Rifts and other zones of lithospheric extension

Rift basics

Extension

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

Transtension and oblique rifting

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

Location of some important rifts and extensional zones

Oceanic rifts - mid ocean ridges

Diagrams showing Mid-Ocean Ridge structures

Continental rift zones

Regional geology

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!

Fault geometries in rifts

Isolated faults at low strain

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.)

single normal fault

In general, the width (distance between tip points) of a fault trace increases as displacement increases.

Growth faults

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.

growth

The sediment pattern in half-grabens is particularly characteristic.

Transfer zones and relay ramps

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

relay ramp

'Drag' folds

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

'Rollover' folds

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

VDPM 16.07 strain compatibility

16.08

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...

chevron construction

The lengths t1 t2 t3... can be used to reconstruct the thickness of the hanging wall and thus fix the shape of the fault.

chevron construction

Other possible modles include simple shear on some other plane: e.g. a conjugate plane to the main fault.

inclined shear construction

 

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

Fault arrays

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 16.08bVDPM 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 16.06VDPM 16.06

vdpm 08.26, 16.04, 16.05, 16.13a

Overall symmetry of rifts

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

Post-rift subsidence

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.

steer's headVDPM 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

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

Evolution from rift to passive margin

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 margin

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.

Post-rift tectonics on passive margins

Gravity tectonics

Some passive margins show long distance (10s of km) gravity slides of large sediment sheets.

slide

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.

Salt tectonics

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

Halokinesis refers to the movement of salt.

Halokinesis is typically due to:

Salt geometries

These effects combine to produce a number of geometries

Salt withdrawal features VDPM 2.20

salt struct

Kinematic styles

Overall configuration of salt tectonic features on a passive margin: Gulf of Mexico

Map VDPM 16.27

Section VDPM 16.28