Date of Award

Spring 1-1-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Charles R. Stern

Second Advisor

Lang Farmer

Third Advisor

Thomas M. Marchitto

Fourth Advisor

Brian M. Hynek

Fifth Advisor

Suzanne P. Anderson

Abstract

Sediment cores from twelve lakes located to the east of the volcanic front of the southernmost portion of the Andean Southern Volcanic Zone (SSVZ) preserve greater than 75 late Pleistocene to Holocene tephras derived from different explosive eruptions of volcanoes in this region. Correlation of the tephra deposits and source volcano identification are based on the stratigraphic position of the tephra within the cores, tephra lithostratigraphic data (deposit thickness, grain size) and petrochemistry. Only 7 of these tephra have been previously identified in outcrops, indicating the importance of lake cores for identifying smaller eruptions. The suggested source volcanoes for these tephra include Hudson (33 events), Mentolat (18 events), Melimoyu, (4 events), and either Macá, Cay, Yanteles, or some of the many minor monogenetic eruptive centers (20 events) in the area. Hudson has been the most active volcano in SSVZ, producing greater than 45 km3 of eruptive material since the late Pleistocene. Radiocarbon age determinations on the lacustrine sediments allow for tephra age estimates using a Bayesian approach. The tephra records preserved in the lake cores and the age constraints indicate that there is no significant temporal change in the frequency of explosive eruptions associated with deglaciaiton. They also constrain changes in sedimentation rate in each lake through time, an important paleoclimate indicator.

For the four southernmost volcanoes along the volcanic front in the SSVZ (Melimoyu, Mentolat, Macá and Hudson) glass compositions of melt inclusion from olivine phenocrysts found in the tephras are used to constrain primitive magma compositions and melt generation parameters such as slab surface temperatures, fraction of mantle melting and mantle water content. The results indicate that magma compositions and melting parameters are similar along the arc front below Melimoyu, Macá, and Hudson, but distinct below Mentolat. These differences may be generated at the source region above the subducted slab, and in the case of Mentolat, may be related to a subducted Nazca Plate fracture zone that projects beneath this center and can transport greater amounts of hydrous phases, seawater and sediments into the sub-arc mantle.

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