3-D seismic imaging of the plumbing system of the Kora Volcano, Taranaki Basin, New Zealand: The influence of syn-rift structure on shallow igneous intrusion architecture

Descriptions of the interactions between volcano plumbing systems and early-stage rift systems dominated by large half grabens and tilted fault blocks are scarce and lack detail. 3-D seismic reflection data across the Early Miocene-age subduction-related Kora Volcano plumbing system in the Taranaki Basin, offshore New Zealand, provides an opportunity to investigate in detail how this system has interacted with a Late Cretaceous-Early Paleogene rift system. The shallow (<8-km-depth) architecture of intrusions are dominated by stacked saucer-shaped, and transgressive sills, and laccolith complexes distributed in an oval shape around the volcano. The thickest sill complexes occur in the thickest (fault-controlled) syn-rift depocenters. There is also local elongation of sill complexes in a N-S direction following structural trends. Numerous sills locally follow segments of large normal faults, or terminate at the faults. Laterally extensive sill complexes tend to lie close to the base of the syn-rift/top of pre-rift contact for distances up to 15 km away from the Kora Volcano. A general pattern is that the lowest-level sills are the most laterally extensive, and the shallower sills lie closer to the volcano. Shallow sills form a complex of concentric sills that dip away from the volcano center (ring sills). Sills are interpreted to have been fed by intrusions following faults that intersect a broad upper crustal magma chamber with a calculated area of similar to 345 km(2). Strong preference for sill/laccolith stacking in the thick syn-rift section appears to have had four effects. (1) In the upper 6 km there probably was not a simple single pipe feeding the volcano, instead the volcano is inferred to have been fed by a network of sills linked by short dikes and pipes. (2) The extensive intrusions resulted in folding/doming of the Paleogene section in several places, but most significantly under the central part of the volcano. (3) Much of the magma was stored in intrusive complexes within the syn-rift section and the underlying magma chamber (volume similar to 124 km(3)), not extruded (volume similar to 40 km(3)). (4) Edifice construction and broad doming related to subsurface intrusions may have caused the crest of the volcano to become subaerial. The consequent absence of phreatomagmatic eruptions and the extensive accumulation of magma in relatively thin sill complexes (that could cool relatively rapidly and solidify) avoided a destructive end to the volcanic edifice.