Arab J Geosci DOI 10.1007/s12517-013-0951-0
ORIGINAL PAPER
Lithostratigraphy, deformation history, and tectonic evolution of the basement rocks, Republic of Yemen: an overview Mohamed Th. S. Heikal & Salah A. Al-Khirbash & Adel M. Hassan & Ahmed M. Al-Kotbah & Khaled M. Al-Selwi
Received: 24 March 2012 / Accepted: 15 April 2013 # Saudi Society for Geosciences 2013
Abstract Basement rocks of presumed Precambrian age, in Yemen Republic (105,000 km2), are exposed in the northwestern and southeastern parts of the country. The basement rocks of southern Saudi Arabia and northern parts of Yemen are almost continuous and similar in the lithostratigraphic succession. In spite of the presence of such common basic characteristics for each, there are slight differences of local structural framework and major tectonic events. The structural complexity, great variety of rock units and types, multiintrusive environments, and multiplicity of metamorphic events in the study basement rocks make the main target of lithostratigraphic analyses, in particular, daunting in the southern Arabian Shield. As reported here, accepting that the southern shield consists of five terranes and suture zones requires a limitation of such tectonic modifications. This led to the renaming of certain formations and groups and the revision of the lithostratigraphic successions for some regions. As a result, new lithostratigraphic relationships and names as well as tectonic events are proposed. Based on M. T. S. Heikal (*) Department of Geology, Faculty of Science, Tanta University, Tanta 31527, Egypt e-mail:
[email protected] S. A. Al-Khirbash (*) Department of Earth Science, College of Science, Sultan Qaboos University, Muscat, Oman e-mail:
[email protected] A. M. Hassan Department of Geology, Faculty of Science, Teba University, El Madenia El Menwra, Kingdom of Saudi Arabia A. M. Al-Kotbah : K. M. Al-Selwi Department of Earth and Environmental Sciences, Faculty of Science, Sana’a University, Sana’a, Yemen
field and space image data, the basement rocks in Yemen exhibit at least six major phases of deformation (D1 to D6) including intensive brittle and ductile deformations that trend NW–SE and NNE–SSW (in major). Neoarchean rocks are well developed and restricted in the southeastern exposures (Al Bayda, Al Mahfid, and Al Mukalla terranes), whereas the final Pan-African cratonization of several rock units is widespread on all terranes, in which the major tectonic events and deformation history were concentrated during pre-Pan-African and early to late Pan-African orogenies. A correlation and evolution of the Precambrian rocks in Saudi Arabia and Egypt are taken into consideration. Keywords Archean–Proterozoic . Lithostratigraphy . Tectonism . Yemen
Introduction The Arabian–Nubian Shield is part of the East African Orogen formed by plate tectonic accretionary events during the closure of Mozambique Ocean and subsequent collision of east and west Gondwana at about 540 Ma (Stern 1994; Johnson et al. 1996; Whitehouse et al. 2001; Kusky et al. 2003; Johnson and Woldehaimanot 2003; Stern and Johnson 2010). Four and a half decades have elapsed the appearance of the first realistic and applicable details of the rock units forming the basement rocks in Yemen (Greenwood and Bleackley 1967; Grolier and Overstreet 1978). The authors feel it is duty and right to revise its synthesis and present a documentation of how exactly it was founded, verified, and tested throughout a period of 45 years that have seen a great revolution in geological thoughts. Traditionally, the basement rocks of Yemen (Fig. 1) are thought as a part of the Proterozoic Arabian–
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Nubian Shield which covers Northeast Africa and the Arabian Peninsula (Stoeser and Camp 1985; Heikal 1987; Sakran 1993; Ba-Bttat 1991; Whitehouse et al. 1993, 1998, 2001; Windley et al. 1996; As-Saruri and Wiefel 2012; Leroy et al. 2012). Windley et al. (1996) classified and identified four gneissic terrains, namely, Asir, Afif, Abas, and Al Mahfid, and two island arcs, Al Bayda and Al Mukalla. Comprehensive studies on the basement rocks of Yemen have been done by few authors (Heikal 1987; Ba-Bttat 1991; Al-Kotbah 1992; Sakran 1993; Whitehouse et al. 1993; Windley et al. 1996; Al-Khirbash et al. 2001, 2008; Al-Selwi 2005; Naman 2010; As-Saruri and Wiefel 2012) on such excellent exposures, Hajjah, Al Bayda, Mukairas, Haifan, Mudiyah–Mukalla, and Eastern Aden areas (Shabwa and SW Hadramawt), that reflect a clean picture to deduce in good agreement for integrating classification and