RESEARCH Mantle earthquakes in the absence of subduction? Continental delamination in the Romanian Carpathians Melvin A. Fillerup1, James H. Knapp1, Camelia C. Knapp1, and Victor Raileanu2 1DEPARTMENT OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF SOUTH CAROLINA, 701 SUMTER ST., EWS 617, COLUMBIA, SOUTH CAROLINA 29208, USA 2NATIONAL INSTITUTE FOR EARTH PHYSICS, P.O. BOX MG-2, RO-077125 BUCURESTI-MAGURELE, ROMANIA ABSTRACT The Vrancea seismogenic zone of Romania is a steeply NW-dipping volume (30 × 70 × 200 km) of intermediate-depth seismicity in the upper mantle beneath the bend zone of the Eastern Carpathians. It is widely held that the source of this seismicity is the remnant of a Miocene-age subduction zone. However, recent deep seismic-refl ection data collected over the Eastern Carpathian bend zone image an orogen lacking (1) a crustal root and (2) dipping crustal-scale fabrics routinely imaged in modern and ancient subduction zones. Here, we use these data to evaluate the lithospheric structure of the Eastern Carpathians as it relates to the Vrancea seismogenic zone. Crustal architecture obtained from these data indicate the 140-km-wide orogen is only supported by ~33-km-thick crust, while the adjacent Transylvanian and Focsani basins have ~37- (possibly up to ~46 km) and 42-km-thick crust, respectively. Because the Vrancea seismogenic zone is located beneath the east side of the thin orogenic crust, we infer that the lower orogenic crust was removed through continental delamination and is now repre- sented by the mantle seismicity observed in the Vrancea seismogenic zone. These data and their interpretation suggest an alternate means of generating mantle seismicity in the absence of subduction processes. LITHOSPHERE; v. 2; no. 5; p. 333–340; Data Repository 2010239. doi: 10.1130/L102.1 INTRODUCTION The association of earthquakes in the upper mantle with the subduc- 48°N tion of oceanic plates at convergent boundaries has been a fundamental N tenet of plate tectonics for decades. Indeed, the recognition that Wadati- Benioff zones of localized seismicity represent the descent of oceanic lith- E. Carpathians osphere into the mantle was a signifi cant contribution to the plate-tectonic paradigm. To date, the scientifi c community has yet to agree on a case 47° where a well-defi ned zone of upper-mantle seismicity is unrelated to the Apuseni subduction process, and yet the geologic history of the Romanian Car- Mountains Transylvanian pathians appears to preclude such an origin for the Vrancea seismogenic Basin zone (Knapp et al., 2005; Houseman and Gemmer, 2007; Lorinczi and DRACULA I 2600 Houseman, 2009).The Vrancea seismogenic zone of Romania is one of 46° the most active seismic regions in Europe (Fig. 1), with two to three large (MW > 7.0) events per century occurring at depths of 70–200 km, and it is widely interpreted to be the result of remnant oceanic subduction (Wortel DACIA-PLAN and Spakman, 2000). Here, we present deep seismic-refl ection profi les S. Carpathians from the Eastern Carpathians that document the lithospheric architecture 45° Elevation (m) Focsani associated with the Vrancea seismogenic zone. Our data provide strong Basin evidence that the Vrancea seismogenic zone seismicity is unlikely to be Bucharest related to subduction, and that the geometry and position of the seismo- 50 km genic volume match closely that of a large void in the lower crust beneath 0 Moesian Platform the Eastern Carpathians. We interpret these relationships to mean that the 44° seismogenic body resulted from a process of delamination of the lower 22°E 23° 24° 25° 26° 27° 28° continental crust beneath the bend zone of the Eastern Carpathians in the absence of subduction. Figure 1. Digital elevation map showing the Eastern Carpathian reentrant and location of the intermediate-depth (70–200 km) seismicity (M >4) As originally proposed by Bird (1979), continental delamination was w of the Vrancea zone (black dots) and combined DRACULA I (heavy red envisioned as an inevitable consequence of lithospheric thickening, but line) and DACIA-PLAN (heavy blue line) deep seismic profi les. Note higher largely as an aseismic process limited to the mantle lithosphere. Subse- elevation of the Transylvanian basin compared to surrounding Pannonian quently, Kay and Mahlburg Kay (1993) suggested that lithospheric delam- and Focsani basins. ination could also include density inversions in the crust, which would LITHOSPHEREFor permission to| Volumecopy, contact 2 | Number [email protected] 5 | www.gsapubs.org | © 2010 Geological Society of America 333 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/5/333/3050127/333.pdf by guest on 29 September 2021 FILLERUP ET AL. trigger the removal of both the lower crust and upper