Neoproterozoic tectonic evolution and exhumation history of transpressional shear zones in the East African orogen: implications from kinematic analysis of Meatiq area, Central Eastern Desert of Egypt

Abstract

The origin, structural history, and tectonic evolution of core complexes and associated shear zones in the East African orogen are fundamental issues in the recent geological studies. Meatiq dome is one of the best exposed complexes in the Arabian–Nubian shield. The dome is occupied by metamorphic rocks with amphibolite-facies grade, tectonites, and deformed granites which are structurally overlaid by low-grade metamorphosed ophiolitic and island-arc rock assemblage. This tectonic pile is intruded by several granite intrusions through their evolution history. Microstructural, strain, and vorticity data from deformed tectonites were used to investigate the kinematics of rock flow in the Meatiq and Atalla shear zones and to reconstruct their deformation and exhumation history. The strain and vorticity analyses of Atalla shear zone (ASZ) indicate that this zone experienced general shear transpression with a monoclinic symmetry. The mean vorticity number (Wm) estimated with the porphyroclast-based methods ranges from 0.65 to 0.72 which indicate a high pure shear contribution in the shear zone. The numerical estimates allow the kinematic modelling of ASZ which introduces valuable implication about the tectonic framework of the Central Eastern Desert during the collision of East and West Gondwana. The results of satellite image interpretation and structural analysis of Meatiq area reveal a poly-deformational structural history including five stages of deformation during the successive convergence and collision of East and West Gondwana continents at around 600 Ma. The first stage is represented by compressional structures and high-grade metamorphism in the amphibolite enclaves preserved in Um Baanib granite. The reconstructed structural history of the shear zones started with the second structural stage. It involves the initiation of Meatiq shear zone and associated ductile structures and fabrics. The third stage was related to the final oblique convergence and collision of East and West Gondwana and represented by the NW–SE thrust faults, NW–SE folds, and the initiation of Atalla shear zone. The fourth and fifth stages were evolved after the crustal thickening. They involve the initiation of the strike-slip faults and the NW–SE extensional structures. The tectonic and regional implications of the structural and kinematic analysis were discussed.

Introduction

East African Orogen (EAO) is a major Precambrian orogenic belt that formed during the Pan-African orogeny (950–550 Ma) by the Neoproterozoic collision of East and West Gondwanaland (Kröner 1984; Stern 1994). It is divided into Arabian–Nubian Shield (ANS) in its northern extent and Mozambique Belt (MB) in their southern extent. The ANS is composed of accreted and cratonized island-arc terranes that amalgamated to each other by ophiolite-decorated suture zones (Abdelsalam and Stern 1996, Johnson et al. 2011). The juvenile crust of the ANS hosts a series of domal structures (e.g., Meatiq, Hafafit, and Qazaz), which described as core complexes of amphibolite-facies metamorphic rocks overlain by low-grade ophiolitic-island-arc assemblages (Fig. 1a) (e.g., Ries et al. 1983; Habib et al. 1985; El Gaby et al. 1984, 1990; Fritz et al. 1996; Loizenbauer et al. 2001; Fowler and Osman 2009). Several structural models were proposed to interpret the geometry and structural evolution of these metamorphic complexes in the ANS including evolution within a mid-crustal compressional shear zone in island-arc or continental margin setting (Sturchio et al. 1983), antiformal stack over fault bend (Greiling 1997), ductile shearing related to extensional gravitational sliding (Fowler and El Kalioubi 2004; Fowler and Osman 2009), metamorphic core complex related to NW–SE wrench corridor and orogen-parallel extension (e.g., Fritz et al. 1996, 2002, 2014; Loizenbauer et al. 2001; Shalaby 2010; Hamdy et al. 2017), crustal-scale sheath folds (Fowler and El Kalioubi 2002), transpressional positive flower structures (Makroum 2017), and extensional metamorphic core complex (e.g., Andresen et al. 2010).

Fig. 1

a Distribution the core complexes in the Arabian–Nubian shield and its relation to the Najd-shear system (M Meatiq, Si El-Sibai, Sh El-Shalul, Ha Hafafit, B Beitan, A Abu Swayel-Haymur, D Abu Hodeid, Fe Feiran-Solaf, K Kid, Q Qazzaz, Aj Ajjaji, WWajiyah, N An Nakhil, Kir Kirsh, Haj Hajizah-Tin, H Habariyah), b location map shows Meatiq area and surroundings in the Central Eastern Desert of Egypt

Meatiq area comprises best example of the domal structures in the Central Easter Desert (CED) of Egypt (Fig. 1b). The CED is dominated by NW–SE structural fabrics that extend northward to the Northern Eastern Desert and truncated southward by the E–W structural domain of El-Barramia—Wadi Mubarak belt (e.g., Greiling et al. 1994; Shalaby et al. 2005; Abd El-Wahed and Kamh 2010). Ductile shear zones are important structural elements in the architecture of the ANS (e.g., El Kazzaz 1996, 2001, 2009, 2012; El Kazzaz and Hamimi 2000; El Kazzaz and Taylor 2001; Abd El-Wahed and Kamh 2010). They extend for several kilometers across the Precambrian basement and represents major crustal boundaries between different structural domains. The Najd-Shear System (NSS) comprises the most pervasive shear system in the ANS. It consists of NW–SE parallel strands of brittle–ductile, sinistral shear zones that extends for about 1100 km in the Arabian shield (e.g., Moore 1979; Stern 1985; Stacey and Agar 1985; Agar 1987). The recent structural studies interpreted several shear zones in the Nubian shield as eastern continuation of the Najd-Shear System, and emphasized their extensive of role in the structural reshaping of the CED of Egypt (e.g., Stern 1985; Sultan et al. 1988; Fritz et al. 1996, 2002; Shalaby et al. 2005; Shalaby 2010). Two main ductile shear zones are observed in the study area through the structural mapping: Meatiq shear zone (MSZ) and Atalla shear zone (ASZ). The first one is well exposed in eastern part of the study area and represents the deep ductile root of major sub-horizontal thrust nappe, while Atalla shear zone (ASZ) is an NW–SE curvilinear sub-vertical strike-slip ductile shear zone that extends along the western side of Meatiq area. Although several shear zones had been identified in Meatiq area, there is still pronounced debate about their structural nature, geometry, and kinematic history. The kinematic analysis aims to reconstruct the incremental deformation history of a definite geological province using the fossilized indicators of the flow parameters operative during deformation (e.g., Simpson and De Paor 1993). Quantifying the strain magnitude and geometry, and vorticity is essential for accurate understanding the kinematics of ductile flow in high-strain zones, determining displacement across them and reconstructing the deformational stages at the tectonic scale (e.g., Ramsay and Graham 1970; Bailey et al. 2004).

The present contribution aims to clarify and reinterpret the structural history and tectonic evolution of the shear zones in Meatiq area using new structural and kinematic data. The study integrates different levels of structural analyses (i.e., analysis of satellite images, field investigation, analysis of orientation data, and kinematic analysis) to reconstruct the structural stages prevailing through the final stages of Pan-African orogeny. Landsat-8, Sentinel-2, and the high-resolution Quickbird Satellite imagery were used to emphasize the large-scale geological structures and trace the major strike-slip faults in the area. The outcomes of microstructures, three-dimensional strain data, and vorticity analysis were integrated to establish a new quantitative kinematic model for ASZ. The proposed model quantifies the kinematics of transpressional shear zones in CED and allows the numerical estimation of the total displacement, as well as shortening across ASZ. The structural evolution and exhumation history of shear zones in Meatiq area are finally discussed.

Geology and tectonic setting

The Precambrian rocks of the CED are classified based on their structural position and configuration into two tectono-stratigraphic units (tiers) (Greiling et al. 1994), which are equivalent to infrastructure and suprastructure units of El Gaby et al. (1990). The structurally lower unit “Tier 1” include the medium-to-high-grade (amphibolite-facies) metamorphic rocks and highly deformed plutonic rocks which exposed in the domal structures (e.g., Sturchio et al. 1983; Fritz et al. 1996, 2002). The upper unit “Tier 2” is buildup of low-grade metamorphic assemblages including dismembered ophiolites (e.g., Fawakhir, El-Rubshi), island-arc metavolcanics, as well as molasse-type sediments (e.g., Hammamat, Um-Esh–Um Seleimat, wadi Kareim, Zeidun). In the present study, the complex tectonic pile of Meatiq area is classified in terms of two main tectono-metamorphic units, namely, Meatiq and Atalla successions (Hassan et al. 2017), which are, respectively, equivalent to Tier-1 and Tier-2 of Greiling et al. (1994). Each succession is subdivided into numerous smaller mappable units which intruded by several granitic intrusion at various stages in the tectonic history (Fig. 2).

