Immature and mature transform zones near a hot spot: The South Iceland Seismic Zone and the Tjörnes Fracture Zone (Iceland)

We describe and compare the two transform zones that connect the Icelandic rift segments and the mid-Atlantic Ridge close to the Icelandic hot spot, in terms of geometry of faulting and stress fields. The E–W trending South Iceland Seismic Zone is a diffuse shear zone with a Riedel fault pattern including N0°–N20°E trending right-lateral and N60°–N70°E trending left-lateral faults. The dominant stress field in this zone is characterised by NW–SE extension, in general agreement with left-lateral transform motion. The Tjörnes Fracture Zone includes three major lineaments at different stages of development. The most developed, the Húsavík–Flatey Fault, presents a relatively simple geometry with a major fault that trends ESE–WNW. The stress pattern is however complex, with two dominant directions of extension, E–W and NE–SW on average. Both these extensions are compatible with the right-lateral transform motion and reveal different behaviours in terms of coupling. Transform motion has unambiguous fault expression along a mature zone, a situation close to that of the Tjörnes Fracture Zone. In contrast, transform motion along the immature South Iceland Seismic Zone is expressed through a more complicate structural pattern. At the early stage of the transform process, relatively simple stress patterns prevail, with a single dominant stress field, whereas, when the transform zone is mature, moderate and low coupling situations may alternate, as a function of volcanic–tectonic crises and induce changes in stress orientation.     

X-ray photoelectron studies of the mechanism of iron silicate dissolution during weathering

Iron silicate minerals (bronzite, fayalite), exposed to aqueous dissolution in the laboratory for up to 60 days at room temperature and pH 1, 1.5, and 6, have been studied for evidence of changes in surface composition, using XPS, and these results compared with those obtained from solution chemical analysis. In the absence of dissolved O₂ or at low pH (1–1.5) dissolution proceeds congruently after the initial formation of a thin (<10 Å) protonated surface layer depleted in Fe relative to Si. This layer is unstable and does not grow with time as attested to by long term congruent dissolution and by the formation of an amorphous silica surficial breakdown product at pH 1 and 1.5. In bronzite the layer is also slightly depleted in Mg but much less than it is in Fe due to the preferential occupation by Fe⁺² of more weakly bonded M₂ sites. The behavior of the layer is similar to that found earlier on iron-free pyroxene (Schottet al., 1981); in other words, because of its thinness and instability it is not diffusion-inhibiting or protective toward dissolution.In the presence of dissolved O₂, as would be the case in most weathering solutions, dissolution of bronzite and fayalite results in the formation of two surface layers whose compositions were deduced by measurements of XPS binding energies. The outer layer, consisting of hydrous ferric oxide, is readily removed by ultrasonic cleaning and, most likely, is not protective toward dissolution. The inner layer consists of Fe⁺³ in a protonated or hydroxylated silicate (Mg-silicate in the case of bronzite) matrix. This layer appears to impede dissolution over the time scale of the experiment as attested to by parabolic dissolution rates. However, the layer does not continue to grow on the time scale of weathering because ultrasonically cleaned soil grains (Berner and Schott, 1982) exhibit surface compositions similar to those found in the present month-long laboratory experiments. In other words, a thick, highly altered, diffusion-inhibiting, protective surface layer does not form at the acidic pH of most soils.     

Clinopyroxene composition as a method of identification of the magmatic affinities of paleo-volcanic series

