TERRANES OVERVIEW 455 Koeberl C and Shirey MB (1993) Detection of a meteorite component in Ivory Coast tektites with rhenium osmium isotopes Science 261: 595 598 McCall GJH (2000) The age paradox revisited Journal of the Royal Society of Western Australia 83: 83 92 McCall GJH (2001) Tektites in the Geological Record: Showers of Glass from the Sky Bath: Geological Society Publishing House McNamara K and Bevan A (2001) Tektites Perth: Western Australian Museum Melosh HJ (1998) Impact physics constraints on the origin of tektites Meteoritics and Planetary Science 33(Supple ment): A104 O’Keefe J (1963) Tektites Chicago: University of Chicago Press Taylor SR (1962) The geochemical composition of australites Geochimica et Cosmochimica Acta 26: 685 722 Taylor SR (1969) Criteria for the source of australites Chemical Geology 4: 451 459 TERRANES OVERVIEW L R M Cocks, The Natural History Museum, London, UK Copyright 2005, Natural History Museum All Rights Reserved Introduction The word ‘terrane’ is used in a specialised sense by geologists, and should not be confused with the samesounding ‘terrain’, which is used by many people, particularly the military, to denote characteristics of the countryside in a particular area To a geologist, terrane is used for a discrete block of continental crust that is moving or has moved in relation to those blocks that surround it Definition The Earth is today, and through geological time, made up of a number of moving plates (see Plate Tectonics) Each plate consists of heavier oceanic crust underlying lighter continental crust Plates are constantly being enlarged through ocean-floor spreading, reduced by subduction or obduction, or displaced laterally by transform faulting, all of which processes affect both oceanic and continental crusts However, because of its lighter density, continental crust tends to remain at the Earth’s surface for far longer periods than does oceanic crust; consequently, very often the continental crust of an old plate remains at the surface today long after the oceanic crust on which it once rested has disappeared within Earth’s interior, perhaps to be later remobilized into fresh crust The oldest ocean crust known today in its original position is only about 160 million years old (Jurassic), whereas the continental crust includes rocks from modern times to more than billion years ago, the oldest known Terranes can be of varied size, ranging today from the vast Eurasian– African block down to the relatively small microplates found in the south-west Pacific within the East Indies The difference between a ‘continent’ (as strictly defined) and a ‘terrane’ is that the former is invariably bounded by one or more oceans, whereas the latter is defined by its surrounding structural discontinuities An accreted terrane is one that has been added to the margin of a larger one Many areas may or may not have been real (i.e., separate) terranes in the past, and geological opinions can often differ widely as to their reality and status When this uncertainty exists, the area is referred to as a ‘suspect terrane’ Boundaries of Terranes The marginal boundaries of old terranes are termed ‘sutures’: when exposed, they are usually faults or fault systems, with the obvious characteristic that the rocks and stratigraphy are completely different on the opposite sides of the faults Because movement of the crust is principally dominated by horizontal components, the suture faults are usually strike-slip or transform For example, a major suture is the Tornquist–Teisseyre Line, or the Trans-European Suture Zone (TESZ), which stretches from the North Sea to the east of Aberdeen, through southern Denmark, north-eastern Germany, south-central Poland, Slovakia, Hungary, and Romania to the Black Sea That suture represents what remains of the south-eastern margin of the old terrane of Baltica and separates that terrane from Avalonia (see later), Perunica (often termed Bohemia), and others to its south The suture was originally formed during the Variscan Orogeny in Late Palaeozoic time, but movements along the TESZ area of crustal weakness have been reactivated during several subsequent geological periods and continue sporadically to the present day Principal Terranes At two recognizable times in Earth history, at about 1000 Ma and 250 Ma, most of the continental