰, followed by a recovery to -3‰, in the youngest carbonate unit (Trezona Formation) beneath the Marinoan glacigenic Elatina Formation. The latter is capped by a thin dolostone, the Nuccaleena Formation, which records a consistent up-section decrease in δ¹³C (- 1 to -3‰). Superimposed on this overall pattern of C-isotopic variation are second and third-order fluctuations attributable to eustatic sea-level change. Pyritic sulphur in the lower Tapley Hill Formation (δ³⁴S = + 9 to + 40‰) is distinctly heavier than that in the underlying Wilyerpa Formation (+ 4 to + 20‰). Minimum ⁸⁷Sr/⁸⁶Sr values obtained from the Brighton Limestone (0.7071) and Etina Formation (0.7076) are consistent with their late Cryogenian age. This Sturtian to Marinoan interglacial succession is more complete than equivalent sequences elsewhere, and thus provides a useful reference for global chemostratigraphic correlation. Notable features of its C-isotope curve are the strong post-Wilyerpa positive excursion, also seen in key sections from Svalbard and western Mongolia; the 'Etina plateau' of ¹³C enrichment which is remarkably similar to maxima in the terminal Proterozoic C-isotope records of western Mongolia, north-western Canada and Brazil; and the spectacular depletion of ¹³C in the lower Trezona relative to similar carbonate facies in the underlying Etina Formation and Enorama Shale. A preglacial offset in δ¹³Ccarb values of such proportions (∼ 14‰) is almost unprecedented, except perhaps for that recently documented in the upper Ombaatjie Formation beneath tillites of the Ghaub Formation, Otavi Group, Namibia. It represents a key piece of evidence in the case presented by the proponents of a Neoproterozoic `snowball' Earth. © 2001 Elsevier Science B.V.
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A comparative study of five reference materials and the Lombard meteorite for the determination of the platinum-group elements and gold by LA-ICP-MS
(Wiley Online Library, 2013) Gilbert, S.; Danyushevsky, L.; Robinson, P.; Wohlgemuth-Ueberwasser, C.; Pearson, N.; Savard, D.; Norman, M.; Hanley, J.
A range of independently characterised reference materials (RMs) for LA‐ICP‐MS, used for the determination of the platinum‐group elements (PGE) and Au in a sulfide matrix, were analysed and compared: 8b, PGE‐A, NiS‐3, Po727‐T1, Po724‐T and the Lombard meteorite. The newly developed RM NiS‐3 was used as the RM for the calibration of all LA‐ICP‐MS analyses and the measured concentrations of the other RMs compared against their published concentrations. This data were also used to assess the consistency of concentrations calibrated against the different RMs. It was found that Po727‐T1 and 8b produced results that were comparable, within uncertainty, for all elements. Po727‐T1 also produced consistent results with NiS‐3 for all elements. All other RMs showed differences for some elements, especially Ru in Po724‐T, and Os, Ir and Au in PGE‐A. The homogeneity of the PGE and Au in each RM was assessed, by comparing the precision of multiple LA‐ICP‐MS spot analyses with the average uncertainty of the signal. Po724‐T, Po727‐T1 and the Lombard meteorite were found to be homogeneous for all elements, but 8b, PGE‐A and NiS‐3 were heterogeneous for some elements. This is the first direct comparison between a range of independently characterised PGE and Au LA‐ICP‐MS RMs.
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A comparative study of in-river geophysical techniques to define variations in riverbed salt load and aid managing river salinization
(Soc Exploration Geophysicists, 2010) Hatch, M.; Munday, T.; Heinson, G.
Increased interest in the character of sediments at the base of waterways, for the purpose of managing river salinization, has led to the application of several geophysical techniques for collecting information from this zone. These instream methods are based on established ground and airborne electrical and electromagnetic technologies, including towed transient electromagnetic systems, towed direct current resistivity array systems, and frequency-domain helicopter electromagnetic systems. Although these systems are individually successful, a systematic examination of their relative effectiveness for identifying variations in substrate conductivity for a common stretch of a river remains lacking. We have compared results obtained from data collected using three instream geophysical techniques for a common stretch of the Murray River in southeastern Australia. The Murray River is an important water resource for drinking and agricultural purposes. Data from these surveys were acquired tolocate areas of significant saltwater accession to the Murray from a saline regional groundwater system that discharges into it. The three methods indirectly inform on those reaches that most likely contribute to higher salt loads in the river, and they do this through the identification of a conductive substrate (a gaining reach). For a [Formula: see text] stretch of the river, the methods identified similar variations in the conductivity structure of sediment substrate, although differences were observed in the modeled response relating to intrinsic differences between each system, including the sampling interval and resolution. The helicopter electromagnetic (EM) system is capable of acquiring hundreds of kil-ometers of data in a day, under any river flow condition, from near the river surface to depths in excess of [Formula: see text]. The other two techniques require safe river flow conditions for acquisition, with as much as 50 river km of data per day possible. The ground-based methods had enhanced lateral and vertical resolving capabilities relative to the helicopter EM system, but their depth of investigation was less (usually only [Formula: see text]).
