The age and nature of Mesozoic-Tertiary magmatism across the Indus Suture Zone in Kashmir and Ladakh (N. W. India and Pakistan)

We report the following new⁴⁰Ar/³⁹Ar ages: 130–150 and 90–100 Ma from monzodiorite and tremolite-actinolite schist of the Kohistan Complex; 44±0.5, 39.7±0.2 Ma from dikes cutting the Ladakh-Deosai Batholith Complex; 130–145 Ma from a diorite in the Shyok melange; and 7.8±0.1 Ma from a late stage monzogranite of the Kärakorum Batholith. A 261±13 Ma age from gneiss of the Karakorum Batholith is of uncertain significance. These dates, previously published ones which we summarize here, and some Sr isotope data suggest the following, (due to subduction switching between the Indian and Asian margins during closing of the Tethys ocean): Late Cretaceous emplacement of the Dras-Kohistan Cretaceous Island arc, followed by rapid cooling between abut 85 and 45 Ma. A quiet phase tectonically on the northern Indian plate during the Palaeocene to early Eocene, when subduction was occurring on the Asian margin. Further southward thrusting of the Indian continental margin associated with the development of an Andean-type arc (the Ladakh-Desosai Batholiths) on the northern Indian margin during the Eocene. An Oligocene Andean arc (the Karakorum Batholiths) on the Asian margin, followed by Miocene collision of the two continents and intrusion of ‘true’ granites derived from partial melting of continental crust.     

北美五大湖区晚更新世劳伦泰德冰盖演化年代学综述及石英光释光测年的初步尝试

北美五大湖区的安大略湖北岸Don Valley Brickyard、Scarborough Bluffs、Bowmanville Bluffs剖面共同构成了北美东北部最长也是最厚的陆地第四纪沉积记录, 较完整地记录了晚更新世劳伦泰德冰盖(the Laurentide Ice Sheet)的演化. 晚更新世劳伦泰德冰盖演化的重建有赖于这些经典剖面中重要沉积地层单元的准确年代学控制. 传统的地层年代学主要是依靠少量¹⁴C年代, 将主要的混杂堆积单元(diamicton)解释为气候变冷环境下的冰川扩张, 并与指示全球冰量变化的深海氧同位素曲线一一比对建立起来的. 这样建立起来的年代学存在很大的不确定性. 20世纪80-90年代的少量热释光年代也不相吻合, 最近的13个长石红外释光定年则只集中于Bowmanville Bluffs的一个分层, 并未建立整个剖面的地层年代学, 使这些经典沉积剖面的年代学一直没有得到系统的建立. 应用石英光释光SAR-SGC法测试了Bowmanville Bluffs剖面Glaciofluvial Sand单元的2个冰水沉积样品, 年代结果分别为(41.6±3.8) ka、(48.1±4.4) ka, 分析表明这一年代结果偏老, 石英颗粒可能晒褪不完全. 由于大测片无法识别晒褪不完全的颗粒, 因此, 测试更多的剖面序列的光释光年代并尝试采用粗颗粒小测片或单颗粒技术解决样品颗粒晒褪不完全的问题将是必要的.     

柴达木盆地北缘古生代超高压带中花岗质岩浆作用

祁连南缘古生代超高压变质带 (榴辉岩年龄为 4 6 6～ 4 95 Ma)上一套中高级变质岩系 (达肯大坂片麻岩 )中存在三类花岗岩组合 : 类 :石英二长闪长岩 -花岗闪长岩 -二长花岗岩 , 类 :二长花岗岩 -二云母花岗岩-含白云母花岗岩 -正长花岗岩 , 类 :花岗闪长岩 -二长花岗岩 -黑云母花岗岩。 类岩石组合中的二长花岗岩锆石SHRIMP年龄为 4 73Ma, 类岩石组合中的正长花岗岩锆石 SHRIMP年龄为 4 4 6 Ma, 类岩石组合中的二长花岗岩锆石 SHRIMP年龄为 397Ma。从三类花岗岩组合的组成矿物来看 , 类和 类的矿物组合主要为斜长石、角闪石、石英、碱性长石、黑云母 , 类的矿物组合为钾长石、石英、白云母、黑云母、斜长石 ;从岩石地球化学特征上看 , 类和 类花岗岩为 型花岗岩 ,岩石的 Si O2 =6 1%～ 6 9% ,Na2 O/ K2 O>1,ANK<1,δEu=0 .7～ 1.0 ; 类花岗岩为 S型花岗岩 ,岩石的 Si O2 =70 %～ 76 % ,Na2 O/ K2 O<1,ANK>1,δEu=0 .1～ 0 .3;从构造环境上看 , 类花岗岩形成于岛弧环境或活动大陆边缘 , 类花岗岩形成于同碰撞 , 类花岗岩形成于碰撞后。结合区域地质特征 ,我们认为 ,早奥陶世 ,南祁连洋板块向北俯冲于祁连陆块之下 ,规模不大的南祁连洋很快闭合 ,但俯冲下去的大洋板块仍拖动柴达木陆块继续俯冲 ,俯冲     

Temperature and humidity flux-variance relations determined by one-dimensional eddy correlation

It may be possible to estimate surface fluxes of scalar quantities from measurement of their variance and mean wind speed. The flux-variance relation for temperature and humidity was investigated over prairie and desert-shrub plant communities. Fluxes were measured by one-dimensional eddy correlation, humidity by fast-response wet-bulb psychrometers and Krypton open-path hygrometers, temperature by fine-wire thermocouples, and mean windspeed by a cup anemometer. The quality of the flux-variance relation proved to be good enough for application to flux measurement. Regressions of flux estimated by the variance technique versus measured flux usually had r ² values greater than 0.97 for sensible heat flux and greater than 0.88 for water vapor flux. More uniform surfaces tended to yield the same flux-variance relations except when fluxes were small. This exception supported the hypothesis that sparse sources of flux may increase variance downwind. Nonuniform surfaces yielded flux-variance relations that were less predictable, although reasonably accurate once determined. The flux-variance relation for humidity was quite variable over dry surfaces with senescent vegetation.     

