Date of Award

2016

Document Type

Open Access Thesis

Department

Earth and Ocean Sciences

Sub-Department

Geological Sciences

First Advisor

Gene Yogodzinski

Abstract

Drilling at Site U1438, located immediately west of Kyushu-Palau Ridge (KPR), the site of IBM subduction initiation, penetrated 1,460 m of volcaniclastic sedimentary rock and 150 m of underlying basement. Biostratigraphic controls indicate a probable age for the oldest sedimentary rocks at around 55 Ma. This is close to the 48-52 Ma time period of IBM subduction initiation, based on studies in the forearc. There, the first products of volcanism are tholeiitic basalts termed FAB (forearc basalt), which are more depleted than average MORB and show subtle indicators of geochemical enrichment due to subduction. Site U1438 basement basalts share many characteristics with FABs, including primitive major elements (high MgO/FeO*) and trace element patterns more depleted than those of normal MORB with respect to Nd, Sm, Lu, and Hf abundances, as well as Lu-Hf, Nd-Lu, and Sm-Nd ratios. Hf-Nd isotopes for Site 1438 basement basalts show a significant range of compositions from εNd of 7.0 to 9.5 and εHf of 14.5 to 19.8 (present-day values). The data define a well-correlated array in Hf-Nd isotope space with relatively radiogenic Hf compared to Nd, which indicates an Indian Ocean-type MORB source. The dominant signature, however, with εHf >16.5, is more radiogenic than most Indian MORB. The pattern through time is from more-to-less radiogenic and more variable Hf-Nd isotopes within the basement section. This pattern culminates in basaltic andesite sills, which intrude the lower parts of the sedimentary section. The sills have the least radiogenic compositions measured so far (εNd ~6.6, εHf ~13.8), and are similar to those of boninites of the IBM forearc and modern IBM arc and reararc rocks. The pattern within the basement suggests modest enrichment of a depleted Indian MORB source over time. These results, combined with additional new Hf-Nd isotope data from ocean drilling sites 1201 and 447, which are also located immediately west of the KPR, indicate that FAB geochemistry was produced not only in the forearc, but also in backarc locations (west of the KPR) at the time of subduction initiation. The data indicate that initiation of IBM subduction was in an extensional setting by seafloor spreading-type processes immediately prior to, and contemporaneous with, initial construction of the KPR. These processes fit the model of spontaneous subduction initiation as proposed by Stern (2004 – Earth & Planetary Science Letters), wherein a change in motion causes a plate to founder and sink below another plate, possibly along a transform boundary or other zone of weakness.

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