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

摘要

Subduction-transform edge propagator (STEP) faults are the locusof continual lithospheric tearing at slab edges, resulting in sharp changesin the lithospheric and crustal thickness and triggering lateral and/ornear-vertical mantle flow. However, the mechanisms at the lithosphericmantle scale are still poorly understood. Here, we present themicrostructural study of olivine-rich lherzolite, harzburgite and wehrlitemantle xenoliths from the Oran volcanic field (Tell Atlas, northwest Algeria). Thisalkali volcanic field occurs along a major STEP fault responsible for theMiocene westward slab retreat in the westernmost Mediterranean. Mantlexenoliths provide a unique opportunity to investigate the microstructures inthe mantle section of a STEP fault system.The microstructures of mantle xenoliths show a variable grain size rangingfrom coarse granular to fine-grained equigranular textures uncorrelated withlithology. The major element composition of the mantle peridotites providestemperature estimates in a wide range (790–1165?~(°)C) but ingeneral, the coarse-grained and fine-grained peridotites suggest deeper andshallower provenance depth, respectively. Olivine grain size in thefine-grained peridotites depends on the size and volume fraction of thepyroxene grains, which is consistent with pinning of olivine grain growth bypyroxenes as second-phase particles. In the coarse-grained peridotites,well-developed olivine crystal-preferred orientation (CPO) is characterizedby orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherentCPO with that of olivine, suggesting their coeval deformation by dislocationcreep at high temperature. In the fine-grained microstructures, along withthe weakening of the fabric strength, olivine CPO symmetry exhibits a shifttowards [010] fiber and the [010] and [001] axes of orthopyroxene aregenerally distributed subparallel to those of olivine. These data areconsistent with deformation of olivine in the presence of low amounts ofmelts and the precipitation of orthopyroxenes from a melt phase. The bulkCPO of clinopyroxene mimics that of orthopyroxene via a topotaxialrelationship of the two pyroxenes. This observation points to a melt-relatedorigin of most clinopyroxenes in the Oran mantle xenoliths.The textural and geochemical record of the peridotites are consistent withinteraction of a refractory harzburgite protolith with a high-Mg no. melt atdepth (resulting in the formation of coarse-grained clinopyroxene-richlherzolite and wehrlite) and with a low-Mg no. evolved melt in the shallowsubcontinental lithospheric mantle (forming fine-grained harzburgite). Wepropose that pervasive melt–peridotite reaction – promoted by lateraland/or near-vertical mantle flow associated with lithospheric tearing –resulted in the synkinematic crystallization of secondary lherzolite andwehrlite and had a key effect on grain size reduction during theoperation of the Tell–Rif STEP fault. Melt–rock reaction and secondaryformation of lherzolite and wehrlite may be widespread in other STEP faultsystems worldwide. Downloadandlinks Article (PDF, 7448 KB) Supplement (19513 KB) How to cite Back to top top How to cite.? Hidas, K., Garrido, C. J., Booth-Rea, G., Marchesi, C., Bodinier, J.-L., Dautria, J.-M., Louni-Hacini, A., and Azzouni-Sekkal, A.: Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle, Solid Earth, 10, 1099-1121, https://doi.org/10.5194/se-10-1099-2019, 2019. 1?Introduction Back to toptop Slab edges are necessary ingredients of plate tectonics, and verticaltearing along the lithosphere at the termination of subduction trenches is ageometric consequence that enables the subduction to continue (e.g.,Wilson, 1965; Millen and Hamburger, 1998; Govers and Wortel, 2005; Nijholt andGovers, 2015). A subduction-transform edge propagator (STEP) fault is thelocus of continual lithospheric tearing at slab edges, which allowssubduction of one part of a tectonic plate, while the juxtaposed partremains at the surface (Govers and Wortel, 2005; Nijholt and Govers, 2015).The evolution of the subduction zone along STEP faults results in sharpchanges in the lithospheric and crustal thickness, which trigger lateraland/or near-vertical mantle flow associated with lithospheric tearing(e.g., Mancilla et al., 2015; Menant et al., 2016). Upwelling melts thatform in this process are likely channelized by the translithospheric STEPfaults and may lead to intense melt–rock reaction in the shallowsubcontinental lithospheric mantle (SCLM).In terms of modal composition, melt–rock reaction processes that occur atshallow mantle depths result in a lithological variation trending fromrefractory peridotites towards lherzolites with a highclinopyroxene/orthopyroxene modal ratio, eventually forming wehrlite(e.g., Peslier et al., 2002; Parkinson et al., 2003; Ionov et al.,2005; Soustelle et al., 2009; Lambart et al., 2012; Bodinier and Godard,2014; Varas-Reus