Crustal magnetization variations across the Iapetus Suture Zone

Abstract

The Iapetus Suture (Solway) line coincides with a magnetic low, which lies between magnetic highs over southwestern Scotland and the Lake District-Isle of Man region. Although topography on deep magnetic basement can account for these long wavelength geophysical variations, an explanation which involves lateral basement magnetization contrasts is preferred on the basis of (a) correlations between inferred magnetization boundaries and major structures delineated from other evidence, and (b) the apparent westward continuation of the Solway low through Ireland and Newfoundland across areas with very different subsidence histories but similar position with respect to the collision of Laurentia and Avalonia. In the preferred model, relatively

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…If the Ballantrae Complex formed through compression of an arc to back-arc zone formed in such a way, it would obviate the otherwise necessary long-distance obduction of the ophiolite across the fore-arc (Stone 2014). Another important assumption in Figure 12a is the inclusion within the fore-arc crust of the magnetic continental basement unit recognised by Kimbell & Stone (1995) as the source of the 'Galloway High' (Figure 11), and identified as 'Novantia' by Armstrong & Owen (2001). The early incorporation of Novantia at the Laurentian margin, perhaps as a part of the processes driving Late Arenig obduction of the Ballantrae Complex ophiolite, would ease the difficulty of subsequently subducting a block of continental crust beneath the Southern Uplands accretionary prism.…”

A review of geological origins and relationships in the Ballantrae Complex, SW Scotland

“…Stone (1984) argued for the strike-slip assembly of the complex, and in possible support of that mechanism it is worth noting that geophysical evidence, in particular the results of magnetic modelling (Kimbell & Stone 1995) At this point in the Ballantrae Complex narrative, with a range of possible development models in play, it is appropriate to consider the various lithological components in greater detail. These are described sequentially in the following sections so that a more informed reassessment of the origins and geological history of the complex can then be attempted.…”

“…However, geophysical evidence seems to be less indicative of a widespread ophiolitic presence at depth. Instead, gravity and magnetic modelling (Kimbell & Stone 1995;British Geological Survey 2006) shows the Ballantrae Complex to have a steeply inclined extension for at least 30 km along strike to the north-east, in a narrow zone adjacent to the north side of the Stinchar Valley -Southern Upland Fault and buried beneath younger rocks (Figure 4, section A-B). This narrow zone of ophiolitic rocks extends downwards to a depth of about 6 or 7 km and appears to separate broadly similar basement blocks most probably composed of metamorphosed Lower Palaeozoic rocks.…”

“…The Dalreoch Formation (part of the Tappins Group) was one of the earliest accreted tracts in the Southern Uplands accretionary complex and is separated from the Ballantrae Complex by the Stinchar Valley Fault, the local expression of the Southern Upland Fault which, on a regional scale, separates the Midland Valley and Southern Uplands terranes. It is possible that the magnetic anomaly farther to the north-east modelled by Kimbell & Stone (1995) as the result of a buried extension of the Ballantrae Complex (Figure 4, section A-B), also arises in part from Tappins Group rocks. However, the Southern Upland Fault as currently mapped lies to the south-east of that magnetic anomaly, making an association with the Ballantrae Complex the more likely interpretation.…”

“…This geometry is perhaps more in keeping with accretion of the ophiolitic rocks within a major strike-slip zone at the margin of Laurentia, than it is with their obduction as thrust sheets onto that continental margin. The apparent presence of dense, (Kimbell & Stone 1995). If the Ballantrae Complex arose as oceanic crust in the Iapetus Ocean, considerable northward displacement must have occurred during obduction.…”