142 Disturbances in Geospace: The Storm-Substorm Relationship A. Surjalal Sharma, Yohsuke Kamide, and Gurbax S. Lakhima (Eds.)

143 Mt. Etna: Volcano Laboratory Alessandro Bonaccorso, Sonia Calvari, Mauro Coltelli, Ciro Del Negro, and Susanna Falsaperla (Eds.)

144 The Subseafloor Biosphere at Mid-Ocean Ridges William S. D. Wilcock, Edward F. DeLong, Deborah S. Kelley, John A. Baross, and S. Craig Cary (Eds.)

145 Timescales of the Paleomagnetic Field James E. T. Channell, Dennis V. Kent, William Lowrie, and Joseph G. Meert (Eds.)

146 The Extreme Proterozoic: Geology, Geochemistry, and Climate Gregory S. Jenkins, Mark A. S. McMenamin, Christopher P. McKay, and Linda Sohl (Eds.)

147 Earth's Climate: The Ocean–atmosphere Interaction Chunzai Wang, Shang-Ping Xie, and James A. Carton (Eds.)

148 Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans Christopher R. German, Jian Lin, and Lindsay M. Parson (Eds.)

149 Continent-Ocean Interactions Within East Asian Marginal Seas Peter Clift, Wolfgang Kuhnt, Pinxian Wang, and Dennis Hayes (Eds.)

150 The State of the Planet: Frontiers and Challenges in Geophysics Robert Stephen John Sparks and Christopher John Hawkesworth (Eds.)

151 The Cenozoic Southern Ocean: Tectonics, Sedimentation, and Climate Change Between Australia and Antarctica Neville Exon, James P. Kennett, and Mitchell Malone (Eds.)

152 Sea Salt Aerosol Production: Mechanisms, Methods, Measurements, and Models Ernie R. Lewis and Stephen E. Schwartz

153 Ecosystems and Land Use Change Ruth S. DeFries, Gregory P. Anser, and Richard A. Houghton (Eds.)

154 The Rocky Mountain Region—An Evolving Lithosphere: Tectonics, Geochemistry, and Geophysics Karl E. Karlstrom and G. Randy Keller (Eds.)

155 The Inner Magnetosphere: Physics and Modeling Tuija I. Pulkkinen, Nikolai A. Tsyganenko, and Reiner H. W. Friedel (Eds.)

156 Particle Acceleration in Astrophysical Plasmas: Geospace and Beyond Dennis Gallagher, James Horwitz, Joseph Perez, Robert Preece, and John Quenby (Eds.)

157 Seismic Earth: Array Analysis of Broadband Seismograms Alan Levander and Guust Nolet (Eds.)

158 The Nordic Seas: An Integrated Perspective Helge Drange, Trond Dokken, Tore Furevik, Rüdiger Gerdes, and Wolfgang Berger (Eds.)

159 Inner Magnetosphere Interactions: New Perspectives From Imaging James Burch, Michael Schulz, and Harlan Spence (Eds.)

160 Earth's Deep Mantle: Structure, Composition, and Evolution Robert D. van der Hilst, Jay D. Bass, Jan Matas, and Jeannot Trampert (Eds.)

161 Circulation in the Gulf of Mexico: Observations and Models Wilton Sturges and Alexis Lugo-Fernandez (Eds.)

162 Dynamics of Fluids and Transport Through Fractured Rock Boris Faybishenko, Paul A. Witherspoon, and John Gale (Eds.)

163 Remote Sensing of Northern Hydrology: Measuring Environmental Change Claude R. Duguay and Alain Pietroniro (Eds.)

164 Archean Geodynamics and Environments Keith Benn, Jean-Claude Mareschal, and Kent C. Condie (Eds.)

165 Solar Eruptions and Energetic Particles Natchimuthukonar Gopalswamy, Richard Mewaldt, and Jarmo Torsti (Eds.)

166 Back-Arc Spreading Systems: Geological, Biological, Chemical, and Physical Interactions David M. Christie, Charles Fisher, Sang-Mook Lee, and Sharon Givens (Eds.)

167 Recurrent Magnetic Storms: Corotating Solar Wind Streams Bruce Tsurutani, Robert McPherron, Walter Gonzalez, Gang Lu, José H. A. Sobral, and Natchimuthukonar Gopalswamy (Eds.)

168 Earth’s Deep Water Cycle Steven D. Jacobsen and Suzan van der Lee (Eds.)

169 Magnetic ULF Waves: Synthesis and New Directions Kazue Takahashi, Peter J. Chi, Richard E. Denton, and Robert L. Lysak (Eds.)

170 Earthquakes: Radiated Energy and the Physics of Faulting Rachel Abercrombie, Art McGarr, Hiroo Kanamori, and Giulio Di Toro (Eds.)

171 Subsurface Hydrology: Data Integration for Properties and Processes David W. Hyndman, Frederick D. Day-Lewis, and Kamini Singha (Eds.)

