Gas hydrates on New Zealand's Chatham Rise: An indicator of Gondwanan thermogenic gas expulsion?

High amplitude bottom simulating reflections (BSRs) interpreted in seismic reflection data are common proxy indicators for gas hydrates, while the spatial relationship of BSRs with fault systems can provide an indication for the source of gas bound within hydrates. The spatial correlation of fault zones with strong BSRs and thick free gas zones (FGZs) has sometimes been recognised as a common feature of thermogenic hydrate systems. We investigate the seismic stratigraphy and reflectivity of New Zealand's Chatham Rise northwestern slope (CRNWS), which is a remnant section of the ancient East Gondwana subduction margin's accretionary wedge. Seismic interpretation indicates the presence of a partially subducted, Cretaceous-aged sedimentary unit beneath the inactive, ∼6 km-thick East Gondwana accretionary wedge. Heat flow estimation and calculation of temperature profiles suggests that the ∼1–2 km-thick downgoing Mesozoic sedimentary sequence (MES) is situated within a thermal window conducive to thermogenic methane generation. As the downgoing MES may contain source rocks similar to the Glenburn Formation, a well-known thermogenic source, which is proposed to span the neighbouring Pegasus and East Coast Basins, we propose the presence of a sub-Gondwanan wedge thermogenic gas zone. Overlying the proposed Gondwanan thermogenic zone by ∼6–8 km, seismically-distinct shallow BSRs and hydrate-related features are interpreted spanning ∼1400 km 2 of the CRNWS. BSRs often terminate against and are associated with deeply-rooted CRNWS faults, which penetrate the Gondwana wedge. These BSRs mark previously-undefined hydrate accumulations situated ∼50–100 km SW from the active Hikurangi subduction margin. We interpret the presence of thermogenic methane within the CRNWS hydrate zone, supplied from the ∼8–10 km deep downgoing MES by Gondwanan thrust faults, and BSR-proximal fault asperities. The close association of BSRs with a shallow, unit-bound polygonal fault complex may be further distributing free gas, up-dip across the CRNWS, resulting in concentrated hydrate accumulations and thick (∼100–150 m) FGZs.