Lithospheric struc ture across the Trans-European Su ture Zone in NW Po land based on grav ity data in ter pre ta tion
Czesław KRÓLIKOWSKI and Zdzisław PETECKI
Królikowski C. and Petecki Z. (2002) — Lithospheric struc ture across the Trans-European Su ture Zone in NW Po land based on grav ity data in ter pre ta tion. Geol. Quart., 46 (3): 235–245. Warszawa.
We pro vide an anal y sis of geo phys i cal and geo log i cal data from the Bal tic seg ment of the Trans-European Su ture Zone (TESZ). The con - struc tion of struc tural and den sity maps of the Zechstein-Mesozoic-Cenozoic com plex has al lowed iden ti fi ca tion of anom a lies of base - ment orgin. As a re sult of in ter pre ta tion of these anom a lies, ma jor struc tural el e ments of the litho sphere have been char ac ter ised.
Ac cord ing to grav ity mod el ling the crustal struc ture is more com pli cated than shown on ve loc ity model along re frac tion and wide-angle re flec tion pro file LT-7. Long-wavelength anom a lies have been mod elled in terms of lat eral het er o ge ne ity within the lower crust and up - per man tle. In or der to achieve a match be tween the ob served and cal cu lated grav ity ef fects, it was nec es sary to as sume dense up per man - tle be neath the TESZ. Grav ity data also in di cate the pres ence of high-density bod ies in the up per crust, and a com plex tran si tional zone be tween crust and up per man tle in the TESZ.
Czesław Królikowski and Zdzisław Petecki, Pol ish Geo log i cal In sti tute, Rakowiecka 4, PL-00-975 Warszawa, Po land; e-mail:
ckro@pgi.waw.pl, zpet@pgi.waw.pl (re ceived: De cem ber 20, 2001; ac cepted: May 6, 2002).
Key words: Trans-European Su ture Zone, grav ity anom a lies, struc tural-density model, grav ity strip ping, lithospheric struc ture
INTRODUCTION
The method of elim i na tion of grav i ta tional ef fects, orig i nat - ing from a rock com plex which has a well-known struc - tural-density char ac ter is tics, is re ferred to as grav ity strip ping (Ham mer, 1963). It has been widely used dur ing the last 15–20 years, mainly in or der to iden tify the geo log i cal struc ture of the deeper base ment of sed i men tary bas ins (e.g. Bojdys and Lemberger, 1986; Her mes, 1986; Grobelny and Królikowski, 1988; Pilkington et al., 1995; Yegorova and Starostenko, 1999). This method is of ten an es sen tial el e ment of com plex in - ter pre ta tion of geo phys i cal data that re lies on mod el ling of the Earth’s crustal struc ture us ing po ten tial field anom a lies.
The pur pose of this pa per is to pres ent the re sults of a re - gional anal y sis of grav ity data across the Trans-European Su - ture Zone (TESZ) in NW Po land. An at tempt is made to sep a - rate grav ity ef fects of the Zechstein-Mesozoic-Cenozoic (ZMC) com plex from the grav ity ef fects of deeper crustal lev - els. In or der to in ves ti gate the pre-Zechstein lithospheric struc - ture, the 3D grav ity strip ping method has been used to elim i - nate the grav ity ef fects of the ZMC com plex from the Bouguer anom aly map. The in ter pre ta tion is based on struc tural and den -
sity maps of the ZMC com plex. Den sity maps have been con - structed with re gard to both lab o ra tory and well log data, in ter - preted us ing com puter tech niques.
This pa per, based on re sults of the re search pro ject no 9 T12B 036 14, fi nanced by the State Com mit tee for Sci en tific Re search (Królikowski et al., 2001), in cludes the anal y sis of geo phys i cal and geo log i cal data from the Bal tic seg ment of the TESZ (Fig. 1). Valu able re sults, from in te grated geo phys i cal and geo log i cal data, achieved pre vi ously in the Pom er a nian (area I) and Kujawy (area II) seg ments en cour aged in ves ti ga - tions far ther into the Bal tic seg ment (Królikowski and Petecki, 1997, 1999; Petecki, 2000).
