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A Strike-Slip Core Complex From The Najd Fault System, Arabian Shield

September 2014
Terra Nova 26(5):387-394

DOI:10.1111/ter.12111

Authors:

Sven Erik Meyer

Johannes Gutenberg University Mainz

Cees W. Passchier

Johannes Gutenberg University Mainz

Tamer Abu-Alam

UiT The Arctic University of Norway

Kurt Stüwe

University of Graz

Metamorphic core complexes are usually thought to be associated with regional crustal extension and crustal thinning, where deep crustal material is exhumed along gently dipping normal shear zones oblique to the regional extension direction. We present a new mechanism whereby metamorphic core complexes can be exhumed along crustal-scale strike-slip fault systems that accommodated crustal shortening. The Qazaz metamorphic dome in Saudi Arabia was exhumed along a gently dipping jog in a crustal-scale vertical strike-slip fault zone that caused more than 25 km of exhumation of lower crustal rocks by 30 km of lateral motion. Subsequently, the complex was transected by a branch of the strike-slip fault zone, and the segments were separated by another 30 km of lateral motion. Strike-slip core complexes like the Qazaz Dome may be common and may have an important local effect on crustal strength.This article is protected by copyright. All rights reserved.

Typical metamorphic core complex formed during regional extension; (b) strike-slip core complex, formed during regional shortening by local extension associated with a jog in a strike-slip shear zone.

…

Overview map of the Najd shear zones and the gneiss complexes in the Arabian-Nubian Shield; modified after Abu-Alam and Stüwe (2009).

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Overview map of the Najd shear zones and the gneiss complexes in the Arabian-Nubian Shield; modified after Abu-Alam and St€ uwe (2009).

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Proposed sequence in the development of the Qazaz complex along the Qazaz strike-slip shear zone.

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a-e) Proposed sequence in the development of the Qazaz complex along the Qazaz strike-slip shear zone.

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Figures - uploaded by Tamer Abu-Alam

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Content uploaded by Tamer Abu-Alam

Content may be subject to copyright.

A strike-slip core complex from the Najd fault system,

Arabian shield

Sven Erik Meyer,

Cees Passchier,

Tamer Abu-Alam

2,3,4,5

and Kurt St€

uwe

Institute of Earth Sciences, Johannes Gutenberg University Mainz, Mainz 55128, Germany;

Institut f€

ur Geowissenschaften, Karl-

Franzens-Universit€

at Graz, Universit€

atsplatz 2, Graz A-8010, Austria;

Norwegian Polar Institute, Hjalmar Johansens gt. 14, Tromsø NO-

9296, Norway;

Geology Department, Faculty of Science, Tanta University, Tanta, Egypt;

Egyptian Institute of Geodynamics, Cairo, Egypt

ABSTRACT

Metamorphic core complexes are usually thought to be asso-

ciated with regional crustal extension and crustal thinning,

where deep crustal material is exhumed along gently dipping

normal shear zones oblique to the regional extension direc-

tion. We present a new mechanism whereby metamorphic

core complexes can be exhumed along crustal-scale strike-slip

fault systems that accommodated crustal shortening. The

Qazaz metamorphic dome in Saudi Arabia was exhumed along

a gently dipping jog in a crustal-scale vertical strike-slip fault

zone that caused more than 25 km of exhumation of lower

crustal rocks by 30 km of lateral motion. Subsequently, the

complex was transected by a branch of the strike-slip fault

zone, and the segments were separated by another 30 km of

lateral motion. Strike-slip core complexes like the Qazaz

Dome may be common and may have an important local

effect on crustal strength.

Terra Nova, 26, 387–394, 2014

Introduction

Metamorphic domes surrounded by

low-grade metamorphic rocks are

commonly formed by the exhuma-

tion of medium- to high-grade meta-

morphic rocks from lower crustal

levels (Davis and Coney, 1979; Crit-

tenden et al., 1980). The most com-

mon mechanism of exhumation is

thought to be regional-scale exten-

sion (Wernicke, 1981; Davis et al.,

1986) and crustal thinning, where

higher grade rocks are brought up in

the footwalls of gently dipping shear-

zone systems oblique to the regional

extension direction (often termed

‘low-angle detachments’) forming so-

called core complexes (Lister et al.,

1984; Tirel et al., 2008; Huet et al.,

2010; Fig. 1a). Here, we present evi-

dence that core complexes can also

be locally exhumed along major ver-

tical strike-slip shear zones in areas

of crustal shortening without regio-

nal-scale crustal thinning, using an

example from the Najd shear-zone

system in Saudi Arabia (Abdelsalam

and Stern, 1996; Fig. 1b).

The Najd shear-zone system

The Arabian-Nubian shield (ANS) in

Egypt, Saudi Arabia and Sudan is

composed of ~870–630 Ma Neopro-

terozoic juvenile arc terranes and

remains of ophiolite belts which

amalgamated during the closing of

the Mozambique Ocean and the

associated assembly of Gondwana-

land (Stern, 1994; Johnson et al.,

2004; Stern and Johnson, 2010).

Most of the ANS consists of

low-grade metavolcanics and metase-

diments with scattered intrusive arc-

type and a few A-type granitoids. All

units are affected by the Najd fault

system (NFS), a network of crustal-

scale sinistral strike-slip zones

2000 km long and 400 km wide,

which cut and partly reactivate older

tectonic elements in the shield (Stern,

1994; Fig. 2). Development of this

shear-zone network during and fol-

lowing the collision of West and East

Gondwana resulted in E–W shorten-

ing with a northwards trend of

escape tectonics (Burke and Seng€

or,

1986; Stern, 1994; Abdelsalam and

Stern, 1996). This was accompanied

by the exhumation of metamorphic

domes (e.g. Blasband et al., 2000;

Fritz et al., 2002; Brooijmans et al.,

2003; Abd El-Naby et al., 2008;

Abu-Alam and St€

uwe, 2009). We

studied one of these domes in NW

Saudi Arabia: the Qazaz Dome,

which is associated with the sinistral

Qazaz strike-slip shear zone, one of

the largest Najd structures with a

length of at least 140 km (Stern and

Johnson, 2010; Genna et al., 2002;

Figs 2 and 3).

