A low-temperature ductile shear zone: The gypsum-dominated western extension of the brittle Fella-Sava Fault, Southern Alps.

Based on structural and fabric analyses at variable scales we investigate the evaporitic gypsum-dominated Comeglians-Paularo shear zone in the Southern Alps (Friuli). It represents the lateral western termination of the brittle Fella-Sava Fault. Missing dehydration products of gypsum and the lack of annealing indicate temperatures below 100 °C during development of the shear zone. Despite of such low temperatures the shear zone clearly exhibits mylonitic flow, thus evidencing laterally coeval activity of brittle and viscous deformation. The dominant structures within the gypsum rocks of the Lower Bellerophon Formation are a steeply to gently S-dipping foliation, a subhorizontal stretching lineation and pure shear-dominated porphyroclast systems. A subordinate simple shear component with dextral displacement is indicated by scattered σ-clasts. Both meso- and microscale structures are characteristic of a subsimple shear type of deformation with components of both coaxial and non-coaxial strain. Shortening in a transpressive regime was accommodated by right-lateral displacement and internal pure shear deformation within the Comeglians-Paularo shear zone. The shear zone shows evidence for a combination of two stretching faults, where stretching occurred in the rheologically weaker gypsum member and brittle behavior in enveloping lithologies.

Introduction

Shallow portions of major continental strike-slip faults are exposed to brittle deformation conditions. At crustal levels shallower than ca. 3–4 km, kakirites (Heitzmann, 1985) dominate such strike-slip faults, the porosity is therefore high, and vertical fluid flow is dominant resulting in fluid–rock interaction and formation of fault gouge (Caine et al., 1996; Wibberley et al., 2008). In terms of fluid motion, this crustal section is considered to represent the hydrothermal convection zone (Saishu et al., 2014). Ductile fabrics respectively plastically formed fabrics are generally missing within quartzofeldspathic fault rocks at these shallow crustal levels. In contrast, evaporitic rocks show plastic behavior even at low temperatures and pressures (Rutter, 1986). Consequently, the presence of viscously behaving rocks like evaporites (halite, gypsum) at such shallow levels may significantly change the fault zone architecture and related porosity and permeability structure. Evaporitic layers frequently act as detachment horizons (e.g. De Paola et al., 2008 and references therein). However, evaporitic strike-slip fault zones are rare (Schorn and Neubauer, 2014) and detailed descriptions of the internal structure of such shear zones are hampered by intense and quick surface weathering of friable mylonitic gypsum. Furthermore, structures and fabrics of secondary tectonic origin within evaporitic sequences are often misinterpreted as primary, depositional features, as outlined by Schreiber and Helman (2005).

Here we present structural observations from the Comeglians-Paularo strike-slip shear zone in the Southern Alps, which propagated entirely through the subvertically tilted gypsum-dominated member of the Lower Bellerophon formation. In such a case the steeply dipping trend of a rheologically weak layer entirely controls propagation and the structural trend of the shear zone, whereas in strike-slip fault systems within gently to moderately dipping sedimentary successions, the lithology virtually does not play a major role. There, fault location in the cover rocks is primarily controlled by the strength of the basement rocks (Mandl, 1987; Wilcox et al., 1973). A change in trend of the Bellerophon Formation also seems to control the position of fault termination. Such terminations are geometrically and kinematically complex and a number of distinct brittle structures such as extensive and compressive horsetails are often associated with fault tips (Sylvester, 1988 and references therein). We demonstrate how the ductilely behaving shear zone accommodates N–S shortening and dextral displacement by two mechanisms, (1) non-coaxial and coaxial deformation and (2) a compressive horse-tail at the western fault termination.

This paper first gives an overview of the geological and tectonic setting of the Comeglians-Paularo shear zone. Then we describe detailed field observations of the internal fault structures and then assess the microscale deformation structures. In the discussion, we focus on the low-temperature fabrics and assess the role of viscously behaving evaporites within the otherwise brittle shallow crust for the hydrothermal convection zone. The Fella-Sava Fault, and its western extension, the Comeglians-Paularo shear zone apparently form the northern boundary of a seismogenic zone, which is affected by intense seismicity with dominant N–S thrusting. On a large scale, the active dextral Comeglians-Paularo shear zone/Fella-Sava Fault represents the Pliocene-recent southern boundary of the east-directed lateral extruding block.

