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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-Universität Mainz

Cees W. Passchier

Johannes Gutenberg-Universität Mainz

Tamer Abu-Alam

UiT The Arctic University of Norway

Kurt Stüwe

Karl-Franzens-Universität 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.

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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 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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Spatio-temporal position of the Ediacaran Thalbah Basin in the Najd Fault System, Arabian Shield

Article

Jan 2015
GEOARABIA

Richard Al-Husseini

This paper starts with a bibliographic review of the lithostratigraphy and radiometric dating of the Ediacaran Thalbah Group in the northwestern Arabian Shield, Saudi Arabia. It seeks to establish the spatio-temporal position of the group in the ongoing compilation and correlation of Ediacaran–Cambrian sedimentary time-rock units in the Middle East Geologic Time Scale (Al-Husseini, 2010, 2011, 2014). The group is defined and described in the Thalbah Basin, which crops out in the Al Wajh Quadrangle, and is approximately 100 km (NW-SE) by 40 km (SW-NE) in extent (Davies, 1985). The basin is situated within the approximately (ca.) 300 km-long, NW-trending Qazaz Fault Zone of the Najd Fault System. The Thalbah Group consists of three siliciclastic units: Hashim Formation (ca. 1,050–1,300 m thick) and likely coeval Zhufar Formation (ca. 600–1,400 m thick), and the younger Ridam Formation (ca. 1,000 m thick). Recently published U-Pb dating of detrital zircons gave ages of ≤ 596 ± 10 Ma for the Hashim Formation, and ≤ 612 ± 7 Ma for the Zhufar Formation (Bezenjani et al., 2014). The maximum depositional ages of the Hashim and Zhufar formations indicate they are approximately coeval to the lower part of the sedimentary and volcanic rocks of the Jibalah Group (≤ 605 ± 5 and ≥ 525 ± 5 Ma). The latter group was deposited in pull-apart basins along the ca. 600 km-long Rika and several other extensive fault zones of the NW-trending Najd Fault System in the northern and eastern parts of the Arabian Shield. The Qazaz Fault Zone left-laterally dislocated ophiolites of the NE-trending Yanbu Suture Zone (≥ 700 Ma) by about 100 km. The strike of the Qazaz Fault Zone projects into the Rika Fault Zone, along which five major pull-apart basins contain the Jibalah Group. The Rika Fault Zone dislocated by about 100 km the NS-trending ophiolite outcrop belts of the Ad Dafinah and Hulayfah fault zones (sometimes interpreted as parts the Nabitah Suture Zone, 680–640 Ma). Based on the time correlation of the Thalbah and Jibalah groups, and the highlighted structural features, the Rika and Qazaz fault zones are interpreted as a continuous 30 km-wide, 1,200 km-long, N63°W-striking fault zone, the “Rika-Qazaz Fault Zone”, which left-laterally dislocated the Arabian Shield by approximately 100 km after 605 ± 5 Ma and before 525 ± 5 Ma.

Najd Shear System in the Arabian-Nubian Shield

Chapter

Jul 2021

The Arabian-Nubian Shield (ANS) was assembled from juvenile crust during a three-stage Neoproterozoic tectonic evolution involving: (1) intra-oceanic subduction and arc accretion stage, (2) orogenic extension stage, and (3) post-extensional compressional stage. Stage 1 is manifested by ophiolite-decorated arc-arc high-strain zones (suture zones) and calc-alkaline magmatism. The orogenic extensional tectonic stage generated dyke swarms, bimodal volcanism, molasse basins, A-type granite magmatism, low angle normal faulting (LANFs) and metamorphic gneiss complexes. The geological features attributed to this stage have been interpreted in terms of continental rifting, gravitational collapse, crustal and mantle delamination, transpression, escape/extrusion tectonism, and gravitational uplift. The post-extensional compressional stage is typified by dominantly NW–SE trending folds and thrusts, E–W transpression, and the N–S shortening zones. The Najd Fault System (NFS) (ca. 630–540 Ma) to be described in this chapter is attributed by some workers to the orogenic extension tectonic stage and by others to the post-extensional compressional stage. Earlier interpretation connects the NFS to the Najd Orogeny (570–520 Ma). The NFS is one of the largest transcurrent shear systems worldwide and deciphering its kinematic history adds considerably to our understanding of the cratonization of Gondwana, and specifically to mechanisms of exhumation of metamorphic complexes in the ANS. The NFS extends in a NW–SE direction across the Arabian Shield (e.g., Ajjaj, Qazaz, Ruwah, Ar Rikah, and Halaban strands of the NFS) for more than 1300 km (~400 km wide) and continues beneath Phanerozoic cover in Yemen. The NFS is believed to extend into the Nubian Shield (Egyptian Eastern Desert and Sinai). The dominant sense of shearing along the NW–SE trending Najd megashears is sinistral, however, evidence exists for an earlier phase of dextral slip. NE- (to ENE-) oriented shear zones (e.g. the Ad-Damm, Fatima, Idfu-Mersa Alam, Qena-Safaga shear zones) could be Najd-related conjugates or earlier fault systems. The shear and volume strain aspects of Najd shears are described, as are the stress controls on the brittle evolution of Najd faults. The role of Najd brittle structures in hydrothermal mineral deposits and ground water flow patterns are also covered in this chapter.

