2.9B Accretion and Orogeny: Definition of Orogenesis: Western Pacific to Alaska

Orogenesis includes the processes that collectively result in the formation of mountains. Some mountain belts develop as a result of the collision and merger of an island arc, or some other small crustal fragment, to a continental block. The process of collision and accretion (joining together) of comparatively small crustal fragments to a continental margin has generated many of the mountainous regions rimming the Pacific.

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The widely accepted view is that as oceanic plates move, they carry embedded oceanic plateaus, volcanic island arcs, and microcontinents to an Andean type subduction zone. When an oceanic plate contains a chain of small seamounts, these structures are generally subducted along with the descending oceanic slab. However, very thick units of oceanic crust, such as the Ontong Java Plateau, or a mature island arc composed of abundant light igneous rock produced by magmatic differentiation, may render the oceanic lithosphere too buoyant to subduct. In these situations, a collision between the crustal fragment and the continent occurs.

A complicated sequence of events occurs when a mature island arc reaches an Andean type margin. Because of its buoyancy, a mature island arc will not subduct beneath the continental plate. Instead, the upper portions of these thickened zones are peeled from the descending plate and thrust in relatively thin sheets upon the adjacent continental block. In some settings, continued subduction may carry another crustal fragment to the continental margin. When this fragment collides with the continental margin, it displaces the accreted island arc further Inland, adding to the zone of deformation and to the thickness and lateral extent of the continental margin.

The North American Cordillera. The idea that mountain building occurs in association with the accretion of crustal fragments to a continental mass arose principally from studies conducted in the North American Cordillera. Here it was determined that some mountainous areas, principally those in the orogenic belts of Alaska and British Columbia, contain fossil and paleomagnetic evidence indicating that these strata once lay near the equator.

It is now assumed that many of the other terranes found in the North American Cordillera were scattered throughout the Eastern Pacific, much as we find island arcs and oceanic plateaus distributed in the Western Pacific today. Since before the breakup of Pangaea, the Eastern portion of the Pacific Basin, the Farallon plate, has been subducting under the Western margin of North America.

Apparently, this activity resulted in the piecemeal addition of crustal fragments to the entire Pacific margin of the continent, from Mexico’s Baja Peninsula to Northern Alaska. In a like manner, many modern microcontinents will eventually be accreted to active continental margins, producing new orogenic belts.