Geological Quarterly, 2000,44 (4): 377-390
Ifi memoriam of hf: DP, ARdm@ flenkiel
Jointing in the Drakla Nappe (Outer Carpathians, Poland): an attempt
at paHaeastress reconstruction
Leonard MASTELLA and Witold ZUCHIEWICZ
Mastella L, and Zuchicwicz W. (2000) - Jointing in the Dukla Nappe (Outer Carpathians, Poland): an attcmpt at palacostrccss rccon- shuction. Geol, Quart., 44 (4): 377-390. Warszawa.
Thc joint network in the portion of thc Dukla Wappc studicd originated in two atagcs showing diffcrent orientations of thc strws ficld axcs. Thc oldcr, compressive stagc I was charactcrisod by a N45'8-orientated (at prcscnt coordinatcs) horizontat a,, and fomlation of the diagonal, shcar (SL and SR) joint systcrn. Thc younger, cxtcnsional stagc I1 was charactmised by vertical tr, and horizontal, N35"E-aricntated (also at prcscnt coordinatcs) a,, soitablc for formation of thc fold-parallcI joints. At thc turn rnfthc first stagc, under di- minisling horizontal compmsion (c, trcndingN35"E), the ctoss-fald Tjoints dcvelopcd. Tbe orientation of the maximum compression axis changcd bchccn stagcs I and [I. The first stagcprobnbly cornmcnced whilc strata which wcrc not fully lithified underwent shortcn- ing, and lnstcd until thc termination of nappc cmpIaccmcnt. The socond stagc, in turn, is assoc~ated with pat-orogcnic colIapsc ofthc Carpathians, whosc onsct in lhc study rcgion bcgan at least in Latc Mimcnc times.
LeonardMastella, Btsh'brr!e ofC;eoIogy. Warsaw Uniwrsiw, 2wi~ki i Wigury 93, PL-02-089 Warszawn, Polai~d: Witold Zuckiewicz, In- s h t ~ ~ t s of Geological Sciences, JugielIonian U)tiversily, Oleandry 2A. PL-30-063 Kmkbw, Poland (received Jut~i 12, 2000; accepted:
September 20, 2000).
Kcy words: Outer Carpathians, Dukla Nappe, jointing, palnwshess reconstruction.
INTRODUCTION
The objective of this paper is to describe and characerise the origin of jointing in the Polish segment of the Dukla Nappe (Fig. I), as well as to show that joints in structurally cornpli- cated settings, such as the Dukla Nappe, are related to Ear-field
streses and that they can be used as key tools in palaeosttess reconshctions. Ths shldy is a continuation of previous re- search, initiated in the PoIish Outer Carpathians by Ksiqkdciewicz (1968), and in the Dukla Nape by one of us in the late 1980s and early 1990s (Henkiel and Zuchiewicz, 1988;
Zuchiewiu and Henkiel, 1995).
Some authors claim that jointing generally precedes fatilt- ing [Sheperd and Huntington, 198 1; Segall and Pollard, 1983) and folding (ia. Cook and Johnson, 1970; Tokarski, 1977), whereas others maintain that systematic jointing in fold-thrust
belts post-dates the main orogenic compressional event (Meere and Rogers, 1999) and that it can rarely be used as a Ear-field
stress indicator (e.g. Pollard and Aydin, 1988). Yet another group of geologists concludes that joints can be initiated before, during and afkr folding mancock, 1964, 1985; Rixon et al., 1983). It has recently been suggested that joints in young fold-and-rhrust belts are related ro far-field stresses and that they can easily be used in paIaeostrem reconstructions (Mastella et a!., 1997; 6wimmewska and Tokarski, 1998). The Dukla Nappe in the Outer Carpathians of Poland, showing fairly complicated shcture, appms to be a perfect area to test this hypothesis.
We have chosen the Dukla Nappe as a test area, since this region has been studied extensively and has a good coverage of detailed geological maps (i.a. Wdowiarz, 193 1 ; ~ l ~ c z k a , 197 1, 1985; Tokmski, 1975a; Slqczka and 2ytk0, 1979; Kosmski, 1985; Kuherek, 1990).
