285-11 Structural inheritance, pluton emplacement, and the making of continental arc crust, Median Batholith, Fiordland, New Zealand

Abstract:

The Fiordland block in SW New Zealand exposes a nearly complete section of Cretaceous continental arc crust from paleodepths of ~5 to at least 50 km and locally ~65 km. The present architecture of the section was assembled mainly at 8-7 Ma when reverse faults inboard of the Alpine Fault uplifted and imbricated slices of lower, middle, and upper crust. These exposures allow direct examination of how arc magmatism built a thick, compositionally stratified column of continental crust above a subduction zone along the Mesozoic Gondwana margin. We combine structural analyses, new mapping of lower crustal plutons, published zircon Hf and O isotopes, and >100 published U-Pb zircon and titanite dates to reconstruct the patterns of pluton emplacement during two periods of high magma addition rates, including a terminal flare-up lasting ~14 m.y.

The results show that three inherited, lithospheric-scale discontinuities, spaced ~25-30 km apart, localized magma influx during emplacement of the Darran Suite at 147-136 Ma and the Separation Point Suite/Western Fiordland Orthogneiss at 128-114 Ma. The inherited structures coincide with distinct zircon Hf- and O-isotope domains, supporting their long-lived control on magma pathways. The younger of the two magmatic events peaked at 118-115 Ma when ~70% of the exposed lower crust was emplaced within ~3 m.y. Our mapping reveals five subvertical feeder zones, each up to ~5 km wide, that localized magma input along two of the discontinuities, forming overlapping mushroom-shaped plutons in the lower crust. Although magmatism migrated regionally from outboard to inboard between the Late Jurassic and Early Cretaceous, successive pulses repeatedly reused and alternated between the same structural conduits. Plutons were emplaced at multiple crustal levels, precluding a simple bottom-up model of arc construction.

Repeated mafic recharge at the base of the crust, recorded by hornblendite and wehrlite sheets emplaced into lower crustal mushes, utilized the same inherited structures and mobilized felsic interstitial melts. These melts were extracted through compaction, channelized flow, and diking, and transferred upward along the inherited discontinuities into the middle and upper crust. These results indicate that inherited structures controlled where mantle-derived magma entered the crust, where lower crustal storage and processing reservoirs developed, and how evolved melts escaped to build a compositionally stratified column of arc crust.

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