Environmental Engineering Reference
In-Depth Information
geomorphological processes with gravitational energy. Plate architecture - literally, the
style, design and construction of plate structures - is the key to global morphotectonic
landforms and the
rock cycle
. Constructive, destructive and conservative styles of plate
margins are translated into
mid
-
ocean ridge
,
subduction zone
and transform faults and
related structures in the oceans and continents. The logical place to start is where new
lithosphere is created at mid-ocean ridges, but this is actually preceded by continental
rifting in the Wilson cycle, which charts the birth and eventual death of the ocean.
RIFT FORMATION AND DEVELOPMENT
Rifting involves the splitting and separation of crustal lithosphere under high shear
stresses. Sustained stress propagates or extends rifts, often along major
lineaments
or
existing linear weaknesses such as sutures and faults. Continental
rift valleys
and oceanic
rifting, in the form of mid-ocean ridges, develop with symmetrical separation on both
sides of the rift.
Structural basins
form on asymmetrical crustal extension (Figure 10.5).
Active rifting
occurs over mantle plumes and leads eventually to the emergence of new
crust. Rifting may still occur in their absence, in which case the necessary
crustal
extension
for
passive rifting
must occur mechanically in various ways. Subduction and
associated slab pull on the far side of a continent may trigger a corresponding
trench
suction force
, and thereby extensional rifting in the continental lithosphere on the near
side. Surface erosion may have a similar extensional effect. In this case the reduced mass
of upper, brittle crust requires an isostatic adjustment. It is achieved by 'inflow' of
underlying ductile (pliable) crust which undermines adjacent brittle crust. Extension then
causes
faulting
as lithosphere is stretched beyond its brittle strength limits and the rift, or
graben
, is formed as crust subsides between inward-facing faults. New crust will form in
the rift only if faulting penetrates the entire lithosphere and/or there is insufficient
resistance to magma flow from underlying asthenosphere (Figure 10.5).