GEOLOGICAL SETTING AND RESULTS OF PREVIOUS GEOPHYSICAL INVESTIGATIONS
The study area cov ers the Bal tic seg ment of the TESZ to - gether with mar ginal ar eas of the Palaeozoic plat form (ad join - ing to the SW) and East Eu ro pean Craton (EEC). Evo lu tion of this wide zone, characterised by a com pli cated geo log i cal struc ture, be gan in the Early Palaeozoic (Dadlez, 1974;
Pożaryski, 1977) and in cluded a Palaeozoic col li sion and de -
for ma tion of ter ranes docked at the bor ders of Baltica dur ing the for ma tion of Pangea (e.g. Pha raoh, 1996). In the Late Car - bon if er ous-Permian, the area was sub jected to ex ten sion, man i - fested by volcanics. The area sub sided dur ing the Perm ian and Me so zoic, re sult ing in the for ma tion of the Mid-Polish Trough (Dadlez et al., 1995). This ba sin was in verted dur ing the Cre ta - ceous and Ter tiary. It is now filled with Perm ian and Me so zoic successions, up to 5–7 km thick. The study area cov ers the fol - low ing tec tonic units (Dadlez ed., 1998; Znosko, 1998):
Szczecin Trough, Pom er a nian Swell and Pom er a nian Trough, to gether with their pro lon ga tions into the area of the Bal tic Sea.
The anal y sis of po ten tial field anom a lies (Grabowska et al., 1991; Królikowski and Petecki, 1995, 1997, 1999;
Królikowski et al., 1996, 1999) shows the pres ence of a num - ber of fault zones in the TESZ, iden ti fied as deep crustal frac - tures, in ter preted as bound aries of crustal blocks, which are con sid ered as rep re sent ing struc tural units of the crys tal line crust. Their par al lel trend along the edge of the plat form may re flect dy namic pro cesses of col li sion and ac cre tion of proto- continents (ter ranes). The TESZ is also transected by trans - verse fault zones — crustal frac tures, re sult ing in the oc cur - rence of a num ber of seg ments that show dif fer ent struc tural and pet ro log i cal fea tures.
The lat est seis mic in ves ti ga tions (Guterch et al., 1994; Grad et al., 1999; Jensen et al., 1999; Środa et al., 1999) in di cate that
thick ness of the Earth’s crust ranges from 32 km in the Palaeo - zoic plat form to 36 km along the TESZ, and up to 42 km in EEC. The up per crust of the Palaeozoic plat form is com posed of Palaeozoic and Me so zoic se quences, 6–7 km thick, and a meta mor phic com plex reach ing to be low 20 km. Sim i lar thick - ness re la tions oc cur in the TESZ, whereas in the mar ginal zone of EEC, the sed i men tary com plex is 5–7 km thick, and is un - der lain by Pre cam brian rocks.
DENSITY MAPS OF THE ZMC COMPLEX
The den sity dis tri bu tion is based on data from the in ter pre - ta tion of well logs as well as the re sults of lab o ra tory mea sure - ments. Where such de ter mi na tions were un avail able, and there were bore holes of known lithostratigraphical sec tions, av er age den si ties were es ti mated (Królikowski ed., 1988). The first method is con sid ered to be the most re li able be cause the data have been ob tained from a com plete depth-converted pro file and scaled bas ing on lab o ra tory mea sure ments of core sam ples.
Den sity de ter mi na tions of rocks were made from well log data for a to tal of 21 bore holes, drilled on the on shore por tion of the study area. These data were used for a con struc tion of den sity maps of the fol low ing strati graphi cal units: Zechstein, Lower
236 Czesław Królikowski and Zdzisław Petecki
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Fig. 1. Lo ca tion of the re search ar eas and DSS pro files
I and II — ar eas of pre vi ous stud ies, III — area un der study; P1, P2, P3, P4, LT-7 and TTZ — DSS (deep seis mic soundings) pro files
and Up per Tri as sic, Lower and Mid dle Ju ras sic, Up per Ju ras - sic, Cre ta ceous and Ce no zoic.
In or der to con struct den sity maps on shore, al ready ex ist ing den sity and thick ness data were used. For off shore ar eas, data from the on shore were ex trap o lated, with ad di tional use of an in ter preted seismoacoustic im age. The ver ti cal gra di ent of gra - v ity maps, map of per cent age con tri bu tion of tills in Qua ter nary de pos its and map of brown coal de pos its and their oc cur rence in Po land (Ciuk and Piwocki, 1990) were also used for the whole area stud ied.
