Thinned crust in southwest Pacific may harbor gas hydrate

Abstract

The Lord Howe Rise (LHR) is a large, complex, and poorly studied fragment of thinned continental crust submerged 750–3000 m beneath the oligotrophic waters of the central Tasman Sea (Figure 1). Deep seismic profiles taken during recent cruises in the region have revealed an intriguing phenomenon: the eastern slope of the LHR has a prominent and extensive bottom simulating reflector (BSR) that cross‐cuts lithology at 0.65–0.75 s two‐way travel time (twt) [Exon et al., 1998; Lafoy et al., 1998]. Given best estimates for sonic velocity (1600 m/s) and thermal gradients (0.9+0.04°C/m) in sediment on the LHR, predicted temperatures and pressures at the depth of the BSR lie on the CH4‐CH4 hydrate‐seawater equilibrium curve. Thus, the BSR on the LHR most likely represents an interface between gas hydrate and free gas [e.g., Dickens et al., 1997]. Although BSRs and gas hydrates are increasingly found in the marine realm, the LHR discovery has raised an interesting conceptual problem c oncerning the origin of gas hydrates.