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Permo-Triassic geodynamics at the Gondwana-Laurasia boundary: interplay between magmatism and tectonics 

The Gondwana-Laurasia boundary was subjected to a complex geodynamic evolution between Late Paleozoic and Early Mesozoic, typified by multiple magmatic cycles developed under different tectonic and thermal regimes. A variety of mantle sources was involved in these tectonic, magmatic and metamorphic events, which induced significant modifications of the continental crust.
In the last decade, detailed studies in petrology, tectonics and stratigraphy have contributed on shedding light on the articulated evolution of this area, stimulating an intense debate about the overall Permo-Triassic geodynamic framework.
A multidisciplinary session is proposed to assess and discuss the recent advancements that contribute to draw an accurate geodynamic picture of this pivotal sector of the Pangea realm in the time span between the Variscan orogeny and the Late Triassic onset of rifting in the Central Atlantic-Alpine Tethys domain. Researches from a broad range of disciplines, such as (but not limited to) petrology/geochemistry, tectonics, geochronology, stratigraphy and basin analysis, are welcome.

Co-organized by GMPV11/TS1
Convener: Federico Casetta | Co-conveners: Zanetti Alberto, Peter Brack, Piero Gianolla, Benoît PetriECSECS
vPICO presentations
| Wed, 28 Apr, 11:45–12:30 (CEST)

Session assets

Session summary

vPICO presentations: Wed, 28 Apr

Chairpersons: Federico Casetta, Zanetti Alberto, Piero Gianolla
11:45–11:47
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EGU21-6547
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ECS
Mario Buehler, Roger Zurbriggen, Alfons Berger, Marco Herwegh, and Daniela Rubatto

Many pre‐Mesozoic basements of the Alpine belt contain kilometre‐scaled folds with steeply inclined axial planes and fold axes. Those structures are referred to as Schlingen folds. They deform polymetamorphic gneisses, often Late‐Ordovician metagranitoids and are cross‐cut themselves by Permian intrusions. However, the structural evolution of such Schlingen is still not completely understood and their geodynamic significance for the Variscan evolution is not clear. To close this gap, this study investigates in detail a well-preserved Schlingen structure in the Gotthard nappe (Central Swiss Alps). This Schlingen fold evolved by a combination of shearing and folding under amphibolite facies conditions. Detailed digital field mapping coupled with petrological and structural investigations reveal local synkinematic migmatisation in the fold hinges parallel to axial planes. U‐Pb dating of zircons separated from associated leucosomes reveal cores that record a detrital country rock age of 450 ± 3 Ma, and rims with a range of dates from 270 to 330 Ma. The main cluster defines an age of 316 ± 4 Ma. We ascribe this Late‐Carboniferous age to peak metamorphic conditions of the late‐Variscan Schlingen phase.

The pre-Schlingen structures are subdivided into three older deformation events, which are connected to the Cenerian and post-Cenerian deformations. In addition, until now unknown, post Schlingen-, but pre-Alpine transpressional deformation have been detected and described. This superimposed deformation produced locally a low-grade foliation and minor undulation of the Schlingen structures.

The detail data of the investigated fold structures are linked with already described Schlingen folds in the wider Alpine realm, which all are concentrated in the most southern parts of the Variscides. From a geodynamic point of view and based on the new tectono-metamorphic constraints, we propose Schlingen formation preceded and concurred the crustal-scale transpressional tectonics of the East Variscan Shear Zone. This scenario separates, at least in a structural sense, the Southern Variscides from more northern parts (also Gondwana derived) inside Pangea, where Schlingen folds are absent.

How to cite: Buehler, M., Zurbriggen, R., Berger, A., Herwegh, M., and Rubatto, D.: Late Carboniferous Schlingen in the Gotthard nappe (Central Alps) and their relation to the Variscan evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6547, https://doi.org/10.5194/egusphere-egu21-6547, 2021.

11:47–11:49
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EGU21-3891
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ECS
Sofia Locchi, Stefano Zanchetta, Marilena Moroni, and Andrea Zanchi

