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Tectonics of Miocene–Pliocene fresh-water molasses in the Carpathian Foredeep (Witów Series, South Poland)
Journal of Geodynamics 41 (2006) 369–384
Tectonics of Miocene–Pliocene fresh-water molasses in the Carpathian Foredeep (Wit´ow Series, South Poland) Marta Rauch-Włodarska a , Witold Zuchiewicz b,∗ , Stanisław Brud b,1 , Galicia T. Group a
Institute of Geological Sciences, Polish Academy of Sciences, Krak´ow Research Centre, Senacka 1, 31-002 Krak´ow, Poland b Institute of Geological Sciences, Jagiellonian University, Oleandry 2A, 30-063 Krak´ ow, Poland Received 16 February 2005; received in revised form 16 September 2005; accepted 23 September 2005
Abstract Normal faults of different orientations appear to be the youngest manifestations of faulting in the Polish Outer Carpathians, composed of the Lower Cretaceous trough Lower Miocene strata, and the related Carpathian Foredeep, which is filled with the Lower to Middle Miocene sediments. In the Outer Carpathians, the folds and thrusts produced by accretion-related shortening were formed between the Paleocene and early Late Miocene. The origin of normal faults is still debatable, since it is not known whether these faults were a result of multidirectional extension produced in a single collapse event, or differently oriented extension proceeding in a series of successive events. Structural studies of the Late Miocene–Pliocene(?) fresh-water molasses of the Wit´ow Series provide a possibility to reconstruct the Late Neogene stress field in the central part of the Polish Carpathian Foredeep and, indirectly, in the central part of the Polish Outer Carpathians. The strata of such an age are unique features in the Polish Carpathian Foredeep, proving thereby a key record of structural deformation during the latest stages of orogenic evolution of the Carpathian orogen. The molasses are cut by joints, and normal and strike-slip faults which were formed in two successive events: (1) a syn-depositional one, proceeding under NNW-SSE to N-S oriented horizontal compression, possibly coeval with reactivation of a NE-striking sinistral fault of the Kurdwan´ow-Zawichost Fault Zone in the basement; and (2) a post-depositional one, during N-S to NE-SW-oriented extension. In the first event, reactivation of the NE-striking sinistral fault led to formation of N-S-oriented joints, as well as NW-striking dextral, and NNW-trending normal faults. This event was probably contemporaneous with sinistral reactivation of some thrusts in the Western Outer Carpathians, induced by eastward-directed extrusion of crustal blocks in the Carpathian internides. In the second event, both W-E and NW-SEoriented joints and WNW-striking normal faults were formed. The latter most probably originated due to reactivation of the Early Paleocene WNW- and NW-striking normal faults in the basement. Therefore, normal faults detected in the Outer Carpathians and Carpathian Foredeep appear to be a result of not a single collapse event but of different successive events. This extensional episode lasted at least to the late Pleistocene. No evidence for the recent NNE-directed tectonic compressive stress, typical for that segment of the Carpathian arc, has been found yet. © 2005 Elsevier Ltd. All rights reserved. Keywords: Poland; Carpathian Foredeep; Miocene–Pliocene molasses; Outcrop-scale tectonic structures
∗ 1
Corresponding author. Tel.: +48 12 6332270; fax: +48 12 6332270. E-mail addresses: [email protected] (M. Rauch-Włodarska), [email protected] (W. Zuchiewicz). Sadly, Stanisław Brud passed away recently.
0264-3707/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2005.09.001
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Fig. 1. Geological sketch of the Carpathians with location of Fig. 2. Dashed line shows the boundary between ALCAPA and TISZA continental fragments in the Intra-Carpathian area (after Fodor et al., 1999).
1. Introduction The Carpathian Foredeep is a foreland basin related to flexure of the continental lithosphere in a continental collision ´ aczka, 1984; Krzywiec, 1997, 2001; Zoetemeijer et al., 1999; Oszczypko, 2004; and references zone (Pescatore and Sl˛ therein). This basin developed on the European plate in front of the Carpathian orogenic belt (Fig. 1) and was related to convergence between the European and African plates. Reconstruction of structural evolution of the Carpathian Foredeep is, therefore, of crucial importance for the understanding of the formation of the Carpathians themselves (cf. Decker and Peresson, 1996; Matenco et al., 1997; Linzer et al., 1998). Along the entire Carpathian arc, trajectories of recent stress change their strike from NNW in the western and medial portions of the Carpathian Foredeep, through NE and E-W in the north-eastern and eastern part, to ESE in the southern and south-eastern parts (Cloetingh et al., 2002). As far as the Polish segment is concerned, these trajectories are arranged fan-like, trending NNW to NE, from the west to the east (cf. Jarosi´nski, 1998). The aim of this paper is to constrain the Late Neogene stress field in the medial portion of the Carpathian Foredeep in Poland, basing on an analysis of small-scale tectonic structures within Late Miocene–Pliocene(?) fresh-water molasses exposed at Wit´ow, a single key exposure of such strata in the entire Polish segment of the Carpathian Foredeep. Since these deposits are not occurring anywhere else, their structural record becomes crucial for the Late Cenozoic paleotectonic reconstruction of the region. 2. Local and regional geological setting The sandy-gravel sediments described by Łyczewska (1948) as the “Wit´ow Series” are exposed at a spur between the Vistula and Szreniawa rivers, in the medial segment of the Polish Carpathian Foredeep, approximately 30 km north of the Carpathian frontal thrust (Figs. 2 and 3). These are poorly cemented sandstones, sands, gravels, and silts which cap Miocene clays and are overlain by differentiated Quaternary sediments (Fig. 4). The series bears numerous examples of young tectonic deformations. The coarse-clastic Wit´ow Series represents fresh-water molasses, whose origin and age have long been a matter of debate. This series marks an episode of late orogenic uplift-induced intensive erosion of the Outer Carpathians (cf. D˙zuły´nski et al., 1968; Starkel, 1972). Lithofacies assemblages present in the Wit´ow Series are characteristic of a braided river, which was flowing from the Carpathians, accumulating Carpathian-derived material alongside that scoured from the Miocene strata of the Foredeep. Previous studies have resulted in contrasting opinions about the age and origin of this series, starting from a concept of marine Miocene strata, through the Eopleistocene, up to the
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Fig. 2. Tectonic sketch of the central part of the Carpathian Foredeep of Poland (after Krysiak, 2000), showing the extent of the Wit´ow Series (Brud, 2003) and location of the Wit´ow pit. The Zgłobice Unit after Połtowicz (1991; simplified).
Middle Pleistocene (cf. Brud, 2002; Brud and Worobiec, 2003; Brud et al., 2003, and references therein). The results of recently conducted integrated sedimentological, petrographic, and palaeobotanical studies indicate that sediments of the Wit´ow Series were deposited in Late Miocene and, possibly, Pliocene times (Brud, 2002; Brud and Worobiec, 2003). The Wit´ow Series rests unconformably upon Langhian-Serravallian (Middle Miocene) strata of the central part of the Polish segment of the Carpathian Foredeep. The Foredeep is superimposed on Proterozoic–Mesozoic rocks of the European Platform (cf. Oszczypko, 1998; Olszewska, 1999) and represents a basin that developed syn-tectonically in front of the Western Outer Carpathians (Fig. 1). The infill of this basin is of Early to Middle Miocene age (Oszczypko, 1998; Olszewska, 1999). The strata are mostly sub-horizontal and only occasionally folded. By contrast, in the southern
Fig. 3. Location sketch of the Wit´ow exposures.
