An intermittent detachment faulting system with a large sulfide deposit revealed by multi-scale magnetic surveys

Magmatic and tectonic processes can contribute to discontinuous crustal accretion and play an important role in hydrothermal circulation at ultraslow-spreading ridges, however, it is difficult to accurately describe the processes without an age framework to constrain crustal evolution. Here we report on a multi-scale magnetic survey that provides constraints on the fine-scale evolution of a detachment faulting system that hosts hydrothermal activity at 49.7°E on the Southwest Indian Ridge. Reconstruction of the multi-stage detachment faulting history shows a previous episode of detachment faulting took place 0.76~1.48 My BP, while the present fault has been active for the past ~0.33 My and is just in the prime of life. This fault sustains hydrothermal circulation that has the potential for developing a large sulfide deposit. High resolution multiscale magnetics allows us to constrain the relative balance between periods of detachment faulting and magmatism to better describe accretionary processes on an ultraslow spreading ridge.

--Basalt (60%) suffered low degree of fragmentation. Chlorite or quartz can be seen in cracks of basalt under the microscope.

P3
Altered pyroxene presents greyish-green to light green at its surface and has wide-spread of fibrous talc particles.
--Serpentine largely replaced pyroxene and the crystal outline of original mineral is not visible. The aggregate of serpentine presents fold shape.
Broken plagioclase and oriented pyroxene is visible.
Chlorite and epidote were filled in crack.
Serpentine with network structure mainly composed by chrysotile.

Talus ramp P5
Basalt has intergranular texture and its phenocryst mainly composes by olivine and plagioclase.
--Altered cataclastic gabbro has cataclastic and gabbro texture.  (Fig. 1b) and little is known about them at this time. Thus, we focus on the better defined LQ-3 and LQ-1 hydrothermal vent fields and discuss below their detailed magnetic settings and how they relate to the OCC structure.
LQ-3 vent field -The inactive vent field LQ-3 appears to have been originally part of the footwall but is now a slipped hanging wall block that is marked by basaltic basement. Rock samples reveal that a significant amount of breccia and/or altered basalt are distributed in the area surrounding the LQ-3 vent, suggesting that hydrothermal fluid has altered the host rock when the vent was active in the past. While we would expect to see a zone of reduced magnetization associated with this hydrothermal area 1 , we do not observe an obvious magnetic low on the magnetic map (Fig. 3a). We speculate that this is because the AUV survey line spacing was too wide (400 m) and the grid interpolation is not sensitive enough to detect features on the scale of ~100 m. However, by extracting the raw profile data (Fig. 3b) that crosses the LQ-3 vent site from east to west (marked by double arrow line in Fig. 3a) we find a narrow but obvious magnetic anomaly low over the LQ-3 vent site with about 200 m width (circled by red dotted line in Fig. 3b). Given the recovery of altered basalt from the site, we conclude that the focused hydrothermal alteration has influenced the basaltic crust locally in the vicinity of the LQ-3 vent site.
LQ-1 vent field -The regional AUV022 survey (see Fig. 3a, Supplementary Figure 2a) shows that a strong magnetic anomaly is present over the hanging wall block below the termination.
The active LQ-1 vent field sits just at the southern edge of this strong magnetic zone on the hanging wall block but just north of the termination. The adjacent OCC-2 fault shows magnetic anomaly lows above the termination that are a continuation of magnetic lows that mark the main OCC-2 exposure. A magnetic high associated with the breakaway B2 wraps around the northern edge of the OCC and stops at the termination just before the LQ-1 vent field (Fig. 3a, Supplementary Figure 2a). The focused 3D magnetization inversion of the LQ-1 vent field area (Supplementary Figure 2b) shows that the zone of highest magnetization forms a bulls-eye positive anomaly just north of the LQ-1 vent field (Supplementary Figure   2b). We infer that this strong magnetization is the result of intrusive emplacement of magma into this block from a nearby heat source. We note that the non-transform discontinuity is nearby and projects into this location suggesting a possible link to recent magmatism (Fig. 1a).
While the 400 m line spacing of the AUV022 survey is too wide to capture the detailed magnetic response of the LQ-1 vent field directly, the vent field was surveyed previously by