Alkaline peridotite, pyroxenite, and gabbroic intrusions in the Oman Mountains, Arabia

1984 ◽  
Vol 21 (4) ◽  
pp. 396-406 ◽  
Author(s):  
Michael P. Searle
Keyword(s):  
Continental Margin ◽  
Late Cretaceous ◽  
Volatile Components ◽  
Titanium Oxides ◽  
Structural Mapping ◽  
Oman Mountains ◽  
Intrusive Rocks ◽  
Alkaline Magmas ◽  
High Level ◽  
Semail Ophiolite

High-level intrusions of highly undersaturated alkalic ultrabasic and gabbroic rocks occur in four areas of the Oman Mountains. They all intrude either the Haybi volcanic – Oman Exotic limestone (Permo-Triassic) thrust slice immediately beneath the Semail Ophiolite (Cenomanian) or the uppermost thrust slice of the underlying Hawasina (Permian to Cenomanian) Tethyan sediments. Detailed structural mapping indicates that the sills were all emplaced prior to the Late Cretaceous thrusting of the Oman allochthon onto the Mesozoic continental margin of Arabia, and therefore in an oceanic setting. These differentiated sills consist of biotite wehrlites at the base and kaersutite-bearing jacupirangites above, with kaersutite gabbros at the top. Olivine occurs only at the base. Titanaugite, kaersutite, titanium phlogopite, apatite, and opaque iron–titanium oxides are common mineral phases.Fractional crystallization and gravity differentiation processes and a rapid increase in volatile components at decreasing pressures all played a part in the petrogenesis of these uncommon intrusive rocks. K–Ar ages on biotites span the mid-Jurassic to Cenomanian, and in the northern Oman Mountains kaersutite jacupirangites are incorporated into the Cenomanian–Turonian amphibolite facies metamorphic sheet beneath the Semail Ophiolite. Alkaline magmas were present at depth along the passive continental margin, right up until Cenomanian times when northeast subduction was initiated and compressional tectonics began. Alkaline volcanism of Cenomanian age in the Dibba Zone indicates that tensional rifting processes were operative along the continental margin at the same time as compressional thrusting was occurring outboard. The alkaline rocks are unrelated to the ophiolite but are artifacts of Mesozoic rifting events in Tethys now preserved in footwall thrust slices beneath the ophiolite.

scholarly journals Salt intrusions in Jabal Qumayrah, northern Oman Mountains: Implications from structural and gravity investigations

GeoArabia ◽  
2013 ◽  
Vol 18 (2) ◽  
pp. 141-176 ◽  
Author(s):  
David J.W. Cooper ◽  
Mohammed Y. Ali ◽  
Michael P. Searle ◽  
Ali I. Al-Lazki
Keyword(s):  
Late Cretaceous ◽  
Host Rock ◽  
Central Complex ◽  
Salt Diapir ◽  
Salt Intrusion ◽  
Salt Domes ◽  
The North ◽  
Oman Mountains ◽  
High Level ◽  
Semail Ophiolite

ABSTRACT The Jabal Qumayrah area, 50 km ESE of Al Ain and Buraimi, preserves a culmination of Jurassic and Cretaceous continental slope deposits (Sumeini Group) that was emplaced during the Late Cretaceous onto the Oman margin with other Neo-Tethyan units and the Semail Ophiolite. Almost uniquely in the Oman Mountains, Jabal Qumayrah also contains outcrops of gypsum and anhydrite that occur as a central complex from which laterally discontinuous linear and arcuate outcrops extend up to 4 km to the northwest and south. The gypsum and anhydrite bodies contain sedimentary clasts and rafts, which show close affinities with the local Sumeini Group host rock. There are no sedimentary features that indicate the evaporites were deposited in situ, either as part of, or unconformably overlying the Sumeini Group. Boundaries with the host rock are either high-angle faults or steep and intrusive, with significant dissolution of host rock limestones. Two gravity transects across the area indicate the areas of gypsum and anhydrite lie on a gravity low, compatible with an elongated, high-level body concentrated along the main N-S axis of the Jabal Qumayrah dome. Taken together, these features point towards an intrusive origin for the evaporite bodies in Jabal Qumayrah. While the sub-surface is poorly constrained, the central complex is interpreted as representing the deeply weathered top of a salt diapir, whose emplacement had a strong tectonic fault-driven component. The smaller, discontinuous exposures to the northwest and south are interpreted as pods of gypsum and anhydrite that were injected along faults. The absence of other evaporite minerals, in particular halite, is attributed to deep weathering and dissolution similar to that seen at the surface-piercing salt domes of the Ghaba Salt Basin in central Oman. In the absence of unequivocal dating evidence, the regional context suggests the intrusion may be derived from evaporites within the Ediacaran–Early Cambrian Ara Group. These form large deposits in the Fahud and Ghaba salt basins to the southwest of the Oman Mountains and the Hormuz Salt Basin to the north. The Jabal Qumayrah area may represent another, smaller basin or an extension to the Fahud Basin. The Jabal Qumayrah intrusion does not contain rafts of Ara Group limestones, which characterise the salt diapirs of the Ghaba Salt Basin, but this is not considered diagnostic. Other regional evaporite units of Permian to Jurassic ages do not extend into the area of the Oman Mountains and are thus unlikely potential sources. There is no evidence to suggest the Jabal Qumayrah culmination was thrust over Cenozoic evaporites and this potential source is also discounted. The timing of intrusion is constrained by the boundary faults, which cut across and thus post-date structures related to the Late Cretaceous emplacement of the Sumeini Group of Jabal Qumayrah. There is no evidence of any movement since the unroofing and exposure of the salt intrusion, which began in the Late Miocene.

