Morphology and Segmentation
Figure 11.2 exhibits the compiled shaded bathymetry map of the Rodriguez Triple Junction (RTJ) area. The present-day ridge axis between the RTJ and 23°S consists of four second-order segments. There is no transform fault in this area and the segments CIR-S1 to S4 are right lateral offset by non-transform discontinuities (NTDs). The axial valley develops at the ridge crest and numerous small, undeformed volcanic cones and flat-topped small volcanic knolls form a neo-volcanic zone.
CIR-S1 is a 20-km segment with a deep axial valley. In contrast to the typical axial morphology, the off-axis area of this segment is highly anomalous. The ridge-parallel abyssal hill pattern is disturbed and several domed highs are distributed along the trace of the NTD between CIR-S1 and S2 (Fig. 11.3a). We recovered deep crust and/or mantle materials on most of these anomalous highs, and interpreted them as modern and remnant NTO (non-transform offset) massifs or oceanic core complexes. They are closely related to the origin of the Kairei hydrothermal field (KHF), and we describe details in Sect. 11.4.4. The segment length shortens, in the farther off-axis, suggesting that the CIR-S1 was newly created during the evolution of the triple junction (Munschy and Schlich 1988; Honsho et al. 1996; Mendel et al. 2000).
CIR-S2 is offset 34 km eastward from CIR-S1. The shallow axial depth around 24°50′S and the concentration of volcanic cones suggest mantle upwelling occurs here. The axial valley appears asymmetric, suggesting a recent jump of neo-volcanic zone eastwards. A unique domed high is located within the southern end of the axial valley, where we recovered fresh and altered peridotite samples (Phoenix knoll in Fig. 11.3a). The off-axis morphology clearly shows that CIR-S2 consisted of two segments in the past. The southern segment progressively shortened and finally vanished, whereas the northern segment lengthened at its expenses. Gac et al. (2006) modeled such a shortening and lengthening of ridge segments. A typical oceanic core complex, the 25°S OCC, was formed in the final phase of the southern segment (Morishita et al. 2009; Sato et al. 2009).
CIR-S3 also shows an anomalous feature at the southern end of the axial valley, where it is filled by the Knorr Seamount, a large triangular-shaped volcano measuring nearly 40-km long (Fig. 11.2). This seamount is volcanic, and a set of parallel rifts has developed at its summit. The off-axis morphology indicates that the segment was also made of two segments in the past, like CIR-S2. The abyssal hill pattern is not symmetric and the magmatic activity appears to be highly variable, particularly in the southern end.
CIR-S4 is only slightly offset from CIR-S3. The axial valley narrows and shallows toward the segment center at 23°20′S, like a typical slow-spreading ridge segment (e.g., Sempéré et al. 1993), suggesting three dimensional mantle upwelling (Lin et al. 1990). Despite the insufficient off-axis coverage, the abyssal hills are shallower in the eastern off-axis near the current segment center.
Magnetics and Gravity
Based on the estimated magnetization distribution (Fig. 11.4), we identified Brunhes/Matuyama (B/M) boundaries (0.78 Ma), the Jaramillo subchron (Chron 1r.1, 0.99–1.07 Ma), Chron 2 (1.77–1.95 Ma) and young and old boundaries of Chron 2A (2.58–3.22 Ma). Chron 2Ao (old) is only recognized in the eastern off-axis of CIR-S1, S2, and in the northernmost segment of SEIR. The neo-volcanic zone in each segment is accompanied by high magnetization stronger than 10 A/m. The neo-volcanic zone of CIR-S1 lies at the center of a positive magnetization zone bounded by the B/M boundaries. In contrast, the neo-volcanic zone with the highest magnetization deviates easterly in CIR-S2, and westerly in CIR-S3 and S4. This observation suggests a recent jump of the volcanic center. In support of this idea, another high magnetization zone is also situated between the eastern B/M boundary and the neo-volcanic zone in CIR-S3 and S4.
The calculated full-spreading rate is 48.5 mm/year in the studied area on average, which is comparable to that of 47 mm/year as predicted by the global plate model (DeMets et al. 2010). A spreading asymmetry is observed for all segments. In CIR-S1, there is no significant difference between our interpretation and that proposed by Mendel et al. (2000). The identification of magnetic isochrones suggests that CIR-S1 was created in ca. Chron 2Ay (2.58 Ma). The off-axis morphology in the northern three segments suggests that the segments each consisted of two segments in the past. Magnetic isochrones support this interpretation, for the isochrones are slightly offset or discontinuous at the traces of the NTDs. Although the exact timing of the amalgamation of the segments differs among the segments, the southern CIR was highly segmented before 1 Ma. This may suggest a change in the mantle upwelling scheme in this region.
