Strike-slip fault bridge fluid pumping mechanism: insights from field-based palaeostress analysis and numerical modelling

We present a finite-element study of stress perturbation in evolving compressive and extensional strike-slip fault bridges. The results are compared with a fracture study of a compressive bridge at St Donats, UK. Horizontally interbedded calcareous mudstone and bioclastic calcilutite at St Donats have a distinct vertical permeability anisotropy. This sedimentary sequence behaves as a set of horizontal aquifers. The fluid flow in these aquifers is sensitive to mean stress gradients. Paleostress analysis of field fracture data, verified by finite-element modelling, indicates a rotation of σ₁ towards parallelism with boundary faults inside the growing compressive bridge. Boundary faults and bridge faults recorded numerous fluid flow events. The modelled mean stress pattern shows a regional maximum within the bridge and local maxima/minima pairs at boundary fault tips.Finite-element modelling of an extensional bridge indicates that σ₃ rotates towards parallelism with boundary faults. The mean stress pattern is similar to the pattern in compressive bridge but with maxima and minima locations interchanged. The stress patterns are reestablished by each stress build-up preceding the rupturation of the boundary faults throughout the development stages of strike-slip fault bridges. Mean stress gradients developed pre-failure control the fluid flow in fractures of the strike-slip fault system at and after the end of each stress build-up and the fluid flow in boundary faults post-failure. Fracture reactivation and new fracture generation within an evolving bridge is a process consisting of multiple successive events that retain the storage capacity of the bridge. Rupture and sealing of the main bounding-faults is a step-wise process that opens and closes fluid conduits between areas with different pressures.     

Cation distribution and structure modelling of spinel solid solutions

 This paper presents an improved generalisation of cation distribution determination based on an accurate fit of all crystal-chemical parameters. Cations are assigned to the tetrahedral and octahedral sites of the structure according to their scattering power and a set of bond distances optimised for spinel structure. A database of 295 spinels was prepared from the literature and unpublished data. Selected compositions include the following cations: Mg²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, V³⁺, Cr³⁺, Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺ and vacancies. Bond distance optimisation reveals a definite lengthening in tetrahedral distance when large amounts of Fe³⁺ or Ni²⁺ are present in the octahedral site. This means that these cations modify the octahedral angle and hence the shared octahedral edge, causing an increase in the tetrahedral distance with respect to the size of the cations entering it. Some applications to published data are discussed, showing the capacity and limitations of the method for calculating cation distribution, and for identifying inconsistencies and inaccuracies in experimental data. Received: 19 February 2001 / Accepted: 1 June 2001     

Post-mid-Cretaceous eastern North Sea evolution inferred from 3D thermo-mechanical modelling

The Late Cretaceous–Cenozoic evolution of the eastern North Sea region is investigated by 3D thermo-mechanical modelling. The model quantifies the integrated effects on basin evolution of large-scale lithospheric processes, rheology, strength heterogeneities, tectonics, eustasy, sedimentation and erosion.

The evolution of the area is influenced by a number of factors: (1) thermal subsidence centred in the central North Sea providing accommodation space for thick sediment deposits; (2) 250-m eustatic fall from the Late Cretaceous to present, which causes exhumation of the North Sea Basin margins; (3) varying sediment supply; (4) isostatic adjustments following erosion and sedimentation; (5) Late Cretaceous–early Cenozoic Alpine compressional phases causing tectonic inversion of the Sorgenfrei–Tornquist Zone (STZ) and other weak zones.

The stress field and the lateral variations in lithospheric strength control lithospheric deformation under compression. The lithosphere is relatively weak in areas where Moho is deep and the upper mantle warm and weak. In these areas the lithosphere is thickened during compression producing surface uplift and erosion (e.g., at the Ringkøbing–Fyn High and in the southern part of Sweden). Observed late Cretaceous–early Cenozoic shallow water depths at the Ringkøbing–Fyn High as well as Cenozoic surface uplift in southern Sweden (the South Swedish Dome (SSD)) are explained by this mechanism.

The STZ is a prominent crustal structural weakness zone. Under compression, this zone is inverted and its surface uplifted and eroded. Contemporaneously, marginal depositional troughs develop. Post-compressional relaxation causes a regional uplift of this zone.

The model predicts sediment distributions and paleo-water depths in accordance with observations. Sediment truncation and exhumation at the North Sea Basin margins are explained by fall in global sea level, isostatic adjustments to exhumation, and uplift of the inverted STZ. This underlines the importance of the mechanisms dealt with in this paper for the evolution of intra-cratonic sedimentary basins.     

Lithosphere structure of European sedimentary basins from regional three-dimensional gravity modelling

A three-dimensional (3D) density model, approximated by two regional layers—the sedimentary cover and the crystalline crust (offshore, a sea-water layer was added), has been constructed in 1° averaging for the whole European continent. The crustal model is based on simplified velocity model represented by structure maps for main seismic horizons—the “seismic” basement and the Moho boundary. Laterally varying average density is assumed inside the model layers. Residual gravity anomalies, obtained by subtraction of the crustal gravity effect from the observed field, characterize the density heterogeneities in the upper mantle. Mantle anomalies are shown to correlate with the upper mantle velocity inhomogeneities revealed from seismic tomography data and geothermal data. Considering the type of mantle anomaly, specific features of the evolution and type of isostatic compensation, the sedimentary basins in Europe may be related into some groups: deep sedimentary basins located in the East European Platform and its northern and eastern margins (Peri-Caspian, Dnieper–Donets, Barents Sea Basins, Fore–Ural Trough) with no significant mantle anomalies; basins located on the activated thin crust of Variscan Western Europe and Mediterranean area with negative mantle anomalies of −150 to −200×10⁻⁵ ms⁻² amplitude and the basins associated with suture zones at the western and southern margins of the East European Platform (Polish Trough, South Caspian Basin) characterized by positive mantle anomalies of 50–150×10⁻⁵ ms⁻² magnitude. An analysis of the main features of the lithosphere structure of the basins in Europe and type of the compensation has been carried out.     

基于DEM的分布式水文模型构建方法

基于 DEM的分布式水文模型是现代水文学同高科技 (如计算机技术、 3S技术等 )相结合的产物 ,是研究变化环境下水文循环与水资源演化规律的理想工具 ,代表了水文模型的最新发展方向。从 DEM的特性出发 ,本文探讨并总结了分布式水文模型的特点、建模思路和模型基本结构框图。在流域离散化方面 ,重点介绍了分布式水文模型常用的三种单元划分方法 ;最后 ,针对分布式水文模型构建问题 ,从“输入模块”、“单元水文模型”、“河网汇流模型”三方面 ,阐述了分布式水文模型微结构构建方法。     

分布式侵蚀预报模型中网格面积的选定——以黄土高原丘陵沟壑区为例

在分布式土壤侵蚀过程模型中 ,承载数据以及进行运算的最小单元 ,即基本地块的选取是非常关键的 ,它直接关系到模型的模拟精度和运算数据量。目前大多数分布式模型都采用平均布设矩形网格的方法 ,这种方法在基本网格的大小选取上存在着盲目性和不统一性。本文以黄土高原丘陵沟壑区为例 ,利用GIS和SPSS分析了黄土高原丘陵沟壑区属性均一的基本地块在面积上的统计规律 ,给出了基本地块选取的合理依据 ,以促进建立更好的分布式模型