Qazaz metamorphic complex

The Qazaz Dome developed in a

low-relief area with nearly continu-

ous exposure in the desert of Saudi

Arabia. It is a triangular dome of

medium- to high-grade gneisses sur-

rounded by low-grade mylonite zones

and very low-grade metapelite, con-

glomerate and volcanic rocks of the

Neoproterozoic Thalbah and Bayda

Groups (Fig. 3a). The Thalbah

group sediments have been deposited

unconformably on the Imdan plu-

tonic complex (660 4 Ma) and

were intruded by the Liban complex

(621 7 Ma), which appears to

bracket deposition between 660 and

620 Ma (Johnson et al., 2011). How-

ever, the age of the group is debated:

new U-Pb dates of detrital zircons

from two of the three formations

that make up the Thalbah group

(Bezenjani et al., 2014) suggest depo-

sitional ages of ≤596 10 Ma (Ha-

shim Formation) and ≤612 7Ma

(Zhufar Formation). The Qazaz

shear zone is locally 3–4 km wide

with a dominance of vertical folia-

tions and gently plunging stretching

lineations. It is an anastomosing

complex of high-strain branches with

Correspondence: Mr. Sven Erik Meyer,

Tectonophysics, Institute of Earth Sci-

ences, Becherweg 21, Johannes Gutenberg

University Mainz, Mainz 55099, Rhein-

land Pfalz, Germany. Tel.:

+49 6131 3920293; e-mail: meyersv@

uni-mainz.de

doi: 10.1111/ter.12111

high-grade mylonitized rocks in the

core (Fig. 3a). Adjacent to the Qazaz

Dome, the shear zone splits into two

strike-slip zones with similar sinistral

shear sense, which ﬂank the dome as

described below. The activity of the

Qazaz shear branches is bracketed

between ~630 and 580 Ma (Calvez

et al., 1984; Kennedy et al., 2009)

based on displacement of dated

granitoids.

The Qazaz Dome is characterized

by a dominant gently SW–NE-dip-

ping mylonitic foliation with NW–SE

or NS trending gently plunging

stretching lineations developed in

granitic gneisses (Fig. 3c). The age of

the gneisses (protolith) in the dome

itself is given by SHRIMP zircon

dating as 725–696 Ma (Johnson and

Woldehaimanot, 2003). Towards the

southern detachment, the gentle

southern dip of the mylonitic folia-

tion increases to a maximum of 40°

(Figs 3c and 4). The detachment

includes parts of the Qazaz gneisses

and metasediments of the Thalbah

group with a strong south-dipping

mylonitic shape fabric (Fig. 3c). The

footwall contains high-strain migma-

tites and high-metamorphic-grade

mylonites with r-type feldspar clasts

and garnet clasts with strain shad-

ows, giving a dominant top to the

south shear sense (Fig. 5). These

high-grade shear sense markers and

shape fabrics are overprinted by

lower grade S-C shear bands and

chlorite veins indicating the same

movement direction and shear sense,

suggesting synkinematic exhumation

of the dome. The hanging wall of the

Qazaz Dome to the south and west

is composed of rocks of the Thalbah

Group. These are weakly to moder-

ately deformed with open upright

folds south of and alongside the

dome and the Qazaz shear zone, but

undeformed further away (Fig. 3c).

To the SE, the Thalbah group is

invaded by small, mostly undeformed

monzogranite bodies that are not

affected by the main strike-slip or

detachment mylonitization, although

some minor shear zones occur at the

contact of the granite and the

metasediments. The gently dipping

mylonitic foliation in the Qazaz

Dome is affected by km-scale folds

with NNW–SSE trending steep axial

planes, which are open in the SE but

become increasingly tight to the NW

(Figs 3c and 6c–e). In the NW apex

of the Dome, a single tight to isocli-

nal antiform dominates the mylonite

zone and grades into the Qazaz shear

zone. Up to 20 km north of the

Qazaz Dome, this fold is still recog-

nizable because the strike-slip Qazaz

shear zone has gently plunging linea-

tions throughout, but a foliation that

changes from subvertical in the

shear-zone limbs to horizontal in the

centre (Figs 3d, 6e and 7 inset). Fur-

ther NW, only vertical foliations are

present in the Qazaz shear zone.

The west side of the Qazaz Dome

is ﬂanked by a sinistral strike-slip

shear zone, which overprints the

high-grade mylonitic fabrics in the

upper/middle crust

lower crust

upper/middle crust

lower crust

(a)

(b)

Fig. 1 (a) Typical metamorphic core complex formed during regional extension; (b)

strike-slip core complex, formed during regional shortening by local extension asso-

ciated with a jog in a strike-slip shear zone.

Fig. 2 Overview map of the Najd shear zones and the gneiss complexes in the Ara-

bian-Nubian Shield; modified after Abu-Alam and St€

uwe (2009).

Strike-slip core complex •S. E. Meyer et al. Terra Nova, Vol 26, No. 5, 387–394

.............................................................................................................................................................

Road

Lithostratigraphy of the Qazaz Complex

Legend

Gneiss

Monzogranite

Litharenite and Conglomerate, Thalbah Group

Syenogranite

Litharenite and Siltstone, Bayda Group

Amphibolite Facies

Amphibolite F. overprinted by Greenschist F.