Fig. 2

Geological map of Meatiq area

Several shear zones have been identified in Meatiq area. The ductile fabrics and structures recorded in the medium-to-high-grade metamorphic rocks were interpreted as vestiges of earlier sub-horizontal shear zone of either extensional (e.g., Fowler and El Kalioubi 2004) or compressional origin (e.g., Sturchio et al. 1983; Wallbrecher et al. 1993). Fritz et al. (1996, 2002) and Loizenbauer et al. (2001) interpreted the steeply dipping eastern and western flanks of Meatiq core complex as two crustal-scale NW–SE strike-slip shear zone-related to Najd-shear system that enclose the Meatiq core complex in a regional wrench corridor with sinistral sense of movement. The Western part of Meatiq area is transected by the pervasive, 8 km-wide Atalla shear zone (ASZ; online source 1). The structural nature of ASZ is still great matter of contrary, where the structural configuration of ASZ has been interpreted in terms of two opposing models. Some researchers interpreted it as NW–SE strike-slip shear zone with a sinistral sense of shearing related to initiation and evolution of the Najd-shear system of the Arabian shield (e.g., Sultan et al. 1988; Greiling et al. 2014; Zoheir et al. 2018). Others interpreted the ductile fabrics in Atalla shear zone to be formed within a sub-horizontal shear zone with a top-to-NW sense of shearing (e.g., Fowler and Osman 2001; Fowler and El Kalioubi 2004). The subsequent NE–SW shortening caused the folding of this sub-horizontal shear zone.

Metamorphic successions

Meatiq succession

This succession represents the structurally lowest rock unit in the area (Ries et al. 1983). It consists mainly of amphibolite-facies schists and mylonite which are structurally overlain by low-grade green-schist facies metamorphic rocks of Atalla succession. This succession is differentiated into three formations based on the petrographic description and structural relations from the lower to the upper: (1) Um Esh- El-Hamra Formation; (2) Abu Zohleiqa Formation; and (3) Abu-Fannani Formation (Fig. 2). These units are exposed in a major elliptical structure called Meatiq dome. The thickness of formations shows a gradual increase toward the west. Um Esh El-Hamra Formation is highly heterogeneous and composed mainly of dark-grey biotite schists, reddish-brown to pale-grey quartzo-feldspathic schists, mylonites, and minor intercalated hornblende schist. This unit is well exposed as circular ridges overlaying Um Baanib deformed granite and constituting the core of Um Esh El-Hamra antiform (Fig. 2). Abu Zohleiqa Formation is made up of phyllonites, muscovite- schist, garnet- muscovite schist, biotite schists, micaceous quartzite, and amphibolites, intercalated with variable cataclased sheets of granite. Abu-Fannani Formation constitutes a transitional unit between Meatiq succession and overlying Atalla succession. This unit comprises a thick section of an interbedded pelitic and semipelitic schistose sequence that consisting of biotite schist, hornblende–actinolite schist, amphibolites, garnet–biotite schist, and graphite schist. A sedimentary origin for the schists and mylonites of Meatiq succession was suggested by Ries et al. (1983), Neumayr et al. (1996, 1998) and Loizenbauer et al. (2001).

Atalla succession

This succession built up of highly sheared low-grade metamorphosed ophiolites, island-arc metavolcanics, and deformed molasse sediments. The rock units of this succession are subjected to low-grade metamorphism under green-schist facies conditions.

Serpentinites and related rocks such as talc-carbonates are the most common unit of ophiolites in the study area. They are best exposed and distributed in several locations including Gabal El-Rubshi, Wadi Um El-Saneyat, Atalla shear zone, and Wadi El-Sid. Serpentinites of Um El-Saneyat area comprising two large separated bodies trending nearly in E–W direction, in addition to a number of elongated small bodies. They are structurally incorporated with Abu-Fannani schists and sheared metavolcanics. These sheets are locally altered into talc-carbonates which occur as pockets and slices near shear planes. In Atalla shear zone, serpentinites, Metagabbro and metavolcanics are represented mainly by elongated lenses and sheets of variable size trending in the NNW–SSE direction, and bounded by a mélange matrix of sheared metavolcanics and molasses metasediments. Serpentinites are usually converted into talc-carbonates and talc schist with net-vein streaky textures and abundant magnesite veinlets. Basic and intermediate metavolcanics blocks in Um Esh area show a characteristic pillow structure that reflects the ophiolitic origin of these metavolcanics. These rocks are of andesitic basalt to basaltic in nature with a tholeiitic affinity (e.g., El-Sayed et al. 1999; Farahat 2010). Molasse sediments form an NW-trending strip extending from Wadi Um Esh in the north to Wadi Um Seleimat in the south and are bordered from all sides by the ophiolitic mélange rocks of Atalla shear zone. They composed mainly of highly deformed conglomerates and greywackes that are intercalated with the surrounding ophiolitic mélange. Conglomerates consist mainly of poorly sorted sand and gravel-size clasts that constitutes mainly lithic fragments of basic metavolcanics, serpentinites, metagabbro, and alkali granite.

Granitoid intrusions

Um Baanib deformed granite

Um Baanib deformed granite crops out in the core of the Meatiq antiform, covering an area of about 100 km², and elongated in NW–SE direction parallel to the axis of the antiform. This granite intruded the Meatiq Succession as well as amphibolites and intruded by the Arieki granite. The intrusive contact is well preserved locally and can be easily recognized, although it is affected by thrust contacts which overprint the intrusive relations. The pluton is buildup of highly deformed granite with a monzogranite composition (e.g., Habib et al. 1985). The mineral crystals are highly stretched in aligned domains which give the rock its gneissose appearance. The granite locally hosts amphibolite xenoliths and enclaves which are up to several tens of meters in size. Amphibolites show a well-developed thin gneissic banding with partial melting in some enclaves. The intrusive relation between granite and amphibolites is overprinted by later thrusting in several localities. Recent geochronological data of Andresen et al. (2009) assigned a 630 Ma for the crystallization of Um Baanib granite.

Abu-Ziran granitoids

This granite is well exposed in wadi Abu-Ziran along the Qift-Quseir asphalt road and occupies an area of about 19 Km². It is oval-shape pluton, elongated in the WNW–ENE direction. The length of the granite outcrop, which is orientated subparallel to S2 foliation in Abu-Fannani Formation, is about 10 km and the width is about 1.9 km. Abu-Ziran granite intrudes the Meatiq Succession parallel to the foliation trend with sharp intrusive contact, while itself is intruded by the younger Arieki granite. It ranges in composition from granodiorite to tonalite varieties. The crystallization age of the Abu-Ziran pluton was dated as 614 Ma (Stern and Hedge 1985), and 609–606 Ma (Andresen et al. 2009).

Fawakhir granitoids

This intrusion occupies the southwest corner of the mapped area along the entrance of Wadi Hammamat. It is a rhomb-shape pluton that covering an area of about 23 km² that intruded the ophiolites of Fawakhir area. The Fawakhir granites are composed of two compositionally granitic phases; an earlier grey tonalitic phase that intruded by a larger pink alkali-feldspar granite. The geochronological data reveal a 638 Ma for the grey tonalitic phase (Zoheir et al. 2018) and 598 Ma for the alkali granite phase (Andresen et al. 2009).

Arieki granitoids

This granite occupies the central part of the Meatiq dome covering a semicircular area of about 35 km². It is intruded the rock units of the Meatiq succession (quartzo-feldspathic and muscovite schists) as well as Abu-Ziran syn-tectonic granites. Arieki granites are made up of alkali-feldspar granite that grades into granodiorite. The estimated crystallization age varies from 590 Ma (Andresen et al. 2009) to 579 Ma (Sturchio et al. 1983).