Chapter 5 Tectonic evolution of the Oman Mountains

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.

Structure of the Hawasina Window culmination, central Oman Mountains

1986 ◽  
Vol 77 (2) ◽  
pp. 143-156 ◽  
Author(s):  
M. P. Searle ◽  
D. J. W. Cooper
Keyword(s):  
Structural Analysis ◽  
Continental Margin ◽  
Cross Sections ◽  
Structural Evolution ◽  
Complex Model ◽  
Shelf Edge ◽  
Oman Mountains ◽  
Stacking Order ◽  
Structural History ◽  
Semail Ophiolite

ABSTRACTDetailed mapping, stratigraphic logging and structural analysis of the Hawasina Window culmination in the central Oman Mountains of Arabia reveals an extremely complex thrust geometry and structural history. The initial thrust sequence involved a southwestward propagating stacking during telescoping of the Arabian continental margin slope (Sumeini complex), and time-equivalent, more distal Tethyan basin (Hawasina and Haybi complexes) facies rocks. The Semail thrust, carrying the 12 km-thick ophiolite sequence, progressively overlaps Haybi and Hawasina duplexes towards the SW. Late stage “leap-frog” thrusts have punched Sumeini duplexes higher up into the earlier thrust stack locally reversing the normal stacking order. SW-directed thrusts and SW-facing folds in the SW and NE-directed backthrusts and NE-facing backfolds in the NE have created a fan structure cored by the Jebel Rais “pop-up” composed of Sumeini slope facies rocks.The palaeogeographic presence of a large NE-facing promontory in the Cretaceous shelf edge is inferred beneath the Window. The frontal ramp of this promontory was sufficiently large to inhibit the overthrusting of large volumes of Hawasina and Haybi complex rocks. The whole central part of the Window shows NE-facing and verging backfolds and backthrusts affecting all duplexes from the lowest Sumeini up to the Semail ophiolite. The promontory is bounded by major lateral ramps to the NW (Wadi Shafan area) and SE (Jebel Milh area). A map and four balanced cross-sections are presented here to promote a complex model for the structural evolution of the Hawasina Window.

scholarly journals Structural Style and Stratigraphy of the Huwayyah Anticline: an Example of an Al-Ain Tertiary Fold, Northern Oman Mountains

GeoArabia ◽  
2000 ◽  
Vol 5 (3) ◽  
pp. 387-402 ◽  
Author(s):  
M. Atef Noweir ◽  
Abdulrahman S. Alsharhan
Keyword(s):  
Late Cretaceous ◽  
Middle Eocene ◽  
Fold Axis ◽  
The North ◽  
Oman Mountains ◽  
Structural Style ◽  
Dammam Formation ◽  
Al Ain ◽  
Mountain Front ◽  
Semail Ophiolite

ABSTRACT Detailed field mapping and structural studies in the Jebel Auha-Jebel Huwayyah area northeast of Al-Ain indicate that folding of neoautochthonous sedimentary rocks produced the north-northwest-trending Huwayyah Anticline. The anticline at the surface is composed of the Maastrichtian Qahlah and Simsima formations unconformably overlain by shallow-marine carbonate rocks that are correlated on faunal grounds with the Middle Eocene Dammam Formation. The investigation of the Huwayyah Anticline has identified three microfacies of bioclastic packstone, nummulitic packstone, and nummulitic packstone-grainstone in the local Dammam Formation. Diagenesis in the form of silicification, cementation, recrystallization, dissolution, compaction and neomorphism is widespread. The Huwayyah Anticline is a fault-propagation fold above a thrust ramp. The ramp developed from a pre-existing Late Cretaceous basal thrust within the Semail Ophiolite on the Oman Mountain Front. The anticline was formed as a result of regional compressive deformation due to rejuvenation of the Late Cretaceous thrust in post-Middle Eocene times. Westward-directed high-angle reverse faults of Jebel Auha trend parallel to the fold axis of the anticline. The Auha faults probably originated as west-dipping thrusts on the western flank of the anticline and were subsequently rotated to their present attitude as the flank of the anticline became steeper due to compression from the east.