378 Lconard Maslelia and Wi told Zuchiewice
-1 X C
,
,L.,r-.-
--Outer Carpathians:
~p"~',"i",~~,~~~~$z~~~, tml
Neogene basinsOutcr Carpalhians)
Aushalpineunits %? Thrust
[-rrl=IQ
Southalpine (Dinaridas)Fig. 1. A - Structural skctch of thc Carpatho-P~nnoninn rcgion @ascd on Nqubaucr el al., 1997, rnodifiod); B - Tcctonic skctch of thc Polish Outer Carpnthians (based on Zylko at al., 1989)
Definition. The term "joint" is used here as a fieM term de- scribing
"...
a barn, closed fracture on which there is no mea- surable slip or dilation at the scale of observation" (Hancock, 1985, p. 445; cJ also Dunne and Hancock, 1994).Only
pene- trative joints which are perpendicular to bedding, systematic, which cut singular beds and whose spacing is roughly equal to the thickness of the host bed, have been taken into accotmnt (Jaroszewski, 1972,1980; Dunne and Hancock, 1994). Follow- ing the classification schemes by Engelder (1985) and Bahat (1 99X), we shall be dealing with tectonic and buriallsyntectonic joints, respectively.STATE OF RESEARCH
Zuchiewicz, 1997a, b, 1998a, b; Tokarski and Swierczewska, 1998). The relation of jointing to regional fold trends has be- come easy to establish owing to recently published calct~lations of imp-scale fold axes within homogeneous domains thro~~gh- o~lt nearly the whole of the Polish Outer Carpathians (Mastella es a]., 1 997; Szczgny, 1998).
Jointing in the Polish segment of the Dukla Nappe and the neighbotuing portion of the Silasian Nappe has been dealt with by Henkiel and Zuchiewicz (1 988), Zuchmicz and I-Ienkiel (1995), Rubinkimvicz (1996, 1998), and Zt~chiewicz (1 997a, 1998a). Inte~pretati~ns contained in papers published before 1996, however, are biassed by the fact that the orientations of the measured joint sets has not been corrected for beddmg and fold plunge attitudes. Such corrections were then difficult to a p ply due to the lack of necessary computer software.
Joints are ubiquitous stn~ctures in the Cretaceo~rs through Tertiary flysch strata of the Polish Outer Carpathians, and have
been stuhed by numerous authors (i.a Bober and Oszczypko, GEOLOGICAL SETTWG 196% KsiqAuewicz, 1968; Tokarski, 1975b, 1977; Lenk, 1981;
Aleksmdrowski, I985a, b, 1989; Mastella,
E
988; MardaS The eastern portion of the Outer Carpathians comprises 1495; Zt~chiewicz and Henkiel, 1995; MastelIa eb a/,, 1997; several nappes that are thlst over each other from the SS W toJointing in thc Dukla Nappc (Outer Carpathians, Poland): an attempt at palamstrcss reconshuctim 379
Fig. 2. Geological skctch of thc Polish scgmcnt o f thc Dukla Nappc {bascd on Slqczka and &ko, 1979)
the NNE (Fig. 1).
The
thrusting proceeded throughout middle and Late Miocene limes, except for the inner part of the Magura Nappe where it had begun in the Paleogene (cJ Tokarski and Swierczewska, 1998; Zytko, 1999). The stratigraphic inventory of these nappes inclt~ddes Cretaceotu through Lower Miocene flysch strata of variable thicknesses (Ksiz@hewicz, 1977;Oszczypko and ~lqczka, 1989). Thin-bedded turbidit@ are sirongly deformed by tight folds and are included in a number of iinbricated thrust sheets, locally forming anti format stackq, whereas thick-bedded turbidites are Iess deformed and are acci-eted in slightly imbricated thrust sheets (Roca eta[,, 1995).
The thrusting propagated gradually with time as a result of oblique convergence between the Eurasian Plate and Alcapa
(c$ Royden, 1 988; Oszczypko and kqczka, 1989; Ellouz and Roca, 1994). Most of the shortening took place between the Late Oligocene and the end of the Sa~matian during an interval of 16-1 3 Ma (Roca et a[., I 995; Oszczypko, 1996).
