Pleistocene-Holocene Monogenetic Volcanism at the Malko-Petropavlovsk Zone of Transverse Dislocations on Kamchatka: Geochemical Features and Genesis

The Malko-Petropavlovsk zone of transverse dislocations (MPZ) was formed on the extension of the deep Avachinsky transform fault in perpendicular relation to the subduction trench. It is a natural boundary between variously aged slabs in Kamchatka (103-105 Ma under Southern Kamchatka and 87-92 Ma under the Eastern volcanic belt). Monogenetic cinder cones in the MPZ are randomly distributed along these long-lived rupture zones. Here we present new geochemical and isotopic results of monogenetic volcanism in the MPZ. Based on whole rock and trace element geochemistry, Pb-Sr-Nd isotopic ratios of monogenetic cinder cone magmas were shown to tap the enriched mantle source (low Nd-143/Nd-l44 isotopic ratios (0.512959-0.512999), and changed as Sr-87/Sr-86 (0.703356-0.703451) and Pb-206/ Pb-204 (18.30-18.45), Pb-208/Pb-207 (38.00-38.12) isotopic ratios). High Nb/Yb and La/Yb ratios, without significant inputs of the slab's components (the lowest Ba, Th content), indicate decompression melting predominately. Calculations of the pressure (9-11 kbar) and temperature (1160-1240 degrees C) conditions using a glass thermobarometer suggest that magma of monogenetic cinder cones resided near the Moho boundary prior to eruption. This correlates with the crustal discontinuity under the MPZ according to geophysical observations (converted-wave seismic exploration and magnetotelluric sounding). The majority of well-preserved monogenetic cinder cones were formed in Holocene, after the last glaciation, but eruptions were not observed historically. This, however, suggests that similar eruptions in the MPZ may occur in the future. Given that the MPZ hosts major population centres of Kamchatka (Petropavlovsk, Elizovo, Vilyuchinks, and Paratunka: similar to 250,000 people or similar to 80% of the whole Kamchatkan population live in the major cities on the coastline of the MPZ), we highlight the urgent need to install a continuous monitoring system around the MPZ cones, geophysical investigation, and more serious attention from the local government and scientists. In particular, a detailed study of the MPZ regarding age, volume, and volcanic hazard assessment (pyroclastic vs extrusive) will help reduce potential risks of eruptions from monogenetic volcanoes for humans and infrastructures.