phanerozoictectonothermalhistoryofthearabian–nubian...

Phanerozoic tectonothermal history of the Arabian–Nubianshield in the Eastern Desert of Egypt: evidence from fission track

and paleostress data

Ana-Voica Bojar *, Harald Fritz, Sabine Kargl, Wolfgang Unzog

Department of Geology and Paleontology, Karl-Franzens University, Heinrichstrasse 26, A-8010 Graz, Austria

Received 15 December 2000; accepted 3 November 2001

Abstract

To constrain the post-Pan-African evolution of the Arabian–Nubian Shield, macro-scale tectonic studies, paleostress and fission

track data were performed in the Eastern Desert of Egypt. The results provide insights into the processes driving late stage vertical

motion and the timing of exhumation of a large shield area. Results of apatite, zircon and sphene fission track analyses from the

Neoproterozoic basement indicate two major episodes of exhumation. Sphene and zircon fission track data range from 339 to 410

Ma and from 315 to 366 Ma, respectively. The data are interpreted to represent an intraplate thermotectonic episode during the Late

Devonian–Early Carboniferous. At that time, the intraplate stresses responsible for deformation, uplift and erosion, were induced

by the collision of Gondwana with Laurussia which started in Late Devonian times. Apatite fission track data indicate that the

second cooling phase started in Oligocene and was related to extension, flank uplift and erosion along the actual margin of the Red

Sea. Structural data collected from Neoproterozoic basement, Late Cretaceous and Tertiary sedimentary cover suggest two stages of

rift formation. (1) Cretaceous strike-slip tectonics with sub-horizontal r1 (ENE/WSW) and r3 (NNW/SSE), and sub-vertical r2resulted in formation of small pull-apart basins. Basin axes are parallel to the trend of Pan-African structural elements which acted

as stress guides. (2) During Oligocene to Miocene the stress field changed towards horizontal NE–SW extension (r3), and sub-vertical r1. Relations between structures, depositional ages of sediments and apatite fission track data indicate that the initiation ofrift flank uplift, erosion and plate deformation occurred nearly simultaneously.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Phanerozoic; Fission track thermochronology; Palaeostress; Arabian–Nubien shield; Egypt

1. Introduction

After Pan-African consolidation, the basement com-plex of the Arabian–Nubian shield was eroded and apeneplain surface formed (Said, 1990). Regionally, twoPhanerozoic major thermotectonic episodes are known:(1) During a major Late Devonian–Early Carboniferousintracratonic event, pre-Early Carboniferous strata wereuplifted, eroded and large scale structures were formed(Bender, 1968; Bellini and Massa, 1980; Beyth, 1981;Weissbrod and Gvirtzman, 1988; Kohn et al., 1992;McGillivray and Husseini, 1992; Schandelmayer et al.,

1997). Little is known about the extent of this eventbecause Paleozoic sedimentary strata in the EasternDesert of Egypt are largely missing.(2) The second major Phanerozoic event in the his-

tory of the Arabian–Nubian shield is the opening of theRed Sea. During this period, extension was associatedwith uplift and erosion which affected the shield alonga belt parallel to the rift strike. The thermal history ofthe rift shoulders, the amount of tectonic versus ero-sion induced uplift, as well as the topographic evolutionof the rifted continental margin, were constrained bymodelling apatite fission track patterns (Kohn and Eyal,1981; Omar et al., 1987, 1989; Bohannon et al., 1989;Steckler and Omar, 1994; Omar and Steckler, 1995; vander Beek et al., 1995). Tectonic studies on Red Sea rif-ting, including geophysical (e.g. Makris and Rihm,1991) and structural work (Cochran and Martinez,1988; Ghebreab, 1998; Moustafa, 1997) highlight the

Journal of African Earth Sciences 34 (2002) 191–202

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9870.

E-mail address: [email protected] (A.-V. Bojar).

