Download Earth Structure - Processes in Structural Geology and Tectonics

Whole Earth Structure
and Plate Tectonics
Processes in Structural Geology & Tectonics
Ben van der Pluijm
© WW Norton+Authors, unless noted otherwise
3/29/2016 2:04 PM
We Discuss …
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Earth’s topography
Earth’s layers
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Tenets of plate tectonics
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Today’s plates
Plate boundaries
Kinematic of plate tectonics
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The crust
The mantle
Linear and angular
velocities
Absolute and relative
motions
Mechanics of plate tectonics
Tectonic cycles
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Wilson Cycle
Supercontinent Cycle
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Earth’s Surface - 3D Topography
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Hypsometric (=cumulative frequency) Elevation Curve
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Earth’s Seismologic and Rheologic Layering
strong
weak
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Crustal Section and Characteristic Rock Types
Gabbro
Granite
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Crustal Thickness
http://earthquake.usgs.gov/data/crust/
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The Crust and Geologic Provinces
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Age of the Crust
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Oceanic vs. Continental Crust
Gabbro
Composition
Continental crust has a mean composition that is less mafic
than that of oceanic crust.
Formation mode Continental crust is an amalgamation of rock that originally
formed at volcanic arcs or hot spots, and then subsequently
passes through the rock cycle. Mountain building, erosion
and sedimentation, and continuedvolcanism add to or
change continental crust. Oceanic crust all forms at mid-ocean
ridges by the process of seafloor spreading.
Thickness
Continental crust ranges between 25 km and 70 km in
thickness. Most oceanic crust is between 6 km and 10 km
thick. Thus, continental crust is thicker than oceanic crust.
Heterogeneity
Oceanic crust can all be subdivided into the same distinct
layers, worldwide. Continental crust is very heterogeneous,
reflecting its complex history and the fact that different
regions of continental crust formed in different ways.
Age
Continental crust is buoyant relative to the upper mantle,
and thus cannot be subducted. Thus, portions of the
continental crust are very old (the oldest known crust is
about 4 Ga). Most oceanic crust, gets carried back into
the mantle during subduction, so there is no oceanic crust
on Earth older than about 200 Ma, with the exception of
the oceanic crust in ophiolites that have been emplaced
and preserved on continents.
Moho
The Moho at the base of the oceanic crust is very sharp,
suggesting that the boundary between crust and mantle is
sharp. The continental Moho tends to be less distinct.
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Granite
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The Mantle - Tomography and Plates
SW California
SE North Carolina
Red is slow, is hot
Blue is fast, is cold
P-waves
S-waves
Ritsema
Grand et al., 1997
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The Tenets of Plate Tectonics
http://denali.gsfc.nasa.gov/dtam/
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Types of Plate Boundaries
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Examples of Plate Boundaries
Convergent: Japan
Divergent: Red Sea
Transform: New Zealand
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Insights from Earthquakes
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The Kinematics of Plate Tectonics
Relative velocity
Absolute velocity
Mantle hotspots as “fixed”
reference frame
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Plate Kinematics on a Sphere
Rotation on Euler pole
(=rotation axis):
a) Displacement follows
small circles
b) Transforms parallel
small circle segments
c) Same angular
velocity (w) between
plates
Different linear
velocity (v) as
function of distance
from Euler pole
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Extra: Triple Junctions and Evolution
(a) Stable ridge-trench-transform triple junction.
(b) With time ridge-trench-transform triple junction
location changes (from T to T’), but the geometry
stays same.
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Today’s Plate Motions (Absolute and Relative Velocities)
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Absolute Motions and
Speed Limit?
Zahirovic et al., 2015
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Mechanics of Plate Tectonics - Driving Forces
Gravitational forces (colored):
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Ridge push: topographic spreading
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Slab pull: negative buoyancy of slab
Resistive forces (black) are relatively
small
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Plate Tectonic Cycles:
The Wilson Cycle
a-b) Continent rifts, such
that crust stretches, faults
and subsides.
c) Seafloor spreading
begins, forming a new
ocean basin.
d) The ocean widens and
flanked by passive margins.
e) Subduction of oceanic
lithosphere begins on one
passive margins, closing
ocean basin.
(f-g) Ocean basin is
destroyed by continental
collision.
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Plate Tectonics Cycles: The Supercontinent Cycle
(a) Continents gradually aggregate over a
mantle downwelling zone.
(b) While supercontinent exists, large-scale
convection in the mantle reorganizes.
(c) Upwelling begins beneath
supercontinent and weakens it, leading
to breakup.
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1 b.y. of Plate Motion (late Precambrian-Future)
Scotese, 2004
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