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Relationship between metamorphism and structure in the Skiddaw Group, English Lake District

Published online by Cambridge University Press:  01 May 2009

N. J. Fortey ,
B. Roberts  and
S. R. Hirons
N. J. Fortey
Affiliation:
British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
B. Roberts
Affiliation:
Department of Geology, Birkbeck College, Malet Street, London WC1E 7HX, U.K.
S. R. Hirons
Affiliation:
Department of Geology, Birkbeck College, Malet Street, London WC1E 7HX, U.K.
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Abstract

Regional variation of white mica (illite) crystallinity in the Skiddaw Group is set against the structural interpretation of Hughes, Cooper & Stone (1993, this issue) in which early- or pre-Ludlow deformation and slaty cleavage development (S1) were succeeded by southward thrusting and an associated development of S1 and S1 crenulation cleavages, possibly during early Devonian times. Kubier index (KI) values are plotted in relation to geological structure for a major part of the Skiddaw Group, and cross-sections constructed. The pattern is interpreted in terms of three processes: (1) diagenetic to low anchizonal burial metamorphism under a relatively high geothermal gradient during the late-arc stage (early to mid-Silurian) which preceded the orogenic phase and formation of S1; (2) upper anchizonal to epizonal metamorphism due to tectonic thickening and slaty cleavage development during the early to pre-Ludlow orogenic phase following closure of Iapetus; (3) late-tectonic uplift of already metamorphosed rocks by southward-directed movement on the Loweswater, Gasgale Gill and Causey Pike Thrusts possibly during early Devonian time. An analogous interpretation is made for the Skiddaw area, though with the addition of major post-S1 contact metamorphism.

Type
Articles
Information
Geological Magazine , Volume 130 , Issue 5 , September 1993 , pp. 631 - 638
Copyright
Copyright © Cambridge University Press 1993

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