Seismic Evidence of Magmatic Rifting in the Offshore Taupo Volcanic Zone, New Zealand

Relationships between extensional tectonics and magmatism are ubiquitous in continental rifts and oceanic spreading centers. Yet few studies document interactions between extensional faults and mantle melts in volcanic arcs. We constrain the crustal structure of the extensional offshore Taupo Volcanic Zone (TVZ) from a marine multichannel and wide-angle seismic experiment. The TVZ crust thins from >26 km to ∼18-19 km across ∼50 km in the Bay of Plenty. Elevated P wave velocities in the lower crust indicate mafic additions. Magmatic sills between 4- and 15-km depth lie beneath listric normal faults in a ∼40-km-wide active rift zone. P wave velocities in the middle and upper crust along the arc front are ∼0.3-0.5 km/s slower than in the adjacent crust, indicating a possible thermal anomaly imparted by heat from magmatic intrusions. We propose that rifting in the offshore TVZ is partially compensated by intrusions and assisted by thermal weakening of the lithosphere.