Geological setting

Overview of the tectonic framework and the Comeglians-Paularo shear zone

The study area is located in the (Eastern) Southern Alps, which are bordered in the north by the Periadriatic Fault, the major strike-slip fault of the Alpine orogen (Handy et al., 2010). The N-ward moving Adriatic microplate and the Southern Alps at its front act as a rigid indenter. North of the Periadriatic Fault, part of the shortening is accommodated by lateral extrusion and tectonic escape to the east along E-trending strike-slip faults and N-trending extensional detachments (Fig. 1) (Ratschbacher et al., 1991a, 1991b). The dextral Periadriatic Fault in the south and the sinistral Innsbruck-Salzburg-Amstetten (ISAM), Salzach-Enns-Mariazell-Puchberg (SEMP) and Mur-Mürz faults in the north (Fig. 1) delineate the extrusion corridor. Eastward extrusion starts in the Oligocene mainly on the ISAM and Periadriatic faults (Wölfler et al., 2011). In the Early Miocene activity migrates to the SEMP Fault, which then defines the northern border of the extrusion block. A successive rejuvenation of fault activity from the northernmost faults (e. g. ISAM Fault) to the more in the SE located faults (e. g. Mur-Mürz Fault) has been suggested by Wölfler et al. (2011). The eastern sector of the Periadriatic Fault is transected by the dextral ca. NW–SE striking Möll Valley and Lavanttal faults, which were active during the Miocene and Pliocene (Pischinger et al., 2008). Southward migration of fault activity has also been shown for the Periadriatic Fault. Based on the opening of the Gorenjska basin since Late Miocene times a displacement shift to the Fella-Sava Fault is evident (Fodor et al., 1998; Vrabec et al., 2006). The Comeglians-Paularo shear zone is the westernmost segment of the Fella-Sava Fault (e.g. Venturini et al., 2009a). Lateral offsets of 30–60 km (Fodor et al., 1998; Pinter et al., 2006; Vrabec et al., 2006) or up to 65–70 km (Placer, 1996) along the Fella-Sava Fault have been inferred. The long-term slip rate for the last 20 Ma has been reported to range from 1 to 5 mm/a (Jamšek Rupnik et al., 2012). Ongoing dextral offset of 1.2 mm/a was reported for the central Fella-Sava Fault in Slovenia, whereas no movement was observed along its eastern part and along the Periadriatic Fault within the scope of the same study (Vrabec et al., 2006). Using GPS geodetic methods shortening can be detected across the Fella-Sava Fault. (e.g. Bechtold et al., 2009; Caporali et al., 2009, 2013). The area around the fault is characterized by NW–SE contraction and large E–W oriented shear strain with strain rates of ∼20 ppb/a (Grenerczy, 2013).

Fig. 1

Simplified tectonic model of the Eastern Alps (modified after Egger et al., 1999; Neubauer and Höck, 2000; Plenicar, 2009) showing major ca. orogen-parallel sinistral and dextral strike-slip faults and resulting east-directed lateral extrusion of the central Eastern Alps. ISAM F. Innsbruck-Salzburg-Amstetten Fault; SEMP F. Salzach-Enns-Mariazell-Puchberg Fault; MÖF, Möll Valley Fault.

The steeply southward dipping Fella-Sava Fault can be traced to the NE corner of the Gorenjska basin (Poljak, 2007), where it disappears beneath Neogene sediments. An eastward continuation of the fault to Celje and to the Croatian border has been assumed (Fig. 1, Placer, 1996; Plenicar, 2009). Between the Periadriatic and the Fella-Sava fault systems en echelon sedimentary basins formed along left-stepping oversteps (Vrabec et al., 2006). The Fella-Sava Fault separates mainly the Ladinian Schlern Formation in the south from the Permian Gröden Formation and part of the Bellerophon Formation in the north (Selli, 1963). Its westernmost segment was introduced by Gortani (1926) as the Comeglians-Paularo line and later mapped in detail by Braga et al. (1971) and Venturini et al. (2009b). In the study area, the Comeglians-Paularo shear zone developed within the gypsiferous member of the Bellerophon Formation (Venturini et al., 2009a) and approximately juxtaposes the older pre-Variscan basement with its Pennsylvanian cover in the north with Permian to Mesozoic post-Variscan sedimentary strata in the south (Venturini et al., 2002). The entire sedimentary succession is steeply tilted to the south (Merlini et al., 2002).

Reconstruction of the evolution of the main river system of Friuli (Tagliamento river) indicates a watershed between the Fella-Sava Fault and the Friuli foreland up to Messinian times (Monegato and Stefani, 2010; Monegato et al., 2010). The supposed river along the Fella-Sava Fault probably drained eastwards into the Gorenjska and similar Slovenian basins (Dunkl et al., 2005; Monegato and Stefani, 2010). In Zanclean times, the watershed migrated northwards and the E–W drainage along the Fella-Sava Fault was captured by the southward dewatering Tagliamento river moneg (Monegato and Stefani, 2010).

