Imaging and quantification of gas hydrate and free gas at the Storegga Slide offshore Norway

Wide–angle reflection seismic experiments were performed at the Storegga slide offshore Norway in 2002 with the goal to quantify the amount of gas hydrate and free gas in the sediment. Twenty‐two stations with Ocean Bottom Hydrophones (OBH) and Seismometers (OBS) were deployed for a 2D and a 3D experiment. Kirchhoff depth migration is used to transform the seismic wide–angle data into images of the sediment layers and to obtain P wave velocity–depth functions. The gas hydrate and free gas saturations are estimated from the elastic properties of the sediment on the basis of the Frenkel–Gassmann equations. There is 5–15% gas hydrate in the pore space of the sediment in the gas hydrate stability zone (GHSZ). The free gas saturation takes the value of 0.8% for a homogeneous distribution of gas in the pore water and 7% for the model of a patchy gas distribution

Fault-controlled hydration of the upper mantle during continental rifting

Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems1. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges1 and are thought to be present at about half the world’s rifted margins2, 3. Serpentinite is also inferred to exist in the downgoing plate at subduction zones4, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults3, 4. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth

Waveform inversion of the S reflector west of Spain: fine structure of a detachment fault.

The S reflection west of Iberia has been interpreted as a low-angle detachment fault separating crustal fault blocks from partially serpentinized mantle. We apply full waveform inversion to investigate the fine structure of S. Our results confirm that S is largely a step increase in velocity (and density), probably from crustal rocks to partially serpentinized mantle peridotites. A ~50 m thick low velocity zone above S might represent a main fault zone of highly serpentinized peridotites or hydrofractured and altered crustal rocks above the main fault zone. Both interpretations imply focused fluid flow along S, raising the possibility that low-angle movement along S was aided by the development of local, transient high fluid pressures