Constraints on the late Quaternary glaciations in Tibet from cosmogenic exposure ages of moraine surfaces

This contribution provides new constraints on the timing of Tibetan glacial recessions recorded by the abandonment of moraines. We present cosmogenic radionuclide ¹⁰Be inventories at 17 sites in southern and western Tibet (32 crests, 249 samples) and infer the range of permissible emplacement ages based on these analyses. Individual large embedded rock and boulder samples were collected from the crests of moraine surfaces and analyzed for ¹⁰Be abundance. We consider two scenarios to interpret the age of glacial recession leading to the moraine surface formation from these sample exposure ages: 1) Erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age; and 2) Erosion progressively exposes large boulders with little prior exposure, and so the oldest sample age records the minimum moraine emplacement age. We found that depending on the scenario chosen, the moraine emplacement age can vary by > 50% for ～100 ka-old samples. We consider two scaling models for estimating the production rates of ¹⁰Be in Tibet, which has an important, although lesser, effect on inferred moraine ages. While the data presented herein effectively increase the database of sample exposure ages from Tibet by ～20%, we find that uncertainties related to the interpretation of the ¹⁰Be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies. A reanalysis of published data from moraines throughout Tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the Tibetan moraine age database. Thus, when viewed in a similar way with other samples collected from this area, uncertainties within moraine exposure ages obscure attribution of Tibetan glacial advances to temperature minima or precipitation maxima. Our work suggests that more reliable chronologies of Tibetan glaciations will come from improvements in production rate models for this portion of the world, as well as a better understanding of the processes that form and modify these geomorphic surfaces.     

Co-seismic and cumulative offsets of the recent earthquakes along the Karakax left-lateral strike-slip fault in western Tibet

The 400 km-long Karakax left-lateral strike-slip fault is the westernmost segment of the Altyn Tagh fault. It separates northwestern Tibet to the south from the Tarim basin to the north. The western section of the Karakax fault exhibits clear co-seismic surface ruptures of past large earthquakes. Geomorphic offset measurements from the field and high-resolution Ikonos images along 1.5 km across the Sanshiliyingfang fan and along 55 km of the fault, range from 3 to 28 m, with distinct clusters at 6 ± 2(3), 14 ± 2, 19 ± 2 and 24 ± 3 m. The cluster of the smallest offsets around 6 m (full range from 3 to 10 m) distributed over a minimum length of 55 km, is attributed to the last largest surface rupturing event that testifies of the occurrence of a magnitude Mw 7.4-7.6 earthquake along the Karakax fault. We interpret the other offset clusters as the possible repetition of similarly sized events thus favoring a characteristic slip model for the Karakax fault. In a 3 m-deep trench dug across the active trace of the fault we can identify the main rupture strands of the last and penultimate events. The penultimate event horizon, a silty-sand layer, has been radiocarbon dated at 975-1020 AD (AMS ¹⁴C age). It is proposed that large Mw 7.4-7.6 events with co-seismic slip of about 6 m rupture the Karakax fault with a return time of about 900 years implying an average slip-rate of about 6-7 mm/years during the late Holocene. These results suggest that the Karakax fault is the largest left-lateral strike-slip fault at the rim of northwestern Tibet accommodating eastward movement of Tibet due to the India-Eurasia collision.     

Large earthquake‐triggered liquefaction mounds and a carbonate sand volcano in the Mesoproterozoic Wumishan Formation,Beijing, North China

Ten well‐preserved, earthquake‐triggered liquefaction mounds and a carbonate sand volcano have been found in the Mesoproterozoic Wumishan Formation (1550–1400 Ma) in the Beijing area, North China. These features crop out in a roadcut near Zhuanghuwa Village. All ten mounds occur in the same sedimentary layer and have rounded shapes with some concentric and radial fissures arising from the centre. They range from 1.5 to 4 m in diameter and from 10 cm to 30 cm in height. The carbonate sand volcano has a diameter of 110 cm and the ‘crater’ at the top has a depth of about 30 cm. Associated with these mounds and the sand volcano are many ‘normal’ sedimentary structures and numerous soft‐sediment deformation structures. The former include ripple marks, cross‐bedding, stromatolites and desiccation cracks, indicating deposition in a stable shallow‐water peritidal platform environment. The latter include intrastratal faults and folds, seismically formed breccias and carbonate clastic dykes. The morphological features and the genesis of these liquefaction mounds are very similar to mounds formed recently by the great Wenchuan Earthquake of China (2008). Detailed thin‐section study of the mounds found no signs of any kind of biological constructional process; instead it reveals some obvious fluidification and liquefaction characteristics. Comparative studies have shown that these features are probably the products of Mesoproterozoic earthquake activity. Copyright © 2013 John Wiley & Sons, Ltd.     

