Oceanic Crustal Structure North Of The Kane Fracture Zone From 87-147 Ma

The 2001 Far-Offset Active-source Imaging of the Mantle (FAIM) experiment was conducted along an 800-km-long transect in the western Atlantic. This transect extends along a plate kinematic flow line which lies on 87-147 Ma lithosphere. The primary objective of this experiment is to image upper-mantle layering associated with Mid-ocean ridge (MOR) processes by recording and modeling direct P-wave phases propagating through the mantle. The main transect of the experiment consisted of 7 pairs of ocean-bottom seismometers (OBSs) spaced 80-120 km apart. Shots were spaced 1 km apart. These data also provide constraints on oceanic crustal thickness at each OBS pair location, and thus a history of MOR melting and melt-extraction processes over a 60-m.y. time-span. Crustal structure observations are relevant to the primary objective of this experiment in that MOR processes expressed in mantle structure should have some corollary in the crust. Crustal models based on travel-time fit and comparison with synthetic seismograms indicate an abrupt change in crustal thickness near the center of the transect, with average crustal thickness of 6.8 km below the western portion of transect and 5.8 km below the eastern portion. These differences in crustal thickness are consistent with the long-wavelength gravity field and with a apparently abrupt change in spreading rate from 18 mm/yr in the west to 7 mm/yr in the east. The crustal thickness change is also correlated with dramatic changes in basement morphology and mantle P-wave propagation. Smooth and rough basement are overlying thicker and thinner crust, respectively. In the west (thick crust), mantle P-wave phases are absent or have very weak amplitudes implying a weak or negative upper-mantle velocity gradient. In the east (thin crust), mantle P-wave phases have strong amplitudes, suggesting a positive upper-mantle velocity gradient. This correlation between crustal thickness and upper-mantle velocity gradient may be related to differences in melt retention at the MOR, suggesting a strong sensitivity of MOR processes to small changes in spreading rate for slow spreading systems.