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Inversion and exhumation of crystalline basement units of passive margins is a critical stage of mountain-building processes. With a multi-methodological approach that combines 3D geological modelling and cross-section restoration at different time steps, we unravel the 4D geodynamic evolution of the Aar Massif (central Alps, Switzerland) during the late-stage Alpine orogeny (22 to 0 Ma). Our results demonstrate how a portion of the crystalline basement units of the European continental margin was exhumed in a non-cylindrical framework with an alternation of vertical and horizontal-dominated tectonics. Rapid exhumation (22 to 12 Ma), with deformation along steep reverse/normal faults, alternated with northwest-directed thrusting of the previously exhumed basement units (16 to 0 Ma). We pay special attention to how shear/fault zone patterns change laterally and temporarily to accommodate the non-cylindrical deformation in dome-like basement rises. Published thermochronometric and metamorphic peak temperature data provide additional insights into the temporal sequence of reverse and thrust faulting perpendicular and parallel to the strike of the orogen. Such variations in the fault geometries and associated offsets were constrained by the detailed reconstruction of the shear zone pattern in 3D. In this study, we show how the basement-involved uplift and shortening controlled the late-stage collisional overprint and mechanics of the northern rim of the central Alps.

The pre-Alpine evolution of the Central Alpine basement is dominated by magmatic and metamorphic events that occurred during an Ordovician orogenic cycle (ca. 480-440 Ma) and the Variscan orogenic cycle (ca. 350-300 Ma). A detailed zircon U-Pb data and Hf-isotope study of a large set of magmatic and meta-magmatic rocks revealed four magmatic pulses (Ruiz et al. 2022): at 340-350 Ma (calc-alkaline diorite and tonalite from the Surselva Group), 330-335 Ma (shoshonitic diorites, monzonites, granites and syenites of the Rötifirn Group), 307-310 Ma (calc-alkaline diorites, ranging from cumulate-like hornblende gabbros to hornblende-diorites and hornblende- or biotite quartz- monzonite, granodiorites and metaluminous weakly peraluminous I-type granites of the Fruttstock Group), and 297-300 Ma (late-orogenic, calc-alkaline I-type granites of the Haslital Group). High precision U-Pb dates from meta-magmatic rocks indicate a minor, but variable impact of Alpine metamorphism on the U-Pb dates (Gaynor et al. 2022, Ruiz et al. 2022). However, given the poly-cyclic metamorphic record of the country rocks, the relative contributions of the Alpine, Variscan and an earlier Ordovician orogenic cycle are difficult to quantify. More specifically, the physical conditions of the Variscan metamorphic overprint are only weakly constrained, and available radio-isotopic ages are not reliable. However, monazite, rutile, titanite and zircon ages of 329-317 Ma in high-grade metapelites and calcsilicate gneiss indicate a major high grade Variscan metamorphism along the northern rim of the massif (Schaltegger et al., 2003). In addition, frequently found U-Pb dates between 478 and 445 Ma on gabbros, metapelitic to metapsammitic gneisses in the northern part of the Aar massif (Schaltegger et al., 2003) show relics of an older metamorphism in these polycyclic basement unitsIn order to understand better the temperature-time evolution of this poly-cyclic basement, we will apply detailed U-Pb geochronology on different minerals together with mineralogical, chemical and textural characterization. Combining the mineralogical data with microstructures and petrological data should give better insights in the link of metamorphism and magmatism for the Variscan orogenic cycle. These data will allow placing the Aar massif evolution in a wider framework of the European Variscan orogen. Moreover, they will reveal the existence of one or several pulses of earlier, Ordovician-age high-grade metamorphism, anatexis and magmatism.References: Gaynor S.P., Ruiz M., & Schaltegger U. (2022) Chem. Geol., 603, 120913; Ruiz M., Schaltegger U., Gaynor S.P., Chiaradia M., Abrecht J., Gisler C., Giovanoli F. & Wiederkehr M. (2022) Swiss J. Geosci., 115, 20; Schaltegger. U., Abrecht J. & Corfu F. (2003) Schweiz. Mineral. Petrogr. Mitt. 83, 183-195