STRUCTURAL-DENSITY MODEL OF THE ZMC COMPLEX AND ITS GRAVITY EFFECTS
A three-dimensional struc tural model of the ZMC com plex is based on struc tural and thick ness maps, con structed by a team led by Prof. Ryszard Dadlez within the frame work of a re search pro - ject fi nanced by the State Com mit tee for Sci en tific Re search (Dadlez et al., 2000). Be cause of in ter pre ta tional re quire ments, the mapped area has been en larged by 20 km wide belts along its mar - gins, us ing data from Mojski ed. (1995) and Dadlez ed. (1998).
The structural-density model was created as a digital database, after digitizing of structural, thickness and density maps. The model was helpful at the first stage of investigation to control the accuracy of structural and thickness maps. The maps were verified by summing up the successive stratigraphical units from the surface down to the base of the Zechstein, comparing the obtained sum with the structural map of the base of Zechstein, and introducing corrections on maps, if necessary. The next stage of verification of the structural and density model was the analysis of gravity effects, originating from the ZMC complex, as well as anomalies caused by the sub-Zech stein basement.
The grav ity ef fect from the ZMC com plex was sub tracted from the ob served Bouguer anom aly field (Fig. 2), giv ing the grav ity anom a lies caused by the sub-Zechstein base ment, the so-called “stripped” anom a lies. They were cal cu lated us ing a Bouguer cor rec tion with vary ing den sity, in or der to elim i nate the ef fect of den sity vari a tions in rocks ly ing above sea level (Królikowski and Twarogowski, 1991).
The first ver sion of the “stripped” grav ity anomaly map re - vealed lo cal dis tur bances of the field, which were not re moved in the pro cess of ini tial ver i fi ca tion of the struc tural and den sity model. In or der to re move these dis tur bances, we used soft ware that en abled in tro duc tion of cor rec tions into the da ta base, and cal cu la tions of grav ity ef fects from the ver i fied ZMX com plex.
The “stripped” grav ity anom a lies, ob tained af ter ver i fi ca - tion of the struc tural-den sity model, were smoothed to elim i - nate anom a lies re lated to small er rors in the model. The
“stripped” grav ity anom aly map, con structed in this way (Fig. 3), is the ba sis of fur ther in ter pre ta tion.
“STRIPPED” GRAVITY ANOMALIES CAUSED BY THE SUB-ZECHSTEIN BASEMENT
For com par a tive pur poses, the Bouguer anom a lies with vari able den sity of re duced hor i zon tal slab (Królikowski and
Petecki, 1995), in ter po lated onto a 2 x 2 km grid, are shown in Fig ure 2 against the back ground of ma jor tec tonic units of the ZMC com plex (Dadlez, 1987; Dadlez et al., 2000). A sig nif i - cant role in the for ma tion of these anom a lies is played by Zechstein-Me so zoic struc tures.
One of the most im por tant fea tures of the “stripped” anom - a ly field (Fig. 3) is the pres ence of a vast grav ity high trending NW–SE across the en tire study area. Its lo ca tion cor re sponds ap prox i mately to the Pom er a nian grav ity high on the Bouguer anom aly map (Królikowski and Petecki, 1995). How ever, con - sid er able dif fer ences, when com pared with the Bouguer anom - aly map, can be ob served both in the trend of gra di ent zones, which bound the grav ity high to the NE and SW, and in its in - ner struc ture. This grav ity high be comes wider, mainly to wards the NE, cov er ing the NE part of the Pom er a nian Swell. The gra di ent zone, bound ing the grav ity high from the SW, is not so dis tinct and lin ear as on the Bouguer anom aly im age. Sig nif i - cant changes oc cur in the in ter nal vari abil ity of the grav ity high. Anom a lies, which had not been seen on the Bouguer anom aly map, are ob served here. The high est am pli tude is re - corded for the anom aly lo cated in the cen ter of the study area, near the Ślepce 3 bore hole (S in Fig. 3). Worth not ing is its me - rid i o nal strike. At its north ern end there is a sim i lar but smaller anom aly near the Dobrzyca 2 bore hole (D in Fig. 3). It trends NW–SE and is sit u ated east of Kołobrzeg. This anom aly, and a strong gra di ent zone ad join ing from the NE, is not ob served on the Bouguer anom aly map. The pos i tive anom aly, vis i ble near the Czaplinek IG 2 bore hole (C in Fig. 3), has also un der gone con sid er able mod i fi ca tion.
Many changes can also be ob served in the Bal tic Sea area.
