Investigation of the strength contrast at the Moho: A case study from the Oman Ophiolite

Homburg, J. M.; Hirth, Greg; Kelemen, P. B.

doi:10.1130/g30880.1

Cited by 27 publications

(15 citation statements)

References 31 publications

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“…An initially slower migration, accelerating in the past 3-4 m.y., is characteristic of a growing instability. Although 5 × 10 20 Pa·s is a relatively low value of viscosity for lithospheric mantle, it is in keeping with either hot or wet mantle lithosphere (Homburg et al, 2010) and consistent with other estimates from tectonically active regions (Billen and Houseman, 2004;Flesch et al, 2000;Freed et al, 2006).…”

Section: Mantle Instability Beneath Western North Islandsupporting

confidence: 88%

Instability of a lithospheric step beneath western North Island, New Zealand

Stern

Houseman

Salmon

et al. 2013

Geology

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Lithospheres of different thicknesses are often juxtaposed by movement on a continental-transform boundary. Such a boundary with a step change in densities may trigger a gravitational instability as lateral pressure gradients are created where normal mantle lithosphere terminates against less dense asthenospheric mantle. Here we show, for plausible values of the lithospheric viscosity, a mechanism by which the thicker mantle lithosphere will drip off into the lower density asthenosphere. As the mantle deforms it also progressively thickens and then thins the overlying crust, creating a topographic wave that migrates in concert with the removal of mantle lithosphere. Within western North Island, New Zealand, geophysical data defi ne a sharp lithospheric step across the Taranaki-Ruapehu line, and geological observations provide a history of uplift and subsidence that has propagated southward in the past 12 m.y. The rate of observed north to south migration of the wave (~30 mm/yr), its wavelength (~250 km), and amplitude (~±1 km) are compatible with it being caused by progressive removal of mantle lithosphere, if the viscosity of the uppermost lithospheric mantle is ~5 × 10 20 Pa·s, providing one of the clearest examples yet of this fundamental geological process.

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Section: Mantle Instability Beneath Western North Islandsupporting

confidence: 88%

Instability of a lithospheric step beneath western North Island, New Zealand

Stern

Houseman

Salmon

et al. 2013

Geology

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“…Strong rheological contrast between plagioclase‐rich lithologies and peridotite would lead to strain localization within the plagioclase‐rich veins (cf. Homburg et al, ). Resultant plastic deformation in the already weakened impregnated zones would (i) lead to mechanical mixing of the two lithologies, which is a requirement for our proposed feedback (see next section) to kick in, and (ii) presumably remain active down‐temperature, thus opening pathways for fluids that could, if penetrated to deep lithospheric levels, induce the here investigated water–rock interactions.…”

Section: Resultsmentioning

confidence: 99%

Melt Impregnation of Mantle Peridotite Facilitates High‐Temperature Hydration and Mechanical Weakening: Implications for Oceanic Detachment Faults

Albers

Schroeder

Bach

2019

Geochem Geophys Geosyst

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The footwalls of oceanic detachment faults commonly expose shear zone rocks that appear to have compositions intermediate between those of mantle peridotite and magmatic rocks. These compositions either reflect metasomatic mass transfers or they relate to the impregnation of lithospheric mantle with basaltic or more evolved melts. We studied chlorite‐amphibole‐rich shear zone rocks from a detachment fault zone in the 15°20′N Fracture Zone area, Mid‐Atlantic Ridge, to examine their origin and role in strain localization. Geochemical compositions of these rocks imply that they formed by mixing between peridotite and gabbro. Textural observations indicate a strong contrast between the deformation intensity of these hybrid peridotite‐gabbro rocks and the host serpentinized peridotite. Geothermometry data give formation temperatures of >500 °C for synkinematic amphibole, zircon, rutile, and titanite. Chlorite appears intergrown with these phases and likely grew at similar temperatures. These results are compliant to thermodynamic computations that predict comparable mechanically weak mineralogies when hydrating hybrid rocks at 500 to 600 °C, whereas secondary assemblages after pure peridotite or gabbro are considerably stronger. Consequently, metamorphic weakening takes place to a much greater extent in rocks with a hybrid ultramafic–mafic composition than in purely ultramafic or gabbroic lithologies. Deformation may enhance fluid flow, which will in turn increase the extent of hydration and mechanical weakening. A positive feedback loop between hydration and strain localization may hence develop and facilitate the concentration of extensional tectonics into long‐lived, high‐displacement faults. We suggest that hybrid lithologies may play a key role in detachment faulting at slow spreading ridges worldwide.

