Fig. 1

Tectonic map of the study area showing multibeam and geophysical survey track lines. Black dotted line indicates the ridge axis and transform fault"

Fig. 2

Shaded topography of the study area. Black boxes outline the subregions in Fig. 6, 7, and 8. Du Toit TF: Du Toit Transform Fault. #cod#x02160;, #cod#x02161;, #cod#x02162; indicate three axial volcanic ridges in the eastern subregion"

Fig. 3

Schematics showing calculations of the axial relief: (a) original topography; (b) filtered topography with wavelengths less than 20 km removed; (c) an example of calculation of the axial relief. Gray and blue lines are the profiles from the original and filtered topography, respectively. TS, TN indicate the axial relief from the southern and northern flanks"

Fig. 4

Schematics showing the calculation of the M factor (a) and an example profile of the SWIR (b). AVR indicate the axial volcanic ridge. (a) Red and blue boxes indicate the interpreted magmatic accretion (Lm) and tectonic extension (Lf), respectively; (b) fault scraps are shown by blue lines"

Fig. 5

Along-axis variations in the axial depth (upper panel) and axial relief (lower panel). #cod#x02160;, #cod#x02161;, #cod#x02162; indicate three axial volcanic ridges in the eastern subregion. Blue, red and black points indicate the calculated axial relief from the northern flank, southern flank and their averaged values, respectively"

Fig. 6

Shaded topography (a), slope (b), RMBA (c) and the along-profile variations of M (upper d), mean fault throw (middle d) and mean RMBA (lower d) in the area of Narrowgate-16#cod#x000b0;E. Values shown in panel d are the averaged data of the four across-axis profiles shown in Panels Fig. 6 a-c. The multibeam bathymetry and gravity data are from Grindlay et al (1998) and Dick et al (2003)"

Fig. 7

Shaded topography (a), slope (b), RMBA (c) and the along-profile variations of M (upper d), mean fault throw (center d) and mean RMBA (lower d) in the area of 17#cod#x000b0;30'E #cod#x02014; 19#cod#x000b0;E. Data resources are the same as Fig. 6"

Fig. 8

Shaded topography (a), slope (b), RMBA (c) and the along-profile variations of M (upper d), mean fault throw (center d) and mean RMBA (lower d) in the area of 22#cod#x000b0;E#cod#x02014; 24#cod#x000b0;E. Data resources are the same as Fig. 6"

Fig. 9

Extensional strains of the study area. (a) Narrowgate #cod#x02014; 16#cod#x000b0;E; (b) 17#cod#x000b0;30'E#cod#x02014;19#cod#x000b0;E; (c) 22#cod#x000b0; E#cod#x02014;24#cod#x000b0;E; (d) synthesis of the three sub-regions shown in Fig. 9 a-c. Red and blue lines indicate the profiles from southern and northern flanks, respectively. Black straight lines indicate the extensional strain of 20% and 50%. Gray shaded regions in Fig. 9 a, b and c show the approximate patches of data coverage of our study area, while the red shaded regions are from Escartin et al (1999) for the Mid-Atlantic Ridge at 29#cod#x000b0;N. The thick dark gray curves in Fig. 9 d are the second-order polynomial fit of all profiles at the two ridge flanks"

Fig. 10

Plots showing correlations of (a) M and mean RMBA, (b) M and mean fault throw, (c) mean RMBA and mean fault throw, (d) M and axial relief. Blue, black and red points are derived from the eastern, middle and western subregions, respectively. Spearman#cod#x02019;s rank correlation coefficient r, and p values are shown at the lower left corner of each panel. Positive r means positive correlation, while a value of 1 indicates the strongest positive correlation, correlation with p less than 0.05 is thought to be significant"