Deciphering shallow paleomagnetic inclinations: 2. Implications from Late Cretaceous strata overlapping the Insular/Intermontane Superterrane boundary in the southern Canadian Cordillera

[1] Significant discrepancies exist between paleomagnetic and geologic estimates of the timing and magnitude of terrane displacement in the southern Canadian Cordillera. Lithostratigraphic, palynological, geochronologic, geochemical, and structural data, to which we add paleomagnetic data, demonstrate that Upper Cretaceous strata in Churn Creek are laterally equivalent facies of the Silverquick/Powell Creek (SPC) succession. These rocks therefore comprise a single overlap sequence linking the Insular and Intermontane Superterranes by Late Albian-Cenomanian time (similar to95 Ma), contradicting previous assertions based solely on paleomagnetic interpretation that major (similar to2000 km) latitudinal displacement separated the two regions. The Churn Creek succession retains primary magnetic remanence, as demonstrated by positive fold, conglomerate and contact tests. The sediments do not hold the anisotropy signature of compaction-shallowed inclinations. The mean inclination of all SPC sites, from both Mount Tatlow and Churn Creek, is 55.6degrees +/- 2.3degrees (N = 36 sites), corresponding to a paleolatitude of 36.1degrees +/- 2.4degrees. This reflects northward translation of the composite Insular/Intermontane Superterrane of 3050 +/- 450 km between 85 and 50 Ma. The Silverquick/Powell Creek succession at Churn Creek unconformably overlies the northern end of the Spences Bridge Group, a late Early Cretaceous volcanic arc assemblage on the Intermontane Superterrane. Paleomagnetic data in the companion paper place it 1050 +/- 450 km south of its current position between 100 and 105 Ma. Integration of these new data sets require that the Insular and Intermontane Superterranes formed a single, enormous crustal block that underwent complex, episodic south-north translation between circa 105 and 50 Ma. The timing, rate, and vectors of translation are problematic in terms of geologic constraints along the continental margin.