Understanding the growth and evolution of conjugate fault systems in compressional settings using Distinct Element Method forward Models and natural growth structures

Benjamin Chauvin and Jessica Don and Yanpeng Sun and John H. Shaw. ( 2019 )
in: 2019 Ring Meeting, ASGA

Abstract

Characterizing faults and their related folds is an essential element of defining structural styles and tectonic histories in fold-and-thrust belts, which has direct applications in seismic hazard assessments and natural resource development. Moreover, it is important to understand the mechanisms of this deformation to infer subsurface geometry and reduce uncertainties in structural interpretation. We focus on conjugate fault systems in compressional domains. These structures are ubiquitous in fold-and-thrust belts, and while we readily understand their origins in the context of Mohr-Coulomb theories, we generally lack insights into their kinematic behaviors. Our motivation is to use numerical modeling to investigate conjugate fault nucleation and growth with the goal of better understanding structures, using examples from the Niger Delta. We produce four two-dimensional numerical forward models representing the emergence and the evolution of conjugate faults. We use the Distinct Element Method (DEM), which is a particlebased numerical technique specialized in solving physical processes for discontinuous media. In our approach, models are cross sections in which geological units are composed of numerous disks interacting using contact laws. Faulting and folding occur during the numerical simulation as a consequence of the horizontal compression of the initially undeformed models. All of the models have a sedimentary growth sequence to track the evolution of faults and to study the kinematics of the deformation. Models have different geometrical and mechanical parameters in order to develop various styles of structures. Using the DEM, we develop conjugate faults, including fore-thrusts and back-thrusts, which present analogies with natural structures. We compare our models to high-quality seismic reflection data from the Niger Delta. Using growth strata geometries, we are able to determine the growth history of conjugate faulting and related structures. We observe structures that initiate as conjugate faults with one fault becoming dominant and carrying the secondary fault in its hanging wall as shortening increases. In these cases, growth strata record an initial period of coeval fault activity, followed by the secondary fault generally becoming inactive. In contrast, other conjugate fault systems develop related to the geometry and displacement on a non-planar fault. Specifically, displacement over a fault bend nucleates an antithetic fault often localized within a synclinal fault-bend fold. This secondary fault is subsequently translated in the hanging wall of the main fault away from the bend, and thus becomes inactive. Often, new antithetic faults develop, with the age of each fault recorded by growth strata. We use the interpretations inferred from the forward models to better understand the mechanisms of deformation occurring in the Niger Delta, and potentially in other compressional areas.

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BibTeX Reference

@INPROCEEDINGS{ChauvinRM2019,
    author = { Chauvin, Benjamin and Don, Jessica and Sun, Yanpeng and Shaw, John H. },
     title = { Understanding the growth and evolution of conjugate fault systems in compressional settings using Distinct Element Method forward Models and natural growth structures },
 booktitle = { 2019 Ring Meeting },
      year = { 2019 },
 publisher = { ASGA },
  abstract = { Characterizing faults and their related folds is an essential element of defining structural styles and tectonic histories in fold-and-thrust belts, which has direct applications in seismic hazard assessments and natural resource development. Moreover, it is important to understand the mechanisms of this deformation to infer subsurface geometry and reduce uncertainties in structural interpretation. We focus on conjugate fault systems in compressional domains. These structures are ubiquitous in fold-and-thrust belts, and while we readily understand their origins in the context of Mohr-Coulomb theories, we generally lack insights into their kinematic behaviors. Our motivation is to use numerical modeling to investigate conjugate fault nucleation and growth with the goal of better understanding structures, using examples from the Niger Delta. We produce four two-dimensional numerical forward models representing the emergence and the evolution of conjugate faults. We use the Distinct Element Method (DEM), which is a particlebased numerical technique specialized in solving physical processes for discontinuous media. In our approach, models are cross sections in which geological units are composed of numerous disks interacting using contact laws. Faulting and folding occur during the numerical simulation as a consequence of the horizontal compression of the initially undeformed models. All of the models have a sedimentary growth sequence to track the evolution of faults and to study the kinematics of the deformation. Models have different geometrical and mechanical parameters in order to develop various styles of structures. Using the DEM, we develop conjugate faults, including fore-thrusts and back-thrusts, which present analogies with natural structures. We compare our models to high-quality seismic reflection data from the Niger Delta. Using growth strata geometries, we are able to determine the growth history of conjugate faulting and related structures. We observe structures that initiate as conjugate faults with one fault becoming dominant and carrying the secondary fault in its hanging wall as shortening increases. In these cases, growth strata record an initial period of coeval fault activity, followed by the secondary fault generally becoming inactive. In contrast, other conjugate fault systems develop related to the geometry and displacement on a non-planar fault. Specifically, displacement over a fault bend nucleates an antithetic fault often localized within a synclinal fault-bend fold. This secondary fault is subsequently translated in the hanging wall of the main fault away from the bend, and thus becomes inactive. Often, new antithetic faults develop, with the age of each fault recorded by growth strata. We use the interpretations inferred from the forward models to better understand the mechanisms of deformation occurring in the Niger Delta, and potentially in other compressional areas. }
}