Paper No. 7
Presentation Time: 9:35 AM
BIASES IN CONSTRUCTING COMPOSITE APW PATHS: REASSESSING TERRANE MOTIONS, TPW IN THE MESOZOIC AND THE LATE PALEOZOIC PANGEA PROBLEM
Most Phanerozoic paleomagnetic results are from sedimentary rocks in which inclination error (I-error) has been increasingly although not universally recognized. Consequently, estimates of paleolatitude for the major continents are too low. Accordingly, Kent & Irving (2010) constructed a composite APW path for the Mesozoic and Cenozoic using poles from igneous rocks and certain sedimentary formations corrected for I-error, all brought into common coordinates using plate reconstructions. Excluded were uncorrected sedimentary data including those from the Colorado Plateau whose rotation relative to stable North America requires separate study. Individual poles from large igneous provinces such as the 201 Ma CAMP are often assigned widely different ages; better documentation shows that these events were short-lived and we have collapsed them accordingly. This more stringent standard reduces the number of independent data points but does allow testing of new and revived interpretations. 1) They show that during the Late Triassic and Early Jurassic, the Wrangellia and Stikinia terranes in Canada that are prominent elements in the Cordilleran Ribbon Continent (SAYBIA of Johnston (2001, 2008) and Rubia of Hildebrand (2009)) have undergone large anticlockwise rotations and were indeed ~1000 km farther south than at present relative to the craton, which is consistent with an exotic Tethyan origin as paleontological and mantle geochemical evidences imply; through the Early Cretaceous they moved northward more slowly than the craton, implying oblique sinistral net convergence over this 130 Myr interval, followed by 2000 km dextral shear in the latest Cretaceous through the Paleocene. 2) Clustering of global poles for 160-190 Ma and especially 60-120 Ma (in North American coordinates) makes whole-Earth rotation (TPW) hardly possible during these intervals but a previously unrecognized candidate for TPW is a 30° polar shift in the Late Jurassic (~145-160 Ma) that is also present in an independent analysis of data from Adria as a promontory of Africa (Muttoni et al., 2013). Yet, Early Permian poles from Adria do not fit a standard Pangea A reconstruction, a misfit that cannot be plausibly explained either by I-error or large nondipole fields; they are fully consistent with a Pangea B model through that interval.