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

Abstract

The northeastern Lau Basin is one of the fastest opening and magmatically most active back-arc regions on Earth. Although the current pattern of plate boundaries and motions in this complex mosaic of microplates is reasonably understood, the internal structure and evolution of the back-arc crust are not. We present new geophysical data from a 290 km long east-west oriented transect crossing the Niuafo'ou Microplate (back-arc), the Fonualei Rift and Spreading Center (FRSC) and the Tofua Volcanic Arc at 17°20′S. Our P wave tomography model and density modeling suggest that past crustal accretion inside the southern FRSC was accommodated by a combination of arc crustal extension and magmatic activity. The absence of magnetic reversals inside the FRSC supports this and suggests that focused seafloor spreading has until now not contributed to crustal accretion. The back-arc crust constituting the southern Niuafo'ou Microplate reveals a heterogeneous structure comprising several crustal blocks. Some regions of the back-arc show a crustal structure similar to typical oceanic crust, suggesting they originate from seafloor spreading. Other crustal blocks resemble a structure that is similar to volcanic arc crust or a "hydrous" type of oceanic crust that has been created at a spreading center influenced by slab-derived water at distances <50 km to the arc. Throughout the back-arc region, we observe a high-velocity (Vp 7.2-7.5 km s −1) lower crust, which is an indication for magmatic underplating, which is likely sustained by elevated upper mantle temperatures in this region.

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

“…Such a pattern of melt flow in the sub-crustal domain would supply the S-FRSC volcanic centers with a surplus of melt and cause a melt deficit in the regions that lie between the volcanic centers, where local depressions in the seafloor occur (Figure 1c; . Schmid et al (2020) presented a 2D P-wave velocity (Vp) model along an east-west oriented geophysical transect crossing the S-FRSC at 17°20'S (Figure 1c). Vp versus depth profiles extracted from the 2D model indicate that the rift shoulders on both sides of the S-FRSC are of similar, volcanic arc-like, composition implying that the S-FRSC has rifted into the arc crust.…”

“…Magnetic anomaly data from a profile crossing the S-FRSC at 17°20'S show a single positive peak near the center of the axial valley but no reversals inside the S-FRSC (Hannington et al, 2019;Schmid et al, 2020). Based on the seismic velocities in the S-FRSC crust, intermediate between the andesitic rift shoulders and typical oceanic crust, and the lack of geomagnetic reversals Schmid et al (2020) proposed that until recently the extensions processes at the S-FRSC were dominated by rifting of arc crust and focused seafloor spreading was not yet active. The overarching research question addressed here is how the distribution of seismic activity and magmatism vary along the axis of the southwards propagating S-FRSC and how they interact.…”

“…Absolute values of station correction terms are provided in the SI Table S2. The 1D velocity model used for event location was extracted from a west-east oriented 2D refraction seismic profile crossing our OBS network at 17°20'S (Schmid et al, 2020; profile location is shown in Figure 1c). The 1D velocity model was constructed by horizontally averaging the velocities from the 2D seismic profile (see Supporting Information Figure S1 for more details).…”

“…Dashed white gives the location of a geophysical transect presented in Schmid et al. (2020). White triangles are locations of ocean bottom seismometers (OBS) recording microseismicity presented in this study.…”

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

“…This process may be quicker when propagation occurs in thinner crust and where back-arc retreat is the greatest (Parson et al, 1990). This structural reconfiguration may have induced the propagation of new rifts, such as FRSC segments, which are interpreted to be the youngest features in the NE Lau Basin (Schmid et al, 2020). Simultaneous northward propagation of the FRSC and southward propagation of the MTJ-S produce a zone where the spreading centers overlap (Fig.…”