Fault gouges of granitoid composition represent the principal non-cohesive tectonites within fault zones in the continental crust. The spatial distribution and strength of granitoid fault gouges is therefore crucial for understanding how weak the upper continental crust could be due to the formation of fault zones. Although several laboratory investigations reported the mechanical weakening of granitoid gouges in shear experiments, the deformation mechanism responsible for such behavior remains not well understood.To address this issue, we conducted two series of shear experiments on granitoid gouges by using a ring shear apparatus. The starting gouge powders were derived from crushed granitoid mylonite with a median grain size of 45 μm. In a first set of experiments, gouges were sheared at a sliding velocity of 100 μm/s for a displacement of 15 mm. Temperatures explored ranged from 20°C to 650°C in order to determine the temperature dependence of gouge strength. The second set of experiments is identical to the first ones, except that the applied sliding velocity was set at 1 μm/s to study how fault slip rate influences the strength of gouges.We observe that differences in gouges strengths as a function of sliding velocity and temperature: At a sliding velocity of 100 μm/s, the steady-state shear stress (τ) remains relatively constant at τ=76-82 MPa over the entire temperature range. Contrastingly, at a sliding velocity of 1 μm/s the steady-state shear stress remains temperature-insensitive with τ≈75 MPa up to tempertures of 450°C, but decreases then to τ≈50 MPa at 650°C (Fig.1 a). Furthermore, the amount of decrease of shear stress is strain dependent (Fig.1 b). At even slower sliding velocity of 0.1 µm/s, the shear stresses decrease further to τ≈38 MPa.Microstructurally, all gouges deformed at T≦450°C show typical cataclastic features, where angular clasts with grain size of ~10 μm are surrounded by a fine-grained matrix. Intergranular fracture arrays in Riedel- and Y-shears are well developed over the entire cross section, indicating homogeneous bulk deformation. In contrast, gouges sheared at 650°C with τ≈50 MPa show strain localization in a principal slip zone. It is shear plane parallel with widths up to ~50 µm. Inside the principal slip zone, all grains are dramaticly reduced to nm-size and tightly packed. No intergranular fracture arrays are observed. Outside the principal slip zone, rounded grains with size of ~5 μm are loosely packed, with meniscus cement growing in between. The aforementioned strain localization is enhanced at temperature above 450°C and slip rate below 1μm/s, suggesting that viscous creep mechanisms (e.g. pressure solution) control the deformation process at slow sliding velocities, which is not the case in fast rate experiments. Our results show that the activation of viscous creep mechanisms leads to significant fault zone weakening, while contrasts in grain size keep deformation localized.Figure 1 Shear stress plotted as a function of temperature. Shear stress data collected at (a) 15mm displacement in steady-state, and at (b) 5mm displacement.

Crustal rocks involved in orogenic processes frequently bear evidence for widespread fluid circulation. The hydration history of the granitic continental crust in inverted passive continental margins is of particular interest, as granitoids experience rheological weakening by fluid-rock interaction processes. Regrettably, it is often unclear if hydration occurs during rifting or during tectonic inversion. Hence, it is difficult to appreciate the interplay of pre- and syn-orogenic fluids inside continental crustal segments of rifted margins. The geochemical fingerprint of ancient hydration events is stored in hydrous minerals that crystallized directly from circulating paleo-fluids. Thus, such minerals can shed light on the nature of these ancient fluids, as well as provide temporal constraints if they can be dated. Hence, advances in geochronological methods applied to hydrous minerals may prove pivotal in untangling the history of fluid circulation in the granitic continental crust in orogens.We applied U–Pb geochronology of epidote [i.e., Ca2Al2(Al,Fe3+)Si3O12(OH)] in hydrothermal veins hosted by a late Carboniferous/early Permian calc-alkaline granodiorite in the inverted Adriatic passive continental margin (hereafter “Err nappe”), both located in the eastern Swiss Alps. During Jurassic rifting leading to the break-up of Pangea, the continental crust in the Err nappe was hydrated, as seawater-derived fluids percolated along syn-rift faults. However, geochronological data of epidote reveal that the hydration of the granitic continental crust in the Err nappe occurred also later during inversion. Epidote U–Pb geochronology returned two age clusters: (1) 85.2 ± 9.7 Ma, related to Late Cretaceous compression; and (2) 59.9 ± 2.7 Ma, related to subsequent Paleocene extension. These age clusters unveil two distict events of fluid circulation, which are consistent with the timing of tectonic inversion and deformation proposed in the literature. As confirmed by Pb–Sr–O–H isotope geochemistry of epidote, Late Cretaceous fluid circulation was likely mediated by fluids released by underlying units undergoing metamorphism during Eo-Alpine compression. Notably, the Paleocene fluids circulating during extension were most likely surficial in origin (i.e., meteoric water and/or modified/connate seawater), and they percolated into the granitic continental crust by exploiting extensional faults.In the context of existing data, our results show that the hydration of the granitic continental crust of the Adriatic passive continental margin was mediated by a repeated series of fluid circulation events. Our work advocates that the use of a multi-methodological approach, combining new geochemical and geochronological, tools provides unprecedented insight into complex processes of fluid circulation in the continental crust, and beyond.

