Michel Guiraud

Seismites in the fluviatile Bima sandstones: identification of paleoseisms and discussion of their magnitudes in a Cretaceous synsedimentary strike-slip basin (Upper Benue, Nigeria)

Abstract The Upper Benue Basin (northeastern Nigeria) is interpreted as a complex strike-slip basin which consists of a set of sub-basins controlled by synsedimentary N60°C E-trending, strike-slip faults which were active during Early Cretaceous times. The oblique planar cross-beds made of the medium- to fine-grained sandstones of the upper member of the Bima Sandstone Formation are affected by various soft-sediment i.e., early diagenetic but soft-sediment deformational structures. These are present not only close to the synsedimentary faults but also on a larger regional scale. Three main types of structures (undulated and recumbent-folded cross-bedded lamination, dislocated convolutions and concordant microbreccias, discordant fluidised intrusions) are described. These are related to processes of sediment deformation favored by sand hydroplasticity, liquefaction and fluidization. The local organization of these soft-sediment intrastratal deformational structures, their spatial distribution with respect to synsedimentary faults, and their independance with respect to marginal paleoslopes, suggest that they cannot be explained by slumping or current drag. All the deformations are related to pore water expulsion and it is proposed that the required sand reorganization was triggered by seismic waves. The spatial and chronologic distribution and intensity of the various types of deformation structures suggest that they were produced around different epicenters repeated by earthquakes of varying magnitudes associated with the synsedimentary strike-slip tectonics.

Double seismic zone of the Nazca plate in northern Chile: High‐resolution velocity structure, petrological implications, and thermomechanical modeling

This paper presents an interdisciplinary study of the northern Chile double seismic zone. First, a high-resolution velocity structure of the subducting Nazca plate has been obtained by the tomoDD double-difference tomography method. The double seismic zone (DSZ) is observed between 80 and 140 km depth, and the two seismic planes is 20 km apart. Then, the chemical and petrologic characteristics of the oceanic lithosphere associated with this DSZ are deduced by using current thermal-petrological-seismological models and are compared to pressure-temperature conditions provided by a numerical thermomechanical model. Our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions. In the seismic upper plane located within the upper crust, these reactions would affect material of basaltic (MORB) composition and document different metamorphic reactions occurring within high-P (>2.4 GPa) and low-T ( 130 km), lawsonite-amphibole eclogite conditions. The lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions. The Vp and Vs characteristics of the region in between both planes are consistent with a partially (~25-30 vol % antigorite, ~0-10% vol % brucite, and ~4-10 vol % chlorite) hydrated harzburgitic material. Discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions. While various geophysical indicators evidence particularly cold conditions in both the descending Nazca plate and the continental fore arc, thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400°C (+/-50°C) isotherm. Lower plane earthquakes are predicted to occur in the slabs flexural neutral plane, where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity. Fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane, thus producing a sheeted layer of free fluids, or a serpentinized layer. Therefore earthquakes may present either downdip compression and downdip tensile characteristics. Numerical tests indicate that the slabs thermal structure is not the only factor that controls the occurrence of inner slab compression. (1) A weak ductile subduction channel and (2) a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab. (3) Decreasing the radius of curvature of the slab broadens the depth of inner slab compression, whereas (4) decreasing upper plate convergence diminishes its intensity. All these factors indicate that if DSZs indeed contour inner slab compression, they cannot be linked only to slab unbending, but also to the transmission of high compressional stresses from the upper plate into the slab.

Late Variscan strike-slip tectonics between the Teplá-Barrandian and Moldanubian terranes (Czech Bohemian Massif): petrostructural evidence

In the Variscan Bohemian Massif (Czech Republic), no agreement has been reached on the nature of the relationship between the Teplá-Barrandian and the Moldanubian terranes. However, the Teplá-Barrandian is interpreted as a central segment separating the Moldanubian and Saxothuringian terranes and its position is therefore essential for any reconstruction of this part of the Variscan belt. Our petrological and structural study in the Domazlice-Klatovy region spans the boundary between the Teplá-Barrandian and Moldanubian terranes. There is no conformity of ductile structures in both domains near their interface and their P–T evolution is separate in time. The development of major structures and associated metamorphism in the southwestern part of the Barrandian was accompanied by syntectonic intrusions of granitoid and diorite bodies, dated at about 520 Ma. The Moldanubian migmatites are characterized by a LP-HT metamorphism that possibly took place at about 320 Ma. The metamorphic record of the Barrandian rocks shows no influence of the Moldanubian partial melting. The contacts are marked by semi-brittle to brittle structures: sinistral strike-slip faulting developed in the Central Bohemian Shear Zone, whereas the West Bohemian Shear Zone acted as a semi-brittle normal fault with a dextral component. Both shear zones may be nearly contemporaneous, accommodating a bulk N-S compression. The Central Bohemian Shear Zone was later reactivated in a dextral regime. Although timing is uncertain, the juxtaposition of both terranes took place probably in the waning stages of the Variscan orogeny, earlier than the late Stephanian. We argue that the late-orogenic (semi)brittle strike-slip tectonics modified considerably the face of the Bohemian Massif and the geodynamic models of the Bohemian Massif have to be reviewed with reference to this essential point.

Opals from Slovakia (“Hungarian” opals) a re-assessment of the conditions of formation

Slovakian opals are found in an andesitic host-rock and believed to have formed by water circulation during a tectonic event. Their physical properties are investigated: X-Ray Diffraction (opal-A), Raman spectra (main Raman peak at 437 cm −1 ) and microstructure (large silica spheres 125 to 270 nm in diameter) surprisingly are properties of opals usually found in sedimentary deposits, and differ from those of opals found in other volcanic deposits. The temperature is proposed to control these physical properties rather than the nature of the host-rock. Some preliminary results of oxygen isotopic composition indicate a high δ 18 O for Slovakian and Australian opals (≈ 31‰) consistent with low temperatures of formation (lower than 45°C); by contrast, Mexican opals-CT show a lower δ 18 O at 13‰ consistent with a formation at a higher temperature, possibly up to 190°C.

Submersible observations of Cretaceous deformations along the Cote d'Ivoire-Ghana transform margin

Abstract The Côte d’Ivoire–Ghana marginal ridge, mainly formed by thick detrital sediments of Lower Cretaceous age, was surveyed by deep dives with the submersible Nautile (Equanaute survey). Samples retrieved during the dives, combined with observations of video and photos records, allowed us to document a compressional event that occurred along the continental margin off Côte d’Ivoire and Ghana, during Early Cretaceous times. Remarkable fracturing, folding, and cleavage characteristics in the sedimentary wedge are best explained by large-scale strike-slip transform activity between the two parting African and South American continents in Cretaceous times.