Chris Goldfinger

1
Transforms
OCE 661 Plate Tectonics
OCE 661 Plate Tectonics
Chris Goldfinger Burt 282
7-5214 gold_at_coas.oregonstate.edu http//activetect
onics.coas.oregonstate.edu Reading for
Tuesday Dickinson, 1996 Atwater and Stock,
1998 Suggested Supplements Chapter 6 and
Chapter 8, strike slip, transform, and triple
junctions.
2
BTW, when we discussed the lack of agreement
regarding models of the Indian Collision I meant
to show this figure, a compilation of numerous
models of just one simple aspect How big was
greater India before the collision. From Ali
and Aitchison, 2007. /The one at lower left is
the definitive one, according to the authors.
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The zone between two plates sliding horizontally
past one another is called a transform-fault
boundary, or simply a transform boundary. The
concept of transform faults originated with
Canadian geophysicist J. Tuzo Wilson, who
proposed that these large faults connect two
spreading centers (divergent plate boundaries)
or, less commonly, trenches (convergent plate
boundaries). Most transform faults are found on
the ocean floor. Fracture zones are the (mostly)
inactive extensions of transform faults.
Transforms commonly offset the active spreading
ridges, producing zig-zag plate margins, and are
generally defined by shallow earthquakes.
However, a few occur on land, for example the San
Andreas fault zone in California. This transform
fault connects the East Pacific Rise, a divergent
boundary to the south, with the Gorda -- Juan de
Fuca -- Explorer Ridge, another divergent
boundary to the north. The San Andreas fault
zone, which is about 1,300 km long and in places
tens of kilometers wide, slices through two
thirds of the length of California. Along it, the
Pacific Plate has been grinding horizontally past
the North American Plate for 10 million years, at
an average rate of about 5 cm/yr. Land on the
west side of the fault zone (on the Pacific
Plate) is moving in a northwesterly direction
relative to the land on the east side of the
fault zone (on the North American Plate).
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Fracture Zones What are fracture zones? They are
formed by plate motion along transform
faults. Fracture zones are dominant features in
the topography of the ocean basins. We define
fracture zones as "extensive linear zones of
irregular topography of the oceanic basement,
characterized by steep-sided or asymmetrical
ridges, troughs or escarpments, caused by a
lateral offset of the plate boundary". They
provide flowlines of plates relative to each
other, and document changes in plate motions, but
their topographic expression is often complex and
not well understood. In order to use fracture
zones quantitatively in conjunction with magnetic
anomalies to calculate reconstruction poles and
their uncertainties for relative plate motions,
understanding the origin of fracture zone
topography and gravity anomalies is a crucial
matter. We distinguish different types of
fracture zones based on the length of the offset
of the mid-ocean ridge along a transform
fault. The above figure illustrates a mid-ocean
ridge that has been spreading at a rate of 20
mm/yr for 15 m.y. Isochrons spaced at 5 m.y. are
shown as well. This ridge is offset by a
transform fault 300 km long. Hence the offset of
the ridge is 300 km. Knowing the spreading rate
of 20mm/yr we can calculate that this offset
corresponds to an age offset of 15 m.y. The age
offset refers to time interval that it takes to
produce 300 km of ocean floor at a given
spreading rate. In this case this time interval
is 15 m.y. therefore the ridge-transform
intersections (RTIs) are characterized by an
active mid-ocean ridge being juxtaposed by ocean
floor that is 15 m.y. old on the opposite side of
the transform.
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Oceanic fracture zones are ocean-floor valleys
that horizontally offset spreading ridges some
of these zones are hundreds to thousands of
kilometers long and as much as 8 km deep.
Examples of these large scars include the
Clarion, Molokai, and Pioneer fracture zones in
the Northeast Pacific off the coast of California
and Mexico. These zones are presently inactive,
but the offsets of the patterns of magnetic
striping provide evidence of their previous
transform-fault activity. (1) All transform
faults are small circles about the pole of
rotation (2) The seafloor spreading rate v
increases as the sine of the distance from the
rotation pole v w rsinq (r radius of the
earth, w angular velocity, qdistance to the
rotation pole) The relative velocity between two
plates is zero at the rotation pole and has a
maximum value at the 90 from the rotation
pole Transform motion is called right-lateral
when something attached to the plate on the other
side of the fault appears to move to the right as
seen from where you are standing on this side of
the fault. If the object appears to move to the
left, the motion is called left-lateral. If you
were to cross to the other side of the fault, in
order to face this side, you would have to turn
around, and so the relative motion would appear
the same. As a result, whether a fault is right
or left-lateral does not depend on which side of
the fault you are on.
