Subduction zone obliquity and rheology dictate global trench-parallel inner forearc deformation

Subduction zones are defined by plate convergence, yet their upper plates exhibit a wide range of deformation styles globally. While various hypotheses have been proposed to explain this global variability, the controlling factors remain poorly understood. We analyzed ~24,000 km of active global subduction zones to investigate how subduction obliquity influences trench-parallel and horizontal deformation in the terrestrial inner forearc overriding plate. Using global datasets of GNSS velocities and active fault catalogs, we examined inner deformation across 13 forearcs on both short (decadal) timescales, captured by GNSS, and long (millennial to million-year) timescales, inferred from trench-parallel active strike-slip faults. Our results reveal a strong link between subduction zone obliquity and both the sense and magnitude of upper plate rotation observed in GNSS data, as well as the sense and rate of deformation along trench-parallel strike-slip faults. Unlike earlier studies suggesting that obliquity influences deformation only above a certain threshold, we find that even low to moderate obliquity affects forearc behavior. High-obliquity margins, such as New Zealand and the Philippines, show the highest GNSS-derived vorticity and cumulative slip rates on trench-parallel strike-slip faults. In contrast, lower-obliquity regions, like portions of Cascadia and Peru, exhibit reduced vorticity and either diffuse strike-slip faulting or broadly distributed deformation. Across all forearcs, we find strong correlations between obliquity and both GNSS vorticity and trench-parallel fault slip rates in the inner forearc.

Beyond controlling the magnitude and sense of inner forearc deformation, our results suggest that rheologic factors also influence how trench-parallel shear is accommodated within the inner forearc. We observe a continuum of trench-parallel strike-slip deformation styles within the inner forearc, ranging from motion accommodated on a single, through-going sliver fault with high slip rates to more distributed deformation expressed across multiple shorter strike-slip faults with lower slip rates. Emerging results suggest that this variability is linked to the distance between the down-dip extent of megathrust locking and the volcanic arc. Subduction segments with a short trench-to–locked-zone distance preferentially develop coherent sliver faulting, whereas those with a greater distance between the down-dip locking limit and the arc tend to exhibit more distributed strike-slip deformation across the forearc. We interpret this pattern to reflect a rheological control on how trench-parallel shear is accommodated. If shear strain is concentrated near the down-dip edge of locking, as predicted by simple elastic models, deformation localizes above this region. Where this localization occurs within or near the arc, conditions favor development of a single, through-going strike-slip fault. In contrast, when the locus of strain concentration lies farther trenchward within the forearc, deformation is more likely to be partitioned across pre-existing structures, resulting in distributed strike-slip faulting. These results suggest that while subduction obliquity exerts a first-order control on the sense and magnitude of inner forearc deformation, additional geometric and rheologic factors govern the style of trench-parallel inner forearc strain accommodation.