ELECTRICAL CONDUCTIVITY ANOMALY OF THE NORTHERN CARPATHIANS AND THE DEEP STRUCTURE OF THE OROGEN
Kazimierz ŻYTKO
S ta te G eological Institute, C arpathian Branch, Skrzatów I, 31-560 K raków , P o la n d
Żytko, K., 1997. Electrical conductivity anomaly o f the Northern Carpathians and the deep structure o f the orogen.
Ann. Soc. Geol. Polon., 67: 25—43.
A bstract: The Carpathian conductivity anomaly, also called geoelectrical anomaly, is constrained by the presence of high-conductivity rock series at depths o f 10 to 25 km, in different segments of the orogen. This paper aims at locating the source of the anomaly in the framework of crustal blocks and at its geological interpretation. The faulted and locally depressed to 20 km surface o f the high-resistivity Precambrian basement o f the Polish Carpathians is presented basing on the results o f magnetotelluric sounding (MTS). The major faults, o f Neogene age, are oriented SW-NE in the west and SE-NW in the east, running symmetrically to the Kraśnik-Rzeszów- Rymanów-Debrecen line (ca. 22° E). The high-conductivity layer overlying the crystalline basement is probably composed o f metamorphosed Palaeozoic strata, including coal-bearing Carboniferous rocks. The sole presence and extent o f these rocks, however, are not sufficient to generate the regional conductivity anomaly whose outline is clearly related to that o f the Tertiary Carpathian orogen.
The results o f MTS sounding have been compared with those o f seismic refraction studies in the Ukrainian Carpathians. Two belts of depressions of variable depth have been distinguished in the basement. The southern belt includes a collision suture, i.e. the contact zone between the lowered Central European block and the blocks o f the Central West and East Carpathians. Between Bardejov-Wysowa and the Marmarosh Massif the source of the geoelectrical anomaly is situated close to the northern margin o f the southern basement depression. Farther to the south, up to 46° N near Sf. Gheorghe, Romanian geologists place this source on a deep-seated fault that borders the Central East Carpathian block from E.
To the east o f the Slovak-Ukrainian border, the southern depression is overlain by folded flysch complexes of the Rakhov, Porkulets, “Black Flysch”, and Ceahlau units, interpreted as the infill of the Outer Dacides rift, as well as by the Dukla unit flysch. The rift was situated in the marginal part o f the European continent. Its western continuation, presently covered by the Dukla and Magura Nappes, is documented by relics o f dark Doggerian and Neocomian flysch strata, as well as by basalts exposed within the Grajcarek Unit, north o f the Pieniny Klippen Belt west of Krynica. The farther continuation of this zone can be found in the Penninicum (Piemontais?) o f the Alps.
The source of the Carpathian geoelectrical anomaly is situated near the collision suture; however, the structure o f the Ukrainian Carpathians basement and a shift o f the anomaly source towards the Carpathian Foredeep in the Focçani region, Romania, may indicate a relationship between the anomaly and the lowered, marginal part o f the continental crust.
It is suggested that an important factor generating the Carpathian geoelectrical anomaly is graphite originated due to post-Oligocene migration and graphitization of organic substance within deeply buried strata o f the Jurassic-Cretaceous rift and within fault zones in the crystalline basement. The near-surface manifestation o f this process is the presence o f veins bearing hydrothermal mineral association, including authigenic quartz (“Marma
rosh diamonds”). This quartz contains inclusions of anthraxolite, a hard bitumen showing traces of incipient graphitization. Such veins are ubiquitous in the Carpathians, close to the axis o f the anomaly.
A bstrakt: Karpacka anomalia przewodności zwana też anomalią geoelektryczną wywołana jest obecnością dobrze przewodzących kompleksów skal w głębokości od 10 do 25 km na różnych odcinkach orogenu. Podjęto próbę zlokalizowania źródła anomalii w układzie bloków skorupy oraz próbę wyjaśnienia jego geologicznej natury. Przedstawiono przybliżony obraz zdyslokowanej i obniżonej lokalnie nawet do 20 km powierzchni wyso- kooporowego fundamentu Kaipat polskich (prekambr w oparciu o wyniki sondowań magnetotellurycznych (MTS). Główne pęknięcia powstały w neogenie; układają się one symetrycznie (SW-NE na zachodzie, SE-NW na wschodzie), skośnie w stosunku do linii Kraśnik-Rzeszów-Rymanów-Debreczyn bliskiej południkowi 22.
Przedstawiono cechy nadścielającej fundament warstwy dobrze przewodzącej - jest to przypuszczalnie zmeta- morfizowany paleozoik z udziałem węglonośnego karbonu. Obecność i zasięg tych utworów nie wystarczają do generowania regionalnej anomalii przewodności, która nawiązuje wyraźnie do trzeciorzędowego zarysu Karpat.
Wyniki badań fundamentu metodą MTS zestawiono z wynikami refrakcyjnych badań podłoża Karpat ukra
ińskich. Wyznaczono dwa pasy depresji podłoża o zmiennej głębokości. W pasie południowym znajduje się kolizyjny szew czyli strefa kontaktu obniżonego bloku skorupy kontynentu europejskiego i bloków centralnych
4 — Annales...
Karpat Zachodnich i Wschodnich. Między Bardejovem-Wysovvą a masywem Marmarosz źródło anomalii geo
elektrycznej znajduje się przy północnym brzegu południowego pasa depresji podłoża. Dalej ku południowi, aż po 46 równoleżnik w pobliżu Sf. Gheorghe według rumuńskich geologów źródło to znajduje się przy rozłamie sko
rupy stanowiącym zewnętrzną granicę bloku centralnych Karpat Wschodnich.
Na wschód od granicy słowacko-ukraińskiej nad depresją południowego pasa znajduje się sfałdowany flisz jednostek rachowskiej, porkuleckiej, Czarnego Fliszu i Ceahläu interpretowany jako wypełnienie lyftu Dacydów zewnętrznych, a także flisz jednostki dukielskiej. Ryft ten zlokalizowany był w brzeżnej strefie kontynentu. Jego zachodnie przedłużenie przykryte płaszczowinami dukielską i magurską dokumentują relikty ciemnego dogger- skiego i neokomskiego fliszu oraz bazaltów w jednostce Grajcarka przy północnej granicy pienińskiego pasa skał
kowego na zachód od Krynicy. Dalsze przedłużenie znajduje się w Penninicum (Piemontais?) w Alpach.
Strefa źródła karpackiej geoelektrycznej anomalii jest bliska kolizyjnego szwu; budowa fundamentu w Karpa
tach ukraińskich oraz przesunięcie źródła anomalii do zapadliska w rejon Foc$ani w Rumunii w skazująna związek anomalii z obniżonym brzeżnym pasem skorupy kontynentalnej.
Istotnym czynnikiem generującym karpacką geoelektryczną anomalię jest przypuszczalnie grafit. Powstał on w wyniku pooligoceńskiej migracji i grafityzacji substancji organicznej w pogrążonych głęboko osadach juraj- sko-kredowego lyftu i w strefach pęknięć fundamentu. Przejawem tego procesu przy powierzchni jest obecność żył z asocjacjąminerałow hydrotermalnych, w tym autigenicznego kwarcu (diamenty marmaroskie). Obecne są w nim wrostki antraksolitu, twardego bituminu o początkowej grafityzacji. Żyły te są częste w Karpatach w są
siedztwie epicentrum anomalii.
Key words: geomagnetic sounding, magnetotelluric sounding, high-resistivity basement, high-conductivity layer, orogen, plate boundary, graphitization, mineral veins, anthraxolite, Carpathians, collision.
