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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.

Fissure monazite-(Ce) (hereafter called monazite) commonly crystallizes during deformation at low metamorphic grade and offers the possibility to date protracted deformation over several millions of years and to identify distinct deformation phases. We performed Th-Pb geochronology at the microscale on 10 samples of fissure monazite from two Alpine crystalline massifs (Aar Massif and Gotthard Nappe). Ion microprobe ages show that the earliest stage of crystallization recorded by fissure monazite domain ages occurred around 15.9 Ma in the Gotthard Nappe and about 1 My later in the Aar Massif, with the latest crystallization event recorded at 6 Ma. Protracted monazite crystallization in fissures indicates that deformation datable with fissure monazite lasted about 10 Ma. Comparison of Th-Pb crystallization ages of fissure monazite to existing thermochronological data shows that early monazite crystallization coincide with zircon fission track ages, whereas the youngest monazite crystallization overlaps with apatite fission track ages. Monazite also grows contemporaneous with muscovite/illite crystallization (K-Ar ages) in fault gouges. As monazite can grow during dissolution/precipitation cycles induced by tectonic activity, their chronology allows to further constrain in the Aar Massif the final Handegg phase at 12–11.5 Ma, and the coeval activity of the Pfaffenchopf (11.5–9 Ma), and the Oberaar and Rhone-Simplon phases (11.5 and 6 Ma). In the Gotthard Nappe, monazite crystallization constrains a major portion of the Chiera backfolding phase at 14–13 Ma, and confirms that the south-western termination of the nappe was affected by the Rhone-Simplon phase.

Epidote – here defined as minerals belonging to the epidote–clinozoisite solid solution – is a low-μ (μ=238U/204Pb) mineral occurring in a variety of geological environments and participating in many metamorphic reactions that is stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a U−Pb geochronometer has been hindered by the commonly high contents of initial Pb, with isotopic compositions that cannot be assumed a priori. We present a U−Pb geochronology of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 µg g−1), Th (0.01–38 µg g−1), and U (2.6–530 µg g−1) contents and with μ values between 7 and 510 from the Albula area (eastern Swiss Alps), from the Grimsel area (central Swiss Alps), and from the Heyuan fault (Guangdong Province, China). The investigated epidote samples show appreciable fractions of initial Pb contents (f206=0.7–1.0) – i.e., relative to radiogenic Pb – that vary to different extents. A protocol has been developed for in situ U−Pb dating of epidote by spot-analysis laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with a magmatic allanite as the primary reference material. The suitability of the protocol and the reliability of the measured isotopic ratios have been ascertained by independent measurements of 238U/206Pb and 207Pb/206Pb ratios, respectively, with quadrupole and multicollector ICP-MS applied to epidote micro-separates digested and diluted in acids. For age calculation, we used the Tera–Wasserburg (207Pb/206Pb versus 238U/206Pb) diagram, which does not require corrections for initial Pb and provides the initial 207Pb/206Pb ratio. Petrographic and microstructural data indicate that the calculated ages date the crystallization of vein epidote from a hydrothermal fluid and that the U−Pb system was not reset to younger ages by later events. Vein epidote from the Albula area formed in the Paleocene (62.7±3.0 Ma) and is related to Alpine greenschist-facies metamorphism. The Miocene (19.2±4.3 and 16.9±3.7 Ma) epidote veins from the Grimsel area formed during the Handegg deformation phase (22–17 Ma) of the Alpine evolution of the Aar Massif. Identical initial 207Pb/206Pb ratios reveal homogeneity in Pb isotopic compositions of the fluid across ca. 100 m. Vein epidote from the Heyuan fault is Cretaceous in age ( 107.2±8.9 Ma) and formed during the early movements of the fault. In situ U−Pb analyses of epidote returned reliable ages of otherwise undatable epidote–quartz veins. The Tera–Wasserburg approach has proven pivotal for in situ U−Pb dating of epidote, and the decisive aspect for low age uncertainties is the variability in intra-sample initial Pb fractions.

Recently, the application of LA–ICP–MS has enabled U–Pb dating of epidote minerals within the epidote–clinozoisite solid solution series (Peverelli et al., 2020). Epidote crystallization ages can provide an absolute time frame of deformation sequences when combined with detailed microstructural and metamorphic P–T analysis. Epidote deformation occurs in a brittle manner over a wide range of conditions below its closure temperature for Pb diffusion (685–750 °C; Dahl, 1997); hence, such deformation will not affect its formation U–Pb age. Nevertheless, the possibility of isotopically resetting epidote via fluid–mineral interaction has to be taken into account even at low deformation temperatures.We investigated the geochemical and Sr–Pb isotopic characteristics of epidote in one hydrothermal vein in the Aar Massif (central Swiss Alps). The vein is associated with an Alpine shear zone and it is composed of aggregates of 0.1–1 mm anhedral to subhedral epidote grains (epidote-A) + green biotite within a quartz matrix. This quartz dynamically recrystallized by subgrain rotation at temperatures above 400 °C (Stipp et al., 2002) along with crystallization of a second epidote generation (epidote-B) made of tiny (< 0.1 mm) anhedral epidote grains in part mantling epidote-A and defining a fold. We address whether interaction with the fluid that precipitated epidote-B chemically affected epidote-A, i.e. whether the U–Pb age measured by LA–ICP–MS in epidote-A still dates its crystallization upon vein formation or displays age disturbance.LA–ICP–MS Sr and Pb concentration data overlap between epidote-A and epidote-B, as do their REE patterns, with (La/Yb)N ratios of 0.03–0.92. Lead and Sr isotopic signatures were measured respectively by solution MC–ICP–MS and by TIMS in epidote-A and in separates mixing different proportions of epidote-A and -B (no pure mechanical separates of epidote-B possible), and they are different. This requires open-system conditions during deformation, i.e., introduction of an external fluid with higher 87Sr/86Sr and 208Pb/206Pb ratios during crystallization of epidote-B. Despite the presence of an external fluid and the incorporation of external Sr and Pb in epidote-B, LA–ICP–MS U–Pb isotopic data for epidote-A define a regression in a Tera–Wasserburg plot indicating an age of 19.2 ± 4.3 Ma, consistent with epidote-A crystallization during original vein opening. The preservation of the crystallization age in epidote-A indicates that interaction with the fluid that formed epidote-B did not geochemically and isotopically affect epidote-A. The consistency in trace element contents between epidote-A and -B hints that the epidote-forming cations were inherited by the fluid from epidote-A, and thus suggests dissolution-precipitation as the formation process for epidote-B. Dahl, Earth Planet. Sci. Lett. 150, 277–290, 1997.Peverelli et al., Geochronology Discuss. [preprint], https://doi.org/10.5194/gchron-2020-27, in review, 2020.Stipp et al., Geological Society, London, Special Publications, 200(1), 171-190, 2002

