THE LAU-HAVRE-TAUPO BACK-ARC BASIN - A SOUTHWARD-PROPAGATING, MULTISTAGE 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-are system include ODP drillhole, swat h bathymetric and side-scan imagery, seismic reflection, rock geochemi stry, magnetic anomaly, heat now, shallow seismicity, gravity and seis mic refraction data. The LHT forms a longitudinal transect from southw ard-propagating true oceanic spreading in the Lau Basin near Tonga, si milarly southward-propagating are rifting in the Havre Trough, and sou thward-propagating continental rifting in the Taupo Volcanic Zone (TVZ ) in northern New Zealand. Four transitional tectonic settings are ide ntified along the LHT, north to south, between the continuum of spread ing to rifting and comprise: (1) oceanic spreading of the Central Lau Spreading Centre (CLSC) propagating into oceanic crust formed by the E astern Lau Spreading Centre (ELSC); (2) oceanic spreading of the ELSC propagating into rifted Lau-Havre are crust; (3) rifting within Havre are crust propagating into rifting New Zealand continental crust; and (4) rifting of continental crust propagating into as yet non-rifted, b ut flexurally subsiding, continental crust. The enhanced spatial and t emporal resolution of observed tectonic and magmatic processes along t he strike of the LHT, is used to constrain a model for the evolution o f back-are rifting to back-are spreading. A sequence of five stages is identified and comprise: (1) a combination of incipient simple and pu re shear; (2) early, distributed and spatially heterogeneous half-grab en rifting with low-angle synthetic and antithetic structures associat ed with rapid graben sediment infilling, ephemeral early MORE-like rif t magmatism, and the initial trenchward migration of the are front; (3 ) a latter phase of half-graben rifting mostly confined to a contiguou s axial zone, with continued ephemeral MORE-like rift magmatism, and f urther trenchward migration of the are front; (4) initial oceanic spre ading with evolving ''steady-state'' MORE magmatism along the axial ri ft graben system; and (5) the reconfiguration of ridge geometry in res ponse 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 similar to 120-km-wide back-are crust by a balance of crustal extension (with an estimated beta factor of 3), wit h rift and constructional are magmatism. In the LHT system, depending on latitude, rates of widening during stages 2 and 3 vary between 15 a nd 45 mm yr(-1), whilst stages 4 and 5 have full spreading rates of si milar to 90 mm yr(-1).