Geology Reference
In-Depth Information
Fig. 2.9
Cross-sections illustrating the kinematics of a
subduction zone. Velocity vectors
v
A
,
v
B
,and
v
S
are
relative to the lower mantle,
v
SB
is the slab velocity relative
to plate
B. Lines
in the upper mantle represent streamlines,
everywhere parallel to mantle velocity vectors (Chap.
other dynamic scenario results from superposition of these
two basic configurations
plate will move at a velocity given by:
v
A
. The velocity
v
B
increases progressively
until the additional frictional resistance that is
generated at the trench balances the trenchward
asthenospheric drag. If the starting dynamic
configuration is that illustrated in Fig.
2.9
b, so
that the overriding plate velocity
v
B
ยค
0 while
v
A
D
0, the induced oceanic corner flow below
A
drags actively this plate trenchwards, thereby
A
will acquire a small velocity opposite to
v
B
.
Therefore, the final equilibrium configuration
will be similar to the previous one. Finally,
hypothetical scenarios such that the two
converging plates move in the same direction
but with different velocity (in the upper mantle
reference frame) cannot be stable geodynamic
configuration. In fact, in this instance the
excess hydrodynamic resistance exerted on the
slab should be supplied by additional torques
other than the known driving forces of plate
geodynamic process determining the progressive
oceanward migration of the trench zone in the
upper mantle reference frame. In fact, in a
reference frame fixed to the subducting plate
A
the hinge line always migrates toward the
foreland of
A
.
The third kind of oceanic plate boundary
is represented by strike-slip faults, which are
characterized by pure or prevalent left-lateral or
right-lateral motion. Differently from transform
faults, which must be considered in the context
of mid-ocean spreading centers, these structures
are truly independent plate boundaries, which
can be linked to mid-ocean ridges, trenches,
or other strike-slip faults. In the present day
v
H
D
v
B
D
v
HA
C
v
A
(2.33)
If the relative hinge line velocity
v
HA
increases, for any reason, then the overriding
plate margin will be subject to back-arc
extension, in order to preserve the coupling
between margin and hinge line, a phenomenon
which is known as
trench retreat
.Conversely,
any decrease of
v
HA
will lead to episodes of
back-arc compression and shortening. In general,
the deformation of active continental margins
and island arcs is an expression of both spatial
and temporal changes of the relative velocity
field between the convergent plates, as well
as of variations in the hinge line velocity
v
HA
.
Regarding the possibility that subducting plates
roll over stationary hinge lines, thereby they
would slide down fixed slots, this is not a wrong
interpretation of the subduction process but one
of two end-member geodynamic configurations
at a subduction zone, as illustrated in Fig.
2.9
a.
In fact, when the velocity
v
B
D
0, a subducting
plate effectively bends and rolls over a fixed
hinge line in the mantle reference frame,
dragging the surrounding asthenosphere, as
conditions, the pull exerted by the sinking
slab is balanced by the resistive viscous drag
and subduction tends to proceed at constant
velocity. However, the active drag exerted by the
corner asthenospheric flow below the overriding
plate
B
pulls this plate trenchward, so that
B
will
acquire a
small
velocity
v
B
opposite to
