Structure of the late Proterozoic Nakasib suture, Sudan

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Structure of the late Proterozoic Nakasib suture, Sudan Powered By Docstoc
					Journal of the Geological Society, London, Vol. 150, 1993, pp. 1065-1074, 13 figs. Printed in Northern Ireland

                      Structure of the late Proterozoic Nakasib suture, Sudan

                                        M.    G.   ABDELSALAM                    & R.      J.   STERN
                      P r o g r a m s in Geosciences, The University o f Texas at Dallas, P O B o x 830688,
                                                R i c h a r d s o n , Texas, 75083-0688, U S A

                    Abstract: The Nakasib suture is a prominent structural belt in the central Red Sea Hills, Sudan. It is
                    one of the ophiolite-decorated sutures along which the late Proterozoic (Pan-African) island arc/back
                    arc terranes and continental micro-plates of the Arabian-Nubian Shield were welded together. It
                    juxtaposes the 900-800 Ma Haya terrane in the south with the 830-700 Ma Gebeit terrane to the
                    north. New structural data from the Nakasib suture show that the suture had evolved through three
                    phases of deformation which gave rise to a fold and thrust belt. During the early two phases of
                    deformation, SE-verging tight folds and thrusts were developed. The third phase of deformation
                    refolded the earlier structures into NE-trending, upright, and horizontal antiforms and synforms and
                    culminated in the development of NW-verging thrusts. The structural data are used, together with
                    previous stratigraphic and geochemical data, to suggest a tectonic model for the evolution of the
                    Nakasib suture. The suture followed a Wilson Cycle. This started with rifting of the Haya terrane,
                    extrusion of rift volcanics and deposition of passive margin sediments, and development of a
                    NW-dipping subduction zone over which the arc volcanics of the Gebeit terrane erupted. The closing
                    of the oceanic basin gave rise to the Nakasib suture with the present structural configuration.
                        The structural styles of other sutures of the Arabian-Nubian Shield are examined in light of
                    structural data from the Nakasib suture. These sutures show a common feature of steepening of the
                    early subhorizontal ophiolite-obduction structures during a late event of upright folding which may be
                    related to collision between terranes.

The A r a b i a n - N u b i a n Shield of Arabia and NE Africa (Fig.          suture, Quick & Bosch 1989; Quick 1991). (2) O t h e r
1) is an example of a late Proterozoic orogen that formed by                  ophiolite belts of the Shield, namely the Hamisana
Phanerozoic-type plate tectonic processes (Kroner et al.                      ophiolites, mark zones of east and west-directed pure shear
1987). The evolution of the Shield is attributed to accretion                 flattening (i.e. maximum shortening semi-axis orientated
of intra-oceanic island arc/back arc basins and continental                   E - W and subhorizontal) due to crustal shortening in the
micro-plates (Almond 1982; Vail 1983; 1985; Camp 1984;                        same direction (Nielsen et al. 1988; Stern et al. 1990). (3)
Embleton et al. 1984; Stoeser & Camp 1985; K r o n e r et al.                 Structures associated with s o m e of the ophiolites of the
1987). These terranes are welded along ophiolite-decorated                    Shield are fold and thrust belts (e.g. the Bir U m q suture,
sutures (Kroner 1985) and disrupted along shear zones                         Ramsay 1986). The above general models will be examined
(Almond & A h m e d 1987).                                                    in light of structural data from the Nakasib suture. These
     The Nakasib suture is an ophiolite-decorated structural                  data will also be used, together with structural data from
belt in the central part of the Red Sea Hills of the Sudan                    other sutures, to examine the deformational styles displayed
(Fig. 1). It is left-laterally offset by the younger, north to                by sutures of the A r a b i a n - N u b i a n Shield.
NNW-trending Oko shear zone (Fig. 1). The Nakasib suture
separates the 900-800 Ma old Haya terrane in the south from
the 830-700 Ma old Gebeit terrane to the north (Vail 1983,                    Geology of the Nakasib suture
1985; Fitches et al. 1983; Klemenic 1985; Reischmann 1986;                    The geology of the Nakasib suture is outlined in Abdelsalam
Kroner et al. 1987; Kroner et al. 1991). The Nakasib suture,                  & Stern (1993). Five tectono-stratigraphic groups were
the Haya terrane, and the Gebeit terrane are thought to                       identified across the suture. These are from south to north;
continue across the Red Sea into Arabia as the Bir U m q                      the Arbaat volcanic rocks, the sediments of the Salatib and
suture, the Asir terrane, and the Hijaz terrane, respectively                 the Meritri groups, the Nakasib ophiolites, and the volcanic
(Fig. 1; Vail 1983, 1985; E m b l e t o n et al. 1984; Camp 1984;             rocks and sediments of the Shalhout group. These groups
Stoeser & Camp 1985; Kroner et al. 1987; A l m o n d &                        are separated by thrusts and are intruded by syn- and
A h m e d 1987; Pallister et al. 1989; Kroner et al. 1991).                   post-tectonic granitoids (Fig. 2).
     This work deals with the structural evolution of the                         The Arbaat volcanic rocks are pre-730 Ma old, tholeiitic,
Nakasib suture. The work is based on structural analysis of                   rift-related mafic volcanic rocks with subordinate felsic
five main areas (Fig. 2). Structural observations along some                  volcanic rocks and siliciclastic sediments. The Salatib and
supplementary traverses (e.g. the Khor Youdib traverse,                       Meritri groups are composed of siliciclastic and marine sedi-
Fig. 2) are also considered. This work also provides a data                   ments as well as felsic tuff, ignimbrite, and rhyolite. The
base to evaluate the tectonic settings of the ophiolites of the               Arbaat, Salatib, and Meritri groups signify d e v e l o p m e n t of a
A r a b i a n - N u b i a n Shield. These ophiolites have been                passive margin prior to the collisional event forming the
characterized in one of three general tectonic settings; (1)                  Nakasib suture. The Nakasib ophiolites are ophiolite frag-
Some of the ophiolites of the A r a b i a n - N u b i a n Shield are          ments which occur along the Nakasib suture and define
associated with strike-slip fault zones (e.g. the Nabitah                     the NE-trending Shalhout antiform (Fig. 2). The Shalhout
 1066                                                                             M.G.           A B D E L S A L A M & R. J. STERN