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importance of left lateral slip during early phases of riftdevelopment.The aim of this study is to put constraints on the

exhumation, erosion and the structural history of theArabian–Nubian Shield in the Eastern Desert of Egyptthroughout the Phanerozoic. To provide a more com-plete picture of these processes, we integrated informa-tion from lithostratigraphy, tectonics and fission trackdating on zircon and sphene. Few of our own and pre-viously published fission track data on apatite allow thedetermination of time relationships between single RedSea rift related deformation increments, basement ex-humation and erosion.

2. Geological setting

2.1. Neoproterozoic

In the Eastern Desert of Egypt, major portions of thePan-African orogen were formed during crustal con-solidation in the Neoproterozoic (e.g. Gass, 1982; Vail,1985; Stern, 1994). In the study area, between Safagaand Marsa Alam, gneissic domes are overthrustedby Neoproterozoic oceanic crust and arc volcanics (El

Gaby et al., 1990; Greiling et al., 1994; Fritz et al., 1996)referred as the ‘‘Pan African nappe complex’’ (Fig. 1).Neoproterozoic molasse sediments discontinuouslycover this complex (Grothaus et al., 1979; Fritz andMessner, 1999). The basement domains (gneiss domes)are exposed within metamorphic core complexes boun-ded by NW trending sinistral shear zones and relatedNE trending normal faults (Sturchio et al., 1983; Gre-iling et al., 1994; Fritz et al., 1996). They form domalstructures arranged in a NW–SE direction at low anglewith the Red Sea strike. These shear zones representzones of lithospheric weakness and are known as theNajd shear system (Stern, 1985).

2.2. Paleozoic

During the Paleozoic, the Trans-African Lineament(Fig. 1) played a major role for the facies distribution inNE Africa. To the north of TAL mostly shallow marineand/or continental sedimentary facies have been devel-oped. By contrast, only continental facies and/or largeerosion areas are known to the south of TAL (Keeley,1989; Schandelmayer et al., 1997). In the Late Ordovi-cian, polar glaciers expanded across Gondwana andcovered also western Arabia. In the Arabian–Nubian

Fig. 1. Simplified geological map of the study area. Apatite fission track ages (A), zircon FTA (Z) and sphene FTA (S) are indicated for each sample.

192 A.-V. Bojar et al. / Journal of African Earth Sciences 34 (2002) 191–202

Shield (north-western Desert, Sinai and Arabia) theSilurian post-glacial transgression is characterised bythick shales and fine-grained marine clastics (Husseini,1991). Devonian sedimentary sequences are missing orare represented by continental deposits.Structural, sedimentological and geochronological

data indicate a major Late Devonian–Early Carbonif-erous uplift and erosion in the Sinai, Israel, Jordan andGulf of Suez areas (Bender, 1968; Beyth, 1981; Gvirtz-man and Weissbrod, 1984; Weissbrod and Gvirtzman,1988; Kohn et al., 1992; Kohn et al., 1997), in the northAfrica (Bellini and Massa, 1980; Keeley, 1989), andArabia (Husseini, 1991; McGillivray and Husseini,1992). During this event, strata older than Early Car-boniferous were tilted and large scale structures, as forexample the Helez anticline (Weissbrod and Gvirtzman,1988) or the Wadi Birk-Haradh syncline–anticline(McGillivray and Husseini, 1992) were formed. Thesestructures are interpreted to reflect the regional intracr-atonic deformation, uplift and erosion that were initiatedduring the Late Devonian. The event, known as the‘‘Hercynian orogeny’’, followed relatively stable condi-tions after the Pan-African orogeny. There are twopossible mechanisms that could have driven the LateDevonian event: (a) regional compression released bythe Paleo-Tethyan oceanic crust at c. 390 Ma duringchange from a passive to an active continental margin(McGillivray and Husseini, 1992) and/or (b) regionalcompression due to the contact between Gondwanaand Larrusia which probably occurred in southernIberia and Morocco during the Famennian (Ziegler,1989). Uplift continued in Early Carboniferous duringprogressive Laurussia/Gondwana continent–continentcollision.In particular, for the study area, only few continental

Palaeozoic deposits have been described. Remnants ofthe Paleozoic cover are present between the northernWadi and Wadi Dakhal (both Wadis are situatednorthward of Safaga). These remnants are representedby Early-Middle Carboniferous fluvial to deltaic de-posits (Klitzsch et al., 1990) with a total thickness of c.200 m.

