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Shear veins observed within anisotropic fabric at high angles to the maximum compressive stress

Nature Geoscience volume 3, pages 482–485 (2010)Cite this article

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Abstract

Some faults seem to slip at unusually high angles (>45°) relative to the orientation of the greatest principal compressive stress1,2,3,4,5. This implies that these faults are extremely weak compared with the surrounding rock6. Laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre-existing frictionally weak surface7,8,9, weakening from chemical reactions10, elevated fluid pressure11,12,13 or dissolution–precipitation creep14,15. Here we describe shear veins within the Chrystalls Beach accretionary mélange, New Zealand. The mélange is a highly sheared assemblage of relatively competent rock within a cleaved, anisotropic mudstone matrix. The orientation of the shear veins—compared with the direction of hydrothermal extension veins that formed contemporaneously—indicates that they were active at an angle of 80°±5° to the greatest principal compressive stress. We show that the shear veins developed incrementally along the cleavage planes of the matrix. Thus, we suggest that episodic slip was facilitated by the anisotropic internal fabric, in a fluid-overpressured, heterogeneous shear zone. A similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings. We therefore conclude that incremental slip along a pre-existing planar fabric, coupled to high fluid pressure and dissolution–precipitation creep, may explain active slip on severely misoriented faults.

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Figure 1: Geometry of the Chrystalls Beach Complex fault-fracture mesh.
Figure 2: Field evidence for shear at high angles to σ1.
Figure 3: Deformation sequence.
Figure 4: Conditions for ‘dilational hydroshear’.

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Acknowledgements

We thank R. J. Norris for discussion. A.F. gratefully acknowledges financial support from a GNS Science Hazards Scholarship and an Otago University Postgraduate Publishing Grant. A. R. Niemeijer and D. Faulkner provided critical reviews, which significantly improved the paper.

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Author notes

  1. Åke Fagereng

    Present address: Present address: Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa,

Authors and Affiliations

  1. Department of Geology, University of Otago, Dunedin 9054, PO Box 56, New Zealand

    Åke Fagereng & Richard H. Sibson

  2. Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Largo S. Eufemia 19, 41100 Modena, Italy

    Francesca Remitti

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Contributions

A.F. and F.R. carried out the field and microstructural studies. R.H.S. designed the study. All authors contributed to the writing.

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Correspondence to Åke Fagereng.

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Fagereng, Å., Remitti, F. & Sibson, R. Shear veins observed within anisotropic fabric at high angles to the maximum compressive stress. Nature Geosci 3, 482–485 (2010). https://doi.org/10.1038/ngeo898

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