a Location of the Dragon Horn area at 49.7°E. b Bathymetry with tectonic and hydrothermal features. The corrugated surface of the OCC extends about 5 km × 5 km to the south in a series of steep scarps typical of breakaways, and to the north ending at the axial valley wall. NTD represent non-transform discontinuities. Location of hydrothermal fields and anomaly sites are shown as colored stars, including active Longqi-1 hydrothermal field (red), inactive Longqi-3 field (orange), and hydrothermal anomalies (black). c Three-dimensional view of the detachment fault zone. It shows the distribution of the epicenters, located within the white rectangle in Fig. 1b. The AA′ seismic velocity profile comes from a wide-angle seismic experiment15. d High-resolution bathymetry and distribution of vents in Longqi-1 field. The bathymetry data were acquired by near-bottom underwater vehicles, human-occupied and autonomous underwater vehicles. DFF3, DFF5, DFF6, and DFF20 represent the vents from which the hydrothermal fluids were sampled in this paper.