2.3. Mesozoic and tertiary

Between Safaga and Marsa Alam, the oldest Meso-zoic deposits are Albian to Cenomanian nonmarineterrigenous sequences which are known as the Nubianbeds (Ward and McDonald, 1979). These beds lie on thePaleozoic strata or directly on the Neoproterozoicbasement. Starting with the Cenomanian the sea ad-vanced gradually from north to the south, crossing theQuena––Hurgada hinge line during Turonian (Klitzschet al., 1990). In the study area the Cretaceous marinedeposits are of Campanian to Maastrichtian age (DuwiFormation) (Fig. 2).

Paleocene and Eocene sediments, deposited in marinefacies conditions, are represented by shales (DaklaFormation); marls, marly limestones and shales (Tara-wan and Esna Formation), and limestone with flintbands and chert concretions (Thebes Formation). TheCretaceous and younger deposits belong to the coastalwedge of the Neo-Tethyan passive margin. They form asouthward thinning wedge from the Eastern Mediter-ranean basin to the northern Red Sea.The Oligocene deposits reflect shift from shallow

marine to continental environment and are representedby the Nakail Formation. They consists of lacustrinesediments interbedded with coarse breccia with pebblesderived from Eocene and Cretaceous sediments. Thesedeposits evolved along Red Sea rift related escarpmentssuggest an early phase of erosion and uplift in the Oli-gocene. These processes continued during depositionof Ranga Formation (Early Miocene). Besides thesedimentary components, the Ranga conglomerate alsocontains Neoproterozoic basement clasts suggestingongoing exhumation and erosion from the Neoprote-rozoic basement.Middle Miocene to Pliocene sediments, mostly depos-

ited in a shallow marine environment, are represented by

Fig. 2. Stratigraphy of the Mesozoic–Cenozoic sediments between

Safaga and Marsa Alam (after Said, 1990).

A.-V. Bojar et al. / Journal of African Earth Sciences 34 (2002) 191–202 193

the ‘‘Evaporite’’ Formation (Langhian–Late Miocene)and the ‘‘Post-Evaporite’’ Formation (Late Miocene–Pliocene) (Said, 1990).In conclusion, Cretaceous-Neogene sedimentation in

the central Eastern Desert can be divided into: (a) pre-rift stage when the Pan-African basement was coveredby platform sediments of Upper Cretaceous to Eoceneages. Prior to rifting, the Gulf of Suez and northern RedSea were shallow water marine platforms. (b) The syn-rift stage initiated in the Oligocene with uplift and ero-sion related to the onset of rift related extension. Novolcanic rocks related to extension are known in thisarea. (c) The post-rift stage is characterized by coastaldeposits (Fig. 2).The topographic relief along the Red Sea coast is

related to extension, tectonic uplift and progressiveerosion (Omar and Steckler, 1995; van der Beek et al.,1995). The actual preserved surface geology exposesprimarily the Neoproterozoic crystalline basementflanked eastward and westward by sequences that rangefrom Cretaceous to Tertiary (Fig. 1).Two major tectonic elements played a major role in

