GeolCarp_Vol68_No3_177

GEOLOGICA CARPATHICA

, JUNE 2017, 68, 3, 177 – 192

doi: 10.1515/geoca-2017-0014

www.geologicacarpathica.com

3D density modelling of Gemeric granites

of the Western Carpathians

JÁN ŠEFARA

†

, MIROSLAV BIELIK

1,2,

, JOZEF VOZÁR

, MARTIN KATONA

VIKTÓRIA SZALAIOVÁ

, ANNA VOZÁROVÁ

, BARBORA ŠIMONOVÁ

JAROSLAVA PÁNISOVÁ

, SABINE SCHMIDT

and HANS-JÜRGEN GÖTZE

Department of Applied and Environmental Geophysics, Faculty of Natural Sciences, Comenius University, Ilkovičova 6,

842 48 Bratislava, Slovakia;



bielik@fns.uniba.sk

Earth Science Institute of the Slovak Academy of Sciences Dúbravská cesta 9, 840 05 Bratislava, Slovakia; jozef.vozar@savba.sk

Horska 9/A, 831 52 Bratislava, Slovakia; katonamartin@gmail.com

Geocomplex, Grösslingová 45, 811 09 Bratislava, Slovakia; szalaiova@geocomplex.sk

Institute for Geosciences, Christian-Albrechts-University, Otto-Hahn-Platz 1, 24118 Kiel, Germany;

sabine@geophysik.uni-kiel.de, hajo@geophysik.uni-kiel.de

(Manuscript received Septebmer 7, 2016; accepted in revised form March 15, 2017)

Abstract: The position of the Gemeric Superunit within the Western Carpathians is unique due to the occurrence of the

Lower Palaeozoic basement rocks together with the autochthonous Upper Palaeozoic cover. The Gemeric granites play

one of the most important roles in the framework of the tectonic evolution of this mountain range. They can be observed

in several small intrusions outcropping in the western and south-eastern parts of the Gemeric Superunit. Moreover, these

granites are particularly interesting in terms of their mineralogy, petrology and ages. The comprehensive geological and

geophysical research of the Gemeric granites can help us to better understand structures and tectonic evolution of the

Western Carpathians. Therefore, a new and original 3D density model of the Gemeric granites was created by using the

interactive geophysical program IGMAS. The results show clearly that the Gemeric granites represent the most significant

upper crustal anomalous low-density body in the structure of the Gemeric Superunit. Their average thickness varies in the

range of 5–8 km. The upper boundary of the Gemeric granites is much more rugged in comparison with the lower

boundary. There are areas, where the granite body outcrops and/or is very close to the surface and places in which its

upper boundary is deeper (on average 1 km in the north and 4–5 km in the south). While the depth of the lower boundary

varies from 5–7 km in the north to 9–10 km in the south. The northern boundary of the Gemeric granites along the

tectonic contact with the Rakovec and Klátov Groups (North Gemeric Units) was interpreted as very steep (almost

vertical). The results of the 3D modelling show that the whole structure of the Gemeric Unit, not only the Gemeric granite

itself, has an Alpine north-vergent nappe structure. Also, the model suggests that the Silicicum–Turnaicum and Meliaticum

nappe units have been overthrusted onto the Golčatov Group.

Keywords: applied geophysics, gravity, 3D density modelling, Gemeric granites, Spiš-Gemer Ore Mts., Western

Carpathians.

Introduction

The Gemeric granites comprise several small intrusions out-

cropping in the western and south-eastern parts of the Gemeric

Superunit, which is one of the principal Alpine tectonic units

of the Central Western Carpathians. They are particularly

interesting not only from the point of their geological struc-

ture, tectonic position, mineralogical and petrographical com-

position but also in terms of mineral deposits occurring in the

Spiš-Gemer Ore Mts. This was one of the reasons why this

mountain belongs to the best geophysically explored regions

of Slovakia (e.g., Filo 1968; Plančár et al. 1977; Grzywacz &

Margul 1980; Husák & Muška 1984; Mikuška 1984; Grecula

et al. 1985; Šefara et al. 1987; Filo & Kubeš 1994; Suk et al.

1996; Vozárová 1996; Mikuška & Marušiak 1999; Vozár &

Šantavý 1999; Szalaiová et al. 2001). Some of these works

deal with the geophysical interpretation of the geological

structure of the Gemeric Unit and the Gemeric granites. These

geological structures are well documented by their outcrops

and in structural boreholes (e.g., SG-2 in the Prakovce locality,

Grecula 1992). They can also be clearly recognized in the seis-

mic and gravimetric images (e.g., Šefara et al. 1987; Tomek

1993; Vozár et al. 1996; Vozár & Šantavý 1999; Bielik et al.

