TRIASSIC-JURASSIC RIFT-RELATED DEFORMATION AND TEMPERATURE-TIME EVOLUTION OF THE FOSSIL ADRIATIC MARGIN: A REVIEW FROM OSSOLA AND STRONA DI OMEGNA VALLEYS (IVREA-VERBANO ZONE)

In the last decade, studies of rifted margins have benefited from an increasing quantity of high-quality data from several disciplines. The Ivrea-Verbano Zone (IVZ), in the Italian Southern Alps, represents a complete section of middle to lower continental crust, which records both the Variscan and subsequent Alpine Tethys rift-related tectonics (Beltrando et al., 2015; Simonetti et al., 2023).One of the most important structures is the Forno-Rosarolo shear zone (Siegesmund et al., 2008). It is a NE-SW-oriented, subvertical shear zone made of metapelites, amphibolites, calc-silicates and granulites involved in anastomosed proto- to ultra-mylonite layers enveloping weakly deformed lenses. Mylonites formation postdate Variscan metamorphism and deformation and predate Jurassic brittle fracturing and faulting.In present day orientation, the kinematic indicators point to a sinistral sense of shear. Removing the Alpine tilt at high angle of the IVZ, this kinematic points to a former extensional shear zone. Investigations on the mylonitic flow kinematic reveal a non-coaxial deformation characterized by dominant pure shear (between 70 % and 50 %) and minor simple shear. Metamorphic conditions of the wall rocks vary from the upper amphibolite (SE, footwall) to the granulite facies (NW, hanging wall). Within the mylonites, PT estimate from mineral assemblage points to amphibolite facies conditions during deformation (~650 °C and ~5.5 kbar).Such kinematic data and metamorphic conditions allow to constrain the development of the Forno-Rosarolo shear zone mylonitic deformation, together with other similar structures of the IVZ, during the intermediate phase of the Tethyan rift (Beltrando et al., 2015; Simonetti et al., 2023) known as “thinning mode” (Manatschal et al., 2007). This stage was characterized by general shear conditions (pure shear between 70 % and 50 %) suggesting a phase of transition from a symmetric to an asymmetric configuration of rift.Beltrando M., Stockli D.F., Decarlis A., Manatschal G., 2015. A crustal‐scale view at rift localization along the fossil Adriatic margin of the Alpine Tethys preserved in NW Italy. Tectonics, 34, 1927–1951. https://doi.org/10.1002/2015TC003973Manatschal G., Müntener O., Lavier L.L., Minshull T.A., Péron-Pinvidic G., 2007. Observations from the Alpine Tethys and Iberia–Newfoundland margins pertinent to the interpretation of continental breakup. Geol. Soc. Spec. Publ., 282, 291–324. https://doi.org/10.1144/SP282.14Siegesmund S., Layer P., Dunkl I., Vollbrecht A., Steenken A., Wemmer K., Ahrendt H., 2008. Exhumation and deformation history of the lower crustal section of the Valstrona di Omegna in the Ivrea Zone, southern Alps. Geol. Soc. Spec. Publ., 298, 45–68. https://doi.org/10.1144/SP298.3Simonetti M., Langone A., Bonazzi M., Corvò S., Maino M., 2023. Tectono-metamorphic evolution of a post-variscan mid-crustal shear zone in relation to the Tethyan rifting (Ivrea-Verbano Zone, Southern Alps). Journal of Structural Geology, 173, 104896. https://doi.org/10.1016/j.jsg.2023.10489

Tectono-metamorphic evolution of a post-variscan mid-crustal shear zone in relation to the Tethyan rifting (Ivrea-Verbano Zone, Southern Alps)

The Lora del Rio metamorphic core complex corresponds to the lowermost, high-grade block below a Hercynian extensional shear zone. A peculiarity of this sector is that exhumation of the metamorphic core was the result of the activity of two low-angle, approximately perpendicular shear zones: the main and the secondary shear zones, both of which are separating three structural levels with distinct tectonometamorphic imprints. The Lora del Rio metamorphic core underwent rapid exhumation due to the combined action of both extensional shear zones. The Hueznar unit, which represents the median block, shows a complex evolution whereby the highest metamorphism occurs in relation to the secondary extensional structure, although most structures appear to be controlled by the main extensional shear zone. Metamorphism and deformation within the upper block (Los Miradores unit) are controlled by the underlying units. Recognition in the Ossa-Morena zone of extensional deformation processes (dated at 340 Ma), spatially and temporally related with the convergent deformations, can help in the establishment of comparisons and correlations with other sectors of the European Hercynian foldbelt.

