Plio cene to Qua ter nary stress field change in the west ern part of the Cen tral West ern Carpathians (Slovakia)
Rastislav VOJTKO, Jozef HÓK, Michal KOVÁÈ, ¼ubomír SLIVA, Pe ter JONIAK and Mar tin ŠUJAN
Vojtko R., Hók J., Kováè M.,Sliva L’., Joniak P. and Šujan M. (2008) — Plio cene to Qua ter nary stress field change in the west ern part of the Cen tral West ern Carpathians (Slovakia). Geol. Quart., 52 (1): 19–30. Warszawa.
Knowl edge of the cur rent tec tonic re gime plays an es sen tial role in nat u ral haz ard as sess ment, es pe cially in the risk as sess ment of fault ac tiv ity. Struc tural anal y sis of brit tle de for ma tions (us ing in ver sion tech niques) was used to de ter mine the stress field state oc cur ring within Plio cene and Qua ter nary de pos its in the west ern part of the Cen tral West ern Carpathians. The de for ma tion pat tern of the re duced stress ten sor showed that all struc tural mea sure ments could be sep a rated into two groups. An older, Late Plio cene fault pop u la tion was ac ti vated un der NNW–SSE ori ented ex ten sion. A youn ger, Qua ter nary fault pop u la tion re flected or i gin in a NE–SW extensional tec tonic re gime and it dis tinctly showed a change the ori en ta tion of the S3 of about 70°. The change in tec tonic ac tiv ity, as well as in the stress field ori en ta tion, is dated to the Plio cene–Pleis to cene bound ary. The Qua ter nary stress field de vel oped dur ing the post-collisional stage of the orogen. Our study shows that the West ern Carpathian in ter nal units doc u ment NE–SW to NNE–SSW ex ten sion in the broader re gion around of the north ern Dan ube Ba sin.
Rastislav Vojtko, Jozef Hók, Michal Kováè, ¼ubomír Sliva, Pe ter Joniak, De part ment of Ge ol ogy and Pa le on tol ogy, Fac ulty of Nat u ral Sci ences, Comenius Uni ver sity, Mlynská dol ina, 842 15 Bratislava, Slovakia, e-mails: vojtko@fns.uniba.sk, hok@fns.uniba.sk, kovacm@fns.uniba.sk, sliva@fns.uniba.sk, joniak@fns.uniba.sk; Mar tin Šujan, EQUIS LTD., Raèianska 57, 831 02 Bratislava, Slovakia, e-mail: equis@equis.sk (re ceived: Oc to ber 3, 2006; ac cepted: De cem ber 3, 2007).
Key words: West ern Carpathians, fault-slip data, Qua ter nary stress field, neotectonics, palaeostress anal y sis.
INTRODUCTION
The youn gest tec tonic his tory and changes in the struc tural evo lu tion of the Al pine–Carpathian–Pannonian do main have been de scribed in many pa pers (e.g. Csontos et al., 1991, 1992;
Ratschbacher et al., 1991; Csontos, 1995; Fodor, 1995; Fodor et al., 1999, 2005; Kováè, 2000; Hók et al., 2000). Re sults of these in ves ti ga tions have pro vided in for ma tion on the Mio cene palaeostress field evo lu tion. This evo lu tion and the changes in the palaeostress field were af fected by the coun terclock wise ro - ta tion of the ALCAPA microplate (ap prox i mately 40–80°) that took place dur ing this time (Márton et al., 1992; Márton and Fodor, 1995; Kováè and Márton, 1998; Márton et al., 1999;
Kováè, 2000). The lat est, Qua ter nary evo lu tion of the stress field and of the as so ci ated tec tonic re gime have only been stud - ied in part, due to the short age of field data. Re sults of com - puter sim u la tion and mod el ling of the re cent stress field ori en - ta tion (Zoback, 1992; Gerner et al., 1995, 1999; Bada et al.,
1996, 2001; Horváth and Cloething, 1996; Bada, 1999; Fodor et al., 2005) at the Al pine–Carpathian–Pannonian junc tion con trasted with those from field mea sure ments in the Dan ube Ba sin and Transdanubian Range in Hun gary (Fodor et al., 2005), the East ern Alps and Vi enna Ba sin in Aus tria (Reinecker and Lenhardt, 1999; Decker et al., 2005), and also in the Outer West ern Carpathians and the Eu ro pean Plat form east ern mar gin in Po land and the Czech Re pub lic (Peška, 1992;
Jarosiñski, 1998, 2005). The main goal of our in ves ti ga tion was to de ter mine the Plio cene to Qua ter nary stress field and its evo - lu tion in the north ern most part of the Dan ube Ba sin in Slovakia (the Cen tral West ern Carpathians).