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Large-offset fracture zones have offsets of
several hundreds of kilometers. In fast spreading
regimes their primary characteristic is a
depth-age step, which develops as a function of
their large age-offsets.
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Based on the length of the ridge offset, we
distinguish 3 classes of fracture zone offsets
small, medium, and large. Small-offset fracture
zones have offsets less than about 30 km (age
offset 2.0 m.y. for slow spreading) and
dominantly represent the off-axis continuations
of non-transform offsets of the mid-ocean ridge.
All discontinuities of the Mid-Atlantic Ridge
with offsets less than 30 km mapped to date fall
in the category of non-transform
offsets. Medium-offset fracture zones, defined
by offsets of 30-200 km (2 m.y. for
slow spreading), are the off-axis traces of
transform faults that and have a well-developed
strikeslip valley. Examples for Atlantic
medium-offset fracture zones are the
Oceanographer, Hayes, Atlantis, and Kane fracture
zones. Pitman Fracture Zone in the South Pacific.
(a) topography, (b) magnetic anomalies (S.
Cande) Large-offset fracture zones have offsets
of several hundreds of kilometers. In
fast spreading regimes their primary
characteristic is a depth-age step, which
develops as a function of their large
age-offsets. In slow spreading regimes they have
complex morphologies that combine a depth/age
step (typical for Pacific type fracture zones)
with rugged topography and/or the presence of a
central valley. For instance the Romanche
Fracture Zone is characterized by a dominant
depth/age step modified by large amplitude
topography. Another morphological element often
observed at North Atlantic medium and
large-offset fracture zones is an asymmetry in
their cross section, expressed as a high
wall (often termed transverse ridge) on the old
side of fracture zones.
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As weve previously discussed, global bathymetry
derived from satellite altimetry is critical in
defining the geometry of fracture zones, fault
slip rates, and spreading rates. This dataset
has revolutionized the construction of global
plate circuit models
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But in reality, the topographic scarps are a
function of not just the age-depth relationship,
but also of local volcanic input, local off-axis
volcanism, and faulting.
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Main causes of complexity in transform fault
systems on a local scale nature of strain in 3-D
within a simple shear couple changes in
orientation of the main trace local compression
uplift at restraining bend local tension
subsidence at releasing bend changes in plate
motion, eg. Alpine Fault, San Andreas
Snapshot of strike slip displacement of an orange
grove along he San Andreas. All faults should be
required to cross cultivated land!
12
Transpression and transtension When existing
lines of weakness or resistant blocks control the
location of fault planes, the fault strike may
divert from a simple linear trend that follows
a small circle on the earths surface. When
transform faults are diverted from small circle
paths, this gives rise to local zones of
convergence and divergence. transpression
combination of strike-slip motion and
compression. -gt exhibit thrust faulting, folding,
and uplift. transtension combination of
strike-slip motion and extension. -gt exhibit
normal faulting, basin extension, and
volcanism. These characteristics can occur along
any strike-slip fault.
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Cagos Lacadive Ridge
What are these?
NinetyEast Ridge
A hypothesis for the formation of LIPs, such as
the once contiguous Kerguelen Plateau and Broken
Ridge in the SE Indian Ocean (Figure 1), is that
they result from the initial impact of a mantle
plume on the lithosphere (a mantle plume is a
localized and typically long-lived buoyant
upwelling in the mantle). The long-term
manifestation of a plume is a linear chain of
volcanoes (hotspot track) formed as a migrating
lithospheric plate moves over the plume location.
The Hawaiian-Emperor Ridge in the north Pacific
Ocean is the best known example, but the 5000 km
north-south trending Ninetyeast Ridge in the
eastern Indian Ocean is an equally good
example. The geochemical characteristics of the
magmas that form a LIP and subsequent volcanic
chain differ significantly from the lavas erupted
at spreading and converging plate boundaries
consequently, plume volcanism provides new
information about the earths interior. Studies
of the basaltic basement cores established that
(1) these lavas are geochemically very different
from lavas erupted at spreading ridge axes. The
basaltic basement of the Ninetyeast Ridge
increases in age from south (38 Ma) to north
(82 Ma) Duncan, 1991 Are the ages and
geochemical characteristics of these lavas
consistent with derivation from a single fixed
plume? The answer is yes. The age progression
of volcanism along the Ninetyeast Ridge is
consistent with rapid northwards movement of the
Indian Plate over the Kerguelen Plume located at
50S. After Frey et al., unpublished MIT.
