Backward modeling of the rifting kinematics in the Upper Rhine Graben: insights from an elastic-perfect contact law on the restoration of a multi-bloc domain

This paper addresses the problem of volume restoration for 3-D sedimentary basin kinematic deformation. The primary purpose is methodological and concerns the use of contact mechanics with the finite element method, in order to deform a geological multi-bloc domain. This approach is applied to backward model the later stage of rifting of a segment of the southern Upper Rhine Graben (France–Germany border). Preliminary results from our modeling demonstrate the ability of the method not only to handle complex geometries, but also to successfully perform retro-deformation of a complex geological domain. In addition, they provide or confirm crucial information on the rifting evolution and tectonic features of this segment of the Upper Rhine Graben, such as the distribution of deformation, the asymmetry of the graben and a significant left-lateral strike-slip component of displacement.     

Fitting the Linear Model of Coregionalization by Generalized Least Squares

In geostatistical studies, the fitting of the linear model of coregionalization (LMC) to direct and cross experimental semivariograms is usually performed with a weighted least-squares (WLS) procedure based on the number of pairs of observations at each lag. So far, no study has investigated the efficiency of other least-squares procedures, such as ordinary least squares (OLS), generalized least squares (GLS), and WLS with other weighing functions, in the context of the LMC. In this article, we compare the statistical properties of the sill estimators obtained with eight least-squares procedures for fitting the LMC: OLS, four WLS, and three GLS. The WLS procedures are based on approximations of the variance of semivariogram estimates at each distance lag. The GLS procedures use a variance–covariance matrix of semivariogram estimates that is (i) estimated using the fourth-order moments with sill estimates (GLS₁), (ii) calculated using the fourth-order moments with the theoretical sills (GLS₂), and (iii) based on an approximation using the correlation between semivariogram estimates in the case of spatial independence of the observations (GLS₃). The current algorithm for fitting the LMC by WLS while ensuring the positive semidefiniteness of sill matrix estimates is modified to include any least-squares procedure. A Monte Carlo study is performed for 16 scenarios corresponding to different combinations of the number of variables, number of spatial structures, values of ranges, and scale dependence of the correlations among variables. Simulation results show that the mean square error is accounted for mostly by the variance of the sill estimators instead of their squared bias. Overall, the estimated GLS₁ and theoretical GLS₂ are the most efficient, followed by the WLS procedure that is based on the number of pairs of observations and the average distance at each lag. On that basis, GLS₁ can be recommended for future studies using the LMC.     

Modeling coastal tsunami hazard from submarine mass failures: effect of slide rheology,experimental validation,and case studies off the US East Coast

Natural Hazards - We perform numerical simulations to assess how coastal tsunami hazard from submarine mass failures (SMFs) is affected by slide kinematics and rheology. Two types of two-layer SMF...     

Contribution of geophysical surveys to groundwater modelling of a porous aquifer in semiarid Niger: An overview

Subsurface geophysical surveys were carried out using a large range of methods in an unconfined sandstone aquifer in semiarid south-western Niger for improving both the conceptual model of water flow through the unsaturated zone and the parameterization of numerical a groundwater model of the aquifer. Methods included: electromagnetic mapping, electrical resistivity tomography (ERT), resistivity logging, time domain electromagnetic sounding (TDEM), and magnetic resonance sounding (MRS). Analyses of electrical conductivities, complemented by geochemical measurements, allowed us to identify preferential pathways for infiltration and drainage beneath gullies and alluvial fans. The mean water content estimated by MRS (13%) was used for computing the regional groundwater recharge from long-term change in the water table. The ranges in permeability and water content obtained with MRS allowed a reduction of the degree of freedom of aquifer parameters used in groundwater modelling.     

Tectonics of the western margin of the Shan plateau (central Myanmar): implication for the India–Indochina oblique convergence since the Oligocene

We present new data and interpretations on the Neogene tectonics of the Shan scarp area (central Myanmar) and its relationship with the India–Indochina oblique convergence. We describe ductile and brittle fabrics associated with the major features in this area, the Mogok Metamorphic Belt (MMB), the Shan scarp and the Sagaing fault. From these data we identify a succession of two tectonic regimes. First, a dominant NNW–SSE-trending extension, marked by ductile stretching that characterizes the MMB, and associated N70E brittle normal faults. Later, from Middle or Upper Miocene to the Present, these structures were cross-cut by brittle right-lateral faults, among which the most important are the N20W transpressive Shan scarp fault zone and the N–S Sagaing fault. To explain this transition from a dominant transtensive to a transpressive stress regime, that occurred during Miocene, we place our data within a larger geodynamic context. We suggest that, like the intraplate deformation in the Indian Ocean, the end of spreading in the South China sea, the opening of the Andaman basin or the end of subduction within the Indo-Burma range, the change in the tectonic regime in central Myanmar could be in response to a major Miocene regional plate kinematic reorganization.     

