The origin of the Bowers and Shirshov Ridges (Bering Sea, NW Pacific)

• Main Author: Wanke, Maren
• Format: Thesis
• Language: English
• Published: 2012
• Subjects: Bering Sea; Bowers; Pacific; Shirshov Ridge
• Online Access: https://oceanrep.geomar.de/id/eprint/19217/; https://oceanrep.geomar.de/id/eprint/19217/1/Diplomarbeit_Wanke_klein.pdf

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 ...