Abundance and distribution of ichthyoplankton in the upwelling zone off Oregon during anomalous El Niño conditions

The abundance and distribution patterns of nearshore ichthyoplankton were investigated during a year of anomalously high sea temperatures off Oregon. Samples collected from 2 to 18 km offshore from April through September of 1983 showed increased occurrences and higher abundances of taxa usually found at distances offshore of 37 km in other years. The dominant species collected, comprising more than half of the total larval fish abundance, was the northern anchovy (Engraulis mordax). Larval anchovy have rarely been collected inshore in previous studies. Many of the dominant taxa normally found inshore, especially osmerids, were present in reduced numbers in 1983. Changes in the hydrographic conditions associated with onshore surface drift and reduced summer upwelling during the 1983 El Niño could explain the distributional patterns observed. The warm inshore waters apparently provided a substantial spatial and temporal expansion of the spawning habitat for E. mordax.     

Phase transitions in leucite,KAISi2O6

An order parameter treatment of the phase transitions in leucite, KAlSi₂O₆, at approximately 950 and 920 K: (cubic) I4₁ acd(tetragonal) I4₁ a(tetragonal) is presented in terms of Landau theory and induced representation theory. The Al-Si order with decreasing temperature is taken as the primary order parameter to which other distortions (K⁺ ion displacements, strain components, etc.) couple linearly. The expected Al-Si ordering behavior and the associated K⁺ ion displacements for both transitions are derived and the resulting twin domain orientations are listed. The sequence of phase transitions results from a coupling of ₃ ⁺ and ₄ ⁺ representations. The Landau free energy for the five-dimensional reducible representation has been simplified to two components resulting in a linearquadratic coupling of the components. Possible phase diagrams are derived by free energy minimization. The cubic tetragonal transition is first-order, whereas the tetragonal-tetragonal transition may be second order. A tricritical point exists at which the first-order transition changes to second-order.     

Frequency domain electromagnetic and ground penetrating radar investigation of ephemeral streams: case study near the Southern High Plains,Texas

A frequency domain electromagnetic (FDEM) and ground penetrating radar (GPR) study was conducted on an ephemeral stream in north-central Texas to determine if FDEM and GPR measurements can be combined to determine the electrical characteristics of current and ancient stream channels. GPR data were collected at several frequencies to image sedimentary structures of different scale lengths, and to determine the formation porosity and water content of stream sediments. FDEM measurements were collected using Geonics EM31 and EM34 loop–loop instruments on a profile along the current stream channel and five profiles perpendicular to the channel. The results indicate that the greater spatial resolution of the EM31 mapped the current and possible ancient channels better than the EM34, however, the EM34 provided depth information on the formations underlying the channel sediments that the EM31 could not image. GPR measurements taken along a point bar deposit with 200, 100 and 25 MHz antennae indicated that the higher frequency antenna better resolved channel structures including laminar bedding, trough scours and cross-bedding, however, lower frequency antenna (25 MHz) imaged sedimentary structures within the underlying channel sediments. Common midpoint GPR measurements collected along the point bar deposit were used to estimate the sediment formation porosity (26%) and the water content, during a dry period, of the unsaturated (12%) and saturated (26 or 100% of the pore space) sediments. The combined results indicated that the FDEM data should be collected first because of the speed and ease of measurements. The FDEM data (especially the EM31) pointed to the locations of possible sedimentary structures, which can then be resolved by using different frequency GPR measurements.     

Headless submarine canyons and fluid flow on the toe of the Cascadia accretionary complex

Headless submarine canyons with steep headwalls and shallowly sloping floors occur on both the second and third landward vergent anticlines on the toe of the Cascadia accretionary complex off central Oregon (45 °N, 125° 30′W). In September 1993, we carried out a series of nine deep tow camera sled runs and nine ALVIN dives to examine the relationship between fluid venting, structure and canyon formation. We studied four canyons on the second and third landward vergent anticlines, as well as the apparently unfailed intercanyon regions along strike. All evidence of fluid expulsion is associated with the canyons; we found no evidence of fluid flow between canyons. Even though all fluid seeps are related to canyons, we did not find seeps in all canyons, and the location of the seeps within the canyons differed. On the landward facing limb of the second landward vergent anticline a robust cold seep community occurs at the canyon’s inflection point. This seep is characterized by chemosynthetic vent clams, tube worms and extensive authigenic carbonate. Fluids for this seep may utilize high-permeability flow paths either parallel to bedding within the second thrust ridge or along the underlying thrust fault before leaking into the overriding section. Two seaward facing canyons on the third anticlinal ridge have vent clam communities near the canyon mouths at approximately the intersection between the anticlinal ridge and the adjacent forearc basin. No seeps were found along strike at the intersection of the slope basin and anticlinal ridge. We infer that the lack of seepage along strike and the presence of seeps in canyons may be related to fluid flow below the forearc basin/slope unconformity (overpressured by the impinging thrust fault to the west?) directed toward canyons at the surface.