Silica cycling in Neoproterozoic oceanic lithosphere: A case study from Wadi Igla carbonate-serpentinite (southern Eastern Desert of Egypt)

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.

Infra-Cambrian Wajid Graben and the Mozambique Suture, Saudi Arabia

Article

Full-text available

Feb 2023

Moujahed Al-Husseini

The fault-bounded, NS-oriented Wajid Graben is imaged by seismic profiles below the reflection from the lower Cambrian pre-Siq unconformity (Angudan unconformity, ca. 525 Ma) in southern Saudi Arabia. It is ca. 350 km long, 60–150 km wide, and the graben-infill rocks (3–5 km thick) are buried at a depth of 7–8 km below sea level, and have not been penetrated by a borehole. A clue to the graben’s origin consists of the outline of a seismically transparent, domal feature (ca. 20 km basal width, ca. 2 km crestal height) below the Angudan reflection. The feature is interpreted as a salt dome and assigned to the Ara Group in the South Oman salt basins (ca. 555–538 Ma). Based on the salt-dome interpretation and regional tectonics, the region spanning Oman and the Wajid Graben shared a common tectono-stratigraphy in the East Arabia Terrane from ca. 580 or before. The collision between West and East Gondwana along the Mozambique Ocean Suture occurred much earlier to the west of the Wajid Graben during the Amar Orogeny (ca. 640–600 Ma). The Mozambique Suture zone is either the Amar Suture if the Mozambique down-going slab was east-dipping below the island arc in the Ar Rayn Terrane; or (2) along the eastern flank of the Ar Rayn Terrane if the slab was west-dipping. In northern Saudi Arabia, where the Ar Rayn Terrane is covered by Phanerozoic sediments, its signature is expressed by the NS-oriented Central Arabia Magnetic Anomaly. In southern Saudi Arabia, the suture zone also passes beneath the Phanerozoic cover rocks to the east of the Arabian Shield, and probably emerges along one of flanks of the island arc in the Al Mukalla Terrane in Yemen.

Drowned in granite - retrieving the tectono-metamorphic history of the Janub metamorphic complex, the northernmost part of the Arabian-Nubian Shield

Article

Dec 2022
PRECAMBRIAN RES

The Janub Metamorphic Complex (JMC) in southern Jordan provides new correlative data constraining the transition from compressional to extensional tectonics in the northern Arabian-Nubian Shield. This constraint comes from the identification of extensional mylonitic shear zones affecting both the JMC and some intruding granitoids. The JMC comprises metamorphosed andesitic-dacitic-rhyolitic flows and pyroclastics, meta-volcanogenic sediments, and hornfels. Volcanism ceased by 618 ± 5 Ma. The volcanosedimentary sequence formed in an intra-arc basin in a mature island arc and was then buried and regionally metamorphosed at lower greenschist facies conditions in the waning accretionary phase between 618 and 615 Ma. This is based on the age of crosscutting plutons of the Rumman Suite, which triggered local high-T/low-P metamorphism at ∼ 615 Ma. Rumman granitoids and JMC rocks were deformed by narrow gently-dipping mylonitic shear zones, interpreted as extensional, that ceased operating by 605 Ma, the age of crosscutting undeformed dikes and plutons of the Yutum Suite. At 596 Ma, a late granite intruded the JMC and thermally metamorphosed some of the meta-sediments. This granite was deformed in a Najd-related brittle shear zone at ∼ 590 Ma as recorded by 40Ar/39Ar geochronology. The JMC can be correlated with the higher-grade Abu-Barqa Metamorphic Complex (ABMC), despite tectonics relations being obscured by younger intrusive rocks. The main metamorphic phase in the ABMC occurred at a depth of 18 km at ∼ 620–615 Ma, followed by gradual uplift and high-T metamorphism at 615–610 Ma and exhumation to the surface at ∼605 Ma. Extensional shearing in the JMC and exhumation of the ABMC probably developed in a core-complex like setting triggered by orogenic collapse. Ductile shearing related to collapse ceased before 605 Ma. The 590 Ma brittle shear zone is the first dated Najd-related structure from the basement of Jordan. The central setting of our data allows correlation of metamorphic complexes in Sinai and Saudi Arabia and is consistent with the final assembly of Gondwana, initiation of orogenic collapse, and transition to extensional tectonics occurring at ∼ 620–610 Ma in the northern Arabian-Nubian Shield.