lithostratigraphy of the basement rocks in Yemen. Most recent work (As-Saruri and Wiefel 2012) studied and subdivided the intrusive rocks in the Mudiyah–Mukalla area into four tectonic stages: the anatectic stage (Ahwar Supergroup), the syntectonic stage (Gharish Supergroup), the post-tectonic stage (Tha'lab Group), and the late orogenic stage (Ghabar Group). In addition, by age determinations (K/Ar) of several samples from Mudiyah–Al Mukalla basement rocks, Windley et al. (1996) recorded an age of 3.03– 2.73 and 2.5–1.2 Ga for Ahwar and Gharish Supergroups, respectively, that are distributed in Al Mahfid and Al Bayda terranes. Sassi et al. (1993) recorded an age of 700–640 for the island arc metavolcanics (Tha'lab Group) in Al Mukalla area. The purpose of the present study is an attempt to establish the space and relative time relationships among the basement rocks in Yemen and thereby classify and identify the deformational phases in combination with the tectonic events of these rocks and the regions in which they occur. In addition, the correlation of the Precambrian rocks in Northeast Africa (Egypt) and Saudi Arabia is taken into consideration. But, in fact, revising lithostratigraphy is a complicated task that involves weighing up the relative merits and reliabilities of geologic sources of new geologic concepts and tectonic events that are beyond the scope of this work.
Lithostratigraphy and structural framework The basement rocks of Yemen (105,000 km2) comprise distinguished rock units in the form of belts and terranes, extending from the north to the south and from the northeast to the southeast (Fig. 1). The structural framework is considered to be a more realistic manner, as the most effective part of the geological fabric of the basement areas. A mere glance at some exposures (Fig. 2a–d) of gneisses and
metasediments conveys the intense deformations that have affected these rocks, where intense faulting, folding, and deformational phases have been observed on these rocks. The more detailed studies concerned with the structural elements and fracture system and fluid flow implications in the Proterozoic basement of Yemen are given by AsSaruri and Wiefel (1998) and Le Garzic et al. (2011). In general, the structural framework of Yemen is characterized by Late Archean–Proterozoic basement fault blocks transected by the NW–SE trending Jurassic–Cretaceous intracratonic rift system of the Ramlat As-Sab'atayn graben, bounded to the south and the west by Tertiary, to present day, the Gulf of Aden and Red Sea rift system (i.e., Robertson Group 1992; Al-Kotbah 1996; Windley et al. 1996; Heikal et al. 2006). It is now suitable to look into successions of the lithostratigraphic units of basement rocks in the present work. These units (Table 1) are distinguished and named in accordance with the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature 2005). On the other hand, the present authors suggested subdividing the basement rocks of Yemen into two main geographic categories, the southeastern exposures and the northwestern ones, which collectively reflect the whole picture of the integrated lithostratigraphy of the basement rocks of Yemen (Table 1). The southeastern exposures of the basement rocks of Yemen include four terranes, namely, Abas and Al Mahfid gneiss terranes and Al Bayda and Al Mukalla island arc terranes (Windley et al. 1996), which represent the most favorable, complicated, and deformed rocks in terms of lithology and structural implications than those found in the northern areas in which they are different in lithology and age. Al Mahfid gneiss terrane (2,900–2,550 Ma; Ba-Bttat 1991; Whitehouse et al. 2001) constitutes infracrustal (continental) rocks. This group comprises Late Archean to Late Proterozoic ortho- and paragneisses and migmatites (in major) (Fig. 2a–c) and amphibolites, banded metataxite migmatites, anatectic granites, and mylonites (in part). These rocks are also found in Lawdar and Shabwa, where they are exposed as NE–SW refolded and faulted sheets. Previously known Aden Metamorophic Group (Greenwood and Bleackley 1967) is partly included in this terrane. Ahwar–Gharish Supergroup is ranging in age from Paleoproterozoic to Neoproterozoic (3.03–1.2 Ga; Schramm et al. 1986; Windley et al. 1996). They comprise highly metamorphosed gneisses up to upper amphibolite facies and show irregular folding and boudinage and subjected to monoclinal and sigmodial folding (As-Saruri and Wiefel 2012). They include five groups that belong to both Al Mahfid (most of Ahwar) and Al Bayda (most of Gharish) terranes (see Table 1).