Manuscript received 25 April 1996, accepted 20 February, 1997
INTRODUCTION
R esults o f geoelectrical sounding point to vertical and lateral differentiation in th e conductivity o f rocks and(or) enclosed m edia, both w ithin the near-surface sedim entary cover, and in th e w hole lithosphere. T he studies o f g eoelec
trical effects induced by changes in the geom agnetic field helped to locate a num ber o f regional conductivity anom a
lies in E urope in the 1970s (Fig. 1; P orstendorfer et al., 1976). S uch anom alies are also called geoelectrical anom a
lies (Jankow ski et al., 1984; P etr e t al., 1994). The axes o f anom alies are indicated b y zero value o f variation o f th e Z com ponent o f the geom agnetic field, as w ell as by divergent pattern o f th e W iese in d uction vectors. V ectors detected by m agnetic sounding (M V ), show ing the directions o f in d u c
tion currents, point outw ards o f the axis o f an elongated body o f high conductivity w hich is situated w ithin inhom o- geneous com plexes o f crustal rocks (R okityansky, 1976;
R okityansky e t al., 1976a). T he shape o f the anom alous field contours is the basis for determ ination o f the m axim um depth to th e to p o f th e co n ductive body. T he depth o f the boundary betw een rocks o f dram atically contrasting con
ductivity m ay also be v erified by m agnetotelluric sounding (M TS; B erdichevski & D m itriev, 1976).
G eological interpretation o f the sources o f electrical anom alies, i.e., zones o f highly conductive strata, is usually controversial. Both, electrolithic (ionic) conductivity associ
ated w ith fluids contained w ithin pores and fissures and electronic conductivity, can be taken into account. The latter possibility holds true - on th e regional scale - in the case o f the presence o f biogenic o r ju v e n ile graphite. F rost et al.
(1989) found intergranular g raphite form ing continuous film around grains in rocks o f th e low er continental crust, and concluded th at graphite originated from C 0 2 -rich fluids dur
ing cooling. T he graphite can cause high electrical conduc
tivity o f rocks and m ay be a source o f conductivity anom aly.
The presence o f graphite w ithin cataclastic fractures and re lated m icroresistivity anom aly have been docu m en ted by the deep K TB borehole near O berpfalz, B ohem ian M a ssif (H aak, 1993).
A n im portant regional conductivity anom aly is related to th e deep zone o f the C arpathian orogen (R okityansky et al., 1976b; Jankow ski e t al., 1984; P inna et a l., 1992; P e tr e l al., 1994). It extends from V ienna to th e South C arpathians
Fig. 1. Map o f electrical conductivity anomalies in Central Europe (compiled after Porstendorfer et al., 1976; Rokityansky et al., 1976b; Pinna et al., 1992; coal basins after Carte Tectonique International de l'Europe, 1982; Pożaryski & Dembowski, 1984;
and Oszczypko et al., 1989). 1 - extent o f Upper Carboniferous coal-bearing strata, 2 - Crystalline-Mesozoic zone o f the East Carpathians (northern part - Marmarosh massif), 3 - conductivity anomalies (NGP - North German-Polish, C - Carpathian, RG - Upper Rhine Graben-Gottingen), 4 - Carpathian frontal thrust, 5 - Pieniny Klippen Belt
(Fig. 1) at a depth ran g in g from 10 to 25 km in different seg
m ents o f th e C arpathian arc. P in n a e t al. (1992) and D.
S tanica & M . S tanica (1993) point to a discontinuity o f the axis o f this anom aly clo se to the 46 parallel in th e R om a
nian E ast C arpathians. It has also been found that long-term (>900 s) m easurem ents o f geom agnetic induction vectors detect deeply-buried, regular anom alies, w hereas short-term m easurem ents reveal highly changeable vector directions and local anom alies (Jankow ski e t al., 1991). D ifferent geo
logical m odels explaining the presence o f highly conductive rocks or solutions, i.e. th e sources o f anom alies, have been constructed. The role o f altered and(or) fractured rocks, saturated w ith hot m ineral w aters, as w ell as the proxim ity o f anom aly sources to boundaries o f different crustal blocks, including that betw een the C arpathians and the adjacent platform s, have been taken into account (A dam & Pospisil, 1984; Jankow ski et al., 1984, 1991; O szczypko & Ślączka, 1985; P in n a e t al., 1992; P etr é ta l., 1 9 9 41.
M ost o f the authors reject the idea o f graphite schists as a possible source o f th e geoelectrical anom aly. H ow ever, T om ek (1988) is inclined to accept S tan ley ’s opinion (pers.
com m ., 1986) that graphite associated w ith buried subduc
tion zones is the m ost p robable source o f conductivity anom alies.
In this paper, a p ossible connection betw een the C arp a
thian anom aly and the presence o f m etam orphosed coal- bearing C arboniferous strata beneath the orogen is d is
cussed; nevertheless, the a u th o r’s opinion is that the source o f the anom aly should be related to graphitized rocks o ccur
ring close to the boundaries o f crustal blocks. The pattern o f the latter is analyzed below , basing on th e results o f m agne- totelluric sounding (M T S) conducted in the Polish C arpa
thians by th e State G eological Institute in 1978-79 and 1986-90 (M olek & O raczew ski, 1988; M olek & K lim kow ski, 1991), as w ell as on published data on the U krainian, Slovak and R om anian C arpathians.
COAL-BEARING AND BITUMINOUS FORMATIONS AS POSSIBLE SOURCES
OF THE CONDUCTIVITY ANOMALY IN CENTRAL EUROPE
D ortm an and T oporec (1984) report on very low electri
cal resistivity o f m etam orphosed coals present in w ell logs o f the Sakhalin and D onbas basins (Fig. 2). The specific re
sistivity o f these coals decreases w ith increasing coalifica- tion, attaining less than 1 O m m in anthracites. Sim ilar prop
erties m ay have m etam orphosed claystones rich in plankton- derived organic substance.
T he northern, G erm a n -P o lish anom aly, occurs in the platform area betw een H am burg and Ł ó d ź (Fig. 1). In a m odel elaborated by P o rstendorfer et al. (1976) for the area south o f R ügen, low -resistivity layers (6 and 25 O m m , rarely <5 O m m ) are found betw een highly resistive Zech- stein salts and the crystalline basem ent, at a depth o f 5 -7 km. H ighly conductive layers also occur in the supra-Z ech- stein sedim entary cover, as w ell as in the low er crust and u p p er m antle. The studies carried out by E R C E U G T -G ro u p (1992) along the A lp s-B a ltic S ea profile, located the d is
0 M- M
A B C
Fig. 2. Relationship between electrical resistivity and degree of coalification, based on well logs (after Dortman & Toporec, 1984).
1 - coal of low ash content, 2 - coal o f high ash content, 3 - coal-bearing shale. Coalification: A - brown coal, B - black coal, C - anthracite. Coal basins: I - Sakhalin, II - Donbas
cussed anom aly betw een the R hine and the Elbe. L ow resis
tivity o f the topm ost part o f sedim ents has been related to electrolites (brines), w hereas the low resistivity o f rocks at depths betw een 5 and 15 km is thou g h t to have been induced by an electronic conductor, pro b ab ly biogenic m eta-anthra
cite/graphite w ithin the horizon o f E arly P alaeozoic black shales.
