image: (a) Geological units and earthquake distribution of an oceanic subduction zone. The orange shadow beneath the volcanic arc represents partially molten areas and magma channels. (b) Thermal structure of an oceanic subduction zone, modified from modeling results of Peacock and Wang (2021) for NE Japan. Black dashed lines are isotherms in ºC. Abbreviations: SSE, slow slip events; ETS, episodic tremor and slip.
Credit: ©Science China Press
How the heat exchange between the subducted plate and the mantle wedge affects the fluid/melt migration and earthquakes in subduction zones is critical for our understanding of material and energy cycles during plate convergence. Earthquakes in the overriding plate often occur along thrust faults in the upper crust under compression. Interplate earthquakes not only include destructive megathrust earthquakes at the subduction plate boundary, but also slow earthquakes with longer source durations and lower frequencies. Intraslab earthquakes take place from the outer rise to the top of the lower mantle, showing variable focal mechanisms and distribution patterns from place to place.
“Because temperature is a controlling factor of mineral dehydration reactions and the brittle-ductile transition in rocks, the key to the contrasting seismicity in different oceanic subduction zones is thermal structure” explains Qin WANG, professor at School of Earth Sciences and Engineering at Nanjing University and corresponding author of the study.
Thermal structure of oceanic subduction zones
Geodynamic modeling results indicate the plate convergence rate is the primary factor controlling the thermal structure of subduction zones. Younger slabs exhibit higher slab surface temperature, and slower plate convergence velocity leads to higher slab surface temperature at depths less than 70 km. The maximum depth of decoupling (MDD) between the slab and the mantle wedge is 70–80 km. Updip of the MDD, the cooling effect of the subducting slab results in a "cold corner" in the forearc mantle wedge. Downdip of the MDD, the subducting slab drags the overlying mantle to move together (fully coupled) and induces the corner flow in the mantle wedge, which heats the slab surface rapidly.
It is interesting to notice that 70–80 km is also the peak exhumation depth of metamorphic rocks along the oceanic subduction plate interface. Mechanisms of the MDD remain unclear. Comparison of rock P-T records with thermal models of subduction zones reveal that exhumation of metamorphic rocks mainly occurs at high geothermal gradients at the beginning or ending subduction stages. Therefore, it is important to consider the effects of “survivorship bias” in geological records. Due to the widespread occurrence of oblique subduction and the thermal structure evolution of subduction zones, it is necessary to integrate global and regional scale observations, 3D thermodynamical modeling of subduction zones and global plate reconstructions.
Structure and deformation of the subduction plate interface
The subduction plate interface, also referred to as the subduction channel, comprises the roof décollement, the basal décollement, and a deformation zone between them. In partial locking areas, strain localization in weak rocks or lithologic contact could cause downward or upward migration of the basal and roof décollements. Consequently, rocks exhumed from the oceanic subduction channel are very complex. They could originate from the subducted slab, the forearc crust, and peridotites and pyroxenites from the mantle wedge.
The mechanical coupling degree and deformation of the subduction plate interface are controlled by temperature and lithology. From the trench to depths of 40–50 km, decoupling between the subducting plate and the relatively fixed upper plate is characterized by megathrust earthquakes along the subduction plate interface. Below depths of 70–80 km, the comparable rheological strength between the subducting plate interface and the mantle wedge causes viscous coupling. As a result, earthquakes distribute within the slab. Between the decoupling and viscous coupling domains, the subducting slab is partially coupled with the mantle wedge. Topography variations, material mixing and heterogeneous fluid activity in the subduction plate interface will affect the fault locking and earthquake nucleation process, resulting in coexistence of short-term brittle deformation (such as earthquakes and interseismic locking) and long-term ductile deformation.
Fluid activity and seismicity in subduction zones
Statistics on earthquakes in global subduction zones show that number of earthquakes decreases with depth and reaches the minimum at depth of ~300 km. In cold subduction zones, most hydrous minerals experience dehydration at subarc depths of 80–200 km, while water in lawsonite and phengite will be totally released until ~300 km. In warm subduction zones, complete dehydration is achieved in most hydrous minerals at subarc depths of <160 km. Earthquake distribution in the present-day cold and warm subduction zones is consistent with dehydration depths of hydrous minerals, demonstrating that dehydration embrittlement of hydrous minerals is the major mechanism of intermediate-depth earthquakes in oceanic subduction zones. Other mechanisms such as thermal runaway instability, eclogitization-related embrittlement, and metamorphism-facilitated instability in orthopyroxene may also contribute to intermediate-depth earthquakes.
The source regions of slow earthquakes in subduction zones are characterized by low effective stress and high pore fluid pressure, which may be caused by dehydration reactions of multiple hydrous minerals. Nominally anhydrous minerals and dense hydrous magnesium silicates in cold slabs can transport water to depths below 300 km, resulting in localized water enrichment in the mantle transition zone. Transformational faulting of metastable olivine is considered as the main mechanism for deep-focus earthquakes, while the role of water in deep-focus earthquakes remain controversial.
“There are still many puzzles in the interplay of metamorphism, seismicity, and fluid/melt activity during plate subduction,” says Qin WANG. Experiments and theoretical calculations of phase stability in an open system, high-precision earthquake locations, as well as integrated studies of fossil earthquakes in ancient subduction zones will provide new insights into dynamic evolution of subduction zones and improve seismic risk assessment in subduction zones.
Journal
Science China Earth Sciences
Method of Research
Literature review