Tectonophysics, 140 (1987) 233-246

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

233

Proterozoic deformation of the East Saharan Craton in Southeast Libya, South Egypt and North Sudan

H. SCHANDELMEIER, A. RICHTER and U. HARMS

SFB 69, TU Berlin, Ackerstrasse 71, loo0 Berlin 65 (F.R. of Germany)

(Received October 1,1986; revised version accepted December 17,1986)

Abstract

Schandelmeier, H., Richter A. and Harms, U., 1987. Proterozoic deformation of the East Saharan Craton in Southeast

Libya, South Egypt and North Sudan. Tectonophysics, 140: 233-246.

The basement areas in Southeast Libya, South Egypt and North Sudan, west of the Nile, between Gebel Uweinat

and the Bayuda Desert, are part of an approximately 1000~km-wide, complexly folded, polymetamorphic zone with a

regional N-NNE-NE-ENE trend of foliation and fold axis. Since this belt extends southwestward into the area of

Zalingei in the southern Darfur block (West Sudan), it is named the Northern Zalingei fold zone. Sr and Nd isotopic

studies suggest that this zone is older than Pan-African and further indicate that, apart from Archean rocks in the

Gebel Uweinat area, this belt is of Early-Middle Proterozoic age. An Early-Middle Proterozoic three-stage deforma-

tional and anatectic event established the present-day fold and fault geometry in the western parts of this zone in the

Gebel Uweinat-Gebel Kamil area. The Pan-African tectono-thermal episode was most effective in the eastern part of

the belt, near the boundary with the Nubian Shield volcano-sedimentary-ophiolite-granitoid assemblages. It caused

migmatization, granite emplacement, mylonitization and large-scale wrench faulting which was related to Late

Proterozoic accretionary and collisional events of the Arabian-Nub&r Shield with the margin of the East Saharan

Craton.

Introduction

Northeast Africa and Arabia are dissected by a number of transcontinental shear zones (Fig. 1). The Tram+Africa Lineament, which extends from the Niger delta in Nigeria to the Nile delta in Egypt, was interpreted by Nagy et al. (1976) to represent probably a Pan-African paleocontinen- tial margin. Calculations of lithospheric thickness for the whole East Saharan Craton (Fairhead and Reeves, 1977) oppose this view. It is more likely that this zone is a Pan-African megashear, related to compressive stress that arose during collision of the East Saharan Craton with the West African Craton (KrSner, 1979). The Central African Lin- eament (Browne and Fairhead, 1983) extends from the Cameroon volcanic line to immediately south

of the Darfur block in West Sudan, and initially formed as a Late Proterozoic dextral megashear (De Almeida and Black, 1967). A marked struct- ural line, occupied by Cenozoic volcanics, strikes from Gebel Marra in the Darfur block northwest- ward into the Central Bayuda Desert (Fig. 1). The Najd wrench fault system of Saudi Arabia is a well-known Late Proterozoic wrench fault system (Davies, 1984).

The Late proterozoic fault systems west of the Nile cut various basement areas of the East Saharan Craton that are characterized by liigh- grade polymetamorphic and granitoid assemb- lages. The largest of these is the Gebel Uweinat-Gebel Kamil basement complex, which is situated at the convergence of the frontiers of Southeast Libya, Southwest Egypt and Northwest

0040-1951/87/$03.50 0 1987 Elsevier Science Publishers B.V.

234

Major structural trends

NE Africa and Arabia

Nubian Desert

Fig. 1. Major Late Proterozoic transcontinental shear zones of northeast Africa and Arabia.

Sudan. Smaller areas in South Egypt and in North Sudan are the Bir Safsaf-Aswan Uplift System (Bernau et al., 1987), the Nukheila-Wadi Howar basement areas north of Wadi Howar, and the Nubian Desert basement area west of the Nile (Fig. 2). All of these areas comprise parts of the pre-Pan-African East Saharan Craton (Schan- delmeier et al., 1987), onto which the volcano- sedimentary-ophiolite and related granitoid as- semblages of the Arabian-Nubian Shield were accreted in the Late Proterozoic (Krbner, 1985; Vail, 1985, 1987).