Regional geology of the Southalpine unit

The Southern Alps are tilted southward and are divided into a basement, the Carnic Alps, which are exposed along the Periadriatic Fault, and the Southalpine cover unit (Fig. 2a). Both parts represent sedimentary mega-sequences: the Carnic Alps show the evolution of an Early Paleozoic passive continental margin of the pre-Variscan cycle (Rantitsch, 1997), whereas the Southalpine cover unit represents the upper post-Variscan cycle (Schönlaub and Heinisch, 1993). The basement of the Carnic Alps contains a nearly continuous geological section ranging from Late Ordovician to Early Pennsylvanian, which is overprinted by both the Variscan and the Alpine orogenies (e.g. Brime et al., 2008). Upper Ordovician clastic sediments are overlain by thick Devonian shelf margin carbonates. During the earliest Viséan, the extensional regime shifted to a contractional regime and Mississippian synorogenic flysch was deposited (Mader et al., 2007). A Pennsylvanian unconformity separates the folded and faulted Variscan basement from the post-Variscan cover. The deposition of the cover started with intracontinental siliciclastic molasse sediments (Leppig et al., 2005) The relative sea level changed as a result of Gondwana glaciation and shelf and later massive limestone were deposited during stages of high sea-level (Krainer, 2007). A hiatus indicates regression at the end of the Lower Permian during which the carbonate platform was partly destructed (Schönlaub, 1992). The semiarid Gröden Formation, with conglomerates, sandstones and fine-clastic layers was deposited (Cassinis et al., 2011). Subsequently, a west to east progressing transgression began (Cassinis et al., 2011) and the Gröden Formation locally interfingered with the marine Bellerophon Formation (Venturini, 1990). The Upper Permian Bellerophon Formation (stratigraphic log in Fig. 2b) was originally a gently to the SE dipping carbonate ramp located between sabkhas in the west and an open marine regime in the east (Schönlaub, 1992 and references therein). The Bellerophon Formation was deposited in the shallow tropical “Bellerophon Sea”, an embayment of the Paleo-Tethys (Posenato, 2010). The Bellerophon Formation is divided into two parts: The lower part contains evaporites and cataclastic dolomite at the base (Massari and Neri, 1997). The gypsiferous member acted as a major décollement horizon in the Southalpine unit (Schönborn, 1999). The upper part of the Bellerophon Formation consists of shoal-water deposits of dolomite, often tectonically brecciated, while the uppermost part is made up of bituminous limestone (Massari and Neri, 1997) deposited during a regression (Scholger et al., 2000 and references therein). In the study area, the Permian Bellerophon Formation is approximately 260 m thick (Discenza and Venturini, 2002) and is concordantly overlain by the Lower Triassic Werfen Formation. During the opening of the Alpine Tethys in Jurassic times, the Southalpine cover unit represents part of the passive continental margin of the Adriatic plate and sedimentation continued until the Eocene (Bertotti et al., 1993; Carminati et al., 2010).

Fig. 2

(a) Stratigraphic log of the Southern Alps. (b) Detailed stratigraphic log of the Bellerophon Formation. The thickness of the stratigraphic units has been taken from Carulli (2000); Discenza and Venturini (2002); Krainer (2007); Schönlaub (1980); Venturini (1990, 2006). Major decollément horizons are indicated; the main lithostratigraphic units are shown; representative field photos illustrate lithologies of the Bellerophon Formation (I) outcrop ∼ 2 km south of Comeglians. (I I) Gypsum opencast near Comeglians.

The Southalpine unit shows no or only a very low-grade metamorphic overprint (e.g. Brime et al., 2008; Rantitsch, 1997 and references therein). In the Variscan basement, the metamorphic grade increases from east to west and from south to north (Sassi et al., 2004 and references therein), and a Cretaceous age (eo-Alpine) for the metamorphism is assumed (Läufer, 1996).

Geomorphologic expression of the Comeglians-Paularo shear zone

The width of the Comeglians-Paularo shear zone varies from 100 to 500 m and depends on the thickness of the gypsum-bearing member of the Bellerophon Formation. The high erodibility of gypsum within the shear zone led to the formation of E–W striking linear valleys and mountain passes in between (Fig. 3a) but also to poor outcrop conditions. Approximately 11 km west of Comeglians the valley changes its strike from E–W to ENE–WSW (Fig. 4). N–S draining confluents of the Tagliamento river cross the shear zone perpendicularly. In the area around Paluzza and Comeglians, the rivers Degano and Bút are dextrally deflected along the shear zone and lateral offsets of ∼0.5 km respectively 1 km of the drainages are evident (Fig. 3b).

Fig. 3

(a) 3D-digital elevation model of the Comeglians-Paularo shear zone area. (b) Detailed map showing the dextral displacement of the rivers Degano and Bút. (c) A field photo with view to the west illustrates the geomorphological situation with E–W striking linear valleys and mountain passes in between.

