National Science Foundation Workshop
December 9-12, 2004
Stanford University, Stanford CA

Report from the Neotectonics Breakout Group

Group Members and Associates:

1. Principal Authors:

Kazuya Fujita
Jeffrey Park
Bernard Coakley
Joanne Bourgeois
Vera Ponomareva

With contributions by

Kevin Mackey
Elizabeth Miller

2. Most important unresolved question:

Where, and what is the nature of, the plate boundary between North America and Eurasia in continental northeast Asia?

The boundary between the North American and Eurasian plates in northeastern Asia remains one of the most poorly known in the world. It transitions from the ultra-slow spreading Gakkel (Arctic) mid-ocean ridge in the Eurasia Basin of the Arctic Ocean, across a diffuse zone of extension in the continental crust of the Laptev Sea, into diffuse, presumed to be transpressional, zones in the Verkhoyansk and Chersky Ranges of northeast Asia which extend to Kamchatka and Sakhalin. In so doing, the plate boundary passes near or through its pole of rotation (in the present-day, the only such example) and creates a zone of diffuse deformation, with possible extrusion tectonics, in the Magadan District and the Sea of Okhotsk (Fig. 1).

Primary unknowns include the location of the boundary (or the partitioning of deformation onto various connected faults), the nature of the transition from oceanic to continental rifting, how the extension in the Arctic is taken up as compression in northeast Asia, the existence (or not) of microplates, and the nature of deformation in the zones of diffuse seismicity. Secondary unknowns include the spatial and tectonic variation of this plate boundary through the Cenozoic, the effects on the boundary of Pacific subduction and far-field effects of the India-Eurasia collision, and the level of seismic hazard in the region.

Figure 1. Index map of northeast Russia showing the zone of deformation between the North American (NA), Eurasian (EU), and Pacific (PA) plates. Other proposed blocks or microplates are shown (AM = Amur, OK = Okhotsk, BE = Bering). Arrows denote proposed motions on the boundaries between the plates and blocks. The purple dot denotes the approximate location of the NA-EU pole of rotation based on Cook et al. (1986), Sella et al. (2002), and Steblov et al. (2003). Red boxes denote study areas discussed in section 5.

Fig. 2. Seismicity map of northeast Russia, 1960-2004, from the MSU-LANL database. Events are color coded by focal depth; size of dots denotes relative magnitude. Insets show plate and block boundaries, historical seismicity (1125-1959), seismic stations and network boundaries and the extent of explosion contamination (for details of explosion contamination see Mackey et al., 2003).

3. Scientific rationale and subsidiary questions:

The plate boundary between North America and Eurasia, two of the Earth’s major plates, can easily be followed along the Mid-Atlantic Ridge and then along the Gakkel Ridge. Beyond the termination of the Gakkel Ridge in the Laptev Sea, the boundary lies within continental northeast Asia. Exactly how and where the North American-Eurasia plate boundary extends from the Arctic through northeast Russia is a first-order question. The seismicity is diffuse (Fig. 2) and defines a cryptic deformational boundary which links the tectonics of the Arctic and North Atlantic to that of the North Pacific. This boundary ultimately links the plate movements in the Arctic to those in the Pacific and is unique in that its Euler pole of rotation lies on, or very close to, the boundary itself, resulting in a transition from extension in the north to compression in the south (Cook et al., 1986; Fujita et al., 1990).

The Gakkel Ridge is a slow spreading center that extends into the northeast Russian continental margin at a right angle, forming a series of basins within the Laptev Sea portion of the continental shelf. Limited seismic reflection and seismicity data (e.g., Drachev et al., 1998) have outlined the basic features of the stretching of the continental crust offshore, but little or no work has been done onshore to study the continuation (past or present) of spreading in the continent proper. Because the Gakkel Ridge is fairly well understood in terms of its age and spreading history, constraints are available on how this rift system continues into continental crust; this is also the only present-day example of a ridge entering a continent at a high angle. This transition from focused extension to distributed deformation is not well understood, but offers important constraints on the rifting process.

Topographically elevated coastal areas along the coast and the New Siberian Islands have been poorly investigated for age and other constraints on the rifting process. Where is the active rifting today? Has it changed location in the recent past? What is the deep structure underneath the continental rift? What does the transition zone to the deep Arctic basin look like? What is the depth of seismicity – and the inferred thickness of the brittle crust? Although there is considerable microseismicity data for the southern part of the Laptev Sea, the geometry of the station distribution and the uncertainty in crustal velocities, preclude high quality hypocentral locations for most of the region.

