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

Abstract

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.

Microstructural Analysis of a Mylonitic Mantle Xenolith Sheared at Laboratory-like Strain Rates from the Edge of the Wyoming Craton

“…This apparent discrepancy between the CPO and the SGB was also recognized in other studies on the Wyoming Craton [42] and at other localities [28,59]. This discrepancy may arise from a change of the thermochemical conditions during deformation [63], transition to grain-size sensitive deformation mechanism [64], a change in the deformation kinematics [28,65], or difference in the recovery rate for different slip systems [7,66]. With a change in the deformation conditions (either with the thermo-chemical conditions or the kinematics of deformation), the subgrain boundaries will reflect deformation from the latest deformation stage.…”

Geodynamic Modeling of Edge-Delamination Driven by Subduction-Transform Edge Propagator Faults: The Westernmost Mediterranean Margin (Central Betic Orogen) Case Study

“…Vertical slab tearing would then put into direct contact thinned continental/oceanic lithosphere and normal or even thickened orogenic continental lithosphere of very different densities. The buoyancy forces associated with this density contrast generate mantle flow that can trigger continental delamination (edge-delamination), as proposed for the Betics STEP (Mancilla et al, 2013;Mancilla et al, 2015a;Heit et al, 2017) and for the Tell-Rif STEP in NW Algeria (Hidas et al, 2019). This scenario of delamination developing after slab tearing is completely different from other studies (e.g., Duggen et al, 2005;Faccenda et al, 2009;Gogus et al, 2011;Ueda et al, 2012;Levander et al, 2014;Baratin et al, 2016) where delamination operates in conjunction with slab sinking.…”

Back‐Arc Dynamics Controlled by Slab Rollback and Tearing: A Reappraisal of Seafloor Spreading and Kinematic Evolution of the Eastern Algero‐Balearic Basin (Western Mediterranean) in the Middle‐Late Miocene

“…During the initial stage of opening of the ABB, the slab sinking has produced asthenospheric upwelling to the base of the fore-arc, contributing to the collapse of a previous orogenic system and explaining the high-temperature Alboran and Kabylides metamorphism and widespread exhumation of the subcontinental mantle in the latest Oligocene-Early Miocene times (Gueydan et al, 2015;Hidas et al, 2015, and references therein). Later on, that is, during the middle-late Miocene, the southest-and westward propagation of the slab tear promoted the subcontinental-edge delamination and associated sub-lithospheric mantle upwelling explaining the post-collisional magmatism along the Algerian and Alboran margins (Booth-Rea et al, 2007;Chazot et al, 2017;Hidas et al, 2019;Roure et al, 2012 and references therein). This upwelling could have also affected the nearby Hannibal domain, an area potentially underlain by a stretched continental crust of an abandoned fragment behind the migration of the GALB during the second phase of the opening of the western Algero-Balearic back-arc basin (Figures 12c and 12d).…”