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Fig. 4. Exposure C, the principal exposure at Wit´ow, viewed from the south and showing position of Figs. 6A and 11. Miocene–Pliocene: 1: sands, gravels, and silts of the Wit´ow Series; 2: massive, grey silts; Quaternary: 3: loessial silts; 4: massive, yellow loess; 5: brown loess; 6: anthropogenic layer; 7: anthropogenic infill of small valleys and ditches; 8: dump heap; 9: faults. Letters refer to lithostratigraphic code of individual strata.
part of the Carpathian Foredeep, close to the Outer Carpathians, the Miocene strata are strongly folded and cut by thrust faults. This zone of folded strata is called the Zgłobice Unit (Kotlarczyk, 1985) (Fig. 2), where axes of map-scale folds and strikes of map-scale thrust faults are W-E to WNW-ESE (Mitura and Moskała-Martini, 1954; Kirchner and Połtowicz, 1974; Połtowicz, 1991, 1998; Krzywiec, 2001). The basement of the central part of the Carpathian Foredeep is cut by map-scale normal and strike-slip faults (Dokt´or and Graniczny, 1983; Krzywiec, 1997, 2001; Krysiak, 2000; and references therein) (Fig. 2). Normal faults of Early Paleocene age strike mostly NW-SE, more rarely NNW-SSE and WSW-ENE. The NW-SE-trending boundary faults enclose horsts and grabens, the latter frequently referred to as depressions. The studied area is situated within the Działoszyce Depression (Fig. 2). The other map-scale faults trend NE-SW to NNE-SSW and are mostly sinistral strike-slip or oblique-slip in character (Krysiak, 2000; and references therein). One of the NE-SW-trending faults of the Kurdwan´ow-Zawichost Fault Zone is located east of the Wit´ow pit (Fig. 2). According to Osm´olski et al. (1978), the Kurdwan´ow-Zawichost Fault Zone was reactivated as a left-lateral fault in the Middle Miocene. On the contrary, Krysiak (2000) suggested that Miocene activity of this zone began by normal faulting during the late Middle Miocene, being replaced by sinistral faulting in the Late Miocene. Jarosi´nski (1992) confirmed a sinistral reactivation of the Kurdwan´ow-Zawichost Fault Zone due to N-S-oriented compression induced by the overthrusting Carpathians during the Serravallian (late Middle and early Late Miocene) or later (up to 11.5 or 9 Ma), although this short episode was subsequently replaced again by normal faulting. Recent 3D seismics and tectonic studies of a gas-bearing structure in the eastern portion of the Carpathian Foredeep in Poland documented intense tectonic deformations within Late Miocene marine strata, including NNW-SSE to N-Strending tight folds, strike-slip and dip-slip faults, along with joints and pencil cleavage. All of these are interpreted in terms of a sinistral transpression with the horizontal maximum compression axis oriented E-W, and a sinistral strike-slip displacement on NW-SE striking faults in the basement (Aleksandrowski et al., 2005). The age of deformed marine strata in the east may be roughly coeval with that of the lower portion of the fresh-water Wit´ow Series in the west. The formation of the Carpathian Foredeep and Miocene reactivation of older faults within its basement were strongly dependent on the Miocene stress field generated within the Carpathians. The Carpathians are part of the European Alpine system that originated from the convergence between the European and African plates, which terminated by collision and emplacement of the continental fragments into irregular, southern European plate boundary (Dewey et al., 1973; Golonka et al., 2000; Neubauer et al., 2001). The Inner Carpathians are composed of two major continental fragments: ALCAPA and TISZA (cf. Fodor et al., 1999), while the arcuate Outer Carpathians represent the external pile of nappes (Fig. 1). The northernmost part of the Carpathians is situated in Poland. The Polish part of the Western Outer Carpathians is a fold-and-thrust belt, mainly composed of Lower Cretaceous to Lower Miocene flysch sediments (Ksi˛az˙ kiewicz, ´ aczka and Kami´nski, 1998), the main structural features of which originated during formation of the Carpathian 1977; Sl˛ ´ aczka, 1984; accretionary prism in the Paleogene and Neogene (Ksi˛az˙ kiewicz, 1977; Tokarski, 1978; Pescatore and Sl˛ Aleksandrowski, 1989; Fodor et al., 1999). The Western Carpathian accretionary prism was connected with the roughly
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northward movement of the ALCAPA and southward subduction of the European plate (Aleksandrowski, 1989; Tomek and Hall, 1993; Fodor et al., 1999). Tectonic evolution of the Polish Outer Carpathians was characterized by superposition of two shortening events connected with folding and thrusting, NNW- (N), and NE- (NNE)-directed ones (Aleksandrowski, 1985, 1989; Decker et al., 1997, 1999a). According to Decker et al. (1999a), the first shortening event occurred between the Paleocene ´ ´ and Early Miocene (cf. Swierczewska and Tokarski, 1998; Swierczewska et al., 2001), whereas the second one was initiated in the Early and Middle Miocene (Decker et al., 1999a), and probably lasted to the early Late Miocene (Decker et al., 1999b; see also W´ojcik et al., 1999, 2001). Folds and thrusts produced by accretion-related shortening are commonly overprinted by normal faults which appear to be the youngest manifestations of faulting in the Polish Outer Carpathians (Decker et al., 1997). These normal faults are characterized by different orientations. According to Zuchiewicz et al. (2002), it is still not clear whether these faults were a result of multidirectional extension (single collapse event) or differently oriented extension proceeding through successive events. Nevertheless, normal faults resulting from NE-SW extension appear to be the youngest ones (Decker et al., 1997; Zuchiewicz and Cieszkowski, 1998). The extensional phase of structural evolution of the Polish Outer Carpathians most probably commenced in the Late Miocene and has continued into the Quarternary (Zuchiewicz et al., 2002; and references therein). The results of geomorphological studies conducted in the medial and eastern portions of the Polish Outer Carpathians have shown that horizontal compressive stresses may have played an important role in the formation of young (PlioceneQuaternary) tectonic deformations, while the western portion is dominated by normal faulting (Zuchiewicz, 1995; Zuchiewicz et al., 2002). A study of the present-day maximum horizontal stress directions inferred from borehole breakouts in the Polish Outer Carpathians and their foreland, in turn, suggests a NNE-SSW orientation of compressive stress trajectories across the study area (Jarosi´nski, 1998; Kł˛ek et al., 2003; see also Cloetingh et al., 2002). 3. State of research into outcrop-scale tectonic structures 3.1. Medial segment of the Carpathian Foredeep Miocene strata of the Carpathian Foredeep are cut by joints and rare normal faults (Krysiak, 1986; Rauch, 1995, 1998; Rauch and Tokarski, 1995). Joints are ubiquitous outcrop-scale structures within the entire area, providing excellent paleostress-field indicators. Strata of the central Carpathian Foredeep are cut by four joint sets that are oriented: (I) N70–110◦ E; (II) N165–185◦ E; (III) N10–60◦ E; and (IV) N125–155◦ E (Rauch and Tokarski, 1995; cf. also Krysiak, 1986). The oldest (I) set strikes parallel to the orientation of normal faults (WNW-ESE and NW-SE) within the basement. An analysis of seismic sections confirms the mid-Middle Miocene, synsedimentary character of these faults (Krzywiec, 2001). Therefore, joint set (I) is interpreted as coeval with normal faulting, produced due to extension in the foreland of the Carpathians exerted by the European Plate plunging into the Carpathian subduction zone (Rauch and Tokarski, 1995). Set (II) is oriented perpendicular to the fold axes and the strike of thrust faults in the folded southern part of the Carpathian Foredeep, within the Zgłobice Unit (Rauch, 1998). Therefore, this joint set is considered to have been connected with folding and thrusting in the Western Outer Carpathians (Rauch and Tokarski, 1995), when horizontal compression was oriented NNW-SSE to N-S. In the Zgłobice Unit, both the orthogonal sets of joints (I) and (II) were interpreted as an effect of the northward thrusting of the Outer Carpathians (Rauch, 1998) during the Langhian-Serravallian times (Rauch and Tokarski, 1995). According to these authors, the third, NE-striking joint set originated due to NE compression active in the Late Miocene, and set IV (NW-SE) was probably formed in the Pliocene owing to NW-directed compression. Strata of the Zgłobice Unit exposed at Zgłobice (Fig. 2) are cut by three fault systems: (1) W-E-striking thrust faults; (2) WSW-ENE-oriented normal faults; and (3) NE-SW-striking thrust faults (Rauch, 1998). Cross-cutting relationships confirm such a succession of faulting events. The orientation of thrust fault system (1) is in line with the first shortening event recorded in the Polish Outer Carpathians (Decker et al., 1997, 1999a,b). At Zgłobice, the hinge part of a W-E-oriented fold is exposed, and normal faulting is interpreted as connected with folding in this area, leading to extension in the hinge zone of the folds. There is no evidence for the second shortening event, which is observed in the Outer Carpathians (see below); and the thrust fault system (3) appears to have resulted from a local change in the orientation of compression. However, according to Rauch and Tokarski (1995), the youngest joint set (IV) striking NW-SE, which occurs in the entire Polish segment of the Carpathian Foredeep, was formed during the NW-SE-oriented compression.