T aking into account the v ertical resistivity distribution in the P o rstendorfer’s e t al. (1976) m odel, one can also con
sider coal seam s and carbonaceous C arboniferous shales o f the V ariscid es’ foreland as a source o f the G erm a n -P o lish anom aly (Fig. 1). T hese strata, if present in the area in ques
tion, should be therm ally m etam orphosed by the E arly P er
m ian volcanism , w hich extended as far east as P oznań—Łódź region (R yka & Pokorski, 1978). B asing on geom agnetic and m agnetotelluric sounding, Jankow ski et al. (1991) con
cluded about the presence o f a deep graben filled w ith highly conductive form ations in th e basem ent o f the N o rth ern C arpathians; porous rocks infilling th e graben, saturated by hot m ineral w aters, have been considered the principal source o f the C arpathian co nductivity anom aly. In relation to the above discussion on geological setting o f the G er
m a n -P o lish anom aly, it seem s necessary to analyze the pos
sible occurrence o f rocks show ing electronic conductivity in th e C arpathian basem ent and th eir location in the deep struc
ture o f the orogen.
MAGNETOTELLURIC SOUNDING (MTS) OF THE POLISH CARPATHIAN BASEMENT: RELIABILITY OF THE
RESULTS
M agnetotelluric sounding has been p erform ed w ith the aim o f locating th e source o f th e C arpathian geoelectrical anom aly (Jankow ski e t al., 1984). In 1975, a survey o f the
top o f the high-resistivity C arpathian basem ent w as initiated (W . B achan, J. S w ięcicka-P aw liszyn & M . M olek, in M ły narski et a l., 1982). This survey detected a depression in the basem ent at a depth o f 17 km near B aligród, as w ell as the overlying low -resistivity layer (0 .4 -5 O m m ). These results led to further sounding in th e years o f 1986-90, throughout the P olish C arpathians.
The sounding w as conducted every 4 - 6 km , along p ro
files perp en d icu lar to the strike o f the orogen. T he data ob tained include depths to the high-resistivity basem ent, the distribution and properties o f the low -resistivity layer. To the w est and south o f K ry n ica and N E o f W ysow a, the M TS resistivity curves are p articularly strongly disturbed. A few soundings betw een R ym anów and Ł upków are characteri
sed by uninterpretable curves. Som e o f the results have al
ready been published by D októr et al. (1990), K uśm ierek (1990) and R yłko & T om aś (1995). The results o f M TS sounding have been used to construct a m ap o f conductivity, as w ell as resistivity m odels o f the tw o transversal C arpa
thian profiles (Jankow ski e t a l , 1991).
Figure 3 show s data concerning the depth to the h igh-re
sistivity horizon. In the zones o f high density o f sounding, only representative values have been show n. These results are confronted w ith w ell-log data. The location o f selected w ells w hich reached crystalline basem ent under the sed i
m entary cover and overthrust flysch nappes (Fig. 3) is also
presented. T he high-resistivity basem ent rocks SW o f W ad
ow ice include P recam brian gneisses, m igm atites and grani
toids o f th e U pper Silesian m assif; those d rilled SE o f K raków are crystalline schists, gneisses and am phibolites o f the R zeszotary block, w hereas east o f that b lock num erous w ells reached poorly m etam orphosed flysch strata o f the C adom ian M ało p o lsk a-C en tral D obrogea orogen. This V en d ia n -(? )L o w er C am brian flysch represents th e m arginal C arpathian basem ent south o f T arnów (Z a k lic zy n -B rzo - zow a) and betw een R zeszow -P rz e m y si and K uźm ina in the south. It should be noted th at th e flysch n ature o f this series w as established by the present author in th e cores from the K uźm ina 1 borehole in the depth interval 7 4 21-7541 m.
It has been found that the high-resistivity horizon drilled in m arginal part o f th e C arpathians is associated w ith the above listed crystalline P recam brian rocks in th e base
m ent. T heir resistivity is a few hundred O m m ; m uch higher than th at o f the overlying sedim entary rocks.
The reliability o f the M TS sounding data is a m atter o f debate. In som e cases, they are com patible w ith w ell-log data (e.g., K uźm ina 1), in other cases they are not. The b o re
hole Z aw oja 1, south o f Sucha, indicates th at th e crystalline basem ent lies deeper than suggested by M TS data; the error exceeds 15%. In the vicinity o f G orlice (boreholes G 11, G 13), O ligocene flysch strata extend at least 300—400 m deeper than the high-resistivity horizon indicated by M TS
Fig. 3. Depth to the high-resistivity horizon (probably top o f the crystalline basement) in the Polish Carpathians, inferred from magnetotelluric sounding (MTS). Dots mark the deepest zone. Boundary between foreland massifs redrawn from Jachowicz & Moryc (1995). / - depth in km, values in brackets refer to sounding data that are inconsistent with well description data or doubtful, 2 - wells that drilled crystalline Precambrian rocks, 3 - wells documenting greater depths to the basement than those indicated by MTS data from the neighbouring areas, 4 - boundaries between blocks of comparable depth to the high-resistivity horizon (most probably faults and erosional scaips on top o f the basement), 5 - boundary of the Upper Silesia Massif (USM) and Małopolski (Little Poland) Massif (MM) beneath the Carpathian Foredeep and marginal part o f the Carpathians, 6 - principal near-surface faults in the Central Carpathians (R - Rużbachy, M - Murań), 7 - Carpathian frontal thrust, 8 - axial zone of the electrical conductivity anomaly, 9 - symmetiy axis o f the pattern o f Neogene faults in the substratum. B - Brzostek, Ba ^ Baligród, Brz - Brzozów, BU - Bukowsko, Cz - Czarna, D - Dukla, G - Gorlice, Gi - Grybów, J - Jasło, Ja -Ja b ło n k i, Jo-Jordanów , K - Krynica, Kk - Krościenko, K r-K ro sn o , K u-K uźm ina, L - Lesko, Li - Limanowa, Ł - Łupków, MD - Mszana Dolna, NS - Nowy Sącz, NT - Nowy Targ, P - Podczerwone, Ra -Rabka, Ry - Rymanów, S - Sanok, St - Strzyżów, Su - Sucha. Tu - Tuchów, Ty - Tylawa, UD - Ustrzyki Dolne, UG - Ustrzyki Górne, U ś-U śc ie Gorlickie, We - Wetlina, Wy - Wysowa, Z - Zakopane, Za - Zakliczyn
sounding. T aking into account these exam ples, the sounding data distinctly too low as com pared to w ell-log data or those o f dubious quality are show n in brackets in Fig. 3. It appears that the relative differences am ong depths o f individual blocks and their boundaries are generally reliable, although not every M TS sounding value show n in Fig. 3 can b e inter
preted as reflecting the actual depth to the crystalline b ase
ment.
Approxim ate depth and main features o f the surface o f high-resistivity basement o f the Polish Carpathians T he overthrust C arpathian nappes are underlain by the E uropean platform , com posed o f crystalline basem ent rocks and overlying, differentiated P alaeo zo ic-M eso zo ic sedi
m entary rocks and M iocene m olasses. These elem ents form together th e C arpathian basem ent. The results o f seism ic re
fraction studies o f the re lie f o f the C arpathian crystalline basem ent have been p resen ted by S łączka (1975); the M TS sounding, how ever, brings num erous changes to this picture.
D ata concerning approxim ate depths to the high-resistivity horizon (Fig. 3) m ake it p ossible to detect supposed bounda
ries betw een areas show ing com parable depths to the crys
talline basem ent. T hese boundaries usually follow different faults and erosional scarps o f canyons. R egular depth changes detected along certain sounding profiles, particu
larly to th e w est and SW o f K rosno and south o f Sanok, p oint to th e presence o f tectonically-tilted and/or erosional surfaces o f basem ent blocks.
The variations in the depth o f the high-resistivity h o ri
zon perm it one to distinguish several depth levels in the top o f the crystalline basem ent (Fig. 3). The upper level is a con
tinuation o f the top o f th e C arpathian Foredeep basem ent beneath th e overthrust flysch nappes. It is a differentiated surface occurring at depths o f 4 - 8 km, averaging at 5 -7 km.