GEOLOGICAL SETTING
The West ern Carpathians are the north-east ern most Al pine Orogen in Eu rope, where they are di vided into three parts (Outer, Cen tral and In ner West ern Carpathians) ac cord ing to
Plašienka (1999) and Kováè (2000) (see also ref er ences therein). The west ern part of the Cen tral West ern Carpathians com prises el e vated moun tain chains and in verted mar gins of the Neo gene Pannonian Ba sin (Fig. 1). Depocentres of the north ern part of the Dan ube Ba sin are sit u ated among ex humed horst struc tures of the Tatra Core Moun tains, from the west to the east: these in clude the Malé Karpaty, Považský Inovec and the Tribeè Mts. (Fig. 2). These moun tains are com posed of four pre-Ter tiary tec tonic units, from bot tom to top: Infratatric, Tatric, Fatric and Hronic units. The pres ent ar range ment of the units was cre ated dur ing Cre ta ceous col li sion. The Považský Inovec Mts. com prises a horst-like mega-anticline struc ture. Its crys tal line base ment be longs to the lat est ex humed pre-Ter tiary base ment unit in the West ern Carpathians. Ap a tite fis sion track (FT) thermochronology has pro vided ages from 14.0 to 25.0 Ma (Kováè et al., 1994; Danišík et al., 2004; Krá¾ in Hók et al., 2005). This sug gests very re cent up lift, as is also doc u - mented by the geo mor phol ogy of its west ern mar gin, where the
N–S run ning Považie fault sys tem di vides the horst from the Blatné De pres sion. These fault dis rup tions are marked by mor - pho log i cal fac ets of the moun tain front.
The Blatné, Rišòovce and Komjatice de pres sions have a fin ger-like con fig u ra tion (Fig. 2) and are filled pre dom i nantly with Mio cene ma rine se quences pass ing up wards into to Plio - cene and Qua ter nary lac us trine, flu vial and al lu vial de pos its (Fig. 3). The cen tral part of the Dan ube Ba sin is rep re sented by the Gabèíkovo De pres sion, and it is filled pre dom i nantly with Qua ter nary flu vial de pos its. The youn gest sed i men tary se - quence of the Dan ube Ba sin con sists of the Up per Mio cene:
Ivánka For ma tion (Fm.), the Hlavina Mem ber (Mb.) and the Beladice Fm., the Lower Plio cene: Volkovce Fm. and Pieš•any Mb., the Up per Plio cene: Kolárovo Fm. and the Syslie vàšky Mb. (Vass, 2002). The Qua ter nary se quence is gen er ally com - posed of the Lukáèovce Mb. and Rissian and Würmian loess de pos its, al lu vial fans and Nitra–Váh ter races (Joniak et al., 2001; Šarinová, 2002; Šarinová and Maglay, 2002).
20 Rastislav Vojtko, Jozef Hók, Michal Kováè, ¼ubomír Sliva, Peter Joniak and Martin Šujan
Fig. 1. Tec tonic sketch of the Al pine–Carpathian–Pannonian junc tion area
The Up per Mio cene sed i men tary se quence be gins with a Pannonian deltaic to shal low lake en vi ron ment (the Ivánka Fm.).
This Pannonian se quence is cov ered by the Up per Pannonian to Pontian fresh wa ter en vi ron ment of ephem eral lakes with lig nite (the Beladice Fm.). In the lower part of this for ma tion, the Hlavina Mb. oc curs. It is char ac ter ized by car bon ate de po si tion, of trav er tine type. Plio cene de po si tion starts with al lu vial, flu vial and lac us trine en vi ron ments (the Volkovce Fm., Kolárovo Fm.
and Syslie vàšky Mb.); a Mast odon fauna at MN15–MN17 con - strains on age of Early to Mid Plio cene (Holec et al., 2002). Sim - i lar con clu sions as to the age of this Mast odon fauna were pub - lished by Gasparik (2001) and by Tóth (2006). Above these for - ma tions, with a ero sive con tact, lies a red bed unit of Early Pleis - to cene age (the Lukáèovce Mb.).
