The Tertiary structures of the Northern Oman Mountains are marked by a discontinuous belt of jebels peripheral to their western margin. Detailed field mapping of the northern Hafit structure in the Northern Oman Mountains indicates that the structures consist of two en echelon anticlines, the main Hafit Anticline to the south, and the Al-Ain Anticline to the north. Both anticlines are related to the same deformation event. Structural analysis, using geometric balancing techniques suggests that the Hafit structure developed over a west-vergent basal thrust. The depth to detachment of the thrust indicates that the basal detachment occurs at different stratigraphic positions and progressively increases northward, while the magnitude of deformation increases southward. The thrust wedges back to the east and propagated upward through the structure during a major Tertiary deformational event resulting in an east-vergent, fault-propagation fold. The recent interpretation that the Hafit structure grew as a detachment fold above a basal decollement and synchronously with sedimentation, is at variance with structural evidence from the Hafit area. It is believed that the Hafit structure formed after the Miocene time.
INTRODUCTION AND REGIONAL GEOLOGICAL SETTING
The Oman Mountains are located in the southeast part of the Arabian Peninsula and form an arcuate belt parallel to the Gulf of Oman (Figure 1). The mountains were deformed initially during Late Cretaceous time when the Semail Ophiolite and associated sedimentary and volcanic rocks of the Hawasina and Haybi complexes were emplaced onto the eastern margin of the Arabian Platform (Glennie et al., 1973, 1974; Graham, 1980; Coleman, 1981; Lippard et al., 1986). A foreland basin formed at the leading edge of the obducted allochthon and this basin was filled by the late Maastrichtian-Tertiary sediments (Patton and O’Connor, 1988; Boote et al., 1990; Warburton et al., 1990).
In the Northern Oman Mountains, a second compressional post-obduction event occurred in the Late Eocene-Miocene. This second compressional event is marked by folding and reactivation of deep-seated faults in the Upper Cretaceous-Tertiary sedimentary rocks in the foreland basin (Boote et al., 1990; Dunne et al., 1990; Searle et al., 1990). The second event is correlated by some authors to the Zagros Orogeny in Iran (Ricateau and Riche, 1980; Searle et al., 1983; Searle, 1985, 1988; Searle et al., 1990).
A discontinuous belt of Jebels outcrops around the periphery of the Northern Oman Mountains, form the first neo-autochthonous deposits on top of the obducted Semail Ophiolite complex (Nolan et al., 1990; Skelton et al., 1990). In recent years attention has focused on the structure and internal geometry of the frontal structures of mountain belts (Vann et al., 1986; Mount et al., 1998). This is because understanding the structural configuration of the mountain fronts is of great interest in locating hydrocarbon accumulations, which are often found in structures developed in the underlying autochthonous sedimentary rocks.
The Hafit frontal structure is located about 20 kilometers (km) west of the Oman Mountain front and south of Al-Ain (Figure 1). It is one of several prominent Tertiary structures in the Northern Oman Mountain front which trend parallel to the mountain range. It forms an elongated mountain, extending for 29 km in length and 4 to 5 km in width, and is about 930 meters (m) above the adjacent plains. It is a doubly-plunging anticlinal structure, plunging north in the United Arab Emirates (UAE) and south in Oman. The present study is an attempt to interpret the origin of the northern Hafit structure, located in the UAE, through detailed field mapping of the area at a scale of 1: 20,000. In particular the mapping focuses on the highly deformed eastern limb, and the construction of balanced cross-sections across the Hafit structure to reveal the shape, size and timing of deformation.
STRATIGRAPHY OF THE HAFIT ANTICLINE
The first stratigraphic division of the rocks of the northern Hafit structure was carried out by Hunting (1979), later by Cherif and El Deeb (1984), Hamdan and Bahr (1992), and recently by Whittle and Alsharhan (1994). The sub-surface stratigraphic succession of the area was also included in some studies (Patton and O’Connor, 1988; Boote et al., 1990, and Woodward, 1994). The stratigraphic sequence of the Hafit structure is composed primarily of carbonates and marl, ranging in age from early Eocene to Miocene. The generalized Cretaceous and Tertiary stratigraphy of the area is shown in Figure 2.
