Paleostress Analysis Of Polyphase Calcite E-twins:Principle And Applications

The paleostress analysis of calcite e-twin is one of the important means in the field of geodynamics,which is very suitable for the structural study in the areas lacking mesoscale deformation structures or poor exposure,such as craton basin,foreland basin and foreland fold-thrust belt.But most of existing methods are designed to deal with monophase twin data,which limits the wider application of this method.On the other hand,there are many remarkable kilometer-scale tectonic domes developed in the reactivated Yangtze platform in Mesozoic.So far the tectonic evolution or formation mechanism of some domes is still controversial,due to the poor exposure and difficulty in identifying secondary deformation structures realted to the dome evolution in the field.The paleostress analysis method of calcite twins is the most suitable tool to address these problems at present.In this thesis,the inversion method of polyphase calcite e-twins and its applications in the dome structure of South China are studied.1)Method.A new inversion method is developed in this paper to estimate deviatoric stress tensors from a certain population of polyphase calcite twins.It is both to minimize the number of the common incompatible(twinned and untwinned)e-planes and to maximize the sum of the resolved shear stresses along the compatible twinned e-planes under each reduced stress.The genetic algorithm is employed to resolve this optimization problem.A series of two-phase,equally-mixed artificial datasets,generated under the prescribed stresses,were taken to demonstrate the feasibility of this method.Results indicate a strong tendency to have an increasing accuracy of reduced stress with the ascent of the prescribed maximum differential stress,the difference between the prescribed stresses,and the number of twinned calcite crystals,respectively.What is more,the maximum differential stress becomes sufficiently large to permit an accurate estimate of reduced stress,even though the prescribed stresses are intermediately similar to each other.For the improved version,maximizing the number of compatible untwinned e-planes shared by the unknown stresses is chosen as a third criterion,which improves the accuracy of stress estimation,especially in the estimation of maximum difference stress.However,the method assumes a constant critical resolved shear stress value to rescale the stress tensors,and does not consider the influence on the critical resolved shear stress value of strain hardening,grain size,and temperature.It takes an enormous amount of time in calculation.2)Application 1: Huangling dome in Western Hubei Province.The north-trending Huangling Dome,located on the northern margin of the reactivated Yangtze Platform,provides an appropriate place to understand the complexity of intraplate deformation of the South China continent since the Early Mesozoic.In this study,palaeostress analysis using calcite twins was carried out to address the deformation mechanism of the dome,poorly understood at present.Ten limestone samples and three marble samples were collected from the platform cover on the eastern and western flanks and the core basement,measured under the universal stage,and inverted for stress analysis using our recently proposed method.All the stress estimates for the samples were classified into two layer-parallel shortening(LPS)subsets and two non-LPS subsets.They represent NW-SE compression,NE–SW compression,N-S compression,and N-S or E-W extension,respectively,in correspondence to principal tectonic events since the Early Mesozoic.They do not verify existing explanations about the genesis of the dome,for example,extrusion-induced E-W compression and differential uplift-related E-W extension.A new formation model was therefore proposed: under the nearly N-S compression,the dome initiates as an E-W trending brachy anticline and gains the current configuration,owing to basement inheritance,variation of regional shortening,and obstruction of basement fault beneath the current western flank.The results of numerical simulation verify the model and determine that the basement fault inclines eastward with a dip angle of 45-55°.3)Application 2: Longshan dome in central Hunan Province.It is generally accepted that during the Mesozoic NE-NNE-trending folds overprinted E-W-trending folds to form the Longshan dome in the central South China continent,although the interference map does not tell the relative ages of the fold sets.In an effort to deepen our understanding the process of reworking the continent,palaeostress analysis using calcite twins was carried out in this study to verify or falsify this model.Ten limestone samples were collected from Upper-Paleozoic limestones on the flanks and were measured under the universal stage for calcite e-twins.E-twins in the samples are divisible into two kinds,thick twin(? 1 ?m)and thin twin(< 1 ?m),indicative of relatively higher and lower deformation temperatures,respectively.Stress estimates obtained using the improved method were grouped into two layer-parallel shortening(LPS)subsets and three non-LPS subsets.These subsets comprise four tectonic regimes: NWW-SEE compression(LPS1 and non-LPS1),NNE-SSW compression(LPS2 and non-LPS2),NW-SE extension(non-LPS3a)and NNE-SSW extension(non-LPS3b).They were further arranged in a temperature-decreasing order to establish a complex deformation sequence of the study area.In the sequence NE-NNE-trending folds have an older age than E-W-trending folds,as different from the model.The approximately N-S regional compression responsible for the former folds should have a profound effect on the intensely deformed continent,and this has been ignored ever since.