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.

Mapping Main Structures and Related Mineralization of the Arabian Shield (Saudi Arabia) Using Sharp Edge Detector of Transformed Gravity Data

Article

Full-text available

Jan 2022

Saudi Arabia covers most of the Arabian Peninsula and is characterized by tectonic regimes ranging from Precambrian to Recent. Using gravity data to produce the lateral boundaries of subsurface density bodies, and edge detection of potential field data, a new subsurface structural map was created to decipher the structural framework controls on the distribution of gold deposits in Saudi Arabia. Moreover, we detected the relationships between major structures and mineral accumulations, thereby simultaneously solving the problem of edge detectors over complex tectonic patterns for both deeper and shallower origins. Analytic signal (ASg), theta map (TM), TDX, and softsign function (SF) filters were applied to gravity data of Saudi Arabia. The results unveil low connectivity along the Najd fault system (NFS) with depth, except perhaps for the central zones along each segment. The central zones are the location of significant gold mineralization, i.e., Fawa-rah, Gariat Avala, Hamdah, and Ghadarah. Moreover, major fault zones parallel to the Red Sea extend northward from the south, and their connectivity increases with depth and controls numerous gold mines, i.e., Jadmah, Wadi Bidah, Mamilah, and Wadi Leif. These fault zones intersect the NFS in the Midyan Terrane at the northern part of the AS, and their conjugation is suggested to be favorable for gold mineralization. The SF maps revealed the boundary between the Arabian Shield and Arabian Shelf, which comprises major shear zones, implying that most known mineralization sites are linked to post-accretionary structures and are not limited to the Najd fault system (NFS).

Origin of the Volcanic-Arc Signature in Late-Orogenic Granitoids from the Arabian–Nubian Shield

Chapter

Full-text available

Sep 2021

Although formed after subduction tectonics have already waned and terrane accretion has concluded, numerous late-orogenic granitoid phases in the Neoproterozoic Arabian-Nubian Shield (ANS) hold geochemical characteristics typical of the volcanic-arc granites (VAG). The latter are accentuated by the high abundances of LILE and LREE relative to the HFSE and HREE, and thus high Ba/Nb, Ba/Zr, Sr/Y, La/Yb ratios along with fractionated REE patterns. Synthesis of bulk-rock geochemical and zircon Hf isotope data of the ANS late-orogenic granitoids, exclusively dated as ~640-600 Ma, is employed herein to reveal the origin of the VAG signature in many of these rocks. Data of widespread granitoid intrusions reveal that these rocks span typically metaluminous to peraluminous, calc-alkalic I-type to alkali-calcic ferroan A-type granite compositions. Mild-to-strong negative Eu anomalies generally correspond to increasing magmatic differentiation, and transition from hybrid to evolved granites is envisaged for the younger phases, irrespective of their distribution. The available bulk-rock geochemical data and zircon Hf isotope compositions weigh with parental magmas produced by high-temperature melting of subduction-related/subduction-metasomatised lower crustal rocks (e.g., TTGs, high-K mafic rocks, and metasediments) and mixing with mantle-derived sanukitoid-like melts. These magmas then experienced plagioclase-rich/poor fractionation (revealed by variable negative Eu anomalies) prior to crystallization and fractionation toward A-type/ferroan granite geochemistry. The heat source and melting trigger were most likely asthenospheric mantle upwelling during the onset of extensional collapse of the ANS. The widespread distribution of the post-collisional granitic intrusions in most of the shield area can be explained by lithospheric delamination generally results in voluminous rather than restricted linear magmatism. The commonly described VAG signature in some ANS late-orogenic granitoids (<640 Ma) does not, therefore, depict the factual tectonic setting of these rocks, but rather reproduces their source materials and petrogenesis.

