Throw variations and strain partitioning associated with fault-bend folding along normal faults

Efstratios Delogkos,Conrad Childs,John J Walsh,Muhammad Mudasar Saqab,Vincent Roche

doi:10.5194/se-11-935-2020

Abstract

Abstract. Normal faults have irregular geometries on a range of scales arising from different processes including refraction and segmentation. A fault with constant dip and displacement on a large-scale will have irregular geometries on smaller scales, the presence of which will generate fault-related folds and down-fault variations in throw. A quantitative model is presented which illustrates the deformation arising from movement on irregular fault surfaces, with fault-bend folding generating geometries reminiscent of normal and reverse drag. Calculations based on the model highlight how fault throws are partitioned between continuous (i.e. folding) and discontinuous (i.e. discrete offset) strain along fault bends for the full range of possible fault dip changes. These calculations illustrate the potential significance of strain partitioning on measured fault throw and the potential errors that will arise if account is not taken of the continuous strains accommodated by folding and bed rotations. We show that fault throw can be subject to errors of up to ca. 50 % for realistic down-dip fault bend geometries (up to ca. 40∘), on otherwise sub-planar faults with constant displacement. This effect will provide irregular variations in throw and bed geometries that must be accounted for in associated kinematic interpretations.

Highlights

We present a new quantitative model for the relationship between down-dip fault bend geometry and strain partitioning along normal faults, and we demonstrate its applicability to different geological examples
A quantitative model has been presented for the throw variations and strain partitioning associated with fault-bend folding along normal faults with fault surface irregularities arising from propagation-related phenomenon
The main feature of this model is that the variations in discontinuous and continuous throws along nonplanar normal faults are complementary given that the displacement and total throw are constant and not affected by the fault bends

Summary

Introduction

Fault-bend folding refers to the folding of layered rocks in response to slip over a down-dip fault bend (e.g. Suppe, 1983), an issue which has been the subject of many studies in both extensional E. Delogkos et al.: Throw variations and strain partitioning derives from the importance of fault bends and associated ramp-flat geometries in thrust systems and from circumstances in which fault-bend folding is often easier to define than the fault displacements that are responsible for its development. Gibbs, 1984; Williams and Vann, 1987; Xioa and Suppe, 1992; Withjack and Schlische, 2006; Xiaoli et al, 2015) in particular, but the recognition that normal faults are often approximately planar in comparison to the ramp-flat geometries in thrust systems has meant that other models are often used to explain the deformation geometries surrounding normal faults, including hangingwall rollover and footwall uplift The local variations in the component of fault throw along fault bends are accommodated by folding (i.e. continuous deformation) and faulting (i.e. discontinuous deformation) and have implications for interpretations of fault growth and for a variety of practical applications, such as (i) across-fault juxtaposition and sealing, (ii) the generation of fault-related traps, both in terms of four-way and three-way dip closures, and (iii) assessments of hazard and earthquake slip

Quantitative model of strain partitioning

Geological examples

Model assumptions

Evolution of fault zones

Implications