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).