Origin of gypsiferous intrusions in the Hawasina Window, Oman Mountains: Implications from structural and gravity investigations

ABSTRACT Gypsiferous intrusions are exposed in road-cuts in the south-central Hawasina Window in the central Oman Mountains. They are located at lower structural levels in the allochthonous Hawasina Complex and lie along faults that cut Upper Cretaceous structures related to the obduction of the Semail Ophiolite and Hawasina Complex deep-water sediments onto the Arabian Plate. The intrusions form gypsiferous pods that are up to 200 m long, in which the gypsum occurs as a dark, fine-grained matrix that contains a pervasive network of anastomosing veins of gypsum and anhydrite. The intrusions contain abundant sub-angular to sub-rounded litharenites, and less common fragments of chert and fine-grained limestone. Although these clast types are undated, their petrographic characteristics suggest they originate from the local Hawasina (Hamrat Duru Group) country rock. Very well-rounded pebbles and cobbles of feldspathic litharenites, some of which show a well-developed cleavage, and rarer cobbles of well-rounded vein quartz appear to have come from the basement. Gravity investigations indicate salt diapirs are not present beneath the Hawasina Window. Instead, the gypsiferous intrusions are interpreted as having been brought up from depth during compression to form disconnected pods along deep-rooted faults, bringing with them small amounts of the basement country rock. Strontium isotope analysis and regional considerations, in particular the distribution, age and nature of other evaporite units on the eastern Arabian Plate, suggest the gypsum may have its origins in the Neoproterozoic (Ediacaran) to lower Cambrian Ara Group evaporites, perhaps from a previously unknown extension of the Fahud Salt Basin beneath the Hawasina thrust sheets.

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Structural architecture of a thin-skinned imbricate fan: Evidence from Mesozoic deepwater sediments in the Jabal Wahrah area of the central Oman Mountains

GeoArabia ◽

10.2113/geoarabia160117 ◽

2011 ◽

Vol 16 (1) ◽

pp. 17-42

Author(s):

David J.W. Cooper

Keyword(s):

Leading Edge ◽

Trailing Edge ◽

Second Stage ◽

Oman Mountains ◽

Third Phase ◽

The Face ◽

Thrust Sheets ◽

Structural Architecture ◽

Semail Ophiolite ◽

Structural Levels

ABSTRACT In the central Oman Mountains, Mesozoic deepwater off-margin sediments of the Hawasina Complex were emplaced from the northeast onto the Oman continental margin during the Late Cretaceous obduction of the Semail Ophiolite. Detailed field mapping and structural investigation have shown that, in the area studied, margin-ward detachment of continental rise sediments (Hamrat Duru Group) created two major thrust units in the face of the advancing ophiolite and subduction zone wedge of sediments from more distal parts of the Hawasina Ocean. The upper unit is preserved in jabals Wahrah and Hurah as a wedge-shaped sheet, restoring to at least 60 km perpendicular to the line of emplacement but only about 500 m thick at its maximum. Its thinner leading edge (Jabal Wahrah) comprises a classic thin-skinned imbricate fan which is divided into five laterally continuous structural zones with finer-grained structures that are influenced by local stratigraphical variations in its Early Jurassic to Early Cretaceous section. The rear part of the thrust sheet (Jabal Hurah) behaved more rigidly, reflecting a thicker and more competent sedimentary sequence spanning the Early Triassic to ?mid Cretaceous. With the exception of a major duplex along its trailing edge, significant internal thrusts are rare and shortening is mostly accommodated by asymmetrical folding. This wedge was emplaced over the trailing edge of a lower thrust unit (Hammat Shulayshil), which formed through forward propagation of the Hawasina sole thrust and which was also deformed primarily through SW-directed folding with limited internal imbrication even after a translation during emplacement of at least 150 km. A second stage of thrusting after the main emplacement phase is linked to renewed locking of the lowest thrust planes in the imbricated Hawasina sediment wedge ahead of the Semail Ophiolite and late-stage motion transferring to higher structural levels closer to the ophiolite as movement of the latter gradually ceased. This resulted in out-of-sequence re-thrusting of higher thrust sheets over lower sheets along existing thrust planes. This was accompanied by the local rotation of parts of the Jabal Wahrah imbricate fan as an effect of the heterogeneous composition of the overlying thrust units, in particular the out-of-sequence emplacement of a mountain-sized thrust block of intra-oceanic reef limestone (Jabal Kawr) over the Hamrat Duru Group immediately to the east. A third phase of compression then folded and locally thrusted this re-thrust stack. The timing of this phase is not well constrained. It may represent the final effects of the Campanian emplacement; alternatively it may be tentatively linked to limited lateral motion (gravity sliding) of the thrust stack along the flanks of the Al Jabal al-Akhdar anticline during its main growth phase in the Oligocene.

