Paraglacial gullying of sediment‐mantled slopes: a case study of Colletthøgda,Kongsfjorden area,West Spitsbergen (Svalbard)

This paper evaluates the paraglacial evolution of a sediment‐mantled slope in a polar maritime environment. The intensity of paraglacial processes is estimated through quantification of erosion and dating of field sectors with the help of photographic archives. Gully erosion has been estimated using morphometric parameters and by surveys of vegetation cover. The rapid melting of dead‐ice cores controls gully formation. This leads to slope form modification: gully profile gradients are reduced from a mean of 35° to a mean ranging between 10° and 15°. Profile evolution results from the collapse of glacier lateral moraine. All data (mean slope angle of individual gullies, frequency distribution of slope angles, fractional distance to the apex, gullying index, volume of debris mobilized, vertical erosion rate) tend to increase with increasing deglaciation age and the duration of paraglacial activity. Vegetation colonization is a response to stabilization of the ground surface and the drying up of the ground surface due to dead‐ice melting. The full sequence of paraglacial slope adjustment (gully incision‐stabilization) may occur rapidly at the study site, i.e. within two decades. Finally, a lateral morphogenic sequence is proposed showing the importance of paraglacial processes at the onset of the deglaciation. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparison of vibrocompaction methods by numerical simulations

Soils can be best compacted by repeated shearing. The strain amplitude plays an important role for the maximum compaction that can be reached. Experimental evidence emphasizes a vital impact of simultaneous multidirectional shear loading on the rate and magnitude of soil compaction. Two different vibrocompaction methods were analysed by the numerical simulations in the light of these findings. In an elastic finite element (FE) analysis, strain paths were determined. A strain amplitude‐dependent stiffness at small strains was introduced by multiple runs of the FE calculation to reach an appropriate stiffness for particular distances from the vibrator. Subsequently, the obtained strain paths were used to control single element simulations using hypoplasticity with intergranular strains. The calculated compaction profiles show three zones known from practical evidence: a limited compaction close to the vibrator, a zone of maximum compaction and a non‐densified zone remote from the vibrator. The deep vibrator produces a faster compaction than the top vibrator, especially in the more distant zone. The more efficient work of the deep vibrator can be attributed to a more general multidirectional shearing. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical simulation of lumpy soils using a hypoplastic model

The lumpy soil is a by product of the open-pit mining. A composite-lumpy material (in which, the lumps are randomly distributed in the reconstituted soil) is being created due to the degradation of the initial granular structure. In the present study, the compression and failure behaviour of an artificial lumpy material with randomly distributed inclusions are investigated using the finite element method. The computation results show that the stress ratio, defined as the ratio of the volume average stress between the lumps and the reconstituted soil within the inter-lump voids, is significantly affected by both the volume fraction and the preconsolidation pressure of the lumps under an isotropic compression path, while the volume fraction of the lumps plays a minor role under a triaxial compression path. Based on the simulation results, a homogenization law was proposed utilizing the secant stiffnesses.     

Tectonique quaternaire et plis de rampe dans le golfe d'Hammamet (offshore tunisien)

The Jriba trough is an Upper Miocene graben located within the Tunisian offshore Gulf of Hammamet area, east of the Atlas front. This distensive structure suffered a compressive event during the Early Quaternary (Villafranchian). The Jriba structure was previously interpreted as ‘flower structure’, which possibly complicated by halokinetics movements. A new analysis of a set of seismic lines crossing the Jriba trough allows us to propose a new tectonic model where the Villafranchian deformation is characterized by (1) occurrence of a decollement level cutting Messinian to Pliocene layers; and (2) the growth of fault-related folds (fault-propagation fold). The NE–SW Miocene, inherited normal faults, locate the position of the ramps and folds whereas the NW–SE inherited normal faults are reactivated as tear faults. These NW–SE tear faults define various domains of different shortening values (one kilometre at maximum). To cite this article: M. Ben Romdhane et al., C. R. Geoscience 338 (2006).     

