Wednesday, March 7th, 2018

Wednesday Seminar

4:10 PM, 55 Roessler
Tea and cookies at 3:45 in the aviary - (2110 EPS)


      – by Dr. Mitch Mihalynuk

Quantitative plate reconstructions rely on ocean isochrons, but constant recycling of ocean lithosphere ensures that only ~50% of ocean floor predates Cenozoic, and almost none is older than Jurassic. Such isochron subduction hampers efforts to reconstruct Cordilleran North America, but fortunately, subducted ocean lithosphere is not destroyed. It persists within the mantle as a folding mass, cooler than ambient mantle, and seismically visible, as fast domains called slabs.

Subducted slabs beneath North America can now be highly resolved thanks to USArray, and we use them in combination with existing isochrons to reveal evolving arc/trench-plate geometries (tomotectonic analysis) back to Pangea breakup. Resultant paleogeographies bear on fundamental questions about the assembly of North America. For example, geological relations show that eastward, Andean-type subduction formed both the native Late Triassic-Early Jurassic arc (rooted in continental crust of southwest USA and shedding clastic strata as far inboard as the Colorado Plateau) and the Cascades, but what about times in between? Most workers embrace the simplicity of an always Andean margin. An alternate viewpoint dates to the advent of Plate Tectonics (Moores, 1970, at UC Davis). It explains multiple arc and ophiolite belts as a product of westward subduction beneath an offshore volcanic archipelago.

We bring Tomotectonic analysis to bear on this debate and show that the archipelago interpretation is correct. Two massive arc complexes originated in the seas west of Pangea during its early fragmentation (~190-170 Ma), at a time when east-directed subduction beneath the continental margin arc was shutting down. Subduction zones reconfigured from EAST-directed beneath the continental margin (during final growth of the Intermontane Superterrane, IMS, also known as AltaBC), to WEST-directed, beneath an intraoceanic, massive arc chevron (MAC). East-pointing MAC was >10,000 km long and located 2000-4000 km off the west coast. Within the mantle reference frame, MAC was stationary, as indicated from the near-vertical slab walls 4-7x as thick as the mature ocean lithosphere. Between west-drifting North America and the MAC apex, all ocean lithosphere was consumed by ~155 Ma, causing a microcontinent that extended >2600 km southwards from the MAC apex, the Insular Superterrane (INS, also known as 'BajaBC'), to collide with the leading edge of North America (IMS). Initial collision of INS, which was comparable in length to the Indian subcontinent, generated 'Nevadan' deformation. As MAC was driven farther into, and raked southward along the continental margin, diachronous Sevier deformation migrated eastward and newly accreted terranes were offset sinistrally with respect to the continent. By ~130 Ma, MAC geometry was breaking down, large segments had accreted to the new North American margin, and subduction was forced to jump outboard, forming the nascent Franciscan accretionary complex: a return to eastward/Andean subduction (now the Farallon plate). Arrival of the Shatsky conjugate plateau on the Farallon plate ~90 Ma strongly coupled with Cordilleran crust, transporting it rapidly northward along a new transform margin (BajaBC > AltaBC), as recorded by upward slab truncation, an extinguished Sierra Nevada arc (80 Ma), subducted sediments underplated far inboard of the continental margin, paleomagnetic measurements, and Laramide deformation.

A remarkably complete analogue for MAC collision at ~130Ma (and nascent Shatsky conjugate collision) can be found in modern Australia's override of the Sunda-Banda / Solomon-New Hebrides arcs.

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