Tectonic structual features of the Fauske conglomerates in the Løvgavlen quarry, Nordland, Norwegian Caledonides, and regional implications
A quarry excavated in a multilayered, carbonate conglomerate formation of apparent Cambrian age in the Fauske Nappe of the Uppermost
Allochthon, Nordland, provides evidence for three principal phases of Caledonian deformation. The 60 m-thick Fauske conglomerates are characterised by a rapid, vertical facies change from carbonate debris and carbonate breccia to rhythmite conglomero-breccia and graded calcareous greywacke. The carbonate clasts were deposited from southeast-directed turbidity currents on the palaeoslope of a basin that deepened to the southeast. Both the basin physiography and the palaeocurrent directions are opposite with respect to features to be expected along the former Baltoscandian margin of Baltica.
The earliest tectonic structures are NW-directed, ramp-and-flat thrust faults and fault-bend folds, that progressed in to a higher-strain regime
involving clast flattening and elongation, and the development of an amphibolite-facies schistosity, S1. The early staircase thrusts and folds therefore record a tectonic reversal of the basinal regime, as well as documenting an opposite structural vergence with respect to thrust -sheet assembly during Scandian orogenesis. The early structures are overprinted by SE-verging folds and small-scale thrusts, and a penetrative, NW-dipping, S2 cleavage. Later minor structures are mainly extensional.
The combined palaeodepositional and early structural picture is highly reminiscent of the geological development that has been recorded along
the Laurentian margin of the Iapetus Ocean in Cambro-Ordovician time, involving an extensive carbonate shelf and adjacent slope-and-rise. We
postulate that the Fauske conglomerates originated along that former passive margin. The staircase thrusting and early folds, and earliest metamorphic fabric, s1 , are Jikely to be Taconian (Mid to Late Ordovician) . The Fauske Nappe itself, on the other hand, is a part of the SE-translated Caledonian allochthon, generated during the Scandian orogeny (Late Silurian-Early Devonian) as a result of Baltica-Laurentia continent-continent collision. The second-generation, SE-verging folds and minor thrusts, and associated NW-dipping cleavage (52), are almost certainly of
Scandian age. Later, extensional structures at Fauske are probably a reflection of the WSW-directed, Early to Mid Devonian shear regime recorded
widely in this part of the Norwegian Caledonides.