40Ar/39Ar age pattern associated with differential uplift along the Eastern Highlands shear zone,Cape Breton Island,Canadian Appalachians     

《Journal of Structural Geology》2001,23(6-7):1031-1042

The Eastern Highlands shear zone in Cape Breton Island is a crustal scale thrust. It is characterized by an amphibolite-facies deformation zone ∼5 km wide formed deep in the crust that is overprinted by a greenschist-facies mylonite zone ∼1 km wide that formed at a more shallow level. Hornblende ⁴⁰Ar/³⁹Ar plateau ages on the hanging wall decrease towards the centre of the shear zone. In the older zone (over 7.8 km from the centre), the ages are between ∼565 and ∼545 Ma; in the younger zone (within 4.5 km of the centre), they are between ∼425 and ∼415 Ma; and in the transitional zone in between, they decrease abruptly from ∼545 to ∼425 Ma. Pressures of crystallization of plutons in the hanging wall, based on the Al-in-hornblende barometer and corresponding to depth of emplacement, increase towards the centre of the shear zone and indicate a differential uplift of up to ∼28 km associated with movement along the shear zone. The age pattern is interpreted to have resulted from the differential uplift. The pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende Ar blocking temperature (∼500°C) until differential uplift occurred. The ⁴⁰Ar/³⁹Ar ages in the zone (∼425–415 Ma) thus date shear zone movement or the last stage of it. In contrast, rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of ∼565–555 Ma plutons in the area, long before shear zone movement. The transitional zone corresponds to the Ar partial retention zone before differential uplift. The ⁴⁰Ar/³⁹Ar age pattern thus reflects a Neoproterozoic to Silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone. A similar K–Ar age pattern has been reported for the Alpine fault in New Zealand. It is suggested that such isotopic age patterns can be used to help constrain the ages, kinematics, displacements and depth of penetration of shear zones.  相似文献   

Deformation path in high-strain zones,with reference to slip partitioning in transpressional plate-boundary regions     

《Journal of Structural Geology》2001,23(6-7):991-1005

The current status of the kinematics and strain geometry of high-strain zone studies is briefly summarized. A general high-strain zone has a triclinic deformation path, and monoclinic shear zones are special end member cases. Fabrics observed in natural shear zones and theoretical considerations based on continuum mechanics are compatible with this conclusion. Non-steady deformation paths remain difficult to deal with, and may ultimately rely on a realistic mechanical treatment of high-strain zones which may be possible when our knowledge of the mechanical behavior of rocks under natural deformation conditions is improved. An examination of the phenomenon of slip partitioning in transpressional plate boundary regions shows that the bulk deformation path in the forearc area (trench-parallel high-strain zone) is generally triclinic. The Alpine Fault in the South Island of New Zealand provides an example of a currently active triclinic shear zone. The Southern Knee Lake shear zone of Manitoba, Canada, provides an Archean example of a triclinic shear zone.  相似文献   

Heavy metal contamination of urban soils and street dusts in Hong Kong     

《Applied Geochemistry》2001,16(11-12):1361-1368

Due to rapid urbanisation and scarcity of land, most of the urban parks and recreational areas in Hong Kong are built close to major roads or industrial areas, where they are subject to many potential pollution sources, including vehicle exhaust and industrial emissions. An extensive soil survey was conducted in urban parks of Hong Kong to study the current conditions of heavy metals contamination in soils. Soil samples and associated street dusts were collected from more than 60 parks and public amenity areas in old urban districts, industrial areas and New Towns of the territory. Soils were also sampled in the remote country parks to establish the baseline conditions. The total concentrations of heavy metals and major elements in the samples were determined. The results indicate that urban soils in Hong Kong have elevated concentrations of Cd, Cu, Pb and Zn. The parks with high metal concentrations are located in old urban commercial districts and industrial areas, indicating that the major contamination sources in these soils are traffic emissions and industrial activities. In addition, the application of Cd containing phosphate fertilizers may be an important source of Cd in urban park soils. The street dusts have highly elevated Zn concentration, particularly along the main trunk roads. The high Zn content in the street dusts may come from traffic sources, especially vehicle tyres. Selective soil and road dust samples were also analysed for potential chemical speciation by a sequential chemical extraction method. The chemical partitioning results show that Pb and Zn are mainly in the carbonate/adsorbed and Fe–Mn oxide phases, while Cu is largely associated with the organic and sulphide fractions. The high exchangeable Cd in urban soils and high concentration of Zn in street dusts need further investigation for their ecological and health implications.  相似文献   

Géochimie et signification géotectonique des volcanites du Cryogénien inférieur du Saghro (Anti-Atlas oriental,Maroc)     