Published under the aegis of the AGU Books Board

Darrell Strobel, Chair; Gray E. Bebout, Casandra G. Fesen, Carl T. Friedrichs, Ralf R. Haese, W. Berry Lyons, Kenneth R. Minschwaner, Andy Nyblade, and Chunzai Wang, members.

Library of Congress Cataloging-in-Publication Data

Volcanism and subduction : the Kamchatka region / John Eichelberger… [et al.], editors.

p. cm. -- (Geophysical monograph ; 172)

ISBN 978-0-87590-436-8

1. Volcanism--Russia (Federation)--Kamchatskaia oblast’. 2. Subduction zones--Russia (Federation)--

Kamchatskaia oblast’. I. Eichelberger, J. C. (John Charles)

QE527.2.R8V65 2007

551.210957’7--dc22

2007041340

ISBN 978-0-87590-436-8

ISSN 0065-8448

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PREFACE

Long before introduction of the subduction paradigm, it was recognized that there was a “Pacific Ring of Fire” characterized by explosive eruptions, devastating earthquakes, and far-reaching tsunamis. This belt of closely coupled tectonism and volcanism girdles a hemispheric ocean. We chose a segment of this ring as the subject of this volume, a choice that deserves some explanation. An astronaut arriving here, had Earth’s oceans gone the way of Mars’ oceans, would certainly be drawn to this deep kinked furrow in the planet’s skin, but there are more reasons than topography.

One reason is the high level of activity. Five of Earth’s ten largest earthquakes of the 20^th century occurred in this segment, and over a span of only 12 years. Volcanism is likewise robust. Exceptional volcanic events include the great Katmai/Novarupta eruption of 1912, by far the largest on Earth in the last hundred years; the Bezymianny and Shiveluch collapse/Plinian events of 1956 and 1964, respectively; and the Great Tolbachik Fissure Eruption of 1975 with a vent span of 30 km. At this writing, 5 volcanoes of the Kurile-Kamchatka system and 3 of Aleutian-Alaska are in continuous to frequent intermittent low-level eruption. Tsunamis of the past century have obliterated whole villages, Severo-Kurilsk in 1952 and Valdez, Alaska in 1964. Here is a place where Earth’s interior dynamics are illuminated dramatically and sometimes tragically by earthquakes, deformation, and melting.

Obviously the activity does not end at the geographic limits of this volume. Vigorous subduction continues uninterrupted south of the Kuriles into Japan. At the other end, volcanism but not seismicity diminishes in southeastern Alaska where the plate boundary becomes the Queen Charlotte transform fault of western Canada. The chosen segment does, however, coincide with relative lack of visibility within the global geoscience community. This is somewhat ironic, because the Aleutian arc is a place where important aspects of the subduction paradigm were first introduced.

One impediment to science in this region is the harsh environment. The weather is often cold and stormy, and supply points are few and far. In most cases, a helicopter or ship or both are required. The high cost of transportation and support are exacerbated by the need for budgeting weather days. Scientists stuck in bad weather and unaccustomed to this fact of northern life have been known to contact their embassies for help in improving flying conditions. Field seasons are generally limited to mid June to mid September, and maintaining operation of geophysical instruments through the long winter is difficult. A team or expedition approach to field work is often needed, though happily this has benefits in encouraging cross-discipline collaboration and cross-culture understanding.

The new driving force toward scientific understanding of this part of the world is the concern shared by all governments about natural hazards. Significant local populations are at risk to earthquakes and eruptions, and the entire northern Pacific basin is at risk to tsunamis generated here. For volcanology, the risk for jet aircraft encountering ash clouds from explosive eruptions has motivated rapid growth of volcano observatories in Alaska, Kamchatka, and in Sakhalin for the Kuriles. Some 25,000 passengers and equally impressive amounts of cargo are carried by roughly 200 large aircraft per day along the Kurile-Kamchatka-Aleutian volcanic line en route between eastern Asia and North America. Approximately one hundred volcanoes in this subduction segment are capable of erupting ash clouds to flight levels.

Before the growth in volcano monitoring, for which a triggering event was the near-disastrous encounter of a wide-body passenger jet with an ash cloud from Redoubt volcano over southcentral Alaska in 1989, only the Soviet Union maintained volcano observatories in the region. Alaska Volcano Observatory (AVO) now employs dense seismic networks on 30 volcanoes, as well as continuously recording, telemetered GPS networks on four of them. The Kamchatka Volcanic Eruption Response Team (KVERT) monitors 10 Kamchatka and northern Kurile volcanoes seismically in real time. Both in Kamchatka and Alaska, a great deal of work has gone into developing stand-alone telemetered geophysical stations that can withstand the rigors of the environment for long periods without expensive helicopter visits.

An important parallel development was the use of satellite-based remote sensing observations to detect and warn of volcano unrest and eruption. Nowhere in the world is satellite data used so intensively for volcano hazard mitigation as at the observatories of Alaska, Kamchatka, and Sakhalin. Rapidly advancing technology has changed not just the resolution of satellite systems but also the kinds of data that can be acquired, including volcano deformation, eruption cloud composition, and estimation of effusion rate. Although seismic data from dense proximal networks remains the preferred means of detecting activity precursory to eruptions, satellite remote sensing makes possible monitoring of volcanoes for which ground installations are prohibitively expensive and provides essential confirmation of explosive ash production where ground stations are present.

