Pseudotachylitic breccias from fault zones in the Witwatersrand Basin, South Africa: evidence of autometasomatism and post-brecciation alteration processes

Summary Pseudotachylitic breccias, associated with either bedding-parallel or normal faults, are abundant in the northern and northwestern parts of the gold- and uranium-rich Witwatersrand Basin in South Africa. They are particularly abundant in a zone tangential to the Vredefort Dome, a structure which is now widely accepted to be the eroded remnant of the central uplift of the originally 250 to 300 km wide Vredefort impact structure.Several of these fault zones were sampled along vertical traverses that included both fault breccias and host rocks. Geochemical analyses were undertaken to investigate the processes involved in fault rock formation. In addition, the temporal relationships of fault activity to regional geological events was studied. Detailed petrographic analysis shows evidence for strong hydrothermal overprints of the faults related to breccia formation (thus termed autometasomatism), as well as post-formational alteration. Mixing between two or more precursor lithologies and fluid alteration affected both the fault zones and their wall-rocks. A wide spectrum of trace elements, including Au and U, has been locally mobilised and redeposited by these processes.These fault zones, some of which are of basin-wide significance, were important channels for fluids that also modified the distribution of base metals in the Witwatersrand Basin. The regionally separated locations of our study areas suggest that these processes affected at least a major part of the basin. However, as the chemical characteristics determined for the fault zones differ from site to site, it is likely that the metals were only locally remobilised in the vicinity of fault zones or to previously hydrothermally altered country rock. Subsequent redeposition of metals was limited to the regimes in and around such fault zones.

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Pseudotachylitische Brekzien aus Störungszonen des Witwatersrand-Beckens: Hinweise auf Autometasomatose und Alteration nach der Brekzienbildung

Zusammenfassung Pseudotachylitische Brekzien, entweder an schicht-parallelen oder normalen Verwerfungen, kommen häufig im nördlichen und nordwestlichen Teil des Gold- und Uranreichen Witwatersrand Beckens in Südafrika vor. Man hat sie hauptsächlich von einem tangentialen Bereich um den Vredefort Dom beschrieben. Der Dom ist der tieferodierte Rest der zentralen Aufwölbung der Vredefort Impaktstruktur, die ursprünglich linen Durchmesser von mindestens 250 Km hatte.Einige dieser Verwerfungszonen wurden entlang von Traversen beprobt, die Verwerfungsbrekzien sowie deren Nebengesteine beinhalteten, und geochemisch mit dem Ziel untersucht, die Prozesse bei der Brekzien-Bildung und die zeitlichen Verhältnisse von Verwerfungen und regionalgeologischen Ereignissen zu erfassen. Petrographische Analysen zeigten, daß die Verwerfungszonen im Zusammenhang mit der Brekzierungsphase stark hydrothermal überprägt wurden (ein Prozess, der daher als Autometasomatose bezeichnet wird) und eine zweite Überprägungsphase erfahren haben, die nach der Brekzierung stattfand. Es wird gezeigt, daß sowohl Mischung von wenigstens zwei Mutter-Gesteinskomponenten und Flüssigkeitsüberprägung, die Verwerfungszonen und Nebengesteine betraf, wichtige Rollen in der Bestimmung der chemischen Charakteristiken der untersuchten Zonen spielten. Viele Elemente, darunter Gold und Uran, wurden durch diese Prozesse lokal mobilisiert und wieder abgelagert.Diese Verwerfungszonen, von denen einige eine beckenweite Bedeutung besitzen, agierten als wichtige Kanäle für Flüssigkeiten, die die Verbreitung von wichtigen Metallen im Witwatersrand Becken modifizieren konnten. Da unsere Untersuchungsorte weit voneinander getrennt liegen, muß angenommen werden, daß diese Prozesse wenigstens weite Teile des Beckens beeinflußten. Da jedoch die chemischen Charakteristika der untersuchten Zonen von Fall zu Fall unterschiedlich sind, liegt die Vermutung nahe, daß Metalle höchstens lokal mobilisiert und umgelagert wurden und daß dies vermutlich im wesentlichen in der Nähe von existierenden Reefs stattfand. Es ist ferner klar, daß die Umlagerung von Metallen auf die unmittelbaren Bereiche dieser Verwerfungszonen beschränkt war.

     

Beach erosion under rising sea‐level modulated by coastal geomorphology and sediment availability on carbonate reef‐fringed island coasts

