Geodynamic modelling of lithosphere subduction and the topographic response to slab tearing. Application to the Gibraltar Arc.

Abstract

Lithospheric slab tearing, the process by which a subducted lithospheric plate is torn apart and sinks into the Earth’s mantle, has been proposed as a cause of significant surface vertical motions. Although this has been linked to the change implied in the isostatic balance in subduction zones, little is known about the mechanisms and rock properties determining the tear propagation and the uplift-subsidence rates involved. This thesis aims to explore the link between the tearing of subducted lithospheric slabs and the associated vertical motions. To this purpose, I first explore the mechanisms controlling the buoyancy of a subducted lithosphere and then, with this understanding, numerically simulate the process of lithospheric tearing upon continental collision, using the Betic Cordillera as a reference scenario where such tearing-uplift interaction has been proposed for this region. With a mineral-physics approach, where a lithospheric mantle can be less dense than the underlying asthenosphere, I explore the controls on lithospheric buoyancy using a 2D thermal- diffusive model of plate convergence. Five chemical compositions and tectonothermal ages were considered, namely Archon (> 2.5 Ga), Proton ( 2.5 − 1.0 Ga), Tecton (< 1.0 Ga), and two oceanic lithospheric plates of 30 Ma and 120 Ma. While the advection of colder rock in oceanic-like plates always results in negative buoyancy, Protons and Tectons exhibit an ability to slowly flip from negative to positive buoyancy at low convergence rates: they first favour the sinking due to advection and then become more buoyant because they are thinner and heat up faster during subduction. In contrast, the lighter density of cratons (Archons) overprints this effect and hinders delamination or subduction, regardless of the convergence rate. This may explain why Archons are more stable during the Wilson Cycle Having gained these insights into the role of lithospheric buoyancy in subduction settings, I then set to explore the characteristics surrounding lithospheric slab tearing and the asso- ciated surface uplift. I used 3D thermo-mechanical numerical modelling to investigate the geodynamic parameters affecting the slab-tearing initiation and its lateral propagation, and to quantify the corresponding surface vertical motions. The Betics-inspired model suggests x that the obliquity of the continental passive margin (relative to the trench axis) is a major influence on the initiation of slab tearing because it promotes a laterally diachronous conti- nental collision which leads to slab tearing. The model illustrates an east-to-west slab tearing (tearing velocity ∼ 37.6 − 67.6 cm/yr with the lower-mantle viscosity of up to 1022 Pa·s), which leads to surface uplift signature of 0.5 − 1.5 km across the forearc region throughout the tearing process. While the fast slab tearing (< 2 Myr over 600 km wide slab) and the lack of arcuate slab in my models limit a direct comparison with the Western Mediterranean, this approach provides a new insight into the link between slab tearing in the mantle and surface uplift. My models yield uplift rates of 0.23 − 2.16 mm/yr, as a result of slab tearing, which is compatible with the uplift rate needed to achieve an equilibrium between seaway-uplift and seaway-erosion which could have led to the closure of marine gateways that reduced the water-flow from the Atlantic Ocean into the Mediterranean Sea during the first stage of the Messinian Salinity Crisis