Differential uplift on the boundary between the Eastern and the Southern European Alps: Thermochronologic constraints from the Brenner Base Tunnel

The Brenner Base Tunnel will connect Innsbruck (Austria) and Franzensfeste (Italy) by piercing two of the most important fault structures of the Alps: the Periadriatic fault system (PFS) and the Southern limit of Alpine metamorphism (SAM). (U‐Th)/He dating (apatite) and fission‐track analysis (apatite and zircon) on samples taken during excavation reveal a complex pattern of exhumation through time. The results yield temporal constraints for relative vertical block movement and fault activity. Furthermore, they indicate differential uplift of the northern block along the ~E–W striking PFS and allow locating the position of the SAM in the overtilted nappe stack south of the Tauern Window. Our data strongly support, for the first time, an ongoing north‐side‐up movement along this section of the PFS until at least the end of Miocene.

the northern tip of the indenting Adriatic crust resulted in their thinning due to lateral extrusion (Ratschbacher et al., 1991) as well as vertical extension. This study aims at localizing major boundaries between blocks of coherent exhumation behaviour and at constraining the timing of their vertical displacement.
The Austroalpine nappe stack overlies the Penninic and Sub-Penninic units (Reiter et al., 2018, the tunnel cross section presented coincides with the trace of their figure 8). Austroalpine F I G U R E 1 Geological overview along the trace of the Brenner Base Tunnel, crosscutting the Tauern Window and the border between Eastern and Southern Alps. The examined cross section (position indicated by the small white rectangle) pierces both DAV and PFS. The map is based on the 'Geological Overview', compiled by the 'Amt für Geologie und Baustoffprüfung der Südtiroler Landesverwaltung', which is derived from published and still unpublished findings of the projects 'CARG' and 'Basiskarte' as well as existing Italian and Austrian geological maps and the 'Geologische Übersichtskarte von Tirol' (Brandner, 1980) [Colour figure can be viewed at wileyonlinelibrary.com] F I G U R E 2 Simplified map and cross sections of the Periadriatic fault system between Passo Tonale (W) and the western part of the Tauern Window (E), based on Pomella et al. (2012); The colour code indicates the tectonic affiliation of single faults to superordinate fault systems. Cross sections modified based on: (A-A′) Reiter et al. (2018), (B-B′) Selverstone (1988), (C-C′ and D-D′) Pomella, Flöss, Speckbacher, Tropper, and Fügenschuh (2016), (E-E′) Viola, Mancktelow, Seward, Meier, and Martin (2003) units can be subdivided into a northern part affected by Eoalpine metamorphism (lower nappes during orogeny) which was juxtaposed to a weakly to non-affected (uppermost nappes) southern one (Borsi, Moro, & Ferrara, 1972) by a set of faults (Figure 2, blue) that mark the 'Southern limit of Alpine metamorphism' (SAM, Hoinkes et al., 1999). This tectonic structure is attributed to post-Eoalpine, latest Cretaceous to Paleogene extension (Froitzheim et al., 1994;Fügenschuh, 1995). In the investigated area, the SAM spatially coincides with the Defereggen-Antholz-Vals fault zone (DAV, Hoinkes et al., 1999), is in steep orientation and incorporated in the southern limb of the western Tauern  (Luth & Willingshofer, 2008;Rosenberg et al., 2018). This pattern indicates the Miocene exhumation process of the Tauern Dome and proves earlier cooling of its overlying, Austroalpine thrust sheet, framing the Tauern Window today. It is remarkable that the SW part of the Austroalpine domain, marked as Passeier unit in Figure 1, shows more affinity to the Tauern Window than to its eastern continuation in both Apatite and Zircon fission-track age pattern (Bertrand et al., 2017).