These sed i men tary se quences mainly rep re sent tec toni cally in verted for ma tions of the north ern mar gin of the Dan ube Ba - sin (the Volkovce and Kolárovo Fms.). The for ma tions were de pos ited in al lu vial and flu vial en vi ron ments of me an der ing
chan nel belts en cased within overbank fines (Kováè et al., 2006). Palaeotransport di rec tion and grain-size anal y sis sug - gest that the south ern part of the Považský Inovec Mts. was bur ied at this time, and not el e vated as to day. At the Plio - cene–Pleis to cene bound ary, the sed i men tary en vi ron ment of the north ern Dan ube Ba sin changed and a dis tinct ero sive sur - face de vel oped. This event was closely re lated to an in crease in tec tonic ac tiv ity and a change in the source of clastic ma te rial (Joniak et al., 2001). The south east ern slopes of the Považský Inovec Mts. were grad u ally el e vated and the Lower Pleis to cene al lu vial fans of the Lukáèovce Mb. were de pos ited (Šarinová and Maglay, 2002; Kováè et al., 2006). The sed i men tary re cord of the en tire area stud ied is gen er ally cov ered by Qua ter nary loessic de pos its, pre dom i nantly of Würmian age.
The struc tural anal y sis was car ried out in dis turbed Plio - cene, Pleis to cene and Ho lo cene strata (cf. Hók et al., 1998;
Joniak et al., 2001; Kováè et al., 2006) within the ba sin, while field mea sure ments were also car ried out on de pos its pres ently
Fig. 2. Tec tonic map of the north ern part of the Dan ube Ba sin and ad ja cent moun tains
in cor po rated into the ex humed young horst struc tures, oc cur - ring mainly in the Považský Inovec Mts.
METHODS
The struc tural re search was di rected towards de ter mining the palaeostress field ori en ta tion dur ing the neotectonic interval. The stan dard pro ce dures used for anal y sis of fault-slip data and palaeostress re con struc tion are well-es tab lished (Angelier, 1979, 1989, 1990, 1994; Etchecopar et al., 1981;
Mi chael, 1984; Célérier, 1995; Célérier and Séranne, 2001;
Delvaux and Sperner, 2003). In this study, we used two meth - ods of palaeostress ten sor com pu ta tion from fault slip data:
— the in ver sion method (Angelier et al., 1982; Angelier, 1984, 1994) which has the abil ity to sep a rate het er o ge neous fault pop u la tions;
— in the case of small sets of con ju gate faults, the Dihedra method was ap plied (Angelier and Mechler, 1977).
The in ver sion method cal cu lates the re duced ten sor which shows palaeostress fields that are de fined by the S1 or s1 (prin - ci pal max i mum com pres sion axis), S2 or s2 (prin ci pal in ter me - di ate compressional axis) and S3 or s3 (prin ci pal min i mum compressional axis. The ra tio be tween these axes de fines the shape of the stress el lip soid and is ex pressed by the R or F pa - ram e ter (for fur ther de tails see Angelier, 1984, 1994). Fault slip datasets are shown in ste reo grams of fault planes with ob served slip striae and his to grams of slip-shear an gles (the Lam bert equiplanar pro jec tion, lower hemi sphere).
22 Rastislav Vojtko, Jozef Hók, Michal Kováè, ¼ubomír Sliva, Peter Joniak and Martin Šujan
Fig. 3. The Plio cene and Pleis to cene sed i men tary for ma tions of the north ern part of the Dan ube Ba sin (ac cord ing to Joniak et al., 2001; Holec et al., 2002; Kováè et al., 2006)
The re sults of the Dihedra method are de rived from fault planes and slip lines and in clude his to grams of count ing de vi a - tions. The re duced stress ten sors were com puted by the TENSOR soft ware pack age de vel oped by Damien Delvaux (Delvaux, 1993; Delvaux and Sperner, 2003).