2006). The seismic reflection measurements along the N–S

trending Transect G (Fig. 1) played perhaps the greatest signi-

ficance for the geological and geophysical studies of the

Gemeric Superunit. It was situated in the eastern part of the

Spiš-Gemer Ore Mts. The explanation of the complicated geo-

logical and tectonic structure of the Gemeric Superunit as

a dominant mega-tectonic unit of the innermost Western Car-

pathians has been the goal of the previous seismic reflection

measurements.

In the last decades, there was unbelievable progress in

development of 3D interpretation of anomalous bodies by

means of gravity field (anomaly). At the beginning the calcu-

lated effect of the anomalous density body has been solved by

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replacing the sum of calculated effects of the geometrically

simple (regular shaped) bodies. The most widely used appro-

ximation consisted of a variable number of rectangular prisms

(e.g., Talwani & Ewing 1960; Grant & West 1965; Cordell &

Henderson 1968; Smíšek and Plančár 1970; Talwani 1973;

Plančár et al. 1977; Starostenko et al. 1997, 2015, 2016;

Starostenko & Legostaeva 1998; Grabowska et al. 1998;

Bojdys 2006 a, b). Currently, the 3D interpretive methods using

the so-called polyhedrons (i.e. the bodies bounded by a poly-

gonal surfaces (facets)) are applied frequently (e.g., Bott 1963;

Nagy 1966; Okabe 1979; Hansen & Wang 1988 in Blakely

1996; Pohánka 1988, 1998). This category also includes the

software IGMAS (Interactive Gravity and Magnetics Applica-

tion System), which is a tool applied for the interpretation of

observed gravity and magnetic fields. The IGMAS program is

an indirect modelling approach using trial-and-error forward

modelling. It works by means of a numerical simulation of

underground structures that are described as closed poly-

hedrons of constant density/susceptibility, the surface of which

is triangulated (Götze 1978; Götze & Lahmeyer 1988; Schmidt

& Götze 1998). Now, the current IGMAS software ranks

among the best in the world (Schmidt et al. 2011, 2015; Alvers

et al. 2014).

The main aim of this work is to apply the interactive IGMAS

program for development of the original 3D density model of

the Gemeric granites, which gives results consistent with

recent geological and geophysical knowledge. The article has

been completed in honour and memory of J. Šefara by the

team of the authors.

Geological overview

According to the classical definition (e.g., Andrusov 1968;

Andrusov et al. 1973), the Gemeric Superunit (Fig. 1) includes

the Early Palaeozoic complexes and Late Palaeozoic–Meso-

zoic envelope sequences. The classical definition changed

fundamentally, as it was proved that the Mesozoic carbonate

rock complexes, originally thought to be its cover sequence

are in an allochthonous position on the nappe units of Silici-

cum, Turnaicum and Meliaticum, which was verified (Kozur

& Mock 1973; Bajaník et al. 1983; Mello et al. 1996). Detailed

investigations of the Early and Late Palaeozoic rock comple-

xes led to the subdivision of the formerly defined Gemeric

Superunit into two tectonic units: the Northern and Southern

Gemeric Units (Bajaník et al. 1983, 1984 a, b; Vozárová &

Fig. 1. Geological map of the Gemeric Superunit — a segment of the studied region (modified after Vozárová et al. 2013 and Geological Map

of the Slovak Republic at scale 1:500,000; Biely et al. 1996 a, b). The course of the approximated Transect G and the interpretative profiles

shown in Fig. 6.

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GEOLOGICA CARPATHICA

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Vozár 1988; Vozár et al. 1996; Vozárová 1996). Both consist

mainly of pre-Carboniferous crystalline rock complexes and

late- to post-orogenic Variscan formations. In the cover

sequence only linking between the Lower Triassic and Permian

is evident. The majority of the Mesozoic part in both cover

complexes was tectonically truncated.

Sporadically, stratigraphic data in pre-Carboniferous forma-

tions were and are the reason of the controversial understan-

ding of the inner structure. According to one group of authors,

there is an asymmetric highly Alpine-reworked mega-

anticline, lined up by granitoid (see the map of Bajaník et al.

1984a). According to Grecula (1982) the inner structure of the

Gemeric Superunit is dominated by a system of Late Variscan

nappes, in which granites are also included. However, this

interpretation is not in agreement with the results of the deep

reflection seismic transect G (Vozár et al. 1993, 1996). It con-

firmed an Alpine north-vergent nappe structure supported by

the mainly Permian age of granitoids (Finger & Broska 1999;

Poller et al. 2002; Kohút & Stein 2005) and/or the Jurassic to

Cretaceous cooling ages of their tectonic overprinting (Kantor

1957; Kantor & Rybár 1979; Kovách et al. 1979). The seismic

interpretation was also supported by that of Hók et al. (1993),

data of contact metamorphism (Vozárová et al. 2001) and the

Alpine age of reworked mica (Breiter et al. 2015).

The Northern Gemeric Unit consists of Lower Palaeozoic

volcanic-sedimentary formations reflecting subduction-

collisional processes of the Variscan orogeny, which were

connected with polyphase, metamorphic events and develop-

ment of the Carboniferous–Permian syn- and post-orogenic

basins (Bretonic, Sudetic, Asturian movements). They contain

pre-Carboniferous high-grade and low-grade metamorphosed

complexes of distinct oceanic affinity, which were amalgama-

ted by polyphase processes in the Early and Middle Carboni-

ferous times. This is confirmed by relicting infillings of the

Lower Carboniferous remnant-basin with olistoliths of ser-

pentinized ultrabasic rocks, metabasalts, dolerites and amphi-

bolites (Ochtiná and Črmeľ Groups — Vozárová 1996), as

well as of a peripheral shallow-marine Westphalian basin

(remaining formations of the Dobšiná Group) and the sedi-

ments, which already superimposed on the Variscan structure.

The post-orogenic transpressional regime was linked with

development of continental Permian sequences. The lagoonal-

sabkha-type Upper Permian to Lower Triassic formations are

connected with the beginning of the Alpine cycle.

The Southern Gemeric Unit is composed, in its major part,

of the Lower Palaeozoic volcanogenic flysch (Gelnica Group

in the sense of Snopko & Ivanička 1978; Ivanička et al. 1989),

probably affected by Late Variscan folding and very low-grade

metamorphism. The origin of this complex is connected with

an active continental margin (Bajaník & Reichwalder 1979;

Vozárová 1993). The Gelnica Group was generally described

as a megasequence of deep-water turbidite siliciclastic sedi-

ments, associated mainly with the rhyolite-dacite volcanic/

volcaniclastic rocks. Acidic to intermediate magmatic arc vol-

canism (Vozárová & Ivanička 1996; Vozárová et al. 2010) was

highly explosive, which resulted in the redeposition of vast

amounts of volcaniclastic material into the sedimentary basin

by a system of gravity and mass currents. Besides them, thin

horizons of metabasaltic volcaniclastics and sparse associated

metabasalts occur. Olistoliths of metabasalts were included in

the binder of gravity sliding and slumping. Their chemical

composition points to mixed tectonic settings of the magmatic

source, similar to CAB, VAB, E- and E-MORB (Ivan et al.

1994).

According to microflora, the stratigraphy of the Gelnica

Group ranges from the Cambrian to Lower Devonian

(Snopková & Snopko 1979). Further biostratigraphical data,

based mainly on agglutinated foraminifers of the family

Psammo sphaeridae and Saccamminidae, prove the Late

Cambrian/Ordovician to Early Silurian ages (Vozárová et al.

1998; Soták et al. 1999). The Late Cambrian-Ordovician in

situ U–Pb sensitive high-resolution ion microprobe (SHRIMP)

concordant average zircon ages, 494 ±1.6 Ma, 465.8 ±1.5 Ma

and 463.9 ±1.7 Ma (Vozárová et al. 2010), of magmatic rocks

confirm the biostratigraphic data.

The Štós Formation is a further pre-Permian low-grade

complex, situated only in the SE part of the Southern Gemeric

surface exposures. The contact of the Gelnica Group and Štós

Formation rock complexes is tectonic. A shallow north-

verging thrust plane is documented by the deep seismic profile

(Vozár et al. 1995). Due to the intense Lower/Middle Creta-

ceous nappe stacking of the Inner Western Carpathians nappe

units, the Southern Gemeric complexes are affected by strong

Early Cretaceous overprinting (chemical Th–U-total Pb iso-

chrone method (CHIME) monazite data (Urban et al 2006;

Vozárová et al. 2014).

The Lower Palaeozoic Southern Gemeric Unit is discon-

formably covered with an angular unconformity at the base by

the Permian continental riftogenic formation (Gočaltovo

Group) prograding into Upper Permian–Lower Triassic

lagoo nal to shallow-marine deposits. This sequence is gene-

tically connected with the beginning of the Alpine geotectonic

cycle.

The Northern Gemeric and Southern Gemeric Units were

probably juxtaposed during latest Pennsylvanian/Permian

transtensional movements, as is documented by detrital zircon

assemblages (Vozárová et al. 2013). This does not exclude

later separation during the Late Permian-Triassic extension or

subsequent Cretaceous juxtaposition during Alpine nappe

stacking. The latter is documented by the 131 Ma newly-

formed zircon rims around older detrital zircons.

