Chaotic rock deposits of the northeastern South Island: implications for the break up of eastern Gondwana

The geological record provides an unparalleled window into past geological events and is critical to understanding Earth processes that may have operated in the past and may change in the future. During the late Early to early Late Cretaceous, a major change in geodynamic regimes occurred on the eastern margin of Gondwana, with the cessation of long-lived subduction and the initiation of rifting. Great thicknesses of well-preserved strata that record this transition are preserved throughout the East Coast Basin, including chaotic mélanges, and have been key to the ongoing debate regarding the timing and processes of the Mesozoic Gondwana subduction shutdown. Within the Cretaceous strata of Zealandia, these chaotic mélanges are highly distinctive, spanning a limited period within the East Coast Basin, which during the late Early to early Late Cretaceous occupied the accretionary wedge front.

Despite the possible importance of these mélange deposits, their formation processes and tectonostratigraphic significance remain uncertain.

This study determines the formation process and possible cause of mélange occurrences within the East Coast Basin by completing a field-based analysis of a mélange located in the Wharekiri Stream, northeastern Canterbury, at the southern terminus of the East Coast Basin. A combination of recently developed mélange diagnostic criteria and traditional sedimentological and structural observations form the foundations of mélange interpretation and understanding of the paleogeographic evolution of the Wharekiri Stream section. Observations of the thick upward fining succession within the Wharekiri Stream illustrate that complex bathymetric relief was present during the Cretaceous, with mélange emplacement occurring via sedimentary processes between ∼108.4-99.5Ma. Paleoslope data suggest that mélange emplacement and subsequent infilling occurred in a deep channel-like structure oriented sub-parallel to the NW-SE oriented Mesozoic Gondwana margin. Diachronous mélange emplacement in the East Coast Basin is hypothesised to have occurred as a result of the introduction of the Hikurangi Plateau into the Cretaceous Gondwana margin starting in the north with clockwise rotation, causing its eventual collision in the southern East Coast Basin. The collision resulted in widespread uplift and instability in the accretionary wedge front, ultimately leading to mass-wasting as the wedge worked to return to a sub-critical taper.

Infilling of the channel structure after mélange emplacement appears to coincide with the activity of the Wharekiri Fault, as indicated by periods of punctuated high-energy deposits within a largely mudstone-dominated succession. Activity on the Wharekiri Fault ceased between ∼98.5 and 95.2Ma, with later reverse faulting occurring on the Limestone Stream Fault during the Neogene. Ultimately, the results of this study accord well with the ∼105-100Ma subduction-subduction model following the Hikurangi Plateau collision.