The re sults are ar ranged in fig ures ac cord ing to the rel a - tive chro nol ogy of the stress stages and ex plained be low in this text. Pa ram e ters of the re duced stress ten sors are shown in Ta ble 1. We dis cuss two as pects of the re sults ob tained; a qual ity as sess ment, ac cord ing to the World Stress Map stan - dards (Sperner et al., 2003) and the nu mer i cal ex pres sion of the stress re gime as the Stress Re gime In dex for re gional com - par i sons and map ping (Delvaux and Sperner, 2003). In the World Stress Map, the qual ity ranges from A (best) to E (worst), and it is pre dom i nantly de ter mined as a func tion of at least two cri te ria (num ber of fault slips with fluc tu a tion of the whole dataset; cf. Sperner et al., 2003). Ori en ta tions of the stress axes (SH — max i mum hor i zon tal com pres sion axis, Sh
— min i mum hor i zon tal com pres sion axis, and Sv — ver ti cal axis) and the stress re gime (R’ = 0–1 for extensional re gime, 1–2 for strike-slip re gime, and 2–3 for com pres sive re gimes) are fully de scribed by the SH az i muth (as de fined in the World
Stress Map by Müller et al., 2000) and by the av er age stress re gime in dex R’ as de fined by Delvaux et al. (1997).
RESULTS OF FAULT SLIP ANALYSIS AND STRESS INTERPRETATION
The re search was car ried out in the west ern part of the Tatric core moun tain zone which has a ba sin and range struc - ture. This struc tural pat tern was formed dur ing the Neo gene, as con strained by ap a tite fis sion track data (Kováè et al., 1994;
Danišík et al., 2004; Krá¾ in Hók et al., 2005) and also by the sedimention his tory of sur round ing de pres sions (Kováè et al., 1994; Šarinová and Maglay, 2002; Kováè et al., 2006). Struc - tural anal y sis of brit tle de for ma tion was car ried out for re con - struc tion of the stress field that af fected Plio cene and Qua ter - nary de pos its. We found only a few sites which were suit able for struc tural anal y sis (Figs. 1 and 2). In the un con sol i dated Plio cene and Qua ter nary de pos its slickenside lineations on fault planes are poorly pre served. In our study, we have taken into ac count well-pre served fault planes, though these are very rare in the West ern Carpathians.
T a b l e 1 Palaeostress ten sors from fault slip data
Site Lat i tude Lon gi tude De scrip tion n nT S1 S2 S3 R a Q R’
Ve¾ké Ripòany sandpit site PI-VR1 N48°30.085’ E017°57.05’ cov ered by the Lukáèovce
Mem ber 10 25 287/87 063/02 153/02 0.2 11.06 C 0.2
PI-VR2 N48°30.085’ E017°57.05’ cut Qua ter nary for ma tions 14 25 173/85 324/04 054/03 0.5 12.77 C 0.5 Dolné Trhovište sandpit site
PI-DT1 N48°26.078’ E017°55.386’ cov ered by the Würmian
loess 24 28 139/85 261/03 351/04 0.17 6.3 B 0.17
PI-DT2 N48°26.078’ E017°55.386’ cut Qua ter nary for ma tions 4 28 210/73 302/01 032/17 0.5 2.82 E 0.5 Lukáèovce sandpit site
PI-LU1 N48°24.443’ E017°55.707’ cov ered by the Lukáèovce
Mem ber 1 5 350/54 175/36 083/02 – – E –
PI-LU2 N48°24.443’ E017°55.707’ cut Qua ter nary for ma tions 4 5 217/71 312/02 043/19 0.3 3.2 E 0.3 Horné Otrokovce quarry site
