Polyphase Tertiary fault kinematics and Quaternary reactivation in the central-eastern Alps (western Trentino)

Polyphase Tertiary fault kinematics and Quaternaryreactivation in the central-eastern Alps (western Trentino)

Maria Giuditta Fellina, Silvana Martinb,*, Matteo Massironic

aDipartimento di Scienze della Terra e Geologico-Ambientali, Universita di Bologna, I-40127, Bologna, ItalybDipartimento di Chimica Fisica e Matematica, Universita dell’Insubria, via Valleggio 11, Como, 22100, ItalycDipartimento di Geologia Paleontologia e Geofisica, Universita di Padova, via Giotto 1, Padova, 35137, Italy

Received 20 November 2000; received in revised form 17 July 2001; accepted 22 November 2001

Abstract

The network of faults and morphotectonic lineaments at the hanging wall of the North Giudicarie fault(western Trentino, NE Italy) is examinated to investigate the Alpine kinematics from Neogene to Qua-ternary times. The faults display an average trend parallel to the North Giudicarie line and are characterisedby mylonites, ultramylonites, cohesive and uncohesive cataclasites and associated pseudotachylytes indi-cating a long tectonic activity in a contractional regime. Calculations of the stress distributions (P–T–B axesmethod, numerical dynamical analysis, direct inversion methods, dihedra calculations) have yielded stressfields which may be attributed to different tectonic steps of the Alpine contraction along the Giudicarielineament. The NNE- to NE-trending faults (e.g. Val Clapa, Val dell’Acqua, Val Burlini and Rumo faults)and the ENE-trending faults (Malga Preghena and Passo Palu’ faults) at the hanging wall of the NorthGiudicarie line indicate a NW- and WNW-oriented compression during Neogene. Some morphologicalfeatures suggest a possible quaternary activity of the NW and NE to ENE faults. # 2002 Elsevier ScienceLtd. All rights reserved.

1. Introduction

The network of Alpine faults at the hanging wall of the North Giudicarie fault (WesternTrentino, NE Italy) has been investigated by means of photo-geological and structural analyses.In this sector of the Alps, the Giudicarie lineament is the most important Alpine fault systembecause it accomodated the shortening induced by the Adria indenter against Europe during theNeogene (Ratschbacher et al., 1991; Castellarin et al., 1992; Martin et al., 1996; Prosser, 1998).The Adria push largely contributed to the unroofing, exhumation and erosion of the Alpine

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Journal of Geodynamics 34 (2002) 31–46

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* Corresponding author.

E-mail address: [email protected] (S. Martin).

nappe stack buried under the Adriatic crust in the Central and Eastern Alps between Oligoceneand Miocene (Schmid et al., 1989; Hurford et al., 1989; Neubauer et al., 1999). In particular,along the North Giudicarie fault (i.e. the central-eastern sector of the Alps), the erosion of theTonale nappe representing at present the uppermost Alpine unit (Austroalpine domain, AU inFig. 1) started at Middle/Late Miocene (Martin et al., 1998a). The erosion occurred as a con-sequence of the uplift of the Austroalpine due to the underthrusting of the Permo–Mesozoicsequences of the Southern Alps beneath the Austroalpine domain (Santini and Martin, 1988;Werling, 1991; Prosser, 1998).Several Alpine faults, dismembering the high grade pre-Alpine basement of the Tonale nappe,

are present in the hanging wall of the North Giudicarie fault and show an average trend parallel

Fig. 1. Tectonic map of the central–eastern Alps. The major faults and related kinematics of the western Trentino areindicated in detail. The studied area is enclosed in the box.

32 M.G. Fellin et al. / Journal of Geodynamics 34 (2002) 31–46

to the North Giudicarie fault. Some faults appear to be coeval with the North Giudicarie faultitself, some show a multiphase Alpine history as documented by the kinematic indicators, othershave been activated only since Quaternary and are still active (Slejko et al., 1987). Often, relevanttrench systems and deep seated gravitational deformations are developed in the fault areas.This paper is a regional overview of the geomorphic and brittle tectonic structures located at

the hanging wall of the North Giudicarie fault, which prove the considerable shortening of thehanging wall of this lineament and his present morphogenetic activity. It also demonstrates thecontinuous compressive to transpressive kinematics at the Austroalpine–Southern Alps boundaryfrom Neogene to the Quaternary, and yields the first stress field analysis of the North Giudicariearea.