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A comparison of diffraction imaging to incoherence and curvature
(Society of Exploration Geophysicists, 2016) Tyiasning, S.; Merzlikin, D.; Cooke, D.; Fomel, S.
Detecting small subsurface features such as faults, fracture swarms, steep reef edges, and channel edges is a routine task in seismic interpretation. Common seismic attributes used for this task, such as curvature and incoherence, are derived from conventional images containing both reflections and diffractions. An emerging alternative approach is to interpret these edge features using the diffraction-imaging technique, which images diffracted energy separately. A diffraction-imaging volume is generated by separating specular reflections from diffractions in an unmigrated volume followed by migration of those diffractions. Different methods can be used to separate diffractions from reflections on unmigrated data. In a case history, they are separated with a plane-wave destruction (PWD) filter. An evaluation of diffraction-imaging performance is applied to a 3D seismic data set from the Cooper Basin of Australia. Diffraction-imaging horizon maps are compared with corresponding maps made from incoherence and curvature volumes. The resolution and the ability to detect faults, fractures, and channel-edge features are compared. The results show that diffraction imaging provides superior vertical and spatial resolution over conventional incoherence and curvature attributes for mapping faults and stratigraphy.
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A complex linear least-squares method to derive relative and absolute orientations of seismic sensors
(Oxford University Press (OUP), 2012) Grigoli, F.; Cesca, S.; Dahm, T.; Krieger, L.
Determining the relative orientation of the horizontal components of seismic sensors is a common problem that limits data analysis and interpretation for several acquisition setups, including linear arrays of geophones deployed in borehole installations or ocean bottom seismometers deployed at the seafloor. To solve this problem we propose a new inversion method based on a complex linear algebra approach. Relative orientation angles are retrieved by minimizing, in a least-squares sense, the l2-norm between the complex traces (hodograms) of adjacent pairs of sensors. This methodology can be applied without restrictions only if the wavefield recorded by each pair of sensors is very similar. In most cases, it is possible to satisfy this condition by low-pass filtering the recorded waveforms. The main advantage of our methodology is that, in the complex domain, the relative orientations of seismic sensors can be viewed as a linear inverse problem, which ensures that the preferred solution corresponds to the global minimum of a misfit function. It is also possible to use simultaneously more than one independent data set (other seismic events) to better constrain the solution of the inverse problem. Furthermore, by a computational point of view, our method results faster than the relative orientation methods based on waveform cross-correlation. After several tests on synthetic data sets we applied successfully our methodology to different types of real data. These applications include the alignment of borehole sensors relative to a Vertical Seismic Profiling (VSP) acquisition and the orientation of Ocean Bottom Seismometers (OBS) relative to a neighbouring land station of known orientation. Using land stations, the absolute orientation of OBS can be retrieved. Finally, as a last application, we checked the correct orientation for land stations of a seismological array in Germany.
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A convenient hydrothermal route for the synthesis of MxVOPO4·yH2O (M=Na and K)
(Elsevier Science B.V., 1998) Ayyappan, P.; Ramanan, A.; Joy, P.; Pring, A.
Monophasic crystals of MxVOPO4 · yH2O (M = Na, K) could be readily synthesized through a hydrothermal reaction (180°C and 10 h) involving alkali borohydrides as reducing agents.
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A critical appraisal of asymptotic 3D-to-2D data transformation in full-waveform seismic crosshole tomography
(Society of Exploration Geophysicists, 2013) Auer, L.; Nuber, A.M.; Greenhalgh, S.A.; Maurer, H.; Marelli, S.
Seismic full-waveform inversion (FWI) is often based on forward modeling in the computationally attractive 2D domain. This implies the assumption of a line source extended in the out-of-plane medium invariant direction, with far-field amplitudes decaying inversely with the square root of distance. Realistic point sources, however, generate amplitudes that decay approximately with the inverse of distance. Conventionally, practitioners correct for this amplitude difference and the associated phase shift by transforming the recorded 3D field data to the approximate 2D equivalent by using simplistic asymptotic filter algorithms. Such filters assume straight raypaths, a constant velocity medium, and far-field recordings. We have assessed the validity of 3D-to-2D data transformation in the context of crosshole seismic full-waveform tomography by propagating 3D and 2D wavefields through 2D media and comparing 2D reference synthetics with their filtered 3D equivalent. The filter performs well in simple situations, which confirms the general applicability of the conventional asymptotic approach. However, we have observed substantial errors in more complex elastic models, associated with overlapping arrivals and strongly curved raypaths. To test if this error translates into deficient model reconstruction in FWI, we performed complementary inversion experiments using a frequency-domain algorithm. Purely acoustic waveform inversions of 3D-to-2D filtered data are only weakly affected, but in the case of elastic FWI, in which an S-wave influence is present, adverse effects increase substantially. Two-dimensional FWI in combination with filtering seems to be an acceptable strategy as long as the model is two-dimensional, the recording geometry is straight and perpendicular to strike, and only slight S-wave energy is contained in the data. The latter two conditions are generally met in exploration-type marine seismic surveys at short offsets and in some crosswell applications using explosive sources and nondirectional pressure receivers.
