Upper mantle structure beneath the eastern Pacific Ocean ridges

摘要

We analyze vertical component body and surface waves for 10 M w > 5 earthquakes, recorded by ocean bottom seismometers at regional and teleseismic distances. Through waveform modeling, we place new constraints on along-axis variation in temperature and partial melt beneath the eastern Pacific ridges. The resulting best fit models show over 9% variation in average lithosphere shear velocities between different ridge segments. We demonstrate that lid velocity correlates with the square root of plate age, consistent with a conductive cooling process, but we find a more rapid dependence on age close to the rise axis. We map the average plate age into a mean lithospheric temperature for each of our profiles using a half-space cooling model, and the temperature derivatives (dVs/dT) determined from least squares linear fits are ?1.1 and ?0.26 m s?1 deg?1, respectively, for temperatures above and below ～1000°C. The former estimate is more negative than values determined by earlier reports (?0.4 to ?0.7 m s?1 deg?1), using global or regional data from a much wider range of seafloor age but with less resolution at young ages. The high ∣dVs/dT∣ value may suggest the presence of limited partial melt at shallow mantle depths, even after accounting for the strong effect of anelasticity and anharmonicity resulting from temperature and grain size variations. Our data also show a strong north-south difference in mantle structure: the surface waves that traverse the southern East Pacific Rise (EPR) experience shear velocities as low as ～3.75 km s?1, more than 0.2 km s?1 slower than the average mantle structure at comparable depths beneath the northern EPR and the Galapagos spreading center. This difference cannot be explained by the simple conductive cooling process or spreading rate variations between ridge segments. The slow seismic speeds in the low-velocity zone (LVZ) appear to require partial melt. The velocity difference might be solely caused by higher temperatures under the southern EPR, but it may also suggest more melt in the LVZ beneath the southern ridge axis due to potential differences in melt production or extraction compared to the north. In addition, differences in width or symmetry of the partial melt zones can affect the observed path-averaged velocities beneath these two segments. Finally, we determine more accurate depths for ocean transform earthquakes by analyzing the amplitude ratios between body and surface waves. The depths of these transform earthquakes are consistent with brittle deformation within the oceanic crust.