The Gemeric granites (Uher & Broska 1996) are exposed in

several massifs which intruded Lower Palaeozoic

metapelites-metapsammites as well as acid metavolcanics

(rhyolites to dacites and their pyroclastic equivalents) of the

Gelnica Group, in the Southern Gemeric Unit. It is assumed

that they are the topmost parts of a granite body, of which the

main part is located at depth. Known surface exposures of

granites are found in the vicinity of Hnilec, Zlatá Idka, Poproč,

Betliar. The granite is also outcropping in the transverse eleva-

tions of three (Hnilec, Lužice and Turecka Hill) anticlinal

bands of the Gelnica Group and in many other smaller

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outcrops. The greatest outcrop around Poproč has dimensions

of 6.5 km to 1.5 km.

These leucocratic biotite and biotite-muscovite granites are

accompanied by granite porphyries (Betliar body), and some-

times by greisens and albitites in granitic cupolas with Sn-W-

(Li-Nb-Ta) mineralization (Hnilec, Dlhá Valley; Malachovský

1983). According to the first monazite electron-microprobe

dating results (Finger & Broska 1999), they are post-orogenic

and of Permian age. Uher & Broska (1996), Petrík & Kohút

(1997), Broska & Uher (2001) and Broska et al. (2002)

assigned these granites to the specialized S-type characte-

ristics, for example, by their high Si, K, Rb, Sn, B, F; and low

Zr and REE contents. Their data indicate a high temperature

(solidus T – 750 °C), dry (1–3 % H

0), and a variable oxygen

fugacity of the magma.

It is not certain today whether the Gemeric granites are

formed by Variscan post-orogenic and/or the Early Alpine

riftogenic processes. Indeed, their age was proven as Permian

(275–251 Ma) by various mineral dating methods (monazite

— CHIME, Finger & Broska 1999; zircon — CC-TIMS —

cathodeluminescence controlled thermal ionization mass

spectrometry, Poller et al. 2002; molybdenite — N-TIMS —

negative thermal ionization mass spectrometry, Kohút & Stein

2005; zircon — SHRIMP Radvanec et al. 2009; zircon — LA

ICP-MS — laser ablation inductively coupled plasma mass

spectrometry, Kubiš & Broska 2010). Their geochemical and

mainly isotopic characteristics suggest sources in the mature

upper crustal material with a contribution from lower crustal

metabasites (Kohút 2012). Generally, granites are emplaced

within the crust during extension/relaxation phases of oro-

geny, albeit the situation in the Gemeric Superunit suggests

rather transition between the post-Variscan subduction/

collision relaxation and the initial Palaeo-Alpine rifting

(Kohút & Stein 2005; Radvanec et al. 2009).

Recent U–Pb zircon SHRIMP/SIMS dating results from the

various Variscan Western Carpathians I/S-types of granitic

rocks (Kohút et al. 2009, 2010; Broska et al. 2013) imply that

they originated between 367–353 Ma, and 340 –332 Ma

respectively, mirroring subduction and collision stages of the

Variscan orogeny. Most probably, they originated in an arc-

related environment within the Galatian superterrane (an assem-

blage of Gondwana derived fragments) in the so-called

“Proto-Tatricum” (Broska et al. 2013). Now, these granitoids

are incorporated as a part of the crystalline basement into the

Alpine tectonic units — Tatric and Veporic Units within the

present West-Carpathian mountain chain.

Specialized Permian granites from the Gemeric unit repre-

sent another family of granitoids, influenced by high contents

of volatiles (F, B, H

O) and highly increased P, Rb, Li concen-

trations. A model of their evolution (Breiter et al. 2015)

involves differentiation into three levels, postmagmatic retro-

gression and a strong Alpine reworking. Their minerals record

intensive low temperature overprint which caused a strong

oxidation of micas and formation of low temperature alumino-

phosphates (Petrík et al. 2014). Both mentioned interpreta-

tions, however, have one common basis, namely the Early

Proterozoic development of the Northern and Southern

Gemeric Units in the framework of one geotectonic domain,

whether already with lateral continuous or vertical connection.

Previous geophysical interpretations of

the Gemeric granites

In the Spiš-Gemer Ore Mts., geophysical research and sur-

veys have been carried out roughly from the middle of the last

century. Regional gravimetric mapping at the scale 1:25,000

(Kadlec 1965; Šefara 1966; Bárta 1969; Grzywacz & Margul

1976, 1980; Obernauer & Stránska 1983; Mikuška 1984) and

detailed at a scale 1:10,000 (Ferenc et al. 1974, 1978; Mikuška

& Špaček 1982; Steiner et al. 1983,1987; Grecula et al. 1985;

Mikuška et al. 1985; Kucharič et al. 1987, 1988, 1989, 1990,

1993; Kucharič 1991) provided a sufficiently high-quality

gravity database that became the basis for defining the gravity

field of these mountains.

