Sediment compaction and fluid migration in the Makran Accretionary Prism

Abstract

The Makran continental margin in the Gulf of Oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland. A recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region. Velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge, as seen in previous sonobuoy wide‐angle data. Accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism; this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment. A prominent “bottom simulating reflector” appears 500–800 m beneath the sea bed. Several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas, which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile. The hydrate reflector appears to shallow in the region of some faults, suggesting a temperature anomaly due to the presence of warm pore fluids. A heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens. Simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly. The inferred presence of overpressured pore fluids in the Makran suggests that accreted sediments have a low permeability. The seismic evidence suggests a two‐stage compaction process, with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones.