Lm. Parson et Ic. Wright, THE LAU-HAVRE-TAUPO BACK-ARC BASIN - A SOUTHWARD-PROPAGATING, MULTISTAGE EVOLUTION FROM RIFTING TO SPREADING, Tectonophysics, 263(1-4), 1996, pp. 1-22
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).