STRUCTURAL STYLE OF HAFIT ANTICLINE
The leading edge of the foreland fold and thrust belt south of Al-Ain is marked by the Hafit structure. Since Jebel Hafit is located between the UAE and Oman, field work was carried out on the northern part only. The northern Hafit structure is about 17 km long and 4 to 5 km in width. It consists mainly of two en-echelon anticlines, the Hafit Anticline to the south, and Al-Ain Anticline to the north (midway between Jebel Al-Ain West and Jebel Al-Ain East in Figure 3). These two anticlines are linked in a characteristic Z-shape by a plunging syncline, comprising the Rwaidhat Syncline (Warrak, 1996), a small plunging anticline and another syncline. Faults are subordinate in the area except for the Tarabat Thrust Fault, in the eastern limb where the fault extends parallel to the Hafit fold axis and three northwest to west-northwest oriented faults cutting the eastern flank of the Hafit Anticline (Figure 3). The following is a brief description of the northern Hafit structure from south to north.
The Hafit Anticline occupies the southern part of this structure, trending generally north-northwest. It is an asymmetric, east-verging anticline with a north-plunging fold axis (Figures 3 and 4). Detailed field mapping, especially along the eastern limb, reveals that the anticline shows two styles of deformation: a mildly deformed western limb, and a strongly deformed eastern limb. This is evident from the contrast between the weak deformation of the gently to moderately dipping rocks of the western limb and the intense and complex deformation of the eastern limb, where the beds are steeply-inclined (Figure 5), sometimes vertical to overturned. The hinge zone of the Hafit Anticline is sub-horizontal, giving rise to an overall box fold. On the eastern limb of the Hafit Anticline, a north northwest-trending, west-dipping thrust is seen, the Tarabat Thrust Fault (Figure 3). This fault, mapped here for the first time, can be traced for about 8 to 9 km between the middle and upper Eocene rocks.
A number of spectacular meso-structures can be seen in the eastern limb within the Eocene rocks. In the lower Eocene rocks near the core of the anticline, a horst structure is observed (Figure 6). Farther north, near the hinge zone, the rocks show small-scale recumbent and open folding of the upper Eocene rocks (Figures 7 and 8). Also, the upper Eocene rocks (D4) are cut by a small-scale thrust fault (Figure 9).
Figure 4 (cross-section C-C’) reveals the following:
(1) an asymmetrical structure characterized by a gentle west flank, a relatively flat crest, and a quite steep and overturned east flank;
(2) a west-vergent basal detachment located in the lower part of the Lower Fiqa Shale that wedges back to the east and ramps up along most of the structure through the entire succession is responsible for the Hafit structure; and
(3) a notable characteristic of the ramp fault in this cross-section is that it does reach the surface.
Rwaidhat Syncline and Subsidiary Folds
Detailed mapping of the central part of the area shows that upper Eocene rocks (D4) have been folded to form an asymmetric, plunging syncline, the Rwaidhat Syncline (Warrak, 1996), and an asymmetric plunging anticline and syncline pair (Figures 3 and 10). The forelimbs have been affected by several northwest-oriented faults where the lower surface of the Upper Eocene rocks are heavily marked by slickensides. These folds are parallel to each other and trending in a northwest direction. The Rwaidhat Syncline has a moderate to steeply-dipping southwest limb and a steeply-dipping, sometimes vertical to overturned northeast limb with a southeast-plunging fold axis. The subsidiary folds, mapped here for the first time, consist of a plunging anticline and a syncline. The anticline has a steeply-dipping sometimes vertical to overturned southern limb and a gently to moderately-dipping northern limb, with a northwest-plunging fold axis. The anticlinal fold axis is displaced by a fault (Figure 3). The syncline has a gently-dipping northern limb and a gentle to moderately dipping southern limb.