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                                                                                                                                     o                         20                                      ~o KM
 Fig. 1. Sketch map of the Arabian-Nubian Shield with the Red Sea
 closed (modified after Vail, 1985).
                                                                                                                       Fig. 2. Structural map of the Nakasib suture.

group occupies the region to the north of the Nakasib
ophiolites (Fig. 2). It is made up of felsic and intermediate
volcanic rocks, tuff, agglomerate, ignimbrite, fine turbidite,                                                            To establish the deformation history of the Nakasib
and minor limestone beds. Geochemical data suggest that                                                               suture, five areas along the belt were investigated (Fig. 2).
these rocks are arc-related (Vail et al. 1984).                                                                       These data will be presented below during the discussion of
                                                                                                                      different phases of deformation.
Structural e v o l u t i o n o f the N a k a s i b suture
The Nakasib suture is exposed as a NE-trending fold and                                                               D 1 deformation
thrust belt which is at least 150 km long, extending from                                                             D1 is characterized by the development of tight to isoclinal
c. 35 ° 30'E to c. 37 ° 00'E (Fig. 2). The average width of this                                                      folds with an associated closely spaced axial planar cleavage
belt is about 40km (Fig. 2). The intensively deformed                                                                 (S1). D1 is interpreted to represent the early phase of
volcano-sedimentary package of the Nakasib suture is                                                                  deformation within the Nakasib suture since folds related to
bounded by two SE-verging thrusts. The southern thrust is                                                             this deformational episode deform the primary bedding
defined by the steeply dipping (70 ° NW) narrow (c. 250 m)                                                            planes. D1 is also characterized, together with D2, by
Arbaat/Salatib melange zone (Fig. 2). Folds related to the                                                            development of thrusts which tectonically stacked imbricate
Nakasib deformation are also present to the south of this                                                             sheets of the Nakasib ophiolites. Moreover, D1 and D2 are
thrust, but they die out southward. The northern thrust is a                                                          also responsible for development of thrusts and melanges
moderately dipping (45 ° NW) wedge of limestone and clastic                                                           separating the major groups of the area. Because D1 and
sediments of the Shalhout group.                                                                                      D2 are coaxial progressive deformations, it is believed that
    The Nakasib fold and thrust belt developed through                                                                thrust development started late during D1 and continued
three phases of deformation (D1, D2, and D3). A fourth                                                                during D2. Hence, these thrusts will be referred to as
phase of deformation is related to the development of the                                                             D1/D2 thrusts and melanges.
Oko shear zone (Fig. 2) and is interpreted to be an                                                                       D1 folds are preserved as mesoscopic isoclinal, tight and
intra-plate deformation in the form of NNW-trending folds                                                             gentle folds (Fig. 3). The orientations of axial surfaces and
and NW-trending left-lateral strike-slip faults. D1, D2, and                                                          axes of these folds depend on their positions with relation to
D3 are the focus of this work.                                                                                        D2 and D3 folds. The largest preserved D1 fold occurs at
                                                  THE NAKASIB S U T U R E , SUDAN                                                          1067

