The impact of slab dip variations, gaps and rollback on mantle wedge flow: insights from fluids experiments

Observed seismic anisotropy and geochemical anomalies indicate the presence of 3-D flow around and above subducting slabs. To investigate how slab geometry and velocity affect mantle flow, we conducted a set of experiments using a subduction apparatus in a fluid-filled tank. Our models comprise two independently adjustable, continuous belts to represent discrete sections of subducting slabs that kinematically drive flow in the surrounding glucose syrup that represents the upper mantle. We analyse how slab dip (ranging from 30 degrees to 80 degrees), slab dip difference between slab segments (ranging from 20 degrees to 50 degrees), rates of subduction (4-8 cm yr(-1)) and slab/trench rollback (0-3 cm yr(-1)) affect mantle flow. Whiskers were used to approximate mineral alignment induced by the flow, as well as to predict directions of seismic anisotropy. We find that dip variations between slab segments generate 3-D flow in the mantle wedge, where the path lines of trenchward moving mantle material above the slab are deflected towards the slab segment with the shallower dip. The degree of path line deflection increases as the difference in slab dip between the segments increases, and, for a fixed dip difference, as slab dip decreases. In cases of slab rollback and large slab dip differences, we observe intrusion of subslab material through the gap and into the wedge. Flow through the gap remains largely horizontal before eventual downward entrainment. Whisker alignment in the wedge flow is largely trench-normal, except near the lateral edges of the slab where toroidal flow dominates. In addition, whisker azimuths located above the slab gap deviate most strongly from trench-normal orientations when slab rollback does not occur. Such flow field complexities are likely sufficient to affect deep melt production and shallow melt delivery. However, none of the experiments produced flow fields that explain the trench-parallel shear wave splitting fast directions observed over broad arc and backarc regions in many subduction zones.