The Lau-Havre-Taupo back-arc basin: A southward-propagating, multi-stage evolution from rifting to spreading
The Lau-Havre-Taupo (LHT) system is an active, 2000-km-long, back-arc basin, associated with Pacific-Australian plate convergence in the SW Pacific. Data from the LHT back-arc system include ODP drillhole, swath bathymetric and side-scan imagery, seismic reflection, rock geochemistry, magnetic anomaly, heat flow, shallow seismicity, gravity and seismic refraction data. The LHT forms a longitudinal transect from southward-propagating true oceanic spreading in the Lau Basin near Tonga, similarly southward-propagating arc rifting in the Havre Trough, and southward-propagating continental rifting in the Taupo Volcanic Zone (TVZ) in northern New Zealand. Four transitional tectonic settings are identified along the LHT, north to south, between the continuum of spreading to rifting and comprise: (1) oceanic spreading of the Central Lau Spreading Centre (CLSC) propagating into oceanic crust formed by the Eastern Lau Spreading Centre (ELSC); (2) oceanic spreading of the ELSC propagating into rifted Lau-Havre arc crust; (3) rifting within Havre arc crust propagating into rifting New Zealand continental crust; and (4) rifting of continental crust propagating into as yet non-rifted, but flexurally subsiding, continental crust. The enhanced spatial and temporal resolution of observed tectonic and magmatic processes along the strike of the LHT, is used to constrain a model for the evolution of back-arc rifting to back-arc spreading. A sequence of five stages is identified and comprise: (1) a combination of incipient simple and pure shear; (2) early, distributed and spatially heterogeneous half-graben rifting with low-angle synthetic and antithetic structures associated with rapid graben sediment infilling, ephemeral early MORB-like rift magmatism, and the initial trenchward migration of the arc front; (3) a latter phase of half-graben rifting mostly confined to a contiguous axial zone, with continued ephemeral MORB-like rift magmatism, and further trenchward migration of the arc front; (4) initial oceanic spreading with evolving "steady-state" MORB magmatism along the axial rift graben system; and (5) the reconfiguration of ridge geometry in response to changes in opening orientation, leading to the development of ridge propagation into newly created oceanic crust. Rifting stages 2 and 3 can produce at least ∼ 120-km-wide back-arc crust by a balance of crustal extension (with an estimated β factor of 3), with rift and constructional arc magmatism. In the LHT system, depending on latitude, rates of widening during stages 2 and 3 vary between 15 and 45 mm yr -1, whilst stages 4 and 5 have full spreading rates of ∼ 90 mm yr -1.