Very limited potential field and geomorphologic investigations suggest that young, presumed to be extensional, tectonic features exist beneath the coastal plain that transfer the extensional deformation into the continent itself. These features have never been studied. The spreading rates in oceanic crust can be used to predict rates of extension on land to the south and convergence even farther inland. What does the end of a continental rift look like? How does the plate boundary change as you approach, and pass through, the pole of rotation? Why did the rifting propagate into the Laptev Sea and did it extend father south at one time?

Intracontinental Deformation Between North America and Eurasia

Farther to the south the tectonic character changes dramatically. Focal mechanisms in the Chersky Range are dominated by strike-slip and thrust (reverse) faulting events and geomorphologic features include higher topography, river captures, uplifted terraces, offset-drainages, etc. Satellite imagery and geologic field mapping also indicate the presence of large strike-slip and reverse fault systems. As a result of the diffuse seismicity and the numerous mapped faults (e.g., Imaev et al., 2000; Kozhurin, 2004), a number of locations for the boundary, as have various configurations which include microplates, blocks, or zones of diffuse deformation, have been proposed.

Many recent models incorporate an independent Okhotsk microplate (Cook et al., 1986; Parfenov et al., 1988; Seno et al., 1996), bounded in continental Asia by zones of active deformation, that is being extruded to the southwest (Riegel et al., 1993). Alternatively, it has been suggested that no Okhotsk microplate exists and that the Sea of Okhotsk and Kamchatka are part of the North American plate (Chapman and Solomon, 1976). Recent GPS data (Kogan et al., 2000; Stebelov et al., 2003) suggest a small motion of Okhotsk relative to North America, barely above the noise, that is consistent with the Okhotsk extrusion hypothesis. The extrusion hypothesis is contradicted by a seismicity gap in the northwest Sea of Okhotsk and the lack of documented transcurrent motion across the Kamchatka Isthmus (see below).

Possible models of the North America – Eurasia interaction include 1) a rigid Okhotsk plate exists, but its motion is too slow (<5 mm/yr) to fill all its plate boundaries with seismicity on a 50-year time scale; 2) the Okhotsk plate region is not rigid, but is subject to distributed deformation; 3) some combination of the two in which there is some rigid core, but it is surrounded by zones of deformation; 4) the region is part of either the North American or Eurasian plates. Recent seismic tomographic data suggest that the lithosphere of the proposed Okhotsk plate is warm and characteristic of a long back-arc thermal feature of the western Pacific, and is likely to be weak. Interestingly, one of the most prominent seismic shear wave velocity minima lies between the Chersky range and the Kamchatka Isthmus under Shelikhov Bay (Levin et al., 2002a). This raises the possibility that semi-rigid block motion in the Magadan region, as indicated by river offsets, diffuses into a softer lithosphere which fails to transfer as much strain into the Isthmus; however significant seismicity exists to and across the Isthmus. A similar local minimum of shear wave speed seems to lie along the western margin of the Sea of Okhotsk, in the baffling seismicity gap in the NW Sea of Okhotsk, which may be another patch of “deformable lithosphere.” Seismic reflection data, however, suggest that a band of faults crosses this region. Clearly, there are “disconnects” between GPS, seismicity, tomographic, and structural data throughout the region (Fujita et al., 2004).

How is the convergence taken up in the region? Is there an Okhotsk plate, and if so, what and where are its boundaries? What is the crustal and upper mantle structure of the region; are the tectonics thick or thin skinned? Which of the many faults in the region are active and what motions occur along them? To date, a paucity of focal mechanisms and a very sparse network of GPS and seismic stations, has made it difficult to determine where, how, and how much,displacement/deformation is presently occurring.

The interpretation of the tectonics of the region is also complicated by a series of sedimentary basins (the so-called “Moma rift system;” Fig. 1) that are superimposed on the Mesozoic and Early Cenozoic structures. The “Moma rift” was previously thought to be the continuation of Arctic extension (e.g., Grachev, 1982), however, these basins appear to be either inactive or under compression (Imaev et al., 2000) in the present time. The “Moma rift” closely follows the presumed present-day plate boundary and therefore may influence its location or deformation. How and why did these basins form? Are they completely inactive today? How does the present day plate boundary relate to them?