The first attempts to estimate the geometry and position of

the Gemeric granite bodies were made by Šefara & Filo (in

Plančár et al. 1977). Their granite-geological model was based

on the results of the gravimetry. To define the model they

applied the method of vertical prisms, in which each inhomo-

geneity was replaced by a system of vertical n-side prisms of

final heights. The output was a map of the surface granite

relief up to a depth of 3000 m (Plančár et al. 1977).

Further research was performed by Grzywacz & Margul

(1980) in the eastern part of the Spiš-Gemer Ore Mts. The 2D

interpretation showed that the relief of the granite would be

more rugged than was expected. The authors of the interpreta-

tion used a combination of the vertical steps, with density con-

trast of − 0.15 g.cm

-3

to the reference density of the Gelnica

Group. This anomalous high-density contrast caused the lower

boundary of the granite body to be interpreted as too shallow

under the surface.

Grecula et al. (1985) performed the interpretation of the

Gemeric granites along forty profiles. Separation of the gravity

field into regional and residual anomalies and a 2D inverse

gravimetric problem have been solved. The gravity effect of

the granite bodies has been calculated by Pohanka’s unpub-

lished formulas for the 2D prismatic bodies with a polygonal

cross-section. The applied density contrast for the granite

body against the Gelnica Group was − 0.11 g.cm

-3

(Husák &

Muška 1984). The lower boundary of the anomalous granite

body was interpreted approximately at a level of 4200 to

5000 m under the surface. The results suggested that the upper

boundary of the granite is discontinuous and that the lower

boundary in the eastern part of the Gemeric Superunit is about

1000 m more shallow in comparison to the west.

The reflection seismic measurements along the Transect G

meant a major benefit for the study of the Gemeric Superunit’s

geology. Its course (Fig. 2a) was situated based on many ter-

rain geological and geophysical works (e.g., Vozárová 1973;

Bajaník et al. 1984b; Fusán et al. 1987; Šefara et al. 1987;

Vozárová & Vozár 1988) and realized in 1991–1992 (Vozár

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1991). The S–N transect crosses the Northern and Southern

Gemeric Units including their cover and nappe formations:

Bôrka nappe, Silicicum, Turnaicum and Meliaticum. Further it

runs across the Palaeozoicum of the Southern Gemeric Unit

— the Štós formation and the Gelnická Group. The northern

part, the Transect G crosses the Northern Gemeric Unit —

the Rakovec and Klátov Groups and their cover formations

(the Dobšina and Krompachy Groups). The measurements

were carried out by ELGI of Budapest in 1992. The first

interpretation of the reflection seismic measurements

(Novotný & Dvořáková 1993) identified the nappe character

of the northern part of the transect. Based on these results,

Vozárová (1996) improved the geology of the Gemeric

Superunit internal structure. A new interpretation of the crustal

elements along the Transect G was presented by Vozár &

Šantavý (1999). It was based on reprocessing done by ELGI

of Budapest in 1996 (Fig. 2b). From the interpretations, it can

be clearly seen that the Gemeric Superunit is overthrusted

on the units of the Northern and Southern Veporicum and

the tectonic basement of the Gemeric Superunit decreases

from the north to south. The Gemeric granites were mani-

fested as the low reflection zone. In terms of deep seated

structure, it is worth mentioning that the significant reflection

zone was found at about 10 seconds. This anomalous zone

probably represents the Moho discontinuity (Vozár et al. 1997,

1998 a, b).

The next model was estimated by Mikuška & Marušiak

(1999). The new element in the process of interpretation was

the introduction of new findings on the bottom boundary of

the granite body, which resulted from interpretation of the

reflection seismic Transect G (Vozár et al. 1998 a, b; Vozár &

Šantavý 1999). The determined depths of the bottom boundary

of the granite body for density contrast − 0.11 g.cm

-3

was about

4 km in the north and 8 km in the south.

Fig. 2. a — Location of the deep reflection seismic Transect G. b — Reprocessing: ELGI Budapešť, 1996; interpretation by Vozár & Šantavý

1999. Legend: T – Tatricum, NV – North Veporicum, SV – South Veporicum, NG – Northern Gemeric Unit, g – granites, Me – Meliaticum inclu-

ding the Bôrka nappe, LC – Lower crust, M – Moho, UM – Upper mantle.

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The latest work, concerning the interpretation of the Gemeric

granites was carried out by Szalaiová et al. (2001). Their inter-

pretation was done by the program GM-SYS 2

1/2

D along the

four profiles going across the Gemeric Superunit. The basic

interpretative profile was coincident with the seismic reflec-

tion Transect G (Fig. 2a). The resultant 2

1/2

D density model is

shown in the Figure 3a. Outside of the interpretative profiles

the values of the upper boundary of the granite body (Fig. 3b)

were estimated by interpolation.