PI-HO1 N48°29.642’ E017°52.482’ cov ered by the Würmian
loess 2 2 355/78 252/03 161/12 – – E –
Teplièky site
PI-TE2 N48°28.383’ E17°50.800’ cut Qua ter nary for ma tions 1 1 216/60 125/00 035/30 – – E – Ducové “Old quarry” site
PI-DO1 N48°37.466’ E17°56.233’ cov ered by the Lukáèovce
Mem ber 8 13 301/85 082/03 172/04 0.5 11.23 D 0.5
PI-DO2 N48°37.466’ E17°56.233’ cut Qua ter nary for ma tions 4 13 138/82 311/08 041/01 0.46 11.02 E 0.46 Dubná Skala site
MF-DS1 N49°07.917’ E18°53.450’ cut Plio cene for ma tions 5 13 198/24 044/64 293/09 0.5 8.94 E 0.5 MF-DS2 N49°07.917’ E18°53.450’ cut Plio cene for ma tions and
MF-DS1 faults 8 13 331/25 090/46 226/34 0.51 4.71 D 0.51
Site — code of lo cal ity; De scrip tion — ad di tional im por tant com ment; n — num ber of faults used for stress ten sor de ter mi na tion; nT — to tal num ber of fault data mea sured; S1, S2 and S3 — az i muth and plunge of prin ci pal stress axes; R — stress ra tio (S2–S3/S1–S3); a — mean slip de vi a tion (in °); Q — qual - ity rank ing scheme ac cord ing to the World Stress Map Pro ject (Sperner et al., 2003); R’ — ten sor type in dex as de fined in the text
24 Rastislav Vojtko, Jozef Hók, Michal Kováè, ¼ubomír Sliva, Peter Joniak and Martin Šujan
Twenty five fault planes were found at the Ve¾ké Ripòany lo cal ity (Figs. 4A, B and 6A, B) which is sit u ated 2.5 km north-west of Ve¾ké Ripòany vil lage (for more pre cise lo cal - isa tion see GPS in for ma tion in Ta ble 1). Sed i ments are faulted with off sets rang ing from 2 to 130 cm. The faults strike al most per pen dic u lar to the NE–SW run ning Majcichov fault which bounds the Považský Inovec Mts. to the south-east (Fig. 2).
Two gen er a tions of faults were mea sured. Faults of the older gen er a tion cut the Plio cene Syslie vàšky Mb. but do not con - tinue into the youn ger strata con sist ing of the Lower Pleis to - cene Lukáèovce Mem ber. The youn ger fault gen er a tion cuts both sed i men tary se quences, and also the Würmian loess. The older, Up per Plio cene set of con ju gate nor mal faults doc u - ments NNW–SSE pure ten sion (Fig. 4A). The youn ger, Qua -
ter nary set of faults shows ten sion in a NE–SW di rec tion (Fig.
4B). The data gained sup port a change in the prin ci pal stress axes ori en ta tion at the Plio cene–Pleis to cene bound ary, of ap - prox i mately 70°. The S1 stress axis is nearly ver ti cal.
In the next ex po sure, the Dolné Trhovište sandpit (1.75 km south-east of Dolné Trhovište vil lage, Fig. 2) twenty seven faults with off sets from sev eral cm to 50 cm were mea - sured (Fig. 6C, D). Two ge netic sets of nor mal faults can be dis - tin guished. (1) Older faults, ob served only in the Plio cene Volkovce For ma tion die out at the base of the Pleis to cene. This set is rep re sented by 23 con ju gate faults cre at ing a char ac ter is - tic horst and graben pat tern (Fig. 4C). The ENE–WSW strik ing nor mal faults in di cate a ver ti cal po si tion of the S1 stress axis and con se quently point to NNW–SSE-di rected ex ten sion.
