Late Proterozoic reconstructions of North-West Scotland and Central Canada: Magnetic fabrics, paleomagnetism and tectonics

Abstract

Red-beds dominate the Stoer Group and the unconformably overlying Torridon Group, having accumulated between ∼1400 and ∼1000 Ma, in a rift of the Hebridean foreland. The sequences were weakly strained and shortened E–W, and each underwent successive diagenetic changes which altered their magnetic properties and caused their different characteristic post-depositional magnetizations. Anisotropy of magnetic susceptibility (AMS) reveals an N–S vertical tectonic foliation superimposed on the bedding-planar AMS sub-fabric. The tectonic AMS sub-fabric was isolated by comparing normalized and non-normalized mean tensors of multiple specimens. The N–S vertical AMS tectonic foliation postdates warps of bedding; thus the high-susceptibility minerals, including those carrying palaeomagnetic signals re-oriented or recrystallized after deposition and diagenesis. Thus characteristic remanence vectors (ChRM) were acquired long after deposition, first in the Stoer Group, then in the Torridon. ChRMs were isolated for 143 Stoer specimens and 94 Torridon specimens using two cycles of low-temperature demagnetization followed by at least 12 steps of thermal demagnetization. For the Stoer Group, two structurally integral groups of sites yield mean magnetizations of 317.0/+43.1 α 95 = 11.3 ( n = 49) and 309.9/52.1 α 95 = 10.6 ( n = 23). Some Stoer specimens bear a Torridon age overprint 133.3/+525 α 95 = 9.6 ( n = 19). The younger Torridon Group yields 138.4/+52.2 α 95 = 7.4 ( n = 42); 134.9/+45.5 α 95 = 16.0 ( n = 15) and 134.4/+55.1 α 95 = 11.3 ( n = 19) from three structurally integral clusters of sites. These late chemical magnetizations postdate bedding warps, faults and tectonic AMS fabrics; they do not warrant local “rigid-body” tilt corrections. However, we restored the ChRM directions for the regional post-Cambrian eastwards tilt which affected the entire foreland from the Lewisian basement to the overlying Proterozoic strata. Paleopoles calculated from the strata's restored vectors were rotated into Laurentian coordinates for comparison with Canadian Proterozoic palaeopoles. The Stoer Group was magnetized in a normal polarity geomagnetic field whereas the Torridon Group was magnetized in a reversed polarity epoch. However, both sequences include at least one polarity transition, indicating long duration chemical magnetizations successfully averaging secular variation. At a minimum, red-bed magnetization processes exceeded 0.1 Ma and may have encompassed several million years. During Stoer Group magnetization, the Hebridean craton was at paleolatitudes of 26–44°N; when the Torridon Group magnetized it was at 23–27°S. Accepting published ages, this defines a minimum southward latitudinal displacement rate of 27–40 km/Ma. Restored Stoer Group paleopoles lie in the present-day north-central Pacific Ocean (∼190E/25N); corresponding to the Laurentian APWP dated at between 1090 and 1109 Ma. Torridon Group paleopoles lie in the south-central Pacific (∼210E/25S) near the poorly documented (<900 Ma) southernmost part of the Laurentian APWP.