2008 Joint Annual Meeting (5-9 Oct. 2008): Syntectonic(?) Mafic Gravel Associated with Miocene Obduction and Subduction Initiation in New Zealand

The East Cape of North Island, New Zealand is an actively deforming forearc overprinted by a south-verging obduction event that upthrust Miocene to Cretaceous sedimentary units over the forearc basin. The thrust-based Allochthon contains deeper water facies equivalents of underlying sedimentary units, as well as an ophiolitic section of Cretaceous age and equivocal origin (Matakaoa) exposed at its northernmost extreme. The ophiolite is thought to have been emplaced during a complex reorientation of the subduction zone from east-west to north-south in the Miocene. Near the frontal thrust of the Allochthon within the Waipaoa River drainage basin there are a few outcrops of an unusual mafic conglomerate (Ihungia). These outcrops lie ~70 km distant from the ophiolite outcrops to the north. Pebble counts show the composition to be roughly 60% mafic volcanic rocks, 30% intermediate/mafic plutonic/hypabyssal rocks, and %10 sedimentary rocks. Some of the igneous rocks are foliated and exhibit shear-sense indicators. Associated sandstones show volcanic, plutonic and amphibolite clasts in addition to detrital serpentine. No bioclasts were noted. Gravel clast shapes are dominantly equant with lesser rods/rollers/discs. Gravel clast shapes and lithologies are consistent with the Ihungia being fluvial gravels derived from the emerging allochthon and perhaps deposited in an associated foredeep.