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Discussion papers
https://doi.org/10.5194/se-2019-32
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2019-32
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 27 Feb 2019

Research article | 27 Feb 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle

Károly Hidas1,2, Carlos J. Garrido1, Guillermo Booth-Rea2, Claudio Marchesi1,3, Jean-Louis Bodinier4, Jean-Marie Dautria4, Amina Louni-Hacini5, and Abla Azzouni-Sekkal5,6 Károly Hidas et al.
  • 1Instituto Andaluz de Ciencias de la Tierra, CSIC & Universidad de Granada, Av. de las Palmeras 4, 18100 Armilla (Granada), Spain
  • 2Departamento de Geodinámica, Facultad de Ciencias, Universidad de Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain
  • 3Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain
  • 4Géosciences Montpellier, UMR 5243, CNRS & Université de Montpellier, Place E. Bataillon, 34095 Montpellier, France
  • 5Faculté des Sciences de la Terre, de Géographie et de l’Aménagement du Territoire - Université des Sciences et de la Technologie Houari Boumédiène - Laboratoire de Métallogénie et Magmatisme de l’Algérie, 32, El Alia, 16111, Bab Ezzouar, Algiers, Algeria
  • 6Faculté des Sciences de la Nature et de la Vie et des Sciences de la Terre et de l’Univers, Université Abou Bekr Belkaïd, BP. 119, 13000, Tlemcen, Algeria

Abstract. Subduction-Transform Edge Propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, NW Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system.

The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with modal variations. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 °C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second phase particles. In the coarse-grained peridotites, well-developed olivine crystal preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high-temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010]-fiber and the [010]- and [001]-axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths.

The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg# melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite), and with a low-Mg# evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt-peridotite reaction – promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing – resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and played a key effect on grain size reduction during the operation of the Rif-Tell STEP fault. Melt-rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

Károly Hidas et al.
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Short summary
Subduction-Transform Edge Propagator (STEP) faults are the locus of continual lithospheric tearing at the edges of subducted slabs, resulting in sharp changes in the lithospheric thickness and triggering lateral and/or near-vertical mantle flow. Here we study upper mantle rocks recovered from a STEP fault context by < 4 Ma alkali volcanism. We reconstruct how the microstructure developed during deformation and coupled melt-rock interaction, which are promoted by lithospheric tearing at depth.
Subduction-Transform Edge Propagator (STEP) faults are the locus of continual lithospheric...
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