2. An overview of the structural setting

The Tonale, Pejo and North Giudicarie faults of the Periadriatic (Insubric) lineament, boundthe triangle-shaped area which is represented in Fig. 1. To the north of the Tonale fault andnorthwest of the North Giudicarie fault, the Austroalpine basement (Tonale nappe s.l.) includespre-Alpine�kyanite�sillimanite-bearing gneisses, migmatites, eclogites and ultramafic lenses(Ulten unit), sillimanite-bearing gneisses, marbles and orthogneisses (Tonale and Mt. Pin units;Martin et al., 1998b). To the south, the Southern Alps domain includes a low grade pre-Alpinebasement, sedimentary covers and the Adamello Tertiary batholith.The compressive to transpressive kinematics represented in Fig. 1 are the result of multiphase

deformations along the Pejo fault (Late Cretaceous–Miocene), along the Tonale fault (Late Oli-gocene–Early Miocene) and along the North Giudicarie fault (Late Oligocene–Late Miocene)(Zingg and Hunziker, 1990; 1993; Martin et al., 1998a; Muller, 1998; Prosser, 1998). Zircon andapatite fission track data constrain the exhumation of the Tonale nappe basement between LateCretaceous and Late Miocene (Martin et al., 1998b; Viola, 2000).The regional stress field calculated in the Southern Alps along the whole Giudicarie fault

changed from NNE (a), to NNW (b), and finally to WNW (c) during the Neogene (Castellarin etal., 1992; Castellarin and Cantelli, 2000). At first (a in Fig. 2), the North Giudicarie fault acted asa dextral transpressive bend of the major Periadriatic lineament during Late Oligocene–EarlyMiocene (Martin et al., 1991; Prosser, 1998, 2000). Then (b in Fig. 2), a NNW–SSE shortening

Fig. 2. Main stress axes orientation since Late Oligocene to Late Miocene times, after Castellarin et al. (1992).

M.G. Fellin et al. / Journal of Geodynamics 34 (2002) 31–46 33

took place since Middle Miocene and evolved from predominant thrusting to left-lateral trans-pressive movement (Laubscher, 1988; Prosser, 1990, 1998; Ratschbacher et al., 1991). To thenorth, the left-lateral transpression included two components: a compressive one mostly devel-oped along the North Giudicarie fault and a left-lateral strike-slip at the foot wall of the Giudi-carie fault. During this phase, the stress field was characterised by an average s1 orientationvarying from NNW to NW (Prosser, 1998, 2000). Later (c), a WNW-directed compression reac-tivated the North Giudicarie fault as a thrust, active during Late Miocene–Early Pliocene (Cas-tellarin et al., 1992).Mylonites of the Tonale fault registered a continuous dextral strike-slip movement con-

temporaneously with the cooling of the Adamello batholith since the Late Oligocene (Martin etal., 1991). After cooling, later deformations produced a few hundred meters thick cataclastic–ultracataclastic belt. The Austroalpine nappe along the fault was uplifted and the previous dex-tral strike-slip deformation planes were reactivated as high angle reverse faults (Martin et al.,1991).The Pejo fault developed along the tectonic contact between the Ortler s.l. to the north and the

