摘要：Thorium–lead (Th-Pb) crystallizationages of hydrothermal monazitesfrom the western, central and eastern Tauern Window provide new insightsinto Cenozoic tectonic evolution of the Tauern metamorphic dome. Growthdomain crystallization ages range from 21.7 ± 0.4 to 10.0 ± 0.2 Ma.Three major periods of monazite growth are recorded between∼ 22–20 (peak at 21 Ma), 19–15 (major peak at 17 Ma) and14–10 Ma (major peak around 12 Ma), respectively, interpreted to berelated to prevailing N–S shortening, in association with E–W extension,beginning strike-slip movements and reactivation of strike-slip faulting.Fissure monazite ages largely overlap with zircon and apatite fission trackdata. Besides tracking the thermal evolution of the Tauern dome, monazitedates reflect episodic tectonic movement along major shear zones that tookplace during the formation of the dome. Geochronological and structural datafrom the Pfitschtal area in the western Tauern Window show the existence oftwo cleft generations separated in time by 4 Ma and related to strike-slipto oblique-slip faulting. Moreover, these two phases overprint earlierphases of fissure formation.Highlights. In situ dating of hydrothermal monazite-(Ce). New constraints on the exhumation of the Tauern metamorphic dome. Distinct tectonic pulses recorded from east to west. Downloadandlinks Article (PDF, 12300 KB) Supplement (288 KB) How to cite Back to top top How to cite. Ricchi, E., Bergemann, C. A., Gnos, E., Berger, A., Rubatto, D., Whitehouse, M. J., and Walter, F.: Cenozoic deformation in the Tauern Window (Eastern Alps) constrained by in situ Th-Pb dating of fissure monazite, Solid Earth, 11, 437–467, https://doi.org/10.5194/se-11-437-2020, 2020. 1 Introduction Back to toptop In situ thorium–lead (Th-Pb) dating of hydrothermal fissure monazite-(Ce) (in the followingsimply monazite) has recently been demonstrated to be a reliable method fordating tectonic activity under retrograde metamorphic conditions(Bergemannet al., 2017, 2018, 2019, 2020; Berger et al., 2013; Fitz-Diaz et al., 2019;Gasquet et al., 2010; Gnos et al., 2015; Grand'Homme et al., 2016a; Janotset al., 2012; Ricchi et al., 2019). These studies conducted through theentire Alpine orogenic belt allowed constraining tectonic activity inrelation with exhumation and fault activity under retrograde lowergreenschist to sub-greenschist facies metamorphic conditions.Hydrothermal fissure monazite, concentrating light rare earth elements (LREE), Th and U, generallycrystallizes in Ca-poor lithologies, outside the stability field of titaniteor epidote ∕ allanite. However, once formed, hydrothermal processes may causedissolution–reprecipitation events leading to resetting of the monaziteTh-Pb decay system in parts of the crystal. Chemically and isotopicallyhomogeneous crystals indicate a single, rapid growth episode (e.g.Grand'Homme et al., 2016a). However, crystals showing differentgrowth domains indicative of successive growth episodes are more common. Inother cases, parts of, or entire, grains display a patchy zoning due todissolution–reprecipitation processes (e.g.Ayerset al., 1999; Grand'Homme et al., 2016b). These processes involve elementfractionation resulting in crystal zones with often distinct Th∕U values(Seydoux-Guillaumeet al., 2012).The advantage of using hydrothermal monazite for dating tectonic activity isrelated to the high closure temperature of monazite of > 800 ∘C,implying that diffusion in monazite is negligible(Cherniaket al., 2004; Gardés et al., 2006, 2007) under P–T conditions at orbelow 450–500 ∘C and 0.3–0.4 GPa (e.g.Mullis et al., 1994; Mullis,1996; Sharp et al., 2005) where hydrothermalfissures form. Fissure monazites date crystallization following chemicaldisequilibrium within a fissure. This causes a dissolution–precipitationcycle that may include dissolution or partial dissolution of existingfissure monazite. This has the consequence that latedissolution–precipitation steps may be well recorded, whereas earlier growthdomains may be completely destroyed. Thus, monazite crystallization due tochemical disequilibrium is interpreted as being related to tectonic activity(e.g. volume change, fissure propagation, exposure of fresh host rock,delamination of fissure wall, seismic activity, fluid loss or gain).Recent studies have shown that fissure monazite typically forms betweengenerally lower to higher 200–400 ∘C(Gnos et al., 2015; Janots et al.,2019). For this reason, fissure monazite ages are generally interpreted