Greenschist Facies

Metamorphic grade of deformation zone

10 km

E36.60°E36.40° E36.80°

N26.90°

N26.70°

N26.50°

Very low grade Facies

E36.38°

E36.09°

N27.11°

N26.92°

Qazaz Dome

14 km

Red Sea

Equal-area

Lower hemi spher e

[L] Qazaz-West-ShearZone-Lineation.txt (poles to lines) n=40

[L] Qazaz-East-ShearZone-Lineation.txt (poles to lines) n=30

[L] Qazaz-Decollement-Lineation.txt (poles to lines) n=29

Lower hemisphere

[P(dd)] Qazaz-West-ShearZone-Foliation.txt (poles to planes) n=44

[P(dd)] Qazaz-East-ShearZone-Foliation.txt (poles to planes) n=29

[P(dd)] Qazaz-Decollement-Foliation.txt (poles to planes) n=30

35 70

Equal-area

Lower hemi spher e

[P(dd)] Qazaz_foliation_granite.txt (poles to planes) n=79

[L] qazaz_lineation granite.txt (poles to lines) n=70

E36.38°

E36.09°

N27.11°

N26.92°

Qazaz Dome

14 km

Red Sea

Legend

Grade of Deformation

Strongly deformed - Mylonized

Medium strain

Low strain

Undeformed rocks

Structural Symbols

10 km

Syncline

Anticline

Foliation S1 / Dip

Lineation L1, / Plunge

Qazaz - Eastern shear zone

Qazaz - Western shear zone

Qazaz - Detachment

Shear zones

pole of foliation

Shear Zones

Lineation

Qazaz dome gneiss - Foliation

Qazaz dome gneiss - Lineation

Pole of foliation / Lineation

Brittle Fault

Sense of movement - Strike Slip

E36.60°E36.40° E36.80°

N26.90°

N26.70°

N26.50°

Road

Detachment

Sample P1

7.5 ± 0.5 kbar

560-640 °C

Sample P3

4.7 ± 0.3 kbar

400-600 °C

Sample P2

7.0 ± 0.5 kbar

570-630 °C

Sample P4

0.7 ± 0.2 kbar

430-450 °C

Geothermobarometry

Gneiss Dome

Geothermobarometry, Sample location

Thalbah Group

Dome

Mylonite zones

n = 103

n = 99

n = 149

P4 P3

(a)

(b)

(c)

(d)

Fig. 3 (a) Lithological and metamorphic map of the Qazaz complex; (b, d) inset maps showing the shear zone northwest of the

Qazaz Dome; (c) structural map of the Qazaz complex.

Terra Nova, Vol 26, No. 5, 387–394 S. E. Meyer et al. •Strike-slip core complex

............................................................................................................................................................

western part of the gneiss dome with

a lower grade shear band cleavage.

The western shear zone shows a

steep mylonitic foliation with a west-

wards dip and stretching lineations

that gently plunge with a NW–SE to

N–S trend (Fig. 3c). In the SW cor-

ner of the Dome, the strike-slip shear

zone changes direction and grades

into the extensional detachment

described above; the stretching linea-

tions in both segments have the same

orientation with a plunge of 10–35

degrees (Fig. 3c). There is no indica-

tion of any overprint, suggesting that

the strike-slip and detachment seg-

ments operated simultaneously as

one continuous shear zone. The

gently dipping mylonitic fabric in the

central parts of the dome gradually

steepens and grades into the western

shear zone and southern detachment

without overprint and without a sig-

niﬁcant change in orientation of the

main lineations (Figs 3c and 7).

The eastern branch of the Qazaz

shear zone is a sinistral shear zone

with steep foliations and gently

plunging stretching lineations similar

to the western branch; it likewise has

a greenschist facies mylonitic fabric

with prominent shear band cleavage

that overprints the gently south-dip-

ping amphibolite-grade mylonites of

the central dome. Locally, the older

mylonitic foliation is folded, and two

parallel stretching lineations of dif-

ferent age can be found. The eastern

branch is therefore younger than the

detachment and formed later than

the other structures. The eastern

branch also transects the contact of

the dome with the Bayda group in

the south, interrupted by a sinistral

brittle fault in the wadi (Fig. 3c).

Depth of emplacement

Mineral exchange thermobarometers

(i.e. garnet-biotite of Hodges and

Crowley, 1985; muscovite-plagioclase

of Green and Usdansky, 1986 and

hornblende-plagioclase of Holland

and Bundy, 1994) and the Al-in-horn-

blende barometer of Johnson and

Rutherford (1989) were used to calcu-

late peak metamorphic conditions for

several samples across the shear zone

to determine the maximum depth of

burial. The hornblende-plagioclase

thermometer and the Al-in-horn-

blende barometer are based on cali-

brations of igneous systems, but can

be used for the metamorphic system

(e.g. Mancini et al., 1996; Sch€

arer and

Labrousse, 2003). The calculated

pressure and temperature conditions

based on hornblende-bearing assem-

blages agree with the conditions that

were calculated using the garnet-bio-

tite and the muscovite-plagioclase

thermobarometers. A sample from

the core of the Dome (Fig. 3a, P1:

26.6956°N, 36.6996°E) attained peak

metamorphic conditions of 560–

640 °C and 7.5 0.5 kbar (crustal

depth of 24–28 km; for an overbur-

den density 2850 kg m

3

and

assuming lithostatic conditions). A

high-grade gneiss sample near the

Qazaz dome Detachment

Thalbah group

metasediments Foliation

Shear zone

mylonite

Bayda group

metasediments Strike-slip shear zone

Gneiss

250 m

2 km

250 m

Shear zone Shear zone

Qazaz dome

–125 m

A A′

B B′

Fig. 4 Profiles through the Qazaz Dome, marked in Fig. 3c.

qtz

100 μm qtz

300 μm

qtz+fsp

300 μm

100 μm

qtz

grt

qtz+fsp

(a) (b)

Fig. 5 (a) Quartz (qtz) with irregular grain boundaries with lobate structures, devel-

oped by grain boundary migration (GBM) recrystallization. Amphibolite facies,

centre of the Qazaz Dome. (b) feldspar (fsp) porphyroclast replaced by epidote

(ep). Quartz shows a typical dynamic recrystallization fabric of subgrain rotation

(SGR). Upper greenschist facies, Western shear zone. (c) Quartz with subgrain

rotation (SGR) recrystallization and garnet crystals with irregular rims of plagio-

clase. Lower amphibolites facies, Western shear zone. (d) Relicts of GBM recrystal-

lization, overprinted by lower temperature bulging recrystallization (BLG). Gneiss,

detachment, S-side Qazaz Dome.

