Morphotectonic evolution of the Jebel Bou Naceur in the South Middle Atlas Fault Zone (Morocco)

International audienceIn eastern Morocco, fault-related folds develop above the South Middle Atlas fault that separates the J. Bou Naceur 'pop-up' structure and the Missour Basin, filled with alluvial sediments. Using a combination of surfaces, field observations and digital topographic information, we have investigated several geomorphic criteria and geometry of depositional surfaces governed by the relief's growth. The propagation of the deformation is recorded by warped segmented fans that moved southeastward and wellpreserved geomorphological features of tectonic origin. Uplift of the anticline hinges and acceleration of valley incision since the Late Pliocene are considered a consequence of the ongoing NW-SE convergence between Africa and Eurasia. A scenario of morphotectonic evolution of this active mountain front is proposed

New geomorphic criteria on lateral propagation of blind thrust-related fold growth accommodating oblique convergence

International audienceThe aim of this contribution is to show the styles of growing anticlines and the ways in which they accommodate oblique convergence through proximal foreland basins and intramontane basins. The four natural examples of individual fault-related-folds are: the Chandigarh anticline in the Siwalik foothills of Himalaya (NW India), the Pakuashan anticline in the western foothills of Central Range (Taiwan), the J. Bou Dhar-Tamdafelt fold belt (Morocco) and the Marand anticline in the North-Tabriz Fault-Zone (NW Iran). The evolution of these geomorphic criteria and drainage patterns shows evidence of lateral propagation of folds accommodating oblique convergence. This paper focuses on the geomorphic indicators that could be used to provide information on the timing of fault-related folding and direction of lateral propagation. Several tectonic scenarios has been analysed and an attempt has been made to delineate the sequential evolution of anticlines accommodating oblique convergence

Geodinámica de las cordilleras del Alto y Medio Atlas : síntesis de los conocimientos actuales

Se presenta una síntesis geodinámica de las cadenas de intraplaca del Alto y Medio Atlas de Marruecos. Estas cadenas derivan de la inversión tectónica de cuencas extensionales Jurásicas, con un acortamiento orogénico modesto (máximo del 24% en el Alto Atlas y del 10% en el Medio Atlas) heterogéneamente distribuido en el tiempo y en el espacio. Las relaciones entre tectónica y sedimentación en el margen sur del Alto Atlas indican que la deformación compresiva tuvo lugar entre el Eoceno medio y el Cuaternario. Consecuentemente con los valores de acortamiento, el espesor de la corteza detectado por gravimetría no supera los 40 km. A pesar de ello, la topografía de las cadenas del Atlas es elevada, y el modelado de campos potenciales sugiere que está causada por un marcado adelgazamiento litosférico, independiente de la tectónica compresiva regional. Un abundante magmatismo alcalino indica que el adelgazamiento litosférico coexistió con el acortamiento en los últimos 15 Ma, aunque los indicadores geomorfológicos sugieren que la mayor parte del levantamiento de origen mantélico es relativamente reciente (últimos 5 Ma). La termocronología indica que la cantidad de erosión es generalmente pequeña en el Atlas, registrándose de manera notable solo en los últimos 20-25 Ma

Crustal thickness and velocity structure across the Moroccan Atlas from long offset wide-angle reflection seismic data: The SIMA experiment

The crustal structure and topography of the Moho boundary beneath the Atlas Mountains of Morocco has been constrained by a controlled source, wide-angle seismic reflection transect: the SIMA experiment. This paper presents the first results of this project, consisting of an almost 700 km long, high-resolution seismic profile acquired from the Sahara craton across the High and the Middle Atlas and the Rif Mountains. The interpretation of this seismic data set is based on forward modeling by raytracing, and has resulted in a detailed crustal structure and velocity model for the Atlas Mountains. Results indicate that the High Atlas features a moderate crustal thickness, with the Moho located at a minimum depth of 35 km to the S and at around 31 km to the N, in the Middle Atlas. Upper crustal shortening is resolved at depth through a crustal root where the Saharan crust underthrusts the northern Moroccan crust. This feature defines a lower crust imbrication that, locally, places the Moho boundary at 40-41 km depth in the northern part of the High Atlas. The P-wave velocity model is characterized by relatively low velocities, mostly in the lower crust and upper mantle, when compared to other active orogens and continental regions. These low deep crustal velocities together with other geophysical observables such as conductivity estimates derived from MT measurements, moderate Bouguer gravity anomaly, high heat flow, and surface exposures of recent alkaline volcanism lead to a model where partial melts are currently emplaced at deep crustal levels and in the upper mantle. The resulting model supports the existence of a mantle upwelling as mechanism that would contribute significantly to sustain the High Atlas topography. However, the detailed Moho geometry deduced in this work should lead to a revision of the exact geometry and position of this mantle feature and will require new modeling effortsThis work has been primarily funded by the Spanish MEC project CGL2007–63889. Additional funding was provided by projects CGL2010–15416, CSD2006-00041, and GL2009–09727 (Spain), CGL2008–03474-E, 07-TOPO_EUROPE_FP-006 (ESF Eurocores) and EAR-0808939 (US, NSF).Peer reviewe

Crustal structure and velocity model of the Moroccan Atlas from refraction/wide angle data. Implications for its tectonic evolution

The Atlas Mountain Range is an intra-continental Cenozoic orogenic belt located at the southern edge of the diffuse plate boundary zone separating Africa and Europe. Its western part, the Moroccan Atlas, has long been under the scope of scientist regarding the origin of its high topographies, locally exceeding 4000 m. Geological studies indicate that this mountain belt has experienced low to moderate shortening. Furthermore, the later decreases as topography increases towards the west. These observations rise the question about the origin of the Atlas Mountains topography. Potential field studies indicate that an astenospheric upwelling supports the Atlas high elevations. However, these models depend strongly on the Moho topography and depth. Refraction/wide angle experiments carried out in the 80¿s suggested that the crust is thin and the Moho relatively flat. However, the proposed crustal structure and velocity inversions are not in agreement with the present models of this mountain belt. With the goal of improving the knowledge of the Moho boundary geometry and the velocity structure of the crust, a refraction/wide angle experiment was carried out in spring 2010 by an international team: the SIMA (Seismic Imaging of the Moroccan Atlas) experiment. A ~700 km long profile, going from Tanger to the Sahara Desert, south of Merzouga, recorded, every 400-1000 m, the energy of 6, 1 tn shots. Even with a low signal/noise ratio, the data allows the identification of crustal phases (Ps, Pg and PiP) and Moho reflected/refracted phases (PmP and Pn). Very weak subcrustal energy appers in some shot gathers. Forward modeling pictures a 3 layers crust and shows the Moho as an asymmetric feature that locally defines a crustal root, suggesting that the crust is imbricated. The crust-mantle boundary is modeled at relatively shallow depths that are in accordance with the results of other geophysical data, thus supporting the idea of a `mantle plume¿ as main contributor to the Atlas mountains topography. P wave velocities are low in the crust and upper mantle. First arrivals/ reflections tomography supports the forward modelling results. Further investigations are needed to unravel the source of the low velocities and to constrain the existence of mantle reflectors.Peer Reviewe