Numerical modelling of the impact crater depth-diameter dependence in an acoustically fluidized target

2D numerical modelling of impact cratering has been utilized to quantify an important depth-diameter relationship for different crater morphologies, simple and complex. It is generally accepted that the final crater shape is the result of a gravity-driven collapse of the transient crater, which is formed immediately after the impact. Numerical models allow a quantification of the formation of simple craters, which are bowl-shaped depressions with a lens of rock debris inside, and complex craters, which are characterized by a structural uplift. The computation of the cratering process starts with the first contact of the impactor and the planetary surface and ends with the morphology of the final crater. Using different rheological models for the sub-crater rocks, we quantify the influence on crater mechanics. To explain the formation of complex craters in accordance to the threshold diameter between simple and complex craters, we utilize the Acoustic Fluidization model. We carried out a series of simulations over a broad parameter range with the goal to fit the observed depth/diameter relationships as well as the observed threshold diameters on the Moon, Earth and Venus.     

Overburden deformation associated with halokinesis in the Southern North Sea: implications for the origin of the Silverpit Crater

Gravitational collapse in response to the creation of salt swells and diapirs is a common phenomenon within regions such as the Southern North Sea. Although collapse faulting and slumping of linear salt swell flanks has previously been documented, the existence of concentric collapse structures surrounding diapirs and isolated salt lows is a recently recognised feature. 3-D seismic data from the Southern North Sea demonstrates that concentric collapse faulting around both diapirs and subcircular salt lows is common. The recognition of concentric collapse structures formed in response to salt withdrawal, and adjacent inflation, provides an alternative mechanism for the origin of the proposed Silverpit impact crater.     

Sulfur speciation with high performance liquid chromatography as a tool for El Chichón volcano,crater lake monitoring

The monitoring of sulfur species in crater lakes has proven to be useful for forecasting episodes of volcanic unrest in certain active volcanoes, including Poás, Costa Rica; Kusatsu-Shirane, Japan; and Mt. Ruapehu, New Zealand. In this study, we have improved the current geochemical monitoring of El Chichón volcano through the setting of optimal high-performance liquid chromatography conditions (HPLC) for the analysis of S²⁻, SO₃²⁻, S₂O₃²⁻, S₄O₆²⁻ and SO₄²⁻ using a common chromatographic system. The procedure was applied to the analysis of lake samples taken in March, July and October of 2014 and April of 2015. The results were promising, since nearly all species were detected (with the exception of S₂O₃²⁻) in measurable amounts, including S²⁻ (<0.85–5.05 mg/L), SO₃²⁻ (<2.77–26.1 mg/L), S₄O₆²⁻ (108.27–303.82 mg/L) and SO₄²⁻ (489.58–676.26 mg/L). The spatial distribution of these species along the lakeshore showed zones of increased concentrations to the east and southeast of the lake, which provides information on the distribution of faults or cracks that feed hydrothermal fluids to the lake. This method thus provides additional information linked to the volcanic and hydrothermal activity of the volcano.     

月球典型撞击坑溅射物研究及对月球地质编图的意义

撞击坑是月表最典型的地质单元,其溅射物作为撞击坑的坑外组成部分可分布到距离坑中心10个直径距离之外的区域,因此撞击溅射物也是月球地质编图中最重要的表达要素之一。本文使用月球勘测轨道器(LRO)的激光高度计(LOLA)数据、广角相机(WAC)影像、窄角相机(NAC)影像以及Clementine的UVVIS多光谱数据,研究了哥白尼纪正面月海区直径31km的Kepler撞击坑和背面月陆区直径30km的Necho撞击坑。哥白尼纪撞击坑溅射沉积物可以分为三个相:连续溅射沉积相(CE)、不连续溅射沉积相(DE)和辐射纹(CR)。连续溅射沉积相分布在最大约2.6个半径范围之内,不连续溅射沉积相分布在最大近11个半径范围之内,辐射纹分布在最大近29个半径范围之内。本文强调了多源数据结合在识别撞击坑溅射沉积物中的作用,对Kepler坑和Necho坑溅射沉积物进行了填图,不对称分布的特征表明这两个坑可能形成于倾斜撞击。     

