2. Geological setting
Eastern Arabia was affected by the Late Paleozoic Pangean Neo-Tethys rifting, forming ~NW-striking extensional basement faults (e.g., Béchennec et al., 1990; Blendinger et al., 1990; Chauvet et al., 2009). Subsequent to rifting, Arabia remained largely a passive margin during the Permian to mid-Cretaceous (e.g., Glennie et al., 1974; Searle, 2007). This resulted in deposition of mostly shallow-marine, shelfal sedimentary rocks on the Arabian platform (Hajar Supergroup; e.g., Glennie et al., 1973, 1974; Searle, 2007). These rocks were overthrust by Neo-Tethys-derived allochthonous Hawasina-Basin and trench-facies rocks from the NE (~1-2 km in thickness) and the Semail Ophiolite during the Late Cretaceous (~10-15 km in thickness; e.g., Glennie et al., 1973, 1974; Béchennec et al., 1992; Nicolas et al., 2000; Blechschmidt et al., 2004; Searle, 2007), forming the Oman Mountains. At the base of the ophiolite, a metamorphic sole formed coeval with the formation of the ophiolite (Rioux et al., 2016).
Post-obductional successions in the Muscat-Seeb area (Figs. 3 and 4) consist of the <860-m-thick late Campanian to Maastrichtian Al-Khod (or Qahlah) Formation, containing alternating fluvial, coastal and deltaic shales, sandstones and conglomerates (e.g., Nolan et al., 1990; Abbasi et al., 2014; Pickford, 2017). The Al-Khod Formation is unconformably overlain by the 250-to-300-m-thick late Paleocene to early Eocene shallow-marine limestone of the Jafanyn Formation (e.g., Nolan et al., 1990; Mattern and Bernecker, 2019). The Jafnayn Formation is conformably overlain by the ~150-m-thick Rusayl Formation, consisting of marl, shallow marine limestone and sandstone of lower Eocene age (e.g., Nolan et al., 1990; Mattern et al., 2021). The Rusayl Formation is conformably topped by the Seeb Formation. The latter measures ~600 m in thickness, consists of middle to upper Eocene shallow-marine limestone (e.g., Nolan et al., 1990; Hersi & Al-Harthi, 2010). The Oligocene MAM or Ma’ahm Beds Formation, consisting of reefal limestone, rests unconformably on the Seeb Formation (Hersi and Al-Harthy, 2010). The thickness of this formation is not known, owing to the poor exposure but assumed by us to measure ~100 m. The latest Oligocene to probably Pliocene ~170-m-thick Barzaman Formation rests stratigraphically on top of the MAM Reefs Formation (Mattern et al., 2020b). The lithology of this formation is highly variable, ranging from conglomerates to sand- and claystones and limestones reflecting terrestrial and marine deposition, intense weathering and erosion of the Oman Mountains during arid to pluvial episodes (Mattern et al., 2020b). Further details on the geology and tectonics of the eastern Oman Mountains are provided in Scharf et al. (2021a, 2021b).
Immediately after and/or slightly coeval with the emplacement of the ophiolite, major exhumation and top-to-the-NE shearing occured in the Saih Hatat Dome, and some gentle exhumation with top-to-the NE shearing in the Jabal Akhdar Dome (e.g., Nolan et al., 1990; Breton et al., 2004; Fournier et al., 2006; Saddiqi et al., 2006; Al-Wardi and Butler, 2007; Searle, 2007; Agard et al., 2010; Hansman et al., 2017, 2021; Grobe et al., 2018, 2019; Scharf et al., 2019b; Figs. 1 and 4). Further convergence of Arabia and Eurasia led to major doming of the Jabal Akhdar and some mild doming/surface uplift in the Saih Hatat during the late Eocene to Oligocene (~43-30 Ma; e.g., Hansman et al., 2017, 2018; Grobe et al., 2018, 2019; Corradetti et al., 2019). Convergence was NE/SW-directed (e.g., Fournier et al., 2006; Fig. 5).
Extensional tectonics associated with doming at the flanks resulted from gravitational collapse (e.g., Hanna, 1990; Mann et al., 1990; Mattern & Scharf, 2018; Scharf et al., 2019a). Extension at the northern margin of the two domes was facilitated along the Frontal Range Fault (Mattern & Scharf, 2018; Figs. 2-4). The area between the northern Jabal Akhdar and Saih Hatat domes in the Muscat-Seeb area was deformed during extension (Rusayl Embayment; Fig. 3). Extension in the Rusayl Embayment was NE/SW-directed during the latest Cretaceous to early Eocene (Tectonic Stage 1) and NE/SW and NW/SE-directed (Tectonic Stage 2A and 2B) during the late Lutetian to possibly Miocene (Fournier et al., 2006; Scharf et al., 2020b; Figs. 4 and 5).
