The redistribution of heat by fluid circulation in subducting igneous crust generates thermal anomalies that can affect the alteration of material both within a subduction zone and in the incoming plate prior to subduction. This hydrothermal circulation mines heat from subducted crust and transports it seaward, resulting in anomalously high temperatures in material seaward of the trench and anomalously low temperatures in the subduction zone. Anomalously high temperatures on the incoming plate are spatially limited; for example, on the Nankai margin of southern Japan, a zone of high temperatures is within ∼30 km of the accretionary prism deformation front. The incoming plate (Shikoku Basin) undergoes the high-temperature anomaly for less than 2 million years; so the alteration of clay minerals in Shikoku basin sediments advances only slightly because of the thermal anomaly. In contrast, subducted material is cooled by hydrothermal circulation, and therefore alteration of subducted sediment and igneous rock is shifted farther landward (i.e., delayed); in the Cascadia and Nankai margins, this includes the seismically inferred locations of the basalt-to-eclogite transition in the subducting crust. In very hot margins, hydrothermal circulation cools the subducting slab and affects where, and if, subducting material may melt. In southern Chile, this cooling helps explain the lack of a basaltic melt signature in arc lavas despite the young subducting lithosphere. Finally, the cooling of the subducting slab via hydrothermal circulation shifts fluid sources from dehydration reactions farther landward, delays metamorphic reactions that tend to reduce permeability, and increases fluid viscosity. The responses to hydrothermal circulation in subducting crust are most pronounced in the hottest subduction zones, where the lateral heat exchange in the subducting basement aquifer is greatest.
Abstract In Northeast Japan and Izu‐Bonin, arc volcanoes form in clusters or as cross‐arc chains. Their occurrence emphasizes the non‐uniform distributions of sub‐arc temperature and fluids that control the spacing of arc volcanoes. Here, using 3‐D numerical models, we show that the cessation of back‐arc spreading promotes volcano clustering by triggering the formation of nascent lithospheric drips – downward protrusions of cold and dense lithosphere‐adjacent to the thinned back‐arc lithosphere. The nascent drips interfere with the flow of the hot asthenospheric mantle from the back‐arc toward the arc, leading to gradual development of alternating hot and cold regions beneath the arc. The results indicate that along‐arc variation in the sub‐arc mantle temperature is largest not during back‐arc spreading but after its cessation, explaining the time offset by several million years between back‐arc spreading and volcano clustering in Northeast Japan and Izu‐Bonin.
The shallow part of the interface between the subducting slab and the overriding mantle wedge is evidently weakened by the presence of hydrous minerals and high fluid pressure. We use a two‐dimensional finite element model, with a thin layer of uniform viscosity along the slab surface to represent the strength of the interface and a dislocation‐creep rheology for the mantle wedge, to investigate the effect of this interface “decoupling.” Decoupling occurs when the temperature‐dependent viscous strength of the mantle wedge is greater than that of the interface layer. We find that the maximum depth of decoupling is the key to most primary thermal and petrological processes in subduction zone forearcs. The forearc mantle wedge above a weakened subduction interface always becomes stagnant (<0.2% slab velocity), providing a stable thermal environment for the formation of serpentinite. The degree of mantle wedge serpentinization depends on the availability of aqueous fluids from slab dehydration. A very young and warm slab releases most of its bound H 2 O in the forearc, leading to a high degree of mantle wedge serpentinization. A very old and cold slab retains most of its H 2 O until farther landward, leading to a lower degree of serpentinization. Our preferred model for northern Cascadia has a maximum decoupling depth of about 70–80 km, which provides a good fit to surface heat flow data, predicts conditions for a high degree of serpentinization of the forearc mantle wedge, and is consistent with the observed shallow intraslab seismicity and low volume of arc volcanism.
Abstract Mineral grain size in the mantle affects fluid migration by controlling mantle permeability; the smaller the grain size, the less permeable the mantle is. Mantle shear viscosity also affects fluid migration by controlling compaction pressure; high mantle shear viscosity can act as a barrier to fluid flow. Here we investigate for the first time their combined effects on fluid migration in the mantle wedge of subduction zones over ranges of subduction parameters and patterns of fluid influx using a 2‐D numerical fluid migration model. Our results show that fluids introduced into the mantle wedge beneath the forearc are first dragged downdip by the mantle flow due to small grain size (<1 mm) and high mantle shear viscosity that develop along the base of the mantle wedge. Increasing grain size with depth allows upward fluid migration out of the high shear viscosity layer at subarc depths. Fluids introduced into the mantle wedge at postarc depths migrate upward due to relatively large grain size in the deep mantle wedge, forming secondary fluid pathways behind the arc. Fluids that reach the shallow part of the mantle wedge spread trench‐ward due to the combined effect of high mantle shear viscosity and advection by the inflowing mantle and eventually pond at 55–65 km depths. These results show that grain size and mantle shear viscosity together play an important role in focusing fluids beneath the arc.
Some recent damaging earthquakes occurred in the lower crust or mantle of warm subducting slabs. They are consistent with a theoretical prediction that larger events tend to be deeper inside the slab as a result of mechanical damage to the crust caused by metamorphic rock densification. The densification begins in a thin layer along the slab surface, inducing a stretching force in it. Fracture spacing scales with layer thickness, resulting in a “shattered” upper crust in which earthquake ruptures have limited propagation distance. In contrast, the more uniform untransformed substrata can host larger ruptures. Often, the lack of compression in warm‐slab mantle is also consistent with a shattered crust.
