Geologic and Structural Evolution of the NE Lau Basin, Tonga: Morphotectonic Analysis and Classification of Structures Using Shallow Seismicity

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.

Spatial Variations in Magmatic Volatile Influx and Fluid Boiling in the Submarine Hydrothermal Systems of Niuatahi Caldera, Tonga Rear‐Arc

“…The volcanic edifice of Niuatahi consists of a ring-shaped caldera wall (∼9 km in diameter, up to 500 m high), showing horst-graben structures and landslide deposits, which are crosscut by a N-S trending lineament (Figures 1b and 1f). Ongoing E-W extension of the caldera along this lineament is indicated by two gaps in the southern and northern caldera wall (Anderson et al, 2021;Baker et al, 2019). Several volcanic cones post-dating the caldera collapse occur along this lineament, including Niuatahi Motutahi in the center of the caldera, as well as the cones of Niuatahi Southwest and North that occur at the southern and northern intersection of the caldera ring faults with the lineament, respectively (Figure 1b).…”

“…The Tonga-Kermadec volcanic arc extends over 2,000 km from New Zealand to the Samoan Islands and forms by the westward subduction of the Pacific Plate beneath the Indo-Australian plate. Subduction rates of up to 240 mm/ yr in the northern Tonga arc are the highest globally observed and result in extensive magmatism and hydrothermal activity along the active island arc and the associated rear-and back-arc regions (Anderson et al, 2021;Baker et al, 2019;Bevis et al, 1995;Lupton et al, 2015;Taylor et al, 1996). The Lau basin is located to the west of the active Tofua arc (northern segment of the Tonga arc) and is the result of extensive rifting in the south and mature seafloor spreading in the north since 5.5-6 Ma (Baker et al, 2019;Taylor et al, 1996).…”

“…Caldera collapse and the formation of associated fault systems have been suggested to be a key process in focusing hydrothermal fluid flow and generating polymetallic sulfide mineralizations in submarine caldera volcanoes (de Ronde et al, 2019;Eddy et al, 1998;Fouquet et al, 2018;Stix et al, 2003). The occurrence of magmatic activity and related extensive hydrothermal venting in the central Lau rear-arc region are controlled by strike slip-dominated faulting (Anderson et al, 2021;Fouquet et al, 2018;Herzig et al, 1998). Accordingly, the magmatic and hydrothermal activity at Niuatahi caldera is likely controlled by the location of the volcanic edifice at the intersection of SWW-NEE and N-S trending lineaments in the vicinity to the NELSC (Figures 1a and 1b; Anderson et al, 2021;Baxter et al, 2020).…”

Trace Element and Isotope Systematics in Vent Fluids and Sulphides From Maka Volcano, North Eastern Lau Spreading Centre: Insights Into Three-Component Fluid Mixing

“…The North Eastern Lau Basin in the SW Pacific Ocean (Figure 1A) is known for its intense hydrothermal activity and complex tectonic setting (Zellmer and Taylor, 2001;Lupton et al, 2015;Embley and Rubin, 2018;Baker et al, 2019;Baxter et al, 2020;Anderson et al, 2021). The North Eastern Lau Spreading Centre (NELSC, Figure 1B) is situated in the western part of the Lau back-arc basin and follows a NEorientation with an estimated spreading rate of ∼42 mm yr −1 (Baxter et al, 2020).…”

“…The North Eastern Lau Spreading Centre (NELSC, Figure 1B) is situated in the western part of the Lau back-arc basin and follows a NEorientation with an estimated spreading rate of ∼42 mm yr −1 (Baxter et al, 2020). It can be subdivided into four segments with the axial Maka volcanic edifice being situated on the southernmost segment (Figure 1B, Baker et al, 2011;Anderson et al, 2021). The geochemical composition of the lavas at the NELSC varies from basaltic to dacitic composition, with basaltic compositions dominating at Maka volcano (Haase et al, 2018;Zhang H. et al, 2019).…”