Structural architecture

A systematic approach was undertaken during structural fabric analysis and synthesis of data. The field-based structural attributes to simplify the outcrop analysis are well documented in recent past (Goswami et al. 2017, 2019; Goswami and Upadhyay 2019; Mukherjee et al. 2018). For practical reason, we considered all the types of outcrops available in the field. Some are two-dimensional planar surface outcrops and some are curved cross sections and vertical planes as well. The overprinting relations at all scales (Macroscopic, Mesoscopic, and microscopic) were uses as an effective tool for ordering the different generations of fabrics and reconstruction of the deformation phases in the light of the criteria proposed by Park (1969) and Williams (1970).

Foliation and lineation

Three different generations of foliation observed in the rocks of the study area. The oldest one (S₁) is restricted to the amphibolitic enclaves within Um Baanib granite and represented by the gneissic banding and schistosity. The orientation of this foliation is varying due to the reorientation of amphibolite enclaves as a result of the emplacement of Um Baanib granite. The second generation of foliation (S₂) is most pervasive in the Meatiq succession rocks. It varies from schistosity to mylonitic fabric in the high-strain zones. The planar fabrics are defined by a strong preferred orientation of biotite, muscovite, hornblende, flattened quartz, and feldspar grains. S₂ varies in dip from 30° to about 65°, and generally shows a systematic variation across the different flanks of Meatiq dome due to the later F₃ folding event which bends the foliation surfaces and change their original orientation. Restoration of the inclined flanks of Meatiq dome to the pre-folding (F₃) state indicates original sub-horizontal attitude of S₂ foliation. L₂ Mineral-stretching lineation is commonly developed along S₂ planes and defined by the elongated quartz aggregates, mica, and garnet crystals. The L_2-stretched mineral lineation plunges shallowly both NW and SE. The third generation of foliation (S₃) is well developed in the ophiolitic mélange and metavolcanics along Atalla shear zone. They mostly striking NW–SE with a sub-vertical to steeply dipping planes (Fig. 3a). The attitude may locally change within deformed blocks due to the F₃ folding. L₃ lineations are defined by the long axes of stretched pebbles in deformed conglomerates (Fig. 3b), stretched mineral lineations in the metavolcanics and amphibolites, and stretched actinolite fibers in highly sheared serpentinites. L₃ lineations are generally sub-horizontal trending NW–SE or plunging shallowly with angles do not exceed 30° toward NW or SE.

Fig. 3

a S3 vertical foliation with boudinaged quartz vein. b The sub-horizontal L3 stretched conglomerates along Atalla shear zone, c recumbent fold in quartzo-feldspathic and muscovite schists, d verturned similar fold, e parasitic folds with the characteristic asymmetric S-, Z- & M-forms. f Overturned to recumbent F3 folds associated with large thrust faults

Folds

Three different generations of folds have been observed in the rock sequence of Meatiq area and they are labeled as F₁, F₂, and F₃ based on temporal relationships. The threefold generations were chronologically ordered based on the cross-cutting relationships. F₁ folds are limited only to amphibolite enclaves in Um Baanib granite and not observed in the medium-grade metamorphosed of Meatiq succession and surrounding low-grade ophiolites. Although it is difficult to accurately measure the exact orientation of F₁ fabrics and folds, as they are re-oriented during the emplacement of the Um Baanib deformed granite and subsequent deformation, in general, they plunge toward ESE with shallow angles not exceeding 20–30°. F₁ folds and associated fabrics represent the older structural fabrics in the area. F₂ folds are one of the most characteristic and spectacular features of Meatiq succession. They are clearly visible at mesoscopic and microscopic scales and well exposed in quartzo-feldspathic schists, as well as biotite and hornblende schists of Abu-Fannani thrust sheet. They are frequently tight to isoclinal folds with an NE–SW steeply inclined to horizontal axial planes and fold axes that plunge moderately both NE and SW (Fig. 3c, d). The F₂ fold axes are nearly perpendicular to the mineral stretching lineation. Three distinct types or styles of F₂ observed in the medium-to-high-grade rocks of Meatiq succession. The first one is characterized by a multi-layer, nearly harmonic folding, with overturned to nearly horizontal axial planes (recumbent folds), and a well-defined curved to subangular hinge zones (Fig. 3d). The second style is typically preserved in quartzo-feldspathic schists, and they are frequently tight to isoclinal folds with subangular to highly angular sharp hinge zones and relatively straight limbs (i.e., chevron folds). Parasitic folds are well-developed in this style, and exhibit the characteristic pattern of asymmetry (Fig. 3e). The third style is the intrafolial folds which are well- developed in foliated rocks as well as quartz veins and igneous dykes. They are commonly shown a well-developed transposition and boudinaged into several fragmented rootless intrafolial folds as a result of the progressive stretching of the fold limbs in direction subparallel to the axial planes. Small circular and eye-shape non-cylindrical sheath folds with highly curved fold axis were observed in the quartzo-feldspathic schists. F₂ folds were developed in Meatiq ductile shear zone.

The F₃ folds occur at scales ranging from mesoscopic outcrop scale to macroscopic kilometer scale (Figs. 2, 3f). They are commonly open to gentle folds with an NW–SE upright to steeply inclined axial planes and fold axes that plunge moderately towards the SE. Two large-scale antiforms (Um Baanib antiform and Um Esh El-Hamra antiform) with intervening synform (Abu Zohleiqa synform) have been deduced in the Meatiq Succession, where the fabrics of Meatiq shear zone have been folded to its present-day orientation. Um Baanib antiform is cored by the oval-shape deformed pluton of Um Baanib granite, while the limbs dip in all directions around the core of antiform with variable dip angles. Abu Zohleiqa synform is a roughly symmetrical plunging synformal structure, where the limbs dip both NE, and SW with average dip angle about 20–25°. Um Esh El-Hamra antiform is relatively asymmetrical plunging antiformal structure. The limbs of the fold dip NE and SW with average dip angle about 25°and 55°, respectively (Fig. 4). The folds of Meatiq successiondie out toward the north, and south, and cannot be traced within the surrounding ophiolitic assemblage. Some styles of F3 folds exhibiting sub-horizontal axial planes with overall recumbent geometry. These folds are usually gentle to open, rounded hinges (Fig. 3f). F₃ folds are the youngest fold generation, where the large-scale F₃ folds folded the earlier Meatiq shear zone and associated F₂ folds.

Fig. 4

Stereograms of the structural elements of Meatiq dome. All of the data contoured at 3%, 6%, 9%, 12%, 15%, 18%, 21%, 24% and 27% contours

Thrust faults

Thrust faults are mainly striking in NW–SE direction, and dipping both NE and SW. They are represented by imbricated thrust sheets that distributed throughout the study area and mark the tectonic boundaries between the different thrust sheets in Meatiq succession, as well as surrounding ophiolitic mélange (Fig. 5). Based on the dipping of fault planes, two main types of NW–SE thrusts observed in the study area, NE-dipping, and SW-dipping thrusts. The thrust faults are well exposed in three main localities; East Meatiq, west Meatiq, and Fawakhir. The thrust faults in the Eastern side of Meatiq dome are trending NNW with moderate dipping (around 35–40°) toward NE. They marked by the imbricated thrust duplexes in the quartzo-feldspathic schists, biotite schists, and quartzites. Several fault planes are curved and characterized by the staircase geometry (Fig. 5e, f).

Fig. 5

Thrust and extensional faults in Meatiq area. a Thrust wedge and duplex structure in Um Baanib granite, b thrust duplex in Abu-Ziran granite, c thrust faults in the Fawakhir serpentinites, d panoramic view of imbricate thrusts in the quartzo-feldspathic and biotite schists, e thrust fault in serpentinites with a flat and ramp geometry, f sketch illustrate the geometry of structures in image e, g normal fault in the southeastern flak of Meatiq dome, h small-scale normal faults in faulted quartz vein

Extensional faults and joints

They are represented by the NE–SW normal faults and joints. The normal faults extended along the northern and southern flanks of Meatiq antiform and they are dipping gently to moderately to the NW and SE (Fig. 5g). They usually truncate the thrust faults and other compressional structures and juxtapose the medium–high-grade rocks of Meatiq succession with the low-grade ophiolitic mélange. Small-scale normal faults observed along the northern and northeastern parts of Meatiq antiforms in faulted quartz veins (Fig. 5h). Joints and vertical fractures observed in association with normal faults. They are oriented in NE–SW trend with a nearly vertical dip.