16
Ridges like the NinetyEast Ridge were formerly
known as aseismic ridges because they have very
few earthquakes. Now they are known as hotspot
tracks.
The Deccan Traps
The Rajmahal Traps
Chagos-Laccadive
The old end of these hotspot tracks are regions
of flood basalts known as he Deccan and Rajmahal
traps. These, much like the Columbia river
basalts, are the massive eruption of the plume
after it burns successfully through the
continental lithosphere.
83 MA
38 MA
17
 Transform Faults and Plate Tectonics A
transform fault is a conservative plate boundary.
No material is created or destroyed. Plates just
slide past each other. True? Well no.
Transforms can leak if they are slightly
divergent. The Eastern Blanco transform leaks in
the form of pull apart basins currently filling
with volcanic material. The Gulf of California
and the Gulf of Elat are two places where a
spreading ridge makes a transition to a transform
fault, and can be considered a leaky transform in
the transition. The Mendocino Fault on the
other hand, is convergent due to the
non-parallellism with the Blanco, the other fault
that bounds the Gorda plate. The result is the
Gorda Ridge, composed of Gorda plate material
faulted against the Pacific Plate.
18
http//earth.leeds.ac.uk/leb/tectonics/
The Dead Sea Transform similar in function to
the Chaman fault in that it accomodates the
collision of the Arabian Plate with Eurasia.
Note the forearc sliver in the Zagros, and the
syntaxis in eastern Turkey and Iran/Iraq. The
Persian Gulf and the Gulf of Oman are the
remnants of the Tethys sea, is the last bit,
about to close.
19
http//earth.leeds.ac.uk/leb/tectonics/
Ok lets look at the transform part of this fault
20
The Gulf of Elat spreading center turns rift,
turns transform There are only two like this,
wheres the other one?
Ben-Avraham et al., Science v. 206, 12 October
1979
21
Klinger et al., 2000
22
http//earth.leeds.ac.uk/leb/tectonics/
The Lebanese segment of the transform is
characterized by a series of faults together with
two main mountain ranges. The ranges run
sub-parallel to the Yammouneh Fault.
Interpretation of the seismicity data is
difficult because not all the earthquakes need
have occurred on the main strike-slip faults -
there is plenty of "off-fault" deformation.
Presumably much of this reflects active crustal
shortening - which generates the uplift ,
particularly of Mt. Lebanon and the modern
coastline.
This scene shows the basins developed in the
southern part of the transform, along the
Israel-Jordan border. The southern edge of the
Dead Sea appears at the top of the image. Note
that there is no through-going fault here -
despite the 100km of bulk displacement on the
transform. Presumably fault strands have
transitory existences - growing and slipping,
perhaps rotating and then being replaced (and cut
by) younger strands. The old fault segments may
be represented by the bed rock lineaments on the
satellite image - but obviously they are obscured
by young sediment in the basins themselves.
23
http//earth.leeds.ac.uk/leb/tectonics/
The notched coast line and exposed platform
testify to uplift of the land relative to the
Mediterranean sea level. As the sea is
essentially non-tidal this landscape remains
constant. It looks like the tide has gone out -
it has, but permanently. The uplift probably
occurred in conjunction with (co-seismic) the
551AD earthquake that devastated coastal cities
(including Beirut).
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Hellenic Trench
Present day velocity field 1990-2003 gps
measurements show that about 2 cm/yr convergence
goes into underthrusting at the frontal Himalayan
arc. While GPS measurements in Pamir and India
have shown that a significant amount of
deformation goes into underthrusting at the
frontal Himalayan arc, the deformation at depth
remains obscure. One may also ask where are the
remaining 3 cm/yr inferred from kinematic
reconstructions at plate tectonic scale. The
focal mechanism map yields some answers.
