Mud volcanoes at the shelf margin of the East China Sea

Roughly circular sediment mounds, varying from tens to hundreds of meters in diameter and from a few meters up to 40 m in height, have been mapped for the first time at the shelf edge of the East China Sea (ECS). High-resolution seismic profiles, together with swath bathymetric and acoustic data, indicate that these features are common on the ECS outer shelf and on the upper western slope of the Okinawa Trough. They are associated with unusually large pockmarks, up to 40 m in depth and 600 m in diameter. On seismic records, bright spots, phase inversions and other acoustic anomalies indicate that gas and/or fluid escape plays an important role in the formation of these mounds. They are often associated with normal faults that occur along the western slope of the Okinawa Trough, these faults acting as conduits for migrating fluid. Based on regional stratigraphic correlation, we propose that the source of mud and fluid is deeper/older than 235 ka. The seepage process is suspected to be very recent, and probably still active, based on the facts that (1) despite high sedimentation rate, the mounds are not buried by recent sediments, and (2) Last Glacial Maximum deposits are reworked at the emplacement of the mounds. Carbonate-cemented sediments and deep-water reefs possibly associated with fluid seepage could be the origin of local patches of very high backscattering acoustic facies, mapped with the swath bathymetric system. (C) 2003 Elsevier Science B.V. All rights reserved.