Interactive gravity and magnetics application system

Our new and original 3D density model of the Gemeric

granites was constructed using the IGMAS software. The inter-

pretation of a potential field (gravity or magnetic) in the

IGMAS system is based on determining shapes, positions and

physical parameters of the geological structures that cause that

particular field in an investigated area (Schmidt 1996; Schmidt

& Götze 1998). The problem of data inversion requires the

application of additional geological and geophysical informa-

tion (constrains), which can be obtained, for example, from

wells, other geophysical methods, and measurements of phy-

sical properties of rocks. The indirect modelling approach

includes calculation of the effects of modelled bodies that

approximate geological structures, followed by matching the

modelled curve with the observed gravity curve. The 3D

structure is achieved in IGMAS by including several vertical

planes, on which geological bodies are geometrically defined

in the form of polygons that are based on all the available data.

The planes are always parallel and should be placed perpen-

dicular to the geological structures that they represent.

Through triangulation, these cross-sections with defined poly-

gons are connected to create the layer boundaries (triangular

facets). They are represented by the shape and form of the

modelled geological structures of constant density or suscepti-

bility. The triangulation between the vertical planes is per-

formed automatically. The data structure in IGMAS, which is

required for the description of 3D model geometry, must be

simple and flexible enough to visualize the results obtained.

The construction of the final 3D modelled structures is done

by the IGMAS system and does not require any knowledge of

the topology of a model and/or the triangulation techniques

(Schmidt 1996). All the processes are done visually and

interactively. The modelled bodies are adjusted by trial and

error method using interactive graphical tools until a good fit

is obtained (Tašárová 2004).

Input data

Gravity anomaly maps

In general, a basic map for interpretation of the gravity field

is represented by the Bouguer gravity anomaly. The map of the

Bouguer gravity anomaly of the Gemeric Superunit was calcu-

lated for the reference density of 2.67 g.cm

-3

by Katona (2007;

Fig. 4a). Since the Bouguer gravity anomaly represents

a superposition of the gravity effects of all the masses located

below the surface it is necessary to separate from it the effects

of the masses which are not the subject of the interpretation. In

our case, it was therefore necessary to determine the so-called

map of the residual gravity anomaly, which should reflect

primarily the gravity effect of the anomalous masses located

in the upper crust. To achieve this residual gravity anomaly

map we corrected the Bouguer gravity anomaly by the regional

gravity field, which represents, on the contrary, the effect of

deep-seated inhomogeneities (masses located approximately

beneath the upper crust). For 3D quantitative gravity inter-

pretation of the Gemeric granites, we used this evaluated

residual gravity anomaly, which is shown in the Figure 4b.

The regio nal field was approximated by using the mathe-

matically defi ned polynomial function of the third degree

(estimated by means of the Least Squares method), the main

requirement of which was that its character would agree

with the regional trend observed on the map of the Bouguer

gravity anomaly. In other words, the determined regional

gravity trend would approximate the regional increasing of

the observed gravity from the Western Carpathian gravity

low area towards the Pannonian gravity high. The resultant

map of the residual gravity anomaly was also compared

with another one that has been calculated, in this region,

by the Fourier transformation using a high-frequency Butter-

worth’s filter (Kubeš et al. 2001). The character and amplitude

of the gravity fields of both residual gravity maps were very

similar.

Analysis of the gravity fields presented by the Bouguer gra-

vity anomaly and residual gravity anomaly maps (Fig. 4a,b)

indicate that the individual anomalous areas correlate well

with the main tectonic units of the geological structure as well

as with their density distribution. In the central part of the

Spiš-Gemer Ore Mts., a significant Southern Gemeric gravity

low (SGGL with maximum amplitude −28 mGal on the

Bouguer gravity anomaly and −9 mGal on the residual gravity

anomaly) dominates. The source of this anomaly is a deep

granite (granitoid) body, the top parts of which reach the

surface and are the sources of local gravity lows. From the

northern part, the gravity low is bounded by a distinct Northern

Gemeric gravity high (NGGH with maximum amplitude

−13 mGal on the Bouguer gravity anomaly and +11 mGal on

the residual gravity anomaly). Its position correlates well with

the occurrence of the Rakovec and Klátov Groups, in which

the relatively heaviest Palaeozoic rocks (basic volcanics and

metamorphites) occur. In the south-eastern direction it conti-

nues towards the sizable gravity high. The zone turns and it

becomes a part of the Košice gravity high (KGH — maximum

amplitude +1 mGal on the Bouguer gravity anomaly and +11

on the residual gravity anomaly) reflecting metamorphic rocks

of the Veporicum in the Čierna hora Mts. In this part, a posi-

tive anomaly occurs induced by the Mesozoic and crystalline

rocks. To the south, the SGGL is bounded again by gravity

high (maximum amplitude −5 mGal on the Bouguer gravity

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GEOLOGICA CARPATHICA

, 2017, 68, 3, 177 – 192

anomaly and +5 mGal on the residual gravity anomaly), which

overlaps with the Slovak Karst.