25
This present day velocity field and pole of
rotation is different than earlier models. Does
this make sense to you? Black is Nuvel 1a,
Green is data from this study, red is model fit
of the Euler pole. From Klinger et al., 2000
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Description This image is an interferogram that
was created using pairs of images taken by
Synthetic Aperture Radar (SAR). The images,
acquired at two different times, have been
combined to measure surface deformation or
changes that may have occurred during the time
between data acquisition. The images were
collected by the European Space Agency's Remote
Sensing satellite (ERS-2) on 13 August 1999 and
17 September 1999 and were combined to produce
these image maps of the apparent surface
deformation, or changes, during and after the 17
August 1999 Izmit, Turkey earthquake. This
magnitude 7.6 earthquake was the largest in 60
years in Turkey and caused extensive damage and
loss of life. Each of the color contours of the
interferogram represents 28 mm (1.1 inches) of
motion towards the satellite, or about 70 mm (2.8
inches) of horizontal motion. White areas are
outside the SAR image or water of seas and lakes.
The North Anatolian Fault that broke during the
Izmit earthquake moved more than 2.5 meters (8.1
feet) to produce the pattern measured by the
interferogram. Thin red lines show the locations
of fault breaks mapped on the surface. The SAR
interferogram shows that the deformation and
fault slip extended west of the surface faults,
underneath the Gulf of Izmit. Thick black lines
mark the fault rupture inferred from the SAR
data. Scientists are using the SAR interferometry
along with other data collected on the ground to
estimate the pattern of slip that occurred during
the Izmit earthquake. This is then used to
improve computer models that predict how this
deformation transferred stress to other faults
and to the continuation of the North Anatolian
Fault, which extends to the west past the large
city of Istanbul. These models show that the
Izmit earthquake further increased the already
high probability of a major earthquake near
Istanbul. March 15, 2001 issue of Geophysical
Research Letters.
29
The topography of California is shaped entirely
by plate interactions. The gross features
evident in the state geologic map overlay on
topography shown here are the Great Valley, and
the Sierras. The Great Valley is a former
forearc basin, now abandoned by northward
migration of the Mendocino Triple Junction. The
coastal range is a former subduction complex
(accretionary wedge and underlying melange) that
was the former forearc high. The Sierra Nevada
range is a former arc system. Most of the arc
volcanics have been eroded, and their roots form
the present mountains as a series of granitic
batholiths. Subsequently, as the triple junction
moves north, the San Andreas transform lengthens,
modifying the landscape.
30
The big bend in the San Andreas is a well known
restraining bend in the trace of the fault. The
left bend results in compression and uplift of
the Transverse Ranges, so names because of their
transverse orientation to the structural grain of
California. They also result from the collision
of one of the rifted blocks pulled away from the
mainland further south, as shown in the Atwater
movies.
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Here, from Dickinson (1996) are the fault systems
and tectonic elements of the transform plate
boundary in western North America.
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Geometry of fault blocks between two shear zones,
dexral (Right-lateral) in this case.
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Geometry of fault blocks between two shear zones,
dexral (Right-lateral) in this case. A schematic
model for the Western Transverse Ranges
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Geometry of fault blocks between two shear zones,
dexral (Right-lateral) in this case. A schematic
model for the Western Transverse Ranges, note
the space problems (holes and overlaps) when
panels rotate horizontally.
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If this continues, you either have large holes,
i.e rifting, or there is rifting perhaps implicit
in the process, i.d more of a driver than a
passive result of the rotating geometry.
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A better model, allowing for variable panel
widths, resolves some of the space problems.
Its also consistent with compression in the
western transverse ranges
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Is there evidence in the California borderland of
these tectonic models? Borderland deformation
included rifting, basin and range style
extension, and later dextral strike slip. Its a
mess to say the least.
What happened here? In map view it is
possible to retrodeform both the extension and
strike slip. Notice that the was extension, not
compression, along the peninsular range faults.
From Goldfinger et al., 2000
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Anything is this look familiar from recent
discussions?
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Summary of offset features along the SAF
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So, what came before the SAF?
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Tanya Atwaters Plate Reconstruction Movies
Stay tuned, Bill gates doesnt want us to watch
Quicktime movies..
North America, 80MA
North America, 20MA
California, 20MA
Southern California, 20MA
With Paleomagnetics, 20MA
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• The main constraints used in these animations,
  and discussed in Atwater and Stock (1998) are
  based on the idea that coast perpendicular
  (assumed to be N60E ) deformation within the
  North American plate must equal or exceed the
  coast perpendicular motion calculated in the
  plate circuit solution.