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

1982 ◽  
Vol 119 (5) ◽  
pp. 497-503 ◽  
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.

scholarly journals Geological and seismic evidence for the tectonic volution of the NE Oman continental margin and Gulf of Oman

Geosphere ◽  
2021 ◽  
Author(s):  
Bruce Levell ◽  
Michael Searle ◽  
Adrian White ◽  
Lauren Kedar ◽  
Henk Droste ◽  
...  
Keyword(s):  
Continental Margin ◽  
Late Cretaceous ◽  
Deep Water ◽  
Large Scale ◽  
Accretionary Prism ◽  
Oceanic Lithosphere ◽  
Seismic Reflection Data ◽  
Gulf Of Oman ◽  
Semail Ophiolite

Late Cretaceous obduction of the Semail ophiolite and underlying thrust sheets of Neo-Tethyan oceanic sediments onto the submerged continental margin of Oman involved thin-skinned SW-vergent thrusting above a thick Guadalupian–Cenomanian shelf-carbonate sequence. A flexural foreland basin (Muti and Aruma Basin) developed due to the thrust loading. Newly available seismic reflection data, tied to wells in the Gulf of Oman, suggest indirectly that the trailing edge of the Semail Ophiolite is not rooted in the Gulf of Oman crust but is truncated by an ENE-dipping extensional fault parallel to the coastline. This fault is inferred to separate the Semail ophiolite to the SW from in situ oceanic Gulf of Oman crust to the NE. It forms the basin margin to a “hinterland” basin formed atop the Gulf of Oman crust, in which 5 km of Late Cretaceous deep-water mudstones accumulated together with 4 km of Miocene and younger deep-water mudstones and sandstones. Syndepositional folding included Paleocene–Eocene folds on N-S axes, and Paleocene to Oligocene growth faults with roll-over anticlines, along the basin flank. Pliocene compression formed, or tightened, box folds whose axes parallel the modern coast with local south-vergent thrusts and reversal of the growth faults. This Pliocene compression resulted in large-scale buckling of the Cenozoic section, truncated above by an intra-Pliocene unconformity. A spectacular 60-km-long, Eocene(?) to Recent, low-angle, extensional, gravitational fault, down-throws the upper basin fill to the north. The inferred basement of the hinterland basin is in situ Late Cretaceous oceanic lithosphere that is subducting northwards beneath the Makran accretionary prism.

Chapter 3 Thrusts, extensional faults and fold patterns of the major units

10.1144/m54.3 ◽  
2021 ◽  
Vol 54 (1) ◽  
pp. 49-60 ◽  
Author(s):  
Andreas Scharf ◽  
Frank Mattern ◽  
Mohammed Al-Wardi ◽  
Gianluca Frijia ◽  
Daniel Moraetis ◽  
...  
Keyword(s):  
Late Cretaceous ◽  
Large Scale ◽  
Oman Mountains ◽  
Semail Ophiolite ◽  
Cylindrical Folds ◽  
Arabian Platform

AbstractThis chapter is concerned with the main faults and folds within the Southeastern Oman Mountains based on available literature. The main, best and most widely exposed thrusts are those related to the SW-directed late Cretaceous obduction of the allochthonous nappes onto the Arabian platform and margin. These thrusts are related to obduction of rocks, which had formed hundreds of kilometres offshore Oman. The thrusts were active from the Cenomanian to the Campanian. Obduction-related thrusts and folds are spectacularly exposed within the rocks of the Arabian platform in the eastern part of the Saih Hatat Dome, including large-scale recumbent cylindrical folds and sheath folds. At least six fold sets can be studied in the Southeastern Oman Mountains. At least two of them had formed prior to obduction and are exposed in the Pre-Permian formations of the Jabal Akhdar Dome. At least three fold sets formed in the course of obduction, while at least one fold set is postobductional in age. Besides the compressional structures, the Oman Mountains expose major post-obductional extensional faults, mostly at the margins of the Jabal Akhdar and Saih Hatat domes. The throw of these faults amounts to a few to several kilometres. Finally, this chapter provides an overview of the enigmatic Batinah Mélange which consists of slivers of Hawasina rocks, resting (unusually) structurally above the Semail Ophiolite.