Connection to the Pacific Plate Boundary

The geometry of plate tectonics requires that the boundary between the North American and Eurasian plates connect with the subduction zones in the northwest Pacific. The plate boundary from continental northeast Russia to the Pacific Ocean has been traced along two branches, one heading to northwest Kamchatka and the Aleutian-Kamchatka corner and the other through Sakhalin to northwest Japan. On the southward striking branch through Sakhalin, which has been the preferred choice for the plate boundary, a serious obstacle is the lack of seismicity between the coast near the settlement of Okhotsk and the northern end of Sakhalin. Deployment of a seismic station at Okhotsk has failed to detect any significant seismicity in the northwest Sea of Okhotsk. If there is an Okhotsk microplate or block, where is its western boundary and why is there a seismicity gap? How do other Cenozoic basins in the northern Sea of Okhotsk (Worrall et al., 1996; Chalyy and Sedov, 2004) relate to the plate boundary in the northern Okhotsk region and to its evolution. Has there been any recent magmatism in the area?

The Pacific plate is actively subducting under the east coast of Kamchatka, south of the Aleutian junction at Cape Kamchatka. Recent studies (e.g., Levin et al., 2002a; Park et al., 2002) have indicated that the Pacific slab does not continue north of Cape Kamchatka and that there is a hanging slab edge with associated volcanism. A clear (although diminished relative to the subduction zone farther south) seismicity trend, however, continues north of the Aleutian-Kamchatka corner, although no Wadati-Benioff zone is observed; the level of seismicity is approximately constant along the entirety of this trend. The larger events in this trend have thrust mechanisms, indicating convergence, but not necessarily subduction. Based on marine terraces along the shore, the region is also undergoing moderate uplift, further indicating the convergence is still continuing (Pedoja et al., 2004; Bourgeois et al., 2004). There is a largely extinct volcanic arc and an offshore accretionary wedge that indicates that subduction was active in the Late Cenozoic. A history of tsunamis of a size comparable to that generated in 1969, with a recurrence interval of 200 years or less, is recorded by deposits in this region (Bourgeois et al., 2004, and submitted). The tectonic implications of these deposits have yet to be resolved.

This seismicity trend could represent the boundary between the proposed Okhotsk and Bering (Mackey et al., 1998) plates. The northern termination of this seismicity trend corresponds to the point where the strongest band of seismicity that crosses the Isthmus of Kamchatka from the Magadan region terminates. However, no clear geologic offsets are known. The seismicity transects the Isthmus in generally east-west striking zones, thus, a major question is whether a boundary (or boundaries) between Okhotsk, North America, and Bering exists in the Kamchatka Isthmus. Are there one or more fault zones? How do proposed plate configurations in the area relate to the thrusting and present-day tectonism (subduction or underthrusting) in northeastern Kamchatka? What is the true strain rate across this zone? Does a Bering plate exist? Is the seismicity and volcanism in Chukotka and Seward Peninsula an active rift system, and is it associated with the existence and motion of the Bering plate or is it due to other causes? While GPS data will soon be acquired to address the large-scale problem of the Bering plate, the nature and causes of tectonism in Chukotka and Seward Peninsula need to be explained in some context.

The Kamchatka Subduction Zone

The subduction zone between the Pacific plate and Kamchatka Peninsula has been extensively studied, seismically and geologically. However, several problems remain that could logistically be associated with other work in the region.

There are two roughly parallel volcanic arcs in southern Kamchatka. The first, called the Eastern volcanic front, lies above the 100 km depth line of the Pacific slab and exhibits magmatic activity related to current subduction. In the southernmost part of Kamchatka, the front runs parallel to the trench, but towards the north it deviates to the northwest to produce the most voluminous volcanic cluster of the arc - Klyuchevskoi group and Shiveluch. This volcanic cluster represents a departure from other arc volcanoes in terms of its geographic, tectonic and geochemical significance and is likely related to the evolution of the Kamchatka-Aleutian junction (Volynets et al., 2000; Yogodzinski et al., 2001). The second volcanic arc is located farther inland, in the Sredinny Range, and is less active and combines volcanic rocks with island arc and within-plate characteristics (Churikova et al., 2001; Portnyagin et al., 2005; Volynets, 1994). How did Kamchatka develop two volcanic arcs? What is the nature of young volcanism north of Shiveluch volcano, which is presumed to overlie the northern boundary of the Pacific slab (Churikova et al., 2001)? The answer to these questions may also be related to the most recent terrane accretion on the Pacific coastline, involving the so-called " Cape" terranes (Fig. 1).