The Rhafas Cave (Morocco): Chronology of the mousterian and aterian archaeological occupations and their implications for Quaternary geochronology based on luminescence (TL/OSL) age determinations

The Middle Palaeolithic chronology of the Maghreb region of North Africa is poorly known because of the paucity of sites with a long stratigraphy and the limited number of available radiometric dates. In this paper, we report the age-estimates obtained by the TL and OSL methods on sediments and burnt lithic samples from the Rhafas Cave in Eastern Morocco. Our results indicate that the Mousterian is largely earlier than 100 ka, that the latest Mousterian dates to OIS 5 between 90–80 ka and the Mousterian to Aterian transition occurred about 70–80 ka, at the end of OIS 5 or during OIS 4. These dates will be most useful in the construction of a chronological framework of this unique sequence and for the interpretation of paleoenvironmental information.     

Calculations of upper-mantle P-wave velocity anisotropies

Discovery of anisotropy for upper-mantle P-wave velocities has resulted recently in many laboratory determinations of velocities for natural mantle material, either by direct measurement on rock samples or through calculations based on single-crystal elastic constants, fabrics and modal compositions Comparison of field and laboratory data provides gross constraints for convection processes and flow mechanisms. As natural peridotites yeidl a wide range of maximum seismic anisotropies, their depth of origin and deformation history are inferred from pyroxene geobarometry and geothermometry, and from stress estimates derived from neoblast grain sizes. Large maximum anisotropies (ca. 1.0 km/s) are typical of samples with fabrics strengthened by low-temperature/highstress deformation related to emplacement. Other highly anisotropic mantle material includes peridotites beneath continental rifts and thinned continental crust. Maximum anisotropies for ordinary suboceanic and infracontinental peridotites are generally in the range of the field data (0.3–0.6 km/s) and support a model with [010]_ol nearly vertical (in accord with observations on ophiolites) and [100]_ol subparallel to the spreading direction. On the basis of fabric data, seismic anisotropy should decrease with depth.     

The vourinos ophiolite complex, Greece: The tectonite suite

Classical structural analysis, in combination with new techniques including pyroxene thermobarometry and recrystallized-grain- and subgrain-size paleopiezometry, has been used to deduce the tectonic history of the Vourinos Ophiolite Complex, Greece. Results can be used to infer the variation with depth of differential stress in the upper mantle and indicate that these rocks were subjected to an initial stress level of 10–100 bar over a depth interval from 100 to 40 km, respectively. Subsequently, they underwent mylonitization, associated with stresses of about 2500 bar at 30 km depth. Rocks equilibrated at the deepest levels commonly have the highest structural positions within the ultramafic tectonites and also show appreciable Al depletion. Lack of annealing recrystallization in the mylonitic rocks, together with low stress levels and an abnormally high pyroxene geotherm for the Complex lead to the interpretation of the Complex being the remnant of a mantle diapir, with an overlying magma chamber, located at a spreading ridge.     

Origin of the Xigaze ophiolite, Yarlung Zangbo suture zone, southern Tibet

The Xigaze ophiolite, (Tibet) displays unusual lithological, petrological, textural, and structural characteristics. There are no large masses of cumulate gabbros, but dolerite intrusives throughout the whole ophiolite sequence, some of which were intruded into already serpentinized peridotites, only minor residual harzburgites and dunites in dominantly Iherzolitic peridotites equilibrated at low temperatures and pressures, and relatively low-temperature deformation structures in the uppermost peridotites. These features suggest a very low heat flow at the spreading center where the Xigaze ophiolite was formed, in good agreement with a discontinuous and slowly-accreting spreading center origin. However, this ophiolite does not represent a typical mid-oceanic ridge ophiolite; rather it was formed in a small basin located at the southern margin of Eurasia, hence within a preexisting oceanic lithosphere. The opening of the nearly N-S Xigaze paleo-ridge resulted from the W-E drift of Africa relative to Eurasia from 180 to 110 Ma. The N-S emplacement onto the continent of the Xigaze ophiolite, formed 120 to 110 Ma ago, can be correlated to changes in direction of motion of the African and Indian plates: a primary intra-oceanic thrusting event probably occurring at 110 or 85 Ma and the final obduction near 50 Ma during the India-Eurasia collision.