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

Accretion, structure and hydrology of intermediate spreading-rate oceanic crust from drillhole experiments and seafloor observations

Downhole measurements recorded in the context of the Ocean Drilling Program in Hole 504B, the deepest hole drilled yet into the oceanic crust, are analyzed in terms of accretion processes of the upper oceanic crust at intermediate spreading-rate. The upper part of the crust is found to support the non steady-state models of crustal accretion developed from seafloor observations (Kappel and Ryan, 1986; Gente, 1987). The continuous and vertical nature of borehole measurements provides stratigraphic and structural data that cannot be obtained solely from seafloor studies and, in turn, these models define a framework to analyze the structural, hydrological, and mineralogical observations made in the hole over the past decade.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43190/1/11001_2005_Article_BF01204282.pd

Explosion seismic sounding of throws and dips in the continental Moho

Using critical‐distance Moho reflections on fan‐profiles, sections of the topography of the crust‐mantle boundary across the Pyrenean mountain range are obtained. At two different places throws of more than 10km in the Moho are detected, they are situated beneath the same surface fault. Together with other features such as local dips, these results have a strong bearing on models of regional evolution. The existence of fractures through the continental lithosphere is documented by that simple technique using the Moho as a marker.Peer reviewe

Crustal structure of the Paleozoic Axial Zone of the Pyrenees and transition to the North Pyrenean Zone

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Lateral variation of the crust in the North Pyrenean Zone

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Relationship between deep structure and seismicity in the western Pyrenees

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Geophysical constraints on the deep structure along the Ecors Pyrenees Line

The structure of the central part of the Pyrenean range, traversed by the ECORS vertical reflection seismic line, is further constrained by a joint interpretation of complementary and reprocessed data. Wide‐angle, low‐frequency profiling of Moho topography indicates an abrupt termination of the northern, European Moho at the North Pyrenean fault and a range of possible geometries of a deeper, Iberian Moho which does not seem to extend significantly to the north of this fault. When reprocessing vertical reflection data to 25 s two‐way travel time, strong clear straight reflections are discovered in the part of the section under the northern foreland, long after the Moho reflection. Their strong northward dip indicates that they originate in fact at reflectors at depth beneath the North Pyrenean fault, which may correspond to the connection between the two levels of Moho, the reflections then being from the northward inclined limit between the wedge of Iberian crust underlying the southern edge of the European mantle. A range of possible shapes is allowed by the conjunction of these seismic data, which is compatible with the results of a detailed gravity profile. This would indicate that the imbrication of European into Iberian lithosphere has been of limited extent with respect to the surface trace of the North Pyrenean fault, provided no strong density contrast in the mantle part of the section contributes significantly to the gravity anomaly.AcknowledgmentFs.u ndingf or this work has been providedin the frameo f the French-SpanisEhC ORS Pyrenees project supported by INSU-CNRS, IFP, Elf-Aquitaine, Esso-Rep, CSIC, IGN,IGME,Hispanoil.Peer Reviewe

Gravity constraints on the deep structure of the Pyrenean belt along the ECORS profile

In order to furnish further constraints to modelling the ECORS Pyrenees deep seismic profile, gravity data were collected along a stripe 20 km wide and 220 km long containing the seismic line. A total of 4020 stations have been used to derive a Bouguer anomaly map of the area, A large and roughly symmetrical negative anomaly ( - 100 mGa1) is centered on the Axial zone. A local positive anomaly over the North Pyrenean zone disturbs the negative regional trend. Somewhat detailed densities were assigned to the sedimentary cover on tbe basis of borehole samples. Refraction data and a velocity-density relationship were used to constrain the densities adopted for the deeper layers. The results of the two-dimensional gravity modelling confirm the asymmetrical shape of the crust/mantle boundary beneath the Pyrenean belt; the downward deflection of the Iberian Moho from the Ebro basin leads to a maximum depth of 65 km under the Paleozoic Axial zone, whereas a depth of 35 km is found for the European Moho under the North Pyrenean zone. Two extreme models are proposed for the northern limit of the Iberian crust, a vertical contact in the downward continuity of the North Pyrenean fault or a 70° northward-sloping interface. The positive anomalies beneath the North Pyrenean zone may be explained by dense intractustal mantle slices or lower crustal sheets. The outcrop of granulitic basic gneiss within the Labourd massif suggests that the anomaly results from lower crustal sheets