Spatial and temporal variations in magma-assisted rifting, Taupo Volcanic Zone, New Zealand

Abstract

Taupo Volcanic Zone (TVZ), New Zealand, is a NNE-trending rifting arc, active for similar to 2 Myr, with a 125-km long central segment characterized by exceptionally voluminous rhyolite volcanism. The volcanic segmentation reflects along-axis variations in magmatism with implications for the thermal state of the crust and consequent rifting dynamics. Along the zone to the north and south of Central TVZ, the limbs of broad monoclines, disrupted to various degrees by normal faults, dip SE against major NW-facing fault zones. In these northern and southern segments, the loci of magmatism (shown by the position of volcanoes) and rifting (manifested by the distribution of seismicity and modern (<61 ka) faulting in the Taupo Fault Belt (TFB)) coincide. Mantle-derived magmas are localized within the crust in a plexus of small bodies, dikes and sills, and dike-assisted rifting operates at times (but not always) as shown by the historic record. In contrast, throughout most of Central TVZ the loci of magmatism and tectonism (shown by the distribution of high-temperature geothermal systems and inferred from geophysical models and surface fault studies) are offset laterally and extensional strain appears to be partitioned accordingly. Geological, geophysical and geodetic Studies indicate the following magma-assisted mechanisms of extension in Central TVZ: 1) mafic dike intrusion of length scale >20 km and width > 1 m oriented perpendicular to the extension direction; 2) fault slips of <2 m on structures along-strike from and coeval with silicic eruptions, some of which were triggered by mafic dike intrusion; 3) rifting episodes associated with regional-scale uplift, multi-fault rupture (slips <2 m) and transient subsidence, arguably driven by changes in state at shallow depths. Volcanic studies of <340 ka deposits demonstrate that an additional, but less frequent. mechanism involves temporally higher rates of fault slip with regional-scale collapse of rift basins in association with large-scale silicic eruptions. TVZ rifting mechanisms thus vary in space and time according to magmatic style and result in unpredictable fault behaviour over millennial time scales. (C) 2009 Elsevier B.V. All rights reserved.