Extension and Magmatism in the Bering Strait Region

Our studies in the Bering Strait region began with the desire to further understand and compare the geotectonic evolution of Alaska with that of North-East Russia and the Arctic. They grew into a project that provides a better understanding of the links between processes in the mantle and crust and the importance of mantle-derived magmatism and heating in lithospheric extension. Funded by NSF EAR-9018922, NSF Continental Dynamics EAR-93-17087 and a variety of other sources. Illustration shows main granitic batholiths (red) and volcanic rocks (grey with white chicken tracks)

Index map of Bering Strait region — Index map of the Bering Strait region. Our studies in this region combine data from gneiss domes, granites, deep crustal xenoliths and seismic to obtain a better idea as to how and why continental lithosphere stretches.

Seismic reflection profile with a x5 vertical exaggeration, showing approximate locations and depths of crustal xenolith suites studied from the region of reflective crust (see inset for a better idea of this reflectivity). Zircon was dated from crustal xenoliths using the U-Pb method with the Stanford-USGS SHRIMP-RG. Both magmatic and metamorphic zircons from the lower crust give ages (histogram)that are the same as supracrustal magmatic belts (grey shaded regions on histogram).

Photos of crustal xenoliths — The photos above show what the crustal xenoliths we dated looked like. They have very strong gneissic fabrics developed at granulite grade conditions: they are two-pyroxene plagioclase gneisses with no amphibole, interpreted as metamorphosed gabbroic rocks. The crustal xenoliths are rare- 98% of xenoliths are mantle-derived and only 2% are crustal. They provide unique "hand-specimens" of the reflective lower crust imaged by the seismic data. The crustal xenoliths studied are also small- no bigger than fist-size. Their zircons were separated very carefully, by methods that lost no material in the process.

research results from Bering Strait region — Combined P-T conditions determined for deep crustal xenoliths (red circles) with P-T paths of rocks in gneiss domes of the Bering Strait region. The implied thermal gradients (30-40°C/km) are too high to be explained without the addition of mantle-derived magmas into the crust.

block diagram of the Bering Strait region — Block diagram of the Bering Strait region showing the coincidence of Cretaceous magmatism (red= plutons, grey v's = volcanic rocks) at the surface with the zone of reflective lower crust at depth.

schematic crustal section beneath the Bering Straits — Schematic crustal section beneath the Bering Straits today, showing evolution from Cretaceous gneiss domes to present-day fossilized stretched and magmatically modified crust as sampled by the xenoliths brought up by neogene magmas. Our studies have effectively dated the MOHO beneath this region as a Late Cretaceous to Paleocene age feature (younger to the south). Although the bedrock geology at the surface dates back to the Precambrian, the lower crust beneath the Bering Strait has been so magmatically modified and re-constituted in terms of melting, granulite facies metamorphism and flow, that it is now Cretaceous to Paleogene crust. We would expect a similar story beneath the Basin and Range province of the USA today- ie the modern MOHO is Tertiary to present in age.

Comparison of Bering Shelf and Basin & Range Crust — Comparison of reflective crust of the Basin and Range with that imaged beneath the Bering Sea. We wish we had more xenoliths in the Basin and Range!

Publications

Amato, J. A., and E. L. Miller. 1998. “Bedrock Geologic Map of the Kigluaik Mountains, Seward Peninsula, Alaska,” Division of Geological and Geophysical Surveys, Public Data File 97-31,

Dumitru, T. A., E. L. Miller, P. B. OSULLIVAN, J. M. Amato, K. A. Hannula, et al. 1995. “CRETACEOUS TO RECENT EXTENSION IN THE BERING STRAIT REGION, ALASKA.” TECTONICS 14 (3). AMER GEOPHYSICAL UNION: 549–63.

Klemperer, S., A Grantz, E. Miller, D. Scholl, and the Bering-Chukchi Working Group. 2002. “Crustal Structure of the Bering and Chukchi Shelves: Deep Seismic Reflection Profiles across the North American Continent between Alaska and Russia.” Geological Society of America Special Paper 360,

Amato, J. M., E. L. Miller, J. E. Wright, and W. C. McIntosh. 2003. “Dike Swarms on Seward Peninsula, Alaska, and Their Implications for the Kinematics of Cretaceous Extension in the Bering Strait Region.” CANADIAN JOURNAL OF EARTH SCIENCES 40 (6). NATL RESEARCH COUNCIL CANADA-N R C RESEARCH PRESS: 865–86. DOI: 10.1139/E03-019

Miller, E. L., V. V. Akinin, J. E. Spencer, and R. Titley. 2008. Geology of the Bering Shelf Region of Alaska-Russia: Implications for Extensional Processes in Continental Crust. Edited by J. E. Spencer and R. Titley. Ores and Orogenesis: Circum-Pacific Tectonics, Geologic Evolution and Ore Deposits . Vol. 22. Arizona Geological Society Digest. Arizona Geological Society.

Miller, E. L., M. Gelman, L. Parfenof, and J. Hourigan. 2002. “Mesozoic Northern Circum-Pacific Magmatism-A Comparison between NE Russia and the North American Cordillera.” Geological Society of America Special Paper 360,

Amato, J., and E. L. Miller. 2002. “Orogenic Mass Transfer and Orthogonal Flow Directions in Extending Continental Crust: An Example from the Cretaceous Kigluaik Gneiss Dome, Seward Peninsula, Alaska,” Geological Society of America Special Paper 360,

Akinin, V. V., E. L. Miller, J. Wooden, R. B. Miller, and A. W. Snoke. 2009. Petrology and Geochronology of Crustal Xenoliths from the Bering Strait Region. Edited by R. B. Miller and A. W. Snoke. Crustal Cross-Sections from the Western North America Cordillera and Elsewhere: Implications for Tectonic and Petrologic Processes. Geological Society of America Special Paper 456. Geological Society of America.

Miller, E. L., T. Ireland, S. Klemperer, K. A. Wirth, V. V. Akinin, et al. 2002. “SHRIMP Zircon Dating of Reflective Crust and Moho beneath the Bering Shelf, Alaska and Russia,” Geological Society of America Special Paper 360.

Hannula, K. A., E. L. Miller, T. A. Dumitru, J. Lee, and C. M. Rubin. 1995. “STRUCTURAL AND METAMORPHIC RELATIONS IN THE SOUTHWEST SEWARD PENINSULA, ALASKA - CRUSTAL EXTENSION AND THE UNROOFING OF BLUESCHISTS.” GEOLOGICAL SOCIETY OF AMERICA BULLETIN 107 (5). GEOLOGICAL SOC AMER, INC: 536–53.

Miller, E.L., A. Grantz, and S. Klemperer. 2002. “Tectonic Evolution of the Bering Shelf-Chukchi Sea-Arctic Margin and Adjacent Landmasses.” Geological Society of America Special Paper 360.