POLONAISE'97 — Seismic and gravimetric modelling of the crustal structure in the polish basin

Pips. C/W,I. Earth (A/, Vol. 25, No 4, pp. 355-363, 2000 ‘I?2000 Elsevier Science Ltd. All rights reserved I464- I895’OOi$ - see front matter

Pergamon

PII: S1464-1895(00)00057-0

POLONAISE’97 the Polish Basin

- Seismic and Gravimetric Modelling of the Crustal Structure in

Lech Krysiriski, Marek Grad and POLONAISE Working Group* *(W. Czuba, E. Gaczytiski, M. Grad, A. Guterch, T. Janik, L. Krysinski, P Sroda (Poland), S. L. Jensen, H. Thybo (Denmark), U. Luosto, J. Yliniemi (Finland), G. Motuza, V Nasedkin (Lithuania), G. R. Keller, K. Miller (USA)) University of Warsaw, Institute of Geophysics ul. Pasteura 7, 02-093 Warsaw, Poland [email protected] Received 28 May 1999; accepted 30 September

1999

Abstract. The main aim of this paper is seismic and gravimetric modelling of the crustal structure in the Polish Basin. Preliminary results of a large seismic experiment POLONAISE’97 which was conducted during May of 1997 and targeted the deep structure of the Trans-European Suture Zone in Poland are presented. Apart of five POLONAISE’97 profiles (Pl+PS) two other deep seismic sounding profiles (LT-7 and TTZ) passing the Polish Basin are discussed. Two-dimensional P-wave velocity models of the crust for these high resolution profiles of a total length of about 3000 km are presented. The actual resolution of the crustal structure recognition gives a new possibility for the study of the gravity field’s morphology. The gravity modelling along the profiles was undertaken in a general form, as the study of the mutual accordance between the geometry of seismic boundaries and gravity anomalies as a mathematical relation and it given suggestion for reformulation of the fit problem. The first results obtained using the new technique and interpretation are presented in the case of the two-dimensional density modelling of the layers for single profiles of the network and estimations of the supracrustal gravity compensation. The analysis of the residue (r.m.s.) and its gradient is proposed in this new technique instead of analysis of density values, which determination is unstable. The supracrustal gravity response was modelled as a field of equivalent masses on the level situated in the lower lithosphere. 0 2000 Elsevier Science Ltd. All rights reserved ___ _~__~

1 Introduction:

sedimentary rocks during a phase of’ extension after the Variscan orogeny. The Polish Basin forms the easternmost part of the Permian Central European Basins. It is situated on the contact zone between and on the edges of the Palaeozoic and Precambrian platforms. From the south-west it is bordered by the Bohemian Massif (Dadlez. 1989: Ziegler, 1990; Pharaoh et al.. 1997). The axis of the basin, called the Mid Polish Trough (MPT). parallels the edge of the East European Craton. The MPT (Fig. I) is a part of the Trans European Suture Zone (TESZ), a first order geotectonic unit, stretching from the British Islands, through Poland and on to the Black Sea region. In the area of Poland this axial zone has its own crustal structure of intermediary type, characteristic for the shelf of a passive continental margin. During the late Cretaceous and Tertiary, the Alpine orogeny caused an uplift that passes through the Permian Basin and extends to the NW into Denmark and Sweden (Berthelsen, 1992a,b). The Holy Cross Mountains in southcentral Poland include exposures of Palaeozoic structures and are the best known evidence of this uplift. The axial area of the basin has a long history of systematic subsidence (probably thermal subsidence of the shelf) from the Early Palaeozoic (and probably from the Late Precambrian). The tectonic nature of the subsidence during the PermianMesozoic episode is interpreted in a slightly different way by Kutek (1997), as a basin formed by an asymmetrical fault-bounded rift structure and a superimposed Upper Cretaceous sag basin. Karnkowski (1999) interprets the Polish Basin as an asymmetric extending rift of Wemicke type (Wemicke, 1981, 1985) with associated volcanism of 290-270 My (Early / Late Permian).