Fig. 4

Digital elevation model, stereograms (Lambert projection, lower hemisphere) and composite diagrams of structural field data (for location of the DEM see Fig. 1). Shear bands are drawn as great circles, striae as arrows pointing in the direction of displacement of the hanging wall (normal or reverse); double arrows indicate the shear sense.

Results

Structural study of the Comeglians-Paularo shear zone

In addition to detailed field observations we performed map interpretation in order to characterize the brittle, large-scale structures associated with the Comeglians-Paularo shear zone. A schematic overview of the whole shear-zone (Fig. 5a) and detailed geological field maps (Fig. 5b–d) of the Rio Turiea valley in the east (Fig. 5b, after Venturini, 1990), the area around Comeglians in the west (Fig. 5c, after Venturini et al., 2002) and the western termination of the shear zone (Fig. 5d, after Cantelli et al., 1960) are presented.

Fig. 5

Overview sketch map and detailed structural maps of the Comeglians-Paularo shear zone and Fella-Sava Fault. Locations of maps (b) to (d) are shown on Fig. 4. (a) Sketch map showing along-strike variation of large-scale structures along the Fella-Sava Fault and its western extension, the Comeglians-Paularo shear zone; including the locations of the detailed structural maps. (b) East: Rio Turiea valley area modified after Venturini (1990). (c) Central: Comeglians area, modified after Venturini et al. (2002). (d) West: termination of the Comeglains-Paularo shear zone after Cantelli et al. (1960). Special structures visible in the maps (b) to (d) are labeled with capital letters and described in detail in the text: (A) Comeglians-Paularo shear zone; (B) Riedel shears; (C) anti-Riedel shears; (D) restraining bend; (E, F) conjugate strike-slip fault system indicating N–S shortening; (G) Horse-tail type thrust fault.

Along the gypsiferous Comeglians-Paularo shear zone (structure A in Fig. 5) dextral NW–SE striking Riedel shears (B in Fig. 5) and sinistral NNW–SSE striking anti-Riedel shears (C in Fig. 5) developed. In the area around the Rio Turiea valley Venturini (1990) described SW- vergent folds with axes plunging to the SE parallel to the Riedel shears (B in Fig. 5). In the area near Comeglians, a restraining bend (D in Fig. 5c, photo II in Fig. 2) with ENE–WSW striking thrusts and folds with SSE-vergency is consistent with the dextral movement of the main shear zone. We further interpret dextral faults (B in Fig. 5c) as possible extensive horsetails. Further to the west (Fig. 5d) parallel to the trend of the valley the E–W gypsum-bearing strike-slip shear zone passes into a ENE–WSW striking thrust fault (G in Fig. 5d) (Cantelli et al., 1960; Venturini et al., 2002). The thrust fault transported sandstones of the Permian Gröden formation over Triassic carbonates and gypsum is lacking along the thrust (Fig. 5d) (Cantelli et al., 1960; Venturini et al., 2002). The thrust accommodates, therefore, dextral displacement along the Comeglians-Paularo shear zone. We interpret the structure as a dextral compressive horsetail (G in Fig. 5d), which terminates the shear zone in the west.

Further structures were formed during a different deformation event: The structures E and F are interpreted as conjugate strike-slip fault system indicating N–S shortening. NW–SE trending folds visible in the southern part of Fig. 5b attest NE–SW shortening.

Field data

We performed detailed structural studies, including measurements of the penetrative foliation S1 and stretching lineations L1, fold-axes (F2) and shear bands between Paularo and the valley west of Comeglians (Fig. 4). Outcrop locations and results of orientation data, including composite diagrams, are given in Fig. 4. Fig. 6 illustrates representative meso-scale structures. Structural investigations along the shear zone revealed that the entire gypsiferous part of the Lower Bellerophon Formation is strongly deformed in a ductile respectively plastic manner representing the shear zone. Brittle behavior is restricted to boudinaged dolomitic layers within gypsum and more competent lithologies of the wall rocks. Although poorly exposed, a sharp contact between foliated gypsum and brittely deformed bedded dolomites in the hangingwall respectively footwall was observed in a few outcrops (e.g., locations 1, 2 and 9 of Fig. 4). The country rocks are fractured and contain small-scale faults and slickensides. These rocks represent the damage zone adjacent to the gypsum-bearing fault core (according to Caine et al., 1996).