The collapse of the Caledonian orogen in SW Norway: Insights from quartz textures

<p>The Sierra de los Filabres mountain range in the Betics system of SE Spain is one of the best natural laboratory to investigate processes associated with nappe stacking and subsequent exhumation of metamorphic rocks during the orogenic evolution. Existing research separates the Iberia-derived high-pressure, amphibolite facies Nevado-Fillabrides complex in a lower tectonic plate position from the lower grade ALCAPECA microcontinent-derived Alpujárride complex in an upper tectonic plate position. Their nappe-stack contact is also defined as an extensional detachment that controls the exhumation of the higher grade Nevado-Fillabrides complex. We have tested this model with a detailed (micro-)structural and lithological analysis complemented by <sup>40</sup>Ar/<sup>39</sup>Ar white mica dating of key shear zones. We aim to define key shear zones that separate different tectonic units, to determine the kinematics and timing of main deformation phases, and to understand the interplay between burial and exhumation structures. The results show that shearing related to the subduction burial up to the amphibolite facies is ~ top-NW in present-day coordinates. Three amphibolite facies nappe units are distinguished, which may correspond to proximal and more distal parts of the former hyper-extended Iberian margin. The bottom and top nappes consist of continental material, while the middle nappe is largely made of mafic and ultramafic rocks. Top-NW shearing was coeval with the isoclinal and tight asymmetric folding of the formations. These structures were overprinted by upright folds and greenschist facies shear zones that still developed under compression. These contractional structures are cross-cut by ~ top-W shear zones associated with exhumation that show evidences of gradually decreasing P-T conditions during extension from ductile shearing to normal faulting. We show that the same protolith can be followed in amphibolite grade below and in low greenschists grade above the main extensional detachment. This demonstrates that the extensional detachment did not follow and reactivate exactly the former nappe contact between the Nevado-Fillabrides and Alpujárride complexes. Our single grain fusion <sup>40</sup>Ar/<sup>39</sup>Ar ages on white micas show a range of 10 to 20 Ma in case of nappe contacts or extensional shear zones, while yield a significantly older, 25-40 Ma age cluster in case of a sample far away from the main shear zones in the core of the Nevado-Fillabrides dome. This age cluster could either represent excess Ar in the sample, or resetting due to a distinct tectono-metamorphic event that occurred prior to the Early Miocene subduction of the Nevado-Fillabrides complex. The latter case would require the reconsideration of recent tectonic reconstructions of the region.</p>

Crustal modelling of the Ivrea–Verbano zone in northern Italy re-examined: Coseismic cataclasis versus extensional shear zones and sideways rotation

Current models for the Oligo-Miocene post-orogenic back-arc extension of the Aegean domain suggest that stretching is accommodated by two bivergent detachment systems of opposing shear sense. The co-existence in the Eocene of a top-to-the-S thrust at the base of the Cycladic Blueschists unit and top-to-the-N extensional shear zones at the roof raises the problem of differentiating synorogenic and post-orogenic deformations with similar directions and shear senses. Based on structural field data, this study shows that the post-orogenic deformation recorded in the Southern Cyclades is extremely asymmetric as the Cycladic Blueschists unit is pervasively affected by top-to-the-N shearing deformation distributed on four main shear zones. All activated in greenschist-facies conditions, some of these shear zones operated in the brittle regime during the final part of the exhumation. The Cycladic Blueschists/Cycladic Basement contact displays clear polyphased deformation with the preservation of top-to-the-S thrust kinematics. Thermal structure of the Cycladic Blueschists unit with regards to position of ductile shear zones was retrieved using the Raman Spectroscopy of Carbonaceous Material peak-metamorphic temperatures. This study shows a series of major metamorphic gaps accommodating an upward and stepwise decrease of more than 200 °C within the Cycladic Blueschists unit. Pressure-temperature estimates show that only lower parts of the Cycladic Blueschists unit recorded ca. 18–20 kbar for 530 °C peak-conditions. While flanking the West Cycladic Detachment System, which shows a top-to-the-S shear sense, the Southern Cyclades are dominated by a top-to-the-N non-coaxial shearing. Deformation is therefore genuinely asymmetric in the center of the Aegean domain.

