Upper mantle beneath Kamchatka: The depth model and its relation to tectonics

Anomalous seismic structure beneath the Klyuchevskoy Group, Kamchatka

“…[17] We propose that the low-velocity inclined layer we infer beneath the KG is analogous to the the zone of high seismic attenuation and low velocities noted in previous studies [Firstov and Shirokov, 1971;Gorbatov et al, 1997;Gontovaya et al, 2008]. We refer to this feature as the Klyuchevskoy Upper Mantle Anomaly (KUMA) and discuss a number of scenarios that can explain its presence in the upper mantle beneath the KG.…”

“…The order of pulses (negative earlier than positive) is consistent with conversions from boundaries of a low-velocity layer. We compute P-to-S phase delays for dipping interfaces embedded in the otherwise homogeneous upper mantle wedge with P-wave speed of 8 km/s [Gorbatov et al, 1999;Gontovaya et al, 2008], and Vp/Vs = 1.75. We vary the strike and dip of the interface to match the sinusoidal pattern, and vary the depth beneath the station to match the timing.…”

“…The Central Kamchatka Depression (CKD), which separates the EVB and SR, contains volcanic complexes composed of basalts, basaltic andesites and andesites which are Late Pleistocene to Holocene in age [Avdeiko et al, 2007]. Volcanoes of the CKD are geographically offset to the west relative to the EVB (Figure 1), a fact often explained by the shallowing of the subducted slab [Gorbatov et al, 1997;Jiang et al, 2009]. Interestingly, while the shallowing is gradual [Avdeiko et al, 2007] the offset is abrupt, with CKD and EVB volcanoes following the same linear trend.…”

“…This feature was found to attenuate P and S waves much more than the surrounding mantle. More recent tomographic imaging studies by Gorbatov et al [1999] and Gontovaya et al [2008] that used local earthquake data from much larger networks identified a dipping low-velocity feature in the upper mantle beneath the CKD, ranging in depth from 50 to over 100 km. Studies utilizing teleseismic data [Lees et al, 2007;Jiang et al, 2009] resolve the lateral extent of the Pacific slab and identify slower seismic speed in the wedge above, but cannot constrain details of the wedge structure.…”

“…However, estimates of the depth to the subducting slab depth beneath the KG area are 150-200 km [e.g., Gorbatov et al, 1997;Avdeiko et al, 2007;Portnyagin and Manea, 2008], exceeding the global average.…”