___._~

the area of investigation

The investigated area of Poland (Fig. 1) is located on the border zone between Precambrian crustal terranes of the East European Craton (EEC) and the younger, Phanerozoic terranes in the south-west (Berthelsen, 1992a,b, 1998). Much of Poland and the northern Germany is covered by a deep (>I0 km) basin, usually called the Permian (PermianMesozoic). which was filled with Permian and Mesozoic

2 Geophysical

investigations

of the study area

The TESZ is clearly visible as a tectonic unit of individual crustal structure (e.g., Guterch et al., 1986), outlined by the morphology of the magnetic and gravity fields (Tomquist, 355

L. Krysinski et al.: POLONAISE’97

356

The area of TESZ is characterised by a lack of magnetic anomalies. On the Precambrian Platform strong and a large scale (of the order of 100 km) oval and belt shape magnetic anomalies correspond to granitoid massifs, metamorphic belts and metamorphic-magmatic complexes of the crystalline basement (Ryka, 1984). The area of the most south-western Poland, Palaeozoic Platform, shows the pattern of the magnetic field characterised by numerous local, small scale (of the order of 10 km) magnetic anomalies (Karaczun et al., 1978; Krolikowski and Petecki, 995; Krolikowski and Wybraniec, 1996). 15 E

I

L120 ’

160

200

Ii

21 E

240

. 5

Fig. 1. The major tectonic units in Poland (compiled from Bogdanov and Khain, 1981; Guterch et al., 1996a.b; Guterch and Grad, 1996). 1. 2, 3 boundaries of the major tectonic units; 4 - Mid Polish Trough; 5 anomalous zone in crustal structure in the marginal zone of the Precambrian Platform; 6 - selected tectonic lines; 7 - deep seismic sounding profile LT-7. Abbreviations: SB = Upper Silesian Coal Basin: SM = Sowie Mtns; SwM = Holy Cross Mtns.

1908; Jankowski, 1967; Grabowska and Raczyuska, I99 1; Grabowska et al., 1991; Krolikowski and Petecki, 1995; Krolikowski and Wybraniec, 1996) as well as the heat-flow data (e.g., Majorowicz and Plewa, 1979; Cermak et al., 1989), in relation to the neighbouring lithospheric blocks. In general, the East European platform in NE Poland is characterised by a 0.5-5 km thick sedimentary cover, 42-47 km thick crust, “cold” lithosphere with relatively low heat flow < 40 mW/m2, and an age of about 2000 to 800 Ma. The Palaeozoic platform in SW Poland is characterised by a sedimentary cover up to about 8 km of thickness. thinner, 28-34 km thick crust, “hot” lithosphere with much higher heat flow of 40-70 mW/m* and an age of about 450-290 Ma (Majorowicz and Plewa, 1979; Guterch et al., 1986; Cermak et al., 1989; Guterch and Grad, 1996; Pharaoh et al., 1997). The deep crustal structure of the MPT is characterised by the crustal thickening beneath the central part of the basin (Guterch et al., 1986) as well as by Moho shallowing beneath the NW part of the basin (Guterch et al., 1991, 1992, 1994).

30

90mGal

lOOkm_

Fig. 2. Location of the POLONAISE’97 profiles Pl+P5, and LT-7 and TTZ profiles plotted on the gravity Bouguer anomaly map of Poland (Wybraniec, 1999). Colour pseudo-relief illuminated from the north-east. Colours correspond to negative anomalies (pink and blue), near normal (green) and positive anomalies (yellow and brown).

The morphology of the gravity field coincides with tectonic structures and has clear lineation in the SE-NW direction (Fig. 2). The basic feature of Bouguer anomalies of the Polish Basin is the presence of an extensive depression down to -60 mGal; on this background, an increase of up to +15 mGa1 is noted corresponding to the Mid-Polish Anticlinorium. The Fore-Sudetic monocline in the south-west and the East European Platform in the northeast are characterised by positive gravimetric anomalies of up to +20 mGa1 and +I0 mGa1, respectively (Krolikowski and Petecki, 1995).

