Chris Goldfinger

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Chris Goldfinger Burt 282 7-5214 gold_at_coas.or
egonstate.edu http//activetectonics.coas.oregonst
ate.edu Reading for Tuesday von Heune et al.
2003 JGR. McCloskey et al., 2005 Zhang et al.,
2004 Stein, 2002 Song and Simon, 2001 Suggested
Supplements Moores and Twiss Chapter 7
Convergent Margins
OCE 661 Plate Tectonics
Never lend a geologist money. They consider a
million years ago to be Recent!
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Plate tectonics tip for Today If you see this,
dont stop to take pictures
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A note about focal mechanisms. Here is an
earthquake from June, 2005, and its aftershocks.
Which nodal plane was the fault plane?
Which way was the directivity? What might the
coulomb stress change be? Would this generate
a tsunami?
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Another note about solving the problem of poor
station coverage in ocean basins Hydrophones

• Compressional waves (sound, earthquakes) move
  very efficiently through water, particular in the
  SOFAR channel.
• Information on seismicity and volcanism, can
  be acquired with hydrophones.
• BONUS! May be useful for detecting submarine
  landslides (not as easy with seismometers).

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Ocean bottom observatories

• Real-time data
• Power feed from shore (problematic for regions
  such as the Aleutians)
• Require extensive cabling

We now return you to plate tectonics.
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Stress transfer and stress triggering. We
dont have time to explore the details, but
youll see some convincing examples in the
reading of the effect of slip on a fault
influencing the stress state in the nearby
region. This example shows the effect of the
Landers earthquake, and the combined effect of
Landers and Big Bear on the subsequent Hector
Mine earthquake 7 years later. The idea that
faults interact is surprisingly new, within the
past 15-20 years.
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Coulomb stress modeling Coulomb stress transfer
has been modeled for a simple source model based
on the earthquake scenario used for ground motion
modeling. The plot shows coulomb stress change in
the fault-near region for optimally oriented
strike-slip faults. The positive stress lobe in
northern Sumatra indicates that stress on the
Great Sumatran Fault has increased after the
December 26, 2004 event. Additional computations
of the coulomb stress change for optimally
oriented thrust faults are in agreement with the
locations of the large earthquakes which took
place on March 28th 2005 (M8.7) and April 10th
2005 (M6.8). Similar results are also obtained
by McCloskey et al. (2005). M. B. Sørensen et
al., 2005 EGU
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Take a look at where coulomb failure stress is
elevated, and where it is reduced. Possible
connection to the aftershock pattern?
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So far weve mostly looked at subduction zones
with accretionary wedges. Lets take a look at
Northern Chile from a surficial perspective
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Subduction Erosion, that other end member. As
much as 100 km of margin has probably been
removed, although the search is still on for the
history What are the implications for volcanism
etc?
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Nitinat Fan Astoria Fan
Meanwhile back in Cascadia. Remember the
landward vergent region, dominated by frontal
accretion and high pore fluid pressures? Lets
look further south, where sediment supply
diminishes due to the thinning of the Astoria
Fan. We can easily see changes on the gross
morphology of the margin, and an odd region of
chaotic slope south of the landward vergent
region.
Mud volcano
Landward vergence
News flash Singapore Airlines announces 2 Yen
price reductions on some routes due to plate
tectonics.
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Meanwhile back in Cascadia. Remember the
landward vergent region, dominated by frontal
accretion and high pore fluid pressures? Lets
look further south, where sediment supply
diminishes due to the thinning of the Astoria
Fan. We can easily see changes on the gross
morphology of the margin, and an odd region of
chaotic slope south of the landward vergent
region.
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Looking more closely at the bathymetry, we can
easily see a failure zone about the size of
Corvallis on the lower slope. But look closely
at the wedge and the deformation front. Does it
look normal? Where is the fold thrust belt
were used to seeing? Why is the deformation
front so irregular?
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Here, in a sidescan sonar view (SeaMarc 1A) we
see a high-resolution view of the deformation
front. What are we looking at? Is there a
thrust fault somewhere in the picture?
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More clues in E-W seismic profiles Upper slope
stratigraphy is truncated Abyssal plain has an
anomalous hummocky reflector package buried in
it. To the south, another anomalous reflector
package, this one much deeper.
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Looking more closely at the bathymetry, we can
easily see a failure zone about the size of
Corvallis on the lower slope. But look closely
at the wedge and the deformation front. Does it
look normal? Where is the fold thrust belt
were used to seeing? Why is the deformation
front so irregular?
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We can now see that much of the southern Oregon
margin is dominated by giant submarine
landslides, the largest one 80 km x 40 km.
They left buried debris in the abyssal plain,
the tops of the block are poking out. The
three main slides get older southward, accounting
for the deeper burial. The southern, oldest
slide formed a seaward dipping backstop against
which landward vergent folds are growing. The
northern slide, 100,000 yrs old, is just barely
beginning to be re-accreted to the margin. No
fold thrust belt is apparent. The older slides
zones are re-forming their fold belts, and
recycling their slide debris.
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Upper crust- A lot of crumbs held together with
dough
So, there seems to be an age-progressive failure
series, moving from south to north along the
margin (Goldfinger et al., 2000). There is a
possible explanation, in the form of a basement
high that has subducted beneath the margin,
moving at about the right rate. Could this
cause the slope failures? Well, probably not.
The margin parallel component of JDF north
America plate convergence is about 20 mm/yr, or
20 km/my. The earliest slide is about 1 my old,
and is 1.5 degrees of latitude to the south, or
165 km away. So more likely these failures are
from something else. Seamount chain? N-S
oriented Ridge? Subduction erosion?
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Hmm, can you have subduction erosion on a margin
with high sediment supply? This image is of
Heceta Bank on the central Oregon shelf. What
marks the boundary between the rough and smooth
topography?
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Good guess, a Pleistocene low-stand shoreline!
Great, a very handy strain marker that was cut
15-20,000 years ago, and was level at the time.

