New paleomagnetic results from the Aegean extensional province

Various Oligocene formations from NE Greece (ignimbrites from the Medousa area, rhyolites from Zagradenia, granodiorites from Elatia) show discordant paleomagnetic signatures, in each case indicating small cw (clockwise) rotation and also inclination flattening. Marls from Pithion were partly remagnetized in a present-day field. Samples that contain ancient magnetization components also indicate small cw rotation and inclination flattening. However, the magnetization of andesites from Peplos reflects a considerably larger rotation, likely owing to local tectonics. In the context of previous work in the area, these results are used to propose a subdivision of NE Greece into four structural zones of distinctive rotational behaviour (from east to west): sites in zone 1, east of the Kavala-Xanthi-Komotini fault (KXK), show various cw and ccw (counterclockwise) rotation angles owing to complex kinematics resulting from the interaction of the KXK and the north-Anatolian fault zone. However, zone 2, between the KXK and the Strymon valley, is structurally homogeneous ( 10° cw rotation). The paleomagnetic signature of the Vertiskos massif (zone 3) implies a larger (> 30°) cw rotation, whereas sites in the Vardar basin (zone 4) contain a paleomagnetic signature similar to that of zone 2. This suggests a motion of the Vertiscos massif, a meta-ophiolitic nappe, relative to underlying strata. Indeed, zones 2 and 4 may be parts of the same structural unit which underlies this nappe.     

Escape models of the Alpine-Carpathian-Pannonian region in the light of palaeomagnetic observations

Recent tectonic models of the Alpine-Carpatho-Pannonian region (ALCAPA) assume a large eastward shift of the Transdanubian Range domain, in the Cenozoic. Since palaeomagnetism is one of the most powerful tools in solving geodynamic processes, the authors present an approach to the escape problem by using all available and relevant palaeomagnetic data. This data set demonstrates consistency with models put forward by geologists for Jurassic and older ages. From the mid-Jurassic on the Northern Calcareous Alps (NCA) did not share the rotations of the Transdanubian Range domain and of the Southern Alps. After individual movements from Neocomian to Miocene, the Transdanubian Range domain must have drifted northward in the mid-Miocene up to the Southern margin of the Northern Calcareous Alps, before starting the escape in the geologists' definition.     

The identification of magnetite in limestones using the low-temperature transition

The low-temperature (K₁) transition has been measured in prepared samples with a low concentration of magnetite to test the validity of the technique for identifying magnetite in weakly magnetized rocks. Using an astatic magnetometer, magnetite concentrations as low as 1 part in 100,000 can be satisfactorily detected.Measurements on natural samples show the presence of magnetite in a variety of limestones which are known to have a stable natural remanent magnetization.     

Paleomagnetic investigations in the Thüringer Forest (G.D.R.)

In the Thüringer Forest, the whole “Rotliegende” is well exposed, and was chosen as beginning of a Permian traverse via Czechoslovakia, Austria into Italy. Rockmagnetic experiments established haematite as main carrier of the NRM, except a few localities, where magnetite was found to be present. Sediments and volcanites show consistent ChRM directions. The comparison with KrŠ's (1978) compilation proves a good Permian pole position.     

Water infiltration characteristics of unsaturated soil slope and its effect on suction and stability

Rainfall has been considered the cause of the majority of slope failures and landslides that happened in regions experiencing high seasonal rainfalls. The mechanism of the failures was mainly due to the lost of matric suction of soils by rainwater. This paper presents the results of a laboratory model study on the effect of slope angle and surface cover on water infiltration into soil and soil matric suction. A field infiltration test is carried out for comparison. A parametric study is also done to examine the effect of permeability ratio, development of perched water table and rainfall intensity on the factor of safety against instability of a soil slope. Results of the model study show that different surface covers on slopes have an effect on the water infiltration. Generally the covered surface (grass or geosynthetic net) has a lower infiltration rate compared with the bare (no cover) surface. On the effect of slope angle, it was observed that water infiltration decrease with increase in the slope steepness. With regards to the movement of the wetting front, it appears that water infiltration is more at the toe compared with the top of the model slope. Based on the parametric study, it is found factor of safety of the slope against instability drops for slope with higher ratio of permeability for the permeable and impermeable stratum. As the perched water table is formed, the factor of safety decreased. The rainfall intensity also has a marked effect on the slope factor of safety. The higher the intensity of the rainfall, the higher is the infiltration rate into the soil, hence the lower is the factor of safety against slope instability.     