The Dukla Nappe belongs to the ForeMagura group of slices ($19czka and Kaminski, 1998; and references therein)
and i s exposed at the surface in the eastern portion of the Polish Carpathians. To the south it is in contact with the m n g l y tectonised Sash Nappe or the Magura Nappe, whereas towards the west it passes under the Magura Nappe into the Grybbw unit which, in turn, is underlain by the Obidowa-Siopnice slice (~lqczka, 1971; Cieszkowski et al., 1985; Slqczka and Kaminski, 1998). The Dukla Nappe itself usually nsts on top of the Silesian Nappe, although locally other tmmic slices, such as the Fore-Diua and Michalczowa units, composed of Cretaceous to Paleogene shalIaw-water successions, occr~r be- neath the DukIa sole thrust (Cieszkowski, 1992; Roca et al,, 1995).
Lithostratigraphy, The Dukla Nappe is composed of folded Cretaceous to PaIeogene deep-water flysch strata (~lqczka, 1 97 1,1985; Koszarski, 1 985; see also Figs. 2,3). The stratigraphc inventory includes a seqltence of alternating in- competent and competent s h a h and medium- to thick-bedded turbidites, dariing from the Senonian Eupk6w beds through to the OligocensLower Miocene - Krosno beds, the latter oc-
380 Leonard Mastclla and Witold Zuchicwicz
KROSNO BEDS
MAJOAN BEDS
ClSNA BEDS
-
L V P K ~ W 100a BEDS
. . u
I,*,1
Sandstonesa
ChartsMarls Variegated shales
Shales
[
Syderites@
Number of stationsFig. 3. Lithoslmtigmphic log of the Dukla Nap c (bascd on Slqczka and
Zytko, 1979; hlqczka, 19!5)
Prz - Fmybysz6w smdstoncs, GS - grccn shalcs, GM - GIobigcrina rnarls, sCcM - sub-Ccrgowa marls, Msz - Maanka aandstoncs, Cc -
Ccrgowa sandstoncs, Mc - mcnilitc shalcs, TR- hnsitionai beds; +- compctcncy, and - - incompctcncy of adjaccnt lithostmitigraphic mcm- bcrs
curring in marginal parts of the DuWa Nappe only (Fig. 2). The total thiclmess of the Dukla sedimentary sequence estimated from study of exposures and borehole data ranges between 4000 and 5700 rn (slWzka and iytko, 1979; Gucik et a]., 1980). The most resistant litbostratigmphic members are repre sented by thick-bedded sandstones of the Cisna (Upper Creta- ceous-Paleocene) and Cergowa sandstones (Oligocene) (Fig.
3). Tectonics. The Dukta Nappe comprises a number of NNW-SSE to NW-SE and WNW-ESE skiking, imbricated folds (Figs. 2, 4) which plunge towards the north-west below the Magura or Jado Nappes (~1zpczka and Zytko, 1979; ~ l ~ c z k a and Karninski, 1998). The unit is separated from the Silesian Nappe by a thrust plane which in the eastern part is locally overturned ( ~ l ~ c z k a and kytko, 1919; Sl$czka, 1985;
Rubinluewicz, 19961, The minimum amount of thrusting of the Dukla Nappe upon the Silesian Nappe has been found to attain 15 lan south of Wetlina (Cieszkowski e6 a/., 1985).
According to ~lqczka (1971, 1985), the Dukla Nappe is subdivided into two subunits, the internal and external ones, separated by a low-anglt: thrust (Fig. 4). The external sr~brmit is composed of several steeply dipping horsm, the internal one showing nearly flat bedding. The lack of the Krosno beds in the internal subunit is tRo11ght to indicate earlier uplift in this do- main (~l~czka, 197 1, 1985). Folded strata of the DukIa Nappe are cut by NNE- to NEorientated, mostly strike-slip ia~~lts, showing both sinistral and dextral components of motion.
The D d l a Nappe with the coeval Grybdw, Obidowa-310- p i c e and Michalcma slices were emplaced during the Early Miocene (Slqczka, 1985; Ciesikowski, 1992). The final em- placement of the nappe passibly post-dates the folding of flysch
seata, as the Dukla sole thrust cuts both most of the Dukla folds and those of the underlying Silesian Nappe (Roca et al., 1995).