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To verfy what this is, submersible dives were
done to collect datable material, such as
barnaces and mussels. Wait a minute arent
those intertidal critters? Yes, and we can
also see shoreline features such as sea stacks
and Pholad borings along the former sea cliffs.
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It turns out that this shoreline, and a similar
one at Coquille Bank to the south, are both
tilted strongly to the west, and to the south as
well. This is the same pattern we see in the
sediment starved northern Chile margin.
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While the outer arc high was going up most of the
time since the Miocene (Kulm et al., 1974), it is
now going down and tilting west, consistent with
subduction erosion.
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Heterogeneous nature of structure in the upper
plate in Cascadia
The heterogeneous structure is more apparent in
this pair of surface and subsurface views. On
the right is a structure contour on a widespread
Pleistocene unconformity.
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Ok, lets look deep again for the mid-level view
of subduction
local underplating of eroded material
With more localized tomographic experiments, we
can begin to see more detailed subduction
structure. The problem of lack of stations
offshore is solved with temporary deployments of
ocean bottom seismometers (OBS).
unusual high Vp/Vs
residuum of former partial melts
subducting Nazca plate
Why the high Vp/Vs Ratio?
Partial Melt! Vs is very slow in a partial fluid
Dr. S. Husen
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• In the framework of plate tectonics, earthquakes
  deeper than about 100 km occur in the cold
  interior of subducted lithosphere. Otherwise,
  deep earthquakes would not be restricted to zones
  of recent convergence e.g., Isacks and Molnar,
  1969, 1971 Isacks et al., 1968 Wadati, 1927.
• The estimated limiting temperature for seismicity
  is approximately 600 to 800C Chen and Molnar,
  1983 Molnar et al., 1979 Wiens and Stein,
  1983. As such, inclined zones of seismicity (the
  Wadati-Benioff zones, WBZ), connecting shallow
  earthquakes near the trench with earthquakes deep
  in the mantle, represent traces of active,
  sinking lithosphere.
• On a global scale, for WBZ that reach depths of
  about 300 km or less, fault plane solutions of
  earthquakes typically show a pattern of downdip
  extension (DDE).
• For WBZ that penetrate toward the bottom of the
  mantle transition zone (down to depths of almost
  700 km), fault plane solutions reveal a dominant
  pattern of downdip compression (DDC) where the
  axes of maximum compression (P axes) are
  sub-parallel to the dip of the slab (Figure 1a)
  e.g., Isacks and Molnar, 1969, 1971.
•    

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• The current paradigm is that these patterns of
  uniform strain reflect internal deformation of
  subducted lithosphere e.g., Apperson and
  Frohlich, 1987 Isacks and Molnar, 1971
  Vassiliou et al., 1984. Specifically, the cold,
  strong slab acts as a stress guide A uniform
  strain
• field develops in the cold slab as it sinks
  because of negative
• buoyancy (slab pull) and encounters resistance
  from a highly
• viscous mantle at greater depths. DDE and DDC
  are patterns of
• internal deformation
• manifestations of the slab either being extended
  under
• its own weight or being compressed by mantle
  resistance
• against slab pull, respectively. In both cases,
  slab pull is the
• instigating factor that provides a natural
  driving mechanism
• for continual accumulation of elastic strain in
  the slab that
• is necessary to cause deep earthquakes.