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X
Hooth
X X
X
Ring Complexes
Fig. 1 Basement outcrop distribution map showing the delineation of infracrustal gneisses and island arc terranes as well as intraplate ring complexes correlated to adjacent areas in the southern parts of Saudi
Arabia (a) and northern Somalia(b) (after Windley et al. 1996; Whitehouse et al. 2001; and Gameil and Heikal 2005)
Al Bayda island arc terrane (2,500–2,000 Ma; after Windley et al. 1996) is composed of supracrustal rocks (mainly of ophiolitic signature) and includes granitoids (2.5–2.0 Ga; Whitehouse et al. 1993), ophiolitic sequence of medium grade metamorphism, as well as intrusive rocks of granodiorite, diorite, gabbros, and late tectonic granites (Sakran 1993; AlKhirbash et al. 2001). Fahman, Al Aswad, and Hatib groups of As-Saruri and Wiefel (2012) Gharish Supergroup mostly belong to this terrane, as mentioned above. Tha'lab Group (syn-rift volcanics) of Neoproterozoic age which was first named and defined by Beydoun (1964) is exposed over extensive areas in Wadi Hajr about 8 km north of Ma'abir Tha'lab (Hadramawt Province) notably at Zulm Ba Tha'lab, after which was named. They consist of thick lava flows including andesites, keratophyres, alkali rhyolites, and less common trachytes. These rocks have been formed during syn-rift volcanic and predated Ghabar Group. The Ghabar Group is located in Wadi Ghabar and Wadi Minhamir, west of Mukalla, and represents an average age
of 520–560 Ma (Greenwood and Bleackley 1967; Sassi et al. 1993). It was first named by Beydoun (1964) and consists of four formations giving rise to slight metamorphosed molasse sediments with minor volcanics. These formations consist of well-bedded sandy dolomites, calcareous quartzite, platy shale, conglomerates, limestones, chert and tuffaceous sediments, as well as crystal-and-lithic tuffs giving rise to molasse facies. These rocks have formed during post-rift in an island arc(s) setting (Greenwood and Bleackley 1967). Ataq–Mudiyah–Al Mukalla late to post-tectonic granite suite is well defined in Mudiyah–Al Mukalla and Ataq areas (Figs. 2e and 3i), comprising syenogranites associated with alkali feldspar granites and alkali amphibole-bearing granites that are emplaced in three tectonic stages on both island arc and intraplate tectonic setting (Flowerdew et al. 2008; As-Saruri and Wiefel 2012; and the present study). The northwestern exposures of the basement rocks of Yemen are pertaining to Asir Terrane. These include Hajjah Group (235 km2), first named by Heikal (1987) and Noweir et
Arab J Geosci Fig. 2 a–e Field photographs showing field observations of some basement rocks in Yemen. a Space image showing highgrade gneiss intersliced by a major strike-slip fault indicating left lateral shear sense of movement (arrows) (Al Bayda area). b Bold exposures of Late Archean gneisses lying across switchback, Mukairas area, Al Mahfid terrane. c Typical exposure of gneiss. The foliation is shown by discontinuous layers of darkand light-colored minerals, AlBayda area, Al Bayda terrane. d An exposure of foliated metasediments. They are an impure marbles, where the foliation is parallel to the bedding. The light-colored quartz fish tail indicates right lateral shear sense of movement, Ain Ali village, Hajjah area, the northern exposure. e A roof pendant of amphibolites (arrows) adjacent to the upper part of granite pluton, Balhaf-Al Mukalla road, the Al Mukalla terrane
a
c
5Km
b
d
e
