Allan Trench

The closure of the Iapetus Ocean and Tornquist Sea: new palaeomagnetic constraints

Early Wenlock volcanic rocks from the East Mendips inlier, SW England, yield primary palaeomagnetic directions which indicate that southern Britain (Eastern Avalonia) occupied a palaeolatitude of 13 ± 5°S in Mid-Silurian (430 Ma) times. When combined with coeval data from Laurentia, the southern Laurentian margin (Scottish terranes), and Baltica, the collective data indicate that: (1) the British and Scandinavian sectors of the Iapetus Ocean were closed, to within the limits of the palaeomagnetic resolution, by the Early Wenlock; (2) Acadian deformation across Britain, which culminated in Early Devonian time, post-dated initial docking of Eastern Avalonia and Laurentia; (3) a previous palaeomagnetic requirement for the Tornquist Sea to remain open into Mid-Silurian times is removed, thereby reconciling palaeomagnetic and biogeographical constraints upon the convergence of southern Britain and Baltica. Recently-acquired palaeomagnetic data from southern Britain and Baltica have facilitated more reliable reconstructions of the early Palaeozoic palaeogeography of Northern Europe (Fig. 1). Palaeomagnetic data now record the progressive drift of southern Britain across the Iapetus Ocean during Ordovician times (Torsvik & Trench 1991; Channell et al. 1992) and a predominant northward drift and counterclockwise rotation of Baltica from Cambrian to Devonian times (Torsvik et al. 1991aTorsvik et al. 1992a; Perroud et al. 1992). Nevertheless, details on the closure history of the Iapetus Ocean and Tornquist Sea (Fig. 1) have remained sketchy due to a sparseness of coeval Late Ordovician–Mid-Silurian data from the bordering continents. These palaeomagnetic uncertainties permit competing geological models. In this contribution, we address two outstanding problems relating to the closure

Palaeozoic palaeomagnetic studies, in the Welsh Basin-recent advances

In the last two years, new palaeomagnetic data fro m Wale s have resulted i n radical revision of the Ordovician palaeogeography of Eastern Avalonia, part of the southern margin of the Iapetus Ocean. Combined with Palaeozoic palaeomagnetic data from Laurentia and Gondwana, these data suggest that Eastern Avalonia was a peri-Gondwanide high latitude continental fragment during at least part of Ordovician time, with a palaeolatitude of about 62° S and 51° S in Arenig and Llanvirn time, respectively. This implies a latitudinal width of the early Ordovician Iapetus Ocean between Eastern Avalonia and Laurentia of at least 30°. Geological evidence for the proximity of Eastern Avalonia and Laurentia suggests that the intervening Iapetus Ocean closed during Silurian time, from late Llandovery to early Ludlow. Recent palaeolatitude data from the Iapetus bordering continents are consistent wit h closure b y middle to late Silurian time . New pre-Acadian early Devonian palaeomagnetic data from the Old Red Sandstone places the Welsh Basin at about 17° S, consistent with a palaeogeography in which Laurentia, Baltica, Avalonia, Armorica, and possibly Gondwana, wer e part o f a singl e supercontinent. Pervasiv e lat e Carboniferous/early Permia n remagnetization affect s the Welsh Basin. The remagnetization i s probably associated with fluids emanating from th e Variscan thrust front. W e do not observe remagnetization associate d with Acadian orogen y and the remagnetizations, which have been studied in more detail in North America, appear to be a unique feature of the Variscan-Hercynian-Alleghenian orogeny.

The palaeogeographic evolution of Southern Britain during early Palaeozoic times: a reconciliation of palaeomagnetic and biogeographic evidence

Abstract Palaeomagnetic, biogeographic and palaeoclimatic evidence together suggest that, in the Early Ordovician, Southern Britain lay off the west African margin of Gondwana at around 60° S. The combination of these disciplines reduces latitude uncertainties to around 5 to 10 degrees. Rifting of Avalonia from Gondwana was followed, during most of the Ordovician, by northward drift as Avalonia and Baltica converged with subduction of Tornquist Sea crust. Both continents had similar moderate northward drift rates (3–6 cm/yr). so their convergence probably had a considerable longitudinal component. Silurian and early Devonian data indicate that northward drift of Avalonia and Baltica continued after their collision, and that Southern Britain reached a latitude of around 20° S by c. 400 Ma.

The polarity of the Silurian magnetic field: indications from a global data compilation

Silurian palaeomagnetic data from all continents are compiled in an attempt to define the polarity history of the Earth’s magnetic field during the period. Data sets from Llandovery, Ludlow, and Pridoli strata often display evidence of field reversals (i.e. the presence of both normal and reverse polarity magnetization directions) despite the fact that the palaeomagnetic sample collections may cover only a limited stratigraphic range in each case. Palaeomagnetic data from rocks of Wenlock age are exclusively of normal magnetic polarity. A comparison of the available Silurian data with published Ordovician palaeomagnetic results suggests that the frequency of magnetic field reversals increased from Ordovician to Silurian times. Palaeomagnetic data sets are not yet available from a continuous Silurian stratigraphic sequence of any great duration, so the Silurian magnetostratigraphy is still at a much more preliminary stage than that for the Ordovician. Apart from the Wenlock, all Silurian series have some intervals with frequent (<2 Ma) polarity reversals. Constant polarity (for durations in excess of 2 Ma) in the Ordovician and Silurian are only apparent during the Arenig (reverse), the early Caradoc (normal), the late Caradoc (reverse), the Ashgill (normal) and the Wenlock (normal). Data are still particularly sparse in Tremadoc and Llandovery rocks.