scholarly journals Geophysical investigation of Al Jaww Plain, eastern Abu Dhabi: Implications for structure and evolution of the frontal fold belts of the Oman Mountains

GeoArabia ◽  
2008 ◽  
Vol 13 (2) ◽  
pp. 91-118 ◽  
Author(s):  
Mohammed Y. Ali ◽  
Manhal Sirat ◽  
James Small
Keyword(s):  
Late Cretaceous ◽  
Sedimentary Basin ◽  
Late Eocene ◽  
Abu Dhabi ◽  
Bouguer Gravity ◽  
Stress Regime ◽  
Oman Mountains ◽  
Thrust Front ◽  
Al Ain ◽  
Semail Ophiolite

ABSTRACT The area to the southeast of the city of Al Ain, Abu Dhabi, United Arab Emirates, is part of an arcuate sedimentary basin whose trend gradually changes from NNW near Al Ain to NNE at Ras Al Khaimah. The basin is bounded to the east by the generally N-trending Oman Mountains and on the west by an arcuate, overall west-verging fold-thrust front that involves Mesozoic carbonates. The fold-thrust front is part of the overall compressional system of Late Cretaceous age (with Late Tertiary reactivation) associated with obduction and emplacement of the Semail Ophiolite, Haybi, Hawasina and Sumeini sheets onto the continental margin of the Arabian Plate. Near Al Ain, the fold-thrust front is expressed as the remarkable, NNW-trending Jabal Hafit that rises one kilometer above the gravel-filled Al Jaww Plain. Gravity and magnetic investigations were carried-out in the Al Jaww Plain, an area of approximately 550 square km. The interpretation of these new data, including measurements of physical properties of rock samples from the area, were integrated with a new interpretation of an industry seismic reflection profile to provide constraints on the modelling of the subsurface structure and evolution of the sedimentary basin beneath Al Jaww Plain. We recognised four major tectono-stratigraphic units in the seismic profiles: autochthonous shelf carbonates, the Hawasina allochthon, Upper Cretaceous foreland basin sediments (primarily Fiqa Formation), and Tertiary neo-authochthonous units. Along-strike variations in the residual Bouguer gravity field were interpreted as being due to either variations in the thickness, or even total absence, of the Hawasina sheet. Comparison of two E-W gravity profiles, one in the southern part of our study area and the other to the north, suggest that the Hawasina sheet underlies little of the southern area but almost all of the northern area. Magnetic anomalies are weak (< 50 nT) over most of the area but peak (> 300 nT) in the easternmost part of the southern profile, where the high-susceptibility rocks of the Semail Ophiolite are exposed. Thus, we interpret that no continuation of the ophiolite extends westward from this outcrop into the subsurface of the study area. The structural geometries described here have resulted from two major tectonic events. The first, a Late Cretaceous phase, emplaced the obduction-related allochthonous thrust sheets of the Oman Mountains westward onto the Mesozoic carbonate platform. This phase primarily affected the eastern part of the study area and contributed to both the high magnetic (> 300 nT) and residual Bouguer gravity (> 14.0 mGal) anomalies. The second event, a Tertiary deformation phase, affected most parts of the area and produced a series of asymmetrical anticlines and synclines trending in a NNW-SSE direction. This phase contributed to the low residual gravity anomaly (< -9.0 mGal) in the center of the study area. We modelled that area as containing a sequence of post-Eocene carbonate sediments with a minimum thickness of 2.0 km. The Tertiary folding and thrusting formed as a result of a regional compressive deformation, whose principal compressive stress axes were sub-parallel to those of the Late Cretaceous compressional stress regime. The younger event reactivated high-angle reverse faults within the Mesozoic platform succession. Precise timing of the Tertiary deformation is debatable; it is most likely that the rejuvenation of the E-W to ENE-WSW Cretaceous stress regime took place in the Late Eocene-Miocene but gradually shifted to become N-S to NE-SW. This shift could be due to the collision of the Arabian and Eurasian plates and the opening of the Red Sea which started during Late Eocene and continues until the present-day.

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

GeoArabia ◽  
2014 ◽  
Vol 19 (2) ◽  
pp. 135-174
Author(s):  
Michael P. Searle ◽  
Alan G. Cherry ◽  
Mohammed Y. Ali ◽  
David J.W. Cooper
Keyword(s):  
Continental Margin ◽  
Granulite Facies ◽  
Continental Collision ◽  
Central Iran ◽  
Early Miocene ◽  
Late Cenozoic ◽  
Shallow Marine ◽  
Oman Mountains ◽  
Thrust Sheets ◽  
Semail Ophiolite

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).

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

GeoArabia ◽  
2007 ◽  
Vol 12 (2) ◽  
pp. 99-130 ◽  
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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