3-DIMENSIONAL ANALYSIS OF GRAVITY-ANOMALIES IN THE MARK AREA, MID-ATLANTIC RIDGE 23-DEGREES-N

We have carried out a three-dimensional analysis of the gravity field over the Mid-Atlantic Ridge at Kane (MARK) area, a well-studied portion of the Mid-Atlantic Ridge south of the Kane fracture zone between 23-degrees-40'N and 22-degrees-30'N. The Mid-Atlantic Ridge in this area is composed of Two distinct spreading segments separated by a small nontransform discontinuity at 23-degrees-15'N. The mantle Bouguer anamalies in the MARK areas are dominated by V-shaped bands of anomalous mass and small-amplitude gravity lows and highs located both on and off-axis, which indicate that the anomaly distribution is more complex than the simple bulls eye pattern described along some other parts of the Mid-Atlantic Ridge. Within the Kane fracture zone, and in the ridge-transform intersection area, mantle Bouguer anomalies do not accurately reflect the extent of crustal thinning that has been documented sesimically. This is attributed to lower average crustal and mantle densities, due to fracturing and alteration, within the fracture zone and the inability of gravity data to resolve the abrupt change to thinner fracture zone crust which is confined to a narrow zone along the fracture zone trough and beneath the nodal basin at the ridge-transform intersection. Outside of the Kane fracture zone the magnitude of the crustal thickness variations predicted solely from the residual gravity anomalies compare reaonabnly well with the crustial structure derived from seismic refraction studies. The largest residual gravity anomalies, implying significant reductions in crustal thickness, are observed over the small nontransform offset at 23-degrees-15'N. The anomalous mass associated with this feature forms a v-shape that can be traced well off-axis suggesting that these discontinuities are fundamental ridge segmentation boundaries that can persist for millions of years. The distinctive V-shaped pattern of gravity anomalies in the MARK area reflects systematic crustal thickness variations related to the northward migration of this nontransform offset over the past 3-4 m.y.