It is the aim of this paper to show, on a regional scale, the time-space relationships of Pro- terozoic deformational periods that, additive in their consequences, created the present-day fold and fault pattern of the East Saharan Craton in Southeast Libya, South Egypt and North Sudan.

Lithology and rn~~o~isrn

The lithology and metamorphic history of the basement gneisses, as well as the lithological and geochernical characteristics of the intrusive rocks of all areas, are described in various publications (Meinhold, 1979; Klerkx, 1980; Schandelmeier et al., 1983; Sch~delmeier and Darbyshire, 1984; Huth et al., 1984; Richter, 1986; Bemau et al., 1987). They are summarized briefly in Tables 1 and 2 and the rock types are listed in their sup- posed chronostratigraphic sequence. Although the lithological assemblages of the metamorphic base- ment of the Bir Safsaf-Aswan Uplift, Wadi Howar and Nubian Desert areas are similar to those described from the Gebel Uweinat-Gebel Kamil area (Klerkx, 1980; Richter, 1986), it is worth stressing at this point that correlation is avoided because the basement units are separated from each other by distances of several hundreds of

235

Fig. 2. Basement areas of the East Saharan Craton (crosses)

and of the Nubian Shield assemblages (stippled) in South

Egypt and North Sudan. Thin lines are directions of metamor-

phic foliations. Solid lines indicate the regional NE-E trend of

the Northern Zalingei fold zone. G&f--Gebel El Asr area:

GUS -Gebel Umm Shaghir area.

kilometers (Fig. 2) and insufficient radiometric age data are available to justify correlation at present. Nevertheless, petrological, structural and metamorphic data are similar in all of the base- ment areas, and indicate that the rocks might have been subjected to the same tectono-magmatic epi- sodes.

Regional structural trends

The first attempt to differentiate the basement of South Egypt and North Sudan and its southern continuation into tectonic domains was by Vail (1976), who recognized, mainly from satellite- image interpretation, a fold belt which strikes in a NE-SW direction through Darfur (West Sudan) into North Sudan. This belt was named the “Zalingei folded zone”, after the town of Zalingei which is located in the southern Darfur block. In Fig. 3, this fold belt is shown to be the continua- tion of the Kibaran belt of Central Africa which

Fig. 3. Tectonic domains in northeast Africa as interpreted by

Vail (1976) and Shackleton (1979).

swings clockwise into Darfur and northern Sudan (Vail, 1976). In the northwest, this belt is bordered by a postulated Eburnean Craton and in the southeast by the so-called Sudan Craton of un- known age (Vail, 1976; Shackleton, 1979). How- ever, it will be shown below that all of the base- ment areas of the East Saharan Craton between Gebel Uweinat and Bayuda Desert (Fig. 2) are characterized by regional N-NNE-NE-ENE trending foliations and fold axis in the metamor- phic basement.

In the Gebel Uweinat-Gebel Kamil area, the main trend of fold axis is NE-SW, but locally this trend swings to NNW-SSE or E-W. The folds are generally tight to isoclinal. On the southeastern slopes of Gebel Uweinat, recumbent folds were observed. The dip of the foliation planes is gener- ally moderately steep to northwest, but varies sometimes owing to superimposed folding. A re- gional southeasterly vergence of folding is indi- cated by the main dip of axial planes. The super- imposed folding led to sigmoidal bending of megafold structures, the most impressive struct- ural features of the Gebel Uweinat-Gebel Kamil basement, conspicuous even on Landsat images.

The regional structure of the basement of the Bir Safsaf-Aswan Uplift is less well-known. The general trend of the foliation in the ~~atitic

236

TABLE 1

Lithology and metamorphic characterization of metamorphic basement rocks of Southeast Libya, Southwest Egypt and Northwest

Sudan

Rock unit Location Lithology Metamorphism

Gebel Uweinnt area (Klerkx, 1980)

Karkur Murr Series SE of Gebel Uweinat

ring complex (Libya)

biotite gneisses, diopside-

hornblende gneisses, meta-

quartzites, noritic to granulitic

gneisses, garnet-granulitic

gneisses, talc-silicates

gram&e facies

I1 Pass0 mylonites SE of Gebel Uweinat

ring complex (Libya)

blastomylonites grant&e facies

Ayn Daw Series SE of Gebel Uweinat

ring complex (Libya)

granitic gneisses, often mig-

matitic, amphibolites, diop-

side-hornblende gneisses,

anatectic granitoids

migmatization under higher

amphibolite facies

Gebel Kamil area (Schandelmeier et al., 1983; Richter, 1986)

Granoblastite

Formation

western part of Gebel

Kamil area, east of

Gebel Uweinat, sur-

roundings of G. Kissu

gneissic granulites and

granoblastites, meta-

quartzites, serpentinites,

amphibolites, pyroxenites,

metagabbro norites.

