Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 232

of Corinth (Fig. 5.43b). The huge strains accompanying
this differential motion are released periodically along
powerful earthquakes on these faults (the seismogenic
layer here ranges from 10 to 15 km thick). The Corinth
gulf is termed a rift or graben . In the east, a half-graben for
the normal faults that define it are only on one side,
causing the prerifting crust to tilt southwards into the
faults (Fig. 5.43c). Detailed GPS surveys also reveal that
the southwest Greece is rotating anticlockwise with respect
to the motion of the northern area. This illustrates that
plates have vorticity, that is, they can spin as solid bodies
about vertical axes.
The continental lithosphere may also deform under
extension over vast areas, exemplified by the high (greater
than 2 km) plateau of the western United States and adja-
cent areas of Mexico. The plateau is known as the Basin
and Range on account of the myriad of individual normal
fault-bounded graben and half-graben that make it up
(Fig. 5.44). The individual ranges are the uplifted footwall
blocks to the normal faults (Section 4.15) while the basins
are the sediment-filled depressions, subsiding hangingwall
ramps between the ranges. Range wavelengths are typically
5-15 km with lengths up to several times this. Today the
GPS-determined velocity field in areas like Nevada is east
to west at about 20 mm year 1 with respect to fixed east-
ern North America. The active normal faulting is located
in distinct belts of high strain at either side of the province,
chiefly associated with the Central Nevada Seismic Belt in
the transition to the more rapidly northwest-moving
California terrain, and to a lesser extent along the western
margin to the Wasatch front in Utah. Although historic
earthquakes have nucleated along steeply dipping normal
faults (Fig. 5.44b) bounding individual range fronts, there
is a record in the tectonic landscape of a previous phase of
low-angle normal faulting (Fig. 5.44c,d). The kinematics
of this kind of extension has given rise to areas of core-
complexes , mid- to lower crustal rocks exposed in the
footwalls to the low-angle faults. It is thought by some
that this phase of low-angle faulting was related to a very
rapid gravitational “collapse” of the over thickened Rocky
Mountain crust some 20-30 Ma, with associated high-
heat flow and volcano-plutonic activity.
Shortening deformation of the continents under
compression occurs at plate destructive and continent-
continent collision boundaries where five physical processes
occur, often combined or “in-series,” that cause the forma-
tion of linear mountain chains like the Pacific arcs, the
Andes, and the Alpine-Himalaya (Fig. 5.45) system:
crustal accretion by thrust faulting (see discussion of thrust
duplexes in Section 4.15) of trench sediments “bulldozer-style”
against the overriding plate - this results in the formation and
rapid uplift of an accretionary prism ;

crustal thickening and buoyancy enhancement of the
crust by the wholesale intrusion of lower density calc-alkaline
magmas as plutonic substrates to volcanic arcs;
whole-lithosphere thickening into a lithospheric mantle
“root” by pure strain, manifest at crustal levels by shear
strain along major thrusts;
buoyant up thrust of crustal mass in thickened
lithosphere resulting from the wholesale detachment of
lithospheric mantle root;
gravitational collapse of the elevated plateau with release
of gravitational potential energy along active normal faults.

5.2.9 The fate of plates: Cybertectonic recycling
and the “Big Picture”

Three possible scenarios concerning the large-scale recy-
cling of plates have been envisaged at different times since
the plate tectonic “revolution” in the late 1960s; they are
sketched in Fig. 5.46.
1 A system of whole-mantle convection in which plates are car-
ried about by applied shear stress exerted at their bases by the
convecting mantle. The plates are thus part of a whole-mantle
plate recycling system. The irregularity of plate areas and vol-
umes compared to the regular system of convecting cells in
Rayleigh-Benard convection (Section 4.20) is a problem with
this idea. Also, the scheme requires rather wholesale mixing of
slabs into the ambient mantle to prevent any lithospheric
chemical signature contaminating the very uniform melt com-
positions represented by midocean ridge basalts (MORB).
2 This recognizes a fundamental physical discontinuity in
the mantle at a depth of about 660 km due to the phase
change of the mantle mineral spinel to a denser perovskite
structure. A two-tier convection/advection system is envis-
aged, involving largely isolated lower mantle convection
cells below the 660 km discontinuity. The upper mantle
tier comprises a separate advecting system with plates
driven by the edge- and top-forces discussed previously and
with no slab penetration into the lower mantle. Separation
of the lower and upper mantle in this way, with plate recy-
cling restricted to the upper mantle, might be expected to
gradually change the composition of the MORB through
time. The scheme does not allow for the buoyant penetra-
tion of lower mantle plumes into the upper mantle and
crust. The scheme was originally supported by the lack of
slab-related earthquake hypocenters below 660 km.
3 This is really a hybrid scheme that has received a degree
of acceptance in recent years. It involves both ongoing
lower mantle convection, upper mantle advection with
plates driven by edge- and top-forces and periodic slab
penetration below the 660 km discontinuity. The model
arose in the 1990s as advances in seismic tomographic