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Zusammenfassung:
The Earth’s mantle exhibits a layered structure seismically characterized by sudden velocity
changes or strong gradients. Several seismic boundaries have been identified in the mantle, and a large
number of theoretical calculations and laboratory experiments have contributed to the debate on their
origin. We analyze P-to-S converted phases generated at such interfaces to image the velocity structure
within the sublithospheric mantle beneath Indonesia. Our study confirms the existence of various layer
boundaries in the upper and lower mantle revealing up to 11 consecutive discontinuities down to ~1,700-km
depth. We detect Ps phases from the Lehmann and the X discontinuities originating at ~245 and ~294 km,
respectively, followed by the top of a low-velocity layer (LVL-410) at ~368 km. The transition zone
discontinuities are imaged at average depths of 408 and 665 km, respectively, which indicates the absence of
significant temperature anomalies. In the midmantle we find vague indications for another interface at
~970-km depth. At ~1,220 km a negative phase is observed followed by a sequence of converting structures
of unknown origin at ~1,320, ~1,460, and ~1,500 km. We interpret these interfaces as compositional
anomalies related to persisting fragments of subducted lithosphere. A further boundary is observed at
~1,700 km. Even though different causes exist to explain the observed seismic discontinuities including
mineral phase transitions, partial melt, and chemical changes, most of them require additional mineral
components. Thus, our findings provide clear evidence for significant compositional alteration of the mantle
beneath Indonesia as a result of recurring subduction.
