Diachronic metamorphic and structural evolution of the Connecticut Valley–Gaspé trough,Northern Appalachians

The Connecticut Valley–Gaspé (CVG) trough represents a major, orogen-scale Silurian–Devonian basin of the Northern Appalachians. From Gaspé Peninsula to southern New England, the CVG trough has experienced a contrasting metamorphic and structural evolution during the Acadian orogeny. Along its strike, the CVG trough is characterized by increasing strain and polyphase structures, and by variations in the intensity of regional metamorphism and contrasting abundance of c. 390–370 Ma granitic intrusions. In southern Quebec and northern Vermont, a series of NW–SE transects across the CVG trough have been studied in order to better understand these along-strike variations. Detailed structural analyses, combined with phase equilibria modelling, Raman spectrometry, and muscovite ⁴⁰Ar/³⁹Ar dating highlight a progressive and incremental deformation involving south–north variation in the timing of metamorphism. Deformation evolves from a D₁ crustal thickening event which originates in Vermont and progresses to southern Québec where it peaked at 0.6 GPa/380°C at c. 375 Ma. This was followed by a D₂ event associated with continuous burial in Vermont from 378 to 355 Ma, which produced peak metamorphic conditions of 0.85 GPa/380°C and exhumation in Quebec from 368 to 360 Ma. The D₃ compressional exhumation event also evolved from south to north from 345 to 335 Ma. D₁ to D₃ deformation events form part of a continuum with an along-strike propagation rate of ~50 km/Ma During D₁, the burial depth varied by more than 15 km between southern Quebec and Vermont, and this can be attributed to the occurrence of a major crustal indenter, the Bronson Hill Arc massif, in the New England segment of the Acadian collision zone.     

Spatio-temporal patterns of the interaction between groundwater and surface water in plains

The study of the dynamics of anthropic disturbances that have an effect on the hydrological systems in plains requires integral simulation tools for their diagnosis. The objective of this article is, first, to analyse and reproduce the spatio-temporal interactions between groundwater (GW) and surface water, net recharge, GW level, surface run-off, and evapotranspiration in the upper creek basin of Del Azul, which is located in the centre of the province of Buenos Aires, Argentina, and second, to obtain insights to apply the methodology to other similar situations. For this purpose, a model coupling the semidistributed hydrological model (Soil and Water Assessment Tool [SWAT]) and the hydrogeological model (MODFLOW) has been used. A simulation was carried out for a period of 13 years (2003–2015) on a daily scale. The application of the SWAT–MODFLOW coupling gave good results based on the adjustment between the calculated flows and levels, reaching a Nash–Sutcliffe of 0.6 and R²0.6 at the Seminario hydrometric station located at the watershed outlet point. According to the annual average balance, out of the total rainfall, evapotranspiration accounts for 85%, recharge accounts for 10.2%, and surface run-off accounts for 4.8%. Annual and monthly trends of the stream–aquifer interaction were determined, obtaining on average an annual GW discharge of 34 mm and an annual average recharge of the stream to the aquifer of 1.4 mm. Monthly GW discharges are higher in winter–spring (July to December with an average of 3.3 mm) and lower in summer–autumn (January to June with an average of 2.8 mm). The monthly average recharge of the stream towards the aquifer varies from 0.02 to 0.36 mm and is higher in March, May, and August, when water excess is produced in the basin. Through the analysis of coupled modelling, it is possible to analyse and reproduce the spatio-temporal transitions of flow existing between the stream, the hyporheic zone, and the aquifer.     

Seeing the trees for the (urban) forest: more-than-human geographies and urban greening

ABSTRACT

Urban spaces have long been places to think through human relationships with nature. The recent shift in thinking from urban green space as outcome to urban greening as a process provides an opportunity to consider more explicitly how we engage with more-than-human worlds in urban spaces, in more differentiated ways, and for what ends. In this paper we contribute to growing interest in improved urban sustainability and well-being by bringing human geography perspectives on more-than-human worlds into conversation with the literature on urban greening. Drawing on key examples oriented around urban trees, we consider two main themes: sensibilities and belonging. We argue for an understanding of urban places as collective achievements that not only involve knowing and living with diverse humans and non-humans but also involve the re/making of sensibilities and belongings. Through this paper, we aim to open dialogue about how more-than-human geographies might help us to differently understand urban trees, contemporary urban greening, and people–plant relations.     