Strike-slip core complex •S. E. Meyer et al. Terra Nova, Vol 26, No. 5, 387–394

.............................................................................................................................................................

periphery of the Dome (P2: 26.7455°

N, 36.6193°E) gave a pressure–tem-

perature range of 570–630 °C and

7.0 0.5 kbar (crustal depth of 22–

26 km). A schist sample from the wes-

tern Qazaz shear-zone branch (P3:

26.7347°N, 36.6034°E) reached peak

conditions of 400–460 °C and 4.4–

5.0 kbar (crustal depth of 15.5–

17.5 km). A sample from the base of

the Thalbah group at a distance of

~120 m from P3, SW of the western

Qazaz shear-zone branch (P4:

26.7331°N, 36.6004°E), shows

greenschist facies conditions of 430–

450 °C and 0.7 0.2 kbar (crustal

depth of 1.5–3.5 km). Clearly, the

local metamorphic gradient is tele-

scoped with signiﬁcant uplift of the

dome with respect to the Thalbah

group due to movement on the shear

zones.

The metamorphic grade of myloni-

tization in the Qazaz Dome and the

shear zones was assessed from the

microstructure using recrystallization

characteristics of quartz and feldspar

as an indicator (Stipp et al., 2002;

Passchier and Trouw, 2005). The

microstructure varies consistently

with the barometry (Fig. 3a). The

highest grade fabrics are found in

the central and southern parts of the

dome, with typical high-temperature

grain boundary migration (GBM)

recrystallization of quartz with lobate

grain boundaries, coarse-grained

(>350 lm) recrystallization of feld-

spars, and bulbous feldspar-quartz

boundaries (Stipp et al., 2002;

Fig. 5a). These fabrics grade to the

south and west into greenschist facies

mylonitic fabrics, with subgrain

boundary rotation or even bulging

recrystallization of quartz, recrystalli-

zation to ﬁne grain-size and brittle

deformation of feldspar, and abun-

dant development of shear band cleav-

age (Stipp et al., 2002; Fig. 5b–d).

The greenschist facies mylonites along

the western, eastern and southern

sides of the core complex show

remains of an earlier high-temperature

fabric (Fig. 5d). The overprinting of

coarse-grained quartz bands with

lobate grain boundaries by shear band

cleavage and bulging recrystallization

indicate exhumation and cooling dur-

ing ongoing mylonitization (Fig. 5d).

Garnet crystals in samples from close

to P3 in the western shear zone show

irregular rims of plagioclase (Fig. 5c)

in a primary volcanic rock, an indica-

tor of decompression during the

growth of the crystals. The calculated

peak metamorphic conditions of P3

classify this part as a middle crustal

segment, which could be the deeper

part of the Thalbah group. The kine-

matics of the shear zone and a steep

metamorphic gradient of over 200 °C

and 4.0 kbar in an E–W proﬁle

between the gneiss and the metase-

diments of the Thalbah group show

that the southern Qazaz complex is

a typical extensional detachment

structure.

Discussion

Field and microstructural evidence

shows that the Qazaz Dome is a

high-grade metamorphic dome that

has been brought into contact with

Jog forms

Exhumation of

lower crust during

strike-slip

New shear zone

transects the

complex

Present state

20 km

(a)

(b)

(c)

(d)

(e)

Fig. 6 (a–e) Proposed sequence in the development of the Qazaz complex along the

Qazaz strike-slip shear zone.

Terra Nova, Vol 26, No. 5, 387–394 S. E. Meyer et al. •Strike-slip core complex

............................................................................................................................................................

adjacent low-grade metasediments by

movement on ductile shear zones.

The emplacement of the footwall

high-grade dome by exhumation

along a gently S-dipping detachment,

and emplacement against low-grade

hanging wall metasediments are typi-

cal of a metamorphic core complex

(Tirel et al., 2008; Huet et al., 2010).

However, the relationship with

strike-slip shear zones is different

from other core complexes (Fig. 1).

From ﬁeld and microstructural data,

we envisage the following scenario

for the development of the Qazaz

Dome (Fig. 6). During development

of the crustal-scale vertical strike-slip

Qazaz shear zone between ~630 and

580 Ma, a 10 km size jog formed as

a gently south-dipping detachment,

probably in response to a local pre-

existing fabric (Fig. 6a). Progressive

strike-slip after the jog formed led to

exhumation of the middle and lower

crust underlying the jog zone

(Fig. 6b), which in turn considerably

changed the local strength proﬁle of

the crust. The developing core com-

plex was affected by regional short-

ening oblique to the strike-slip

shear-zone segments. The uplifted

high-temperature dome underwent

ductile E–W internal shortening lead-

ing to folding of the previously

gently dipping planar fabric of folia-

tion in the dome, at some distance

from the detachment (Fig. 6c). Since

the earlier exhumed part underwent

lateral constriction for a longer time,

folding is tightest furthest away

from the detachment zone, leading to

the triangular shape of the dome

(Figs 6e and 7). This process created

an unusual strike-slip shear-zone seg-

ment with an internal antiformal

folded foliation in its core (Figs 3c

and 7 inset).

If the detachment shear zone has a

dip of 40°throughout, as observed

at the surface, 25 km of vertical

exhumation may have been accom-

modated by 30 km of horizontal slip

on the Qazaz shear zone. The trian-

gular core complex that formed in

this way alongside the strike-slip

zone was subsequently transected by

a newly developing eastern branch of

the Qazaz shear zone, which sepa-

rated and displaced its NE side

(Fig. 6d,e). This may have happened

in response to cooling and strength-

ening of the distal side after slip on

the western shear zone ceased. Sinis-

tral displacement on the eastern

branch is estimated to be at least

30 km, based on strain intensity in

the shear zone and displacement of

marker horizons in satellite images

(Fig. 6d). The accumulated displace-

ment along the Qazaz shear zone is

therefore at least 60 km including

the estimated 30 km of strike-slip

displacement associated with exhu-

mation of the central Qazaz Dome.

Finally, a brittle sinistral strike-slip

fault offset the eastern branch itself

in a late stage of Najd shear-zone

activity (Figs 3c and 6e). The defor-

mation pattern in the low-grade

Thalbah and Bayda Group metasedi-

ments surrounding the Qazaz Dome

(Fig. 3c) also supports a model

where the Qazaz Dome formed in a

regime of strike-slip linked detach-

ment, accommodating regional E-W

crustal shortening (Figs 2 and 3c). It

is even conceivable that deposition of

part of these metasediments was con-

nected with development of the

extensional jog.

