The Motion of Allochthonous Terranes Across the North Pacific Basin

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Abstract
In order to establish southern and western limits to possible points of origin of terranes, we calculated the routes or trajectories by which terranes were carried aross the Pacific basin and along the margin of North America. These trajectories were then tested for internal consistency with paleomagnetic results. Beginning with specific plate tectonic models that describe the motion of adjacent oceanic plates relative to North America, we found a set of terrane trajectories that show the position of terranes as a function of time as the terranes moved with the oceanic plates or were driven by them tangentially along the continental margin. Elements used to define a trajectory are (1) the stage poles describing the motion of the oceanic plates relative to the continent, (2) the sequence of plates carrying the terrane, (3) the time of docking of the terrane, and (4) the coordinates of the point of docking. Additional constraints are that terranes are not permitted to migrate across ridges and that the oceanic plate carrying a terrane cannot be younger than the terrane. The plate model of Engebretson and others (1985) and several of its variants were used in our analysis. For docking times of 30 Ma, trajectories are short because the Pacific-Farallon spreading system is close to the margin. For docking times of 60 Ma, terrane trajectories indicate northward transport by as much as 60° of latitude. For docking times of 120 and 90 Ma, trajectories indicate easterly transport across as much as 60° of longitude. For the Wrangellia, Central Salinia, and Point Arena terranes and the Laytonville Limestone, paleolatitudes were found as a function of time for different plate models, and these results were compared with paleomagnetically determined paleolatitudes from the terranes. The two sets of paleolatitudes were generally consistent only for plate models in which, when the Farallon plate rifted at 85 Ma to form a Kula plate in the north, leaving a smaller Farallon plate in the south, the newly formed Kula plate occupied a large region adjacent to North America, so that the Central Salinia and Point Arena terranes and the Laytonville Limestone were all located north of the Kula-Farallon rift. In addition, a rapid rate of spreading between the Farallon and Pacific plates during the Cretaceous normal superchron is required to model the paleolatitude of the Laytonville Limestone but not that of the other terranes. No terrane trajectory for Wrangellia was found that placed this terrane in the southern hemisphere during Jurassic time, as has been suggested on the basis of paleomagnetic data. The Wrangellia trajectories are consistent with the model of Irving and others (1985) in which Wrangellia arrived at the continental margin at the present latitude of Baja California at ∼100 Ma, was intruded at that latitude by the Coast Plutonic Complex, and then moved tangentially along the coast driven by oblique convergence of the Farallon and Kula plates.

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