Geologic tests of hypotheses for large coastwise displacements - A critique illustrated by the Baja British Columbia controversy

We present two mutually contradictory hypotheses for the mid- and Late Cretaceous paleogeography of the western margin of North America. In Hypothesis A, representative of a family of models based on geologic evidence, the Insular and Intermontane superterranes and the Intervening mid-Cretaceous Coast Mountains orogen were situated in their present positions relative to cratonic North America, 90 my ago. In the alternative Hypothesis B, based on paleomagnetic data, the Insular superterrane and the Coast Mountains orogen along: its eastern edge were situated 3000 km south of their present positions, 90 my ago. This crustal element, Baja British Columbia, then moved northward between 90 and 70 Ma along the hypothetical, coast-parallel, Baja B.C. fault system. At 70 Ma, Baja British Columbia was Juxtaposed with the western part of the Intermontane superterrane, which lay 1100 km south of its present position. Between 70 and 50 Ma, the composite block moved northward an additional 1100 km along the hypothetical Intra-Quesnellia fault, located within the Intermontane superterrane. A goal of the hypothetico-deductive scientific method is the elimination of false hypotheses using empirical tests. A review of the logical arguments governing this method shows that if two contradictory hypotheses are judged to have non-negligible prior probabilities-if both have a significant likelihood of being true-then a crucial test can potentially lead to the refutation of one hypothesis and provide empirical support for the alternative. A crucial test is possible when an observational prediction of one model is prohibited by an alternative model. To evaluate whether hypothesis A or B can be ruled out by geologic evidence, we propose four crucial tests independent of paleomagnetic tests. To test the Baja British Columbia hypothesis, potentially useful geologic evidence includes: (1) the provenance of detritus in mid-Cretaceous strata in the Methow and Tyaughton basins; (2) the provenance of detritus in pre-late Campanian strata in the Nanaimo Group; (3) offset geologic features in BaJa British Columbia matching counterparts in southwestern California and northwestern Mexico; and (4) Late Cretaceous or older geologic features limiting offset across the transcurrent Baja B.C. fault to < 1000 km.