Tonale nappes to the south (Fig. 1). This fault is characterised by a mylonitic–ultramylonitic beltdipping about 35� to the SSE and is overprinted by a steeper (about 60�) cataclastic belt with thesame dipping direction (Andreatta, 1948; Martin et al., 1991). The Tonale and Ortler nappes (theformer shows a weak Alpine metamorphic overprint, the latter a more pervasive one) have beenjoined along the ductile fault since Late Cretaceous (Thoni, 1981). Mylonites related to this eventshow sinistral transtensive indicators dating back to Late Cretaceous (Martin et al., 1991, 1998b;Muller, 1998; Viola, 2000). Lately (Late Oligocene–Miocene?), the hanging wall Pejo fault planeshave been reactivated as N- to NNW- vergent ductile-to-brittle thrusts (Cima Grande thrust,Andreatta, 1948). This last deformation contributed to transform the Pejo mylonitic belt into ahundred meters thick wide high angle cataclastic belt.To the NE, in the Pejo fault hanging wall, the Rumo fault forms a NE-trending some tens of

meters thick mylonitic–cataclastic belt (Morten et al., 1976; Prosser, 1990) extended from the Pejo(Cima Grande) to the North Giudicarie faults (Fig. 1). In the Bresimo valley this fault runs alonga mylonitic orthogneiss body (Herzberg et al., 1977; Morten et al., 1986; Figs. 1 and 3a) whichbounds the high grade Ulten unit to the north. Along the Rumo fault a dextral transtensionalmotion has been described by Prosser (1990, 1998) and a Paleocene age has been establishedthrough 40Ar–39Ar dating on the pseudotachylytes showing transtensional indicators (Muller,1998). According to this author, the transtensional event recorded by both the Rumo and Pejofaults might be linked to a slow down or a standstill of the early Alpine convergent tectonics.The Val Clapa fault was briefly described by Hammer (1902) and Andreatta (1948) who defined

it as a secondary mylonitic horizon of the Pejo fault; Morten et al. (1976) pointed out an align-ment of mylonitic marble lenses along this fault.

3. The faults of the Bresimo valley

The approach to the geology of this area is based on field structural analysis and on morpho-logic observations in order to overview the young fractures and faults before evaluating theireffect on the mountain topography.


Two new sets of faults have been recognized between the Giudicarie and the Pejo lineaments: aENE-trending one (Malga Preghena and Passo Palu’ faults; Fig. 3b) and a NNE-to NE-trendingsystem (Val Clapa, Val dell’Acqua, Val Burlini, Rumo and North Giudicarie faults).The faults are characterised by the occurrence of ultramylonites (over 90% of matrix, nomen-

clature according to Passchier and Trow, 1996), mylonites (50–90% of matrix), cohesive anduncohesive cataclasites and associated pseudotachylytes. In the field mylonites were less evidentbecause of the pervasive cataclastic overprint, therefore mylonites were mainly studied bymicroscope (oriented samples) using asymmetric porphyroclasts, S–C fabrics and shear bands as

Fig. 3. Sketch map of the faults in the Bresimo valley: (a) geological map with the main lithologies and (b) morpho-

logical and tectonic lineaments.


striking indicators (Fig. 4). On the contrary, the analysis of brittle kinematics was performed inthe field; quartz and chlorite slickensides, steps and conjugate fault plane geometries yielded goodkinematic information.

3.1. The ENE-trending fault system

The Passo Palu fault is a thin ENE-trending deformation belt (<5 m). It displays a transitionfrom ductile (at WSW) to brittle (at ENE) deformation which produced poorly exposed SSE-dipping mylonites and ultramylonites (Fig. 4a), cohesive cataclasites and pseudotachylytes, andfinal fault planes with slickensides. Asymmetric porphyroclasts in greenschist fine-grained mylo-nites (Fig. 4b) indicate a right-lateral transtensive shear sense. In the Passo Palu area, two main

Fig. 4. Fault planes and associated shear sense: (a) Passo Palu fault ultramylonites; (b) asymmetric porphyroclastsfrom the Passo Palu fault; (c) pseudotachylytes with plagioclase porphyroclasts partly replaced by scapolite of theMalga Preghena fault; (d) S–C fabric and antithetic microfaults in micrograin from the Rumo fault mylonites; (e) S–C

structures and later shear-planes in the mylonites of the Malgazza fault; (f) Pseudotachylyte injection and fault veinsalong the Rumo fault. Arrows show the sense of shear.


