Down-dip and Along-strike Stress in Subducting Slabs

Abstract:

The reconstructed stress fields for intermediate depths in slabs (between 100 and 300 km) remain enigmatic. Slab pull should enhance downdip tension, but downdip compression is often observed.
Based on hotspot tracks (indicating a relative motion between the lithosphere and the mantle) several researchers proposed a westward drift of the lithosphere with debated velocities (2-13 cm/yr). Available data indicates that dominant down-dip tension occurs along E-directed subductions (e.g., Chile) sustained by the eastward relative mantle flow and down-dip compression along W-directed subductions (e.g., Tonga) encroached by the mantle motion. In this perspective, 3D viscoelastic models (FEM) were performed to investigate the origin of stress in slabs at intermediate depths. Geometries inspired to real subduction zones, were used to analyze the effect of the principal geodynamic forces (slab pull, mantle flow, plate convergence), the correlation between subduction polarity (i.e., how the mantle flow approaches the slab) and geometry, the nature of down-dip and along-strike stresses (poorly investigated in the literature).
We conclude that, although the stress in slabs is controlled by several geodynamic processes, down-dip compression is favored by mantle flow opposing the slab, whereas down-dip tension is enhanced by mantle flow sustaining the slab. These predictions are in agreement with available geophysical observations.
Substantial along-strike deformation affects intermediate and deeper regions of the slab when mantle flow is enhanced. Down-dip and along-strike axes form couples that tend to rotate approaching the slab sides. The larger the lateral extent of the plate the most this effect is evident. According to these findings, along-strike tension or compression comes out in response to the flexure (forward or backward) of the lithosphere depending on the slab longitudinal curvature. Along-strike tension occurs in case of slabs curved towards the mantle flow direction while along-strike compression occurs if the slab has an opposite curvature relative to the mantle flow direction. 3D results suggest that the stress axes direction is largely influenced by the subduction geometry, but down-dip and along-strike stress axes regime is controlled by the mantle flow direction.