PALEOMAGNETIC CONSTRAINTS ON PALEOGEOGRAPHIC RECONSTRUCTIONS OF CORDILLERAN TERRANES

Paleomagnetic results from rocks are commonly used to determine their location relative to the spin axis at the time of their magnetization. Using rocks from both tectonostratigraphic terranes and those associated with cratonic portions of tectonic plates, paleogeographic reconstructions can be made using this paleomagnetic reference frame. The ability to determine the paleolatitude of rocks from a given terrane, and to compare this with predicted locations derived from a given tectonic model makes paleomagnetism an interesting tool for terrane analysis. This presentation will provide a review of paleomagnetic methods, and a discussion of the paleomagnetic datasets available for North America, and for many of the Cordilleran terranes, with a focus on Mesozoic time.

For North America, a significant number of results from cratonic rocks are available for most of Mesozoic time, with some important intervals (early-middle Jurassic, and early Cretaceous) being poorly represented. To augment these data several composite apparent polar wander path (APWP) models for NA have been constructed using results from other plates rotated into NA coordinates. These composite APWPs (Torsvik et al 2001, Besse and Courtillot, 2002, Enkin 2007) will be compared with APWPs that rely only on NA-derived data (e.g. Beck and Housen, 2003). All models place the Cordilleran margin of NA at low paleolatitudes during the Triassic, and at very high latitudes during mid-late Cretaceous time. Significant differences (arising from data selection criteria) exist for middle-late Jurassic time.

Data from tectonostratigraphic terranes are abundant for some of the larger terranes, but are poor or non-existent for many others. Complications include the common association between remagnetization and deformation, and the often poor paleomagnetic behavior of the clastic sediments that are common to many of these terranes. Despite these complications, large sets of data are available, and indicate that many terranes have experienced significant translation relative to North America. Some key questions, such as the timing and amount of dextral-then-sinistral translation models, are dependent on the NA APWP model used. Others, such as the mid-late Cretaceous “Baja-BC” models, are clearly supported by all of the available paleomagnetic data.