Prom i nent anom a lies and gra di ent zones, as so ci ated with the Trzebiatów and Wisełka syn clines, and Kołobrzeg and Kamień Pomorski anticlines, have been al most com pletely elim i nated.
In the SW part of the study area, gen er ally cov er ing the Szczecin Trough, a prom i nent anom aly can be ob served in the vi cin ity of Szczecin. The anom aly is sep a rated from the Pom er - a nian high by a nar row, NW–SE trending anom aly lo cated NW of Nowogard. This anom aly can not be ob served on the Bouguer anom aly map, ei ther. The neg a tive anom aly sit u ated in the Szczecin Trough be comes con sid er ably weaker.
This brief de scrip tion of the “stripped” grav ity anom a lies im age points to some es sen tial el e ments of both the geo log i cal struc tures of the sub-Zechstein base ment and of deeper crustal zones which are not re corded in the Bouguer anom aly im age.
FILTERED “STRIPPED” GRAVITY MAPS
The fil tered ver sions of the “stripped” grav ity map have been used to en hance the re gional and lo cal fea tures of the TESZ and the sur round ing base ment do mains. Gen erally, long-wavelength grav ity anom a lies are at trib uted to deep sources in the crust and the up per man tle, whereas short-wavelength anom a lies are re lated to shal low sources in the up per crust. In ad di tion, max ima in the hor i zon tal gra di ent of the fil tered grav ity data were iden ti fied (Blakely and Simpson, 1986). The gra di ent max ima are rep re sented on the fil tered maps by points of sizes pro por tional to the gra di ent
Fig. 3. Map of the “stripped” grav ity anom a lies caused by the sub-Zechstein base ment
A — pro file of grav ity mod el ling; C, D and S — pos i tive anom a lies dis cussed in the text; other ex pla na tions as in Fig. 2
Koc1
-20 -15 -10 -5 0 5 10 15 20 25 30 35 40 -5 2
-20 -10 0 10 20 30 40 x10 m/s (mGal)
Fig. 2. Map of the Bouguer anom a lies at the back ground of the main tec tonic units
mag ni tude. Plots of these max ima were used to lo cate the edges of grav ity sources and to trace tec tonic and lithological dis con - ti nu ities (Cordell and Grauch, 1985; Grauch and Cordell, 1987;
Thurston and Brown, 1994).
REGIONAL “STRIPPED” GRAVITY ANOMALIES
Re gional anom a lies, as so ci ated with deep-seated sources, were de fined by up ward con tin u a tion of the “stripped” grav ity field by 20 km (Fig. 4). This trans for ma tion was per formed in the wavenumber do main us ing the Geosoft MAGMAP soft - ware.
The most prom i nent and best-doc u mented el e ment is the Pom er a nian high, ex tend ing across the study area. The pres ence of this el e ment in re gional “stripped” grav ity anom aly map (Fig. 4) shows that it is sourced in deeply rooted struc tures. The con sid er able length of the anom aly and its high val ues in di cate that there is a large crustal block of high den sity. It ap pears to have very deep foun da tions as so ci ated with den sity changes in the lower crust and up per man tle, as well as with the Moho dis - con ti nu ity. In this map the Pom er a nian high is boun ded to the NE by a wide high gra di ent zone, cross ing the whole area ana - lysed. It sug gests the oc cur rence of a deep frac ture which, in the LT-7 pro file, cor re sponds ap prox i mately to the frac ture as sumed to rep re sent the edge of the EEC (Guterch et al., 1994).
NE of the Pom er a nian high there is an area show ing lower val ues of the field. Two re gional anom a lies, bounded by prom - i nent gra di ent zones, are ob served here. In the study area they can be traced only partly. A nar row zone of higher val ues of the field sep a rates these anom a lies. This is the bound ary be tween the Bal tic and Pom er a nian high (Królikowski and Petecki, 1995; Królikowski et al., 1996) prob a bly rep re sent ing a lithological dis con ti nu ity sep a rat ing the Pom er a nian gran - ite-gneiss mas sif from heavier meta mor phic or in tru sive rocks of the Bal tic area (Kubicki and Ryka, 1982).
In the area ad join ing the Pom er a nian high to the south-west, an in ter est ing pos i tive anom aly is ob served, lo - cated in the vi cin ity of Szczecin but traced only as frag ments (Fig. 3). It may have a sim i lar or i gin to the East Elbe Mas sif grav ity anom a lies. The Szczecin Block was dis tin guished within the sub-Zechstein base ment by Pożaryski (1987) who sug gested con nec tions of the study area with this mas sif.