3 Seismic models The main aim of this paper is seismic and gravimetric modelling of the crustal structure in the Polish Basin. The very large seismic experiment POLONAISE’97 conducted in May 1997 targeted the deep structure of the TESZ in north-western Poland (Guterch et al., 1997, 1998) and its preliminary results are presented in the special issue of Tectonophysics (Guterch et al., 1999; Jensen et al., 1999;

L. Krysiriski et ul.: POLONAISE’97

357

Sediments Comp. Sed. + ++ “V

Granitoid

”

Basalt Gabbro Peridotite

-100

-200

0

300

P3 profile

PI orofile

0

100

200

0

300

200

100

300

P5 profile

TTZ profile

0

s g

v

V

v

P

V

25

$ 50 0

0

100

100

P4 profile

0

100

200

300

400

500

600

700

Model scale [km] Fig. 3. Collection of two-dimensional

crustal models along POLONAISE’97

profiles PI 1P3. P4. P5 and LT-7 and TT% profiles in Poland.

800

358

L. Krysiliski et al.: POLONAISE’97

Sroda and POLONAISE Working Group, 1999; WildePiorko et al., 1999). Apart of POLONAISE’97 profiles two other deep seismic sounding profiles LT-7 and TTZ crossing the Polish Basin are discussed (Guterch et al., 1994; Grad et al., 1999).

velocities of P waves, the successive layers in the models were classified to a certain type of rock: “sediments” (Cainozoic and Mesozoic), “compact sediments” (metasediments, Palaeozoic and older), “granitoid” (upper crust), “basalt” (middle crust), “gabbro” (lower crust) and “peridotite” (uppermost mantle) type. All cross-sections clearly show differentiation of the seismic structure, strong asymmetry of the Polish Basin both in the basement and in the Moho shape, difference in crustal layering and thickness, as well as difference in the sub-Moho velocities beneath both platforms.

4 Aims of the gravimetric f

w/

b

50

3 : hc n M ’ Level of formal compensative sources $ 702 I- 7” ______-____--_____---__ ‘- Y” x0e w~--r-----r,-Tr~~T~-1-7-- ~rr7,i -77-1 ry . -~‘9” -240.xX,-,m -120-80 41 ” m 80 12016”20024”28” 120 Distance x along the profile @I] Fig. 4. Geometry of the system of the formal compensative sources (situated on the level of depth z) for a chosen seismic layer (Ag is the Bouguer anomaly).

The crustal structure of the East European Platform is represented by profiles P3, P5 and north-eastern parts of profiles P4 and LT-7. All models of the crust for this area are characterised by nearly horizontal uniform structure. The crystalline crust consists of three parts: upper, middle and lower with P-wave velocities of 6.1-6.4, 6.5-6.7 and 7.0-7.2 km/s, respectively. The crystalline basement lies at the depth OS-5 km and strongly declines into SW direction, almost perpendicularly to the edge of the craton. In the north-western part of the profile P5 a body with high seismic velocities of about 6.6 km/s was found at depth range 2-10 km, which coincides with the rapakivi-like and anorthosite Mazurian complex, well known from geology. The depth of the Moho boundary ranges corn 39-45 km in the north-eastern Poland, reaching 50 km beneath Lithuania. The sub-Moho P-wave velocity is 8.05-g. 1 km/s. The crustal structure of the Polish Basin and Palaeozoic Platform is represented by profile P 1, south-western parts of profiles P2, P4 and LT-7, as well as profile TTZ. Profiles Pl and TTZ run on the area of Polish Basin parallel to the general lineament; profile Pl is situated in the zone of direct Variscan influences. Profiles P4, P2 and LT-7 cross the above mentioned profiles and the edge of the East European craton almost perpendicularly. In general, the P-wave velocities of the upper crust in the Polish Basin are low (< 6.1 - 6.2 km/s) down to 20 km of depth. It can be interpreted as an evidence for their metasedimentary type with a low grade metamorphism only. The lower crust has a P-wave velocity of 6.5 - 6.8 - 7.3 km/s, a high velocity gradient, and strong, ringing reflectivity. The velocity of the sub-Moho uppermost mantle is high (> 8.2 - 8.3 km/s). A gallery of crustal cross-sections is presented in Fig. 3. Using the geometry of seismic boundaries and seismic

