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Neoproterozoic shear zone tectonics within the Icartian basement of Guernsey and Sark, Channel Islands

Published online by Cambridge University Press:  01 May 2009

I. R. Tribe ,
R. A. Strachan  and
R. S. D’Lemos
I. R. Tribe
Affiliation:
Geology and Cartography Division, School of Construction and Earth Sciences, Oxford Brookes University, Oxford 0X3 OBP, UK
R. A. Strachan
Affiliation:
Geology and Cartography Division, School of Construction and Earth Sciences, Oxford Brookes University, Oxford 0X3 OBP, UK
R. S. D’Lemos
Affiliation:
Geology and Cartography Division, School of Construction and Earth Sciences, Oxford Brookes University, Oxford 0X3 OBP, UK
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Abstract

The Channel Islands of Guernsey and Sark are amongst the few localities within the Neoproterozoic, Cadomian orogenic belt where Palaeoproterozoic basement is exposed. Basement units collectively referred to as ‘Icartian’ comprise orthogneisses (e.g. the c. 2000 Ma Icart granite gneiss), metasediments and amphibolites. On Guernsey, the protolith to the Icart granite gneiss was intruded into metasediments already deformed during a D1 deformation event. Both were variably deformed during a D2 event within an approximately north—south trending, steeply dipping, oblique dextral shear zone. On Sark, metasediments and amphibolites carry a D1 fabric (not necessarily correlatable with that on Guernsey) which was deformed during D2 to form recumbent—gently inclined folds and tectonic fabrics which locally carry top-to-the-south kinematic indicators. Early Cadomian quartz diorites which intruded the Icartian basement carry variably developed magmatic and moderate temperature (400–550 °C) LS solid-state fabrics which are similarly oriented to D2 basement structures. This indicates that the quartz diorites were emplaced and deformed during a protracted D2 event. Published 40Ar/39Ar amphibole cooling ages of c. 600 Ma from Guernsey and Sark quartz diorites and a host Icartian amphibolite are consistent with reworking of basement units during the Cadomian Orogeny. The contrasting kinematic pattern during Cadomian deformation implies that a flat-lying shear zone on Sark may have accommodated oblique dextral strike-slip movement on Guernsey with an overall southerly direction of tectonic transport. Deformation most likely occurred in the hanging-wall of a major south-dipping Neoproterozoic subduction zone.

Type
Research Article
Information
Geological Magazine , Volume 133 , Issue 2 , March 1996 , pp. 177 - 192
Copyright
Copyright © Cambridge University Press 1996

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Footnotes

*

Present address: Z & S Geology Ltd, Campus 2, Aberdeen Science and Technology Park, Aberdeen, AB22 8GU, UK

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