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3.2. The Wit´ow Series Deformation of the Wit´ow Series sediments was described first by Łyczewska (1948), who found small-scale normal faults and folds in the upper part of the Wit´ow Series and suggested their glacitectonic origin. Krysiak (1986, 1987) documented the occurrence of normal faults which cut the entire exposed portion of the Wit´ow Series and are characterized by listric geometry. The normal faults were to dip 45–70◦ and strike mostly N130–170◦ E, rarely N100–110◦ E. This author (Krysiak, 1987) found no indication of synsedimentary origin of these faults and concluded that the lack of upward-increasing degree of deformation contradicted the inferred glacitectonic origin (Łyczewska, 1948). She took into account the similarity between the orientation of the studied small-scale normal faults and regional normal faults detected in the basement, and interpreted the origin of the former as resulting from earthquakes induced by glacioisostatic rebound. Similar conclusions were drawn by Zuchiewicz (1995) who measured two sets of joints, trending W-E and NNW-SSE, and two sets of normal faults, striking NE-SW and NNW-SSE in the northern and western part of the Wit´ow pit, respectively. According to Krysiak (2000), the NW-SE-striking, synsedimentary faults were formed during the Late Neogene, while an episode of E-W to ENE-WSW-oriented extension was to take place in the Quaternary, when listric faults (of steepening upwards planes) cutting the entire series were supposed to have been formed. The quoted author also speculated on a synsedimentary origin of normal faults. 4. Small-scale tectonic structures in the Wit´ow Series – observations Our detailed studies focused on the Wit´ow exposures, labelled as: Wit´ow A, B, C, D, E, and F (Figs. 3 and 4). They are situated on a spur undermined by the Vistula and Szreniawa rivers. Sediments of the Wit´ow Series are cut by numerous joints, normal and strike-slip faults, infrequent thrust faults, and minor folds that were examined in more detail at exposures C, D, and F. The strata are nearly horizontal. 4.1. Joints At exposures B and C, joints strike mostly NW-SE, whereas subsidiary sets trend N65◦ E, N40◦ E, and N105◦ E. Exposure D, characterized by predominantly W-E-oriented joints, displays as well minor differences in their orientation between the upper and lower, and the northern and southern parts (Fig. 5). Joint sets in the northern and southern lower parts are usually oriented W-E, rarely N55◦ E and N45◦ W, whereas the northern upper part is dominated by a set oriented N105◦ E. Some of the W-E-striking joints probably represent master joints, since they cut the entire, exposed part of the series. Between exposures D and F, W-E-oriented joints are ubiquitous (subordinately striking NNW-SSE and NNE-SSW) and, in the overlying Late Pleistocene loessial loams, infrequent, W-E-trending joints are present as well. At exposure F, in turn, joints tend to form two sets, oriented roughly W-E and N-S (Fig. 5). Subvertically-dipping joints, usually about 85◦ (Fig. 5), are ubiquitous throughout the Wit´ow pit, except for gravel intercalations. The joint spacing is irregular; closely spaced joints occur only close to fault surfaces (see below). 4.2. Faults The Wit´ow Series is cut mostly by normal faults; the strike-slip faults occurring at few places only. 4.2.1. Normal faults Normal faults accommodated displacements ranging from a few millimeters to 2 m. The largest slip was determined along the NW-SE-striking faults at exposure C (Fig. 4). Normal faults commonly form two conjugate sets which bound grabens or, rarely, horsts (Figs. 4 and 6). Their fault planes dip very steeply, more than 60◦ , and commonly 85◦ (Fig. 7A and C). Individual normal faults usually dip at a constant angle, but some of them show listric-like (Fig. 8C; NNE-dipping fault L) or step-like geometry (Fig. 8C; south-dipping fault S). Variable dip angles of a single fault surface are characteristic for normal faults striking W-E, WNW-ESE, and NW-SE. The step-like geometry of fault surfaces displays nearly vertical segments. Such geometry is observed along faults striking W-E and WNWESE.
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Fig. 5. Joint sets within the Wit´ow Series (1) and at individual Wit´ow exposures (2); lower hemisphere plots. (A) Contour diagrams of poles to joint surfaces; (B) great circle plots; (C) rose-diagram of joint surface strikes; (D) rose-diagram of joint surface dips.
The strikes of normal faults usually vary between N85◦ E and N195◦ E, showing two prevailing orientations: N115◦ E and N175◦ E (Fig. 7A and B). The last orientation is most common. Both the NNW- and WNW-striking fault sets usually occur at each exposure (Fig. 9). The NNW-striking fault set dominates in the western exposures (C and F), whereas the WNW-ENE one prevails at the eastern B and D exposures. At exposures D and C, the NNW-striking faults occur mostly below gravel-filled channels of the same orientation, but they usually do not cut the channel fill (Figs. 10 and 11). Nevertheless, the fault planes dip mostly towards axial zones of channels (Figs. 7D and 10). Channels oriented more or less W-E occur at exposure C (Fig. 7D). The NNW-striking faults below such channels do not cut the infilling sediments
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Fig. 6. Examples of normal faults cutting sediments of the Wit´ow Series: (A) graben at exposure C, and (B) horst at exposure F; (C and D): lower hemisphere plots.
(Fig. 11). At exposure F, in turn, NNW-striking faults cut the channel infill (Fig. 6B). The axis of this channel is oriented ENE-WSW, i.e. exactly perpendicular to these faults (Fig. 7D). The NNW-striking faults frequently accompany gravel-filled channels; they occur below the channels, tend to dip towards axial zones of the latter, but do not cut the channel fill except at exposures F and C (Figs. 6B and 11). Sediments underlying the channels have usually been eroded during channel formation (Fig. 11; left part of the channel). However, the occurrence of steps bounded by NNW-striking faults in those channel bottoms (Fig. 11; right part of the channel) which survived erosion suggests that these faults were formed when the channel started to be filled. The even roof of this gravel-filled channel indicates that faulting had been completed before the channel was filled completely. The silt
Fig. 7. Orientation of normal faults (A–C), and axes of gravel-filled channels (D) in the Wit´ow Series. (A) Lower hemisphere plot; (B) rose-diagram of fault plane strikes; (C) rose-diagram of dip angles. Upper case letters in sketch D refer to individual Wit´ow exposures.
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Fig. 8. Negative flower geometry of strike-slip faults at exposure C (A), showing orientation of the principal dextral fault (B). Lower diagram C shows listric-like “L” and step-like “S” geometry of normal faults at exposure D, including fault orientation (D; lower hemisphere plot).
layer overlying channel fill is flat, confirming that faulting affecting the channel bottom was no longer active at the time of silt deposition. The lineation on normal fault planes is usually imperceptible because the studied sediments are mainly poorly indurated sand and gravel. We have recognized slickensides on 10 fault plains, which enabled determination of the extension directions in the study area (Fig. 12). To do this, we applied the numerical-dynamic (NDA) calculation method, using computer program Tectonics FP1.5 (Reiter and Acs, 2000; Ortner et al., 2002). Three faults of the NNW-striking system occurring at exposure C suggest WSW-ENE orientation of the minimum principal stress axis (Fig. 12A), whereas six faults of the WNW-striking fault system at exposure D and one fault at exposure C indicate SSW-NNE orientation of this axis (Fig. 12B). Closely spaced joints usually cluster along WNW- and W-E-striking normal faults (Fig. 8C), and can rarely be found close to NNW-oriented normal faults (Fig. 5). 4.2.2. Strike-slip faults At exposure “C”, two parallel vertical faults accompanied by few minor faults have been found (Fig. 13). Minor faults occur in between the first-order faults and are arrested at their surfaces. Horizontal slickensides are visible on all the fault surfaces, suggesting a strike-slip displacement. The main faults cut the subsidiary ones at low angles, ca. ◦ 30 . These minor faults probably form secondary synthetic shears (“R” Riedel shears) in respect to the main faults, indicating dextral movement which proceeded under N-S-oriented maximum horizontal compression axis (Fig. 13C).
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Fig. 9. Comparison of normal faults orientations at particular exposures. (A) Lower hemisphere plot; (B) rose-diagram of fault plane strikes; (C) rose-diagram of fault plane dips.
Small number of measurements required application of the right-dihedra calculation method, used with the Tectonics FP1.5 program (cf. Ortner et al., 2002). At exposure C, the sub-vertical WSW-dipping fault and associated, mostly ENE-dipping, faults form together a negative flower structure which is oriented NNW-SSE (Fig. 8A and B). This structure is accompanied by joints (Fig. 8A), unlike the NNW-striking normal faults (Fig. 5).
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Fig. 10. NNW-striking normal faults below the gravel-filled channel at exposure D.
Fig. 11. Small-scale tectonic structures at exposure C. (A) Succession of two fault systems: (1) older, NNW-striking, and (2) younger, WNW-striking one; (B) lower hemisphere plot.
Fig. 12. Reconstruction of principal stress axes orientation for normal faults at exposures C (A) and D (B).