Fig. 4. Fault slip data and palaeostress re con struc tion
All ste reo grams have Lam bert lower hemi sphere pro jec tions. Tec tonic re gime is ex pressed by rel a tive val ues of prin ci pal stress with R-ra tio (R = S2– S3/S1–S3) and stan dard de vi a tions char ac ter ized by the “a°” value are sit u ated on the left side of the ste reo grams. Graph of slip de vi a tions (a° = y axis) ver - sus fre quency of weight (x axis) of the fault slip data is lo cated on the lower left side of the ste reo grams. A — stereogram of the fault planes with ob served slip striae and ori en ta tion of prin ci pal palaeostress axes, and his to gram of slip-shear an gles for the older deformational stage at the Ve¾ké Ripòany site; B
— stereogram of the fault planes with ob served slip striae and ori en ta tion of prin ci pal palaeostress axes, and his to gram of slip-shear an gles for the youn ger deformational stage at the Ve¾ké Ripòany site; the fault pat tern is con ju gate; C — older deformational stage at the Dolné Trhovište site; D — youn ger deformational stage at the Dolné Trhovište site; E — older deformational stage at the Lukáèovce site; F — youn ger deformational stage at the Lukáèovce site; G — older deformational stage at the Dubná Skala site; H — youn ger deformational stage at the Dubná Skala site
Fig. 5. Fault slip data and palaeostress re con struc tion A — fault slip data and palaeostress reconstructionat the Horné Otrokovce quarry; stereogram of the fault plane with ob served slip striae and ori en ta tion of prin ci pal palaeostress axes, and his to gram of slip-shear an gles for the older deformational stage; B — fault slip data and palaeostress re con struc tion at the Teplièky lo cal ity; stereogram of the fault plane with ob served slip striae and ori en ta tion of prin ci pal palaeostress axes, and his to gram of slip-shear an gles for the youn ger deformational stage; C — fault slip data and palaeostress re con struc - tion at the Ducové “Old quarry”; stereogram of the fault planes with ob served slip striae and ori en ta tion of prin ci pal palaeostress axes, and his to gram of slip-shear an gles for the youn ger deformational stage; the fault pat tern is con ju gate; for ex pla na tions see Fig ure 4
(2) The sub or di nate set is rep re sented by 4 nor mal faults dip - ping at less than ~60 de grees to wards the NE, which in di cates ex ten sion in a SW–NE di rec tion. The faults are re stricted to the Pleis to cene (Lukáèovce Fm.) (Fig. 4D).
The next lo cal ity with faulted Plio cene and Qua ter nary de - pos its is sit u ated 1 km north-west of Lukáèovce vil lage. Five nor mal faults were mea sured. One fault cuts the older Plio cene strata com posed of the Volkovce Fm. and does not con tinue into the youn ger Lukáèovce Fm. This fault in di cates N–S-di - rected ex ten sion (Fig. 4E). The re main ing four faults cut the en tire sed i men tary se quence be low the Ho lo cene col lu vium.
The NE-dip ping faults in di cates that is ver ti cal and re flect NE–SW-di rected ex ten sion (Fig. 4F).
The Horné Otrokovce aban doned quarry with dark do lo - mite of the Fatric Unit is sit u ated 100 m north-west of Horné Otrokovce vil lage. This do lo mite is cov ered by tec toni cally un - af fected Qua ter nary de bris and by Würmian loessic de pos its (Joniak et al., 2001). At this lo cal ity, two faults were ob served which were prob a bly ac ti vated dur ing the Late Plio cene, as they have a sim i lar pat tern to the faults mea sured at the Ve¾ké Ripòany and Dolné Trhovište quar ries. The strike of the faults is ENE–WSW, with dips to wards the south. These faults were ac - ti vated in the NNW–SSE ori ented ex ten sion (Fig. 5A).
The Teplièky lo cal ity is sit u ated in a deep gully 1200 m north-west of Teplièky vil lage. The lower part of this ex po sure
is formed of sand and gravel. These de pos its be long to the Syslie vàšky Mb. The up per part con tains authochtonous and re de pos ited Würmian loessic ma te rial, and over lies the Syslie vàšky Mb. At this lo cal ity, one fault struc ture with a nor mal slip of 60 cm was ob served. This fault strikes to the NW–SE, where the NE block sub sided and the fault was most prob a bly ac ti - vated in an NE–SW extensional tec tonic re gime dur ing the Up - per Pleis to cene to Ho lo cene in ter val (Fig. 5B).
The Ducové “Old quarry” is sit u ated south of the vil lage of Ducové (Fig. 2). The Mid dle Tri as sic do lo mite of the Hronicum nappe is ex posed in this quarry. This do lo mite is cov ered by poorly-sorted de bris and a small Pleis to cene al lu vial fan. Thir - teen faults were ana lysed at this lo cal ity (Fig. 5C) and five of these did not con tinue into the Qua ter nary de pos its. The faults were gen er ally ob served to have two di rec tions. The first fault gen er a tion is rep re sented by those faults that do not con tinue into the Qua ter nary suc ces sion. Their age can not be de ter mined more ex actly than “pre-Qua ter nary”. These faults were gen er ated dur - ing the im po si tion of pure NE–SW to E–W ten sion. Thus, these faults are not de scribed in this pa per. The sec ond and neotectonically im por tant fault gen er a tion was de vel oped dur ing the Up per Pleis to cene to Ho lo cene be cause these faults have dis - turbed Würmian loessic de pos its. The Qua ter nary de for ma tion is char ac ter ized by N–S ten sion which has a slightly dif fer ent ori - en ta tion to that seen at the pre vi ous lo cal i ties (Fig. 5C).
