Reappraisal of Displacement of Northern Cordilleran Terranes Since the Triassic and Jurassic

Details Reappraisal of Displacement of Northern Cordilleran Terranes Since the Triassic and Jurassic Reappraisal of Displacement of Northern Cordilleran Terranes Since the Triassic and Jurassic

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp44a..01k&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #GP44A-01

Mathematics

Logic

1525 Paleomagnetism Applied To Tectonics: Regional, Global, 1527 Paleomagnetism Applied To Geologic Processes

Scientific paper

Triassic-Jurassic paleomagnetic data from the Cordillera of Canada had initially indicated displacements of about 1000 km from the south. Later, as a result of revisions in the APW path for North America, it was contended that there had been no such displacements. The Cordilleran data are from Wrangellia (Nicolai/Karmutsen (225 Ma) and Bonanza (195 Ma) formations) and Stikinia (Hazelton (195-180 Ma) and Stuhinni (~210 Ma) formations). These are two largest exotic superterranes in the Cordillera. Data all are from massive lavas (mainly) or diabase sills whose bedding attitudes are well controlled and whose magnetizations are believed to record accurately the field direction. On the other hand, several of the APW paths determined from North America that were sometimes used for reference are dominated by results from sedimentary rocks, which recently published work using directional distributions and anisotropy measurements has demonstrated are commonly affected by inclination flattening, for example, Late Triassic and Early Jurassic sedimentary rocks in rift basins of eastern North America. When corrected for inclination flattening, sedimentary data agree with inclinations from coeval igneous rocks when these data are available, for example, 200 Ma CAMP volcanics, but don't support the J1 cusp, previously a cornerstone of many APW paths for North America. In order to avoid possible shallow bias in latitude determinations, we make a special effort to avoid errors arising from inclination-flattening in sedimentary rocks by (1) using only data from sedimentary rocks that have been corrected for inclination-flattening, and by rejecting all other sedimentary data, and (2) incorporating igneous data from all other major continents. Relative to present geography the global APW path relative to North America begins in Mongolia in the Early Triassic, trends north-northwest towards the estuary of the Ob River by the end of the Early Jurassic, where it lingers before rapidly moving to the vicinity of Nunivak Island by the end of the Jurassic (and then boomerangs to the Cretaceous standstill position in the Chukchi Sea). Cordilleran magnetizations always have inclinations that are shallower than expected from the reference path. There is therefore no ambiguity about the sense of displacements - they are always from the south relative to cratonic North America. However, within these structurally disrupted terranes, only individual poles and not their paths can be constructed and there are uncertainties in relating individual results to the global polarity time-scale, and so we cannot say, from the paleomagnetic evidence alone, whether the terranes were in the southern or the northern hemisphere during the early Mesozoic. Assuming the closest (northern hemisphere) position, all the Cordilleran terrane strata nevertheless give displacements of about 1000 km or more. The Nicolai of Alaska yields a displacement of ~2500 km which reflects the strong northward motion of SE Alaska in the Tertiary. We believe this settles the debate: Triassic and Jurassic rocks of Wrangellia and Stikinia all have been displaced significantly from the south. Notable also are the strong anticlockwise rotations of 40°-60° from the Nicolai (Triassic), Bonanza and Hazelton (Jurassic), which have been observed also by Enkin throughout the stratigraphy of the Skeena fold- belt of central Stikinia. The Karmutsen (Triassic) of Vancouver Island shows a huge 150° anticlockwise rotation, perhaps a composite of several deformation phases.

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