Centrifuge Experiments of the Initiation of Self-Sustaining Subduction

<p>The post-Triassic age of all oceanic lithospheres indicates the efficiency and the sustainability of lithospheric subduction, which consumes the basaltic seafloor and recirculates it in the upper mantle. Since at present the initiation of subduction is very rare, comprehension of this cardinal process should be carried through modeling – numeric or analog. While deciphering processes through numeric modeling is commonly comprehensive, the analog models can determine major factor that constrain a tectonic procedure. Analog centrifuge experiments were applied to initiate self-sustained modelled subduction, trying to determine the critical factors that trigger its early stages.</p><p>Analytically we presumed that where densities of two lithospheric plates, juxtaposed across a weakness zone, exceed a critical value, then the denser lithosphere eventually will drive underneath the lighter one, provided the friction across the interface is not too high. Consequently, analog experiments were carried out in a centrifuge at acceleration of ca. 1000 g., deforming miniaturized models of three layers representing the asthenosphere, the ductile and the brittle lithosphere. The lithospheres were modeled to include lighter and denser components, juxtaposed along a slightly lubricated contact plane, where the density difference between these components was ca. 200 kg/m<sup>3</sup>. No mechanism of lateral force was applied in the experiment (even though such a vector exists in nature due to the seafloor spreading at the oceanic ridges), to test the possibility of subduction in domains where such a force is minor or non-existent.</p><p>The analog experiments showed that the penetration of the denser modeled lithosphere under the lighter one led to extension and subsequent break-up of the over-riding plate. That break-up generated seawards trench rollback, normal faulting, rifting, and formed proto-back-arc basins. Lateral differential reduction of the friction between the juxtaposed plates led to the development of arcuate subduction zones. The experimental miniaturization, and subsequent numerical and analytical modeling, suggest that the observed deformation in the analog models could be meaningful to the planet as well.</p><p>Constraints of the analog experimentation setting did not enable the modeling of the subduction beyond the initial stages, but there is ground to presume that at depths of 40-50 km, metamorphic processes of the generation of eclogites would change the initial mineralogy on the subducting plate. Reactions with water would convert basalts into metamorphic serpentinites and schists. Higher temperatures and pressures would melt parts of the subducted slab to generate felsic magmas, which would ascend towards the surface diapirically due to their lighter density. Alternately, low availability of H<sub>2</sub>O would gradually alter the oceanic basalt and gabbro into eclogite, which would sink into the mantle due to its increased density.</p>

Application of the Electrical Resistivity Method for Site Investigation in University of Anbar, Ar-Ramadi City, Western Iraq

The 2D resistivity imaging technique was applied in an engineering study for the investigation of subsurface weakness zones within University of Anbar, western Iraq. The survey was carried out using Dipole-dipole array with an n-factor of 6 and a-spacing values of 2 m and 5 m. The inverse models of the 2D electrical imaging clearly show the resistivity contrast between the anomalous parts of the weakness zones and the background resistivity distribution. The thickness and shape of the subsurface weakness zones were well defined from the 2D imaging using Dipole-dipole array of 2 m a-spacing. The thickness of the weakness zone ranges between 9.5 m to 11.5 m. Whereas the Dipole-dipole array with a-spacing of 5 m and n-factor of 6 allocated the geoelectrical stratigraphic layers sequence in low-accuracy of weakness zones, but deeper than the inverse model of 2 m a-spacing. This survey was made to explain the correlation between the weakness zone and the deeper layers in the study area. It points out that the deeper layers were not affected in the weakness zones. The inverse model was produced using the Standard Least-Squares Inversion Method and the Robust Inversion Model Constraints Method. The first method had a gradational boundary of the weakness zones and the second had sharper and straighter boundaries of fractures and voids within the weakness zones.

The geodynamics of oceanic core complexes: would subduction occur at ridge-transform intersections?

<p>Oceanic core complexes are lithological assemblages of peridotites and serpentinites, embedded in the basaltic oceanic crust at active or dormant intersections of several slow-spreading oceanic accreting rifts with fracture zones. These occurrences are presumed to derive from the upper mantle, emplaced by low-angle and large-throw normal detachment faults. The abundant serpentinites are attributed to alteration of the ultramafic peridotites during its long ascent from the upper mantle. However the absence of both high-pressure lithologies in the oceanic core complexes and the rareness of earthquakes generated by low-angle normal faulting cast doubt on the validity of this conventional model. Alternately, analog tectonic experiments showed that subduction is a probable process for the generation of oceanic core complexes, because it could develop between two juxtaposed tectonic slabs if their density contrast will exceed 200 kg/m<sup>3</sup> with no lateral converging pressure, if the friction between the slabs were low. Indeed oceanic core complexes occur in unique oceanic domains where two basaltic slabs of contrasting densities are juxtaposed across a weakness zone of low friction. Density of fresh basalt at the accreting ridge is approximately 2700 kg/m<sup>3</sup> and that of the older basalts, juxtaposed across the fracture zone, is ca. 2900 kg/m<sup>3</sup>. Slow spreading rates of some ridges would set slabs of significant density contrast across the fracture zone even if the transform offsets are not large. Furthermore, the thermal gradient under the ridge is some 130<sup>0</sup>/km, enabling the metamorphism of the oceanic basalts either to serpentinites or to peridotites at similar P-T constraints, depending on the availability of water. Therefore, it seems that the serpentinites are not secondary products of source-rock alteration, but genetic equivalents to the peridotites. It is presumed therefore that the pliable serpentinite would ascend diapirically through cracks in the over-riding basaltic slab and reach the seafloor, carrying along large blocks of peridotite to produce the serpentinite-peridotite petrology, that lithological association of oceanic core complexes.</p>

Why 3D seismic data are an asset for exploration and mine planning? Velocity tomography of weakness zones in the Kevitsa Ni-Cu-PGE mine, northern Finland

Kevitsa is a disseminated Ni-Cu-PGE (platinum group elements) ore body in northern Finland, hosted by an extremely high-velocity ([Formula: see text]) ultramafic intrusion. It is currently being mined at a depth of approximately 100 m with open-pit mining. The estimated mine life is 20 years, with the final pit reaching a depth of 500–600 m. Based on a series of 2D seismic surveys and given the expected mine life, a high-resolution 3D seismic survey was justified and conducted in the winter of 2010. We evaluate earlier 3D reflection data processing results and complement that by the results of 3D first-arrival traveltime tomography. The combined results provide insights on the nature of some of the reflectors within the intrusion. In particular, a major discontinuity, a weakness zone, is delineated in the tomography results on the northern side of the planned pit. Supported by the reflection data, we estimate the discontinuity, likely a thrust sheet, to extend down approximately 600 m and laterally 1000 m. The weakness zone terminates prominent internal reflectivity of the Kevitsa intrusion, and it is associated with the extent of the economic mineralization. Together with other weakness zones, a couple of which are also revealed by the tomography study, the discontinuity forms a major wedge block that influences the mine bench stability on the northern side of the open pit and likely will cause more issues during the extraction of the ore in this part of the mine. We argue that 3D seismic data should routinely be acquired prior to commencement of mining activities to maximize exploration efficiency at depth and also to optimize mining as it continues toward depth. Three-dimensional seismic data over mineral exploration areas are valuable and can be revisited for different purposes but are difficult to impossible to acquire after mining has commenced.