Kamchatka: Edge of the Plate
IRIS News Letter, Vol. 2000, Num. 1
Jonathan M. Lees
University of North Carolina,
Chapel Hill
Mark Brandon, Jeff Park, Vadim Levin
Yale University
Alexei Ozerov, Evgenii Gordeev
Russian Academy of Sciences
New PASSCAL data
have been acquired along the extent of the Kamchatka Peninsula to examine the
interaction of the Pacific Plate and the mantle in the corner junction of the
Aleutian and Kamchatka trenches. The project, called the Side Edge of Kamchatka
Slab, is a collaborative effort between researchers at Yale University and the
Russian Academy of Sciences Institutes in Petropavlovsk-Kamchatski, in
particular the Institute of Volcanology (Alexei Ozerov) and KOMSP (Evgenii
Gordeev). In Russia, Kamchatka is known
as the caboose, the last car on the train.
It is practically the farthest one can get from Moscow, the center of
cultural life in Russia. Being nine
time zones away from the financial centers creates a sense of isolation and
liberation. For Russians, Kamchatka is
a land of dreams and possibilities, much as Alaska is the last frontier for
Americans. This gives one the sense that being in Kamchatka is like being on
the edge of the world. But the feeling
has more significance than the mere waywardness of the place. The majestic volcanoes, the continuous
seismicity and the incredible geology are omnipresent reminders that this is a
special place. The relentless
subduction of the Pacific plate plunges beneath Kamchatka, 70mm per year. Kamchatka is home to the most magnificent
volcanoes in the Pacific Rim (28 active volcanoes) and neighbor to the Bering
strike-slip fault, marking the western end of the Aleutian-Komandorsky
Islands. This land mass provides an
exceptional platform for investigating the interactions of volcanism,
tectonics, and mantle dynamics.
Deployment
Map of the SEKS experiment, 1998-1999
We installed 15
broadband PASSCAL instruments along the length of the Kamchatka Peninsula. Each station included a Guralp 3T sensor,
housed in an established short-period seismic station or other existing
structures to protect against the harsh environment. Nearly all stations had backup power systems, including dry cell
batteries, solar panels and local, though intermittent electricity
outlets. During the duration of this
installation, Russia suffered significant financial crises, which translated
into erratic fuel supply to power stations and considerable chaos in heating
and power to individual and institutional alike. In spite of these difficulties, we are fortunate to announce at
least an 80% return on the seismic signals for most of the duration of the
experiment. The high level of technical
expertise in the Russian Institutes was critical to the success of our
experiment.
Description of the SEKS
experiment
In this PASSCAL
experiment we address issues related to the transform-trench double junction in
Kamchatka: What are the implications for the crust and upper mantle? The surficial manifestation of the
connection is the massive Bering strike slip fault, extending from Attu Island
westward towards Kamchatka. In
Kamchatka, the margin between the Pacific Plate and North America takes a sharp
turn south, towards the Kurile trench and Japan. How does the Pacific plate accommodate this sharp apparent
bend? We have proposed a new model for
the northern extent of the subducting Pacific Plate and outline the
implications for flow in the upper mantle.
There are two
possibilities to explain the Aleutian/Kamchatka connection. The first employs a laterally continuous
slab, the Pacific plate, which subducts uninterrupted below both the Aleutians
and Kamchatka. This model requires the
Pacific Plate north of Bering fault to move laterally through the mantle and
bend around the Kamchatka corner. It
predicts the presence of a large body of cold slab in the upper mantle,
northwest of the Kamchatka/Bering intersection. A second, alternative, model is that subduction in Kamchatka is
parallel to subduction in the Aleutians, and there is no connection between the
slab below the Bering Sea and the slab in the Kamchatka region. In this model a
tear between Kamchatka and the Aleutian subduction zones isolates and separates
these bodies. Furthermore, the upper mantle would be open to flow through the
tear beneath the Komandorsky Basin. It
is primarily the second, the tear-model that we consider in this paper. The purpose of our study is to test these
alternative models.
Magnetic Anomalies on the Ocean Floor in the Aleutians and
Kamchatka
The main surface
observation connecting the Aleutians and Kamchatka is a very long strike slip
fault that starts in the Aleutians and ends in central Kamchatka, slightly
north of the Klyuchevskoy group of volcanoes.
The Bering fault is over 1,000 km long and has been studied in some
detail by Russian and western researchers.
