Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle
Authors: Hidas Károly , Garrido Carlos J. , Booth-Rea Guillermo , Marchesi Claudio , Bodinier Jean-Louis , Dautria Jean-Marie , Louni-Hacini Amina , Azzouni-Sekkal Abla 
Published: July 2019

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, 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 ∘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 no. melt at
depth (resulting in the formation of coarse-grained clinopyroxene-rich
lherzolite and wehrlite) and with a low-Mg no. 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 had a key effect on grain size reduction during the
operation of the Tell–Rif STEP fault. Melt–rock reaction and secondary
formation of lherzolite and wehrlite may be widespread in other STEP fault
systems worldwide.