Fig. 1 Terminology of rift-basin stratigraphy used by Bosence (1998)

Tectono � sedimentary models of rift basins: the Gulf of Suez and the Northern Red Sea

vorgelegt von: Frank Müller / Betreuer: Sven Egenhoff

Abstract This report reviews models for the tectonical, sedimentary and stratigraphic evolution of rift basins. At first pure shear, simple shear, heterogeneous stretching and plume �related rift basin models are presented at the basin and the sedimentary facies model scale. The different stratigraphic responses according to the tectonic evolution are discussed in terms of the occurrence of pre-rift strata, syn-rift unconformity, syn-rift strata, post-rift unconformity and post-rift strata. The created stratigraphic classification is done based on the uniqueness of sedimentary signatures occurring within the different geotectonic settings. In the second part the northern Red Sea and Gulf of Suez stratigraphy is reviewed. The geometry and kinematics of the rift are analysed and it is shown that the pure shear model is applicable for these areas, in consideration of the inherited geotectonic structures and therefore the formation of zigzag fault pattern.

Classification of rift basin stratigraphy (Figure 1)

Rift basins A rift basin is defined as long, narrow continental trough that is bounded by normal faults; a graben of regional extend. It marks a zone in which the entire thickness of the lithosphere has ruptured under extensional stress. Therefore rift basis are generated within the time span from initial rifting and rift-drift transition from which on oceanic lithosphere is created at a spreading centre.

Pre-rift strata In general strata deposited prior to rifting according to the paleo-environment (Bradley et al., 1984). The upper surface of the pre-rift strata is the syn-rift unconformity or a superimposed post-rift unconformity according to the geotectonic evolution of the basin.

Syn-rift unconformity (SRU) The unconformity between pre-rift and syn-rift strata resulting from footwall uplift respectively sea level fall, is either a local or widespread erosion surface. It is defined

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by an unconformity surface on rotated fault blocks with an overlap of syn- rift strata characterised by the regular and progressive pitching out towards the margins. The syn-rift unconformity is generated prior to and during rifting and so underlies syn-rift strata.

Syn-rift strata Sediments deposited in the active fault-controlled depocentres of the evolving rift. Mechanical extension and subsidence of the basin (equivalent: rift phase of Falvey (1974); synrift megasequence of Hubbard(1988)) also exert a strong control on sedimentation and distribution of the syn-rift facies. The strata can show growth into the active fault respectively repeated thinning into hangingwall basins (=fanning) and facies variations (e.g. footwall derived fans) adjacent to faults. Furthermore beds of the downthrown block may dip towards the footwall respectively the fault surface in an orientation opposite to that produced by the drag (=rollover). These movements generate soft sediment deformation structures (see �Classification of soft sediment structures� SEDIMENTARY STRUCTURES, John R.L. Allen, 1984). The term syn-rift should be used in a local better than in a time-stratigraphic sense and therefore show the above features indicating a syndepositional fault movement (Bosence, 1998).

Post-rift unconformity (PRU) Erosional surface that marks the base of strata deposited during thermal or post-rift subsidence phase of a basin (Badley et al., 1984).

Post-rift strata Strata deposited during post-rift subsidence. These sediments gradually bury any remaining rift-related topography of the basin and fill the cooling controlled expanding accommodation space. Increase in density of lithosphere and astenosphere, load of sediment and water do also have a large effect on the post-rift subsidence rate which occurs spatially wider than the one induced by mechanical extension. The strata Hubbard (1988) called �passive margin� or the �passive wedge� is marked by thick onlapping and offlapping. Stratigraphic sections which may be initially saw tooth-shaped according to the unfilled rift-related topography. Differential subsidence can be accommodated by planar and vertical normal fault movement.

Geotectonic rift-basin models and their stratigraphic response The following four different geotectonic models of rift basins are divided in two groups: active (plume-related) and passive (pure shear, simple shear and heterogeneous stretching) rift models. The passive type is characterised by tensional stresses causing extensional plate movements and thus the input of hot asthenospheric material is a passive response to lithospheric thinning. With the

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Fig. 2 Pure shear model modified by BOSENCE (after McKENZIE)

involvement of hot astenosphere caused by decompression melting, active rifting, domal uplift and convective thinning occurs. The impingement of a thermal plume or sheet on the base of the lithosphere also triggers these movements. The heating of the lithosphere leading to rift uplift can only be realized by convection because conduction and magma generation do not generate such high amounts of heat flow from the astenosphere. The following models illustrate the different stratigraphic relation and degree of development of rift-related strata and unconformities in axial and lateral portions of the rift basin.