Fig. 6

Typical mesoscale structures; orientation of structures is shown on top; number of the outcrop at the bottom (for locations, see Fig. 4); interpretative sketches are given in the lower left corner. (a) Steep mylonitic foliation S1; subhorizontal stretching lineation L1; rigid grains show eye-shaped pattern with tails consistent with the principle matrix lineation). (b) Close-up of an asymmetric clast with pressure shadow indicating top-to-the-SW transtensional dextral shear. (c) Steep reverse shear bands indicating reverse N–S shortening. (d) Boudinage of dark dolomite within schistose banded gypsum forming ductile layers; the boudins are well-aligned, pinch-swell shaped and indicate extension parallel to the foliation. (e) Boundary of cleaved dolomite (left) and ductilely-deformed gypsum (right side). (f) Two different stages of folding. The younger deformation folds (F2) comprise a moderately S-dipping axial plane foliation S2. (g) Quarter fold with a small reverse fault above a rigid object indicating ca. NE–SW shortening. (h) Top view of C'-type shear band indicating dextral movement to SW; pinch-swell shaped boudins are visible on the right side.

The subvertical mylonitic foliation S1 and the stretching lineation L1 are the most prominent micro- and meso-scale structure (Fig. 6a). The foliation of gypsum-rich rocks generally dips steeply to the S to SSE (Fig. 4). In the vicinity of Comeglians (location 9; Fig. 4), the foliation dips steeply to the ESE consistent with its location within the restraining bend. Gypsum attains its maximum thickness and is mined commercially in an opencast (photo II in Fig. 2).

L1 dominantly trends WSW–ENE and plunges mostly eastward with an angle between 10° and 25°. Only at locations 1–3 (Fig. 4), a WNW–ESE trend is found. The stretching lineation L1 is defined by stretched and/or elongated millimeter-sized gypsum grains and rigid dolomite clasts. These often form porphyroclasts belonging mainly to the ϕ-type and subordinately to the σ-type. The latter indicate a non-coaxial deformation component and show asymmetric pressure shadows clearly indicating dextral sense of shear (Fig. 6b). Observed step faults and asymmetric quarter folds (Fig. 6g) indicate the same sense of displacement. Fig. 6d illustrates deformation partitioning of schistose banded gypsum ductilely wrapped around dolomitic boudins. The brittle boudins are well aligned and pinch-and-swell shaped. No significant variations of type and strain of fault rocks were observed along-strike of the shear zone.

Shear bands dip moderately to steeply to the SSW, the associated fault striae plunge gently to the W and show dextral or normal movement (Fig. 4). In the western part of the Comeglians-Paularo shear zone, subvertical shear bands with subhorizontal lineation (Fig. 6h) of C'-type (Berthe et al., 1979a, 1979b) also indicate dextral movement of the northern block to the SW. In the trail of the shear bands, the dark dolomite formed pinch-and-swell shaped boudins with visible dextral displacement. Additionally, steep reverse shear bands indicating N–S shortening (Fig. 6c) were found.

Gently plunging fold axes show a wide scatter with a preferential trend to the E to SE (Fig. 4). At a smaller scale, two different stages of folding are evident (Fig. 6f). The older event is associated with the foliation of the gypsum. The folds are isoclinal, containing a cm-spaced axial plane foliation S1 and a stretching lineation L1. The foliation S1 and the stretching lineation L1 are refolded during the younger event during which open folds (F2) with moderately S-dipping axial plane foliation S2 were formed. The same E–W trending folds are developed at the map-scale (Fig. 5b).

Hand-specimen structures and microfabrics

The structural and fabric elements are generally consistent in style and orientation at all scales. Structures visible in hand specimen are illustrated in Fig. 7 and microstructures in Figs. 8 and 9. All thin sections were cut parallel to the lineation and perpendicular to the foliation and are composed of mylonitic gypsum. The samples are fine-grained (on average ca. 0.05–0.2 mm) and matrix supported, with gypsum as matrix (Figs. 7 and 8). The gypsum grains are mostly subhedral, subrounded, inequigranular and elongated parallel to the stretching lineation. Generally, alteration to bassanite, anhydrite and annealing structures are missing. This is consistent with the preservation of micro-shear zones as shown in Fig. 7c.

Fig. 7

Representative structures visible in hand specimen; number of outcrop/sample and the scale are shown on the right. (a–b) Steep mylonitic foliation S1; subhorizontal stretching lineation L1; rigid grains show eye-shaped pattern with tails consistent with the principle matrix lineation. (c) Micro-shear zone (gray) in the upper section; recrystallized gypsum band (white) in the lower section. (d) Mylonitic foliation S1, recrystallized. (e) Mylonitic foliation S1 with normal faults indicating top-to-the-SW displacement. (f) Mylonitic foliation S1, recrystallized.

