Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle
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Hidas, Károly; Garrido, Carlos J.; Booth Rea, Guillermo; Marchesi, Claudio; Bodinier, Jean-Louis; Dautria, Jean-Marie; Louni Hacini, Amina; Azzouni Sekkal, AblaEditorial
European Geosciences Union
Fecha
2019-07-10Referencia bibliográfica
Hidas, K., Garrido, C., Booth-Rea, G., Marchesi, C., Bodinier, J. L., Dautria, J. M., ... & Azzouni-Sekkal, A. (2019). Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle.
Patrocinador
This research has been supported by the Agencia Estatal de Investigación (grant nos. CGL2016-75224-R, CGL2016-81085-R and CGL2015-67130-C2-1-R), the Junta de Andalucía research groups RNM-131 and RNM-148, and the International Lithosphere Program (grant no. CC4-MEDYNA).Resumen
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, northwest 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 lithology. The major
element composition of the mantle peridotites provides temperature
estimates in a wide range (790–1165 ºC) 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.