We analyzed seafloor morphology using ship-based multibeam depth data displayed as gray scale and color-coded shaded relief maps, with and without superimposed contours, together with seafloor gradient maps (e.g.,Figure S3), and profiles (Figure S1). We interpreted the side scan data in map view and draped over bathymetry, using visual appearance, texture, and quantitative backscatter levels to distinguish volcanic and tectonic features similar to those we have used elsewhere [Searle et al., 2010]. These are listed in Table S1 and illustrated in Figures S5 and S6. We combined all these data sets to make our final interpretations. For example, fault scarps were identified from a combination of side scan texture, bathymetry, slope maps, and high-resolution gradient.Figure 4 presents a geological map interpreted from these acoustic terrains.
Figure 4. Geological interpretation, based mainly on acoustic textures, supplemented by bathymetry and seafloor gradients. Details of the terrains are given in Table S1 and Figures S5 and S6. Blue dashed lines show prominent oblique-trending valleys, and black dotted lines outline isolated areas of inferred magmatic terrain. Black letters identify features discussed in the text.
Download figure to PowerPoint
3.2. Regional Morphology
 Our study area lies within an ∼100 km wide region of blocky terrain between 13°50′N and 12°50′N (Figure 1). The ridge axis generally trends N-S, offset by a 20-km-long second-order non-transform offset (NTO) between 13°35′N and 13°44′N. The V-shaped boundary between the 14N lineated and the 13N blocky segment lies at the northern end of the NTO. From the azimuth of this boundary we calculate that the 14N magmatic segment has expanded southwards at ∼15 km Ma−1 for the last 1.8 Ma.
 Two small, isolated exposures of lineated terrain associated with numerous small, closely spaced faults lie within the blocky region (A and B, Figures 1 and 4). Region A parallels the southern boundary of the 14N segment, and is bounded to north and south by bathymetric lows trending NW-SE (blue lines inFigures 1 and 4).
 We recognize a neovolcanic zone (NVZ) containing the most recent seafloor volcanism and characterized by the brightest sonar backscatter levels (terrain V1, Table S1 and Figure S5). A similar, slightly older, terrain is V1a. These terrains are composed of thousands of small volcanic hummocks [Yeo et al., 2012], often aligned in rows up to 8 km long (V5, V6). Progressively older versions of this terrain, with lower backscatter and inferred greater sediment cover, are mapped as V2 and V2a. Hummocky seafloor constitutes approximately 90% of the visible volcanic seafloor, similar to the MAR between 27°N and 30°N [Briais et al., 2000].
 Several large, volcanically constructed ridges occupy the axial valley (e.g., C, Figures 1 and 4). These ridges comprise numerous, coalesced volcanic cones as observed elsewhere on the MAR [Karson et al., 1987; Lawson et al., 1996; Searle et al., 2010; Smith and Cann, 1990] and identified as axial volcanic ridges (AVRs) [Parson et al., 1993]. However, at 13N they are generally sediment covered and thus relatively old compared with the brighter NVZ, which contains mostly minor volcanic lineations. The large ridges may be mature AVRs, now spread off-axis [MacLeod et al., 2009]. These ridges lie to the east of the current NVZ, implying a relatively recent shift of volcanic activity ∼6 km westward in the region of the two active OCCs, which may be related to their development as we discuss later. Between OCC1320 and OCC1330, the highest acoustic backscatter occurs not on the major volcanic ridge (Figure 1, C), nor even on the minor volcanic lineaments at 44° 52.5′W (Figure 1, D), but at the foot of the E-facing normal fault scarp at 44° 53.5′W (Figure 1, E).
 The robust NVZ near inactive OCC1348 (Figures 1 and 4, F) contains no major ridge, but comprises many minor volcanic lineaments spanning the axial valley floor. This suggests either that volcanism here produces low-relief flows, that melt emplacement has rapidly switched back and forth across the axial valley so there is no preferential zone of volcanic construction, or that melt emplacement at the MAR axis has recently resumed after a prolonged hiatus with insufficient time to form a full AVR.
 There are relatively minor occurrences in the NVZ and its flanks of terrains exhibiting a more uniform backscatter with superimposed hackly texture, labeled V3, V3a and V4. Such terrains are usually associated with relatively smooth seafloor, though not necessarily built from sheet flows [Cann and Smith, 2005; Searle et al., 2010]. As elsewhere on the MAR, there are significant numbers of flat-topped seamounts (V7).
 The width of both the NVZ and other volcanic terrains varies systematically along axis. The detailed distribution of these components along an axial section is given in Figure S7. Because we have used backscatter level as an additional discriminant in this study, our measures of NVZ width are more robust and precise than those of MacLeod et al. , who used a qualitative visual comparison. Although details differ, we confirm that the NVZ is widest between active OCCs and narrows or disappears adjacent to them. The average NVZ width is 2.5 km, with a maximum of 6 km at 13°22′N (immediately north of OCC1320), and vanishing entirely adjacent to OCC1320 and OCC 1330.
 There are gaps of 2 km and 4 km respectively in V1 and the slightly older V1a adjacent to OCC1320, and gaps of 10 km and 7 km at OCC1330. These gaps contain the oldest volcanic terrain V2a and additionally, at OCC1330, smooth terrain V3 and V3a. The narrowing exposures of V1towards the OCCs imply propagation along the MAR axis at rates of ∼13 km Ma−1 (Figure S8). Minor volcanic lineaments (V6) swing from roughly N or S to NW and SW, toward the OCCs, as they approach them from south and north respectively. In contrast, there is a continuous, 6 km wide NVZ opposite the inactive OCC1348 comprising V1and V3.
 We estimate the age of the neo-volcanic hiatuses by noting that the adjacent axial seafloor has texture V2a, with average acoustic backscatter level 370 (Table S1). Assuming that the sonar signal penetrates a maximum of 5 m [Lawson et al., 1996], that the mean backscatter level over unsedimented basalt and thick sediment is 1270 and 100 units, respectively (Table S1), that intensity falls off linearly with increasing sediment cover, and that the sedimentation rate is 5 m Ma−1 [Mitchell et al., 1998], we obtain a maximum age of 0.77 Ma for V2a. If the sedimentation rate were 10 m Ma−1 and the sonar penetration only 2.5 m, the age of V2a would be 0.19 Ma. Thus, the hiatus has lasted at least several hundred thousand years. If OCC1320 and OCC1330 have taken up the full plate separation by slip on their detachments, their widths imply minimum ages of 0.38 Ma and 0.41 Ma, respectively; if they take up only half the plate separation by tectonic slip, they have maximum ages of 0.77 Ma and 0.82 Ma. Since these estimated ages of OCCs and volcanic hiatuses overlap, we are unable to say whether waning axial volcanism preceded (perhaps causing) OCC initiation or followed it.