Inversion of passive margins and their transportation into fold-and-thrust belts is a critical stage of mountain building processes and their structural interpretation is fundamental for understanding collisional orogens. Due to the multitude of parameters that influence their formation (e.g. the interaction between sedimentary cover and basement, the mechanical stratigraphy or the rheology of different rock types) as well as along-strike internal variations, a single cross-sectional view is insufficient in exploring the 3D evolution of a fold-and-thrust belt. Hence, a 3D geological characterization is required to better comprehend such complex systems. Based on a detailed digital map, a 3D structural model of the current tectonic situation and sequential retrodeformation, we elaborate the 3D evolution of a part of the former European passive continental margin. In this setting, we focus on the Doldenhorn Nappe (DN) and the underlying western Aar massif (external Central Alps, Switzerland). The DN is part of the Helvetic nappe system and consists of a large-scale recumbent fold with a thin inverted limb of intensively deformed sediments (Herwegh and Pfiffner 2005). The sedimentary rocks of the DN were deposited in Mesozoic-Cenozoic times in a small-sized basin, which has been inverted during the compression of the Alpine orogeny (Burkhard 1988). Along NNW-SSE striking geological cross-sections, restoration techniques reveal the original asymmetric triangular shape of the DN basin and how the basin has been exhumed from ~ -12 km (Berger et al. 2020) to its present position at 4km elevation above sea level throughout several Alpine deformation stages. Moreover, the model allows to visualize the current structural position of the DN and the massif as well as the geometric and overprinting relationships of the articulated deformation sequence that shaped the investigated area throughout the Alpine evolution. Here we document that: (i) the DN is a strongly non-cylindrical recumbent fold that progressively pinches out toward the NE; (ii) significant along-strike (W-E) stratigraphy thickness variations are reflected in structural variations from a single basal thrust deformation (W) to an in-sequence thrust deformation (E); and (iii) the progressive exhumation of the basement units towards the E and thrusting towards the N. In this context, special emphasis is given to illustrate how three-dimensional geometry of inherited pre-orogenic structures (e.g., Variscan-Permian and rifting related basement cover structures) play a key role in the structural style of fold-and-thrust belts. In summary, today’s structural position of the DN is the result of the inversion of a small basin in an early stage of thrusting, which was followed by sub-vertical buoyancy driven exhumation of the Aar massif and subsequent thrust related shortening. All three stages are deeply coupled with an original non-cylindrical shape of the former European passive continental margin.

More than half a century of investigations on the chemical and isotopic compositions and on geochronological data of the Cenozoic magmatic rocks in the Alps and the transition to the Apennine will be summarized. The Alps itself are dominated by a calc-alkaline series between ~42 and 30 Ma, which we summarized as Periadriatic magmatism. This magmatism includes also eroded volcanic parts and several dykes in the Southern Alps and Tyrol. In addition, Sesia Zone magmatic rocks are characterized by ultrapotassic, shoshonitic and calc-alkaline rocks between 33 and 30 Ma. Two other magmatic provinces are located in between the Alps and the Apennine: (1) Veneto volcanic province (=VVP; nephelinites, basanites and alkali basalts between 52 and 30 Ma); (2) Mortara volcano (~28 Ma). Another group is the Esterél magmatic province, which is located in the Alps and their direct foreland, but are not related to Alpine geodynamics. These are basalts, andesites and dacites with mantle signature developed between 40 and 20 Ma. In the hanging plate of the early Apennine geometry, some minor volcanic activity is preserved in Sardinia. The major volume of Apennine magmatism itself (Elba etc.) is Late Miocene-Pleistocene in age and is related to roll back dynamics of the Apennine.The Eocene/Oligocene Periadriatic magmatism of the Alps requires significant melt production in the crust combined with some ACF processes. This is possible by infiltration of fluids in the mantle wedge and the lower crust and a change of P-T conditions in the mantle. Their calc-alkaline character is related to Na-dominated input in the mantle and crust, which is commonly inferred to result from subduction of oceanic units. Ultrapotassic melts in the Sesia-unit most likely result from infiltration of K-dominated fluids, related to dehydration of continental material. The dynamics of Apennine and possible related forearc extension would allow an extensional related magmatism in the Esterél. This magmatism overlap in time with Alpine magmatism, and require a small-scale mantle dynamic due to the development of two slabs. In addition, the VVP and the Mortara volcano are located on the non deformed continental fragment of Adria between the Alps and Apennine. This area is characterized by overfilled basins and local magmatism inside the Adriatic continental plate.The sometimes minor preserved volumes, but well constrain timing of magmatic rocks at the interaction between Alps and Apennine give insights in the lower crust/mantle dynamics at Oligocene/Early Miocene times. These interpretations may differ from models based on upper crustal tectonics, due to the decoupling between upper crust and lower crust/mantle.