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Fig. 13. Orientation of strike-slip faults at exposure C: (A) block-diagram; (B) lower hemisphere plot; (C) reconstruction of principal stress axes orientation.
5. Small-scale tectonic structures in the Wit´ow Series – interpretation Joint surfaces in the Wit´ow Series tend to strike NW-SE (exposures B and C) and W-E (exposure D), as well as both N-S and W-E (exposure F). The NW-striking joints are parallel to the Early Paleocene faults in the basement which bound a series of horsts and grabens to the west of the Kurdwan´ow-Zawichost Fault Zone, whereas the W-E joints coincide with a normal fault which runs parallel to the Vistula River valley (cf. Fig. 2). Some of these joints have probably been later reactivated as normal faults (see Section 6). On the other hand, N-S-oriented master joints most probably originated due to reactivation of the sinistral NE-striking Kurdwan´ow-Zawichost Fault Zone in the substratum. Joints are spaced irregularly in the Wit´ow Series. The zones of closely spaced joints are associated with W-E and WNW-ESE-oriented normal faults (Fig. 6C), and are also connected with a NNW-striking negative flower structure (Fig. 8A). Joint surfaces disappear on fault surfaces, but there is no evidence for their offset. The syn-faulting origin of these joints is confirmed by their increased density close to the fault surfaces. Therefore, the W-E and NNW-SSEstriking joints can represent secondary fractures generated by faulting (see also below). The formation of extensional fractures connected with normal faulting is a well known feature (Stewart and Hancock, 1990; Kattenhorn et al., 2000). Such joints strike mostly parallel to the fault, although the perpendicular ones can also be found. Fault-parallel joint sets are dominant in the Wit´ow Series. Normal faults in the Wit´ow Series usually occur in pairs of two conjugate faults that bound grabens, half-grabens, and rarely horsts. The faults tend to cluster along two orientations: (1) NNW-SSE, and (2) WNW-ESE, the former being predominant. Cross-cutting relationships suggest that NNW-trending normal faults are older than the WNWstriking ones (Fig. 11). The older faults are probably of synsedimentary origin, since they are strongly connected with gravel-filled channels. Dip angles of normal faults are usually constant. Nevertheless, normal faults striking NW-SE, WNW-ESE, and W-E show different kinds of geometry, some of them displaying either listric- or step-like geometry. Ferrill and Morris (2003) described steep segments in a normal fault as caused by the occurrence of less competent layers in the deformed host rocks. In the Wit´ow Series, however, the sub-vertical and less steep segments occur in single silt-sandy beds (Fig. 8C). The host beds are poorly indurated, laminated, but nearly homogeneous. We interpret such steps as remnants of the primary pre-existing joints. These primary fractures were characterized by vertical en echelon geometry and occurred as right-down stepping or left-down stepping planar, parallel joints. Such en echelon geometry of shear joint zones has been described by Myers and Aydin (2004). Sediments exposed at Wit´ow are cut by two NW-SE-oriented right-lateral faults that are accompanied by NNWSSE-striking Riedel shears. The occurrence of strike-slip faults is also indicated by the presence of a negative flower structure (Fig. 8A and B), whose principal fault, oriented NNW-SSE, is also a dextral one. A map-scale sinistral strike-slip fault has been recognized in the basement of the Carpathian Foredeep nearly beneath the study area. Theoretically, small-scale dextral strike-slip faults connected with this major sinistral fault should have the same orientation as those found in the Wit´ow Series, i.e. NW-SE (Fig. 14), and point to reactivation of the basement fault at a NNW- to N-S-oriented maximum compression axis. During such reactivation, normal faults could have also
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Fig. 14. Scheme showing the pattern of structures formed above an active sinistral strike-slip fault (after Wilcox et al., 1973) (A); orientation of the measured small-scale strike-slip (B); and normal faults (C and D) at Wit´ow, and orientation of the reconstructed principal stress axes (D). The principal fault is a NE-SW trending fault that belongs to the Kurdwan´ow-Zawichost Fault Zone.
form in the sedimentary cover, and their strikes should be oriented NNW-SSE (cf. a model of Wilcox et al., 1973), i.e. exactly as those of the most common normal faults exposed at the Wit´ow pit. The reconstructed horizontal extension directions for the studied normal faults are oriented NNE-SSW and WSWENE (Fig. 12), the second one being predominant and associated with NNW-striking faults. Such an orientation is also compatible with the pattern of structures occurring in sediments overlying a sinistral strike-slip fault in the substratum (Fig. 14). 6. Discussion and conclusions We have recognized two events of brittle deformation in fresh-water sediments of the Wit´ow Series, unconformably overlying older marine molasses of the medial segment of the Carpathian Foredeep of Poland. The first, syndepositional event (1) took place in the latest Late Miocene to Pliocene time, and was associated with NNW- to N-directed horizontal compression axis, and was probably coeval with transtensional reactivation of a NEtrending sinistral fault of pre-Paleocene age in the basement. This episode led to formation of NW-striking right-lateral faults, NNW-trending normal faults, and N-S joints. Some of the NNW-SSE faults could have utilized the pre-existing joints. The second, post-depositional event (2) was typified by N-S to NE-SW-oriented extension, producing both W-E and NW-SE-trending joints, and WNW-striking normal faults. The latter most probably originated due to reactivation of the Early Paleocene WNW- and NW-striking normal faults in the basement. The stress pattern of event (1) differs slightly from that associated with the last event of shortening in the northern Outer Carpathians, characterized by NNE- to NE-oriented horizontal compression (Aleksandrowski, 1989; Decker et al., 1999a,b). It is worth to note, however, that this shortening predated deposition of the Wit´ow Series, and that
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the generally NE-directed push of the advancing Carpathians, which terminated at ca. 11.5-9 Ma in this part of the Carpathian arc (cf. Decker and Peresson, 1996), could have variably reactivated different structures within rigid basement of the foreland basin. Thrusting in the NE part of the Polish Carpathians between ca. 17 and 11.5 (9?) Ma was linked to a major NE-striking sinistral tear fault which joins the fault system of the Vienna Basin. The fault originated due to sinistral reactivation of some thrusts in the Outer Western Carpathians, as a result of lateral displacement of crustal blocks which extruded from the Eastern Alps (cf. Ratschbacher et al., 1991; Decker and Peresson, 1996). We suppose that sinistral reactivation of the Kurdwan´ow-Zawichost Fault Zone in the basement of the medial Carpathian Foredeep could have been associated with this extrusion. Particular attention, however, should be put to the question of a recently recognized transpressional episode of E-W-oriented maximum horizontal compression axis, which affected the Late Miocene strata that terminated marine deposition in the eastern portion of the Carpathian Foredeep basin (Aleksandrowski et al., 2005), more than 100 km east of Wit´ow. This episode could have been contemporaneous with that of sinistral reactivation of some Outer Western Carpathians thrusts, pointed out by Decker and Peresson (1996). Further research will be required to solve this problem, though. The extensional event (2) was probably coeval with an episode of normal faulting in the Western Outer Carpathians. Normal faults resulting from NE-SW-oriented extension appear to be the youngest structural features both in the Polish Western Outer Carpathians (Decker et al., 1997; Zuchiewicz and Cieszkowski, 1998), and in their Foredeep (Jarosi´nski, 1992). We infer, therefore, that normal faults detected in the Outer Carpathians and the Carpathian Foredeep appear to be a result of not a single collapse event, but of different successive events. On the other hand, recent stress pattern in the study area, inferred from borehole breakouts, points to N-S to NNEoriented trajectories of horizontal compressive stresses (Jarosi´nski, 1998; Zuchiewicz et al., 2002). Field evidence of compressional structures resulting from such stresses have not yet been found in the medial segment of the Polish Carpathian Foredeep. In conclusion, following the retreat of the Miocene remnant marine basin, the medial portion of the Carpathian Foredeep of Poland underwent several episodes of deformation. These included: an episode of sinistral transtension related to NNW- to N-trending maximum compression axis in Late Miocene–Pliocene(?) time, preceded or accompanied by an event of sinistral transpression at W-E-oriented maximum compression axis in the eastern part of the Foredeep (cf. Aleksandrowski et al., 2005), and an episode of N-S to NE-SW-oriented extension which was active at least until the late Pleistocene. The recent compressive event, indicated by the results of borehole breakouts and geodetic studies and typified by NNE-directed horizontal compressive stress (cf. Jarosi´nski, 1998; Cloetingh et al., 2002), left no traces in the examined strata. Acknowledgements Detailed comments by K. Benn and an anonymous reviewer improved the manuscript considerably. We extend our thanks to G. Ranalli for his tedious and patient editorial work. References Aleksandrowski, P., 1985. 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Tectonics of Miocene–Pliocene fresh-water molasses in the Carpathian Foredeep (Wit´ow Series, South Poland) Marta Rauch-Włodarska a , Witold Zuchiewicz b,∗ , Stanisław Brud b,1 , Galicia T. Group a
