Orphan Slabs at Mid-Mantle Depths
From UCL, London, UK
Tomographic models show that the Earth’s mantle is full of different slab morphologies. Some are flat lying and temporally stagnating while others are seen to penetrate into the lower mantle. Fragmented and broken slivers of slab material in the lower mantle have also been consistently imaged for older subduction systems such as the Tonga and the Farallon systems. These slab remnants, identified as fast anomalies in seismic tomography, lie directly beneath patches of slow mantle material. The latter is often sandwiched between the fast lower-mantle anomalies and the upper-mantle slab connected to the surface at the trench. This implies a clear break of the slab remnant from the ‘parent’ slab above it, through either a slab tear, slab window or a full slab gap leading to an orphan slab at depth.
In this study we propose a new mechanism for slab orphaning at depth. We suggest that slab orphaning occurs directly at mid-mantle depths, without the involvement of complicated surface kinematics and drastic plate organisations. The mechanism proposed here can also explain the orphan slabs’ often unorthodox orientations and locations with respect to the parent slab and the current subduction regime. Our numerical experiments suggest that slab orphaning and abandonment, is the result of competing buoyancy forces that encourage penetration into the lower mantle and opposing forces that favour stagnation. Orphaning is thus, an expression of the slab’s relative strength with respect to that of the ambient mantle.
Slab orphaning brings to the forefront, the crucial importance of the local slab strength at the Upper-Lower Mantle transition zone, in shaping the slab morphologies. The slab orphaning mechanism as proposed here, also highlights the fact that slab break-off does not necessarily, always mean, continental collision and subduction termination. This has direct implications for the understanding of complex subduction regimes and paleo-plate movements, many of which are reconstructed using a slab reference frame and the assumption that slabs always directly underlie their trenches.