M/HP-HT granulite facies

(- 800°C, - 8.5 kbar)

retrograde amphibolite facies

retrograde greenschist facies

Anatexite

Formation

eastern part of Gebel

Karnil area, Peneplain

area

migmatic gneisses, meta-

texites, nebulitic diatexi-

tes, marbles, talc-silicates,

amphibolites.

LP-HT granulite facies

(700 o-8OO o C, - 6 kbar)

retrograde anatexis

retrograde greenschist facies

Metasedimentary NW Sudan hornblende gneisses, bio- almandine amphibolite to

Formation tite gneisses, etc. greenschist facies

Bir Safsaf-Aswan uplift system, Nubian Desert, Nukheila and Wadi Howar area.v (Huth et al., 1984; Bernau et al., 1986)

Migmatic gneisses

and migmatites

all areas in S

Egypt and N

Sudan

granitic to tonalitic

gneisses, often mig-

matized, amphibolites,

marbles, talc-silicates

granulite facies

(800’-1000°C, 8-9 kbar)

amphibolite facies

(650 O-700 o C, - 6 kbar)

retrogressive greenschist facies

gneisses and migmatites is F-W to WNW-ESE

with strongly varying dips. Deviations from these

trends are common due to intrusive plutons, late-

stage doming and fracturing. Mylonites, which are

abundant in the Gebel El Asr and Gebel Umm

Shaghir areas, strike generally also E-W to

WNW-ESE and the mylonitization postdates

migmatization in these areas (Bemau et al., 1987).

The Nubian Desert area, west of the Nile (Fig.

2), is made up in its western part of grey granitic

gneisses with intercalated metasediments. The

gneisses are isoclinally folded along N-S-trending

fold axis and dip steeply to the east. The central

and eastern parts of this area are occupied by a

synclinal structure, more than 30-km wide, with a

N-S-striking fold axis. The western limb dips

steeply to the east, while the eastern limb dips

moderately to the west; thus, the axial plane prob-

ably has a vergence to the west.

The eastern and central parts of the Wadi

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TABLE 2

Lithology, chemical classification and possible structural setting of intrusive rocks of Southeast Libya, Southwest Egypt and

Northwest Sudan

Rock unit Location Lithology Chemical classification Structural setting

Gebei Uweinat-Gebel iamil areas (Richter, in press)

grey-green grani-

toids

Peneplain area, Gebel Kamil area,

ring-structure com- plex (NW Sudan)

diorites to trondhjemites

pre- to syntectonic

red granites ring-structure com-

plex (NW Sudan), Gebel Kamil area

rapakivi granites, alkali granites

syn- to posttectonic

Bir Safsaf-Aswan uplift system, Nubian Desert and Wadi Howar areas (Huth et al., 1984; Bernau et al., 1986)

leucogranites Gebel Umm Shaghir, granites S-type syncollisionaf

Nubian Desert, Wadi Howar areas

granodiorites Bir Safsaf, Nubian Desert; Wadi Howar areas

granodiorite to

tot-halite

Caledonian I-type postcollisional

biotite granites Bir Safsaf, Nubian Desert areas

biotite granite Caledonian I-type postcollisional

Howar area (north of Wadi Howar) are occupied by extensive, mostly unfoliated granite and struct- urally diffuse migmatites (Huth et al., 1984). Only limited areas are underlain by well-foliated rocks. Fold geometry is difficult to reconstruct, but a preliminary analysis of structures shows a regional NE-SW trend of the gneissic layers (A. Huth, pers. comrnun., 1986). In the western part of the Wadi Howar area, the basement is overlain by folded, mostly weakly metamorphosed metasedi- ments. The stratigraphic position of this structural unit is not yet known and the regional foliation trends in a NE-SW direction (Fig. 2) refer only to the older basement, which is often exposed in the deeply eroded cores of the folded cover rocks.