                                                                            the junction between Khor Meritri and Khor Salatib (Fig.
                                                                            2). This fold verges SE and plunges to the SW (Fig. 4).
                                                                            Associated with D1 is a strong closely spaced axial planar
                                                                            cleavage (S1) (Fig. 3). The lineation associated with D1 (L1)
                                                                            is in the form of mineral lineations and boudin structures.
                                                                            Both lineation types seem to parallel the D1 minor fold
                                                                            axes. However, the original orientation of (S1) and (L1) is
                                                                            modified by the younger D2- and D3 deformational events.
                                                                                Within the Nakasib ophiolites, D1/D2 are characterized
                                                                            by the development of ophiolitic melanges. The best
                                                                            preserved of these melanges is the Arbaat ophiolite in the
                                                                            NE part of the Nakasib suture (Fig. 2). In this melange,
                                                                            lenses and fragments of serpentinite, gabbro, pillowed
                                                                            basalt, chert and jasper, and limestone are enclosed in a
                                                                            highly sheared matrix of lithic wacke and slate (Fig. 5).
                                                                            Other ophiolite occurrences along the Nakasib suture (e.g.
Fig. 3. A photograph showing the tight nature of D1 folds and
                                                                            Meritri, Shalhout, and Igariri ophiolites, Fig. 2) show
associated axial planar cleavage in a lithic wacke from the Meritri
                                                                            similar, but less intensive, ophiolitic melange development.
                                                                                 The boundary between the Arbaat volcanics and the
                                                                            Salatib group is defined by the D1/D2-Arbaat/Salatib
                                                                            melange zone (Fig. 2) in which lenses and fragments of the
                                                                            Arbaat volcanics are sheared and enclosed in a matrix of
                                                                            carbonate and clastic sediments from the Salatib group. The
                                                                            foliation in the matrix strikes NE and dips steeply NW (Fig.
                                                                            6). This melange is interpreted to be developed as a result of
                                                                            thrusting of the Salatib group over the Arbaat volcanic
                                                                            rocks. The zone was later steepened due to folding by
                                                                            D3-upright folds. This interpretation is based on the
                                                                            following. (1) Most of the long axes of the deformed clasts
                                                                            and fragments are parallel to the dip of the foliation which
                                                                            encloses them (Fig. 6). This indicates that the bulk of the
                                                                            displacement was dip-slip. (2) Kinematic indicators such as
                                                                            bookshelf sliding of broken clasts and rotated feldspar grains
                                                                            indicate a reverse-slip displacement. (3). The fold
                                                                            geometries and relationships within the melange zone
                                                                            indicate folding of early subhorizontal foliations and folds by
                                                                             a late upright phase of folding which is consistent with the
                                                                            interpretation that the Arbaat/Salatib melange zone
                                                                             developed as a low-angle D1/D2 thrust which was steepened
                                                                             during D3-upright folding event.
                                                                                 Other D1/D2 thrusts of the Nakasib suture (Fig. 2) are
                                                                             marked by narrow zones (generally less than 40 m thick) of
Fig. 4. Plot of poles to bedding from a mesoscopic D1 fold at the            intense NE-trending foliations with steep NW-directed dips.
junction of Khor Meritri and Khor Salatib (solid circles), poles to          Kinematic indicators similar to those of the Arbaat/Salatib
axial planar cleavage (S1) (open circles), and minor fold axes (small        melange zone are also present. This leads us to conclude
stars) on equal area stereonet. The large star represent the fold            that D1/D2 thrusts were originally low angle SE-verging
axis. The shaded plane is the fold axial surface.                            planes which were folded during D3 deformation.

                                                                                                                 J   FOLDEDTHRUSTS
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Fig. 5. Geological map of the Arbaat
ophiolitic melange (for location see Fig.                                                                                           5 km
1068                                           M. G. A B D E L S A L A M                                    & R. J. S T E R N

                                                                                                           D2 deformation
                                                                                                           D2 is generally coaxial with D1, and is characterized by the
                                                                                                           development of tight folds in which both bedding planes
                                                                                                           (SO) and Sl-foliation are folded around NE-trending fold
                                                                                                           axes. However, the orientations of axial surfaces of these
                                                                                                           folds are modified by D3 folds. Thrusts which developed
                                                                                                           during D1 deformation continued to be active during D2.
                                                                                                                Two major D2 folds were mapped along the Nakasib
                                                                                                           suture. These are the Salatib antiform and the Samadi
                                                                                                           structure (Figs 7 and 8). The Salatib antiform occurs in the
                                                                                                           NE part of the Nakasib suture (Fig. 7a). It is a SE-verging,
                                                                                                           NE-plunging antiform (Fig. 7b, c, and d). The axial surface
                                                                                                           of the fold dips steeply to the N W (Fig. 7b). The southern
                                                                                                           limb of the Salatib antiform is truncated against the
                                                                                                           D 1 / D 2 / A r b a a t / S a l a t i b melange zone (Fig. 7a). The surface
                                                                                                           exposure of the northern limb of the antiform roughly
                                                                                                           parallels the D 1 / D 2 thrust trace which separates the Salatib
                                                                                                           group from the Meritri group (Fig. 7a). The axial planar
                                                                                                           cleavage associated with this fold is mainly crenulation
                                                                                                           cleavage which is folded by D3 deformation (Fig. 7e). The
                                                                                                           Samadi structure is exposed on the SW part of the Nakasib
Fig. 6. Equal area stereonet plot of poles to S1 foliation (dots) and                                      suture (Fig. 8a). The structure is nearly upright with
the long axes of the deformed clasts (open circles) from the                                               inferred axial surfaces that dip steeply to the NW. The fold
Arbaat/Salatib melange zone.                                                                               axes of the antiforms and synforms of the structure plunge

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Fig. 7. (a) Structural map of the D2 Salatib antiform (for location see Fig. 2). (b) A section across the structure. (e) z-diagram of bedding.
n = 58 and contouring is at interval of 1.6% per unit area. (fl) zr-diagram of (S1) cleavage, n = 79 and contouring is at interval of 1.25% per
unit area. (e) z-diagram of ($2) crenulation cleavage which is folded by D3 deformation, n = 31 and contouring is at interval of 3% per unit
                                                   THE NAKASIB SUTURE, SUDAN                                                                               1069

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Fig. 8. (a) Structural map of the Samadi structure (for location see Fig. 2). (b) A section across the structure. (¢) ~r-diagram of bedding, n -- 24
and contouring is at interval of 4% per unit area. (d) ~r-diagram of ($1) cleavage, n = 51 and contouring is at interval of 2% per unit area. (e)
Plot of 82 axial planar cleavage (dots) and D2 fold axes (open circles).

