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Tectonophysics 410 (

Crustal constraints on the origin of mantle seismicity in the

Vrancea Zone, Romania: The case for active continental

lithospheric delamination

James H. Knapp a,*, Camelia C. Knapp a, Victor Raileanu b, Liviu Matenco c,

Victor Mocanu c, Cornel Dinu c

a Department of Geological Sciences University of South Carolina Columbia, SC 29208, United Statesb National Institute of Earth Physics, Magurele, Bucharest, Romania

c Faculty of Geology and Geophysics, University of Bucharest, Bucharest, Romania

Received 15 April 2004; received in revised form 30 September 2004; accepted 10 February 2005

Available online 17 November 2005

Abstract

The Vrancea zone of Romania constitutes one of the most active seismic zones in Europe, where intermediate-depth (70–200

km) earthquakes of magnitude in excess of Mw=7.0 occur with relative frequency in a geographically restricted area within the

1108 bend region of the southeastern Carpathian orogen. Geologically, the Vrancea zone is characterized by (a) a laterally

restricted, steeply NW-dipping seismogenic volume (30�70�200 km), situated beneath (b) thickened continental crust within the

highly arcuate bend region of the Carpathian orocline, and (c) miscorrelation of hypocenters with the position of known or inferred

suture zones in the Carpathian orogenic system. Geologic data from petroleum exploration in the Eastern Carpathians, published

palinspastic reconstructions, and reprocessing of industry seismic data from the Carpathian foreland indicate that (1) crust of

continental affinity extends significantly westward beneath the external thrust nappes (Sub-Carpathian, Marginal Folds, and

Tarcau) of the Eastern Carpathians, (2) Cretaceous to Miocene strata of continental affinity can be reconstructed westward to a

position now occupied by the Transylvanian basin, and (3) geologic structure in the Carpathian foreland (including the Moho) is

sub-horizontal directly to the east and above the Vrancea seismogenic zone. Taken together, these geologic relationships imply that

the Vrancea zone occupies a region overlain by continental crust and upper mantle, and does not appear to originate from a

subducted oceanic slab along the length of the Carpathian orogen. Accordingly, the Vrancea zone appears to potentially be an

important place to establish evidence for active lithospheric delamination.

D 2005 Published by Elsevier B.V.

Keywords: Delamination; Vrancea zone; Romania; Tectonics; Carpathians

1. Introduction

Ever since the recognition of deep focus earthquakes

(Wadati, 1928), the cause of mantle seismicity has been

0040-1951/$ - see front matter D 2005 Published by Elsevier B.V.

doi:10.1016/j.tecto.2005.02.020

* Corresponding author. Tel.: +1 803 777 6886; fax: +1 803 777

6082.

E-mail address: knapp@geol.sc.edu (J.H. Knapp).

a long-standing problem in the Earth sciences. While

early workers observed that earthquake focal depths

generally increase with distance from the trench, form-

ing dipping seismogenic planes in the mantle (Wadati,

1928, 1935; Benioff, 1949), it was many years before

such seismicity was correlated with the subduction of

plates of oceanic lithosphere. Development of plate-

tectonic theory in the late 1960s subsequently provided

2005) 311–323

J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323312

a fundamental new framework in which to explain the

origin and geodynamic significance of earthquakes

occurring at depths of 70–700 km, along so-called

Wadati–Benioff zones (Isacks et al., 1968). We now

commonly understand such intermediate-to deep-focus

seismicity as brittle deformation of descending, rigid

oceanic slabs at subduction zones.

In the past two decades, delamination of thickened

continental lithosphere has gained favor as an important

geodynamic process in orogenic settings (e.g., Nelson,

1991), and has been proposed as an alternative mech-

anism for generation of mantle seismicity (e.g., Seber et

al., 1996). Such an interpretation seems plausible where

earthquake foci do not align along a dipping Wadati–

Benioff plane, but cluster in a more concentrated vol-

ume. dDelaminationT was first introduced in the late

1970s within a theoretical context (Bird, 1978, 1979),

and was originally conceived as detachment of thick-

ened lithospheric mantle from overlying crust during

continental collision (Bird, 1978). Subsequently, the

concept has come to encompass a much broader

range of processes, including removal of material

from the base of the lithosphere due to gravitational

instability (Bird, 1979), detachment of oceanic slabs

(Sacks and Secor, 1990), or foundering of mafic

lower crust and upper mantle driven by phase changes

(Kay and Kay, 1993; Nelson, 1991).

Regardless of the specific mechanism, the removal

of continental lithosphere into the underlying mantle

has been routinely invoked to explain the geologic

evolution of orogenic systems (e.g., Furlong and Foun-

tain, 1986; Dewey, 1988; Austrheim, 1991; Baird et

al., 1996). If such a process has operated regularly in

the development of mountain belts, it has far-reaching

implications for the evolution of continental litho-

sphere (e.g., Nelson, 1991), the dynamics of the

crust/mantle (Moho) and lithosphere/asthenosphere

boundaries, and perhaps most significantly, the long-

term geochemical budget of both crust and mantle. The

tectonic and geodynamic consequences of lithospheric

delamination are generally agreed to include (1) post-

orogenic extensional collapse, (2) regional uplift, (3)

deep-seated alkaline magmatism, (4) elevated heat

flow, and (5) subcrustal seismicity. Accordingly, most

workers have looked to the geologic record to docu-

ment evidence for these collective phenomena in the

geologic past, however, unequivocal proof for a mod-

ern example of lithospheric delamination has yet to be

documented. Such evidence could presumably only

come from a setting where mantle earthquakes are

now occurring in the demonstrated absence of a sub-

ducted slab.

The Vrancea zone of Romania constitutes one of the

most active seismic zones in Europe, where mantle

(70–200 km) earthquakes of magnitude in excess of

Mw=7.0 occur with relative frequency in a geograph-

ically restricted area (Fig. 1). For centuries, these seis-

mic events have resulted in a high toll of human

casualties and property damage, and make Bucharest

today one of the largest population centers in Europe at

significant seismic risk. Geologically, the Vrancea zone

is characterized by (a) a laterally restricted, steeply NW-

dipping seismogenic volume (30�70�200 km; Wen-

zel et al., 1998), situated beneath (b) thickened conti-

nental crust within the highly arcuate bend region of the

Carpathian orocline (Radulescu, 1981), (c) miscorrela-

tion of hypocenters with either documented or inferred

suture zones in the Carpathian orogenic system (Linzer,

1996), and (d) pronounced and localized Late Miocene/

Pliocene subsidence in the Focsani basin of the fore-

land (Artyushkov et al., 1996; Matenco et al., 2003), an

area that also exhibits (e) active surface deformation

and crustal seismicity. These geologic relationships are

for the most part specific to the bend region, and as

such, do not appear to support the premise of a sub-

ducted oceanic slab along the length of the Carpathian

orogen. Accordingly, the Vrancea zone may be the

unique place to establish evidence for active lithospher-

ic delamination.

Despite these considerations, and several recent seis-

mological investigations focused on the mantle source

region of the Vrancea zone (Fan et al., 1998; Wenzel et

al., 1998; Wortel and Spakman, 2000; Hauser et al.,

2001), the weight of scientific opinion remains over-

whelmingly in favor of a subducted slab origin for

Vrancea seismicity. Both active- and passive-source

studies in Romania in the last decade have resulted in a

substantially improved understanding of the velocity

structure of the crust and upper mantle. Similarly, the

spatial relationship of Vrancea mantle seismicity to ac-

tive surface deformation in the foreland suggests that

these regions are mechanically coupled through the crust

and upper mantle, which would be a fundamental con-

straint on the competing geodynamic models.

Here we present interpretations of existing data from

petroleum exploration and reprocessing of deep seismic

reflection data that document the continuity of conti-

nental crust beneath the external nappes of the Eastern

Carpathians, and accordingly, above the Vrancea seis-

mic zone. These surface and subsurface data appear to

preclude the possibility that a slab, either still attached

or now detached, was subducted either in place within

the Carpathian foreland (e.g., Wortel and Spakman,

2000) or beneath the Eastern Carpathians (e.g., Wenzel

Fig. 1. Color-shaded DEM of Romania, emphasizing the highly arcuate nature of the Carpathian orogen, and location of intermediate-depth

seismicity of the Vrancea zone at bend joining Eastern and Southern Carpathians. Focal mechanisms of earthquakes (MwN5.0, 1977–2001) from the

CMT catalogue (Harvard, USA) displayed as beach balls. Crustal seismicity (mbN4) from the ISC catalogue displayed as black dots. Note the

elevated topography of the hinterland (Transylvanian) and foreland basins of the Eastern Carpathians, and Quaternary extensional basins within the

interior of the bend region. Topographic data from USGS GTOPO 30 (Smith and Sandwell, 1997).

J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323 313

et al., 1998; Girbacea and Frisch, 1998; Gvirtzman,

2002). Accordingly, the NW-dipping seismogenic

body of the Vrancea zone may more likely have orig-

inated as delaminating continental lithosphere (Diaco-

nescu et al., 2000, 2001; Knapp et al., 2001), or through

some as yet unidentified geodynamic process for gen-

erating spatially restricted intermediate-depth seismicity

in the absence of subducted oceanic lithosphere.

2. Geologic background

Formation of the Carpathian orogen (Figs. 1 and 2)

can be broadly understood in the framework of Meso-

zoic and Cenozoic closure of the Tethys ocean during

continental collision of the Eurasian and African plates.

Two main periods of compressional deformation are

recognized in the Eastern Carpathians, one during

Late Cretaceous time that was responsible for emplace-

ment of large crystalline thrust sheets (Getic, Supra-

getic, and Bucovinian nappes) now exposed in the

westernmost Eastern Carpathians, and a second phase

during Early and Middle Miocene time that involved

imbrication of a Cretaceous through Miocene strati-

graphic sequence in the external nappes (Sandulescu,

1988; Fig. 2). This sedimentary section is tectonically

detached from the basement upon which it was depos-

ited, and records Miocene-age thin-skinned shortening

of as much as 180 km (Roure et al., 1993; Ellouz et al.,

1994). Neogene strata within the external nappes show

a clear stratigraphic affinity with the East European and

Moesian continental basement on which they now rest

tectonically (e.g., Sandulescu, 1980). Final nappe em-

placement in the Eastern Carpathians was mid-Miocene

(11–9 Ma) in age, and was followed by continued

compression and backthrusting in Pliocene time (Stefa-

nescu, 1986; Sandulescu, 1988; Horvath and Cloetingh,

1996; Matenco, 1997; Matenco and Bertotti, 2000;

Ciulavu et al., 2000).

Tectonic elements of the Carpathian system that are

critical to the evaluation of competing geodynamic

models for the origin of Vrancea seismicity are high-

lighted in Fig. 2 and include (1) the Cretaceous (Trans-

ylvanide) suture zone of the hinterland, (2) the crustal

structure of the Transylvanian basin, (3) an enigmatic

Neogene volcanic arc developed behind and on top of

the fold and thrust belt, (4) foreland subsidence in the

Focsani basin, (5) basement structure in the foreland as

it pertains to active deformation there, and (6) Quater-

nary basins which are forming within the bend region

of the Carpathian fold belt and hinterland.

Fig. 2. Geologic map of Romania (modified after Sandulescu et al., 1978), emphasizing (1) position of mid-Cretaceous suture zone (Transylvanides) in the Transylvanian basin, (2) late Tertiary

volcanic arc (red) and (3) thrust nappes of Eastern Carpathians developed in Cretaceous to Miocene strata (green, orange, and yellow units) deposited on the East European/Moesian continental

plates. Thrust deformation in the Eastern Carpathians was primarily Early to Middle Miocene in age. Locations of geologic sections in Figs. 3(Section 1) and 4(Section 2) indicated by heavy black

lines. Sections A and B are deep seismic reflection profiles projected into the cross-section X–XV (light grey) in Fig. 5.

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J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323 315

Burchfiel (1976) and Sandulescu (1988) first sug-

gested that a marginal ocean basin once occupied the

area of the present-day Carpathian/Pannonian system.

Evidence for closure of an ocean basin during Creta-

ceous time is preserved in ophiolites (Transylvanides)

between the Southern Carpathians and northern Apu-

seni Mountains (Sandulescu, 1988; Royden and Baldi,

1988; Fig. 2). Using gravity and magnetic data, this

ophiolitic complex was traced toward the NE beneath

the Transylvanian basin and was interpreted to be the

root zone of a Transylvanian nappe (Fig. 2; Sandulescu,

1980).

The case for closure of an ocean-floored basin in

Miocene time is far less compelling. Scattered occur-

rences of Mesozoic mafic rocks occur along the inner

(western) margin of the Eastern Carpathians, structur-

ally above the crystalline basement rocks of the Getic

and Supragetic nappes of the Eastern Carpathians (Fig.

2). Many studies have interpreted these mafic rocks as

evidence for subduction of oceanic lithosphere during

Miocene formation of the Eastern Carpathians (Balla,

1987; Sandulescu, 1988; Csontos et al., 1992; Csontos,

1995; Ratschbacher et al., 1993; Linzer, 1996). In turn,

it is this putative Miocene slab that many workers now

interpret as the source for Vrancea seismicity (e.g.,

Linzer, 1996; Wortel and Spakman, 2000). It remains

unclear whether these rocks of oceanic affinity are

rooted in the crust, or whether the eastern Transylva-

nian basin is underlain, at least in part, by continental

crust of East Europe and Moesia.

Probably one of the strongest arguments for subduc-

tion of an oceanic slab beneath the Eastern Carpathians

is the presence of a linear arc of Neogene volcanism

within the hinterland (Fig. 2). This volcanic chain,

comprised of both calc-alkaline and alkaline magmas,

was active from Middle Miocene to Quaternary time

(13.4–0.2 Ma), and migrated successively from north to

south (Mason et al., 1998). Although major- and trace-

element geochemistry of the calc-alkaline lavas suggest

they are subduction-related (e.g., Pecskay et al., 1995;

Mason et al., 1998), (1) this magmatic activity largely

post-dated the final stages of deformation in the Eastern

Carpathians, and (2) the volcanic chain actually cross-

cuts the surface trace of the putative Miocene subduc-

tion zone from which it was presumably derived (Fig.

2). Salters et al. (1988) offered an alternative suggestion

that both calc-alkaline and alkaline lavas of the Car-

pathian hinterland could be derived from a single man-

tle source, and reflect differing degrees of crustal

contamination.

Several hypothetical models for the geodynamic

setting of the Vrancea zone have been posed, revolving

primarily around variations on subduction of oceanic

lithosphere. Numerous authors (e.g., Balla, 1987; Cson-

tos et al., 1992) have suggested that oceanic lithosphere

attached to the East European craton (Fig. 1) was

subducted west- and southwestward along the entire

Carpathian arc during Miocene time, and now coincides

with the Vrancea zone. In this model, the subducting

slab presumably began to progressively tear once thick

continental crust entered the subduction zone at ~70 km

depth (e.g., Wortel and Spakman, 2000). Alternatively,

some authors (Linzer, 1996; Girbacea and Frisch, 1998;

Mason et al., 1998) have proposed that the subducting

basin was attached to the Moesian platform (Fig. 1),

and was subducted northwestward beneath the Car-

pathian orogen. Subsequent large scale roll-back from

NW to SE over a distance of ~130 km resulted in

positioning of this detached slab with the Vrancea

zone. Such a model is in agreement with (1) migration

and diminution of Neogene calc-alkaline magmatism in

the Eastern Carpathians from northwest to southeast

along the arc (Mason et al., 1998), and (2) the NW-

dipping geometry of the Vrancea seismic body (Girba-

cea and Frisch, 1998).

3. Discussion

Existing geological and geophysical data from the

Eastern Carpathians and the adjacent foreland place

critical constraints on the geodynamic setting of the

Vrancea zone. Both surface and subsurface data along

the length of the Eastern Carpathians argue that conti-

nental crust of the East European and/or Moesian plat-

forms can be traced, as a minimum, beneath the

external nappes, and above the locus of Vrancea seis-

micity. On this basis, any model explaining the origin

of Vrancea seismicity must account for the existence of

this seismogenic zone beneath continental lithosphere.

Shown in Fig. 3 is a geologic section through the

frontal nappes of the Eastern Carpathians based on

petroleum industry data (Dicea, 1996). Deep wells

from this transect in the Moldova River valley (see

Fig. 2 for location) penetrate deformed strata associated

with the Tarcau, Marginal Folds, and Sub-Carpathian

nappes, before sampling Cenozoic (Sarmatian and

Badenian) and Mesozoic formations of the autochtho-

nous cover of the East European platform. While the

westernmost well on this section is situated more than

10 km west from the exposed thrust front of the Eastern

Carpathians, the Mesozoic and underlying Paleozoic

platform strata at depth appear to project considerably

further to the west. Based on such data, it follows that

the autochthonous continental basement on which these

Fig. 3. Geologic cross-section (WSW–ENE) through the external nappes of the northern Eastern Carpathians, based on control from petroleum

exploration wells (modified after Dicea, 1996). Location of wells shown by vertical lines. Major thrust faults shown by heavy lines at base of thrust

nappes. Medium weight lines indicate faults; lighter lines indicate formational contacts. Legend (below) shows stacking order of nappes, emplaced

from west to east. Note that the projection of the VSZ along structural strike of the Eastern Carpathians clearly corresponds to a position overlain by

continental crust. See Fig. 2(Section 1) for location.

J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323316

strata were deposited extends a considerable distance

westward beneath the external nappes of the Car-

pathians. Minimum figures reported by Dicea (1996)

are 20 km in the Moldova Valley, and 30 km in the

Bistrita and Trotus valleys further to the south.

Unfortunately, such seismic and well data sampling

the rocks beneath the external nappes near the Vrancea

zone are not currently available. Given the highly cy-

lindrical structure of the Eastern Carpathians, however,

projection of this crustal configuration southward along

orogenic strike seems reasonable. While this projection

is admittedly more than 200 km removed from the

Vrancea zone, the surface geology exposed within the

nappes of the Eastern Carpathians is remarkably later-

ally consistent, and implies a continuity of geologic

structure since Mesozoic time. Similarly, the stratigra-

phy of the Cretaceous to Miocene stratigraphic section

exposed in the Eastern Carpathians is quite continuous

along strike (Contescu, 1974). Faults such as the Trotus

fault have been interpreted as major crustal boundaries

within the Carpathian foreland (Matenco, 1997; Tara-

poanca et al., 2003), but such structures are not mani-

fest in either the stratigraphy or structure of the external

Carpathian nappes (Fig. 2), implying that the basement

upon which these strata were deposited and subsequent-

ly emplaced behaved uniformly along strike. As shown

in Fig. 3, projection of Dicea’s (1996) section, con-

trolled by well penetrations of the autochthonous base-

ment of the East European platform, implies continental

crust lies both updip of and above the steeply northwest

dipping Vrancea zone.

In the central portion of the Eastern Carpathians,

Burchfiel (1976) produced a palinspastic reconstruction

of deformation, extending from the autochthonous plat-

form in the east to the crystalline thrust nappes (Bucov-

inian) exposed in the hinterland (Fig. 4). While no one

thrust nappe contains the entire Cretaceous to Tertiary

sequence, a contiguous stratigraphic section from Late

Cretaceous through Pliocene age can be reconstructed

across the successive nappes of the Eastern Carpathians.

Of critical importance are the observations that (1) this

stratigraphic section can be tied to Neogene strata that

were clearly deposited on the autochthonous continental

platform, and (2) such stratigraphic continuity across the

Eastern Carpathians appears to preclude a significant

intervening basin floored by oceanic crust during Neo-

gene time. Published seismic reflection sections at this

same latitude (Dicea, 1995) image the platform stratig-

raphy extending a minimum of 10 km (and presumably

much further) westward beneath the frontal edge of the

Sub-Carpathian nappe. Furthermore, Burchfiel’s (1976)

section results in an estimate of ~125 km of shortening

Fig. 4. Palinspastic reconstruction (below) through central portion of the Eastern Carpathians (modified after Burchfiel, 1976), based on W–E structural cross-section (above). Reconstruction is

pinned at eastern end, within the autochthonous crust of the East European craton. A contiguous Cretaceous to Pliocene stratigraphic section can be traced from the autochthon to the western end of

the section (crystalline Bucovinian nappes) without interruption. See Fig. 2(Section 2) for location.

J.H.Knappet

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J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323318

within the central Eastern Carpathians, implying that at

the latitude of this section, crust of continental affinity

extends a similar distance westward beneath the Eastern

Carpathian nappes.While the rocks of the Ceahlau nappe

may be an exception, it is certainly plausible on the basis

of this analysis that East European and/or Moesian con-

tinental crust projects tens of kilometers west of the

current known extent, and did so prior to shortening in

Miocene time.

As with the exposed stratigraphy and shallow geo-

logic structure of the Eastern Carpathians, the crustal-

scale structure of the Carpathian foreland provides

additional constraints on the origin of Vrancea seismic-

ity. Deep seismic reflection data from the Carpathian

foreland (Ramnicu Sarat profile) and the Transylvanian

hinterland (Targu Mures I profile) are combined in a

lithosphere-scale cross section across the Vrancea zone

(Fig. 5; see Fig. 2 for location). While some dipping

structure is evident at the basement-cover contact with-

in the deep Focsani basin, crustal fabrics and structure

at the Moho are comparatively sub-horizontal, and

inconsistent with a significant bending of the crust

into the Vrancea seismic zone. Indeed, strata presum-

ably deposited on continental crust (based on palinspas-

Fig. 5. WNW–ESE transect (~320 km) along cross-section X–XV showing i

dots). Topography across the profile (above) shown in m, and depth below se

reprocessed (Ramnicu Sarat) versions of deep profiles (A and B in Fig. 2; se

from refraction and gravity modeling (Radulescu, 1981).

tic reconstruction of the fold and thrust belt) occupy the

high topography of the Eastern Carpathian directly

above the Vrancea zone. While it may yet be permis-

sible that the continental crust imaged by these deep

seismic data in the foreland rolls over essentially verti-

cally into the adjacent Vrancea zone (Fig. 5), such a

geometry does not appear consistent with geologic data

presented here.

Based on these geological and geophysical observa-

tions, three contrasting models have been or can be

posed for the geodynamic setting of the Vrancea region

(Fig. 6): (A) oceanic slab break-off and retreat, (B)

oceanic slab subduction within the foreland, and pro-

gressive lateral tearing of the slab, or (C) lithospheric

delamination due to continental underthrusting and oro-

genic thickening. Each of these envisions a relatively

cold and dense body of lithospheric material that is

presently situated in the upper mantle beneath the

bend region generating seismicity in the Vrancea re-

gion. The first two models require subduction of oce-

anic lithosphere to explain Vrancea seismicity, and can

be distinguished on the basis of where the proposed

subduction occurred within the orogen, and how the

subducted slab evolved (break-off and trenchward re-

nterpreted crustal structure above Vrancea zone (hypocenters in black

a level (below) shown in km. Insets show industry (Targu Mures I) and

ctions plotted ~1:1 at 6 km/s). Moho beneath Carpathians determined

Fig. 6. 3-D perspective lithosphere-scale block models (view towards NNW), illustrating contrasting scenarios for geodynamic setting of the Vrancea zone. (A) Oceanic slab subduction and break-

off. (B) Oceanic slab subduction and progressive lateral tear within the Carpathian foreland. (C) Continental lithospheric delamination. Green = Moesian/East European crust; yellow =

Transylvanian crust; pink = continental mantle lithosphere; purple = oceanic lithosphere; grey = asthenosphere. Vrancea zone is located in lower front corner of models. See text for discussion.

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J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323320

treat vs. migration of a lateral tear along the orogen).

The third model could be generated through closure of

an intracontinental basin and lithospheric thickening,

and does not require the former presence of an ocean-

floored basin, or the subduction of oceanic lithosphere.

Each of these models has implications for the asso-

ciated crustal structure, and is readily testable through

integration of surface and subsurface observations. In

particular, if an ocean-floored basin was consumed

during the formation of the Eastern Carpathians, evi-

dence for the former boundary between two distinct

continental plates should be recorded in both the sur-

face geology (suture zone) and the underlying crustal

structure. The predicted position of such a boundary

differs by more than 150 km in the two subduction

models (Figs. 5 and 6A and B). To date, no conclusive

evidence has been documented for such a crustal (and

lithospheric) boundary in either position. Furthermore,

the tectonic affinity of Neogene strata exposed in the

Eastern Carpathians should be diagnostic. Models A

(slab break-off) and C (continental delamination) clear-

ly imply an East European/Moesian crustal affinity for

these Neogene age strata, whereas model B (slab tear

model) requires that these strata were separated from

the East European/Moesian margin by an intervening

ocean-floored basin.

Although widely accepted in the scientific literature,

the subduction model for the Vrancea zone presents

several clear weaknesses. The Vrancea zone, if now

represented by a remnant slab, does not coincide updip

with the surface expression of a suture zone. Similarly,

Neogene volcanism west of the Eastern Carpathians

occurs in such close proximity (b50 km) to the zone of

modern seismicity (Fig. 2) that any subducting slab

would not likely be at depths to dewater and induce

melting of the overlying mantle. Vrancea zone earth-

quakes are located beneath the external fold and thrust

belt and the adjacent Focsani depression. Furthermore,

eruption of Quaternary alkaline basalts in eastern

Transylvania appears to be unrelated to subduction

volcanism. Mason et al. (1998) appealed to late-stage

slab break-off and asthenospheric upwelling to gener-

ate these basalts. Perhaps most importantly, if such a

detached slab is present, it did not sink vertically

through the mantle, but moved by ~130 km horizon-

tally to the southeast relative to the suture zone iden-

tified in Transylvania. Moreover, development of

extensional sedimentary basins of Miocene–Quaternary

age in the vicinity of the Vrancea zone, as well as

extensive development of uplifted fluvial terraces in

the foreland, would appear to be at odds with subduc-

tion geodynamics.

While continental delamination was proposed as a

mechanism to produce mantle seismicity in the Alboran

Sea (Seber et al., 1996; Mezcua and Rueda, 1997),

Vrancea zone seismicity has not been widely viewed

in this light. Some workers (Csontos, 1995; Girbacea

and Frisch, 1998; Gvirtzman, 2002) have appealed to

foundering of lithospheric mantle as the final stages of

slab subduction beneath the Eastern Carpathians, but

these models still require the consumption of oceanic

lithosphere in Miocene time. A number of observations

appear consistent with active delamination of continen-

tal mantle lithosphere in the southeastern Carpathians

including: (1) the narrow, cylindrical shape of the seis-

mogenic zone, and implicitly, the lack of a well-defined

Benioff plane, (2) the miscorrelation of hypocenters

(spatially) and volcanism (both spatially and temporal-

ly) with the expected position of a bnormalQ descendingslab (Girbacea and Frisch, 1998), (3) the eruption of

Quaternary alkali basalts in the vicinity of the Vrancea

zone, (4) the presence of an aseismic gap between 40

and 70 km depth, with low P-wave velocity (Fuchs et

al., 1979; Oncescu, 1984; Lorenz et al., 1997; Fan et al.,

1998), and high attenuation (Q; Mocanu et al., 1999),

(5) a weak zone beneath the crust based on lithosphere

strength modeling (Lankreijer et al., 1997), (6) the late

Miocene to Pleistocene subsidence of the Carpathian

foreland basin in the absence of major surface loads

(Artyushkov et al., 1996), (7) the development of Qua-

ternary extensional basins in the vicinity of the Vrancea

zone (Ciulavu et al., 2000), and (8) active deformation

in the Carpathian foreland, concentric about the locus

of Vrancea zone seismicity, and recording displacement

rates of up to 15 mm/yr.

To a first order, the presence or absence of a

through-going, dipping crustal fabric beneath the pur-

ported Miocene suture would alternatively substantiate

or eliminate the subduction-model origin for Vrancea

seismicity. Similarly, demonstration of a geometric as-

sociation of foreland deformation with the Vrancea

mantle source region would imply a mechanical cou-

pling of the Vrancea seismogenic body with the over-

lying crust, and hence argue against a detached slab.

Deep seismic reflection data are uniquely suited to

address these competing hypotheses by providing a

high-resolution image of the structural geometry of

the crust, and a direct link of surface geology to

upper mantle seismicity.

4. Conclusions

The origin of intermediate depth seismicity in the

Vrancea zone of Romania continues to be a subject of

J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323 321

debate. While most workers consider that the Vrancea

seismogenic body consists of oceanic lithosphere, ei-

ther resulting from detachment and lateral migration of

an oceanic slab (e.g., Girbacea and Frisch, 1998;

Gvirtzman, 2002), or subduction and lateral tearing of

a slab beneath the eastern edge of the Eastern Car-

pathians (e.g., Wenzel et al., 1998; Wortel and Spak-

man, 2000), such interpretations appear to be

inconsistent with geologic constraints from the Eastern

Carpathians and adjacent foreland. In particular, petro-

leum exploration data document that crust of continen-

tal affinity extends significantly westward beneath the

external thrust nappes (Sub-Carpathian, Marginal

Folds, and Tarcau) of the Eastern Carpathians. In addi-

tion, Neogene strata of the Eastern Carpathians can be

reconstructed much further westward to a position now

occupied by the Transylvanian basin. Finally, geologic

structure in the Carpathian foreland (including the

Moho) is sub-horizontal directly to the east of and

above the Vrancea seismogenic zone. Taken together,

these geologic relationships imply that the Vrancea

zone occupies a region overlain by continental crust

and upper mantle, and does not appear to originate from

a subducted oceanic slab along the length of the Car-

pathian orogen.

An alternative model for Vrancea zone seismicity,

proposed here, involves active continental lithospheric

delamination, resulting from Miocene closure of an

intra-continental basin and attendant lithospheric thick-

ening. Such a model is consistent with the presence of

seismicity beneath thick continental crust, including the

foreland of the Carpathian system, and honors surface

and subsurface geologic constraints that appear to pre-

clude the presence of remnant oceanic crust in Tertiary

time in the region.

Acknowledgements

We have benefited greatly from discussions with M.

Sandulescu, R. Russo, and G. Yogodzinski, as well as

insightful comments by J. McBride and L. Csontos,

and an anonymous reviewer, however we remain re-

sponsible for any misstatements or errors of fact. This

work was supported by an award to JHK from the

National Research Council COBASE program, and

award EAR-0310118 to JHK and CCK from the Tec-

tonics Program of the National Science Foundation.

CCK acknowledges the donors of The Petroleum Re-

search Fund, administered by the ACS, for partial

support of this research. Seismic data processing was

facilitated through a grant from the Landmark Graphics

Corporation.

References

Artyushkov, E.V., Baer, M.A., Moerner, N.A., 1996. The East Car-

pathians; indications of phase transitions, lithospheric failure and

decoupled evolution of thrust belt and its foreland. Tectonophy-

sics 262, 101–132.

Austrheim, H., 1991. Eclogite formation and dynamics of crustal

roots under continental collision zones. Terra Nova 3, 492–499.

Baird, D.J., Nelson, K.D., Knapp, J.H., Walters, J.J., Brown, L.D.,

1996. Crustal structure and evolution of the Trans-Hudson oro-

gen: results from seismic reflection profiling. Tectonics 15 (2),

416–426.

Balla, Z., 1987. Tertiary palaeomagnetic data for the Carpatho–Pan-

nonian region in the light of Miocene rotation kinematics. Tecto-

nophysics 139 (1–2), 67–98.

Benioff, H., 1949. Seismic evidence for the fault origin of

oceanic deeps. Geological Society of America Bulletin 60,

1837–1856.

Bird, P., 1978. Initiation of intracontinental subduction in the Hima-

laya. Journal of Geophysical Research, A, Space Physics 83

(B10), 4975–4987.

Bird, P., 1979. Continental delamination and the Colorado Plateau.

Journal of Geophysical Research 84 (B13), 7561–7571.

Burchfiel, B.C., 1976. Geology of Romania. Geological Society of

America, Special Paper 158 (82 pp.).

Ciulavu, D., Dinu, C., Szakacs, A., Dordea, D., 2000. Neogene

kinematics of the Transylvanian Basin (Romania). AAPG Bulletin

84 (10), 1589–1615.

Contescu, L.R., 1974. Geologic history and paleogeography of East-

ern Carpathians: example of Alpine geosynclinal evolution.

AAPG Bulletin 58, 2436–2476.

Csontos, L., 1995. Tertiary tectonic evolution of the Intra-Carpathian

area; a review. Acta Vulcanologica 7 (2), 1–13.

Csontos, L., Nagymarosy, A., Horvath, F., Kovac, M., 1992. Tertiary

evolution of the intra-Carpathian area; a model. Tectonophysics

208 (1–3), 221–241.

Dewey, J.F., 1988. Extensional collapse of orogens. Tectonics 7,

1123–1139.

Diaconescu, C.C., Knapp, J.H., Mocanu, V., Raileanu, V., Dinu, C.,

Matenco, L., Prodehl, C., Hauser, F., Wenzel, F., 2000. Active

subduction or delamination?: lithospheric structure of the south-

east Carpathian orogen, Romania. Eos, Transactions, American

Geophysical Union 81, 1090.

Diaconescu, C.C., Knapp, J.H., Keller, G., Stephenson, R., Mocanu,

V., Raileanu, V., Matenco, L., Bala, A., Prodehl, C., Hauser, F.,

Dinu, C., Wenzel, F., Harder, S., 2001. Intermediate depth seis-

micity in the Vrancea Zone of Romania: a geodynamic paradox.

Eos, Transactions, American Geophysical Union 82 (47) (Fall

Meet. Suppl., Abstract S51A-0587).

Dicea, O., 1995. The structure and hydrocarbon geology of the

Romanian East Carpathian border from seismic data. Petroleum

Geoscience 1, 135–143.

Dicea, O., 1996. Tectonic setting and hydrocarbon habitat of

the Romanian external Carpathians. In: Zeigler, P.A., Horvath,

F. (Eds.), Peri-Tethys Memoir 2: Structure and Prospects of

Alpine Basins and Foreland, Mem Mus. Natn. Hist. Mat., vol.

170, pp. 403–425.

Ellouz, N., Roure, F., Sandulescu, M., Badescu, D., 1994. Balanced

cross sections in the eastern Carpathians (Romania): a tool to

quantify Neogene dynamics. In: Roure, F., Ellouz, N., Shein, V.S.,

Skvortsov, I. (Eds.), Geodynamic Evolution of Sedimentary

Basins. Technip, Paris, pp. 305–325.

J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323322

Fan, G., Wallace, T.C., Zhao, D., 1998. Tomographic imaging of deep

velocity structure beneath the Eastern and Southern Carpathians,

Romania; implications for continental collision. Journal of Geo-

physical Research, B 103 (2), 2705–2723.

Fuchs, K., Bonjer, K.-P., Bock, G., Cornea, I., Radu, C., Enescu, D.,

Jianu, D., Nourescu, A., Merkler, G., Moldoveanu, T., Tudorache,

G., 1979. The Romanian earthquake of March 4, 1977: II. After-

shocks and migration of seismic activity. Tectonophysics 53,

225–247.

Furlong, K.P., Fountain, D.M., 1986. Continental crustal underplat-

ing; thermal considerations and seismic–petrologic consequences.

Journal of Geophysical Research, B 91 (8), 8285–8294.

Girbacea, R., Frisch, W., 1998. Slab in the wrong place: lower

lithospheric mantle delamination in the last stage of the Eastern

Carpathian subduction retreat. Geology 26, 611–614.

Gvirtzman, Z., 2002. Partial detachment of a lithospheric root under

the southeast Carpathians: toward a better definition of the de-

tachment concept. Geology 30, 51–54.

Hauser, F., Raileanu, V., Fielitz, W., Bala, A., Prodehl, C., Polonic,

G., Schulze, A., 2001. VRANCEA99: the crustal structure be-

neath the southeastern Carpathians and the Moesian Platform

from a seismic refraction profile in Romania. Tectonophysics

340 (3–4), 233–256.

Horvath, F., Cloetingh, S., 1996. Stress-induced late-stage subsidence

anomalies in the Pannonian Basin. Tectonophysics 266 (1–4),

287–300.

Isacks, B., Oliver, J., Sykes, L.R., 1968. Seismology and the new global

tectonics. Journal of Geophysical Research 73, 5855–5899.

Kay, R.W., Kay, S.M., 1993. Delamination and delamination magma-

tism. Tectonophysics 219 (1–3), 177–189.

Knapp, J.H., Diaconescu, C., Hauser, F., Prodehl, C., Raileanu, V.,

Matenco, L., Bala, A., Keller, G.R., Stephenson, R., Mocanu, V.,

Dinu, C., 2001. The Vrancea seismogenic zone, Romania: inter-

mediate depth seismicity in search of a viable subduction zone.

Eos, Transactions, American Geophysical Union 82 (47) (Fall

Meet. Suppl., Abstract S42D-11).

Lankreijer, A., Mocanu, V., Cloetingh, S., 1997. Lateral variations in

lithosphere strength in the Romanian Carpathians; constraints on

basin evolution. Tectonophysics 272 (2–4), 269–290.

Linzer, H.-G., 1996. Kinematics of retreating subduction along the

Carpathian arc, Romania. Geology 24, 167–170.

Lorenz, F.P., Wenzel, F., Popa, M., 1997. Teleseismic travel-time

tomography of the compressional-wave velocity structure in the

Vrancea Zone, Romania, 1997. Eos, Transactions, American Geo-

physical Union 78 (46 Suppl.), 497.

Mason, P.R.D., Seghedi, I., Szakacs, A., Downes, H., 1998. Magmatic

constraints on geodynamic models of subduction in the East

Carpathians, Romania. Tectonophysics 297, 157–176.

Matenco, L., 1997. Tectonic evolution of the Outer Romanian Car-

pathians: Constraints from kinematic analysis and flexural mod-

elling. Ph.D. Thesis, Vrije Universiteit, Faculty of Earth Sciences,

Amsterdam, 160 pp.

Matenco, L., Bertotti, G., 2000. Tertiary tectonic evolution of the

external East Carpathians (Romania). Tectonophysics 316 (3–4),

255–286.

Matenco, L., Bertotti, G., Cloetingh, S., Dinu, C., 2003. Subsidence

analysis and tectonic evolution of the external Carpathian–Moe-

sian Platform region during Neogene times. Sedimentary Geology

156, 71–94.

Mezcua, J., Rueda, J., 1997. Seismological evidence for a delamina-

tion process in the lithosphere under the Alboran Sea. Geophys-

ical Journal International 129 (1), F1–F8.

Mocanu, V., Russo, R.M., Wenzel, F., Dinu, C., 1999. Seismic

Attenuation in the Vrancea region, Carpathians, Romania,

American Geophysical Union Fall Meeting. EOS, Transactions

80, 46.

Nelson, K.D., 1991. A unified view of craton evolution motivated by

recent deep seismic reflection and refraction results. Geophysical

Journal International 105 (1), 25–35.

Oncescu, M.C., Burlacu, V., Smalbergher, V., Anghel, M., 1984.

Three-dimensional P-wave velocity image under the Carpathian

Arc. Tectonophysics 106, 305–319.

Pecskay, Z., Edelstein, O., Seghedi, I., Szakacs, A., Kovacs, M.,

Crihan, M., Bernad, A., 1995. K–Ar datings of Neogene–Quater-

nary calc-alkaline volcanic rocks in Romania. Acta Vulcanologica

7 (2), 53–61.

Radulescu, F.A., 1981. Crustal seismic studies in Romania. Revue

Roumaine de Geologie, Geophysique et Geographie. Serie de

Geophysique 25, 57–74.

Ratschbacher, L., Linzer, H.-G., Moser, F., Strusievicz, R.-O., Bede-

lean, H., Har, N., Mogos, P.-A., 1993. Cretaceous to Miocene

thrusting and wrenching along the central South Carpathians due

to a corner effect during collision and orocline formation. Tec-

tonics 12 (4), 855–873.

Roure, F., Roca, E., Sassi, W., 1993. The Neogene evolution of the

outer Carpathian flysch units (Poland, Ukraine and Romania);

kinematics of a foreland/fold-and-thrust belt system. Sedimentary

Geology 86 (1–2), 177–201.

Royden, L.H., Baldi, T., 1988. Early Cenozoic tectonics and paleo-

geography of the Pannonian and surrounding regions. In: Royden,

L.H., Horvath, F. (Eds.), The Pannonian Basin; a Study in Basin

Evolution, AAPG Memoir, vol. 45, pp. 1–16.

Sacks, P.E., Secor, D.T. Jr., 1990. Delamination in collisional orogens.

Geology 18 (10), 999–1002.

Salters, V.J.M., Hart, S.R., Panto, G., 1988. Origin of late Cenozoic

volcanic rocks of the Carpathian Arc, Hungary. AAPG Memoir

45, 279–292.

Sandulescu, M., 1980. The molasse formations and the geotectonical

evolution of the Carpathians. In: Benek , R., Paech, H.J., Schwab,

G., Hasert, K. (Eds.), Veroeffentlichungen des Zentralinstituts fuer

Physik der Erde, vol. 58, pp. 147–153.

Sandulescu, M., 1988. Cenozoic tectonic history of the Carpathians.

In: Royden, L.H., Horvath, F. (Eds.), The Pannonian Basin, a

Study in Basin Evolution, AAPG Memoir, vol. 45, pp. 17–25.

Sandulescu, M., Krautner, H., Borcos, M., Nastaseanu, S. Patru-

lius, D., Stefanescu, M., Ghenea, C., Lupu, M., Savu, H.,

Bercia, I., Marinescu, F., 1978. Geologic Atlas, Bucharest

(in Romanian).

Seber, D., Barazangi, M., Ibenbrahim, A., Demnati, A.,

1996. Geophysical evidence for lithospheric delamination

beneath the Alboran Sea and Rif–Betic mountains. Nature

379, 785–790.

Smith, W.H.F., Sandwell, D.T., 1997. Global seafloor topography

from satellite altimetry and ship depth soundings. Science 277,

1956–1962.

Stefanescu, M. 1986. Elaborarea profilelor la scara 1 :200,000 pentru

completarea imaginii geologice a teritoriului R.S.R.: Institutul de

Geologie al Romaniei.

Tarapoanca, M., Bertotti, G., Matenco, L., Dinu, C., Cloetingh,

S.A.P.L., 2003. Architecture of the Focsani Depression: a 13

km deep basin in the Carpathians bend zone (Romania). Tec-

tonics 22.

Wadati, K., 1928. Shallow and deep earthquakes. Geophysical Mag-

azine 1, 162–202.

J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323 323

Wadati, K., 1935. On the activity of deep-focus earthquakes in the

Japan Islands and the neighborhoods. Geophysical Magazine 8,

305–325.

Wenzel, F., Achauer, U., Enescu, D., Kissling, E., Russo, R., Mocanu,

V., Musacchio, G., 1998. Detailed look at final stage of plate

break-off is target of study in Romania. Eos, Transactions, Amer-

ican Geophysical Union 79 (48), 589.

Wortel, M.J.R., Spakman, W., 2000. Subduction and slab detachment

in the Mediterranean–Carpathian region. Science 290 (5498),

1910–1917.