The basement of the Bayuda Desert was subdi- vided by Vail (1971), Meinhold (1979) and Dawoud (1980), and despite controversy on the stratigraphic interpretation of individual series (Ries et al., 1985), three major structural units can be recognized. There is a high-grade gneissic unit in the western and central parts, which is the oldest (Harris et al., 1984), an amphibolite facies

metasedimentary unit in the eastern part of the area, and a greenschist metavolcanic series which occurs in a narrow strip along both sides of the Nile. In Figs. 3 and 4, the approximate boundary between East Saharan Craton and Nubian Shield assemblages are shown in a way which includes the metasedimentary group within the Nubian Shield, because these rocks are certainly younger than the high-grade gneiss unit (Harris et al., 1984). This contrasts with the approach of Vail (1987), who restricted the Arabian-Nubian Shield strictly to the volcano-sedimentary-ophiolite- granitoid basement. The regional structural trend in the older gneiss unit is NE-SW to NNE-SSW and the fold axis plunge mainly to the southwest, whereas the axial planes of these isoclinally folded series dip to the northwest (Meinhold, 1979).

Sr and Nd isotopic evidence for a major tectonic event in the Early-Middle Proterozoic

From Rb/Sr whole-rock isochrons (Klerkx and Deutsch, 1977) and Rb/Sr model age calculations

238

Gebd Kamll

Fig. 4. Rb/Sr whole-rock ages and model Nd ages from basement rocks of Southeast Libya, South Egypt and North Sudan.

A-Archeau Rb/Sr whole-rock age; r-Archean model Nd age; O-Middle Proterozoic Rb/Sr whole rock age; @-Late Proterozoic

(Pan-African) Rb/Sr wholerock ages and model Nd ages ranging from 2300 to 1800 Ma; +-Late Proterozoic (Pan-African) Rb/Sr

whole-rock ages and model Nd ages ranging from 1700 to 1200 Ma. Data from Meinbold (1979), Cahen et al. (1984), Harris et af.

(1984) and Schandelmeier et al. (1987). Rocks from the Nubian Shield have Positive cNd values; rocks from the East Saharan Craton have negative cNd values.

obtained from gram&tic gneisses from the Gebel Uweinat area (Table l), Cahen et al. (1984) con- cluded that granulite facies metamorphism oc- curred around 2900 Ma ago. The tectonic episode which produced the N-S-striking relic folds in the Karkur Murr Series (Table 1) was probably con- temporaneous with this gram&e event. Retrogres- sive rnet~o~~srn under amphibolite facies con- ditions affected these rocks around 2650 Ma ago (Cahen et al., 1984). Nd model ages from two

samples of the grant&tic suite confirm their Archean age. Moreover, negative fNd values from these granulites at 2650 Ma indicate crustal re- working at that time (Harris et al., 1984).

This older structure is overprinted by a younger tectonic event that probably produced the present- day regional NE-SW to E-W foliation trend. The Ayn Daw Series (Table 1) of the Gebel Uweinat area is intensively folded along this direction as is the Karkur Murr Series, and the migmatization in

239

the Ayn Daw Series seems to be connected with this phase of deformation. Anatectic granitoids from this series were intruded around 1800 Ma and their Sr isotope systematics (Klerkx, 1980) indicate a derivation from older crustal material.

On the basis of detailed microfabric, mineral- paragenetic, structural and geochemical investiga- tions, Richter (1986) correlated the basement of the Gebel Kamil area (Fig. 4) with the basement of the Gebel Uweinat area in the following way: Metasedimentary Formation-no equivalent, Anatexite Formation-Ayn Daw Series, Grano- blastite Formation-Karkur Murr Series.

Nd model ages from various basement areas (Gebel El Asr migmatites, Nubian Desert gneisses, Gebel Kamil migmatites, see Fig. 4) range from 2300 to 1800 Ma (Schandehneier et al., 1987) and

suggest that this was a crustal formation event. However, their Rb/Sr wholerock isochron ages are in the range 918-670 Ma. This confirms that a Late Proterozoic event caused migmatization and resetting of the Sr isotopic system. This event involved considerable crustal reworking as indi- cated by the negative eNd values of these rocks at the time of their formation (- 18.9 to - 14.7).