Another societal imperative motivating geoscience investigations of active processes is the need for economical, clean, reliable energy for isolated communities. Important use of geothermal energy has been a reality in Kamchatka and the Kurile Islands for some time, and is under serious consideration in Alaska. With concern about oil spills in rich fisheries and rising oil prices, geothermal will likely grow so that northern coastal communities can remain viable.

In order to view the geophysics of this region as a whole and to encourage development of international and interdisciplinary investigations, workers from Hokkaido, Kamchatka and Alaska formed the Japan-Kamchatka-Alaska Subduction Processes Consortium (JKASP). Five biennial meetings, each attracting 100 to 200 scientists and students, have taken place to date: 1998 in Petropavlovsk-Kamchatsky, 2000 in Sapporo, 2002 in Fairbanks, 2004 in Petropavlovsk-Kamchatsky, and 2006 in Sapporo. The present volume is an outgrowth of the birth of this geoscience community.

The contents of the volume span a broad range of disciplines within the general theme of subduction processes. Students will rapidly appreciate that this classic subduction zone lacks the classic simplicity of textbook cartoons, wonder at the relationship of present-day topography to tectonic history, and find that the crowning volcano at Earth’s sharpest subduction corner is not andesite but basalt. For scientists of more southern experience, we hope that the book will serve as a stimulating and useful introduction to the research and the researchers of the far north Pacific. For those who have worked here, we hope that the papers herein will point the way to new connections, collaborations, and directions. Most of all, we hope that through these and other efforts the window of opportunity for collaboration that has opened among Japan, Russia, and the US will remain open; that the Kurile-Kamchatka-Aleutian-Alaska subduction system will be a shared natural geodynamic laboratory of our countries, and indeed of the world.

The editors thank the US National Science Foundation, the Russian Academy of Sciences, and the US Geological Survey for support that has made this volume possible.

John Eichelberger

Evgenii Gordeev

Pavel Izbekov

Minoru Kasahara

Jonathan Lees

Section I: Tectonics and Subduction Zone Structure

Section II: Earthquakes

Section III: Volcanism

Introduction: Subduction’s Sharpest Arrow

John C. Eichelberger¹

¹ Currently at Volcano Hazards Program, USGS, Reston, VA.

Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA

In the center of the 6000-km reach of Kurile-Kamchatka-Aleutian-Alaska subduction is arguably Earth’s most remarkable subduction cusp. The Kamchatka-Aleutian junction is a sharp arrowhead mounted on the shaft of the Emperor Seamount Chain. This collection of papers provides context, definition, and suggestions for the origin of the junction, but a comprehensive understanding remains elusive, in part because of the newness of international collaborations. Necessary cross-border syntheses have been impeded by the adversarial international relations that characterized the 20^th century. For much of this period, Kamchatka and the Kurile Islands were part of the Soviet Union, a mostly closed country. The entire region was swept by World War II, abundant remnants of which are wrecked ships and planes, unexploded ordnance, and Rommel stakes.

Of the three countries with a direct interest in this region, Russia has the longest presence. Russia established settlements in Kamchatka beginning in the early 18^th century, then colonized the Aleutians, Kodiak, and southeast Alaska following A. Chirikov’s and V. Bering’s discovery voyage from Petropavlovsk-Kamchatsky in 1741. Hokkaido was the last territory area added permanently to Japan, during the latter half of the 19^th century. Similarly, the United States purchased the Aleutians and Alaska from Russia in 1867 in the interest of territorial expansion, whaling, and harvesting of fur.

The strategic importance of the region to the US and Russia increased dramatically with World War II, when Japan began launching military operations from the northern Kuriles and occupied the American Near Islands (so named because they are close to Kamchatka) of the Aleutians. At the same time, Alaska and Kamchatka airfields were needed to ferry materiel in support of the Soviet Union’s war effort against Nazi Germany. An immediate American response to these events was to build a road through Canada to Fairbanks, providing what is still Alaska’s only land link to the rest of the United States. In contrast to Alaska, Kamchatka is geographically continuous with Russia but still lacks a land transportation connection. During the war, American soldiers wrote of the Aleutian Islands as a sort of cold, damp hell, while American school teachers more often described them as a flower garden with an advanced native culture. In any case, they were a remote and exotic place to those Americans who even knew they existed. This is still the largely true, and few Americans are aware of the hardships the Aleuts endured during the war, nor of the rich legacy of Russian culture that persists in Alaska among Native people.