Strike-slip faults

Strike-slip faults are represented by three main trends including N–S, NW–SE, and NE–SW faults (Fig. 6). The N–S faults are steeply dipping faults with a right-lateral strike-slip movement as inferred from displacement marker horizons. These faults are readily recognized in the high-resolution imagery and post-date all ductile fabrics of the region. A major N–S fault cross-cutting the western part of the study area, runs for a distance of about 20 km, and extends further to the south beyond the mapped area. The measured displacement along the fault trace attains a range of about (1–1.3 km). The NW–SE and NE–SW faults are major map scale faults cross-cut the N–S dextral faults. The NW–SE exhibits a left-lateral sense of movement, while the other NE–SW indicates a right-lateral sense. The NW–SE faults are commonly sinusoidally curved with strikes mainly NW–SE and deflect to a WNW–ESE along their termination. NE–SW right-lateral strike-slip faults are less common in the area. The two fault trends are cross-cutting each other with an acute angle between the two main trends are about 60° (Fig. 6). Based on their geometrical and angular relationships, the two fault trends are interpreted as complementary conjugate shears.

Fig. 6

Brittle strike-slip faults of Meatiq area. a Major dextral N–S strike-slip fault. b NWSE sinistral strike-slip fault, c NE–SW dextral strike-slip faults, d the conjugate sets of NWSE and NE–SW faults along the western boarder of Meatiq dome, e Landsat 8 false color composite 7–6–4 showing Atalla shear zone and their cross-cutting strike-slip faults, f sketch of the major strike-slip faults in the image e, g the geometrical relation of the different sets of strike-slip faults

Methodology

The kinematic history of shear zones in Meatiq area is evaluated through the integration of microstructure analysis, strain analysis, and vorticity analysis of mylonite from Meatiq and Atalla shear zones coupled with field-based mapping and characterization. The results were used to characterize the deformation behavior of ductile flow in the shear zones and to reconstruct their deformation history.

Strain analysis

Shear zones are mostly zones of localized and concentrated strain in the lithosphere (e.g., Fossen and Cavalcante 2017). Different strain analysis methods were proposed in the literature (e.g., Ramsay and Huber 1983) depending on the type of strain marker used in the measurement. Strain analysis seeks to understand the spatial variations in the strain intensity and symmetry and to evaluate their kinematic significance in the context of regional tectonic framework (Ramsay and Huber 1983; Mookerjee and Peek 2014; Fossen 2016).

In the present study, finite strain analysis is performed on the deformed conglomerates in the Atalla shear zone and stretched crystals of Um Baanib granite to characterize the strain state in both shear zones and to determine the main direction of principal strain axes. Three different techniques are used in this analysis based on strain markers; R_f/ф method (Ramsay 1967; Lisle 1985), Flinn diagram (Flinn 1962), and Nadai-Hsu diagram (Lode 1926; Nadai 1963; Hsu 1966; Hossack 1968). The strain magnitude parameters are calculated based on the equations of Nadai (1963) and Watterson (1968).

Vorticity analysis

Natural shear zones deviate from the ideal progressive simple shear conditions and exhibits varies degree of non-coaxiality (e.g., Ghosh and Ramberg 1976; Passchier 1987; Simpson and De Paor 1993; Wallis et al. 1993; Xypolias 2010; Fossen and cavalcante 2017). The quantitative assessment of the relation between vorticial and stretching components of ductile flow is important to characterize their degree of non-coaxiality (Xypolias 2010). In conditions of plane strain, the relative contribution of pure shear and simple shear can be quantified in terms of the kinematic vorticity number W_k (e.g., Means et al. 1980). Natural shear zones display a spectrum of the relative contribution of pure and simple shear that range from simple shear domains with W_k values between (1.0–0.95) and pure shear domain with W_k values between ranges between (0.3–0.0). Shear zones of the intermediate contribution of both types termed general shear zones with W_k values (0.95–0.3).

The mylonites from Meatiq and Atalla shear zones are well examined under the microscope to determine the best methods to quantify the kinematic vorticity number for each one. Three different approaches are used to evaluate the kinematic vorticity of Meatiq and Atalla shear zones. The abundance of porphyroclasts which meet the criteria of Passchier (1987) favors their usage as vorticity gauge. All porphyroclast-based methods are based on the theoretical work of Jeffery (1922) and Ghosh and Ramberg (1976) which emphasize the relation between the rotational behavior of rigid elliptical porphyroclasts in high-strain rocks and the relative amount of simple and pure shear. In the present study, two different porphyroclasts-based methods are applied to the examined samples; the Porphyroclast Aspect Ratio (PAR) method (Passchier 1987; Wallis et al. 1993), and the Rigid Grain Network (RGN) method (Jessup et al. 2007). In both methods, the outlines of porphyroclasts were determined and the best-fit ellipse shapes were chosen for the measurement. The major (m₁) and minor (m₂) axes were measured for calculating the aspect ratio and the angle between the major axis and macroscopic foliation (ф). The results were plotted on the Porphyroclast Aspect Ratio (PAR) graph (Wallis et al. 1993; Law 2010) and Rigid Grain Net (RGN; Jessup et al. 2007).

The third approach is R_XZ/θ′-method (e.g., Tikoff and Fossen 1995; Bailey et al. 2004; Xypolias 2010) which uses the strain ratio (R_s) and the orientation of finite strain ellipse (foliation plane) with respect to the shear zone boundary (θ′).

Deformation mechanisms

The microstructural features and deformation mechanisms of Meatiq and Atalla shear zones were assessed under the microscope from thin sections cut normal to foliation and parallel to lineation (XZ section of strain ellipsoid) from oriented samples. The deformational behavior of quartz and feldspar minerals were used as indicator for the operating deformation mechanisms and dynamic recrystallization (e.g., Schmid 1982; Tullis et al. 1982; Tullis and Yund 1991; Passchier and Trouw 2005). Microscopic deformation mechanisms act as a thermometer for the deformation processes (e.g., Stipp et al. 2002; Faghih and Soleimani 2015; Sarkarinejad et al. 2015, 2016). The mineral assemblage was used to confirm the estimated deformational temperatures.

Shear-sense indicators

The shear-sense indicators are asymmetric elements that developed in faults and shear zones as a response to shearing processes (e.g., Passchier and Trouw 2005). Several forms of mesoscopic and microscopic features were used to interpret the sense of movements in both ductile and brittle shear zones including; porphyroclasts (e.g., Passchier and Simpson 1986), mineral fishes (e.g., ten Grotenhuis et al. 2003), C–S fabric (e.g., Berthé et al. 1979), intrafolial folds (e.g., Trouw et al. 2009), and C-axis fabrics (e.g., Law 1986).

The shear-sense indicators of Meatiq and Atalla shear zones were assessed both in the field from macroscopic structural features and under the microscope from thin sections cut normal to foliation and parallel to lineation (XZ section of strain ellipsoid) from oriented samples.

Results

Strain analysis

Meatiq shear zone

The strain symmetry of MSZ is evaluated based on the stretched quartz grains and mica aggregates in Um Baanib deformed granite. Rock samples are polished in three mutually perpendicular faces parallel to the three main strain axes (X, Y, and Z). The shape and dimensions of the quartz grains and stretched mica aggregates were obtained by measuring their lengths. The axial ratios of the stretched grains were plotted on Zingg (1935) diagram to characterize their three-dimensional shape. The results reveal the dominance of blade and rod shapes (Fig. 7a). R_xy and R_yz are calculated for the stretched grains and plotted on Flinn Diagram (Fig. 7b), whereas the ν and ε_s values are plotted to produce a Nadai–Hsu diagram (Fig. 7c). The results show that all samples are concentrated in the prolate with a relatively higher strain intensity. The averaged k values are found to be ranging from 18 to 2.3 with mean value 4.9 and lode’s factor (ν) values ranging from (− 0.99) to (− 0.25) with mean value about (− 0.65) that reflects a prolate strain ellipsoid. The ε_s estimates vary from 5 to 0.9 with average value of about (2.6). These results indicated that at least parts of Um Baanib deformed granites according to tectonites fabric classification are mostly L to LS tectonites domains with a high-strain intensity.