Red Sea opening. The NW trending Najd Fault Systemrepresents a zone of lithospheric weakness that devel-oped in Neoproterozoic times (Stern, 1985). This pre-existing structure acted as a stress guide and controlledthe initial geometry of the rift line (Dixon et al., 1987).The initial Red Sea is interpreted to have formed asa narrow discontinuous graben since the Cretaceous(Makris and Rihm, 1991). Change from shear controlledearly rift formation to north-eastward extension oc-curred in the early Miocene and may be related to theonset of sinistral strike slip movement in the Gulf ofAquaba. North-eastward extension was accompanied byNW trending normal faults and block tilting (e.g. Favreand Stampfli, 1992; Bosworth, 1994). Ongoing extensionis evident from normal faults that offset coastal sedi-ments along the Red Sea coast, as well as geophysicaldata (e.g. Bosworth and Strecker, 1997).In contrast to the Cretaceous to recent rift related

structures, no direct access to Paleozoic structural ele-ments is available since no Paleozoic strata are exposedin the study area. Indirect evidence arouse from thePaleozoic sedimentary facies distribution as describedabove.

3. Methods

3.1. Fission track dating

This method is based on natural fission decay of 238Uin minerals. During the decay process two heavy nucleiare released. The nuclei traverse the mineral lattice inopposite directions, damaging the crystalline structureand leaving fission tracks behind. The dating is based on

the determination of fission track density on polishedsurface, under an optical microscope after controlledchemical etching (Fleischer et al., 1964; Wagner andReimer, 1972; Naeser, 1979; Gleadow, 1981; Green,1985; Green et al., 1989).Sphene, zircon and apatite were separated using

standard crushing, heavy liquid and magnetic methods.Apatite and sphenes were mounted in epoxy resin andetched at 20 �C in HNO3 (7% solution) for 35–40 s, andin a mixed HNO3–HCl–HF solution for 20–30 min(Gleadow, 1978), respectively. Zircon crystals weremounted in PFA teflon and etched at 220 �C in aNaOH–KOH eutectic melt (Gleadow et al., 1976).Determination of fission track ages (FTA) was made

using the external detector method, samples were irra-diated together with mineral standards and CN glassesat the reactor of the ‘‘Atominstitut der €OOsterreichischenUniversit€aaten’’, Vienna. The mineral standards usedwere: (a) zircons from the Fish Canyon tuff and Tardeerhyolite; (b) sphenes from the Mt. Dromedary igneouscomplex; (c) apatites from Fish Canyon tuff and fromDurango. Muscovite solid state detectors were etched at20 �C for 30 min in 48% HF to reveal induced tracks.Tracks were counted under transmitted light using a dry�100 objective at a total magnification of �2000. Ageswere calculated using the standard FTA equation(Hurford and Green, 1982) and the zeta calibrationrecommended by Hurford (1990). Errors were calcu-lated using the method of Green (1981) and are ex-pressed at one standard deviation level.In comparison with other dating methods, for which

a closing temperature is defined, fission tracks aremetastable in a temperature interval known as the par-tial annealing zone (PAZ). Literature data report: (a)zircon PAZ between 160 and 225 �C, for cooling rates ofc. 1 �C/Ma (Zaun and Wagner, 1985) and (b) sphenePAZ between 260 and 320 �C for cooling rates of 10 �C/Ma (Harrison and McDougall, 1980). More recent datasupport higher values for zircon, with PAZ limits be-tween 210 and 310 �C for cooling rates of 10 �C/Ma(Yamada et al., 1995; Tagami and Dumitru, 1996).Moreover comparative studies have shown that spheneFTA are usually older or concordant with zircon FTA,the base of the sphene PAZ being slightly higher thanthat for zircon, and are concordant with the closingtemperature of the K–Ar on biotite (Fitzgerald andGleadow, 1988; Andriessen and Reuter, 1994). For ap-atite, the PAZ limits are well documented and are be-tween 60 and 120 �C (Naeser and Faul, 1969; Gleadowand Duddy, 1981; Green et al., 1989).