Three different types of microfabrics are evident: Type 1 (Fig. 8a–c) shows one generation of gypsum whereas in type 2 (Fig. 8)d–f and 3 (Fig. 8g–i) two generations of gypsum appear. In type 2, only the older generation is deformed, in type 3 both gypsum generations are deformed and recrystallization is common. Macroscopically, the mylonitic foliation and the stretching lineation are visible in all microfabric types. Microscopically, the foliation is only evident in type 1 (Fig. 8b) and type 2 (Fig. 8e). The grain size of gypsum in type 1 varies from 10 to 50 μm, in type 2 from 30 to 220 μm and in type 3 from 30 to 150 μm. In type 1 (Fig. 8c) the matrix is inequigranular, the grains are elongated and undulatory extinction is visible, the grain boundaries are diffuse and interlobate. Zonation in gypsum is visible and is interpreted to represent growth stages. Type 2 (Fig. 8f) has an inequigranular matrix and subhedral grains, the grain boundaries are polygonal and triple points are visible. Two generations of gypsum are present in type 2, the older grains are fine-grained with sutured grain boundaries, the younger grains are large, elongated and show undulatory extinction. Fig. 8h illustrates a disintegrated grain next to grains of the older generation with polygonal grain boundaries and triple points. Type 3 (Fig. 8i) is foliated, the matrix is fine-grained and inequigranular, the grain boundaries are interlobate and few triple points were found. The majority of the porphyroclasts is symmetric, consisting of carbonate and showing eye-shaped patterns with tails consistent to the principal matrix lineation. In some porphyroclasts (Fig. 9a), extension veins with walls perpendicular to the lineation are visible. Φ -clasts (Fig. 9d) with symmetrical tails and ϴ-clasts (Fig. 9b) lacking tails do not show any sense of simple shear movement. Few σ-clasts (Fig. 9e) with stair stepping tails indicate dextral shear sense consistent with observations at outcrop-scale. In fringes around many porphyroclasts, two stages of crystal growth are visible; the fringe at the strain cap (Passchier and Trouw, 2005) is even-grained and blocky and fibrous at the tails (Fig. 9c–f). The fibers are straight, parallel to the lineation and represent the X-direction of the finite strain. The existence of fibrous structures indicates conditions with a high fluid pressure and solution transfer during deformation (Passchier and Trouw, 2005). The blocky even-grained fringe is perpendicular to the grain surface. We interpret this as a result of extension parallel to the foliation S1 and lineation L1.

Fig. 8

Three distinct microfabric types of deformed gypsum (thin section); number of thin section and the scale are shown on the right; microphotographs are taken in the dark field (crossed polarizers). (a–c) Type 1 shows one generation of gypsum. (a) Overview microphotograph. (b) Microphotograph of the matrix with microscopic foliation of elongated undulose gypsum grains. (c) Inequigranular matrix with elongated grains, undulous extinction and interlobate diffuse grain boundaries. (d–f) Type 2, two generations of gypsum are observed, deformation is restricted to the older phase. (d) Overview microphotograph. (e) Partially recrystallized gypsum grains with abundant crystals crossing the trend of the microscopic foliation (visible by some parallel-elongated grains). (f) Inequigranular matrix with subhedral grains, the grain boundaries are polygonal, triple points are visible. (g–i) Type 3, two gypsum generations, both are deformed and recrystallization is common. (g) Overview microphotograph. (h) The foliation is scale dependent and microscopically invisible. (i) Fine-grained, inequigranular, foliated matrix, the grain boundaries are interlobate and few triple points are visible.

Fig. 9

Microphotographs of clasts; number of thin section and the scale are shown on the right. (a) Disintegrated clast next to grains of the older generation with polygonal grain boundaries and triple points. (b–f) Microstructures, mainly porphyroclasts. (b) ϴ-clasts without tails; extension veins parallel to the lineation; close-up of a vein top left. (c) Clast with two generations of fringes; older fibrous tails straight parallel to the lineation; younger even-grained, blocky fringe at the strain cap. (d) Symmetric Φ-clast. (e) σ-clast indicates dextral displacement (f) Close-up of the fringe; fibrous tail; blocky cap.

Discussion

The new data on the Comeglians-Paularo shear zone show ductile fabrics along the entire length of the subvertical to steeply ca. S-dipping thick gypsum-rich part of the Bellerophon Formation. The country rocks in the North and South are less affected by strike-slip deformation. There mainly Riedel-type brittle fault zones, oversteps and bends can be observed. For simplification, the gypsum-bearing part of the Comeglians-Paularo shear zone can be considered as the fault core and the adjacent brittely deformed country rocks as damage zones applying the model of Caine et al. (1996). The above mentioned Riedel-type brittle fault zones are considered as separate systems, each with a separate fault core (without gypsum) and a separate damage zone.

In the following discussion we first focus on the low-temperature ductile deformation of the gypsum rocks and its potential implications on fluid flow. Then we discuss how strain is accommodated along the shear zone. Finally we address the role of the Fella-Sava Fault and Comeglians-Paularo shear zone for earthquake distribution and propagation.