modelling and the method

The analysed network consists of seismic models Tom 7 profiles: 5 profiles of the POLONAISE’97 experiment (Pl+PS) and 2 profiles which were modelled earlier (LT-7 and TTZ). The high resolution of the velocity field cross sections determined in seismic models is an inspiration for study of the mutual relation between the geometry of seismic boundaries and the gravity anomalies shape. Thus, the new formulation of the fit problem arisen as a result of this study. The detailed problems were formulated during the study. Actually these are: ?? mutual accordance, especially the adequacy of the assumption of 2-D medium’s structure; ?? investigations of the isostatic conditions; ?? possibilities of estimations of the density values or contrasts. At first the fundamental category of models, namely 2-D models for single profiles of the network was considered. The consequent formulation of the problem as one, universal, objective calculation procedure applied for each of the seismic model is proposed. The geometry of n,-1 layers (defined by n, seismic boundaries) is exactly taken from the seismic models without any modifications. The space-constant densities p, are fitted for each of the layers in the frames of the assumed limits: A&, I=f,0 &;g,,,

,=,

(1)

where J; = dg(x,) is the observed Bouguer anomaly in the j-th point xI along the profile; p1 is a mass density attributed to the i-th layer (i = 1 + ~~-1);g,,j is a sum: g,., = g;y

(2)

+ ST

of the gravity field of the i-th layer of density 1 g/cm3:

and the compensative field (the field of antiprojection of the density distribution of the layer onto compensation level of depth z), = +2G Id< ajd< g ‘C”mp .i 4

-

(4)

L. Ktysiriski

rr trl.: POLONAISE’97

in the j-th point at distance x, along the profile having topography -~ilr,~(here -:,““’ was assumed as equal to 100 m). Here x, denotes characteristic function of the i-th layer: JI: where (<,1’, is inside the i - th layer x,(5.4’) = 1 . 0, where (<,n is outside the i - th layer

(5)

For i = n, the tield g,,, is a constant reference Limitations of density values for each layer:

level.

p, E[p:Un,p:“a\] (6) were taken from drilling cores as a pairs of values. different for each of the shallow sedimentary layers (generally of values I .8 + 2.9 g/cm’ and corresponding P-wave velocity values I .8 + 6.0 km/s: Grabowska et al.. 1998; Fajklewicz, 1972) and by help of Nafe-Drake relation (Fowler, 1995; Plewa and Plewa, 1992) for deeper layers classified in the way given by Table I. Also limitation for the density contrasts can be simply introduced to the extended numerical procedure, but it was not done here for clarity of the method presentation.

Table

I. Velocity

and density classifcation

ofthe

petrological

types.

The following concept for the study of isostasy is proposed. The depth z of the level of formal compensative sources (Fig. 4) was introduced as the additional parameter (substitutive isostatic compensation). For each layer the mass contained in the thin column was also put on the I level with opposite sign (antiprojection; Fig. 4. eq. 4) and it is important that the ‘tit problem (eq. 1) continues to be linear in densities p, (the infinite value of the depth z means the model without compensation). In other words, the equivalent model of the compensating masses in a depth 2 is used to account for the contributions to the surface gravity signal, which is generated in parts of the subsurface not covered by the seismic information. This way we have formally an exact isostatical compensation in each thin vertical column, what can be treated as a definition of the isostatic equilibrium. Such an exact equilibrium is not realistic but practically this way gives a good representation of the smooth field origin from supracrustal sources ordered generally by isostatic principles in horizontal length scales of order 100 km. In calculation practice the value of z (estimated as being about 100 km) is a space-scale of horizontal smoothing in transformation (eq. 4) of the mass density of the crust (eq. 9) into the compensative field (eq. 10) and thus, this estimation of the compensative field should not differ much from the real subcrustal response observed on the Earth’s surface. The better model can be given for example by formal compensation on a surface with varying depth along the profile and representing the layer in the lower lithosphere, comprising the dominant

359

density stratification or vertical density contrasts, but in the preliminary studies this variation would introduce additional unknown parameters. Such a better model represents practically a general situation. when rather only one such a layer of dominant density stratification can be expected. The parameter z (or function Z(X)) is treated here phenomenologically and this model can simulate successfully sharp bottom of the lithosphere as well as - the more realistic hypothesis - continuous stratification of density in the uppermost mantle, thanks to the mentioned smoothing. The basic task is to find the densities for each of layers in the frame of assumed upper and lower limits, giving the minimal residue R of the fit: (7) where IV is the number of measuring points (uniformly distributed along profile). The minimal value of the residue is also called r.m.s. (root mean squares) of the residuals, but the residue is considered here also as a function of the depth I and the densities p, (i = I . .... n,). In geometrical language the solution is a local minimum of R inside the cubicoid of densities limitations in n, dimensional space which is also a connected, convex set and unique absolute minimum in the cubicoid. This minimum is generally a simplex set, but usually in practice it is one point on the border of the cubicoid. The determination of densities (or their contrasts) is strongly unstable, because usually most of the eigen values of the matrix of the fit problem has low values and consequently most of the densities have values fitted at the assumed limits. Thus the densities values are not an object of a successive interpretations and they have status of formal parameters only. When the density p, was fitted at the boundary value p,“‘” or p,“““. then the value of the respective i-th component ?Ri?p, of the residue gradient is of non zero value (positive or negative, respectively) and this value is significant for the fit’s analysis. Usually some parts of the seismic cross section are not sufficiently covered by the seismic rays. The high absolute value of the 3Rlap, can mean inadequacy of the assumption of the layer homogeneity or inadequate shape or position of the layer’s boundary in the seismic model. or inadequacy of 2-D medium structure assumption. Generally, the analysis of the minimal value of the residue (r.m.s. residuals). the values of the components of the residue gradient c?RiGp,, the total value of the residue gradient at the minimal point and their dependence on z and on the density limitations seems to be more significant results than the fitted values of the densities or densities contrasts.

5 Results

The dependence of the fit’s minimal residue on the depth of the level of formal compensative sources (Fig. 5) shows the general difference between the two classes of profiles. The

360

L. Krysiliski et al.: POLONAISE’97

good fits (small value of the residue R) were obtained for profiles going nearly perpendicular to the Precambrian craton’s edge (profile LT-7) and for profiles distant to this edge (profiles Pl and P5). Much worse fits (large values of the residue R) were obtained for profiles running parallel and close to the edge of the craton (profiles P3 and TTZ) and for profiles of unfinished seismic processing (profiles P2 and P4). The difference in the minimal residue value defines a quantitative measure of the adequacy of the assumption of 2-D medium’s structure or mutual seismicgravity accordance with critical level of residue (for discrimination) about ‘a value of 10 mGa1. The good fit (R < 10 mGa1) means that the general shape of the gravity anomalies along the profile can be explained by the geometry of the layers in the seismic model in the frame of the chosen category of the models. The bad fit (R > 10 mGa1) means that there is no hope for such a modelling. The presented example confirms quantitatively known qualitative rule as to the adequate localisation of the 2-D profile for the purpose of the gravity study.

Profile P3 can be an example of the bad
Profile

P3

z=75km

b

25 .

??

P3

P2 P4 TTZ

LT-7 P5 PI

0

-,--‘-?--

7-----rT~‘~

__

.__7. .__ -

0

50 100 150 200 250 Depth z of the level of formal compensative sources [km] 50

Fig. 5. The dependence of the minimal residue value (here R = R,,, ) of the tit on the depth z of the level of formal compensative sources for each of the profiles. Circles are the results of the calculation and lines are polynomial interpolation only.

Some curves (Fig. 5) have minima suggesting the adequate value of the depth z of the level of formal compensative sources between 40 km and 120 km, but of about 100 km in the case of the successfil fit for the profile LT-7. Such shallow (as for the continental lithosphere) supracrustal position corresponds to the genetic concept of the lower lithosphere as being created by diffusive cooling and solidification of the sub-lithospheric matter of the asthenosphere. Then the field sources associated with different degree of cooling of the lower lithosphere and with supracrustal isostatical (and nonisostatical) response related to the vertical dislocations of segments are located mostly in the upper part of the lower lithosphere.