Institute of Geological Sciences, Polish Academy of Sciences, Krak´ow Research Centre, Senacka 1, 31-002 Krak´ow, Poland b Institute of Geological Sciences, Jagiellonian University, Oleandry 2A, 30-063 Krak´ ow, Poland Received 16 February 2005; received in revised form 16 September 2005; accepted 23 September 2005
Abstract Normal faults of different orientations appear to be the youngest manifestations of faulting in the Polish Outer Carpathians, composed of the Lower Cretaceous trough Lower Miocene strata, and the related Carpathian Foredeep, which is filled with the Lower to Middle Miocene sediments. In the Outer Carpathians, the folds and thrusts produced by accretion-related shortening were formed between the Paleocene and early Late Miocene. The origin of normal faults is still debatable, since it is not known whether these faults were a result of multidirectional extension produced in a single collapse event, or differently oriented extension proceeding in a series of successive events. Structural studies of the Late Miocene–Pliocene(?) fresh-water molasses of the Wit´ow Series provide a possibility to reconstruct the Late Neogene stress field in the central part of the Polish Carpathian Foredeep and, indirectly, in the central part of the Polish Outer Carpathians. The strata of such an age are unique features in the Polish Carpathian Foredeep, proving thereby a key record of structural deformation during the latest stages of orogenic evolution of the Carpathian orogen. The molasses are cut by joints, and normal and strike-slip faults which were formed in two successive events: (1) a syn-depositional one, proceeding under NNW-SSE to N-S oriented horizontal compression, possibly coeval with reactivation of a NE-striking sinistral fault of the Kurdwan´ow-Zawichost Fault Zone in the basement; and (2) a post-depositional one, during N-S to NE-SW-oriented extension. In the first event, reactivation of the NE-striking sinistral fault led to formation of N-S-oriented joints, as well as NW-striking dextral, and NNW-trending normal faults. This event was probably contemporaneous with sinistral reactivation of some thrusts in the Western Outer Carpathians, induced by eastward-directed extrusion of crustal blocks in the Carpathian internides. In the second event, both W-E and NW-SEoriented joints and WNW-striking normal faults were formed. The latter most probably originated due to reactivation of the Early Paleocene WNW- and NW-striking normal faults in the basement. Therefore, normal faults detected in the Outer Carpathians and Carpathian Foredeep appear to be a result of not a single collapse event but of different successive events. This extensional episode lasted at least to the late Pleistocene. No evidence for the recent NNE-directed tectonic compressive stress, typical for that segment of the Carpathian arc, has been found yet. © 2005 Elsevier Ltd. All rights reserved. Keywords: Poland; Carpathian Foredeep; Miocene–Pliocene molasses; Outcrop-scale tectonic structures
∗ 1
Corresponding author. Tel.: +48 12 6332270; fax: +48 12 6332270. E-mail addresses: [email protected] (M. Rauch-Włodarska), [email protected] (W. Zuchiewicz). Sadly, Stanisław Brud passed away recently.
0264-3707/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2005.09.001
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Fig. 1. Geological sketch of the Carpathians with location of Fig. 2. Dashed line shows the boundary between ALCAPA and TISZA continental fragments in the Intra-Carpathian area (after Fodor et al., 1999).
1. Introduction The Carpathian Foredeep is a foreland basin related to flexure of the continental lithosphere in a continental collision ´ aczka, 1984; Krzywiec, 1997, 2001; Zoetemeijer et al., 1999; Oszczypko, 2004; and references zone (Pescatore and Sl˛ therein). This basin developed on the European plate in front of the Carpathian orogenic belt (Fig. 1) and was related to convergence between the European and African plates. Reconstruction of structural evolution of the Carpathian Foredeep is, therefore, of crucial importance for the understanding of the formation of the Carpathians themselves (cf. Decker and Peresson, 1996; Matenco et al., 1997; Linzer et al., 1998). Along the entire Carpathian arc, trajectories of recent stress change their strike from NNW in the western and medial portions of the Carpathian Foredeep, through NE and E-W in the north-eastern and eastern part, to ESE in the southern and south-eastern parts (Cloetingh et al., 2002). As far as the Polish segment is concerned, these trajectories are arranged fan-like, trending NNW to NE, from the west to the east (cf. Jarosi´nski, 1998). The aim of this paper is to constrain the Late Neogene stress field in the medial portion of the Carpathian Foredeep in Poland, basing on an analysis of small-scale tectonic structures within Late Miocene–Pliocene(?) fresh-water molasses exposed at Wit´ow, a single key exposure of such strata in the entire Polish segment of the Carpathian Foredeep. Since these deposits are not occurring anywhere else, their structural record becomes crucial for the Late Cenozoic paleotectonic reconstruction of the region. 2. Local and regional geological setting The sandy-gravel sediments described by Łyczewska (1948) as the “Wit´ow Series” are exposed at a spur between the Vistula and Szreniawa rivers, in the medial segment of the Polish Carpathian Foredeep, approximately 30 km north of the Carpathian frontal thrust (Figs. 2 and 3). These are poorly cemented sandstones, sands, gravels, and silts which cap Miocene clays and are overlain by differentiated Quaternary sediments (Fig. 4). The series bears numerous examples of young tectonic deformations. The coarse-clastic Wit´ow Series represents fresh-water molasses, whose origin and age have long been a matter of debate. This series marks an episode of late orogenic uplift-induced intensive erosion of the Outer Carpathians (cf. D˙zuły´nski et al., 1968; Starkel, 1972). Lithofacies assemblages present in the Wit´ow Series are characteristic of a braided river, which was flowing from the Carpathians, accumulating Carpathian-derived material alongside that scoured from the Miocene strata of the Foredeep. Previous studies have resulted in contrasting opinions about the age and origin of this series, starting from a concept of marine Miocene strata, through the Eopleistocene, up to the
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Fig. 2. Tectonic sketch of the central part of the Carpathian Foredeep of Poland (after Krysiak, 2000), showing the extent of the Wit´ow Series (Brud, 2003) and location of the Wit´ow pit. The Zgłobice Unit after Połtowicz (1991; simplified).
Middle Pleistocene (cf. Brud, 2002; Brud and Worobiec, 2003; Brud et al., 2003, and references therein). The results of recently conducted integrated sedimentological, petrographic, and palaeobotanical studies indicate that sediments of the Wit´ow Series were deposited in Late Miocene and, possibly, Pliocene times (Brud, 2002; Brud and Worobiec, 2003). The Wit´ow Series rests unconformably upon Langhian-Serravallian (Middle Miocene) strata of the central part of the Polish segment of the Carpathian Foredeep. The Foredeep is superimposed on Proterozoic–Mesozoic rocks of the European Platform (cf. Oszczypko, 1998; Olszewska, 1999) and represents a basin that developed syn-tectonically in front of the Western Outer Carpathians (Fig. 1). The infill of this basin is of Early to Middle Miocene age (Oszczypko, 1998; Olszewska, 1999). The strata are mostly sub-horizontal and only occasionally folded. By contrast, in the southern
Fig. 3. Location sketch of the Wit´ow exposures.
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Fig. 4. Exposure C, the principal exposure at Wit´ow, viewed from the south and showing position of Figs. 6A and 11. Miocene–Pliocene: 1: sands, gravels, and silts of the Wit´ow Series; 2: massive, grey silts; Quaternary: 3: loessial silts; 4: massive, yellow loess; 5: brown loess; 6: anthropogenic layer; 7: anthropogenic infill of small valleys and ditches; 8: dump heap; 9: faults. Letters refer to lithostratigraphic code of individual strata.
part of the Carpathian Foredeep, close to the Outer Carpathians, the Miocene strata are strongly folded and cut by thrust faults. This zone of folded strata is called the Zgłobice Unit (Kotlarczyk, 1985) (Fig. 2), where axes of map-scale folds and strikes of map-scale thrust faults are W-E to WNW-ESE (Mitura and Moskała-Martini, 1954; Kirchner and Połtowicz, 1974; Połtowicz, 1991, 1998; Krzywiec, 2001). The basement of the central part of the Carpathian Foredeep is cut by map-scale normal and strike-slip faults (Dokt´or and Graniczny, 1983; Krzywiec, 1997, 2001; Krysiak, 2000; and references therein) (Fig. 2). Normal faults of Early Paleocene age strike mostly NW-SE, more rarely NNW-SSE and WSW-ENE. The NW-SE-trending boundary faults enclose horsts and grabens, the latter frequently referred to as depressions. The studied area is situated within the Działoszyce Depression (Fig. 2). The other map-scale faults trend NE-SW to NNE-SSW and are mostly sinistral strike-slip or oblique-slip in character (Krysiak, 2000; and references therein). One of the NE-SW-trending faults of the Kurdwan´ow-Zawichost Fault Zone is located east of the Wit´ow pit (Fig. 2). According to Osm´olski et al. (1978), the Kurdwan´ow-Zawichost Fault Zone was reactivated as a left-lateral fault in the Middle Miocene. On the contrary, Krysiak (2000) suggested that Miocene activity of this zone began by normal faulting during the late Middle Miocene, being replaced by sinistral faulting in the Late Miocene. Jarosi´nski (1992) confirmed a sinistral reactivation of the Kurdwan´ow-Zawichost Fault Zone due to N-S-oriented compression induced by the overthrusting Carpathians during the Serravallian (late Middle and early Late Miocene) or later (up to 11.5 or 9 Ma), although this short episode was subsequently replaced again by normal faulting. Recent 3D seismics and tectonic studies of a gas-bearing structure in the eastern portion of the Carpathian Foredeep in Poland documented intense tectonic deformations within Late Miocene marine strata, including NNW-SSE to N-Strending tight folds, strike-slip and dip-slip faults, along with joints and pencil cleavage. All of these are interpreted in terms of a sinistral transpression with the horizontal maximum compression axis oriented E-W, and a sinistral strike-slip displacement on NW-SE striking faults in the basement (Aleksandrowski et al., 2005). The age of deformed marine strata in the east may be roughly coeval with that of the lower portion of the fresh-water Wit´ow Series in the west. The formation of the Carpathian Foredeep and Miocene reactivation of older faults within its basement were strongly dependent on the Miocene stress field generated within the Carpathians. The Carpathians are part of the European Alpine system that originated from the convergence between the European and African plates, which terminated by collision and emplacement of the continental fragments into irregular, southern European plate boundary (Dewey et al., 1973; Golonka et al., 2000; Neubauer et al., 2001). The Inner Carpathians are composed of two major continental fragments: ALCAPA and TISZA (cf. Fodor et al., 1999), while the arcuate Outer Carpathians represent the external pile of nappes (Fig. 1). The northernmost part of the Carpathians is situated in Poland. The Polish part of the Western Outer Carpathians is a fold-and-thrust belt, mainly composed of Lower Cretaceous to Lower Miocene flysch sediments (Ksi˛az˙ kiewicz, ´ aczka and Kami´nski, 1998), the main structural features of which originated during formation of the Carpathian 1977; Sl˛ ´ aczka, 1984; accretionary prism in the Paleogene and Neogene (Ksi˛az˙ kiewicz, 1977; Tokarski, 1978; Pescatore and Sl˛ Aleksandrowski, 1989; Fodor et al., 1999). The Western Carpathian accretionary prism was connected with the roughly
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northward movement of the ALCAPA and southward subduction of the European plate (Aleksandrowski, 1989; Tomek and Hall, 1993; Fodor et al., 1999). Tectonic evolution of the Polish Outer Carpathians was characterized by superposition of two shortening events connected with folding and thrusting, NNW- (N), and NE- (NNE)-directed ones (Aleksandrowski, 1985, 1989; Decker et al., 1997, 1999a). According to Decker et al. (1999a), the first shortening event occurred between the Paleocene ´ ´ and Early Miocene (cf. Swierczewska and Tokarski, 1998; Swierczewska et al., 2001), whereas the second one was initiated in the Early and Middle Miocene (Decker et al., 1999a), and probably lasted to the early Late Miocene (Decker et al., 1999b; see also W´ojcik et al., 1999, 2001). Folds and thrusts produced by accretion-related shortening are commonly overprinted by normal faults which appear to be the youngest manifestations of faulting in the Polish Outer Carpathians (Decker et al., 1997). These normal faults are characterized by different orientations. According to Zuchiewicz et al. (2002), it is still not clear whether these faults were a result of multidirectional extension (single collapse event) or differently oriented extension proceeding through successive events. Nevertheless, normal faults resulting from NE-SW extension appear to be the youngest ones (Decker et al., 1997; Zuchiewicz and Cieszkowski, 1998). The extensional phase of structural evolution of the Polish Outer Carpathians most probably commenced in the Late Miocene and has continued into the Quarternary (Zuchiewicz et al., 2002; and references therein). The results of geomorphological studies conducted in the medial and eastern portions of the Polish Outer Carpathians have shown that horizontal compressive stresses may have played an important role in the formation of young (PlioceneQuaternary) tectonic deformations, while the western portion is dominated by normal faulting (Zuchiewicz, 1995; Zuchiewicz et al., 2002). A study of the present-day maximum horizontal stress directions inferred from borehole breakouts in the Polish Outer Carpathians and their foreland, in turn, suggests a NNE-SSW orientation of compressive stress trajectories across the study area (Jarosi´nski, 1998; Kł˛ek et al., 2003; see also Cloetingh et al., 2002). 3. State of research into outcrop-scale tectonic structures 3.1. Medial segment of the Carpathian Foredeep Miocene strata of the Carpathian Foredeep are cut by joints and rare normal faults (Krysiak, 1986; Rauch, 1995, 1998; Rauch and Tokarski, 1995). Joints are ubiquitous outcrop-scale structures within the entire area, providing excellent paleostress-field indicators. Strata of the central Carpathian Foredeep are cut by four joint sets that are oriented: (I) N70–110◦ E; (II) N165–185◦ E; (III) N10–60◦ E; and (IV) N125–155◦ E (Rauch and Tokarski, 1995; cf. also Krysiak, 1986). The oldest (I) set strikes parallel to the orientation of normal faults (WNW-ESE and NW-SE) within the basement. An analysis of seismic sections confirms the mid-Middle Miocene, synsedimentary character of these faults (Krzywiec, 2001). Therefore, joint set (I) is interpreted as coeval with normal faulting, produced due to extension in the foreland of the Carpathians exerted by the European Plate plunging into the Carpathian subduction zone (Rauch and Tokarski, 1995). Set (II) is oriented perpendicular to the fold axes and the strike of thrust faults in the folded southern part of the Carpathian Foredeep, within the Zgłobice Unit (Rauch, 1998). Therefore, this joint set is considered to have been connected with folding and thrusting in the Western Outer Carpathians (Rauch and Tokarski, 1995), when horizontal compression was oriented NNW-SSE to N-S. In the Zgłobice Unit, both the orthogonal sets of joints (I) and (II) were interpreted as an effect of the northward thrusting of the Outer Carpathians (Rauch, 1998) during the Langhian-Serravallian times (Rauch and Tokarski, 1995). According to these authors, the third, NE-striking joint set originated due to NE compression active in the Late Miocene, and set IV (NW-SE) was probably formed in the Pliocene owing to NW-directed compression. Strata of the Zgłobice Unit exposed at Zgłobice (Fig. 2) are cut by three fault systems: (1) W-E-striking thrust faults; (2) WSW-ENE-oriented normal faults; and (3) NE-SW-striking thrust faults (Rauch, 1998). Cross-cutting relationships confirm such a succession of faulting events. The orientation of thrust fault system (1) is in line with the first shortening event recorded in the Polish Outer Carpathians (Decker et al., 1997, 1999a,b). At Zgłobice, the hinge part of a W-E-oriented fold is exposed, and normal faulting is interpreted as connected with folding in this area, leading to extension in the hinge zone of the folds. There is no evidence for the second shortening event, which is observed in the Outer Carpathians (see below); and the thrust fault system (3) appears to have resulted from a local change in the orientation of compression. However, according to Rauch and Tokarski (1995), the youngest joint set (IV) striking NW-SE, which occurs in the entire Polish segment of the Carpathian Foredeep, was formed during the NW-SE-oriented compression.