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You can also get downdip compression here due to
unbending of the previously bent slab
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Brudzinski et al. JGR, VOL. 110, B08303,
doi10.1029/2004JB003470, 2005 Earthquakes and
strain in subhorizontal slabs
But we also know that we should be expecting
dehydration reactions at stages in the subduction
process. How do we separate the two?
Acting alone, density differences that drive
subduction seem neither necessary nor sufficient
to cause deep earthquakes. An observational basis
for this reasoning comes from the abundance of
deep seismicity in zones that are subhorizontal
e.g., Araujo and Suarez, 1994 Barazangi and
Isacks, 1976 Chen and Brudzinski, 2001
Hamburger and Isacks, 1987 Hasegawa and Sacks,
1981 Okal and Kirby, 1998. Since the downdip
component of slab pull is approximately
proportional to the sine of slab dip, such a
driving force is greatly diminished in
subhorizontal slabs, indicating that slab pull is
not necessary to cause earthquakes. The big
orogenic bend of S. America induces flat slab
subduction, providing contrasts for investigation
of slab seismicity patterns.
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Color-coded fault plane solutions to visually
identify earthquakes that are exceptions to the
more typical case of down-dip compression (deep)
and down dip extension (shallow) DDC red DDE
black beach balls.
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Double seismic zones
Recognition of double seismic zones didnt seem
to fit a kinematic model of plate flexure.
Tension is expected, and observed, in the outer
rise as the plate begins to bend. Compression is
expected in the lower part of the slab as it goes
around this radius. But the double seismic zone
is hard to explain with this simple model.
Hasegawa et al., 1978
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Vertical slices of (A) Vp values, (B) Vs values,
and (C) Vp/Vs ratios through Honshu region
structure. Earthquakes within 25 km of section
are plotted. This model shows a n
interpretation that includes hypothesized zones
of dehydrations and phase reactions that may
explain the double seismic zone. Numbered
areas in C are interpreted as (1) partial
melting, (2) transformations of metagabbro and
metabasalt to blueschist, (3) partially hydrated
mantle harzburgite, and (4) serpentine
dehydration Zhang et al., 2004
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And now, what youve all been waiting for,
forearc slivers!
Forearc slivers are driven by oblique subduction,
and changes in strike fo the subduction zone
result in internal extension or compression of
the sliver. The following slides show the
classic examples.
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Ok last topic for today, forearc slivers
In the Kuril arc, the oblique driving force runs
out as the trench changes strike, causing
compression where the sliver has no place to go.
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Both the Kuril and Sunda systems have a graben at
the starting end of the sliver, where obliquity
reaches a critical value. In the Sunda system,
obliquity becomes greater along strike, and so
the sliver accelerates and splits into multiple
faults to handle the change. At the extreme end,
it merges with backarc spreading.
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Sumatra the neotectonic setting. The figure has
been oriented on the main fault direction. The
India-Southeast Asia convergence vector changes
significantly in both direction and magnitude
over the length of the island, from 52 mm/yr
directed at N100E (at 20N, 950E) to 60 mm/yr
directed at N170E (at 60S, 1020E). Convergence
data (and mainland structural domains) are from
Sieh and Natawidjaja (2000). The seismic image of
Line 42-43 is shown in Figure 2.7. The volcanic
Barisan mountains (shaded above 1000 m) run
virtually the entire length of the island along
the line of the Sumatran Fault. The thick grey
line on the west coast of Nias indicates the
location of possible catastrophic slope collapse.
I.F.Z. Investigator Fracture Zone.
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GPS vectors and the Great Earthquake of June
2000. The upper diagram shows overall movement
vectors relative to Southeast Asia and their
trench-parallel and trench-orthogonal resolved
components. The lower diagrams compare these
components individually. Vector 1 is the regional
convergence vector, after DeMets et al. (1990).
The remaining vectors are GPS vectors from the
1991 - 1993 campaign at sites at the bases of the
arrows, after Prawirodirdjo et al. (1997).
'Beachballs' show the locations of the two
subevents proposed by Abercrombie et al. (2002)
for the June 2000 earthquake.
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(a) Interpreted single-channel seismic reflection
sections across the Mentawai Fault in the
southern part of the Sumatra forearc basin (after
Diament et al. 1992). Line locations as shown.
(b) Multi-channel seismic reflection section
across the Mentawai Fault south of Enggano, after
Schlüter et al. (2002). Location shown on Figure
2.6a. The greater penetration achieved on the
more recent survey suggests a transcurrent origin
for the feature which, in the nearby southernmost
single-channel section, appears to be a simple
faulted anticline.
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The Median Tectonic Line (MTL) on Honshu Island
in Japan results from increasing obliquity south
of about 35N where the Nankai Trough begins.
It ends where the trench swings to the east,
and obliquity is lost.
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When you look closely at and plate margin, you
begin to see unusual things. Cascadia and
perhaps two other margins, shows evidence of an
unusual form of strain partitioning. Recall
from the slides and readings that Cascadia
doesnt show much evidence of arc parallel strike
slip faulting, unusual for an obliquely
convergent margin. It does have a set of
oblique faults in the slab, that also deform the
upper plate.
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The model is that of a set of R shears in the
slab, perhaps induced by hydrodynamic mantle drag
(oblique component).
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1 km
The model is that of a set of R shears in the
slab, perhaps induced by hydrodynamic mantle drag
(oblique component).
Slip rate, determined by offset trench deposits,
is 5-7 mm/yr
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Cascadia does not have an arc-parallel sliver
fault as most subduction zones with 25 or more
degrees of obliquity do. Why not?
Conventional wisdom The Siletzia terrane
effectively acts as a backbone in the forearc,
making it too strong to respond with brittle
failure.
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Lets finish with a little tour of Cascadia.