Thermomechanical modelling of lithospheric slab tearing and its
topographic response in the Gibraltar Arc (westernmost Mediterranean
Sea)
Abstract
Lithospheric slab breakoff can occur in various styles including a
horizontal ‘tearing’, where an initial weakness develops into tearing
and laterally propagates along the slab. Slab tearing has been invoked
to explain changes in plate kinematics in the Western Mediterranean and
the tectonic uplift that led to the Messinian Salinity Crisis. However,
this process remains debated regarding its surface signature and the
physical parameters controlling its initiation and dynamics. Here, we
use 3D thermo-mechanical modelling to investigate geodynamic parameters
affecting the slab-tearing initiation and its lateral propagation, and
to quantify the corresponding surface vertical motions. We find that an
oblique convergence introduces an asymmetry that favors the initiation
of one-sided slab tearing. The tectonic configuration of the overriding
plate has little effect on the trench migration rate, and slab tearing
can results purely from the negative buoyancy of the subducted slab.
This force and the slab retreat it causes are enough to generate an
arcuate plan-view shape to the orogen. The slab-tear propagation rate
varies from 37-67 cm/yr. During propagation, the slab tearing depth
increases along the subducting slab, with a shallow initial tear (80-150
km) and a deeper tear (170-200 km) on the opposite end. The time needed
for the slab to detach completely is geologically fast (<2
Myr). The slab tearing can cause a prominent surface uplift of 0.5-1.5
km throughout the forearc region with an uplift rate of 0.23-2.16 mm/yr,
which is consistent with the situation during the first stage of the
Messinian Salinity Crisis.