5. Interpretation and discussion
5.1 Wrench-fault assemblage and satellite image analysis
Field analysis of 15 sites (Supplementary Material S2) and map interpretations indicate that all formations/units (except the Quaternary deposits) in northeastern Oman were affected by NW-striking sinistral faults, associated with ~WNW-striking sinistral Riedel faults, ~N/S to ~NW/SE-oriented compressive structures (i.e., folds, reverse faults, thrusts) and ~E/W-oriented normal faults. At Site 2 (Halban), a SW-striking dextral fault has been mapped. This dextral fault is interpreted as an anti-Riedel fault (antithetic to the main fault). All these structures combined indicate a wrench-fault assemblage of an overall NW-striking major sinistral shear zone (see Harding Strain Ellipse in Fig. 8).
Most of the >10,000 identified lineamentary features within the study area are WNW/ESE to NW/SE-oriented (Fig. 6). The orientation of the lineamentary features differs outside the study area (i.e., along the eastern margin of the Jabal Akhdar Dome and adjacent ophiolite rocks, with a mostly NNE/SSW-orientation; Scharf et al., 2019a). The general orientation of lineamentary features within different sub-areas of the study area does not change significantly (Fig. 6). Thus, most of the identified linear features formed during the same deformation/shearing event.
The WNW-oriented linear features are interpreted as sinistral Riedel faults with respect to the ~NW-striking main fault, while ~E/W-striking features are probably normal faults (see Harding Strain Ellipse of Fig. 8). The ~NE/SW-oriented lineaments are mostly found in the Batain area (Fig. 6). Only the Batain area was affected by ~WNW-directed obduction of the Masirah Ophiolite. Thus, we conclude that these features are related to the thrusting of the Masirah Ophiolite (i.e., these features present largely thrusts of the Masirah fold-and-thrust belt).
5.2 The Hajar Shear Zone
Wrench-fault assemblage (~NW-striking sinistral faults, ~N/S-oriented compressive structures, ~E/W-oriented extensional structures, ~WNW-striking sinistral fault – Riedel faults and ~SW-oriented dextral faults – anti-Riedel faults) and the WNW to NW-oriented lineamentary features are distributed over a wide area with a minimum length and width of ~250 km and ~50 km, respectively, from the eastern Batinah Coastal Plain to the Batain area (Fig. 8). This entire zone underwent NW-directed sinistral shear and E/W-shortening after the mid-Eocene. We reference his zone as “Hajar Shear Zone” (HSZ). Some parts along the HSZ are active, indicating ongoing ~E/W-shortening (e.g., seismicity near the Qalhat Fault and uplifting marine terraces; Fig. 1). Numerous faults, compressive and extensional structures are present within this area, indicating widely distributed strain. However, most of the sinistral shear is concentrated along major faults, probably because they reactivated older strucutres. These include a segment of the Frontal Range Fault near Fanja, the Wadi Mansah Fault Zone, possibly the Wadi Tayin Fault, the Issmaiya Fault Zone, the Coastal Parallel Shear Zone and the Ja’alan Fault (Fig. 8). In our model, we connect/link all these faults to one major sinistral fault zone, along which most of the shearing of the HSZ has been accommodated.
In the southeast, Quaternary deposits cover large parts of the Batain region, which makes it challenging to trace/map the continuation of the HSZ. The width of the HSZ could exceed 50 km because rocks to the SW of the Wadi Mansah/Issmaiya/Ja’alan faults display widely distributed WNW to NW-oriented sinistral faults, too (Fig. 8). However, most of this area consists of the Semail Ophiolite, where shearing is difficult to detect and date.
5.3. Time-constraining the deformation
5.3.1 Interval I
Ages of several structures within the HSZ have been constrained by different geological approaches to be Oligocene or younger. Most of the ~N/S-oriented structures (faults and folds) deformed late to mid-Eocene formations but also some Oligocene/Miocene formations in the Rusayl Embayment and Sur area. Thus, ~E/W-shortening must be younger than mid-Eocene.
