Abstract Field observations, and an analysis of the strain field, have been carried out in Archaean terranes of the Hebei province in the Dharwar craton of India, the Sino-Korean craton in China, and the Man shield in the Ivory Coast. Two broadly different situations can be recognized. In the Dharwar and Man Shields the deformation shows a range of characteristic features. The foliation trajectories outline dome-and-basin structures in which supracrustal rocks of greenstone belts invariably occupy the basins. The foliation trajectories are perturbed only by bands of transcurrent shearing. Variations in deformation directions depend essentially on the geometry of plutonic or migmatitic bodies within which the foliation invariably has a domal form. Changes in the intensity of deformation are essentially limited to boundaries between granitoids and supracrustal series. These changes are largely horizontal. Characteristics of the finite strain ellipsoid (the parameter k) vary extremely rapidly. The second situation is manifested in the Hebei province where the Archaean crust has deformed without the preservation of domes and basins. Here the deformation is characterized by the presence of a foliation and a very strong vertical flattening which extends into catazonal domains. This deformation is homogeneous. To explain these observations it is proposed that this homogeneous deformation was superimposed on pre-existing dome-and-basin structure, obliterating the original geometry. The Hebei example represents a stage of deformation greater than that of the Ivory Coast, which in turn is greater than that in the India craton. It is apparent that the Archaean crust has specific characteristics that controlled the manner in which it deformed. During accretion, or reworking, it was dominated by body forces related to differences in the densities of the two major Archaean lithological units: granitoid and greenstones. Deformation resulting from boundary forces then led to a second type of behaviour controlled by a vertical planar anisotropy induced during the first stage. These characteristics are never reported in modern orogenic belts, which generally are marked by a uniform structural trend and vergence. It is proposed that Archean cratons completely lacked a rigid element during the accretion events responsible for their formation, that the thermal regime in these periods was quite unlike that of today, and that the continental crust formed or was reworked during episodes of enhanced mantle plume activity.

Soft lithosphere during periods of Archaean crustal growth or crustal reworking

Synchronising EDML and NorthGRIP ice cores using δ18O of atmospheric oxygen (δ18Oatm) and CH4 measurements over MIS5 (80–123 kyr)

Calc-alkaline magmatism in the Grampian Terrane started at c . 430 Ma, after subduction of the edge of continental Avalonia beneath Laurentia, and it then persisted for at least 22 Ma. Isotope dilution thermal ionization mass spectrometry U–Pb zircon dating yields 425.0 ± 0.7 Ma for the Lorn Lava Pile, 422.5 ± 0.5 Ma for Rannoch Moor Pluton, 419.6 ± 5.4 Ma for a fault-intrusion at Glencoe volcano, 417.9 ± 0.9 Ma for Clach Leathad Pluton in Glencoe, and, in the Etive Pluton, 414.9 ± 0.7 Ma for the Cruachan Intrusion and 408.0 ± 0.5 Ma for the Inner Starav Intrusion. The Etive Dyke Swarm was mostly emplaced during 418–414 Ma, forming part of the plumbing of a large volcano (≥2000 km 3 ) that became intruded by the Etive Pluton and was subsequently removed by erosion. During the magmatism large volumes (thousands of km 3 ) of high Ba–Sr andesite and dacite were erupted repeatedly, but were mostly removed by contemporaneous uplift and erosion. This volcanic counterpart to the ’Newer Granite9 plutons has not previously been fully recognized. The intermediate magmas forming both plutons and volcanoes originated mainly by partial melting of heterogeneous mafic to intermediate lowermost crust that had high Ba–Sr derived from previous melting of large ion lithophile element (LILE)-enriched mantle, possibly at c . 1.8 Ga. This crustal recycling was induced by heat and volatiles from underplated small-degree melts of LILE- and light REE-enriched lithospheric mantle (appinite–lamprophyre magmas). The post-collision magmatism and uplift resulted from breakoff of subducted oceanic lithosphere and consequent rise of asthenosphere. Supplementary material: Geochronological methods, tabulated data and additional figures are available at http://www.geolsoc.org.uk/SUP18343.

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Timing, relations and cause of plutonic and volcanic activity of the Siluro-Devonian post-collision magmatic episode in the Grampian Terrane, Scotland

Publisher Summary This chapter focuses on the magnetic stratigraphy in paleoceanography. Magnetic stratigraphy rests on the idea that the recorded magnetization of a rock reflects the behavior of the geomagnetic field. In the simplest case, the natural remanent magnetization (NRM) of sediment is aligned with the (geo)magnetic field and is a function of its intensity and direction at the time of deposition. In practice, many factors may work to modify the original geomagnetic input signal. Under favorable circumstances and with detailed diligence, some of these effects can be separated and others avoided so that an accurate paleomagnetic record is recovered. Paleomagnetism has been working to keep pace and a significant new understanding of geomagnetic field behavior during times of constant polarity has begun to emerge. Essentially, it has been found that high amplitude, high frequency variations in the Earth's magnetic field occur over large spatial scales even during times of constant polarity. New magnetostratigraphic opportunities over a range of temporal and spatial scales are emerging. The chapter also outlines the practical aspects of reconstructing the paleomagnetic record from marine sediments, and discusses some of the recent observations on the Quaternary geomagnetic record that are being made and their uses as a stratigraphic tool for paleoceanographic research.

M. Hesselman

Papers

At the Age