Rifts are often described as extension zones. By now we know enough about strain to realize that almost all strains involve extension in some direction or other. By 'extension' we here mean that S1 is ~ horizontal and S3 is ~ vertical.
In many rifts it seems that movement is roughly perpendicular to the rift axis, and we can therefore assume plane strain.
Gravity regime vdpm 08.27
Note that this is not necessarily the case. When rifting is oblique then plane strain does not apply: we refer to the strain in an oblique rift as transtension
Transtension vdpm 19.18
Diagrams showing Mid-Ocean Ridge structures
The most influential example in the present-day world is the East African Rift, which shows the follwing features:
Many of these features are explained by thinning of the lithosphere. Lithospheric stretch is typically represented by 'beta'. The characteristic topography of rifts is an isostatic response to high heat flow (produces regionally elevated surface), coupled with the localized thinning of the axial region (produces the rift valley).
About 30% of the world's giant oilfields are in rifts!
In homogeneous rocks fractures initiate at points of slightly elevated shear stress or slightly lower strength rock and gradually expand. At low bulk strains, we observe isolated fractures that terminate at a generally elliptical tip line.
In map view these faults show decreasing heave towards fault tips at either end. (Heave is the horizontal component of the fault offset.)
In general, the width (distance between tip points) of a fault trace increases as displacement increases.
If there is sedimentation in an area of active normal faulting the thicknesses of units change across the faults. This is known as growth and the faults are called growth faults.
The sediment pattern in half-grabens is particularly characteristic.
Within a basin isolated faults may overlap in arrangement, and displacement is transferred from one fault to another through zones of ductile deformation. Gentle folding in such zones produces relay ramps between one fault and the next
As a fault tip propagates through a sediment package, it may be preceded by a zone of creep deformation. This may be preserved in the walls of the fault as a drag fold. (The name drag fold is a poor one because the folding probably occurs before faulting; it would be better to call these fault propagation folds by analogy with structures in thrust belts.)
photo of drag fold
Folds showing the opposite kinematics to drag folds are common adjacent to normal faults. This phenomenon used to be referred to as 'reverse drag' but is now commonly known as 'rollover'.
Rollover is a variety of fault-bend folding. To see how rollover works, consider a normal fault with concave-up and convex-up segments. To maintain strain compatibilty, the hanging wall must deform to accomodate changes of shape in the footwall.
VDPM 16.07 strain compatibility
VDPM 16.08 rollover
There is a close relationship between folding of the beds and the shape of the fault.
Flats tend to be overlain by anticlines and the ramps by synclines.
In detail, the relationship depends on the type of strain developed in the hangingwall as it accommodates to the footwall.
The simplest assumption is of vertical simple shear.
If the horizontal component of displacement is h (for heave) then we can make a construction called the Chevron construction, that enables us to predict the fault trace from the bedding trace or vice versa.
For example, given this geometry...
The lengths t1 t2 t3... can be used to reconstruct the thickness of the hanging wall and thus fix the shape of the fault.
Other possible modles include simple shear on some other plane: e.g. a conjugate plane to the main fault.
Fault-bend folds related to listric normal faults are very important as petroleum traps in a number of fields on passive margins, including both margins of the Atlantic. Several big fields including Hibernia have this general geometry.
Hibernia structure
Many common configurations of normal faults are variations on the theme of conjugate faults: multiple normal faults in 2 families, with ~60° angle between them.
VDPM 16.08
Smaller faults may be described as synthetic and antithetic in relation to larger faults
Normal faults bound horsts and grabens
In general, there is also a tendency for normal faults to flatten into shear zones at depth, producing listric geometries, though the extent of flattening in individual rifts may be controversial. Tilting may be explained either by listric faults or by fault blocks that behave like books on a shelf
VDPM 16.06
vdpm 08.26, 16.04, 16.05, 16.13a
At larger scale rifts vary from symmetric to asymmetric
In symmetric rifts, lower crustal deformation is probably approximately coaxial, pure shear
In asymmetric rifts ('Wernicke' model) simple shear occurs at depth. The locus of lower crustal extension may be displaced from the upper crustal rift. Highest heatflow and volcanic activity may be offset from rift axis.
[ a and b] VDPM 16.09
Sense of asymmetry may shift along strike, producing transfer zones
VDPM 16.15, 16.10
If rifting ceases without the development of an ocean basin (sometimes called a failed rift) thermal subsidence may lead to development of a steer's head basin that has two parts, a fault bounded rift basin and an overlying sag basin.
VDPM 16.19
Subsidence rates after the end of rifting gradually slow, exponentially. There is a relationship between post-rift subsidence rate and the stretching factor beta
Predicted subsidence curves for different beta - from Allen and Allen
Passive continental margins are margins of continents that do not coincide with a plate boundary. A pair of passive continental margins forms when rifting gives way to ocean-floor spreading. Passive margins are also called 'rifted margins' or 'Atlantic-type margins'
In addition to the 30% of the world's giant oilfields that are contained in rifts, passive margin post-rift successions contain about another 30%; thus 60% or more of the world's oil is located in rifts and passive margins.
Rift system that led to the formation of Atlantic continental margins
Passive continental margins are rift remnants, left behind when sea-floor spreading starts.
Diagrams showing evolution from rift to passive margin
VDPM 16-18
Passive continental magins characteristically subside because the heat source is removed to the new spreading centre.
Characteristically, passive margins show:
Passive margins typically show an initial rapid phase of subsidence (rift phase) followed by a period in which the subsidence curve is exponential, with gradually slowing subsidence. As in a failed rift, it's possible to predict the cooling rate (and therefore the subsidence rate) associated with a given amount of crustal thinning. Hence it's possible to take a subsidence curve and calculate how much the crust must have been extended.
Some passive margins show long distance (10s of km) gravity slides of large sediment sheets.
The up-slope areas show structures similar to those of asymmetric rifts
Down-slope region shows thrust belt structures
Particularly large slides are known from the Niger delta off Africa where the structures are potentially petroleum bearing.
Rifts in tropical climates may accumulate thick evaporite deposits. As spreading occurs, these evaporites may undergo mobilization within the passive margin succession under the influence of gravity. Salt structures are introduced in Ch 2 of VDPM's text.
Halokinesis refers to the movement of salt.
Halokinesis is typically due to:
These effects combine to produce a number of geometries
Salt withdrawal features VDPM 2.20
Overall configuration of salt tectonic features on a passive margin: Gulf of Mexico
Map VDPM 16.27
Section VDPM 16.28