-c-~l_.

0

VT,

SO

/

100

I

IS0

4

m-mi, 200

2.50

300

Distance x along the profile [km] Fig. 6. Gravimetric modelling for the profile P3: a) anomalies of the crustal load (A.c.1.) and surface field from formal compensative sources (C.f.); b) observed Bouguer anomaly (continuous line) and fits (dashed lines) for different values of the depth z of the level of formal compensative sources (75, 150, 00km); c) geometry of shallow layers (abbreviations: Q - Quaternary, Tr - Tertiary, K - Cretaceous, J - Jurassic, T - Triassic, Z, PI - Permian, C - Carboniferous, D - Devonian, S Silurian, Or - Ordovician, Cm - Cambrian; LP - Lower Palaeozoic and/or older); d) general view of the cross-section (petrological trpes specified also in fig. 3 and Tablel). The numbers denote densities p1 (in g/cm3) and on the right of them (in squared parentheses [ 1) values of the respective components of the residue gradient ~R/c~P, (in mGal cm3/g) at the point of minimal residue R value for the depth z = 100 km (abbreviations: Cs compacted sedimentary type, Gr - granitoid type, B - basalt type, G gabbro type, Pr - peridotite type).

The profile LT-7 is an example of a good fit (Fig. 7) and advanced tectonic interpretation. The value of the residue (Fig. 5), which is not very low, probably corresponds to the

L. Krysitiski ef al.: POLONAISE’97

lack of a subtle stripping. The upper diagram (Fig. 7a) shows anomaly of mass density per unit of surface (strongly varied lines; gravity units) in the complex of layers above the level of compensation sources (anomaly of the crustal load; A.c.I.) for different depths z (75, 150 and 00km): /l.c./.(x,) = l&jpcx,,nd<-

(8)

const ;

where z,,, (zsr, < z) is a constant formal depth of the seismic cross section model and consr is the mean reference level of the first part. These anomalies show structures in different

Profile

LT-7

361

load in the lower lithosphere below the level of depth zsrc. Both functions A.c. I.(x) and C.~(X), representing mass loads in the columns above and below the level <= =.YrL respectively, are referred to their mean values and their sum should give a zero if isostatic equilibrium is present. The sum can be non zero but a smooth function of x if the estimation of the subcrustal load by C.$‘(x) is not exact. Meanwhile the sum has interesting and significant shape’s details of horizontal scales 30 + 100 km. The sum of surface density anomalies (crustal load; A.c.I.) and the supracrustal compensative field (Cf:) can be treated as a candidate for good provisional measure of regional deviations of lithospherical segments from isostatic equilibrium (Fig. 8). For example, the axial region (Mid Polish Trough; distance n = 140 + 190 km) of the Polish Basin beneath the LT-7 profile has unequilibrated isostatically uplift visible as positive anomaly about 40 mGal, both - uplift and anomaly - in the relation to the neighbouring regions.

a +/ I

I tt

9

0

chwwt,<,n

b

2 -10 ’

E -20 zo g -30 m

-40

1

I 0

40

80

Distance

I?0

160

x along

?OO

the protile

240

.1 280

[km]

C Fig. 8. Modelling

of the Polish part of profile

isostatic compensation

on the level of depth

LT-7

with substitutive

100 km: a) sum of the

anomaly of the crustal load (A.c.1.. in the gravity units) and the field from formal compensative sources (C.f., z = 100 km) observed at the surface for the profile LT-7;

the sum gives a provisional measure of the regional

deviations from isostatical equilibrium

in horizontal scales 30 + 100 km:

b) observed Bouguer anomaly and the tit.

40

Pr

0

40

80

?l,IW

120

I60

200

240

180

320

Drstance x along the profile [km) Fig. 7.

Gravimetric

modelling

for

the

Polish

part

of

profile

I

‘-1.

