Velocity Structure of the Ultraslow-Spreading Southwest Indian Ridge at 64°30'E from First Arrival Traveltime Tomography: Evidence of an Active Axial Detachment Fault
Abstract
Our study provides insights into the velocity structure of the crust and uppermost mantle at the eastern Southwest Indian Ridge (SWIR) at 64°30'E. In this area, the SWIR has a full-spreading rate of <14 mm/yr thus representing an ultraslow spreading and, therefore, a magma-poor end-member mid-ocean ridge system. At the seafloor, exhumed variably serpentinized mantle peridotites are exposed with little interference of igneous rocks that can mislead their identification and hamper their geophysical characterization. We perform travel time tomographic inversion of Pg and Pn refracted arrivals recorded by 32 ocean bottom seismometers (OBS) evenly deployed over two long (~150 km) orthogonal profiles, one along the ridge valley (EW direction) and one across it (NS direction). We also carry out Monte Carlo analyses to evaluate the models' robustness and checkerboard tests to investigate the models' capability to resolve geological features. The EW velocity model shows much smaller lateral velocity variations in comparison to the NS profile, which suggests a more complex crustal and mantle structure for the latter. Seismic velocities in both models increase rapidly with depth, changing from 3.5-4 km/s at the seafloor to 7 km/s at 2-5 km depth and the vertical gradient becomes smoother for velocities greater than 7 km/s. Similar velocity-depth profiles have been observed in other geological areas where serpentinized mantle domains have also been suggested, such as the Mid-Cayman Spreading Centre. We define the 7.5 km/s velocity contour as the top of the unaltered peridotites, consistent with observations from ultraslow-spreading ridges and centers elsewhere, and thus we estimate a "crustal thickness" ranging from 4 km to 7.2 km on the EW profile. The "crustal thickness" for the NS profile ranges from 2.2 km to 5.5 km. We interpret a sharp lateral change in the velocities and a high vertical velocity gradient on the NS profile around the highest topographic feature as the seismic expression of an active axial detachment fault. Moho reflected arrivals (PmP) are not identified in the data likely due to the lack of a distinct and seismically well-characterized Moho, which we interpret as indicative of a gradual transition from serpentinized to fresh mantle peridotites.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T12C..03C
- Keywords:
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- 3035 Midocean ridge processes;
- MARINE GEOLOGY AND GEOPHYSICS;
- 8034 Rheology and friction of fault zones;
- STRUCTURAL GEOLOGY;
- 8135 Hydrothermal systems;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS