A paleomagnetic test for northward tectonic transport of the eastern Selwyn Basin, northern Canadian Cordillera: data from the mid-Cretaceous Ragged Pluton

Ragged Pluton is a member of the Tungsten Plutonic Suite in the eastern Selwyn Basin. This circular massive 97 Ma granitic pluton is ∼5 km in diameter. It intrudes deformed and metamorphosed clastic strata of the Neoproterozoic to Lower Cambrian Vampire Formation. Thermal and alternating field step demagnetization data for 216 specimens from 21 plutonic sites isolated a stable characteristic remanent magnetization (ChRM) direction at 20 sites of declination D = 325.9°, inclination I = 80.4° (α95 = 2.7°, k = 142). Thermal demagnetization and saturation remanence analysis show that the ChRM is carried by both single-domain pyrrhotite and magnetite. Ragged Pluton’s paleopole at 73.9°N latitude, 191.4°E longitude (dp = 5.0°, dm = 5.2°) is concordant with its co-eval 97 Ma North American reference paleopole, showing at 95% confidence that the pluton has not been significantly rotated or translated relative to North America. This paleomagnetic result favours tectonic models in which the eastern Selwyn Basin is autochthonous, or nearly so, since the mid Cretaceous.

Paleomagnetism of the Wintering Lake pluton and the Early Proterozoic tectonic motion of the Superior Boundary Zone, Manitoba

This Lithoprobe-funded paleomagnetic study of the Early Proterozoic Wintering Lake granitoid body supports tectonic models that suggest continental accretion of the Trans-Hudson Orogen with the Superior Craton occurred at ~1822 Ma. Thermal demagnetization data for the granitoid specimens suggest that the magnetic remanence carriers are coarse-grained magnetite or titanomagnetite, and saturation isothermal remanence tests suggest that the magnetite is mostly multidomain. Six of seven paleomagnetic contact tests were negative, indicating that the host rocks have been remagnetized and that the granitoid body may have been partially remagnetized near its margins. Acceptable site mean remanence directions for 20 of 21 granitic sites yield a paleopole at 46.8°N, 102.2°W (with semi-axes of the 95% ellipse of confidence about the paleopole of dp = 11° and dm = 11°). The paleopole fits on the extrapolated apparent polar wander path (APWP) for the Superior craton at ~1822 Ma, which is the interpreted emplacement age of the pluton close to the peak of the Trans-Hudson orogeny. This is the first well-constrained paleomagnetic result from the Superior Province that provides direct evidence from concordant paleopoles for the Early Proterozoic accretion of the orogen to the craton. Further, the paleomagnetic results from the pluton's host rocks, along with other recent results from the Superior Boundary Zone, fill in a gap in the APWP for the craton between ~1780 and ~1720 Ma. The Superior path is now shown to form a hairpin as the craton moves from mid to polar paleolatitudes from ~1880 to ~1830 Ma, suffers a stillstand from ~1830 to ~1770 Ma during the peak of the Trans-Hudson orogeny, returns to mid-paleolatitudes from ~1770 to ~1740 Ma, and then moves on to subequatorial paleolatitudes by ~1720 Ma.

Indication for counterclockwise declination deviation in the Cretaceous of the North Paris Basin

Paleomagnetic sampling was carried out in Mesozoic exposures of Northwest France and Southeast Belgium. Cretaceous localities in Hainaut, Boulonnais and Normandie yielded statistically well defined paleomagnetic directions. These localities fall into two groups. Hainaut and Boulonnais are characterized by declinations suggesting 30° counterclockwise (CCW) angular deviation of declination with respect to the present North, while Normandie exhibits moderate (12°) clockwise (CW) angular deviation with respect to the present North. In Normandie, we also observed occasionally very weak signals of a CCW deviated component, which, however, could not be treated statistically. Fold test suggests that both groups of samples were remagnetized during deformation. The overall mean paleomagnetic declination of the first group is westernly, that of the second group practically coincides with post-Eocene European reference directions, From assessing structural inversion and relevant paleostress-directions in Normandie, remagnetization may be connected to tectonic inversion in late Eocene-Oligocene times. While the statistically meaningful paleomagnetic result for the second group fit the synthetic stable European apparent polar wander path (APWP) at about 30 Ma, the overall mean paleomagnetic direction of the first group (based on 5 localities, representing 42 samples) defines a pole which is significantly offset from it, at any time following the deposition of the studied sediments. One explanation of this offset could be intraplate rotation on a small (Brabant Massif) scale. However, the angle of deviation in declination seems to be too large for a tectonic solution. Furthermore the data obtained from the Paleozoic do not show such declination deviation. It seems, therefore, that we need to increase considerably the direct stable European paleomagnetic database for the late Cretaceous-Tertiary, in order to further improve the late Cretaceous-Paleogene segment of the European APWP.