sets of conjugate fault planes with quartz slickensides are visible (Fig. 5a). The kinematic indica-tors yield right-lateral transpression along the NE trending set, left-lateral transtension along theNW trending faults. Along the NW trending planes, pseudotachylytes display macroscopictranstension structures indicating top-to-the-NW motion. Another set of NNE-trending andsteeply ESE-dipping faults, probably related to the sinistral transpressive Val Clapa fault, hasyielded a left-lateral transpression (quartz slickensides) (see grey slip arrows in Fig. 5a).The Malga Preghena fault consists of a few meters thick, ENE-trending and SSE-dipping cat-

aclastic belt with clear-cut borders. It is composed of discontinuous cohesive cataclasites and

Fig. 5. Lower hemisphere, Wulff nets of fault surfaces and related slickenside plots. (a) Intersection zone of the Val

Clapa and Passo Palu faults (grey slip arrows are left-lateral transpressive faults probably related to the Val Clapasystem); (b) Malga Preghena normal fault planes (Oligocene extensional event); (c) strike-slip planes of the MalgaPreghena fault (second phase); (d) Val Burlini and Val dell’Acqua reverse fault planes (second phase); (e) left-lateral

strike-slip planes of the Rumo, Clapa and Val dell’Acqua faults (first phase); (f) strike-slip planes of the Val Burlinifault; (g) North Giudicarie reverse fault planes.


pseudotachylytes developed inside some tens of meter thick horizon of retrogressed gneisses.Anthophyllite–asbestos veins related to hydrothermal circulation of possible Oligocene ageformed in the serpentinised peridotite body along the Preghena fault (Morten et al., 1986). In theMalga Preghena cataclastic belt two main events have been recognised: the first one showschlorite slickensides indicating a normal movement (asbestos veins, Fig. 5b), the second one ischaracterised by quartz slickensides indicating a right-lateral strike-slip displacement (Fig. 5c).Pseudotachylyte veins show plagioclase porphyroclasts partly replaced by scapolite (Fig. 4c) anda pervasive foliation parallel to the fault planes which cut the veins and the cataclastic host rock(Fig. 4c). This cataclastic belt is evidenced in the field by 500 m long trenches due to a post-glacialtectonic reactivation.In the Bresimo valley the most important lineament is the Rumo fault. It splits into two

deformation belts: i.e. the Rumo fault s.s. and the Malgazza fault (Fig. 3b). Both belts are madeup of strongly retrogressed orthogneisses, mylonites and pseudotachylyte veins overprinted by adense subvertical fracture cleavage. Between the two belts orthogneisses show a pervasive folia-tion parallel to the Rumo fault. Mylonites display stretching lineations defined by chlorite andsericite, antithetic microfaults and S–C fabric (Fig. 4d). The Rumo fault s.s. dips 60� to the NWand displays a right-lateral transtensive shear sense (see also Prosser, 1990). Along the Malgazzafault orthogneisses show a subvertical mylonitic foliation and S–C fabric indicating a left-lateraltranspressive shear sense (Fig.4e). This deformation is also confirmed by later low angle syntheticfault planes recognised at the microscopic scale (Fig. 4e). The left-lateral transpression is alsorecorded along the Rumo fault s.s. by subvertical brittle fault planes with pseudotachylytes cut-ting the mylonites (Figs. 4f and 5e). Therefore, the early (Paleocene) right-lateral transtensionalphase is recorded only by the Rumo fault s.s., while the later left-lateral transpressional phase isshown by both deformation belts.