This study addresses gaps in understanding the relative roles of sea‐level change, coastal geomorphology and sediment availability in driving beach erosion at the scale of individual beaches. Patterns of historical shoreline change are examined for spatial relationships to geomorphology and for temporal relationships to late‐Holocene and modern sea‐level change. The study area shoreline on the north‐east coast of Oahu, Hawaii, is characterized by a series of kilometre‐long beaches with repeated headland‐embayed morphology fronted by a carbonate fringing reef. The beaches are the seaward edge of a carbonate sand‐rich coastal strand plain, a common morphological setting in tectonically stable tropical island coasts. Multiple lines of geological evidence indicate that the strand plain prograded atop a fringing reef platform during a period of late‐Holocene sea‐level fall. Analysis of historical shoreline changes indicates an overall trend of erosion (shoreline recession) along headland sections of beach and an overall trend of stable to accreting beaches along adjoining embayed sections. Eighty‐eight per cent of headland beaches eroded over the past century at an average rate of ?0·12 ± 0·03 m yr^?1. In contrast, 56% of embayed beaches accreted at an average rate of 0·04 ± 0·03 m yr^?1. Given over a century of global (and local) sea‐level rise, the data indicate that embayed beaches are showing remarkable resiliency. The pattern of headland beach erosion and stable to accreting embayments suggests a shift from accretion to erosion particular to the headland beaches with the initiation of modern sea‐level rise. These results emphasize the need to account for localized variations in beach erosion related to geomorphology and alongshore sediment transport in attempting to forecast future shoreline change under increasing sea‐level rise.     

Textural and geochemical discrimination between xenotime of different origin in the Archaean Witwatersrand Basin, South Africa

Xenotime (YPO₄) of detrital, diagenetic, and hydrothermal origin within siliciclastic rocks of the Archaean Witwatersrand Basin, South Africa, has been identified on the basis of petrography and in situ ion microprobe (SHRIMP) age data. The chemical composition of xenotime, determined by in situ electron microprobe analysis, can be correlated with its origin. This allows the origin of any xenotime grain to be assessed by a non-destructive microanalytical method prior to ion microprobe geochronology. The main chemical discriminators are MREE-HREE abundance, normalised HREE slope and Eu anomaly, and, in some cases, U and Th contents. Igneous-detrital xenotime (> 2800 Ma) is distinguished from diagenetic (∼2780 Ma) and hydrothermal (< 2780 Ma) xenotime in having lower Eu, Dy, and Gd concentrations and a distinctively lower Gd/Yb ratio. Hydrothermal xenotime has distinctively lower U and Th concentrations when compared to igneous-detrital and diagenetic xenotime. Three separate hydrothermal fluid events and episodes of post-diagenetic xenotime growth are recognised in the geochemical and geochronological data, which correspond in time to the extrusion of the Ventersdorp lavas at ∼2720 Ma, the emplacement of the Bushveld Igneous Complex at ∼2061 Ma, and an event of unknown affinity at ∼2210 Ma. Although geochemical discrimination of the xenotime types from the Witwatersrand Basin, in combination with careful petrography, appears achievable, universal application of these discriminators to xenotime in other sedimentary basins remains untested.     

Distribution of chloride in a transgressive sequence of fine-grained sediments

Two vibracores taken in a transgressive coastal lagoon in Delaware show an unusual distribution of the chloride anion (Cl). Considerations of the geological processes responsible for the stratigraphy in these cores provide clues to the problem of Cl diffusion. A partial cloistering or contribution to the initial Cl^– content of the buried sediments byin situ organic material may explain the unusual distribution of the anion.     

(Re)making space for kiwi: beyond ‘fortress conservation’ in Northland

Mainstream conservation has been long dominated globally by the protected area paradigm. This approach has been widely challenged in recent years, and new conservation initiatives have emerged. The situation is mirrored in New Zealand, where ongoing biodiversity loss has prompted reappraisal. Within this context, we highlight the plight of kiwi, focusing on Northland and efforts there to (re)make space for this iconic bird which is at risk of extinction. While the state has primarily responded by fortifying ‘islands’ on public conservation land, Far North communities are working in a variety of ways and localities, both within and beyond ‘the fortress’, to secure a future for the ‘people's bird’.     

A new view of the solar outer atmosphere by the Transition Region and Coronal Explorer

The Transition Region and Coronal Explorer (TRACE) – described in the companion paper by Handy et al. (1999) – provides an unprecedented view of the solar outer atmosphere. In this overview, we discuss the initial impressions gained from, and interpretations of, the first million images taken with TRACE. We address, among other topics, the fine structure of the corona, the larger-scale thermal trends, the evolution of the corona over quiet and active regions, the high incidence of chromospheric material dynamically embedded in the coronal environment, the dynamics and structure of the conductively dominated transition region between chromosphere and corona, loop oscillations and flows, and sunspot coronal loops. With TRACE we observe a corona that is extremely dynamic and full of flows and wave phenomena, in which loops evolve rapidly in temperature, with associated changes in density. This dynamic nature points to a high degree of spatio-temporal variability even under conditions that traditionally have been referred to as quiescent. This variability requires that coronal heating can turn on and off on a time scale of minutes or less along field-line bundles with cross sections at or below the instrumental resolution of 700 km. Loops seen at 171 Å (～1 MK) appear to meander through the coronal volume, but it is unclear whether this is caused by the evolution of the field or by the weaving of the heating through the coronal volume, shifting around for periods of up to a few tens of minutes and lighting up subsequent field lines. We discuss evidence that the heating occurs predominantly within the first 10 to 20 Mm from the loop footpoints. This causes the inner parts of active-region coronae to have a higher average temperature than the outer domains.