The petrogenesis of 30–20 Ma basic and intermediate volcanics from the Mogollon-Datil Volcanic Field,New Mexico,USA

In the western USA calcalkaline magmas were generated hundreds of kilometres from the nearest destructive plate margin, and in some areas during regional extension several Ma after the cessation of subduction. The Mogollon-Datil Volcanic Field (MDVF) in southern New Mexico was a centre of active magmatism in the mid- to late-Tertiary, and a detailed field, petrographic and geochemical study has been undertaken to evaluate the relations between extensional tectonics and calcalkaline magmatism in the period 30–20 Ma. The rocks comprise alkalic to high-K calcalkaline lavas, ranging from basalt to high silica andesitc. Most of the basaltic rocks have relatively low HFSE abundances, elevated ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd, similar to many Tertiary basalts across the western USA, and they are inferred to have been derived from the continental mantle lithosphere. Two differentiation trends are recognised, with the older magmas having evolved to more calcalkaline compositions by magma mixing between alkalic basaltic andesites and silicic crustal melts, and the younger rocks having undergone 30–40% fractional crystallisation to more alkalic derivatives. The younger basalts also exhibit a shift to relatively higher HSFE abundances, with lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd, and these have been modelled as mixtures between an average post-5 Ma Basin and Range basalt and the older MDVF lithosphere-derived basalts. It is argued that the presence of subduction-related geochemical signatures and the development of calcalkaline andesites in the 30–20 Ma lavas from the MDVF are not related to the magmatic effects of Tertiary subduction. Rather, basic magmas were generated by partial melting of the lithospheric mantle which had been modified during a previous subduction event. Since these basalts were generated at the time of maximum extension in the upper crust it is inferred that magma generation was in response to lithospheric extension. The association of the 30–20 Ma calcalkaline andesites with the apparently anorogenic tectonism of late mid-Tertiary extension, is the result of crustal contamination, in that fractionated, mildly alkaline, basaltic andesite magmas were mixed with silicic crustal melts, generating hybrid andesite lavas with calcalkaline affinities.     

Modelling the effect of vegetation on channel pattern in bedload rivers

We modify a simple numerical stream‐pattern model to examine the effect of sediment stabilization by roots on the channel pattern of bedload rivers. In the model, vegetation enhances bank resistance to erosion, causing the development of a single channel instead of a rapidly changing, multiple channel (braided) pattern. Net aggradation resulting from a high sediment supply, however, causes frequent avulsions that destroy vegetation locally, leading to the development of a multiple‐channel pattern. A stability diagram representing multiple model runs predicts whether a river will exhibit single or multiple channels, based on plant‐enhanced bank strength, and on the time scale of plant development relative to a time scale for change in unvegetated channels. A second stability diagram predicts the way in which the amplitude and period of a fluctuating imposed sediment load influence whether a single or multiple‐channel pattern develops. Copyright © 2003 John Wiley & Sons, Ltd.     

新疆叶城晚新生代山前盆地演化与青藏高原北缘的隆升--Ⅱ沉积相与沉积盆地演化

叶城盆地属于塔里木盆地的西南坳陷 ,在晚新生代沉积了巨厚的磨拉石建造。盆地的演化具有阶段性 ,反映了西昆仑山不断的隆升。中新世 ,盆地的沉积环境为曲流河和辨状河等河湖相环境 ,到上新世早期变为冲积扇的远端。晚上新世 (～ 3.6 Ma)开始 ,盆地的沉积环境发生了质的变化 ,沉积物以粗颗粒砾岩为主 ,沉积环境为干旱气候条件下的冲 -洪积扇近端。沉积相的变化 ,反映了昆仑山在晚上新世有强烈的隆升。     

火山灰的提取及测试技术

同一次火山爆发产生的火山灰 ,能在广阔的区域内形成等时标志层 ,成为地层定年和对比的极有价值的工具。文中介绍了提取肉眼不可见火山灰的 4种方法 :烧失法、稀碱法、重液浮选法和酸化法 ,还介绍了火山玻璃的显微镜下识别技术及电子探针测试技术。     

Late Neogene loess deposition in southern Tarim Basin: tectonic and palaeoenvironmental implications

Uplift of the Tibetan Plateau during the late Cainozoic resulted in a thick apron of molassic sediments along the northern piedmonts of the Kunlun and Altyn Mountains in the southern Tarim Basin. Early Neogene sediments are characterised by sandstone, siltstone and red mudstone, representing floodplain to distal alluvial fan environments. The Early Pliocene Artux Formation consists of medium-grained sandstone and sandy mudstone with thin layers of fine pebbly gritstone. The Late Pliocene to Early Pleistocene Xiyu Formation is dominated by pebble to boulder conglomerate typical of alluvial fan debris flow deposits. Sedimentological investigation, together with grain size and chemical analyses of siltstone bands intercalated with sandstone and conglomerate in the Xiyu and Artux Formations, point to an aeolian origin, suggesting desertic conditions in the Tarim Basin by the Early Pliocene. The onset of aeolian sedimentation in the southern Tarim Basin coincided with uplift of the northern Tibetan Plateau inferred from the lithofacies change from fine-grained mudstone and sandstone to coarse clasts. Tibetan Plateau uplift resulted in the shift of sedimentary environments northwards into the southern Tarim Basin, and could well have triggered the onset of full aridity in the Taklimakan region as a whole.     

