The tectonic evolution of pre-Permian rocks, Central and Southeastern Oman Mountains

Abstract

The structural history of pre-Permian basement rocks exposed in the domal uplifts of Jebel Akhdar, Jebel Nakhl and Saih Hatat, central and southeastern Oman mountains, is dominated by Late Palaeozoic and Late Cretaceous deformation and metamorphism. In Jebel Akhdar pre-Permian rocks have been deformed by large NE-SW trending thrusts and folds and intense cleavage during Late Palaeozoic compression. Later reactivation of these structures accompanied by a major flattening deformation caused by loading may be related to thrusting and folding in the overlying Permian to Cenomanian platform carbonates. These structures are ascribed partly to Late Cretaceous deformation. The complex internal structure of Saih Hatat is attributed to overprinting of possible Proterozoroic and Late Palaeozoic tectonic events by Late Cretaceous thrusting, folding and associated blueschist metamorphism. The identification of a major basement-cover duplex involving pre-Permian rocks and platform carbonates in Saih Hatat suggests that the pre-Permian basement and platform carbonates did not remain as autochthonous units during Late Cretaceous deformation. Instead, both basement and cover have been thrust southwards during emplacement of the Tethyan thrust sheets onto the Arabian continental margin. Sedimentological evidence from the Tertiary sequence reinforces the development of the Saih Hatat dome as a Late Cretaceous culmination above a deep seated thrust which may have been rejuvenated during the Tertiary. In Jebels Akhdar and Nakhl there is no evidence of a basement-cover duplex. Nevertheless, reactivation of pre-Permian structures and imbrication of cover rocks (Mesozoic platform carbonates) in Jebel Akhdar has important implications to the timing of its structural growth. An alternative model of under-thrusting of the N Arabian margin beneath a subduction trench is also proposed to explain the formation and subsequent elevation of the Oman blueschists.

Tethyan oceans

“…The geometry of this Permian unconformity, its age compared to the rifting period, the geochemistry of associated basalts, the sedimentary record and the geodynamic context, imply a synrift thermal uplift and not a contemporaneous orogenic event, as clearly demonstrated in all these areas by thorough field work done in the last ten years (e.g. Mann & Hanna 1990;Vannay 1993;Garzanti et al 1994Garzanti et al , 1996Pillevuit et al 1997). Similarly, Ricou (1974) and Braud (1987) never spoke of Variscan deformation or metamorphism concerning the Sanandaj-Sirjan Zone (the Iranian part of the Podataksasi Zone); they regarded the metamorphic rocks of this region as a retrogressed epi-Baikalian basement [this is also indicated on the 1:1 000 000 map of Iran (Huber & Eftekhar-Nezhad 1978)].…”

A new model for the late Palaeozoic—Mesozoic tectonic evolution of Iran and implications for Oman

“…Recently, both Michard (1982Michard ( , 1983 and Mann & Hanna (1990) studied the late Palaeozoic deformation, documenting the presence of a NE-SW trending foldbelt. In the Jebel Akhdar, Jebel Nakhl, and Saih Hatat, an intense slaty cleavage dips generally to the SW at angles between 20 ~ and 50 ~ .…”

“…In the Jebel Akhdar, Jebel Nakhl, and Saih Hatat, an intense slaty cleavage dips generally to the SW at angles between 20 ~ and 50 ~ . Late Cretaceous deformation in the NW part of the Saih Hatat window has overprinted and largely obscured the Late Palaeozoic structures (Mann & Hanna 1990). Stretched grain and pressure shadow orientations in the slaty cleavage plane indicates a predominant stretching direction trending S to SW. Michard (1982) indicated that fold vergence seems N and NW in many places where the cleavage/bedding relationship can be observed, and that this is commonly corroborated by parasitic fold overturning, but he also stressed that vergence of late Palaeozoic folds is inconsistent, as, locally, are the trends of fold axial traces.…”

“…is known of the late Palaeozoic deformation and subsequent rifting has been described in some detail elsewhere (Michard 1982(Michard , 1983Mann & Hanna 1990, Glennie et al 1974Lipard et al 1986;Robertson 1986;Bernoulli & Weissert 1987;Bechennec 1988), I deal with them only briefly, but sufficient for the reader to follow my arguments. Instead, I describe at some length the late Palaeozoic to end-Mesozoic tectonic evolution of the entire Middle Eastern Tethysides, of which Oman is a part, because without that knowledge it is not possible to understand the tectonic significance of the late Palaeozoic to early Mesozoic events in Oman.…”

Overview of tectonic settings related to the rifting and opening of Mesozoic ocean basins in the Eastern Tethys: Oman, Himalayas and Eastern Mediterranean regions

“…The in situ continental margin exhibits a variably complete Late Precambrian-Early Palaeozoic prerift sedimentary sequence that was unconformably overlain by a Late Permian transgressive succession, beginning with thin clastics and passing directly into much thicker shallow-marine carbonates (Glennie et al 1973;Rabu et al 1990). The unconformity was exploited as a Late Cretaceous emplacement related thrust in some areas, whereas in others a spectacular angular unconformity is exposed (Mann & Hanna 1990). The underlying Pre-Permian sequence is deformed, cleaved and folded and was subjected to regional low-grade metamorphism in contrast to the overlying Upper Permian and younger sequence.…”