E xtensive basem ent blocks w hose surfaces represent this upper level extend far beneath the C arpathians, as far as Ż yw iec, farther w est into the U pper V istula drainage basin (D októr e t al., 1990), as w ell as southw ards o f Przem yśl, as far as U strzyki D olne area. The southernm ost extent o f the u pper level is m arked by the S u ch a-M sz an a D o ln a-K ry n ica fault system . South o f R zeszów , an equivalent o f this boundary lies closer to the C arpathian front. It is represented by the Ja slo -K ro sn o -L e sk o faults w hich border on the north the subsided basem ent blocks, extending betw een the W y- so w a -Ja sto and S an o k -L e sk o -U strz y k i G óm e faults.
C lose to the upper level are relatively sm all areas w here the top o f the basem ent occurs at depths o f 8 to 10 km, aver
aging at 9 km (Fig. 3). T h ey are incorporated into the upper level N E o f M szana D olna, south o f N ow y Sącz, N W o f G orlice, and close to K rosno and Sanok; they also form large grabens to the north and east o f Lesko. The top o f the basem ent, occurring at 9 km depth, is also represented by a large crustal block, extending betw een S ucha and R abka and including an additional ho rst close to Jordanów , by subsided fragm ents o f the upper level betw een K rynica and W ysow a and east o f Baligród, and by th e R ym anów horst. The last horst m arks a distinct salient o f the upper level in the south, sim ilarly to a sm all horst occurring N E o f K rościenko and another one close to C zam a, n ear U strzyki D olne in the east.
The areas show ing low er-situated top o f th e high-resis- tivity basem ent occur south o f the M szana D o ln a-K ry n ica and Ja slo -K ro sn o -L e sk o lines (Fig. 3). In the w est, they are represented by a vast N ow y T arg -K ry n ic a depression, w here the top o f the basem ent occurs at a depth o f 12-14 km. This level is dissected b y a m inor graben (dotted area in Fig. 3), w herein the high-resistivity horizon has been de
tected at depths 15-16 km and in the eastern p art even at 18-20 km. F arther to th e east, w ithin the bi-partite D u k la - B aligród depression, the high-resistivity horizon occurs at depths ranging from 15-16 km to 18 -2 0 km. T he depression is subdivided in tw o parts by th e R ym anów salient.
T he southern m argin o f the tw o above depressions only partly lies in Poland and only there it w as sounded. D iffer
entiated depths to the basem ent, a general shallow ing and the presence o f blocks w hose top occurs at 7 -9 .5 km, have been detected near G ronków SE o f N ow y Targ, south o f K rynica (M uszyna) and D ukla (T ylaw a), as w ell as betw een Ł upków and W etlina (R oztoki G óm e).
The N ow y T arg -K ry n ic a and D u k la-B a lig ró d depres
sions form a subparallel belt w hich is bordered by the Pod- c z erw o n e-M szan a D olna fault on the w est and th e S a n o k - L esko-U strzyki G órne fault on the east. T he depression belt is also cut by the already m entioned W y so w a -Jaslo fault, w hose southern continuation in th e C entral C arpathians (ac
cording to the tri-partite subdivision o f th e W est C arpa
thians into Outer, C entral and Inner C arpathians; cf. M ahel, 1986) is the near-surface, p ost-P alaeogene M urań fault (Pospisil et a l , 1989). The P o d cze rw o n e -M sza n a D olna fault does also have its southern continuation in po st-P a
laeogene faults th at subdivide the V elka Fatra and N izk e T a try blocks and m ark th e w estern boundary o f th e Stare H ory m a ssif ( vide M ahel e t al., 1967).
L eaving aside a detailed analysis o f this young-A lpine system o f faults in the basem ent, one should take notice o f a peculiar sym m etric pattern, w hose axis is m ark ed by a line extending betw een K raśnik in the C arpathian foreland, via R zeszó w -R y m an ó w tow ards Z em plin and D ebrecen in the Pannonian B asin in the south (Fig. 3). In this pattern, the W ysow a-Jaslo fault is paired by a conjugate S an o k -L esk o -U strz y k i G óm e fault, both o f th em being reverse faults.
Som e 7 5 -8 0 km aw ay o f these faults, tw o conjugate fault system s can be detected: in the C arpathian basem ent be
tw een R u żo m b ero k -M szan a D o ln a-Jo rd an ó w in the west, and in the C arpathian Foredeep basem ent betw een K rako- v e ts-S try y -G o ro d o k in the east (see Figs. 3 and 5 for loca
tion). This N eogene pattern o f a com plem entary set o f shears is follow ed by the extent o f th e M iocene C arpathian F oredeep and, arranged sym m etrically to this axis, Little P annonian B asin and T ransylvanian D epression.
Properties o f the highly conductive layer in the basement of the Polish Carpathians T he M TS sounding have confirm ed the presence o f the highly-conductive layer, frequently called th e low -resistiv- ity layer, at different depths above th e high-resistivity hori
zon in the southern part o f the P olish C arpathians. T his layer is absent in the northern part o f th e orogen, and in the San drainage basin, far in the south (Fig. 4). It has already been
Fig. 4. Location and properties o f high-conductivity rocks in the substratum of the Polish Carpathians inferred from magnetotelluric sounding (MTS). / - selected wells which proved the presence (a) and lack (b) of coal-bearing Upper Carboniferous rocks, 2 - extent of proven coal-bearing Carboniferous strata in the basement o f the Carpathians and Carpathian Foredeep, 3 - extent o f high-conductivity rocks found by MTS sounding in the Polish Carpathians, 4 - approximate thickness (in km) o f the high-conductivity complex overlying high-resistivity basement, 5 - resistivity o f high-conductivity rocks in Omm, 6 - extent o f high-conductivity rocks in the Carpathian basement, detected by geomagnetic (MV) and magnetotelluric (MTS) sounding, 7 - areas where resistivity of rocks overlying high-con
ductivity complex is lower than 10 Omm, 8 - axial zones o f segments of the Carpathian gravity low. For other explanations - see Fig. 3
detected by previous M V and M TS studies (Jankow ski et al., 1984) w hich m arked th e C arpathian conductivity an o m aly (Figs. 1, 4). B asing on geoelectrical data, L efeld and Jankow ski (1985) concluded about the allochthonous nature o f th e T atra m assif, bein g underlain by sedim entary rocks.
B etw een Z akopane and N o w y Targ, as w ell as SW o f N ow y Sącz there are areas w here the highly-conductive layer has not been found. Further to the east, the layer extends co n tinuously o v er a large area, w ith a m inor interruption near R ym anów .
T he low resistivity o f the layer, its m ost im portant char
acteristic, is changeable throughout the study area (Fig. 4).
W est o f K ry n ica m eridian, the resistivity ranges betw een a few and 13 O m m , exceptionally attaining 17.5 O m m . E ast o f K rynica, how ever, th ese values do not exceed 6 -8 O m m and, locally in the southern part o f the area, they drop even below 1 Om m .
T he h igh-conductivity layer is overlain by rocks o f re
sistivity ran g in g betw een 20 and 3 0 0 -4 0 0 O m m , rarely at
ta ining 500 O m m . East o f W y so w a -Jasło line, in the zone o f the m axim um depression o f th e high-resistivity basem ent (Fig. 3), the resistivity o f rocks overlying the highly-con
ductive layer dim inishes to ca. 10 O m m and even 3 O m m (Fig. 4). T hese are the areas w here th e resistivity o f the u n derlying highly-conductive layer drops below 1 Omm.