3.2. The NNE to NE trending fault system

The Val Clapa fault is a 30-m thick ca. NNE-trending steeply ESE-dipping subvertical mylo-nitic to cataclastic belt extending from the Val Clapa (a right-lateral valley to the Ultental) to theRabbi valley. It separates the Binasia ridge migmatites (Ulten unit) to the E (Fig. 3a) from ret-rogressed and strongly deformed gneisses (Tonale or Ortler s.l. nappes) to the W. The NE-trending and SE-dipping mylonites consist of sericite and scarce chlorite surrounding quartz andfeldspar porphyroclasts whereas cohesive cataclasites show relics of pre-Alpine fibrolitic sillima-nite and biotite. Close to the Passo di Val Clapa, retrogressed paragneisses are characterised by apervasive NNE-trending subvertical fracture cleavage. The deformation front changes from duc-tile in the inner zone (cohesive cataclasites with frequent pseudotachylyte veins and mylonitichorizons) to brittle in the outer zone (cataclastic marble lenses and polished fault planes). Innermylonites display chlorite stretching lineations and related kinematic indicators showing a left-lateral transpression. Outer cataclasites show quartz slickensides on the fault planes with thesame kinematics (Fig.5a).The Val dell’Acqua fault is a NE-trending NW-dipping fault, which is composed of a few

meters thick mylonites and a wide area of cataclasites. The ductile and brittle horizons of the Valdell’Acqua fault display different geometries: greenschist mylonites dip 50� to N260, while thebrittle fault planes are nearly subvertical and dip to N290. Asymmetric porphyroclasts in mylo-


nites and synmylonitic folds have yielded left-lateral transpressive kinematics. The brittle hor-izons show a left-lateral strike-slip deformation (corresponding to the first phase) characterizedby chlorite slickensides (Fig. 5e), and later reverse fault planes (second phase) with quartz slick-ensides (Fig. 5d).The cataclasites of the Val Burlini fault crop out along two parallel smaller valleys just near the

tectonic contact of the Austroalpine paragneisses with the Southern Alps sedimentary lithologies(i.e. the North Giudicarie fault). The roughly 100-m thick main deformation belt consists ofcohesive cataclasites derived from gneisses and amphibolites associated to pseudotachylyte veins,fault planes with chlorite and quartz slickensides, and gouge. The foliation of the cohesive cata-clasites and the main fault plane trend NE and dip to the SE, in the opposite direction in respectto the North Giudicarie fault. Chlorite slickensides and drag folds suggest a NW–SE contrac-tional phase (Fig. 5d). The same kinematics are shown by calcite slickensides on sets of conjugatefault planes observed within deformed horizons of the Cretaceous Flysch and Dolomia along theNorth Giudicarie fault (Fig. 5g; see also Prosser 1998, 2000). Minor NE and NW-trending faultplanes display two types of strike-slip kinematic indicators (chlorite and quartz slickensides) witha left-lateral strike-slip movement on NW- trending planes and a right-lateral strike-slip move-ment on the NE- trending fault planes (Fig. 5f) respectively.

4. Paleostress inversion and ductile–brittle tectonic phases

Events with the same kinematics and rock deformation behaviour observed on different faultshave been combined together, in order to reconstruct the regional paleostress field and test thehomogeneity of the yielded paleostress fields. Four different methods have been applied for thecalculations of the stress axes orientations: (i) the P–T–B axes method (Turner, 1953), (ii) thenumerical dynamical analysis (Spang, 1972), (iii) the dihedra calculation (Angelier and Mechler,1977) and (iv) the direct inversion method by Angelier and Goguel (1979). The first three calcu-lation methods yielded similar stress axis orientations, while the stress orientations derived fromthe direct inversion method by Angelier and Goguel (1979) are similar to the others in one caseonly. The same kind of methodological problems have already been experienced and discussed bySperner et al. (1993), Sperner (1996) and Bistacchi (1999).The ductile kinematic indicators observed along the Rumo fault have confirmed the right-lat-

eral transtension already known from the literature (Prosser, 1998, 2000). This deformation is notcompatible with the ductile kinematics of the other faults analysed in this area. Therefore, itcould be assimilated to the Cretaceous–Paleocene extensional activity of the Pejo fault (Muller,1998; Viola, 2000). Similarly, the extensional kinematic indicators on the SW-dipping mylonitesof the Passo Palu could be consistent with the early transtensional kinematics observed on thePejo and Rumo mylonites.The kinematic indicators of the mylonites from the Malgazza branch of the Rumo fault, the Val