LOCAL “STRIPPED” GRAVITY ANOMALIES
In or der to ob tain lo cal anom a lies, whose sources are as so - ci ated with shal lower struc tures lo cated in the up per crust, up - ward-con tin ued anom a lies (Fig. 4) were sub tracted from
“stripped” grav ity anom a lies (Fig. 3).
In the lo cal “stripped” grav ity anom aly map (Fig. 5), where the ef fects of deep crustal struc tures and the Moho are fil tered out, anom a lies re sult ing from den sity dif fer en ti a tion of the im - me di ate sub-Zechstein base ment in the Pom er a nian Trough are ev i dent. Their dis tri bu tion and trend of gra di ent zones in di cate that, in the area lo cated SW of the Pre cam brian Plat form, the sub-Zechstein base ment shows a block struc ture, de ter mined by deep tec tonic lin ea ments. Three prom i nent anom a lies (C, D and S in Fig. 5) in di cate the oc cur rence of ad di tional rock
masses in the up per crust, most prob a bly in tru sive bod ies with dom i nantly ba sic rocks.
The north east ern bound ary of the Pom er a nian high cor re - sponds over a long dis tance to the Biesiekierz Fault Zone (Dadlez and Marek, 1997). Many other faults, ob served in the Zechstein-Me so zoic com plex of the area (e.g. Rzeczenica, Sianów-Polanów), cor re spond to grav ity gra di ent zones. The SW part of the Pom er a nian high shows a more com pli cated struc ture. It is bounded by a few prom i nent gra di ent zones (Fig. 5) which oc cur at the bor der of the Pom er a nian Swell (Fig. 2). It is prob a ble that they mark fault zones of the sub-Zechstein base ment, which show a re la tion ship with the tec ton ics of the over ly ing com plex.
Off the Pol ish coast, the lo cal anom aly pat tern is less vari - able (Fig. 5). Only the NE bound ary of the Kołobrzeg Block (Fig. 2) man i fests it self strongly in the form of hor i zon tal gra di - ent max ima. The Trzebiatów Fault Zone (Fig. 2) is less dis tinct.
The dif fer ence be tween off shore and on shore lo cal anom aly im ages (Fig. 5) can be as so ci ated with the oc cur rence of lower den sity Up per Car bon if er ous de pos its in the sub-Perm ian base - ment of the Gryfice and Kołobrzeg blocks, as com pared with the cen tral part of the Pom er a nian Swell (Fig. 2), where the sub-Perm ian base is un der lain by higher den sity Lower Car - bon if er ous and De vo nian de pos its (Dadlez, 1987).
THREE-DIMENSIONAL (3D) INVERSION OF THE “ŚLEPCE” GRAVITY ANOMALY
The “Ślepce” lo cal pos i tive grav ity anom aly is lo cated in the cen tral part of the area (S in Fig. 5). The en larged im age of the anom aly is shown in Fig ure 6a. It was in ter preted with the use of the 3D in ver sion method, as sum ing that an in tru sive body causes this anom aly. It en abled de ter mi na tion of the ap - prox i mate depth of oc cur rence, thick ness and den sity con trast of the body. A depth to its top was de ter mined us ing the power spec trum method (Spector and Grant, 1970). The depth ob - tained (ap prox i mately 9 km) shows that the top of the anom a - lous body oc curs within the up per crust, which, ac cord ing to the LT-7 pro file (Guterch et al., 1994) and TTZ pro file data (Grad et al., 1999), is char ac ter ised by rel a tively low seis mic ve loc i ties Vp < 6.0 km/s (com pare Fig. 7). A high ve loc ity body has been in ter preted at a sim i lar depth in the TTZ pro file in the Pom er a nian seg ment of the TESZ, SW of the in ter sec tion with the LT-7 pro file.
The “Ślepce” anom aly (Fig. 6a) was mod elled us ing a 3D grav ity in ver sion mod el ling pro gram (Petecki, 2000). The mod - elled body is as sumed to have a flat bot tom. The model con - structed with the power spec trum depth con trol is shown in Fig - ures 6b and c. The re sults of the anal y sis in di cate that the “Ślepce”
pos i tive “stripped” grav ity anom aly may be caused by a source lo - cated at a depth of about 9 km, and it has a max i mum thick ness of ap prox i mately 7 km. A con sid er able den sity con trast (0.22 g/cm3) sug gests that the source is an in tru sion of lower crustal and/or up - per man tle or i gin. The mod el ling re sults show that the fi nal model fit the ob served data to a sat is fac tory de gree (Fig. 6d) and can be used to pro vide con straints on deep den sity struc ture.
