The structure, fluid distribution and earthquake potential of the Makran Subduction Zone, Pakistan

Abstract

The Makran subduction zone (offshore Pakistan and Iran) has the largest accretionary prism of any margin worldwide, formed due to the thick incoming sediment section of up to 7.5 km. This margin has been relatively understudied, and this thesis presents a new, detailed structural and hydrological interpretation and seismogenic hazard assessment for the Makran. The accretionary prism is dominated by simple, imbricate thrusts which form seaward verging, anticlinal ridges up to 200 km long. The prism has a low average taper angle of 4.5°. Two oceanic basement features intersect the deformation front: The Little Murray Ridge (LMR), a discontinuous, largely buried seamount chain, and the Murray Ridge, a large transtensional ridge. The subduction of the LMR causes an increase in fault spacing, a seaward step in the position of the deformation front, and may segment earthquake rupture. The Murray Ridge influences the incoming sediment stratigraphy and reduces sediment thickness in the east. Fault activity in the Makran is widely distributed within the prism, with over 75% of faults showing some evidence for recent activity. This may be the result of the high levels of frontal accretion causing the Makran to behave as a sub-critical prism. The décollement in the outer prism occurs within the sediment section and is unreflective. There is extensive evidence for fluid and fluid migration in the Makran, with a widespread hydrate BSR, high amplitude gas zones in the shallow sediment, reflective fault sections (indicating high porosity and likely high pore pressure), and surface seeps. The spatial distribution of these features appears to be controlled by changes in the incoming section and fault activity, and significant fluids are trapped within anticlinal hinge zones. Reflective fault sections are concentrated in the upper sediments, and there is no evidence for a significant fluid contribution from the deeper (>4 km) sediment section. This may indicate that the lower sediment section is largely dehydrated, prior to accretion.

The Makran experiences low seismicity compared to many global subduction zones, but produced a Mw8.1 tsunamigenic earthquake in 1945. Thermal modelling suggests that temperatures at the plate boundary are over 150°C at the deformation front due to the thick sediment section. These results suggest that the plate boundary may have the potential to be seismogenic to shallow depths. Thermal modelling also indicates that the shallow dip of the subducting plate produces a wide potential seismogenic zone, which when combined with along-strike rupture scenarios produces potential earthquake magnitudes of Mw8.7-9.2 with significant regional hazard implications.

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