Author:
BEACOM L. E.,ANDERSON T. B.,HOLDSWORTH R. E.
Abstract
Clastic infills of fractures (here termed clastic veins) in basement rocks immediately underlying
sedimentary cover sequences can be used to date fault movements if these demonstrably
occurred at the time of infilling or prior to the lithification of the entrained clastic material. This
allows reconstruction of the syn-rifting palaeostress system using stress inversion techniques. During
Riphean intracontinential rifting of Laurentia, the Torridonian Stoer Group sediments of northwest
Scotland were deposited in half-graben basins controlled by faults, e.g. the Coigach and Clachtoll
faults. At Clachtoll, northeast–southwest oblique sinistral normal faulting in the underlying basement
is associated with extensive development of shear, hybrid and tensile clastic veins filled with Stoer
Group sediment, infilled and deformed prior to sediment lithification. Clastic veins initially formed by
gravitational infilling of sediment from above, followed by tectonically-driven, forceful hydraulic injection
of fluidized sand into new fractures and reactivated pre-existing basement faults. Palaeostress
axes, determined from fault lineation data and tensile fracture extension directions in the Clachtoll
Fault zone, indicate west-northwest–east-southeast directed extension during rifting. On a regional
scale, this implies oblique-dextral extension on the north- to north-northeast-trending Coigach Fault
during Stoer Group deposition. Similar orientations, age relationships and kinematics have been
obtained from pre-Torridon Group fault arrays developed in the Lewisian basement near Gairloch
and Loch Maree. Overall, the faulting patterns reflect a three-dimensional strain (k≠1) formed by
east-southeast–west-northwest-directed extension during deposition of the Stoer Group. More speculatitively,
asymmetric density patterns of sinistral and dextral faults may indicate that rifting occurred
in a regional zone of broadly north–south-oriented dextral transtension.
Publisher
Cambridge University Press (CUP)
Cited by
35 articles.
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