PhD manuscript - J. Perret - 2021

Thesis

Full-text available

Jun 2021

Julien Perret

Gold is a siderophile element preferentially concentrated in the Earth’s mantle and core. The understanding of mechanisms leading to its transfer towards the crust requires the study of the geodynamical evolution of juvenile crust portions, i.e., crystallized from melt directly extracted from the mantle. The gold mineral system thus combines the deciphering of crustal fertilization geodynamical processes and subsequent transient tectono-metamorphic setting(s) favorable for remobilization of this metal-enriched reservoir and formation of mineralized occurrences. This approach is applied to the Keraf and Atmur-Delgo sutures which are part of the Arabian-Nubian shield, the world-largest track of Neoproterozoic juvenile crust and one of the main Pan-African gold provinces. Although these structures are considered as insightful windows into the assembly of Gondwana in Sudan and host major gold deposits, their coupled geodynamical-metallogenic evolution remains poorly understood. We report the existence of two magmatic flare-ups with suprachondritic Hf and Nd signals, recording the build-up and maturation of island arcs along these suture zones between 840-810 Ma and 760-655 Ma. The ~185 Myr-long lifespan of the island arc described along the Keraf suture and its predominant juvenile nature likely accounted for crustal gold fertilization at the regional scale. The earliest crustal growth stage, only recorded along the Keraf suture, is coeval to a minor magmatic-hydrothermal gold event. Both the Keraf and Atmur-Delgo sutures keep record of the second island arc flare-up. The matching between field and laboratory data exemplifies the district- to microscopic-scale spatial continuity of structural control on later gold-bearing structures described in this study. On one hand, a gold event occurred between 755-725 Ma, during high-strain progressive deformation expressed by sheath folding and the formation of linear ore shoots under lower amphibolite facies metamorphism. It likely relates to tectonic accretion of the sedimentary wedge at the Atmur-Delgo intra-oceanic subduction site. At the microscopic scale, the ore formation involved the syn-metamorphic remobilization of the gold budget contained in earlier sulfide generations. On another hand, several orogenic gold deposits display intrusion-hosted mineralized extension veins, highlighting a strong rheological control on ore deposition. The fault-valve mechanism induced the formation of mineralized veins, triggered by episodic and localized reactivation of crustal strike-slip shears within the Keraf suture zone until 550 Ma. Gold events described along the Atmur-Delgo and Keraf sutures are the oldest vein-type gold and youngest orogenic gold episodes reported throughout the Arabian- Nubian shield to this day, respectively. The mineral system approach therefore enables district-scale ore targeting by translating magmatic processes and tectono-metamorphic settings into environments and geological features (e.g., relics of an island arc and/or an accretionary wedge, presence of late collisional strike-slip shears) favoring the formation of gold occurrences with very distinct structural, geochemical and timing characteristics in the western Arabian-Nubian shield. This alternative view of ore deposits sharply contrasts with the typological strategy which targets a single class of ore occurrence with an effectiveness limited to the deposit scale.

Genesis of the gneissic core complexes in the Arabian-Nubian Shield and its tectonic implications: A regional overview

Article

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

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

Jan 2004

Geologic evolution of western, central, and eastern parts of the northern Precambrian Shield, Kingdom of Saudi Arabia

Article

Jan 1982
PRECAMBRIAN RES

Structural evolution of the Whipple and South Mountains shear zones, southwestern United States

Article

Jan 1986
GEOLOGY

The Whipple and South mountains of the southwestern United States have undergone a strikingly similar sequence of deformations. In both ranges, gently dipping mylonitic fabrics have been overprinted by successively more brittle structures associated with a low-angle detachment fault. Kinematic indicators reveal that the mylonitic rocks, brittle structures, and detachment faults are kinematically coordinated and were all formed by top-to-the-northeast shear. The structural evolution of both areas can be explained in terms of major, shallow-dipping shear zones that accommodated Tertiary crustal extension. We suggest that detachment faults and associated zones of brecciation, cataclasis, and seismic slip were originally continuous downdip along the low-angle shear zones into mylonitic gneisses formed below or near the ductile-brittle transition. As the mylonites were drawn out from beneath the brittlely extending upper plate, they were progressively uplifted above the ductile-brittle transition and were overprinted by successively more brittle structures.

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

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