mantle. Our data zone in pre-Miocene time, and underwent detachment and retreat toward suggest that the combination of continental delamination and introduc- the southeast to correspond with the zone of mantle hypocenters. tion of lower-crustal material to mantle depths (1) is the source of mantle The presence of a linear Miocene–Pleistocene volcanic chain located seismicity in the Vrancea seismogenic zone, (2) represents an important in the East Carpathian hinterland has been interpreted as further evidence alternative process to explain mantle seismicity in the absence of subduc- for subduction. The age of this volcanism decreases from older (11 Ma tion, and (3) suggests that lithospheric delamination may potentially be in the northwest) to younger (0.03 Ma in the southeast) toward the East seismogenic when lower crust is involved. Carpathian bend zone (Pécskay et al., 2006). Calc-alkaline rock types are mainly andesites and dacites, while the alkaline rocks found in the Persani Delamination versus Subduction in the Vrancea Seismogenic Mountains near the bend zone are basalts (Peltz et al., 1973; Downes et Zone al., 1995; Mason et al., 1996). However, the relationship of these volcanic rocks to the proposed subduction is not without complication. The most Historically, geodynamic models proposed for the Vrancea seismicity problematic geodynamic example presented by the subduction hypoth- have been subduction related (Fuchs et al., 1979; Girbacea and Frisch, eses is that the magmatism continued for >8 m.y. after the purported sub- 1998; Linzer et al., 1998; Wortel and Spakman, 2000; Sperner et al., 2001; duction and shortening had ceased around ca. 11 Ma. Contemporaneous Cloetingh et al., 2004; Schmid et al., 2008). Subsequently, much of the alkaline basalt and andesitic eruptions in the bend region during the Pleis- research conducted during the past 20 yr has utilized the various subduc- tocene further complicate the subduction models because these extrusive tion models in interpreting observations of basin evolution (Tarapoanca et magmas are >100 km west of the near-vertical purported slab. The lat- al., 2003; Leever et al., 2006; Matenco et al., 2007), volcanism (Seghedi est alkaline eruptions in the Persani Mountains occurred ~500,000 yr ago et al., 2004, 2005; Downes et al., 1995; Falus et al., 2008), uplift (Sanders (Pécskay et al., 2006, and references therein), and the timing and relation et al., 1999; Fugenschuh and Schmid, 2005; Krezsek and Bally, 2006), of this Pliocene–Pleistocene volcanism to the inferred subduction in the geophysical investigations of the crust (Hauser et al., 2007; Martin et al., southeast Carpathians are some of the many questions not fully satisfi ed 2005; Diehl et al., 2005), and mantle (Fan and Wallace, 1998; Weidle by existing subduction models for the Vrancea seismogenic zone. et al., 2005; Ismail-Zadeh et al., 2005, 2008; Ivan et al., 2008), which Deep seismic-refl ection profi les (Fig. 2) collected in collisional orogens ultimately have infl uenced the prevailing ideas about the regional tec- clearly show the suture and fabric imparted to the crust from subduction tonic evolution of the Carpathian-Pannonian system (Wortel and Spak- and collision. The suture between the Adriatic plate and European plate man, 2000; Ustaszewski et al., 2008; Schmid et al., 2008). However, new (Fig. 2A) dips steeply east through the crust and upper mantle, imparting a models for Vrancea seismicity have also been forthcoming. Knapp et al. prominent dipping fabric (Marchant and Stampfl i, 1997). Similarly, Con- (2005) suggested continental lithospheric delamination as a probable sortium for Continental Refl ection Profi ling (COCORP) lines Georgia 13 mechanism for the generation of seismicity beneath the East Carpathians, and 14 (Fig. 2B) image a wide band of dipping refl ectivity imparted to while Houseman and Gemmer (2007) suggested that the seismic activ- the crust from what is interpreted as the collision of Africa with North ity resulted from gravitational instability caused by lateral thinning of the America (Nelson et al., 1985). Zhao et al. (1993) showed the result of the mantle lithosphere beneath the Pannonian basin, resulting in thickening collision of the Indian plate beneath Asia where the Himalayan crust over- and downwelling beneath the Eastern Carpathians. Lorinczi and House- rides the north-dipping Indian crust (Fig. 2C). In each case in Figure 2, man (2009) proposed that Vrancea seismicity is the consequence of a there is a predominant crustal-scale through-going dipping fabric asso- Rayleigh-Taylor instability affecting the continental mantle lithosphere ciated with the suture indicating collision. Accordingly, Deep Refl ection and supported this view by showing that the seismic