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Structural geometry, style and timing of deformation in the Hawasina Window, Al Jabal al Akhdar and Saih Hatat culminations, Oman Mountains

GeoArabia ◽

10.2113/geoarabia120299 ◽

2007 ◽

Vol 12 (2) ◽

pp. 99-130 ◽

Cited By ~ 17

Author(s):

Michael P. Searle

Keyword(s):

High Pressure ◽

Late Cretaceous ◽

Late Stage ◽

Arabian Plate ◽

Structural Geometry ◽

Northern Margin ◽

Oman Mountains ◽

Al Jabal Al Akhdar ◽

Thrust Sheets ◽

Semail Ophiolite

ABSTRACT The Al Jabal al Akhdar and Saih Hatat culminations in the central Oman Mountains expose the complete mid-Permian to Late Cretaceous (Cenomanian) passive shelf and margin carbonate sequence beneath the allochtonous slope (Sumeini Group), basin (Hawasina complex), distal ocean-trench (Haybi complex) facies rocks, and the Semail ophiolite thrust sheets that were emplaced from NE to SW during the Late Cretaceous. Reconstruction of the pre-thrust sequences shows that time-equivalent rocks occur in successively stacked thrust sheets from shelf to slope to basin. The Al Jabal al Akhdar structure is a 60 km wavelength anticline plunging to the northwest beneath the Hawasina Window and with a fold axis that curves from WNW-ESE (Jabal Shams) to NNE-SSW (Jabal Nakhl). The structure shows little internal deformation except for minor intra-formational thrust duplexing within the Cretaceous shelf stratigraphy along the northern margin. The upper structural boundaries around the flanks of the shelf carbonate culminations have been re-activated as late stage normal faults. The Semail thrust formed a passive roof fault during late-stage culmination of al Al Jabal al Akhdar such that the ophiolite rests directly on Wasia Formation top-shelf with the entire Sumeini, Hawasina and Haybi thrust sheets displaced around the margins. NE-directed backthrusting and intense folding in the northern part of the Hawasina Window affects all allochtonous units and is related to a steep ramp in the Late Cretaceous shelf margin at depth. The Saih Hatat culmination is another 40 km half-wavelength anticline in the central Oman Mountains, but shows extreme deformation in the form of recumbent folds, sheath folds with NNE-trending axes and thrusting along the northern margin. High-pressure carpholite, blueschist and eclogite facies rocks are exposed at successively deeper structural levels, separated by high-strain normal sense shear zones. There is no evidence for a separate ‘North Muscat microplate’ or an intra-continental subduction zone, as previously proposed; all high-pressure units can be restored to show their pre-deformation palaeographic positions along the northern margin of the Arabian Plate. Both Al Jabal al Akhdar and Saih Hatat are Late Cretaceous culminations, folded after obduction of the Hawasina, Haybi and Semail ophiolite thrust sheets from northeast to southwest during the period Turonian to Campanian-Lower Maastrichtian. Maximum compressive stress along the central Oman Mountains was oriented NE-SW, parallel to the ophiolite emplacement direction, but a second compressive stress axis was oriented WNW-ESE, either concurrently or slightly later in time, resulting in a dome and basin structural geometry. The biaxial fracture pattern in the foreland, southwest of the Oman Mountains could be explained as a result of the WNW-directed emplacement of the Masirah ophiolite belt and Batain mélange during the Campanian-early Palaeocene. Both Al Jabal al Akhdar and Saih Hatat were positive topographic features at the end of the Cretaceous with Upper Maastrichtian and Palaeogene sediments onlapping both flanks. In Jabal Abiad, these Palaeogene sediments have been uplifted by at least 2 km since the Late Miocene-Early Oligocene associated with minor NNE-SSW compression. Tertiary shortening, folding and thrusting increases to the north in the Musandam peninsula where the first effects of the Arabian Plate-Eurasian Plate (Zagros belt) continent-continent collision are seen.