Clapa cohesive cataclasites and mylonites, the reactivated left-lateral Rumo fault and the left-lateral strike-slip planes of the Val dell’Acqua fault (first phase) have been attributed to the sametectonic phase on the basis of their common structural characters, i.e. to the left-lateral trascur-rence which developed under ductile to brittle conditions (Fig. 5a and e). The average s1 (max-imum stress axis) of this phase appears oriented NNW ca. (Fig. 6a). In particular, the mylonites


from the Valle dell’Acqua fault have yielded left-lateral kinematic ductile indicators consistentwith the brittle ones.The Val dell’Acqua reverse fault planes (second phase) present the same kinematics and geo-

metry of the Burlini fault (Fig. 5d) and therefore they can be attributed to the same tectonicphase. They are consistent with a NW-trending compression developed under brittle conditions(Fig. 6b).The kinematic analysis of all (ENE-, NE- and NW-trending) brittle fault planes in the intersec-

tion zone of the Passo Palu and Val Clapa faults has yielded an average WNW-trending horizontal

Fig. 6. Lower emisphere stress axes plots (s1 in red, s2 in blue, s3 in grey) obtained by: DIH=right dihedra plot;

sigma=inversion method of Angelier and Goguel (1979); lambda l=numeric dynamic analysis; PT=P–T–B axesmethod; calculations by TectonicsFP program by Reiter F. and Acs P. Main stress axes obtained from (a) Malgazza, ValClapa, Rumo and Val dell’Acqua (first phase) faults, (b) Val dell’Acqua (second phase) and Val Burlini reverse faults, and

(c) from Malga Preghena and Val Burlini strike slip faults, and from Val Clapa and Passo Palu intersection zone.


s1 (Fig. 6c). Moreover, the right-lateral strike-slip fault planes measured along the NE MalgaPreghena fault (second event), the NW- and NE-trending strike-slip faults in the Val Burlinideformation zone (Fig. 5f) have confirmed a brittle reactivation under the same stress field(Fig. 6c).

Fig. 7. Photogeological interpretations of the Bresimo valley: (a) Val dell’Acqua fault and related trenches (scale ’

1:70.000); (b) Castel Pagano DSGD, Cima Binasia WNW-trending double ridge and uphill-facing counterscarps andtwo main ENE- and NNE-trending alignments (scale ’ 1:75.000).


5. Morphotectonic overview of the Bresimo valley

In the upper sector of the Bresimo valley the morphology is constrained by NNE-, ENE-, NW-and WNW- trending lineaments represented by aligned trenches, ridges and lateral valleys(Figs. 3b, 7a and b), as suggested by geo-morphologic and photo-geological surveys. Some ofthese lineaments could have a tectonic origin (morphotectonic lineaments), others are mostly dueto gravitational processes (morphologic lineaments). The upper Bresimo valley trends ENE par-allel to the ENE fault trend, in the middle portion the valley orientation changes abruptly andtrends WNW and NW, presumably due to the influence of other lineaments. The lower valley isparallel to the main drainage system of the contiguous wide Sole valley. The lateral valleys of theBresimo valley (Fig. 3b) also trend ENE and NE.In the upper valley, the intersection of the Val Clapa e Passo Palu faults (morphotectonic

lineaments) produced a wide morphological basin which controlled the confluence of local glaciertongues and transfluences during the glacial maxima (20,000 B.P. ca.) and influenced the post-glacial erosion.

5.1. The NNE and ENE morphotectonic lineaments

No NNE-trending morphological lineaments parallel to the North Giudicarie fault are notevident in the Bresimo valley. A NNE-trending alignment of trenches is present along the con-tinuation of the Val Clapa (NNE-trending) fault in the southern side of the Clapa valley (Ultenregion) to the NE, and it could interpreted as a morphologic lineament influenced by faultactivity (morphotectonic lineament). Similarly, in the Val dell’Acqua area, a ENE-trending 1-kmlong, 2–3 m deep trench occurs (Figs. 3b and 7a). The gentle morphology, the low topographicalstress (Engelen, 1963; Dramis, 1984) and the absence of deep seated gravitational deformation(DSGD), suggest a post-glacial (re)activation of an earlier ENE-trending tectonic lineamentrather than a gravitational origin. Similarly, the ENE-trending ridge along the Malga Preghenafault, which is a 500-m long trench with a maximum depth of 2–3 m (Figs. 3b and 7b), is inter-preted as a quaternary reactivation of the Preghena fault. Other ENE alignments of trenches,glacial steps and lateral valleys present in the southeasternmost area between the Rabbi and theBresimo valleys (Figs. 3b and 7b) provide further evidence for a post-glacial brittle reactivation ofthe ENE tectonic systems.

