Deep resistivity cross section of the intraplate Atlas Mountains (NW Africa): New evidence of anomalous mantle and related Quaternary volcanism (original) (raw)
Related papers
2009
Cratonic margins have played a major role in shaping the modern Southern African tectonic landscape, and central to our knowledge is information about the lithospheric-scale structures and geometries using geophysical methods. Understanding the evolution of the younger orogenic belts around Archean cratons using electrical resistivity data is one of the primary objectives of the Southern African Magnetotelluric Experiment (SAMTEX); the largest ever land-based magnetotelluric (MT) project. MT data were acquired along four profiles crossing the enigmatic Rehoboth Terrane, the Neo-Proterozoic Ghanzi-Chobe/Damara belts (collectively termed the Damara Mobile Belt, DMB) and the southern Angola craton. The extended Groom and Bailey distortion decomposition technique was applied to the MT data and analyses show significant depth and along-profile variations in geo-electric strike and dimensionality on all transects crossing these three tectonic units (i.e. Rehoboth Terrane, Angola craton and the DMB). Geo-electric strikes are generally parallel to the north-east trending tectonic fabric, as inferred from the magnetic data, but the significant strike variations with depth are expressions of heterogeneity in lithospheric structure. Electrical resistivity models derived from the data provide the first pseudo three-dimensional tectonic structure of the Damara orogen and adjacent terranes. Regional-scale resistivity models constructed from two-dimensional inversions of the MT data indicate significant variations in lithospheric resistivity structure along and across strike from the younger orogen to the older adjacent cratons. The Damara belt lithosphere, although generally more conductive and significantly thinner than adjacent Angola craton and Rehoboth terrane, exhibits upper resistive upper crustal features tentatively interpreted to be caused by igneous intrusions emplaced during Pan-African magmatic event. Upper crustal listric faults are imaged as conductive features and are interpolated and correlated on the surface with structures mapped with magnetic data. The southern margin of the Angola craton is inferred as a very resistive feature extending to depths of approximately 100 km; as such these results constrain the geographical position of craton boundary margin and its geometry at depth.
The Atlas Mountains in Morocco are considered as type examples of intracontinental mountain chains, with high topography that contrasts with moderate crustal shortening and thickening. Whereas recent geological studies and geodynamic modelling suggest the existence of dynamic topography to explain this apparent contradiction, there is a lack of modern geophysical data at the crustal scale to corroborate this hypothesis. To address this deficiency, magnetotelluric data were recently acquired that image the electrical resistivity distribution of the crust from the Middle Atlas to the Anti-Atlas, crossing the tabular Moulouya plain and the High Atlas. All tectonic units show different, distinct and unique electrical signatures throughout the crust reflecting the tectonic history of development of each one. In the upper crust, electrical resistivity values and geometries can be associated to sediment sequences in the Moulouya and Anti-Atlas and to crustal scale fault systems in the High Atlas developed likely during Cenozoic times. In the lower crust, the low resistivity anomaly found below the Moulouya plain, together with other geophysical (low velocity anomaly, lack of earthquakes and minimum Bouguer anomaly) and geochemical (Neogene-Quaternary intraplate alkaline volcanic fields) evidences, infer the existence of a small degree of partial melt at the base of the crust. Resistivity values suggest a partial melt fraction of the order of 2–8%. The low resistivity anomaly found below the Anti-Atlas may be associated with a relict subduction of Precambrian oceanic sediments, or to precipitated minerals during the release of fluids from the mantle during the accretion of the Anti-Atlas to the West African Supercontinent during the Panafrican orogeny (ca. 685 Ma).
Journal of Geophysical Research: Solid Earth, 2013
Archean cratons, and the stitching Proterozoic orogenic belts on their flanks, form an integral part of the Southern Africa tectonic landscape. Of these, virtually nothing is known of the position and thickness of the southern boundary of the composite Congo craton and the Neoproterozoic Pan-African orogenic belt due to thick sedimentary cover. We present the first lithospheric-scale geophysical study of that cryptic boundary and define its geometry at depth. Our results are derived from two-dimensional (2-D) and three-dimensional (3-D) inversion of magnetotelluric data acquired along four semiparallel profiles crossing the Kalahari craton across the Damara-Ghanzi-Chobe belts (DGC) and extending into the Congo craton. Two-dimensional and three-dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger DGC orogen to the older adjacent cratons. We find Damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent Congo craton. The Congo craton is characterized by very thick (to depths of 250 km) and resistive (i.e., cold) lithosphere. Resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during Pan-African magmatism. Graphite-bearing calcite marbles and sulfides are widespread in the Damara belt and account for the high crustal conductivity in the Central Zone. The resistivity models provide new constraints on the southern extent of the greater Congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south.
