Transition from rifting to seafloor spreading behind the tip of the westward propagating Cocos-Nazca spreading center

At the Galapagos triple junction, the Cocos-Nazca (C-N) Rift propagates westward towards the East Pacific Rise (EPR) breaking into 0.5 Ma crust accreted on the east flank of the EPR. Rifting transitions to full magmatic spreading in the wake of the propagating tip. In April/May 2018, we collected bathymetry, magnetic, and gravity data over the sequence of 9 rift segments extending 360 km east of the rift tip and off-axis to their points of initiation at the north and south scarps of the Galapagos gore. Our goal was to characterize how full magmatic spreading is established and how segmentation evolves. We also collected rock samples along the axis of each segment and adjacent features (66 dredges, see abstract by Klein et al.). The transition from rifting to spreading occurs in stages over a distance of 60 km from the gore tip. A 12-km-long basin represents the earliest stage of rifting (s1) and appears to be amagmatic. The next segment eastward, offset south 3 km, is the 25-km-long Hess Deep rift (s2), a transitional segment from rifting to spreading. A deep basin in the western half of s2 reaches water depths of 5400 m, while in the east, eruptions have built an axial volcanic ridge 11 km long and up to 500 m high; water depths shallow to 4100 m. Full magmatic spreading begins at segment s3, which is 20 km long and offset 2 km south of s3. The morphology of s3 is typical of spreading segments farther east (shallow graben containing small volcanic cones), and average water depth is just a few hundred meters deeper than at s4. Starting at segment s5, both segment length and offset between segments increase. In places, segments with small initial offsets appear to have linked over time to form a single longer segment, such as at segment s5, which is about 60 km in length. The largest offset mapped is between segments s7 and s8. Segment s8 is offset 28 km to the north of s7 and a rotated, intra-segment high, about 1000-km in relief is present within the offset. The off-axis bathymetry and magnetic anomaly data of segments s5-s8 show a complex pattern of spalled off overlap basins and rotated overlap highs, indicating westward propagation of these segments. We present the detailed history of the initiation and development of the spreading segments in the wake of the westward propagating tip of the C-N Rift.