Nearly all earlier studies suggest that the Bering fault is strike slip,
with relatively shallow seismicity. The
Bering fault separates the Komandorsky part of the North American plate from
the Pacific plate. Young Komandorsky basin is juxtaposed against much older
Pacific slab. The Komandorsky basin contains an older spreading center, which
ceased approximately 10 million years ago.
There is very high heat flow in the Komandorsky basin, as opposed to
heat flow in the adjacent Pacific slab.
Gravity studies show that the Komandorsky basin is relatively thin
compared to the much thicker, older Pacific plate.
Mantle Dynamics
The implications
for a tear in the subducting Pacific plate are numerous and essential for our
understanding of mantle dynamics. In
the past, researchers have primarily concentrated on two-dimensional models of
the flow in the mantle, where slabs are typically represented as infinite
planes in a half space. The
introduction of a tear, however, forces us to consider the three-dimensional
nature of flow in the mantle. What
happens near the edge of the exposed end of a subducting plate? How is flow near the edge modified by the
more complicated boundary conditions and how do these affect the very nature of
the slab itself? Our initial studies
have focused on heating of slab by conduction and convective ablation. The shoaling of the seismicity in the
northern part of the Kamchatka Arc suggests parts of the slab are potentially
missing or have thinned considerably.
Anisotropy
If slab rollback
and trench-parallel asthenospheric flow are important, then a northward pattern
of mantle flow should be clearly apparent in observations of velocity
anisotropy. Initial results from
studies of shear wave splitting in the mantle below Kamchatka suggest that
there observable trench parallel anisotropy is small below the broadband array
or offshore. Several observations in
our northernmost stations, however, show significant anisotropy trending
northwest-southeast, nearly parallel to the direction of the Pacific
Plate. These observations seem to agree
with the model that mantle is straining past the edge of the exposed slab north
of the Aleutian-Kamchatka juncture.
Volcano Geochemistry
Most melts at
volcanic arcs are formed in the mantle wedge, but experimental and geochemical
research has identified subduction zones where the crust of the slab has
apparently melted. Oceanic crust,
however, is refractory and much higher temperatures are required to melt it
relative to the more liable mantle.
These crustal melts, or slab melts, were first described in the
Aleutians where they are now called adakites.
Their distinctive geochemical signature has been recognized in many
places, including the Kamchatka arc.
Factors favoring slab melts include subduction of a young (hot) plate,
slow subduction allowing heating at shallow levels, and slab edges where
asthenospheric heating can produce sufficient temperatures for crustal
melting. Volcanoes above the Kamchatka
corner show evidence of slab melts.
Seismic data provide an opportunity to study the slab for evidence of
hot spots where crustal melting might be occurring.
Karymsky Volcano
Opportunities
for exploring various aspects of geophysics abound in Kamchatka. During the SEKS experiment, we were
fortunate to witness the ongoing explosive activity of Karymsky volcano. Karymsky is located in a vast caldera
complex, comparable in size and activity to Long Valley Caldera in
California. On January 1, 1996, a M7.0
earthquake triggered massive eruptions of Karymsky Volcano and nearby Karymsky
Lake. Since the initial eruptions,
Karymsky has continuously erupted in Strombolean mode, exploding every 5 to 15
minutes for nearly 4 years. The
numerous, repetitive small explosions provide excellent multiplicity of
volcanic events, recorded annually by the members of the SEKS working group and
students. In addition, observations of
infrasonic acoustic (Jeff Johnson, University of Washington), COSPEC (Phil
Kyle, New Mexico Inst. Tech; Toby Fischer, Berkeley), gravity (Emily Brodsky,
Caltech), GPS (Dan Johnson, Central Washington University) and petrologic
analyses (Alexei Ozerov, Institute of Volcanology, RAS) are being combined to
distinguish models of conduit dynamics that control explosive activity. Our efforts in Kamchatka thus span several
orders of magnitude in scale, from mantle to volcano conduit dynamics.
Karymsky
volcano in 1999
Conclusion
While the
challenges were daunting, we found the overall experience of working in this
remote part of the world rewarding and exhilarating. Initial inspection of the
data suggests we have a unique data set illuminating the subduction zone, the
volcanic front and the tectonic relations between the active Kamchatka arc and
the Bering-Aleutian transform.
Photographs
of our deployments
Lees Collecting Data at Klichevskoi Volcano
Katrin Baterau and Jeff Johnson resting after strenuous climb
Seismic Data and Raypaths of an Earthquake in the Aleutians recorded on the SEKS array