Low-temperature mylonitic deformation of gypsum

The microfabric study of gypsum mylonites reveals extremely small grain sizes and the lack of significant annealing and recrystallization structures. Most gypsum grains of microfabrics type 1 and 2 are undulose and amoeboid grain boundaries indicate bulging as the main deformation mechanism. Even in the partly recrystallized gypsum grains of microfabrics type 3 amoeboid/interlobate deformation microstructures prevail and grain sizes remained small in contrast to what is observed in many other gypsum-dominated evaporitic deformation zones (De Paola et al., 2008). The absence of annealing indicates that no thermal overprint following deformation occurred and that no hydrous, hot fluids invaded the Comeglians-Paularo shear zone.

We expected both plastic and cataclastic behavior of gypsum as described by De Paola et al. (2008) for faults with evaporitic rocks. Within the Comeglians-Paularo shear zone with dominant gypsum, low-temperature ductile respectively plastic deformation occurred while in the adjacent damage zone brittle deformation prevailed. In all investigated outcrops, gypsum was found as the sole sulfate mineral, and no bassanite or anhydrite was observed. In experiments, gypsum dehydrates at ∼100 °C to bassanite, at temperatures of ∼140 °C a further alteration to anhydrite takes place (Brantut et al., 2011). The absence of evidence for dehydration and annealing structures in thin-sections indicates deformation temperatures well below 100 °C. We did not find any indication (e.g. coarse, undeformed gypsum grains, loss of foliation and lineation) for rehydration processes as observed within many other deformed gypsum-dominated deformation zones (De Paola et al., 2008; De Paola et al., 2007; Schorn and Neubauer, 2011). This implies dry country rocks and limited fluid flow also in the damage zones adjacent to the shear zone. The presence of gypsum results in low porosity and low permeability at shallow crustal levels. The core of the shear zone acts as a barrier and impedes fluid flow as indicated by the absence of rehydration products. Thus it potentially separates two different convection zones of thermal waters within the adjacent damage zones, which dissolve gypsum from shear zone boundaries. Sulfur-bearing thermal waters are known at Comeglians and at the Fella-Sava Fault in the east (Fig. 4) (e.g. Italiano et al., 2009; Petrini et al., 2011). Applying the model of Caine et al. (1996), the shear zone is a barrier for fluid flow, whereas the adjacent damage zones represents a conduit. The missing annealing is in contrast to gypsum-rich shear zones observed elsewhere, e.g., in the Appennines (De Paola et al., 2008; De Paola et al., 2007) or Northern Calcareous Alps (Leitner and Neubauer, 2011; Schorn and Neubauer, 2011), where higher temperature conditions are assumed and access of hydrous fluids resulted in pervasive recrystallization of gypsum rocks.

Displacement and strain accommodation of the Comeglians-Paularo shear zone

The parallel strike of adjacent units and the resulting lack of convincing piercing points renders determination of the amount of structural offset along the Fella-Sava Fault difficult. Based on structurally offset Oligocene and Triassic rocks, Fodor et al. (1998) and references therein inferred a displacement of 30–40 km for the easternmost part. This value seems too high considering a maximum displacement of ca. 20 km as indicated by the northern margin of the Gorenjska (Ljubljana) basin. Vrabec et al. (2006) suggest a transfer of parts of the displacement to the pull-apart Gorenjska basin (Fig. 1) near Ljubljana and to the Sava folds (Fig. 1) (Placer, 1999), presenting a transition zone toward the Slovenian basin and the Bosnian basin in the N and E (Plenicar, 2009). The Comeglians-Paularo shear zone has been assumed to be the western extension of the Fella-Sava Fault, (Placer, 1999; Plenicar, 2009). A contact of the Comeglians-Paularo shear zone with the Valsugana thrust has also been discussed (Venturini et al., 2009a and references therein). As a matter of fact displacement along a fault decreases towards its tips (e.g. Barnett et al., 1987). No movement will be observed at the tip and fault internal deformation will accommodate strain (e.g. Froitzheim et al., 2006).

Further to the west of Comeglians, the gypsum member of the Bellerophon Formation changes its strike from E–W to ENE–WSW and is thrusted onto the Triassic Werfen Formation (G in Fig. 5c, Cantelli et al., 1960; Venturini et al., 2002). We interpret this structure as a dextral compressive horsetail (G in Fig. 5c), which accommodated part of the dextral displacement and consider it to be the western tip of the shear zone. If this is correct, dextral splay faults close to the western end around Comeglians (B in Fig. 5b) could be considered as extensive horsetails, which partly accommodated the lateral displacement, too. The late-stage N–S trending folds are interpreted to have partly accommodated E–W shortening. Such shortening has been found by kinematic and paleostress analyses by Peresson and Decker (1997) who ascribed a Late Miocene age to this compressional event.