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Chapter 5 Tectonic evolution of the Oman Mountains

Geological Society London Memoirs ◽

10.1144/m54.5 ◽

2021 ◽

Vol 54 (1) ◽

pp. 67-103

Author(s):

Andreas Scharf ◽

Frank Mattern ◽

Mohammed Al-Wardi ◽

Gianluca Frijia ◽

Daniel Moraetis ◽

...

Keyword(s):

Subduction Zone ◽

Late Cretaceous ◽

Tectonic Evolution ◽

Oceanic Lithosphere ◽

Passive Margin ◽

Arabian Plate ◽

Continental Lithosphere ◽

Oman Mountains ◽

Semail Ophiolite ◽

Arabian Platform

AbstractThe tectonic evolution of the Oman Mountains as of the Neoproterozoic begins with a major extensional event, the Neoproterozoic Abu Mahara rifting. It was followed by the compressional Nabitah event, still during the Neoproterozoic, in Oman but possibly not in the study area. During the earliest Cambrian, the Jabal Akhdar area was affected by the Cadomian Orogeny, marked by NE--SW shortening. It is unclear, whether the Saih Hatat area was exposed to the Cadomian deformation, too. Still during the lower Cambrian, the Angudan Orogeny followed, characterized by NW--SE shortening. An episode of rifting affected the Saih Hatat area during the mid-Ordovician. During the mid-Carboniferous, both dome areas were deformed by tilting and large-scale open folding in the course of the ‘Hercynian’ event. As a consequence, a major unconformity formed. As another Late Paleozoic event, the Permian break-up of Pangaea and subsequent formation of the Hawasina ocean basin, are recorded in the Southeastern Oman Mountains. As a result, a passive margin formed which existed until the mid-Cretaceous, characterized by deposition of mostly shelfal carbonates. This interval of general tectonic quiescence was interrupted during the early Jurassic by uplift and tilting of the Arabian Platform. The platform collapsed during the late Cretaceous, related to the arrival of the obducted allochthonous nappes including the Semail Ophiolite, transforming the passive margin to an active margin.The Semail Ophiolite formed most likely above a subduction zone within the Neo-Tethys Ocean during the Cenomanian while parts of the Arabian Plate were subducted to the NE. Formation of oceanic lithosphere and SW-thrusting was broadly coeval, resulting in ophiolite obduction onto the Hawasina Basin. The Semail Ophiolite and the Hawasina rocks combined were thrust further onto the Arabian Plate. Their load created a foreland basin and forebulge within the Arabian Platform. Once the continental lithosphere of the Arabian Platform was forced into the subduction zone, a tear between the dense oceanic lithosphere and the buoyant continental lithosphere developed. This led to rapid uplift and exhumation of subducted continental lithosphere of the Saih Hatat area, while obduction was still going on, causing in multiple and intense folding/thrusting within the eastern Saih Hatat Dome. Exhumation of the Saih Hatat Dome was massive. The emplacement of the ophiolite was completed during the Campanian/Maastrichtian. For completeness, we also present alternative models for the developmental history of the Semail Ophiolite.Immediately after emplacement, the Arabian lithosphere underwent intense top-to-the-NE extensional shearing. Most of the Saih Hatat Dome was exhumed during the latest Cretaceous to Early Eocene, associated with major extensional shearing at its flanks. Further convergence during the late Eocene to Miocene resulted in exhumation of the Jabal Akhdar Dome and some gentle exhumation of the Saih Hatat Dome, shaping the present-day Southeastern Oman Mountains. In the coastal area, east and SE of the Saih Hatat Dome, some late Cretaceous to present-day uplift is evident by, e.g., uplifted marine terraces. The entire Oman Mountains are uplifting today, which is evident by the massive wadi incision into various rock units, including wadi deposits which may form overhangs.