Hardly better for science than World War II was the Cold War. The situation changed from the US and Soviet Union allied against Japan to the US and Japan allied against the Soviet Union. Kamchatka and Alaska became armed camps, with the US testing its largest nuclear weapons on Kamchatka’s doorstep in the western Aleutians and with Kamchatka off-limits even to most Russians. The Soviet Union did, however, maintain a robust geoscience effort in Kamchatka. Likewise, the United States, in part in support of defense activities, conducted extensive geological and geophysical work in the Aleutians.

The end of the Cold War brought an end to most travel prohibitions. A lingering border dispute over the southern Kuriles is now being addressed in a positive way by Russia and Japan in terms of access for hazard monitoring and science. But easing of tensions did not make travel easy, only possible. Issues of expense, language, culture, and cumbersome visa procedures remain. Air routes are inconvenient and expensive.

For Russians, the Kurile-Kamchatka-Aleutian-Alaska region is a fabled part of their history, and Kamchatka is the one place in their vast country where spectacular volcanism and the greatest earthquakes can be studied firsthand. It is perhaps not surprising, then, that only recently did the state of knowledge of Aleutian/Alaska volcanoes reach the level of knowledge about Kamchatka volcanoes. The record of eruption from historical documents and careful tephrochronology in Kamchatka, some of which is presented here in the overview paper on volcanism by V. Ponomereva and coauthors, still surpasses that of Aleutia/Alaska. The Kurile Islands, posing transportation and telemetry difficulties in their central portion and lingering international tension in the south, remain the least known.

The positioning of the Emperor seamount chain as the shaft at the Kamchatka-Aleutian arrowhead may or may not be a coincidence, but what seems not a coincidence is the prodigious rate of magma production inboard of this junction, represented by the largely mafic Kliuchevskaya group and its more silicic northern neighbor, Shiveluch. In this volume, M. Portyangin and coauthors suggest an answer in large-scale slab melting, as the Pacific slab, torn open under the western Aleutians, dives into hot mantle under Kamchatka. In tectonic overview papers, G. Avdieko and coauthors and D. Scholl wrestle with the meaning of the cusp from vantage points from the west and east of it, respectively. A. Lander and M. Shapiro focus on constraining the onset of the modern volcanic and subduction regime of Kamchatka with seismic data. Intriguing related problems are the welding of arc fragments to Kamchatka as the eastern capes, the origin and behavior of neighboring microplates, and the apparent double arcs of Kamchatka, one young and robust and the other old and dying.

On either side of the arrow’s point are two almost matching arc pairs: continental Kamchatka Peninsula with oceanic Kurile Islands, and continental Alaska Peninsula with oceanic Aleutian Islands. We use the term “arc” for the volcanic expression of subduction in deference to history and to economy of letters, but the volcanoes more properly comprise “supra-subduction zone volcanism”. Much of the segment of interest is not an island arc because, except for the Aleutians, the arrangement is not arcuate and, except for the Aleutians and Kuriles, the volcanoes are not islands. The arcuate shape seems irrelevant and to call continent-sited volcanoes “islands” is even worse. Continental margin subduction faithfully follows the shape of the unsubductable continental margin, as is clear along Kamchatka and Alaska. Kurile subduction is a straight line pinned to continental margins at both ends, Hokkaido and Kamchatka. D. Scholl suggests that the Aleutians, a true arc and perhaps an inspiration for the term, “budded” off continental margin subduction of the Alaska Peninsula and progressed westward, turning to the right as it went until it was parallel to Pacific Plate motion and became a transform fault. We should perhaps view the western end as “free”, unconstrained by a continental margin because it is perpendicular to it, and hence able to migrate in either direction. Confusingly, arguments can be made for migration in either direction: southward because older “supra-subduction zone volcanism” extends north of the current junction and northward if the east coast capes of the Kamchatka Peninsula represent prior positions of the junction. Indeed, the current plate boundary, the “corner” representing the northern limit of the subducting Pacific plate, is not where the Aleutian arc/trench pair meets Kamchatka but north of this at a back-arc shear zone. Bering Island seems destined to become another cape on the east coast of Kamchatka.

Scholl argues that the Aleutians, and Avdeiko and coauthors and Lander and Shapiro argue that the eastern volcanic front of Kamchatka, record a large forward jump in volcanism due to jamming of subduction by arc fragments. For the Aleutians, this resulted in capture by North America of the Bering microplate. But now the western Aleutians are being fritted and torn from the North American/Bering plate. For Kamchatka, the postulated jump caused the death of Sredinny Range volcanoes and rise of the prolific and caldera-rich volcanism of the eastern Kamchatka Peninsula. It would seem then that the only steady state subduction regimes are the Kuriles and the Alaska Peninsula, though the latter has a relative dearth of older volcanic rocks on its Mesozoic basement, giving the impression of a very recent start to volcanic growth. These interpretations remain speculative. For example, Scholl observes that the age of Bering microplate crust is poorly known and Ponomareva and coauthors show that the Sredinny Range can be viewed as back-arc volcanism arising from the modern subduction configuration.