asdating crystallization or re-crystallization. Monazite geochronology canthus be utilized to constrain shear and damage zone activity undergreenschist and very low-grade metamorphic conditions at least down to200 ∘C (e.g.Bergemannet al., 2017, 2018; Gnos et al., 2015).Fissures and clefts develop close to the brittle–ductile transition(< 450 ∘C; Mullis, 1996) and areusually oriented perpendicular to the foliation and lineation of thehost rock (Gnos et al., 2015). Fissures aregenerally straight when they form and either became enlarged by subsequenttectonic activity to form fluid-filled decimetre- to metre-sized clefts, displaying amore open shape with rounded surfaces (e.g. Ricchi et al., 2019) when thestress field retains the same orientation, or become completely filled toform mineral veins. However, they may show a complex shape when the stressfield direction changes during deformation. Fluid inclusion studies (e.g.Mullis, 1996) show that clefts generally suffered several deformation episodes.Interaction of the fluid that fills the fissures with the surrounding rockleads to dissolution of minerals in the wall rock and mineral precipitationin the fissure. As long as deformation continues, fluid-filled clefts willreact to deformation via dissolution–precipitation cycles due todisequilibrium between fluid, rock wall and mineral assemblage within thecleft (e.g. Putnis, 2009). Thus, hydrothermal mineralslike monazite do not only grow following the initial fissure formation butform, continue to grow or dissolve during subsequent deformation stages.The timing of these growth or alteration stages may not always be resolvablewith the precision of currently available geochronological methods, butdifferent growth stages may be distinguishable through differences in thechemical composition (Grand'Homme et al., 2018). Incontrast to the surrounding country rock, the fissures and clefts remainhighly reactive at low temperature due to the presence of fluids. For thisreasons, deformation steps during brittle deformation may be registeredthrough mineral growth or recrystallization in clefts (e.g. Berger et al.,2013) down to conditions where clay minerals form in fault gauges.The Tauern Window (TW) is a thermal and structural dome of the eastern Alps(Fig. 1) exhumed over a period of about 30 Ma starting from the EarlyOligocene (e.g. Rosenberget al., 2018; Schmid et al., 2013). Previous monazite crystallization agesobtained in the eastern subdome of the TW record tectonic activity between19.0 ± 0.5 and 15.0 ± 0.5 Ma(Gnos et al., 2015). In the current study,monazite geochronology is extended to the entire TW in order to investigateits Cenozoic deformation history. We particularly aim to establish achronological record for the younger exhumation history recorded by fissuremonazite crystallization, to be compared with known deformation phases.Figure 1Tectonic map of the TW dome modified after Bertrand et al. (2017),Scharf et al. (2013), Schmid et al. (2013) and Schneider et al. (2013).Yellow stars on the map represent sample locations, and numbers inside thestars refer to samples listed in Table 1. Range of weighted mean growthdomain ages are indicated for each grain from this study and Gnos et al. (2015),labelled in black and green, respectively, on the map (see Table 4 for an exhaustivesummary of all the ages). Only the spot date range is indicated for grains 1,4 and 6. Locations of AA', BB' and CC' cross sections are indicated by blacklines, and individual cross sections are presented in Fig. 6 together withmonazite crystallization ages. Two normal faults delimit the western andeastern borders of the TW, the Brenner normal fault (BNF) and the Katschbergnormal fault (KNF), respectively. Note that the KNF prolongation results indextral and sinistral strike slips in the north and south, respectively(KSZS: Katschberg shear zone system). Several sinistral strike-slip faults(AhSZ: Ahrntal shear zone; ASZ: Ahorn shear zone; DAV: Defereggen–Antholz–Valsfault; GSZ: Greiner shear zone; InF: Inntal fault; MüF: Mur–Mürz fault;NF: Niedere Tauern southern fault; OSZ: Olperer shear zone; SEMP:Salzach–Ennstal–Mariazell–Puchberg fault; SpSZ: Speikboden shear zone;TSZ: Tuxer shear zones; ZWD: Zwischenbergen–Wöllatratten and Drautal faults),dextral shear zones (HoF: Hochstuhl fault; IsF: Iseltal fault; KLT: Königsee–Lammertal–Traunseefault; Mölltal fault (MöF); PF: Pustertal fault) and a reverse fault(MM: Meran–Mauls fault) are also visible in red on the map.