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K—Ar ages of alkaline igneous rocks in the northern Oman mountains, NE Arabia, and their relations to rifting, passive margin development and destruction of the Oman Tethys

Geological Magazine ◽

10.1017/s0016756800026844 ◽

1982 ◽

Vol 119 (5) ◽

pp. 497-503 ◽

Cited By ~ 15

Author(s):

S. J. Lippard ◽

D. C. Rex

Keyword(s):

Upper Cretaceous ◽

Oceanic Lithosphere ◽

Passive Margin ◽

Igneous Rocks ◽

Oman Mountains ◽

Intrusive Rocks ◽

Igneous Activity ◽

Ophiolite Emplacement ◽

Alkaline Activity ◽

Semail Ophiolite

SummaryK–Ar ages of biotites from a variety of alkaline volcanics and minor intrusive rocks in the nothern Oman mountains allochthon give a range of ages from Triassic (230 Ma) to mid Cretaceous (92 Ma) and represent igneous activity on the Oman continental margin throughout the Mesozoic. This was a passive margin destroyed by the emplacement across it of a pile of nappes in the late Cretaceous, including a largely intact thrust sheet of Upper Cretaceous oceanic lithosphere (the Semail ophiolite). Biotite ankaramite dykes, cutting compositionally similar volcanics, in the thrust complex immediately beneath the ophiolite, give Triassic ages and are related to the rifting and break-up of the northeast Arabian margin at the beginning of formation of the Oman Tethys. Mid Cretaceous (Cenomanian–Turonian) ages are mostly recorded from the northern part of the mountains where there are alkaline tuffs in a sedimentary melange. They are approximately the same age as the ophiolite and may be related to tectonic instability of the Oman margin immediately prior to ophiolite emplacement. Alkaline sills, intrusive into a variety of rocks, including Triassic volcanics, give Jurassic and Cretaceous ages and are interpreted as periodic alkaline activity on the Oman margin throughout passive margin development.

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Structure and metamorphism of rocks beneath the Semail ophiolite of Oman and their significance in ophiolite obduction

Transactions of the Royal Society of Edinburgh Earth Sciences ◽

10.1017/s0263593300013614 ◽

1980 ◽

Vol 71 (4) ◽

pp. 247-262 ◽

Cited By ~ 162

Author(s):

M. P. Searle ◽

J. Malpas

Keyword(s):

