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Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones

Abstract

The feedback between fluid migration and rock deformation in mid-crustal shear zones is acknowledged as being critical for earthquake nucleation, the initiation of subduction zones and the formation of mineral deposits1,2,3. The importance of this poorly understood feedback is further highlighted by evidence for shear-zone-controlled advective flow of fluids in the ductile lower crust4 and the recognition that deformation-induced grain-scale porosity is a key to large-scale geodynamics5,6. Fluid migration in the middle crust cannot be explained in terms of classical concepts. The environment is considered too hot for a dynamic fracture-sustained permeability as in the upper crust7, and fluid pathways are generally too deformed to be controlled by equilibrium wetting angles that apply to hotter, deeper environments8,9,10. Here we present evidence that mechanical and chemical potentials control a syndeformational porosity generation in mid-crustal shear zones. High-resolution synchrotron X-ray tomography and scanning electron microscopy observations allow us to formulate a model for fluid migration in shear zones where a permeable porosity is dynamically created by viscous grain-boundary sliding, creep cavitation, dissolution and precipitation. We propose that syndeformational fluid migration in our ‘granular fluid pump’ model is a self-sustained process controlled by the explicit role of the rate of entropy production of the underlying irreversible mechanical and chemical microprocesses. The model explains fluid transfer through the middle crust, where strain localization in the creep regime is required for plate tectonics, the formation of giant ore deposits, mantle degassing and earthquake nucleation. Our findings provide a key component for the understanding of creep instabilities in the middle crust.

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Figure 1: The investigated hand specimen from a mid-crustal shear zone, and the microstructural evolution with increasing shear strain.
Figure 2: Porosity distribution across a strain gradient in the investigated mid-crustal shear zone.
Figure 3: Three-dimensional visualization of porosity from the shear-zone centre, and grain boundary pores in quartz/K-feldspar mixtures.
Figure 4: Granular fluid pump model for the VGBS mylonite in the shear-zone centre.

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Acknowledgements

This work was supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract number DE-AC02-06CH11357. The work was supported by iVEC through the use of visualization resources and expertise provided by the WASP and ARRC facilities. We acknowledge the Centre for Microscopy, Characterization and Analysis at the University of Western Australia for the use of its FESEM. Our work was financially supported by the Western Australian Premier’s Research Fellowship program and the University of Western Australia through a research grant. The Multiscale Earth System Dynamics group as well as H. Stuenitz, R. Heilbronner and D. Healy participated in discussions. C. Schrank assisted with data processing.

Author Contributions F.F. did the field work, the FESEM analyses and part of the synchrotron experiment, interpreted the data and co-wrote the paper. K.R.-L. designed the study and co-wrote the paper. J.L. processed and analysed the tomographic data. R.M.H. designed and did part of the synchrotron experiment and co-wrote the paper. F.D.C. did part of the synchrotron experiment and processed tomographic data. All authors discussed the results and commented on the paper.

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Correspondence to F. Fusseis.

Supplementary information

Supplementary Information

This file contains a Supplementary Discussion, Supplementary Data, Supplementary Methods, Supplementary References, Supplementary Figures 1-3 with Legends and Legends for Movies 1-2 and Supplementary Movies 1-2. (PDF 951 kb)

Movie 1

In this movie we see porosity (red, lowest absortion of X-rays), a mica phase (blue, intermediate absorption) and oxides plus epidote (yellow, high absorption), in subsample B01 from the low-strain shear zone margin ('a' in Supplementary Figure 1). See file s1 for full Legend. (MOV 33303 kb)

Movie 2

In this movie we see isosurfaces representing porosity (red, lowest absortion of X-rays), a mica phase (blue, intermediate absorption) and oxides plus epidote (yellow, high absorption), in subsample H03 from the high-strain shear zone centre ('b' in Supplementary Figure 1). See file s1 for full Legend. (MOV 32156 kb)

Supplementary Movie 1

Supplementary Movie 1 zooms into pores comprising pore sheets in about the centre of subsample H03, which is shown in Figure 3d and movie 2. See file s1 for full Legend. (MOV 12123 kb)

Supplementary Movie 2

Supplementary Movie 2 zooms into pores forming an intracrystalline porous skeleton in a plagioclase clast in the peripheral low-strain sample J01. See file s1 for full Legend. (MOV 17080 kb)

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Fusseis, F., Regenauer-Lieb, K., Liu, J. et al. Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones. Nature 459, 974–977 (2009). https://doi.org/10.1038/nature08051

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