1. Extensional Rift Basins
BY:
Adrian Jones
Kirstin McBeath
Sotonye Okujagu
Mai Afifi
Supervisor : Dr. Noelle Odling
School of Earth Science,
Leeds University,
December,2005

2. Lithospheric extension
Evolutionary sequence
Mechanisms involved:
i. Brittle extension of the crust
fault-controlled subsidence
Thermal relaxation of lithosphere
regional post-rift subsidence
End-member rifting processes:
Active - lithosphere stretching in response to an active thermal process in the asthenosphere.
Passive - lithospheric tensional stresses cause thinning; up-welling is secondary.
After Allen and Allen, Basin Analysis (2005).

3. McKenzie model (1978) - 1st quantitative model
1. Uniform, instantaneous thinning of lithosphere:
Tc > 18 km initial subsidence (Si),
where Si = d ( 1 – 1/β ),
Moho rise thermal anomaly.
Gradual decay of thermal anomaly:
bulk ρ increases time-dependent,
thermal subsidence (ST),
where ST ≈ E r ( 1 – exp t/τ ).
Simple but explains much subsidence behaviour.
Assumption: maintenance of isostatic equilibrium.
Salveson (1978) provided a qualitative model of passive, mechanical extension.
After Roberts and Yielding (1994).

4. Modifications to McKenzie (1978) model
Time-dependent (finite-rate) stretching
Previous assumption: stretching is geologically instantaneous all thermal
effects occur post-stretching.
Jarvis & McKenzie (1980) show that for relatively fast stretching:
Si and ST ≈ instantaneous stretching,
however, for relatively slow stretching ST may accompany stretching:
Si amplified, ST reduced and STotal remains constant.
Fast vs. slow: dependent on average strain rate relative to conductive heat loss of stretched lithosphere.
stretching time < 60/β2 : time-depnt models v. similar to inst-stretch models,
stretching time > 60/β2 : predictions of models deviate.
Previously assumed stretching is geologically instantaneous; all thermal effects occur post-stretching.
practically, a weakly stretched basin with stretching less than c. 30 Ma, McKenzie model OK. Highly stretched then only good if lasts less than c. 10 Ma.

5. Modifications to McKenzie (1978) model
2. Lateral heat-flow
Previous assumption: decay of thermal anomaly through vertical conduction only, i.e no heat loss to the rift margins.
Alvarez et al. (1984) found a number of effects:
heat conduction lowers bulk ρ lithosphere surface rises isostatically
i. Erosion accompanies uplift: unconformity in geological record and margins subside to below their initial elevation.
ii. No erosion: uplift is totally recoverable and margins return to initial elevation.
Si and ST also affected, however, McKenzie equations remain valid if:
basin width > lithosphere thickness.
margins lowers the bulk density and causes the surface of the lithosphere to rise isostatically. May then get erosion preserved as an unconformity. Total thermal recovery will cause margins to subside below their initial, pre-stretch elevation.
May then get erosion preserved as an unconformity. Total thermal recovery will cause margins to subside below their initial, pre-stretch elevation.

6. Modifications to McKenzie (1978) model
3. Depth-dependent stretching
Previous assumption: uniform stretching.
Varying β with depth:
i. Discontinuous – upper and lower lithosphere are decoupled at depth d.
– uplift can occur if d ≈ yc.
ii. Continuous – extension as a function of depth.
– for large Φ Si is increased but
ST decreases.
– uplift of ridge shoulders.
Due to rheological properties changing within the lithosphere the lithosphere extends inhomogeneously and discontinuously. Decoupling at a depth d.
After Allen and Allen, Basin Analysis (2005).

7. FAULT SYSTEM IN RIFT BASIN

8. University of Oxford web site
sahsjaaskakskkakaka

9. Schlumberger

10. Rifting and Faulting

11. Schlumberger

12. Schlumberger Listric Fault

13. DURING SUBSIDENCE (i.e. the SYN-RIFT PHASE):
Lithosphere is thinned and the base of the lithosphere (the 1330 ºC isotherm)
is elevated
Heat flow through the crust and mantle are now higher than they were prior to
stretching
Subsidence creates space for early syn-rift sediments to be deposited
Footwall uplift above base level may result in erosion of the fault block crests
BASEMENT DEPTH (km)
RIFT AXIS
RIFT MARGIN
SYN-RIFT SUBSIDENCE UPLIFT
POST-RIFT THERMAL SUBSIDENCE
SUBSIDENCE 1 2
Q (mWm-2) 40 60 80
HEAT FLOW
100 Ma

