A systematic analysis of the Mid-Atlantic Ridge morphology and gravity between 15 degree N and 40 degree N: Constraints of the thermal structure

Multibeam bathymetry data obtained along a 2400 km long section of the Mid-Atlantic Ridge (MAR) from 15 degree N to the Azores platform (40 degree N) and satellite-derived gravity data were used to calculate the mantle Bouguer anomaly (MBA) along this portion of the MAR. Both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference. A long-wavelength influence is characterized by a gentle, continuous slope of the average ridge axial depth and a general gradient in the along-axis MBA profile. This thermal influence of the Azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 degree 30'N. South of 26 degree 30'N, the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments. Except on the Azores platform, the systematic along-axis 11-90 km long wavelength segmentation is independent of the long-wavelength influence of the Azores. At the segment center, the axial morphology is linked to the thermal state of the segments between: (1) "Hotter segments" characterized by a smooth axial morphology, a well-defined shallow "inner valley", high Delta MBA and a long length; (2) "colder segments" which present a rough axial morphology with a deep, wide and well-defined rift valley, a low Delta MBA and a small length. For "hotter segments" the formation of the abyssal hills is mainly due to a magmato-tectonic cycle over periods of 0.3 to 1 Myr, whereas on "colder segments" the axial morphology is mainly controlled by a tectonic rift valley formation. We propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years.