6 research outputs found
Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle
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.This research has been supported by the
Agencia Estatal de InvestigaciĂłn (grant nos. CGL2016-75224-R,
CGL2016-81085-R and CGL2015-67130-C2-1-R), the Junta de
AndalucĂa research groups RNM-131 and RNM-148, and the International
Lithosphere Program (grant no. CC4-MEDYNA)
Rhyolite petrogenesis and meteoric–hydrothermal alteration at the Maghnia volcanic massif, Northwest Algeria
International audienceVolcanic rocks from the Maghnia region of northwest Algeria consist of fresh to altered rhyolitic units that were emplaced during late Miocene time. Petrographic observations and geochemical data indicate that rhyolitic rocks were derived from a hybridized peraluminous magma. Petrogenetic modeling, REE contents and oxygen isotope data of quartz and feldspar and spessartine-rich garnet allows the reconstruction of the fractional crystallization between 680 and 780 °C and pressure of less than 0.5 GPa. Rhyolitic units are partially weathered and form an economic deposit of montmorillonite. Spectroscopic data and stable isotopes (O, H) of glass and montmorillonite indicate that the residual glass was partially hydrated by meteoric water at temperatures between 31 and 54 °C. The chemical mass balance indicates that rhyolitic rocks were altered to montmorillonite by meteoric water enriched in Ca, Mg and Sr. The combination of the chemical mass balance and the 87Sr/86Sr initial ratios of rhyolite and montmorillonite indicate that continental water/rock interaction was related to the surrounding Miocene lacustrine limestone
Evidence of seismites in coastal Quaternary deposits of western Oranie (northwestern Algeria)
Coastal Quaternary deposits of western Oranie show typical soft-sediment deformations including sedimentary dykes, sand volcanoes, sismoslumps, thixotropic bowls, thixotropic wedges, diapir-like structures, and faults grading. Field observations indicate that these deformations exist at several levels of the studied deposits along the west Oranian coast. This study demonstrated that these structures are earthquakes-related, by analysis of potential trigger sources. Several arguments demonstrate the seismic origin: the depositional environment rich in water that located in an active tectonic region, the thixotropic nature of deformations and their large vertical and horizontal diffusion in the Quaternary series consistent with a seismic recurrence. This allows characterizing these Quaternary soft-sediment deformations as seismites that were triggered by earthquakes
Sr, Nd, Pb isotope systematics of the neogene volcanism in the algerian tell belt: new constraint on the geodynamic evolution of the westernmost mediterranean
Workshop Alboran domain and Gibraltar Arc: geological research and natural hazards, Granada (Spain), 16-18 october, 201