Ophioliticlastic Ihungia igneous conglomerate (Early Miocene), North Island, New Zealand: evidence for an island source related to subduction initiation and deposition within a slump-generated submarine slope re-entrant

Abstract

In the eastern North Island, the Early Miocene Tolaga Group contains some small isolated outcrops of distinctive basaltic–clast conglomerate and serpentine-bearing sandstone, the Ihungia igneous conglomerate. The shape, texture and composition of ultramafic to mafic gravel and sand components of the conglomerate indicate that they were likely delivered via submarine continental slope canyons from emergent island(s) on which ophiolitic peridotite, gabbro and basalt of the East Coast Allochthon were exposed. We speculatively propose that, following subduction initiation and ophiolite emplacement, and just prior to deposition of these units, eastward (downslope) displacement of part of the allochthon formed a large re-entrant in the continental slope in which the gravel subsequently accumulated. This Miocene Ihungia re-entrant has similarities to modern re-entrants along the continental shelf edge of the Raukumara Peninsula. Thus, subduction-related submarine mass transport processes may have been active along the New Zealand continental margin for the last 20 Ma. This study documents the sedimentary signature of an ophiolite likely created, uplifted and subaerially exposed in association with subduction initiation.