According to well-established hypotheses based on field observations of natural faults, viscous deformation may localize following pre-existing brittle fractures. The weak behaviour can be explained by brittle grain size reduction and phase mixing, which may activate grain size sensitive processes in the viscous field. To prove this hypothesis, it is necessary to perform experiments to observe the strain and stress evolution in faulted and non-faulted rocks. Pec et al. (2012) performed experiments on granitic rocks by shearing manually crushed granitic powder between coarse solid granitic forcing blocks. However, in their study, there are unavoidable boundary conditions between the forcing blocks and the gouge, and a comparison to an intact rock without fracture is difficult.In our study, we reduce the boundary conditions to a minimum and can directly compare the stresses and microstructural evolution during deformation of intact and fractured granitic ultramylonites at 650°C, confining pressure of 1.2GPa, and a constant displacement rate of 10-8m/s. We perform these experiments on initially solid cylindrical samples in two experimental sets: In set A, we slowly apply the load and confining pressure, to ensure an intact rock sample is deformed. In set B, we create fractures before the experiment starts but already in the closed system of the experimental setup. Once experimental P/T conditions are reached, both experimental sets are deformed to different finite strains to investigate the associated microstructural evolution. The deformation is disseminated in the set A experiments, but localizes strongly along the fracture in experimental set B. The strain is accommodated by viscous granular flow incorporating an impressive grain size reduction of up to 1000x and dissolution/precipitation processes. In addition, the stress records show that in experiments A, initially a 30% higher yield stress has to be overcome before steady state flow, while in set B steady state flow is reached directly without a strain softening increment. In both sets, steady state stresses range around 300MPa, i.e. far below the confining pressure.Applying microstructural observations and mechanical data of our experiments to deformation of granitoid crust in nature reveals that fractures serve to reach mechanical steady state earlier compared to non-fractured crust. As a matter of strain, however, both settings may yield at the same mechanical strengths of resulting shear zones. It is important to note that polymineralic fine-grained ultramylonites are up to four times weaker than monomineralic quartz, presenting an important behaviour of efficient strain localization and rheological properties substantially below those of the end member minerals. Pec, M., Stünitz, H. and Heilbronner, R., 2012. Semi-brittle deformation of granitoid gouges in shear experiments at elevated pressures and temperatures. Journal of Structural Geology, vol. 38, pp. 200-221. https://doi.org/10.1016/j.jsg.2011.09.

Deformation of polymineralic aggregates can be accommodated by viscous granular flow, a process mediated by the interplay among intracrystalline plasticity and dissolution–precipitation, each active in specific minerals under given P–T conditions. Some rock-forming minerals like quartz and feldspars have been intensively studied in terms of deformation processes. Instead, the deformation behavior of epidote and its role during viscous granular flow is not well investigated, although this mineral is ubiquitous in granitic rocks deforming under greenschist-facies conditions. In this contribution, we provide microstructural and geochemical evidence for the occurrence of dissolution–precipitation of epidote during deformation of an epidote–quartz vein. The main part of the vein is deformed, producing a fold, which is visible due to relicts of primary-growth layering inside the vein. The deformation mechanisms active during deformation include dynamic recrystallization of quartz by subgrain rotation recrystallization, producing grain size reduction in the primary vein quartz. Recrystallization occurs contemporaneously with dissolution and (re)precipitation of epidote and quartz grain boundary sliding, leading to a combined process described as viscous granular flow. The combination of grain boundary sliding and dissolution locally and repeatedly produces creep cavities. These represent not only loci for nucleation of new epidote grains at the expense of dissolved ones, but they also allow fluid-mediated transport of elements. The same trace element patterns between old epidote relicts and newly formed grains, with much narrower variability in the latter, indicate a process of chemical homogenization. The nature of the fluid that mediates deformation is investigated using Pb–Sr isotope data of epidote, which suggest that deformation is assisted by internally recycled fluids with the addition of a syn-kinematic external fluid component.

