Harzburgite–dunite–orthopyroxenite suite as a record of supra-subduction zone setting for the Oman ophiolite mantle

Geological and geochemical evidence suggest that the Oman ophiolite is a fragment of a submarine arc-basin complex formed above a short-lived subduction zone in the mid-Cretaceous. Detailed studies of the lava stratigraphy and the intrusive relationships of dykes, sills and high-level plutons provide further evidence for the magmatic and tectonic development of the complex in question. Four consecutive events can be recognized to have taken place before emplacement: (1) eruption of basalts of island arc affinity onto pre-existing (Triassic) oceanic crust; (2) creation of new oceanic crust by backarc spreading; (3) intrusion of magma into this back-arc oceanic crust accompanied by eruption of basalts and andesites from discrete volcanic centres; (4) further intrusion of magma accompanied by uplift and eruption of basalts and rhyolites in submarine graben. A combined structural and geochemical analysis of the dyke swarm indicates that extension took place in approximately a N-S (ridge) and an ESE-WNW (leaky transform) direction relative to an inferred direction of subduction to the NE, and that a small but significant proportion of the sheeted dykes were injected during the ‘arc’ rather than the earlier ‘back-arc spreading’ episode. These various observations can be explained in terms of the progressive response of a non-isotropic lithosphere to the stresses induced during subduction.

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Unravelling the Remagnetization of the Oman Ophiolite

10.5194/egusphere-egu2020-2063 ◽

2020 ◽

Author(s):

Louise Koornneef ◽

Antony Morris ◽

Michelle Harris ◽

Christopher MacLeod

Keyword(s):

Subduction Zone ◽

Large Scale ◽

Oceanic Lithosphere ◽

Magnetic Fabric ◽

Oman Ophiolite ◽

Mantle Peridotites ◽

Important Exception ◽

Magnetic Techniques ◽

Tectonic Rotation ◽

Lower Crustal

The Oman ophiolite is a natural laboratory for the study of processes operating above a nascent subduction zone. It formed in the Late Cretaceous by supra-subduction zone spreading and shortly afterwards was emplaced onto the Arabian continental margin. Twelve massifs in the ophiolite expose complete sections of the Neotethyan oceanic lithosphere, including upper mantle peridotites, lower crustal gabbros, and upper crustal sheeted dykes and lava flows.&#160;Previous palaeomagnetic studies have suggested that the southern massifs of the ophiolite were affected by a large-scale remagnetization event during emplacement, that completely replaced original remanences acquired during crustal accretion. In contrast, primary magnetizations are preserved throughout the northern massifs. This study aimed to: (i) apply palaeomagnetic, magnetic fabric and rock magnetic techniques to analyse crustal sections through the southern massifs of the Oman ophiolite to investigate further the extent and nature of this remagnetization event; and (ii) use any primary magnetizations that survived this event to document intraoceanic rotation of the ophiolite prior to emplacement.&#160;Our new data confirms that remagnetization appears to have been pervasive throughout the southern massifs, resulting in presence of shallowly-inclined NNW directions of magnetization at all localities. An important exception is the crustal section exposed in Wadi Abyad (Rustaq massif) where directions of magnetization change systematically through the gabbro-sheeted dyke transition. Demagnetization characteristics are shown to be consistent with acquisition of a chemical remanent overprint that decreased in intensity from the base of the ophiolite upwards. The top of the exposed Wadi Abyad section (in the sheeted dyke complex) appears to preserve original SE-directed remanences that are interpreted as primary seafloor magnetizations. Similar SE primary remanences were also isolated at a control locality in the Salahi massif, outside of the region of remagnetization. Net tectonic rotation analysis at these non-remagnetised sites shows an initial NNE-SSW strike for the supra-subduction zone ridge during spreading, comparable with recently published models for the regional evolution of the ophiolite.

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