The Interrelationships between Faulting and Volcanism in the Okataina Volcanic Centre, New Zealand
Continental rifts show close spatial relations between faulting and volcanism, however the interrelations between each process and their roles in the accommodation of regional extension are not well understood. The geometric and kinematic relations between an active silicic caldera complex and active faults in the upper 3-4 km of the crust (i.e. Taupo Rift) are investigated using regional gravity data, digital elevation models, outcrop mapping, seismic reflection lines, focal mechanisms and an historical account of the 1886 AD Tarawera eruption adjacent to, and within, the Okataina Volcanic Centre, New Zealand.The location and geometry of the Okataina Caldera were influenced by pre-existing faults. The caldera is elongate north-south, has a maximum subsidence of 3 +/- 0.5 km at the rift axis and occupies a 10 km hard-linked left step in the rift. The principal rift faults (55-75 degrees dip) define the location and geometry of the northwest and southeast margins and locally accommodate piecemeal caldera collapse. Segments of the east and west margins of the caldera margin are near vertical (70-90 degrees dip), trend north-south, and are inferred to be faults formed by the reactivation of a pervasive Mesozoic basement fabric (i.e. bedding, terrane boundaries, and/or faults). Measured displacements along the Paeroa and Whirinaki Fault zones in, and adjacent to, the Okataina Volcanic Centre took place over time periods ranging from 60 to 220 ka (together with historical accounts of the 1886 AD Tarawera eruption). These indicate that neither dike intrusion nor caldera collapse have a measurable influence on fault displacement rates outside the volcanic complex. Within the volcanic complex, vertical displacement along the Whirinaki Fault zone increases by up to 50% between the caldera topographic margin and inner collapse boundary. This increase in vertical displacement is predominantly due to the collapse of the caldera 60 ka ago. In the Okataina Volcanic Centre, extension is accommodated by a combination of tectonic faulting, dike intrusion, and gravitational caldera collapse. Gravitational caldera collapse is however, superimposed on regional extension without contributing to it. Rift-orthogonal extension dominates across the Taupo Rift with a minor (= 20 degrees) component of right-lateral slip increasing northwards. The regional principal horizontal extension direction rotates 30 degrees clockwise south to north along the rift. The modal principal horizontal extension direction for the Okataina Volcanic Centre trends ~145 degrees, approximately normal to northeast striking rift faults and intra-caldera linear vent zones, and oblique to north-south faults. Zones of crustal weakness, brittle deformation, and dilation at the intersections of northeast-southwest dip slip and north-south oblique slip active fault sets are inferred to locally promote the ascent of magma. Preliminary examination of volcanism outside the Okataina Volcanic Centre suggests that intersecting northeast-southwest and north-south fault sets may also play a role in defining the geometry of calderas and locations of volcanic centres throughout the Taupo Volcanic Zone. Outside these volcanic centres (e.g. Taupo and Okataina) active extension is primarily accommodated by normal faulting which is driven by tectonic processes (e.g. far-field plate motions) and is not attributed to dike intrusion. The Taupo Rift has not yet reached the stage where it is dominated by magma-assisted extension and is primarily a young tectonic rift in an arc environment.