Heat Source ◽

Upper Cretaceous ◽

Frictional Heating ◽

Metamorphic Rocks ◽

Polyphase Deformation ◽

Oman Mountains ◽

Wide Range ◽

Semail Ophiolite ◽

Low Pressures ◽

Residual Heat

ABSTRACTMetamorphic rocks showing an inverted metamorphic zonation from upper amphibolite fades immediately beneath the peridotite to greenschist fades at lower levels, crop out discontinuously along the base of the Semail ophiolite thrust sheet in the Oman Mountains of eastern Arabia. These metamorphic rocks show polyphase deformation, mylonitic fabrics, and have been disrupted, folded and imbricated, and in places form tectonic inclusions in a serpentinite melange. In the more intact sequences, garnet-clinopyroxene amphibolites, with rare hornblende-bearing marbles and banded quartzites, occur at the higher levels, whilst a wide range of metasedimentary and metabasaltic rocks occur in the greenschist fades. Low glaucophane content of amphiboles and low jadeite content of clinopyroxenes suggest relatively low pressures of crystallisation. The distribution coefficient KD for co-existing garnet and clinopyroxene suggests a temperature range of 670 to 750°C. Residual heat from the recently formed ophiolite probably provided the dominant heat source for metamorphism, although frictional heating could have supplemented this. The metamorphic rocks were produced during Cenomanian-Turonian (late Cretaceous) times.The sub-ophiolite metamorphic rocks, together with basal serpentinite, Triassic alkaline and tholeiitic basalts (Haybi volcanic group), mountain-sized ‘exotic’ limestones and an upper Cretaceous sedimentary melange, comprise a distinct thrust slice termed the Haybi complex, which always overlies marine sediments of the allochthonous Hawasina complex and underlies the Semail ophiolite. The rocks of the Haybi complex are bounded by major thrust planes, the Semail thrust above and the Haybi thrust beneath, which truncate all schistosities, fold axes, imbricate thrusts and associated features. A model for ophiolite obducton is presented based on a palinspastic reconstruction of the allochthonous rocks on the Oman continental margin.

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Volcanic clays in the Cretaceous of southern England and Northern Ireland

Clay Minerals ◽

10.1180/claymin.1982.017.1.10 ◽

1982 ◽

Vol 17 (1) ◽

pp. 105-156 ◽

Cited By ~ 72

Author(s):

C. V. Jeans ◽

R. J. Merriman ◽

J. G. Mitchell ◽

D. J. Bland

Keyword(s):

Northern Ireland ◽

Western Europe ◽

Upper Cretaceous ◽

Isotope Analysis ◽

Trace Element Geochemistry ◽

Element Geochemistry ◽

Southern England ◽

Fine Grained ◽

The North ◽

The North Sea

AbstractThe mineralogy, petrology and trace element geochemistry of volcanogenic glauconites and smectite-rich clays are described and related to clay assemblages in Lower and Upper Cretaceous sediments of southern England and Northern Ireland. Volcanogenic glauconite grains represent argillized lava particles of predominantly mafic composition and may have been derived from submarine basaltic magmatism; they occur in all the sediments examined (Aptian-Senonian), and are particularly abundant in the Cenomanian-Campanian Hibernian Greensand of Antrim. The smectite-rich clays in southern England have developed by the argillization of predominantly acid or alkaline ash during early diagenesis. Three types of volcanogenic deposit are recognized. Primary bentonites are thin ash-falls deposited in quiet, brackish and marine waters (Speeton Clay, Ryazanian; Weald Clay, Barremian). Secondary bentonites are local accumulations of ash transported into the Cretaceous seas by rivers draining ash-blanketed, local land areas (London Platform, Portsdown Axis). These deposits are well-developed in the Sandgate Beds, Folkestone Beds and their contiguous deposits, and the lower part of the Gault (Upper Aptian-Middle Albian). The ash originated from penecontemporaneous, subaerial vulcanism located in the southern part of the North Sea. The most conspicuous phase of activity occurred during late Aptian times and has been dated by 40Ar/39Ar isotope analysis at 112 m.y. Bentonitic clays and marls are widespread accumulations of argillized ash that occur as a fine-grained fringing facies to glauconitic quartz sand facies. They make up the upper part of the Atherfleld Clay (Aptian) and the upper part of the Gault (Upper Albian), and they are associated respectively with the Hythe Beds (Aptian) and the Upper Greensand (Upper Albian). They may also occur in the lower part of the Lower Chalk (Cenomanian). The distribution pattern of these smectite-rich clays in southern England is related to the changing palaeogeography of the area in Cretaceous times, and the general coincidence of extensive glaueonite deposits and smectite-rich clays in the Middle and Upper Cretaceous of western Europe and along the eastern seaboard of North America is briefly discussed.