Fig. 7

The results of two-dimensional and three-dimensional strain analysis. The data from stretched minerals of Um Baanib granite plotted on a Zingge plot, b Flinn diagram and c Nadai-Hsu diagram. The data from stretched pebbles from ASZ plotted on d Zingge plot, e Flinn diagram and f Nadai-Hsu diagram. R_f/φ plots for six samples in orientations parallel to the XZ and XY planes are presented from g to l

Atalla shear zone

The dominance of deformed conglomerates along the western part in ASZ makes their clasts the best choice for the strain state assessment. The clasts in the conglomerates are composed mainly of polymineralic lithic fragments of metavolcanics, serpentinites, and granites with subordinate quartz clasts.

The pebbles show a significant shape change with elongation toward NW. Under microscope, most of pebbles show a considerable amount of ductile deformation and mylonitization, especially in metavolcanics pebbles. The competence contrast between the pebbles and their matrix are relatively low, as most of the matrix composed mainly of mineral composition similar to lager pebbles. The measurements were made in four different locations along the molasse sediments strip in ASZ.

The three-dimensional strain state is estimated directly from the in situ measurements of the length of the long, intermediate, and short axes of pebbles, as well as the trend and plunge of their long and short axes. Twenty-five deformed pebbles were extracted from their matrix, and their shape was obtained by measuring the three mutually perpendicular shape axes (X, Y, and Z). Plotting of the (b/a) and (c/b) ratios for pebbles on the Zingg (1935) plot reveal that most of pebbles are of blade and rod shapes (Fig. 7d). The R_xy and R_yz for pebbles were plotted to produce a Flinn Diagram (Fig. 7e), whereas the ν and ε_s values were used to produce a Nadai-Hsu Diagram (Fig. 8f). The results showed that the pebbles are concentrated in the prolate field with minor samples fallen in the oblate and plane strain fields.

Fig. 8

The results of vorticity analysis. a Cartoon illustrative image describing and the main parameters of a porphyroclast system used in vorticity measurement. b RXZ/θ′ diagram show the vorticity estimation for several locations in ASZ. c–e Data results of porphyroclast s aspect ratio and orientation in the PAR diagram for mylonite samples from ASZ. The R_c is the critical aspect ratio which is function of kinematic vorticity. f–h Data results of porphyroclast s aspect ratio and orientation in the in the RGN diagram for mylonite samples from ASZ. i Data of mica fish aspect ratio and orientation in the in the RGN diagram for MSZ mylonite

The averaged k values are found to be ranging from 0.5 to 4 with mean value 1.5, and lode’s factor (ν) values ranging from − 0.83 to 0.25 with mean value about − 0.24 that reflects a prolate strain ellipsoid. The strain intensity evaluated using the octahedral shear strain (ε_s) of Nadai (1963) and the (r) parameter of Watterson (1968). The εs estimates vary from 2.6 to 0.28 with average value of about (1.2), while the r values range from 8 to 2 with average of about (3.7).

A number of oriented field images of deformed conglomerates with orientations parallel to the XY plane and XZ plane were taken and imported to EllipseFit software. The two-dimensional finite strain was determined using the R_f/ф method, where the elliptical shape of the pebbles is digitized to estimate the axial ratio of pebbles and plot them against the ɸ values (Fig. 7g–l). The results reflect a moderate to high-strain values in both XZ and XY directions with finite strain (R_XZ and R_XY) range of about 4.7 and 3.67, respectively. Integration the strain data of R_XZ and R_XY values reveal a strain ellipsoid of a proper prolate shape with a major X axis oriented NW.

The finite strain values increase spatially toward the north, where attain maximum value at the northern branch of wadi Atalla (Rs = ~ 4.7), while the values decrease toward the south, where it reaches average value about (Rs = ~ 2.37) in El-Qash basin (Fowler and Abdeen 2014). The results also confirm the increase of strain intensity along ASZ from Wadi Hammamat in the west (Rs values of about 1.716–2.692) toward the east (e.g., Greiling et al. 2014). The strain state of the deformed conglomerates in ASZ reveals intermediate strain magnitude with a prolate strain ellipsoid. The orientation of the major, intermediate and minor axes of pebbles from ASZ were measured and the resulting average orientations of the principal axes of the strain ellipsoids for the ASZ are plotted on equal area stereonet. The stereograms show that the overall orientation of strain ellipsoid as inferred from strain state of Atalla conglomerates is that the X axis of ellipsoid oriented 321°/7°, intermediate Y axis is 160°/83°, while the Z axis is oriented 51°/2°.

Vorticity analysis

Meatiq shear zone

The mylonites from MSZ are evaluated using the Rigid grain network (RGN) method only. The samples are checked under a microscope to choose the best samples representative for the analysis. The absence of the common rigid porphyroclasts such as plagioclase and quartz in the high-grade mylonites led to searching for other minerals to quantify the kinematic vorticity number of Meatiq shear zone. Several studies used mica fishes as clasts for estimation of vorticity, as they behave as lubricant rigid clasts like feldspar porphyroclasts in mylonites (e.g., ten Grotenhuis et al. 2002; Johnson et al. 2009). The obtained B* and ф values of white-mica fishes were plotted on RGN net show a high-density cluster of data points between W_k values of 0.80 and 1, with a mean value of about 0.85 (Fig. 8i). These values of the kinematic number indicate a high contribution of simple shear about 80–75% of the total strain in the Meatiq shear zone that reflects a sub-simple shear zone.

Atalla shear zone

ASZ samples are checked under the microscope to choose best representative samples with a high amount of porphyroclasts with proper elliptical or semi-elliptical shape and with the least amount of recrystallization (Passchier 1987). The outlines of porphyroclasts were drawn by hand in (ImageJ) software, and the best-fit ellipse shapes are chosen for the measurement (Fig. 8a). In PAR method, the samples define a minimum and maximum Rc values of (2.5) and (3), respectively. This indicate a mean kinematic vorticity number range between 0.7 and 0.83. These estimates reflect transpressional conditions with a high pure shear contribution of about 40–50% (Fig. 8c–e).

For the RGN method, the obtained major (m₁) and minor (m₂) axes were used to calculate the B* and the results are plotted against ф values of porphyroclasts on RGN nets (Jessup et al. 2007). The mean kinematic number (W_m) value for samples range from 0.66 to 0.8. Pure shear contributes to about 50–40% of total shear in Atalla shear zone based on the RGN method (Fig. 8f–h).

The third method of vorticity quantification is the R_XZ/θ′-method. Atalla shear zone runs along the contact between the Fawakhir serpentinites and the Um-Esh deformed molasse sediments which separates the ductile fabrics of Atalla shear zone from the brittle thrust faults which dominate the Fawakhir area. The average strike of the boundary is about N40°W was taken as our reference line for measurement. The angle of θ′ estimated from strain ellipsoid shows a nearly parallel orientation to the shear zone boundary with a slight deflection angle between 7 to 12°. The mean values of R_XZ range between 3.65 and 4.8. Hence, the Wk values lay in a range of 0.5–0.7 (Fig. 8b). These W_k values indicate 70–60% pure shear and 30–40% simple shear for the total deformation in the study area. All the results of kinematic vorticity number from ASZ reflect a general shear conditions with a high contribution of pure shear (transpressional shear zone).

Deformation mechanisms

Meatiq shear zone

As quartzo-feldspathic rocks are the most dominant rock type in MSZ, the deformational behavior of quartz and feldspar minerals was used to characterize the dominant deformation mechanisms and to estimate the physical conditions prevailing throughout their evolution. The crystal–plastic deformation processes are dominant as evidenced from the intracrystalline deformation features as well as by grain elongation and stretching (Fig. 9a). Quartz and feldspar occur as stretched, medium-to-coarse-grained crystals dominate the fabric with highly sutured grain boundaries as result of grain boundary migration recrystallization (GBM) (Figs. 9b–d). They are pinned on the foliation plane by aligned muscovite crystals (Fig. 9c).