The character of the gravity anomalies is also accompanied

by linear gravity features representing a zone of maximum

gravity gradients, which indicate the presence of vertical

(inclined) density boundaries located at different depth levels.

The zones of maximum gravity gradients have four predo-

minant orientations: N–S, W–E, NW–SE, NE–SW.

Density of the rocks

It is well-known that the quality of the gravity field interpre-

tation also depends on the quality of our knowledge about the

density of rocks. The densities applied in our interpretation

were obtained by means of analysis of rock samples coming

from surface outcrops, mining and drilling works (e.g., Plančár

et al. 1977; Husák & Muška 1984; Mikuška & Marušiak 1999;

Szalaiová et al. 2001 and references therein). In the 3D model,

the following geological units and their average densities were

defined:

• Neogene sediments (2.40 gcm

-3

)

• Inner Carpathian Palaeogene (2.61 gcm

-3

)

• Silicicum, Turnaicum and Meliaticum (2.73 gcm

-3

)

• Southern Gemeric Units

◦ Golčatov Group (2.68 gcm

-3

)

◦ Gelnica Group (2.77 gcm

-3

)

◦ Gemeric granite (2.65 g.cm

-3

)

• Northern Gemeric Units

◦ Rakovec and Klatov Groups (2.82 g.cm

-3

)

◦ Dobšina and Krompachy Groups (2.73 g.cm

-3

)

• Veporic Units (2.68 g.cm

-3

)

Results

The input model of the granites was based on the interpre-

tation of the seismic reflection Transect G (Vozár & Šantavý

1999). The value of this profile is that in the north-south direc-

tion it runs perpendicularly across all geological units, which

allowed us to define the positions and geometries of the geo-

logical units forming the Gemeric Superunit. Within this con-

text, the granite body was modelled. The seismic results

allowed us to define also the lower boundary of the granite

body, which decreases in depth from north to south. The basic

input shapes of the individual bodies and their physical

characteristics were taken from the interpretation of gravity

field along this profile (Szalaiová et al. 2001). There is no

doubt that it is very likely that the granite body consists of

several smaller bodies located in different positions. But for

effective modelling it is necessary to approximate realistic

geological units in a simplified model. The modelled area and

boundaries of the geological units on the relief, we obtained

by digitalization of the geological map of the Gemeric Super-

unit with the scale 1:500,000 (Biely et al. 1996 a, b).

The initial model in the vicinity of the reference Transect G

was created by increasing of the number of parallel profiles on

both sides of this transect (its approximated course is identical

to the profile with co-ordinates X = 4490000) in order to create

a resultant model in the 3D space. It can be assumed that in

more distant parts from this reference seismic transect the

approximation accuracy of the geological structure is going

down (the absence of constraints). Finally, 21 north–south

profiles were defined (twelve profiles on the left and eight on

the right of the reference Transect G). Along each of them the

model (the shape of the inhomogeneities) was adjusted by the

method of trial and error until a good fit between the calculated

effect and the residual gravity anomaly map was obtained.

The results of the 3D density modelling in IGMAS yield

a model showing the simplified geological structure of the

studied region (Fig. 5) with the main emphasis on the inter-

preted granite body. The resultant model shows clearly the

tectonic position of the granite body in relation to adjacent

geological units. Figure 6 shows the geometry and location of

the anomalous granite body along the selected four 2D

cross-sections (3, 8, 9, 11). We present a better and clearer 3D

view of the tectonic position of the Gemeric granites in rela-

tion to the Veporic unit basement in Figure 7a.

The Gemeric granites form the most significant low-density

anomalous body in the structure of the Gemeric Superunit. Its

average thickness varies in the range 5–8 km, with the lower

boundary sloping downwards from north to south. In the

north, the lower boundary of the Gemeric granites is located at

depths of only about 5 –7 km, while in the south it is 9 –10 km.

The upper boundary of the Gemeric granites is much more

rugged. There are areas where the granite body is very close to

the surface (these places correlate very well with known sur-

face outcrops of the granites, e.g., Hnilec, Betliar, Zlatá Idka)

and places where the depth of its upper boundary is deeper (on

average 1 km in the north and 4–5 km in the south). A hori-

zontal slice through the density model at 1.0 km depth (Fig. 7b)

indicates that the Gemeric granites cannot be represented by

a unified body. It can be divided into smaller blocks, each dif-

ferently offset (Grzywacz & Margul 1980).