《Geodinamica Acta》2001,14(6):373-385

The Early Cryogenian groups of Sidi Flah, Kelaat Mgouna and Boumalne (Saghro, Anti-Atlas, Morocco) are constituted by turbiditic deposits and interbedded lavas, accumulated in tectonic basins. At Sidi Flah, volcanics are transitional showing initial rift tholeiites (IRT) fingerprint and alkali basalts of oceanic island basalt (OIB) compositions. At Kelaat Mgouna, volcanics consist of low-Nb continental tholeiites. At Boumalne, basalts are of IRT composition. The volcanic and sedimentary formations belong to a nascent rift caused by thermal doming along a SW-NE axis, the Saghro rift. The continental break-up occurred in the early Neoproterozoic and during the Rodinia supercontinent dislocation, within a continent called “Ibero-saharian Craton” which was in front of the West-African Craton located near the South Pole. The Saghro rift is contemporaneous to the opening of an oceanic domain represented by Central Anti-Atlas ophiolites and related to the extension of the Brazialiano Ocean.  相似文献   

Plio–Quaternary sedimentation processes and neotectonics of the northern continental margin of the South China Sea     

《Marine Geology》2001,172(3-4):331-358

Analyses of over 6600 km of reflection seismic profiles on the northern continental margin of the South China Sea permit the recognition of four Quaternary high-frequency type 1 sequences of the 4th order, deposited during the past ca. 690 kyr. At the present-day shelf edge, only lowstand systems tracts characterised by a prograding clinoformal internal reflection pattern are preserved. The prograding complexes can be considered as regressive units accumulated during relative sea-level falls. They exhibit internal discontinuities which might point to minor sea-level fluctuations of the 5th order. A preliminary regional relative sea-level curve for the past 630 kyr is established using the present positions of the delta fronts mapped. The neotectonics curve derived by subtracting eustatic sea-level changes from the relative sea-level curve shows that the depths of the delta fronts today are controlled primarily by regional tectonic movements and the global sea-level.Our seismo-stratigraphic interpretation documents that the area off Hong Kong and around the Dongsha Islands experienced two uplift episodes during the past 5 ma, namely at the Miocene/Pliocene boundary and at the end of the lower Middle Pleistocene, respectively. These uplift events which are centred on the Dongsha Rise led to its subaerial exposure and to the erosion of the Pliocene and most of the Pleistocene strata. The change from thermal subsidence of the continental margin initiated at the end of the drift phase to the phase of magmato-tectonic uplift was caused by a reorientation of the tectonic regime.The Recent depositional environment on the northern continental margin of the South China Sea is dominated by sediment accumulation within the inner shelf and the Zhujiang (Pearl River) estuary. The outer shelf and upper slope, especially around the Dongsha Islands, are characterised by bypass of terrigenous material.The sedimentary column in the deepsea basin has a thickness of more than 2 km and comprises 14 depositional units starting with terrestrial rift deposits. It overlies oceanic as well as transitional crust.  相似文献   

南海北部不同涡旋对内潮的影响     

张宇  陈旭  刘娟  宁珏 《海洋与湖沼》2024,55(2):306-317

南海北部吕宋海峡是内潮最为活跃的区域之一,且涡旋种类繁多,不同特性的涡旋对内潮的影响不同。基于近岸与区域海洋共同模式(coastal and regional ocean community model,CROCO),模拟探究理想涡旋存在时,涡旋位置、极性、峰值流速和半径对内潮的影响。结果表明:涡旋位置是影响内潮的直接因素,位于涡旋区域内的内潮是主要影响对象,涡旋中心以西内潮方向变化的角度是以东的3倍。气旋涡和反气旋涡分别使潮能通量的方向向南和向北偏转,最大偏转角度超过12°,当涡旋所致背景流与内潮传播方向一致时,内潮群速度增强,反之减弱。涡旋对内潮的影响范围和幅度随着涡旋的半径和峰值流速的增大而变大。当涡旋峰值速度变大时,反气旋涡心以北的潮能通量增长量超过15 kW/m。当涡旋半径增大时,涡旋峰值速度的位置发生变化,涡旋的峰值流速和半径共同影响潮能通量水平分布结构,使其呈现纬向单峰或多峰结构。  相似文献   

The influence of background winds and attenuation on the propagation of atmospheric gravity waves     

