3-D imaging of two episodes of Hikurangi Plateau subduction in the southern South Island of New Zealand

The Hikurangi Plateau (originally part of the Ontong Java large igneous province) has been subducted beneath New Zealand twice - firstly at ca. 108-105 Ma during north-south convergence with Gondwana, and currently during east-west convergence between the Pacific and Australian plates. We have investigated the southern limit of this subducted plateau by supplementing the sparse GeoNet permanent seismograph network in the southeastern South Island with a forty-station broadband portable seismograph network. We have then used local earthquake tomography to determine detailed 3-D Vp and Vp/Vs structure in the southern South Island. We track a region of Vp 8.5 km/s (which has previously been associated with an eclogite layer at the base of the Hikurangi Plateau from studies in the North Island) across most of the southwestern South Island. Its southeastern edge runs diagonally from near Christchurch to northern Fiordland. It dips both to the northwest and the southwest, and impacts the subducted Australian plate in northern Fiordland, where it currently bends the subducted Australian slab to vertical. The plateau and its leading oceanic crust are distinguished by low Vp/Vs, consistent with extensive dehydration of the thick (ca. 35 km), buoyant plateau during ca. 500 km of flat subduction at the Gondwana margin. The plateau is also revealed by dipping zones of relocated earthquakes. The backstop of Gondwana subduction appears to be the ophiolitic Maitai terrane, which extends through the crust and forms the trenchward boundary of the ca. 60-65 km-thick Median Batholith. We image the low Vp crustal root associated with orogeny at the Gondwana margin in the southeastern South Island, as well as the crustal root resulting from the current convergent episode in the western South Island. The shapes of both crustal roots are controlled by the Hikurangi Plateau.