The residual mantle Bouguer anomaly map (RMBA, Fig. 11.5) shows a remarkable difference between segments. CIR-S1 is characterized by a high RMBA of around 30 mGal in the off-axis area. This suggests a thin crust and/or a dense material beneath the seafloor, supporting the idea of melt-starved, tectonism dominant, spreading processes. It is also consistent with the existence of an oceanic core complex and other topographic highs with exhumation of mantle materials. The RMBA in CIR-S2 is generally higher in the southern part, and the 25°S oceanic core complex accompanies a very high RMBA. The RMBA is also high at the southern segment end of the present-day ridge axis. CIR-S3 shows a low RMBA along the axial valley. Although we cannot deny the possibility of an inappropriate estimation of plate cooling, we prefer the interpretation that the crust is thicker beneath the Knoll Seamount and recent CIR-S3, and in support of this the abyssal hill morphology also suggests recent robust magmatism in this area. The trace of NTD between CIR-S3 and S4 is clearly accompanied by a high RMBA, suggesting a thinner crust along the segment boundary. CIR-S4 shows relatively a lower RMBA than other segments and clear along-axis variation of RMBA. Although we do not have enough data for the northern segment end, CIR-S4 is a magmatically active slow-spreading type segment, where the mantle upwells at the segment center.
Kairei Hydrothermal Field and Surroundings
The Kairei Hydrothermal Field (KHF) is located about 7 km east of the neo volcanic zone of CIR-S1, on the eastern rift-valley wall (Fig. 11.3a). Due to the presence of a couple of normal faults, the rift-valley wall shows a step-like morphology, and the KHF is located 1,700 m higher than the valley floor on one of these steps. Although the KHF is developed on a typical ridge-parallel structure, the unusual geological features suggest that tectonism-dominant ridge processes spread over the boundary between CIR-S1 and S2.
An anomalous domed high with a corrugated surface was first reported northwest of a deep nodal basin in the northern CIR-S1 (Mitchell et al. 1998) (25°S OCC in Fig. 11.3a). The survey that followed revealed less-deformed serpentinized peridotite exposed on the steep inward slope, and highly deformed rocks widely distributed on the top (Morishita et al. 2009). This observation, and the higher gravity anomaly, support the idea that this domed high is an oceanic core complex (OCC) exposed along a detachment fault. A detailed magnetic analysis showed that the oceanic core complex, known as 25°S OCC, was initiated at the southern inside corner of CIR-S2 during the Matsuyama reversal polarity chron (Sato et al. 2009).
We also discovered another OCC, about 15 km east of the KHF (Fig. 11.3a). Known as Uraniwa Hills, this OCC consists of two domed highs (Kumagai et al. 2008): the northern hill is a rectangular-shaped dome, whereas the southern hill is a more elongated massif with corrugations on the surface. A prominent breakaway ridge bounds the eastern end of this massif. Troctolites and olivine gabbros were sampled at Uraniwa Hills, in addition a dunite (Morishita et al. 2009; Nakamura et al. 2009). A NW-SE trending minor ridge (Nakaniwa Ridge in Fig. 11.3a) where gabbroic rocks were also recovered is situated between the Uraniwa Hills and the abyssal hills where the KHF develops.
We observed two deep nodal basins at both the northern segment end of CIR-S1 and at the southern segment end of CIR-S2. Between the two nodal basins, a shallow domed-like massif, has developed. It is 1,700 m high from the rift valley floor and lies oblique-to-parallel to the ridge. High RMBA (Fig. 11.5) and the peridotite exposure on the surface, indicate a very thin magmatic crust beneath this massif and the tectonism-dominant extension of the area. Similar massifs are often observed at NTDs along the Mid-Atlantic Ridge and, are known as NTO (non-transform offset) massifs. Low-angle detachment faults are likely to be the mechanism for the exhumation of deep materials at NTO massifs (Eulalia et al. 2000). The Rainbow and Saldanha hydrothermal sites are located at NTO massifs, where serpentinized peridotite hosts the hydrothermal system supplying hydrogen-rich fluid (Eulalia et al. 2000; Miranda et al. 2003). We named the NTO massif between CIR-S1 and S2 Yokoniwa Rise. During the recent submersible dive, relatively fresh inactive hydrothermal chimneys were discovered on the top. Although the dense CTD tow-yo surveys could not detect any hydrothermal plume anomaly around Yokoniwa, an ultramafic-hosted hydrothermal system probably existed in the recent past.
