Structure of remanent magnetization in some skye lavas,NW Scotland

In the British Tertiary igneous province one commonly observes reversed magnetizations with an abnormally large range of inclinations. Two of the Skye lava sequences which are of Early Eocene age have been chosen to investigate why this range of inclinations exists. Various laboratory studies of the natural remanence reveal a composite palaeomagnetic record. There are two axes of magnetization present: on steeply inclined (～ 75°) and one with an intermediate inclination (～35°). There is an excess of reversed polarity components in the bulk fossil remanence of most lavas and the inclination spread seems basically caused by superposition of these components. The experimental problem of splitting the polyphase magnetization into its separate sub-components is demonstrated by many examples. It is concluded that processes of low-temperature mineral alteration (which strongly overprints the high-temperature exsolution structures) and remagnetization must have been active for a minimum time span of 20 m.y. after the original cooling of the lavas, involving both polarity inversions and a major geomagnetic axis shift in mid-Tertiary times. As a conseqence, the original TRM has probably been erased to a major extent and replaced by CRM's in subsequent times. The polar estimate based on the shallow magnetization group agree well with suggested Lower Tertiary palaeopoles from Northern Ireland and from the Faeroe Irelands. The multivectoral nature of the remanent magnetization in the Skye lavas signifies that even for geologically very young rocks it is necessary to employ much more rigorous analysis techniques than are currently being used in many palaeomagnetic laboratories.