Shallow Seismicity and the Classification of Structures in the Lau Back‐Arc Basin

Abstract

 Centroid Moment Tensors, calculated for large, shallow seismic events were compared to seafloor lineaments mapped throughout the Lau Basin.  The most likely fault plane solutions were chosen, thus classifying the faults and establishing the nature of the basin's stress regimes.  Stress regimes are highly variable, but dominated by both rigid and non-rigid transcurrent deformation, and extension at spreading centers.

Crustal Structure of the Niuafo'ou Microplate and Fonualei Rift and Spreading Center in the Northeastern Lau Basin, Southwestern Pacific

“…This model proposes a three‐plate configuration with the Niuafo'ou Microplate (N) in the center separated by the Peggy Ridge—Lau Extensional Transform Zone (LETZ) from the Australian Plate in the West and the Mangatolu Triple Junction—FRSC boundary from the Tonga Plate (T) in the East (Figure 1). The southern boundary of the Niuafo'ou Microplate is diffuse, and a local seismicity study did not show distinct seismic activity between the FRSC and the LETZ (Baxter et al, 2020; Conder & Wiens, 2011). The Northwestern Lau Spreading Center (NWLSC) marks the western portion of the northern boundary of the Niuafo'ou Microplate, but farther East, the plate boundary is not well defined (Figure 1).…”

“…The FRSC constitutes the plate boundary between the Niuafo'ou and Tonga plates. The current tectonic setting of the FRSC is characterized by pure extension (Baxter et al, 2020; Zellmer & Taylor, 2001).…”

“…Arrows are not to scale. The hatched area highlights the aseismic diffuse southern boundary of the Niuafo'ou Microplate (Baxter et al, 2020), and the white rectangle indicates the Eastern Lau Spreading Center (ELSC) region investigated by Dunn and Martinez (2011) and Arai and Dunn (2014). Blue circles show dredge locations from Keller et al (2008) yielding basalts and basaltic andesites.…”

“…In contrast, little is known about the crustal structure of the northeastern Lau Basin including the Niuafo'ou Microplate and the Fonualei Rift and Spreading Center (FRSC; Figure 1). Unique to the Lau Basin is the synchronous coexistence of two parallel zones of extension—the Lau Extension Transform Zone (Baxter et al, 2020) and the FRSC (Sleeper et al, 2016; Sleeper & Martinez, 2016)—that both accommodate E‐W extension (Figure 1). Such a configuration of two parallel zones of extension does not exist in any other back‐arc system.…”

Microseismicity and Lava Flows Hint at Magmato‐Tectonic Processes Near the Southern Tip of the Fonualei Rift and Spreading Center in the Lau Basin

“…Generally, focal mechanisms of micro‐earthquakes below our OBS network yield normal faulting solutions or solutions with a strong normal faulting component (Figure 4a). Similarly, focal mechanism of teleseismic events from the global centroid‐moment‐tensors catalog (Ekström et al., 2012), near the southern tip of the FRSC, show exclusively normal faulting mechanisms (Baxter et al., 2020).…”