The most likely candidates that

may show a similar development to

the Qazaz Dome are other gneiss

domes in the Najd Fault system such

as the Hafaﬁt complex (Fowler and

Osman, 2009) and the Kirsh gneiss

Dome (Al-Saleh, 2012; Fig. 2). The

Sha’it-Nugrus shear zone of the Ha-

faﬁt dome shows an identical transi-

tion from a strike-slip to a

detachment fault as does the Qazaz

Dome (Fowler and Osman, 2009).

Gneiss domes superﬁcially similar

to the Qazaz Dome also occur in a

number of other tectonic settings. A-

type domes in the Aegean (Jolivet

et al., 2004b, 2010; Le Pourhiet

et al., 2012) and the gneiss domes in

the central Pyrenees (van den Eeckh-

out and Zwart, 1988; Den

ele et al.,

2007) superﬁcially resemble the

Qazaz Dome, but have a more com-

plex history and no permanent link

to crustal-scale strike-slip shear

zones. Core complexes along the

Lewis and Clark fault zone in the

Rocky Mountains (Foster et al.,

2007) differ from the Qazaz Dome,

in that the core complexes are the

dominant structure, with strike-slip

faults playing an accommodating

role. The Ni

gde massif along the

Central Anatolian fault zone in Tur-

key (Whitney et al., 2007) and

domes along the Red River Shear

Zones in Southern China and Indo-

china may have formed in a similar

way to the Qazaz Dome, but are

reported to have formed with a

more complex history including a

transtension phase (Jolivet et al.,

2001). Further research on these

structures, and along major strike-

slip fault systems, will show whether

10 km

E36.80°E36.40°

°09.62N °

Fig. 7 Annotated satellite image and 3-D cartoon of the present structure of the

Qazaz complex. Black dashes indicate the orientation of the stretching lineation in

the mylonites. Folding in the internal part of the dome is highlighted by an EW

section.

Strike-slip core complex •S. E. Meyer et al. Terra Nova, Vol 26, No. 5, 387–394

.............................................................................................................................................................

other isolated metamorphic domes

formed in a similar way to the

Qazaz Dome.

Conclusions

The development of metamorphic

core complexes is generally thought

to involve crustal-scale extensional

processes of crustal thinning with

exhumation of the lower crust. The

Qazaz Dome shows that core

complexes can form along crustal-

scale strike-slip shear zones which

accommodated crustal shortening.

The synchronous activity of strike-

slip shear zones and a detachment

jog is an extremely effective way to

exhume deeper crustal rocks under

constrictional conditions. Strike-slip

core complexes similar to the Qazaz

Dome may therefore be present,

unrecognized, along many other

crustal-scale strike-slip shear zones

where they have been transposed by

ongoing strike-slip deformation.

Development of a strike-slip core

complex will locally change the ther-

mal proﬁle of the crust and can have

far reaching effects on local crustal

strength and the functioning of crus-

tal-scale strike-slip shear zones.

Acknowledgements

This project was ﬁnanced by the Geocy-

cles and SRFN programs at the Univer-

sity of Mainz. We thank the Saudi

Geological Survey (SGS) for logistic sup-

port in the ﬁeld and further cooperation.

Special thanks to the President of the

SGS, Dr Zohair Nawab and to the direc-

tor, Dr Khalid Kadi. We thank Saad M.

S. Al-Garni, Mubarak M. M. Al-Nahdi

and Wiesiek Kozdr

oj for their support.

This project is part of and was supported

by the Swedish JEBEL research initiative.

We gratefully acknowledge support by

FWF Project P22351-N22 and by the

Sch€

urmann Foundation.

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A GIS-based mineral prospectivity analysis of the Neoproterozoic Arabian Shield

Conference Paper

Full-text available

Aug 2023

The Neoproterozoic Arabian Shield formed following three major tectono-magmatic events during the Cryogenian-Ediacaran Nabitah orogenic cycle, including the pre-accretion, syn-orogenic and late-to post-orogenic stages, representing fertile environment for various mineral systems related to precious, base and rare metals. At the shield scale, the mineral prospectivity analysis that was performed, based on the selective review and reclassification of multiple geological and geophysical datasets, identified a series of mineral belts correlated with suture and shear zones concentrating the majority of mineral deposits and occurrences. (i) Pre-accretion arc-related porphyry, epithermal, VMS mineral systems and magmatic deposits related to ultramafic rocks are predominantly distributed along the Nabitah, Al Amar, Bi'r Umq and Yanbu suture zones. (ii) Orogenic gold veins mainly developed in the N-trending Nabitah shear zone that is coaxial with the Nabitah and Al Amar sutures. Gold was remobilised from source rocks during this syn-collisional tectonic event associated with a peak M1 of low-grade metamorphism. Orogenic gold mineralisation also occurred sporadically along the NW-trending Najd strike-slip fault system developed during late orogenic extension and associated with a peak M2 of high-grade metamorphism, locally. (iii) Finally, magmatic-hydrothermal rare metal deposits formed in association with late-to post-orogenic alkaline, peralkaline and peraluminous granites.

Structure of the Barud Dome Complex in the Eastern Desert of Egypt: Insights from Crustal Origins in the Northern Basement Terrains of the Arabian-Nubian Shield

