Flow in the western Mediterranean shallow mantle: Insights from xenoliths in Pliocene alkali basalts from SE Iberia (eastern Betics, Spain)

Abstract

Mantle xenoliths in Pliocene alkali basalts of the eastern Betics (SE Iberia, Spain) are spinel ± plagioclase lherzolite, with minor harzburgite and wehrlite, displaying porphyroclastic or equigranular textures. Equigranular peridotites have olivine crystal preferred orientation (CPO) patterns similar to those of porphyroclastic xenoliths but slightly more dispersed. Olivine CPO shows [100]‐fiber patterns characterized by strong alignment of [100]‐axes subparallel to the stretching lineation and a girdle distribution of [010]‐axes normal to it. This pattern is consistent with simple shear or transtensional deformation accommodated by dislocation creep. One xenolith provides evidence for synkinematic reactive percolation of subduction‐related Si‐rich melts/fluids that resulted in oriented crystallization of orthopyroxene. Despite a seemingly undeformed microstructure, the CPO in orthopyroxenite veins in composite xenoliths is identical to those of pyroxenes in the host peridotite, suggesting late‐kinematic crystallization. Based on these observations, we propose that the annealing producing the equigranular microstructures was triggered by melt percolation in the shallow subcontinental lithospheric mantle coeval to the late Neogene formation of veins in composite xenoliths. Calculated seismic properties are characterized by fast propagation of P waves and polarization of fast S waves parallel to olivine [100]‐axis (stretching lineation). These data are compatible with present‐day seismic anisotropy observations in SE Iberia if the foliations in the lithospheric mantle are steeply dipping and lineations are subhorizontal with ENE strike, implying dominantly horizontal mantle flow in the ENE‐WSW direction within vertical planes, that is, subparallel to the paleo‐Iberian margin. The measured anisotropy could thus reflect a lithospheric fabric due to strike‐slip deformation in the late Miocene in the context of WSW tearing of the subducted south Iberian margin lithosphere.

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

“…5b and S1). Similar correlation between olivine and orthopyroxene CPOs in orthopyroxeneimpregnated dunites from the Bay of Islands ophiolite (Suhr, 1993), in ultramylonitic shear zones from the Othris ophiolite (Dijkstra et al, 2002) and in fluid-assisted ductile strain localization from the Ronda massif (Hidas et al, 2016b), has been interpreted as having formed due to synkinematic melt/fluid-facilitated dissolution-precipitation processes. In the Oran equigranular peridotites, we interpret the CPO of interstitial small orthopyroxenes similarly, and we propose that they record constrained synkinematic crystallization from a melt phase.…”

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

“…Beneath the Betics, a sharp and prominent lithospheric step at the termination of the Iberian lithosphere is interpreted as a near-vertical STEP fault structure associated with lateral lithospheric tearing (Mancilla et al, 2015;Mancilla et al, 2018). Deformation pattern and seismic anisotropy signal of mantle xenoliths from the eastern Betics indicate steeply dipping foliation and subhorizontal lineations in the shallow SCLM, which are consistent with WSW tearing of the subducted south Iberian margin lithosphere along the STEP fault (Hidas et al, 2016a). Moreover, in the same volcanic field, wehrlite lithologies are interpreted as an interaction with SiO2understaturated magmas, similar to the alkali basalts that host the mantle xenoliths (Marchesi et al, 2017).…”

“…In the Oran equigranular peridotites, we interpret the CPO of interstitial small orthopyroxenes similarly, and we propose that they record constrained synkinematic crystallization from a melt phase. As in coarse-grained peridotites, the lack of intracrystalline deformation in equigranular peridotites points to a post-kinematic recovery stage, possibly enhanced by the presence of melts as proposed by Rampone et al (2010) and Hidas et al (2016a) in mantle xenoliths from Tallante.…”

“…Late Miocene to Pliocene topographic uplift and coeval Si-K-rich magmatism and Quaternary alkali basalts in SE-Iberia and the Maghrebian margins have been related to asthenospheric mantle upwelling after late Miocene edge delamination (Duggen et al, 2003;Duggen et al, 2005;Duggen et al, 2008) and/or earlier slab break-off in the Algerian-Rifean margin (Spakman and Wortel, 2004;Chazot et al, 2017). Mantle xenoliths in Plio-Quaternary alkali basalts have been interpreted to have registered part of this tectonic evolution in the mantle underlying the Betics, preserving the originally subvertical tectonic foliation (Hidas et al, 2016a) probably related to mantle flow during slab tearing along the Betic STEP (Mancilla et al, 2015;Mancilla et al, 2018).…”

Fluid‐Enhanced Annealing in the Subcontinental Lithospheric Mantle Beneath the Westernmost Margin of the Carpathian‐Pannonian Extensional Basin System

“…A decrease in M2M values indicates a reduction of intracrystalline misorientations within olivine grains. It allows, therefore, estimating the degree of recrystallization, which can be static (annealing) or dynamic in the xenoliths (e.g., Hidas, Konc, et al, ; Tommasi & Ishikawa, ).…”