湖北铜绿山矿床石英闪长岩的矿物学及Sr-Nd-Pb同位素特征

铜绿山是长江中下游鄂东南矿集区最重要的、大型夕卡岩型Cu-Fe(Au)矿床。本文对该矿区中与成矿密切的石英闪长岩进行了详细的矿物成分、地球化学及Sr-Nd-Pb同位素研究。结果表明:岩石中斜长石主要为更长石(An=21～31);角闪石贫Ti(0.2),高Mg/(Mg+Fe)(0.5),属于富镁角闪石;而黑云母为镁质黑云母。岩石的地球化学具有高硅(58.86%～67.71%),富碱(Na2O+K2O=5.67%～9.63%),富集轻稀土元素(LREE)和大离子亲石元素(LILE),并强亏损元素Nb、Ta、Ti等特征。岩石的(87Sr/86Sr)i为0.7055～0.7069,εNd(t)为-7.65～-3.44;(206Pb/204Pb)i=17.66～18.00,(207Pb/204Pb)i=15.49～15.56,(208Pb/204Pb)i=37.73～38.19。矿物成分、地球化学和同位素特征说明,铜绿山岩体与阳新岩体为同源岩浆的产物,源区为深度大于40km的富集地幔,经下地壳的混染及分离结晶作用形成。岩浆熔体形成的温度应大于889℃。角闪石和黑云母的温度计估算岩浆结晶温度分别为650～800℃和500～630℃,黑云母开始结晶温度略低于角闪石结晶结束温度,压力为1.49kbar,对应侵位深度约4.9km。岩浆具有利于Cu、Fe、Au等成矿元素进入熔体的条件,可能与板块俯冲作用相关。     

Palaeomagnetic investigations on the East African rift in Northern Tanzania

Near Lake Manyara and north of Monduli, Northern Tanzania, Precambrian gneisses and Neogene basalts on the East African rift were sampled. Rock magnetic and microscope investigations established coarse and fine grained magnetite as carrier of the NRM in the basalts, and hematite in the gneisses. In all three sections only reversed magnetization directions were found. The Precambrian gneisses show very hard magnetizations which is known to be prefolding and possibly of Proterozoic age.     

Geophysical investigations of large landslides in the Carnic Region of southern Austria

The area under investigation for the past two decades is in the vicinity of the Gailtal lineament, which is the most dominant tectonic feature of the eastern Alps of southern Austria. An area of about 8 km² is in a state of constant instability, as documented by movement of road tracks of several centimetres per year. Geotechnical and surveying techniques have been used to measure these movements in the past but without solving the problem of the mechanism of these failure processes. Geophysical methods (seismic refraction, geoelectrics, and electromagnetics) were applied in order to determine the validity of one of the discussed movement models. In-situ velocity measurements were used to identify different lithologies beneath surficial talus deposits. The thickness of these talus deposits, of about 4–30 m, found by seismic refraction clearly demonstrates that huge ‘blocks’ (i.e. more or less undisturbed lithologic units) within the talus/debris are in close contact with the basement. This basement, which shows lower seismic velocities in different parts combined with low electric resistivities, is obviously strongly disturbed by different failure surfaces. The different gliding velocity of the blocks and the talus/debris deposits leads to a geological model in which huge rock blocks move slowly in relation to the disintegrating basement, whereas the talus/debris deposits move over the surface of these blocks at a higher velocity. The interpretation of these landslide studies is not a straightforward analysis. It is a complex problem with a complex solution, including all information from geotechnical, geophysical, and surveying investigations.     

华北地台南缘张士英岩体的锆石SHRIMP U-Pb测年、Hf同位素组成及其地质意义

张士英岩体位于华北地台南缘,岩石类型包括钾长花岗岩、似斑状花岗岩和石英斑岩脉,其中只在钾长花岗岩中发育有暗色岩石包体,在包体和寄主岩中发育反映岩浆混合作用的岩石结构。钾长花岗岩、似斑状花岗岩和石英斑岩脉的SHRIMP锆石U-Pb年龄分别为107.3±2.4Ma、106.7±2.5Ma和101±3Ma。锆石Hf同位素分析结果显示,钾长花岗岩的锆石εHf(t)为-15.96～-20.80,单阶段模式年龄(tDM1)为1396～1643Ma,两阶段模式年龄(tDM2)为1880～2018Ma;似斑状花岗岩的锆石εHf(t)为-18.97～-22.18,tDM1为1512～1640Ma,tDM2为1925～2080Ma;除了一粒年龄为2.6Ga的锆石具有εHf(t)为-0.71、tDM1为2943Ma和tDM2为3036Ma的组成,石英斑岩的锆石εHf(t)为-23.41～-27.95,tDM1为1678～1896Ma,tDM2为2144～2330Ma。这些数据暗示,除了存在少量太古宙地壳物质的贡献外,张士英岩体的物质来源可能主要为1.9～2.3Ga期间形成的新生地壳,但也不排除古老地壳与富集地幔源混合的可能。综合分析前人研究成果表明,在太平洋板块俯冲方向发生转变的过程中,先存断裂带发生拉张。张士英岩体与中国东部同期岩浆活动一起可能形成于这种受断裂带控制的伸展环境。