Abstract
The transition from subduction to transform motion along horizontal
terminations of trenches is associated with tearing of the subducting
slab and wrench tectonics in the overriding plate. One prominent example
is the northern Tonga subduction zone, where the influence of wrench
tectonics is indicated by abundant strike-slip faulting in the NE Lau
back-arc basin. We explore the back-arc dynamics of this region for the
first time through structural lineament analyses and kinematic analyses
interpreted from ship-based multibeam bathymetry and Centroid-Moment
Tensor data. Our results indicate two distinct sets of Riedel shear
structures that are associated with a counter-clockwise rotation in the
stress field. We propose that this rotation is driven by the collision
of the previously unstudied Capricorn Seamount(s). The strain of this
collision was accommodated by right-lateral slip along the adjacent
crustal scale fault, known as the Fonualei Discontinuity, which
segmented the fore-arc. Internal deformation of the northern tectonic
block may have been enhanced by friction along the northern boundary
imparting westward-directed stress. This study highlights the importance
of non-rigid plate kinematics and extensive re-activation of
pre-existing faults in this region. Importantly, these structures
directly control the development of complex volcanic-compositional
provinces, which are characterized by variably-oriented spreading
centers, off-axis volcanic ridges, extensive lava flows, and
point-source rear-arc volcanoes that sample a heterogenous mantle wedge,
with sharp gradients and contrasts in composition and magmatic affinity.
This study adds to our understanding of the influence of
subduction-transform motions and terrane collisions on the structural
and magmatic evolution of back-arcs.