Download Deformation of the Plates

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Deformation of the Plates
Rocks behave in either a brittle or a plastic way depending upon temperature, pressure and
environmental conditions (most importantly the presence of water). Brittle deformation
occurs by faulting and fracturing in the upper parts of the plates. It is important to
note that faulting and earthquakes are two sides of the same coin. Plastic deformation
is pervasive in the lower parts of the plates, but ductile folding of rocks may also occur
towards the top of the plates, often intimately associated with faulting.
Brittle deformation
When rock samples are deformed in the laboratory, three modes of fracture can be distinguished and related to the principal stress components σ1 (the direction of maximum
stress) and σ3 (the direction of minimum stress). The two most important modes are (a)
tensile failure, involving parting on a surface approximately normal to the σ3 axis; and
(b) in uniaxial and triaxial compression, shear on a surface inclined at an acute angle to
σ1 . A third mode, longitudinal splitting under low confining pressure, is less important
in nature.
Brittle failure in tension
In terms of naturally occurring rock structures, tensile failure produces joints. Good
examples are hexagonal cooling joints in basalt and block jointing in limestone, which
leads to formation of a limestone pavement if exposed at Earth’s surface. Joints which
have been filled with minerals are known as veins.
Three modes of fracture observed in laboratory experiments: (a) tensile fracture; (b) faulting in
a compresion test; (c) splitting observed in a compression test at low confining pressure.
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Brittle deformation in compression
Rock in compression fails macroscopically by the formation of shear fractures inclined at
an acute angle to the σ1 direction. Under the low temperature and pressure conditions
of the upper parts of the plates, silicate rock responds to large strains by brittle fracture.
The mechanism of brittle behaviour is by the propagation of cracks, which may occur on
all scales. Shear fractures that occur when rock in compression fails macroscopically are
known as faults.
Orientation of Faults Geologists describe the orientation of planar features such as
faults in terms of strike and dip. The strike is the azimuth of a horizontal line in the
plane of the fault. The dip is the largest angle of the fault plane with respect to the
horizontal (i.e. the angle between the horizontal and a line perpendicular to the strike).
Classification of Faults Geologists describe faults in terms of three end-members: normal faults, strike-slip faults and reverse faults/thrusts. Normal faults accommodate extension, strike-slip faults accommodate lateral motion parallel to Earth’s surface and reverse
faults/thrusts accommodate shortening. Faults which have both a strike-slip component
and a component of slip in the dip direction (i.e. either normal or reverse faults) are
known as Oblique Slip faults.
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Characteristic features of faults
Fault Zones Some faults consist of a single plane. Others exhibit a damage zone consisting of many anastomosing fault strands. Lines representing faults on geological maps
do not usually convey the real width of the damage zone.
Fault Rocks Each phase of slip on a fault (i.e. each earthquake) causes wear of the
rocks cut by the fault. Fault rock is the term for the crushed rock that is found along
the fault plane. A rich variety of fault rock types is observed, depending on the pressure/temperature conditions when the fault moved, the availability of water and the type
of rock cut by the fault.
Fault Populations Faults occur on a continuum of scales, from microscopic cracks to kmscale fault segments. A fault marked as a single, straight line on a large-scale geological
map may well represent a hierachy of sub-fault segments in the field.
Length of Normal Faults The longest normal faults have the greatest displacement.
Measurements of large numbers of normal faults spanning 7 orders of magnitude in
displacement show that the maximum displacement D is related to fault length L by
D ∼ L/100.
Lateral Termination of Normal Faults Faults do not go on for ever. The maximum
displacement along a normal fault is usually close to the centerer of the fault segment.
Displacement profiles along the strikes of normal faults (i.e. parallel to their surface traces)
taper to zero at each end. The rate of this taper depends on whether the fault is isolated
or whether it overlaps with other fault segments.
Fault Interaction Adjacent fault segments in a fault zone may overlap in plan view
without actually intersecting (known as soft linking). Alternatively, two adjacent and
overlapping fault segments may be linked by a third fault segment which intersects both
the other two (known as hard linking).
Faults as Fluid Conduits Some faults act as fluid conduits, channelling water from
deep to shallow levels. Many globally important mineral deposits were formed when
mineralising fluids were channelled along faults and then precipitated valuable minerals
as veins along the fault or within the adjacent rock. Other faults are sealing, i.e. they
act as barriers to fluid flow. Whether or not a fault is sealing depends on the natures of
the damage zone and fault rock, which are related in turn to the pressure/temperature
conditions and the rock type cut by the fault.