The origin of low δ18O granites and related rocks from the Seychelles

The 750-Ma Seychelles granites have whole-rock '¹⁸O values that range from -1.2 to +7.5‰. Differences in '¹⁸O values between quartz, feldspar and whole-rock for samples that have a range of '¹⁸O values suggest that these minerals are in magmatic equilibrium, and that the whole-rock '¹⁸O values were little affected by post-crystallization interaction with fluids. Two suites of granites (the Mahé type and the Praslin type) have previously been recognized on the basis of chemical and radiogenic isotope composition. The former have a mean whole-rock '¹⁸O value of 5.25ǂ.65‰ (1C), whereas the latter are much more variable, with a mean whole-rock '¹⁸O value of 3.00Dž.27‰. Biotite and amphibole separates from both granite types range in 'D from -69 to -116‰, and show a positive correlation with whole-rock '¹⁸O values. Dolerite dykes that intruded the granites during or shortly after their crystallization also have whole-rock '¹⁸O values (mean 2.24ǃ.93‰) lower than that expected for mantle-derived basaltic rocks, but these values appear to be the result of fluid-rock interaction. We suggest that the Mahé-type granites are derived mainly from juvenile mafic to intermediate crust with 'normal' 'D and '¹⁸O values, whereas the Praslin-type granites are mixtures of this source and older crust that acquired its low 'D and '¹⁸O values by extensive interaction with meteoric water at high temperature. It is unlikely that meteoric water sufficiently depleted in D and ¹⁸O was available at 750 Ma because of the relatively low latitude of the region at that time. We suggest that alteration of the source took place significantly before production of the granite magmas. Depletion in ¹⁸O of the Seychelles granites does not necessarily require a regional extensional tectonic setting at 750 Ma, as has been proposed.     

Multivariate Monitoring with GPS Observations and Auxillary Multisensor Data

In a multisensor measurement or monitorin environment, p variables are measured simultaneously. The measured data are correlated and can be monitored to identify special causes of variation in order to establish control and to obtain reference samples to use as a basis in determining the control limits for future observations. One common method is to construct a multivariate control chart based on Hotelling's T² statistic. When the monitoring process is at the start-up stage, F and chi-square distributions are used to construct the necessary control limits. An example from a seven-day GPS data sample measured concurrently with the accelerometer response, wind velocity and temperature illustrates that this technique can improve the interpretation of GPS results. Moreover, the computational complexity is reduced through a reduction in the data dimensionality. ? 2002 Wiley Periodicals, Inc.     

Role of water in the origin of podiform chromitite deposits

We report experiments in basalt oversaturated with water to duplicate the nodular ore textures of podiform chromitite ores. In immiscible basalt-water systems saturated with olivine and chromite, olivine will reside in the melt while chromite will collect in the fluid phase. Fractionation is physical and is driven by differential wetting properties of melt and fluid against silicate and oxide surfaces. There is no need to suppress olivine from the liquidus of a primitive basaltic melt as suggested by Irvine [Irvine, T.N., Geology 5 (1977) 273-277], to achieve chromite accumulations as observed in natural podiform ore deposits. The results imply that podiform chromitite ores will form where a primitive olivine-chromite-saturated mantle melt is sufficiently water-rich to exsolve a fluid phase during passage through the uppermost mantle. The most likely geodynamic environment for podiform chromite mineralization to take place is a supra-subduction zone setting.     

Estimating past leaf-to-air vapour pressure deficit from terrestrial plant δ13C

δ¹³C was determined in lignin extracted from present-day cladodes of Phyllocladus alpinus (a small coniferous tree) from seven well-lit sites across New Zealand. The δ¹³C values ranged from −30.9‰ to −23.6‰ and were compared with monthly means of temperature, precipitation, relative humidity and vapour pressure deficit from the nearest recording stations. Of these parameters, the leaf-to-air vapour pressure deficit of the first month of cladode growth and expansion proved to be the most significantly correlated with lignin δ¹³C, over a range of 0.3 to 0.8 kPa, confirming the importance of atmospheric moisture content on stomatal conductance. The carbon isotopic signature of lignin from fossilised cladodes preserved under the Kawakawa Tephra (22.6 k ¹⁴C yr BP) on the North Island is identical to that of the whole tissue, suggesting that for this species at least, fossil material can be used to approximate the lignin δ¹³C. The δ¹³C of species- and organ-specific fossil terrestrial plant material therefore provides an excellent method to quantify past changes in leaf-to-air vapour pressure deficit. Copyright © 1999 John Wiley & Sons, Ltd.     

A power-law-based approach to mapping COVID-19 cases in the United States

This paper examines the spatial and temporal distribution of all COVID-19 cases from January to June 2020 against the underlying distribution of population in the United States. It is found that, as time passes, COVID-19 cases become a power law with cutoff, resembling the underlying spatial distribution of populations. The power law implies that many states and counties have a low number of cases, while only a few highly populated states and counties have a high number of cases. To further differentiate patterns between the underlying populations and COVID-19 cases, we derived their inherent hierarchy or spatial heterogeneity characterized by the ht-index. We found that the ht-index of COVID-19 cases persistently approaches that of the populations; that is, 5 and 7 at the state and county levels, respectively. Mapping the ht-index of COVID-19 cases against that of populations shows that the pandemic is largely shaped by the underlying population with the R-square value between infection and population up to 0.82.