al. (1994). It is represented by regionally metamorphosed sediments and volcanics. It comprises two distinct formations, named Ain Ali Formation and Mawr Formation. The Ain Ali Formation (25 km2) is the lowermost rock unit in the Hajjah Group and forms irregular elongated outcrops showing an eroded relatively small anticline. It consists mainly of lowgrade metasediments, including plitic schist, marble, and quartzite. It possesses a structural contact with the upper Mawr Formation. The Mawr Formation (210 km2) is the uppermost rock unit within the Hajjah Group and named after Wadi Mawr. It consists mainly of mafic metavolcanics, amphibolites, and hornfelses and is characterized by strong foliation trending NE and NW. On the basis of their geochemical signatures, these rocks originated from a transitional calc-alkaline to tholeiitic magma and seem to be found in a mature island arc setting (Aly and Heikal 1991). These rock assemblages are also found in Sa'dah (Asir Terrane) and Marib areas (Abas Terrane) that form island arcs of metavolcanics including metabasalts, metaandesites, metadacites, and their associated metatuffs, whereas metasediments include graphite schist, hornblende schist, biotite–muscovite schist, metagreywacke,
and marble (Fig. 2d) indicating calcareous, siliceous, and pelitic origin. The sam metagabbro–diorite complex (old gabbros) was first named by Heikal (1987) and Noweir et al. (1994). It composed mainly of metgabbros together with diorite and mafic schist. These rocks are also well defined around Sabah village in Al Bayda area. At-Tur syntectonic granites (after At-Tur village, Hajjah area) include monzogranite, granodiorite, tonalite, and quartz diorite association. Exfoliation, cavernous structure, and spheroidal landforms characterize its geomorphic appearance. These rocks are well exposed in Sa'dah and Al Bayda areas (Geomin 1985; Sakran 1993; Beydoun et al. 1998). Denèle et al. (2012) studied the age dating of some basement rocks exposed in Socotra Island which belongs to the southern rifted margin of the Gulf of Aden and occupied, in Neoproterozoic times, a key position to constrain the age and the nature of the largely hidden Neoproterozoic rocks of the Arabian plate. Their geochronological studies (U-Pb data) of the syntectonic granites showed that felsic intrusions mainly composed of calc-alkaline and pinkish granites dated between 840 and 780 Ma. On the other hand, Whitehouse et al. (2012)
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concluded that the earliest arc magmatism in the Socotra basement was at about 870–860 Ma, most magmatic intrusions occurred at about 800 Ma, and the late granitic intrusive phase (Hadibo K feldspar granite) was at about 720 Ma. La'ah young gabbros (after Wadi La'ah, Hajjah area), first named by Heikal (1987) and Noweir et al. (1994), comprises unmetamorphosed calc-akaline to mild alkaline gabbros and norites in the form of small hillocks and cones. These rocks are also found in Al Bayda area (Sakran 1993). Late to post-tectonic granite suite is pertaining to Ataq–Mudiyah–Al Mukalla granite suite. They embrace many plutons of island arcs to intraplate tectonic settings. They comprise more fractionated granitic masses of monzogranite, syenogranites, alkali feldspar granites, and finally amphibole-bearing granites. This rock suite is situated in Hajjah and Sa'dah areas (Geomin 1985; Heikal 1987) and around Al-Hikhel town in Al Bayda area (Sakran 1993; Al-Khirbash et al. 2001; As-Saruri and Wiefel 2012). Hooth ring complex and new volcanics, first named by Gameil and Heikal (2005), represent the final phase of tectonism, after unconformity episode, giving rise to syenitic composition of alkaline nature that evolved within intraplate environment. Other ring complexes comprise syenites, and carbonatites are well recognized in the areas toward SW Taiz town (Fig. 1). Also, new volcanics including trachyte plugs and ring dikes that are dominantly alkaline in nature are well preserved along strike-slip faults passing through SW Taiz. On the basis of field data and tentative chronological sequence as well as geochemical interpretation (Heikal 1987; Ba-Bttat 1991, 1993; Sakran 1993; Whitehouse et al. 1998; Al-Khirbash et al. 2001; As-Saruri and Wiefel 2012), the classification of the basement rocks of Yemen can be reconstructed as in Table 1.
