Structural Geometry, Style and Timing of Deformation in the Hawasina Window, Al Jabal Al Akhdar and Saih Hatat Culminations, Oman Mountains
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GeoArabia, Vol. 12, No. 2, 2007 Structural deformation, Oman mountains Gulf PetroLink, Bahrain Structural geometry, style and timing of deformation in the Hawasina Window, Al Jabal al Akhdar and Saih Hatat culminations, Oman Mountains Michael P. Searle ABSTRACT The Al Jabal al Akhdar and Saih Hatat culminations in the central Oman Mountains expose the complete mid-Permian to Late Cretaceous (Cenomanian) passive shelf and margin carbonate sequence beneath the allochtonous slope (Sumeini Group), basin (Hawasina complex), distal ocean-trench (Haybi complex) facies rocks, and the Semail ophiolite thrust sheets that were emplaced from NE to SW during the Late Cretaceous. Reconstruction of the pre-thrust sequences shows that time- equivalent rocks occur in successively stacked thrust sheets from shelf to slope to basin. The Al Jabal al Akhdar structure is a 60 km wavelength anticline plunging to the northwest beneath the Hawasina Window and with a fold axis that curves from WNW-ESE (Jabal Shams) to NNE-SSW (Jabal Nakhl). The structure shows little internal deformation except for minor intra-formational thrust duplexing within the Cretaceous shelf stratigraphy along the northern margin. The upper structural boundaries around the flanks of the shelf carbonate culminations have been re-activated as late stage normal faults. The Semail thrust formed a passive roof fault during late-stage culmination of al Al Jabal al Akhdar such that the ophiolite rests directly on Wasia Formation top-shelf with the entire Sumeini, Hawasina and Haybi thrust sheets displaced around the margins. NE-directed backthrusting and intense folding in the northern part of the Hawasina Window affects all allochtonous units and is related to a steep ramp in the Late Cretaceous shelf margin at depth. The Saih Hatat culmination is another 40 km half-wavelength anticline in the central Oman Mountains, but shows extreme deformation in the form of recumbent folds, sheath folds with NNE-trending axes and thrusting along the northern margin. High-pressure carpholite, blueschist and eclogite facies rocks are exposed at successively deeper structural levels, separated by high-strain normal sense shear zones. There is no evidence for a separate ‘North Muscat microplate’ or an intra-continental subduction zone, as previously proposed; all high-pressure units can be restored to show their pre-deformation palaeographic positions along the northern margin of the Arabian Plate. Both Al Jabal al Akhdar and Saih Hatat are Late Cretaceous culminations, folded after obduction of the Hawasina, Haybi and Semail ophiolite thrust sheets from northeast to southwest during the period Turonian to Campanian-Lower Maastrichtian. Maximum compressive stress along the central Oman Mountains was oriented NE-SW, parallel to the ophiolite emplacement direction, but a second compressive stress axis was oriented WNW-ESE, either concurrently or slightly later in time, resulting in a dome and basin structural geometry. The biaxial fracture pattern in the foreland, southwest of the Oman Mountains could be explained as a result of the WNW- directed emplacement of the Masirah ophiolite belt and Batain mélange during the Campanian-early Palaeocene. Both Al Jabal al Akhdar and Saih Hatat were positive topographic features at the end of the Cretaceous with Upper Maastrichtian and Palaeogene sediments onlapping both flanks. In Jabal Abiad, these Palaeogene sediments have been uplifted by at least 2 km since the Late Miocene-Early Oligocene associated with minor NNE-SSW compression. Tertiary shortening, folding and thrusting increases to the north in the Musandam peninsula where the first effects of the Arabian Plate-Eurasian Plate (Zagros belt) continent-continent collision are seen. 99 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/12/2/99/4570063/searle.pdf by guest on 24 September 2021 Searle INTRODUCTION Three major culminations of Permian - Cenomanian shelf carbonate rocks occur along the Oman Mountain range, from NW to SE the Musandam, Al Jabal al Akhdar and Saih Hatat culminations (Figure 1). Each is a very large-scale antiformal fold that also affects the structurally overlying Sumeini, Hawasina, Haybi and Semail ophiolite thrust sheets. Palinspastic reconstruction of the Oman continental margin has shown that the allochthonous thrust sheets contain time-equivalent units but of progressively more distal facies structurally up-section (Figure 2; Glennie et al., 1973, 1974; Searle et al., 1983; Rabu et al., 1993). The reconstructed shelf margin and basin for the Central Oman Mountains is shown in Figure 3. Several basin-wide stratigraphic markers have been traced from the proximal Sumeini Group to the most distal Hawasina radiolarian cherts (Searle et al., 1983; Figure 3). 