Abstract
The transition from subduction to transform motion along horizontal terminations of trenches is associated with tearing of the subducting slab and strike-slip tectonics in the overriding plate. One prominent example is the northern Tonga subduction zone, where abundant strike-slip faulting in the NE Lau back-arc basin is associated with transform motion along the northern plate boundary and asymmetric slab rollback. Here, we address the fundamental question: how does this subduction-transform motion influence the structural and magmatic evolution of the back-arc region? To answer this, we undertake the first comprehensive study of the geology and geodynamics of this region through analyses of morphotectonics (remote-predictive geologic mapping) and fault kinematics interpreted from ship-based multibeam bathymetry and Centroid-Moment Tensor data. Our results highlight two notable features of the NE Lau Basin: 1) the occurrence of widely distributed off-axis volcanism, in contrast to typical ridge-centered back-arc volcanism, and 2) fault kinematics dominated by shallow-crustal strike slip-faulting (rather than normal faulting) extending over ∼120 km from the transform boundary. The orientations of these strike-slip faults are consistent with reactivation of earlier-formed normal faults in a sinistral megashear zone. Notably, two distinct sets of Riedel megashears are identified, indicating a recent counter-clockwise rotation of part of the stress field in the back-arc region closest to the arc. Importantly, the Riedel structures identified in this study 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. This study adds to our understanding of the geologic and structural evolution of modern backarc systems, including the association between subduction-transform motions and the siting and style of seafloor volcanism.
Highlights
Back-arc basins are extensional features formed behind subduction zones by progressive rifting of volcanic arcs or behind the arc fronts in pre-existing oceanic basins until passive mantle upwelling creates new oceanic crust (Karig, 1970)
We focus on the structures of the NE Lau Basin in greater detail, complemented by morphotectonic analyses, in order to better understand the origin and evolution of strike-slip faulting and the possible influence on back-arc magmatic-hydrothermal processes
Based on detailed morphotectonic analyses and additional ground-truthing data, we present new remote-predictive geological and structural maps of the NE Lau Basin at high-resolution (Figures 5–7; Supplementary Table S2), which highlight the wide variety of rock types, extensive volcanism, and complex structural fabrics resulting from the dynamic evolution of the area
Summary
Back-arc basins are extensional features formed behind subduction zones by progressive rifting of volcanic arcs or behind the arc fronts in pre-existing oceanic basins until passive mantle upwelling creates new oceanic crust (Karig, 1970). Their initiation is triggered by processes of either hinge-rollback (Chase, 1978; Scholz and Campos, 1995), and/ or upper plate migration away from the trench associated with slab anchoring (Uyeda and Kanamori, 1979; Scholz and Campos, 1995; Heuret and Lallemand, 2005). Seismologic data alone cannot fully resolve the types of faulting, and the stress regimes that lead to the emergence of these structures often remain enigmatic
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