Emplacement processes and magma flow geometries of the Whin Sill complex
The emplacement processes of the Whin Sill complex and its associated dykes have been
studied using a combined approach of detailed field and magnetic investigations.
Regional palaeodirectionai results show two different but consistent palaeomagnetic signatures
and allow the sill complex to be subdivided into three geographically separate intrusions: the
'Holy Island Sill', the 'Alnwick Sill' and the ' Hadrian's Wall-Pennine Sill', The Little Whin
Sill has been identified previously as a fourth, separate sill on petrological evidence. On the
basis of the palaeodirectional results it is also possible to relate the exposed contemporaneous
dykes to the individual intrusions.
The magma flow geometries within the individual Whin Sill intrusions were detennined by
AMS analyses, and both traditional and newly identified magma flow indicators. For the Holy
Island Sill the results suggest that the magma flow was homogeneously southwards directed
and that the Holy Island Dyke acted as the feeder to this intrusion. In the Alnwick Sill the
magma flow was homogeneously westwards directed and it is proposeU that an offshore, en
echelon segment of the High Green Dyke fed the Alnwick Sill. The magma flow geometry for
the Hadrian's Wall-Pennine Sill is complex. The results suggest that this intrusion was fed by
the Hett Dyke and that the magma flow within the sill was generally north and north westwards
directed. It is proposed that the intrusion was emplaced during a phase of late Variscan, E-W
compression and that pre-existing faults at a high angle to this compression direction acted as a
magma flow barrier along which the magma became deflected.
The geometry of the sills is approximately that of a quarter- to half-saucer-shape, with the
dykes being situated at the saucer truncation. From the feeder dykes magma was injected into
the individual sill bodies and the magma flow was generally up dip or parallel to the strike of
the host rock bedding, thus out of the basin centres and into levels of lower lithospheric
It is proposed that a compressional stress field and locally overpressured horizons had a
significant impact on the initiation and emplacement of the Whin Sill complex.