The Yokoniwa Rise is a current NTO massif, but we also recognized past NTO massifs along the NTD trace between CIR-S1 and S2. One of them is located just south of the 25°S OCC where Hellebrand et al. (2002) reported the exposure of mantle peridotites. The other smaller massifs (white stars in Fig. 11.3a) are also accompanied by ultramafic rocks on the surface, suggesting a common origin of NTO massifs.
Another unusual massif, hereafter known as Phoenix knoll, is located within the axial valley floor of CIR-S2 (Fig. 11.3a). This massif is ~6 km in diameter, with a steeper slope on its western side. We tentatively recognized an ambiguous surface corrugation on the top, but cannot completely deny the possibility of artifact. The dredged samples are mostly serpentinized peridotites. The formation process of the Phoenix knoll is still discussed: Morishita et al. (Chap. 14) propose serpentinite diapirism, whereas the asymmetric structure with its corrugated surface supports the hypothesis of a detachment fault. Ridge-parallel fault scarps cut the massif on its northern slope and undeformed volcanic knolls adjoin the massif. These observations indicate that the segment is now under a normal magmatic spreading phase. The chaotic structure at the current segment boundary and along the NTD trace, strongly suggests an important temporal and spatial variation of the melt supply in this area.
Tectonic Evolution and Hydrothermalism
The hydrothermal fluid of the KHF is unique in its high H2 and low CH4 contents (Van Dover et al. 2001; Gallant and Von Damm 2006; Kumagai et al. 2008). In general, the high content of H2 in the hydrothermal fluid is explained by the generation of H2 through the serpentinization of ultramafic rocks. Methane is also a product of a series of the serpentinization process, so most H2 rich hydrothermal fields spout both high H2 and high CH4 fluids and are located above the exposure of ultramafic rocks due to tectonic extension. The KHF is just on the ridge-parallel volcanic knoll and is surrounded by basalt lava flows. However, ultramafic rocks are widely exposed around the KHF. Nakamura et al. (2009) proposed that the interaction of seawater with troctolites beneath the Uraniwa Hills explains the composition of the KHF fluids. Their theoretical model predicts that the high H2, low CH4, and high Si contents of the KHF fluid can be attributed to serpentinization of the troctolites and the subsequent hydrothermal reactions with basaltic wall rocks under the KHF. Although such an idea explains the KHF fluid geochemistry, it is, however, rather hard to suppose a large scale, across axis circulation from the Uraniwa Hills to the KHF. It is considered likely that a detachment fault exhuming mantle materials on the surface continues to the shallow subsurface beneath the basaltic flow at the KHF, although the fault geometry has to be confirmed. The troctolites and/or other olivine-rich materials may exist in the shallow subsurface beneath the KHF, where the serpentinization process goes on, closely related to the detachment faulting. The relatively high RMBA beneath the KHF supports this idea. The vents of the KHF appear to stand at the foot of an inward facing normal fault scarp. It is likely that the surface normal faults control the position of the upward fluid flow, and that the circulation is driven by the magmatic heat of the neo-volcanic zone. The KHF differs from both the “magma-driven” basalt-hosted hydrothermal fields and the “tectonic-controlled” ultramafic-hosted fields. It is in another category, and is, a magma-assisted, ultramafic hosted, hydrogen-rich hydrothermal system (hydrogen TAIGA).
The Edmond hydrothermal field, located near the northern end of CIR-S3, is similar in that the vent site is located on the eastern rift-valley wall near the NTD (Fig. 11.3b). However, the fluid shows a typical sulfur-rich content, and there is no evidence of ultramafic exposures around the Edmond field (Van Dover et al. 2001; Gallant and Von Damm 2006; Kumagai et al. 2008). Unlike the other hydrothermal fields on fast-spreading ridge, the Edmond filed is not directly linked with the neo-volcanic zone, and the upward fluid path may be controlled by the normal faults of the rift-valley. It is likely that this explains the longevity of the hydrothermal activity (Van Dover et al. 2001), because the circulation path has been constrained by the fault system. The Edmond field is a sulfur-rich hydrothermal system (sulfur TAIGA), hosted by basalt lava and driven by the magmatic heat of the ridge axis, and its circulation path is tectonically controlled.