3.2. Paleostress data

In order to constrain directions of principal stresses,we analysed fault planes and slickenside striations


Table 1

Fission track analytical data

Sample Rock type Mineral/

Number of

crystals

Dosimeter qd(�106 cm�2) (Nd)

Spontaneous track

density qs(�106 cm�2) (Ns)

Induced track

density qi(�106 cm�2) (Ni)

Pooled

Age� 1r(Ma)

Proba-bi-

lity PðX 2Þ(%)

U (ppm) Mean track

length (lm)Standard de-

viation (lm)

SNM3 Amphibolite S/10 0.961 (3488) 0.892 (278) 0.436 (136) 339� 37 76 4

SNM96 Amphibolite S/16 0.961 (3488) 1.715 (726) 0.784 (332) 362� 27 99 8

SED85 Amphibolite S/16 0.676 (2864) 3.014 (892) 0.966 (286) 372� 28 6 13

SAB280 Amphibolite S/6 0.961 (3488) 2.984 (269) 1.298 (117) 380� 44 52 12

SWK9 Granite S/7 0.961 (3488) 12.274 (794) 4.839 (313) 418� 31 46 47

SED97 Gneiss S/12 0.448 (5428) 6.344 (715) 1.287 (145) 380� 37 77 26

ZMPJ239 (�) Amphibolite Z/8 0.961 (3533) 15.609 (979) 1.91 (120) 363� 43 99 66

ZED85 (�) Amphibolite Z/8 0.961 (3533) 26.25 (926) 4.56 (161) 342� 38 16 117

ZMMS7 Sandstone Z/7 1.255 (5316) 12.183 (788) 2.355 (151) 366� 42 65 51

ZWK16 Granite Z/8 1.255 (5316) 14.662 (740) 3.15 (159) 327� 37 95 69

ZWK9 Amphibolite Z/7 1.255 (5316) 15.004 (794) 3.175 (168) 332� 37 65 70

ZMB307 Gneiss Z/7 1.255 (5316) 17.650 (787) 3.386 (151) 366� 42 81 74

ZNM90 Amphibolite Z/17 1.255 (5316) 9.918 (1419) 2.034 (291) 343� 33 44 44

ZMPJ242 Gneiss Z/20 1.255 (5316) 9.350 (2781) 2.088 (621) 315� 27 92 45

AGML82b Gneiss A/20 0.676 (2864) 0.101 (109) 0.552 (593) 23� 2 98 6 12:83� 0:52ð16Þ 2.1

AED85 Amphibolite A/15 0.676 (2864) 0.072 (53) 0.243 (179) 36� 6 94 3 11:67� 0:63ð12Þ 2.2

q is track density (�106 tracks cm�2); N is number of tracks counted; s, i and d denote, spontaneous, induced tracks and tracks in the fluence monitor glass, respectively. Samples were dated using the

external detector method (Hurford and Green, 1983). Age determinations: (a) for zircon (Z) using f ¼ 122� 4 and CN2 for the ages followed by (�), and f ¼ 115� 8 and CN1; (b) for sphene (S)using f ¼ 354� 10 and CN5; (c) for apatite (A) using f ¼ 357� 10 and CN5.

A.-V

.Bojaret

al./JournalofAfrica

nEarth

Scien

ces34(2002)191–202

195

(commonly known as paleostress analyses), tension ga-shes and offset of lithological boundaries at regional andoutcrop-scale. Data on fault planes and relative move-ment between faulted blocks are displayed in stereonetdiagrams. Inferred orientation of principal stresses wasobtained by the ‘‘right dihedra technique’’, as first de-scribed by Angelier and Mechler (1977). Data have beenevaluated using the computer program GEFUEGE ofWallbrecher and Unzog (2000). To obtain a progressiveevolution on stress directions, special attention was paidby collecting data from different stratigraphic sequences(i.e., to resolve overprinted stress regimes). Further-more, regional variation of stress fields have been ob-tained by the systematic analysis of outcrops from theRed Sea coast towards the western hinterland.