T he M TS sounding h av e also supplied a w ealth o f infor
m ation on the thickness o f the low -resistivity layer th at co
vers the consolidated basem ent (Fig. 4). This thickness u su
ally ranges betw een 2.0 and 3.5 km; som e m easurem ents suggest 1.5 km and - in exceptional cases (south o f N ow y Sącz and near Ł upków ) - even 4 .5 -7 .0 km. D epths to the low -resistivity layer are conform able w ith differentiated
depths to the top o f the high-resistivity basem ent (cf. Figs. 3 and 4).
A n analysis o f nearly 400 M T sounding perfo rm ed by J.
P aw liszyn (in Jankow ski e t al., 1991, Fig. 6) clearly show s th at the longitudinal electrical co nductivity in the Polish C arpathians increases SE o f N ow y Sącz. T he highest values have been found near K rynica, in a zone o f a basem ent high south o f D ukla, and in the B aligród depression. T he influ
ence o f regional basem ent lows, i.e., the D u k la and B aligród depressions, upon the increase in conductivity is notable.
Geological interpretation o f the highly conductivc layer in the Polish Carpathians basem ent A possibility, resulting from th e P o rsten d o rfer’s et al.
(1976) m odel, th at highly conductive layer in the G e rm a n - Polish anom aly zone com prises P alaeozoic and, p articu larly, m etam orphosed coal-bearing C arboniferous strata should be taken into account w hen discussing th e C arpa
thian anom aly. T he W est C arpathian foreland includes the U pper Silesia C oal Basin w hose U p p er C arboniferous coal m easures are overlain in the south by M iocene m olasses o f the C arpathian F oredeep and o verthrust flysch nappes o f the Carpathian m argin (see O szczypko e t al., 1989). These m easures have been drilled as far south as in basem ent de
pressions south o f O strava and in th e S kaw a river drainage basin (Figs. 1, 4). T he southernm ost borehole Z aw oja 1 drilled C arboniferous coal-bearing strata in a depth interval o f 4858 m to 5023 m (Jaw or, 1989; K am kow ski, 1989).
T hese rocks are also know n from the basem ent o f the M oravian C arpathians N W o f H odonin, w here they have been drilled by N em ćićky 1 b orehole at a depth o f 4 0 0 0 -
4500 m (see O szczypko e t al., 1989).
D ata p resented in Figs. 3 and 4 clearly show th at the highly conductive layer detected by M TS sounding in the C arpathian basem ent could rep resen t a continuation o f coal- b earing C arboniferous ro ck s o f the Skaw a drainage basin, docum ented by boreholes S ucha IG-1, Jachów ka 1 and Z a
w oja 1 (Fig. 4), and low ered by a few to even m ore than 10 km. T he total thickness o f th e coal-bearing U pper C arboni
ferous strata, drilled by a borehole near C hełm ek, attains 2.1 km (Jureczka & K otas, 1986), approaching th at o f the geo
p h y sically detected highly conductive layer. Such a re la tionship suggests th at th e h ighly conductive layer in the C ar
p ath ian basem ent south o f R abka is also com posed o f U pper C arboniferous strata.
T he abnorm ally high thicknesses o f this layer south o f N o w y S ącz and n ear Ł upków m ay result from tectonic cou
plin g or steeply dipping o f strata, w hereas the eastw ard de
crease in resistivity could be associated w ith th e increasing depth, and an additional decrease east o f W ysow a - w ith dy- n am om etam orphism and therm al influence o f the Slova
kian T ertia ry volcanic centres.
It should be also taken into account th at betw een the top o f the high-resistivity basem ent and the low -resistivity coal- b earing sequence, m ay occu r older Palaeozoic, D evonian and L ow er C arboniferous siliciclastic and carbonate rocks, saturated w ith m ineral w aters. Such strata are know n from the basem ent o f the m arginal part o f the C arpathians w here their thickness is 600 m like, for instance, in boreholes near L achow ice, SW o f S ucha (K am kow ski, 1989), or greater.
T hese strata have not been distinguished in the interpretion o f the M TS curves (M olek & O raczew ski, 1988; M olek &
K lim kow ski, 1991; Jankow ski e t al., 1991). T his, how ever, does not contradict a co n c ep t w hich ascribes the high-resis
tivity horizo n in the P olish C arpathians to the top o f P re
cam brian rocks, and the overlying highly-conductive layer to m etam orphosed coal-bearing C arboniferous strata.
T he d ata presented in Figs. 3 and 4 suggest th at the h ig h ly-conductive layer occurs both w ithin the m ajor b ase
m en t depressions and in th e southw ard-located, uplifted blocks o f M uszyna, T y la w a and R oztoki G órne. This points to th e relation betw een th e P alaeozoic infill o f the depres
sions and uplifts in th e basem ent. S E o f R abka, near K roś- c ie n k o -K ry n ic a and close to R ym anów (Fig. 3), the highly- conductive layer has also been found on the high southern m argin o f the platform . S im ilarly as the C arboniferous strata, d ocum ented in the S kaw a drainage basin (Fig. 4), this m ay indicate initial p roxim ity o f the platform and the low ered crustal blocks in the C arpathian basem ent, although tectonic shortening betw een these regions cannot be ex cluded. T herm obaric conditions associated w ith a low ering o f crustal blocks in th e K ry n ic a -D u k la -B a lig ró d zone caused increase in the conductivity o f th e low er part o f their sedim entary cover.
O ne should also take n o tic e o f the fact th at variously m etam orphosed coal-bearing C arboniferous strata are know n in outcrops in th e C entral W est C arpathians, nam ely in Slovenske R udohorie and farther east, n ear Z em plin (M a
hel e t al., 1967), i.e. south o f the conductivity anom aly axis.
M etam orphosed C arboniferous rocks, ca. 250 m thick, have also been found in N W part o f the M arm arosh (R om anian
M aram ureç) M a ssif (Zhukov, 1968), em erged w ithin the E ast C arpathian flysch (Fig. 1).
T he above data indicate th at extensive, h ighly-conduc
tiv e layers found by M TS sounding w ithin the low ered zone o f th e N orthern C arpathian b asem en t are pro b ab ly represen
ted by m etam orphosed, coal-bearing C arboniferous strata.
H ow ever, neither the belt o f b asem en t depressions filled w ith these deposits, no r th e te cto n ic graben filled w ith coal- bearing C arboniferous strata w ithin the platfo rm basem ent o f the C arpathians in th e S kaw a drainage basin, do generate th e changes in th e orientation o f the W iese induction vec
tors. The axis o f the C arpathian geoelectrical anom aly, lo
cated w ith a precision o f a few kilom etres (P raus & Pëcova, 1991), occurs farther south (Figs. 3 and 4), an d it is there w here one should look for geological explanation o f this anom aly. H ence, let us review th e crustal p roperties o f the anom aly zone.
MAJOR CRUSTAL BLOCKS OF THE NORTHERN CARPATHIANS
T he basem ent o f the C arpathian F o red eep and, as fol
low s from the above m entioned m agnetotelluric sounding, o f the O uter C arpathians as w ell, is rep resen ted by crustal blocks th at belong to the C entral E uropean P latform . T hese blocks are overlain by overthrust flysch n ap p es o f an accre- tionary prism . South o f the P ieniny K lippen B elt, th e S lo
vak, i.e., Central W est C arpathian block, extends from the V ienna B asin to th e U zhgorod area (Fig. 1). F urther east
w ards lies an obliquely oriented block o f the C entral East C arpathians th at crops out from beneath flysch strata N E o f the P ieniny K lippen B elt as th e M arm arosh M assif. B oth these Central C arpathian blocks are tectonically com pressed and uplifted; they also differ from one another in th e age o f the m ain folding. T he blocks consist o f nappes w hich com prise, apart from m ostly carbonate M esozoic strata, p re-A l
pine rocks, including crystalline ones. P ost-collisional li- thospheric suture betw een the subsided p latfo rm blocks and th e uplifted C entral C arpathian blocks o riginated in a sub
duction zone o f th e prim ary basem ent o f flysch nappes and the P ieniny K lippen Belt.
