Deep Free‐Gas Accumulation Beneath the Chatham Rise, New Zealand—An AVO Study

Subduction zones serve as carbon recycling centers, where vast amassments of geologic carbon accrete or subduct through thermogenic gas windows over millions of years. We focus on New Zealand's Chatham Rise, a fossilized accretionary wedge remnant of the ∼400 Myr‐active East Gondwanan margin. We undertake an amplitude‐variation‐with‐offset (AVO)‐based seismic analysis of the abiogenic Mesozoic sedimentary sequence (MES) and overlying Sequence Y chalk interval, which span the Chatham Rise's northwestern slope. Two‐term AVO attribute analysis resulted in the interpretation of distinct AVO Class III–IV reflection anomalies, which demarcate the siliciclastic MES from overlying Sequence Y chalks. Unified through their strong negative intercept amplitudes, Class III anomalies increase in absolute amplitude with angle, while Class IV anomalies decrease in absolute amplitude with angle of incidence. Simultaneous AVO inversion of seismic data highlighted the presence of P‐impedance anomalies, which directly underlie the regionally occurring Sequence Y chalk interval. Class III anomalies are modeled and interpreted as the result of a previously undefined coarse‐grained lithofacies, bearing low saturations (2%–10%) of free gas. Co‐occurring ClassIV AVO anomalies are modeled to provide evidence for a fine‐grained upper MES, bearing similarly low saturations of free‐gas in pore space. We speculate on the gas' origin, which could be from the Hikurangi subduction margin, in situ ancient microbial activity, or a new undetermined source related to the ancient East Gondwanan subduction margin and accretionary wedge. Plain Language Summary Active subduction zones (aka subduction margins) are globally occurring zones of geologic interest, defined by the subduction of one tectonic plate under another. Subduction margins host vast amassments of geologically bound carbon, which eventually subducts or accretes to the overriding plate. Carbon amassment and subsequent gaseous carbon‐escape (e.g., methane escape) from subduction zones into the Earth's exosphere necessitate an understanding of the dominant sources of gas generation within a subduction zone. Here, we present a seismic reflection‐based study of the northwestern Chatham Rise, which is a fossil remnant of the ancient East Gondwanan subduction zone. We provide seismic evidence for a previously undetected deep accumulation of carbon‐bearing free gas over a large area ∼50 km east of the active Hikurangi subduction margin. T