PRE-SPREAD RIFTING AND HETEROGENOUS VOLCANISM IN THE SOUTHERN HAVRE TROUGH BACK-ARC BASIN

The Havre Trough is an active back-arc basin associated with Pacific-Australian plate convergence. Two segments within the southern Havre Trough (SHT), surveyed with GLORIA, SeaBEAM and SYS09 swath data, now provide the first detailed data to reliably document the structure and morphology of this complex back-arc region. At the southern segment (35-degrees-40'-37-degrees-S), along a strike length of some 120 km, the SHT structure comprises a series of segmented, en echelon, generally flat floored, sediment filled, axial rift grabens, flanked by ridge and basin rift block topography. Prominent fault escarpments, with relief of 500-1000 m and a marked zig-zag arrangement, mark the outer margins of the rift system. Within the rift grabens extensional tectonism is pervasive. Extensive shallow seismicity within the SHT can only partly be associated with the rift graben system at present. Youthful, constructional volcanic terrains within the rift grabens are not common, and invariably have a restricted areal extent and a relief < 400 m. Neovolcanism is fault controlled, being apparently restricted to the rift escarpments. Lavas within the rifts display a marked source heterogeneity as evinced by the eruption of back-arc basin and island arc basalts on the western and eastern escarpments, respectively, even though separated by as little as 14 km. An identical rift morphology has been identified at the northern (33-degrees-34-degrees-S) site. Data from both sites show individual rifts are 15-35 km in length, 6-10 km wide, and have axial depths of 3100-4000 m. Apart from at least one 15-20 km wide, cross trough ridge, available data indicate the rift graben system is contiguous between 33-degrees-S and 37-degrees-S. This rift graben morphology, by analogy with other known rifting back-arc basins, is considered here not to be consistent with true oceanic back-arc spreading. Rather it is argued that the SHT, possibly indeed all of the Havre Trough, is undergoing back-arc rifting prior to true spreading. Further, the rifting tectonic fabric within the SHT, and in the onshore Taupo to the south, extension is consistently dextrally oblique relative to the bounding Colville and Kermadec Ridges. The degree of obliquity increases from south to north. If accepted that the SHT is rifting rather than spreading, magnetic anomaly data within the trough do not record the accretion of new oceanic crust, and consequently can no longer be used to establish the ''spreading'' history of the SHT. The SHT magnetic anomalies are here interpreted as ''pseudo-linear'' magnetic anomalies resulting from the generally irregular spatial and temporal emplacement of magnetic sheeted lava and dike intrusives between older, low magnetisation arc basement rift blocks flanking the axial rift. Proxy data from onshore North Island, New Zealand, and offshore SHT suggest that the rate of extension and age of initiation of back-arc rifting are, between 35-degrees and 37-degrees-S, respectively, 15-20 mm yr-1 and 5 Ma.