Deformation history In order to visualize a framework for deformation history of the above two terranes and based on structural data and space images, it is clear that the presumed Precambrian belts in Yemen have evolved through six deformational phases (D1–D6). The first phase of deformation, D1, is considered to be pre-Pan-African (pre-thrust) and is dominated by regional isoclinal major and minor parallel folds (F1) (Fig. 3a), regional schistosity, and gneissosity (S1, Fig. 2c), where the rocks were formed principally of granitic gneisses and migmatites. The migmatites occur as preserved in Al Bayda, Mukairas, and Mudiyah–Mukalla areas (Ahwar Supergroup, Al Mahfid gneiss terrane, in major).
Structures in these rocks comprise convolute and ptygmatic folding and swirling pattern, and axes of minor folds of migmatitic relics trend NNE. The second phase of deformation, D2, is related to thrust structures. It most probably occurred during early PanAfrican and is dominated by compression, refolding (F2), and strike-slip faulting (Fig. 2a) related to early Najd Fault System (NFS) (e.g., Nehlig et al. 2002). The island arc development causes refolding and remobilization of the infracrustal rocks and thrusting of the newly formed oceanic crust and island arcs on the regional scale. In most tectonic units, the thrusting was accompanied by mesoscopic structures such as recumbent, chevern, and overturned folds (Fig. 3b–c). In addition, deformed prophyroclasts, S-C fabrics, and σσ are toward NNE. In addition, the field data collected from Al Bayda rocks and Al Aswad Group (Al Bayda terrane) (Sakran 1993; Al-Khirbash et al. 2001; AsSaruri and Wiefel 2012) indicated that this belt is an island arc that is sutured to the adjacent Al Mahfid gneiss terrane by an oblique continental collision that took place at about 760 Ma (Whitehouse et al. 1998,2001). The third phase, D3, is probably of late Pan-African, related to wrenching tectonics. It is dominated by major sinistral strike-slip and dextral strike-slip faults which trend NW–SE (in major) and is related to movement along the NFS. These faults were accompanied by mesoscopic structures such as kink folds, Z-folds (F3), boudinage structures, quartz σ-type object, and dynamic recrystallization (Fig. 3d, e). Cratonization of the accreted old continental and ophiolitic island arc (Abas and Al Bayda) terranes onto the newly formed craton is well developed. It concludes that the deformation phases D2 and D3 represent wrenching (strikeslip faults only) followed by transpression (strike-slip faults plus thrusts) ending by wrenching. The fourth deformation D4 is of brittle structure in the basement complex in Yemen (Abas terrane). It is represented by major and minor normal faults that are trending NE– SW and less commonly NW–SE and NNE–SSW and NNE. These faults are accompanied by mesoscopic structures such as local shear zones, graben and horse blocks, mega and minor joints, and quartz veins (Fig. 3f–h). Deformation phase (D5) evolved during post-thrust, characterized by compression only giving rise to major and minor folds (F5) which are trending NNW and NNE (Hajjah area, Asir Terrane) (Fig. 3i). These folds were accompanied by foliation and mineral lineations. This event also marked rifting giving rise to extrusion of syn-rift volcanics that are weakly to unmetamorphosed calc-alkaline arc, followed by the titling to drastic folding of Ghabar Group (post-rift), giving rise to the tectonic depressions and down faulted blocks marked by the present exposures of the Ghabar Group. This is in good agreement with AsSaruri and Wiefel (2012).
Arab J Geosci Table 1 Integrated lithostratigraphy of the basement rock units of Yemen and their tectonic events
ROCK UNITS Hooth Ring Complex & New Volcanics
TECTONIC EVENTS Intra-Plate Magmatism
----------- Unconformity--------Ataq-Mudiyah-Al-Mukalla Late-to-Post Tectonic Granites Suite Highly fractionated granites