During the accretion of foreland basin sediments into an accretionary or orogenic wedge, the sediments dehydrate and deform. Both dehydration and deformation intensity increase from the outer to the inner wedge and are a function of metamorphic processes and strain. Here, we study the microstructural evolution of slates from the exhumed Flysch units making up a paleo accretionary wedge in the European Alps. With classic SEM imaging and synchrotron X-ray fluorescence microscopy, we document the evolution of slate fabrics and calcite veins and aim at understanding the role of the evolving slate fabrics for strain localisation and fluid flow at the micro-scale.The investigated slate samples are from a NW-SE transect covering the outer and inner wedge from 200 to 330 °C. The metamorphic gradient directly correlates with an increasing background strain gradient. With the use of the autocorrelation function, we quantify the evolution of the microfabrics along the metamorphic gradient and document deformation stages from a weakly deformed slate without foliation in the outer wedge to a strongly deformed slate with a dense spaced foliation in the inner wedge. The foliation mainly forms by dissolution-precipitation processes, which increase with increasing metamorphic gradient.The slate matrix reveals two main sets of veins. The first vein set includes micron-scaled calcite veinlets with very high spatial densities. The second vein set includes layer parallel calcite veins that form vein-arrays (couple of metres thick) in the inner wedge. Both vein sets could have moved large amounts of fluids through the wedge. The spatial distribution of the micron-veinlets reveals that fluids were moved pervasively. In the case of the layer parallel veins forming vein-arrays, fluid flow was localized, supported by the dense spaced foliation formed in the slate matrix in the inner wedge. This way we now establish a direct link between the microstructural evolution in the slate matrix and associated dehydration, where fluids become increasingly channelled towards the inner wedge. Knowing that the vein-arrays have length dimensions in the order or hundreds of metres to kilometres, these structures are important for larger-scale fluid flow, the feeding of fluids into megathrusts and for related seismic activity in the wedge.

Journal of structural geology 150, 104404 (2021). doi:10.1016/j.jsg.2021.104404, Published by Elsevier Science, Amsterdam [u.a.]

White mica has been widely used to date microstructures and tectonic events in faults, shear-zones and folds because of its suitability for radiogenic dating. However, complex (i) microstructural evolution, (ii) individual chemical evolution of the K-bearing phases, (iii) mixing of ‘detrital grains’ with newly formed and/or recrystallized or chemically reset grains as well as (iv) volume diffusion may result in apparent K-Ar ages. Here, specimens from a prograde sediment sequence of the exhumed fossil European Alpine accretionary wedge were used to investigate resetting processes of white mica by the type and intensity of deformation as well as peak metamorphic conditions. We combine the K-Ar system with mass and mineral quantities from grain size fractions to calculate the amount of recrystallized white mica in each grain size fraction along the metamorphic gradient. Increasing recrystallization with increasing metamorphic grade is related to thermally activated pressure solution and dissolution-precipitation creep, as seen by the formation of a spaced cleavage of recrystallized phyllosilicates documented through Synchrotron X-ray Fluorescence Microscopy and Scanning Electron Microscope imaging techniques. Increasing recrystallization by dissolution-precipitation processes induces chemical resetting of the isotopic system, resulting in a prograde decrease of apparent K-Ar ages. We demonstrate that Ar volume diffusion does not play a significant role for the low-temperature samples, promoting recrystallization as the important physico-chemical process for age resetting. However, white mica chemistry reveals that no simple relation between isotopic resetting and grain size exists along the prograde path. Reliable age information can therefore only be obtained in the case of (nearly) complete resetting, which accounts only for the smallest grain size fraction at the highest metamorphic temperature. These findings could shed new light on accurate dating of mica-rich fault rocks, where the time constraints depend not only on the temperature, but also on the amount and type of deformation.

Monoclinic epidote is a low-µ (µ = 283U / 204Pb) mineral occurring in a variety of geological environments, participating in many metamorphic reactions and stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a U–Pb geochronometer has been hindered by the commonly high contents of initial Pb with isotopic compositions that cannot be assumed a priori. We present U–Pb geochronology of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 µg g−1), Th (0.009–38 µg g−1) and U (2.6–530 µg g−1) contents and with µ values between 7–510 from the Albula area (eastern Swiss Alps), from the Grimsel area (central Swiss Alps) and from the Heyuan fault (Guangdong province, China). The investigated epidote samples show appreciable fractions of initial Pb that vary to different extents. A protocol has been developed for in situ U–Pb dating of epidote by spot-analysis laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) with a magmatic allanite as primary reference material. The suitability of the protocol and the reliability of the measured isotopic ratios have been ascertained by independent measurements of 238U / 206Pb and 207Pb / 206Pb ratios respectively by quadrupole and multicollector ICP–MS applied to epidote micro-separates digested and diluted in acids. For age calculation, we used the Tera–Wasserburg (207Pb / 206Pb–238U / 206Pb) diagram, which does not require corrections for initial Pb and provides the initial 207Pb / 206Pb ratio if all intra-sample analyses are co-genetic. Petrographic and microstructural data indicate that the calculated ages date the crystallization of vein epidote from a hydrothermal fluid and that the U–Pb system was not reset to younger ages by later events. Vein epidote from the Albula area formed in the Paleocene (62.7 ± 3.0 Ma) and is related to Alpine greenschist-facies metamorphism. The Miocene (19.1 ± 4.0 Ma and 16.9 ± 3.7 Ma) epidote veins from the Grimsel area formed during the Handegg phase (22–17 Ma) of the Alpine evolution of the Aar Massif. Identical initial 207Pb / 206Pb ratios reveal homogeneity in Pb isotopic compositions of the fluid across ca. 200 m. Vein epidote from the Heyuan fault is Cretaceous in age (108.1 ± 8.4 Ma) and formed during the early movements of the fault. In situ U–Pb analyses of epidote returned reliable ages of otherwise undatable epidote-quartz veins. The Tera–Wasserburg approach has proven pivotal for in situ U–Pb dating of epidote and the decisive aspect for low age uncertainties is the variability in intra-sample initial Pb fractions.

