Palaeomagnetism of Palaeogene volcanic series of the KamchatskyMys Peninsula, East Kamchatka: the motion of an active island arc

Abstract

The Paleocene Tarkhov and Eocene Baklan Formation are the lower and upper members, respectively, of a continuousisland-arc sequence at the Kamchatsky Mys Peninsula on the Pacific side of the Kamchatka Peninsula. We studied Paleocene tuffs at five localities and Eocene tuffs and basalts at nine localities from beds of various attitude. Thermal demagnetization revealed consistent directions of the characteristic component (ChRM) of natural remanent magnetization in both formations. Normal and reversed ChRM directions in the Paleocene rocks are roughly antiparallel thus constituting a positive reversal test; at the same time, the fold test (McFadden and Jones, 1981) is not straightforward as locality-means display a banana-shaped distribution, most probably because of local rotations. Inclination-only data, however, show considerable decrease in dispersion after tilt correction thus constituting a positive fold test and implying a prefolding and most prprobably primary age of the ChRM in these rocks. The formation-mean inclination of 57.5 ± 3.8° calculated with the aid of inclination-only statistics corresponds to a palaeolatitude of 38.1 ± 4.1°N. ChRM directions isolated from the Eocene Baklan Formation are well grouped and pass both the fold and reversal tests; the formation-mean inclination of 65.0 ± 4.9° corresponds to a palaeolatitude of 47.0 ± 6.4°N. Geological data point to uninterrupted accumulation of the studied island-arc complex and, hence, to continuous activity of the subduction zone and related island arc. The difference between the Paleocene and Eocene palaeolatitudes of 8.9 ± 6.3° is statistically significant, thus implying an absolute motion of the active subduction zone and related island arc. During the Paleocene to Eocene, no other active subduction zone existed between the studied area and Eurasia. We think that the studied island arc belonged to the Pacific plate and was moving northwestward, while the oceanic part of the Eurasian (or North American) plate was subducted southward underneath this arc; this island arc became inactive in the Late Eocene and was transported with the Pacific plate which started subducting under the Eurasia margin since that time.