Palaeomagnetic study of the (late Mesoproterozoic) Torridon Group, NW Scotland: Age, magnetostratigraphy, tectonic setting and partial magnetic overprinting by Caledonian orogeny

The (late Mesoproterozoic) Torridonian Supergroup is a predominantly fluviatile ∼7 km thick succession of red sediments deposited onto (Palaeoproterozoic) Lewisian metamorphic terrane of the Hebridean Craton in NW Scotland. Although a subject of investigation since the earliest days of palaeomagnetism, the age and constitution of the remanent magnetisation in the Upper Torridonian (Torridon Group) have remained unclear. In this study we undertake detailed statigraphically controlled sampling through the succession and conduct progressive thermal demagnetisation to resolve component structures. Rock magnetic analysis shows that magnetite, although volumetrically small, is a significant component of the ferromagnetism in these sediments in addition to specularite and pigmentary hematite. Unlike the Lower Torridonian (Stoer Group), which was fully lithified prior to late Grenville deformation (∼1100 Ma), the Torridon Group was not influenced by tectonism until Early Palaeozoic times and records a complex history of partial, and sometimes total, later overprinting. The A remanence is extensively overprinted by a B remanence with E to S directed intermediate to steep positive directions and subordinate reversed representatives and less commonly by a C remanence with NE+ direction. B is resolved mainly in the magnetite spectrum and incorporates a swathe of directions coincident with mid Ordovician palaeofields predicted from Baltica and adjoining Caledonian terranes. It is therefore interpreted as a remanence residing in small amounts of authigenic magnetite acquired during burial of this margin of the Hebridean Craton by nappe stacking during the Grampian Orogeny (∼470-430 Ma) following final closure of the Iapetus Ocean. Due to the merging of the B population with the A+ direction, A has not been defined as an antiparallel axis by former studies and can be inseparable by cleaning. However, defining A to include components with I < 50° at 140 sites yields a mean of D/I = 115/+31° (α95 = 2.9°) for A+ and D/I = 293/−25° (α95 = 3.5°) for A−. The poles at 238°E, 1°S (dp/dm = 2.0/3.8°) and 235°E, 1°N (dp/dm = 1.8/3.3°) are close to the Laurentian APWP at 1039-1025 Ma on the pre-drift reconstruction and constrain sedimentation within the 1060 to 994-977 Ma range of previous evidence. Only minor polar movement is detected through the entire thickness of the Torridon Group and suggests a sedimentation rate of the order of 0.5 m/1000 years for this clastic succession. A magnetic stratigraphy in A suggested by early work is confirmed in at least 13 polarity zones and identifies a rapid reversal rate preserved in specular hematite fixed by post-depositional mechanisms postdating slumping and sand volcano formation in wet sediment. The palaeomagnetic age of the A remanence links Torridonian sedimentation to rifting and subsidence accompanying break up of the Laurentian and Fennoscandian Shields at ∼1050-950 Ma following the Grenville Orogeny. Rifting between the two shields was followed by rotation and rewelding into a Neoproterozoic configuration. These events correlate with transition in Torridon Group palaeoenvironments from enclosed basin, to open basin with river systems feeding onto a passive margin, to marine transgression, and finally return to enclosed basin conditions. Diagenesis continued in post-Palaeozoic times and a D remanence with N-S shallow axis is sporadically preserved in authigenic hematite and interpreted as mainly a Variscan acquisition.