Explanations the same as for fig. 6.

horizontal scales. The anomalies having scales 30 + 100 km can be interpreted as deviations of the lithosphere segments The supracrustal from the isostatic equilibrium. compensative field (C.f.; referred to its mean level const’): n,-t

C.f.(x,

)=

cp,gf:“” -const’

(10)

1=I

represents long-scale isostatical response for the crustal load and it represents a shape of function -A.c.I.(x) smoothed in length scales of value z. Thus the C$(x) is a provisional but probably realistic estimation of the anomal

This result correlate with geological data (drilling cores) and shallow seismic reflection sections. The uplift is of Alpine age. The amplitude of this anomaly can be interpreted by density contrast between eroded Cretaceous-Jurassic rocks (2.0 + 2.3 g/cm”) and uplifted top of the mantle (3.4 g/cm?) as segment’s uplift of value about 780 m (the geometrical uplift is greater than 1 km, but uplifted Cretaceous and Upper Jurassic sediments of low densities were eroded). The axial region is bounded by two impressive fault zones (deep fractures) at x = 100 km and at x = 200 km (Koszalin-Chojnice Zone). The dynamic reaction of the axial region seems to be generally rigid (relative vertical movement and small rotation) and forced by interaction with the two neighbouring segments (also rigidly reacting), where the deep fractures acted as reverse faults during this incident. The systematic trend of compensative field along the profile (Fig. 7; uppermost) we can try to interpret as a manifestation of different thermal

362

L. Krysinski er al.: POLONAISE’97

state of the lower lithosphere beneath the “hot Europe” and “cold Europe”. The increase of the compensative field value of about 40 mGa1 = 27tGpcxdTdh @a for peridotite) corresponds to the horizontal temperature contrast dT of 200 K, if it is referred to the layer of thickness dh = 100 km, or 100 K for 200 km of thickness. This result generally confirms statements of Grabowska et al. (1998) as to the clear presence of this horizontal density contrast in the lower lithosphere.

6 Summary and conclusions The new method of the study of the mutual relation between geometry of boundaries in 2-D seismic models and morphology of the gravity anomalies in the area of Polish Basin is presented. The consequent formulation of the fit problem was proposed (search for the minimum of the fit residue inside the cubicoid of densities limitations) with a special concept of discussion of isostatical equilibrium (level of formal compensative sources). The method gives the following results. ?? The general difference between two classes of cross sections was found. The good tits were obtained for profiles orthogonal to the axis of tectonic structures (profile LT-7) and for profiles distant from the boundary of East European Platform (profiles Pl and P5). Much worse tits in gravity modelling were obtained for profiles running closely to the platform’s boundary (profiles P3 and TTZ) and profiles for which seismic processing is not yet finished (profiles P4 and P2). The difference of the fit residue is a measure of adequacy of 2-D medium structure assumption. The critical level is about 10 mGal. ?? The residue R(z) dependence on depth of formal compensative sources gives preliminary suggestions as to the depth z of about 100 km. ?? The modelling along the profile LT-7 suggests a possibility of estimation of the density contrast of the lower lithosphere between “cold” and “hot” Europe. If the difference (about 40 mGal) of subcrustal field given by its systematic trend along the profile - is referred to the layer of thickness about 100 km, then the temperature contrast is 200 K. ?? Anomalies of the surface mass density of the crust of middle horizontal scale 30 km
We thanks to Prof. Teresa Grahowska for the fruitful discussion and to Dr. Stanisiaw Wybraniec for providing the gravity map of Poland.

Acknowledgemenfs.

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120. 223-230.

Bogdanov. A.A. and Khain, V.E. (eds), 1981. International Tectonic Map 0s Europe. IGC Sub-commission of the Tectonic Map of the World, Moscow. Cermak, V., Safanda. J. and Guterch, A., 1989. Deep temperature distribution along three profiles crossing the Teisseyre-Tomquist tectonic zone in Poland. Tectonophysics, 164, IS 1-I 63. Dadlez. R., 1989. Epicontinental Permian and Mesozoic basins in Poland. Kwart. Geol., 33, 175-198.

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