Sinistral WNW to W-striking faults at Wadi Ja’arah, Taww and Fanja (sites 1, 2 and 5) cut all structures including top-to-the NE extensional structures (e.g., compare Al-Wardi and Butler, 2007; Grobe et al., 2018; Mattern and Scharf, 2018). Furthermore, sinistral strike-slip faults are vertical or sub-vertical and were not tilted during the main doming interval of the Jabal Akhdar, which lasted from the late Eocene to Oligocene (Hansman et al., 2017; compare Levell et al., 2021 for offshore folding). We suggest that these NW-striking sinistral faults are part of the HSZ with the ~E/W to WNW-striking sinistral faults representing Riedel faults (synthetic faults). Strike-slip faults at Taww post-date Jabal Akhdar doming and top-to-the-NNE extensional shearing and, thus, occured during the Oligocene or later.
At Fanja, the strike-slip faults cut all extensional structures of the southern Fanja Half Graben. The first extensional interval at this half graben lasted from the latest Cretaceous to early Paleogene (Tectonic Stage 1 of Fournier et al., 2006) and the second interval was post-mid-Eocene (probably Oligocene; Tectonic Stage 2 of Fournier et al., 2006; Fig. 5). Therefore, strike-slip faults at Fanja are also Oligocene or younger in age.
The sinistral fault at the southwestern margin of the Saih Hatat Dome, at Wadi Al Khabbah (site 14; Fig. S22) displaces autochthonous Jurassic and Cretaceous rocks, which were tilted during the late Eocene to Oligocene doming of the Saih Hatat (Hansman et al., 2017). Thus, this WNW-striking fault must be Oligocene or younger, too.
A key outcrop is the syndepositional thrust at the Sultan Qaboos University Campus (site 7a; Fig. S13). This thrust occurs within the Upper Oligocene to Lower Miocene section of the Barzaman Formation (age based on foraminfer genera; Mattern et al., 2020b), attesting that ~E/W-directed shortening was active during the latest Oligocene to Early Miocene. This designated age agrees with computed paleostress information derived from fault slip data by Fournier et al. (2006), indicating that E/W-compression occurred during the early Miocene.
A similar and slightly younger (Miocene) age for ~E/W-shortening has inferred by Fournier et al. (2006) and Schreurs and Immenhauser (1999) for the Batain/Sur area (Fig. 5). Filbrandt et al. (1990) assigned a post-mid-Eocene age to ~E/W-shortening in the Jabal Ja’alan area. The inversion of the Abat Basin started during the late Burdigalian (compare Wyns et al., 1992).
GPlates reconstructions document ~E/W-shortening between Arabia and India. Interval I is related to the counterclockwise rotation of India between 32.5 and 20 Ma (Rupelian to Burdigalian; Cohen et al., 2013). This deformation created the HSZ and its wrench-fault assemblage. Owing to coeval counterclockwise rotation of India and northward motion of India, the direction of shortening also rotated counterclockwise from ~E/W to ~ENE/WSW. This rotation and the coeval northward movement of India resulted in non-coaxial deformation (simple shear) at northeastern Oman.
Furthermore, U-Pb dating of synkinematic calcite proofs a latest Oligocene/earliest Miocene age for sinistral deformation in the Barzaman Formation (22.31 ±2.15 Ma; site 7b; Fig. S14). The age of 22.31 Ma corresponds to the Aquitanian (Cohen et al., 2013). This age indicates that deformation ensued during deposition of the Barzaman Formation, consistent with the age of the synsedimentary thrust at site 7a (Fig. S13).
A further precise U-Pb age for carbonate was obtained from the Halban area (30.08 ±0.47 Ma; site 2; Fig. S3-6). This early Oligocene age (Rupelian; Cohen et al., 2013) dates dextral motion along a ~SW-striking anti-Riedel Fault of the HSZ and within the Jafnayn and Rusayl formations.
