A BASEMENT CULMINATION IN THE SCANDINAVIAN CALEDONIDES FORMED BY ANTIFORMAL STACKING (BANGONAIVE, NORTHERN SWEDEN)

The Bangonaive basement culmination, a doubly plunging antiform trendi ng SW-NE in its southern part and SSW-NNE in the north, is part of a m ajor Caledonian antiform in north-central Scandinavia. Crystalline, Pr oterozoic basement rocks (mainly syenite) are unconformably overlain b y a sedimentary cover including tillites at the base, passing up into arkoses, quartzites and shales, capped by black phyllites. This sequen ce is correlated with the Varangian to Cambrian succession of the Balt oscandian platform farther east. Detailed mapping revealed a successio n of five basement-cover horses, which represent the accessible part o f an antiformal stack exposed beneath the Middle and Upper Allochthons and taken here as the Lower Allochthon structural level. Caledonian d eformation varies in intensity from penetrative near thrusts and in pe litic rocks to very weak in the more internal parts of the horses. A p enetrative foliation is associated with the growth of white mica and r are biotite. This early fabric is overprinted by a mylonitic foliation related to localized shear zones, which separate the structural units within the Lower Allochthon. Stretching and mineral lineations trend WNW-ESE and related shear-sense criteria indicate transport (top) towa rds the ESE. Structural units (horses) are thrust into an antiformal s tack and folded around the lowermost horse exposed, which is itself fo lded into an anticlinal lift-off fold. Towards the northeast, the anti formal stack is overprinted by a pop-up and an out-of-sequence thrust. The latter breached the roof of the Lower Allochthon and transported part of it over the Middle and Upper Allochthons. Further folds are as sociated with lateral and oblique ramps in the Lower Allochthon. These structures relate very well with the complex fold pattern previously observed in the higher structural units and thrust tectonics provides a straightforward genetic explanation for these folds. Therefore, earl ier genetic models of the Bangonaive basement culmination as a simple imbrication of basement into higher units, as a buckling structure or as a gravitational dome structure are rejected here. The structural in formation, supported by gravimetric data, is consistent with an essent ially flat regional detachment surface (2-degrees dip) extending from the present external Caledonian margin to the base of the Bangonaive a ntiformal stack.