The volume is divided into three themes: tectonics, earthquakes, and volcanism. Each section begins with one or more overview papers that not only provide background and context, but also new ideas. They are followed by topical studies focusing on specific features or processes. Of course, the ultimate goal should be a holistic view that encompasses all these manifestations of subduction. It is clear that we are far from that. But although the discussions of tectonics are highly speculative, they pose hypotheses that are clearly testable with more data on age and origin of terranes and on current rates of deformation. Perhaps the greatest progress is evident in seismology, with an understanding of earthquake distribution in time and space based on slab age, convergence rate, stress distribution, and formation of asperities. The diversity of volcanic expression of subduction, in contrast to all other tectonic domains, seems most resistive to solution, though increasingly under attack by new sophisticated geochemical techniques and synthesis with geophysical results. An accompanying DVD provides a view of eruptions in Kamchatka and of the style of field work conducted there not previously available outside Russia.

If there is one place where tectonic, seismic, and volcanic interpretations seem to be converging, it is the arrow itself. The subduction of the torn Pacific plate corner, the seismically inferred rounding of its leading edge, and geochemical inference of a large slab component in resultant eruption products are internally consistent. It is towards such a synthesis of geological, geophysical, and geochemical techniques at the micro and macro scales that this volume strives.

Viewing the Tectonic Evolution of The Kamchatka-Aleutian (KAT) Connection With an Alaska Crustal Extrusion Perspective

David W. Scholl

Department of Geophysics, Standford University, Standford, California, USA, and College of Natural Science and Mathematics, University of Alaska, Fairbanks, Alaska, USA

The Kamchatka and Aleutian (KAT) arc-trench systems meet orthogonally at Cape Kamchatka Peninsula. The KAT connection is the intersection of the NE-striking Kamchatka subduction zone and the NW-striking, transform setting of the western, Komandorsky sector of the Aleutian Ridge. Deciphering the origin and evolution of the KAT connection is challenging because of the paucity of constraining information about the age and latitude of formation of major crustal blocks of the deep water Bering Sea Basin.

It is proposed that in the late early Eocene (∼50 Ma) the combined tectonic machinery of subduction zone obstruction and continental margin extrusion created the tectonic and rock architecture of the Aleutian-Bering Sea region. Accretion of the Olyutorsky arc to the north Kamchatka-Koryak subduction zone forced the offshore formation of the Aleutian subduction zone (SZ), added a sector of Pacific crust—Aleutia—to the North America plate, and established the KAT connection. Subsequently, but also in the middle Eocene, extrusion of Alaska crust southwestward across the Beringian margin connecting Alaska and NE Russia buckled Aleutia and forced the offshore formation of the Shirshov and Bowers SZs. Extrusion was driven by northward oblique underthrusting beneath British Columbia and SE Alaska.

In the early Tertiary the Aleutian and Kamchatka SZs, linked at the KAT connection, thus consumed northwest moving crust of the Pacific Basin and within the Bering Sea the Beringian, Shirshov, and Bowers SZs accommodated SW extruding Alaska and captured Aleutia crust. Since the early Miocene extrusion space has been provided by the Aleutian SZ. Arc-arc collisions at the KAT connection have been guided by the right-lateral Bering-Kresta shear zone, which lies at the Bering Sea base of the Komandorsky section and terminates at Cape Kamchatka Peninsula. In the past the tectonic connection with Kamchatka may have been farther to the north.

1. INTRODUCTION

The northwestern corner of the Pacific basin, as widely recognized, is an unusual right-angle confluence of two lengthy arc-trench systems, those of the Kamchatka and Aleutian subduction zones. Their tectonic contact is, for the convenience of this paper, dubbed the KAT connection (Fig. 1A). The KAT connection is widely believed to be an intermittent collision zone between the arc massifs of the two subduction zones. Collision began in the late Neogene or earlier in the Tertiary as terranes or blocks of arc crust of the far western or Komandorsky sector of the Aleutian Ridge entered the Kamchatka subduction zone (SZ) from the east. The eastward projecting promontory of the Cape Kamchatka Peninsula physiographically marks the collision zone (Fig. 1B; Watson and Fujita, 1985; Zinkevich et al., 1985; Zoneshain et al., 1990; Baranov et al., 1991; Geist and Scholl, 1994; Seliverstov, 1998; Gaedicke et al., 2000; Freitag et al., 2001; McLeish et al. 2002). In the past, the collision zone may have been farther to the north.

Collision at the KAT connection is the kinematic consequence of the circumstance that the Cape Kamchatka Peninsula is the intersection of the NE-striking Kamchatka SZ and the right-lateral transform plate boundary striking NW along the Komandorsky sector of the Aleutian Ridge (Fig 1B). The transform boundary along the far western Aleutian Ridge is a complex, distributed shear zone that embraces the width of the arc massif from trench floor to the backarc (Cormier, 1975; Geist and Scholl, 1994; Gaedicke et al., 2000, Freitag, 2001; Kozhurin, this volume).

