Abstract-Isotope diffusion in a mineral is strongly temperature dependent but is also a function of grain size. Deformation must therefore be an important consideration in the interpretation of isotopic data because it provides a means of modifying grain size and shape.We illustrate the range of different deformation mechanisms common in micas and use simple models to investigate the relationship between these and isotope diffusion. We consider three different thermal scenarios with deformation taking place during a) the prograde heating path, b) at the closure temperature of the deforming mineral and c) at temperatures significantly below the closure temperature. We have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages. This modelling illustrates that obtaining deformation ages is critically dependent on an understanding and recognition of the different deformation mechanisms that have affected the sample. In the cases where deformation causes a change in grain size, it is important to characterise both the temperature at which deformation takes place and the closure temperature of grains formed during the deformation. The development of grains with Tc greater than the deformation temperature may record a deformation age. Examples of this condition include i) neocrystallisation; ii) grain size reduction occurring at temperatures below Tc (of the reduced grain size) where the deformation mechanism has reset the grains; and iii) deformationinduced grain coarsening.
ZusammenfassungDie Nanga Parbat Konvergenz im nordwestlichen Himalaya ist eine noch im Wachsen begriffene nach Norden streichende antiformale Struktur im Krustenmaf3stab. In ihrem Inneren wurden Gneisse der Indischen Platte aus der N~ihe der t~berschobenen Gesteine des Kohistan Inselbogens herausgehoben. Isotopen-und Spaltspurengeochronologie zeigen, dab das HeraushebungsmaB innerhalb der Konvergenz zu der heutigen Rate yon > 6 mrrda angestiegen ist. Die Heraushebung hat sich angepa.g.t an die Kombination yon initialer NW-vergenter Uberschiebung auf den westlichen Konvergenzrand, die gefolgt wurde von einer krustalen Faltung und lateral yon dextraler antithetischer Verwerfung des westlichen Randes. Diese Deckent~berschiebung, Faltung und Verwerfung ist das Resultat der Deformation des nordwestlichen Ausstriches der Himalaya-Hauptt~berschiebung, wo sie in Wechselwirkung mit der SSE-vergenten Oberschiebung des NW-Himalayas steht.
AbstractThe Nanga Parbat syntaxis, in the NW Himalaya, is a still growing crustal-scale north-trending antiformal structure in the core of which Indian Plate gneisses have been uplifted from beneath the overthrust rocks of the Kohistan island arc. Isoto-
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