Aeromagnetic imaging of the Dun Mountain Ophiolite Belt in northern New Zealand: insight into the fine structure of a major SW Pacific terrane suture

Abstract

A new aeromagnetic survey of the Junction Magnetic Anomaly in northern New Zealand reveals that it is associated with up to 10 subparallel lineaments. The Junction Magnetic Anomaly delineates the Dun Mountain Ophiolite Belt, a remarkably linear though sigmoidally bent major boundary zone traceable through most of New Zealand, which separates terranes sutured onto the New Zealand Gondwana margin in late Mesozoic time. Modelling of the lineaments, in conjunction with a new review of structures along the Dun Mountain Ophiolite Belt throughout New Zealand, indicates that the Dun Mountain Ophiolite Belt in Auckland consists of a number of imbricate slices, most of which are likely to dip steeply to the east (i.e. overturned away from the former trench) and extend to considerable depth in the crust. The source bodies for the central high-amplitude magnetic lineaments are interpreted to be serpentinite shear or mélange zones, cutting a large lensoid body of ultramafic rocks, similar to those occurring elsewhere along the length of the Dun Mountain Ophiolite Belt. Our data have revealed that a formerly postulated offset of the Dun Mountain Ophiolite Belt in the region instead resembles a duplex or thrust imbricate-like bundling of shear zones. These shear zones were probably initiated as accretion thrusts, but complexities in their pattern indicate subsequent shearing, including ‘backfolding’ to an overturned attitude and possibly strike slip to produce the linear nature of the Dun Mountain Ophiolite Belt. The outer lineaments mostly mark accretion thrusts in the adjacent terranes. This study demonstrates that detailed aeromagnetic surveys can delineate the fine structure in ophiolite complexes and thereby give valuable insights into the effects of micro-continental collision on an accretionary margin.

Os isotopic constraints on crustal contamination in Auckland Volcanic Field basalts, New Zealand

“…The crust underlying the AVF is 20-30 km thick, and composed of Waipapa and the Murihiku terranes ( Fig. 1.C) (e.g., Kermode 1992;Eccles et al, 2005;Horspool et al, 2006;Mortimer et al, 2014), which are overlain by up to 1-2 km of Miocene Waitemata terrane sediments. The western Waipapa and eastern Murihiku terranes mainly consist of late Triassic to late Jurassic low-grade meta-sediments, separated by the Dun Mountain terrane.…”

“…The JMA traverses the Auckland area as a narrow (ca. 2-5 km), linear series of positive magnetic anomalies interpreted to be eastward-dipping serpentinised shear zones, extending throughout the 20-30 km thick crust beneath the AVF (Eccles et al, 2005). Direct evidence of the Dun Mountain Ophiolite Belt (DMOB) beneath the AVF occurs in the form of serpentinite xenoliths within volcanic rocks in tuff rings from Pupuke, St. Heliers and Taylors Hill volcanoes (e.g., Searle, 1959;Bryner, 1991;Jones, 2007;Spörli et al, 2015).…”

“…Direct evidence of the Dun Mountain Ophiolite Belt (DMOB) beneath the AVF occurs in the form of serpentinite xenoliths within volcanic rocks in tuff rings from Pupuke, St. Heliers and Taylors Hill volcanoes (e.g., Searle, 1959;Bryner, 1991;Jones, 2007;Spörli et al, 2015). The basement rocks that make up the region of the study area have been extensively investigated (e.g., Bryner, 1991;Kermode, 1992;Eccles et al, 2005;McCoy-West et al, 2013), allowing potential sources of crustal contamination to be well characterised.…”

Post-Miocene faults in Auckland: insights from borehole and topographic analysis

“…1) is a greywacke and argillite assemblage of the Waipapa (composite) Terrane (Spo¨rli 1989). Separating the two is the Dun MountainÁMaitai terrane, made up largely of variably serpentinised ultramafic ophiolitic rocks that are inferred to be the source of the strong NNW-trending gravity and magnetic anomalies known collectively as the Junction Magnetic Anomaly (JMA) (Woollaston 1996;Sivell & McCulloch 2000;Eccles et al 2005;Williams et al 2006;Cassidy & Locke 2010).…”

“…In the Auckland region it widens abruptly to form a complex zone of subparallel lineaments up to 14 km wide, extending from the Auckland isthmus northwards for about 30 km. This unusual widening is thought to represent a pod of up to 10 NNW-trending serpentinised shears (Eccles 2003;Eccles et al 2005;Cassidy & Locke 2010) with a flat upper surface reaching to within 500 m of the ground surface (Williams 2003).…”