Characteristics of Romanian lithosphere from deep seismic reflection profiling

TECTONOPHYSICS EL-SEWER

Tectonophysics

Characteristics

239 (1994) 165-185

of Romanian lithosphere from deep seismic reflection profiling

Victor Rgileanu,

Camelia

Diaconep,

Florin RSdulescu

National Institute for Earth Physics, P.O. Box MG-2, Bucharest-Migurele, Received

4 March

1993; revised version

accepted

Romania

18 July 1994

Abstract The tectonic units of Romania contain both Alpine erogenic regions and platforms. The investigations of crustal structure by near-vertical reflection seismic profiling are based on less than 100 lines unequally distributed and of limited lengths (5-30 km). The seismic features studied are: the crustal seismic reflectivity and transparency; dipping events and diffractions; and the Moho and subcrustal reflectivity. The major common features are: a reflective sedimentary cover of mostly Neogene layers; a weak signal from the sedimentary-basement boundary; a quasi-transparent upper crust; a mid-crust with short and subhorizontal events; a transparent or layered lower crust; and a Moho that is either poorly visible or evidenced by a zone with reflection bands and some subcrustal reflections. The Moesian platform, together with the adjacent Carpathian foredeep, show a mid-crust with short and subhorizontal events, a crust-mantle transition zone in the west, an unclear or prominent Moho and some subcrustal reflectivity. The northern part of the Carpathian foredeep shows strong seismic layering of the lower crust. The central part of the Transylvanian depression is characterized by short and subhorizontal events at mid-crustal level, a slightly reflective lower crust with diffractions and a weak Moho signal. The southern sector of the Pannonian depression is generally marked by a reflective lower crust. The tectonic significance of these seismic signatures is interpreted as follows: a brittle upper crust; a decoupling mid-crustal zone; a lower crust with brittle to ductile behaviour; a possible extensional or isostatic rebalanced Moho and laminated zones with reduced viscosities at the lower crust and upper mantle levels. A series of crustal models based on the reflection seismic sections are presented for several areas of Romania.

1. Introduction Near-vertical deep seismic reflection profiling has developed in Romania during the last 18 years rather as a by-product of oil and gas prospecting than as national program, such as COCORP, DEKORP, ECORS or BIRPS. The deep seismic lines, therefore, are only distributed in areas of interest for hydrocarbons. They are of limited 0040-1951/94/$07.00 0 1994 Elsevier SSDZ 0040-195 1(94)00145-6

Science

length ( < 30 km) and the field acquisition parameters have been adjusted for a good resolution in the sedimentary cover, except for the record time that has been lengthened to lo-17 s two-way travel time (TWT). Of the over 1000 km deep seismic lines recorded on the Romanian territory (Fig. 11, most of the data come from the Moesian platform and Carpathian foredeep (- 750 km). The Pannonian and Transylvanian depressions

B.V. All rights reserved

are poorly

represented and the Carpathian Range and Dobrudja are almost without seismic reflection data. Although the individual deep seismic Iines are of limited length, their analyses have shown some correlations between the crustal reflectivity and tectonic structure (Cornea et al., 1978, 1981; Ta102 et al., 1979; F. Rgdulescu, 1981; F. Rsdulescu et al., 1984; RBileanu et al., 1992, 1993). We used only unmigrated seismic sections, since the migration process can often produce ‘poor results’ and destroy the diffractions (Warner, 1987; Sadowiak et aI., 1991a). The analyses of the seismic sections were aimed mostly at the features of the crys-

talline crust, expressed by the distribution and character of the reflectivity (length, density and dip of the reflection events over the whole crust). We payed special attention to the position and character of the Moho, as well as to the subcrustal reflectivity. Following Wever et al. (19871, the statistical analyses of the reflections Ied us to establish the reflectivity pattern for some stacked sections (Figs. 10 and 11). Finally, some crustal models based on the seismic sections, reflectivity histograms and other previous deep data are presented. Of the tens of deep seismic reffection lines recorded on Romanian territory (Fig. 11, the au-

Fig. 1. Map showing locations of the deep seismic reflection lines recorded in Romania. Boundaries between tectonic units after Srindulescu (19844).1 = seismic line; 2 = seismic line described in the text; 3 = boundary between tectonic units; 4 = crustal fault; S.P. = Scythian platform; N.D.O. = North Dobrudjan orogen; S.F. = Sob fault; P.C.F. = Peceneaga-Camena fault: 1.M.F. = Intramoesian fault.

V. Ra’ileanu et al. / Tectonophysics 239 (1994) 165-185

thors have selected for discussion most representative lines distributed tectonic units.

2. Geological

and geophysical

eight of the on the main

setting

The geology of Romania comprises a large variety of tectonic units, some of them old, such as the Moldavian and Moesian platforms, and others young, such as the Carpathian orogen, the North Dobrudjan orogen, the Apuseni Mountains, the Carpathian foredeep, the Transylvanian depression and the easternmost part of the Pannonian depression (Fig. 1). The Moldavian platform is located at the western margin of the East European platform. It consolidated in the Proterozoic and suffered oscillating movements during the erogenic phases of the adjacent, unstable areas. The Moldavian platform has a heterogeneous basement that is made up of Mesozoic-Proterozoic formations 1,OOO- 1,600 My old east of Solca fault and Caledonian-Hercynian rocks to the west (Visarion et al., 1988; Mutihac, 1990). The Solca fault is considered to be the prolongation of the TornquistTeysseire fault on Romanian territory Wndulescu, 1984). A package of Palaeozoic-Quaternary sedimentary layers, varying in thickness and age, discordantly and transgressively cover the basement of the platform. The Moesian platform consolidated as a result of the Cadomian orogeny during the Early Palaeozoic and now occupies the southern part of Romania Windulescu, 1984). It is separated from the North Dobrudjan orogen in the northeast, by the Peceneaga fault (PCF, Fig. 1). The basement of the Moesian platform is complex and heterogeneous, being composed of mesometamorphic schists of Meso-Proterozoic age (over 1,000 My) westward of the Intramoesian fault (IMF, Fig. 1) and pre-Baikalian greenschists (over 500 My old), together with Sveco-Karelian crystalline schists (1,600-1,900 My old) eastward of the Intramoesian fault (Mutihac, 1990). All these schists have been reworked by the Cadomian orogeny (Sgndulescu, 1984). The entire Moesian basement is pierced by acid and basic magmatites of Palaeo-

I67

zoic age. The cover of the Moesian platform starts with Cambrian and Ordovician sediments and continues, with some stratigraphic hiatuses, up to the Neogene and Quaternary. The Alpine movements caused strong subsidence of both platforms (Moldavian and MoeSian) in a step-like fashion under the Carpathian orogen. The basement of these platforms is brittle and has a fault system that separates them into more blocks which suffered vertical oscillations. This feature can be identified in the sedimentation cycles of the sedimentary cover. The Carpathian erogenic structures in the Romanian territory belong to the central and southeastern European Alpine units. Two sectors with different evolution lie on either side of these: the Dacides, which include areas of Cretaceous tectonic origin, and the Moldavides. with areas of Neogene tectonic origin (Gmdulescu, 1984). The Eastern Carpathians are made up of a system of cover and basement nappes which are folded and overthrust from the west to east. The Neogene magmatites occur in the westernmost part of the Eastern Carpathians. The Southern Carpathians are a thrust nappe structure that contains both basement and sedimentary formations. The Apuseni Mountains are composed of basement nappes (in the northern half) and flysch deposits associated with ophiolites (in the southern half) Wmdulescu, 1984). The Carpathian foredeep, located in front of the Eastern and Southern Carpathians, is a posterogenic subsidence zone. Its basement belongs to the platform units which were subducted under the Neogene molasse. The sedimentary cover reaches the greatest thickness between the cities of FocSani and Rsmnicu-Sarat (- 18 km) (Cornea et al., 1981). The common depths to the basement vary between 6 km in the east to more than 10 km in the northwest and southwest. The Transylvanian depression is the greatest molassic Neogene depression in Romania and contains a basement of Dacidic origin. It is composed of metamorphic rocks which are covered by Mesozoic sedimentary deposits and underlie Neogene post-tectonic sediments in the central and eastern part Wndulescu, 1984; Ionescu et al., 1986; Mutihac, 1990). The basement structure

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V. Ra’ileanu et al. / Tectonophysics 239 (1994) 165-185

shows a deep central depression (N 10.5 km deep) bounded by faults of the secondary raised and subsided areas. Geophysical data show an uplift of the lower crust in the central area (Ionescu et al., 1986). The easternmost part of the Pannonian depression consists of a Neogene cover on a basement that is made up of metamorphic rocks and pre-Neogene sedimentary deposits. The structure shows a system of fractured and unlevelled blocks (Visarion and Sandulescu, 1979; Ionescu, 19811. The thickness of the sedimentary cover varies from about 1 to 4-6 km (Ionescu, 19811. Neogene deposits overlie the crystalline basement on the uplift blocks. The basement of subsided blocks is covered by Mesozoic, sometimes Palaeogene and Neogene formations (Ionescu, 1981). Crustal structure investigations on a large scale started in Romania with seismic refraction studies. Since 1970, two important geotraverses were made as part of an international project for Eastern Europe. The trans-Carpathian profile (XI) is over 500 km long and traverses the cities of Gala& Foqani, Targu Mure?, Cluj and Oradea. Profile II connects the cities of GalaSi and Calara$, a distance of about 150 km. Profile XI, oriented NW-SE, crosses some of the main tectonic units and shows the evolution of the crustal thickness from the North Dobrudjan orogen (4345 km), through the Carpathian foredeep (40-47 km), Carpathian orogen (45-47 km), Transylvanian depression (28-35 km) and Apuseni Mountains ( - 35 km) to the eastern part of the Pannonian depression (- 27 km) (D.P. Radulescu et al., 1976; Cornea et al., 1981). The second geotraverse, trending NNE-SSW, crosses the North Dobrudjan orogen and the Moesian platform, which are separated by the Peceneaga-Camena fault (Fig. 1). The crustal thickness is 48-49 km in the northern block of the Peceneaga-Camena fault but only 40-42 km south of it (Cornea et al., 19811. Two shorter seismic profiles west of Craiova (XII) and south of Foqani (XI,) provide new data on the crustal structure (F. Radulescu et al., 1977; F. Radulescu, 1979; Cornea et al., 1981). Profile XII, together with some wide-angle seismic soundings, shows crustal thicknesses greater

than 30 km for the western part of the Moesian platform. Profile XI, shows crustal thicknesses of about 42 km in the Foqani depression. The refraction and reflection seismic studies revealed a sedimentary cover with a thickness ranging between a few hundred meters in the platform-type areas to about 18 km in the FocSani depression. The basement surface was traced by head waves and sometimes by reflected waves at a wide angle (Cornea et al., 19811. Other deep seismic soundings carried out in the 1980s in Dobrudja (between the Danube and the Black Sea) reveal increasing crustal thicknesses from the south (30 km) to the north (40 km) (Pompilian et al., 1991). Seismological data recorded by the Romanian seismological network were used for a systematic study of the Earth’s crust (Enescu, 1987; Enescu et al., 1988, 1992). These data point out crustal and lithospheric thicknesses as well as average velocities of the P and S waves for the main tectonic units. The authors established average values of crustal thicknesses and velocities as follows: the Moldavian platform, 43 km and 6.22 km/s; the Moesian platform, 34 km and 6.21 km/s; the Carpathian Orogen together with the foredeep bend zone, 44 km and 6.18 km/s; the Transylvanian depression 33 km and 6.11 km/s; and the Pannonian depression, 25-30 km (velocity unknown). An overall image of the deep structure of the Romanian crust and lithosphere can be compiled using the Conrad and Moho maps of F. Radulescu (1988) and the Moho and lithosphere base maps of Enescu et al. (1992). Other geophysical data on the deep structure have been provided by geothermal studies (Andreescu et al., 1989; Demetrescu et al., 1991; Andreescu, 1993; Demetrescu and Andreescu, 19941; geomagnetic soundings (Soare et al., 1980); magnetotelluric soundings (St&id et al., 1986) and by integrative studies (Visarion et al., 1984, 1988). 3. Reflection seismic

data acquisition

and pro-

cessing

As we showed before, the deep seismic reflection lines were recorded in the sedimentary ar-

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K Rciileanu et al. / Tectonophysics 239 (1994) 165-185

eas, with acquisition parameters proper for oil prospecting. Of the possible options, we chose those lines that could offer a higher signal-tonoise ratio through their topographic position (a gentle relief) and acquisition parameters (high fold coverage, much explosive for shooting, longer spreads, greater lengths of lines). Most of the lines belong to platform and depression areas where the sedimentary cover played a negative role in signal penetration due to the great thicknesses (more than 4-5 km). At the same time, the complex shallow tectonics of the orogen has weakened the signal penetration and complicated the seismic sections because of interference (Mayer and Brown, 1985). The geometry and recording parameters of the reflection seismic lines were: 12-24-fold coverage; record spreads with 24-96 channels and 25-50 m spacing; shots with charges of 5-3.5 kg in one or more boreholes; geophones with eigenfrequencies higher than 8 Hz; and a recording time of lo-17 s TWT. Data were processed with standard programs for oil prospecting. F-k filtering and time-variable frequency filtering increased the signal-to-noise ratio and enhanced some deeper reflections. Owing to the short spreads (N 2.5 km), the RMS stacking velocities are reliable only to 4-5 s TWT. In the past few years we have tried to use some other attributes of the seismic trace (e.g., amplitudes) to identify the deep reflections (Raileanu et al., 1992). For interpretation we generally used unmigrated versions of the seismic sections but sometimes the migrated sections provided some information as well. The quality of the seismic sections varies because of the seismogeological conditions (geological heterogeneity) and the acquisition parameters (the power and penetration of the signal). While some of the seismic sections show clear deep events that prove a real geological heterogeneity, others are noisy because of either the acquisition parameters (weak energy generation or insufficient penetration) or the relative homogeneity of crust (Mayer and Brown, 1985). A statistical analysis of reflectivity has been carried out, which resulted in the histograms of reflectivity densities and of the average length of

reflections (Wever et al., 1987). The first histogram shows the distribution of the reflectivity with depth; that is, the sum of the reflection lengths correlated in a 1 s window and divided by seismic line lengths (Figs. 10a and 1 la). The second histogram shows the average length of the reflections over the same time interval (Figs. lob and lib). Seismic sections, reflectivity histograms and amplitude analyses, together with other deep data (D.P. Radulescu et al., 1976; Cornea et al., 1981; F. Radulescu, 1981, 1988; Enescu et al., 1988, 1992) allowed separation of the main crustal domains bounded by interfaces or transition zones: the crystalline basement, Conrad (as a boundary between the upper and lower crust, often unclear on the seismic sections) and the Moho. Based on these data, some crustal models were compiled and added to Figs. 10 and 11. Although the limits of the main crustal domains could not always be rigorously established, these patterns give an image of the thickness of the main domains (sedimentary, upper and lower crust) in the areas surveyed. The specific references for each line (Fig. 1) are as follows: north Strehaia (F. Radulescu et al., 1984); west Craiova (F. Radulescu et al., 1977; Talo$ et al., 1979); northwest Pitesti (Raileanu et al., 1991); east Titu (F. Radulescu, 1979); north Urziceni (Raileanu et al., 1992); east Buzau and south Comanesti (Stanchevici, 1992); west Falticeni (Raileanu et al., 1991), Targu MureS (F. Radulescu and Raileanu, 1981); Valea lui Mihai (Raileanu et al., 1992); Beba Veche and Teremia Mare (Raileanu et al., 1991). Furthermore, Figs. 10 and 11 show values of the surface heat flow and temperatures at the 20 km depth for each study line (Demetrescu et al., 1991; Demetrescu and Andreescu, 1994).

4. Seismic

reflection

data

The large amount of deep seismic reflection data accumulated in different tectonic areas of the world allow demonstration of the peculiarities of crustal reflectivity and their grouping into specific patterns such as: seismic lamellae; bands of reflections; diffractions; reflectivity decreasing

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V. Ra’ileanu et al. / Tectonophysics

with depth; seismic duplex; ‘crocodiles’; and ramp and flat structures (Brown, 1987, 1991; Meissner, 1989; Sadowiak et al., 1991a,b). In addition, some features of the Moho (Brown, 1991) and subcrustal reflectivity have been noted too (Fuchs, 1986; Posgay et al., 1990). Some of the crustal peculiarities shown before have also been recognized on the deep seismic reflection profiles recorded in Romania. A significant variability in the crustal and Moho reflectivity is observed not only among the different tectonic units but also within the same unit, suggesting a relatively separate evolution of the crust in each sector. In the following, we shall present some of the significant seismic reflection lines. Most of the lines belong to the Moesian platform and Carpathian foredeep. The name of each line is assigned

239 (1994) 165-185

according to the nearest town and the lines are indicated by bold lines in Fig. 1. The geological identification given of the most prominent of the sedimentary seismic horizons on each seismic section is based on the original seismic sections and well data interpretations of the oil geologists. West Craioca line (C, Fig. 1): This is seated in the western part of the Moesian platform. Deep seismic soundings achieved in this area in the 1970s show a sedimentary cover of about 5 km, while the Conrad (less clear) and Moho (well pointed) are located at depths of about 15 and 30 km, respectively (F. Radulescu et al., 1977; F. Radulescu, 1979). The seismic section (Fig. 2) shows a reflective sedimentary cover with flat and near-horizontal reflectors. Three, more intensive, reflection bands at 0.8, 2.0 and 3.5 s TWT (on the southeastern edge) are generated by the strong

TWTkl

TWT Fig. 2. Seismic sectlon of the west Cralova

line (C, Fig. 1).

V: Ra’ileanu et al. / Tectonophysics 239 (1994) 165-185

acoustic impedance contrasts of the NeogeneMesozoic, Triassic-Palaeozoic and basal Palaeozoic interfaces. The upper crust is nearly transparent under the sedimentary cover (3.5-5.0 s TWT). The mid-crust (5-7 s TWT) is pointed out by short, subhori~onta~ or dipping events, some of them being diffractions. Down to 8 s TWT the density and length of the reflections increase and two bands with prominent and near-horizontal reflections can be seen over the 9-11 s TWT interval (Fig. lOa). The two bands express the crust-mantle transition zone proved by refraction studies CF. Radulescu, 1981). In the central part of the seismic section, and over the same time interval, some sideswipes cross the deep reflections from the northwest to the southeast. The northwestward prolongation of these diffractions disappear into a transparent area below about 6 s TWT. Some subcrustal reflections are also noted,

171

the most prominent appearing at 14 s TWT over a distance of N 3.5 km. Nor& Strehaiu line 6, Fig. 1): This is one of the deep seismic reflection lines recorded in a small area around Strehaia (F. Radulescu et al., 1984) which, although belonging to the Carpathian foredeep, has a Moesian piatform type basement (Sandulescu, 1984). Former deep seismic soundings in this area (F. Radulescu et al., 1977, 19841 brought some information on the deep structure, especially on the depths to the basement (7-9 km), Conrad (L-18 km) and Moho (32-33 km). The seismic section (Fig. 3) displays a reflective sedimentary cover over a O-4 s TWT interval. The Neogene deposits are thick and show some continuous reflections down to 1 s TWT. Two important geological boundaries: the SarmatianBurdigalian and Burdigalian-Mesozoic are

TWT(s)

TWT(s) Fig. 3. Seismic section

of the north Strehaia

line (S. Fig. 1).

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V. Ra’ileanu et al. / Tectonophysics

reached at 2.7 and 3.2 s TWT, respectively, in the northern part (- 2.5 km from the northern edge of the seismic section). The central part of the seismic section shows a more raised sector at the Sarmatian level, which can be correlated with the Strehaia-Vidin uplift (Mutihac, 1990). Beneath 4 s TWT the upper crystalline crust appears to be transparent. A few short, dipping events around 5-6 s TWT show a brittle medium in the northern part of the seismic section. The reflectivity of the lower crust appears to increase with depth down to 8 s TWT (Fig. lOal. While in the top of the lower crust there are weak, short and subhorizontal events, their length and strength increase towards its base. They gather in two bands of multi-cyclic reflections at 9.5 and 11.5 s TWT (Cornea et al., 1978). The two reflection bands dip gently to the north, showing a thickening of the crust towards the Carpathian Orogen.

239 (1994) 165-185

East Titu line (Ti, Fig. 1): This is located in the southern part of the Carpathian foredeep (with platform basement), near the Intramoesian fault. Previous deep seismic studies for this area indicate thicknesses of the sedimentary cover of 8-10 km (Barbu, 19801 and depths to Conrad and Moho of 18 km and 33-35 km, respectively (F. Radulescu, 1988; Enescu et al., 1992). Other reflection data show a decrease in the reflectivity with depth (Raileanu, 1990). In the upper part, the seismic section (Fig. 4) shows a good reflectivity within the sedimentary layers. A seismic marker (Sarmatian-Cretaceous boundary) appears at 2.2-2.4 s TWT. Other shorter and weaker reflections belonging to Mesozoic and Palaeozoic layers can be seen down to * 5 s TWT. The reflectivity histogram (Fig. 10a) shows a minimum value that can be correlated with the base of the sedimentary cover (8-9 km).

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I/: Ra’ileanu et al. / Tectonophysics 239 (1994) 165-185

The image of the crystalline crust differs in the east and the west. While in the eastern half horizontal events with a lateral correlation of > 1 km can be seen over a time interval of 6-12 s TWT, the density and length of the events decrease over the same interval in the western half. A decrease in the reflectivity down to 10 s TWT and a minimum reflective zone over the lo-12 s TWT interval are observed on the reflectivity histogram in Fig. 10a. Below 12 s TWT the reflectivity increases and a band of multi-cyclic reflections (- 13 s TWT), together with other reflections (- 14 s TWT) can be observed. The base of the crust should be at - 11 s TWT (in the eastern end of the section) and at - 34 km depth, for a mean crustal velocity of 6.2 km/s in the Moesian platform (Enescu et al., 1992). If the reflections at 13-14 s TWT are considered to be the base of the crust then the corresponding depths are 40-44 km. Such depths are not known for this area but only for the southern edge of the Carpathian orogen (- 50 km to the north). Consequently, these can only be subcrustal reflections.

Other subcrustal reflections can be noticed in the adjacent areas (Raileanu, 1990). North Urziceni and East Buza’u KJ and B, Fig. 1): These lines are placed southeast of the Carpathian Arc bend. Fig. 5 is the seismic section of the North Urziceni (U) line and Fig. 6 shows only a segment of the East Buzau (B) line. The basement of both areas belongs to the Moesian platform. The sedimentary cover increases in thickness from the south to the north and northwest; from - 8 km north of Urziceni (Raileanu et al., 1993) and - 13 km east of Buzau (Stanchevici, 1992) to - 18 km south of FocSani (Cornea et al., 1981). The base of the upper crust (Conrad) has been determined by refracted and wide-angle reflected waves (Cornea et al., 1981) and shows depths of 16-24 km (F. Radulescu, 1988). The lower crust shows less reflectivity than the upper crust and the Moho appears to be better marked by wide-angle than by near-vertical reflections (Cornea et al., 1981). The depth of the Moho is 35-40 km (F. Radulescu, 1988; Enescu et al., 1992).

NHW

.s

Fig. 5. Seismic section

of the north Urziceni

line (li, Fig. 1).

,_

SSE ‘” ._ _

V. R&am

174

et al. / Tectonophysics

8

16 TWT ts) Fig. 6. Seismic section

TWT(s) of the east BuzSu line (B, Fig. 1).

The two seismic sections display good reflectivity at least down to the Tertiary-Cretaceous boundary (- 3 s TWT and - 4.5 s TWT in the southeastern ends of the lines). The reflection coefficient reaches a value of 0.26 at this level on line U (Raileanu et al., 1993). The Tertiary cover thickens toward the north on line U, while on line B the same deposits appear to be quasi-

239 (1994) 165-185

horizontal. The Mesozoic and Palaeozoic layers generate rare and shorter reflections as well as some diffractions. Because the base of the sedimentary cover does not appear as a seismic marker, its position was established by correlation with the previous refraction data (F. Radulescu, 1981) and the reflectivity histograms (Fig. lOa), at 4-5 s TWT for line U (Raileanu et al., 1993) and at - 6.5 s TWT for line B (Stanchevici, 1992). The mid-crust is marked by a moderate increase in the reflectivity in the 6-8 s TWT interval on the North Urziceni line (Figs. 5 and 10a) and by a relative transparency on the East Buzau line. Both lines show a decrease in the reflectivity in the lower crust (Fig. lOa). Some diffractions cut the whole crust and one of them is very prominent in the central part of the North Urziceni line, having its apex in the 11-12 s TWT interval. Because its great curvature is uncommon for these depths, we consider this to originate from a shallower and laterally located diffractor. The base of the crust is poorly expressed by a slightly higher reflectivity in the 1l-13 s TWT on line U (see reflectivity histogram, Fig. lOa), while it can be seen as a reflection band in the 12.5-13 s TWT interval on line B (Fig. 6). These estimates of the Moho are supported by the previous seismic studies (Cornea et al., 1981; F. Radulescu, 1981, 1988; Enescu et al., 1992). A common feature of both lines is the subcrustal reflectivity, better expressed on line B, where a very clear band of reflections fills the 15-16 s TWT interval. In addition, some short reflection segments are noticed at - 14 s TWT on line B. West FGlticeni he (F, Fig. 11: This is located in the transition zone between the Carpathian foredeep and the orogen (the Moldavides). It crosses the Solca fault, which separates the East European (Moldavian) platform with Middle Proterozoic basement in the east from the post-Palaeozoic platform with Caledonian-Hercynian basement in the west. The latter unit is overthrust on the former and the Solca fault points out the front line of this thrust (Visarion et al., 1988). This fault also separates the Subcarpathian Nappes (Carpathian orogen), in the west, from the Moldavian platform to the east.

I/. Ra’ileanu et al. / Tectonophysics 239 (1994) 165-185

The magnetotelluric sounding in this area shows crustal thicknesses of 42-44 km to the west and 32-36 km to the east of the Solca fault

NW

SOLCA FAULT

175

(Visarion et al., 1988). A refraction profile between Bicaz and Roman shows thicknesses of - 10 km. - 18 km and - 42 km for the base-

,_2Km

I

SE-

‘0

10.0

TWT (s) Fig. 7. Seismic section

of the west FBlticeni

line (F, Fig. I).

K Rriileanu et al. / Tectonophysics 239 (1994) 165-185

176

ment surface, Conrad and Moho, respectively, in the Piatra-Neamt area (Pompilian et al., 1990). A seismic geotraverse 200 km north of line F crosses the Carpathians across the Ukraine and Hungary. Some very interesting deep data have been derived from this profile (Sollogub et al., 1988). The pre-Carpathian and Carpathian areas show a sedimentary cover lo-12 km thick, with a maximum value of 23 km. Two transition zones can also be observed; the first, from the upper to the lower crust, at depths between 18 and 33 km (with velocities of 6.8-7.0 km/s) and the second, between the crust and the upper mantle, at depths

from 33-39 km to 54-60 km (with velocities of 7.5-7.6 km/s). Around line F the sedimentary cover reaches a thickness of 9-12 km in the area of maximum subsidence, westward of the Solca fault (Visarion et al., 1988), and a thickness of 6 km to the east of the Solca fault (Barbu, 1980). The seismic section (Fig. 7) shows a strong reflective horizon in the upper part ( - 1.2 s TWT at the south-southeast end), known as the Badenian anhydrite horizon. It is a seismic marker for the Moldavian platform. Its interruptions and gradual deepening illustrate the stepwise subsi-

TWT (sl

TWT (s 1 Fig. 8. Seismic section of the east TCrgu Mure? line (Tg, Fig. 1).

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V. Ra’ileanu et al. / Tectonophysics 239 (1994) 165-185

dence of the platform toward the orogen (at least up to the Solca fault). Under this level, the Mesozoic and Palaeozoic sediments show less pronounced reflectivity. A coherent horizon, which is assumed to be the base of the sedimentary cover, appears at 2.8 s TWT (- 6.5 km depth) in the south-southeast zone. It sinks slightly to the northwest and can be followed up to the Solca fault. The crustal image differs between the west and eastern sides of the Solca fault. While coherent energy is poorly expressed to the west, there are several zones with strong coherent energy to the east, mostly in the lower crust. The dipping events within the basement (3-4 s TWT), in the central part of the seismic section, suggest a thrust structure composed of nappes. Other dipping events with steep slopes in the western half (3-4 s TWT) can be interpreted as diffractions generated by the Solca fault. Even if the upper crust is relatively transparent it still contains a few diffractions. The mid-crust (6-9 s TWT) shows an increasing reflectivity with depth, which is accompanied by some dipping events. A prominent reflectivity, with dense and multi-cyclic ,lKm

lamellae like those delineated by Meissner (1989) and other researchers (Klemperer, 1987; Brown, 1991; Sadowiak et al., 1991b) can be seen along the entire lower crust. In the central part, a wave group dipping southeastward overlaps the seismic lamellae. This group seems to be of a diffractive nature and can be generated by the prolongation of the Solca fault at depth. The base of the crust cannot be exactly established because the layered structure of the lower crust extends down to 15 s TWT. Nevertheless, the reflectivity histogram (Fig. lla) shows a decrease in the reflection density around 13 s TWT ( - 42 km), which is supposed to be the base of crust. The subcrustal domain presents weak reflections below 13 s TWT. T&p Mure~ line (Tg, Fig. 1): This is placed in the central area of the Transylvanian depression, eastward of Targu MureS and northward of the Transcarpathian geotraverse XI. Refraction data collected along this geotraverse have revealed thinnings both of the whole crust, from a depth of 32-33 km east of Targu Mures_ (- 35-40 km distance) to a depth of 28 km to the west (- 25 km distance), and of the upper crust, from 13-14 ,

TWT(s)

TWT(s I Fig. 9. Seismic section

of the Teremia

Mare line (Tr, Fig. 1).

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K Ra’ileanu et al. / Tectonophysics 239 (1994) 165-185

km east of Targu MureS to 10 km to the west CF. Radulescu, 1981). Fig. 8 displays only the eastern half of the seismic section because its quality is better. The seismic section shows a Neogene cover with several reflections, which have a good coherence down to 1.0 s TWT. Below, the coherence decreases and two more prominent reflections are seen at - 1.6 and 2.0 s TWT. They belong to the Badenian complex (known as the Dej tuffl and the base of the Neogene cover (- 3.5 km depth in accordance with the morphostructural map at the basement level by Ionescu et al., 1986). The upper crust (2-4 s TWT) appears to be void of reflections but it is crossed by some diffractions. Some short, subhorizontal events can be noticed in the central part of the middle crust (4-6 s TWT). While the top of the lower crust (6-8 s TWT) shows some diffractions generated at the section ends, its base appears to be more reflective, with dense, shorter, subhorizontal events. The Moho is not clearly delineated, but it would have to be around 11 s TWT, based on the refraction data CF. Radulescu, 19811 as well as the histogram of reflectivity (Fig. lla). On the western half of the seismic section a few short and subhorizontal events are observed over the 9.5-11.0 s TWT interval (Fig. lla). This part of the line covers the uplifted area of the lower crust (Ionescu et al., 1986). Teremia Mare line (Tr, Fig. 1): This is located in the westernmost Romanian area of the Pannonian depression. It lies on a small tectonic block (with deepened basement) that is delimited by the adjacent raised blocks by basement fractures with a north to south orientation (Visarion and Sandulescu, 1979). The seismic section (Fig. 9) displays a very reflective Neogene cover down to - 3 s TWT. Its structure looks like a syncline with the apex in the central part. Borehole and seismic data show a thickness of the Neogene cover of - 5 km in the central part of the seismic section (Ionescu, 19811. Under the Neogene cover, the thin Mesozoic layer (- 0.3 s TWT) belonging to the basement has only poor coherent energy. The crystalline crust shows different patterns from the west to the east. In the west almost the whole crust seems to be reflective, while in the eastern part only a few reflections can be seen. The

western half shows a group of horizontal reflections with a good coherence in the 4.5-5.5 s TWT interval. After an interval of - 1 s TWT devoid of reflections, the density and strength of reflections increase in the interval 6.5-10.0 s TWT. Two bands of reflections, at - 8 s and 9-10 s TWT, on the western edge can be followed to the central part. This image of the seismic section is similar to the southern end of the Pannonian Geotraverse (Posgay et al., 1990), a reflection profile that runs across Hungary up to the Romanian boundary, with a northwest to southeast orientation. It ends to the northwest of the city of Arad. Both the Teremia Mare section and the Pannonian Geotraverse (in the southeasternmost end) show a group of reflections at - 5 s TWT in the mid-crust and a reflective lower crust down to 10 s TWT. Another Hungarian seismic profile, Geotraverse II, that runs 40 km to the west of the Teremia Mare line shows depths of 25-28 km for the base of the crust and depths of 18-20 km for a midcrustal horizon (K2), the latter having velocities of 6.6-6.9 km/s (Posgay et al., 19881. On the same profile, the horizon Kl can correspond to the 4.5-5.0 s TWT reflective interval of line Tr and horizon K2 can be identified with the top of the lower crust at - 6.5 s TWT (14-15 km). A neighbouring seismic reflection line, Beba Veche, which is seated on a raised tectonic block (Figs. 1 and 11) shows a group of reflections at the base of the crust at - 9.5 s TWT (RZleanu et al., 1991). Another reflection line (Valea lui Mihai line, in the northern part of the Pannonian depression, Figs. 1 and 11) shows the base of the crust at 9.3-9.4 s TWT (Raileanu et al., 1992).

5. Results and conclusions In the interpretation of the deep crustal structure we have used geological and geophysical data, the character of crustal reflectivity, comparison with other areas and, to a lesser extent, the deep velocity data. The lines are located only in the sedimentary areas of platforms and depressions. The sedimentary cover shows a reflectivity which decreases

K Ra’ileanu et al. / Tectonophysics 239 (1994) 165-185

with depth, the more contrasting level being the Neogene deposit, where the presence of many reflectors suggests an undeformed structure. Some seismic markers are known in the lower part of the Neogene (Badenian) package. They are: the Badenian tuff, extending over the Moldavian platform (Fig. 71, with a strong reflection coefficient of 0.16-0.225 (Ovessea and Iordan, 19901, and the Dej tuff, encountered in the Transylvanian depression. The transition from the Tertiary to the Mesozoic deposits (an erosion relief) is very reflective, with a reflection coefficient of 0.13-0.36 (Ionescu, 1979). Strong reflections are also encountered in the Pannonian depression at the level of the pre-Pliocene surface (Miocene or Mesozoic) (Ionescu, 1979). Under these levels, the reflectivity decreases and a suite of reflections can be seen at the erosion surface levels or within the Mesozoic and Palaeozoic stacks of the Moesian platform (Ionescu, 1979). None of the seismic sections presented shows the basement surface as a seismic marker. The best seismic data on the basement surface were obtained in the areas where the sedimentary cover has a thickness of less than 2-3 km (Ionescu, 19791, mostly in the Pannonian depression. The base of the sedimentary cover is often evidenced by short groups of reflections with variable correlation, as is shown on Fig. 2 at - 3.5 s, Fig. 7 at - 2.8 s, and Fig. 8 at - 2 s. The transition from the sedimentary cover to the basement is marked by a sharp decrease in the reflectivity (Figs. 10 and 11 for lines S, Ti, U and B). The upper crystalline crust appears to be transparent, with some diffractions (lines S, B, Tg and Tr) or with rare, short and subhorizontal events (lines C, Ti and U). A possible structure in the basement, such as thrust nappes, is perhaps visible on line F in its central part at 3-4 s and on the seismic reflection line northwest of Pitesti in the foredeep depression (Raileanu et al., 1991). The overall image of the upper crust is that it is either rather homogeneous relative to the seismic wavelengths used and/or it has a brittle structure which can generate diffractions. The brittle structure of some lines is also proved by the lower temperature values at 20 km (Demetrescu and

179

Andreescu, 1994): 200°C for lines Ti and U, and 300°C for lines C, S and B, based on the correlation between rigidity, viscosity and temperature of Meissner and Kusznir (1987) and Wever et al. (1987). The mid-crust seems to be relatively transparent (line B) but short and subhorizontal events can often be seen (lines C, Ti, U, F, Tg and Tr). Such events could belong to the mylonite zones created by mid-crustal decoupling (Valasek et al., 1989). The lower crust shows a different crustal signature for the various tectonic units: from a transparent crust or a decreasing reflectivity with depth (often crossed by diffractions), as in the lines Ti, U and B, to a crust where either the reflectivity increases toward its base, as in lines C, S and Tg, or becomes very rich in reflections, illustrating the ‘classical’ seismic layering (mostly on line F and, to a lesser extent, on line Tr). The character of the Moho fits the patterns delineated by Brown (1991); beginning with its absence or poor appearance (lines Ti and U) to the clear image of a transitional crust/mantle zone (lines C and S) and a well marked reflection band (line B). A special case is encountered on lines F, Tg and Tr. Here, the Moho signal is part of the continuous reflections from the lower crust to the upper mantle. The Moho position is proved by refraction data (the Bicaz-Roman line for line F, the Geotraverse XI for line Tg, and the Hungarian Geotraverse II for line Tr). A consistent subcrustal reflectivity appears on some of the lines recorded with travel times greater than 14 s TWT. Some clearer reflections can be observed on lines C ( - 14 s), Ti (- 13 and 14.5 s>, U (- 14 s), B (15-16 s> and F (> 13 s). In the following we shall try to point out the reflectivity characteristics of the tectonic areas that have been investigated by deep seismic reflection profiling. Lines C, S, Ti, U and B show the seismic crustal structure on the northern flank of the Moesian platform. This flank has been affected by the Alpine orogeny (Mutihac, 1990). Thus, in our opinion, the mid-crustal zones, marked by short and subhorizontal events, are crustal decoupling zones. The accommodation of the different

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182

K Ra’ileanu et al. / Tectonophysics 239 (1994) 165-185

horizontal movements of the upper and lower crust may have taken place at this level (Valasek et al., 1989). The same level could also correspond to the brittle-ductile transition zone (Meissner and Kusznir, 1987). The lower crust shows specific features in the eastern and western parts of the Moesian platform. In the west, the top of the lower crust appears to be relatively transparent and/or is crossed by some diffractions, while the base of the crust contains two multi-cyclic reflection bands over the 1.5-2.0 s TWT interval, designating a crust-mantle transition zone like other areas of Europe and the USA (Berry and Mair, 1977; Prodehl, 1977; Klemperer et al., 1986). Sadowiak et al. (1991b) show that such a reflectivity type could be specific to the Palaeozoic extensional areas. Klemperer et al. (1986) and Brown (1987, 1991) consider that “Moho reflectivity may be due to magmatic underplating associated with rifting” or that the “Moho may have served as an intra-lithospheric detachment”. A correlation between the Moho reflectivity and its present temperatures (> 300°C) shows that both hypotheses are possible. The eastern part of the Moesian platform has a lower crust with poor reflections and/or diffractions and a Moho which has either an unclear expression (lines Ti and U> or shows prominent reflection bands (line B). The subcrustal reflectivity seems to be a common feature of the western and northern parts of the Moesian platform. Among the possible causes we can mention isostatic adjustment, which moved the Moho from an older position (13-16 s) to the present one (9-13 s) (Klemperer et al., 1986; Meissner and Wever, 1986) or a reduced viscosity interval, where the horizontal shear stress can produce laminated zones (Fuchs, 1986; Meissner and Kusznir, 1987). A third hypothesis could be a larger crust-mantle transition zone in the 9 (13)13 (16) s TWT interval. Line F has a special position, since it is placed in the transition area between the foreland and orogen and crosses the Solca fault. Two convergent reflections to the south-southeast can be seen in the central part of the seismic section and in the 3-4 s TWT interval. They suggest an underthrust at basement level. The mid-crust (6-9

s) shows dense and short reflections which could be a mid-crustal decoupling zone. The lower crust presents a reflectivity which may be caused by seismic lamellae, as in other areas (Meissner and Wever, 1986; Meissner, 1989; Brown, 1987, 1991; Sadowiak et al., 1991b). The lower crust appears to be ductile mostly in young mountain areas and in young extensional zones (Meissner et al., 1991). The western margin of the Moldavian platform seems to have suffered the tectonothermal influences of the Alpine orogeny. This is evidenced by the seismic layering, an effect of the ductile state of the lower crust (Meissner and Kusznir, 1987; Wever et al., 1987). The existing temperatures of 300-400°C at a depth of 20 km (Demetrescu and Andreescu, 1994) prove that the conditions necessary for their preservation have been maintained up to the present (Klemperer, 1987). Line Tg is located in the hinterland of the Carpathians. The short and subhorizontal events at the mid-crustal level (4-6 s TWT) can perhaps be related to a possible decoupling zone and a certain reflectivity in the lower crust, overprinted by some diffractions. The various reflectivity patterns along the seismic line, from a transparent crust disturbed by diffractions in the west (missing in Fig. 8) to a crust having a certain reflectivity in the eastern part, prove that line Tg crosses two crustal sectors with different seismic signatures. Line Tr (in the Pannonian depression) shows two zones of high reflectivity in the western half in the 4.5-5.5 s and 6.5-10.0 s TWT intervals. Based on the surface heat flow ( * 70 mWm_‘) and temperatures higher than 600°C at 20 km depth (- 8 s), it is possible that within these two depth intervals the viscosity may be < lo*” Poise, the threshold for the ductile behaviour of the crustal rocks and, thus, for the formation of layering (Meissner and Kusznir, 1987). This fact could provide evidence for the extensional origin of the Pannonian depression (Polonic, 1985). The reflectivity histograms illustrate the distribution of crustal reflectivity with depth (Figs. 10a and lla). They show the different reflectivity patterns from a continuous decrease in the reflec tivity toward the base of the crust and an increase of the reflectivity in the lower crust (lines S and

I/. Rn’ileanu et al. / Tectonophysics

F). The lengths of the correlated events were relatively short, with a mean value of l-2 km (Figs. 10b and lib). The crustal structural patterns show the relations between the main lithospheric domains: sedimentary cover, upper crust, lower crust and upper mantle (Figs. 1Oc and 11~).

Acknowledgements

This research was supported by the Prospectiuni S.A. company, which collected and processed the deep seismic reflection lines. This is also a good opportunity for the authors to thank Dr. V. Varodin and Dr. 0. Dicea, as well as their collaborators, for their most helpful work. Special thanks are due to Dr. Th. Wever, Dr. F. Horvath and an anonymous reviewer for the remarks and suggestions which helped us to improve the paper.

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