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Sep 2024

The Barud gneissic dome complex is situated along the ENE trending dextral shear zone of the Qena-Safaga Line that serves as a significant tectonic boundary between the basement terrains of the Northern and Central Eastern Desert. These terrains exhibit distinct differences in crustal composition and deformation style. The Northern Eastern Desert and its extension into Sinai are predominantly composed of gneissic granites that are intruded by large batholiths of calc-alkaline and alkaline granites. Conversely, the Central and Southern Eastern Desert are commonly blanketed by a carapace of ophiolite-bearing volcano-sedimentary rocks of the Pan-African cover nappes. These northern terrains, just north of the Barud dome complex, the crust underwent significant NW-SE regional crustal extension across the Qena-Safaga Line, which sharply delineates the northern limit of the transpressional deformations linked to the Najd Fault System in the Central and Southern Eastern Desert. Through comprehensive geological mapping and the integration of various geophysical, geochemical and geochronological data, this paper offers explanations for the contrasting geological features of the basement terrains on both sides of the Qena-Safaga Line and its analogous Fatira Shear Zone that plays a significant role in tectonic modeling of the Barud dome complex region. The Barud gneissic protolith experienced crustal shortening approximately 697 million years ago in the NW-SE direction, initiating dextral motion along the Fatira Shear Zone. Large batholiths of granodiorite/tonalite complex intruded the Barud gneissic dome protolith around 630 million years ago along the Qena-Safaga Line, at relatively shallow crustal depths, following the same orientation as the earlier shortening direction. Ongoing magmatic activity along the Qena-Safaga Line indicates intense magmatic underplating, resulting in significant intrusions of granodioritic melts into the early rifted crust of the Northern Eastern Desert and Sinai terrains. The crust of these northern terrains likely underwent isostatic compensation through uplifting and subsequent erosion. The disappearance of ophiolite-bearing belts and the presence of Paleo- to Mesoproterozoic continental-derived cobbles and ignimbrites in Sinai metasedimentary belts and Northern Eastern Desert molasse basins suggest that the northern terrains, located north of the Qena-Safaga Line, originated as a cohesive, thin continental crust that rifted off the eastern passive margin of the Sahara Metacraton during the early Neoproterozoic rifting of the Rodinia supercontinent.

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Article

Mar 2023
PRECAMBRIAN RES

Serpentinites play a pivotal role in carrying fluids and different elements into the Earth’s mantle. However, their role in exchanging silica (Si) between the marine environment and the mantle remains a matter of investigation. The Wadi Igla serpentinite (southern Eastern Desert of Egypt) formed at the expense of abyssal harzburgite by ∼15–22 % melting. It contains abundant Cr-spinel with sub-microscopic serpentine and chlorite-rich pores providing a base to explore Si cycling during serpentinization-carbonatization processes. The low-grade metamorphism of the harzburgite protolith started on the ocean floor, forming lizardite and chlorite (250–300 °C). Increasing the temperature (400–450 °C) caused the formation of brucite and antigorite. With the subduction in the fore-arc and the interaction with subducting sediments-related CO2-rich fluid, the Wadi Igla serpentinite underwent metasomatism, producing chlorite (300 °C), antigorite, tremolite, dolomite, and ferritchromite rims around Cr-spinel (Type 1), with brucite loss. In the upper greenschist-lower amphibolite facies (ca. 500 °C), CO2-rich hydrothermal fluids (with XCO2 of ∼0.55) penetrated a large volume of the protolith leading to full serpentinization together with abundant magnesite replacement. The resultant silica-rich fluids percolated in the Type 1 Cr-spinel from the outward to cores through microfractures and pores, producing Type 2 and Type 3 Cr-spinel with serpentine ± chlorite along cleavages, diminished Al-cores and growing outer ferritchromite zone and/or Cr-magnetite to magnetite zones. The suprachondritic NbN/LaN (up to 39.35) and NbN/BaN (up to 13.37) of whole rock implies for HFSEs metasomatism by subduction sediments input components, while slight enrichment in LREEs (LaN/YbN = 2.5–3) and FMEs (Li, Pb, Sr, and Ba) may have resulted from serpentinization-related hydrothermal alteration. The Wadi Igla serpentinite indicates silica cycled in a closed system, suggesting that the altered Neoproterozoic oceanic lithosphere may not have shared their main components with the surrounding environment whether to the ocean floor or the subduction zone.

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Article

Full-text available

Feb 2023

Moujahed Al-Husseini

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Article

Dec 2022
PRECAMBRIAN RES

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Remote sensing techniques and geochemical constraints on the formation of the Wadi El-Hima mineralized granites, Egypt: new insights into the genesis and accumulation of garnets

Article

Full-text available

Aug 2022
INT J EARTH SCI

The Wadi El-Hima Neoproterozoic I- and A-type granites in the Southern Eastern Desert of Egypt are rich in garnets (up to 30 vol%) and are cut by NW–SE strike-slip faults, as confirmed from structure lineament extraction maps. These mineralized granites and garnet mineralization zones can be successfully discriminated using remote sensing techniques. Spectral angle mapper and matched filtering techniques are highly effective for mapping garnet-rich zones and show that the highest garnet concentrations occur along the intrusive contact zone of NW–SE striking faults. El-Hima granites have high SiO2 (73.5–75.1 wt%), Al2O3 (13.4–15.3 wt%) and total alkali (6.7–8.7 wt%) contents, suggesting that they were sourced from peraluminous (A/CNK > 1) parental magmas. Garnet-bearing trondhjemites are metasomatic in origin and formed after I-type tonalite-granodiorites, which originated in a volcanic arc tectonic setting. Garnet-rich syenogranites and alkali-feldspar granites are both post-collisional A-type granites: the syenogranites formed from peraluminous magmas generated by partial melting of lower crustal tonalite and metasedimentary protoliths during lithospheric delamination, and the alkali-feldspar granites crystallized from highly fractionated, felsic and alkali-rich peraluminous magmas in the upper crust. Garnets in El-Hima mineralized granites occur in three forms: (1) subhedral disseminated crystals, (2) vein-type crystals, and (3) aggregated subhedral crystals, reflecting different mechanisms of accumulation. All are dominantly almandine in composition (Alm76Sps10 Prp7Grs6Adr1) and have high average concentrations of heavy rare earth elements (HREE) (ΣHREE = 1636 ppm), Y = (3394 ppm), Zn (325 ppm), Li (39.17 ppm) and Ga (34.94 ppm). Garnet REE patterns show strong negative Eu anomalies with HREE enriched relative to LREE, indicating a magmatic origin. These magmatic garnets are late-stage crystallization products of Al-rich hydrous magmas, and formed at low temperature (680–730 °C) and pressure (2.1–2.93 kbar) conditions in the upper continental crust. Peculiar garnet concentrations in syenogranites near and along contact zones with alkali feldspar granites are related to peraluminous parent hydrous magma compositions. These garnets formed by in situ crystallization from A-type granite melts, alongside accumulation of residual garnets left behind after partial melting of the host garnet-rich granites along the intrusive contact. Magmatic-fluid flow along the NW–SE striking fault of Najd system enhanced garnet accumulation in melts, which formed clots and veins of garnet.

