Tectonics of the Musandam Peninsula and northern Oman Mountains: From ophiolite obduction to continental collision

Abstract

The tectonics of the Musandam Peninsula in northern Oman shows a transition between the Late Cretaceous ophiolite emplacement related tectonics recorded along the Oman Mountains and Dibba Zone to the SE and the Late Cenozoic continent-continent collision tectonics along the Zagros Mountains in Iran to the northwest. Three stages in the continental collision process have been recognized. Stage one involves the emplacement of the Semail Ophiolite from NE to SW onto the Mid-Permian–Mesozoic passive continental margin of Arabia. The Semail Ophiolite shows a lower ocean ridge axis suite of gabbros, tonalites, trondhjemites and lavas (Geotimes V1 unit) dated by U-Pb zircon between 96.4–95.4 Ma overlain by a post-ridge suite including island-arc related volcanics including boninites formed between 95.4–94.7 Ma (Lasail, V2 unit). The ophiolite obduction process began at 96 Ma with subduction of Triassic–Jurassic oceanic crust to depths of > 40 km to form the amphibolite/granulite facies metamorphic sole along an ENE-dipping subduction zone. U-Pb ages of partial melts in the sole amphibolites (95.6– 94.5 Ma) overlap precisely in age with the ophiolite crustal sequence, implying that subduction was occurring at the same time as the ophiolite was forming. The ophiolite, together with the underlying Haybi and Hawasina thrust sheets, were thrust southwest on top of the Permian–Mesozoic shelf carbonate sequence during the Late Cenomanian–Campanian. Subduction ended as unsubductable cherts and limestones (Oman Exotics) jammed at depths of 25–30 km. The Bani Hamid quartzites and calc-silicates associated with amphibolites derived from alkali basalt show high-temperature granulite facies mineral assemblages and represent lower crust material exhumed by late-stage out-of-sequence thrusting. Ophiolite obduction ended at ca. 70 Ma (Maastrichtian) with deposition of shallow-marine limestones transgressing all underlying thrust sheets. Stable shallow-marine conditions followed for at least 30 million years (from 65–35 Ma) along the WSW and ENE flanks of the mountain belt. Stage two occurred during the Late Oligocene–Early Miocene when a second phase of compression occurred in Musandam as the Arabian Plate began to collide with the Iran-western Makran continental margin. The Middle Permian to Cenomanian shelf carbonates, up to 4 km thick, together with pre-Permian basement rocks were thrust westwards along the Hagab Thrust for a minimum of 15 km. Early Miocene out-of-sequence thrusts cut through the shelf carbonates and overlying Pabdeh foreland basin in the subsurface offshore Ras al Khaimah and Musandam. This phase of crustal compression followed deposition of the Eocene Dammam and Oligocene Asmari formations in the United Arab Emirates (UAE), but ended by the mid-Miocene as thrust tip lines are all truncated along a regional unconformity at the base of the Upper Miocene Mishan Formation. The Oligocene–Early Miocene culmination of Musandam and late Cenozoic folding along the UAE foreland marks the initiation of the collision of Arabia with Central Iran in the Strait of Hormuz region. Stage three involved collision of Arabia and the Central Iran Plate during the Pliocene, with ca. 50 km of NE-SW shortening across the Zagros Fold Belt. Related deformation in the Musandam Peninsula is largely limited to north and eastward tilting of the peninsula to create a deeply indented coastline of drowned valleys (rias).

Tectono‐Stratigraphic Evolution of the Foreland Fold‐and‐Thrust Belt of the United Arab Emirates

“…Nonetheless, the three main shelf carbonate culminations in the Oman Mountains (Musandam, Jabal Al Akhdar, and Saih Hatat) exhibit different structural geometries, degrees of thrusting and internal folding, and timing of culmination and uplift (Searle, 2019). The timings and strikes of the thrust and folds observed in the study area correlated well with those documented in the Musandam culmination (Ali et al., 2018; Ricateau & Riché, 1980; Searle, 1988a; Searle et al., 2014).…”

Sequence Stratigraphy, Palaeogeography and Petroleum Plays of the Cenomanian – Turonian Succession of the Arabian Plate: An Updated Synthesis

“…This unconformity marks a transition from a passive to an active margin along the eastern edge of the Arabian Plate. The initiation of a subduction zone within the central NeoTethys and progressive closure of the southern branch of the ocean resulted in compression within the southern and eastern Arabian Plate (Robertson and Searle, 1990) as a precursor to Late Cretaceous ophiolite obduction (Searle et al, 2014). As a result of this uplift, the preservation of Turonian stratigraphy, and in some areas Cenomanian stratigraphy, is variable across the southern part of the plate (Fig.…”

“…However, the mid-Cenomanian (K130 SB) and Cenomanian-Turonian boundary (probable Tu1 SB) unconformities pre-date the transition to an active margin which occurs within the mid-Turonian (K150 SB) (Sharland et al, 2001). As such, it seems likely that only the mid-Turonian unconformity is associated with the compressional phase that occurred as a precursor to obduction in the Late Cretaceous (Searle et al, 2014).…”

Crustal structure of the UAE-Oman mountain range and Arabian rifted passive margin: new constraints from active and passive seismic methods

“…Taking these factors into account, it is possible that folding and thrusting during obduction of the Semail ophiolite has thickened the crust beneath the mountain belt by about 16 km (current maximum thickness 51 km minus 35 km, which is the thickness of stretched crust plus the load of ophiolite), thereby creating a substantial crustal root beneath the mountains. Evidence for pervasive folding and thrusting of the lower crust beneath the obducted Semail ophiolite is derived from the Bani Hamid thrust sheet in UAE (Figure 9), where a ca 1 km thick unit of tight to isoclinally folded granulite facies meta-carbonates and meta-quartzites has been thrust into the ophiolite mantle sequence by late-stage out-of-sequence thrusting (Searle et al, 2014). These granulite facies rocks have PT conditions of 850 ± 60 °C and 6.3 ± 0.5 kbar and 206 Pb/ 238 U zircon dates of…”

“…Metamorphic soles accreted beneath the mantle sequence peridotites, exposed in tectonic windows beneath the ophiolite, have provided important clues for subduction initiation (Rioux et al, 2016). Folded and thrusted granulite facies meta-sedimentary rocks exposed in the Bani Hamid thrust sheet have been key for making inferences on how the lower continental crust deforms, and was buried to depths of ca 30-40 km and then exhumed (Searle et al, 2014(Searle et al, , 2015. Unsurprisingly, the Semail ophiolite (Glennie et al, 1973;Lippard et al, 1986), largely considered as the finest example of its kind, has been the focus of numerous petrological, geochemical, thermobarometric, and structural studies (e.g., Ambrose & Searle, 2019;Boudier et al, 1995;McLeod et al, 2013;Rioux et al, 2013Rioux et al, , 2016Searle & Cox., 1999;Searle et al, 2015Searle et al, , 2014Tilton et al, 1981).…”