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“…This includes evidence in the Sumail massif, adjacent to the Wadi Tayin massif, for propagation of a new ridge or diapir into older lithosphere [ Amri et al , 1996; Boudier et al , 1997; Ceuleneer et al , 1996; Godard et al , 2000] and off‐axis, polyphase magmatism [ Jousselin and Nicolas , 2000]. In the Wadi Tayin massif, the Makhibiyah shear zone [ Homburg et al , 2010; Nicolas and Boudier , 2008] and the large scale rotation of sheeted dike orientations from N‐S in the east to NW‐SE in the west [ Bailey , 1981; Nicolas et al , 2000a] could also result from plate reorganization. The abundance of propagating rifts in the geologic record of the ophiolite and paleomagnetic evidence for rotations between the different massifs have been attributed to plate rotations prior to the onset of intraoceanic thrusting and ophiolite emplacement [ Boudier et al , 1997; Luyendyk and Day , 1982; Luyendyk et al , 1982; Nicolas et al , 2000a; Perrin et al , 1994, 2000; Weiler , 2000].…”

Section: Discussionmentioning

confidence: 99%

Rapid crustal accretion and magma assimilation in the Oman‐U.A.E. ophiolite: High precision U‐Pb zircon geochronology of the gabbroic crust

et al. 2012

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[1] New high-precision U/Pb zircon geochronology from the Oman-United Arab Emirates (U.A.E.) ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust. The new data come from a well-preserved and exposed crustal section in the center of the Wadi Tayin Zircon dates from upper-level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half-rates of 50-100 km/Ma. Dates from tonalites/trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper-level gabbros, suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates. Three of the dated upper-level gabbros and a single gabbroic pegmatite from the base of the crust have >1 Ma intrasample variability in single grain dates, suggesting assimilation of older crust during the formation or crystallization of the magmas. Whole rock ɛ Nd (t) of seven samples, including the upper-level gabbros with variable zircon dates, have tightly clustered initial values ranging from ɛ Nd (96 Ma) = 7.59 AE 0.23 to 8.28 AE 0.31. The ɛ Nd values are similar to those from other gabbros within the ophiolite, suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas. The new dates suggest that the studied section formed at a fast spreading mid-ocean ridge between $96.4-95.5 Ma. The large intrasample variability in zircon dates in some samples is unexpected in this setting, and may be related to propagation of a younger ridge into older oceanic lithosphere.Citation: Rioux, M., S. Bowring, P. Kelemen, S. Gordon, F. Dudás, and R. Miller (2012), Rapid crustal accretion and magma assimilation in the Oman-U.A.E. ophiolite: High precision U-Pb zircon geochronology of the gabbroic crust,

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Investigation of the strength contrast at the Moho: A case study from the Oman Ophiolite

Cited by 27 publications

References 31 publications

Instability of a lithospheric step beneath western North Island, New Zealand

Instability of a lithospheric step beneath western North Island, New Zealand

Melt Impregnation of Mantle Peridotite Facilitates High‐Temperature Hydration and Mechanical Weakening: Implications for Oceanic Detachment Faults

Rapid crustal accretion and magma assimilation in the Oman‐U.A.E. ophiolite: High precision U‐Pb zircon geochronology of the gabbroic crust

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