Dynamics of oceanic slab tearing during transform-fault
horizontally-oblique subduction: Insights from 3D numerical modeling
Abstract
Oceanic-plates vertical tearing is seismically-identified in the
present-day Earth. This type of plate tearing is frequently reported in
horizontally-oblique subduction zones where transform-faulted oceanic
plates are subducting (or subducted). However, the mechanisms behind
vertical slab tearing are still poorly understood, thus we utilize 3D
time-dependent Stokes’ flow thermo-mechanical models to further study
this problem. We find that (i) the age offset of transform fault and
(ii) the horizontal obliqueness of subduction fundamentally control the
tearing behavior of two generic, materially-homogeneous oceanic slabs
separated by a low-viscosity zone. The overriding-continental plate
bends one slab first, which combined with the age-thickness difference
between slabs, causes the differential sinking of them. Based on the
modeling results, well-developed slabs vertical tearing would happen
when the oblique angle of subduction is ≥30° or the age ratio of the
secondly-bent to firstly-bent slab being
~<0.6. Quantifying the horizontal
distance-vector between sinking slabs, we find that subduction at
medium-low horizontal-obliqueness angles (≤30°) of young lithosphere
(slabs-average ~15 Myr) tends to produce
fault-perpendicular tearing. Contrastingly, old-age slabs (average ≥ 30
Myr) with medium-large obliqueness angles
(~>20°) tend to produce fault-parallel
tearing, related to differential slab-hinge retreat or rollback.
Correlations between slabs’ (i) computed tearing horizontal-width and
(ii) scaling-theory forms of their subduction-velocity differences, are
reasonable (0.76-0.97). Our numerically-predicted scenarios are
reasonably consistent with plate-tear imaging results from at least 4
natural subduction zones. Our modeling also suggests that continual
along-trench variation in subduction dip angle may be related to a
special case of oblique subduction.