Mud volcanoes in an active fore-arc setting: A case study from the Makran Coastal Belt, SW Pakistan

The Makran coastal belt is over 1000 km long stretching from Iran to east of Karachi in Pakistan. A major active subduction zone known as Makran subduction zone defined by the under thrusting of the Arabian plate beneath the Eurasian plate runs parallel to the coastline. The subduction zone is associated with a thick accretionary sedimentary wedge deposited in an active fore-arc basin containing very thick detrital sediments contributed by the accretion of the subducting plate since Late Eocene time. The sediments in the fore-arc basin are fine-grained usually of clay size fraction deposited in highly fluidized conditions trapping methane gas. The fluidized mud diapirically moves upward along weak zones as mud volcanoes due to high sedimentation rates and escaping gas pressure piercing through the overlying sediment layers. The development of mud volcanoes shows a close relationship between the sedimentation rates, gas escape from sediments and tectonic activity. Mud volcanoes are found in abundance both onshore and offshore of the Makran Coast. Most of the onshore mud volcanoes are associated with active fault zones and are believed to be triggered by tectonic activity. Located in the hanging wall of an active subduction zone, the region is seismically highly active and occurrence of major earthquakes exceeding magnitude 7 is a common phenomenon (e.g., Mw = 7.7 2013 Awaran, Mw = 8.2 1945 Makran). The seismic activity of this scale is likely the major triggerer for the emergence of new islands off the Makran coast in the Arabian Sea on regular basis in the past history. The latest of these emerged in 2013 immediately after the Mw = 7.7 Awaran earthquake. In this chapter we review major mud volcanoes formed along the Makran coastal region by describing their distribution, evolutionary history and mechanism of their formation. The chapter also describes the sudden appearance of islands off Makran coast since 1945 major earthquake and their importance in the geological history of the coastal regions.