Tuesday, March 27, 2012

Future Exotic Terranes

The continents of planet Earth are composed of central, "stable" platforms to which smaller "exotic terranes" have accreted and from which other segments of crust have rifted away. In fact, the central platforms (or cratons) are, themselves, composed of ancient segments that fused together early in geologic history and possess aborted rift zones that could reactivate in the future.

Exotic terranes are segments of continental crust that either rifted from a larger land mass or formed above oceanic hotspots, mid-oceanic ridges or subduction zones as volcanic islands. Moving with the surrounding oceanic crust, they accrete to another continent where the intervening oceanic crust subducts into a trench. Looking at the current geologic map of our planet, we can thus identify land masses that will become exotic terranes in the future (as long as the current plate movements and subduction zones do not change).

Examples include Southern California and the Baja Peninsula, which are moving NNW with the Pacific Plate toward the Aleutian Chain of Alaska, the Hawaiian Islands, which are moving toward the Kamchatka Peninsula of Siberia, and the Galapagos Islands, which are moving eastward on the Nazca Plate toward Ecuador. If the East African Rift continues to develop, southeastern Africa will become a large exotic terrane that could swivel northeastward to join India or might drift eastward and attach to Southeast Asia. Of course, all of these projections will be wrong if the current oceanic spreading zones shut down or if new continental rifts develop; we should find out in 80 million years or so.