Fluid assisted Alpine fissure-vein and cleft formation starts at prograde, peak or retrograde metamorphic conditions of 450–550 °C and 0.3–0.6 GPa and below. Early-formed fissures become overprinted by subsequent deformation, locally leading to a reorientation. Deformation that follows fissure formation initiates a cycle of dissolution, dissolution/reprecipitation or new growth of fissure minerals enclosing fluid inclusions. Although fissures in upper greenschist and amphibolite facies rocks predominantly form under retrograde metamorphic conditions, this work confirms that the carbon dioxide fluid zone correlates with regions of highest grade Alpine metamorphism, suggesting carbon dioxide production by prograde devolatilization reactions and rock-buffering of the fissure-filling fluid. For this reason, fluid composition zones systematically change in metamorphosed and exhumed nappe stacks from diagenetic to amphibolite facies metamorphic rocks from saline fluids dominated by higher hydrocarbons, methane, water and carbon dioxide. Open fissures are in most cases oriented roughly perpendicular to the foliation and lineation of the host rock. The type of fluid constrains the habit of the very frequently crystallizing quartz crystals. Open fissures also form in association with more localized strike-slip faults and are oriented perpendicular to the faults. The combination of fissure orientation, fissure quartz fluid inclusion and fissure monazite-(Ce) (hereafter monazite) Th–Pb ages shows that fissure formation occurred episodically (1) during the Cretaceous (eo-Alpine) deformation cycle in association with exhumation of the Austroalpine Koralpe- Saualpe region (~ 90 Ma) and subsequent extensional movements in association with the formation of the Gosau basins (~ 90–70 Ma), (2) during rapid exhumation of high-pressure overprinted Briançonnais and Piemontais units (36–30 Ma), (3) during unroofing of the Tauern and Lepontine metamorphic domes, during emplacement and reverse faulting of the external Massifs (25–12 Ma; except Argentera) and due to local dextral strike-slip faulting in association with the opening of the Ligurian sea, and (4) during the development of a young, widespread network of ductile to brittle strike-slip faults (12–5 Ma).

Fault gouges of granitoid composition represent the principal non-cohesive tectonites within fault zones in the continental crust. Their velocity-dependent friction is crucial for understanding earthquake nucleation and the depth distribution of fault-related seismicity in granitoid shear zones (Wehrens et al. 2016; Blanpied et al. 1998). In the framework of rate-and-state friction laws (RSF), the friction parameter (a-b) is measured in sliding experiments to describe the velocity dependence of friction in fault gouges (Scholz, 1998). A velocity-strengthening system is frictionally stable, (a-b) >0, whereas a velocity-weakening system can be frictionally unstable, (a-b) To address this issue, we conducted velocity stepping sliding experiments under hydrothermal conditions by using a ring shear apparatus. The powdered starting gouge was derived from a granitoid mylonite collected at the NAGRA Grimsel Test Site (Central Swiss Alps). The applied velocity steps were 1-3-10-30-100 μm/s. Pore fluid pressure and the effective normal stress were 100 MPa. Temperatures explored ranged from 20-650 °C. Values of (a-b) were obtained from RSF model inversions of the evolution of friction coefficients at mechanical steady state conditions. Our experiments showed pronounced changes in (a-b) values with across the full range of temperatures up to 650 °C and velocities investigated. At temperatures below ~100 °C and above ~400 °C, we observed mostly velocity strengthening with positive (a-b). In contrast, velocity weakening with negative (a-b) was observed between ~100 °C and ~400 °C. Samples deformed at a sliding velocity of 100 μm/s deviated slightly from this trend, as (a-b) values were negative between ~200 °C and ~400 °C.The presented experimental study demonstrates a significant influence of temperature and sliding velocity on velocity-dependence during deformation of granitoid gouge. We suggest that the observed transitions in velocity dependence reflect an interplay of interactions. In terms of crustal faulting, our data suggest the existence of a seismogenic window that limits the depth distribution of earthquakes on faults in granitoid shear. REFERENCESWehrens, P. C., Berger, A., Peters, M., Spillmann, T., Herwegh, M. 2016: Deformation at the frictional-viscous transition: Evidence for cycles of fluid-assisted embrittlement and ductile deformation in the granitoid crust, Tectonophysics, 693, 66-84.Blanpied M. L., Tullis T. E., Weeks J. D. 1998: Effects of slip, slip rate, and shear heating on the friction of granite.Scholz, C. H. 1998: Earthquakes and friction laws, Nature, 391, 37-42.