56°E 57° 58° TURKEY59° Caspian Sea N 0 km300 Semail Ophiolite Med SYRIA Sea IRAN Metamorphic sole IRAQ 26° 26°N JORDAN Musandam Amphibolites, Greenschists KUWAIT Arabian Peninsula Gul Haybi, Hawasina thrust sheets f BAHRAIN Permian-Mesozoic shelf carbonates QATAR EGYPT Arabian UAE Shield OMAN Dibba High pressure metamorphic units Hagab Thrust SAUDI ARABIA Asimah-Masafi Carpholite zone Red SUDAN Sea YEMEN Bani Hamid Garnet, glaucophane, blueschists Arabian Sea granulites ERITREA Wadi Ham 25° N Semail Thrust 0 50 Wadi Hatta km Sumeini Wadi Jizi Sohar Haybi Figure 12 Hawasina Window 24° Figure 11 24° Figure 13 Figure 14 As Judi Al Ajaiz Muscat As Sifah eclogites Al Jabal Hulw Hawasina al Akhdar Saih Hatat ap G Jabal Nakhl ail Thrust Jabal Abiad 23° Sem 23° Ras Al Sem Hadd ail Thrust OMAN Jabal Ja’alan 56° 57° 58° 59° Figure 1: Geological map of the Oman Mountains, after Glennie et al. (1974). Locations of the three cross-sections of the Hawasina Window, Al Jabal al Akhdar and Saih Hatat are shown. 100 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/12/2/99/4570063/searle.pdf by guest on 24 September 2021 Structural deformation, Oman mountains Interior Oman and Stratigraphy Shelf Slope Hawasina Basin Haybi Trench Al Jabal al Akhdar Exotics Maastrichtian Campanian Juweiza Santonian Fiqa Group Aruma Qumayrah Fm Late Coniacian Muti Turonian Natih Cenomanian Wasia Nayid Fm Albian Group Nahr Umr Safil Aptian Shu’aiba D Barremian Kharaib CRETACEOUS Lekhwair Hauterivian Early Habshan C Group Valanginian Kahmah Salil Berriasian Rayda B Sidr Fm Tithonian Jubaila Nadan Kimmeridgian Hanifa Late Oxfordian Tuwaiq Callovian Bathonian Bajocian Dhruma A Middle Aalenian Toarcian Sahtan Group JURASSIC Mafraq Guwayza Formation Wahrah Formation Pliensbachian Mayhah Formation Sandstone - Limestone Sinemurian Early Hettangian Unconformity Rhaetian Jabal Wasa Fm Triassic Norian Hamrat Duru Group Exotics Sumeini Group Misfah Late E Fm (limestone) Carnian n (limestone) Dhera Fm Haybi Ladinian Mahil Volcanics OIB D Haybi Volcanic Group Middle Anisian TRIASSIC WPB Zulla Fm Maqam Formatio Olenekian C Akhdar Group Early Induan B 3 A Baid Fm Saiq/ 2 Yankul PERMIAN Khuff Exotics 1 Al Jil Fm OIB Figure 2: Generalised Permian and Mesozoic stratigraphy of the Oman Mountains including the shelf carbonates (Glennie et al., 1973, 1974; Sharland et al., 2004) and allochthonous slope and basin facies rocks (Robertson and Searle, 1990; Blechschmidt et al., 2004). WPB – Within-plate volcanics; OIB – Ocean island volcanics. Regional mapping of the Oman Mountains was carried out first by the Shell team of Glennie et al. (1973, 1974) who also defined the stratigraphy, and proposed a thrust emplacement model for the Semail ophiolite and Hawasina complex rocks, a model that is still widely accepted today. During the 1970–1980s, a USGS team mapped a strip across Saih Hatat and the Ibra ophiolite block from Muscat south to Ibra (Bailey, 1981) and an Open University team mapped much of the northern Oman Mountains, concentrating mainly on the Semail ophiolite complex (Lippard et al., 1986). The BRGM carried out detailed mapping at 1:100,000 scale of much of the Oman Mountains and summaries of their work have been published by Rabu et al. (1993) and Le Métour et al. (1990). The northern Saih Hatat region has been studied in some detail following the discovery of high-pressure (HP) eclogites, blueschists and carpholite-bearing rocks (Le Métour et al., 1986; Goffé et al., 1988; El-Shazli et al., 1990; Searle et al., 1994, 2004; Miller et al., 1998, 1999). Recently, debate has centered on the timing of uplift of the Al Jabal al Akhdar and Saih Hatat culminations (Late Cretaceous or Tertiary) and the tectonic evolution of the Oman Mountains and the foreland. 101 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/12/2/99/4570063/searle.pdf by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/12/2/99/4570063/searle.pdf Searle Sedimentary profile of the restored Mesozoic Arabian continental margin, Central Oman Mountains Southwest Northeast Shelf carbonates Sumeini Group Hawasina complex Kawr-Misfah + Haybi complex Semail Al Jabal al Akhdar slope facies distal slope and basin facies platform facies distal ocean-trench ophiolite (Umar basin) complex (Hamrat Duru Group) Age System Formation Ma Period slumps 92 Natih CRETA- Nahr Umr Qumy (slope facies Shu’aiba conglomerates) CEOUS Lekhwair Huwar 149 Mayhah JURASSIC Sahtan Fm 182 Chert horizons 102 Nadan Fm (chert) TRIASSIC Mahil Nay Cenomanian id Fm Maqam Sidr Fm Mid Jurassic Fm Hallstatt Guwayza Fm U Saiq/Khuff facies U. Triassic 257 PERMIAN 95 Ma Haushi L oceanic crust Off-axis alkaline volcanics Triassic alkali volcanic substrate Haybi volcanics Triassic 0 100 km Metamorphic sole approximate scale only amphibolites greenschists Figure 3: Palinspastic reconstruction of the Oman shelf, slope and basin in the Late Cretaceous. The shelf, slope (Sumeini Group), basin (Hawasina and Haybi complexes) are time-equivalent stratigraphic units bounded by major thrusts. Structural deformation, Oman mountains Two recent models in particular have drawn attention to the key questions of timing of high pressure metamorphism and subduction, and the polarity of the subduction system. Gregory et al. (1998), Gray and Gregory (2000) and Gray et al.