Deep-focus earthquakes, occurring at depths between 300 and 700 kilometers within subducting oceanic lithosphere, present a fundamental paradox in geophysics. At such immense pressures and temperatures, rocks should undergo ductile deformation rather than brittle failure, which is the mechanism responsible for shallow crustal quakes. To reconcile this discrepancy, two competing hypotheses have been advanced: transformational faulting and dehydration embrittlement. Transformational faulting posits that metastably preserved olivine within the cold core of a subducting slab undergoes a prompt phase transformation to a denser polymorph—wadsleyite or ringwoodite—under non-hydrostatic stress. This phase transition nucleation generates localized zones of fine-grained, superplastic material, leading to runaway shear instability. Conversely, dehydration embrittlement suggests that hydrous minerals, such as serpentine, release pore fluids upon thermal breakdown, thereby reducing effective normal stress and facilitating brittle shear.
However, recent high-pressure mineral physics experiments conducted by Dr. Aris Thorne and his team have qualified the scope of dehydration embrittlement. Thorne’s experiments demonstrated that at depths exceeding 400 kilometers, the ambient overburden pressure inhibits the volumetric expansion required for free fluid liberation. Consequently, while dehydration embrittlement remains a viable trigger for intermediate-depth events (70–300 km), it cannot account for hypocenters located in the lower mantle transition zone. Furthermore, acoustic emission spectroscopy during the olivine-to-ringwoodite transition revealed that phase-transformation shear instability operates independently of fluid pressure, producing stress drops consistent with seismic observations of deep subduction zones. Thus, while fluid-driven faulting dominates shallower subduction regimes, transformational faulting stands as the primary physical driver of failure at extreme depths.
According to the passage, Thorne’s experiments led to which of the following conclusions regarding dehydration embrittlement?
- It is physically incapable of initiating seismic failure at depths greater than 400 kilometers because high ambient pressures restrict the volume expansion necessary for fluid release.Cevap
- BIt serves as the predominant trigger for shallow crustal earthquakes occurring at depths of less than 70 kilometers.
- CIt generates localized zones of fine-grained, superplastic material through prompt phase transformations under non-hydrostatic stress.
- DIt operates independently of ambient overburden pressure when hydrous minerals such as serpentine undergo thermal breakdown.
- EIt accounts for the acoustic emissions and stress drops observed across all subduction zones regardless of hypocenter depth.