The Aar Massif is a mid-crustal basement section of the European plate and it was intensely deformed during the Alpine orogeny. Alpine deformation of Aar Massif granitoids is expressed by a pervasive network of ductile shear zones consisting of fine-grained polymineralic (ultra)mylonites dominated by viscous granular flow processes (Wehrens et al., 2016). Fluid circulation and hydration reactions are recorded by both granitic protoliths and shear zones. In particular, they are made evident by the alteration of feldspar into hydrous minerals like epidote and mica. The timing of this hydration event and, consequently, whether Alpine deformation was initiated in already altered granitoids or in fresh ones were unclear. This lack of time constraints led to a pivotal question: did deformation initiate in rocks that were altered by pre-kinematic hydration, or was hydration syn-kinematic and driven by the formation of Alpine shear zones? Laser ablation ICP-MS U–Pb geochronology applied to epidote in hydrothermal veins provides new evidence for pre-Alpine hydration of granitoids in the Aar Massif. Two veining events are recognized: 1) one at ca. 276 Ma occurring during Permian transtension and rifting, and 2) another at ca. 14 Ma related to late Alpine exhumation phases. Initial 207Pb/206Pb ratios of all Permian epidote samples overlap within uncertainty, suggesting only one fluid source and equilibration path. Also, these ratios are more radiogenic than those of the host rocks at the time of vein formation. The hydrogen isotopic composition of the Permian fluids was calculated from measurements in bulk epidote separates by high-temperature conversion elemental analyzer. With a temperature range of epidote crystallization estimated between 200–300 °C, the calculated δDfluid value is -57 to -44 ‰. An external source for the Permian fluids is suggested by the disequilibrium of Pb isotopes between hydrothermal epidote and host granitoids. Percolation of meteoric water along transtensional faults and interaction with syn-rift sediments before reaching the granitoids underneath is suggested by the Permian transtensional geodynamics and supported the hydrogen isotopic composition of the Permian fluids. The occurrence of Permian fluid circulation in the Aar Massif granitoids indicates that these rocks were altered before the onset of Alpine deformation. In fact, it can be inferred that fluid circulation caused not only veining, but also pervasive flow and thus the alteration of magmatic feldspar into fine-grained hydrous minerals throughout the granitoids of the present-day Aar Massif. This enabled pre-Alpine storage of water and the creation of numerous additional grain boundaries, both favoring viscous granular flow during Alpine deformation. In this context, the localization of strain in polymineralic aggregates containing hydrous minerals can recycle stored pre-kinematic (i.e., Permian) water. Thus, the initiation of Alpine deformation did not necessarily require the addition of syn-kinematic (i.e., Alpine) fluids, although their presence is confirmed by this and previous studies (e.g., Ricchi et al., 2019; Peverelli et al., 2021).

Rheological models of Earth’s granitoid mid- to upper crust are commonly based on the physico-chemical properties of the most abundant rock forming minerals quartz and feldspar. However, there is increasing field evidence that deformation in these rocks localizes in ultrafine-grained polymineralic shear zones, which are weaker than any of the end member minerals. Especially at the brittle to viscous transition, the localization and deformation mechanisms, i.e. the role of incipient brittle deformation vs. continuous viscous strain localization, is not yet fully understood.To fill this gap in knowledge, ultramylonite samples with granitic composition from the Central Aar Granite (Aar Massif, Central Switzerland) were deformed using a Griggs type apparatus. The foliation of the ultramylonitic starting material was oriented 45° to the compression direction, to investigate the influence of grain size and composition on strain localization in the different mylonite bands. Two types of coaxial experiments were conducted at 650°C, and 1.2 GPa confining pressure: A) Discrete fractures were created before the shear deformation starts; B) No fractures were induced during an early stage of the experiment.All experiments have in common that strain is accommodated in 20-100 µm wide viscous shear zones with elongated grains and minor grain size reduction. In these shear zones, most strain is further localized in 10-20 µm wide zones, showing dramatic grain size reduction down to few tens of nanometres. In the experimentally generated shear zones, both, brittle and viscous processes are active. In terms of overall rock strength, all newly formed ultrafine-grained shear zones are up to three times weaker than comparable experiments on pure quartz or coarser grained granites – which agrees well with field observations. Furthermore, pre-fractured type A) is up to two times weaker than the non-fractured type B), and the orientation and number of shear zones is also fundamentally different between the two experiment types.This study confirms two weakening factors promoting different types of strain localization at the brittle to viscous transition: 1) The existence of fractures and their interconnectivity – facilitating highly-localized grain size reduction; 2) Initial sample heterogeneity by polymineralic composition and ultrafine grain size – generating grain size reduction along strain gradients by activating viscous processes. Further quantitative microstructural analyses will reveal the role of chemistry and the deformation mechanisms on the localization behaviour.