East Carpathians - Ukrainian and N-Rom anian sector T o characterise crustal blocks o f th e N orthern C arpa
thians in greater detail, the results o f M T S sounding o f the Polish C arpathian basem ent (Fig. 3) have been com pared w ith those o f the seism ic refraction studies, including deep sounding, in the U krainian C arpathians. T he latter have been confronted w ith w ell-log data and reinterpreted to show features o f the autochthonous, P ala eo z o ic-M e so zo ic basem ent, called “ sub-flysch substratum ” by B u ro v e t al.
(1986) and G lushko & K ruglov (1986). D espite different m ethods (M TS, seism ic refraction) o f various p recision, a general picture o f basem ent highs and low s has been o b tained for a large segm ent o f the C arpathians (Fig. 5). The nam es o f the tectonic elem ents are introduced by the present author.
It is evident from this p ictu re th a t a belt o f depressions
in the P olish C arpathians (N ow y T a rg -K ry n ic a -D u k la - B aligród) has its continuation to the east in the Skole depres
sion, 1 3 -1 4 km deep, show ing longitudinal fractures in its axial and southern parts. T he S W -shifted continuation o f this depression can be found east o f Rakhov, w ithin the u p p er reaches o f the B lack and W hite C herem osh rivers. F u r
ther to the SE, in C âm pulung M oldovenesc, Rom anian C ar
pathians (see Figs 5 and 6 for location purposes), the top o f the basem ent o f this depression occurs at a depth o f 14 km, as show n by M TS data (S tanica et al., 1986).
The B aligród and S kole depressions are separated by a transversal C zarna high (depth to the basem ent ca. 7 km), south o f U strzyki D olne. T he values o f m agnetic Z com po
nen t (see Ż ytko, 1985) suggest that this high is a salient o f the platform th at constitutes the basem ent o f the C arpathian Foredeep, sim ilarly as the already m entioned R ym anów sa
lient (horst). N E o f R akhov, another shallow ing o f the d e
pression belt to ca. 9 km, as w ell as the transversal Shopurka fault can be seen.
The uplifted T ylaw a and R oztoki G órne blocks th a t sur
round the depression on th e south have their continuation in the seism ic refraction pattern in the U krainian E ast C arpa
thians. It is the longitudinal U z h o k -U st’ C horna uplift o f variable h eight w hich rises to th e SE, from - 1 4 km near
U zhok to - 7 km north o f R akhov. F arther to the SE, in R o
m anian C arpathians, the top o f this uplift betw een V atra D ornei and C âm pulung M oldovenesc (cf. Figs. 5 and 6) lowers again to ca. -1 1 km (S tanica et al., 1986).
SW o f this belt o f basem ent highs, w ithin the “sub- flysch substratum ” o f th e P orkulets and D ukla nappes and the F ore-D ukla zone, a deep depression has been found (G lushko & K ruglov, 1986), called the C h e rn o g o lo v a- K rasna depression. It shallow s tow ards SE, from - 1 7 km in the Uh river valley to -1 1 km N W o f Rakhov.
M TS sounding perform ed at “ R akhov” locality have show n that the top o f the h ig hly-conductive layer lies at a depth o f ca. - 1 6 km (R okityansky e t al., 1976b); it w as sup
posed th at this depression is filled w ith altered flysch sedi
m ents underlain by high-resistivity rocks. This m ay indicate a low ering o f the continuation o f th e southern basem ent de
pression under th e overthrust M arm arosh m assif, east o f the S hopurka transversal fault.
A n analysis o f seism ic refraction profiles has show n th at the C h e m o g o lo v a -K rasn a depression has a com plex structure. To the w est o f R akhov it is cut by a longitudinal fault o f dow nthrow n southern side. This fau lt continues through M ezhg o ry e-V o lo v ets tow ards Zboj in eastern S lo
vakia (Fig. 5). It is this fault w hich is follow ed by the axis o f
Fig. 5. Map o f the top o f high-resistivity basement of the Polish Carpathians, concordant with Fig. 3, supplemented by the pattern of seismic refraction-studied basement of the Ukrainian Carpathians (based on Glushko & Kruglov, 1986). Selected surficial geological boundaries shown for location purposes. 1-6 - northern extent of the East Carpathian nappes ( / - Stebnik-Sambor, 2 - Borislav-Pokutie, 3 - Skiba: further to the west - sub-Silesian and Skole frontal thrusts. 4 - Chernohora, 5 - Dukla, 6 - Porkulets), 7 - Rakhov Unit, 8 - Marmarosh massif (Crystalline-Mesozoic zone of the East Carpathians), 9 - Magura Nappe (its outer part in the west has been omitted), 10 — Pieniny Klippen Belt (segments near Nowy Targ and Presov have been omitted), 11 -young-A lpine volcanic rocks, 12 - boundaries o f uplifted (+) and subsided (—) basement blocks, 13 - depth contours (in km) and faults in the flysch substratum, 14 — boundary between the margin o f the Central European Platform and the subsided crustal slab, 15 - basement high within the subsided crustal slab (M - Muszyna, T - Tylawa, RG - Roztoki Górne), 16 - depth contours (in km) of the sub-Palaeogene surface o f Triassic dolomites in the Central West Carpathians. Bv - Bardejov, C - Chernogolova, Kr - Krasna, M - Mezhgorye, R - Rakhov, Sv - Svalava, U - Uzhok, UCh - Ust’ Chorna, V - Volovets, Zb - Zboj. For other explanations see Figs. 3 and 4
Fig. 6. Relation between geoelectrical anomaly and tectonic elements o f the East Carpathians (vectors compiled after Rokit- yansky et al., 1976b and Pinna et al., 1992;
geology based on Sändulescu et a l, 1978;
Sàndulescu, 1984; Glushko & Kruglov, 1986). 1 - major thrusts o f the Moldavides, 2 - inner flysch units (Outer Dacides): Rak
hov, Porkulets, “Black Flysch”, Ceahläu; 3 — Magura Unit (Monastyrets, Petrova and Botizei flysch zones, as well as “Wild- flysch”), 4 - Pieniny Klippen Belt and Poiana Botizei klippes, 5 - Crystalline- Mesozoic zone o f the East Carpathians (Mid
dle Dacides), 6 - Transylvanian nappes, 7 - pre-Tertiary rocks o f the hinterland, 8 - young-Alpine volcanic rocks, 9 - northern extent o f Neogene sediments in the Transyl
vanian Basin, 10 - axis o f geoelectrical anomaly and X -Y displacement, I I - Wiese vectors
geoelectrical anom aly. T he C h e m o g o lo v a -K rasn a depres
sion is bordered on the SW by the longitudinal Svalava fault, beneath the inner part o f th e P orkulets unit (G lushko
& K ruglov, 1986). T he fau lt probably form s th e N W con
tinuation o f the outer b o undary o f the M arm arosh m assif, i.e. the C entral E ast C arpathians.
T he SE continuation o f the Svalava fault is a deep (dow n to M oho) crustal fau lt in the R om anian C arpathians near V atra D om ei (S tan ica e t al., 1986). The fault-related depression is, how ever, v ery narrow ; a possible continu
ation o f th e graben o ccurring w ithin the deepest part o f the C h e m o g o lo v a -K ra sn a depression, betw een Svalava and Z b o j-M ez h g o ry e faults, is here buried beneath the block o f C ry sta llin e-M eso z o ic units o f th e C entral E ast C arpathians th at are th ru st upon flysch units. S tanica e t al. (1986) and D.
