Transition of Ridge Axis Gravity with Spreading Rate: The Effect of a Weak Lower Crust

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The principal investigator's preliminary analysis of Geosat altimetry observations reveal an abrupt change in mid-ocean ridge axis gravity with spreading rate that is more pronouced than the topographic variations. below a spreading rate of 60-70 mm/yr the ridge-axis gravity signature has high amplitudes and is variable where as above this rate the gravity depends upon both the topography and the compensation mechanism, the abrupt change in the gravity signature must signify an abrupt change in the isostatic compensation mechanism, there being no abrupt change in the topography. The principal investigator's interpretation is that there are two dynamically distinct modes of seafloor spreading. This work will address the problem of ridge-axis morphology and its variation with spreading rate, crustal thinckness, and mantle temperature from both a modeling and observational stand point. The objectives are to develop a quantitative model of ridge-axis dynamics which in corporates a realistic non-linear flowlaw as well as a rheologically distinct crust and mantle to characterize ridge exis/flank gravity and ridge axis/flanktopography in terms of spreading rate, crustal thickness and inferred mantle temperature, and, by comparing model gravity/topography with observations, to establish the role of the weak lower crustal layer in the transition between spreading types.