Wrench faults,pull-apart basins,and volcanism in central Oregon: A new tectonic model based on image interpretation

A tectonic study of the Newberry Crater region of central Oregon has been based on the interpretation of Landsat Thematic Mapper imagery. Two major faults, the Brothers-Tumalo and Eugene-Denio Faults, pass NW-SE through the region and step to the right at the eastern margin of the Cascades Range. Dextral wrench faulting on these structures during the Tertiary controlled the formation of the La Pine Basin, a pull-apart structure containing Tertiary and Quaternary sediments and volcanics. Tertiary wrench faulting appears to have been associated with rotations of crustal blocks at a plate margin, but was superseded in the Quaternary by extensional faulting of the Basin and Range province. Newberry Crater and other major bimodal volcanic centres in the NW Cordillera (Crater Lake, Medicine Lake, Mt. St. Helens) seem to have a similar tectonic setting in crustal pull-aparts. A relationship between magma type and fault trend at Newberry and Medicine Lake is suggested.     

Development and sensitivity analysis of a model for assessing stratification and safety of Lake Nyos during artificial degassing

To prevent the recurrence of a disastrous eruption of carbon dioxide (CO₂) from Lake Nyos, a degassing plan has been set up for the lake. Since there are concerns that the degassing of the lake may reduce the stability of the density stratification, there is an urgent need for a simulation tool to predict the evolution of the lake stratification in different scenarios. This paper describes the development of a numerical model to predict the CO₂ and dissolved solids concentrations, and the temperature structure as well as the stability of the water column of Lake Nyos. The model is tested with profiles of CO₂ concentrations and temperature taken in the years 1986 to 1996. It reproduces well the general mixing patterns observed in the lake. However, the intensity of the mixing tends to be overestimated in the epilimnion and underestimated in the monimolimnion. The overestimation of the mixing depth in the epilimnion is caused either by the parameterization of the k-epsilon model, or by the uncertainty in the calculation of the surface heat fluxes. The simulated mixing depth is highly sensitive to the surface heat fluxes, and errors in the mixing depth propagate from one year to the following. A precise simulation of the mixolimnion deepening therefore requires high accuracy in the meteorological forcing and the parameterization of the heat fluxes. Neither the meteorological data nor the formulae for the calculation of the heat fluxes are available with the necessary precision. Consequently, it will be indispensable to consider different forcing scenarios in the safety analysis in order to obtain robust boundary conditions for safe degassing. The input of temperature and CO₂ to the lake bottom can be adequately simulated for the years 1986 to 1996 with a constant sublacustrine source of 18 l s^–1 with a CO₂ concentration of 0.395 mol l^–1 and a temperature of 26 °C. The results of this study indicate that the model needs to be calibrated with more detailed field data before using it for its final purpose: the prediction of the stability and the safety of Lake Nyos during the degassing process.Responsible Editor: Hans Burchard     

Chemostratigraphy of Lago Albano sediments (Central Italy):geochemical evidence of palaeoenvironmental changes in lateQuaternary

The results of the geochemical investigation on bulk sediment fromthree cores collected in Albano crater lake (Central Italy) are reported, andthe main markers of the palaeoenvironmental changes in the last 26ka are discussed. The sediment of Lago Albano consists of greysilt and mud, and is made of volcanogenic, calcareous, siliceous and organicmaterial. Some tephra layers provide a chronological framework for thesequence. The chemical features of the sediments are strongly impacted by theAlban Hills volcanism (Roman volcanic area), but there are also strong changesof organic/inorganic matter ratios, owing to variations in biologicalproductivity and terrigeous supply from the catchment. Six chemostratigraphiczones record the main steps of the transition from the cold and dry climate ofGlacial Maximum to the warmer and more humid Holocene climate. The most usefulgeochemical indices are: biogenic silica, CO₂, Br and organiccontent (OM) for biological productivity; Al, Y, CIA (Chemical Index ofAlteration), Al/Rb, Ti/Zr and Y/Al ratios for terrigenous clasticmaterial; OM/Al ratio for organic/terrigenous ratio; S/Fe andMo/Fe ratios and Authigenic U for redox diagenetic conditions of the lakesediment. The geochemical records agree rather well with those of thelithological and paleomagnetic studies, and despite the information obtainedare less detailed than those acquired by the study of biological remains, thepalaeoenvironmental significance inferred is consistent. A comparison of thechemostratigraphic results of Lago Albano with those obtained on coevalsediments from Central and Southern Italy lakes supports the role of thegeochemical investigation as important complement to more sophisticatedtechniques in the palaeoenvironmental reconstructions.     

