Initiation of Subduction Along Oceanic Transform Faults: Insights From Three-Dimensional Analog Modeling Experiments

摘要

Three-dimensional analogue experiments are employed to explore how self-sustaining subduction may initiate along an oceanic transform fault. The models include a realistic spatial distribution of plate, thickness, strength and buoyancy of the lithosphere near an oceanic transform fault characteristic of the spreading rate. Convergence is imposed across the transform fault and strain in the model lithosphere is quantified using a surface Particle Imaging Velocimetry system. A force sensor is employed to defined when a self-sustaining subduction regime is generated. Cylindrical experiments reveal that subduction polarity is controlled by the buoyancy gradient and the strengths of the plates. With no inclined weak zones, imposed orthogonal compression results in the nucleation of a new fault in the weakest plate leading to the young and positively buoyant plate subducting. However, with an inclined weak zone,the buoyancy contrast controls subduction polarity with the most negatively buoyant plate subducting and a self-sustaining subduction regime obtained after $sim$300,km of imposed shortening. This situation is obtained when including an inverted triangular weak zone on top of the transform fault associated with the serpentinization of the crust and mantle. In non-cylindrical experiments, taking into account the change along strike of plate strength and buoyancy, the capacity of the transform fault to generate a self-sustaining subduction regime is greatly reduced. Subduction initiates simultaneously with opposite polarity at the two extremities of the transform segment and, at depth, a lithospheric tear is produced that separates the two subducting slabs. In the center of the transform fault, the lack of buoyancy or strength contrast between the two plates leads to multiple thrusts with variable polarities, overlapping each other, and each accommodating too little shortening to become the new plate boundary. This indicates that additional mechanical work is required in the center of the transform fault which prevents the establishment of a self-sustaining subduction regime.