Four scenarios for the development of the Cape terranes have been proposed. 1) Subduction of the Pacific plate caused the arc volcanism in the Sredinny Range until the collision of the Cape terranes, which either rode passively on the Pacific plate or formed part of a small, independent, subduction zone. After the Cape terranes accreted to Kamchatka, subduction and arc volcanism jumped seaward to their present position (Volnyets, 1990, 1994). 2) The Cape terranes were originally volcanic islands in what is now the western Aleutian arc system. The Pacific plate motion reconstruction of Wessel and Kroenke (1997), when applied to the Aleutian-Kamchatka junction (AKJ), predicts that prior to a change in plate motion at ~6 Ma the AKJ had been moving along the coastline for 20 million years to its present position. During this time, active subduction of a Bering/Kommandorsky ocean basin sustained arc volcanism in the Sredinny Range. At ~6 Ma, changes in Pacific plate motion caused this convergence to cease. 3) Subduction subsequent to the accretion of the Cape terranes was shallow-dipping and sustained arc volcanism in the Sredinny Range up until the arrival of the plume head of the Emperor seamount chain. The subduction of the Emperor plume is hypothesized to have caused a substantial erosion of the Cape terranes. A coeval disruption of the subduction process is hypothesized to have induced a steepening of subduction angle to its present position, and a coeval eastward step of arc volcanism. 4) The exposed Cape terranes are the areas of greatest uplift while intervening areas are either undergoing subsidence or have been removed as in scenario three.

 

4. Broader scientific and societal impact:

The complexity of the Eurasia/North America plate boundary is one of the major unresolved issues of global tectonics. This is a frontier region about which very little is known, but that links the tectonics of the Arctic to those of the North Pacific. The interactions of these regions are important for a number of reasons including the determining history of oceanic connections between these two ocean basins, the tectonic evolution of Alaska, the nature of the circum-Pacific margins over time, and the plate motions of both North America and Eurasia. In a more general context, study of this region can address problems of continental deformation, the effect of Euler poles near plate boundaries, progressive rifting and rupturing of continental crust, development of superposed basins (both in rifting and compressional areas), evolution of large strike-slip faults, and extrusion tectonics.

Neotectonic studies in this area will also provide information on natural hazards (primarily seismic, but also landslides, tsunamis, and volcanism) that affect a resource rich region important to the economy of the Russian Federation. Tsunamis and volcanic eruptions in this area have potential to affect the northwestern Pacific, including aviation passing through the region and coastal settlements in Alaska and northern Japan. These studies may also contribute to the study of petroleum potential of the eastern Russian Arctic. The cooperative study of these problems will also train graduate students and benefit Russian scientists.

5. Approaches to questions:

Clearly, northeast Russia has a wide range of unresolved neotectonic problems. The working group selected four specific areas and topics on which to focus research on the near term: The extensional boundary in the Laptev Sea region, the Okhotsk plate question as applicable to the Magadan district, Northeast Kamchatka, and terrane accretion on the Pacific front of Kamchatka. Studies are in progress to examine the Bering Plate question and there is considerable Russian interest in examining the seismicity of Chukotka. These ongoing and proposed future studies will both establish the present plate tectonic framework for the Arctic Ocean and improve our understanding of how deformation is transferred along plate boundaries.

Many of the projects are seismology driven. This is due to the “relative” ease of deploying a limited number of passive recording stations, the need for a comprehensive regional characterization, and the necessity to determine the deep structure of the crust and upper mantle. Our understanding of seismicity in northeast Russia has greatly improved in recent years with the compilation of a seismicity catalog and map for eastern Russian (Mackey et al., 2004; Fig. 2) and the deployment of permanent and temporary digital seismic networks in Kamchatka (e.g., Levin et al., 2002b) and mainland Russia (Mackey and Fujita, 2001). GPS data are beginning to become available in the Magadan district and Kamchatka (e.g., Kogan et al., 2000; Steblov et al., 2003; Gordeev et al., 2001). Neotectonic and paleoseismological studies are beginning to contribute to key questions of tectonics (Bourgeois et al., submitted). Thus the time is appropriate to prepare to initiate more detailed studies of the neotectonics of northeast Russia. The projects suggested here, taken as a whole, would test a variety of hypothesis about the plate boundary and processes occurring along it. The study areas are shown by the red boxes in Figure 1.