The geology and tectonics of the Batain area differs from that of the Oman Mountains. The Batain area is characterized by mostly deep-sea sediments derived from the Batain Basin whose fill was thrust to the WNW onto Arabia during the course of the obduction of the Masirah Ophiolite at the Cretaceous/Cenozoic boundary (e.g., Mountain & Prell, 1990; Shackleton & Ries, 1990; Schreurs & Immenhauser, 1999; Fig. 1). Emplacement of the Masirah Ophiolite and Batain nappes followed deposition of the upper Maastrichtian flysch-type Fayah Formation on the oceanic lithosphere of the Batain Basin (Immenhauser, 1996).
After the emplacement of the Masirah Ophiolite, the Batain area underwent extensional and compressional deformation (Schreurs & Immenhauser, 1999). The earlier extension is characterized by NNE-SSW and E/W-orientated conjugated extensional faults within Cenozoic sedimentary rocks (Fig. 5). Extension is probably linked to Cenozoic reactivation of the Cretaceous Masirah Graben, which is located beneath the Batain area (Beauchamp et al., 1995; Fig. 1). Deformation in the Batain area may correlate with extension in the Gulf of Aden starting during the late Eocene (e.g., Hempton, 1987). Basanite/alkaline intrusions in the Batain area of late Eocene age may be associated with this extension (e.g., Peters et al., 2001).
Extension was followed by Miocene-Pliocene compression, which affected late Paleocene to early Miocene rocks (Schreurs & Immenhauser, 1999). A ~NE/SW to ~E/W-oriented shortening interval produced NW/SE to N/S-oriented open folds (Roger et al., 1991). The Miocene Tahwah and Sur formations are intensely folded near the Qalhat Fault (Wyns et al., 1992). In the footwall of the Qalhat Fault, the uppermost beds of the Miocene Sur and Salmiyah formations cut across lower beds of the same formations, forming an angular unconformity (Wyns et al., 1992). The Miocene Salmiyah Formation is folded within an open N/S-trending syncline (Wyns et al., 1992).
Uplift and inversion in the Jabal Ja’alan area is post-mid-Eocene in age (Fournier et al., 2006). Uplift is associated with N/S-trending asymmetric anticlines and occurred during E/W-compression (Filbrandt et al., 1990). All these field observations indicate that the area near the Qalhat Fault underwent ~E/W-shortening from the Miocene to the present.
The transition between the Oman Mountains and the Batain area is broadly localized along the Qalhat Fault (Filbrandt et al., 1990; Figs. 1, 2). This major ~N/S-striking fault has a long-lasting history and may have originated during Pan-African terrane accretion, similar to the Semail Gap Fault Zone at the eastern margin of the Jabal Akhdar Dome (Romine et al., 2004; Scharf et al., 2019a; Mattern & Scharf, 2019; Weidle et al., 2021; Fig. 2). During the Phanerozoic, the Qalhat Fault was likely reactivated several times in different ways (Fournier et al., 2006). During the early Cenozoic, the transtensional Qalhat Fault bounded the Abat Basin and the future Salma Plateau to the West, with the Salma Plateau being uplifted the Abat Basin inverted during the Miocene (e.g., Wyns et al., 1992; Fournier et al., 2006). Details on the stratigraphy of the Abat Basin and the Salma Plateau are provided in Fournier et al. (2006) and Figure 4. The rocks west of the Qalhat Fault (Tiwi Platform or Salma Plateau) mostly consist of late Paleocene to mid-Eocene shallow-marine limestone, now uplifted to ~2000 m of elevation. This area is still uplifting, as indicated by a historical earthquake at Qalhat and uplifted marine terraces (e.g., Wyns et al., 1992; Fournier et al., 2006; Mattern et al., 2018a; Moraetis et al., 2018; Ermertz et al., 2020; Hoffmann et al., 2020). In contrast, no or only little Quaternary tectonic activity has been reported from the Central Oman Mountains (Scharf et al., 2019a; Moraetis et al., 2020). Mid-Eocene to Oligocene shallow-marine rocks east of the Qalhat Fault are not currently being uplifted and represent the footwall of the Qalhat Fault (Wyns et al., 1992).
In the Northern Oman Mountains, the Oligocene to early Miocene Hagab Thrust is a prominent N/S-striking compressional feature, interpretd to be associated with the Arabia/Eurasia convergence (Searle et al., 2014), although the thrust is higly obliquely oriented relative to the WNW-ESE-striking convergence zone. Further to the South, at the Oman/UAE border, the sizable NNW/SSE-oriented anticline of Jabal Hafit occurs (Fig. 1). This anticline grew during the late Oligocene to early-middle Miocene and is related to WSW-directed shortening (Hansman & Ring, 2018).