Geophysical investigation of the Honningsvåg igneous complex, Scandinavian Caledonides

The Honningsvåg igneous complex within the Mageroy nappe, northern Norway, has marked magnetic and gravimetric signatures. Palaeomagnetic studies reveal a Siluro-Devonian (Scandian) dual-polarity remanence (NE and up—SW and down; mean declination = 045°, inclination = -38° and a95 = 8.4°). Rock-magnetic and petrological studies indicate a pure magnetite host for the remanent magnetization component. Magnetic-fabric ellipsoids delineate Dl and D2 Scandian structures affecting the area, and demonstrate an internal reclined fold-structure of the Honningsvåg igneous complex. Forward-modelling demonstrates that a short-wavelength, poúitive aeromagnetic anomaly (c. 700 nT) over the northwestern part of the complex is associated with an area of reverse remanent polarity (SW and down). Conversely, areas of normal remanence polarity (NE and up most common) are associated with magnetic ‘lows’ or ‘quiet’ zones. The gravity field in the vicinity of the complex displays a symmetrical positive anomaly of approximately 200 g.u. (wavelength 15 km). This can be attributed to a density contrast of approximately 0.20 Mg/m3 between the gabbros of the Honningsvåg complex (2.93) and surrounding metasediments (2.73). An optimal geophysical and geological model demonstrates that the complex has been tilted through 90°, and now has the form of a steeply dipping reclined fold-structure extending to a depth of at least 6 km. Palaeomagnetic remanence acquisition post-dates emplacement of the Magerøy nappe (late Dl), and probably originated during post-Dl uplift. Brittle, upper-crust, D2 deformation which continued into Devonian times, folded the Honningsvåg igneous complex and introduced a small but systematic dispersion in the palaeomagnetic remanence directions. The magnetization is, therefore, post-Dl but pre-D2 in age, and can be termed ‘syn-Scandian’. Palaeomagnetic and isotopic evidence suggests a syn-tectonic magnetization age of c. 410 Ma. The palaeomagnetic pole (N7° and E344°) resembles that from the underlying Kalak nappe complex. It is therefore inferred that the Scandian emplacement of the Magerøy nappe was synchronous with a thermal event which affected the older and underlying Finnmarkian-defor-med Kalak nappe complex.

An assessment of the potential for transformational market growth amongst the critical metals

A ‘critical metal’ is one that has important economic uses, but which also faces supply risks for geopolitical or environmental and sustainability reasons. The constrained nature of critical metals supply means proposed solutions to the problem commonly involve reducing demand and therefore reliance, via recycling, substitution and thrifting. However, most critical metals are presently only small markets and therefore such an approach ignores the potential of transformational market growth to reduce supply risk, by creating large, diverse, transparent markets with multiple sources of primary mine supply, akin to modern base metals markets. Research is therefore required into which critical metals have the greatest potential for such transformational market growth. This study therefore conducts an evaluation of 49 critical metals to determine which are nearest to the combined breakthroughs in discovery, supply and demand that may lead to transformational market growth. The study concludes that 13 markets from the 49 critical metals, being magnesium, silicon, barium, boron, lithium, cobalt, chromium, vanadium, gallium, strontium, cerium, lanthanum and scandium have the highest potential for transformational market growth and thus efforts to resolve supply risk in these markets may be better focussed on overcoming current market constraints and growing these markets, rather than lessening reliance by reducing demand.

Geophysical exploration for Archaean gold; a case study from the Southern Cross Greenstone Belt, Western Australia

Abstract Ground magnetometer and gravity surveys have identified a coincident magnetic (400 nT) and gravity (0.5 mGal) anomaly on the western limb of the Caudan Antiform in the Burbridge area of the Southern Cross Greenstone Belt, Western Australia. The anomaly corresponds with the postulated along-strike continuation of a sulphide-facies iron formation ( if ) which locally hosts gold mineralisation. Gold concentrations in surface laterites (up to 100 ppb) occur in the vicinity of the geophysical anomalies. Two-dimensional modelling of the magnetic data, following filtering to remove short wavelength noise originating in the near-surface, predicts that magnetic if (susceptibility contrast 120×10 −3 SI units) occurs at a depth of 100 m, dips sleeply to the west, and is overlain by a non-magnetic “oxidised” iron formation facies in the zone of deep weathering. The stratigraphic thickness of the fresh if is up to 60 m. This model is consistent with the observed gravity anomaly, assuming a density contrast of +0.4 g/cm 3 between the magnetic if and the surrounding lithologies. Magnetic susceptibility measurements of surface laterite failed to reveal anomalous values associated with presence of the if . After completion of the initial geophysical investigation, a reverse circulation drilling program confirmed the inferred position of the if on the western limb of the Caudan Antiform and showed it to be associated with gold mineralisation.

Teaching Geophysics To Students with Various Scientific Backgrounds

Geophysical classes often include students with widely different experience in mathematics, physics and geology. This causes problems with both course content and assessment. At the University of Western Australia introductory geophysics courses, taken by many students with low levels of numeracy, concentrate on the principles behind the various geophysical methods, qualitative interpretation of geophysical data and the integration of geophysical and geological data sets. More advanced, optional, geophysics units (courses) are taken by students specialising in geology, physics and mathematical geophysics. In order not to discriminate against any one of these groups a range of assessment procedures are utilised. These include structured essay examination questions where the examinee can choose the weighting given to each part of the question, assignments which are sufficiently open that the student can attempt them within the framework of their particular knowledge, and seminar presentations where the mult...