to the SW (Fig. 8b, c, and d). The southern limb of the                         (1) the vergence of D1/D2 thrusts is generally SE whereas
antiform is truncated by the D1/D2 thrust zone which                            that of D3 thrusts is NW; (2) D1/D2 thrust planes are
separates the Salatib group from the Arbaat volcanics.                          relatively steep (due to folding during D3) compared to the
    D1/D2 thrusts are locally accompanied by development                        shallow D3 thrusts; (3) D3 thrust planes terminate the
of strong mylonite fabric along thrust planes. Good                             continuation of D1/D2 thrust planes.
examples of this mylonite fabric can be observed along the                          Plot of poles to bedding (n-diagrams) from the NE part
thrust which separates the Salatib group from the Meritri                       of the Nakasib suture (Fig. 9) indicate that the area is
group and along the thrusts which imbricate the Meritri                         occupied by a doubly-plunging synform, referred to as the
ophiolite.                                                                      Olig synform. It is an interference pattern which developed
                                                                                as a result of refolding of the NE-trending, upright and
D3 deformation                                                                  horizontal D3 synform by a NNW-trending, upright and
                                                                                horizontal open synform related to the younger deformation
D3 refolded both D1 and D2 structures and fabrics to                            mentioned above. The axial trace of the Olig synform
produce gentle, nearly upright and horizontal, NE-trending                      coincides with the central part of the Meritri group (Fig.
antiforms and synforms (Fig. 2). The planar fabric ($3)                         9a). The fold is slightly asymmetrical and its axial surface
associated with D3 is a widely spaced, NE-trending, upright                     dips steeply to the SE (Fig. 9b). The southern limb of this
to steeply SE-dipping crenulation cleavage. L3 lineation is                     synform steeply dips to the NW and is represented by the
mainly subhorizontal NE-trending intersection and crenula-                      Arbaat volcanic rocks, the Salatib group, and the southern
tion lineations. D3 is also responsible for the development                     part of the Meritri group. The northern limb dips
of NW-verging thrusts which emplaced the Meritri group                          moderately to the SE and is occupied by the northern part
back over the Nakasib ophiolites. Distinction between                           of the Meritri group and the Meritri ophiolite. The SW part
D1/D2 thrusts and D3 thrusts was based on the following:                        of the synform plunges to the NE while its NE part plunges
1070                                                              M. G. A B D E L S A L A M                                        & R. J. S T E R N

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Fig. 9. (a) Structural map of the NE part of the Nakasib suture (for location see Fig. 2). (b) A section across the structure. (¢) n-diagram of
bedding from the SW part of the area. n = 39 and contouring is at interval of 2.5% per unit area. (d) ~r-diagram of bedding from the NE part
of the area. n = 42 and contouring is at interval of 2.5% per unit area. (e) ~r-diagram of S1 cleavage from the SW part of the area. n = 54 and
contouring is at interval of 2% per unit area. (f) Plot of $3 axial planar cleavage (dots) and D3 fold axes (open circles) from the entire area.

to the SW (Fig. 9c, d, e and f). T h e Olig synform is                                                                               T h e rock units of the Meritri g r o u p to the N E define a
responsible for steepening D1 and D2 structures in the south                                                                     mappable SW-trending, D3 synform, the A d e n a i a b synform
(i.e. structures within the Salatib and Meritri groups) and                                                                      (Fig. 10a). This synform plunges S W and verges N W (Fig.
reversal of dip from N W to SE in the n o r t h e r n part of the                                                                10b, c, and d). T h e N W limb of this synform is t r u n c a t e d
area (i.e. structures within the Meritri ophiolite).                                                                             against the D3 thrust which s e p a r a t e s the Meritri g r o u p

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                                                                                                                                                                        Adenaiab synform (for location see Fig.
                                                                                                                                                                        2). (b) A section across the structure. (¢)
                                                                                 ~                     //I                    :;22222:'2+2+222"2
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                                                                                                                                                                        contouring is at interval of 3% per unit
                                                                                                                                                                        area. (d) n-diagram of (SI) cleavage.
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                         SKM                          I+]+~+ ;+ ;+ ;+ ;+ ;i:                    ~:::: ~ : " ~ -.\\~'1 Hli//i,+ +. "+++++". ++++"+++"+++l                planar cleavage (dots) and D3 fold axes
                                                        +++ +++ + + + + + + + +~ :: ;;7{::7777: :7' :7;5 : ;77 :: \ \ ~ / i / i, /~/J+ +++++ + ++++++++++++++++
                                                      L ~ ]                               :I]
                                                                            : ''ill ]I]I]I] ]~]    ]] ]]]I ~ " "
                                                                                                          I                    7 ', , , ...........             1       (open circles) of the area.
                                                                   THE NAKASIB S U T U R E , SUDAN                                                  1071


                                                    •          0     A           /
                               •           0        • 0                  °°O/
                                       •                800                  /

                                               "°         o"             /           Fig. 12. A photograph showing deformed conglomeratic clasts at
                                                                                     the base of D3 thrust which separates the Meritri group from the
                                               o                                     Meritri ophiolite. Looking NE at a rotated clast which suggests
                                                                                     thrust from SE to NW.