Fig. 9

Photomicrographs showing the deformation mechanisms in Meatiq and Atalla shear zones. a Stretched quartz grains indicating crystal–plastic deformation mechanisms, b highly sutured quartz grains due to high-temperature grain boundary migration, c window microstructure (WMS) and muscovite inclusions in quartz crystals indicate GBM recrystallization, d sillimanite fibers aligned parallel to the main foliation, e and f Synkinematic garnet porphyroblasts with internal spiral quartz inclusions. g Microfaulting and cracks within plagioclase crystal in Um Baanib deformed granite, h cataclastic shear zone with well-defined sharp boundaries in Um Baanib deformed granite, i mylonic foliation in Atalla mylonites with quartz and feldspar porphyroclasts, j bulging recrystallization in quartz grains. k Undulose extension in large quartz porphyroclast consumed by grain size reduction and development of subgrains, l parent quartz crystal fully consumed by dynamic recrystallization

The schists and mylonites of MSZ hosting different generations of garnet porphyroblasts. The textural relationship between the porphyroblasts and the penetrative matrix foliation, where used to determine the relative timing of growth with respect to deformation. The first generation of garnet is represented by stretched crystals that aligned parallel to (L₂) stretching lineations. The second generation of porphyroblasts is represented by elliptical to irregular-shaped crystal that commonly enclosing crenulated quartz inclusion trails that are oriented either inclined at a high angle to the surrounding external S₂ foliation or exhibit a spiral-shaped (Fig. 9e, f). These porphyroblasts are interpreted to grow syn-kinematically through the evolution of MSZ.

The ductile fabrics of MSZ were overprinted by subsequent cataclastic brittle shear zones. They are a relatively narrow zone with frictional sliding and grain fracturing processes. The shear zone boundaries are sharp and filled with a large number of fragmented and fractured grains of quartz and feldspar (Fig. 9g, h).

Atalla shear zone

Mylonites from Atalla shear zone show a great variation in the lithological composition and the degree of deformation. They range from the highly sheared ophiolitic mélange which exhibits general steep foliation planes with poor shear-sense indicators to the highly mylonitized felsite and metavolcanics along the northwestern boundary of the shear zone.

Quartz and plagioclase minerals are the most common constituents of ASZ mylonites. They occur as large porphyroclasts embedded in a fine-grained groundmass of quartz, plagioclase, chlorite, muscovite and epidote. The mylonitic foliation is delineated by the alignment of muscovite and chlorite that wrapping around the large porphyroclasts. Quartz porphyroclasts are sub-circular or elongated grains that heterogeneously deformed and display characteristic features of crystal–plastic deformation such as undulose extinction (Fig. 9i). A number of porphyroclasts are consumed by grain size reduction in response to dynamic recrystallization (Fig. 9j–l). Bulging (BLG) and subgrain rotation (SGR) are the common types of dynamic recrystallization observed in the quartz porphyroclasts. Plagioclase crystals commonly occur as rounded to surrounded porphyroclasts that show evidence of rotation and fracturing and developed core-and-mantle structures.

Deformation temperature

Meatiq shear zone

Mylonites of MSZ contain metamorphic mineral assemblages and microstructural features that documented that main part of deformation occurred under amphibolite-facies conditions. The mineral assemblages of quarzt-feldspathic schist and mylonites consists of quartz, plagioclase, muscovite, garnet and sillimanite (Fig. 9d). This mineral assemblage indicates a medium-to-high-grade metamorphic conditions. The Quartz and feldspar occur as stretched, medium-to-coarse-grained crystals with highly sutured grain boundaries as result of high-temperature grain boundary migration recrystallization (GBM). This fabric is typical for tectonites that deformed in temperature range 500–650 °C (e.g., Stipp et al. 2002). The high-temperature conditions which documented from the microstructures and of MSZ was subsequently replaced by brittle deformation which evolved under low temperature conditions (less than 300° c) in the upper crustal levels. through the exhumation of the shear zone.

Atalla shear zone

The microstructural features of quartz grains display undulose extinction, bulging (BLG) and subgrain rotation (SGR) recrystallization and minor, while the feldspar deformed in a more brittle fashion with common fractures and core-and-mantle structures. These observations suggesting near deformation conditions near the brittle–ductile transition conditions of feldspar minerals with a temperature estimate about 300–450 °C (e.g., Pryer 1993; Stipp et al. 2002). The temperature estimates from deformation mechanisms in ASZ is conformable with the mineral assemblage of ASZ mylonites (quartz, feldspar, muscovite, epidote, biotite and hornblende) and with the previous estimates based on petrographical methods (Fowler and El Kalioubi 2004).

Shear-sense indicators

Meatiq shear zone

Several shear-sense indicators are present in the mylonites of MSZ that used to assess the sense of movement across shear zone. Micaceous quartzites from the eastern flank of Meatiq dome hosts well-developed white-mica fishes embedded in coarse-grained quartz with sutured boundaries (Fig. 10a, b). The fishes exhibit a characteristic lozenge to perfectly lensoidal shape with monoclinic asymmetry indicate an apparent sinistral sense of shearing. The vergence direction of intrafolial folds and folded quartz veins indicates both dextral sense of shearing from the western limb of Um-Esh El-Hamra antiform and sinistral sense of movement from its eastern limb (Fig. 10c). In comparison to the microscopic indicators, macroscopic asymmetric boudins and sigmoids form the northern and southern flanks of Meatiq dome indicate a top-to-NW sense of shearing. Also, C–S fabric is well developed in mylonitic schists and phyllonites with a top-to-NW shear sense (Fig. 10d).

Fig. 10

Photomicrographs showing the shear-sense indicators in Meatiq and Atalla shear zones. a and b Lensoidal to rectangular-shape white-mica fish structures indicating a top-to -left shear sense. c asymmetrical folded quartz vein with vergence indicate a top-to-right shear sense. d C´ Shear bands indicate a sinistral sense of shearing observed using gypsum plate. e and fδ-Type porphyroclast s in Atalla felsite indicating a sinistral sense of shearing, gσ-type porphyroclast of plagioclase indicating a sinistral sense of shearing, h sigmoidal clast in deformed conglomerates indicate a sinistral sense of shearing, i and j quartz fish structure in Atalla felsite with a sinistral sense of shearing. k Synthetic fracture in plagioclase clast with a sinistral shear sense. The crack is filled with mineral fiber which indicate crystallization coeval with crack opening, l C´ shear bands indicate a sinistral sense of shearing

Atalla shear zone

The samples of ASZ shows a great variability in shear-sense indicators as compared with the mostly MSZ. The investigated samples were deformed under green-schist facies conditions, wherever numerous porphyroclasts of quartz and feldspar were well preserved. Several shear-sense indicators are well-documented in ASZ including: (1) delta- and sigma-type core-and-mantle structure in quartz and feldspar porphyroclasts with asymmetric recrystallized tails from mylonitized felsite (Fig. 10e–g) and deformed conglomerates (Fig. 7h); (2) mineral fish from mylonitized felsite (Fig. 10i, j); and (3) synthetic and antithetic microfaulting from plagioclase porphyroclasts (Fig. 10k); (4) macroscopic and microscopic shear bands, such as S–C mylonitic fabric from the basic metavolcanics and mylonites along the eastern part of shear zone fabrics (Fig. 10l). These indicators exhibit dominant sinistral shear sense at both the macroscopic and microscopic scales of observation indicating a substantial component of non-coaxial flow within the shear zone.

Discussion

Structural and tectonic history of shear zones

The structural events of the Meatiq area were deduced based on the results of structural analysis and placed in a successive time frame. This is achieved by synthesizing the results of structural analyses such as orientation data, style of the different structures, microstructures and kinematic indicators. The spatial and temporal relationships between the different deformation phases are determined based on overprinting relationships between the main fabric elements, microstructures and field relationships between large-scale structures. Detailed structural analysis of Meatiq area revealed five stages for the structural evolution and exhumation of shear zones in the Meatiq area (Fig. 11).

