Location and characteristics of the X-discontinuity beneath SW Morocco and the adjacent shelf area using P-wave receiver functions

Abstract

SUMMARY Receiver function approaches have proven to be valuable for the investigation of crustal and upper mantle discontinuities whose sharp changes in seismic velocities cause wave conversions. While the crustal and mantle transition zone discontinuities are largely understood, the X-discontinuity at 250–350 km depth is still an object of controversial debate. The origin and global distribution of this structure with a velocity jump of 1.5–4.8 % for compressional and shear waves is still unexplained. Although the crustal and mantle transition zone discontinuities beneath SW Morocco and surroundings have been investigated, only a few studies observed the X-discontinuity and place the depth at 260–370 km beneath the region of western Morocco. In order to better locate and characterize the X-discontinuity beneath southwest Morocco, we create P-wave receiver functions using data recorded by the Morocco–Münster array and detect the X-discontinuity at apparent depths of 285–350 km. In the western part of our study region we find apparent depths of ∼ 310–340 km. The eastern part of the study area appears more complex: we locate two velocity jumps at apparent depths of around 285–295 km and 330–350 km in the northeast, and in the southeast we find a discontinuity at apparent depths of 340–350 km. Due to the large depth range and the twofold appearance of the X-discontinuity, we suggest that two different phase transitions cause the X-discontinuity beneath SW Morocco. The velocity contrasts at larger depths likely point to the coesite–stishovite phase transition occurring in deep eclogitic pools. The shallower depths can be explained by the transition from orthoenstatite to high-pressure clinoenstatite which requires the reaction between eclogite and peridotite to form orthopyroxene-rich peridotite. This reaction is likely related to previously proposed small-scale mantle upwellings beneath SW Morocco. Since both phase transitions require eclogite occurrence, the location of the X-discontinuity in this region can be used to indicate the location of recycled oceanic crust.