Erosion in northwest Tibet from in‐situ‐produced cosmogenic 10Be and 26Al in bedrock

Concentrations of in‐situ‐produced cosmogenic nuclides ¹⁰Be and ²⁶Al in quartz were measured by accelerator mass spectrometry for bedrock basalts and sandstones located in northwest Tibet. The effective exposure ages range between 23 and 134 ka (¹⁰Be) and erosion rates between 4·0 and 24 mm ka^?1. The erosion rates are significantly higher than those in similarly arid Antarctica and Australia, ranging between 0·1 and 1 mm ka^?1, suggesting that precipitation is not the major control of erosion of landforms. Comparison of erosion rates in arid regions with contrasting tectonic activities suggests that tectonic activity plays a more important role in controlling long‐term erosion rates. The obtained erosion rates are, however, significantly lower than the denudation rate of 3000–6000 mm ka^?1 beginning at c. 5‐3 Ma in the nearby Godwin Austen (K2) determined by apatite fission‐track thermochronology. It appears that the difference in erosion rates within different time intervals is indicative of increased tectonic activity at c. 5–3 Ma in northwest Tibet. We explain the low erosion rates determined in this study as reflecting reduced tectonic activity in the last million years. A model of localized thinning of the mantle beneath northwest Tibet may account for the sudden increased tectonic activity at c. 5–3 Ma and the later decrease. Copyright © 2006 John Wiley & Sons, Ltd.     

北京地区暖季对流天气的气候特征

对北京地区最近12年暖季(5—9月)雷暴、冰雹、暴雨和大风等各种对流天气进行了气候统计和分析。统计结果表明:北京地区暖季发生对流的概率很高,按日数统计的气候概率达47.77%,大风、暴雨和冰雹气候概率分别为27.29%、10.84%和6.29%。暴雨多发季节为7月中旬到8月上旬。冰雹集中于6月中、下旬。在对流天气的地理分布上,北京西北部、东北部山区及西南部山区多对流天气,中心区和东南部平原地区对流天气较少。暴雨呈西南—东北方向带状分布,东北部山区、中部和东南部平原地区多发生暴雨,而西北部和西南部山区很少发生暴雨。山区冰雹明显多于平原。西北部和东北部山区大风偏多。暴雨有明显的夜发性。冰雹集中发生在午后到傍晚,占冰雹总站次的76.72%。     

Compositions of heavy minerals in Northeastern China sandlands and provenance analysis

Sand samples deposited since the Last Glacial Maximum (LGM) from the Hulun Buir, Horqin and Otin-dag sandlands were measured for their assemblages of heavy minerals and chemical compositions of detrital garnets and tourmalines. Heavy mineral assemblages of these three sandlands consist mainly of garnet, ilmenite, epidote and minor amphibole and magnetite. Garnets consist mainly of high Mg type-A ones (58% on average), and minor type-B ones. Toumalines are mainly composed of Mg-rich and minor Fe-rich ones. Compared with those of central-southern Mongolia and central Tarim, these three sandlands are derived from the Phanerozoic rocks of the Central Asian Orogenic Belt and the Archean and Paleoprotorozoic basement rocks of the North China Craton. Our results provide direct evidence that the present-day sands may be reworked from LGM ones. Differences of heavy mineral composi-tions occur between the samples of the three sandlands and Taklimakan Desert, indicating that the material of the western arid regions contributes little to the eastern sandlands.     

Occurrence of a White-Light Flare Over a Sunspot and the Associated Change of Magnetic Flux

A white-light flare (WLF) on 10 March 2001 was well observed in the Hα line and the Ca ii λ8542 line using the imaging spectrograph installed on the Solar Tower Telescope of Nanjing University. Three small sunspots appeared in the infrared continuum image. In one sunspot, the infrared continuum is enhanced by 4–6% compared to the preflare value, making the sunspot almost disappear in the continuum image for about 3 min. A hard X-ray (HXR) source appeared near the sunspot, the flux of which showed a good time correlation with the profile of the continuum emission. In the sunspot region, both positive and negative magnetic flux suffered a substantial change. We propose that electron precipitation followed by radiative back-warming may play the chief role in heating the sunspot. The temperature rise in the lower atmosphere and the corresponding energy requirement are estimated. The results show that the energy released in a typical WLF is sufficient to power the sunspot heating.     