S tanica & M . S tanica (1993) report on the presence o f a deep-seated fault also further south, close to M iercurea Ciuc, in a M TS profile b etw een O dorhei and T ârgu O cna {cf. Fig. 6). C rystalline p latform basem ent o f the F lysch C ar
pathians outw ards o f th e fau lt form s there a vast depression o f poorly differentiated bottom , w hose depth attains - 1 0 km.
P in n a et al. (1992) accep t that the crustal fault betw een V atra D o m ei and M iercu rea C iuc continues as far as 46°
parallel n ear Sf. G heorghe and th at it is associated - geo
graphically - w ith th e C arpathian electrical conductivity anom aly, induced by th e presence o f highly conductive rocks close to the fault.
G eoelectrical anom aly is disrupted close to 46° parallel and appears again ca. 100 km to the east, near Focçani, from w here it continues to the SW and w est into the C arpathian F oredeep (Fig. 1). This disruption approaches as w ell a left- lateral shift o f the axis o f regional gravity low by about 25 km along the Trotuç river valley; the shift occurs in the w es
tern continuation o f the northern boundary o f the C im m erian orogen o f N orthern D o b ro g e a -C rim e a (V isarion et a l , 1988).
W est Carpathians - Polish and E-Slovak sector D ata p resented in Fig. 5 p o in t to the presence in the sub- flysch basem ent o f the U krainian C arpathians o f tw o belts o f orogen-parallel depressions. It is possible th at th e C her- n o g o lo v a -K rasn a -R ak h o v depression continues to the N W under flysch units o f the Slovak C arpathians, as indicated by data from the P re so v -D u k la region.
M TS sounding south o f D u k la detected a basem ent high ( -1 0 km ), called the T ylaw a h o rst (Figs. 3, 5). S eism ically- detected (M orkovsky e t a l., 1992) surface o f T riassic dolo
m ites o f the C entral W est C arpathians, co v ered by P alaeo gene flysch strata, dips to the north u nder th e P ien in y K lip pen B elt and the inner part o f fly sch nappes, at least to a depth o f - 8 km (Fig. 5). W ell-log data collected by the State G eological Institute betw een th e T atras and th e P ieniny K lippen Belt, close to N o w y T arg, do also reveal a sim ilar feature. O ne can infer, therefore, the presence o f a basem ent depression betw een the T y law a and R oztoki G órne horsts in the north and the northern slope o f th e C entral C arpathians in the south. H ence, the C h e rn o g o lo v a -K ra sn a -R a k h o v de
pression probably continues tow ards N W , from the Uh drainage basin to B ardejov.
It has already been m entioned th a t th e deep basem ent o f the northern belt o f depressions betw een N o w y T arg and B aligród, together w ith the horsts w hich surround them on the south, probably represents a low ered part o f th e C entral E uropean Platform . T aking into account m agnetic p ro p er
ties (AZ) o f the C zarna high, south o f U strzyki D olne, one can suppose as w ell that the Skole depression and the sur-
5 — Annales...
Fig. 7. Geoelectrical anomaly in relation to tectonic elements o f the West Carpathians (vectors compiled after Jankowski et al., 1984; Lefeld
& Jankowski, 1985; geology based on Książkiewicz, 1962; Mahel etal.,
1967). I - major thrusts o f the Outer Carpathians (SW - Świątkowa), 2 - /one o f probable occurrence of slices composed o f the black Neo- comian flysch (possible continuation o f the Outer Dacides), 3 - Magura Unit, 4 - Pieniny Klippen Belt, 5 - pre-Tertiary units o f the Central West Carpathians, 6 - young-Alpine volcanic rocks, 7 - axis o f geoelec
trical anomaly and the possible X -Y displacement, 8 - Wiese vectors. R - Ruźbachy fault, M - Murań — Wy
so w a -Ja sło fault
rounding U z h o k -U s t’ C h o m a high could also represent a low ered, m arginal part o f the C entral European Platform . This conjecture is additionally supported by sim ilarities be
tw een the boundaries o f th e subsided and uplifted parts o f the crust, particularly to th e east o f the W y so w a-Jasło fault.
T he R ym anów and C zam a salients, belonging to the up
lifted part, face recesses in the southern belt o f basem ent highs (Fig. 5).
T he above m entioned tw o C entral C arpathian crustal blocks occur SW o f the B a rd e jo v -C h e rn o g o lo v a - K ra sn a - R akhov depression and V atra D o m ei-M iercu rea C iuc fault.
CARPATHIAN GEOELECTRICAL AND GRAVITY ANOMALIES VERSUS SURFICIAL AND DEEP STRUCTURE OF THE NORTHERN CARPATHIANS
It should be stressed th a t east o f N o w y T arg the axis o f the gravity low lies north o f the outer b elt o f basem ent de
pressions and it is related to th e N eogene pattern o f sym m e
try (see Figs. 4, 5). The basem ent depressions do n o t show up in the pattern o f the grav ity field. B etw een the T atras and Żilina region, the axis o f th e anom aly is situated w ithin the C entral C arpathians. F urther to th e w est it cuts the P ieniny K lippen B elt, and then proceeds along the flysch nappes to w ards V ienna (Figs. 1, 7). Sim ilarly, to the east o f the T a tras, the anom aly axis protrudes into the flysch Carpathians.
T he arrangem ent o f W iese induction vectors (Fig. 7) indi
cates a disruption o f a generally linear trend o f th e anom aly betw een the T atras and K rynica, probably due to the p res
ence o f the R uźbachy and M u ra n -W y so w a -Ja sło fault sys
tem. L efeld and Jankow ski (1985) hypothesized about the overthrust o f the T atra crystalline m a ssif upon a series o f sedim entary and/or m etam orphic rocks th at are the carrier o f a ionic source o f the anom aly.
East o f K rynica, the anom aly axis occurs w ithin the
M agura N appe area and then w ithin rocks o f th e structurally low er D ukla nappe and even F o re-D u k la zone. SE o f K ras
na, even up to Sf. G heorghe (Fig. 6), th e axis o f th e electri
cal anom aly appears to be associated w ith the O uter D acides flysch sequences (Porkulets, R akhov, “ B lack F ly sch ” , Cea- hläu N appes) that are overlain, to a large extent, by over- thrust units o f the C entral E ast C arpathians (P inna et al., 1992). It is a feature sim ilar to th a t inferred for the W est C arpathians in the T atra region.
To explain th e relation betw een th e basem ent and the overlying sedim entary rocks o f the accretionary prism in the geoelectrical anom aly zone, the area n ear the M arm arosh m a ssif o f the Central E ast C arpathians near R akhov (Fig. 5) should be taken as the best exam ple. T he depth to the top o f the highly-conductive layer under th e R akhov and P orkulets flysch units is about 16 km (R okityansky et al., 1976b). A n association o f grey and b lack flysch strata, to gether w ith m afic eruptive rocks w ithin these units (R akhov, B elaya Tisa, B urkut F orm ations), alongside w ith th eir R om anian counterparts in the “B lack F lysch” and C eahläu U nits (S i
naia and B istra Form ations) represents sedim entary infill o f the Ju rassic-E arly C retaceous p alaeo rift o f the O uter D aci
des (vide Sändulescu, 1984, 1989). T his rift, originally situ
ated in a m arginal zone o f the E uropean continent, has been com pared to the A fa r-R e d Sea rift, and considered to be an equivalent o f V alais, an outer rift in the A lpine P enninicum zone. It is likely th at th e O uter D acides rift, w hose very con
cept is based on an analysis o f allochthonous orogenic com plexes, is associated w ith the C h e m o g o lo v a -K ra sn a -R a k - hov basem ent depression and its continuations, both to w ards B ardejov and V atra D ornei.