《Journal of Atmospheric and Solar》2003,65(7):857-869

The influence of background winds and energy attenuation on the propagation of atmospheric gravity waves is numerically analyzed. The gravity waves, both in the internal and ducted forms, are included through employing ray-tracing method and full-wave solution method. Background winds with different directions may cause ray paths of internal gravity waves to be horizontally prolonged, vertically steepened, reflected or critically coupled, all of which change the accumulation of energy attenuation along ray paths. Only the penetrating waves propagating against winds can easily reach the ionospheric height with less energy attenuation. The propagation status of gravity waves with different periods and phase speeds is classified into the cut-off region, the reflected region and the propagating region. All the three regions are influenced significantly by winds. The area of the reflected region reduces when gravity waves propagate in the same direction of winds and expands when propagating against wind. In propagating region, the horizontal attenuation distances of gravity waves increase and the arrival heights decrease when winds blow in the same direction of gravity waves, while the attenuation distances decrease and the arrival heights increase when gravity waves propagate against winds. The results for ducted gravity waves show that the influence of winds on waves of lower atmospheric modes is not noticeable for they propagate mainly under mesosphere, where the wind field is relatively weak. However, strong winds at thermospheric height lead to considerable changes of dispersion relation and attenuation distance of upper atmospheric modes. Winds against the wave propagating direction support long-distance propagation of G mode, while the attenuation distances decrease when winds blow in the same direction of the wave. The distribution of TIDs observed by HF Doppler array at Wuhan is compared with the simulation of internal gravity waves. The observation of TIDs shows agreement with our numerical calculations.  相似文献   

Preliminary results of long-term slip rates of 1999 earthquake fault by luminescence and radiocarbon dating     

《Quaternary Science Reviews》2003,22(10-13):1213-1221

For the purpose of deriving the long-term slip rates of different segments along the surface rupture of 1999 Chi-chi earthquake, radiocarbon and luminescence methods (TL and OSL) were applied to the tectonically influenced geomorphic features and deposits. We obtained six age estimates on pottery, driftwood and fluvial sediments. We found that the OSL signals of the samples collected from the coarse fluvial deposits had not been totally bleached due to rapid deposition. Based on the cumulative vertical offsets, the vertical slip rates were computed for each site. Our results reveal that the slip rate of the northern segment is relatively higher than that of the southern segment, which is consistent with the coseismic offsets that occurred in the 1999 earthquake.  相似文献   

Migration of seismic activity during the 1998 volcanic unrest at Iwate volcano,northeastern Japan,with reference to P and S wave velocity anomaly and crustal deformation     

《Journal of Volcanology and Geothermal Research》2002,113(3-4):399-414

We describe the seismicity at Iwate volcano, northeastern Japan, during the volcanic unrest of 1998 with reference to a three-dimensional P and S wave velocity model from tomographic analysis. The abnormal seismic activity beneath Iwate volcano started under the caldera in February, 1998 and migrated westward in the period February to August, 1998. Previous geodetic modeling [Sato and Hamaguchi, Chikyu Monthly 21 (1999) 312–317] suggested the growth of a dike in the time of the seismic activity. Comparing the seismicity and dike extension with the tomographic images of the P and S wave velocity structure, we find that the trace of the growing dike coincides with the region of the high V_p and high V_p/V_s ratio beneath the volcano. The seismic and geodetic data are consistent with an intrusion of magma or other fluid under the caldera in 1998. Another pressure source causing the predominant crustal deformation at Iwate volcano was detected from geodetic data, which was located in the region with high V_p/V_s ratio under the western end of the volcano through the period from February to August. It is suggested that the activation of the point pressure source probably associated with the inflation of a hot fluid reservoir relate to a geothermal region adjacent to the western edge of the volcano.  相似文献   

Quaternary phosphorites off the southeast coast of India     

《Chemical Geology》2002,182(2-4):483-502

Detailed petrological, mineralogical, geochemical and radiogenic (U, Sr, Nd) and stable isotope (C, O, S) studies have been carried out on the Quaternary phosphorites of the continental margin off Chennai, southeast coast of India. These phosphorites are formed as a result of trapping and binding of sediments by microbial mats and are similar to phosphate stratiform stromatolites. Detrital and biogenic constituents enclosed in the phosphorites controlled the major and minor element composition. Except for Sr and U, the concentrations of most of the trace metals are lower than those in average shale and phosphorite. Middle rare earth element (MREE)-enriched patterns are the characteristic feature. The U–Th dating method indicates that the ages of the phosphorites are beyond 300,000 years. ⁸⁷Sr/⁸⁶Sr ratios of the phosphorites are higher than that of present-day seawater and εNd values are more negative than those of seawater of the Atlantic Ocean. Carbon isotope ratios are within the range expected for the oxic/suboxic zone but sulfur isotope ratios indicate suboxic conditions during phosphorite formation. These results imply that the benthic microbial mats thrived on the shallow shelf during the Quaternary low sea level conditions. Periodic or episodic sedimentation onto the mats led to their death. The bacteria associated with decaying microbial mats utilised phosphorus supplied by continental sources and rapidly precipitated phosphate. The availability of a high percentage of phosphorus in seawater seems to be an important controlling factor for the formation of phosphate stromatolites. The composition of these phosphorites differs from the modern phosphorites in upwelling regions, but are similar to Cambrian apatite stromatolites. These phosphorites provide evidence that the replicates of ancient phosphate stromatolites do exist in the Quaternary.  相似文献