A. Laptev Sea: How does an oceanic rift orthogonally enter a continent?

The Laptev Sea is the one region on the Earth’s surface where sea-floor spreading presently, or in the recent past, propagates into the edge of a continent at a very high angle. This establishes an important limit on the efficacy of rifting processes that can not be observed anywhere else. At the point where the Gakkel Ridge disappears under the continental slope at the seaward edge of the Laptev Sea, its full spreading rate is ~0.5 cm/yr. How this deformation is absorbed and transferred by the deformation of the adjacent continent is an outstanding question for geodynamics. Answering this question will dramatically improve our understanding of how the crust-upper mantle system deforms and the mechanical difference between continental and oceanic lithosphere.

The primary objective is to study the rifting below the Laptev Shelf using a deployment of 6 – 10 broadband seismometers around the margins of the Laptev Sea. These stations would supplement the IRIS GSN station at Tiksi (TIXI) and it would be desirable, if possible, to make them permanent; this may be possible for stations at settlements in northern Yakutia. Additional objectives, such as the geology of the New Siberian Islands could be added for relatively low cost because of common logistics.

Four different data sets can be obtained through this means to examine the structure and evolution of the Laptev Rifts and the Gakkel Ridge.

1) The current estimated threshold for complete detection of seismic events on the Laptev Shelf is between magnitude 3 and 4 (Engen et al., 2003). The proposed seismometer deployment would reduce that substantially, enabling higher resolution studies of active shallow structures.

2) Focal mechanisms of more events can be determined; Fujita et al. (1990), Avetisov (1996), and Imaev et al. (2000) have suggested that a wide variety of mechanisms occur on the shelf and, especially, along its margins.

3) Records of teleseismic activity for tomographic studies of the deeper rift structures.

4) Refraction records for study of the crust and upper mantle.

The seismicity data should be augmented with GPS; this would add little additional expense. However, motions are likely to be small, perhaps below detection, over the one to two year span envisioned for primary data acquisition.

The logistics of deploying seismometers on the New Siberian Islands, Severnaya Zemlya, and Taimyr Peninsula are difficult due to the lack of permanent facilities. It will be necessary to provide power and a satellite datalink. Due to the high latitude, solar power can not be used for a full year deployment.

Deployment of the seismometers on the islands and other remote localities using a small ship opens up several possibilities:

1) Ocean Bottom Seismometers could be deployed at and beyond the shelf edge to constrain the seismic activity on the slowest spreading ridge in the world.

2) Shooting to and from the broadband stations with a large volume

airgun to obtain refraction records as we sail away from each site after installation and sail towards them for recovery.

3) Collecting relatively shallow-penetration seismic reflection records (1-2 km) so that we can relate local, shallow structure to the deeper results from refraction and tomography.

The deployment of additional seismometers to the south to augment stations operated by the Yakutsk regional network could provide information on the southern extent of rifting and the change from extension to compression. Reanalysis of Russian gravity and magnetic data over the coastal plain may help constrain the extent of the rift basins underlying the Laptev shelf; although logistically difficult, seismic reflection profiles in the coastal plain would provide considerable new information on the nature of continental rifting. Measurement of contemporary rates of deformation (would require a long GPS deployment due to presumed low strain rates) on land and study of earthquake focal mechanisms of small events would be key to establishing how this major plate boundary progresses into continental crust and how it links up with other belts of seismicity in NE Russia.

B. Magadan District: Is there an Okhotsk Plate and how is the convergence between North America and Eurasia accommodated in this area?

The Magadan district and northeastern Yakutia encompass the area where the extension in the Arctic Ocean changes to convergence. In order to accommodate this convergence, crustal shortening, lithospheric extrusion, and/or extensive crustal deformation must occur.

The primary objective here is to evaluate models for Okhotsk existence, motion, and deformation.

1) Structural mapping and determination of the timing of faulting in the Magadan region to document the extent to which deformation in the "escape" corner is distributed or localized. An important target would be splay faults from the Ulakhan fault, a significant strike-slip fault bordering the Chersky range with 24 km of motion indicated by river offsets. Also important is the identification of areas of uplift and determination of uplift rates. Additional g eomorphologic and neotectonic field studies, combined with satellite imagery, aerial photos, etc., would also help identify faults with significant recent motion, and the nature of that motion. Examination and dating of terraces and other level surfaces will help constrain uplift rates. Insights may also be gained into the geochemistry and sediment content of rivers flowing into the Arctic Ocean, and the evolution of drainage systems affecting the Arctic and North Pacific.