• If it can be assumed that shear is caused only by
  the plate interactions (i.e. the ridge is not
  still spreading), and if an unknown quantity of
  that shear is accommodated offshore then the
  total deformation parallel to the to the coast
  must be less than that calculated for the Pacific
  North American motion. When the two become
  fully coupled ( 18Ma ) it should be expected
  that the coast parallel components should be
  equal. With these restrictions in place the
  following local assumptions have been made
• The Colorado plateau is assumed to be rigid block
  with little internal deformation.
• This is also held true for the Sierra
  Nevada-Great Valley block although to accommodate
  variation in Basin and Range extensional it is
  assumed that some deformation in the Southern
  Sierras did occur.
• Basin and Range extensional values can be seen to
  vary both through time and spatially with a range
  between 150 235 km in a coast perpendicular
  direction since 24 Ma.
• Extension within the Rio Grande rift can be seen
  to be small relative to the other deformations. A
  value of 15km of extension is found in a coast
  perpendicular direction for the past 24 Ma.
• The San Andreas fault has seen 315 10 km of
  strike slip displacement within the last 23 ma
  based on reconnecting piercing points ( the
  Neenach volcanics ). The majority of this slip
  budget occurred later than 8 Ma ( 200km )
• Independent estimates of Californian deformation
  can be found using the rotation of the Transverse
  Ranges ( 110 degrees clockwise Dickinson, 1996
  ) and the partly associated extension ( 100km )
  in the Inner Californian borderland ( Crouch and
  Suppe, 1993 )
• There are other assumptions concerning the
  rotations of the Tehachapi mountains and the
  Oregon Coast ranges along with the translation of
  Baja California.

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What happens when an active spreading center
(the East Pacific Rise in this case ) reaches a
subduction zone. The figure below shows several
options
Do spreading centers continue to spread as they
are subducted beneath continents? Or since there
would be little removal of heat due to the
thermal blanket effect of the crustal mass above,
would accretion at the spreading center stop?.
Is there geologic evidence for slab windows or
gaps?
46
Dickinson and Snyder (1979) suggest that as the
spreading center approaches North America and
stalls, the young slab detaches to form a slab
window. This slab window continues to grow as
the Mendocino and Rivera triple junctions
propagated north and south respectively. The
exact aerial extent of the window through time is
a function of the plate vectors of the three
plates involved ( NA-PAC-Farallon ) but it is
usually simplified to be triangular. This process
neatly explains the apparent magmatic arc shut
down as the San Andreas developed. At the time of
initiation ( 28Ma ) the arc was active again
after its previous dormancy during the Laramide
orogeny but began to switch off in good agreement
with the proposed slab window widening. A
synthesis of this is shown above. Dickinson and
Snyder ( 1979 ) also linked the slab break away
to upwelling asthenosphere which may have heated
the base of the lithosphere ( Dickinson 1997 )
and caused (or at least aided) extension in the
Basin and Range.
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Here is an alternate model from Atwater and
Sveringhaus, 1990. What is the difference
between a slab gap, this paper, and a slab
window?
Severinghaus and Atwater ( 1990 ) focused on the
thermal properties of the subducting plate and
its variation along the North American plate
boundary. They argue that as the small Monterey
and Arguello microplates, approached and slowed
near the subduction zone, their downgoing slabs
were getting progressively younger and warmer.
These young slabs take a proportionally short
time to heat up to asthenospheric values as is
observed at the present day Juan de Fuca
subduction slab. They argue that instead of a
slab window developing west to east (while also
propagating N-S), a slab gap develops from the
east which then also propagates north and south
with the Mendocino and Rivera triple junctions.
This is because at a certain time after
subduction the slab would become
indistinguishable from the aesthenosphere
surrounding it.
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Quite a bit of the initial deformation was
offshore, though it has probably slowed
considerably.
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Nan desu ka? Japanese lesson for today What is
this?
There are rift fragments and failt spreading
centers scattered in the southern California
borderland. These are poorly known, and under
current investigation. The following slides are
from Jason Chaytors in review publication.
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An attempt to fit the magnetic profiles may work,
but oen mag expert believes this is noise.
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LARSE seismic experiment images the crust in
Southern California
The LARSE wide angle reflection/refraction
surveys have attempted to fill in the basic
crustal architecture of the southern California
rift/transform/borderland system. A very complex
one for sure