Fig. 11

Structural evolution of Meatiq area through the Pan-African orogeny

This initial stage of deformation is best exposed in the amphibolite enclaves enclosed in Um Baanib pluton. It is represented by ductile fabrics and folds. These structures considered as vestiges of an early compressional stage (thrusting and folding) that pre-date the emplacement of Um Baanib deformed granite.

The reconstructed structural history of the shear zones started with the second structural stage. This structural stage involved the thrusting and NW-ward translation of the ophiolitic nappes over the rock units of Meatiq succession. This compressional regime led to the initiation of a sub-horizontal ductile shear zone in the mid-crustal levels. The ductile structures (S₂ foliation, L₂ lineations and F₂ folds) and shear-sense indicators observed within Meatiq succession reveal a top-to-NW sense of shearing. The observed microstructures indicate dynamic recrystallization of quartz by GBM under high-temperature conditions (550 °C; Stipp et al. 2002). The emplacement of Um Baanib granite was synchronized to this stage as inferred from NW-trending penetrative lineation in Um Baanib granite. Andresen et al. (2009) dated the crystallization of Um Baanib granite as 630 Ma, which represents based on the present model, the age of the NW–SE shortening.

The third structural stage includes an NE–SW transpressional event. The structural features of this stage including the conspicuous Atalla shear zone, NW–SE thrust faults, and F₃ folds at all scales. These structural elements can be interpreted in terms of oblique convergence model between East and West Gondwana. The first stage of this deformation phase is represented by the ductile fabrics of NW–SE Atalla shear zone including mylonitic foliation (S₃) and stretched mineral lineation (L₃). This stage is followed or synchronized with the F₃ folding and thrusting. D₃ structures reflect a shallow structural level that indicates a gradual exhumation of the Meatiq rocks through this compressional phase into shallower crustal depths. The observed microscopic cataclastic shear zones revealed that a considerable exhumation was achieved through this stage of deformation.

The fourth structural stage is characterized by an overall NE–SW shortening regime which produced brittle strike-slip faults of various orientations including; N–S, NW–SE and NE–SW. These strike-slip faults cross-cutting the ductile fabrics of Atalla shear zone. Paleostress analysis reveals that the maximum principle compressive stress σ₁ changes their orientation from E–W at the third stage to a nearly ENE–WSW direction in the fourth stage.

The last structural stage involves the NW–SE extension associated with the gravitational collapse of the amalgamated thickened crust and the emplacement of the post-collision A-type granitic plutons (e.g., Arieki and Um Had granites). This stage is represented by extensional structures including NE–SW normal faults and joints. The normal faults extended along the northern and southern flanks of Meatiq antiform and they are dipping gently to moderately to the NW and SE. The gravitational collapse and associated extension of the thickened crust promotes the final, last stage of the long exhumation process of Meatiq area. This extension probably extends to the beginning of Cambrian period (Stern 1985).

Kinematic modelling of Atalla shear zone

Evidences of the simple shearing component of deformation

Atalla shear zone (ASZ) is an integral part of the structural configuration of Meatiq area. Several studies were performed to reveal their structural nature and evolution (e.g., Sultan et al. 1988; Fritz et al.1996; Kamal El-Din et al. 1996; Bregar et al. 1997; de Wall et al. 1998; Fowler and Osman 2001; Fowler and El Kalioubi 2004; Akawy 2007; Greiling et al. 2014; Zoheir et al. 2018). The debate between the contradicting models concerning with the structural modelling of ASZ arise mainly from the geometrical interpretation of the complex spatial and temporal relationships between structural elements. However, the dominance of folds and thrusting within the deformed blocks of ASZ support the significant role for the NE–SW compressional structures in the shaping of the area, but several structural features along ASZ which cannot be interpreted without the contribution of ductile wrench shearing component in the total deformation. These features include;:(1) the lensoidal stretched outcrops of ophiolites and its high disturbance in a tectonically mélange suit; (2) the high-strain values recorded in the deformed conglomerated in comparison to the recorded in the nearby basins (e.g., Hammamat basin; Greiling et al. 2014); (3) the deflected and stretched strand of Fawakhir serpentinites along its northern connection with Atalla shear zone; (4) the dominance of steep to vertical foliation in association with zones of protomylonites and mylonites; (5) on satellite images, Um-Esh molasse sediments seem to be a northern continuation of El-Qash molasse basin which displaced laterally along the zone of left-lateral sense of shear toward NW; and (6) the dominance of kinematic indicators of sinistral shear sense. All of these observations indicate the existence of important simple shear component associated with NE–SW shortening.

The kinematic model of ASZ

Based on the orientation of structural elements mapped in the field, microstructural and kinematic analyses, ASZ is interpreted a sub-vertical NW–SE strike-slip shear zone with a characteristic sub-vertical foliation and sub-horizontal lineation. The kinematic indicators reveal a dominant sinistral sense of shear. The ductile deformation was mainly operated under a low-to-medium grade metamorphic conditions (around 300–450 °C) at relatively shallow crustal levels as inferred from mineral assemblage, dynamic recrystallization features (Bulging and subgrain rotation) and deformational behavior of quartz and feldspar minerals. Evidence for overprinting brittle deformation is abundant both in the field and in thin section, indicating later brittle strike-slip shearing with both sinistral and dextral sense of shearing and extensional faulting.

The quantitative assessment of strain and vorticity data allows the new kinematic modelling and structural interpretation of the Atalla shear zone and provide regional implications for this deformation stage in the ANS. All the obtained numerical results are used to model the kinematics of ASZ using the free software Strain & Shear Calculator 3.1(produced by Rod Holcombe). The estimated results of vorticity and strain show that ASZ is characterized by a nearly equal contribution of coaxial and non-coaxial strain accommodating simultaneous sinistral simple shear in NW–SE direction and shortening across the shear zone represented by the folds and thrust faults, coupled with elongation and extrusion in the shearing direction (direction of maximum strain axis). The measured two-dimensional average strain (Rs) in the XZ direction reflects a relatively high-strain values up to 4.7, while the three-dimensional strain measurements indicate a prolate strain ellipsoid with mean K values (1.5) and lode’s factor (− 0.23). The proposed kinematic model predicts an oblique convergence angle α (~ 45°) which is equal to the angle between the two flow apophysis of the ASZ. The maximum instantaneous stretching axis (ISA_max) oriented with an angle θ (~ 22.5°) to the shear zone boundary and minimum instantaneous stretching axis (ISA_min) with an angle θ_p (~ 67.5°) (Fig. 12). The (ISA₂) is parallel to the vorticity axis. The direction of (ISA₁) is coincident with the direction of (σ₃) of the principal stress ellipsoid relative to the lineation direction, while (ISA₃) is corresponds to the direction of Maximum principal stress (σ₁). As folds generally nucleate parallel to ISA_max and normal to the ISA_min, so they have been predicted to initiate at an angle of about (~ 22.5°) from the shear zone boundaries. In real, the axes of folds in ASZ oriented with lower angles than predicted by the kinematic model (~ 10°). This can be explained by takeing into consideration the rotation of fold axes through the ongoing non-coaxial components of deformation, so the fold axes tend to rotate into lower angles with respect to the shear zone boundaries.

Fig. 12

The kinematic modelling of Atalla Shear zone. a Calculation of shortening value (S) measured perpendicular to flow plane taking into account both strain magnitude and vorticity of flow (after Law 2010). b The relative contribution of pure shear in ASZ based on the kinematic vorticity from mylonite samples. c Relationships between strain ratio (R_xz), shear strain (γ) vorticity number (W_m) for ASZ samples (d) Kinematic model of ASZ illustrate the different kinematic parameters of ductile flow within the shear zone, e cartoon illustrative model for ASZ based on the numerical kinematic parameters

The shortening across ASZ is calculated based on the mathematical relation of Wallis et al. (1993) which relate the vorticity number (W_m) and the finite strain ratio (R_xz) to the shortening (Fig. 12a). The estimates indicate about 0.5 which corresponding to 50% shortening. The shear strain in ASZ is estimated based on the values of Kinematic vorticity number (W_m) and the finite strain ratio (R_xz) using the plot of (Law 2010; Fig. 12b). The results indicate shear strain range between (1.3–1.9) with an average of about 1.5.