In experiments designed to understand deep shear zones, we show that periodic porous sheets emerge spontaneously during viscous creep, forming a hydro-mechanical anisotropy that influences mass transfer. These findings challenge the current paradigm of viscosity in solid rocks. In particular, they showcase how shear zones may actively focus mass transport and highlight the possibility that viscous rocks could locally transition from flow to fracture. Our work demonstrates that viscosity in solids is not directly comparable to viscosity in fluids and this is consequential for a range of important solid Earth topics, like slow earthquakes, the flow of glacial ice and the tectonics of exoplanets.

The seismic-interseismic cycle strongly relates to the interplay between dilation owing to fracturing and frictional granular flow on one hand side and hydrothermal cementation processes on the other side. This study investigates different fault rocks of a crustal-scale fault zone in the Central Alps (Switzerland). We combine microstructural with geochemical approaches to decipher the interaction of grain size reduction via frictional processes with precipitation and resulting particle size increases. The three major fault rocks, i.e. (1) cockade-bearing breccias, (2) cataclasites, and (3) fault gouges, differ in their microstructure. The chemical data clearly demonstrate a decreasing gain of volume along this group of tectonites. Their different precipitation volumes most likely relate to dynamic changed of the local permeability of these rocks. The fluid pathways control the precipitation at different localities and times, which affect the healing of these fault rocks inducing a gain in rock strength. During the next deformation event, the extent of healing therefore directly controls the mechanical behavior of the rock. The estimated volume gain (~+110%) in cockade-bearing breccias is consistent with the seismic dilatant behavior of these frictional rocks as already proposed from other arguments (Berger and Herwegh 2019). This is in contrast to the fault-gouges with only minor gains in volume and mass resulting in a predominantly non-cohesive deformation style. This example indicates that permeability evolution (and related hydrothermal processes) strongly influences the mechanical behavior of such faults. This shows the highly dynamic behavior with time in long-lived fault systems. These dynamic changes in precipitation and resulting different strengths occur at different timescales from minutes (seismic events) to thousands of years.Ref.: Berger, A., Herwegh, M., 2019. Cockade structures as a paleo-earthquake proxy in upper crustal hydrothermal systems. Nature Scientific Reports, 9, 9209.

A paleohydrothermal giant quartz reef (at least 75 m wide, 40 km long) and abundant hot springs at the Heyuan fault, South China, provide an excellent opportunity to investigate hydrothermal flows from the Mesozoic through to present-day.The giant quartz reef has formed in the extensional regime initiated in the Mesozoic, while a change to compressional stress on the Heyuan in the Cenozoic led to the development of cross-cutting strike-slip faults and associated vertical fracture network. Here, we present multiscale observations and analyses from the earlier long-term extensional phase.Detailed microstructural analyses identified a 'quartz-reef window' of formation occurring between ~200-350˚C, linking in both quasi-static criteria (accommodation space; massive fluid sources; and a cap rock/seal) and dynamic mechanisms (episodic-dynamic permeability; brittle-ductile cycles; and fluid injection though brittle-ductile equivalent of Sibson's 'fault-valve' behaviour.This oscillatory brittle-ductile fault-valve is recorded in the field through its apparent contradiction between idiomorphic 5 cm long quartz crystal growth in mode-I fractures, embedded at large-scale inside far from equilibrium fault zones with mylonitic and cataclastic microstructures. Another characteristic feature is the increasing quartz vein frequency towards the core shown by enrichment of SiO2, with depletion of K2O and Na2O in tectonites during alteration from the host granite; a reaction partly sourcing the SiO2 for the quartz reef.We present a first theoretical model compatible with the observation of oscillatory macroscale far from equilibrium conditions, followed by long periods of micro-scale local equilibrium. The model can in particular describe mechanisms of abundant SiO2 dominated fluid release reaching episodically above hydrostatic pressures followed by long periods of SiO2 precipitation, allowing growth of up to 5 cm long idiomorphic quartz crystals in subparallel open channels, which presumably were held open by high fluid pressures. In this interpretation, the observations instabilities are seen to stem from the multiscale and multiphysics of the mineral reactions at the brittle-ductile transition, promoted by a slow extensional geodynamic driver at the Heyuan fault.The new approach allows interpretation of rock physics properties in terms of recently discovered Thermo-Hydro-Mechanical-Chemical (THMC) multiscale wave-like instabilities. In the model short wavelength chemical dissolution-precipitation reaction waves are bouncing between the phyllonitic cap rock and the mylonitic shear zone below. A resonance phenomenon of constructive interference in a finite width around the future quartz-reef triggers the long-time scale steady-state attractor allowing quartz reef growth over geodynamic time scales. We show that this solitary wave limit forms a standing wave matching the characteristic periodic pattern of mode-I quartz veining around the reef and also explaining the fluid overpressures leading to local hydro-fracturing.