Figure 4 (cross-section B-B’) reveals the following:
(1) the Hafit Anticline is an asymmetrical structure, which is characterized by a relatively gentle west flank, a flat crest, and a moderately steep east flank;
(2) the origin of the structure is a step in a deep detachment thrust in the Thamama Group that dies out in the Eocene Dammam Formation (D1D2-D3D4 boundary); and
(3) the thrust fault does not reach the surface (blind thrust).
The Al-Ain Anticline occupies the extreme northern part of the Hafit structure, south of Al-Ain. The anticline extends for about 5 km in a north-south to north northwest direction with the axial trace located midway between Jebel Al-Ain East and Jebel Al-Ain West. The hinge zone has nearly sub-horizontal dips, with a gently-dipping western limb and steeply-dipping eastern limb, giving rise to a box fold (Figure 4 (cross-section A-A’)). The Al-Ain Anticline is cored by upper Eocene rocks (D4) overlain by Oligocene rocks that are steeply-dipping to overturned in the eastern limb of the structure and gently-dipping in the western limb. Warrak (1996) used the name Al-Ain Anticline, but his description differs from the one given here, with the hinge surface trace located just to the west of Jebel Al-Ain East. It should be noted that the extreme northern part of Al-Ain Anticline, the area between Jebel Al-Ain East and Jebel Al-Ain West, has been leveled and is now occupied by Batan Village. This part of the map has been taken from Hunting (1979).
Figure 4, balanced cross-section A-A’ shows the following:
(1) Al-Ain Anticline is an asymmetrical structure with a gentle west flank, a flat crest and a steep east flank;
(2) the fold was formed above a ramp fault located in the Thamama Group that dies out in the Natih-Nahr Umr formations; and
(3) the ramp fault does not reach the surface (blind thrust).
ORIGIN OF HAFIT ANTICLINE
Boote et al. (1990) proposed that the folds along the Oman margin, including the Hafit structure, resulted from southward-directed transpressional deformation along deep basement fractures. The problem with the model of Boote et al. (1990) is that there is no field evidence for strike-slip faults in the Hafit structure to support the strike-slip origin. Warrak (1996) concluded that the Hafit structure grew as a detachment fold above a decollement surface, and the reversed vergence, back-thrust and the folds superposed on the limbs of the Hafit folds are due to simple shear modification of the original structure. The problem with Warrak’s model is that it proposes a westward thrust propagation from a basal detachment in the Lower Fiqa Shale and the later development of westward dipping back-thrust. However, he presents no field or seismic evidence to confirm the presence of these thrusts.
The structural model in this study interprets the formation of the fold to occur above an upward propagating thrust. Fault-propagation folding described by Suppe and Medwedeff (1990) and Mitra (1990), is a widely accepted kinematic explanation for folds that form during lateral and up-section propagation of thrust faults (Figure 11), where the fold develops over a propagation fault tip. The fault-propagation fold interpretation (Figure 4) is based on the asymmetry of the structure with steeply-dipping east limb and moderately-dipping west limb. This interpretation is also supported by the presence of the Tarabat Thrust Fault in the eastern limb and confirmed in the sub-surface underneath the steep eastern limb of the structure (Hunting, 1979). The geometry of this type of fold requires the presence of a detachment horizon between the folded and nonfolded layers.
