Paleogeographic and Morphologic Reconstroction of a Buried Monogenetic Volcanic Field

Abstract

Zealand 5 alan.bischoff@canterbury.ac.nz *corresponding author 6 7This is a non-peer reviewed preprint submitted to EarthArXiv. Abstract 10Technological advances of our modern society motivated an unprecedented necessity to 11 find natural resources in the subsurface of our planet. The search for these valuable 12 resources has revealed an unexpected number of ancient volcanoes buried and preserved 13 within sedimentary basins globally. Continuous improvements in remote sensing 14 techniques such as seismic reflection can provide a valid opportunity to observe these 15 extinct buried volcanic systems. In this paper, we present the Maahunui Volcanic Field 16 (MVF), a cluster of Miocene volcanoes and shallow intrusions currently buried by more 17 than 1000 m of sedimentary strata of the Canterbury Basin, New Zealand. This 'fossil" 18 volcanic field was imaged by high-quality 2D seismic lines and representative igneous 19 rocks were recovered by the exploration borehole Resolution-1. Here, we present the 20 reconstructed regional paleogeography in which eruptions and shallow (<2 km) 21 intrusions occurred, as well as the original morphology of the volcanoes found in the 22 MVF. Volcanism in the MVF occurred over an area of ca 1,520 km 2 , comprising at 23 least 31 crater-and cone-type volcanoes. Eruptions in the MVF typically produced 24 small-volume volcanoes (< 1 km 3 ), controlled by a plumbing system that fed magma to 25 disperse eruptive centres, a characteristic of monogenetic volcanic fields. The MVF 26 plumbing system emplaced a number of shallow intrusive bodies up to 2.5 km 3 in 27 volume, typically within the Cretaceous-Paleocene sedimentary strata. In many cases, 28 these intrusions have served as a shallow stationary magma chamber that possibly fed 29 eruptions onto the paleo-middle Miocene sea-floor. Eruptions were entirely submarine 30 (500 to 1500 m), producing deep-water morphologies equivalent of maar-diatreme and 31 tuff cones. The morphology of the volcanoes is interpreted to be primarily controlled by 32 high-energy pyroclastic eruptions, in which coeval thermogenic gases and CO2 33 incorporated in the magmatic system could have had an important role in the 34 fragmentation and dispersion of erupted material. In addition, post-eruptive degradation 35 has changed the original volcanic morphology, which was controlled by the height of 36 the edifices and by their location in relation to a major base-level fall. By the late 37Miocene, high volcanic edifices (> 200 m) located in a neritic setting were possibly 38 emergent at the paleo-sea surface, forming an archipelago of nine small extinct volcanic 39 islands. This study demonstrates that despite a number of perceived limitations, the 40 geological history of ancient volcanoes now buried and preserved in sedimentary basins 41 can be reconstructed by detailed seismic stratigraphic mapping and analysis of borehole 42 data. 43 44