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3.2. The Wit´ow Series Deformation of the Wit´ow Series sediments was described first by Łyczewska (1948), who found small-scale normal faults and folds in the upper part of the Wit´ow Series and suggested their glacitectonic origin. Krysiak (1986, 1987) documented the occurrence of normal faults which cut the entire exposed portion of the Wit´ow Series and are characterized by listric geometry. The normal faults were to dip 45–70◦ and strike mostly N130–170◦ E, rarely N100–110◦ E. This author (Krysiak, 1987) found no indication of synsedimentary origin of these faults and concluded that the lack of upward-increasing degree of deformation contradicted the inferred glacitectonic origin (Łyczewska, 1948). She took into account the similarity between the orientation of the studied small-scale normal faults and regional normal faults detected in the basement, and interpreted the origin of the former as resulting from earthquakes induced by glacioisostatic rebound. Similar conclusions were drawn by Zuchiewicz (1995) who measured two sets of joints, trending W-E and NNW-SSE, and two sets of normal faults, striking NE-SW and NNW-SSE in the northern and western part of the Wit´ow pit, respectively. According to Krysiak (2000), the NW-SE-striking, synsedimentary faults were formed during the Late Neogene, while an episode of E-W to ENE-WSW-oriented extension was to take place in the Quaternary, when listric faults (of steepening upwards planes) cutting the entire series were supposed to have been formed. The quoted author also speculated on a synsedimentary origin of normal faults. 4. Small-scale tectonic structures in the Wit´ow Series – observations Our detailed studies focused on the Wit´ow exposures, labelled as: Wit´ow A, B, C, D, E, and F (Figs. 3 and 4). They are situated on a spur undermined by the Vistula and Szreniawa rivers. Sediments of the Wit´ow Series are cut by numerous joints, normal and strike-slip faults, infrequent thrust faults, and minor folds that were examined in more detail at exposures C, D, and F. The strata are nearly horizontal. 4.1. Joints At exposures B and C, joints strike mostly NW-SE, whereas subsidiary sets trend N65◦ E, N40◦ E, and N105◦ E. Exposure D, characterized by predominantly W-E-oriented joints, displays as well minor differences in their orientation between the upper and lower, and the northern and southern parts (Fig. 5). Joint sets in the northern and southern lower parts are usually oriented W-E, rarely N55◦ E and N45◦ W, whereas the northern upper part is dominated by a set oriented N105◦ E. Some of the W-E-striking joints probably represent master joints, since they cut the entire, exposed part of the series. Between exposures D and F, W-E-oriented joints are ubiquitous (subordinately striking NNW-SSE and NNE-SSW) and, in the overlying Late Pleistocene loessial loams, infrequent, W-E-trending joints are present as well. At exposure F, in turn, joints tend to form two sets, oriented roughly W-E and N-S (Fig. 5). Subvertically-dipping joints, usually about 85◦ (Fig. 5), are ubiquitous throughout the Wit´ow pit, except for gravel intercalations. The joint spacing is irregular; closely spaced joints occur only close to fault surfaces (see below). 4.2. Faults The Wit´ow Series is cut mostly by normal faults; the strike-slip faults occurring at few places only. 4.2.1. Normal faults Normal faults accommodated displacements ranging from a few millimeters to 2 m. The largest slip was determined along the NW-SE-striking faults at exposure C (Fig. 4). Normal faults commonly form two conjugate sets which bound grabens or, rarely, horsts (Figs. 4 and 6). Their fault planes dip very steeply, more than 60◦ , and commonly 85◦ (Fig. 7A and C). Individual normal faults usually dip at a constant angle, but some of them show listric-like (Fig. 8C; NNE-dipping fault L) or step-like geometry (Fig. 8C; south-dipping fault S). Variable dip angles of a single fault surface are characteristic for normal faults striking W-E, WNW-ESE, and NW-SE. The step-like geometry of fault surfaces displays nearly vertical segments. Such geometry is observed along faults striking W-E and WNWESE.
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Fig. 5. Joint sets within the Wit´ow Series (1) and at individual Wit´ow exposures (2); lower hemisphere plots. (A) Contour diagrams of poles to joint surfaces; (B) great circle plots; (C) rose-diagram of joint surface strikes; (D) rose-diagram of joint surface dips.
The strikes of normal faults usually vary between N85◦ E and N195◦ E, showing two prevailing orientations: N115◦ E and N175◦ E (Fig. 7A and B). The last orientation is most common. Both the NNW- and WNW-striking fault sets usually occur at each exposure (Fig. 9). The NNW-striking fault set dominates in the western exposures (C and F), whereas the WNW-ENE one prevails at the eastern B and D exposures. At exposures D and C, the NNW-striking faults occur mostly below gravel-filled channels of the same orientation, but they usually do not cut the channel fill (Figs. 10 and 11). Nevertheless, the fault planes dip mostly towards axial zones of channels (Figs. 7D and 10). Channels oriented more or less W-E occur at exposure C (Fig. 7D). The NNW-striking faults below such channels do not cut the infilling sediments
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Fig. 6. Examples of normal faults cutting sediments of the Wit´ow Series: (A) graben at exposure C, and (B) horst at exposure F; (C and D): lower hemisphere plots.
(Fig. 11). At exposure F, in turn, NNW-striking faults cut the channel infill (Fig. 6B). The axis of this channel is oriented ENE-WSW, i.e. exactly perpendicular to these faults (Fig. 7D). The NNW-striking faults frequently accompany gravel-filled channels; they occur below the channels, tend to dip towards axial zones of the latter, but do not cut the channel fill except at exposures F and C (Figs. 6B and 11). Sediments underlying the channels have usually been eroded during channel formation (Fig. 11; left part of the channel). However, the occurrence of steps bounded by NNW-striking faults in those channel bottoms (Fig. 11; right part of the channel) which survived erosion suggests that these faults were formed when the channel started to be filled. The even roof of this gravel-filled channel indicates that faulting had been completed before the channel was filled completely. The silt
Fig. 7. Orientation of normal faults (A–C), and axes of gravel-filled channels (D) in the Wit´ow Series. (A) Lower hemisphere plot; (B) rose-diagram of fault plane strikes; (C) rose-diagram of dip angles. Upper case letters in sketch D refer to individual Wit´ow exposures.
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Fig. 8. Negative flower geometry of strike-slip faults at exposure C (A), showing orientation of the principal dextral fault (B). Lower diagram C shows listric-like “L” and step-like “S” geometry of normal faults at exposure D, including fault orientation (D; lower hemisphere plot).
layer overlying channel fill is flat, confirming that faulting affecting the channel bottom was no longer active at the time of silt deposition. The lineation on normal fault planes is usually imperceptible because the studied sediments are mainly poorly indurated sand and gravel. We have recognized slickensides on 10 fault plains, which enabled determination of the extension directions in the study area (Fig. 12). To do this, we applied the numerical-dynamic (NDA) calculation method, using computer program Tectonics FP1.5 (Reiter and Acs, 2000; Ortner et al., 2002). Three faults of the NNW-striking system occurring at exposure C suggest WSW-ENE orientation of the minimum principal stress axis (Fig. 12A), whereas six faults of the WNW-striking fault system at exposure D and one fault at exposure C indicate SSW-NNE orientation of this axis (Fig. 12B). Closely spaced joints usually cluster along WNW- and W-E-striking normal faults (Fig. 8C), and can rarely be found close to NNW-oriented normal faults (Fig. 5). 4.2.2. Strike-slip faults At exposure “C”, two parallel vertical faults accompanied by few minor faults have been found (Fig. 13). Minor faults occur in between the first-order faults and are arrested at their surfaces. Horizontal slickensides are visible on all the fault surfaces, suggesting a strike-slip displacement. The main faults cut the subsidiary ones at low angles, ca. ◦ 30 . These minor faults probably form secondary synthetic shears (“R” Riedel shears) in respect to the main faults, indicating dextral movement which proceeded under N-S-oriented maximum horizontal compression axis (Fig. 13C).
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Fig. 9. Comparison of normal faults orientations at particular exposures. (A) Lower hemisphere plot; (B) rose-diagram of fault plane strikes; (C) rose-diagram of fault plane dips.
Small number of measurements required application of the right-dihedra calculation method, used with the Tectonics FP1.5 program (cf. Ortner et al., 2002). At exposure C, the sub-vertical WSW-dipping fault and associated, mostly ENE-dipping, faults form together a negative flower structure which is oriented NNW-SSE (Fig. 8A and B). This structure is accompanied by joints (Fig. 8A), unlike the NNW-striking normal faults (Fig. 5).
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Fig. 10. NNW-striking normal faults below the gravel-filled channel at exposure D.
Fig. 11. Small-scale tectonic structures at exposure C. (A) Succession of two fault systems: (1) older, NNW-striking, and (2) younger, WNW-striking one; (B) lower hemisphere plot.
Fig. 12. Reconstruction of principal stress axes orientation for normal faults at exposures C (A) and D (B).
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Fig. 13. Orientation of strike-slip faults at exposure C: (A) block-diagram; (B) lower hemisphere plot; (C) reconstruction of principal stress axes orientation.