5.2. The NW and WNW morphologic and morphotectonic lineaments

Some NW-trending morphologic and morphotectonic lineaments are present in the area.They are: (i) the Cima Binasia–Monte Pin ridge and a pervasive NW-trending fracture sys-tems (Fig. 7b), (ii) the trend of the middle Bresimo valley itself, (iii) the orientation of theCastel Pagano deep seated gravitational deformation (DSGD), i.e. a nearly 1500-m long and60-m deep system of trenches (Fig. 7b) and (iv) several trenches developed along the CastelPagano south-facing side. The Castel Pagano DSDG has been interpreted as a morphologicalstructure possibly related to the post-glacial gravitational (re-)activation of a NW-trendingfault (Cima Sternai lineament) (Fig. 3b) which is developed along the Rabbi valley (Massironiet al., 2000).


An evident WNW-trending morphological lineament is developed on the western slope of CimaBinasia–Monte Pin ridge. It is characterised by WNW-trending double ridges and uphill-facingcounterscarps (Fig. 7b). In this case, the main morphogenetic factors seem to have been both thehigh topographical stress of the slope and the intense brecciation of the rocks, whereas the tec-tonic influence seems to have been insignificant.The photo-geological analysis and the field test suggest that the morphology of the upper

Bresimo valley is mainly influenced by the intersection of the NW- and NE-trending faultand fracture trends, whereas the WNW-trending structures represent minor morphologicalignments.

6. Discussion

Three main deformation phases have been recognised in the hanging wall of the North Giudi-carie fault. The first event was a right-lateral transtension evidenced by the mylonitic horizons ofthe Rumo fault related to the extensional event of Paleocene age (Muller, 1998; Viola, 2000). Thisevent is older than the extensional brittle structures (faulted asbestos-vein) observed in the Preg-hena serpentinites along the Malga Preghena fault (Fig. 5b).The second event (a in Fig. 6) was a NNW-oriented compression under ductile–brittle condi-

tions responsible for the dominant sinistral displacements along the Rumo, Val Clapa, Malgazzaand Val dell’Acqua (first phase) faults (grey arrows in Fig. 5a and e). In the Giudicarie area,during this phase, the average orientation of the s1 was NNW ca. and this compression re-acti-vated the hanging wall of the North Giudicarie fault as a top-to-the-SE thrust, while the foot wallaccomodated a dominant left-lateral strike-slip (Prosser, 1998). The late reactivation of theRumo fault as a sinistral strike-slip structure is in agreement with the kinematic informationgiven by the fission track data. In fact, dating across the fault constrained the vertical movementsof this fault to Paleogene (Muller, 1998) and the strike-slip activity to Neogene (Martin et al.,1998a).The third event was a NW- to WNW-oriented compression developed under brittle conditions,

responsible for the dominant thrusting along the North Giudicarie, Val dell’Acqua (secondphase) and Val Burlini faults (Figs. 5c and d–f and 6b and c). This event reactivated the PassoPalu and Val Clapa faults with a WNW-oriented s1 (Figs. 5a and 6c).According to apatite fission track data carried out in the Bresimo valley, rapid exhumation of

the Ulten gneisses and migmatites (Tonale nappe) began since Middle Miocene (15 Ma, Martin etal., 1998a). The final cooling below a T of about 100 �C involved firstly the uppermost eclogiticand migmatitic gneisses of the inner Ulten sector (15 Ma, Fig. 3a) and thus the lowermost MontePin gneisses of the outer sector located along the North Giudicarie fault (11 Ma). The structuralobservations reported above suggest that the exhumation and related cooling were facilitated bytwo contraction events with different s1: the former trending NNW and the latter trending NWto WNW.Between the Pejo fault and the Passo Palu–Clapa fault system (Figs. 1 and 3a), the Tonale