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Faults and Principal tectonic stresses
EM Anderson developped a theory which relates the class of fault to the principal tectonic
stresses σ1 > σ2 > σ3 . He reasoned that one of the three principle stresses must be oriented
perpendicular to the free surface of the Earth. The other two are orthogonal and lie in
the plane of Earth’s surface.
Normal Faults For normal faulting, the maximum principal stress sigma1 is oriented
vertically, i.e. gravity is the maximum principal stress. According to this view, normal
faulting and associated extension of the plates is driven by gravity, which causes regions
of high topography to collapse. Imagine books collapsing and spreading out along a
bookshelf if the bookend is removed. The horizontal extension direction is parallel to the
least principal stress component σ3 .
Thrust Faults The minimum principal stress is oriented vertically, i.e. gravity is the
minimum principal stress. This can be understood because the thrust faulting acts to
thicken the crust by stacking up sheets of rock, which does work against gravity. The
horizontal shortening direction is parallel to the maximum principal stress component σ1 .
Strike-Slip Faults The maximum and minimum principal stresses lie in the horizontal
plane and σ2 is oriented vertically.
Conjugate Faults In each mode of faulting, there are two conjugate fault place orientations at around 30◦ either side of σ1 .
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Faults and Earthquakes
Faulting and earthquakes are two aspects of the same dynamical system: the former is its
long-timescale manifestation and the latter its short timescale manifestation. Earthquakes
are a result of an instability in faulting; most of the slip on most faults occurs during
earthquakes. Neither faulting nor earthquakes behaves in an isolated manner but interact
with other faults and earthquakes through their stress fields, sometimes stimulating the
activity of neighbouring faults, sometimes inhibiting it.
The Seismic Cycle In the periods between earthquakes, strain accumulates steadily in
the general region surrounding a fault. An earthquake suddenly releases this accumulated
strain. In the simplest case of an elastic model, all the strain accumulated between faulting
is released during the main earthquake, together with any fore-shocks and after-shocks.
Measuring plate deformation This can be done using standard surveying techniques.
Synthetic Aperture Radar interferometry is also a hot topic in geophysics at the moment.
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The brittle-plastic transition
Purely brittle behaviour in crystalline materials gives way, at sufficiently elevated pressures
and temperatures, to crystalline plasticity. There is usually a broad transition between
these distinct regimes in which the deformation is semi-brittle, involving on the microscale
a mixture of brittle and plastic processes, and the rheology is macroscopically ductile.
Thus brittle faulting at shallow depths gives way, over a depth range that depends largely
on the lithology and thermal environment, to plastic shearing at greater depth. However,
the brittle-plastic transition for rock is only qualitatively understood.
Pressure and Temperature Brittle deformation occurs by propagation of cracks, which
are associated with and volume increase and frictional work. Hence, brittle deformation
is inhibited by increasing pressure. Plastic deformation involves internal deformation
of mineral crystals, which is enhanced by thermal activation, but there is no associated
volume change. Hence plastic flow is favoured by both high temperature and high pressure.
Quartz and Feldspar The behaviour of quartz and feldspar is important because the
continental crust is dominated by these minerals. Fully plastic flow in quartz appears
at about 300◦ C and for feldspar at about 450–500◦ C. Between these two states quartzofeldspathic rocks behave as composite materials, with the quartz flowing and the feldspar
responding in a rigid, brittle manner. This occurs over the depth range 10–20 km, given
typical geothermal gradients of around 30◦ C/km.
Effect of Water The brittle plate can be assumed to be almost universally permeated
with fluids, mainly water. This produces two very important but unrelated effects on rock
strength and fracturing that have a strong influence on geological processes. Both effects
act to reduce the strength of the rock.
Pore Fluid Pressure This is a purely physical effect, owing to the pressure the fluid
transmits to the pore space. The pore fluid pressure counteracts the confining pressure,
reducing the overall strength of the rock. In fracture and friction, pore fluid pressure is
thus akin to certain types of lubrication.
Physicochemical Effects Certain chemically active species in the environment react
with the solid at a rate that is enhanced by the tensile stress field at crack tips and at
the same time. This reaction locally reduces the fracture toughness of the solid. Water
is the principal chemically active species in the case of silicates. The presence of water
also enhances plasticity in silicates (hydrolytic weakening). In this case, the reaction takes
place within the mineral crystals rather than at a crack tip.
Time Dependence The physicochemical effects make the strength of rock inherently
time-dependent. For example, if quartz is subjected to uniform compression well below
its instantaneous breaking strength, it will fail spontaneously after a characteristic time.
This behaviour is called static fatigue and depends on the chemical activity of water and
the exponential of the temperature.