The northern boundary of the Gemeric granites along the

tectonic contact with the Rakovec and Klátov Groups was

interpreted as very steep (in some places up to subvertical). The

importance of the presented 3D model goes beyond the scope

of the individual Gemeric granite bodies, since it expresses the

overall structure of the Gemeric Superunit, its internal structure

and its relationship to the underlying Veporic unit. The model

also shows that the Silicicum-Turnaicum and Meliaticum

nappe units are overthrusted onto the Golčatovo Group. The

whole 3D model clearly indicates that not only the Gemeric

granite body but also the whole structure of the Gemeric Super-

unit represents an Alpine north-vergent nappe structure.

Discussion

For the purpose of the transformation of the Bouguer gra-

vity anomalies to the residual and regional gravity anomalies

we applied the classical method of approximating regional

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, 2017, 68, 3, 177 – 192

field by using the mathematically defined polynomial function

of the third degree and digital filtering. This approach is sup-

ported by the fact that the Gemeric Superunit area is very

small. The courses of the Moho and lithosphere-asthenosphere

boundary are very smooth and do not change. From this point

of view it can be suggested that their regional gravity effects

will not influence the results of the interpretation of the

Gemeric granites.

In recent years, new data on the deep physical boundaries,

such as the boundary between the upper and lower crust,

Moho discontinuity and lithosphere–asthenosphere boundary

have been obtained (e.g., Zeyen et al. 2002; Dérerová et al.

2006; Grad et al. 2006, 2009; Środa et al. 2006; Alasonati

Tašarová et al. 2008, 2009, 2016; Hrubcová et al. 2008;

Csicsay 2010; Janik et al. 2011; Grinč et al. 2013). Therefore,

if the density modelling of the studied area on a regional scale

will be done in the future, then it will be necessary to take into

account the above mentioned lithosphere discontinuities, since

it can be expected that their influences on the observed gravity

field will play an important role.

The post-orogenic Permian Gemeric granites are specia-

lized (tin-bearing), SS-type granites that are interpreted as

products of partial melting of a sedimentary protolith due to

magmatic underplating during the post-Variscan orogenic col-

lapse and crustal stretching (e.g. Broska & Uher 2001). Despite

voluminous Variscan granite magmatism in the Western

Carpathian basement complexes, this type of granite is spatially

restricted to the Gemeric Unit. On the surface, the Gemeric

granites only occur as comparatively small bodies with narrow

contact aureoles (see geological map in scale1:50 000, Bajaník

et al. 1984a). A question may arise whether or not these are

only apophyses of a large subsurface plutonic body as it could

be indicated from the resultant 3D density model. Here, it

necessary to emphasize that the geophysical modelling in 3D

space is very difficult and the 3D model represents a major

simplification. Therefore, it may seem that the interpreted

model of the Gemeric granites generates a unified massive

body at a depth. On the other hand, this does not exclude the

assumption that the granites may consist of smaller single

bodies. Moreover, the Gemeric granites have the shape of

relatively thinner intrusions and apophyses and they are well

defined to its surrounding. In a seismic image they are not

reflective (Novotný & Dvořáková 1993; Vozár & Šantavý

1999; Vozárová 1996). The Veporic granites in contrast to the

Gemeric ones form the large masses of granite bodies (more

metamorphosed) with large thickness and are highly reflective

(Tomek et al. 1987, 1989).

An alternative model was presented by Lexa et al. (2003), in

which the low-density body underlying the Gemeric Palaeo-

zoic metasedimentary formations might represent a pre-

Variscan (possibly Cadomian) crystalline basement sheet that

originated from the foreland lower plate of the ancient Variscan

orogen. This interpretation takes into account the general

southern tectonic polarity of the Variscan orogen in the

Western Carpathians (Plašienka 1991; Putiš 1992; Vozárová

1996; Plašienka et al. 1997; Bezák et al. 1997; Vozárová et al.

1998; Putiš et al. 2009) with the Gemeric complexes forming

the frontal fold-and-thrust belt overriding a Gondwana-

derived Cadomian terrane. Later on, during the Alpine oro geny

with a distinct opposite — northern vergency, the Gemeric

thrust sheet might have incorporated a part of this basement,

which is likely composed of felsic rocks like granitoids and

migmatites.

Conclusion

For the first time, a new 3D density model of the Gemeric

granites in the Gemeric Superunit was created by using the

interactive geophysical program IGMAS.

Fig. 5. The resultant 3D density model of the Gemeric granites showing their tectonic position in relation to the surrounding tectonic units.

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