Ds. Wilson et Rn. Hey, HISTORY OF RIFT PROPAGATION AND MAGNETIZATION INTENSITY FOR THE COCOS-NAZCA SPREADING CENTER, J GEO R-SOL, 100(B6), 1995, pp. 10041-10056
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.