Delamination or slab detachment beneath Vrancea? New arguments from local earthquake tomography

Vrancea, located at the southeastern Carpathians Arc bend, is one of the areas in the Alpine-Himalayan belt that features strong earthquakes occurring at intermediate depths (60-200 km). In this study we investigated the crustal and lithospheric structure beneath the Vrancea seismic area using a local earthquake tomography approach. We used an updated and revised catalog, spanning from 1982 to 2006 that uses data from both permanent and temporary networks in the target area. Simultaneous tomographic inversion for the Vp and Vs anomalies and the Vp/Vs ratio and source locations was done using the LOTOS code. The reliability and robustness of the results were rigorously checked using various tests (e. g., by studying the role of different parameters on the results of the inversion, performing the inversion using random data subsets, and synthetic modeling). The tomography results clearly indicate the presence of a high-velocity material beneath Vrancea at a depth interval of about 60-200 km that coincides with the distribution of intermediate-depth seismicity. This result agrees generally with previous tomographic studies. We compare two scenarios leading to this structure: (1) subduction and slab detachment and (2) "drop forming'' or delamination. The latter mechanism presumes that the thickening of the crust due to continent-continent collision causes transformation of the mafic lower crust into denser eclogite. This material accumulates until it reaches a critical mass, at which point it forms a large drop that begins to fall down. We propose that the high-velocity anomaly we observe in our tomogram might represent the descending eclogitic lower crust material enveloped by the entrained lithosphere. It is possible that a similar delamination process can be observed in other parts of the Alpine-Himalayan belt, such as in the Pamir Hindu-Kush area.