14. SYN-RIFT SEDIMENTS:
Deposited in active, fault-controlled depocentres of the evolving rift
Often show roll over and growth (thickening) into the active faults
Differential subsidence across the extensional faults may exert a strong
control on facies distribution
Dominated by coarse-grained continental and shallow-marine sediments
Rapid sedimentation during the syn-rift phase is a result of isostatic adjustment
to the lithosphere stretching
Syn-rift sediments are easily identifiable from reflectors on seismic sections. They have a distinct wedge-shaped geometry and show growth into the active, basement-involved rift faults.

15. SYN-RIFT SEDIMENTS:
Deposited in active, fault-controlled depocentres of the evolving rift
Often show roll over and growth (thickening) into the active faults
Differential subsidence across the extensional faults may exert a strong
control on facies distribution
Dominated by coarse-grained continental and shallow-marine sediments
Rapid sedimentation during the syn-rift phase is a result of isostatic adjustment
to the lithosphere stretching
Syn-rift sediments are easily identifiable from reflectors on seismic sections. They have a distinct wedge-shaped geometry and show growth into the active, basement-involved rift faults.

16. POST-RIFT SEDIMENTS:
When the stretching stops, heat flow is reduced and the lithosphere cools,
returning to its original thickness (c. 125 km)
Cooling induces contraction and further subsidence creates more space for post
rift sediments to be deposited
The post-rift sediments infill any remnant rift-related topography; fill the subsiding
basin; and onlap the basin margins
Deposits are dominated by progressively deeper-water, fine-grained sediments;
deposition is gradual, as the asthenosphere cools

17. POST-RIFT SEDIMENTS:
When the stretching stops, heat flow is reduced and the lithosphere cools,
returning to its original thickness (c. 125 km)
Cooling induces contraction and further subsidence creates more space for post
rift sediments to be deposited
The post-rift sediments infill any remnant rift-related topography; fill the subsiding
basin; and onlap the basin margins
Deposits are dominated by progressively deeper-water, fine-grained sediments;
deposition is gradual, as the asthenosphere cools

18. Such stratigraphic evolution results in a “Steer’s Head” (or “Texas Longhorn”) geometry, typical of rift basins, and is achieved when the rift basin is stretched over a wider region than the crust (but with equal amounts of extension to avoid space problems), and progressive stratigraphic onlap develops on the basin margin over previous rift shoulders during the post-rift thermal subsidence phase of extension.
Post-rift stratigraphic onlap can not be accounted for simply from the lithospheric stretching model (McKenzie 1978). Explanations include:
Global or eustatic sea-level rise;
Flexural rigidity of the continental lithosphere after rifting, and;
A two-layer lithospheric stretching model (when (i) and (ii) are inconsistent with observations)
SYN-RIFT SEDIMENTS
PRE-RIFT SEDIMENTS
POST-RIFT SEDIMENTS

19. The Play Elements What is a Play? Reservoir Seal(s) Source Rock
Trap
Geometry
Maturation
&
Plumbing
Timing
The Play Elements
A PLAY FAIRWAY is a model of how a producible reservoir, petroleum charge system and traps may combine to produce petroleum accumulation at a specific stratigraphic level
**10% rifted or graben basins

20. CASE STUDY 1: Faroe / Shetland Basin, North Atlantic Margin
up to 125km wide and 600 km long with five times prospective parts of N. Sea basin
Play-fairway distribution can be classified into: pre-, syn- and post-rift plays.
Pre-rift play: most successful plays of the titled fault blocks with Jurassic and pre-Jurassic reservoirs
Syn-rift play: Lower Cretaceous submarine fan sandstones (basin floor fan and slope-apron fan)
Post-rift play: centred around Palaeocene deep-water/submarine fan sandstone

21. CASE STUDY 2: Gulf of Suez, Red Sea
Models involving an active asthenoshperic heat source should predict uplift before rifting.
Rift appears to have initiated by Miocene times
Two main causes of vertical movements during rifting are:
1- Uplift caused by heating of the lithosphere
2-subsidenenc caused by thinning of the crust
Peripheral Bulge
Syn-Rift
Post-Rift 1
Post-Rift 2
C.S Not to scale

22. Thank You