Fig. 5. Map of the lo cal “stripped” grav ity anom a lies
Pos i tive anom a lies C, D and S are dis cussed in the text; other ex pla na tions as in Figs. 2 and 3 Fig. 4. Map of the “stripped” grav ity anom a lies con tin ued up wards to 20 km
Other ex pla na tions as in Figs. 2 and 3
The other two pos i tive anom a lies, “Dobrzyca” and
“Czaplinek” (D and C in Fig. 5), pre sum ably of sim i lar or i gin, have not been ana lysed us ing the above-de scribed meth ods due to ei ther un fa vour able po si tion with re spect to other anom a lies or their in com plete re cord. The source of the “Dobrzyca”
anom aly, lo cated north of the “Ślepce” anom aly, is seated prob a bly at shal lower depths. The “Czaplinek” anom aly, sit u - ated in the south east ern bor der of the area, is prob a bly caused by a source of sim i lar pa ram e ters as the “Ślepce” anom aly.
TWO-DIMENSIONAL (2D) MODELLING OF THE ”STRIPPED” GRAVITY DATA
The pur pose of the grav ity mod el ling was to study the crust and up per man tle struc ture across the con tact zone of the Palaeozoic plat form and EEC in the study area. Deep seis mic sur veys, per formed in Po land (Guterch et al., 1994, 1999; Grad et al., 1999; Jensen et al., 1999; Środa et al., 1999; Janik et al.,
Fig. 6. 3D in ver sion of the “Ślepce” lo cal “stripped” anom aly
a — “Ślepce” lo cal “stripped” anom aly, b — depth of the source body in km, c — source body thick ness in km, d — grav ity ef fect from source body
2000), re vealed that this re gion is char ac ter ised by anom a lous geo log i cal struc ture of the Earth’s crust and up per man tle.
Grav ity for ward mod el ling was per formed along pro file A, cross ing the ana lysed area nearly per pen dic u lar to the ma jor grav ity anom a lies (Fig. 3). Grav ity pro file A co in cides with part of the re frac tion and wide-an gle re flec tion pro file LT-7 (Guterch et al., 1994). The mod el ling was car ried out us ing the GM-SYS™ soft ware that al lows cal cu la tion of 2D grav ity ef - fects orig i nat ing from bod ies per pen dic u lar or oblique to the pro file’s strike. The bod ies are as signed a fi nite strike length.
Grav ity mod el ling is based on the 2D seis mic ve loc ity model along the LT-7 pro file (Guterch et al., 1994) shown in Fig ure 7. Seis mic data in di cate that, be neath the Palaeozoic plat form and in the TESZ, the up per crust has an oma lously low ve loc i ties (be low 6.0 km/s) down to a depth of ap prox i mately 20 km. The thin ner lower crust (10–15 km) is com posed of two lay ers, show ing ve loc i ties of 6.5 and 7.15–7.25 km/s. Its great - est thick ness is ob served in the TESZ, and the ve loc ity be low the Moho is 8.26 km/s. In the EEC, the crys tal line crust con sists of three lay ers with ve loc i ties of 6.2–6.3, 6.5–6.6 and 7.0–7.15 km/s, re spec tively, and the depth to the Moho is about 42 km. Seis mic ve loc i ties be neath the Moho in crease to 8.33 km/s.
Ve loc i ties for in di vid ual crustal blocks and the Earth’s man tle were av er aged and con verted into den si ties us ing the den sity/ve loc ity re la tion of Christensen and Moo ney (1995).
