Influence of the geometry of continental to oceanic lithospheric transition on slab break-off
Abstract
Slab break-off is generally proposed to explain different features recognized at the Earth’s surface such as uplift
of the upper plate (due to isostatic rebound after break-off and unloading of the subducted plate), extension in the
inner higher portion of the rising orogenic belt, or magmatism and metamorphism related to the inflow of sub-slab
mantle material. In addition, slab break-off is generally identified on tomographic images as a gap, or anomalously
slow region, within the subduction slab. However, these tomographic images only give information on the
present-day state of a subduction zone. Analogue and numerical models have proven useful to understand the time
evolution of the slab break-off process. They have shown that slab break-off occurs after a less positively-buoyant
material enters in the sub-lithospheric mantle, increasing the overall buoyancy of the subducting lithosphere and
leading to internal slab deformation under stretching. If localization of deformation is important enough, slab
stretching can lead to slab break-off within a few m.y, which can eventually propagate laterally. However, most of
the models done so far are either 2D or 2.5D, the transition between the negatively and positively buoyant domains
being linear and parallel to the trench, while in Nature more complex geometries may have existed.
We present here the first results of laboratory models that simulate the arrival at trench of a continental
lithosphere following oceanic subduction into the upper mantle by using a two-layer system made of silicone
(PDMS) and glucose syrup. The transition between oceanic and continental lithospheres (COT) is represented
as a thin layer of PDMS silicone homogenously seeded with high viscous glass beads. This combination allows
the oceanic part to detach from the continental one because of the high local strain obtained around the seeds
that makes the mixture to behave as a non-Newtonian medium. We obtain with our initial 2.5D set-up depths of
break-off (120-300km depth) and time periods for break-off after continental subduction (20-35 m.y) in agreement
with previous models. We then studied the influence of the angle between the trench and the COT on the evolution
of trench velocity, timing of necking and break-off, velocity of lateral propagation and surface topography changes
obtained by using photogrammetric analysis. For instance, we show that increasing the angle between the COT
and the trench from 0 to 40 increases the timing for slab break-off initiation by 5-10 m.y. We finally tested the
influence of different geometries (from linear to sinusoidal shapes) for the COT..