Brittle–ductile transition zone

The depth interval in Earth’s crust where rocks shift from fracturing and frictional sliding to ductile flow under increasing temperature and pressure.

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Brittle deformation: Rock deformation by cracking, frictional sliding, or other permanent failure without sustained flow. This is the shallow deformation style that gives way across the transition.
Geothermal gradient: The rate at which temperature increases with depth in Earth. It determines how quickly rocks reach temperatures that permit ductile deformation.
Seismogenic zone: The part of a fault system capable of generating earthquakes through unstable slip. Its lower boundary often lies near the transition to stable ductile deformation.
San Andreas Fault: A major right-lateral strike-slip fault system in California marking part of the boundary between the Pacific and North American plates. Its earthquake depths and creeping sections illustrate lateral variation in brittle behavior.
Laboratory rock mechanics: Experimental study of rock deformation under controlled pressure, temperature, fluid, and strain-rate conditions. Experiments constrain flow laws used to estimate the transition in the crust.
Ductile deformation: Permanent change in a material’s shape through continuous flow or distributed crystal deformation rather than macroscopic fracture. This is the deeper deformation style that dominates beyond the transition.
Differential stress: The difference between the greatest and least principal stresses in a material. Its magnitude influences whether rocks fail by faulting or deform by creep.
Earthquake focal depth: The depth below Earth’s surface where an earthquake begins. The distribution of focal depths can reveal the brittle layer’s lower limit.
Alpine Fault: A major oblique plate-boundary fault in New Zealand’s South Island between the Pacific and Australian plates. Its steep thermal gradient helps produce a comparatively shallow transition.
Frictional-Viscous Transition: The change in rock deformation from frictional faulting to viscous creep as conditions vary. Its predicted location can differ from field estimates of the brittle–ductile boundary.