4. Data presentation and discussion

4.1. Paleozoic thermotectonic event

Because the Phanerozoic sedimentary cover is largelymissing, the thermotectonic history after the Pan-Afri-

can event has been constrained by fission track data onzircon and sphene. Sample locations and individualFTA are displayed in Fig. 1 (for analytical data seeTable 1). Sphene FTA range from 339 to 410 Ma whilezircon FTA range from 315 to 366 Ma. Despite the largenumber of samples prepared, only two samples hadapatite with 36 and 23 Ma. The time scale used to relatethe FT data to the stratigraphic age is that of Gradsteinand Ogg (1996).At this stage it is important to remember that in the

immediate vicinity of the study area, Early Carbonifer-ous sedimentary strata lies directly over the Neoprote-rozoic basement. Thus, immediately prior to depositionof the Lower Carboniferous the region was near theearth’s surface at temperatures less than 100 �C. Asdiscussed before structural and sedimentological dataindicate a ‘‘Hercynian event’’ within the Arabian–Nu-bian shield, although the timing of this event is poorlyconstrained (Schandelmayer et al., 1997). In the Sinaiand Gulf of Suez areas geochronological evidence in-cludes FTA on zircon, sphene and apatite and 40Ar/39Ardata on K-feldspars. FTA on zircon and sphene fromthe Neoproterozoic basement and Permian/Mesozoic

Fig. 3. (a) Comparison of the fission track data from the Easter Desert with data by Kohn et al. (1992) from the Sinai. (b) Distribution of confined

length measurements in apatite. (c) Plot of mean track length versus FT ages. The sample symbols are as follows: (open squares) samples from Omar

et al. (1987, 1989) for the western flank of Gulf of Suez and northern Red Sea; (closed squares) our own samples.


sediments of Sinai support a Paleozoic thermotectonicepisode (Kohn et al., 1992, 1997 ) (Fig. 3a). Apatite fromthe Gulf of Suez and northern Red Sea have been re-ported by Omar et al. (1987, 1989), Steckler and Omar(1994), Omar and Steckler (1995), and Kohn et al.(1997). Although the data show a much younger thermalhistory we emphasize that only few pooled apatite agesare older than 350 Ma (Fig. 3c). The data are best in-terpreted as Late Devonian–Early Caboniferous reset-ting of the early thermal history recorded in apatite.Moreover, 40Ar/39Ar on K-feldspar from the crystallinebasement of the Suez Golf suggest partial argon lossduring a heating event at �350–330 Ma (Kohn et al.,

1997). Recent 40Ar/39Ar data on hornblende and micaseparated from the same area as our zircon samplesshow argon loss in low temperature heating experi-ments. These data are interpreted as reheating around400 Ma (Fritz et al., 2002).Because in our study area the maximum temperatures

since the Carboniferous has not exceeded �150 �C for aheating duration of c. 106–107 years, the zircon data areconsidered to record a pre-Carboniferous cooling event.The question remains whether the ages represent only aperiod of accelerated erosion or a period of high thermalflux followed by cooling and erosion. Considering thebase of the PAZ for zircon of c. 300 �C and a normal

Fig. 4. Late Creaceous strike slip tectonics in the study area resolved by orientation of fault planes and fault plane solution data (left row). Vein

orientations (right row) indicate both NW–SE and later NE–SW extension. Age relation between sets are based on overprint criteria. Note the en-

echelon alignment of basin axes.


geothermal gradient between 20 and 30 �C, our datasuggest that the actual section was buried at depth of c.10–15 km before Late Devonian–Early Carboniferouserosion. Subsequently rocks cooled down below 100 �Cby removal of a c. 6–10 km thick sedimentary pile.Another scenario is that the fission track data indicate ahigh thermal gradient and a thermal peak predatingcooling and erosion. Rocks could have been reheated inLate Devonian–Early Carboniferous times by, for ex-ample, magmatic activity to c. 300 �C for short time (c.106 years) and subsequently could have experiencedcooling below c. 100 �C. Unfortunately no structuraldata are available to relate FTA with tectonics because

no Paleozoic sedimentary cover is exposed within thestudy area.

