9:30 AM - 9:45 AM
[SCG46-03] Serpentine influences both triggering mechanisms and focal mechanisms within the Vrancea seismic slab
Keywords:Intermediate-depth earthquakes, Vrancea slab, Subduction, Triggering mechanism, Focal mechanism, Dehydration
Hydrous minerals and free fluids can play major roles in deformation processes at any scale in various tectonic settings. Importantly, they are known to significantly influence intermediate-depth seismicity within actively subducting oceanic lithospheres (e.g. Kita & Ferrand, 2018). However, the extent of their contribution to seismicity has remained unclear because the parameters controlling the process are not mutually independent. The Vrancea slab, Eastern Romania, is a subducted remnant of the Tethyan lithosphere characterized by a significant intermediate-depth seismicity (60-170 km i.e. 2-6.5 GPa). The particular setting of this locked slab makes it a natural laboratory to study the effect of isobaric thermal equilibration on post-subduction seismicity. A debate has lasted for decades regarding whether the seismic volume would correspond to subducted Tethyan lithosphere or delaminated Carpathian lithosphere. Using the entire seismicity dataset (≈ 10,000 events from Mw 2 to Mw 7.9) beneath Vrancea for P > 0.55 GPa (> 20 km) since 1940, we estimated the P-T conditions associated with each hypocenter based on seismic tomography. We recently showed that most hypocentral conditions match the thermodynamic stability limits of minerals typical of the uppermost mantle (serpentine and its high-pressure destabilization products; Ferrand & Manea, 2021). Most triggering conditions match antigorite dehydration between 2 and 4.5 GPa; at higher pressures, the dehydration of the 10-Å phase (high-pressure hydrated talc) provides the best fit. This demonstrates that the Vrancea intermediate-depth seismicity is evidence of the current dehydration of an oceanic slab. The recent rollback of a W-dipping oceanic slab associated with limited delamination of the Moesian lithosphere would explain its current location beneath Vrancea, i.e. 100 km away from the Tethyan suture zone.Furthermore, we investigate the potential link between the triggering mechanisms and the retrieved focal mechanisms of 940 earthquakes, which allows interpreting the depth distribution of the stress field (Craiu et al., 2022). Reactive fluid flows can influence the stress field at the scale of either a fault zone (reactivation) or a subducted slab. We observe a switch from horizontal compression to vertical extension between 100 and 130 km depth, where the Clapeyron slope of serpentine dehydration is negative. The negative volume change within dehydrating serpentinized faults, expected mostly sub-horizontal in the verticalized slab, could well explain the vertical extension recorded by the intermediate-depth seismicity. This apparent "slab pull" is accompanied with a rotation of the main compressive stress, which could favor slab detachments in subducting slabs. Considering an open systems thanks to the possibility of horizontal fluid percolation and vertical connected pathways (former interface) up to the surface (mud volcanoes), volume changes up to 20% can be reached, which would significantly modify the stress field in within the peridotite volumes in between the dehydrating faults and thus explain the observed focal mechanisms.
- Kita, S. & Ferrand, T. P. (2018). Physical mechanisms of oceanic mantle earthquakes: Comparison of natural and experimental events. Scientific Reports 8(1), 1-11.
- Ferrand, T. P. & Manea, E. F. (2021). Dehydration-induced earthquakes identified in a subducted oceanic slab beneath Vrancea, Romania. Scientific Reports 11(1), 10315.
- Craiu, A., Ferrand, T. P., Manea, E. F., Vrijmoed, J. C. & Mărmureanu, A. (2022). A switch from horizontal compression to vertical extension in the Vrancea slab explained by the volume reduction of serpentine dehydration. Scientific Reports 12(1), 22320.
- Kita, S. & Ferrand, T. P. (2018). Physical mechanisms of oceanic mantle earthquakes: Comparison of natural and experimental events. Scientific Reports 8(1), 1-11.
- Ferrand, T. P. & Manea, E. F. (2021). Dehydration-induced earthquakes identified in a subducted oceanic slab beneath Vrancea, Romania. Scientific Reports 11(1), 10315.
- Craiu, A., Ferrand, T. P., Manea, E. F., Vrijmoed, J. C. & Mărmureanu, A. (2022). A switch from horizontal compression to vertical extension in the Vrancea slab explained by the volume reduction of serpentine dehydration. Scientific Reports 12(1), 22320.