What is Trishear?

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Authors

Alexander James Coleman, Oliver B Duffy, Christopher Aiden-Lee Jackson 

Abstract

The kinematics of fault-propagation folds, formed above the tips of upward propagating normal faults, is typically inferred from numerical and physical models. Trishear is a forward kinematic model in which deformation occurs in a triangular zone in front of the propagating fault tip, with the geometry of this zone, and the geometry and growth of the resulting fold, related to several parameters (e.g. fault dip, trishear angle, trishear symmetry, concentration factor, cover thickness). Trishear is powerful as it can model fold growth through time, allowing us to assess how natural structures identified in the field or in seismic reflection data evolved. However, the geological significance of trishear is poorly understood, and the effects of trishear parameters on the overall fold geometry and the stratigraphic architecture of synkinematic deposits remain poorly constrained. In this study we vary trishear parameters independently to demonstrate how they control the temporal variability in fold geometry and size, and how this is recorded in the architecture of synkinematic strata. We show that the propagation-to-slip ratio (the ratio between upper tip propagation into the cover and slip increment at the fault centre) is the most important factor in fold growth. When this ratio is relatively low, other parameters, such as the trishear angle and symmetry, concentration factor, more strongly control fold shape and size, with fault dip arguably and perhaps surprisingly being the least important. When this ratio is relatively high, the cover is breached rapidly, leaving little time for folding. Our analysis predicts that fault-propagation folds widen rapidly and establish their near-final width early during fold growth, whereas fold amplitude develops gradually with fault slip. Fold shape therefore significantly changes throughout fold growth. During early fold growth, folds are wide with initially small amplitudes but gradually amplify as folding progresses so that amplitudes and widths become increasingly similar towards the later stages of growth folding, folds have similar widths but have large amplitudes. We also speculate on the geological significance of the propagation-to-slip ratio, trishear angle, concentration factor and trishear symmetry, and under what scenarios these parameters may correspond to in extensional basins. Our results have implications for understanding the geometry and growth of extensional fault-propagation folds, and for estimating the best-fit parameters (and related geological controls) for natural examples.

DOI

https://doi.org/10.31223/osf.io/uzhkr

Subjects

Earth Sciences, Geology, Physical Sciences and Mathematics, Tectonics and Structure

Keywords

Fault kinematics, fault-propagation folding, normal fault, normal faulting, fault-related folding, fault-tip folding, fold kinematics, kinematic modelling, trishear

Dates

Published: 2019-07-20 18:38

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License

CC BY Attribution 4.0 International

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