The cal cu lated grav ity ef fect from the den sity model that cor re - sponds to the ve loc ity model de scribed above dif fers con sid er - ably from the ob served curve. Com par i son of the mod elled and
“stripped” grav ity ef fects re veals a ma jor dis crep ancy of 70 mGal around the TESZ (Fig. 7). The lower grav ity val ues com puted from the ve loc ity model demostrate a mass de fi - ciency in the ini tial in ter pre ta tion. It in di cates of the oc cur rence of high-den sity sources, which have not been re corded in the ve loc ity model. The anal y sis of grav ity anom a lies from the sub-Zechstein base ment in the Pom er a nian seg ment of the TESZ (Królikowski and Petecki, 1997) shows two causes of the ob served dis crep ancy: the in creased den sity of the up per man tle in the TTZ and the oc cur rence of ba sic rock in tru sions (of high den sity) in the up per crust. These con clu sions, re cently con firmed by a seis mic sur vey per formed along the TTZ pro - file (Grad et al., 1999), were taken into con sid er ation dur ing the grav ity mod el ling. Re sults of in ter pre ta tion of seis mic pro - files, car ried out within the POLONAISE’97 pro ject (Fig. 1) (Guterch et al., 1994; Jensen et al., 1999; Środa et al., 1999;
Janik et al., 2000), have also been em ployed. They have pro - vided more de tailed data on the geo log i cal struc ture of the litho sphere in the vi cin ity of the study area.
An im por tant re sult of the grav ity mod el ling (Fig. 8) was the con fir ma tion of the oc cur rence of a high-den sity (~ 2.90 g/cm3) in tru sive body in the up per crust of the TTZ area. The pres ence of ba sic rock in tru sions in the up per crust of the Pom er a nian seg ment of the TESZ was ear lier pos tu lated on the ba sis of in ter pre ta tion of grav ity anom a lies from the sub-Zechstein base ment (Królikowski and Petecki, 1997).
Seis mic data along the LT-7 pro file do not in di cate the oc cur - rence of any in tru sion, maybe due to its rel a tively small di men - sions. In the Pom er a nian seg ment of the TESZ on the TTZ pro -
file, how ever, a high-ve loc ity body has re cently been in di cated at a depth of about 10 km (Grad et al., 1999).
The lower crust of the TESZ also has a com plex struc ture.
The grav ity mod el ling in di cates the oc cur rence of a tran si tional zone be tween the crust and up per man tle. Its for ma tion can be re lated to pen e tra tion of man tle mat ter into the lower crust. The max i mum depth of the Moho dis con ti nu ity (ap prox i mately 48 km) pre sum ably oc curs be neath the outer edge of the EEC crust. The con tact of this com plex tran si tional zone with the crys tal line crust of the Palaeozoic plat form dips north - eastwards, and may rep re sent a south east ern con tin u a tion of the Trans-European Fault (TEF) (Królikowski et al., 1999).
The den sity model of the crys tal line crust of the EEC along the pro file ana lysed shows a rel a tively sim ple, three-layered struc ture, not al tered sig nif i cantly by grav ity data mod el ling in re la tion to the seis mic model.
Den sity vari abil ity within the up per man tle is nec es sary to ex plain long-wavelength grav ity anom a lies. The lat est in ter - pre ta tions of deep seis mic ex per i ments also in di cate much ve - loc ity vari abil ity within the up per man tle (Środa et al., 1999;
Grad et al., 1999; Jensen et al., 1999).
DISCUSSION
Based on the 3D struc tural-den sity model of the ZMC com - plex in the Bal tic seg ment of the TESZ in NW Po land, the grav - ity anom a lies caused by the struc tures be low the Zechstein were cal cu lated. The “stripped” grav ity im age (Fig. 3) shows a sig nif i - cant pos i tive anom aly in the cen tral part of the study area, the NW–SE trending Pom er a nian grav ity high. Ac cord ing to the model pre sented here, the re gional com po nent of this grav ity high (Fig. 4) is caused by dense lower crust and up per man tle in the TESZ. The high-den sity lower crustal zone that ex tends be - neath the cratonic edge (Fig. 8) is in ter preted as be ing com posed of mafic ma te rial that may have re sulted from underplating of man tle ma te rial. This pro cess could play an im por tant role in the evo lu tion of the Mid-Pol ish Trough (Dadlez et al., 1995).
The con tact of the crys tal line crust of the TESZ with the crys tal line crust of the Palaeozoic plat form dips to the NE (Fig. 8), and may rep re sent a south east ern con tin u a tion of the TEF (Królikowski et al., 1999). On the ba sis of grav ity and mag netic mod el ling along the LT-7 pro file Petecki (2002) pointed out that it may in di cate a su ture formed due to the Cal - edo nian col li sion of East ern Avalonia and Baltica with the pre - sumed par tic i pa tion of an ad di tional crustal block of un clear prov e nance.