A second set of rocks from the Bir Safsaf, Wadi Howar and Nubian Desert areas (S-type leucogranites and LILE-enriched granitoids; Schandelmeier et al., 1987) yielded Rb/Sr whole- rock isochron ages between 686 and 564 Ma. Their model Nd ages are in the range 1700-1200 Ma and their eNd values at time of formation are moderately negative (- 8.0 to -5.3, Schan- delmeier et al., 1987). This indicates that the Pan-

TABLE 3

Radiometric age data from all rocks shown in Fig. 4

Rock type T(Rb/Sr WR) Sr, Nd model age cNd(T) Reference

(Ma) TDM Ofa)

Nubian Desert

red granite 565 f 8 0.7065 f 2 1190 -5.3 3 Bir Safsaf

granodiorite 564i 77 0.7055 f 3 1445 - 7.2 3 Wadi Howar

microgranite 585 f 19 0.7086 f 8 1720 - 8.0 3 Bir Safsaf

orthogneiss 600” 1479 -7.4 3 Nubian Desert

microgranite 623 f 31 0.7053 f 18 1451 - 6.6 3 Gebel El Asr

migmatite 680* 1839 - 13.4 3 Gebel El Asr

migmatite 680* 2399 - 14.7 3 Gebel Kamil

migmatite 673 f 56 0.7049 * 4 2020 - 18.9 3 Wadi Howar

migmatite 686 f 26 0.7064 f 8 1520 -6.7 3 Rahaba

metasediments 874 f 33 1900-1600 -3.7, -3.0 4,2 Nubian Desert

gneiss 918 f 40 0.7167 f 40 2097 - 15.8 3 Ayn Daw ana-

tectic granitoids 1784 f 126 0.7081 f 14 1 Karkur Murr

granulites 2656k 71 0.7019 f 15 3200-3000 -6.6, -5.2 1,2

* Assumed ages. References: (1) Cahen et al. (1984), recalculated from Klerkx and Deutsch (1977), (2) Harris et al. (1984), (3)

Schandelmeier et al. (1987), and (4) Meinhold (1979)

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African event which generated these intrusives involved reworking of older crust, but the 1700-1200-Ma model Nd ages can at present not

be attached to any significant tectonic episode in that definite rock ages of this time range are not known. Isotopic studies on the Rahaba metasedi- ments from the Central Bayuda Desert (Fig. 4) reveal a similar data pattern: Rb/Sr whole-rock tges of about 870 Ma (Meinhold, 1979) and model Vd ages between 1900 and 1600 Ma with negative Nd values (-3.7 to -3.0) at time of formation

(Harris et al., 1984). U/Pb ages obtained from detrital zircons from

Late Proterozoic metasediments of the Eastern Desert of Egypt yielded ages between 2100 and 1100 Ma. These zircons are interpreted to be derived from continental areas west of the Nile, indicating the age of the crust there (Abdel Monem and Hurley, 1979; Dixon, 1981).

Based on the available data (see Table 3) the following schematic geological history for the Gebel Uweinat-Gebel Kamil area is proposed (Table 4):

TABLE 4

Inferred evolution of the Gebel Uweinat-Gebel Kamil area

Time (Ma) Event

670 Late Proterozoic (?tectono)-thermal

event, formation of migmatites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1750 anatectic event, formation of anatectic

granitoids

2000-1750 D, deformation along NE-SW to

E-W-trending fold axes, deformation

of fold axes (D,) and sigmoidal bend-

ing of metamorphic foliation

2000 partly formation of new crust ;Bsb. . . . . . .. . . . ., . . : ,. . . .

retrogressive amplubohte facles meta-

morphism (relict N-S folds)

2900 crustal reworking and gram&e facies

metamorphism, D, deformation along

N-S-trending fold axes (relict N-S

folds)

3200-3000 crustal formation event

(1) Sr and Nd isotopic data show that Archean crust is present only in the Gebel Uweinat area proper. This Archean event is structurally repre- sented by relic N-S-striking Fl folds.