| ME THODOLOGY
Fission-track analysis is based on the spontaneous fission of 238 U while the associated formation of latent destruction trails in solid media (Wagner & van den Haute, 1992). Apatite (AFT) and zircon (ZFT) preserve these tracks at temperatures below 60°C and 180°C respectively Hurford & Green, 1983) for geologically significant time scales. Above these temperatures, fission tracks shorten in a time-and temperature-dependent manner within the partial annealing zone .
(U-Th)/He on apatite (AHe) allows for dating of rock cooling below 60-70°C (Wolf, Farley, & Silver, 1996), as He diffuses and abandons the system at temperatures higher than 80°C and remains quantitatively steady below 40°C.
Thermal histories of the samples were modelled using the HeFTy ® modelling software (Ketcham, 2005) and the annealing model of Ketcham, Carter, Donelick, Barbarand, and Hurford (2007 values (Donelick, 1993) were included in the modelling as kinematic parameter indicating the chemical composition of the individual grains.

| THERMOCHRONOLOGY RE SULTS
18 ZFT, 16 AFT (Table 1) and 11 AHe ( reproducibility of single grains, a weighted mean age was calculated for each sample. Based on spatial association and similar cooling history, six domains were identified (Figure 4). These 'groups' are briefly described in Table 3 and discussed in the following paragraph.
Group I-Three samples roughly overlap with AFT ages (1s) but the AHe single grain age dispersion is quite high and yields inverted AHe-AFT age correlation. This could result from, for example long residence in the PRZ or PAZ as suggested by the old ZFT ages. F I G U R E 5 Visualization of thermochronologic data showing snap shots in time during Alpine orogeny, each significant for a state of regional tectonic evolution. Tunnel section and position of the samples are shown on the x-axis, whereas temperature is given on the y-axis. 32 Ma: The Mauls tonalitic lamellae starts to exhume after its emplacement, tectonically requiring a relative upward movement on both the northern and southern border. 26 Ma: Main phase of cooling in the Mauls tonalitic lamellae, most likely related to exhumation. The bend in temperature distribution within the Mauls tonalitic lamellae is conspicuous. 17 Ma: Differential uplift within the cross section right after incipient Giudicarie activity. Levelling out N and S SAM is conspicuous for this stage. The wide range of uncertainty for temperature and depth is noticeable. 6 Ma: Differential vertical movement is accommodated by the southern branch of the PGF and the SAM, which coincides with the DAV here *modelled path; **path modelled by Pomella et al. (2012) for sample F1032; all other samples could not be modelled and were fit to the paths of the proper modelled ones; for samples SA7CC-320 and SA7CC-320 cooling paths are straight as no modelling could be performed) [Colour figure can be viewed at wileyonlinelibrary.com] that no single grain was measured at a younger age than earliest Paleocene.
Group Vb-The over-dispersion of sample SAV11-8 is caused by two young grains.
Group Va-Sample SAV20-42 was used for a HeFTy modelling of the cooling path.
Group III and IV contained no apatite suitable for dating due to the intense cataclastic overprint. Within group Va + b apatite was present and suitable for AFT analysis but did not pass requirements for AHe dating.

| D ISCUSS I ON
Our new cooling ages and paths constrain the timing of post-Cretaceous, Alpine processes at the boundary between the Eastern and Southern Alps. The results are discussed in time slices ( Figure 5) of cooling behaviour to visualize differential movements during large-scale tectonic events.
We put special focus on the PGF, where differential vertical movement is only known for the strike-slip duplex of the Eder unit in the Carnian Alps (Läufer, Frisch, Steinitz, & Loeschke, 1997) and the Karawanken (positive) flower structure (Heberer et al., 2016;Nemes, 1996). Kinematics of the PGF's westernmost segment comprise dextral strike-slip and a north-side-up component during a late brittle stage .
The following stages are derived from existing literature. However, our data constrain timing and occurrence of expected processes:

| Upper Cretaceous-Late Eocene: low angle detachment within the Austroalpine nappe stack
The step in ZFT data between Groups Va/b (Oligocene-Miocene) and IV (Mesozoic) clearly indicates the SAM in the presented cross section (Figures 4 and 6f). The presumable process driving the exhumation of Group Va/b is the partial collapse of the Eoalpine nappe stack (low-angle detachment during E-W extension in the Brenner region senso Fügenschuh, 1995). Late Cretaceous to early Paleocene extension along low-angle detachments in Austroalpine units was also described by Froitzheim et al. (1994). However, we cannot provide a well-constrained cooling path as strong cataclastic overprint severely decreased the number of countable apatite grains. The formerly south-directed extensional structures were incorporated in the steeply N-dipping, overtilted southern limb of the western Tauern sub-dome during Neoalpine updoming Stöckli, 1995). Mancktelow et al. (2001) Pomella et al. (2011) dated the Mauls lamellae using LA-ICP-MS U/Pb on zircon and report Oligocene intrusion ages (33-32 Ma) (Figure 6d,e). The intrusion depths of these plutonic rocks range from 5-12 km (Castellarin et al., 2005) up to 15-25 km in the Rensen area near Mauls (Trepmann, Stöckhert, & Chakraborty, 2004). We suggest the heat advection related to Periadriatic intrusions as a possible reason for Oligocene ZFT ages in Alpine non-metamorphic Austroalpine rocks (Group III). These ages are in the range of the adjacent Mauls lamellae (Group II). The extent of thermal reset has been quantified using the software 4DTherm (Fu & McInnes, 2006) which describes the cooling history of igneous bodies from their assumed emplacement temperature.

| Late Eocene-Oligocene: periadriatic intrusions and strike-slip movement along the PFS
Assuming a country rock temperature of <200°C, a rectangular shape (2 × 1 km) of the intrusion and a total exhumation of 4 km, it shows complete resetting of ZFT samples up to a distance of ca. 300 m from the intrusion.

| Miocene: Giudicarie fault activity, northward movement of Dolomites Indenter, main phase of Tauern Window compression
The data presented (Figures 4 and 6c) suggest differential vertical movement at the SAM (later cooling of Group Va/b compared to IV).
This indicates that a successor of the DAV is still active in Miocene.
Time-temperature paths (Supporting Information) derived from the Austroalpine unit north of the SAM (Group Va) indicate fastest cooling from 18 to 9 Ma. This step represents the main exhumation pulse which almost compensates the temperature difference between today's adjacent Austroalpine units ( Figure 5). The temperature step across the PGF between Group I and II is still noticeable. F I G U R E 6 Compilation of main deformation stages connected to the tectonic evolution of the Tauern Window in cross section and map view, illustrated using present-day's position of the BBT cross section (Figure 3) and the tonalitic lamellae as a spatial reference; horizontal black lines illustrate lateral movement and cannot be scaled but give an indication of paleogeographic position (abbreviations attached provide information of fault segments that compensate next step lateral movement); Abbreviations see Figure

| Late Miocene-today: ongoing backthrusting
Top-to-south, brittle reverse faults, kink bands and folds (Figure 6a,b), interpreted as backthrusts related to ongoing indentation, were mapped by Stöckli (1995) and Mancktelow et al. (2001). These are the structures which most likely accommodate the north-side-up movement indicated by our data. They crosscut all structures except the top-to-east normal faults ( Figure 3) and therefore indicate a very late stage in orogenic compression.   are most likely assigned to this tectonic phase.

| CON CLUS IONS
New AHe, AFT and ZFT data based on samples derived from the Brenner Base Tunnel and modelled time-temperature paths allow for the following conclusions: The narrow Austroalpine corridor near Mauls accommodates a significant amount of vertical displacement.
Younger AFT and AHe ages north of the PGF indicate northside-up movement in the Miocene. Thus, the PGF (i) is not a pure strike-slip fault but has an important north-side up kinematic and (ii) was active at least until Pliocene times.
A striking change in ZFT ages localises the position of the Southern limit of Alpine metamorphism within the narrow, overtilted Austroalpine nappe stack and confirms Miocene activity at the DAV.

ACK N OWLED G EM ENTS
We acknowledge support by the Brenner Basistunnel BBT SE and are especially grateful for Ulrich Burger's counseling. All tunnel samples and a basic version of the cross section were kindly provided. Matteo Massironi supported the authors with valuable input during discussion. We thank Andrea Eberhöfer for the preparation of the samples. The University of Innsbruck provided student scholarship funding.

DATA AVA I L A B I L I T Y
The data that support the findings of this study are presented in Tables 1 and 2. Thermal history models are available from the corresponding author.