GRAVITY EVIDENCE OF VERY THIN CRUST AT THE GAKKEL RIDGE (ARCTIC-OCEAN)

Gakkel Ridge, the active spreading center in the Arctic Ocean, is the slowest spreading portion of the global mid-ocean ridge system. Total spreading rates range from 0.6 cm/yr in the east where the ridge disap pears beneath the Laptev shelf to 1.3 cm/yr in the west near Greenland . Bathymetry and gravity surveys of four sections of the Gakkel Ridge were carried out in 1996 by the U.S. Navy nuclear submarine USS POGY a s part of SCICEX 96 in order to sample variations in seafloor morpholo gy and gravity anomalies as a function of spreading rate. The ridge ax is throughout the survey area is characterized by a continuous axial r ift valley similar to that observed at other slow spreading ridges. Th e continuous rift axis suggests that well-organized seafloor spreading is occurring at total Spreading rates of less than 1 cm/yr. In three faster spreading (1.13-1.24 cm/yr) western survey areas located betwee n 7 degrees E and 54 degrees E, the Gakkel Ridge is deep compared with other ridge axes. Axial depths range between 4600 and 5100 m and ridg e flanks at about 3200 m. The ridge flank morphology is very blocky an d is characterized by large scarps and deep fault-bounded troughs. Ver y large amplitude free-water anomalies with peak-to-trough amplitudes of 85-150 mGal are observed centered on the axis of the Gakkel Ridge. Modeling of the free-water anomalies by varying the crustal thickness and average crustal density, including the gravity effect of the cooli ng of the mantle away from the axis, implies that if the average crust al density is less than 2900 kg/m(3), the crustal thickness must be le ss than 4 km. The axial rift valley at the fourth survey area, near 98 degrees E where the total spreading rate is 0.99 cm/yr, is buried by sediments. The axis in this region is associated with a continuous 70 mGal gravity minimum implying the presence of a large buried rift vall ey. The rift flanks at 95 degrees E are at a depth of greater than 380 0 m, 600 m deeper than the average depth at the Gakkel Ridge axis west of 60 degrees E. Simple isostatic calculations suggest that the crust in this region may be vanishingly thin beneath the sediment cover. Th ese observations indicate a relationship between melt production and s eafloor spreading rate at very slow spreading rates,suggesting that ul tra-slow spreading may suppress melt production or delivery at the Gak kel Ridge. (C) 1998 Elsevier Science B.V. All rights reserved.