Anisotropic effect of shear zone on groundwater potentiality in crystalline hard rock terrain

Article

Aug 2024

Assessment of potential groundwater recharge sites and sustainable water resource management in semi-arid crystalline rock terrain is a challenging task. Globally, analysis of remote sensing satellite imagery data for delineation of groundwater potential zones over sheared crystalline hard rock terrains has been fairly successful. But there is no existing study present at our disposal which discusses the factors controlling the inconsistent groundwater potentiality that exits along the shear zones. This study attempts to analyse the major geological factors controlling the irregular groundwater potentiality of shear zones within older crystalline rock terrain. Therefore, the study area selected for this analysis is the Purulia district of West Bengal, NE India, composed mostly of Precambrian metamorphic rocks i.e., quartzite, granite gneisses, porphyroclastic granite-gneiss, quartzo-feldspathic-granite-gneiss, mylonitic granites, quartz-biotite-granite gneiss, quartzites, carbonatites and phyllites. Satellite imagery study of IRS-P6 LISS IV standard FCC image reveals the presence of two bifurcating shear zones namely North Purulia Shear Zone (NPSZ) and South Purulia Shear Zone (SPSZ) over the study area. Careful analysis of rock structure, different lithotypes, soil thickness, electrical resistivity tomography data and water table data with an emphasis on high water table fluctuation, shows a strong spatial relation between the potentially good groundwater recharge zones and the branching/confluence sites of shear zones present in the study area. The study constructs an attempt to demonstrate the relationship between shear zone conjunctions and significant groundwater recharge sites in Precambrian crystalline fractured-rock aquifer system.

Strain Analysis and Microstructural Study of Hamadat Core Complex, Northwestern Arabian Shield, Saudi Arabia

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Apr 2024

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Jul 2022
J ASIAN EARTH SCI

Metamorphic core complexes (MCCs) are a domed structures cored by high grade gneiss overlain by low grade supracrustal rocks. They are characterized by some common features such as extensional fabrics, ultra high-grade metamorphic facies, low angle normal fault (detachment fault), strike-slip shear zones surrounded the core complexes and ductile mylontitic shear zone (thrust zone) that separate the overlying low-grade rocks from the lower high-grade rocks. There is a debate about the presence or absence of metamorphic core complexes in the Arabian-Nubian Shield (ANS) especially in its northern part. The gneissic complexes in the ANS are considered as strike-slip core complexes like the Qazaz and Krish Domes whereas, those in the Egyptian Nubian Shield (ENS) are interpreted as antiformal stacks (e.g. Meatiq and Hafafit) formed during thrusting, or core complexes formed during orogen-parallel crustal extension. Some metamorphic core complexes are domes or contain gneiss domes within them, but not all gneiss domes possess the essential elements of a true metamorphic core complex. The most important points that negate the existence of MCCs in the ANS are absence of ultra-high grade metamorphic facies, absence of real low-angle normal faults, not all the gneisses have a domal structures, adjacent syn-extensional basins have fill that is older than the gneissic complexes, and models of ANS core complex exhumation include strike-slip faults with slip senses recently found to be inconsistent with the models. The gneissic complexes in the ANS differ from the Cordilleran-type or Aegean-type metamorphic core complexes. The origin of gneiss domes in the ANS is controversial, and many of them are presumably produced by mechanisms other than horizontal extension. During the oblique convergence of East and West Gondwana, the gneissic complexes in the ENS evolved from pure shear to simple shear-dominated transpression due to oblique convergence between East and West Gondwana along the Mozambique belt.

Detrital Zircon Geochronology and Provenance of the Neoproterozoic Hammamat Group (Igla Basin), Egypt and the Thalbah Group, NW Saudi Arabia: Implications for Collision Tectonics

Article

Full-text available

Jan 2013
PRECAMBRIAN RES

Detrital zircon U–Pb SIMS dating is used to evaluate the provenance of two correlative basins in the Arabian-Nubian Shield (ANS). The Wadi Igla Formation in the Central Eastern Desert (CED) of Egypt and the Thalbah Group in the Midyan Terrane (MT) of NW Saudi Arabia are considered to be post-amalgamation terrestrial basins, developed during closure of the Mozambique Ocean and amalgamation of the ANS in Cryogenian-early Ediacaran time. The analytical results indicate that the upper-part of the Wadi Igla Formation has a maximum depositional age of 628 ± 6 Ma, contains 98% Neoproterozoic zircon with ages between 815 and 628 Ma, and has two distinct peaks at 690 Ma and 652 Ma. A rhyolite clast from the upper-part of the Wadi Igla Formation gives a U–Pb age of 700 ± 6 Ma. This age significantly pre-dates Dokhan volcanism, indicating that the dominant rhyolitic clasts within the Wadi Igla Formation are not from the Dokhan Volcanics, as previously believed. Analytical results from the Thalbah Group suggest multiphase basin formation and development. The lower part of the Thalbah Group is intruded by monzogranites of the Liban complex, has a minimum depositional age of 635 ± 5 Ma, resembling that of the Wadi Igla Formation. Its middle part has a maximum age of 612 ± 7 Ma and is comprised of 90% Neoproterozoic zircon with ages ranging from 820 to 612 Ma. The upper part of the Thalbah Group has a maximum age of 596 ± 10 Ma and contains a wider range of Neoproterozoic detritus with ages between 985 and 596 Ma. The basement of the Thalbah Group is represented by metasediments and metavolcanics of the Zaam Group. The sample collected from the uppermost part of the Zaam Group (Um Ashsh Formation) contains zircon of mostly Cryogenian age (ca. 812–697 Ma) and has a maximum age of 700 ± 4 Ma, suggesting that the Zaam Group might be correlative with the subduction-related metavolcanic and metasedimentary rocks that are overlain unconformably by the Wadi Igla Formation in the CED.

