ПРОДОЛЬНЫЕ СДВИГИ В ОСТРОВНОЙ ДУГЕ ПРИ НОРМАЛЬНОМ ПОДДВИГЕ ОКЕАНИЧЕСКОЙ ПЛИТЫ: ПРИМЕР КАМЧАТКИ

Abstract

Crustal deformation in Kamchatka since the Middle Pleistocene is driven by two processes: transverse extension as a result of its eastern part displacement towards the ocean following the submerged part of the oceanic plate, and transverse shortening just north of the subduction zone as the Komandor block of the Aleutian arc gets closer to Kamchatka. The areas of differently directed movements and deformations of Kamchatka are separated by the left-lateral horizontal flexure appearing in plan as curved uplifts of the Eastern Range, the deep water trench, and consequently, the axis of the East volcanic belt. Kinematically, the flexure replaces the expected transverse left-lateral strike-slip fault between the two differently deformed parts of Kamchatka. The revealed right-lateral displacements along the longitudinal faults in the northern part of the uplift of the Eastern Range in Kamchatka result from bedding-plane slip in the flexure limb. The high alluvial terrace of the Kamchatka River, displaced landforms, and faulted layers of the soil-pyroclastic cover were dated by the tephrochronological method. Age determinations imply that the recurrence of displacements along the faults of the flexure is about once every 3-5 thousand years, and the average rate of lateral displacements during the second half of the Holocene is about 1 mm per year. Comparison of the fault movement rates and the maximum value of the accumulated lateral displacement of the Kamchatka River valley suggest that horizontal movements began no later than the second half of the Middle Pleistocene. This also means that the Kamchatka River valley already existed at that time, and its antecedent part was formed when down-cutting into the bedrock of the uplifting Eastern Range. The total amount of displacement along the faults of the flexure since the second half of the Middle Pleistocene is estimated at about 1 km. On the whole, the results suggest the possibility of lateral movements along longitudinal faults in an island arc with the oceanic plate direction being normal to it.