Age of the Shirshov Submarine Ridge Basement (Bering Sea) Based on the Results of Investigation of Zircons Using the U-Pb SHRIMP Method

摘要

The Shirshov Ridge holds an important position in the structure of the Bering Sea Basin. Stretching from north to south for over 500 km, it divides the Bering Sea into two deep water basins, the Aleutian and Komandorsky basins. The age of oceanic crust of the Aleutian basin based on linear magnetic anomalies is conventionally considered Early Cretaceous, of the Komandorsky basin — Miocene, according to the K—Ar dating of the basalts (9.8 Ma) exposed in 191 deep water drill borehole. Rocks belonging to the basement of the Shirshov Ridge were dredged during the 29th cruise of research vessel Dmitrii Mendeleev and are represented by amphibolitic gabbro whose composition is similar to that of gabbroids of mid-oceanic ridges. The age of metamorphism based on the results of K—Ar dating of amphibole is 47 ± 5 Ma. The U-Pb zircon dating method was used to determine the age of gabbro. Zircons were extracted from a ~5 kg combined amphibolitic gabbroid sample, and the age of zircons was determined using a SHRIMP—II sensitive high resolution secondary ion microprobe (Center of Isotopic Studies, A.P. Karpinskii Russian Geological Research Institute, St. Petersburg). The average concordant age value for the 25 determinations performed based on 20 points for 18 grains is 72 ± 1.4 Ma (Late Campanian). For 5 grains, the measured age values are within the range of 88 ± 3.5 Ma to 126.5 ± 4.5 Ma. Given the western vergence of the thrust structure of the Shirshov Ridge, the acoustic basement of the Shirshov Ridge is most probably a complexly deformed oceanic crust of the Aleutian Trench, which most likely dates from the Early to Late Cretaceous.%Geological Institute, Russian Academy of Sciences, 7 Pyzhevskiiper., Moscow, 119017 Russia;Geological Institute, Russian Academy of Sciences, 7 Pyzhevskiiper., Moscow, 119017 Russia;International Institute of Earthquake Prognosis Theory and Mathematical Geophysics, Russian Academy of Sciences, 84/32 ul. Profsoyuznaya, Moscow, 117997 Russia;A. P. Karpinskii Russian Geological Research Institute, 74 Sredniipr., St. Petersburg, 199106 Russia;A. P. Karpinskii Russian Geological Research Institute, 74 Sredniipr., St. Petersburg, 199106 Russia;