U-Pb dating of zircon fromthe bed parallel anatectic granitic intrusion in the Baoban group in Hainan Island and the tectonic implication

1 Geological setting Hainan Island is situated in the conjunction region between the Euro-Asian plate, the Indian-Australian plate and the Pacific plate, its tectonic setting and evolution is implicated in understanding the continen-tal margin accretion and evolution of East Asia and the formation of the South China sea. The Jiusuo-Lingshui fault zone divides Hainan Island into the Yaxian Pa-leozoic massif in the south and the Qiongzhong Pa-leozoic massif in the north (Fig. 1), they con…     

龙门山断裂带的性质与活动性研究

沿龙门山构造带两侧，在9个100～500m深的钻孔中进行了水压致裂地应力测量，获得了断裂带两侧地表浅部构造应力的大小、方向和分布特征。据这些实测的应力资料，用库仑摩擦滑动准则分段研究了断裂带的性质和活动性。结果表明：试验得到的摩擦强度(0．6～1．0)似乎适用于浅部断层构造，地壳浅部地应力的大小和方向与断裂的性质及活动性密切相关，应力状态与断裂带的逆冲性质相一致；在断裂带东段的广元地区．中段的平武、汶川和茂县地区，实测的最大水平主应力值达不到逆断层活动要求的临界状态；而西段与鲜水河断裂带、南北向构造带复合部位的二郎山地区，现今构造应力状态比较复杂．实测的最大水平主应力值满足逆断层活动要求的临界状态。     

弱磁化等温薄吸积盘的轴对称脉动不稳定性

本文在不考虑粘滞律的条件下，较详细地讨论了弱磁化等温薄吸积盘的轴对称脉动不稳定性．结果表明，对轴对称扰动，盘内存在４种轴对称振荡模式．仅当Ｂｚ和Ｂφ同时存在时磁场才能影响这些振荡模式的不稳定性．Ｂｚ和Ｂφ对其中２种角频率较高的振荡模式（对应于ωＩ）表现为非稳因素，而对其中２种角频率较低的振荡模式（对应于ωＩ）表现为致稳因素．     

青藏高原地震波三维速度结构的研究

根据青藏高原及其邻区的模拟地震台站和宽频带数字地震记录资料,采用区域体波层析成你和瑞利面波层析成像,反演得到了青藏高原地区的三维地震波速度结构。两种层析成像方法得到的地壳上地幔P波和S波速度结构的结果非常相似,它们显示,青藏高原南部的拉萨块体的上地壳存在明显的低速区,青藏高原北部的羌塘地区的下地壳和上地幔顶部相对速度较低。这些结果与青藏高原南、北部处于碰撞过程中的不同阶段有关。     

1998年南海季风爆发时期中尺度对流系统 的研究：Ⅱ中尺度对流系统对大尺度场的作用

通过对1998年南海季风爆发过程中大尺度风场、温度场、厚度场、地面气压场以及视热源与视水汽汇的演变分析研究了对流活动对大尺度场的作用,结果表明:大尺度环流与中尺度对流活动之间可能存在着一种正反馈机制。在季风爆发早期,大尺度背景与中尺度对流活动的关系主要表现为前者为季风爆发以及中尺度对流活动的发生提供有利的天气和动力条件;季风爆发后期持续的大范围中尺度对流活动反过来会对大尺度环流存在明显的反馈作用。由对流活动强烈发展产生的凝结潜热释放在南海北部造成了显著的大气加热,使对流层中上层出现一明显的加热中心,这导致:(1)南海上空经向温度梯度由高层向低层发生反向,形成北高南低的温度梯度,从而使大尺度环流发生季节性改变;(2)相应南海北部地面气压不断加深,形成宽广的季风槽和明显的减压区,促使副热带高压从南海地区最后撤离;(3)随着中低层低压环流的不断发展,对流系统和降水区进一步加强并向南扩展,有利于南海季风在南海中、南部地区爆发和维持;(4)季风槽的加深使其南侧的季风气流与水汽输送进一步加强,促使季风爆发过程达到盛期。