Download Hilliker 1 The Effects of the Farallon Plate Subduction Frank Hilliker

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The Effects of the Farallon Plate Subduction
Frank Hilliker
Dalton State College
Dr. Jean Johnson
GEOL 1122k
21 April 2012
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The Earth is an ever-changing planet. For millions upon millions of years sections of this
planet’s lithosphere have drifted across the inner aesthenosphere. These sections (plates) form at
a three-pronged junction with the ensuing cracks running until they intersect another threepronged point. At these cracks the now-divided plates function in one of three ways: they
diverge, converge, or slide laterally against each other. When these plates converge the older,
heavier plate subducts beneath the younger, lighter plate. If this subducting plate consists
entirely of volcanic material (the ocean floor) eventually it will be completely subducted with
another plate located on its far boundary coming in contact with the convergent boundary. This
is the case with the Farallon Plate. This plate’s almost complete subduction contributed to the
current state of various rock formations in what is now the western United States and possibly
influence the earthquake activity in the New Madrid Seismic Zone.
http://geology.isu.edu/Digital_Geology_Idaho/Module6/mod6.htm/PlateMovement.pdf (DeGray)
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The Farallon plate formed in the western portion of the Earth during the breakup of
Pangea during the Jurassic Era. This plate began subducting beneath the western shore of what
would eventually become North and South America. As the Farallon continues to subduct, a
divergent boundary formed between North and South America ~150 million years ago. By ~130
B.C. Africa begins to rift from South America (below).
http://www.uwgb.edu/dutchs/CostaRica2008/CR-PT.HTM/Carib150.gif
The dark blue portion is the Farallon Plate
http://www.uwgb.edu/dutchs/CostaRica2008/CR-PT.HTM/Carib100.gif
The dark blue portion is the Farallon Plate
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By the Cretaceous period another plate formed, containing what is present day Mexico
and part of California. This plate moved in a southeasterly direction along a transform boundary
with the rest of North America. The Farallon Plate subducted beneath it to the west while
subducting beneath another, newly created, plate to the southeast.
The Farallon and North American plates converged in a motion similar to the closing of a
door. The “hinge” is located in the north. When the northernmost part of the Farallon plate
subducted beneath North America, the Pacific Plate came into direct contact with the North
America plate. However, the Pacific Plate’s motion is such that this became a transform
boundary, not another convergent boundary or a divergent boundary.
Tomographic search for missing link between the ancient Farallon subduction and the present Cocos subduction
Tomogrpahic Search.pdf (Gorbatov, 2005)
The image above shows the movement of the Farallon Plate beneath North America and
the tear in the plate which eventually forms the Cocos and Nazca Plates. These two plates
continued to subduct in the typical manner while the parent (Farallon) plate sank, unhindered,
deeper into the mantle (Gorbatov, 2005).
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http://www.uwgb.edu/dutchs/CostaRica2008/CR-PT.HTM/CaribPac20.gif
The lower dark blue portion is the Farallon Plate; the dark blue portion in the upper right is the Pacific Plate
Approximately 20 million years ago, the northern section of the Farallon Plate had
completely subducted beneath the North American continent. This brought the North American
and the Pacific Plates into direct contact for the first time. The shape of the Farallon Plate is
such that portions of it are completely subducted while leaving remnants. These remnants are
known as the Cocos, and Nazca Plates. In time, these too will become completely subducted.
This will make the west coast of North and South America a transform boundary with the Pacific
Plate.
The subduction of the Farallon Plate has had profound effects upon the western portion of
the North America Plate. It has been established that volcanic arcs occur along convergent plate
boundaries; the evidence is readily apparent. However, subducting plates affect the nonsubducting plates far inland from the boundary. These effects range from alterations of various
rock formations, generation of batholiths far from the boundary, and the creation of new rock
strata. There is also evidence that the subduction of the Farallon Plate has influenced seismic
activity in the New Madrid Seismic Zone.