The independently obtained age constraints for ~E/W-shortening from map interpretation, cross-cutting relationships, syndepositional thrusting, GPlates reconstruction and U-Pb dating of synknematic carbonate, demonstrate shortening along the HSZ between 32.5 and 20 Ma and define our Interval I.
5.3.2 Interval II
GPlates reconstructions indicate that no E/W-shortening between the Arabian and Indian plates occurred between 20 to 11 Ma. Further convergence with an E/W-shortenig component started at 11 Ma (early Tortonian, Cohen et al., 2013) and lasting until present-day associated with ~60-75 km of shortening (Interval II). This ongoing shortening interval may have reactivated the Qahlah Fault (recent seismicity), is responsible for activities at the transpressional Coastal Parallel Shear Zone, inverted the Abat Basin and uplifted marine terraces NW of the Qalhat Fault (Fig. 13). However, further structural work and dating are needed confirm the GPlates results.
5.4 Amount of deformation in eastern Arabia and the HSZ
According to GPlates reconstructions, the E/W-convergence between the Arabian and Indian plates amounts to ~100-135 km from 32.5-20 Ma. Thus, the combined amount of shortening of all described structures from Musandam to the Batain area cannot exceed this amount, assuming that the pre-Oligocene and present-day coastline of eastern Oman were at the same position and of the same shape as today.
The amount of Oligocene to early Miocene westward translation along the Hagab Thrust measures >15 km (Searle, 1988). The Oligocene/early Miocene shortening along Jabal Hafit Anticline and the Suneinah area has not been quantified but is assumed by us to amount to a few tens of kilometers (compare Boote et al., 1990; Hansman & Ring, 2018).
The amount of sinistral shearing (lateral displacement during Interval I) along the HSZ and ~E/W-shortening is unknown, due to a lack of markers. Numerous small structures occur within the HSZ (folds, thrusts, reverse faults, normal faults and strike-slip faults), and these structures are widely distributed throughout our study area of ~250 km by ~50 km. Folds are mostly open and range in size from several meters to kilometers in wave length. Mapped faults have a displacement of a few kilometers. We acknowledge that not all structures within this large area were measured and mapped.
A conservative estimate for sinistral shearing and ~E/W-shortening within the HSZ amounts to a few to several tens of kilometers. Some unknown amount of sinistral shearing and ~E/W-shortening may have been absorbed (1) in the non-mapped areas of the HSZ (Batinah Coastal Plain and Batain area), (2) within the Arabian Plate, i.e., the Central and Northern Oman Mountains, (3) the Omani margin offshore Batain (compare Rodriguez et al., 2014; 2016), (4) the former plate boundary of Arabia and India and (5) within the Indian Plate.
The amount of ~ENE/WSE-oriented shortening within eastern Oman (Interval II) remains unknown due to a lack of markers. The amount of plate convergence between India and Arabia is ~85 km since 11 Ma, resulting in 60-75 km of E/W-shortenig (Fig. 12). The amount of dextral motion between the Arabian and Indian plates for the last 11 Ma (~7.5 mm/a; based on GPlates) exceeds the reported present-day dextral motion at the Owen Fracture Zone of 3 ±0.4 mm/a (Fournier et al., 2008; DeMets et al., 2010). Thus, other active faults/plastic deformation/uplift may have absorbed the difference in plate motion (i.e., ~4.5 mm/a). Potential “absorbing structures” are the Owen Ridge with an uplift of >2 km (Rodriguez et al., 2014), the active Qalhat Fault and the uplifting marine terraces of the coastal Salma Plateau (e.g., Wyns et al., 1992; Mattern et al., 2018a; Moraetis et al., 2018).