Late stage Alpine collision is the result of collision of the European and Adriatic continental plates. For this stage, particularly the External Crystalline Massifs (ECM) and their forelands provide information on deformation style/kinematics, exhumation history and geodynamic driving forces. Using the ECMs as marker for the non-thinned European passive continental margin, the margin’s paleogeography with its curved geometries controls in parts later compressional tectonics. During closure of the Valaisan in Eocene times, a major NW-SE trending sinistral transfer zone must have acted as lateral ramp between the NNW migrating Penninic front and the westerly situated European margin units (Argentera and Maritime Alps, stage 1 of Schmid et al. 2017). Hence, first transpressive movements were documented by transpressional strike-slip faults in the case of Argentera and southern Belledonne s.l. Massifs. The SW-NE trending ECMs became affected during a first stage of horizontal tectonics in Oligocene, when first the margin sediments were scrapped off their substratum (Helvetics, Chaînes Subalpines, Dauphinois) followed by thick-skinned thrusting. Transport directions gradually changed from W to N from the Western towards the Eastern ECMs. This spread in transport direction is the consequence of an Alp-internal vertical uplift (Internal Crystalline Massifs (ICM), Lepontine dome) induced by indentation of Adriatic mantle into European lithosphere (stage 2 of Schmid et al. 2017). With further down-bending of the European crust, a major change to a vertical tectonic deformation style occurred in Mid to Late Miocene. Steep reverse faults in the ECMs, in parts with oblique slip components (Mont Blanc), witness this stage with its enhanced vertical rock uplift component. The latter is most pronounced in the Aar Massif and gradually decreases towards the West (Belledonne s.l.). With continuous ICM exhumation in Late Miocene, deformation style switches again to horizontal tectonics, leading to ‘en bloc’ exhumation above basement thrusts of all massifs (Belledonne to Aar Massifs). Progressive shortening induced thrust propagation into the foreland sediments as well as the Jura mountains. In the case of the Argentera Massif, oroclinal bending probably led to a substantial anticlockwise rotation, which goes in hand with the rotation of the entire SW Alpine arc (stage 3 of Schmid et al. 2017). In a geodynamic context, the ‘Adriatic push model’ could explain aforementioned stages of classical horizontal tectonics. Not so, however, the observed severe components of vertical tectonics in the case of ECMs. Here, lower crustal delamination, with a loss of lithospheric negatively buoyant forcing and consequent strong increase in positive buoyancy explains generation and activity of steep reverse faulting in the ECMs. In this context, the ‘orogeny slab rollback’ model provides a physically more consistent framework to explain the observed deformation sequences of late-stage continent-continent collision.Schmid, S.M., Kissling, E., Diehl, T., van Hinsbergen, D.J., Molli, G., 2017. Ivrea mantle wedge, arc of the Western Alps, and kinematic evolution of the Alps–Apennines orogenic system. Swiss Journal of Geosciences 110, 581-612.

In plate-convergent settings, fluid-saturated sediments dehydrate during subduction. The sediments are subsequently accreted to the upper plate. Along their dehydration-deformation path, the initial unconsolidated soft marine sediments become thick, foliated, impermeable meta-sedimentary sequences. Fluid flow through such ‘non’-porous low-permeability rocks is concentrated in fracture networks, ranging from the mm- to the km-scale. We study the interplay between ductile and brittle deformation processes and fluid flow by investigating calcite veins in slates from the exhumed European Alpine accretionary wedge across scales (µm to km). These slates experienced peak metamorphic temperatures between 200°C and 330°C and represent the transition between the upper aseismic and seismic zone. With the use of Synchrotron X-ray Fluorescence Microscopy (SXFM), we investigate the slates by visualizing trace-element distributions. This technique shows that alternating cycles of slow pressure-dissolution processes and brittle fracturing persist over long time scales. At the micron-scale, pressure solution of the initial carbonate-rich slates is indicated by an enrichment of newly recrystallized phyllosilicates on cleavage planes and in pressure shadows. These ductile deformation features are mutually overprinted by calcite veins (aperture 10 µm), which are nicely visualized with Sr-SXFM maps. Increasing compaction and recrystallization in the slate-rich matrix leads to progressed dehydration resulting in an increased pore fluid pressure and subsequent hydrofracturing. The micron-sized fractures are immediately filled in with minerals, which are oversaturated at that time in the fluid, resulting in the formation of (i) micron-veinlets. Micron-veinlets collect (ii) into mm-cm sized veins, which themselves form (iii) vein arrays and (iv) mega-arrays, respectively at the 50-100 m and 300-400 m scale. This upscaling of fluid pathways indicates a localised fluid transport through the accretionary wedge, which has important implications for the understanding of the mechanical stability of the accretionary wedge and related seismic activity.

3D modelling of complex and irregular geological bodies is an expanding discipline that combines two-dimensional cartographic and structural data managed with GIS technology. This study presents a complete workflow developed to process geological information to build a 3D model of major stratigraphic, structural and tectonic boundaries. The investigated area is located in the western part of the Aar Massif (external Central Alps, Switzerland) characterized by pronounced topographic (600–(1) Generation of 2D polylines in a map view: a two-dimensional dataset of sequences of polylines has been generated in ArcGIS (10.3.1) defining the starting dataset for the major stratigraphic and tectonic boundaries of the bedrock units. This dataset has been compiled and integrated by using: (i) GeoCover vector datasets 1:25 000 of the Swiss Geological Survey; (ii) The Geological Special Map 1:100 000 of the Aar Massif and the Tavetsch and Gotthard Nappes of the Swiss Geological Survey; (iii) data from literature; and (iv) additional field work conducted for this study in key-locations.(2) Projection of 2D information onto 3D digital elevation model: with the 3D structural modelling software Move (Petex/Midland Valley; 2019.1) the boundaries have then been projected on a digital elevation model (swissALTI3D) with 2 m resolution.(3) Construction of tectonic cross sections: the use of geometric arguments as well as structural measurements allows for projection of these boundaries into a dense regularly spaced network of 2D cross-sections.(4) Interpolation of 3D surfaces: the surface and cross-sections boundaries can be interpolated by applying 3D projection and meshing techniques resulting in a final 3D structural model.Generally, steps (2–4) require iterative adaptations particularly in the case of surface areas being covered by glaciers or unconsolidated Quaternary sediments. In the model, special emphasis is given to visualize the current structural disposition of the western Aar Massif as well as the relative geometric and overprinting relationships of the deformation sequence that shaped the investigated area throughout the Alpine deformation. Finally, since in the investigated area underground data are scarce, an assessment of the relative uncertainties related to input data and is intended to be performed following the approach proposed by Baumberger (2014) and Ferńandez (2005). The workflow presented here offers the chance to gain validation approaches for domains only weakly constrained or with no surface data available, by generating a 3D model that integrates all accessible geological information and background knowledge. REFERENCESBaumberger, R. (2014): Quantification of Lineaments: Link between internal 3D structure and surface evolution 328 of the Hasli valley (Aar massif, central alps, Switzerland), University of Bern, PhD Thesis, unpublished.Ferńandez, O. (2005): Obtaining a best fitting plane through 3D georeferenced data, Journal of Structural Geology 27, pp. 855–858