Towards Stochastic Time-Varying Geological Modeling

The modeling of subsurface geometry and properties is a key element to understand Earth processes and manage natural hazards and resources. In this paper, we suggest this field should evolve beyond pure data fitting approaches by integrating geological concepts to constrain interpretations or test their consistency. This process necessarily calls for adding the time dimension to 3D modeling, both at the geological and human time scales. Also, instead of striving for one single best model, it is appropriate to generate several possible subsurface models in order to convey a quantitative sense of uncertainty. Depending on the modeling objective (e.g., quantification of natural resources, production forecast), this population of models can be ranked. Inverse theory then provides a framework to validate (or rather invalidate) models which are not compatible with certain types of observations. We review recent methods to better achieve both stochastic and time-varying geomodeling and advocate that the application of inversion should rely not only on random field models, but also on geological concepts and parameters.     

Do paleoclimate proxies agree? A test comparing 19 late Holocene climate and sea‐ice reconstructions from Icelandic marine and lake sediments

Geochemical, mineralogical and biological indicators preserved in sediments are widely used to reconstruct past climate change, but proxies differ in the degree to which their utility as climate indicators has been validated via laboratory experiments, modern spatial calibrations, or down‐core comparisons with instrumental climate data. Multi‐proxy studies provide another means of evaluating interpretations of proxies. This paper presents a multi‐proxy assessment comparing 19 sub‐centennially resolved late Holocene proxy records, covering the period 300–1900 AD, from seven Icelandic marine and lacustrine core sites. We employ simple statistical comparisons between proxy reconstructions to evaluate their correlations over time and, ultimately, their utility as proxies for regional climate. Proxies examined include oxygen isotopic composition of benthic and planktonic foraminifera, abundance of the sea‐ice biomarker IP₂₅, allochthonous quartz in marine sediments (a proxy for drift ice around Iceland), marine carbonate abundance, total organic carbon concentration, chironomid assemblages, lacustrine biogenic silica and carbon/nitrogen ratios in lake sediments. Most of the examined proxy records, including temperature and sea‐ice proxies, correlate strongly with each other over multi‐centennial timescales, and thus do appear to record changes in regional climate. Copyright © 2011 John Wiley & Sons, Ltd.     

Accounting for uncertainties and time preference in economic analysis of tackling climate change through forestry and selected policy implications for Scotland and Ukraine

The paper discusses the development of economic techniques for dealing with uncertainties in economic analysis of planting trees to mitigate climatic change. In consideration of uncertainty, time preference and intergenerational equity, the traditional cost-benefit analysis framework is challenged with regard to the discounting/non-discounting of carbon uptake benefits, and because it usually uses a constant and positive discount rate. We investigate the influence of various discounting protocols on the outputs of economic analysis. The idea of using the declining discount rate is also considered. Several numerical examples dealing with the analysis of afforestation for carbon sequestration in Scotland and Ukraine are provided. We show that the choice of discounting protocols have a considerable influence on the results of economic analysis, and therefore, on the decision-making processes related to climate change mitigation strategies. The paper concludes with some innovative insights on accounting for uncertainties and time preference in tackling climate change through forestry, several climate policy implications of dealing with uncertainties, and a brief discussion of what the use of different discounting protocols might imply for decision making.     

A seasonal dipolar eddy near Ras Al Hamra (Sea of Oman)

Trajectories and hydrological data from two Argo floats indicate that warm and salty water at 200–300-m depths was ejected from the coast of Oman, near Ras al Hamra, in spring 2008, 2011, and 2012. This warm and salty water, Persian Gulf Water (PGW), once ejected from the coast, recirculated cyclonically in the western Sea of Oman, but also flowed eastward along the Iranian and Pakistani coasts. There, it was expelled seaward by mesoscale eddies as shown by other float data. Seasonal maps of salinity were computed from all available Argo floats; they showed that, in spring, PGW is present in the middle and north of the Sea of Oman, contrary to fall, when the salinity maxima lie southeast of Ras al Hadd. The ejection of PGW from Ras al Hamra is related here to the influence of a mesoscale dipolar eddy which often appears near this cape in spring. The time-averaged and empirical orthogonal functions of altimetric maps over 11 years for this season confirm the frequent presence and the persistence of this feature. From surface currents and hydrology, deep currents were computed via thermal wind balance, and the associated shear and strain fields were obtained. This deformation field is intense near Ras al Hamra, with an offshore direction. This flow structure associated with the mesoscale dipole explains PGW ejection from the coast. This observation suggests that PGW distribution in the Northern Arabian Sea can be strongly influenced by seasonal mesoscale eddies.     

The snowball Earth aftermath: Exploring the limits of continental weathering processes

Carbonates capping Neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball Earth aftermath. It is commonly assumed that post-snowball climate dominated by CO₂ partial pressures several hundred times greater than modern levels, would be characterized by extreme temperatures, a vigorous hydrological cycle, and associated high continental weathering rates. However, the climate in the aftermath of a global glaciation has never been rigorously modelled. Here, we use a hierarchy of numerical models, from an atmospheric general circulation model to a mechanistic model describing continental weathering processes, to explore characteristics of the Earth system during the supergreenhouse climate following a snowball glaciation. These models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse. Indeed, constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation. Even at 400 times the present day pressure of atmospheric CO₂, continental runoff is only 1.2 times the modern runoff. Under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event, the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux. Consequently, it takes millions of years for the silicate weathering cycle to reduce post-snowball CO₂ levels to background Neoproterozoic levels. Regarding the origin of the cap dolostones, we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume.