A 3RD VOLCANIC CHAIN IN KAMCHATKA - THERMAL ANOMALY AT TRANSFORM CONVERGENCE PLATE BOUNDARY

The Kamchatka volcanic arc, which is located at the northern edge of the Kurile arc, consists of three volcanic chains, all parallel to the trench axis. In contrast, most subduction zones have only two subparallel volcanic chains. The third chain in Kamchatka, which is farthest from the trench, is characterized by the occurrence of voluminous plateau lavas; volcanoes in the two chains closer to the trench are stratovolcanoes typical in arc magmatism. The third chain magmatism is also unusual in that lavas show concentrations of incompatible elements intermediate between those in the two trenchward chains. Both the unusual occurrence of the third volcanic chain and the unusual lava chemistry could be caused by partial melting of K-amphibole bearing periodotites in the downdragged hydrous layer at the base of the mantle wedge under anomalously high-temperature conditions associated with the characteristic tectonic setting of transform/convergence transition in the region.