4.2. Late cretaceous-neogene opening of the red sea rift

Early formed Duwi and Umm Gheig basins recordthe sedimentation history from Late Cretaceous up tothe Miocene, whereas sedimentation along the Red Seacoastal plain continued to recent times. During the de-position of the Nakeil and Ranga formations withinDuwi and Umm Gheig basins (Fig. 4), the early sedi-ments of Upper Cretaceous to Eocene age as well as theNeoproterozoic basement were progressively uplifted

Fig. 5. (a) Fault planes, striations and veins for the younger set of brittle structures in the study area. Fault plane solution, as well as vein orientation,

indicate SW–NE extension during Red Sea opening. (b) Step, normal faults within Eocene sediments northwest of Quseir.


and eroded, as seen from pebble spectra in conglomer-ates. Progressive uplift in Oligocene and Miocene is alsosupported by the already published apatite ages fromthe Neoproterozoic basement. Our own data (Fig. 3b;Table 1) has been plotted together with the alreadypublished results (Omar et al., 1987, 1989) from thewestern flank of Suez Golf and the Northern Red Sea.The pattern of age versus track length are interpreted toindicate that the samples have been exhumed from abroad range of depths. The young ages with high meantrack length (to the left in Fig. 3c) indicate almost totalannealing prior to uplift and erosion. Ages of thesesamples indicate approximately the time of major riftrelated cooling between 21 and 25 Ma for the Gulf ofSuez and northern Red Sea (Omar and Steckler, 1995)and support two distinct Miocene and Oligocene un-

roofing episode. This is also suggested from structuraldata as discussed later.Length distributions of the two measured samples

show values below 12–13 lm (Fig. 3b). A rapid coolinghistory between 60 and 120 �C can be excluded becausefast cooling rocks never have tracks less than 12–13 lm(Andriessen, 1995). However, due to low U content ofapatite and young ages, only few confined tracks havebeen found. The age of 23 Ma with track length around12.8 lm is interpreted to date approximately the onset ofMiocene cooling. The older apatite age of 36 Ma withlower mean track length of 11.7 lm belong to thecooling path that had already started in the Oligocene(Fig. 3c). This cooling trend is discussed by Omar andSteckler (1995) and corresponds to a period of rapidunroofing during the Early Miocene.

Fig. 6. Possible Phanerozoic time-temperature path for the Pan-African basement in the Eastern Desert of the Egypt. Major cooling and erosional

phases occurred in Carboniferous and Oligcene/Miocene times. Two possible cooling scenarios may be assumed from Cambrian to Early Carbo-

niferous (paths a, b).

Fig. 7. Summary of the regional stress field rotation in the Red Sea rift system. (a) Upper Cretaceous: strike-slip regime with evolution of en-echelon

oriented pull-apart basins; (b) NE–SW extension and opening of the Red Sea.


As indicated by the apatite FTA, prior to uplift anddeposition of the Nakail Formation, some of the base-ment blocks were still buried at temperatures exceeding120 �C. This implies that for a geothermal gradient of20 �C (Morgan et al., 1985), a section of at least 5 kmwas eroded since the onset of Miocene extension (con-sidering a present day surface temperature of c. 20 �C).The eroded section comprises pre-rift sediments andbasement itself, as indicated for example by crystallinebasement pebbles within the Ranga Formation. Similarvalues for unroofing were obtained by Kohn and Eyal(1981) in the Sinai region, Omar et al. (1987, 1989) andOmar and Steckler (1995) in the Eastern Desert. Thusthe regional lithostratigraphic data, as well as ourown, and already published FT ages indicate that thefirst pulse of uplift and erosion had already begun inOligocene.In contrast to the Paleozoic cooling phase, the

thermal history related to Red Sea opening can belinked to structural data. Field and paleostress datasuggest a two stage tectonic evolution:(1) The basins next to Quseir are en-echelon oriented