Optimal design of snow stake networks to estimate snow depth in an alpine mountain range

Monitoring and estimation of snow depth in alpine catchments is needed for a proper assessment of management alternatives for water supply in these water resources systems. The distribution of snowpack thickness is usually approached by using field data that come from snow samples collected at a given number of locations that constitute the monitoring network. Optimal design of this network is required to obtain the best possible estimates. Assuming that there is an existing monitoring network, its optimization may imply the selection of an optimal network as a subset of the existing one (if there are no funds to maintain them) or enlarging the existing network by one or more stations (optimal augmentation problem). We propose an optimization procedure that minimizes the total variance in the estimate of snowpack thickness. The novelty of this work is to treat, for the first time, the problem of snow observation network optimization for an entire mountain range rather than for small catchments as done in the previous studies. Taking into account the reduced data available, which is a common problem in many mountain ranges, the importance of a proper design of these observation networks is even larger. Snowpack thickness is estimated by combining regression models to approach the effect of the explanatory variables and kriging techniques to consider the influence of the stakes location. We solve the optimization problems under different hypotheses, studying the impacts of augmentation and reduction, both, one by one and in pairs. We also analyse the sensitivity of results to nonsnow measurements deduced from satellite information. Finally, we design a new optimal network by combining the reduction and augmentation methods. The methodology has been applied to the Sierra Nevada mountain range (southern Spain), where very limited resources are employed to monitor snowfall and where an optimal snow network design could prove critical. An optimal snow observation network is defined by relocating some observation points. It would reduce the estimation variance by around 600 cm² (15%).     

Coupling of P–T–t–D histories of eclogite and metagreywacke—Insights to late Ordovician–Silurian crustal folding events recorded in the Beishan Orogen (NW China)

The Beishan complex is composed of orthogneiss and metagreywacke that both enclose bodies of eclogite and serves as a unique example for comparative petrological study of all these lithologies. The rocks show the earliest regional steep N-S striking fabric (S2) preserved in low strain domains that are reworked by ubiquitous steep N-NE dipping cleavage (S3). The eclogite shows an almost isotropic fabric defined by an M1 assemblage of Grt–Cpx–Amp–Qz–Rt–Ilm that is locally retrogressed to M2-3 amphibolite facies mineral assemblages, with P–T peak at 20–21 kbar and 750–775°C and retrogression to 2–3kbar and 530–550°C. The typical mineral assemblage of the host metagreywackes is Bt–Ms–Pl–Qz−Chl–Ilm±Grt. Rare Al-rich metagreywacke layers are composed of Grt–Ky–St±Sil−And–Bt–Ms–Pl–Qz±Chl±Rt–Ilm giving a P–T path with peak at 8–8.5kbar and ~670°C correlated with the S2 fabric and retrogression to ~2.5kbar and 525–550°C correlated with the S3 foliation. In two eclogite samples, the garnet-whole rock-clinopyroxene Lu–Hf isochrons give ages of 461.9±1.6 Ma and 462.0±6.2 Ma interpreted as reflecting average age of garnet formation, and Sm–Nd isochrons give ages of 453.6±2.7 Ma and 452.8±3.0 Ma interpreted as dating near-peak metamorphism. In metagreywacke, in-situ U–Pb dating of monazite gives two groups of ages of 445–440 Ma (Mnz cores) and 436–429 Ma (Mnz rims), interpreted as reflecting the metamorphic peak and retrogression. Our results show that eclogite was formed during Ordovician by subduction of a continental crust (D1). Eclogite and metagreywacke underwent partly decoupled P–T–t–D paths until their juxtaposition at mid-crustal levels during a first late Ordovician–early Silurian D2 shortening. Coupling of their P–T–t–D paths occurred during exhumation in the Silurian and a second and orthogonal D3 shortening event. The data from the Beishan Orogen are consistent with a collisional intra-Gondwanan orogen located south of the Central Asian Orogenic Belt.     

Relative pollen productivities of the major plant taxa of subtropical evergreen–deciduous mixed woodland in China

Quantifying the relationship between pollen and vegetation is an essential step in the pollen-based quantitative reconstruction of past vegetation cover. In this study, we use the Extended R-Value (ERV) model and a modern dataset of pollen (collected from moss polsters) and related vegetation from 50 sites in the Daba Mountains (subtropical China) to (i) estimate the relevant source area of pollen (RSAP) of the moss samples and the relative pollen productivities (RPPs) of nine major plant taxa-characteristic of the region, and (ii) evaluate the obtained RPPs. The RSAP estimates of moss polsters vary between 225 and 610 m depending on the ERV submodels and models of pollen dispersal and deposition used. The RPP estimates are different from values published in previous studies from temperate and subtropical China. This may be explained by differences in methodology, climate and vegetation (species composition and spatial distribution), of which vegetation is probably the most important factor. The ranking of the RPP estimates for the nine taxa is Pinus > Juglandaceae > D − Quercus (deciduous Quercus) > Poaceae > Rosaceae > Cyperaceae > Anacardiaceae > Castanea > Fabaceae. We use a ‘leave-one-out’ cross-validation strategy and the Landscape Reconstruction Algorithm (LRA) for pollen-based reconstruction of regional and local plant cover to evaluate the ERV model-based RPP estimates. Both the REVEALS (Regional Estimates of VEgetation Abundance from Large Sites)-based and the LOVE (LOcal Vegetation Estimates)-based plant cover using the RPP estimates are closer to the modern vegetation composition than pollen percentages, thus confirming the applicability of the ERV model and the LRA approach in subtropical China.     

