The structure and fault activity of the Makran accretionary prism

The Makran Subduction Zone has the highest incoming sediment thickness (up to 7.5 km) of any subduction zone. These sediments have formed a wide accretionary prism (similar to 400 km). Seismicity in the Makran is generally low; however the margin experienced an M-w 8.1 earthquake in 1945 which generated a significant regional tsunami. Seismic reflection data and swath bathymetry data from offshore Pakistan are used to analyze the structure and fault activity of the outer accretionary prism. The outer prism has a simple structure of seaward verging imbricate thrust faults, many continuous for over 100 km along strike. Fault activity is analyzed using basin stratigraphy and fault geometry, revealing a frontal continuously active zone, a central intermittently active zone, and a landward inactive zone. Over 75% of the faults in the seaward similar to 70 km of the prism show evidence for recent activity. The decollement occurs within the lower sediment section, but steps onto the top-basement surface in regions of elevated basement topography. Fault spacing (6 km) and taper (4.5 degrees) are comparable to other margins such as S. Hikurangi, Cascadia and Nankai, suggesting that high sediment input is not leading to an unusual prism structure. The decollement is unreflective, which is unexpected considering other prism characteristics predicting a weak surface, and may suggest a potentially stronger decollement than previously predicted. This study provides a significant advance in our understanding of the structure of an end-member convergent margin and demonstrates that systematic analyses of accretionary prism structure can help to elucidate subduction zone dynamics with ultimate relevance to seismogenic potential.