(2) A Middle Proterozoic D, deformation over- printed the older structures, followed by refolding, D,, establishing the regional NE-SW trend in the Gebel Uweinat-Gebel Kamil area. This event ended with intrusion of anatectic granitoids at around 1800 Ma ago.

(3) One set of model Nd ages indicates an Early-Middle Proterozoic crust (about 2000 Ma old), another set indicates either new additions to the crust between 1700 and 1200 Ma ago, or mixed ages between Pan-African and older Pro- terozoic material.

(4) eNd values of all rocks west of the Nile confirm that the tectonic process during the Pan- African in the East Saharan Craton was one of crustal reworking.

(5) U/Pb zircon ages support a Middle Pro- terozoic age for the crust (between Gebel Uweinat and the Nile.

(6) The lithology and the metamorphic history of all basement areas is very similar (see.Tables 1 and 2).

(7) The structural trends in all subareas (Fig. 2) fit on a regional scale into a lOOO-km-wide, com- plexly folded, polymetamorphic zone that gener- ally strikes NE-SW and swings clockwise north of the Egyptian-Sudanese border to E-W, and fi- nally disappears under the Late Proterozoic as- semblages of the Nubian Shield.

(8) A second major well-defined event oc- curred in the East Saharan Craton in the Late Proterozoic between about 900 and 520 Ma ago, resulting in migmatization, resetting of the Sr iso- topic system, mylonitization and intrusion of granitoids.

Based on the above arguments, it may be con- cluded that at present no indications are available to subdivide the area between Gebel Uweinat and Bayuda Desert into different tectonic domains as shown by Vail (1976, see Fig. 3). We propose for this reason to name this polymetamorphic mega- structure the Northern Zalingei fold zone, which is the eastern part of the East Saharan Craton in Southeast Libya, South Egypt and North Sudan.

241

Early-Middle Proterozoic deformational history of the Gebel Uweinat-Gebel Kamil metamorphic complex

As mentioned earlier, the most striking struct- ural feature in the Gebel Uweimat-Gebel Kamil basement is a sigmoidal bending of megafold structures that originally had a NNE-SSW and NE-SW trend of fold axis. For this reason, mea- surements of poles of foliation planes over the whole area show a large scatter. The principal trend of deformation becomes clearer from an analysis of P&rears (Richter, 1986). The plot of p&rears reveals a double-branched spiral geome- try, suggesting a later clockwise rotation and tilt- ing of pre-existing fold axes (Fig. 5). Based on statistical evaluation of the /I-linears, the mea- sured shear and fault zones (Fig. 6), combined with the available radiometric age data (Klerkx and Deutsch, 1977; Cahen et al., 1984; Harris et al., 1984; Schandelmeier et al., 1987), the follow- ing three stages of deformation are proposed for the Gebel Uweinat-Gebel Kamil area:

(1) Primary folding occurred around 2000 Ma ago under a principal stress direction 6, of about 145 O. This caused regional fold axis trending about 55 o (6,). Related to this early phase of folding, a

Winears N N = 223

Fig. 5. Isoline diagram of weighted distribution of &linears

(B-axes) from the eastern part of the Gebel Uweinat basement

inlier. A two-branch spiral distribution is obvious, indicating

rotation and tilting of pre-existing fold axes.

150 NUMBER LENGTH 969 3696km

,574 km

S

Fig. 6. Azimuthal distribution of the fault pattern from the

eastern part of the Gebel Uweinat inlier. Length and number

per azimuth interval are presented.

pair of first-order conjugate shear zones developed at 175 o (sinistral) and 115 o (dextral). This kind of fault system created a mosaic of blocks within the upper crust. (Fig. 7A).

This first phase of deformation might have been related to initial southeast motion of Gondwana 2100-2000 Ma ago (McWilliams, 1981). At around 2000 Ma ago the polar wander swathe is characterized by a distinct loop, indicat- ing a change in the drift direction of Gondwana to the northeast (combined with a slight synchronous clockwise continent rotation), which went along with the creation of large dextral intracontinental transform or megashear across Northeast Africa. The dextral wrench movement along a direction of about 55” (Fig. 7B) induced a clockwise rotation of the individual crustal blocks of about 35” be- tween the large shear zones. This resulted in the clockwise rotation of pre-existing fold axis of the smaller crustal blocks. At the same time, owing to wrench movement along 55 O, a new major prin- cipal stress, S;, was established at about llO” and this external rotation tended to create new fold axes directions 8; at about 20 O. Both combined tendencies, anticlockwise rotation of the fold axis from 55 o N (8,) to 20° (6;) in the whole wrench zone and clockwise rotation of fold axes within the individual crustal blocks, resulted in a signifi- cant sigmoidal bending of the early fold axes and foliation planes (Fig. 7B). New shear planes were developed at 80” (dextral) and the former direc-