Freeze–thaw processes degrade post-fire water repellency in wet soils

Wildfires are a cause of soil water repellency (hydrophobicity), which reduces infiltration whilst increasing erosion and flooding from post-fire rainfall. Post-fire soil water repellency degrades over time, often in response to repeated wetting and drying of the soil. However, in mountainous fire-prone forests such as those in the Western USA, the fire season often terminates in a cold and wet winter, during which soils not only wet and dry, but also freeze and thaw. Little is known about the effect of repeated freezing and thawing of soil on the breakdown of post-fire hydrophobicity. This study characterized the changes in hydrophobicity of Sierra Nevada mountain soils exposed to different combinations of wet–dry and freeze–thaw cycling. Following each cycle, hydrophobicity was measured using the Molarity of Ethanol test. Hydrophobicity declined similarly across all experiments that included a wetting cycle. Repeated freezing and thawing of dry soil did not degrade soil water repellency, but freeze–thaw cycles decreased hydrophobicity in wet soils. Total soil organic matter content was not different between soils of contrasting hydrophobicity. Macroscopic changes such as fissures and cracks were observed to form as soil hydrophobicity decayed. Microscopic changes revealed by scanning electron microscope imagery suggest different levels of soil aggregation occurred in samples with distinct hydrophobicities, although the size of aggregates was not clearly correlated to the change in water repellency due to wet–dry and freeze–thaw cycling. A 9-year climate and soil moisture record from Providence Critical Zone Observatory was combined with the laboratory results to estimate that hydrophobicity would persist an average of 144 days post-fire at this well-characterized, typical mid-elevation Sierra Nevada site. Most of the breakdown in soil water repellency (79%) under these climate conditions would be attributable to freeze–thaw cycling, underscoring the importance of this process in soil recovery from fire in the Sierra Nevada.     

Quantifying the contribution of climate and underlying surface changes to alpine runoff alterations associated with glacier melting

Climate variability and underlying surface changes are strongly associated with runoff alterations. The Yarlung Zangbo River Basin (YZRB) is a typical alpine region located in the southeast Qinghai–Tibet Plateau, where runoff is particularly sensitive and vulnerable to climate and environmental changes. Here, we conducted a quantitative assessment of the contributions of climate variability and underlying surface changes to runoff alterations from 1966 to 2015 in the upper, middle, and lower regions of the YZRB. The year 1997 was identified as the runoff breakpoint in all three sub-regions, which divided the runoff time series into the baseline period (1966–1997) and change period (1998–2015). An adjusted Budyko framework accounting for glacier runoff was developed to conduct a runoff alteration attribution analysis. The results indicated that the increase in runoff in the upper region was dominated by changes in the underlying surface and glacier runoff, whose contribution accounted for 59.61 and 49.18%, respectively. The runoff increase in the middle and lower regions was mainly attributed to the increase in precipitation, accounting for 39.36 and 129.21% of the total runoff alteration, respectively. Moreover, due to the little variation in vegetation and degradation of permafrost in the upper region, increases in runoff might be largely attributed to increases in subsurface runoff caused by the melting of permafrost. In the middle region, in addition to increased precipitation, vegetation degradation had positive effects on runoff increases. The lower region exhibited far higher water consumption rates due to its extensive and dense vegetation coverage accompanied by rising temperature, which resulted in a negative contribution (−58.74%) to runoff alteration. Our findings may therefore have important implications for water resource security and sustainable development in alpine regions.     

全断面盾构机异质岩土掘进载荷等效模型研究

由于岩土结构的变化,全断面盾构机在掘进过程中推进力和扭矩需要进行适当的顺应性设计以获得良好的掘进效率,以减少刀具磨损和损伤破坏事故的发生。针对盾构机在掘进过程中遇到的典型地质结构,建立地质结构的圆形百分比模型,分析刀盘上不同刀具在掘进过程中载荷受力,建立不同刀具及总扭转载荷的数学表述方法,基于该方法对盾构机在异质岩土结构中旋转时刀盘总扭矩随旋转角度的变化特性进行研究,结果表明,在软岩与硬岩比例大约为30%时刀盘的扭矩载荷波动最大,容易造成刀盘堵转及损伤事故,该可以解释云南上公山隧道掘进时频繁堵转产生机理,为异质岩土掘进时盾构机冗余驱动电机的顺应性设计提供指导。     

Soil–structure interaction for a SDOF oscillator supported by a flexible foundation embedded in a half-space: Closed-form solution for incident plane SH-waves

A closed-form analytical solution is presented for the dynamic response of a SDOF oscillator, supported by a flexible foundation embedded in an elastic half-space, and excited by plane SH waves. The solution is obtained by the wave function expansion method. The solution is verified for the special case of a rigid foundation by comparison with published results. The model is used to investigate the effect of the foundation flexibility on the system response. The results show that the effect is significant for both foundation response and structural relative response. For a system with more flexible foundation, the radiation damping is smaller, the foundation response is larger, especially for obliquely incident waves, while the structural relative response is smaller, and the system frequency shifts towards lower frequencies. This simple model may be helpful to obtain insight into the effects of soil–structure interaction for a slim structure on an extended flexible foundation.