Paleostress analysis

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Introduction

A structural model is an important part of the geotechnical model of a mine development. The purpose of the structural model is to illustrate the location, orientation and character of major geologic structures, as well as identify the spatial distribution of features that make up the rock mass fabric (i.e minor faults, folds, foliation). The identification of patterns or spatial trends in rock mass fabric allows the selection of structural domains. [1] Developing domains simplifies the structural evaluation of the block to be mined, by illustrating the interaction of major structures and rock fabric, which may derive a particular overall stress state within that block. [2] Comprehensive structural evaluation includes geologic structures at all scales – regional mine scale to drill core scale – with enough detail to produce a representative model that reflects the mining block’s entire geological history, formally referred to as tectonogenesis (Figure 1). Illustrating a block’s structural features and unique fault-related movement data contributes to paleostress analysis methods by identifying the controlling structural features and associated stresses. Completing a sequenced paleostress analysis as a means to back-calculate stress through geological time is of significant value to mine sequence planning. This allows assessment of the possibility of remobilizing old structures or veins, resulting in a fault-slip burst or seismic event during mining. Paleostress analysis methods are recommended to be applied at all burst or seismic event prone mine-sites. Using the paleostress back-analysis to estimate the state of stress yields invaluable data for helping develop efficient and safe excavation sequencing during mine-planning. [2]

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Figure 1: Block model reconstruction of geological history and associated paleostress inversion analysis results for inferred stress states based on evaluation of fault slip data. [2]

Tectonogenesis Assessment and Palinspastic Reconstruction

Tectonogenesis is the unraveling of a block’s geologic tectonic history.[2] Palinspastic reconstruction is the recreation of geo-historical depositional geologic events and kinematic influences. Coupling these together allows the identification of the most influential and controlling structural features of a mining area, which plays a significant role in stability analysis. In the past, tectonogenesis and palinspastic reconstruction have been commonly used for the resolution of geologic history within the petroleum industry, locating oil reservoirs within structurally complex geology. With sufficient research and funding in this particular industry, numerical methods have been established, allowing more rigorous modeling.[2] This rigorous numerical modeling, however, is only effective with good understanding of the area that is being modeled, further necessitating the procedure of tectonogenesis and palinspastic reconstruction. The following steps are recommended to complete a tectogenetic assessment:

  • Literature review of the area for regional and local geology, including surface geologic mapping
  • Establish the chronological sequence of tectonic events, and regional and local scale deformation (i.e. faulting).
  • Examine available drill core with particular attention paid to major structural feature intersections with the drill holes
  • Modeling in three-dimensions the identified drill hole intersections with major structural features, and possibly project them in 3D space relative to the geological model.
  • Display structural data representing rock mass fabric and major structures on stereonets to define structural domains at the regional scale.
  • Isolate block models and domain stereonets to define the sequence of events at the various scales.
  • Link, where possible, inferred structural paleostress with principal ground stresses. This will be a preliminary estimate until mining occurs and stress measurements are collected.

Paleostress Analysis

The goal of paleostress analysis is to derive the direction of slip on a fault, using basic fault data collected in the field.[3] There are many methods of paleostress inversion, which outline different approaches to the problem, however they are all based on similar assumptions:

  • Slip on a fault plane occurs in the direction of the resolved shear stress
  • Individual faults do not interact – movement on one fault is independent of another
  • The blocks bounded by the fault planes do not rotate
  • The stress field activating the faults is time-dependent and homogeneous

Fault Data

Data pertaining to a particular fault includes the fault plane orientation, slip direction and sense of slip, as well as location and character of fault attributes, the sense of slip also referred to as striations, indicate the direction of fault movement (Figure 2).[4]

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Figure 2: Fault data pertaining to the paleostress tensor. [5]

The data pertaining to the fault movement is used to calculate the paleostress tensor, which indicate the stress orientation interpretation relative to kinematic movement of a fault. [5] It is important to collect as many fault data measurements as possible, as most paleostress methods are statistic. The following criteria exist to distinguish fault rock types and their relation to tectonic regimes, contributing to the reconstruction process:

  • Geometrical relationships of unconformities, indicating whether separate faults are from the same event, or are superimposed.
  • Qualitative aspect of fault planes, indicating similar or different geologic events (i.e. alteration, mineral coatings, weathering)
  • Influence of lithology
  • Size of the fault plane and relative movement

Classification of Faults

Faults are most often classified by their stress state, as developed by E.M. Anderson in the early 1900’s. [6] Anderson derived the belief that the magnitude of horizontal stresses (σ2 and σ3) relative to that of the vertical stress (σ1) can change, which gives rise to three main types of faults: thrust fault, normal fault and strike-slip fault (Figure 3). Principal stress directions are represented by the three unit vectors S1, S2 and S3, for their respective principal stresses σ1, σ2 and σ3. This also derives the three main tectonic regimes:

  • S1 vertical: extensional tectonic regime
  • S2 vertical: strike-slip regime
  • S3 vertical: compressional regime

The basics of faulting and stress geometry assumes that the body of rock is homogeneous and isotropic, and that the shear surface of the fault follows Mohr-Coulomb shear failure criterion, that is, a fault occurs on the plane which intersects the failure envelope. Lineations on a slip surface indicate the movement direction, and are assumed to have the same direction and sense as the resolved shear stress on the fault plane. Striations represent the intersection of the fault surface with the S1-S3 plane. The intersection of conjugate faults defines the intermediate principal stress direction S2, and the acute angle between conjugate faults is bisected by the largest principal stress S1. The aforementioned is represented graphically by Mohr-Coulomb failure envelope (Figure 4).

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Figure 3: Interpretation of faults - Anderson's "standard" relationship between stresses and ideal faults.[5]

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Figure 4: Mohr-Coulomb failure criteria representing conjugate faults and associated principal stress orientations.[5]

Direct Graphical Methods of Stress Inversion

Numerous methods of graphical stress inversion using stereonet or coordinate geometry have been developed. [4] [7] With increased success of numerical analysis, many of these methods have been coded to produce solutions in a fraction of the time.

The P-T Method

References

  1. Read, J. & Stacey, P. (2010). Guidelines for Open Pit Mine Design. Collingwood, Australia CSIRO Publishing.
  2. 2.0 2.1 2.2 2.3 2.4 Golder Associates Ltd. (2011). Structural Geology Guidelines for Aiding Characterization of Deep Mining Fault Behaviour, Second Edition. The Center for Excellence in Mining Innovation.
  3. Zalohar, J. & Vrabec, M. (2007). Paleostress analysis of heterogeneous fault-slip data: The Gauss method. Journal of Structural Geology, 29, 1798-1810.
  4. 4.0 4.1 Allmendinger, R.W., Gephart, J.W. & Marrett, R.A. (1989). Notes on fault slip analysis. Geological Society of America Short Course on Quantitative Interpretation of Joints and Faults.
  5. 5.0 5.1 5.2 5.3 Burg, J.P. (2013). Structural Geology: Tectonic Systems. ETH Zurich and University of Zurich. Retrieved from http://www.files.ethz.ch/structuralgeology/JPB/files/English/
  6. Scholz, C.H. (2004). The Mechanics of Earthquakes and Faulting (Second Edition). Cambridge, UK. Cambridge University Press.
  7. Lisle, R.J. (1994). Detection of zones of abnormal strains in structures using Gaussian curvature analysis. AAPG Bulletin 78, 245-252.