26 Rastislav Vojtko, Jozef Hók, Michal Kováè, ¼ubomír Sliva, Peter Joniak and Martin Šujan
Fig. 6. Field pho tos of the fault struc tures mea sured
The Ve¾ké Ripòany lo cal ity with ob served con ju gate nor mal fault sys tem and drag folds (A) and a com plex nor mal fault with extensional du plex struc tures (B); the Dolné Trhovište lo cal ity with nor mal faults in the loessic sed i ments (C and D)
The Dubná skala lo cal ity is an aban doned quarry which is sit u ated ap prox i mately 5 km NW from Mar tin town on the left side of the Váh River (Fig. 1). Fresh wa ter lime stones, travertines and sandy clays to clays are pres ent in the base of the quarry. Ac cord ing to biostratigraphical ev i dence, this rock se quence be longs to the Plio cene Dubná skala For ma tion (Hók et al., 1998; Rakús and Hók, 2002).
Thir teen faults were ob served and mea sured at this lo cal ity (Fig. 4G, H). These faults were ac ti vated pre dom i nantly as transpressional strike-slips with a gen eral E–W-strike. A poly - gen etic fault pop u la tion was pres ent, which can be di vided into two gen er a tions. The older gen er a tion com prises five con ju - gate fault struc tures which are ori ented to wards the WSW–ENE and NNE–SSW (Fig. 4G). These strike-slip faults were de vel oped dur ing NNE–SSW com pres sion and ten sion
per pen dic u lar to this. The next, youn ger, ho mo ge neous fault pop u la tion was ac ti vated un der a strike-slip tec tonic re gime.
The prin ci pal max i mum stress axis S1 was com puted trend NW–SE, and the prin ci pal min i mum stress axis S3 to be NE–SW (Fig. 4H). The chro nol ogy of both fault sets was de - duced on the ba sis of the cross-cut ting re la tions (Ta ble 1).
EVOLUTION OF STRESS FIELD SINCE THE PLIOCENE TO QUATERNARY — A DISCUSSION
The re cent stress field is of ten de duced from earth quake fo - cal mech a nisms, bore hole break-out anal y sis and mod el ling of crustal pro cesses (Gerner, 1992; Jarosiñski, 1998, 2005;
Fig. 7. Sim pli fied map of the Al pine–Carpathian–Pannonian junc tion area with the in ter preted tra jec to ries of re cent hor i zon tal stress SH and Sh
Com piled from struc tural mea sure ments, in ter pre ta tion of the fo cal mech a nism so lu tion and from bore hole break out anal y sis (ac cord ing to Peška, 1992; Gerner, 1992; Pospíšil et al., 1992; Labák et al., 1997; Jarosiñski, 1998, 2005; Reinecker and Lenhardt, 1999; Gerner et al., 1999; Hók et al., 2000; Havíø and Stráník, 2003; Havíø, 2004) and own dataset dem on strated by black ar rows
Gerner et al., 1999; Bada et al., 2001). Struc tural mea sure - ments of fault slip data in Plio cene and Qua ter nary deposits have, by con trast, rarely been made.
Mea sure ments on 87 faults in Plio cene and Qua ter nary strata per mit re con struc tion of the palaeostress field evo lu tion in the west ern part of the Cen tral West ern Carpathians. The fault slip anal y sis and stress re con struc tion al lowed rec og ni tion of two stress phases dur ing the Plio cene to Qua ter nary in ter val.
The Plio cene stress field is char ac ter ized by NNW–SSE ten - sion and a sub-ver ti cal ori en ta tion of the prin ci pal max i mum stress axis. The nor mal faults are pre dom i nantly ori ented in an E–W or NE–SW di rec tion form ing con ju gate sets (Figs. 4A, C, E, G and 5A, C). The Plio cene palaeostress field had a sta ble di rec tion with small fluc tu a tions seen at in di vid ual lo cal i ties (stan dard de vi a tion á < 20°).