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Compaction history of Upper Cretaceous shale and related tectonic framework, Arabian Plate, Eastern Oman Mountains

Arabian Journal of Geosciences ◽

10.1007/s12517-018-3781-2 ◽

2018 ◽

Vol 11 (16) ◽

Cited By ~ 1

Author(s):

F. Mattern ◽

A. Scharf ◽

M. Al-Sarmi ◽

B. Pracejus ◽

A.-S. Al-Hinaai ◽

...

Keyword(s):

Upper Cretaceous ◽

Arabian Plate ◽

Oman Mountains ◽

History Of ◽

Tectonic Framework

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Structural and Stratigraphical Setting of the Faiyah Range, Northwestern Oman Mountain Front, United Arab Emirates

GeoArabia ◽

10.2113/geoarabia0303387 ◽

1998 ◽

Vol 3 (3) ◽

pp. 387-398

Author(s):

M. Atef Noweir ◽

Abdulrahman S. Alsharhan ◽

Mohamed A. Boukhary

Keyword(s):

United Arab Emirates ◽

Marine Environment ◽

Upper Cretaceous ◽

Sedimentary Rocks ◽

Fold Axis ◽

Oman Mountains ◽

Mountain Front ◽

Dextral Strike Slip Fault ◽

Semail Ophiolite ◽

Dextral Strike Slip

ABSTRACT The Faiyah Range belongs to a group of regional ridges that formed by post-obduction folding of the Upper Cretaceous-Tertiary sedimentary rocks exposed along the western margin of the Northern Oman Mountains. The Faiyah Anticline, generally trends north-northeast to south-southwest with thrust faults striking parallel to the fold axis. The anticlinal hinge was later displaced by a dextral strike-slip fault, named here as the Faiyah Fault, into two segments. The northeastern segment includes Jebels Rumaylah, Faiyah and Mulayhah, and the southwestern segment includes Jebels Buhays and Aqabah. The anticline is interpreted to result from northeast-southwest compression during the Tertiary. In the Faiyah Range the neoautochthonous sedimentary rocks are the Maastrichtian Qahlah and Simsima formations, and the Eocene Dammam Formation. Stratigraphic evidence shows that the lower part of the Qahlah was deposited in a non-marine environment while the upper part was deposited during a marine transgression. The Simsima was deposited in a shallow-marine environment. These units unconformably overlap the allochthonous Semail Ophiolite. The microfaunal content of the so-called Muthaymimah Formation (?Tertiary), of earlier authors, indicates that it is of Maastrichtian age in the Faiyah Range. This sequence is also conformable to the Simsima and therefore it is considered to be the upper member of the Simsima in this area.

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Chapter 2 Tectonostratigraphy of the eastern part of the Oman Mountains

Geological Society London Memoirs ◽

10.1144/m54.2 ◽

2021 ◽

Vol 54 (1) ◽

pp. 11-47 ◽

Cited By ~ 1

Author(s):

Andreas Scharf ◽

Frank Mattern ◽

Mohammed Al-Wardi ◽

Gianluca Frijia ◽

Daniel Moraetis ◽

...

Keyword(s):

Deep Sea ◽

Upper Cretaceous ◽

Sedimentary Rocks ◽

Passive Margin ◽

Quaternary Deposits ◽

Oman Mountains ◽

Tethys Ocean ◽

Carbonate Formations ◽

Semail Ophiolite ◽

Stratigraphic Chart

AbstractThis chapter provides comprehensive descriptions of 52 numbered formations/rock units of the Southeastern Oman Mountains, based on available literature. The oldest eight siliciclastic and carbonate formations are positioned below the ‘Hercynian’ Unconformity. The overlying formation (9–16) mostly represent carbonates which accumulated in a passive margin platform setting during or after the opening of the Neo-Tethys Ocean. The passive margin slope and platform collapsed during the late Cretaceous because of the obduction of the Semail Ophiolite along with the deep marine Hawasina sedimentary rocks. The collapsing passive margin interval was recorded within the syn-obductional Aruma Group (17; Muti Formation). Above this formation are the allochthonous units (18–42) of the tectonically lower Hawasina deep-sea basin and the structurally overlying Semail Ophiolite. The former contains Permian to Upper Cretaceous formations, while the latter is Cenomanian in age. Above the allochthonous rocks, the Neo-autochthonous formations were deposited, starting with the post-obductional uppermost Cretaceous Aruma Group (43; Al-Khod Formation) until the Quaternary deposits (52). All these formations/rock units are depicted on an accompanying map and stratigraphic chart.

