TESTING ALTERNATIVE MODELS FOR THE LATE MESOZOIC PALEOGEOGRAPHY OF THE CORDILLERA

Diverse kinds of evidence have been used to formulate and test paleogeographic models, including a conceptualization that places Baja British Columbia thousands of kilometers south of its present position in Late Cretaceous time. Each kind of test involves assumptions and limits on resolution. The criteria for evaluating paleomagnetic data are well established; paleolatitudes are typically stated with accuracies of +/- 500 km. Paleobiogeographic analyses are subject to assumptions about paleotemperature and circulation models that may themselves depend on paleogeography. Geologic tests, some of which were proposed by Cowan et al., in 1997, include identifying the provenance of detritus in strata deposited on possibly displaced terranes and matching geologic units along the continental margin. Such crucial geologic tests are difficult because sources of detritus—magmatic belts, formerly accreted terranes—are disposed for hundreds or thousands of km along the margin, parallel to the putative coastwise displacements. Needed are new provenance methods (some now viable) that work to discriminate between similar potential sources of sediment along the continental margin. Moreover, almost nothing is known about Late Cretaceous paleo-drainage patterns in the western Cordillera. Restoration of geologically well-controlled offsets along known faults indicates that much of Baja BC was located about 800 to 1000 km farther south in early Paleogene, and possibly earlier in Late Cretaceous, time. Recent analysis of paleomagnetic data shows that the upper limit of translation may be ~2000 km, not the previously thought 3000 km. We argue that (i) the restoration using known faults makes ~800 km translation a new minimum position for Baja BC, (ii) virtually all tests applied to the problem since 1997 either have ambiguous results or are compatible with conceptual models that place much of Baja BC at least 2000 km south of its expected paleolatitude in early Late Cretaceous time, and (iii) none of the tests is compatible with a Late Cretaceous paleogeography in which Baja BC is approximately situated in its present position with respect to continental North America. These developments in analyzing Baja BC raise a major problem with conventional convergent-margin models for California.