The spin zone: Transient mid-crust permeability caused by coseismic brecciation

• Published in: Journal of structural geology Vol. 87; pp. 47 - 63
• Main Authors: Melosh, Benjamin L., Rowe, Christie D., Gerbi, Christopher, Bate, Charlotte E., Shulman, Deborah
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.06.2016
• Subjects: Breccia; Brittle-plastic transition; Ductile brittle transition; Earthquake; Faults; Fluid flow; Fluids; Pofadder Shear Zone; Rock; Surface layer; Texture; Walls; South Africa; Namibia; Pofadder Shear Zone; Earthquake; Breccia; Brittle-plastic transition; Fluid flow
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2016.04.003

Pore fluids migrating through the deep section of continental strike-slip fault zones have been invoked to explain such phenomena as tectonic tremor, stress transfer across the brittle-ductile transition, and short timescales of co-seismic healing. In this contribution, we describe a coseismic mechanism for forming transient vertical fluid conduits within dilational jogs in strike-slip faults. We present field observations of breccias that formed coseismically at dilational stepovers in the dextral Pofadder Shear Zone, a ∼1Ga exhumed continental strike-slip fault in South Africa and Namibia. These breccias are interpreted to have formed when tensile fractures emanating from rupture tips intersected mylonitic foliation parallel to the rupture surface, which then failed, disaggregating the rock. We used quartz textures in the mylonites determined by electron backscatter diffraction to uniquely compare the orientation of each clast to the neighboring wall rock and constrain finite clast rotation within breccia bodies. Comparison of two- and three-dimensional rotation patterns show that clast trajectories are highly scattered when decoupled from wall rock, suggesting that Pofadder breccias were not formed by gradual plucking of clasts during slip. The dilational breccia bodies have sub-vertical geometries and high porosities relative to the host mylonites. We infer that the opening of these breccias may have created instantaneous, temporary vertical pathways for fluid draining through the brittle-plastic transition. These pathways healed post-seismically by cementation or ductile creep along the fault. The connection of many adjacent and overprinting breccia bodies through time provides a mechanism for fluid transport on a 10 s of km scale though the middle crust. •A new mechanism for fluid flow in the mid-crust.•A new method involving using electron backscatter diffraction to measure the 3D rotation of mylonite breccia clasts.•Time-dependent cyclic nature to coseismic brecciation – interseismic static healing of breccia bodies.