Where do subducted slabs go?

Geophysicists and geochemists are generally opposed on what happens to subducted lithosphere.  Seismic tomography of the deep mantle shows convincing evidence for slab-like cold bodies down to the core-mantle boundary, yet differences in trace-element and isotopic signatures of volcanic rocks formed at ridges from shallow mantle and ocean islands that relate to deep plumes persuades geochemists that restriction of convection within the upper mantle, at the 660 km deep discontinuity, best explains the differences.  There are other models that might account for geochemical differences, such as heterogeneities throughout a poorly stirred mantle or because material in slabs subducted to the bottom of the mantle rarely rises again, but displaces more pristine materials upwards.

The more earthquakes that seismographs detect and locate, the better geophysicists are able to map in 3-D the zones on which they take place.  One destructive margin long known to have aberrant seismicity is the northern part of the Tonga system in the Pacific Ocean.  This is where the fastest subduction anywhere consumes lithosphere that has little time to warm up while it descends – surely a site for slabs to fall steeply into the deep mantle.  Much of the Tonga system shows the expected zone of steeply plunging Earthquakes, yet west and north-west of Fiji there are earthquakes that do not fit the regional pattern.  They are far too shallow to result from motion on the main subduction zone.  By detailed analysis of seismic data Wang-Ping Chen and Michael Brudzinski have revealed a strong possibility that a piece of old subducted slab has slid to the 660 km discontinuity since it parted company with the now rapid and steep motion at the Tonga trench (Chen, W-P. and Brudzinski, M.R. 2001.  Evidence for a large-scale remnant of subducted lithosphere beneath Fiji.  Science, v. 292, p. 2475-2478).  If such behaviour turns out to be more widespread, large volumes of old lithosphere may indeed sit at the discontinuity, satisfying many geochemists as a means to maintain very old differences in composition of the mantle.  The problem is, increasingly good resolution in seismic tomography has so far failed to detect the tell-tale high seismic velocity signature of such cold slabs.  Chen and Brudzinski suggest that they may be “invisible” to this method, because of their mineralogy – perhaps the crustal lithosphere has not equilibrated to eclogitic materials, or is given neutral buoyancy by being heavily hydrated.

Between a rock and a hard place

Plate theory stems from the notion that the lithosphere is overwhelmingly rigid and deforms only at the boundaries between plates, particularly at destructive margins.  The Earth’s seismicity is overwhelmed by earthquakes at discrete boundaries, and the mapping of seismic events along narrow lines by the world-wide network of seismographs (set up as a means of pinpointing nuclear weapon tests) formed on of the main planks in developing the theory of plate tectonics.  The plate whose evolution drove India into Asia bucks this definition.  It has long been known to host seismicity well inside its boundaries.  Oceanographic work has slowly built up a means of relating Indian Ocean seismicity to plate structure, whereas analysis of earthquake first motions from seismographs reveals that the deformation differs between various block of the ocean floor.  The plate suffers folding and thrusting, and transcurrent motions along ancient transform faults, such as the Ninety East Ridge.  The most likely explanation for the Indian plate’s aberrance is that sea-floor spreading from the ridge separating the Indian Plate from that carrying Antarctica can no longer be accommodated by subduction of the subcontinent beneath Asia, whereas it can be taken up by subduction beneath the Java-Sumatra island arc.  The Central Indian basin is being compressed, and must deform in some way, perhaps eventually to become a new subduction zone.

Source:  Deplus, C.  2001.  Indian Ocean actively deforms.  Science, v. 292, p. 1850-1851.

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