Wanke, Maren (2012) The origin of the Bowers and Shirshov Ridges (Bering Sea, NW Pacific). (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Mathematisch-Naturwissenschaftliche Fakultät, Kiel, 82 pp.

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Abstract

The Bering Sea holds two prominent submarine ridges whose origin to date has not yet been clearly defined. Most notably, Bowers Ridge resides to the north of the Aleutian Arc while Shirshov Ridge lies in the western part of the Bering Sea, forming a morphological link to the Russian main land. During the RN SONNE cruises S0201-1b and -2 in 2009, samples of both structures were obtained, which form the basis of this study. Whole rock major and trace element analyses as well as Sr, Nd and Pb isotope measurements and 40Ari39Ar age dating allowed the development of hypotheses of formation for both structures. Bowers Ridge is a -700 km long arcuate ridge behind the central Aleutian Arc in the Bering Sea. The lack of age and geochemical data for the ridge has hampered the development of geodynamic models for the evolution of the North Pacific and the Aleutian-Bering Sea region. I present the first geochemical and 40Ar/39Ar age data for the volcanic basement of Bowers Ridge and a seamount from the western end of the ridge sampled during R/V SONNE cruise SO201-1b. The northern Bowers Ridge basement (26 - 32 Ma) consists of mafic to intermediate calcalkaline rocks with adakite-like (SrN = 33 - 53, LaN/YbN = 3.3 - 7.8), high field strength element (HFSE)-depleted (e.g., NbN/LaN = 0.07 - 0.31) trace element patterns and Sr-Nd-Pb isotope compositions within the Western Aleutian Arc array, implying magma generation above an obliquely subducting slab. The seamount samples (22 - 24 Ma) are HFSE-rich alkaline olivine basalts (LaN/YbN = 3.3 - 3.9, NbN/LaN = 1.0 - 1.4) with minor arc-type trace element signatures (PbN/CeN = 1.4 - 1.6, KN/NbN = 1. 7 - 1.9) but with Pacific mid-ocean-ridge basalt (MORB)-like isotopic compositions, pointing to an origin by small-degree decompression melting from slightly subduction-modified mantle. The geochemistry of the recovered rocks can be explained by highly oblique subduction along the northern part of Bowers Ridge in its present-day configuration, consistent with an in-situ origin of Bowers Ridge as a Cenozoic island arc. Shirshov Ridge is a -500 km long, N-S-striking, submarine ridge behind the western Aleutian Arc in the Bering Sea. It forms the border between the Cenozoic Komandorsky Basin in the west and the Cretaceous Aleutian Basin in the east. For a long time, the absence of age and geochemical data of the Shirshov Ridge basement has limited geodynamic models for the evolution of the Aleutian-Bering Sea region while a variety of different hypotheses on the origin of the ridge was developed. In this study, completely new age and geochemical data of two different magmatic rock series dredged during the R/V SONNE cruise SO201-2 are provided that indicate an origin of some parts of Shirshov Ridge as a Late Cretaceous to Early Paleocene (65 - 69 Ma) island arc. The mafic rock series has strongly enriched LREE relative to HREE and fractionated HREE patterns, pointing to a deep garnet-bearing magma source. The rocks exhibit high LILE, Pb and U, a slightly adakitic component and isotope compositions within the Aleutian Arc array. The silicic series has less enriched LREE patterns, high LILE, Pb and U, a strongly pronounced negative Eu anomaly and low Sr, indicating an origin of a shallow magma reservoir. Both series show strong geochemical and isotopical similarities to rocks from Bowers Ridge, the western Aleutian Arc and rocks from Karaginski Island and the Kumroch Range in Kamchatka. Geochemical and age data indicate an allochthonous origin of Shirshov Ridge, possibly as a part of the Late Cretaceous - Early Tertiary Olyutorsky Arc that rifted into the Bering Sea from further south prior to the development of the western Aleutian Arc.

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