We revisit large shear strain deformation experiments on Carrara marble and observe that anisotropic porous domains develop spontaneously during shearing. Specifically, as samples are deformed periodic porous sheets are documented to emerge and are found to transfer mass. These results imply that viscous shear zones may naturally partition fluids into highly anisotropic bands. As this hydro-mechanical anisotropy is produced by creep, each porous sheet is interpreted to represent a transient dynamic pathway for fluid transport. It is unclear how long each porous domain is uniquely sustained but it is clear that sheets are persistently present with increasing strain. Our results forward the idea that viscous shear zones have dynamic transport properties that are not related to fracturing or chemical reaction. We believe these new results provide experimental foundation for changing the paradigm of viscosity in rocks to include dynamic permeability. In our view making this change in perspective could alter many classical interpretations in natural banded mylonite zones, for example shear zone parallel syn-kinematic veining may be the result of pore sheet instability and ductile fracturing.

The widespread presence of epidote-bearing veins and hydrous minerals such as micas in meta-granitoid rocks attests to the large extent of hydration of the exhuming continental crust. The ability of epidote (Ca2Al3Si3O12(OH) – Ca2Al2Fe3+Si3O12(OH)) to incorporate a wide variety of trace elements renders this mineral a promising geochemical tracer of circulating fluid(s).We report trace element and microstructural data on epidote-bearing veins from the Aar Massif (Central Alps) and from the Albula Pass (Eastern Alps). We characterized and classified the epidote-bearing veins based on their extent of deformation, shape and size of the epidote grains, coexisting minerals, and degree of dynamic recrystallization of associated quartz. Laser ablation ICP-MS data of individual epidote crystals reveal prominent zoning, confirmed by electron probe maps for Sr and Mn. Overall, low to very low Th/U ratios (down to 0.0005 in the Aar Massif veins and 0.001 in the Albula ones) with Th often below limits of detection (< 0.1 µg/g at 16 µm beam size) go along with variably LREE-depleted patterns (and CI Chondrite-normalized LaN/YbN ~0.35 in the Aar Massif veins and ~0.60 in the Albula Pass veins). Strontium contents are variable (hundreds to thousands of µg/g) and mostly high (up to 10100 µg/g in the Aar Massif samples and 12800 µg/g in the Albula Pass samples). The in-situ geochemical data are linked to the microstructures in order to assess whether microstructures can be related to variations in trace elements, also considering the role of coexisting phases. Moreover, trace element data of samples from the Aar Massif are compared to metamorphic host-rock epidotes and cleft epidotes from the same massif.We find that REE patterns of Aar Massif vein epidotes are clearly different than those of metamorphic host-rock epidotes and of cleft epidotes. In addition, REE patterns vary based on the microstructural characteristics of veins. Overall REE patterns of the Albula Pass vein epidotes resemble those from the Aar Massif. Different veins and microstructures define clusters in Sr vs. Y, Eu anomaly vs. Th/U ratios, and Eu anomaly vs. U values. Geochemical heterogeneities are observed among sampling localities within the Aar Massif.The fact that the geochemical characteristics of retrograde hydrothermal vein epidotes are clearly different than those of high-grade metamorphic and metamorphic host-rock epidotes, and the relationship between geochemical characteristics and microstructures support the hypothesis that the deformation did not alter the original geochemical record through neomineralization. Our data argue for the potential of epidote as a powerful fluid tracer in the granitoid continental crust.

Creep cavities are increasingly recognized as an important syn-kinematic feature of shear zones, but much about this porosity needs investigation. Largely, observations of creep cavities are restricted to very fine grained mature ultramylonites, and it is unclear when they developed during deformation. Specifically, a question that needs testing is should grain size reduction during deformation produce creep cavities? To this end, we have reanalyzed the microstructure of a large shear strain laboratory experiment that captures grain size change by dynamic recrystallization during mylonitization. We find that the experiment does contain creep cavities. Using a combination of scanning electron microscopy and spatial point statistics, we show that creep cavities emerge with, and because of, subgrain rotation recrystallization during ultramylonite formation. As dynamic recrystallization is ubiquitous in natural shear zones, this observation has important implications for the interpretation of concepts such as the Goetze criterion, paleopiezometery, and phase mixing.

Groundwater flow in granitic bedrock is of major interest for underground projects such as radioactive waste disposal. It is generally accepted that granitic rocks of the upper crust are characterized as faulted low-porosity rocks showing fault-related permeability. In this study, the influence of existing faults on the present-day water flow in the Grimsel Test Site (Switzerland), an underground rock laboratory situated in granitoid rocks, was investigated by mapping water discharges. As a result, the link between water flow and faults considering slip-tendency analysis and fault intersections is evaluated. Water-conducting features were combined in a structural-permeability favorability map. Faults and dykes occur as three orientation groups, NE–SW, E–W, and NW–SE trending, all steeply dipping southwards with fault intersections also steeply plunging southwards. In total, 100 water discharges were mapped in summer 2014 and 85 in winter 2016, which are located along faults or fault intersections. A comparison of water discharges with structures showed that high-slip-tendency metabasic dykes and fault or dyke intersections represent the dominant flow paths. Further, it could be demonstrated that higher slip-tendency tends to lead to enhanced average hydraulic conductivity and therefore more constant water flow. Based on water fluxes, fault intersections are inferred to represent first-order locations of water percolation followed by high-slip-tendency metabasic dykes. The combination of all water-conducting features into a structural-permeability map results in covering all water discharges. Therefore, the structural-permeability favorability map can serve as suitable representation for constraining water inflow in fractured granitoid host rocks.

