Multi-Disciplinary Evidence for a Large, Previously Unrecognized Caldera in the Islands of Four Mountains, Central Aleutian Arc, Alaska
1) Welded PDC ignimbrites occur on Tana and Carlisle consistent with local caldera-forming eruptions.
2) A ring-shaped free air gravity anomaly connecting Cleveland, Tana, Kagamil, Uliaga, and Carlisle volcanoes.
3) A circular distribution of closely spaced subaerial volcanic centers relative to adjacent portions of the arc.
Additional observations consistent with calderas worldwide include:
4) Complicated and arc-parallel alignments of volcanic vents and fissures between Cleveland and Tana.
5) Swarms of micro-earthquake with normal mechanisms at 5 - 10 km depth beneath Holocene aged vents between Cleveland and Tana that are suggestive of magmatic fluid migration.
6) High and sustained SO2 flux at Mount Cleveland that requires connectivity to a larger magma source.
7) Gas samples on Tana Volcano, that are hydrothermal in character (CO2>H2S; no SO2) with C and He isotopic signatures indicative of a magmatic source.
The northern extent of this possible structure is difficult to evaluate with these observations. Seismic tomography (Ps-P, from receiver functions) using a small aperture network suggests a narrow zone of strong low velocity beneath Cleveland that extends to 20 – 30 km depth. Additional geologic and geophysical studies including a broader network and bathymetry could reveal additional evidence for a caldera and areas of volcanic unrest in the IFM.