Relative Motions Between Oceanic and Continental Plates in the Pacific Basin

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Abstract
A model is presented for the displacement history between western North America, eastern Eurasia, and adjacent oceanic plates (Pacific, Farallon, Izanagi, Kula, and Phoenix) for the past 180 million years. The model is based on the assumption that the hotspots in the Atlantic region have remained fixed relative to the hotspots in the Pacific basin (but not necessarily relative to the spin axis). The model uses a new determination for relative motion between the oceanic plates of the Pacific basin. The results show that in a broad sense the Kula and Izanagi plates moved in a general south to north direction through the Pacific basin, implying rapid subduction beneath Eurasia and right lateral oblique subduction with respect to North America. In contrast, the Farallon plate swept in a general west to east trajectory across the basin and was accompanied by rapid subduction beneath North America. The Kula and Izanagi plates were capable of transporting allochthonous terranes rapidly northward toward the paleopole. The Farallon plate was capable of transporting terranes bearing Tethyan fauna eastward across the Pacific basin and juxtaposing those terranes against the western edge of North America, with moderate displacements toward the paleopole. A set of maps showing reconstructed plate boundaries for the past 140 Ma provides the basis for interpreting terrane displacement histories. The reconstructions are also used to estimate the ages of the plates that were consumed at convergent plate boundaries throughout the late Mesozoic and Cenozoic eras. The following events and relationships are noted: (1) fast (greater than 100 km/m.y.) convergence of the Farallon plate with respect to North America during Late Cretaceous and early Tertiary times (75 to 40 Ma); (2) rapid trenchward migration of western North America over the hotspots during this same time interval; (3) rapid decrease in age of the subducting Farallon plate within this interval of fast convergence; (4) synchroneity of these three processes with the Laramide deformation; (5) decrease in Farallon–North America and North America–hotspot velocities at about 40 Ma as the age of the subducting Farallon lithosphere decreased rapidly to less than 30 m.y. Our analysis shows that the age and bathymetry of the descending plates varied markedly along strike of the trenches. Fracture zones on the Farallon plate, across which large age offsets occur, were characterized by shallow (young) and deep (old) ocean floor on opposite sides of the fracture zones. As the fracture zones migrated north with the Farallon plate along the continental margin, buoyant young lithosphere capable of producing uplift and erosion existed immediately adjacent to dense old lithosphere capable of forming a bathymetric low. We speculate that some of the basins that formed along the continental margin during late Cretaceous and Tertiary times may have originated in these source-sink pairs. The relative velocities between continents and adjacent oceanic plates are shown on a series of maps as arrows representing velocity vectors at selected points of tectonic interest around the Pacific margin. Significant changes through time in these relative plate velocities offer insights into the mechanisms that control the diversity of tectonic styles found in the geologic record at the margins.

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