gneisses are characterized by a fine-grained texture and a pervasive retrogression under greens-chist conditions, rather different from the coarse-grained, sillimanite-bearing Tonale gneissescropping out to the E and to the S of this fault system (Fig. 3a). To the E of the Val Clapa fault,


the Tonale gneisses show a transition from sillimanite-bearing to kyanite-bearing (typical of theUlten unit) types. Therefore, the brittle re-activation of Passo Palu and Clapa lineaments as acompressive (early transpressive) fault system is likely to have accomplished the uplift of a fewkm of the preserved Tonale and Ulten gneisses and migmatites cropping out to the SE.The occurrence of transfer faults in the hanging wall of the North Giudicarie fault is less rele-

vant than in the foot wall (Prosser, 1998; 2000). Prosser has estimated a Neogene shortening of7.5–11 km along the left-lateral faults of the North Giudicarie lineament and a shortening of 12–13 km due to pure thrusts. The hanging wall contraction is confirmed by the presence of thrustshaving opposite vergences in the field: e.g. NW-vergent Clapa and Burlini faults, the SE-vergentreverse faults of the Val d’Acqua and the North Giudicarie (Fig. 1).The morphological analysis suggests a recent re-activation of the NE- and ENE-trending frac-

tures. The NW- and WNW-trending morphologic and morphotectonic lineaments observed inthe field are confirmed by satellite image analysis (Massironi et al., 2000). These lineaments areusually related to dense, NW-trending dilatant joints which do not display clear kinematic indi-cators. The NW dilatant joints with a more or less extensional kinematics are compatible with aNW-directed s1 and therefore these lineaments could have been developed during the LateMiocene–Early Pliocene tectonic phase. Recent geodetic and GPS measurements have effectivelyshown a N-oriented contraction which would represent the last increment of the deformation(Caporali and Martin, 2000).

7. Conclusion

The structural analysis corroborated by available fission track data and morphological studiesconfirms active contractional kinematics in the central–eastern Alps since Oligocene. These kine-matics became active in the western Trentino only lately due to a change in the stress field alongthe Southern Alps–Austroalpine boundary because of the Adria–Europe indentation (see tectonicmodels proposed by Neubauer et al., 1999). The western Trentino represents a peculiar areawhich was characterised in the Late Cretaceous–Paleocene time by extensional tectonics whichperformed a low-structural morphology (graben-like) to the west of the Tauern area preservingthe Austroalpine nappe from early exhumation and erosion. The Giudicarie area was chosenfor this structural work because of its special position in relation to the Tonale and the NorthGiudicarie faults to check the presence of contractional and strike-slip deformations supportingthe exhumation and cooling evolution of the Tonale nappe basement.The paleostress vectors orientation (see paragraph 4 and Fig. 6) calculated for the composite

system of faults of the Bresimo valley change from NNW to NW/WNW between Middle Mio-cene and Late Miocene/Early Pliocene. This change of stress vector orientation as calculated forthe faults in the Bresimo valley is in agreement with the vergence of the youngest thrusts observedelsewhere along the North Giudicarie fault and in its foot wall (Castellarin et al., 1992; Prosser,1998).Some morphological lineaments observed in the Bresimo valley appear due to the re-activation

of brittle structures (NE- to ENE- and NW-trending) which should be also consistent with thepresent N-orientated contraction in the central-eastern Alps at regional and European scales(Bressan et al., 1998; Caporali and Martin, 2000).


Acknowledgements

The authors are grateful to anonymous referees and to the Associate Editor, Professor N. Rast,for their comments and suggestions which improved the earlier version of this manuscript. NicolaMichelon and Stefano Castelli are acknowledged for their graphical support; Sandra Speed forher English revision.

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Polyphase Tertiary fault kinematics and Quaternary reactivation in the central-eastern Alps (western Trentino)

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