VARIATION IN CROSS-SECTIONAL AREA OF THE AXIAL RIDGE ALONG THE EAST PACIFIC RISE - EVIDENCE FOR THE MAGMATIC BUDGET OF A FAST SPREADING CENTER

Along the fast and ultrafast spreading East Pacific Rise, the cross-se ctional area of the axial ridge varies significantly over length scale s similar to its morphologic segmentation. Using an automated method, we measure the ridge area (volume per kilometer along axis) where ther e is complete bathymetric coverage. Along 3500 km of the northern and southern East Pacific Rise, our two study areas encompass numerous tra nsform faults, large overlapping spreading centers, small overlapping spreading centers, and smaller discontinuities (first-, second-, third -, and fourth-order discontinuities). The cross-sectional area variati on mimics the undulation of the ridge crest depth; local area maxima o ccur toward the middle of segments, and the axial area decreases by 40 % or more at first- and second-order discontinuities. Third-order disc ontinuities are generally marked by smaller disruptions in the ridge a rea, and fourth-order offsets do not systematically correspond with fe atures of the cross-sectional area profile. The correlation between sh allower ridges and larger ridge areas breaks down at some locations be cause axial cross-sectional area represents a longer term average of t he ridge's magmatic state than axial depth. A correlation between larg e ridge areas and negative residual gravity anomalies indicates that i nflated ridges are underlain by low-density crust and mantle. Also, a correlation between larger area and higher MgO content of axial basalt s suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone. The cross-s ectional area of the axial ridge appeals to correlate with the width o f the axis-centered low-velocity zone in the crust. These observations , as well as the absence of large, relict axial ridges off-axis, indic ate that the axial ridge originates from bouyancy due to thermal expan sion and the presence of melt in the crust and mantle within about 10 km of the rise axis. Portions of the northern East Pacific Rise underl ain by a magma chamber reflector generally occur where the ridge cross -sectional area is greatest; this supports the connection between proc esses which inflate the axial ridge and those which heat the crust and upper mantle and produce melt. Thus, while the axial high on fast spr eading centers resembles a constructional volcano is cross section, it is more like a long, narrow balloon whose cross-sectional area is a s ensitive indicator of magma supply. Using this relationship, we predic t with 75% confidence that at least 45% of the unsurveyed northern Eas t Pacific Rise (18-degrees-N to 13-degrees-N and 9-degrees-N to 5-degr ees-N) and at least 60% of the southern East Pacific Rise (4-degrees-S to 23-degrees-S) is underlain by a magma chamber reflector.