International audience; Low-temperature chronometers offer potential to gain insights into the temporal evolution of hydrothermal systems. The long-lived fault-bound Grimsel pass hydrothermal system (including a fossil and an active part) in the Central European Alps serves here as a key site to test such an application. Zircon and apatite grains were separated from samples collected along a fault transect. The resulting zircon (U-Th)/He ages are homogenous along the profile at 8-9 Ma and thus record the regional cooling evolution, remaining unaffected by the younger hydrothermal activity. In contrast, the apatite (U-Th)/He ages show three age groups: One Group (1) of ca. 5 Ma inside and outside the hydrothermal zone matches the low-temperature part of the regional cooling trend, while group (2) with ages as young as 1-2 Ma occurs in a central narrow zone associated with hydrothermal activity. One sample (group 3) displays older apparent ages compared to the regional cooling trend. Group (2) apatite samples reveal a different cathodoluminescence texture and trace-element chemistry, which we interpret together with the young age as apatite growth or re-crystallization within the hydrothermal system. Forward 1D modelling of He diffusion indicates that apatite (U-Th)/He ages should always be reset when exposed to hot thermal waters (up to ~140 • C) present over ka timescales or to intermediate temperature waters (~90 • C) over Ma timescales. Combining our measured apatite (U-Th)/He ages with forward modelling results highlight that, besides regional cooling trends, local heat anomalies within hydrothermal zones are very variable in space and time. Combined trace-element geochemistry and (U-Th)/He dating shows local occurrence of newly-formed apatites crystals, which are best described as geochronometers rather than thermochronometers. Such information is important to explore the longevity of hydrothermal systems and associated spatial distributions of heat anomalies.

Solid earth 8(5), 987 - 1002 (2017). doi:10.5194/se-8-987-2017, Published by Copernicus Publ., Göttingen

Detrital garnet is a promising candidate to reliably fingerprint sediment sources in the Alps, which has so far been complicated by the wide range and similarity of some of the lithologies. Garnet is present in most Alpine sediments, is easy to identify, is fairly stable and, most importantly, reflects the type and the metamorphic grade of its source rock in its chemical composition. This study aims to establish fingerprints based on detrital garnet composition for the most important tectonic units of the Central Alps, including European, Penninic and Adriatic basement rocks and their respective metasedimentary covers. Sediments collected from modern rivers, which drain representative portions of the individual tectonic units, contain a natural mixture of the various garnet populations present in each unit. We selected six catchments in southwestern Switzerland draining the External massifs, Helvetic sediments and the Penninic nappe stack at the transition of Alpine greenschist- to amphibolite-facies metamorphism in order to test the variability of Alpine garnets and the role of inherited (pre-Alpine) garnets. Extraordinary grossular- and spessartine-rich garnets of the External massifs, which experienced greenschist facies metamorphism, are clearly distinguishable from generally almandine-rich garnets supplied by the higher-grade metamorphic Penninic nappe stack. The variable pyrope, grossular and spessartine components of these almandine-rich garnets can be used to further distinguish pre-Alpine, Alpine eclogite-facies and low-grade metasedimentary garnets. This provenance proxy has the potential to be used for reconstructing sediment sources, transport and dispersal patterns in a variety of settings throughout the Alpine sedimentary record.

Th-Pb age dating of zoned hydrothermal monazite from alpine-type fissures/clefts is a powerful tool for constraining polyphase deformation at temperatures below 350°C and presents an alternative to K/Ar and 40Ar/39Ar dating techniques for dating brittle tectonics. This study considers the relationship between cleft orientations in ductile shear zones and cleft mineral crystallization during subsequent brittle overprinting. In the Grimsel area, located in the Aar Massif of the Central Alps, horizontal clefts formed during a primary thrust dominated deformation, while younger and vertically oriented clefts developed during secondary strike-slip movements. The change is due to a switch in orientation between the principal stress axes σ2 and σ3. The transition is associated with monazite crystallization and chloritization of biotite at around 11.5 Ma. Quartz fluid inclusion data allow a link between deformation stages and temperatures to be established and indicate that primary monazite crystallization occurred in both cleft systems at 300–350°C. While cleft monazite crystallization ceases at ~11 Ma in inactive shear zones, monazite growth, and/or dissolution-reprecipitation continues under brittle deformation conditions in vertical clefts during later deformation until ~7 Ma. This younger shear zone activity occurs in association with dextral strike-slip movement of the Rhone-Simplon fault system. With the exception of varying Th/U values correlated with the degree of oxidation, there is only limited compositional variation in the studied cleft monazites.

Thorium-Pb crystallization ages of hydrothermal monazites from the western, central and eastern Tauern Window provide new insights into Cenozoic tectonic evolution of the Tauern metamorphic dome. Growth domain crystallization ages range from 22.3 ± 0.6 Ma to 7.7 ± 0.9 Ma. Three major periods of monazite growth are recorded between ~ 22–19 (peak at 21 Ma), 19–15 (major peak at 17 Ma) and 13–8 Ma (major peaks at 12, 10 and 8 Ma), respectively interpreted to be related 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 tracks data. Besides tracking the thermal evolution of the Tauern dome, monazite dates reflect episodic tectonic movement along major shear zones that took place during the formation of the dome. Geochronological and structural data from the Pfitschtal area in the western Tauern Window show the existence of two cleft generations separated in time by 4 Ma and related to strike-slip to oblique-slip faulting. Moreover, these two phases overprint earlier phases of fissure formation.

