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Basic concept:
The outermost layer (LITHOSPHERE) is divided in a small number
of “rigid” plates in relative motion one respect to the other
Basic assumption:
 The generation of new plate material occurs by sea floor spreading.
 The new oceanic lithosphere form part of a rigid plate that may or may
not include continental material.
The Earth’s surface area remains constant; this means that seafloor
spreading must be balanced by consumption of plate elsewhere.
Lithospheric plates are capable of transmitting stress over great
horizontal distances. In other words plates are rigid and the deformation is
concentrated along the boundaries.
From: Fowler, the solid Earth
Lecture 6 May 24th 2005
Lecture 6 May 24th 2005
Lecture 6 May 24th 2005
Lecture 6 May 24th 2005
Lecture 6 May 24th 2005
Lecture 6 May 24th 2005
FLAT EARTH
Transform LL
Convergent
Constructive
Transform RL
Lecture 6 May 24th 2005
Great Circles
Eulerian Vector
Eulerian Pole
Small Circles
Lecture 6 May 24th 2005
Eulerian Theoreme:
Every finite motion on the
surface of a sphere
can be described as a
rotation around an axis
passing throught the
center of the sphere.
The intersection of the
axis with the sphere is
calledEulerian Pole.
Lecture 6 May 24th 2005
On a sphere:
• Plates move along small
circles. I.e. the velocity is
tangential to small circles and
perpendicular to great circles.
•Transform fault are following
small circles
•Constructive and convergent
plate boundaries are directed
as great circles
Lecture 6 May 24th 2005
On a sphere:
• Velocity increase while the
distance from the pole of
rotation increase.
Lecture 6 May 24th 2005
If we know the velocity or slip
direction of a plate we can find the
pole as the intersection of the great
circles perpendicular to the velocity
vector
If we know the direction of
transform faults the pole is at the
intersection of great circles
perpendicular to the transform
The pole is also at the intersection of
great circles indicated by ridge
segments
Lecture 6 May 24th 2005
If we assume rigid plated the motion
can be described by Eulerian poles.
Magnetic anomalies and bathymetry
give us some of the data we need to
find the Eulerian pole that describe
the plate motion.
Since Mag. Anomalies give a time
scale we can compute also the
velocity.
Lecture 6 May 24th 2005
Uncertainty using:
Ridges and ttransform directions
Slip or velocity directions
From Cox and Hart
Lecture 6 May 24th 2005
Seismicity and slip direction
give another constrain
From DeMets et al 1990
Lecture 6 May 24th 2005
From DeMets et al 1990
Velocity as distance from the
pole give a constrain on the
pole position
From Cox and Hart
Lecture 6 May 24th 2005
A good constrain for global plate is plate closure
Do it by hand
Lecture 6 May 24th 2005
Lecture 6 May 24th 2005
Do it by hand
Lecture 6 May 24th 2005
Lecture 6 May 24th 2005
Lecture 6 May 24th 2005