Folding in the Dukla Nappe was restricted, most probably, to Late Oligocene times and was postdated by an episode of strike-slip and, during post-orogenic collapse, normal faulting (Ksiqkkiewicz, 1977; hlqcfka and Kaminski, 1998). Recently published pilot studies made at one Iocality
in
the NW portion of the Dukla Nappe and at a number of stations located in the Silesian and Magura Nappes (Decker et al., 1999) appear to in- dicate two episodes of thrusting, namely the Palmgene-Early Miocene N-directed in-seqr~ence thrust shortening, and an Early-Middle Miocene NE-directed out-of-sequence episode, followed by post-Middle Miocene dextraI strike-slip event. The inferred .timing ofthese events is, however, poorly constraind.MATERLAL AND METHODS
We have analysed joint patterns at 60 stations, individilal singular large exposures or groups of exposures, spread un- evenly tho~~ghout the Polish segment of the Dukla Nappe. The stations studied represent Senonian-Paleocene (22%), Eocene (1 7%) and Oligocene (ca. 62%) smta (cf: Pig. 31, most of the data coming from medium- to thin-bedded (47%) and thick-bedded (33%) flysch sequences. We have measured the attitude of bedding and joint surfaces, as well as the orientation of small-scale shchlres associated with joints, including: en
Jointing in the Dukla Nappc [Outer Carpathians, Poland): an attcmpt at palamtress reconstruction 38 1
NW Cisna
S DUKLA NAPPE
Fig. 4. Simplified geological cross-sections through thc Dukla Nappc (bascd on $14czka, 197 1, 1985)
0 - Istcbna bods, I - Cisna beds, l a - Lupkbw bcds, 2 - himogIypRic beds, 2a- Majdan beds, 3 - Pnybyszbw sandstones, 4 - Mszanka sand- stones, 5 - sub-Cergowa mads, 6- Cergowa sandstones, 7 - mcnilite beds, 8 - transitional bods, 9 - Krosno bcds, t - diffcrcnt ardcr thrusts
echelon fractures, feather fractures, and tectonic fringes, At each station 50 to 120 joint surfaces have been measured, a fig- ure statistically representative for the region (MasteWa, 1988;
Zuchieuvicz, 1 997a; Rubinkiwicz, 1998). Measurement reso- lution was
a".
The data have been pIotted on lower hemi- sphere Schmidt prqections and then bedding- and fold plunge-corrected, using the STEREONET program.Following Price (1 959, 1966), Ksiqikiewicz (1 868), and Jaroszewski (19721, we accept a prefolding - at incipient stage -origin of the majority ofjoint sets. This is indicated by thc fact that joints belonging to a single set and showing vari- abIc orientation in fold limbs, attain the same orientation after rotating foId limbs into hmimntal position about the bedding strike. Moreover, observations carried out in other parts of the Outer Carpathians indicate that some of the joints which were formed in poorly lithifid strata (Tokarski, 197%; Mastella, 1988; Mastella et al., 1997; kwierczewska and Tokarski,
1998), are displaced by foId slip (Mastella and Ozimkowski, 1979) and form tectonic ledges (cf. Jaroszewski, 1968) at inter- sections with bedding planes (Mastella, 1988).
Therefore, apart from standard statistical procedures (Fig.
5A), we have also studied the joint pattern after bedding correc- tion, followhg the suggestions of Murray (1967) and Ksiqkluewicz (1 968). As a result, we have obtained consider- able unification of orientations of individual joint sets (Fig. SB, C) that enabled further calculations pig. XI).
The regional joint network comprises five sets (Fig. 6). At individual exposures, however, usually two to four sets can be encountered (Figs. 3-59, The sets maintain a relativeIy stable orientation
in
respect to the strike of mapscale folds. Sets382 Loonard Mastella and Witold Zuchicwicz
Fig. 5. Tcchniquc o f dctmining joint pattcrn parameters, cxemplificd by statistical processing of data rneasurcd at stations Iocatcd on thc Panna strcam (cj: Fig. 2; thin-beddcd sandstones, mcnilite bcds)
A - stcrcogram of joints bcforc bcdding correction, lowcr hemisphcn:
Schmidt ncc B - thc same stcrcogram aflcr hdding correction; C - in- tcrpretative diagram showing position of main joint sets; arrows portray tendcncy to strikc-slip motion; D - seIectod paramctcrs of thc joint net- work; N - numbcr of data; cross-fold: T - transversal, SR - diagonal dextral, SL - diagonal sinistml; L and L' - foId-parallel: great circle (bodding plane) and dot (normal to bcdding planc) mark the attitudc of bcd- ding
showing the same regional orientation display both similar
&cia1 features and the type of intersection with bedding sur- faces.
