The Comoros Archipelago as a diffuse dextral fault zone of the East African Rift System
Abstract
The volcanism of the Comoros archipelago (Mozambique channel) is the subject of vigorous scientific debate. Based on their isotopic signature and apparent westward younging, the volcanoes have been interpreted as the surface expression of a mantle plume. However, this volcanic progression does not match the modelled absolute motion of the Somali plate, and given the repartition of regional seismicity, it is also possible that the Comoros delineate the yet-undermined northern boundary between the Lwandle and Somali plates. To help understanding the origin of the Comoros, we combined a microtectonic field survey of three islands of the archipelago (Mayotte, Anjouan, and Mohéli) with a morphologic study of the repartition of volcanism.
Mayotte displays ample field evidence of thrust and strike-slip faults consistent with a generalized NW-SE shortening. This shortening is in stark contrast with the deformation of intraplate volcanoes usually dominated by gravity-driven extension. Faults are found even in the most recent tephra of Mayotte, suggesting a tectonic activity younger than 200 ka. The average orientation of dikes is parallel to the NW-SE shortening direction. The same average direction of shortening and diking is also evidenced on the islands of Anjouan and Mohéli. Moreover, faults and dikes are remarkably consistent with focal mechanisms of earthquakes in the archipelago, mostly in strike-slip regime with a NW-SE compression axis. This suggests that the tectonics responsible for field-observed structures is still active. In tandem, the study of topographic and bathymetric features shows that volcanic structures follow a preferential repartition as asymmetric lenses inclined NW-SE, organized into an E-W or NNW-SSE en-échelon fashion. This en-échelon array of volcanism suggests that dextral pull-apart structures control the magmatic activity. From the NW-SE shortening direction indicated by faults, dikes and earthquake focal mechanisms, and given the en-échelon repartition of volcanism, we interpret the Comoros archipelago as built on a diffuse zone of right-lateral shear deformation. Strike-slip tear of the lithosphere beneath the Comoros is still be in a growing stage of localization, explaining why the northern boundary between the Lwandle and Somali plates has remained undetected until now.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43I0223M
- Keywords:
-
- 8499 General or miscellaneous;
- VOLCANOLOGY