First deep electrical resistivity structure of the southern Congo craton
2010
The southern African tectonic fabric is made up of a number Archean cratons flanked by Proterozoic and younger mobile belts, all with distinctly different but related geological evolutions. Of these cratons the southern extent of the Congo craton is one of the least-constrained tectonic boundaries in the African tectonic architecture and knowledge of its geometry is crucial for understanding geological process of formation and deformation prevailing in the Archean and later. In this work, which forms a component of the hugely successful Southern African MagnetoTelluric EXperiment (SAMTEX), we present the first-ever lithospheric electrical resistivity image of the southern boundary of the enigmatic Congo craton and the Neoproterozoic Damara-Ghanzi-Chobe (DGC) orogenic belt on its flanks. The DGC belt is highly complex and records the transpressive collision between the Congo to the north and Kalahari craton to the south. Magnetotelluric data were collected along a profile crossing al...
Crustal resistivity structure of the southwestern transect of the Rif Cordillera (Morocco)
Geochemistry, Geophysics, Geosystems, 2011
1] A NE-SW magnetotelluric 110 km-long profile including 18 sites was acquired across the western Rif Cordillera along the Eurasian-African plate boundary, allowing to constrain its poorly known deep structure. It extends from the Internal Zones, close to the Alboran coast, crossing the External Zones and up to the Gharb foreland basin. The periods recorded range from 0.001 s to 1000 s. The combination of magnetotelluric data with available geological data provides new insight regarding the relationship between deep and shallow crustal structures of the Rif Cordillera. Analyses of structural dimensionality suggest a preferential NW-SE direction, and a 2D joint inversion was performed. A 3D inversion extending the 2D model along the strike confirmed the reliability of this approach. The magnetotelluric model shows a heterogeneous upper crust in agreement with the geological structures observed at surface. The Internal Zones correspond to resistive (metamorphic rocks) and conductive (peridotites) bodies, while the External Zones and the foreland basin are characterized by large conductive bodies of variable thickness. A crustal detachment level separating shallow geological units from a probable variscan basement was inferred. At depth, the most relevant feature consists of large resistive bodies with a shallow irregular top, located below the frontal part of the Rif. The outcrops of exotic metapelitic, granitic and gneissic blocks in the frontal part of the Cordillera suggest that these large resistive bodies may correspond to a gneissic or granitic basement surrounded by metapelitic rocks. Late contractive thrust and diapiric processes were responsible for their uplift and shallow emplacement. The Rif constitutes an active southwestward vergent orogenic wedge, oblique to the present-day NW-SE convergent Eurasian-African plate boundary.
Journal of Geophysical Research: Solid Earth
The Main Ethiopian Rift is part of the East African Rift with its unique geological setting as an active continental breakup zone. The Main Ethiopian Rift includes a number of understudied active volcanoes with potentially high risks for this densely populated part of Ethiopia. Using newly recorded (2016) magnetotelluric data along a 110 km long transect crossing the whole rift, we present a regional 2-D model of electrical resistivity of the crust. The derived model endorses a previous study that drew the surprising conclusion that there was no highly conductive region associated with a magma chamber directly under the central rift volcano Aluto. This has implications for the estimation of the amount of magma present, its water content, and the storage conditions, as the volcano is actively deforming and results from seismicity and CO 2 degassing studies all indicate magma storage at about 5 km depth. Additionally, the existence of a strong conductor under the Silti Debre Zeyt Fault Zone approximately 40 km to the northwest of the rift center is confirmed. It is located with a slight offset to the Butajira volcanic field, which hosts a number of scoria cones at the boundary between the NW plateau and the rift. The magnetotelluric model reveals different electrical structures below the eastern and western rift shoulders. The western border is characterized by a sharp lateral contrast between the resistive plateau and the more conductive rift bottom, whereas the eastern flank shows a subhorizontal layered sequence of volcanic deposits and a smooth transition toward the shoulder.