5. Small-scale tectonic structures in the Wit´ow Series – interpretation Joint surfaces in the Wit´ow Series tend to strike NW-SE (exposures B and C) and W-E (exposure D), as well as both N-S and W-E (exposure F). The NW-striking joints are parallel to the Early Paleocene faults in the basement which bound a series of horsts and grabens to the west of the Kurdwan´ow-Zawichost Fault Zone, whereas the W-E joints coincide with a normal fault which runs parallel to the Vistula River valley (cf. Fig. 2). Some of these joints have probably been later reactivated as normal faults (see Section 6). On the other hand, N-S-oriented master joints most probably originated due to reactivation of the sinistral NE-striking Kurdwan´ow-Zawichost Fault Zone in the substratum. Joints are spaced irregularly in the Wit´ow Series. The zones of closely spaced joints are associated with W-E and WNW-ESE-oriented normal faults (Fig. 6C), and are also connected with a NNW-striking negative flower structure (Fig. 8A). Joint surfaces disappear on fault surfaces, but there is no evidence for their offset. The syn-faulting origin of these joints is confirmed by their increased density close to the fault surfaces. Therefore, the W-E and NNW-SSEstriking joints can represent secondary fractures generated by faulting (see also below). The formation of extensional fractures connected with normal faulting is a well known feature (Stewart and Hancock, 1990; Kattenhorn et al., 2000). Such joints strike mostly parallel to the fault, although the perpendicular ones can also be found. Fault-parallel joint sets are dominant in the Wit´ow Series. Normal faults in the Wit´ow Series usually occur in pairs of two conjugate faults that bound grabens, half-grabens, and rarely horsts. The faults tend to cluster along two orientations: (1) NNW-SSE, and (2) WNW-ESE, the former being predominant. Cross-cutting relationships suggest that NNW-trending normal faults are older than the WNWstriking ones (Fig. 11). The older faults are probably of synsedimentary origin, since they are strongly connected with gravel-filled channels. Dip angles of normal faults are usually constant. Nevertheless, normal faults striking NW-SE, WNW-ESE, and W-E show different kinds of geometry, some of them displaying either listric- or step-like geometry. Ferrill and Morris (2003) described steep segments in a normal fault as caused by the occurrence of less competent layers in the deformed host rocks. In the Wit´ow Series, however, the sub-vertical and less steep segments occur in single silt-sandy beds (Fig. 8C). The host beds are poorly indurated, laminated, but nearly homogeneous. We interpret such steps as remnants of the primary pre-existing joints. These primary fractures were characterized by vertical en echelon geometry and occurred as right-down stepping or left-down stepping planar, parallel joints. Such en echelon geometry of shear joint zones has been described by Myers and Aydin (2004). Sediments exposed at Wit´ow are cut by two NW-SE-oriented right-lateral faults that are accompanied by NNWSSE-striking Riedel shears. The occurrence of strike-slip faults is also indicated by the presence of a negative flower structure (Fig. 8A and B), whose principal fault, oriented NNW-SSE, is also a dextral one. A map-scale sinistral strike-slip fault has been recognized in the basement of the Carpathian Foredeep nearly beneath the study area. Theoretically, small-scale dextral strike-slip faults connected with this major sinistral fault should have the same orientation as those found in the Wit´ow Series, i.e. NW-SE (Fig. 14), and point to reactivation of the basement fault at a NNW- to N-S-oriented maximum compression axis. During such reactivation, normal faults could have also
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Fig. 14. Scheme showing the pattern of structures formed above an active sinistral strike-slip fault (after Wilcox et al., 1973) (A); orientation of the measured small-scale strike-slip (B); and normal faults (C and D) at Wit´ow, and orientation of the reconstructed principal stress axes (D). The principal fault is a NE-SW trending fault that belongs to the Kurdwan´ow-Zawichost Fault Zone.
form in the sedimentary cover, and their strikes should be oriented NNW-SSE (cf. a model of Wilcox et al., 1973), i.e. exactly as those of the most common normal faults exposed at the Wit´ow pit. The reconstructed horizontal extension directions for the studied normal faults are oriented NNE-SSW and WSWENE (Fig. 12), the second one being predominant and associated with NNW-striking faults. Such an orientation is also compatible with the pattern of structures occurring in sediments overlying a sinistral strike-slip fault in the substratum (Fig. 14). 6. Discussion and conclusions We have recognized two events of brittle deformation in fresh-water sediments of the Wit´ow Series, unconformably overlying older marine molasses of the medial segment of the Carpathian Foredeep of Poland. The first, syndepositional event (1) took place in the latest Late Miocene to Pliocene time, and was associated with NNW- to N-directed horizontal compression axis, and was probably coeval with transtensional reactivation of a NEtrending sinistral fault of pre-Paleocene age in the basement. This episode led to formation of NW-striking right-lateral faults, NNW-trending normal faults, and N-S joints. Some of the NNW-SSE faults could have utilized the pre-existing joints. The second, post-depositional event (2) was typified by N-S to NE-SW-oriented extension, producing both W-E and NW-SE-trending joints, and WNW-striking normal faults. The latter most probably originated due to reactivation of the Early Paleocene WNW- and NW-striking normal faults in the basement. The stress pattern of event (1) differs slightly from that associated with the last event of shortening in the northern Outer Carpathians, characterized by NNE- to NE-oriented horizontal compression (Aleksandrowski, 1989; Decker et al., 1999a,b). It is worth to note, however, that this shortening predated deposition of the Wit´ow Series, and that
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the generally NE-directed push of the advancing Carpathians, which terminated at ca. 11.5-9 Ma in this part of the Carpathian arc (cf. Decker and Peresson, 1996), could have variably reactivated different structures within rigid basement of the foreland basin. Thrusting in the NE part of the Polish Carpathians between ca. 17 and 11.5 (9?) Ma was linked to a major NE-striking sinistral tear fault which joins the fault system of the Vienna Basin. The fault originated due to sinistral reactivation of some thrusts in the Outer Western Carpathians, as a result of lateral displacement of crustal blocks which extruded from the Eastern Alps (cf. Ratschbacher et al., 1991; Decker and Peresson, 1996). We suppose that sinistral reactivation of the Kurdwan´ow-Zawichost Fault Zone in the basement of the medial Carpathian Foredeep could have been associated with this extrusion. Particular attention, however, should be put to the question of a recently recognized transpressional episode of E-W-oriented maximum horizontal compression axis, which affected the Late Miocene strata that terminated marine deposition in the eastern portion of the Carpathian Foredeep basin (Aleksandrowski et al., 2005), more than 100 km east of Wit´ow. This episode could have been contemporaneous with that of sinistral reactivation of some Outer Western Carpathians thrusts, pointed out by Decker and Peresson (1996). Further research will be required to solve this problem, though. The extensional event (2) was probably coeval with an episode of normal faulting in the Western Outer Carpathians. Normal faults resulting from NE-SW-oriented extension appear to be the youngest structural features both in the Polish Western Outer Carpathians (Decker et al., 1997; Zuchiewicz and Cieszkowski, 1998), and in their Foredeep (Jarosi´nski, 1992). We infer, therefore, that normal faults detected in the Outer Carpathians and the Carpathian Foredeep appear to be a result of not a single collapse event, but of different successive events. On the other hand, recent stress pattern in the study area, inferred from borehole breakouts, points to N-S to NNEoriented trajectories of horizontal compressive stresses (Jarosi´nski, 1998; Zuchiewicz et al., 2002). Field evidence of compressional structures resulting from such stresses have not yet been found in the medial segment of the Polish Carpathian Foredeep. In conclusion, following the retreat of the Miocene remnant marine basin, the medial portion of the Carpathian Foredeep of Poland underwent several episodes of deformation. These included: an episode of sinistral transtension related to NNW- to N-trending maximum compression axis in Late Miocene–Pliocene(?) time, preceded or accompanied by an event of sinistral transpression at W-E-oriented maximum compression axis in the eastern part of the Foredeep (cf. Aleksandrowski et al., 2005), and an episode of N-S to NE-SW-oriented extension which was active at least until the late Pleistocene. The recent compressive event, indicated by the results of borehole breakouts and geodetic studies and typified by NNE-directed horizontal compressive stress (cf. Jarosi´nski, 1998; Cloetingh et al., 2002), left no traces in the examined strata. Acknowledgements Detailed comments by K. Benn and an anonymous reviewer improved the manuscript considerably. We extend our thanks to G. Ranalli for his tedious and patient editorial work. References Aleksandrowski, P., 1985. 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