Detrital Zircon Provenance of Neogene Forearc Sub-basin Sandstones: East Coast Basin of New Zealand

Abstract:

The East Coast Basin (ECB) of the North Island, New Zealand, is an active forearc basin that features bathymetric sub-basins segregated by low-angle reverse faults. Understanding the spatial and temporal contribution of sediment from distinct source terranes to Neogene deep-water deposits in the ECB may constrain the degree of basin segregation during evolution of the modern tectonic regime. Furthermore, the study of Neogene dispersal patterns may permit delineation of active source areas and source-to-sink boundaries between terrestrial and marine deposition.

Detrital zircon U-Pb dating was used to evaluate thirteen Neogene sandstones from two transects (in the Raukumara and Wairarapa regions) that are hypothesized to represent paleo-sub-basins of the ECB. Age spectra from this suite of Miocene–Pliocene sandstones are interpreted in the context of ongoing and published work. All samples have robust Permian-Triassic and Cretaceous age peaks, except for two lower Miocene samples from the Wairarapa sub-basin which have minor Cretaceous peaks. Middle-upper Miocene Raukumara samples contain contemporary age peaks, and the lower Pliocene sample has a late Miocene age peak. Kaweka terrane likely contributed to both sub-basins by early Miocene time, but Pahau sediment did not appear in the Wairarapa sub-basin until middle Miocene time. Active volcanic arc sources contributed to the Raukumara sub-basin during middle-late Miocene time. Furthermore, no Wairarapa samples contain contemporary age peaks, suggesting that 1) flow of sediments between these sub-basins was not straightforward and there was likely a barrier to sediment dispersal, or 2) the volcanic source was geographically restricted and did not contribute sediment to the southern ECB. Ultimately, presence of a Pahau age signature in both sub-basins, and younger contemporary ages in only the Raukumara sub-basin to the north, indicate independent sediment dispersal pathways contributing Pahau sediment to both basins, with the northern basin receiving additional input from a volcanic source.