Distribution and development of submarine mud volcanoes on the Makran Continental Margin, offshore Pakistan

Abstract

Abstract In 2018, China Geological Survey and National Institute of Oceanography of Pakistan conducted a comprehensive geological survey on the Makran Continental Margin offshore Pakistan using the R/V Haiyangdizhi10. Seismic profiles, multibeam bathymetric and water column data, and seafloor observations were obtained during the expedition. Twelve mud volcanoes and three pockmarks were discovered in the surveyed area on the Makran Continental Margin. The heights of the mud volcanoes and the depths of the pockmarks are 12–121 m and −108 to −26 m, respectively, with diameter of 500–1400 m. The mud volcanoes are distributed on top of the W-E trending ridges at 1200–2800 m water depth. Six mud volcanoes and one pockmark were active during the investigation as indicated by the gas flares mapped in the multibeam water column data. Acoustic blanking zones, faults and gas chimneys are distributed ubiquitously beneath the mud volcanoes on the seismic profiles. Their spatial correlation with mud volcanoes indicates that the faults and gas chimneys provide pathways for fluid migration. In addition, the Bottom Simulating Reflectors (BSR), interface of gas hydrate and free gas, were also discovered in multiple seismic profiles. Based on the BSR discontinuity and piercing of the gas chimneys, it is inferred that mud volcano activities might cause gas hydrate dissociation, leading to the upward movement of the BSR. Tectonic uplifting, seismic-induced shock and fluid overpressure induced by rapid sedimentation are the major driving forces for the mud volcano formation and development. Gases from deep strata and gas hydrate decomposition are suspected to provide additional forces for fluid migration. Pressure decrease in the deep subsurface and gas hydrate sealing of the faults and fracture are speculated to be important in controlling the intermittent mud volcano activities and potential pockmark formation. The new model of mud volcano activities depicted in this study might be applicable to other mud volcanoes elsewhere.