Two ore zones, one enclosed within a shear zone (zone B) and the other in a subsidiary structure (zone D). At onset of the shear zone, subsidiary fracture patterns are developed in second order faults in the following sequence. At peak strength Riedel shears are formed which propagate out into the walls of the shear zones producing the zone D structure. After peak strength, restraint (P) shears are developed in the thrust attitude within the shear zone. Principal displacement shears, (D) developed toward residual strength in the direction of movement. The continuation of the shear displacement gives rise to schistosities within the main shear zone. While the main shear zone and the P and D shears within the main shear zone were in continual movement, the subsidiary zone D structure, once formed retained a simple fracture pattern and moved little in comparison to the main shear zone. Mineralizing fluids introduced into this shear environment show a similar pattern of evolution, with a wider range of homogenization and halite disappearance temperatures in the more active zone B than within zone D. Systematic variations of Ca/Na and Ca/Mg indicate migration. --Modified journal abstract.

Large extensional structures developed during emplacement of a crystalline thrust sheet: the Mondoñedo nappe (NW Spain)

From two examples of orogenic domains, some general mechanisms significant of late orogenic tectonic processes in mountain belts are characterized. The Basin and Range province and the Variscan belt in the French Massif Central have both suffered important compressional orogenic crustal thickening, and the results of late orogenic processes can be observed in the field. Both areas are covered by deep seismic profiling providing constraints on the geometry of a crust which has been restored to a normal thickness. Late orogenic features from the two domains are compared at different scales and their tectonic significance for extension mechanisms is discussed. At the scale of the orogenic domains, the most prominent tectonic features are the metamorphic core complexes (MCC) which expose deformed rocks from the middle crust generally affected by high‐temperature, low to medium pressure metamorphism, partial melting, and widespread granite emplacement. In these MCC, large‐scale extensional shear zones present an intense mylonitic deformation characterized by low dipping foliations and pervasive stretching lineations. They show a complete evolution from early deep‐seated ductile deformation (generally achieved under high‐temperature, low to medium pressure metamorphism) to a late shallow brittle stage characterized by cataclastic deformation. The late detachment stage generally controls the development of asymmetrical extensional sedimentary basins filled by continental deposits. Two main geometries of MCC are defined that are characterized by differing geometry and kinematics of low‐angle shear zones. In the first case, two low‐angle shear zones with opposite vergence develop along the flanks of a roughly symmetrical MCC (often one system is dominant over the other). The second geometry characterizes asymmetrical MCC bounded by a single normal shear zone which is upwarped during uplift and doming of the core caused by tectonic denudation. Detailed strain analysis performed in several extensional shear zones shows that the deformation regime is heterogeneous and results from general noncoaxial flow. Deformation along the shear zones evolves progressively from slight homogeneous pure shear strain to intense heterogeneous noncoaxial shear strain. Strain distribution within the lower crust is less well constrained by field observation; however, analogies between COCORP and ECORS deep seismic reflection profiles give important constraints on crustal structure. Wide zones of highly subhorizontally layered lower crust and a flat high‐amplitude reflection Moho characterize both evolved orogenic domains suggesting that major deformations and flow occur within the lower crust during extension. A kinematic model involving heterogeneous crustal deformation and regional scale flow fits relatively well with late orogenic structures observed in continental domains. A weak, hot upper mantle allows large‐scale flow of lower crust material from zones of deep ductile extension to uplifted domains of upper crustal denudation. Heterogeneous strain is accommodated by low‐angle extensional shear zones from localized zones of extension in the brittle crust to ductile lower crust. Combined pure and simple shear occurs along localized shear zones, whereas at the scale of the whole lithosphere, deformation nearly corresponds to a vertical pure shear. Such deformation processes which affect a thick and hot crust seem to be common in both compared domains suggesting that late orogenic extensional processes are slightly dependent of the type of contractional tectonics. Thus, as much in the Andean‐type west American Cordilleran belt as in the collision‐type Variscan belt, late orogenic processes produced similar extensional features.