Rock Damage Structure of the South Longmen-Shan Fault in the 2008 M8 Wenchuan Earthquake Viewed with Fault-Zone Trapped Waves and Scientific Drilling

This article is to review results from scientific drilling and fault-zone trapped waves(FZTWs) at the south Longman-Shan fault(LSF) zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan, China. Immediately after the mainshock, two Wenchuan Fault Scientific Drilling(WFSD) boreholes were drilled at WFSD-1 and WFSD-2 sites approximately 400 m and 1 km west of the surface rupture along the Yinxiu-Beichuan fault(YBF), the middle fault strand of the south LSF zone. Two boreholes met the principal slip of Wenchuan earthquake along the YBF at depths of 589-m and 1230-m, respectively. The slip is accompanied with a 100-200-m-wide zone consisting of fault gouge, breccia, cataclasite and fractures. Close to WFSD-1 site, the nearly-vertical slip of ~4.3-m with a 190-m wide zone of highly fractured rocks restricted to the hanging wall of the YBF was found at the ground surface after the Wenchuan earthquake. A dense linear seismic array was deployed across the surface rupture at this venue to record FZTWs generated by aftershocks. Observations and 3-D finite-difference simulations of FZTWs recorded at this cross-fault array and network stations close to the YBF show a distinct low-velocity zone composed by severely damaged rocks along the south LSF at seismogenic depths. The zone is several hundred meters wide along the principal slip, within which seismic velocities are reduced by ~30–55% from wall-rock velocities and with the maximum velocity reduction in the ~200-m-wide rupture core zone at shallow depth. The FZTW-inferred geometry and physical properties of the south LSF rupture zone at shallow depth are in general consistent with the results from petrological and structural analyses of cores and well log at WFSD boreholes. We interpret this remarkable low-velocity zone as being a break-down zone during dynamic rupture in the 2008 M8 earthquake. We examined the FZTWS generated by similar earthquakes before and after the 2008 mainshock and observed that seismic velocities within fault core zone was reduced by ~10% due to severe damage of fault rocks during the M8 mainshock. Scientific drilling and locations of aftershocks generating prominent FZTWs also indicate rupture bifurcation along the YBF and the Anxian-Guangxian fault(AGF), two strands of the south LSF at shallow depth. A combination of seismic, petrologic and geologic study at the south LSF leads to further understand the relationship between the fault-zone structure and rupture dynamics, and the amplification of ground shaking strength along the low-velocity fault zone due to its waveguide effect.     

基于地理空间分析和AHP的线路通道滑坡风险性评价

输电线路常需要跨越地质灾害易发区域,为了保证电力的正常、安全输送,对输电线路通道所经区域进行滑坡地质灾害的空间分布及潜在风险评价具有重要意义。本文结合GIS和AHP方法进行了某输电线通道滑坡灾害危险性评价,选取地形地貌、气候条件、水文条件、人为活动条件、植被条件、地质岩组条件等影响因子,建立评价指标体系,利用层次分析法(AHP)确定各因子权重,基于GIS地理空间分析评价了输电线路通道地质灾害危险性,划分了5类区域:极高危险区、高危险区、中危险区、低危险区、较低危险区。危险性评价结果可为输电线路电力设施通道滑坡灾害防治、安全设计和施工提供科学依据。     

基于雷电监测数据的雷电流幅值累积概率分布特征

利用江西省2006—2015年闪电定位系统监测资料,通过数理统计、回归分析等方法,分析江西省雷电流幅值概率以及雷电流幅值累积概率分布特征。结果表明:基于雷电监测数据获得的雷电流幅值累积概率分布拟合公式,一定程度上反映了江西省雷电流幅值累积概率变化特征。正、负极性雷电流幅值平均概率分布特征均表现出明显的堆积效应,且正极性的集中中心小于负极性;同时,正极性闪电相对负极性闪电出现大幅值的概率更高。负闪电实测曲线与拟合曲线完全一致,总闪电实测曲线与拟合曲线基本一致,拟合效果较负闪电稍差,较正闪电好。推导出的江西省正、负闪电和总闪电的雷电流幅值累积概率分布公式,对解决电力系统中由于雷电流幅值造成的雷击输电线路故障并采取有效的解决方案具有一定参考价值。     

The shoshonitic volcanic rocks at Hongliuxia: Pulses of the Altyn Tagh fault in Cretaceous?

Two periods of magma activities, i.e. 106–112 Ma shoshonitic eruption and 82 Ma trachytic intrusion, are recognized at the intersection of the Altyn Tagh fault and the northern marginal fault of the Qilian terrane, northeast of the Tibetan Plateau, which were previously considered to be Quaternary magma activities. The two rock-types have significant differences in mineral assemblage and chemical composition, and formed during two different pulses of magma activity. The Cretaceous magma activities and deformation in or near the Altyn Tagh fault zone suggest a possibly tectonic reactivation after the intense activity of the fault during Indusinian.

     

基于小型无人艇的海洋磁力测量精度评价

为了验证基于小型无人艇进行海上磁力测量的可行性,研制了船载海洋磁力测量系统,将其搭载于小型无人艇上,进行了海上磁力测量试验,在海上测量中获得了重复测线和交叉测线数据;对海上测量数据进行了处理,并按照相关规范和方法计算了重复线和交叉点的内符合精度,计算结果显示两条重复线的内符合精度分别达到1.06 nT和1.71 nT,...     

MODIS 1B 影像条带检测与去除

MODIS 1B 影像中存在横向和纵向条带, 基于均衡化曲线补偿技术, 提出了一种快速检测和去除条带的 算法, 给出了条带检测判定参数, 即: 如果A帧和B 帧的10 组传感器校正系数Ci值分别位于0 的两侧, 且ABS(Ci)> 0.008, 则存在宽条带; 如果10 组传感器的校正系数MAX(Ci)>0.05 时, 则存在单行条带。去除宽条带时, 需以当前 列为首列, 从其后连续的100 列影像中提取各传感器相对当前列的校正系数。结果表明, 条带检测模型能有效识别 单行条带和宽条带, 多行条带需目视识