Single-channel and multichannel seismic reflection data show evidence for recent movement of faults on 5.9 Ma crust near Ocean Drilling Program (ODP) Hole 504B on the southern flank of the Costa Rica rift, >200 km from the ridge axis. These faults, which are associated with N85°W-trending, ridge-parallel basement escarpments, can be traced upward into the thick, overlying sedimentary section that blankets volcanic crust in this area. The offset of sedimentary horizons indicates the style and history of faulting. It consistently shows the downdropped side to the north, signifying inward-facing normal faults or grabenlike structures indicative of crustal extension perpendicular to the ridge axis. Although most of the movement on these faults appears to have occurred in young crust near the ridge axis, many of these faults have a long history of activity, with cumulative displacements of several tens of metres occurring over the past several million years. These results are inconsistent with inferences from borehole stress measurements made in Hole 504B that the crust in this area is in horizontal compression or with previous assumptions that crustal extension at mid-ocean ridges is limited to within 10–20 km of the spreading axis. Although the broad zone of crustal extension on the south flank of the Costa Rica rift could reflect anomalous stresses within the Nazca plate, several independent lines of evidence suggest that the active “tectonic zone” of crustal extension and normal faulting at mid-ocean ridges may be significantly wider than previously suspected.

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