So how did the KAT connection come to be? This paper explores an hypothesis that Pacific rim tectonism created, in the Aleutian-Bering Sea region, the geometry of the offshore subduction zones of the Aleutian-Shirshov-Bowers system that led to arc-arc collision at the KAT connection (Fig. 1A). The scenario outlined in this paper is based on the published ideas and data of many colleagues as cited. The tectonic sketch advance is speculative because constraining information is lacking to test assumptions that must be made about the ages and origins of key crustal blocks and terranes that construct the Aleutian-Bering Sea region. Compounding matters, the paucity of regional paleomagnetic data east of Kamchatka means that assumptions have also to be made about the paleogeographic origins of the crustal blocks of the Bering Sea Basin and the Cenozoic style(s) of north Pacific rim tectonism that brought the key pieces of the KAT junction together. A spotty but improving GPS data set for the Aleutian Ridge, however, apparently reveals the fundamental nature of westward block transport toward collision with Kamchatka (Oldow et al. 1999; Avé’ Lallemant and Oldow, 2000); Gordeev et al., 2001; Steblov et al., 2003, Cross and Freymueller, 2007). Nonetheless, the troubling circumstances of insufficient regional information to constrain models apply equally to all evolutionary schemes that have been suggested for the origin of the Aleutian-Bering Sea region and thus the KAT connection.

Three tectonic models for the early Tertiary genesis of the Aleutian SZ have been posited. The most accepted of these is the clogging or occlusion model describing the tectonic jamming of the north Kamchatka-Koryak SZ then occupying the northwestern-most or Cape Navarin corner of the Pacific Basin (Fig 1A). Southwest of the corner, subduction was obstructed by docking or accretion of the northward migrating Olyutorsky arc complex (Fig. 2A; Zonenshain et al., 1990, Garver et al., 2000). Suturing of the exotic arc complex to the north Kamchatka-Koryak margin forced formation of a new offshore subduction zone—the Aleutian SZ. The driving force was continuing slab pulls beneath Alaska and Kamchatka, west and east, respectively, of the obstructed north Kamchatka-Koryak SZ.

The colliding arc complex is also referred to as the Achaivayam-Valagin or Olyutor-Valaginskii arc complex (see discussion in Park et al., 2002; Sukhov et al., 2004; Chekhovich et al., 2006, Chekhovich and Sukhov, 2006). This Late Cretaceous–early Tertiary arc complex is commonly viewed as having formed well south of it present location either proximal to the NW margin of the Pacific Basin or far offshore. The arc complex would thus be exotic to the Aleutian-Bering Sea Region. This arc complex is usually shown as including the Shirshov and Bowers Ridges of the Bering Sea Basin (Fig. 1A).

In various forms, models for the formation of the Aleutian-Bering Sea region as a consequence of terrane(s) accretion and subduction zone occlusion have been explored and thought through by many authors, for example Ben- Avraham and Cooper (1981), Cooper et al. (1987), Zonenshain et al. (1990), Stavsky et al. (1988; 1990), Seliverstov (1998), Worrall (1991), Scholl et al., (1992), Zinkevich and Tsukanov, (1992), Baranov et al. (1991), Park et al. (2002), Sukhov et al. (2004) and Garver et al. (2000, 2004).

A contrasting model ascribes the origin of the Aleutian-Bering Sea region to the plate-boundary-driven deformation of the north Pacific margin (Scholl et al., 1989). Formation in place of the offshore Aleutian-Shirshov-Bowers subduction zone systems is linked to the SW extrusion of the Beringian margin that trends southeastward from the Koryak margin at Cape Navarin, NE Russia, to the western tip of the Alaska Peninsula (Fig. 1A). Before the formation of the offshore subduction zone system, probably in the late early Eocene about 50 Ma (Jicha et al., 2006), the Beringian margin is inferred to have been the northernmost sector of the dominantly transform boundary separating the North America plate and that of either the Kula or Pacific plate (Moore, 1972; Scholl et al., 1975, 1986; Cooper et al., 1987a; Haeussler et al., 2003; Nokelberg et al., 2005). Figure 2 suggests that the Beringian margin was highly obliquely underthrust by Pacific Basin crust.

Figure 1A. Physiographic and bathymetric index maps of northeastern Russia, Alaska, and the north Pacific-Aleutian-Bering Sea region. Mercator-projection map derived from This Dynamic Planet (http://www.minerals.si.edu/tdpmap/index.htm). Major tectonic elements are identified on companion Figure 1B.

Seaward displacement and deformation of the Beringian margin is conjectured to have been effected by the extrusion of western Alaska toward the Bering Sea region. Tectonic push-out or extrusion of Beringian crust toward and along the Beringian margin is hypothesized to have buckled the oceanic crust then residing in the area of the modern Bering Sea Basin leading to establishment of the Shirshov, Bowers, and Aleutian SZs (Fig. 2B). The extrusion or escape model for the genesis of the Aleutian-Bering Sea region has been outlined by Scholl and Stevenson (1989,1991), Scholl et al. (1992, 1994), Scholl, (1999), and Lizarralde et al. (2002) and most completely by Redfield et al. (in press). Southwestward extrusion of western Alaska and Bering Sea crust is on-going today as the Bering block identified with regional seismic and geologic data assembled and interpreted by Mackey et al. (1997) and Fujita et al. (2002). Enhanced movement of the Bering block southward toward the Aleutian SZ and westward toward the Kamchatka-Koryak margin is presently driven by the late Neogene collision of the Yakutat block with the eastern end of the Alaska SZ (Fig. 1B; see also Mazzotti and Hyndman, 2002; Eberhart-Phillips et al. 2006).