Crossfold johts comprise a single set (T; F 1 5 . 6 , g ) strik- ing perpendicular or sub-perpendicular to mapscale fold axes (80-90°), and two diagonal sets (SR, SL) striking at high angles (60-80') to these axes (Figs. 6, 7A, 8). The acute bisector b e tween these two sets is orientated perpendicular to map-scale folds. Fold-parallel joints
(L)
(Figs. 6-59, (La) (Figs. 6, 7A) strike parallel or at mall angles to mapscale fold axes and are perpendicular or subperpendicular to bedding (70-90").CROSS-FOLD JOINTS
The diagonal system comprises set SR orientat4 N4OW-N25"E, clustering between NP-I SOE, and set St orien- tated, respectively, N56"-8S0E and N75"E (Fig. 1 0). Some of the SR joints terminate on the SL joints and vice versa. No re gona1 variability of joint set orientation has been found. Both sets, although orientated differently at individual exposures, in- tersect one another at an acute angle of 6&75' pigs. 5,7A, 8), whose bisector &Ices SW-NE throughout the study area. This property points to a conjugate character of these two sets.
Joints of the two sets are morphologicalIy similar. Fissures associated with both the sets, a few millimetres wide, De filled in places by calcite. Both surfaces of a singular, non- mineralised joint resemble a am-cast coupling or are accom- panied by plumose structures of axes parallel to the jointbed- ding surface intersection. Acute terminations of plumose s&uctures are, wen on surfaces belonging to the same joint set, variably orientated, pointing to a differenfly directed sense of initial propagation of joints (Parker, 1942). The presence of such structures appears to indicate an extensional mode of joint opening (l3&tz, 1965, 1966; Engelder, 1985).
The surfaces of SR and SL joints are planar and theit traces on bedding surfaces are usually rectilinear (Figs. 7h, 8). Nu- merous joints of the SR and SL sets are accompanied by minor feather frachlras, striking at 30' v e m the joint surface (Pigs.
Fig. 6. Joint pattcrn in a sandstonc,bcd (based on Ksiqkkicwicz, 1968, mcd~ficd)
For cxpIanations scc Fig. 5
Jointing in thF DuklaNqpe (OuteF Cerpathians, Poland): an attempt at pdawsiress reconstruction 383
Fig. 7. A
-
hints on top of a thI& smdstone Cism beds (station no. 58, Solinka &am, 2 Irm south of Cisna); dashed line marks tho axtcbisebtor beiwem conjugate &car join& (m& B
-
Close-up of f d m hctum associatmi with pet SR For dm exphuiiom sw Fig, 5Pi 8. loht pa- in a Mck Cinna sandstone beds (WctIinka stream) EbFclepIanatlons m Fig. 5
384 L w n d Mastclln and Witold Zuchicwicz
Fig. g. Joints in silicified, medium-bcddcd sandstoncs of thc menilitc bcds (statioi~ no. 44, Solinka strcam south of hIbraczc villagc) For cxplanations sac Fig. 5
7B,
X),
that represent low-angle Riedel shears (sensli Riedel, 1929; Bartlett et al., 1981). Locally, instead of a linearlrace, an en ecJ~eioa array composed of low-angle Riedel shears can be encountered, passing sometime~ into a continuous joint sur- face. Such shears are probabIy incipient f m s preceding the formation of continuous diagonal joints, Riedel shears pene aate only a few miIlirnetres into the host bed. Joints opened along en echelon mays of shear fractures bear hnge sklctures on their surfam. Some of the nz echelon cracks are filled by material derived from cornby rocks or by calcite contaminated by this materid.The sb3ke of T joints varies along the segment of the h k l a Nappe studied from N24"E to N46'E, cluskrhg at N35'E (Fig.