2) Densification of GPS sites in the Magadan/Chersky region. A 3-5 year baseline of measurements will be needed to capture small variations of motion that will distinguish deformation types. GPS sites should be added to the Kamchatka Isthmus to help evaluate whether (and how much) Chersky deformation extends to the western Bering Sea and the Aleutian-Kamchatka corner.

3) Add broadband seismic stations to Magadan region and the Kamchatka Isthmus to document better the seismicity of the region of the proposed “plate” boundary. The goal is not to improve catalogs per se, but to identify active faults and lower the threshold for estimating earthquake source mechanisms (down to M~4) that will allow us to characterize whether the brittle deformation of the crust follows the GPS and/or the kinematic predictions. This work is closely linked to the structural studies, above.

This deployment would co-locate broadband seismometers with existing high-frequency Russian network stations, typically sited in towns with significant cultural noise. However, this noise tends to be short period in nature, thus for crustal surface wave observations at periods of ~10 sec, this noise should be less of a problem. Therefore, broadband data will definitely improve what can currently be monitored with current Russian networks. The option of locating stations outside towns is also to be considered. Much better noise characteristics would trade off with somewhat higher maintenance costs and deployment, power, and security issues.

Data from broadband seismic stations will also aid in determining regional lithospheric/asthenospheric mantle structure via surface wave tomography with periods of 10-50 sec. Such tomography will help to define the precise extent of slow-velocity, shallow, mantle in the Okhotsk region; e.g., does it extend into the Magadan/Chersky corner. Because slow shear wave velocity is associated with higher temperatures and weaker lithosphere, such tomography is essential to the goal of assessing the rigidity of Okhotsk region lithosphere. Also, broadband data can be used to estimate shear-wave splitting and P-SH converted waves, both of which would constrain mantle anisotropy and lithospheric strain. The region has also been of interest to global seismologists because of it location antipodal to the South Sandwich Islands which makes seismic stations in the area useful for core and inner core studies, and for studies of subducting slabs in general.

Miscellaneous studies which will contribute to the overall understanding of the region could include systematic heat flow measurements in the region, identification of recent magmatism (if any), and field and seismic studies to determine the origin and nature of the basins superposed on the proposed plate boundary.

 

C. Northeast Kamchatka: Is there a transcurrent boundary that crosses the isthmus? What is the nature of convergent seismicity and tectonics in northeast Kamchatka and how do these relate to proposed plate models?

 The primary objective here is to determine the causes of seismic activity and active compression north of the Aleutian-Kamchatka junction and the strain occurring in the Kamchatka Isthmus. Deployment of additional seismometers, co-located with GPS stations, would improve recording of events in this region and help to identify active faults crossing the Kamchatka Isthmus. Field work and analysis of air and satellite images is also required to establish the nature of faults on the Isthmus. Active faulting can be examined and offset quantified by trenching and the use of marker tephra (e.g., Ponomareva et al., 2004).

The earthquake history of this region can be elucidated by further tsunami and paleoseismology studies (Bourgeois et al., 2004 and submitted). More complete analysis of the 1969 Ozernoi earthquake and tsunami will help quantify active deformation in the region by linking the tsunami record to the source (earthquake rupture). That information can then be extrapolated to the paleoseismic record to generate rates of shortening. A key question is whether the convergence can be explained by either Okhotsk or Bering plate motions, or requires both or none. The torn slab model for the Pacific plate would seem to exclude the last possibility.

Continued study of uplift (e.g., Pedoja et al., 2004) and quantification of active faulting in the region (Kozhurin, 1990; Kozhurin et al., submitted), will help to constrain the extent and rate of convergence in northeast Kamchatka. Ongoing studies of Kamchatska and Ozernoi peninsulas should be extended north to Karaginsky Island and beyond. The proposed boundary between the Bering block and North America trends north from northeast Kamchatka and lies offshore north of Karaginsky Island. Is there a record of tsunamigenic events in this area as well? Is uplift in this region less than farther south?