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A crosswell seismic experiment for nickel sulphide exploration
(Elsevier Science BV, 2003) Greenhalgh, S.; Zhou, B.; Cao, S.
A crosshole seismic tomography experiment was conducted in the Kambalda nickel district to image the space between two dipping exploration boreholes separated by 80 m. A downhole electromagnetic (EM) survey had disclosed a conductor (possible mineralisation) between the holes. Excellent data quality was obtained from explosive sources at 2-m increments and recorded on a hydrophone array at 2-m spacing. Over 3200 travel times were converted into a velocity tomogram. A velocity low was found to coincide with the EM anomaly. Laboratory measurements on rock and core from Kambalda suggested that the velocity low was massive nickel sulphide. A subsequent drillhole was targeted on the seismic and EM anomaly. Unfortunately, no mineralisation was found. However, the sonic log of the hole gave an almost identical velocity profile to that extracted from the tomogram, and an inductive log run in the hole revealed a high conductivity. The cause of the velocity and conductivity anomaly was found to be due to chemical alteration within the host ultramafics.
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A curious case of agreement between conventional thermobarometry and phase equilibria modelling in granulites: new constraints on P-T estimates in the Antarctica segment of the Musgrave-Albany-Fraser-Wilkes Orogen
(Wiley, 2017) Morrissey, L.; Hand, M.; Kelsey, D.
The Windmill Islands region in Wilkes Land, east Antarctica, preserves granulite facies metamorphic mineral assemblages that yield seemingly comparable P–T estimates from conventional thermobarometry and mineral equilibria modelling. This is uncommon in granulite facies terranes, where conventional thermobarometry and phase equilibria modelling generally produce conflicting P–T estimates because peak mineral compositions tend to be modified by retrograde diffusion processes. In situ U–Pb monazite geochronology and calculated metamorphic phase diagrams show that the Windmill Islands experienced two phases of high thermal gradient metamorphism during the Mesoproterozoic. The first phase of metamorphism is recorded by monazite ages in two widely separated samples and occurred at c. 1,305 Ma. This event was regional in extent, involved crustally derived magmatism and reached conditions of ~3.2–5 kbar and 690–770°C corresponding to very high thermal gradients of >150°C/kbar. The elevated thermal regime is interpreted to reflect a period of extension or increased extension in a back‐arc setting that existed prior to c. 1,330 Ma. The first metamorphic event was overprinted by granulite facies metamorphism at c. 1,180 Ma that was coeval with the intrusion of charnockite. This event involved peak temperatures of ~840–850°C and pressures of ~4–5 kbar. A phase of granitic magmatism at c. 1,250–1,210 Ma, prior to the intrusion of the charnockite, is interpreted to reflect a phase of compression within an overall back‐arc setting. Existing conventional thermobarometry suggests conditions of ~4 kbar and 750°C for M1 and 4–7 kbar and 750–900°C for M2. The apparent similarities between the phase equilibria modelling and existing conventional thermobarometry may suggest either that the terrane cooled relatively quickly, or that the P–T ranges obtained from conventional thermobarometry are sufficiently imprecise that they cover the range of P–T conditions obtained in this study. However, without phase equilibria modelling, the veracity of existing conventional P–T estimates cannot be evaluated. The calculated phase diagrams from this study allow the direct comparison of P–T conditions in the Windmill Islands with phase equilibria models from other regions in the Musgrave–Albany–Fraser–Wilkes Orogen. This shows that the metamorphic evolution of the Wilkes Land region is very similar to that of the eastern Albany–Fraser Orogen and Musgrave Province in Australia, and further demonstrates the remarkable consistency in the timing of metamorphism and the thermal gradients along the ~5,000 km strike length of this system.
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A Damara orogen perspective on the assembly of southwestern Gondwana
(Geological Society Publishing House, 2008) Gray, D.; Foster, D.; Meert, J.; Goscombe, B.; Armstrong, R.; Trouw, R.; Passchier, C.