I 1A). The surfaces of T joints are not planar, and their traces on brxlding surfaces are usually curvilinearear Tbe T joints are not aceompanid by feather and en echelon fractures, and are lined by mineral (tlsuaIly calcite) veins less fiquently in comparison to the SR and St diagonal joints. Joht surfaces are usually de- void of fringe smmires. Piss~lres associated with this set are commonly open, ~lnlike those related to the diagonal sets. The
T
joints are, in turn, frequently accompanied by plumose and pinnate strucnres that do not show any preferred orientation.Fold-parallel (longitudinal) joints (L,
L')
strike sub-parallel to the map-scale folds and comprise two sets of different orientation (Figs. 5,7-9, 12). Joints of set L are usu- ally parallel to map-scale fold axes, whereas joints of set L' strike at low angles (up to 20') to these axes.The most frquent joints of set L skikc N 100°-1 30°E, clus- tering at N125"E (Fig. 121, and are nearly always parallel to the bedding stnke. Joints of set L', in turn, strike N145°-1550E (Fig. 12) and are usually independent of map-scale fold orienta- tion.
Both joint surfaces and traces oftheir intersection with bed- ding planes are similar in the two sets. Traces of intersection with beddig planes are discontinr~ot~ or fading, sometimes curvilinear (Fig. 7A). S~ufaces of a single joint are us~ially of the ''formcast" type. Plumose sh~~chlres are particularly abun- dant upon most of the L' joint surfaces, and very rare on
L
joints. This is partict~larly visible in large fald hinges where such joints resemble textbook example of radial Gnctures (Jaroszewski, 1980; Price and Cosgrove, 1994). It is important to note that the origin of fold-parallel joints in the Outer Carpathians has n d been dealt with except in preliminary com- ments by Ksiqikiewicz (1 968) and A1 eksandrowski (1 989), mostly regarding tbe Magura Nappe, This description is a yet
Jointing in the Dukla Nappe (Outer Carpathians, Poland): an attempt at palacoshess reconstruction 385
preliminary attempt to assess the importance of these joint sets in the Dukla Nappe.
JOINTING
-
MTERPRETATIONCross-fold joints. Morphological properties of the diago- nal (SR, SL) joints indicate that the incipient stage of their devel- opment was by shear, whereas their firfher opening proceeded in extensional mode (Ptice, 1959,1966; Ksi@ewicz, 1968;
Saroszewski, 1972; Engelder, 1985). The pattern of en echelon arranged gashes and feather h cshows ~that the SR and SL sets represent, respectively, dextral and sinistral. shears (Figs. 5, 7,8), whose acute bisector is orientated
SW-NE
throughout Zhe D ~ k l a Nappe, Furthermore, abutting relationships suggest that the Sa and Stjoints are roughly coeval and were f m e d as "pa- tmtial shear s~rfaces" in a kiaxial stress field (Mandl, 1988;Engelder, 1989; Dadlez and Jaroszewski, 1994), and that the acute dihedral angle comprised between these sets represents a double value of the angle of shear, 2 0 (Fig. 7A). The occur- rence of pIurnose struchues, in turn, points to subsequent extensional opening of these joints pastella, 1988).
We suppose, feIlowing also o ~ u preview study (cf.
Mastella et al., 1997), that these joints are shear fractures wan- cock, 1985) that form a conj~lgate system. Moreover, some
of
the joints were formed when the host strata were poorly lithified, as indicated by contamination of mineral veins that fill the joints by material derived f?om the host strata (Mastella,
1988).
Analysis of the angle of shear. The acute angle comprised between the SR and SL sets (double value of the angle of shear, 20) changes at individual exposures from 50 (Fig. 8) to 72"
pig. 5D). Nearly 80% of our data, however, fall into an interval of 60-70" ('Figs. 13, 14), typical for conjugate joint sets (Handin et al., 1963; Hancock, 1985). Them is a regional ten- dency towards the increase of 2O from the west (60'3 the east (70") This tendency is not tectonically-induced, but it is rather controlled by lithological properties of the host rocks
(cf:
Mandl, 1988). In the eastern part, most measurements have been made in poorly lithified sandstones of the Cisna beds, as well as in mudstones of the wamitional beds, both showing much lower values of cohesion and internal friction as com- pared, for instance, to those of siliceous sandstones of the hiero- glyphic or menilite beds. Therefore, the angle of shear in the Cisna and bansitional beds attains highcr values than that in the hieroglyphic and meniIite beds (Fig. 14).