Gneiss domes, metamorphic core complexes, and wrench zones: Thermal and structural evolution of the Nigde Massif, central Anatolia

Article

Full-text available

Oct 2007

The Nigde Massif, a migmatite-cored structural dome in south central Turkey, was metamorphosed, deformed, intruded, and exhumed in an oblique tectonic regime involving a switch from transpression (prograde metamorphism) to transtension (exhumation). An 80 Ma history of transpression and transtension is recorded in metamorphic rocks of the massif, and the last ~50 Ma are recorded in both basement and overlying basin deposits. Nigde metamorphic rocks record a high-temperature, top-to-north, syntranspression ductile fabric that was overprinted by lower-grade, top-to-south, syntranstension ductile shear zones. The 40Ar/39Ar data for hornblende, muscovite, and biotite document rapid Late Cretaceous cooling, with the oldest Ar hornblende ages (88-79 Ma) overlapping U-Pb ages in monazite and zircon (92-78 Ma). Muscovite Ar ages are dominantly 81-79 Ma, suggesting a short time between crystallization of high-temperature metamorphic rocks and their cooling. Biotite Ar ages are 79-74 Ma, with the exception of disturbed samples from the eastern margin of the massif and from Eocene granitic stocks in the dome core. The eastern margin, adjacent to the sinistral Central Anatolian Fault Zone, records another cycle of burial and exhumation that overprinted earlier fabrics and disturbed isotopic systems. These two cycles of burial-exhumation took place repeatedly along the same zone; this may be characteristic of highly oblique tectonic regimes. In the Nigde Massif, burial and exhumation became more localized and of lower amplitude over time, suggesting stabilization of the crust.

Geological Society of America

Article

Cordilleran Metamorphic Core Complexes

Article

Jan 1980

Cordilleran metamorphic core complexes.

Article

This volume, dedicated to Peter Misch, includes 18 contributions (separately abstracted in the Petrology section), grouped into four parts as follows: 1) Introduction (by P. J. Coney) and Overview, pp 3-31, 2) Southern Basin and Range, pp 35-283, 3) Eastern Great Basin, pp 287-423, 4) The Northwest, pp 427-490. A separate list of references cited is provided with each contribution; there is no index. -A.P.

Magnesian hornblende from a metamorphosed ultramafic body in Southwestern Finland: Crystal chemistry and petrological implications

Article

Aug 1996

A synkinematic sample of magnesian hornblende from a Svecofennian ultramafic body in southwestern Finland, metamorphosed under conditions of the upper amphibolite - lower granulite facies, has been structurally refined to R = 0.027 for 1082 reflections. The distribution of cations in M(1), M(2) and M(3) is inferred from site-scattering refinement and the relations between mean bond-length and ionic radius. The Fe2+-ordering pattern is M(1) > M(3) ≫ M(2). Fe2+-Mg partitioning between the M(1) and M(3) sites (KM(1)-M(3)D = 0.85) is consistent with high-temperature hornblende (800° < T < 900°C), whereas Fe2+-Mg distribution between the M(1) and M(2) sites (KM(1)-M(2)D = 0.45) is in better agreement with a lower inferred peak of metamorphism (T = 700°C). Owing to uncertainties in the Mg(M4) estimates, the Fe2+-Mg partitioning between M(2) and M(4), (KM(2)-M(4)D = 4.7 · 10-2) corresponds to an unrealistically high temperature of equilibration (800° < T < 900°C). Compared with other structurally refined calcic amphiboles from ultramafic parageneses, the extent of solid solution toward cummingtonite, 9.8%, is consistent with pressure and temperature being the main controlling parameters. The refined distribution of cations correlates with the physicochemical conditions of peak and post-peak metamorphic equilibration. Thus: (1) the low contents of M4Na and AlT, which indicate a low pressure (<3 kbar), combined with the high temperatures (680° < T < 720°C) required by the amphibole-plagioclase exchange equilibrium, are consistent with a period of decompression and uplift accompanied by heating; (2) the low oxidation ratio, the absence of detectable Cl, and the limited extent of F-for-OH substitution, imply that hydration of the ultramafic body resulted from hydrothermal fluids of low f(O2), near the magnetite-wüstite buffer, and low fugacity of halogens, and was therefore likely unrelated to dehydration of a downgoing slab.

Ternary-feldspar mixing relations and thermobarometry

Article

Jan 1986

Kinematics and timing of exhumation of Eocene metamorphic core complexes along the Lewis and Clark fault zone, northern Rocky Mountains, USA

Chapter

Jan 2007

The Priest River, Clearwater, Bitterroot, and Anaconda metamorphic core complexes of the northern Rocky Mountains were exhumed in Eocene time by crustal extension, which was linked via dextral displacement on the Lewis and Clark fault zone. Detailed geochronology and thermochronology (U-Pb, 40Ar/39Ar, and fission- track) from the Bitterroot complex indicates that extension started at 53 ± 1 Ma and continued until after 40 Ma. New U-Pb zircon and 40Ar/39Ar data from the Anaconda complex and published geochronology from the Priest River complex indicate a similar timing for the onset of major extension and exhumation. 40Ar/39Ar data from the Clearwater complex, which formed within a relay between strike-slip splays of the Lewis and Clark fault zone, are consistent with exhumation during the same time span. The Lewis and Clark fault zone separates ENE-directed extension in the Priest River complex from ESE-directed extension in the Bitterroot and Anaconda complexes. Large-scale extension was transferred eastward on the south side of this fault zone, where stretching lineations in core complex mylonites are oriented ∼104°-110° and coincide with the general trend of the transcurrent faults. Extension and exhumation of middle crustal rocks along the Lewis and Clark fault zone was concentrated in areas that also experienced voluminous Eocene midcrustal magmatism. Extension was probably initiated by a change in plate boundary conditions combined with the rapid influx of heat from the asthenosphere as a slab window opened beneath the western Cordillera, which led to collapse of the Cordilleran orogenic wedge and widespread early Eocene magmatism.

Progressive strain localisation, boudinage and extensional metamorphic complexes, the Aegean Sea case, in Gneiss domes in orogeny

Chapter