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Hardy, Stuart, and Richard W. Allmendinger,
Trishear: A Review of Kinematics, Mechanics, and Applications
Stuart Hardy,1 Richard W. Allmendinger2
1Institucio Catalana de Recerca i Estudis Avancats (ICREA) and Grup de Geodinamica i Analisi de Conques (GGAC) Departament de Geodinamica i Geofisica, Facultat de Geologia, Universitat de Barcelona, Barcelona, Spain
2Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York
This review grew out of discussions during the International Conference on Theory and Application of Fault-related Folding held in Beijing, June 2005. The authors are grateful to the conference organizers for hosting such a stimulating meeting and for providing financial support to attend. We are most grateful to our many colleagues with whom we have explored trishear-like folding in the past, including Mary Ford, Ernesto Cristallini, Nestor Cardozo, Alan Zehnder, Emma Finch, Kate Cooper, Asdrubal Bernal, John Shaw, Kavi Bhalla, and Juli Morgan. Reviews by John Suppe and Ken McClay are greatly appreciated. Additionally, we thank several colleagues who have had considerable impact on our understanding of fault-related folds, including Ken McClay, John Suppe, Ray Fletcher, and Karl Mueller. Allmendinger acknowledges support for his trishear research from the U.S. National Science Foundation (EAR-9814348, EAR-0125557) and the donors to the Petroleum Research Fund of the American Chemical Society (ACS-PRF 34882-AC2). Hardy acknowledges support by Institucio Catalana de Recerca i Estudis Avancats (ICREA), the Centre de Supercomputacio de Catalunya (CESCA), the Geomod 3-D project (CGL2004-05816-C02-01/BTE), and the Geomodels program.
Trishear is a kinematic model of fault-propagation folding in which the decrease in displacement along the fault is accommodated by deformation in a triangular shear zone radiating from the tip line. This model has garnered increasing acceptance, particularly for cases where parallel kink-fold models do not work (e.g., footwall synclines, lateral and vertical changes in bedding thickness, and orientation). The articulation of the model in terms of velocity fields has enabled systematic explorations of the parameters controlling the trishear geometry; rapid, objective application of trishear to the simulation of real structures; and application of the model in three dimensions. The model has highlighted the importance of a parameter not unique to trishear, the propagation to slip ratio, which has a profound effect on fold geometry and is fundamental to understanding all types of fault-related folds. The drive to understand the significance of such parameters has instigated the application of several mechanical modeling strategies. Block-motion viscous, finite-element, and discrete-element analyses have all provided insight into trishearlike fault-propagation folds. Clearly, from these models, trishear most successfully simulates fold geometries where significant layered anisotropy is absent and the material is incompressible. Despite these modeling efforts, the significance of the trishear apical angle remains elusive. Trishear has been applied to a variety of real-world problems, including growth strata analysis, potential fracture distribution, paleoseismology, and even seismic hazard analysis.
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