CENTRAL ASIAN OROGENIC BELT AND THE CALIFORNIA CORDILLERA: SIMILARITIES AND CONTRAST

The Central Asian Orogenic Belt (CAOB) and much of the North American Cordillera have long been recognized as long-lived orogenic belts formed primarily as a result of subduction without continental collision. Juvenile materials formed during the subduction process dominate both belts, and they lack large internal bodies of older (pre-orogenic) continental crust. Accretionary complexes in both appear to comprise mostly accreted trench sediments (clastic sedimentary rocks) with subordinate imbricates of ocean plate stratigraphy that represents material scraped off/underplated from a subducted oceanic plate. Both orogenic belts, in common with many orogenic belts of the world, appear to record a history of multiple episodes of subduction initiation and termination without continental collision, and both feature high-pressure and/or ultrahigh pressure metamorphic rocks with a span of ages. In spite of their similarities, notable differences exist, one of which is the nature of ophiolite exposures. The Cordillera of California features many mafic-ultramafic ophiolitic sheets of different ages, the largest ones of which are several km in thickness and extend for hundreds of km along strike. In contrast, ophiolitic bodies of the CAOB are much smaller in along-strike continuity and thickness, generally making up scattered exposures with long dimension lengths of <5 km, and commonly exhibiting the character of an ophiolitic mélange, rather than the coherent ophiolite pseudostratigraphy seen in many of the large Cordilleran ophiolite sheets. In this respect ophiolitic occurrences in the CAOB more closely resemble much of Japan than the Cordillera, in spite of the fact that Japan has been shown to be a more continental orogenic belt than the Cordillera or CAOB. This difference may reflect a fundamental difference in ophiolite formation and/or emplacement processes between the CAOB and Japan compared to the Cordillera.