The Urry Knolls, late Neogene alkaline basalt extrusives, southwestern Chatham Rise

Abstract

A little-studied group of submarine volcanoes, the Urry Knolls, lies on the southern flank of Chatham Rise some 200 km ESE of Banks Peninsula, South Island. Individually, the volcanoes form small cones a few kilometres across and several hundred metres high. Some straddle small ridges, implying eruption from fissures. Buried lava flows or shallow intrusions are common. Vesicular olivine basalts dredged from one of the peaks are hawaiites of “within-plate alkaline” and “oceanisland alkaline” affinities, and similar in mineralogy and chemistry to basalts from Chatham Island and Banks Peninsula. They may have been active as early as upper Late Miocene time and as late as Mangapanian to basal Castlecliffian (latest Pliocene — early Pleistocene) time.

Keywords:

• alkaline basalt
• volcanism
• Chatham Rise
• Miocene
• Pliocene
• Pleistocene
• Urry Knolls
• New Zealand
• seismic stratigraphy
• chemical analyses
• foraminifera