Significance

Rocks close to the surface fracture, whereas rocks at depths flow implying the existence of a “brittle-to-ductile” transition (BDT) region. This region is key in several Earth science applications and yet remains poorly understood. Whether rocks fracture or flow is fundamentally determined by the activity of various microscale defects. Given that these defects are microscopic and typically need high pressures and temperatures to be activated, it remains a challenge to observe the defect dynamics in situ. Here, we present observations of such defect dynamics in rocks undergoing a BDT. Using ultrasound probes, we document that the nature, size, and propagation velocity of defects profoundly changes as the rocks cross their BDT, providing constraints for micromechanical models of semi-brittle deformation.

Abstract

Deformation of all materials necessitates the collective propagation of various microscopic defects. On Earth, fracturing gives way to crystal-plastic deformation with increasing depth resulting in a “brittle-to-ductile” transition (BDT) region that is key for estimating the integrated strength of tectonic plates, constraining the earthquake cycle, and utilizing deep geothermal resources. Here, we show that the crossing of a BDT in marble during deformation experiments in the laboratory is accompanied by systematic increase in the frequency of acoustic emissions suggesting a profound change in the mean size and propagation velocity of the active defects. We further identify dominant classes of emitted waveforms using unsupervised learning methods and show that their relative activity systematically changes as the rocks cross the brittle–ductile transition. As pressure increases, long-period signals are suppressed and short-period signals become dominant. At higher pressures, signals frequently come in avalanche-like patterns. We propose that these classes of waveforms correlate with individual dominant defect types. Complex mixed-mode events indicate that interactions between the defects are common over the whole pressure range, in agreement with postmortem microstructural observations. Our measurements provide unique, real-time data of microscale dynamics over a broad range of pressures (10 to 200 MPa) and can inform micromechanical models for semi-brittle deformation.

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Data, Materials, and Software Availability

All study data are contained in the article and/or supporting information. These data can also be found in digital form in ref. (51).

Acknowledgments

H.O., M.P., and T.M. would like to thank Ben Holtzman for stimulating discussions on “machine listening” during numerous group meetings. Discussion of an early version of the paper with T. Grove, O. Jagoutz, B. Hager, and G. Dresen helped us to improve the paper. We also thank Alejandra Quintanilla Terminel for providing the gridded cylinders used for microstructural analysis and Camilla Cattania for discussion of source models. T.M. acknowledges support by the Crosby Fellowship at MIT, M.P. acknowledges funding by MIT’s J.H. and E.V. Wade Fund. “CORD” Laboratory technician support was provided by NSF-2054414 to M.P. and B.E.

Author contributions

H.O. and M.P. designed research; H.O., M.P., T.M., U.M., and H.C. performed research; H.O., T.M., and H.C. contributed new reagents/analytic tools; H.O., M.P., T.M., U.M., H.C., and B.E. analyzed data; and H.O., M.P., and T.M. wrote the paper.

Competing interests

The authors declare no competing interest.

Supporting Information

Appendix 01 (PDF)
Movie S1.
Slow propagation of a defect front in deformation of a calcite single crystal.
Movie S2.
Fast propagation of river-like defects in deforming calcite single crystal.

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Information & Authors

Information

Published in

The cover image for PNAS Vol.120; No.42
Proceedings of the National Academy of Sciences
Vol. 120 | No. 42
October 17, 2023
PubMed: 37812718

Classifications

Data, Materials, and Software Availability

All study data are contained in the article and/or supporting information. These data can also be found in digital form in ref. (51).

Submission history

Received: April 7, 2023
Accepted: August 18, 2023
Published online: October 9, 2023
Published in issue: October 17, 2023

Keywords

  1. rock deformation
  2. brittle–ductile transitions
  3. defect dynamics
  4. ultrasound probes

Acknowledgments

H.O., M.P., and T.M. would like to thank Ben Holtzman for stimulating discussions on “machine listening” during numerous group meetings. Discussion of an early version of the paper with T. Grove, O. Jagoutz, B. Hager, and G. Dresen helped us to improve the paper. We also thank Alejandra Quintanilla Terminel for providing the gridded cylinders used for microstructural analysis and Camilla Cattania for discussion of source models. T.M. acknowledges support by the Crosby Fellowship at MIT, M.P. acknowledges funding by MIT’s J.H. and E.V. Wade Fund. “CORD” Laboratory technician support was provided by NSF-2054414 to M.P. and B.E.
Author contributions
H.O. and M.P. designed research; H.O., M.P., T.M., U.M., and H.C. performed research; H.O., T.M., and H.C. contributed new reagents/analytic tools; H.O., M.P., T.M., U.M., H.C., and B.E. analyzed data; and H.O., M.P., and T.M. wrote the paper.
Competing interests
The authors declare no competing interest.

Notes

This article is a PNAS Direct Submission.

Authors

Affiliations

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Tushar Mittal
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Present address: Department of Geosciences, Pennsylvania State University, University Park, PA 16802.
Ulrich Mok
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139

Notes

1
To whom correspondence may be addressed. Email: [email protected] or [email protected].

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