The Pan-African Damara orogenic system records Gondwana amalgamation involving serial suturing of the Congo–São Francisco and Río de la Plata cratons (North Gondwana) from 580 to 550 Ma, before amalgamation with the Kalahari–Antarctic cratons (South Gondwana) as part of the 530 Ma Kuunga–Damara orogeny. Closure of the Adamastor Ocean was diachronous from the Araçuaí Belt southwards, with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580–550 Ma in the Kaoko Belt and at 545–530 Ma in the Gariep Belt. Peak deformation/metamorphism in the Damara Belt was at 530–500 Ma, with thrusting onto the Kalahari Craton from 495 Ma through to 480 Ma. Coupling of the Congo and Río de la Plata cratons occurred before final closure of the Mozambique and Khomas (Damara Belt) oceans with the consequence that the Kuunga suture extends into Africa as the Damara Belt, and the Lufilian Arc and Zambezi Belt of Zambia. Palaeomagnetic data indicate that the Gondwana cratonic components were in close proximity by c. 550 Ma, so the last stages of the Damara–Kuunga orogeny were intracratonic, and led to eventual out-stepping of deformation/metamorphism to the Ross–Delamerian orogen (c. 520–500 Ma) along the leading edge of the Gondwana supercontinental margin.
Open Access
A detective duo of apatite and zircon geochronology for East Avalonia, Johnston Complex, Wales
(Geological Society, 2023) Clarke, A.; Kirkland, C.; Glorie, S.
The Johnston Complex represents a rare inlier of the Neoproterozoic basement of southern Britain and offers a window into the tectonomagmatic regime of East Avalonia during the assembly of Gondwana. This work presents in-situ zircon (U-Pb, Lu-Hf), apatite (U-Pb), and trace element chemistry for both minerals from the Complex. Zircon and apatite yield a coeval crystallisation age of 570 ± 3 Ma, and a minor antecrystic zircon core component is identified at 615 ± 11 Ma. Zircon Lu-Hf data imply a broadly chondritic source, comparable to Nd data from East Avalonia, and TDM 2 model ages of ca. 1.5 Ga indicate source extraction during the Mesoproterozoic. Zircon trace element chemistry is consistent with an ensialic calcalkaline continental arc setting and demonstrates that magmatism was ongoing prior to terrane dispersal at 570 Ma. Apatite trace element chemistry implies a sedimentary component within the melt consistent with voluminous S-type granite production during the formation of Gondwana. The similarity of ɛHf and geochemistry between both zircon age populations suggest derivation from a uniform source that did not undergo significant modification between 615 – 570 Ma. Time-constrained apatite-zircon chemistry addresses complexities in dating S-type granitoids (zircon inheritance) and permits inferences on post-magmatic thermal histories.
Open Access
A diachronous record of metamorphism in metapelites of the Western Gneiss Region, Norway
(Wiley, 2022) March, S.; Hand, M.; Tamblyn, R.; Carvalho, B.; Clark, C.; White, R.
In this study, data from garnet-kyanite metapelites in ultrahigh-pressure (UHP) domains of the Western Gneiss Region (WGR), Norway is presented. U–Pb geochronology and trace element compositions in zircon, monazite, apatite, rutile and garnet were acquired, and pressure–temperature (P–T) conditions were calculated using mineral equilibria forward modelling and Zr-in-rutile thermometry. Garnet-kyanite gneiss from Ulsteinvik record a prograde evolution passing through ~690–710 °C and ~9–11 kbar. Zircon and rutile age and thermometry data indicate these prograde conditions significantly pre-date Silurian UHP subduction in the WGR and are interpreted to record early Caledonian subduction of continental-derived allochthons. Minimum peak conditions in the Ulsteinvik metapelite occur at ~28 kbar, constrained by an inferred garnet + kyanite + omphacite + muscovite + rutile + coesite + H2O assemblage. The retrograde evolution passed through ~740 °C and ~7 kbar, firstly recorded by the destruction of omphacite and followed by the partial replacement of kyanite and garnet by cordierite and spinel. Garnet-kyanite metapelite from the diamond-bearing Fjørtoft outcrop document a polymetamorphic history, with garnet forming during the late Mesoproterozoic and limited preservation of high-pressure Caledonian assemblages. Similar to the Ulsteinvik metapelite, zircon and rutile age data from the Fjørtoft metapelite also record pre-Scandian Caledonian ages. Two potential tectonic scenarios are possible: (1) the samples were exhumed at different times during pre-Scandian subduction of the Blåhø nappe, or (2) the samples do not share a history in the same nappe complex, instead the Ulsteinvik metapelite is a constituent of the Seve-Blåhø Nappe, whilst the Fjørtoft metapelite shares its history within a separate nappe complex.

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