COLLISION AND MOUNTAIN BUILDING ALONG NORTH PACIFIC MARGINS

Along the North Pacific ocean margin, the Yakutat Terrane in the Gulf of Alaska, and Cocos Ridge beneath Costa Rica collide with the continent. Reconstructing the subducted Yakutat Terrane involves knowing its relation to the Pacific plate. Its geologic history is traditionally constructed mostly from onshore geology with the terrane partially decoupled from the Pacific plate. However from a marine perspective explaining the growth of 2 huge deep sea fans involves a terrane coupled to the plate. Earthquake seismology indicates a vast subducted ocean plateau with crust up to 22 km thick If the terrane connected North America with the heads of the fans the terrane was connected with the continent during its northward migration. This necessity also provides an estimate of terrane extent. Apparently a 400km wide terrane subducted coeval with uplift of the high coastal mountains. After 5 Ma the uplift supported mountain-valley glaciers.

Subducting beneath Costa Rica is Cocos Ridge formed from Galapagos hot-spot volcanism. Under its crest the crust is 25 km thick and from it Galapagos lavas extend 400 km laterally. The ridge flanks support numerous seamounts and subduction of seamounts has severely eroded the adjacent continental slope. Opposite the ridge are 5-7 Myr old mountains, among the highest of Central America that presumably formed during ridge collision. Active arc volcanism terminated about 2-3 Myr ago and uplift exposed the Miocene volcanic arc. Folding occurs above the seismogenic zone consistent with movement indicated by satellite geodesy. Continuing coastal uplift is evidenced by many elevated coastal terraces.

Collision at both locations involves subduction of buoyant crust ~25 km thick. Despite similar duration of subduction, two very different kinds of oceanic features collide and two different scales of coeval continental uplift and trench sedimentation are observed. The older and colder Yakutat Terrane is associated with an on-land collision suture, larger mountains, and larger earthquakes than along Costa Rica. The elevated temperature of Cocos Ridge is associated with an unusually shallow up-dip seismic limit but lower mountains. Not only the size but also the character of the subducting relief are associated with different slope morphologies and modern earthquake patterns.