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Geophysical survey of Precambrian terranes in SW Lleyn, Caernarvonshire, Wales
Author(s) -
Lodge R. J. E.,
Moore M. A.,
Evans A. D.,
Goulty N. R.
Publication year - 1997
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/(sici)1099-1034(199709)32:3<265::aid-gj745>3.0.co;2-r
Subject(s) - terrane , geology , precambrian , shear zone , outcrop , magnetic anomaly , gravity anomaly , anomaly (physics) , geophysics , seismology , shear (geology) , tectonics , paleontology , physics , condensed matter physics , oil field
The geology of southwest Lleyn comprises two Late Precambrian terranes: the Gwna Mélange in the west and the Sarn Complex in the east, separated by the Lleyn Shear Zone. The location of the terrane boundary is poorly constrained due to the limited exposure. We undertook a ground magnetic survey and also measured four gravity profiles with the original intention of investigating the cause of a positive aeromagnetic anomaly previously recorded close to the terrane boundary at grid reference [SH 2200 3000]. This ‘Sarn anomaly’ appears to be associated with a shallow body within the Sarn Complex, which is known to be a heterogeneous plutonic igneous unit. Of greater interest was the identification of a much larger‐amplitude magnetic anomaly striking roughly N–S for over 7 km close to the mapped position of the Lleyn Shear Zone. It can be modelled as a near‐vertical discontinuous body of overall dyke‐like form, and is too narrow to have been resolved by the aeromagnetic survey. We discovered a previously unreported outcrop of gabbro at Brynhunog Bach [2100 3127] located on this high‐amplitude ‘Brynhunog anomaly’. It seems likely that the whole anomaly is due to a gabbroic body which is an original constituent of the Sarn Complex, but an alternative possibility is that it is a later intrusion along the Lleyn Shear Zone. © 1997 John Wiley & Sons, Ltd.