Pure shear model stratigraphy (Figure 2)

Uniform and instantaneous extension assumes the crust to fail by brittle fracture. The subcrustal lithosphere is flowing plastical as reac-tion on the extensional stress. The arising isostatic disequilibrium leads to a compensating rise of the astenosphere, and conse-quent regional uplift, so the homogeneous pure shear activates faulting and rota-tion of strata in the brittle

upper crust. The generated two rift border faults are dipping towards each other and the syn-rift unconformity (SRU) develops over the footwall highs of rotated fault blocks. The amount of initial fault controlled basin subsidence depends on the original thickness of the crust and the amount of stretching. This regulation of the accommodation space, together with the amount of sediment supply mainly controls the development of the syn-rift strata thickness and fanning. The post-rift unconformity (PRU) develops as a prominent surface extending laterally over the basin shoulders. The second longer period of subsidence is thermally induced and caused by the relaxation of lithospheric isotherms to their pre-stretching position. This basin scale interaction generates the sag in which the post-rift strata occur. The thermal subsidence decreases exponentially as a result of diminishing heat flow with time. So the crust and fault-controlled subsidence is permanent, while the thinning of mantel lithosphere is transient.

Simple shear model stratigraphy (Figure 3) The lithospheric extension is accomplished by displacement along a large scale, gently-dipping shear zone which traverses the entire lithosphere. The displacement develops an asymmetric graben structure, partitioned into a proximal and distal area. Rift flank uplift separating the two sites is related to an unloading and flexural isostacy, which is synchronous with axial extension and sub-si-dence (BRAUN & BEAUMONT, 1989).

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Fig. 3 Simple shear model modified by BOSENCE (after WERNICKE and BURCHFIEL, 1982)

Fig. 4 Heterogeneous model modified by BOSENCE (after COWARD, 1986)

The subsiding and rotated fault blocks over the lithos-pheric detachment gen-erate syn-rift strata accom-modation space located in the proximal site. Syn-rift unconformity is locally developed on those blocks of the mechanically rifted region. The distal region will be uplifted in response to mantel heating and upwel-ling (WERNICKE, 1985). Thus the deposited syn-rift strata are eroded and the SRU

only appears in a local scale within this area. The PRU now affects the whole area and represents a long-lived break in deposition superimposed on the syn-rift unconformity in the distal area. This site is dominated by well developed widespread post-rift strata in response to thermal subsidence, whereas the thermal subsidence affecting the proximal area is smaller because of limited upper mantel thinning, so the post-rift strata are thin.

Heterogeneous stretching model stratigraphy (Figure 4) In the heterogeneous model mechanical and thermal subsidence are not spatially separated like in the simple shear model. The upper crustal zone is characterized by rotated fault blocks propagating away from the rift laterally. The displacement executes along a low angle lithos-pheric detachment. The underlying lower ductile

crust is affected by extension and thinning. This leads to an isostatic, local footwall uplift and erosion of fault blocks within the inner zone. Syn-rift strata may be present in the inner zone, but well preserved in the outer parts caused by the propagating half-graben and therefore missing erosion. The widespread PRU and post-rift strata may cut SRU in the formerly uplifted inner zone, which accumulates large amounts of post-rift strata; whereas the sub-basins not underlain by thinned lower crust/ upper mantel do only preserve poorly developed post-rift strata as a response to slight thermal cooling and subsidence.

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Fig. 5 Plume related model modified by BOSENCE (after WHITE and McKENZIE, 1989)

Plume related model stratigraphy (Figure 5)

The plume related model is based on a primarily rifted and thinned continental lithosphere. This passing over a plume or a region of hot astenosphere may lead to decompression and partial melting from small rises in temperature terminating in massive out-pourings of basaltic lave and related rifting. Plume related rifting has a large effect because of the uplift of extensional basins

through magmatic underplating and the dynamic support. The upwelling plume causes lateral temperature gradients on the base of the lithosphere resulting in horizontal variations in the normal stress (HOUSEMAN and ENGLAND 1986). Uplift then occurs instantaneously, extending each rheological layer according to its own deformation mechanism, caused by the imparting of excess potential energy in the updomed area. The stratigraphic relations in this model are controlled by the timing of the plume affecting the stretching margin. Therefore two examples are given, when the plume is synchronous with rifting and when it is prior to rifting (WHITE and McKENZIE, 1989). The former characterized by SRU on footwall blocks covered by thick syn-rift volcanics and limited thermal subsidence leads to relatively thin post-rift strata. If the plume affects the system prior to rifting, the lithospheric doming begins and raises the affected area up to 1-2 km high and 1000-2000 km broad accompanied by synchronous extensional tectonism. This tectonic setting creates a significant SRU followed by thick volcanic deposits. The post-rift strata also develops relatively thin caused by the same limited thermal subsidence.

