Slab dynamics in the transition zone
Introduction
The Japanese subduction zone is characterized by long-term subduction of old lithosphere (125•132 Ma) at rates of 86•93 mm/yr (Lallemand et al., 2005), suggesting that the sinking slab has more than enough negative buoyancy to pull the subducting plate and to sink deep into the mantle. Yet seismological observations show that the slab is subducting at a shallow dip of 19•26° extending more than 800 km laterally before reaching the transition zone and lies horizontally within the transition zone for another 600 km (Niu et al., 2005). While trench retreat is often suggested as a cause of stagnant slabs in the transition zone, the present-day trench retreat rate in Japan is only 20 mm/yr contributing less than 3° of slab shallowing per million years.
The Japan slab is also characterized by scattered seismicity to depths of 670 km with down-dip compression axis at all depths (Isacks and Molnar, 1969) indicating that the slab remains stiff enough to transmit stresses up-dip. Seismic data indicate that the 410-km phase change is elevated in the slab contributing to the negative buoyancy of the slab, while the 660-km phase change is depressed over a broad region below the horizontal slab, which appears to rest on this boundary (Niu et al., 2005). However, there is also new evidence for a metastable olivine wedge extending from 400 to 560 km at the core of the slab (Jiang et al., 2008).
So why has the Japan slab failed to sink deeper into the lower mantle? Is the positive buoyancy of a metastable olivine wedge or delayed transformation to perovskite at 660 km to blame? Has intense deformation locally weakened the slab and prevented it from pushing into the higher viscosity lower mantle? Did transient trench roll-back in the past cause shallowing of the slab and therefore setting the stage later trapping of the slab in the transition zone?
While the Japan slab is a particularly well-studied stagnant slab (Fukao et al., 2010), these questions can be asked of any of the present-day slabs that appear to be stagnant in the transition zone (e.g., Tonga, Java, Hebrides). Is the stagnation due to a lack of sufficient negative buoyancy, changes in slab strength, the background mantle viscosity structure, geometric effects, or all of these, possibly with each subduction zone exhibiting a unique combination? Are stagnant slabs a transient feature of all subduction zones (Fukao et al., 2001) or does a particular combination of conditions need to occur?
Isolating the first order cause of stagnant slabs is a difficult task given the limited number of observations, and in particular the limited constraints on the evolution of many of these subduction zones. Instead, here I attempt to eliminate possible causes by combining analysis of subduction characteristics with comparisons to predictions from analytical models and the results of recent analog and numerical simulations of subduction.
Section snippets
Enigmatic subduction characteristics
The simplest models consider first order effects of the driving forces of subduction in combination with the effects of coupling a sinking slab with mantle flow, and provide basic intuition on the expected behavior of slabs. For example, we expect a slab with more negative buoyancy (e.g., older and/or longer) to sink more quickly, or a stiffer slab (e.g., colder, dryer, and/or less deformed) to have a more shallow dip. However, as many studies have shown before subduction zone characteristics
Dynamics of slab descent
The evolution of a slab as it descends through the mantle can be thought of as occurring through perturbations of the slab's vertical descent by the evolution of the rheologic structure of the slab. In other words, without the strength of the slab, dense things sink vertically down through the mantle: changes in buoyancy (phase changes) or changes to the background viscosity can only slow down or speed up the descent. A stiff slab, however, can redirect the sinking motion into lateral motion of
Two scenarios for stagnant slabs
The analysis of the slab buoyancy forces, viscous resistance to sinking and slab strength demonstrate that in the absence of trench motion it is difficult for these processes to modify the behavior of an initially steeply dipping slab. In addition, the observations of slab dip show that slabs tend to steepen as they lengthen in the upper mantle. Therefore, in order to trap slabs in the transition zone, it is necessary to shallow the slab by other means either before or after it reaches the
Conclusions
Observations of subduction zone characteristics provide many important insights on slab dynamics both outside and inside the transition zone, by illustrating the ways in which slab behavior deviates from simple model predictions, and by making it possible to isolate combinations of parameters contributing to observed behavior. By comparing these observations to a range of model predictions, two scenarios for the formation of stagnant slabs are proposed: episodic trench retreat of young slabs
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