Geometry and Kinematics: Plates
(Chapter 10 in D & R)
Today's lecture
• 1) Building Blocks of the Earth: What's
getting deformed?
• 2) Geometry + Kinematics = Plate Tectonics
• 3) Kinematic evolution
Earth Structure: defined by
seismic discontinuities;
concentric shells of rock or
liquid with contrasting
chemical and physical
properties
Core: iron alloy
Mantle: ultramafic
Crust: mafic to felsic
Note the figure that is to scale!
Boundary between mantle and crust is called the Moho,
short for Mohorovicic discontinuity
Oceanic Crust
- forms at mid-ocean
ridges by partial melting
of mantle
- basaltic (mafic) in
composition
- igneous extrusion and
intrusion
- 5 to 10 km-thick
- Oceanic crust is no
older than ~200 Ma.
Why??
Continental Crust
- 5 to 10 times thicker than
oceanic crust- 40 km avg.
- This is a simplified sketch!
Continental crust is very
heterogeneous
- Long and complex deformation
history. Majority of continental
crust formed during the
Precambrian (before ~570 Ma)
The oldest known rock is ~4 Ga!
Why so much older than
oceanic crust??
Contours of crustal thickness
-Most of the continental surface lies within 1 km of sea level.
-Most of ocean floor lies at depths ~4 km below sea level.
Rheology (behavior during deformation) of the Earth
Lithosphere: "lithos" = rock, implying strength. It exhibits a
component of elastic strength over geological time scales.
Includes crust + uppermost mantle! Varies in thickness.
Moves as a plate- exceptions are orogens.
Asthenosphere: Weak. It is solid, but behaves like a viscous
fluid (convective flow) over geological time scales.
Plates of the Earth
A plate can include both oceanic and continental lithosphere
oceanic vs. continental lithosphere can be distinguished with
bathymetry (oceanic equivalent of topography)- WHY?
How do we know that plates move?
- Earthquakes localized along plate
boundaries
Most folks were convinced when spreading was
demonstrated at mid-ocean ridges
What is it?
Geometry: Sketch
Kinematic models
1)
2)
Predictions?
Tectonic significance?
Transform faults: more evidence for plate
motion
Not observed!
Observed!
Oceanic transform faults: Spreading occurs
perpendicular to the ridge axis and any curvature is taken
up by transform faults. Different rates of spreading.
GPS (Global Positioning
System)
Satellite-based geodesy: A
new revolution in Earth
Sciences!
(more near end of semester!)
InSAR
Interferometric
Synthetic Aperture
Radar
Continents also break- to form new oceans
So far: Opening of oceans and creation of
oceanic crust
Is the Earth expanding?
Spreading at mid-ocean ridges must
be compensated by subduction
Ocean-Continent convergent margins
Continent-Continent convergent margins
An example:
The Indo-Asian
Collision
Digital elevation map (DEM) of the HimalayanTibetan orogen
Warmer colors represent higher elevations
Transform plate
boundaries
An example: San
Andreas
Velocity vector
diagrams
On a sphere, plate movement is described by a rotation at a
specified angular velocity (w) around an imaginary axis
(rotation pole, also known as Euler pole). Not related to N or
S or magnetic poles!
Oceanic plates move
perpendicular to
ridges and parallel to
transform faults.
Euler poles can be
determined for
oceanic plates by
drawing lines
perpendicular to
transform faults (and
fracture zones).
Thought experiment: How much time did it take to
produce the present-day area of oceanic crust?
The surface area of the Earth is = 4pR2. With R = 6,378
km, area = 5.1x108 km2. 55% of Earth surface is underlain
by oceanic crust (0.55 x 5.1x108 km2 = 2.8x108 km2).
On average, crust is created at mid-ocean ridges at a rate
of 2 to 3 cm/year. The length of ridges is 60,000 km.
Thus, oceanic crust is created at a rate of 2 to 3 cm/year x
60,000 km = 1.2 to 1.8 km2/year.
The time required to produce the present-day area of
oceanic lithosphere is 2.8x108 km2/ (1.2 to 1.8 km2/year) =
155 to 230 Ma.
What does this imply about the plate configuration of Earth
~200 m.y.a?
Pangea
supercontinent
and Tethys
Ocean
http://www.scotese.com/earth.htm
Concept of the supercontinent cycle
RodiniaPangea
mantle
convection
Wilson cycle: opening and closing of oceans at
roughly similar positions
DEM of Aleutian trench
Extraterrestrial plate
tectonics?
DEM of Artemis Corona on
Venus; same scale as above
Plates are rigid! Kinematics described in terms of
translations and rotations
Next: Geometry and Kinematics of Deformation
Starting with Fractures
(Read D&R, p. 38-50; 204-226)
Important terminology/concepts
- Crust: continental vs. oceanic
- Mantle
- Moho
- Rheology: Lithosphere vs. Asthenosphere
- Earth's plates
- Bathymetry
- Evidence for plate motion?
- Mid-oceanic ridges
- Transform faults and fracture zones
- Ocean-continent convergent margins
- trench, accretionary wedge, forearc
- Continent-continent convergent margins
- Plate kinematics
- Euler Pole
- Velocity vector diagrams
- Kinematic history
- Supercontinent cycle