Drilling during IODP NanTroSEIZE Expedition 316 led to the recovery of cores from the basal décollement in the frontal part of the Nankai accretionary prism and from a splay fault branching from the décollement at 25‐km landward of the prism toe. The core from the splay fault shows a main shear zone and two secondary shear zones. The main shear zone can be divided into two subzones. The upper subzone consists of a 1.2‐mm thick foliated gouge zone truncated downward by a through‐going fault encompassing a 0.4‐mm thick weakly foliated gouge interval. A nearby 200‐μm thick granular injection vein is interpreted as derived from the fault. The lower subzone consists of a foliated clayey gouge. A 70‐μm thick granular injection vein is also observed along this subzone. In the basal décollement core, microstructures consist of foliated gouge along a flat‐lying shear zone and seven flat‐lying or gently dipping secondary or incipient shear zones above. A redox front lies beneath the main shear zone. The shear zone and the redox front are truncated by a fault surface outlined by microbreccia developed at the expense of the overlying foliated gouge. Foliated gouge from the shear zones is tentatively interpreted as resulting from slow slip or aseismic creep. The weakly foliated gouge, the microbreccia, and the granular injection veins are interpreted as resulting from coseismic slip. The presence of the redox front beneath the main shear zone of the décollement fault core is interpreted as a consequence of oxidizing fluid flow along the microbreccia‐bearing fault.

Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)

Microstructural observations of shear zones in sensitive clay

We conducted structural and geochronological studies on the Hongzhen metamorphic core complex in the eastern Yangtze craton. The hanging wall of the Hongzhen metamorphic core complex consists of Neoproterozoic to Permian unmetamorphosed strata, which were folded during NW-SE shortening in the Middle Triassic, and Cretaceous terrestrial basins. The exposed Paleoproterozoic Dongling Complex in the footwall is widely overprinted by a detachment ductile shear zone, which has consistent SW-plunging mineral elongation lineations and top-to-the-SW shear indicators. Analysis of the mesostructures, microstructures, and quartz C-axis fabrics indicates that they formed within a low-angled, SW-dipping, extensional shear zone at midcrustal levels. Four muscovite grains separated from mylonites within the shear zone yielded 40 Ar/ 39 Ar plateau ages ranging from 128.5 ± 1.0 Ma to 126.1 ± 1.1 Ma. They are interpreted as cooling ages of the shear zone associated with the Hongzhen metamorphic core complex. We propose that the Hongzhen metamorphic core complex was initiated as a midcrustal, low-angle extensional shear zone with top-to-the-SW shear sense at ca. 145 Ma, and the shear zone was then warped and uplifted by the emplacement of the Hongzhen granite at 122 Ma. Brittle normal faulting and basin rifting in the hanging wall also developed during the formation of the Hongzhen metamorphic core complex, but these processes continued until the Late Cretaceous. This study of the Hongzhen metamorphic core complex demonstrates that the eastern Yangtze craton was also involved in the Early Cretaceous extension widely occurring in the eastern China continent. A NE-SW extensional direction during the Early Cretaceous is indicated by the Hongzhen metamorphic core complex.

Dating the shear zone activity remains challenging and depends on geochronometer reactivity. We investigate the Forno-Rosarolo Shear Zone (Ivrea-Verbano Zone, Italy), developed in the intermediate-low continental crust under amphibolite-facies conditions. Sheared paragneisses and calc-silicates were dated using in situ U–(Th–)Pb monazite and titanite geochronology. Three monazite generations (MNZI-III) were identified based on microstructural position, internal features, chemical zoning (Th, Y) and isotopic data. Deformation was mainly recorded by MNZII, with high-Y domains yielding Triassic dates (average ages of: 238±8 and 222±8 Ma). Rare, highly fractured or porous MNZIII grains provided younger dates (202±8 to 184±6 Ma). MNZI, abundant in protomylonites, retains regional metamorphism, linking monazite U–Th–Pb data to fabric evolution. Titanite shows different zoning features and chemistry as a function of the surrounding mineral assemblage: (i) strongly zoned grains are mostly associated with silicate-rich layers; (ii) homogeneous grains are generally within the silicate-poor layers. Both types show a decoupling between chemistry, almost completely related to the peak metamorphism, and U–Pb isotopes. Deformation microstructures promoted a total reset of the U–Pb dataset at the beginning of deformation and a subsequent volume diffusion through the grains: the innermost domains of both titanite types provide a Triassic lower intercept age (240±5 Ma) while the rims/tips, locally coinciding with high strained portions, define an alignment of isotopic data with a Jurassic lower intercept age (186±6 Ma). This study highlights how combining monazite and titanite geochronology refines the timing and duration of deformation, particularly in large-scale shear zones involving different lithologies.