Application of rift models to the Gulf of Suez / northern Red Sea

General Setting (Figure 6) The Gulf of Suez and the northern Red Sea as part of the Red Sea rift was initiated at the end of the Oligocene located between the African and Arabian shield. The Precambrian basement belongs to several superimposed orogenic cycles, the latest being the �Pan-African� tectono-magmatic phase (about 500 Ma) which formed the African craton, followed by early and late Palaeozoic magmatic events.

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Fig. 6 General setting of the Gulf of Suez and Red Sea .1. Cenozoic volcanics; 2. basement/sedi-mentary cover boundary; 3. thrust front; 4. fold axis; 5. oceanic spreading zone; 6. relative movement of the Arabian shield; AS-Syrian folded Arc; E.A.R-East Africa rift; NJ-Nadj fault system (after GENEBASS group, published in MONTENAT, 1986)

The surrounding Neogene tectonic setting includes (Figure 6):

Taurus and Zagros orogenic belt in the north and north-east the Syric arc folds (AS) • Gulf of Aden oceanic spreading zone to the south-east • the Ethiopian rift to the south (E.A.R.) Red Sea rifting succeeded the formation of the folded Syric arc in the late Cretaceous to Eocene (CHOROWICZ and LYBERIS, 1987). The inherited structural settings of this paleo tectonics are: • north-west/south-east faults, parallel to the basin axis (HUME, 1921), the so called

�Clysmic� trend, • north-north-east/south-south-west �Aqaba� trend, well developed in the Gulf of

Suez, but not that significant on the Red Sea margins (THIRIET et al., 1986), • the sub east-west or �Duwi� trend comprises large corridors of faults (JARRIGE et

al.,1986), • perpendicular to the fault axis, the �cross� trend, only occurs in the Gulf of Suez

and plays a minor role in the rift structure (OTT d`ESTOVOU et al., 1986).

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Fig. 8 Relations be-tween different sedi-mentary units at the Abu Ghusun are (see Fig. 7), defined by GENEBASS and pub-lished by MONTENANT et al., 1986

Fig. 7 surveyed areas, including geological mapping by the GENEBASS group (MONTENAT, 1986)

Settings of the Abu Ghusun area

Pre-rift strata The pre-rift sedimentary cover is unconformably overlaying the Precambrian basement. Those basement series include various crystalline, metamorphic, volcanic and sedimentary rocks with outcrops along the rift shoulders and within the fault blocks. These units are related to the Pan-African magmatic events (about 500 Ma) and have been rejuvenated by evolving Cenozoic rifting. Overlaying sediments of the pre-rift strata begin with a continental terrigenous formation of sandstones, followed by shaley and carbonate marine deposits. The Cretaceous transgression advanced southwards into the Red Sea and therefore the open marine sedimentation exists earlier in the Gulf of Suez than in the northern Red Sea. For this reason the pre-rift cover varies from about 2000 m in the northern Gulf of Suez to 400�800 m in the south respectively in the northern Red Sea. The rejuvenated east/west to north-east/south-west orien-tated paleogeographic features are related to the Thetyan influence and shown in the data of the French group GENEBASS (Figure 8). The SRU shows only slight angularity and there is no major hiatus on the eastern margin of the Gulf, so there is no evidence for widespread pre-rift doming in this region.

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Fig. 10 Syn-depositional tilt-block structures. A Schematic map of block faulting which occurred during the deposition of evaporitic A2 unit. 1 basement, 2 evaporites, 3 fault, 4 foliation of metamorphic basement, 5 dip of evaporites beds, c-d location of sections B & C.

Fig. 9 Synsedimentary deformation of the A1group related to contem-poraneous wrench (regional strike-slip) faulting along N010 sinistral fault at Gebel Tarbul. A Outcropping structure 1 fault zone, 2 Eocene over-turned limestone (Thebes Fm.), 3 basial breccia of the red clastic Fm., 4 red clastics composed of reworked Nubian sandstone. B Synthetic view of the structure. (C.MONTENAT et al.1988)