Uranium contents and²³⁴U/²³⁸U activity ratios have been determined for groundwaters from the Lincolnshire Limestone artesian aquifer in eastern England. Changes in the quantitative and isotopic chemistry of the dissolved uranium are explained in terms of a mixing model involving the rapidly moving fissure water and much older water stored in the pore system of this oolitic limestone. The western part of the aquifer, closest to recharge, is dominated by oxidising groundwaters which then enter a reducing zone towards the east, where there is an abrupt decrease in Eh and the chlorinity of the groundwaters begins to increase. Uranium contents in the oxidising zone range from 0.7 to 3.4 μg kg^?1 and²³⁴U/²³⁸U activity ratio of this dissolved uranium is close to unity, the equilibrium value. The uranium content decreases abruptly when the grounwaaters enter the reducing zone, averaging 0.04 μg kg^?1 east of the oxidation/reduction barrier. Simultaneously with the decrease in uranium content, there is an increase in²³⁴U/²³⁸U activity ratio and this ratio increases to a maximum within 7 km of the oxidation/reduction barrier. This increase in activity ratio is attributed to enhanced²³⁴U solution due to²³⁴Th recoil from uraniferous fissure surfaces east of the oxidising zone. The activity ratio of dissolved uranium in the ancient pore waters could in principle reach high values due to²³⁴Th recoil from the oolith surfaces. However, the activity ratio actually declines further east and this can only be explained as a consequence of mixing with pore waters in which the uranium activity ratio is closer to equilibrium.²³⁴Th recoil from the oolith surfaces has probably been inhibited by sealing of the uranium-bearing surfaces in the process of oolith cementation.     

Trench triple junction off Central Japan—preliminary results of French-Japanese 1984 Kaiko cruise, Leg 2

Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the trench triple junction off central Japan, where the Japan, Izu-Bonin and Sagami Trenches intersect. The Izu-Bonin Trench is deeper than the Japan Trench and filled by a thick turbiditic series. Its anomalous depth is explained by the westward retreat of the edge of the northwestward moving Philippine Sea plate. On the contrary to what happens in the Japan Trench, horst and graben structures of the Pacific plate obliquely enters the Izu-Bonin Trench, suggesting that the actual boundary between these two trenches is located to the north of the triple junction. The inner wall of the Izu-Bonin Trench is characterized in the triple junction area by a series of slope basins whose occurrence is related to the dynamics of this area. The northernmost basin is overthrust by the edge of the fore-arc area of the Northeast Japan plate. The plate boundary is hardly discernible further east, which makes it impossible to locate precisely the triple junction itself. These features suggest that large intra-plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge-shaped margin of the overriding plates.     

Late Permian/early Triassic orogeny in Japan: piling up of nappes, transverse lineation and continental subduction of the Honshu block

Field surveys in the Oga-Atetsu and Yamaguchi areas of Southwest Japan have been conducted in order to precise the structure of the Permian orogen. A stack of nappes is recognized comprising from top to bottom: (1) the Oga nappe which is considered to be a seamount complex, (2) HP Sangun metamorphics, (3) the Permian Yakuno ophiolite, and (4) the Permian detrital Maizuru group which is interpreted as the sedimentary cover of a continental block, called here the Honshu block, outcropping as the Older Granite. This stack of nappes is overthrust by the Paleozoic Hida basement consisting of HT gneisses, granites and late Carboniferous shallow-water sediments. Microtectonic analysis of the Sangun schists shows that the subhorizontal schistosity bearing a submeridian lineation was formed during the synmetamorphic phase. Asymmetric pressure shadows, shear bands and sigmoidal minerals show that the synmetamorphic deformation corresponds to a ductile shear from north to south. The Permian/early Triassic orogeny is interpreted as the result of a collision between the Hida gneiss (or South China block) and the Honshu block, the intervening oceanic area gave rise to southward directed nappes. The Permian orogenic belt extends at least from Taiwan to central Japan.     

Coupes sismiques des structures superficielles dans les petites antilles—I: Guadeloupe

Summary In 1976 and 1977, seismic profiles were carried out in Guadeloupe. Two profiles were established in the area of La Soufriére volcano and one profile through the northern part of Guadeloupe and southern part of Grande Terre. The two first profiles were occupied from 1 to 30 km and the third profile between 5 and 50 km.The interpretation shows that the superficial structures are characterized by a three-layers model: the compressional velocity is about 2.7 to 3.0 km/s down to a depth from 1 to 3 km. Below this, the velocity is between 4.0 and 4.5 km/s in a layer whose thickness varies from 1 to 2.5 km. Under this layer we find a 6.0–6.1 km/s layer which is one of the two known crustal layer under Lesser Antilles. The boundary between the old and new are which form the Lesser Antilles arc, is marked by a thicker layer of sediments on the eastern flank of recent volcanic chain.