Strike-slip faulting and block rotations in the McConnell Creek area, north-central British Columbia : structural implications for the interpretation of paleomagnetic observations

Structures associated with dextral transpression on Finlay-Ingenika fault (FIF) in the McConnell Creek area, north-central British Columbia, are dominated by subvertical to vertical strike-slip faults, as indicated by a variety of kinematic indicators. Position-gradient tensors determined from strain analysis of deformed volcanic fragments in a succession of Late Triassic Takla Group rocks abutting one of the north-northwest trending faults indicate that they are dextrally transpressive, in accord with the common occurrence of subvertical intersections of the fault planes with other planar fabrics and subhorizontal stretching lineations within the fault zones. Principal directions of the magnetic susceptibility ellipsoids from Early Jurassic to Cretaceous dioritic rocks are consistent with those of the strain ellipsoids in their adjacent volcanics, suggesting that the plutonic rocks experienced the same deformation as the Late Triassic Takla Group volcanics. The age of the strike-slip faulting is unknown but was probably not earlier than mid Cretaceous as the Early Jurassic to Cretaceous dioritic rocks were involved in the faulting in many places.Some of the major faults cut the area into discrete, fault-bounded blocks several kilometres wide. As the dextral motion on the FIF progressed, deformation was concentrated within the previously formed fault zones while the fault-bounded rocks remained weakly deformed. Stress tensors for 24 sites were inverted from the regional cleavage within the blocks, which predates the block-bounding faults. The site-mean stress tensors indicate that the fault-bounded blocks were rotated clockwise about a subvertical axis during progressive dextral motion on the FIF. The amount of rotation varies systematically over the area with maximum value (58.7 $ pm$ 3.3$ sp circ$) close to the FIF and minimum value (0.0 $ pm$ 1.6$ sp circ$) about 20 kilometres away from it.Paleomagnetic samples were collected from 13 sites in the widespread Early Jurassic to Cretaceous plutonic rocks. Interpretable magnetic components of the Late Cretaceous, isolated by both stepwise AF and thermal demagnetization from 6 sites, were used to calculate the paleopole. The observed paleopole has a low value of Fisher's precision k = 48, and is significantly different from the Late Cretaceous reference pole for cratonal North America (CNA). After corrections for the local block rotations, the observed paleopole has a high value of k = 383 and moves closer to the reference pole but is still significantly different from it. Rotation about the Eulerian pole for the best-fitting small circle to the Tintina trench and northern Rocky Mountain trench fault zone, however, brings the observed pole into coincidence with that for CNA, and requires only $ sim$670 kilometres of dextral displacement on the fault zone. It is likely, therefore, that local structures associated with dextral strike-slip faults in the western Canadian Cordillera may account for at least part of the paleomagnetic disparity between some western parts of the Canadian Cordillera and CNA