Figure 1B. Major tectonic elements of the north Pacific rim and offshore Aleutian-Bering Sea region and north Pacific Basin. Active and inactive strike-slip faults (F) and shear zones (SZ), and major Beringian shelf basins (B).

A third model merges or couples the tectonic machinery of the first two scenarios as forcing the formation of the Aleutian-Bering Sea region and its three offshore SZs (see Fig. 10; Scholl et al., 1992; Cooper et al., 1992; Scholl 1994). The occlusion-extrusion or coupled model is explored in this paper as the working hypothesis principally because it is evident that the Late Cretaceous-early Tertiary Olyutorsky arc complex was accreted to the Kamchatka margin of the western Bering Sea at the same time as, or just before, the arc volcanic construction of the Aleutian Ridge began to form in the middle Eocene (Levoshova et al., 1997, 2000; Zonenshain et al., 1990; Seliverstov, 1998; Garver et al., 2000; Garver et al., 2004, Jicha et al., 2006, Chekhovich and Sukhov, 2006). Paleomagnetic data also document that the Aleutian Ridge, which is not known to include pre-Tertiary rock, is not an exotic terrane but formed effectively in place (Harbert, 1987) as a western addition to the much older continental crust of the Alaska Peninsula (Burk, 1965). Similarly, the Aleutian SZ is viewed as a westward continuation of the Alaska SZ (Scholl et al., 1975; 1986; 1987). Farther west, the newly formed Aleutian SZ was presumably connected by a NW-trending, right-lateral shear zone or transform system to the older Kamchatka SZ (Lonsdale, 1988). In this way the KAT connection was first established that ultimately led to the collisions of blocks of the far western sectors of the Aleutian arc massif with the landward slope of the Kamchatka Trench (Fig. 2).

Figure 2. Two general models, (A) subduction zone (SZ) occlusion of the Kamchatka-Koryak margin, and (B) plate boundary extrusion of the Beringian margin, have been proposed for the origin of the offshore Aleutian SZ, accretion of a sector of Pacific lithosphere (Aleutia) to the North America plate, and consequent formation of the Aleutian-Bering Sea region. (A) Accretion of a Pacific-basin born, and thus exotic to the Bering Sea, Olyutorsky-Shirshov-Bowers arc complex to the Kamchatka-Koryak margin occludes (jams) the SZs of the Pacific’s northwestern rim and forces offshore formation of the Aleutian SZ. (B) Plate-boundary driven lateral crustal streaming and extrusion of the Pacific’s northeastern rim forces the in-place formation of the offshore Aleutian-Shirshov-Bowers SZ system.

This paper presents a model that merges both plate-boundary modifying forces to create the Aleutian Bering Sea region and the Kamchatka-Aleutian or KAT tectonic connection.

In the coupled model, the Bowers and Shirshov Ridges would have formed in-situ as a consequence of extrusion of western Alaska and Bering shelf crust toward the Beringian plate margin. Woven into the working hypothesis are the recent findings in the north Pacific concerning the age, origin, and tectonic implications of the prominent change in trend or bend in the Hawaiian-Emperor seamount chain (Figs. 1A and B; Tarduno et al., 2003, Pare′s and Moore, 2005; Sharp and Clague, 2006; Steinberger and Gaina, 2007). The beginning age of the sweeping bend, which required about 8 Myr to complete, is ∼50 Ma (Sharp and Clague, 2006). This age matches the final docking time of the Late Cretaceous-early Tertiary massif of the Olyutorsky arc complex (Garver et al., 2000; Garver et al., 2004; Chekhovich and Sukhov, 2006) and the best estimate of the origin of the Aleutian SZ (Jicha et al., 2006).

The coupled model considers that the Bering block of Mackey et al. (1997) and Fujita et al., 2002) is the westernmost part of a laterally moving crustal track that extends northwestward from NW British Columbia in a broad counterclockwise curving arc through central Alaska to the KAT connection (Fig. 2). The northward and westward moving track of British Columbia, Alaska, and Bering crust is collectively referred to (see discussion below) as the North Pacific Rim orogenic stream or NPRS (Redfield et al. in press). The plate boundary forces and suprasubduction zone setting described by Mazzotti et al (in press) that have long mobilized a Pacific rim orogenic float (Oldow et al., 1990) are those involved in laterally moving the NPRS. The expanse of Bird’s (1996) computer simulations of north Pacific rim tectonism is effectively that of the NPRS.