High precision locations of LP events on Mt. Etna: Reconstruction of the fluid-filled volume

During 1991–93 at Mount Etna, long-period (LP) events occurring in swarms characterized the evolution of the eruption. The presence of multiplets i.e. groups of events with similar waveform signatures, has been recognized within this activity. Traditional techniques for locating LP events do not allow obtaining reliable hypocenters, which have only succeeded in placing earthquakes in a roughly 1 km² area slightly east of the Mt. Etna Northeast Crater. Hypocenters have been relocated in two steps: the absolute location has been improved using Thurber’s code and a complex 3D velocity model; a highly precise relative location has been applied on multiplets to define the source geometry. 3D locations and high precision analysis suggest that during the 1991–93 eruption the resonator producing LP events was a part of the uppermost Northeast Crater conduit, measuring 210 meters in height and 45–50 meters in diameter.     

12,000-Year, preliminary results of the stable nitrogen and carbon isotope record from the Empakai Crater lake sediments, Northern Tanzania

The stable isotope compositions of organic carbon and nitrogen, the contents of organic carbon and nitrogen and C/N ratios for two cores recovered from the Empakai Crater at water depths of 11 and 20 m are used to document climatic changes in northern Tanzania. Eight ¹⁴C AMS dates determined on total organic matter (OM) indicate that the sedimentation rate in this lake is about 30 cm/ka for the late Pleistocene to early Holocene period. There are differences in the δ¹³C values of organic carbon between the two cores, which may be a result of differences in location from the present shoreline and of different water depths. In the deeper-water core the δ¹³C values show a general downcore decrease to the base of the core with a sharp change to lower values of about 4‰ at a depth of 100 cm (8.7 ka). The general trend of downcore decrease in ¹³C values can be attributed either to a systematic decrease in the relative proportion of C₄ type of OM, owing to an increase in precipitation and change in vegetation cover from grassland to forest, or to utilization of isotopically enriched carbon during photosynthesis. The δ¹⁵N values show a general downcore increase with again a sharp change of about 5‰ to lower values at about 8.7 ka. A sharp change of about 5‰ and 4‰ to more depleted values at a depth of 100 cm of both ¹⁵N and ¹³C, respectively, suggests either hiatus or abrupt change in climatic condition from wetter conditions to drier conditions. There is enhanced preservation of OM in the lake as depicted by high mean values of organic carbon and nitrogen at both sites.     

月球辐射状撞击坑经度方向分布特征与同步旋转轨道状态模拟分析

月球目前的同步旋转轨道状态使得形成的撞击坑分布满足一定的不对称性.本文利用最新的LRO影像和地形数据,结合早期的Clementine影响数据,分析了月球辐射状撞击坑经度方向成坑率分布,结果表明所识别的辐射状撞击坑的.年龄为O.9 Ga内,西一东半球方向存在明显不对称性,比值约为1.35~1.53;同时利用辐射状撞击坑模拟了0.9 Ga内的同步旋转轨道的平均状态,得到该时间内月球的向点-背点为70°W—110°E附近;最后利用撞击坑数据库资料对月球形成以来各地质世纪时间尺度内的撞击坑进行了成坑率分布分析,结论表明月球在大爆炸中后期间可能处于过近似的同步轨道旋转状态,但是与现在的轨道运行状态相反,月球在之后的地质时期内经历了翻转,其诱因可能是雨海和东方海盆地遭受的撞击.