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

Abstract

[1] 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.

New Seismic and Tomography Data in the Southern Part of the Harghita Mountains (Romania, Southeastern Carpathians): Connection with Recent Volcanic Activity

“…We applied the LOTOS-09 code proposed by KOULAKOV (2009) andKOULAKOV et al (2010) for tomography inversion using local earthquake data in the broader study region, including both the Vrancea seismic zone and the inner part of the Carpathian bend zone. The main feature of this code is the possibility to invert for Vp and the Vp/Vs ratio which can be performed in parallel with the Vp-Vs inversion.…”

“…The subcrustal earthquakes are generated mainly in a confined and seismically active nest characterized by an extreme density of foci within a welldefined high velocity lithospheric body (e.g., MARTIN and WENZEL, 2006;RADULIAN et al, 2008;KOULAKOV et al, 2010). Events with magnitude M w [ 7 are generated here two to three times per century.…”

Mechanisms of Earthquakes in Vrancea

“…The part of the descending lithosphere which is seismically active is confined in the depths between 70 and 170 km, with no seismicity practically detected below this range. This volume fits well within a high seismic wave velocity body, evidenced by all tomographic studies (e.g., Oncescu 1984;Koch 1985;Wortel and Spakman 2000;Sperner et al 2001;Martin and Koulakov et al 2010). The mantle seismogenic volume is separated from the sub-Carpathian crust by a relatively aseismic zone between 40 and 70 km depth.…”

“…Oceanic-type models involve break-off process (Fuchs et al 1979;Wortel and Spakman 2000;Sperner et al 2001), lateral migration of an oceanic slab (Girbacea and Frisch 1998;Gvirtzman 2002), or subduction and lateral tearing of a slab (Martin and Rietbrock 2006;Wenzel et al 1998;Wortel and Spakman 2000). Continental-type models involve active delamination (Knapp et al 2005;Koulakov et al 2010) or gravitational instability (Lorinczi and Houseman 2009;Ren et al 2012). The recent results of Bokelman et al (2014) regarding the dispersion of P waves and the hypothesis of a "double seismic zone" (Bonjer et al 2005;Radulian et al 2007;Cărbunar and Radulian 2011) bring evidence in favor of an oceanic nature of the subducting lithosphere.…”

Vrancea slab earthquakes triggered by static stress transfer

“…This is largely adopted by many researchers, but there are also opinions in favor of a continental delaminated lithosphere in Vrancea (e.g. Knapp et al, 2005;Koulakov et al, 2010). For the purposes of this study the difference concerns the mechanical properties of the deforming material (i.e.…”