                                                                                     the Nakasib suture (west of the Oko shear zone, Fig. 2)
                                                                                     contains ENE-trending D3 antiform and synform defined
                                                                                     here as the Shalhout antiform and Luggag synform (Fig. 2).
                                                                                     These folds are upright and horizontal (Fig. l l b ) . The limbs
                                                                                     of the Shalhout antiform are defined by the Shalhout
                                                                                     ophiolite in the north and the Igariri ophiolite to the south
                                                                                     (Fig. 2). The southern limb of the Luggag synform is defined
                                                                                     by the Samadi structure while its north limb is defined by
                                                                                     the Igariri ophiolite (Fig. 2).
                                                                                         D3 deformation culminated in the development of
                                                                                     thrusts which truncated the northern limbs of the Adenaiab,
                                                                                     Olig, and Luggag synforms and thrust the Meritri group
                                                                                     back over the Nakasib ophiolite (Fig. 2). At the base of
                                                                                     some of these thrusts, conglomeratic clasts are deformed
                                                                                     into pancakes and ellipsoids (Fig. 12).

                                                                                     Younger deformation
                                                                                     Younger, NNW- to NW-trending and steeply-dipping
                                                                                     cleavages (nearly vertical) cross-cut all of the previous
Fig. 11. Equal area stereonet plot of bedding (dots), S1 cleavage                    fabrics. These cleavages are related to a complex structural
(open circles), $3 axial planar cleavage (triangles), and D3 fold axis               history of NNW-trending upright folding and NW-trending
(stars) from (a) along Khor Youdib. (b) The SW part of the                           left-lateral strike-slip shearing which are associated with,
Nakasib suture, west of Oko shear zone. The large stars represent                    and localized along the Oko shear zone (Fig. 2). The
fold axes.
                                                                                     structural evolution of the Oko shear zone and associated
                                                                                     fabrics will be discussed in a separate article.
from the Shalhout group. The SE limb of the fold is
intruded by the Arbaat pluton (Fig. 10a). The closure of the                         Tectonic and structural evolution
Adenaiab synform is partially occupied by isolated outcrops                          The Nakasib ophiolites separates two distinct terranes. The
of felsic tufts and intraformational conglomerate which                              one to the south is represented by the Arbaat, Salatib and
 resemble rocks of the Shalhout group to the north (Fig.                             Meritri groups. These assemblages are rift/passive margin
 10a). This outcrop shows discordant structural relationships                        volcanics and sediments deposited on the north margin of
to the underlying rocks of the Meritri group and is                                  the Haya terrane. The terrane to the north is represented by
interpreted to represent a D1 klippe which detached from                             the Shalhout group which constitutes part of the extensive
the main outcrop to the north during the SE-verging D1/D2                            arc volcanic field of the Gebeit terrane (Abdelsalam & Stern
thrusting event. This klippe, thereafter, was folded around                          1993).
D3 synform together with the rocks of Meritri group.                                     A plate tectonic model is proposed to explain the
    Structural data from along Khor Youdib (Fig. 2) show                             evolution of the Nakasib suture (Fig. 13). In this model, the
that the section is dominated by a synform which represents                          Arbaat volcanics were formed in a rift which was developed
the continuation of the Olig synform. This is flanked by a                           in the Haya terrane (Fig. 13a). The Salatib and Meritri
major D3 antiform to the NW. These are almost horizontal,                            groups were deposited as alluvial sedimentary fans and
NE-trending tight to gentle folds (Fig. 11a). The SW part of                         channel deposits above the Arbaat volcanics (Fig. 13b).
1072                                                      M. G. A B D E L S A L A M & R. J. STERN