The uplifted M arm arosh m a ssif and the regional shal
low ing o f the C arpathian basem ent N E o f R akhov (Fig. 5), as w ell as the proxim ity o f th e anom aly axis to the Z b o j- M ezhgorye fault in the northern m argin o f the basem ent de
pression, all point to a possibility o f a causal relationship be
tw een th e geoelectrical anom aly and the O uter D ac id es’ rift,
o r - d irectly - its N E boundary. A shift o f the anom aly axis into th e flysch o f the D ukla N appe tow ards N W and the p o sition o f th e northern b o u n d ary o f the P orkulets U nit at the surface in respect to basem ent depression (Fig. 5) do not n ecessarily testify to th e lack o f initial interrelationship ex
isting betw een the depression and P orkulets flysch. Such a situation appears to be sim ilar to th e “ fan idea” discussed by A ndrusov (1968), nam ely th e occurrence o f post-P alaeo- gene, south-vergent folds and slices in the inner part o f the M agura U nit, P ieniny K lip p en B elt or even Central C arpa
thians (e.g. the south-vergent T atra m ega-anticline). Back- thrusting L aram ian and S avian m ovem ents have also been accepted b y B irkenm ajer (1986) for th e G rajcarek Unit.
Such features are also d ocum ented by detailed geological m aps o f th e M agura U nit in E ast S lovakia (M atëjka et al., 1964) and th e N -dipping top o f M esozoic strata o f the C en
tral C arpathians (Fig. 5). B ackthrusts occur as w ell in the F ore-D ukla zone in the San river drainage basin (see Żytko, in: M łynarski et al., 1982; Fig. 6). H ence, there is a bulk o f evidence pertaining to subduction and shortening o f the b asem ent w ithin the B a rd e jo v -C h e rn o g o lo v a depression, as w ell as p ointing to a shift o f allochthonous elem ents tow ards SW in relation to the southern boundary o f the depression.
Starting from K rynica, this depression is placed south o f the M uszyna high and co ntinues further w est under the over- thrust T atra m assif, follow ing the centre o f the geoelectrical anom aly.
ON POSSIBLE PRESENCE AND LOCATION OF THE JURASSIC- CRETACEOUS RIFT OF THE OUTER DACIDES IN THE POLISH CARPATHIANS
T he C arpathian geoelectrical anom aly coincides both w ith the Z b o j-M ez h g o ry e basem ent fault in the northern part o f th e C h e m o g o lo v a -K ra sn a depression, and w ith the R akhov, Porkulets, “ B lack F lysch” and C eahlau inner flysch units o f the E ast C arpathian allochthon. T hese units are com posed o f the infill o f th e O uter D ac id es’ palaeorift, situated outw ards o f th e P ieniny K lippen B elt, M agura flysch and M arm arosh M a ssif (Figs. 5, 6). T he P orkulets unit flysch is know n both from the surface and the C hem o- golo v a d eep borehole, close to the S lovak/U krainian bound
ary. T h ese strata plunge tow ards N W under the M agura N ap p e (Fig. 5), and th e ir traces can probably be found far
ther w est. O ne should also take into account the affiliation o f O ligocene strata o f the P orkulets and D ukla U nits (Dus- ina, T uritsa and M alyy V yzhen Form ations) to the M eni- lite -K ro sn o rather than M agura flysch lithofacies (B urov et al., 1986).
In th e 1960s, W. S ikora expressed an opinion about the presence o f L ow er C retaceous black flysch strata, called the S ztolnia B eds, w ithin a separate zone distinguished betw een th e M agura U nit and th e P ieniny K lippen B elt. This zone is also called th e H ulina U nit or G rajcarek U nit (see Sikora, 1971; B irkenm ajer, 1986). B irkenm ajer and M yczyński (1977) docum ented a M iddle Jurassic age o f these dark, m i
caceous S ztolnia Beds and renam ed them as the S zlachtow a Form ation.
N evertheless, there appeared new data on th e presence o f L ow er C retaceous dark flysch deposits betw een th e P ien iny K lippen B elt and the D ukla U nit. B irkenm ajer et al.
(1979) described dark, H a u te riv ian -A lb ia n flysch strata, ca.
200 m thick, from the G rajcarek unit (w ell PD 9 at S zczaw nica) and called them the B ry jark a F orm ation. T hese strata are overlain by C e n o m a n ian -T u ro n ian v ariegated shales and sandy U pper S enonian Jarm u ta F orm ation. T he sam e unit com prises as w ell black and green shales o f the W roni- ne Form ation o f A lbian age, also co ntaining variegated shales at the top (B irkenm ajer & D udziak, 1987). T he pro x im ity o f T ith o n ian -N eo co m ian pelitic, chert-bearing lim e
stones indicates th at the unit is ch aracterised by tw o differ
ent N eocom ian facies, i.e. flysch and lim estones.
G olonka and R ączkow ski (1984) d escribed from Szczaw nica dark flysch strata contain in g E arly C retaceous foram inifer assem blages (det. J. B laicher) and upheld the nam e S ztolnia B eds for these deposits. A cco rd in g to B. O l
szew ska {pers. com m .), the assem blage com prises, am ong others, the species A m m obaculoides carpathicus G eroch, indicative o f N eocom ian age.
In B iała W oda, in the sam e zone, a b lock o f basalts (olistholite?) has been found, w hose age has been deter
m ined at 140 ±8 Ma, i.e. Jurassic/C retaceous transition (A rakelyants, in B irkenm ajer & W ieser, 1990). N um erous clasts o f m afic, spilitic rocks have also been found in Senonian conglom erates.
Further to the north, in the m argin o f the M szana D olna tectonic w indow , dark A lb ia n -C en o m a n ia n flysch strata o f the L gota B eds occur w ithin a series that overlies th e D ukla and G rybów units (B urtan & Ł ydka, 1978; B urtan et al., 1992). C lose to these beds occurs a com plex o f alternating siliceous sandstones and black clayey shales lacking in fora- m inifers; they also com prise an interlayer o f extrusive rocks (B urtan e t al., 1978, p. 27). T he strata in question have been assigned to th e M agura N appe series, although strong tecto- nization m akes such an assignm ent p roblem atic. Som e o f the L gota Beds have been associated w ith the G rybów U nit (M astella, 1988). B oth the W ronine F orm ation strata o f the G rajcarek unit, and the “ Lgota B e d s” close to M szana D olna are im pregnated w ith copper m ineralization.
The above data on the H au te riv ian -A lb ia n age o f the B ryjarka Fm , as w ell as the p resen ce o f N eocom ian foram i- nifers w ithin the S ztolnia B eds near S zczaw nica (cf. G olon
ka & R ączkow ski, 1984) point to a possibility o f deposition o f flysch strata and volcanic rocks (basalts from B iała W oda) in an E arly C retaceous anoxic basin north o f the Pi
eniny K lippen basin (Fig. 7). Such a basin could have ex
isted already in D oggerian tim es (S zlachtow a Form ation), as accepted by B irkenm ajer (1986). This basin m ight have been a continuation o f the O uter D a c id e s’ rift and, strictly speaking, the “ Black F ly sch ” U nit. M ost o f its sedim entary infill has been tectonically reduced; its relics are visible in the G rajcarek Unit.
It should be added that both, th e C retaceous and P alaeo gene strata o f the P ieniny K lippen B elt and the C entral C ar
pathian and M agura flysch, com prise ab undant detritus o f ultram afic rocks, particularly chrom ium spinels (Starobova, 1962; M isik e t al., 1980; W inkler & Ś lączka, 1992, 1994).
T hey indicate the presence o f ophiolitic com plexes o f oce-