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http://geology.isu.edu/Digital_Geology_Idaho/Module6/mod6.htm
Idaho batholiths (DeGray)
The first location to be examined in regards to Farallon Plate subduction resides in Idaho.
During the subduction in this location the partial melting of the plate released superheated water
along with material with relatively low melting temperatures. This magma’s upward flow
stopped approximately 4 to 25 km below the surface. There it slowly cooled forming a granite
batholith. The age of the Idaho batholiths range from 100 Ma to 65Ma (DeGray).
Migration of dynamic subsidence across the Late Cretaceous United States Western Interior Basin in response to Farallon plate subduction,
late cretaceous Farallon.pdf (Shaofang, 2011)
As the Farallon Plate continued to subduct beneath North America, the weight of the
subducting plate caused basins to be formed in the Utah, Colorado, and Wyoming area. As the
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plate subducted, the North American plate atop it flexed in response to the displaced mantle
material. Into these basins sediments accumulated creating definable layers of sandstone, shale,
and mudstone. These conditions persisted until ~80mya – this is when the direction and depth of
the Farallon Plate’s subduction changed by shifting northeastward and the depth of the plate
increased (Shaofang, 2011).
Figure 8. Tectonic model, modified after Humphreys (2009) for the late Eocene–early Oligocene.
Cross sections are roughly E-W at ~34°N. CP—Colorado Plateau; RGR—Rio Grande Rift; B&R—Basin and Range.
Oligocene Laramide-new mexico.pdf (Copeland 2011)
In the New Mexico region, the subducting Farallon plate remained close to the overlying
North American Plate. The Farallon plate began to drag the overlying plate atop of it due to the
two plates being locked together in this location. Being in this state, the top plate began to
bunch up and bow in certain places. To picture this clearly, imagine a board being pushed
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underneath a layer of carpet. In some places the carpet will get hung on the board underneath
and begin to fold and bend following the direction of the underlying board. In the instance
concerning these two plates, slabs of the Farallon Plate broke off ~28mya. The overlying North
American Plate remains contorted in this rippled fashion while the Farallon continued to subduct,
yet this time in a more traditional manner. The North American Plate retains these drag ripples,
known as the Silver City Range in New Mexico (Copeland, 2011).
Figure 1. Map of western United States showing major plate boundaries and past position of Mendocino triple junction (Atwater and Stock,
1998), Quaternary Cascade volcanic arc (Hildreth, 2007), Eocene to Pliocene western Cascade arc (du Bray et al., 2006), and inferred ancestral
Cascades in California and Nevada (du Bray et al., 2009).
Oligocene Laramide-california.pdf (Colgan, 2011)
The Cascade Mountain Range in the western portion of North America (Washington,
Oregon, and California) is a result of the subduction of the Farallon Plate beneath North
America. There are two sections of the Cascade – the Ancestral Cascades and the Western
Cascades. Both sections are considered to be typical of plate subduction – volcanic arcs which
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form on the over-riding plate relational to where the subducting plate begins to sink down into
the mantle. The age of these two arcs range from 45-36 m.y. for the northern portion of the
Western Cascades in Washington, to 25 to 4 m.y. along the Ancestral Cascades (Colgan, 2011).
The predicted rates of horizontal (arrows) and vertical (contours) flow beneath North America (Forte, 2007)
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One of the most mysterious seismic zones in the world is the New Madrid Seismic Zone
located in the central section of the United States. One of the unique factors about this zone is
that it is not located near any known plate boundaries. One proposal set forth to explain this
seismic zone is as follows: the Earth’s crust is slowly rebounding from the removal of weight
(glaciers). Another proposal suggested by A. M. Forte is that the area is responding to unusual
mantle activity due to the subducted Farallon Plate. The figure on page 4 (section D) shows the
relative position of the Farallon Plate beneath the North American Plate as of this time.