                                                                                characterized by development of ophiolitic melanges within
                                                                                the Nakasib ophiolites in the northern part of the suture
                                                                                (e.g. the Arbaat ophiolite, Fig. 5). On the other hand,
                                                                                SE-verging folds and thrusts dominate the Salatib and
                                                                                Meritri groups to the south. Melange development and
                                                                                pervasive shearing within the Salatib and Meritri groups are
                                                                                restricted to narrow zones marking thrusts which separate
                                                                                the major groups (e.g. the Arbaat/Salatib melange zone).
                                                                                This suggests systematic decrease in deformational intensity
                                                                                from north to south across the suture. This leads to the
                                                                                proposition that the ophiolitic melanges within the Nakasib
                                                                                ophiolites represent the accretionary prism which tentatively
                                                                                marks the palaeo-trench location. This is supported by a
                                                                                regional gravity profile over the Arbaat ophiolitic melange
                                                                                which was interpreted in term of a paired Bouger gravity
                                                                                anomaly (i.e. the Arbaat ophiolitic melange define a
                                                                                fossil-plate boundary) (Sadig et al. 1987). If the Arbaat
       H41~47E,9/~4.._   l          ~
                                        ~'~ "tO"
                                          ~ 40~'-                               melange and its SW extensions represent the root of the
                                                                                D1/D2 nappe, this nappe may have extended for more than
                                                                                30 km in a SE-direction prior to folding by D2 and D3.
                                                                                    D3 manifests the final stage of collision between the
                                                                                Haya and Gebeit terranes and resulted in refolding of the
                                                                                D1 and D2 structures into the upright Shalhout antiform
                                                                                and Olig-Luggag synform (Fig. 13d). The exposure of the
                                                                                Nakasib ophiolites as two belts of dismembered ophiolites is
                             HAYA                                               due to folding of the D1/D2 ophiolitic nappe by D3
                                                                                Shalhout antiform (Fig. 2, 13d). The absence of ophiolite
                                                                                fragments on the southern limb of the Olig-Luggag synform
                                        SHALHOUT          /~                    suggests that the D1/D2 nappes did not travel far to the
                                                                                south. A line which roughly coincides with the axial trace of
                         D                                                      the present Olig-Luggag synform (Fig. 13c) is envisioned to
                                                                                be the southern extent of the ophiolitic nappe. D3
                                                                                culminated in the development of NW-verging thrusts which
                                                                                resulted in emplacement of the passive margin sediments
                                                                                back over the Nakasib ophiolites. Following the end of
                                                                                collision deformation, the younger deformations evolved as
                                                                                a product of post-collisional, intra-plate deformation.
Fig. 13. A model to explain the structural and tectonic evolution of
the Nakasib suture. (a) Rifting of the Haya terrane and extrusion of
the Arbaat volcanic rocks. (b) Development of a passive margin                  Discussion
and deposition of the Salatib and Meritri groups. Development of a
NW-dipping subduction zone over which the arc volcanic rocks of
the Gebeit terrane were extruded. (e) Beginning of closing of the               Tectonic settings o f ophiolites of the Arabian-Nubian
basin. Development of D1/D2 SE-verging nappes. (d) Refolding of                 shield
D1/D2 nappes by D3 folds. Development of D3 NW-verging                          The tectonic significance of ophiolite belts of the
                                                                                Arabian-Nubian Shield and their settings are subjects of
                                                                                large uncertainties. Three general settings were suggested
Sediments came mainly from erosion of the Arbaat                                for these ophiolites. (1) Some of these ophiolites are
volcanics and more distal parts of the Haya terrane. The                        associated with strike-slip fault systems (e.g. the Nabitah
complete opening of the rift basin culminated with the                          suture, Quick & Bosch 1989; Quick 1991). These strike-slip
formation of an oceanic crust, which was flanked to the                         simple shear regimes are attributed to oblique collision
north by a NW-dipping subduction zone (Fig. 13b). A                             between arc terranes (Quick 1991). (2) Other ophiolite belts
volcanic arc developed over this subduction zone (Fig. 13b).                    of the region, for example the Hamisana shear zone, mark
As the basin closed, fragments and sheets of the oceanic                        zones of strong E-W-directed pure shear flattening
crust were obducted over the passive margin sediments as a                      deformation due to crustal shortening in the same direction
SE-verging nappe (Fig. 13c). The complete closing of the                        (Nielsen et al. 1988; Stern et al. 1990). Crustal attenuation
basin resulted in collision between the terranes and                            underneath the Hamisana zone is suggested to explain strain
development of progressive D1, D2, and D3 structures.                           localization along its N - S extent. (3) Structures associated
    D1 and D2 are closely related to subduction/obduction                       with some of the ophiolites of the Shield are interpreted as
process which resulted in emplacement of the tectonically                       fold and thrust belts (e.g. the Bir Umq suture, Ramsay
disrupted ophiolite fragments and sheets as a SE-verging                        1986).      These    fold    and    thrust    belts     manifest
nappe over the passive margin sediments (Fig. 13c). D1 and                      subductional/collisional tectonic history between arc
D2 structures record differences in style across the Nakasib                    terranes.
suture from north to south. These deformational phases are                         It is accepted that some of the ophiolite belts of the Shield
                                                       THE NAKASIB S U T U R E , SUDAN                                             1073

