The southern Havre Trough: Geological Structure and Magma Petrogenesis of an active Back-arc Rift Complex

Associated with Pacific-Australian plate convergence, the Havre Trough-Kermadec arc-backarc system (SW Pacific) is an archetypal example of an “island arc” plate boundary. Swath data from the southern Havre Trough between 33–34°S and 35°30′–37°S reveal that within an active, young backarc basin highly complex and heterogeneous tectonic and magmatic processes can be associated with a technically “simple” subduction system. The compressed across arc-backarc length scale would appear to be a significant control on the complexity of the tectonism and magmatism. A near-contiguous, segmented axial rift system, flanked by marginal rifts and subbasins, and horst ridges form the dominant backarc morphology. Arc edifices of the present frontal arc lie within the “backarc region,” sited 15–25 km west of the Kermadec frontal ridge. At its southern extremity, the axial rift system is propagating southward toward New Zealand, penetrating the continental-oceanic arc crustal boundary. Contrary to previous models, the entire Havre Trough is now interpreted to be undergoing backarc rifting prior to true spreading, with the attendant tectonic fabric dextrally oblique to the bounding Colville and Kermadec ridges. “Pseudo-linear” magnetic anomalies are interpreted to result from the generally irregular spatial and temporal emplacement of sheeted lava and dike intrusives between older arc basement. Significant magma source heterogeneity is observed both along and orthogonal to the frontal Kermadec arc. Basalts from the northern and southern sectors of the Kermadec arc define two distinctive isotopic arrays, with the latter overlapping with primitive basalts from the onshore Taupo Volcanic Zone. Across the arc-backarc system, lavas within the axial rift system have a marked spatial asymmetry and heterogeneity, with mid-ocean ridge basalts (MORB)-like and arc basalts being separated by as little as 15 km.