The morphology of T joints indicates that their develop- ment proceeded without the initid "shear" stage and that they are extension (mode I) fractures (Bankwitz, 11965, 1965;
Ksiajkiewjcz, 1468; Engelder, 1985; Price and Cssgrove, 1994).
Fold-parallel joints. Both the L and
I,'
sets are devoid of properties that wouId point to their shear origin, The morphol- ogy of joint surfaces, occurrence of plumose struchua Cpartic- ularly common in the L' set), and characteristic discontinuousFig. IO. Histogram she-~ng thc distribution of Sn and SL joint scts in thc portson of thc Dukla Nappc studied
For othcr explanations scc Fig. 5
or fading, in places nonlinear, traces of intersection with bed- ding planes appear to indicate a tensional origin of join& of the L set and of those L' joints which are parallel to the bedding s h k e (Ksiqkkiewicz, 1968; Aleksandrowski, 1 989).
Taking into account the tensional character of all
L
and some of L' joints, as well as their associationwith
hinge zones of folds one can hypothesize, following AIeksandrowski (1 989), that these joints originated during early stages of fold- ing.The review above leads us to conclude a differentiated ori- gin of individual sets and systems of the regional joint network, formed in different stress fields and at different times. The type and orientation of these fields can be reconstructad from the properties of joints (Ksiqduewicz, 1968; Jmszewsh, 1972;
Aleksandmwski, 19856 b, 1989; Zuchiewicz 1997a, b;
Mastella et al,, 1997).
Stages of joint development. The onset ofjointing should be related to a period when flysch seats of the Outer Carpathians were lithified enough to accumulate stresses re- sulting from regional compression induced by subduction of the Eurasian Plate under Alcapa (i-a. Peseatore and ~l~czka, 1484; Oszczypko and ~ y t k o , 1987; Tomek and Hall, 1993;
Zoetemeijer et a!., 1999; and referenctx therein). A triaxial stress field with horizontal, NE-SW orientated a, (Tapponnier, 1977; Tokarski et
QI.,
1 999) and vertical dominated at that time, leading to initiation of the shear joint system within hori- zontal strata, Only locally, when the shear stress strength of host rocks was attained, SR and SL sets of this system couId have been formed.386 Leonard Mastella and Witold Zuchicwjcz
Fig. 1 1. Maps and histograms showing thc spatial disbibution of T (A, a), cq (B, b), and amount of rotation of ol vcrsus T (C, c); black left-circlc nnd
-- anticlockwise, black right-circlc and
+-
clockwise, whitc circlw - T parallcl a,Jointing in thc Dukla Nappe (Outcr Carpathians, Poland): an attempt at palaeosimss rcconshuction 387
FoIding commenced following the increasing W Whor- izontal compressional stress. In broad hinges of large-scale open folds, tensional joints of set
L,
parallel to fold axes, stated to deveIop. At the same b, with gradual steepening of folded strata, the SR and SL sets became open.These processes lasted throughout
the
shortening of the Outer Carpathians until the beginning of post-omgenic COI- lapse when, under diminishing horizontal compression (cT,), perpendicular to it and also horizontal n3axis attained a nega- tive value,This
change led to formation of extensional joints of set T. Later an, according to the Price's (1959) model, fie- re- maining extensionalL
joints (perpendicular to T) developed, being locally post-dated by the youngest L' joints.Our observations of recently opening joints in rocky river beds, undermined valley sides or quarries indicate that they ase fiquently predisposed by directions of stress relaxation in host rock massifs (cf: also Boretti-Onyszkiewia, 1 968).
ATTEMPT AT PALAEOSTRESS RECONSTRUCTION
Diagonal system. The oldest, diagonal joint system was initiated within subhorizontal strata in a shear lxiaxial stTess
field, charackrised by positive axes of a* > u2>
s.
The orien- tation of this field w a s reconstructed after bedding cmmtion pig. 5B, C), by placing crl along the acute bisector of 2 0 (Bucher, 1920, 192 1 ; Ramsay and Huber, 1987; see also Figs.5D, 7A). at, strikes at individ~ial exposures betweenNZSOE and N62OE (Fig. 24), although nearly BDDh of data fall within the limit of N34"-52"E, cl~istering at N45"E (Fig. 11I3, b).
