The effect of oblique trenches on temperature in subduction zones

The geotherm of a subduction zone is thought to vary as a function of subduction rate and the age of the subducting lithosphere (Kirby et al., 1991; Peacock and Wang, 1999). Along a single subduction zone the rate of subduction can strongly vary due to changes in the angle between the trench and the plate convergence vector, namely the subduction obliquity. This phenomenon is observed all around the Pacific (i.e, Marianna, Sunda-Sumatra, Aleutian...) and is supposed in the geological record of Turkey (van Hinsbergen et al., 2016). However due to observed differences in subducting lithosphere age or lateral convergence rate in nature, the quantification of temperature variation due to obliquity is not obvious and need to be better constrained (Bengtson and Van Keken, 2012; Morishige and van Keken, 2014).
Modelling approach:
In order to investigate this effect, 3D generic numerical models were carried out using the finite element code ELEFANT (Thieulot, 2014). We designed a simplified setup to avoid interaction with other parameters. An ocean/ocean subduction setting was chosen and the domain is represented by a 400 × 500 × 200 km Cartesian box. The trench geometry is prescribed by means of a simple arc- tangent function. The mantle flow is computed in the mantle wedge by solving the equation of mass conservation. The energy conservation equation is solved in the entire domain and the results are analysed after steady state is reached. Depths-temperature trajectories along are computed in order to quantify the influence of obliquity on the temperature of the subduction interface.
Results:
First results show that the effect of the trench curvature on the geotherm with respect to the convergence direction is not negligible (Figure 1). A small obliquity yields isotherms that are slightly
deflected upwards where the obliquity is maximum. With an angle of ∼30°, the isotherms are deflected upwards of about 10 kilometres. Strong obliquity (i.e, angles from 60° to almost 90°) reveals extreme effects of the position of the isotherms. Further model will include other parameter as the dip of the slab and convergence rate to highlight their relative influence on the geotherm of subduction zone.