HISTORY OF RIFT PROPAGATION AND MAGNETIZATION INTENSITY FOR THE COCOS-NAZCA SPREADING CENTER

Analysis of magnetic anomaly profiles collected nearly parallel to tec tonic flow lines allows detailed interpretation of the complicated tec tonic history of the Cocos-Nazca spreading center. Forward models of t he magnetic anomalies accounting for spreading rate variations, ridge jumps, asymmetric spreading, magnetization intensity variations, and b athymetry show excellent agreement with observed anomalies. Spreading rates can be constrained to a common finite rotation history through a nomaly 4A with three changes in rates. Rate changes at about 1.5 Ma an d 4.1 Ma correspond to changes in rate gradients and occur during the well-calibrated part of the reversal timescale, so they-can unquestion ably be identified as true changes in plate motion. A similar to 15% r ate decrease at about 5.2 Ma could be interpreted either as a change i n plate motion or as an artifact of poor calibration of the older part of the timescale. The change at 4.1 Ma is especially important becaus e many timescales are based on the assumption of constant spreading ra te for this plate pair for 0-6 Ma. Rift propagation has played a domin ant role in the continuous reorganization of the geometry of the ridge axis. Propagation has been predominantly away from the hotspot, with jumps predominantly southward. Propagation rates have ranged from 30 t o 120 mm/yr, commonly near 70 mm/yr. Origin of most propagation sequen ces is difficult to interpret, but many appear to involve discrete sou thward ridge jumps forming a new segment near the hotspot. Magnetic an omaly amplitude appears to be a reliable tracer of Fe content of lavas . Several generalizations can be drawn about along-axis variations in magnetization intensities since 8 Ma: high magnetizations are only obs erved at the far ends (relative to the Galapagos hotspot) of segments at least 150 km long; offset at the end of a high-magnetization segmen t is at least 15 km; and there are no offsets larger than 30-45 km bet ween high-magnetization segments and the reconstructed position of the hotspot. We interpret these patterns to indicate that fractionated la vas erupt where gradients in magma supply cause along-axis flow of evo lved magma. The gradients in supply result from subaxial flow of hotsp ot-derived asthenosphere in a narrow conduit. This flow is only partly obstructed by an offset of 20-30 km but entirely blocked by an offset of 50 km.