At the Plio cene–Qua ter nary bound ary in the west ern part of the West ern Carpathians the di rec tion of ex ten sion changed from NNW–SSE to NE–SW. This change is doc u mented by dif fer ent re la tion ship be tween faults ob served in the Plio cene and Pleis to cene sediments.
The Qua ter nary stress field of NE–SW ten sion (Sh) is ori - ented par al lel to the West ern Carpathian range. This tec tonic stress field was ob served at sev eral lo cal i ties (Figs. 4B, D, F, H and 5B). Stan dard de vi a tion of the prin ci pal stress axis ori en ta - tions is 15°. A sim i lar Qua ter nary stress field was sup posed by Marko et al. (1995) and Marko and Kováè (1996) in the Al - pine–Carpathian tran si tion zone and by (Hók et al., 1995) in the Horná Nitra De pres sion. The fault-slip data of these au thors were col lected from rocks older than Plio cene.
Bore hole break-out and earth quake fo cal mech a nism anal y - sis in di cate NW–SE ori ented SH com pres sion in the Bo he mian Mas sif and in the west ern part of the Outer West ern Carpathians (e.g. Peška, 1992; Jarosiñski, 1998, 2005; Havíø and Stráník, 2003; Havíø, 2004). The com pres sion is gen er ally per pen dic u lar to the front of the Outer West ern Carpathians (Fig. 7).
A com pres sive tec tonic re gime pre vailed in the Outer West ern Carpathians and in the ad ja cent Bo he mian Mas sif. A strike-slip tec tonic re gime had been de ter mined along the Pieniny Klippen Belt and the Mur–Murz–Leitha fault sys tem (Labák et al., 1997; Reinecker and Lenhardt, 1999; Fig. 7).
The dom i nant extensional tec tonic re gime in the Cen tral West ern Carpathians can be ex plained as hin ter land ex ten sion
caused by con tin u ing con ver gence of the ALCAPA with the Eu - ro pean plat form (cf. Csontos et al., 1991; Gerner, 1992; Peška, 1992; Pospíšil et al., 1992; Fodor et al., 1999; Hók et al., 2000).
A NE–SW ori en ta tion of SH has been doc u mented in the south ern part of the Dan ube Ba sin (Fig. 7) and the Transdanubian Range (Gerner, 1992; Gerner et al., 1999;
Fodor et al., 2005). It is most prob a bly the re sult of in ter ac tion be tween the ALCAPA and the Adria microplate push ing to the NE (e.g. Bada et al., 1999; Gerner et al., 1999).
CONCLUSIONS
This study has fur ther con strained the de vel op ment of the stress fields and the shape of SH and Sh tra jec to ries in the Al - pine–Carpathian–Pannonian junc tion area dur ing the Plio cene and Qua ter nary. The anal y sis of fault slip data indicated extensional tec tonic re gimes in the west ern part of the Cen tral West ern Carpathians.
Two main extensional phases can be dis tin guished: the older, rep re sented by a Plio cene fault pop u la tion, was gen er - ated dur ing NNW–SSE ori ented ex ten sion, and con sists pre - dom i nantly of nor mal faults. The youn ger, Qua ter nary fault pop u la tion in di cates or i gin dur ing NE–SW ori ented ex ten - sion. The re sult of palaeostress anal y sis clearly showed a 70°
change of stress ten sor ori en ta tion at the Plio cene–Pleis to cene bound ary.
This study has shown that the west ern part of the Cen tral West ern Carpathians and the broader re gion around the north - ern Dan ube Ba sin area have re cently undergone NE–SW to NNE–SSW di rected ex ten sion.
Ac knowl edge ment. This work was sup ported by the Slo - vak Re search and De vel op ment Agency un der the con tract No.
APVV-0158-06, APVV 51-011305. The au thors also wish to thank EQUIS LTD. for their fi nan cial and tech ni cal help and Damien Delvaux for the TENSOR soft ware ap pli ca tions. Our thanks also go to M. Jarosiñski and G. Bada for con struc tive and help ful re views.
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