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Structural evolution of Jabal Qumayrah: A salt-intruded culmination in the northern Oman Mountains

GeoArabia ◽

10.2113/geoarabia1702121 ◽

2012 ◽

Vol 17 (2) ◽

pp. 121-150 ◽

Cited By ~ 4

Author(s):

David J.W. Cooper ◽

Michael P. Searle ◽

Mohammed Y. Ali

Keyword(s):

Continental Slope ◽

Structural Evolution ◽

Deep Ocean ◽

Passive Margin ◽

Salt Intrusion ◽

Thrust Sheet ◽

Carbonate Sedimentation ◽

Oman Mountains ◽

Thrust Sheets ◽

Semail Ophiolite

ABSTRACT The Jabal Qumayrah area of the northern Oman Mountains records the evolution and subsequent destruction of a Mesozoic passive continental margin in the Oman segment of the Neo-Tethys Ocean, followed by the re-establishment of a passive margin, punctuated by phases of Tertiary compression. Almost uniquely along the Oman Mountains, it also contains intrusions of salt. Detachment of oceanic sediments and volcanics during the early phases of NE-directed subduction beneath the nascent Semail Ophiolite created an in-sequence stack of imbricated thrust units comprising distal trench units (Haybi Complex), and deep-ocean and continental rise sediments derived from the Mesozoic Oman margin (the Hawasina Complex). These were emplaced onto the depressed margin beneath and ahead of the ophiolite during its obduction in the Cenomanian– Coniacian. The Mesozoic continental slope sediments of the Sumeini Group had already been largely over-ridden by the more distal thrust sheets when the Hawasina sole thrust propagated into those sediments. This detached a Sumeini Group thrust sheet, which was transported westward for at least 7 km, carrying with it the overlying Hawasina thrust stack. Structurally lower parts of the Hawasina thrust stack (Hamrat Duru Group) also extended ahead of the Sumeini Group thrust sheet, but they were not restacked with it, indicating motion continued along this part of the Hawasina sole thrust. Further footwall collapse detached at least one more imbricate within the Sumeini Group and the combined thrust stack was then folded along a N-S axis, possibly above a frontal ramp. This was associated with complex out-of-sequence forward and back-thrusting at the lower structural levels. A right-lateral scissors fault developed at right angles to the direction of nappe transport, associated with normal faulting down-to-south. Late-stage culmination within the nappe pile created an asymmetrical west-facing dome, around which the structurally overlying Hawasina thrust sheets are folded. Passive margin sedimentation was re-established in the Campanian–Maastrichtian following subsidence of the locally emergent nappe pile and was dominated by carbonate sedimentation with little clastic input from the ophiolite or Hawasina sediments. Stable sedimentation persisted until Oligocene–Miocene compression, synchronous with the Zagros compressional event in Iran, resulted in west-facing folding along the western side of the northern Oman Mountains and their subsequent uplift. The Jabal Qumayrah massif preserves a salt intrusion composed of gypsum and anhydrite, the top of which is now exposed in the centre of the culmination. The origin of the salt remains unclear and investigations continue. Possible sources include the extension of the major regional salt basins found in the foreland, in particular those at the Ediacaran/Cambrian boundary (Ara Group), beneath the Hawasina Nappes and Semail Ophiolite. Alternatively, evaporitic basins may have developed locally along the edge of the proto Neo-Tethyan margin during the earliest rifting phase, beneath what became the continental slope deposits, although there is little evidence for these elsewhere in the autochthonous shelf succession.

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