Syn-rift strata The syn-rift deposits are separated in three parts A to C by the French Group GENEBASS (Figure 8). Deposits of the group A are divided in the sub-unit A1, the Red series with radiogenic K-Ar ages ranging be-tween 26 and 22 Ma (late Oligocene to Aquitanian), in-cluding red terri-genous sediments (2 / 3, Fig. 8) and rare basaltic dykes and flows. Those fine-

grained sediments indicate a low relief hinterland and were deposited on an extensive low-relief region with shallow, poorly drained depressions. PLAZIAT et al. (1998) describes seismites (convoluted structures, breccias, sedimentary dykes, clay diapers, liquefaction) induced by strong and related earthquakes. Those indicators for synsedimentary deformations can only be preserved in flat depositional environ-ments (Figure 9). The strike-slip deforma-tion of the initial stage did not generate high relief blocks and therefore had no major influence on sedimentation (Figure 11 A). The second upper series A2 includes late Aquitanian to early Burdigalian evapo-rates and related anoxic sediments (4 / 5 / 6, Fig. 8). This series is tectonically induced by antithetic normal move-ments on Clysmic-trending (north-west / south-east) faults (Figure 11 B). The con-temporaneous ending of the strike-slip movement resulted in the generation of antithetical tilted blocks occurring in the Gulf of Suez and the northern Red Sea (Figure 10). Due to the panels often being separated by paleo-trending

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Fig. 10 Block diagrams showing the polyphase tectonic evolution of the rift (afterOTT d`ESTEVOU et al., 1989). A Wrench-faulting stage with slightly subsident partitioned rhombic panels. B Tilt-block stage, antithetic Clysmic normal faults cutting the panel. C Horst and graben stage, split blocks strongly subsided by synthetic Clysmic faults.

faults, a zigzag pattern arose and groups of opposite dipping tilt-blocks are noted (OTT d`ESTEVOU et al., 1987). The group A sediments are in general weakly uncon-formable to conformable with the pre-rift strata. The second group are the open marine deposits of the B formation, unconformably overlaying the Group A or even the basement. Biostratigraphic data (PLAZIAT et al., 1998) shows the late Burdigalian to Langhian age of these varying sedimentary facies which are generated within distinct but spatially related environments (7 / 8 / 9 / 10, Fig. 8). The large amount of clastics within Group B sediments are an indicator for the rejuvenation of fault blocks and reactivation of erosion. So these are not due to persistent rotation of tilt-blocks along listric faults but to the generation of a horst and graben structure by the dislocation along predomi-nantly synthetic faults (Figure 10). Thereby reefs are built on structural highs and developing large car-bonate talus are covering fault-scarps (PURSER et al., 1998), while pelagic muds are accumulated in highly subsident graben. The predominance of synthetic faults result in the increase of the basinwards dip marking the beginning of the flexuration stage. This also indicates the centripetal migration of the subsidence and coincides with maximum subsidence of the rift (MORETTI and COLLETA, 1987). The following unconformably overlying Group C is generated within the mayor evaporation event during rifting from Serravallian to late Miocene. Those deposits mainly consist of sulphates locally associated with stromatolitic carbonates. The thickness increases significantly to the centre of the basin where halite was deposited (11 / 12 / 13, Fig. 8). The horst and graben patterns are sealed during deposition, especially the algal mats and stromatolitic carbonates preceding the evaporites and therefore seal morphologies (PRAT et al., 1986).

Post-rift unconformity and strata Group D following these successions overlie the evaporites unconformably or conformably depen-ding on the structural position. If there is important continental siliciclastic sedimentation (sand and con-glomerates), the peripheral basement is uplifted. Areas with open marine carbonates, calcarenites, reefs and peri-reef sediments, were protected from detrital sediments. The main events during the deposition are the halo-kinetic movement of Group C evaporites and therefore strongly deformed Plio-Pleistocene sediments (MART and RABINOVITZ, 1987). Because of the varying signatures of salt tectonics the beginning of the post-rift strata can not exactly be determined in this Red Sea area.

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Conclusion A number of described features such as inherited structures, formation of zigzag fault pattern, initial strike-slip structural stage and polyphase extensional structures can be seen as typical attributes of rift dynamics. The absence of extensive volcanism in the early syn-rift phase, together with a widespread and significant pre-rift unconformity argue against pre-rift updoming and therefore against an active rift model. The symmetry of rift shoulders and basin, the formation of the rift shoulders synchro-nous with rifting suggests that McKenzie`s (1978) pure shear model may be the most appropriate.

References ALLAN, Phillip A. (1997) Earth surface processes, Blackwell Science Publications. ALLAN, Phillip A. and ALLAN, John R. (1993) Basin Analysis � Principles & application,

Blackwell Science Publications. PURSER, B.H. and BOSENCE D.W.J. (1998) Sedimentation and tectonics in rift basins �

Red Sea � Gulf of Aden, Chapman & Hall. READING, H.G. (2000) Sedimentary environments � Processes, facies and stratigraphy,

third edition, Blackwell Science. WINDLEY, Brian F. (1997) The evolving continents, second edition, John Wiley & Sons.