This study investigates strain distribution in granitoid rocks formerly in the middle crust in the Central Aar massif, Switzerland and places the deformation behavior in the tectonic framework of the Alpine orogeny. Strain is heterogeneously distributed in terms of strain partitioning forming several hundreds of closely spaced shear zones (SZ) (>80 SZ/km with SZ thicknesses The majority of the steep shear zones preferentially accommodated upward motion by the southern block leading to an increase in peak metamorphic conditions from 250 °C in the North to 450 °C in the South of the Aar massif. The shear zones initiated at about 18–20 km depths during a stage of crustal thickening (Handegg phase). Subsequent deformation reactivated some shear zones with a gradual transition from reverse dip-slip over oblique-slip to strike-slip shear zones under local transpressional conditions (Oberaar phase).

We present structural observations from foreland basin sediments that were incorporated into the orogenic wedge of the central European Alps during early stages of continental collision. Our analysis focuses on the prograde evolution and considers the full history of the sediments ranging from their deposition in the basin to deep burial and metamorphism at temperatures of ~320°C. The tectonic evolution is matched with constraints on the diagenetic alteration of the sediments. For this purpose, we calculate the temperatures and depths of sediment compaction and illitization as well as the associated fluid liberation. The data set highlights that the tectonic incorporation of the sediments into the orogenic wedge was strongly controlled by their diagenetic state. Earliest deformation took place during imbrication and frontal accretion of unconsolidated and fluid-saturated sediments. Ductile folding of the sediments occurred already at this stage and was assisted by particulate flow. With the progressive consolidation of the sediments the elastic strength increased, which resulted in an overall embrittlement. This rheological change is recorded by the onset of out-of-sequence thrusting, brittle faulting, and the formation of massive quartz-calcite veins, which took place in the approximate temperature range of the seismogenic zone (i.e., ~150–350°C). Moreover, widespread pressure solution resulted in the formation of a penetrative cleavage and records slow but long-lasting deformation at low background strain rates. In summary, the prograde tectonic evolution of the frontal Alpine wedge exhibits many similarities with the structural and mechanical evolution of accretionary wedges at active subduction zones.

Dolomitic marble on the island of Naxos was deformed at variable temperatures ranging from 390 °C to >700 °C. Microstructural investigations indicate two end-member of deformation mechanisms: (1) Diffusion creep processes associated with small grain sizes and weak or no CPO ( c rystallographic p referred o rientation), whereas (2) dislocation creep processes are related with larger grain sizes and strong CPO. The change between these mechanisms depends on grain size and temperature. Therefore, sample with dislocation and diffusion creep microstructures and CPO occur at intermediate temperatures in relative pure dolomite samples. The measured dolomite grain size ranges from 3 to 940 μm. Grain sizes at Tmax >450 °C show an Arrhenius type evolution reflecting the stabilized grain size in deformed and relative pure dolomite. The stabilized grain size is five times smaller than that of calcite at the same temperature and shows the same Arrhenius-type evolution. In addition, the effect of second phase particle influences the grain size evolution, comparable with calcite. Calcite/dolomite mixtures are also characterized by the same difference in grain size, but recrystallization mechanism including chemical recrystallization induced by deformation may contribute to apparent non-temperature equilibrated Mg-content in calcite.

One–component, cementitious, flexible waterproofing membranes bridge cracks because their polymer-dominated matrix takes up the deformation. The sensitivity of the deformation of polymers to temperature and strain rate raises interest to the influence of these two parameters on the crack–bridging ability of the waterproofing membranes. Temperature–controlled crack–bridging and dog-bone experiments with in–situ monitoring of the sample's surfaces show a decrease in the crack–bridging ability (CBA) of the waterproofing membranes and a decrease in plastic deformation at testing temperatures above the glass transition temperature (Tg) of the used polymer. At temperatures below the Tg, the membranes deform in a brittle manner. Rate–controlled crack–bridging and dog-bone tests with in–situ monitoring of the sample's surfaces reveal a decrease in crack–bridging ability and plastic deformation with decreasing displacement rate. Hence, temperature and displacement rate are crucial parameters governing the crack-bridging performance of waterproofing membranes.

Zoned monazite-(Ce) from Alpine fissures/clefts is used to gain new insights into the exhumation history of the Central Alpine Lepontine metamorphic dome, and timing of deformation along the Rhone-Simplon fault zone on the dome's western termination. These hydrothermal monazites-(Ce) directly date deformation and changes in physiochemical conditions through crystallization ages, in contrast to commonly employed cooling-based methods. The 480 SIMS measurement ages from 20 individual crystals record ages over a time interval between ~ 19 and 5 Ma, with individual grains recording ages over a lifetime of 2 to 7.5 Ma. The age range combined with age distribution and internal crystal structure help to distinguish between areas whose deformational history was dominated by distinct tectonic events or continuous exhumation. The combination of this age data with geometrical considerations and spatial distribution give a more precise exhumation/cooling history for the area. In the east and south of the study region, the units underwent monazite-(Ce) growth at 19–12.5 and 16.5–10.5 Ma, followed by a central group of monazite-(Ce) ages at 15–10 Ma and the movements and related cleft monazites-(Ce) are youngest at the western border with 13–7 Ma. A last phase around 8–7 Ma is limited to clefts of the Simplon normal fault and related strike slip faults as the Rhone and Rhine-Rhone faults. The large data-set spread over significant metamorphic structures shows that the opening of clefts, fluid flow and monazite-(Ce) stability is direct linked to the geodynamic evolution in space and time.