The lat er ally het er o ge neous up per man tle (Fig. 8) with an an oma lously high den sity (3.42 g/cm3) zone be neath the TESZ is dif fi cult to ex plain. How ever, the grav ity mod el ling of large pos i tive anom a lies above Dnie per-Donets Ba sin also re vealed in - creased den sity in the up per man tle be neath the ba sin (Lobkovsky et al., 1996). Lobkovsky et al. (1996) pre sented a quan ti ta tive model of the Dnie per-Donets Ba sin evo lu tion based on a sub si - dence of a heavy eclogite lens in the asthenosphere. The model fits well the tec tonic sub si dence data. It may be spec u lated that in - creased den sity in the up per most man tle of the study area is caused by meta mor phic trans for ma tion of mafic rocks to eclogite.
242 Czesław Królikowski and Zdzisław Petecki
Fig. 7. P-wave ve loc ity model of pro file A (af ter Guterch et al., 1994) (bot tom) and the com par i son be tween the cal cu lated and “stripped” grav ity anomalies along pro file A (top)
Fig. 8. Grav ity mod el ling along pro file A Ex pla na tions as in Fig. 7
The lo cal “stripped” grav ity map (Fig. 5) in di cates the com - plex tec tonic pat tern of the pre-Zechstein base ment. The strong lo cal pos i tive anom a lies (C, D and S in Fig. 5) clearly in di cate the pres ence of high-den sity sources in the up per crust. The power spec trum anal y sis, 3D in ver sion and 2D mod el ling of grav ity data show that the depth to the top of these bod ies is about 9 km and their thick nesses reach 5–6 km. These high-den sity (2.90–2.92 g/cm3) sources may be in tru sive bod ies (2.90–2.92 g/cm3) with dom i nant ba sic rocks. Their pres ence may be linked to the de vel op ment of the Mid-Pol ish Trough.
CONCLUSIONS
1. The im age of the “stripped” grav ity anom a lies in the Bal - tic seg ment of the TESZ is characterised by the oc cur rence of a vast pos i tive anom aly that cor re sponds to the grav ity unit of the Pom er a nian high.
2. The im ages of the re gional and lo cal com po nents of the
“stripped” grav ity anom a lies of the Pom er a nian high show that the anom a lies are con trolled by both deep and shal lower (sub-Zechstein) sources.
3. The long-wavelength pos i tive anom aly lo cated within the TESZ is caused by sources sit u ated in the lower crust and up per man tle. 2D mod el ling of grav ity data in di cates in creased den si ties of the lower crust and up per man tle within the TESZ and a com plex struc ture of the lower crust (tran si tional zone).
The bound ary be tween this tran si tional zone and the up per man tle de scends to a max i mum depth of ap prox i mately 48 km
near the mar ginal zone of the EEC. This high-density lower crustal zone may be caused by mag matic underplating.
4. The in creased den sity of the up per most man tle be neath the TESZ may be caused by meta mor phic trans for ma tion of mafic rocks to eclogite.
5. The con tact of the crys tal line crust of the TESZ with the crys tal line crust of the Palaeozoic plat form ap pears to dip north-eastwards and may be a south east ern con tin u a tion of the TEF. It may rep re sent a su ture formed dur ing the Cal edo nian col li sion of East ern Avalonia and Baltica.
6. Lo cal pos i tive “stripped” grav ity anom a lies can be as so - ci ated with in tru sions of dense ba sic rocks from the lower crust and/or up per man tle; 3D in ver sion anal y sis shows that the depth to the top of these bod ies is about 9 km and their thick - nesses reach 5–6 km; grav ity mod el ling in di cates that these may be high-density in tru sive bod ies (2.90–2.92 g/cm3) with dom i nantly ba sic rocks. Their pres ence may be linked to extensional and co eval mag matic events dur ing the Mid-Polish Trough for ma tion and evo lu tion.
Ac knowl edge ments. The au thors ex press their cor dial thanks to Prof. Ryszard Dadlez for valu able co op er a tion in both the last re search pro ject (Bal tic seg ment) and pre vi ous pro jects cov er ing the Pom er a nian and Kujawy re gions. With - out struc tural and thick ness maps of the Zechstein-Mesozoic com plex, com piled by a team of spe cial ists un der the lead er - ship of him, it would have been im pos si ble to study the geo log - i cal struc ture of the sub-Zechstein base ment us ing the 3D grav - ity strip ping method. Care ful re views by Z. Fajklewicz and R.
Stephenson helped to im prove the pa per. Ed i to rial re view by M. Narkiewicz is also ap pre ci ated.
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