242

tion of major compressive stress of 145 o (6,) was

now reactivated as a sinistral shear zone. Ductile

deformation south of the Egyptian-Sudanese

border probably terminated with the end of this

process around 1900 Ma ago (Richter, 1986).

(2) Between about 1900 and 1800 Ma ago,

rotation continued for about 25 o in the individual

crustal blocks north of the Egyptian-Sudanese

border, and this caused further kinking of B-axes.

S,l was rotated to S;’ at about 85 O, and the major

direction of maximum extension, a,, was estab-

lished along the earlier sinistral shear zones at

Fig. 7. Kinetic model for steady ductile and unsteady brittle deformation in the time range between 2100 Ma and about 1800 Ma (for

descriptions of A, B and C, see text).

TABLE 5

Main structural trends in the Gebel Uweinat-Gebel Kamil

area, in the Bir Safsaf-Aswan Uplift and in North Sudan (west

of the Nile) and their genetic significance. *

Tie Gebel Uweinat- Bir Safsaf-Aswan North Sudan

(Ma) Gebel Kamil area Uplift area

-2000 145O8,

55O 8,

175O sinistral

shear

115 o dextral

shear . . . . . . . . . . . . . . . . - 2000 140° sinistral

-1900 shear

55 o dextral

wrench

llo” s;

2o” sj

80 a dextral

shear . . . . . . . . . . . . . . . . - 1900 55 o dextral

-1800 wrench

175O 8;‘

115O sinistral

shear

85O 8; L.75b. . . . . . . . . . . .

-520

. ed d ,~ . . ...& . s;. he,hi . .

shear

100° dextral 145O ministral

shear shear

170 o sinistral 115 o St,,

shear 25.O 83NS

135O 8lBSS 70° main

45 o s3BSS wrench

* Figures in italics are trends which were newly generated

during the respective episode of deformation.

175 O. Again, new shear zones were developed at 115“ (sinistral), which was a former dextral shear zone, and at 55” (dextral), which was the earliest direction of maximum extension (Fig. 7C). The 55“ direction is the present-day first order wrench-fault direction in the Gebel Uweinat- Gebel Kamil area, and is easily recognized on Landsat images.

(3) At around 1800 Ma ago, the principal stress direction Sr was stabilized at 85 o N, south of the Egyptian-Sudanese border. North of the border, thrusting and compression of sigmoidal structures

243

continued for some time. Updoming of structures was probably related to the formation of anatectic granitoids (Klerkx, 1980; Richter, 1986) and re- sulted in tilting of part of the fold axis. Finally, ductile deformation changed into brittle deforma- tion, resulting in further thrusting and shearing with a persistent preference for dextral shear faults of 55 o owing to E-W-directed compression. Table 5 shows all structural trends and their genetic significance; it can be seen that despite three different stages of deformation, only a few new directions (numbers in italics) formed after the earliest phase of deformation.

According to this interpretation, the most im- portant feature has been reactivation of structural trends with a different genetic significance during each phase of deformation. The fold geometry of the other basement areas, and especially the age of the basement rocks, are not yet well-known enough to allow interpretation of the structural evolution.

Late Proterozoic (Pan-African) deformation

Late Proterozoic deformation of the East Saharan Craton seems to be negligible in the Gebel Uweinat-Gebel Kamil area (Schandehneier et al., 1987), but increases considerably to the east near the boundary with the Late Proterozoic volcano-sedimentary-ophiolite assemblages of the Nubian Shield. Dextral wrench-fault systems of North Sudan and particularly of the Bir Safsaf-Aswan Uplift in South Egypt (Fig. 1) are probably related to plate rearrangement processes at the margin of the East Saharan Craton (El Gaby, 1983; Bemau et al., 1987). El Ramly et al., (1984) and Schandelmeier et al. (1987) have sug- gested a relationship between accretion and thrusting of the Nubian Shield assemblages in South Egypt onto the margin of the East Saharan Craton with related deformation of the marginal continental plate.