The impressive width of ASZ suggests a significant displacement along the zone. While, there is no direct evidence for the total amount of horizontal slip across the zone, an approximate calculation of displacement is possible by the integration of the estimated shear strain over the width shear zone (average about 8 km). This yields an estimate about 12–16 km for the total displacement across ASZ.

The development of the ASZ is inferred to represent an inboard response to the oblique convergence between East and West Gondwana at the Neoproterozoic around 605 Ma. This stage of deformation is characterized by transpressional regime which involving shortening perpendicular to the orogen and strike-slip shearing parallel to the orogen (e.g., Harland 1971). The initiation of Atalla shear zone is probably controlled by the rheological conditions which created through the earlier stages of Meatiq area exhumation. The emplacement of Um Baanib granitic intrusion and the partial exhumation of the deformed metasediments to higher levels juxtapose a hot domain with high heat flux “Meatiq dome” to surrounding cold ophiolitic nappes. These conditions possibly created different rheological domains that thermally soften the rocks and triggered the initiation of ASZ. The initiation of strike-slip shear zones along hot–cold rheological boundaries is very common in the literature (e.g., Cao and Neubauer 2016 and the references cited therein). These heat sources are probably the reason for the relatively higher metamorphic conditions (up to biotite zone) which observed along in ASZ (Fowler and El Kalioubi 2004).

Applicability and limitations of ASZ kinematic model

Shear zones are a complex system that involves deformation of heterogenous rocks of different mechanical properties. The deformation is also affected by the inherited structural elements within the shear zone and the prevailing physical conditions. The kinematic modelling of such complex geological systems requires proposing a number of assumptions for simpler calculations. The applicability of kinematic model is a function of reliability and accuracy of structural observations and the proposed assumptions about the deformation in the shear zone. The present kinematic model assumes a monoclinic symmetry for the shear zone, where the vorticity axis is perpendicular to the stretching lineations as evidenced from thin sections, where highly asymmetrical kinematic indicators were observed in sections parallel to lineations. The model also assumes a constant volume deformation as there are no evidences for volume loss or gain through the ongoing deformation (e.g., stylolites or dissolution features). One of the main issues that challenges the transpressional conditions of ASZ is the orientation of fabrics within the shear zone (i.e., foliation and lineation). In the ideal transpressional model (e.g., Sanderson and Marchini 1984; Dewey et al. 1998) the lineation deviate from horizontality based on the kinematic vorticity, except for zones of low strains values or with small convergence angles (less than 20°; Robin and Cruden 1994; Teyssier et al. 1995; Teyssier and Tikoff 1999). For the proposed kinematic model of ASZ, the estimated convergence angle is about 45° and the finite strain attains relatively high values (R_s ~ 4.5), so at these conditions the sub-vertical foliation and widespread horizontal lineation would be unstable. Teyssier and Tikoff (1999) suggested that the combination of transpression with lateral extrusion component parallel to the shear zone boundary stabilizes the vertical foliation and horizontal lineation attitudes in transpressional shear zone over a wide range of convergence angles and strain intensities. We conclude that the proposed transpressional model combined with lateral extrusion kinematic model fit the structural elements and microstructures of ASZ.

Regional and tectonic implications of ASZ kinematic model

The kinematic model of ASZ introduce important implications for the convergence angle between east and west Gondwana prior to their collision. Fossen (2016) pointed that the convergence vector between any two colliding blocks is parallel to the second flow which makes an angle (α) with the flow direction in the shear zone. As estimated from the kinematic model, the second flow apophysis was oriented with angle 45° to the main flow direction (average N40°W–S40°E). The convergence vector of East and West Gondwana is approximately oriented (N85°W) which indicates a nearly East–West convergence.

The role of Najd fault system in the exhumation of Meatiq area

The spatial concurrence of metamorphic complexes and Najd–related shear systems (especially in the Arabian shield) encouraged several authors to relate the exhumation of these complexes in the ANS to the initiation and evolution of Najd-Shear System (e.g., Fritz et al. 1996, 2014; Shalaby 2010; Hamdy et al. 2017).

There are several attempts to relate the evolution of these gneissic structures to the proposed extent of the Najd-Shear System in the Eastern Desert of Egypt. One of these widespread hypotheses is the wrench corridor model which proposed by Fritz et al. (1996, 2002) to explain the exhumation of Meatiq, El-Sibai and Hafafit complexes. They relate the formation and exhumation of medium-to-high-grade metamorphic successions of these complexes to their enclosure in a Najd-related regional scale NW–SE wrench corridor with a left-lateral sense of shearing, that caused the exhumation of lower crustal levels as large core complexes in a low-grade suit of ophiolites and metavolcanics.

In this section, we will discuss, based on the present structural observations, the arguments proposed by Fritz et al. (1996, 2014) and others about the existence of crustal-scale wrench corridor bounding Meatiq dome, and generally the role of the Najd-related Shear System in the exhumation of metamorphic complexes in the CED of Egypt.

The structural observations at all scales are not compatible with the NW–SE corridor model for several reasons:

• 1. The proposed left-lateral shear zone along the western flank of the dome is not documented through the fieldwork, or through the analysis of satellite images.

• 2. The steeply inclined foliation and associated microscopic shear-sense indicators along the western border of Meatiq structure generally reveal a dextral sense of shearing (similar to macroscopic and mesoscopic indicators documented by Andresen et al. 2010). This shear sense is interpreted to be resulted from the subsequent F₃-folding of the sub-horizontal Meatiq shear zone around NW–SE folding axis which act as a flipping axis that displays a contradicting shear sense along the limbs of folds (e.g., Goscombe and Trouw 1999; Fig. 13).

  Fig. 13

  

  Illustrative sketches showing the main structural features of Meatiq shear zone. a Hypothetical reconstruction of the Top-to-NW shearing of Meatiq shear zone based on the structural analysis. b Illustrative sketch shows the effect of folding on the shear-sense indicators. c The observed shear-sense indicators around a folded sub-horizontal shear zone with a top-to-NW sense

• 3. The Eastern flank of Meatiq antiform possesses moderately dipping foliated rocks with a sinistral shear-sense indicators. This sense of shearing also interpreted as false sinistral sense, due to the folding of top-to NW Meatiq shear zone.

Based on the previous notes, the present work concludes that most of the observed structural features recorded by Fritz et al. (1996, 2002) as crustal-scale wrench corridor are mostly the inclined limbs of the folded (MSZ).

The initiation and evolution of strike-slip faults and shear zones similar to Najd-trend are mostly postdated the ductile shear feature of Meatiq succession as well as after a considerable part of exhumation had been occurring. The microscopic fabrics recorded in Meatiq high-grade rocks reflect later deformation phases through the exhumation of rocks and all of these features pre-date the initiation of the Najd-shear system. All these lines of evidence argued against the role of Najd-shear zones on the exhumation of Meatiq area.

Conclusion

Meatiq area experiences a complex history of deformation, metamorphism and exhumation. The structural configuration of the area can be interpreted in terms of five successive structural stages coeval with three metamorphic events and gradual exhumation of the lower crustal levels during the final stages of East and West Gondwana collision. Two main shear zones (Meatiq and Atalla shear zones) were developed through the structural evolution of Meatiq area. The earlier deformation stages are related Island-arc accretion stage characterized by NW–SE shortening. Meatiq shear zone initiated during this stage as a ductile thrust shear zone with a top-to-NW sense of shear. The later structural stages are mostly related to the E–W transpressional regime prevailing during the oblique convergence between East and West Gondwana. Atalla shear zone is a large-scale sinistral strike-slip shear zone that was developed in response to these transpressional conditions.

Microstructural, strain and vorticity data from deformed tectonites were used to investigate the kinematics of rock flow in the Meatiq and Atalla shear zones and to understand their structural and exhumation history. The Strain and vorticity analyses of Atalla shear zone (ASZ) indicate that these zones experienced general shear conditions with equal contribution of pure and simple shear. The mean vorticity number (W_m) estimated with the porphyroclasts-based methods ranges from 0.65 to 0.72, while the strain analysis reveals a prolate strain ellipsoid with a moderate strain intensity. The numerical estimates allow the kinematic modelling of ASZ which introduce valuable implication for the tectonic framework of the Central Eastern Desert and the oblique convergence between East and West Gondwana.