Along the Comeglians-Paularo shear zone we observe accommodation of N–S shortening and dextral displacement by a major pure shear and a subordinate simple shear component.

The subsimple shear zone and shows evidence for a combination of two stretching faults (Means, 1989, 1990) as illustrated by an interpretative sketch in Fig. 10. Symmetric boudins, Φ- and ϴ-porphyroclasts and additional extension veins within the clasts (Fig. 9b) as well as a ca. E–W subhorizontal lineation (Fig. 6a) indicate E–W stretching in a pure shear regime within the shear zone. A subordinate simple-shear component is evidenced by asymmetric clasts, shear bands, micro-shear zones, en échelon structures and block rotation with internal antithetic shear. At the surface, an offset fluvial network indicates ongoing dextral displacement.

Fig. 10

Interpretative sketch of (a) general model of a subsimple shear zone (modified after Fossen, 2011). (b) model of the Comeglains-Paularo shear zone explained as a combination of two stretching faults after Means (1989, 1990).

Seismicity patterns and the Comeglians-Paularo shear zone

The Comeglians-Paularo shear zone delimits a zone of widespread seismic activity in the South from a zone of insignificant seismicity in the North (Fig. 11). Along the shear zone itself, present-day or historical seismicity is restricted to a few minor earthquakes (e.g. Paluzza Ml 4.8, 1956; De Panfilis, 1959; Feliziani and Marcelli, 1965, 1966; Reinecker and Lenhardt, 1999). Along its eastern extension, the Fella-Sava Fault, the large historical earthquake of 1348 A. D. (intensity of ca. 10) was recently placed on the fault (Tiberi et al., 2012). In 1895, Ljubljana was destroyed by a major earthquake (Mm 6.1 earthquake; Ribarič, 1982), which was rather located within the Gorenjska basin than on the Fella-Sava Fault itself. Ongoing N–S thrusting activity within the Gorenjska basin has recently been documented by folded Quaternary sediments and the active Vodice reverse thrust fault N of Ljubljana (Jamšek Rupnik et al., 2013 and references therein). We speculate that the occurrence of gypsum along the western fault trace (Comeglians-Paularo shear zone) may have absorbed stresses by viscous behavior and thus may have prevented earthquake nucleation. In contrast, the predominance of carbonate-dominated sequences along the eastern segment of the fault may have favored earthquake nucleation. South of the Comeglians-Paularo shear zone along a parallel fault in the area of Tolmezzo few larger earthquakes (International Seismological Centre, 2011) with dextral focal mechanisms (Bressan et al., 1998; Willingshofer and Cloetingh, 2003) are registered. This observation supports the theory of further southward migration of the displacement due to the younging of fault activity from the Periadriatic Fault to southern parallel faults (Vrabec et al., 2006). Thus the Fella-Sava Fault represents the southernmost and the SEMP and Mur-Mürz Faults the northern boundaries of the extruding blocks during Late Miocene/Pliocene reactivation and ongoing lateral extrusion (Caporali et al., 2013; Grenerczy, 2013).

Fig. 11

Recent seismicity of Friuli (1900–2014), earthquake data (Mw > 3) are taken from the International Seismological Centre (2011); (for location of the map see Fig. 1).

Conclusions

Structural observations along the Comeglians-Paularo shear zone allow the following major conclusions:

We corroborate that the Comeglians-Paularo shear zone represents the westernmost extension of the Fella-Sava Fault. It terminates west of Comeglians with a horse-tail type thrust.

The rheologically weak, gypsum-dominated lithology determines shear zone propagation and termination. Fault rocks are gypsum-mylonites with a consistently developed steep E–W striking foliation and a subhorizontal stretching lineation.

Missing dehydration products of gypsum and the lack of annealing indicate unusually low temperatures <100 °C during mylonitic deformation of the gypsum. Resulting low porosity and low permeability may lead to the core of the shear zone acting as a barrier, impeding fluid flow.

The structural and fabric elements are generally consistent in style and orientation at all scale and suggest pure shear E–W extension during N–S shortening and subordinate dextral displacement along the shear zone.

The subsimple shear zone is confined by two stretching faults with stretching occurring within the rheologically weak gypsum layer in between.

Both internal stretching within the gypsum and brittle structures like the horsetail west of Comeglians accommodated portions of the displacement at the Comeglians-Paularo shear zone. The displacement migrated from along the Periadriatic Fault southward to the Fella-Sava Fault and as earthquake data suggests, even further to the south during recent times.

The Comeglians-Paularo shear zone impedes fluid flow and possibly earthquake propagation.