EVIDENCE FOR ACTIVE NORMAL FAULTING ON 5.9 MA CRUST NEAR HOLE 504B ONTHE SOUTHERN FLANK OF THE COSTA-RICA RIFT

Single-channel and multichannel seismic reflection data show evidence for recent movement of faults on 5.9 Ma crust near Ocean Drilling Prog ram (ODP) Hole 504B on the southern bank of the Costa Rica rift, >200 km from the ridge axis. These faults, which are associated with N85 de grees W-trending, ridge-parallel basement escarpments, can be traced u pward into the thick, overlying sedimentary section that blankets volc anic crust in this area. The offset of sedimentary horizons indicates the style and history of faulting, It consistently shows the downdropp ed side to the north, signifying inward-facing normal faults or graben like structures indicative of crustal extension perpendicular to the r idge axis, Although most of the movement on these faults appears to ha ve occurred in young crust near the ridge axis, many of these faults h ave a long history of activity, with cumulative displacements of sever al tens of metres occurring over the past several million years. These results are inconsistent with inferences from borehole stress measure ments made in Hole 504B that the crust in this area is in horizontal c ompression or with previous assumptions that crustal extension at mid- ocean ridges is Limited to within 10-20 km of the spreading axis. Alth ough the broad zone of crustal extension on the south flank of the Cos ta Rica rift could reflect anomalous stresses within the Nazca plate, several independent lines of evidence suggest that the active ''tecton ic zone'' of crustal extension and normal faulting at mid-ocean ridges may be significantly wider than previously suspected.