Gloria observations of the propagating rift at 95.50W on the Cocos-Nazca Spreading Center

In this paper we describe a Gloria long-range sidescan sonar survey of the area around 95.5°W on the Cocos-Nazca Rise, where a propagating rift has been predicted to occur. The sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1-Ma -old crust. The new crust is up to 800 m deeper than the surrounding seafloor, although the elevation difference appears to decay with time. The `pseudofaults' which bound this new crust have been observed from the rift tip back to crustal isochron 0.65 Ma. Near the rift tip they consist of scarps several hundred meters high with narrow, shallow valleys at their feet. The `sheared zone' between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and NNW-SSE trending lineaments. These may have been produced by the rotation of tectonic fabric formed at the dying rift, perhaps accompanied by other deformation. The youngest tectonic and volcanic lineaments on both spreading segments strike 273°, but on the dying rift, short spreading segments are offset en echelon so that the regional trend of the rise axis is 265°. Older lineaments throughout the area strike 265°. The propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction, and it is suggested that this is occurring in response to a recent change in spreading direction from 355° to 003°. Variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273° direction, which we take to be normal to the new spreading direction.