supporting early basin formation in a sinistral strike slipregime (Fig. 4). Timing of strike slip deformation isevident from observations at outcrop scale along amajor shear zone at the western margin of the DuwiBasin. Here sheared conglomerates from the NubiaFormation have been incorporated into a major verticalsinistral shear zone including vertical foliation andhorizontal lineation. These rocks are discordantlyoverlain by Upper Cretaceous and Paleocene sedi-ments (Fig. 4). This suggests that in the study area, theLate Cretaceous basins nucleated as small pull-apartbasins by reactivating the Najd lineament. For thisstage, principal stress directions with sub-horizontalr1 (ENE/WSW) and r3 (NNW/SSE), and a sub-verticalr2 have been found from paleostress analyses usingfault plane solution techniques and orientation oftension gashes. These results are in accordance withDixon et al. (1987), who suggested that around the RedSea, the formation of en echelon oriented basins wascontrolled by Pre-cambrian structures that acted asstress guides. Moreover, from other regions such asSudan there is evidence for growth of NW trendingbasins during the Cretaceous (Almond, 1986), andfor reactivation of the Najd lineament (Sultan et al.,1988).(2) Flank uplift with onset in Oligocene was accom-

panied by changed stress field with NE–SW extension(r3) and sub-vertical r1 (Fig. 5a and b). Extensionaltectonic occurs within the Duwi Formation and youngersediments. Conjugate sets of faults, already describedby Moustafa (1997), and north-eastward dipping highangle normal faults occur preferentially in sedimentswest of the Red Sea coast (Fig. 7b). These faults are partof a horst-and-graben tectonics; however, a basal de-

tachment zone related to large scale extension is notexposed in the study area.

5. Conclusions

At the end of the Neoproterozoic, the Arabian–Nu-bian shield was consolidated and basement rocks cooledclosed to surface temperature conditions as indicated byoccurrences of late Neoproterozoic molasses type sedi-ments. FTA of zircon were completely reset during theLate Paleozoic; some of the sphene were only partiallyreset. This indicates that temperatures predating theonset of cooling and erosion were around 300 �C. ForLate Paleozoic cooling history we suggest that (Fig. 6):(1) Basement rocks cooled after Neoproterozoic

orogeny were later re-heated for a short period duringLate Devonian–Early Carboniferous times, followed byrock exhumation and cooling.(2) Basement rocks had been covered by a sedimen-

tary pile of 10–15 km that eroded during Late Devonian.We can relate the Late Devonian–Early Carbonifer-

ous event to regional compression released by the Paleo-Tethyan oceanic crust during change from passive toactive continental margin followed by the first contactbetween Gondwana and Laurussia which probably oc-curred during the Famennian. The clockwise rotationalconvergence between Gondwana and Laurussia, con-tinued during Early Carboniferous times. The study alsosuggests that the Paleozoic event is much more wide-spread than previously known. After this phase theshield remained under relatively stable conditions untilthe onset of Red Sea rifting.Zircon and sphene fission track data suggest that

from Late Devonian–Early Carboniferous to the presenttime, a maximum of 15 km of overburden was strippedoff. From this overburden, approximately 5 km waseroded between the Oligocene and present. The olderperiod of deformation was related to strike slip defor-mation during Late Cretaceous. At that time en-echelonoriented basin formed, following pre-existing litho-spheric structures. The second exhumation period goesalong with NE–SW extension, normal faulting as well assedimentation of syn-rift deposits. The NE–SW exten-sion corresponds with the onset of uplift cooling anderosion along the rift flanks.

Acknowledgements

E. Wallbrecher (Graz) is thanked for interesting dis-cussions concerning the geology of the Nubia basement.This paper has also benefited from constructive criticismby B. Kohn (Melbourne University) and E. Willing-shofer (Vrije University). Angela Poli is thanked as wellfor general text revision. The ‘‘Atominstitut der €OOster-


reichischen Universit€aaten’’ is kindly thanked for helpingwith sample irradiation. This study was supported byFWF project P09703-Geo and by FWF project P13029-Geo.

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