PLATE-KINEMATIC MODEL OF THE NEOPROTEROZOIC-EARLY CAMBRIAN BREAKUP OF THE SIBERIA-LAURENTIA CONNECTION AND THE ESTABLISHMENT OF THE CORDILLERAN MIOGEOCLINE

Evidence for Lower Cambrian sedimentological linkages between the southern Canadian Cordillera and the Olenek promontory-Kharaulakh Mountains of Siberia support a detailed plate-kinematic model of the Neoproterozoic-Early Cambrian breakup of a NE Siberia-SW Laurentia connection, and the establishment of the Cordilleran and Verkoyansk miogeoclines. Initial rifting at about 740-720 Ma zigzagged across Paleoproterozoic and Archean basement terranes and Mesoproterozoic intracratonic basins of the supercontinent, forming a south-tapering, asymmetric aulacogen with Laurentia in the uplifted footwall. Neoproterozoic and Vendian siliciclastics derived from Laurentia, and allodapic limestones derived from Siberia accumulated in the rift basin. Most of the rift-fill remained with Laurentia during Early Cambrian separation of the cratons.

In the Early Cambrian, relative movement between the plates shifted to NW-SE, parallel with a series of concentric NW-trending transform faults that we recognize along the matching zigzag margins of west Laurentia and the northern Siberian craton. The rate of seafloor spreading may be uniquely recorded by sedimentological evidence for the drift of the Olenek promontory-Kharaulakh Mountains past a spreading ridge that is preserved in the Eagle Bay assemblage of British Columbia. Earliest Cambrian Kesseyusa sandstone and conglomerate on the Olenek promontory may have spilled over onto the Siberian side from Laurentian siliclastics that have similar detrital zircon ages. A bouldery, fluvial volcanic conglomerate in the Kharaulakh Mountains dated at 534 Ma (Bowring et al., 1993) may have been derived from correlative, bouldery Eagle Bay volcanics on the spreading ridge. The Cordilleran archeocyathan reef that fringed the Eagle Bay spreading ridge at about 525-530 Ma may have bridged the young ocean basin to the Olenek promontory where correlative archeocyathans are preserved. By 520-525 Ma, when the last link between Siberia and Laurentia was severed, rapid subsidence and black shale deposition occurred on both conjugate margins. The model suggests spreading rates of about 5-10 cm/yr, and is consistent with published thermal subsidence models for the Cordilleran and Verkoyansk miogeoclines, corrected for the revised Cambrian-Ordovician time scale.