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, NW 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 modal variations. 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# melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite), and with a low-Mg# 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 played a key effect on grain size reduction during the operation of the Rif-Tell STEP fault. Melt-rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

Ongoing Inversion of a Passive Margin: Spatial Variability of Strain Markers Along the Algerian Margin and Basin (Mediterranean Sea) and Seismotectonic Implications

“…Recent studies of the westernmost Algerian margin (Badji et al, 2015), the Betics and the Rif (Garcia-Castellanos and Villaseñor, 2011;de Lis Mancilla et al, 2015 have shown that the structure of the southern and northern margins of the Balearic and Alboran basins is characterized by an abrupt transition between continental and oceanic domains, in agreement with a STEP (Subduction-Transform-Edge-Propagator) fault origin (Govers and Wortel, 2005). This slab tear propagation is therefore assumed to have induced, from 16 to 8 Ma, the opening of the central and western Algerian basin along a left-lateral sub-vertical fault, thus forming a transform-type margin (Medaouri, 2014;Medaouri et al, 2014;van Hinsbergen et al, 2014;Hidas et al, 2019).…”

Polyphase Tectonic Evolution of Fore‐Arc Basin Related to STEP Fault as Revealed by Seismic Reflection Data From the Alboran Sea (W‐Mediterranean)

“…Under the central Betics to the north and under Morocco to the south, a sharp lithospheric and crustal step can be recognized in the receiver function computations, tomographic images, and gravimetric data (e.g., Spakman & Wortel, 2004; Bezada et al, 2013; Levander et al, 2014; Mancilla et al, 2015; Petit et al, 2015; Diaz et al, 2016; Molina‐Aguilera et al, 2019) In northern Morocco in particular, a sharp change in crustal thickness is visible at the city of Al Hoceima, where the crust is more than 40 km thick to the west but less than 30 km thick to the east (Diaz et al, 2016). These abrupt offsets are interpreted as the termination of the Iberian and Nubia lithosphere caused by a vertical STEP fault (e.g., Spakman & Wortel, 2004; Levander et al, 2014; Mancilla et al, 2018; Hidas et al, 2019). These lithospheric STEP faults along the North African margins and along the Balearic margin accommodate the westward migration of the Gibraltar slab (e.g., Spakman & Wortel, 2004).…”

“…The North African plate overrides the Alboran domain leading to the present‐day surface expression of the STEP fault (Alboran Ridge, Yusuf fault, and Jebha and Nekor faults) that does not coincide with the present‐day deep structure (Figure 14d). The actual boundaries of the slab tear are determined by mapping the missing orogenic root under the Betics and Rif (e.g., Mancilla et al, 2015; Hidas et al, 2019). The convergence motion is oblique to the E‐W striking folds and major conjugate fault sets causing the reactivation and rotation of the Miocene SAR‐Alboran Ridge and Yusuf fault (Figure 14d).…”

Metastability and Nondislocation‐Based Deformation Mechanisms of the Flem Eclogite in the Western Gneiss Region, Norway

“…While solid line is the fit line using a Zener parameter of 0.52 adopted from Tasaka et al (2014). The background of the figure is modified after Hidas et al (2019).…”