Geoscience Reference
In-Depth Information
(a)
PLATE A
U
A
=
U
B
U
A
A
A
A
B
U
B
Transform
U
A
U
B
A
B
B
Transfo
rm
U
A
U
B
A
A
A
B
PLATE B
(b)
Transform
fault with
earthquake
locii
Ocean floor fracture zone
with no relative strike slip
motion (old transform trace)
Spreading
ridges
U
A
= 0
U
B
U
B
Velocity vectors defining velocity field for
plates A and B.
B
Fig. 5.37
Sketch to illustrate ridge : transform relationships between
two moving plates.
U
B
Fig. 5.36
Sea floor spreading is a continuous process; magnetic min-
erals in the oceanic lithosphere record the orientation of the mag-
netic field that existed at the time of solidification. Here, the black
shading depicts periods of normal magnetic polarity and the white
shading reversed polarity. (a) Shows the conventional view of sym-
metrical spreading about a fixed midocean ridge axis, (b) shows an
alternative scenario in which plate A is held fixed and the spreading
ridge migrates away from it at half the spreading rate. In both cases,
a symmetrical pattern of magnetic anomalies results.
11
Identification by Forsyth and Uyeda of the “self-
propelled” theory of plate driving forces, chiefly involving
slab pull.
5.2.3 Magnitude of plate motion: Rates of sea-floor
spreading and other statistics
Sea-floor spreading is the evocative name given by Vine and
Mathews in 1963 to the discovery that midocean ridges
were the center of creation of ocean crust. They were able to
say this because accurate shipboard magnetic surveying
revealed geomagnetic reversals as symmetrical strips of nor-
mal and reversed ocean crust situated either side of the
ridges (Fig. 5.36). The accurately dated continental record
of reversals was already established and it was then possible
to correlate the oceanic record with this and to establish the
precise time of creation of known widths of ocean crust,
something eventually traceable over 150 My. The speed of
present plate motion, mostly derived from this sea-floor
spreading data, varies over about an order of magnitude,
from 11 to 86 mm y
1
. The speed of motion is related to
the magnitude of the driving forces and resisting forces asso-
ciated with particular plates. Table 5.1 gives relevant
statistics for the major plates and some of the minor ones.
6
Recognition of the particular structural features of a
type of oceanic strike-slip fault, termed a transform fault
(Fig. 5.37).
7
Identification of Benioff-Wadati zones of deep earth-
quakes along tilted interfaces under the oceanic trenches;
8
the seismological recognition of plate boundaries along
(1) midocean ridges (extensional first motion earthquake
mechanisms), (2) subduction zones (compressional first
motion earthquake mechanisms).
9
The McKenzie-Parker kinematic theory of “tectonics
on a sphere” (simply defined in Fig. 5.38) from magnetic
anomaly and transform fault data, with the concept of
Euler poles of rotation.
10
The parameterization of a Rayleigh Number (Section
4.20) well above critical for the existence of convection in
the asthenospheric mantle.
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