Reactivated lithospheric-scale discontinuities localize dynamic uplift of the Moroccan Atlas Mountains: COMMENT

Abstract

The Atlas Mountains of Morocco have received considerable attention in the past decade regarding the lithospheric structure and the uplift mechanisms of this structurally simple mountain belt. Miller and Becker (2014) have recently added to the discussion with new results from receiver functions and shear-wave splitting analysis. These results confirm the mantle upwelling hypothesis for the Atlas uplift that was postulated 10 years ago and we now see highlighted in several forums. The relationship between modest tectonic shortening and high topography in the Atlas was first addressed by Teixell et al. (2003). They concluded that crustal thickness is insufficient to account for the observed elevations, and proposed a mantle-driven (upwelling) component of uplift, supported by low seismic velocities in the Atlas mantle reported already by Seber et al. (1996). Subsequent efforts focused on modeling of gravity, the geoid, and topography that indicated a prominent lithospheric thinning, placing the lithosphere-asthenosphere boundary (LAB) at 70-90 km beneath the Atlas Mountains (e.g., Teixell et al., 2005;Missenard et al., 2006;Fullea et al., 2010). New contributions on the basis of active and passive seismology, partly under the Picasso project frame, are on their way (e.g., Bezada et al., 2013;Carbonell et al. 2013;Palomeras et al., 2013;Miller and Becker, 2014).Miller and Becker present a receiver functions profile that also shows a thinned crust and lithosphere beneath the Atlas Mountains. However, the thinned domain is bounded by abrupt steps coinciding with the location of the South Atlas thrust front; the authors propose offsets of 9 km of the Moho and 26 km of the LAB across the boundary thrust fault at Errachidia (Miller and Becker, 2014, their figure 2), arguing for the important role of lithosphericscale discontinuities in the Atlas mountain building. As for the crust, this is intriguing because early refraction studies by Wigger et al. (1992) and a more dense wide-angle survey by Carbonell et al. (2013) failed to detect a crustal step in this position, but especially because there is ample evidence that the South Atlas fault is a low-angle thrust zone (Saint Bezar et al. 1988;Teixell et al., 2003). Moreover, Miller and Becker imply that the same fault discontinuity goes down to offset the LAB, which means that the South Atlas thrust projects vertically to a depth of 90 km. This has doubtful geological sense even assuming that the present-day thrusts derive from the inversion of earlier extensional faults: those would also have a dip, and at a depth of 90 km they would be off the vertical projection of the fault surface exposure. Miller and Becker recognize that previous authors document shallowly dipping thrust faults from outcrop and seismic reflection studies, but they do not further discuss this issue. We are left questioning whether there is an enigmatic geological feature in the Atlas that escapes our understanding, or an issue of seismic data resolution.Miller and Becker compare the detected positio...

A depositional model for seismo-turbidites in confined basins based on Ionian Sea deposits

“…Our results, similar to other studies imply that location of coring sites is a key point and has to be considered carefully Sumner et al, 2013;Talling, 2014;Goldfinger et al, 2014;Patton et al, 2015). Not all earthquakes trigger widespread slope failure, turbidite distribution may vary in different types of turbidite systems, and the threshold strength for an earthquake to generate an ST will vary in different geologic settings and thus the ST record may be incomplete in Fig.…”

“…Another key issue in ST studies is evaluating the contribution of tsunami wave generation related to the deposition of STs. The catastrophic 2004 Sumatra and 2011 Tohoku earthquakes triggered devastating tsunamis whose effects have been studied using the stateof-the-art methods applied to earthquake geology (Arai et al, 2013;Ikehara et al, 2014;Patton et al, 2013Patton et al, , 2015Sumner et al, 2013;Goldfinger et al, 2014). Associated STs have been identified, some of which could have been deposited by tsunami backwash (Arai et al, 2013).…”