Scaling the Section D map on page four to the dimensions of the maps on page nine will
demonstrate that the NMSZ will line up near the inside tear edge of the subducted Farallon Plate.
Computer-aided modeling of mantle flow and convection in this location demonstrates that the
downward drop of the Farallon Plate would create the highest disturbance of the mantle around
the NMSZ. This disruption of mantle flow could be responsible for the seismic activity in the
New Madrid location. Plate Tectonics occurs in all three dimensions. Since Plate Tectonics are
the force behind building mountain ranges it stands to reason that subduction of plates can have
displacement effects far below the surface. This displacement could result in earthquakes in
non-traditional places (Forte, 2007).
The subduction of the Farallon Plate has caused major changes to the structure and
topography to the North American Plate. These changes include the creation of volcanic arcs on
the coasts of Washington, Oregon, and California. Batholiths were formed due to upwelling of
mantle material into the strata located in Idaho. The effect of the Farallon Plate dragging the
North American Plate caused the “shortening” of the surface lithosphere (demonstrated in New
Mexico). The complete subduction of the Farallon Plate in certain locations has brought a
rotating plate (Pacific Plate) into contact with a margin which was a subduction zone; creating an
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unstable region (the San Andreas Fault). The tear which formed in the subducted plate has the
potential to create unstable conditions deep in the Earth’s mantle (New Madrid Seismic Zone).
Studying all of these effects and results enables a better understanding of how and why the New
Madrid Seismic Zone functions the way it does and could help to plan contingencies for dealing
with similar conditions which could occur in the future.
Plate tectonics has been and will remain a constant process for the Earth. The effects of
plate tectonics are far reaching and evident throughout the Earth’s surface. Evidence of the
effects of the Farallon Plate’s subduction exists throughout North America. The locations of the
evidence include the batholiths located in Idaho, the sedimentary basins formed in Wyoming,
Utah, and Colorado, the shortening of the lithosphere – ripples – found in New Mexico, and the
volcanic arcs running from Washington through Oregon to Northern California. Research is still
underway in regards to sections of the now-subducted Farallon Plate influencing/causing the
seismic activity in the New Madrid Seismic Zone.
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References
Baldridge, W. S. (2004). Geology of the American Southwest: A journey through two billion
years of plate-tectonic history. Cambridge ; New York: Cambridge University Press.
Colgan, J. P., Egger, A. E., John, D. A., Cousens, B., Fleck, R. J., & Henry, C. D. (2011).
Oligocene and Miocene arc volcanism in northeastern California: Evidence for postEocene segmentation of the subducting Farallon plate. Geosphere, 7(3), 733-755.
doi:10.1130/GES00650.1
Copeland, P., Murphy, M. A., Dupré, W. R., & Lapen, T. J. (2011). Oligocene Laramide
deformation in southern New Mexico and its implications for Farallon plate
geodynamics. Geosphere, 7(5), 1209-1219. doi:10.1130/GES00672.1
DeGrey, L., M. M. A. P. K. L. (n.d.). http://geology.isu.edu/. Retrieved from
http://geology.isu.edu/Digital_Geology_Idaho/Module6/mod6.htm
Forte, A. M., Mitrovica, J. X., Moucha, R., Simmons, N. A., Grand, S. P. (2007). Descent of the
ancient Farallon slab drives localized mantle flow below the New Madrid seismic zone.
Geophysical Research Letters Vol. 34 (L04308). doi:10.1029/2006GL027895
Gorbatov, A., & Fukao, Y. (2005). Tomographic search for missing link between the ancient
Farallon subduction and the present Cocos subduction. Geophysical Journal
International, 160(3), 849-854. doi:10.1111/j.1365-246X.2005.02507.x
Shaofeng, L., Dag, N., & Lijun, L. (2011). Migration of dynamic subsidence across the Late
Cretaceous United States Western Interior Basin in response to Farallon plate subduction.
Geology, 39(6), 555-558. doi:10.1130/G31692.1