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The strength of lithospheric plates is a central component of plate tectonics, governed by brittle processes in the shallow portion of the plate and ductile behavior in the deeper portion. We review experimental constraints on ductile deformation of olivine, the main mineral in the upper mantle and thus the lithosphere. Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding (GBS), and diffusion-accommodated grain-boundary sliding (diffusion creep). Deformation in most of the lithosphere is dominated by GBS, except in shear zones—in which diffusion creep dominates—and in the brittle-ductile transition—in which low-temperature plasticity may dominate. We find that observations from naturally deformed rocks are consistent with extrapolation of the experimentally constrained olivine flow laws to geological conditions but that geophysical observations predict a weaker lithosphere. The causes of this discrepancy are unresolved but likely reside in the uncertainty surrounding processes in the brittle-ductile transition, at which the lithosphere is strongest. ▪ Ductile deformation of the lithospheric mantle is constrained by experimental data for olivine. ▪ Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding, and diffusion creep. ▪ Observations of naturally deformed rocks are consistent with extrapolation of olivine flow laws from experimental conditions. ▪ Experiments predict stronger lithosphere than geophysical observations, likely due to gaps in constraints on deformation in the brittle-ductile transition.
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Continuum approximation of dyking with a theory for poro-viscoelastic–viscoplastic deformation
SUMMARY To reach Earth’s surface, magma must ascend from the hot, ductile asthenosphere through cold and brittle rock in the lithosphere. It does so via fluid-filled fractures called dykes. While the continuum mechanics of ductile asthenosphere is well established, there has been little theoretical work on the cold and brittle regime where dyking and faulting occurs. Geodynamic models use plasticity to model fault-like behaviour; plasticity also shows promise for modelling dykes. Here we build on an existing model to develop a poro-viscoelastic–viscoplastic theory for two-phase flow across the lithosphere. Our theory addresses the deficiencies of previous work by incorporating (i) a hyperbolic yield surface, (ii) a plastic potential with control of dilatancy and (iii) a viscous regularization of plastic failure. We use analytical and numerical solutions to investigate the behaviour of this theory. Through idealized models and a comparison to linear elastic fracture mechanics, we demonstrate that this behaviour includes a continuum representation of dyking. Finally, we consider a model scenario reminiscent of continental rifting and demonstrate the consequences of dyke injection into the cold, upper lithosphere: a sharp reduction in the force required to rift.
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- PAR ID:
- 10414412
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 234
- Issue:
- 3
- ISSN:
- 0956-540X
- Page Range / eLocation ID:
- p. 2007-2031
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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