mark zones of pure shear flattening deformation and/or                        The Bir Umq suture (Fig. 1) is a NE-trending belt of
simple shear strike-slip fault systems. However, these                   imbricate thrusts, isoclinal folds and overthrust nappes
deformational belts are younger N- to NW-trending                         (Camp 1984). It is defined by the Jebel Thurwah ophiolite in
post-accretionary structures which cross-cut the earlier                 the SW (Nassief et al. 1984; Ramsay 1986) and the Bir Umq
subductional/collisional structures of the Shield. The                   complex to the NE (A1 Rehaili & Warden 1980). Nassief et
apparent alignment of some of the ophiolite fragments                    al. (1984) mapped the Jebel Thurwah ophiolite as imbricate
parallel to these younger post-accretionary structures is due            thrust sheets which are dipping towards each other in a
to dragging during shearing and flattening (e.g. Oko shear               synform geometry.
zone, Almond & Ahmed 1987; Hamisana shear zone, Stern                         The NE- to E-trending Allaqi-Heiani-Onib-Sol
et al. 1990).                                                            Hamed-Yanbu suture (Fig. 1) is defined by the
    The interpretation of the Nakasib suture as a fold and               Allaqi-Heiani-Gerf (Kroner et al. 1987; Stern et al. 1990)
thrust belt does not agree with the suggestion that it                   and the Onib-Sol Hamed (Fitches et al. 1983, Hussein et al.
represents a strike-slip fault zone with significant right-               1984) ophiolites in N Sudan and S Egypt, and the Jebel Ess
lateral displacement (Almond & Ahmed 1987). The latter                    (Shanti & Roobol, 1979) and the Jebel al Wask (Bakor et al.
proposition was based on local presence of upright and steep              1976) ophiolites in Saudi Arabia. The Allaqi-Heiani suture
foliations, and presence of subhorizontal lineations. It was             represent a S-verging nappe which was refolded around a
demonstrated that local steep foliations along the Nakasib               subhorizontal, E-trending axis that produced upright
suture are either $3 axial planar cleavage or S1 and $2                  antiforms (Stern et al. 1990). The Sol Hamed ophiolite dips
foliations that had been folded during D3. The subhorizon-               steeply to the SE. This led Fitches et al. (1983) to interpret
tal lineations are L3 intersection and mineral lineations.               ophiolite obduction to have been to the NW over a
Extensional lineations (Fig. 7) show large pitch angles and              SE-dipping subduction zone. Stern et al. (1990) objected
are sometimes parallel to the dip direction of foliations                this interpretation on the basis that Fitches et al. (1983)
containing them. This indicates extension at high angle to               documented at least two phases of deformation within the
strike rather than parallel to strike. Moreover, some of the             ophiolites, hence the present dip of the ophiolite cannot be
earlier subhorizontal foliations are well preserved along the            used to infer the original subduction-zone dip. Shanti &
Nakasib suture and localized within thrust planes. These                 Roobol (1979) and Pallister et al. (1988) described the
subhorizontal foliations can not be reconcile with strike-slip           structures associated with the Jebel Ess ophiolite as a folded
displacement.                                                            allochthonous thrust sheet that suggest a refolded synform
    The suggestion that some of the ophiolites of the                    geometry.
Arabian-Nubian shield mark suture zones is debated by                         The above review suggests the following. (1) The
Church (1988, 1991). He suggested that these ophiolites are              Arabian-Nubian Shield sutures are marked by fold and
remnants of nappe complexes that might have travelled far                thrust belts. (2) Local steeply dipping structures are
from the actual suture zone. This work on the Nakasib                    observed from most of these sutures. Subhorizontal
suture suggests that the Nakasib ophiolites occur close to the           structures related to ophiolite obduction were steepened
suture zone. This suggestion is based on: (1) the structural             during younger deformational episodes in the form of
style of the Nakasib ophiolites and the surrounding                      upright folding which may be related to collision between
sediments is continuous; (2) the Nakasib ophiolites and                  terranes. (3) The sutures of the Arabian-Nubian Shield
associated localized strain separate two geochronologically              have traditionally been drawn as lines connecting ophiolite
distinct terranes as discussed earlier; (3) regional gravity             outcrops which implies steeply dipping to vertical planes. It
investigations across the suture shows that the Nakasib                  is suggested here that most of these suture 'planes' are
ophiolites closely coincides with a proposed fossil plate                actually undulating surfaces with varying dips. This, in turn,
boundary (Sadig et al. 1987).                                            suggests that a single suture may crop out along more than
                                                                         one structural level by intersecting the surface more than
                                                                         once, or may show a curved outcrop. This explains the
                                                                         observed differences in dips and apparent vergences along
O n the structural styles o f s u t u r e s o f the A r a b i a n -
                                                                         the strike of some of the Arabian-Nubian Shield sutures.
N u b i a n shield                                                            Although the above structural similarities between
The steep structures associated with sutures of the                      sutures of the Arabian-Nubian Shield are appealing to
Arabian-Nubian Shield, namely the Red Sea Hills of the                   generalize a model for their structural evolution, there are
Sudan, were noted by Kroner (1985). These steep structures               still some differences between the Heiani-Allaqi-Onib-Sol
led Almond & Ahmed (1987) and Almond et al. (1989) to                    Hamed-Yanbu suture in the north and the Nakasib-Bir
suggest that the sutures of the Red Sea Hills were                       Umq suture to the south. (1) The Heiani-Allaqi-Onib-Sol
overprinted by a younger strike-slip faulting event which                Hamed-Yanbu suture is characterized by abundant
was induced during oblique collision between arcs. The                   ophiolites relative to the Nakasib-Bir Umq suture. The
outcome of this work is that the local steep to upright                  ophiolites of the northern suture are less deformed
structures are due to refolding of the earlier SE-verging                compared to those of the southern suture. (2) The
structures during the collisional event. The structural                  deformation along the Nakasib-Bir Umq suture involves the
scenario suggested for the Nakasib suture will be tested                 surrounding sediments and volcanics unlike that along the
against other sutures of the Arabian-Nubian Shield. In so                Heiani-Allaqi-Onib-Sol Hamed-Yanbu suture where there
doing, two well accepted sutures will be examined; the Bir               is less involvement of the surrounding rocks in the
Umq suture, the continuation of the Nakasib suture into                  deformation accompanied suturing. These differences do not
Arabia, and the Heiani-Allaqi-Onib-Sol H a m e d - Y a n b u             prejudice the general model proposed above. However, the
suture, which is the next suture to the north of the                     tectonic significance of these structural differences await
Nakasib-Bir Umq suture (Fig. 1).                                         more comparative work on sutures of the Shield.
    1074                                                        M.    G.    ABDELSALAM            & R.     J.   STERN

