Abstract In the present study the seismic structure of oceanic core complexes (OCC) in 
the Parece Vela Basin, Philippine Sea have been imaged. Together with recent work on the 
Atlantis Massif OCC on the Mid-Atlantic Ridge, including deep drilling, this work provides 
an unprecedented opportunity to advance our understanding of OCC internal structure. A 
continuous, strong and relatively smooth reflection that was ca 0.15 s (two way time) below 
the sea floor of an OCC in the Chaotic Terrain of the Parece Vela Basin was identified. This 
reflection, termed the D-reflector, is similar to that observed beneath Atlantis Massif. A 
faster P-wave velocity (>6 km/s) is observed very shallow beneath the Chaotic Terrain 
OCC, suggesting that the core of these OCC is dominantly gabbroic. The D-reflector might 
be common beneath OCC, owing to localized alteration along fractured zones within 
gabbro. We further observed a series of three detachment events in the Chaotic Terrain. 
The first and second detachments exhumed shallow basaltic crust to deeper gabbroic core, 
whereas the last one only exhumed shallow basaltic crust.