Additional analysis and radiometric dating of volcanism in northeast Kamchatka will also elucidate the nature and history of plate motions here (e.g., Portnyagin et al., 2005). Although the geochemistry of volcanism north of Shiveluch has been examined in some cases (Portnyagin et al., 2005), this region is understudied compared to southern regions, both in terms of geochemistry and chronology. These studies are important for defining the end of subduction in the region (postulated to be Miocene), the tearing of the slab (postulated to have happened c. 2 Ma), and the current plate tectonic setting.

In terms of the Bering plate, the Koryak Highlands are a huge gap in terms of (M < 4) detection (M < 4), although offsets of valleys and faults visible in satellite images indicate that there is active tectonics in the area (Fujita et al., 2002).

 

D. Kamchatka: Why Are There Two Volcanic Arcs in Kamchatka and Where do the Cape Terranes Originate?

This topic has more of a Pacific focus than the others discussed as the geographic area lies dominantly to the south of the Aleutian-Kamchatka corner. As described above, three models have been proposed for the accretion of the Cape terranes. These can be discriminated in part by examining the detailed geochronology of volcanism and whether the Cape terranes have been tectonically eroded.

1) Models one and three should show distinct differences in the pattern of volcanism. Model one predicts a more or less simultaneous die out of the Sredinny Ranges volcanism and the inception of the Eastern Arc. Model two suggests that volcanism in the Eastern Volcanic Front should have begun in the south and progressed gradually up the coastline, as the steeper subduction of Pacific plate advanced north. At the same time, Sredinny Range volcanism is predicted to change character and decrease in volume. At the time of Pacific plate motion change, subduction volcanism in the Kamchatka Isthmus would have changed character and decreased volume, mostly all at once. Quantification of the ages of volcanism in the arcs, from 30 Ma to present, will help resolve this problem.

2) Offshore seismic profiling to study whether the accretionary wedge and Cape terranes have been tectonically eroded will constrain whether model three is viable.

 

6. Existing data sets and projects:

Seismicity catalogs and bulletins, focal mechanisms – Michigan State University, Geophysical Survey of Russia, Institute of Diamonds and Precious Metal Geology (Yakutsk) and Yakutsk EMSD, Sevmorgeo (St. Petersburg), Magadan EMSD, Kamchatka EMSD

Digital seismic data – IRIS DMC (for GSN stations), Michigan State University (for joint stations in Magadan and Yakutsk districts), Magadan EMSD, Yakutsk EMSD

Upper Mantle Structure – Yale University, University of Colorado

GPS data – Columbia University (northeast Russia), University of Alaska Fairbanks (Alaska), Kamchatka EMSD (Kamchatka)

Arctic aeromagnetic and other potential field data – Sevmorgeo (VNIIOkeanologiya), St. Petersburg

Laptev Sea seismic reflection data – BGR

Sea of Okhotsk seismic reflection data – Russian institutes (Magadan), petroleum industry

Active fault mapping data - Institute of Diamonds and Precious Metal Geology (Yakutsk), Geological Institute (Moscow)

Geologic data (including Neotectonics), geologic maps – SVKNII (Magadan), Institute of Diamonds and Precious Metal Geology (Yakutsk)

Paleoseismology (Tsunami) data and marine terrace data in Kamchatka – University of Washington, Institute of Volcanology and Seismology (Petropavlovsk)

Volcanism data – Institute of Volcanology and Seismology (Petropavlovsk)

Analog 1:200,000 topographic maps – Stanford, Michigan State University

Elevation data – GTOPO30

7. Other comments:

The difficulty of logistics in the study area forces as many different investigations as possible to be conducted simultaneously. Many areas of interest are remote and difficult to access. Some target areas should be studied cooperatively with projects suggested by other groups. In order to achieve our goals, we will need the cooperation and support of the seismic network operators and research institutes in Yakutsk, Magadan, and Petropavlovsk-Kamchatsky who have been extremely helpful with previous programs.

On the other hand, the time is right to address the primary question and learn about the “last frontier” in delimiting boundaries between major plates.

8. References:

Avetisov, G. P., 1996. Seismoactive Zones of the Arctic: VNIIOkeanologiya, St. Petersburg, 185 pp. (in Russian)

Bourgeois, J., Pedoja, K., Pinegina, T., and Ponomareva, V., 2004. Variability in coastal Neotectonics along the Kamchatka subduction zone (abstract): Transactions of the American Geophysical Union (Eos), v. 85(47), Supplement, p. F1740-1741.

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