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

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).

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).

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.

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.

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).

FAULT SYSTEM IN RIFT BASIN

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Rifting and Faulting

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Schlumberger Listric Fault

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

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.

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.

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

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

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

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

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

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

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