Detachment positions must be zones of relative weakness (Gretener, 1972). Balanced cross-sections indicate the basal detachment occurs at different stratigraphic positions in the Hafit structure. The depth to the detachment level has been calculated by applying the method of Woodward et al. (1985) to the balanced cross-sections of Figure 4 assuming plane strain deformation. The results indicate that the depth to detachment in Figure 4 is about 5, 4 and 3 km below sea-level in cross-sections A-A’, B-B’ and C-C’, respectively. These estimated depths indicate that the Lower Cretaceous Thamama Group is involved in the deformation. Also, these results indicate a progressive northward increase in the detachment depth. This conclusion is in agreement with the detachment level of the Margham structure, which is located further north of the Hafit structure, where the detachment occurs at a deeper structural level in the Triassic formations (Mount et al., 1995). The amount of shortening has been calculated as 15%, 26% and 35% in cross-sections A-A’, B-B’ and C-C’, respectively. The results indicate that the rate of deformation increases southward.
The origin of the Hafit structure can be interpreted in two structural ways (Figures 12a and 12b), both involving a basal detachment at depth. In the first, a simple east-vergent thrust interpretation matches the observed structural relationships, but is problematic in that it requires thrusts to initiate in the foredeep and verge into the foreland fold-thrust belt (Figure 12a). In the second, west-verging wedge interpretation is consistent with the structural geometry of the surface folds and cross-section balancing. Most orogenic belts have some faults which move in a direction opposed to the regional movement direction, these are back-thrusts (Butler, 1987). Back-thrusting is a characteristic feature in many thrust belts; e.g. the Mackenzie Mountains, Canada (Vann et al., 1986). A similar model of a west-verging wedge, where a thrust fault propagates from a basal detachment with opposite transport direction from west to east, has been described by Mount et al. (1995) from the frontal Margham structure of the Northern Oman Mountains, and Hall and Cook (1998) from the foldbelt of northwestern Canada.
The recent discoveries of several gas and condensate fields such as Margham and Saja’a which are located at the leading edge of a thrust front, have increased the area’s prospectivity. The study of the Hafit structure and its similarity in position and style of deformation with the Margham structure contributes to our understanding of the structural evolution of this hydrocarbon habitat.
Despite the similarity in the structural style of Jebel Hafit and other Tertiary folds along the Northern Oman Mountains, one difference exists between these Tertiary folds. The Hafit structure has a west-dipping thrust fault exposed in the east limb resulting in an asymmetric anticline with the steep limb being on the east side. Most of the other Tertiary folds have the steep limb on the west side. The deformation during the second orogenic event started from the east and migrated westwards. This observed structural relationship honors the interpretation that the fault initiated in the foldbelt (east side) and verged out towards the foredeep (west side). The fault wedged back to the east and propagated upward resulting in an east-vergent fault-propagation fold (Figure 12b).
The Jebel Akhdar and Saih Hatat anticlines in the central and southern Oman Mountains, southeast of the Hafit structure, extend along the same northwest-southeast trend, and are interpreted by Mount et al. (1998) as basement-involved compressional structures created in an early-to-middle Tertiary deformation event.
TIMING OF DEFORMATION
Since the Permian rifting stage and the creation of the Neo-Tethys Ocean, the Oman Mountains were deformed during two major orogenic events in the Late Cretaceous and Tertiary time. The first event was dominated by the obduction of the Semail Ophiolite onto the Arabian continental margin (Glennie et al., 1973, 1974; Lippard et al., 1986). In the Mid-Tertiary time the Oman Mountains were further affected by the second compressional event which was mainly responsible for the folding of the Maastrichtian and Tertiary neo-autochthonous units (Ricateau and Riche, 1980; Searle et al., 1983). These compressive movements are considered to have culminated in Early Miocene (Ricateau and Riche, 1980) or in Late Oligocene to Miocene time (Searle et al., 1985).