5.5. Tectonic implications
Eastern Arabia contains ~NW-striking basement faults, which formed during the Pangean/Neo-Tethys rifting. One of these faults or fault zones is below the HSZ at the southwestern margin of the Saih Hatat Dome. It is likely that this structure was reactivated during the Oligocene/early Miocene ~E/W-convergence between Arabia and India. Thus, the pre-existing basement structure plays a major role in the localization of the HSZ, especially the localization and concentration of sinistral slip and absorption of ~E/W-shortening-related strain along the aligned Wadi Mansah, Wadi Tayin, Issmaiya and Ja’alan faults (Figs. 2 and 13).
GPlates modeling of motion between the Arabian and India plates shows that India rotated counterclockwise from ~32.5 to 20 Ma by ~8° with coeval northward motion of India; Fig. 11). Consequently, the direction of shortening during this stage rotated counterclockwise, too, resulting in simple shear deformation in eastern Oman and activity along the HSZ (Interval I). The overall ~E/W-convergence/shortening between the Arabian and Indian plates amounts to a minimum of ~100-135 km.
The eastern Arabian Plate was affected by this deformation, evident by the wrench-fault assemblage associated with the HSZ (Figs. 8 and 13A). The amount of sinistral slip along the HSZ, including its associated wrench-fault assemblage, is not well constrained but could amount to a few to several tens of kilometers. Thus, the easternmost tip of continental Arabia was located further to the east by a few to several tens of kilometers prior to the Oligocene than today (Fig. 13). This interpretation is supported by the outline of the present-day shelf break (i.e., ~200 m water depth). The shelf break of eastern Oman may have been more or less straight before the Oligocene (Fig. 13A) and must have changed during the Oligocene due to left-lateral shearing. Correspondingly, the present-day shelf break changes direction. It kinks or veers to the North in the Batain area (Fig. 13). Most of this directional change occurs in the realm of the HSZ (Fig. 13). A former pre-Oligocene straight shelf break implies that easternmost Oman was left-laterally sheared for a few to several tens of kilometers (Fig. 13). Factoring in this aspect E/W-shortening during Interval I must have amounted to more than the estimated ~100-135 km of E/W-convergence/shortening, because GPlates uses the present-day countour of the shelf break which is post-deformational.
In addition, the present-day Qalhat Fault strikes NNW at its northern end, while it is oriented ~N/S to NNE/SSW further to the South at Jabal Ja’alan (Fig. 2). This directional change is gradual. The latter orientation is similar to that of other major faults within the Oman Mountains such as the Semail Gap Fault Zone (Fig. 8; Weidle et al., 2021). We suggest that during sinistral shearing, the former ~N/S to NNE/SSW-oriented Qalhat Fault was passively rotated or dragged in a counter-clockwise way to a NNE-strike at its northern end. A consequence of shortening and left-lateral shearing is that the Saih Hatat and Jabal Akhdar domes may have been further apart from one another in map view prior to shearing. In addition, we conclude that the ~N/S to NW/SE-striking compressive structures in the northern Oman Mountains (e.g., Hagab Thrust, Jabal Hafit Anticline) also formed during Oligocene/early Miocene Arabia/India-shortening. This interpretation is supported by the orientation and age of these structures. The ~N/S-oriented compressive structures unlikely formed during the NNE/SSW-directed Arabia/Eurasia-convergence as previously interpreted (e.g., Boote et al., 1990; Searle et al., 2014), because this convergence would have produced differently oriented compressional structures.
If Interval II is accepted as factual, the sinistral faults were not reactivated during Interval I because the direction of shortening (~ENE/WSW) is at a high angle to the orientation of the NW-striking HSZ. Shortening during Interval II rather produced conjugate strike-slip faults, which are indicative for pure shear deformation (e.g., Fournier et al., 2006). Thus, Interval II deformation likely overprinted elements of the wrench-fault assemblage of the HSZ such as the compressive structures at the Coastal Parallel Shear Zone, responsible for Neogene to Quaternary uplift of marine terraces between Quriyat and Sur, Neogene inversion of the Abat Basin and active tectonics at the Qalhat Fault (Fig. 13).