TEPHROCHRONOLOGY OF THE SOUTHERNMOST ANDEAN SOUTHERN VOLCANIC ZONE, CHILE

Correlations among and identification of the source volcanoes for over 60 Late-Glacial and Holocene tephra preserved in eight lacustrine sediment cores taken from small lakes near Coyhaique, Chile (46°S), were made based on the stratigraphic position of the tephra in the cores, lithostratigraphic data (tephra layer thickness and grain size), and tephra petrochemistry (glass color and morphology, phenocryst phases, and bulk-tephra trace-element contents determined by ICP-MS). The cores preserve a record of explosive eruptions, since ~17,800 cal yrs BP, of the volcanoes of the southernmost Andean Southern Volcanic Zone (SSVZ). Suggested source volcanoes for 55 of these tephra include Hudson (32 events), Mentolat (10 events), and either Macá, Cay or some of the many minor monogenetic eruptive centers (MEC; 13 events) in the area. Only four of these eruptions had been previously identified in tephra outcrops in the region, indicating the value of lake cores for identifying smaller eruptions in tephrochronologic studies. The tephra records preserved in these lake cores, combined with those in marine cores, suggest that that no significant temporal change in the frequency of explosive eruptions was associated with deglaciation. Over this time period, Hudson volcano, one of the largest and longest lived volcanoes in the southern Andes, has had >55 eruptions, four of them very large, and has produced >45 km³ of pyroclastic material, making it also one of the most active volcanoes in the SVZ in terms of both frequency and volume of explosive eruptions. Based on the correlation of the tephra amongst the cores combined with radiocarbon age dates, sedimentation rates within the cores were rapid from 18,000 to 15,000 cal years BP, followed by slower rates up to 7,500 cal years BP, after which relatively rapid more rapid sedimentation rates prevail. These changes in sedimentation may be the result of regional environmental changes in southern Chile throughout the Holocene.