The dextral wrench-fault system within the Bir Safsaf-Aswan Uplift, which is probably a con- jugate shear zone to the Arabian Najd fault sys- tem (El Gaby, 1983; Bemau et al., 1987) strikes 80”, and the main trend of faults in North Sudan west of the Nile is 70”. These data were obtained from a statistical evaluation of a total of 1592

244

mapped lineaments from Landsat images (Bemau et al., 1987). A number of these faults were checked in the field to ensure their relative sense of move- ment and their time equivalence. K/Ar whole-rock and mineral ages from dykes which occupy con- jugate shear zones (Bemau et al., 1987) as well as microfabric investigations, confirmed a relation- ship between Pan-African migmatization and sub- sequent mylonitization in South Egypt, and prob- ably also in North Sudan.

Middle Proterozoic initial phase of folding, and these were the 6, trends in South Egypt and North Sudan during the Late Proterozoic dextral wrenching. Finally, it is worth mentioning that all of the Phanerozoic rift structures in South Egypt and central and North Sudan follow these Pre- cambrian meridional trends (Salamaa, 1985).

Conclusions

Owing to the dextral sense of shear within the The present-day fold and fault pattern in the

80 “-striking wrench zone, a pair of conjugate shear East Saharan Craton in Southeast Libya, South

faults developed at 100” (dextral) and 170” Egypt and North Sudan (west of the Nile) is the

(sinistral). The major principal stress direction was result of a polyphase deformational history which

about 135 o (S,,,,) and the direction of maximum began in the Middle Proterozoic around 2100 Ma

extension (6,,,,) occurred at 45 o (Fig. 8a). In ago and ended at around 1800 Ma ago. This

North Sudan, the main wrench direction was 70 o N structural episode can be best reconstructed and

and conjugate shear faults opened originally under correlated with Sr and Nd isotopic results in the

an angle of about 60” at 85 o (dextral) and 145 o Gebel Uweinat-Gebel Kamil area, but a similar

(sinistral). 6,,, was about 115O and a,, about tectonic history might be accepted for the entire

25”. The synthetic 85” shear planes are very Northern Zalingei folded zone. Pan-African

weakly expressed, suggesting that most of them tectonics and isotopic rejuvenation was minor in

were, owing to internal rotation, anticlockwise the Gebel Uweinat-Gebel Kamil area, but was rotated for about 15 o into the main wrench direc- highly effective from the margin of the Northern tion. Apart from the 80”-oriented in the Bir Zalingei folded zone to the Late Proterozoic Safsaf-Aswan Uplift and the 70”-oriented main Nubian Shield, and was expressed by large-scale wrench faults in North Sudan, the most prominent brittle shearing in dextral wrench zones. At the structural trend in both regions lies around 135O end of the Pan-African, the structural pattern of to 145 O. This is approximately the trend of the the Craton was complete. Phanerozoic structural present-day Red Sea rift axis. It was the dominant differentiation into basins, rifts and domal uplifts, trend of maximum compression (6,) in the Gebel as a result of intraplate tectonics, followed the Uweinat-Gebel Kamil area (Fig. 7A) during the older structural trends.

Bir Safsaf - Aswan (BSS) main wrench SO0

Northern Sudan (NS)

main wrench 70’

Fig. 8. Strain ellipses for simple parallel wrenching in the Bir Safsaf-Aswan Uplift (BSS) and in North Sudan (NS).

245

Acknowledgements

This work is part of the Special Research Pro- ject “Arid Areas” financed by the German Re- search Foundation (DFG). We would like to thank our colleagues from the General Petroleum Com- pany, Egypt, and from the Ministry of Energy and Mining, Sudan, for their logistic support and sci- entific discussions during several field trips. Spe- cial thanks are due to two anonymous reviewers who improved the first version of the manuscript. The assistance of many colleagues from the Spe- cial Research Project and from D.P.F. Darbyshire, British Geological Survey, is gratefully acknowl- edged. Drawings are by E. Susin and photographic work by B. K&berg and H. Glowa.

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