Conclusions                                                                                           for Proterozoic crustal growth. Journal of the Geological Society,
    (1) The Nakasib suture is a fold and thrust belt which                                            London, 148, 600-606.
                                                                                                 EMBLETON, J. C. B., HUGHES, D. J., KLEMENIC,P. M., POOLE, S. t~ VAIL, J.
evolved through three phases of deformation. The early two
                                                                                                      R. 1984. A new approach to the stratigraphy and tectonic evolution of
phases are SE-verging folds and thrusts. The third phase of                                           the Red Sea Hills, Sudan. Faculty of Earth Sciences Bulletin, King
deformation is in the form of NE-trending, upright, and                                               Abdulaziz University, Jeddah, 6, 113-126.
horizontal antiforms and synforms which refolded D1 and                                          FITCHES, W. R., GRAHAM, R. H., HUSSEIN, I. M., RIES, A. C., SHACKLETON,
D2 structures. D3 culminated in the development of                                                    R. M. & PRICE, R. C. 1983. The late Proterozoic ophiolite of Sol
                                                                                                      Hamed, NE Sudan. Precambrian Research, 19, 385-411.
NW-verging thrusts.                                                                              HUSSEIN, I. M., KRONER, A. & DURR, ST. 1984. Wadi Onib-a dismembered
    (2) The Nakasib suture evolved during a complete                                                  Pan-African ophiolite in the Red Sea Hills of Sudan. Faculty of Earth
Wilson cycle orogeny which started with rifting of the Haya                                           Sciences Bulletin, King Abdulaziz University, Jeddah, 6, 320-327.
terrane, development of a passive margin that is                                                 KLEMENIC, D. M. 1985. New geochronological data on volcanic rocks from
                                                                                                      northeast Sudan and their implication for crustal evolution. Precambrian
characterized by extrusion of rift volcanic rocks and                                                 Research, 30, 263-276.
deposition of clastic and carbonate sediments, development                                       KRONER, A. 1984. Late Precambrian tectonics and orogeny: A need to
of a NW-dipping subduction zone over which the arc                                                    redefine the term Pan-African. In: KLERKX, J. & MICHOT, J. (eds)
volcanic rocks of the Gebeit terrane were extruded, and the                                           African Geology. Musee Royal de l'Afrique Central Tervuren, Belgium,
closing of this oceanic basin into a suture decorated by the                                            1985. Ophiolites and the evolution of tectonic boundaries in the late
Nakasib ophiolites.                                                                                   Proterozoic Arabian-Nubian Shield of northeast Africa and Arabia.
    (3) The sutures of the Arabian-Nubian Shield share a                                              Precambrian Research, 27, 277-300.
common structural scenario of steepening of the early                                           --,      GREILING, R., REISCHMANN, T., HUSSEIN, I. M., STERN, R. J., DURR,
                                                                                                      S., KRUGER, J. d~ ZIMMER, M. 1987. Pan-African crustal evolution in
subhorizontal ophiolite-obduction structure by a younger
                                                                                                      northeast Africa. In: KRONER, A. (ed.) Proterozoic Lithospheric
event which is likely to be an upright folding episode that                                           Evolution. American Geophysical Union, Geodynamic Series, 17,
was associated with collision between terranes.                                                       235-257.
                                                                                                ~,       STERN, R. J., LINNEBACKER, P., MANTON, W., REISCHMANN, T. &
                                                                                                      HUSSEIN, I. M. 1991. Evolution of Pan-African island arc assemblages in
    T h e first field phase (four weeks) of this w o r k was s p o n s o r e d by the                 the southern Red Sea Hills, Sudan, and in southwestern Arabia as
    G r a d u a t e College of the University of K h a r t o u m . T h e second field                 exemplified by geochemistry and geochronology. Precambrian Research,
    trip (ten weeks) was s p o n s o r e d from N A S A grant to R . J . S . T h e T M                53, 99-118.
    muitiband and b a n d 5 black and white landsat images were                                  NASSIEF, M. O., MACDONALD, R. & GASS, I. G. 1984. The Jebel Thurwah
    d e v e l o p e d i n W a s h i n g t o n University at St. Louis with the help of                 Upper Proterozoic ophiolite complex, western Saudi Arabia. Journal of
                                                                                                       the Geological Society, London, 141, 537-546.
    M. Sultan. T h e G e o l o g i c a l R e s e a r c h A u t h o r i t y of the Sudan
                                                                                                 NIELSEN, K. C., OSMAN, M. M. M., MILLER, M. M.& STERN, R. J. 1988.
    provided             the   authors        with the      topographic     maps,   aerial             Structural evolution of the Hamisana shear zone, NE Sudan. Geological
    p h o t o g r a p h s and aerial p h o t o m o s a i c s of the area. T h e a u t h o r s         Society of America A bstracts, 20, 126-127.
    would like to t h a n k K. Nielsen for his constructive c o m m e n t s during               PALLISTER, J. S., STACY, J. S., FISCHER, L. B. & PREMO, W. R. 1989.
    the p r e p a r a t i o n of this paper. T h e a u t h o r s would also like to t h a n k          Precambrian ophiolites of Arabia: Geologic settings, U-Pb geochronol-
    M. P. C o w a r d and R. C. Greiling for reviewing the manuscript. T.                             ogy, Pb-isotope characteristics, and implication for continental accretion.
                                                                                                       Precambrian Research, 38, 1-54.
    Belo was of great help during the field phase of this work.
                                                                                                 QuicK, J. E. 1991. Late Proterozoic transpression on the Nabitah fault
                                                                                                      system--implications for the assembly of the Arabian Shield.
                                                                                                       Precambrian Research, 53, 119-147.
                                                                                                --      & Boscll, P. S. 1989. Tectonic history of the northern Nabitah fault zone,
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                                                         Received 4 August, 1992; revised typescript accepted 1 March 1993