The balloon Melange, northwest Nelson: Origin, structure, and emplacement

Abstract

New work on the Balloon Melange, a regional scale body of disrupted sandstone and mudstone that encloses a number of exotic blocks, suggests that the characteristic fabric has a two-stage origin. The first stage is dominated by layer parallel extension in weakly lithified rocks, and the second by further strain and cleavage development in lithified rock. Location of intrusive contacts between melange and arc assemblage rocks suggests that initial disruption was followed by a diapiric intrusion into a Cambrian volcanic arc plus arcderived sediment host, probably in the latest Cambrian. Mesoscopic and microscopic structural examination shows the fabric was extensively modified during the development of S 1 slaty cleavage. Cleavage formation postdates diagenesis in both melange and host and probably developed in the mid Paleozoic.Petrology, geochronology, and geochemistry confirm that quartzofeldpathic sediments similar to the Junction Formation were the dominant contributors to the melange. The quartzofeldspathic suite (Junction Formation and Balloon Melange protolith) has an Australian-Antarctic provenance and is thought to be part of a submarine fan that may have been incorporated into an accretionary complex developed near the trench at a convergent plate boundary. Further crustal shortening is required to place the volcanic arc assemblage above the accretionary complex prior to diapiric injection of the melange. Subcretion of water-rich sediments above a subduction zone would create an environment predisposed to diapiric intrusion. Scale, and the juxtaposition of rocks of backarc, arc, and accretionary complex origin, points to significant shortening after arc volcanism ceased.The architecture of the Southwest Pacific sector of the Cambrian Gondwana margin and plate boundary is not well understood. Crustal shortening and thrusting are widely developed in the Late Cambrian to Early Ordovician RossDelamerian orogen, and the events leading to the emplacement of the Balloon Melange suggest that the RossDelamerian orogeny affected northwest Nelson.

Structure of the Buller and Takaka Terrane rocks adjacent to the Anatoki Fault, northwest Nelson, New Zealand

“…2), suggesting the melange formation postdates F 1 folding. However, the melange body here is thought to have a diapiric intrusive relationship with surrounding lithologies (Jongens et al 2003). As a result, the discordant and presumably near-vertically orientated contacts of the Balloon Melange will not follow the normal outcrop patterns of folded strata.…”

“…As a result, the discordant and presumably near-vertically orientated contacts of the Balloon Melange will not follow the normal outcrop patterns of folded strata. In addition, S 1 slaty cleavage is pervasively developed in the Balloon Melange and overprints the already formed pre-D 1 melange texture (Jongens et al 2003).…”

“…The second is a melanging event that formed the Balloon Melange. The deformation occurred while the Balloon Melange protolith was predominantly unlithified and is thought to have occurred in the latest Cambrian (Jongens et al 2003). The deformation does not recognisably affect other lithologies of the study area, and for the purposes of this paper can be referred to as pre-D1.…”

“…Ankerite habit is commonly porphyroblastic with inclusion trails. In the Balloon Melange, S 1 is similarly defined by a penetrative slaty cleavage with orientations and microstructures that match those of adjacent lithologies (Jongens et al 2003). Distinctive lithologic horizons, together with numerous way-up indicators, help to delineate the major F 1 fold structures east of Boulder Lake.…”

“…Carbonate commonly fills the interstices of pillows and breccias. The Balloon Melange consists of disrupted quartzofeldspathic sandstones and mudstones with exotic blocks of chert (Jongens et al 2003). Immediately east of the Anatoki Fault, narrow slivers of limestone (Fig.…”