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

To 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. Plain Language Summary: It has been well established that the vast majority of the world's volcanoes not associated with spreading centers are located along volcanic arcs, ultimately originating from dehydration of water‐bearing minerals brought down by subducting oceanic slabs. Some volcanoes occur in areas away from the volcanic arcs and are usually geochemically different from arc volcanoes. The intraplate volcanoes in Central Java are potassic (K)‐rich and have been hypothesized to be caused by a supply of volcanic material in the deeper asthenosphere compared to the arc volcanoes. The most frequently cited mechanism is mantle upwelling through a slab window. The window was imaged by some of the seismic tomography studies, but its location is debated due to diminishing resolution at greater depth and limited station coverage. More importantly, whether there is indeed a flow system traversing the slab window remains unrevealed. Here we conducted seismic azimuthal anisotropy investigation using splitting analyses of two kinds of shear waves that are originated at different depths. Our results indicate that mantle flow from the subslab enters the mantle wedge by traversing a slab window, and suggest that the vertical component of this flow system accounts for the formation of the potassic volcanoes in the area. Key Points: Azimuthal anisotropy beneath Central Java is delineated using teleseismic and local S wave splitting with unprecedented spatial resolutionMost results are consistent with subslab entrained flow traversing a slab window into the mantle wedge and flows parallel to trenchThe vertical component of the escaped flow through the slab window accounts for producing the potassic volcanoes in the back‐arc region [ABSTRACT FROM AUTHOR]