Potassic Volcanism Induced by Mantle Upwelling Through a Slab Window: Evidence From Shear Wave Splitting Analyses in Central Java

Abstract

AbstractTo delineate the mantle flow fields in the vicinity of a previously proposed slab window and the possible roles that they may play in the formation of potassic volcanism in Central Java, we conduct shear wave splitting analyses using both local S and teleseismic XKS waves (including SKS, SKKS, and PKS) recorded by 121 onshore stations and two ocean bottom seismometers (OBSs). The XKS fast orientations from the OBSs are trench normal and in accord with previous subslab anisotropy measurements. In the eastern part of Central Java, the XKS and local S fast orientations from the onshore stations are mostly trench‐parallel; in contrast, in the western part of Central Java, the XKS fast orientations are trench‐normal while the local S measurements are spatially varying. The observations can be attributed to four flow systems including (a) subslab trench normal mantle flow in areas away from the trench which is entrained by the Australian Plate, (b) trench normal flow that goes into the mantle wedge from the subslab area through a slab window beneath the western part of the study area, (c) trench‐parallel subslab flow near the trench beneath the eastern part of the study area which is driven by slab‐rollback, and (d) dominantly trench‐parallel flow system in the mantle wedge reflecting the horizontal component of the escaped flow system through the slab window. We propose that the vertical component of the escaped flow is responsible for the formation of the potassic volcanoes in the adjacent oceanic area.