Morphotectonic characteristics of a propagating spreading system in the northern Lau Basin

Abstract

Bathymetric Sea Beam mapping of a southward propagating rift system in the Lau Basin (SW Pacific) shows a morphotectonic segmentation of the two competing axes: Over a distance of 70 km the propagating axis develops from a narrow rift graben at the propagator tip, via a leaky graben, into a juvenile volcanic ridge, and finally into a fully developed spreading ridge. The competing dying spreading axis 70 km farther east shows steady deepening towards the north, combined with a change from a small volcanic ridge to a narrow depression before it is buried by sediments. The relay zone between the two axes contains two deep N‐S striking grabens; the eastern one, 3200 m deep, may act as a short‐lived spreading center. A NE‐trending string of slightly offset deep sediment‐filled basins is interpreted as fossil analoges to these grabens, originally generated in the southward migrated relay zone. The string of basins suggests an episodic rather than steady state southward migration. Propagation and opening of grabens led to a rotation of the inherited ridge‐parallel crustal fabric. However, the propagating tip and the grabens cut obliquely through the fabric thus generating small‐scale structural and morphological inhomogeneities of the crust. At the propagating tip, old and new tectonic lineaments closely interfinger.