Orthogonal system. Thls system is younger than t h ~ diag- onal one. Its older set T originat4 in the nl>cF2*3 stress field, the axes rsl and -03 being horizontal. The orientation of such
Fig. 12. Summary rose diagmms showing thc frcquency o f occurrcncc of fold-parallcl Land L'joint scts, as wcll as thc strikcof beddingmeasurcdat individual stations
N - numbcr of data
amount
Fig. 13. Histogram showing thc distribution of 2 0 valucs in thc studicd porlion of thc nukla Nappc
joints
t
parallel to 01 (Price, 1959; Mastella and S z y n k a 1998). At the incipient stage of development of the orthogonal system, the axis a, was orientated N35"~10°E (Fig. I lA, a), and joints T opened due to extension (q) aligned roughly N12SoE.ColIapse-related extension paralleI to the structural grain of that portion of the Carpathian belt induced a profound change in orientation of the s k s s field: 02 axis became horizonkd and parallel to the axis of collapse, 0 , attained a vertical attitude and its former orientation (N35"E horizontal; at present coordi- nates) was taken over by q. In such a stress field most of extensional joints of set
L
originated. Joints belonging to the L' set were formed in a field of similar configuration of principal stress axes, although orientated differently, probably due to a Iocal change of the collapse-related extension.It is important to note that the orientation of crl recon- structed for both shear joints and T joints is independent of the age of host strata (Fig. 14). This suggestq that, since the onset of diagenesis of the Cisna W, the shear joint-related stmss field has maintained a constant orientation, similar to the stress field in which T joints developed (cf. also Rubinkiewicz, 1998). At present, in turn, a marked right-lateral rotation by cu. 10" of shar joint-related rrl (Fig. 1 1 B, b) in respect to that of
T
joints (Fig. 1 1A, a), is notable (Fig. 1 lC, c). This tendency applies to both individual lithostmtigraphic members (Fig. 14), and indi- vidual exposures (Fig. 1 1 C). Since shear joints and T joints are not: coeval, we suppose that they orignated under diffcr- ently-orientated skess fields.Summing up, the joint network in the studied portion of the Dukla Nappe originated in two stages showing different orien- tations of the s m s field axes. The older, compressive stage I was characterised by N45°E-orientslted (at present coordinates) horiaontal a[, and formation of the diagonal, shear joint system.
The younger, extensional stage
IT
was characterised by vertical a1 and horizontal, N3SoE-orientated q, suitable for formation of the fold-paraIlel L joints. At the turn of the first stage, under diminishing horizontal compression (ol -ding N3S0E), the388 Leonard Mastclla and WitoId Zuchiewicz
Fig. 14. Simplified lithostratigraphic log of thc DuMa Nap c showing variability of thc mc,onst~ctcd valucs of 2 0 (anglc of shear), s, (principal ax]$[ and T (cross-fold cxtcns~onal jolnts)
9, 10, 18,10, 13 - numbcr of stations; for othcc ~cplan~tions scz Fig. 3
cross-fold
T
joints developed. The orientation of the maximum compression axis changed between stages I and 11. Such changes have been described from a number of regions (cf.Angelier, 1994), and also from the Carpathians (NemEok, 1993; NemEok et al., 1998; ;Twigel, 1998).
The fmt stage probably commenced when strata which w m not fully lithified undemnt shortening, and lasted until the termination of nappe emplacement. The second stage, in tum, is associated with past-orogenis collapse of the Carpatbians, whose onset in the study region began at least in Late Miocene times (Henkief, 1977; Oszczypko, 1996; ~ 1 ~ c z k a
and Kaminski, 1998; Decker et d., 1999; ~ y t k o , 1999; and ref- erences therein).
Acknowledgements. This study has been sponsored in part (W. 2.) by the Committee for Scientific Research of Poland through p tno. 9 T12B 020 0!9 (to A. K Tokarskg and by the Faculty of Gology Research Fund, Warsaw University (to
L.
M.), through grant no. BW 148419. We are indebted to Jacek Rubinkiewicz for critical reading of an early
dtaR
of the manu- script, and to Marek Jarosi6sk1 and Antmi K. Tokarski fox thorough reviews and comments.Jointing in the Dukla Nappc (Outer Carpathianq Poland): an attempt at palmstress nxonstruction 389
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