2. MAJOR CRUSTAL BLOCK

2.1 The Bering Block

2.1.1 Observations.

Seismic and geological observations of Mackey et al. (1997) and Fujita et al. (2002) define the Bering block as the largest tectonostratigraphic element of the KAT connection. The Bering block includes much of western Alaska and the whole of the Aleutian Ridge including the KAT connection (Fig. 3). The Bering block rotates clockwise and deforms the Koryak and northern Kamchatka margins of the Bering Sea.

The Okhotsk microplate exists west of the Bering block and, as documented by Bourgeois et al. (2006) and Pedoja et al. (2006), the westward extrusion of the Okhotsk lithospheric slab (Riegel et al., 1993) toward the Kuril-Kamchatka SZ is importantly involved in the KAT collisional processes. Extrusion of the Okhotsk microplate forces Kamchatka to overrun the Komandorsky Basin that forms the western side of the deep water Bering Sea Basin (Figs. 1A and 3).

Figure 3. Upper diagram (A) sketches outline of the Bering Block of Mackey et al. (1997) and Fujita et al. (2002). Lower diagram (B) sketches outline of the CCW flowing north Pacific orogenic stream the leading front of which is the Bering Block (Redfield et al., in press). Abbreviations of strike-slip faults match full names shown on Figure 1B.

2.1.2 Interpretations.

Kinematically, the Bering block can be viewed as the westward or leading edge of the NPRS as described by Redfield et al. (in press) and that embraces the regional tectonic concepts of the orogenic float of Oldow et al. (1990), Mazzotti and Hyndman (2002), and Mazzotti et al. (in press) (Fig. 3). The Bering block can be tectonically added, as the leading edge, to the laterally moving crust included within the great family of strike-slip faults that strike northwestward along the British Columbia margin and coastal region (e.g., Tintina, Denali, Fairweather, Queen Charlotte, etc fault systems) and curve through the so-called Alaska oroclinal bend to fan out southwestward toward the Bering Sea (e.g., the Kobuk, Kaltag, Iditarod-Nixon Fork, Denali, Farwell, Castle Mountain, Bruin Bay and Border Ranges, etc fault systems; Fig. 3B; Redfield et al., in press). This view is based on the observation that the major shear systems of the NPRS are or have been active in the late Cenozoic (Mackey et al. 1977; Page et al., 1991; Fujita et al., 2002); Haeussler et al., 2004). In western Alaska the southwestward expanding pattern of strike-slip faults implies the faults are slip lines bordering differentially extruding gores of Alaskan and Bering shelf crust. The pattern is similar to that of the differentially extruding and rotating crustal slices of SE Asia (Tapponnier et al., 1982, 1986) and Anatolia (Nyst and Thatcher, 2004).

2.1.3 Issues.

The notion that the northward and westward moving North Pacific Rim orogenic stream existed in the early Tertiary is based on the absence of significant interior Alaska mountain building yet the recording of hundreds of kilometers of offsets across, and localized deformation along, the curving pattern of regional strike-slip faults that tectonically connect western Alaska and British Columbia (Redfield et al., in press). The concept of a currently extruding western crustal track is based on the seismically defined Bering block, deformation along its western or NE Russian edge, and a limited but growing inventory of epicentral and fault mechanism data stretching eastward back into central Alaska (Page et al., 1991, 1995; Eberhart-Phillipps, 2006). To test this idea an extensive network of GPS stations is needed in western Alaska and on the Bering shelf. Extrusion or tectonic escape can be identified if crustal blocks are detected moving westward and southwestward toward the Aleutian SZ at a rate exceeding a component of motion in these directions generated by convergence between the North America and Pacific plates. The complex movement of blocks of crust documented by Nyst and Thatcher (2004) for the extruding Anatolian microplate implies that similar patterns of crustal shearing, extension, and both large and small scale rotations will be true for the hypothesized NPRS (Mackey et al. 1997; Fujita et al. (2002).

2.2 Komandorsky Basin

2.2.1 Observations.

The broad expanse of the Komandorsky Basin, the far western part of deep water Bering Sea Basin, lies north of the KAT connection (Fig. 1B). The Bering-Kresta shear zone trending along the southern edge of the basin most likely separates its oceanic crustal framework from that of the arc massif of the Komandorsky sector of the Aleutian Ridge (Seliverstov, 1984; Baranov et al, 1991; Geist and Scholl, 1994). To the east, the arc crustal mass of Shirshov Ridge separates the oceanic basement of the Komandorsky Basin from that of the Aleutian and Bowers Basins (Figs. 1A and 1B).

Muzurov et al. (1989), Baranov et al. (1991) and Valyasho et al. (1993), established that the Komandorsky basin was formed by a style of rear or backarc spreading in a direction parallel to the axis of the arc. Baranov et al. (1991) note that opening of the Komandorsky Basin in a NW-SE direction took place parallel to the active Pacific-North America (PAC/ NAM) transform boundary of the Bering-Kresta shear zone. Opening was thus parallel to relative plate motion (Plate 1 and Fig. 4Fig. 1BPlate 1Fig. 7