Lippard et al. (1986) considered the Maastrichtian-lower Tertiary sediments were deformed during mid-Tertiary, post-middle Eocene time. In the Northern Oman Mountains, a discontinuous belt of jebels bordering the western margin marks the Tertiary structures. These Tertiary structures, including the Hafit structure, formed as a result of the Tertiary deformational event and are correlated with the deformation of the Zagros Foldbelt in southwest Iran (Ricateau and Riche, 1980; Searle et al., 1983; Searle, 1985). Mann et al. (1990) concluded that the Tertiary compressional deformation identified on the Musandam Peninsula dies out southeastwards away from the north Oman Mountains and the effects of Zagros compression diminishes southwards. Boote et al. (1990) proposed that the age of the second deformational event in the Suneinah area, including the Hafit structure, is post-early Miocene. Warrak (1996) concluded that the Hafit structure was initiated just before the middle Eocene and grew synchronously with sedimentation till the end of Miocene time.
Field mapping of the Hafit structure indicates that the fold developed in Tertiary neo-autochthonous sedimentary rocks. The evidence that folding is thrust-related, and the relatively steep dips of the Miocene rocks exposed in the eastern limb, suggests that these rocks were deformed in post-Miocene time.
Noweir and Eloutefi (1997) based on a structural and stratigraphical study of Jebel Malaqet-Jebel Mundassa, located about 20 km east of Jebel Hafit near the Omani border, concluded that this Tertiary structure must have formed in the post-middle early Eocene (Ypresian) time. These two Jebels developed sequentially with the Hafit structure from east to west. Noweir et al. (1998) conclude that the Faiyah Range, 100 km north of Al-Ain, is deformed in post-Middle Eocene time. Due to the similiar positions of Jebel Hafit and Faiyah Range, relative to the Oman Mountains and foredeep and their timing of deformation, these two structures share a similar tectonic evolution. It is believed that the Hafit structure formed after the Miocene as a result of the second Late Tertiary deformational event.
SUMMARY AND CONCLUSIONS
Detailed field mapping and structural analysis of the Northern Hafit structure suggest that:
(1) The Hafit structure is composed of two en echelon anticlines, the main Hafit Anticline to the south and Al-Ain Anticline to the north. Both anticlines are linked in a zig-zag pattern by a plunging syncline and subsidiary folds.
(2) The Hafit structure has mainly formed by the processes of fault-propagation folding. The fold developed over a west-vergent thrust initiated in the foldbelt and wedges back to the east and propagated upward causing an east-vergent fault-propagation folding.
(3) These fault-propagation folds are characterized by strong asymmetry with a nearly flat crest, gently dipping western limb and steeply, sometimes vertical to overturned eastern (forward) limb. The steep forelimb of the asymmetrical fold is cut by thrust fault. Both anticlines grew at the same time and are related to the same deformational event.
(4) The depth to detachment of the thrust (as calculated by the method of Woodward et al. (1985) indicates that the Lower Cretaceous Thamama Group is involved in the deformation and a progressive northward increase in the depth to detachment. The magnitude of deformation (shortening) along the Hafit structure is variable, with stronger activity in the south (Hafit Anticline) and less deformation in the north (Al-Ain Anticline).
(5) The Hafit structure was initiated just after the related structures (Jebel Malaqet-Jebel Mundassa) to the east were formed in the middle Early Eocene (Ypresian) until the Miocene.
I would like to acknowledge with deep appreciation Professor Abdulrahman S. Alsharhan (United Arab Emirates University) for critical reading of the manuscript and the UAE University for providing the research facilities during this work. The comments of two anonymous reviewers and GeoArabia’s editors have greatly improved the manuscript.
ABOUT THE AUTHOR
M. Atef Noweir has recently returned to Tanta University, Egypt, where he is Lecturer of Structural Geology. Prior to that, Atef was with the Department of Geology at United Arab Emirates University on secondment basis between 1993 and 1999. In 1977 and 1983, he received BSc and MSc degrees in Geology from Tanta University, respectively. He obtained a PhD in Structural Geology from the University of Missouri-Rolla in 1990. Society affiliations include the GSA, AAPG, IASTG, MAS and GSE. Atef is interested in field mapping, structural analysis and balanced cross-sections of foreland fold and thrust belts.