At the end of the Variscan orogeny, several episodes of crustal extension starting in the Early Permian occurred in central Southern Alps (cSA), affecting the Adria passive margin (Handy et al., 1999). During this period, a megashear zone with dextral kinematics led to the transition from Pangea A to Pangea B configuration (Muttoni et al., 2003). The transtensional to extensional deformation regime led to the development of intra-continental basins infilled by Upper Carboniferous to Lower Permian sedimentary successions (Cadel et al., 1996). Crustal shortening related to Alpine compression was responsible for a partial or complete inversion of favourably oriented normal faults inherited from the Permian tectonics (Blom & Passchier, 1997). Despite this, SSE-dipping Early Permian Low-Angle Normal Faults (LANFs) are well-preserved because they exceptionally escaped most of the Alpine deformations. Their surfaces are within the Lower Permian sedimentary cover, or at the interface between the sedimentary cover and the Variscan basement, passing to intra-basement shear zones.
Two major Permian LANFs (Aga-Vedello and Masoni faults) are recorded in the Pizzo del Diavolo Fm. along the northern border of the Permian Orobic Basin (N Italy). They are “non-Andersonian” normal faults whose surfaces are characterized by cataclastic bands usually sealed by centimetric to metric layers of dark grey to black aphanitic tourmalinites (Zanchi et al., 2019). Tourmalinites indicate fluids circulation channelled along high permeability fault zones and are related to magmatic-hydrothermal fluids that produced metasomatic tourmalines with different compositions at different distances from the fluid source, i.e. the crystallizing intrusive bodies. In addition to Aga-Vedello and Masoni faults, further exposures of Permian LANFs occur in other sectors of the cSA and they are always associated with the presence of tourmalinites. Several authors (De Capitani et al., 1999; Slack et al., 1996; Cadel et al., 1996) link the cSA tourmalinites with the U mineralization of Novazza - Vedello district but this correlation could not be so direct and clear, due to the low concentration of Uranium in tourmalinites coming out from our whole-rock analyses.
The main purpose of this research is to better characterize the entity and the genesis of this regional hydrothermal event and relate it to the role played by the structural setting on hydrothermal circulation in intracontinental extensional settings. Fieldwork and observations combined with microstructural and geochemical analyses of tourmalinites coming from different sectors of the cSA have been performed to reach this goal.

 

Blom, J. C., & Passchier, C. W. (1997). Geologische Rundschau, 86, 627-636.
Cadel, G., et al. (1996). Memorie di Scienze Geologiche, 48, 1-53.
De Capitani, L., et al. (1999). Periodico di Mineralogia, 68, 185-212.
Handy, M., R., et al. (1999). Tectonics 18, 1154-1177.
Muttoni, G., et al. (2003). Earth Planet Science Letters, 215, 379–394.
Slack, J., F., et al. (1996). Schweiz. Mineral. Petrogr. Mitt., 76, 193-207.
Zanchi A. et al. (2019). Italian Journal of Geosciences, 138, 184-201

How to cite: Locchi, S., Zanchetta, S., Moroni, M., and Zanchi, A.: Interaction between low-angle normal faults and hydrothermal circulation during Early Permian extensional tectonic in the central Southern Alps, N Italy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3891, https://doi.org/10.5194/egusphere-egu21-3891, 2021.

11:49–11:51
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EGU21-8056
Andrea Zanchi, Sofia Locchi, and Stefano Zanchetta

The occurrence of synsedimentary tectonics during the beginning of the Permian has been largely documented all cross the present-day region of the central Southern Alps. Evidence of active faults has been generally established based on facies variations often associated to coarse-grained deposits, a characteristic feature of the Laghi Gemelli Group, which was deposited during the Early Permian. Nevertheless, poor attention has been devoted to the reconnaissance and description of the mesoscopic fault record developed during the deposition of the Lower Permian successions, except for a few works (Berra et al., 2011) describing local synsedimentary features such as liquefaction or slumping due to seismic shaking.

Working across the Orobic Alps, we identified several key areas where the occurrence of dewatering structures testify to the activity of synsedimenary faults together with sedimentary dikes, ball and pillars, and small slumps occurring along hundreds of mesoscopic faults showing meter-scale displacement along high-angle conjugate systems as well as domino-style faults, often accompanied by growth structures. These faults mainly affect the Pizzo del Diavolo Formation, which was deposited on top of the volcaniclastic succession of the Ca’ Bianca Volcanite.

According to our structural observations, these high-angle Andersonian normal faults are often associated with low-angle normal faults, which developed along the interface between the Permian cover and the Variscan basement (Bloom & Passchier, 1997; Zanchi et al., 2019). LANF systems are responsible for significant tectonic elision of the volcaniclastic lower successions and for diffuse hydrothermal circulation, resulting in widespread tourmaline deposition along the fault surfaces.

Our analyses point to the definition of tectonic setting characterized by pure extension dominated by ENE-WSW striking normal faults all across the central Southern Alps, which were later inverted during the Alpine shortening as high-angle reverse faults (Zanchetta et al., 2015). It is important to stress that in the considered area the strikes of the Early Permian structure are at odds with the Early Jurassic normal faults which generally show a N-S strike and were reactivated as strike-slip faults, pointing to an independent tectonic extensional event occurring 80 My after the Permian extension.

 

Berra F. et al. (2011). Sedimentary Geology, 235, 249-263

Blom, J. C., & Passchier, C. W. (1997). Geologische Rundschau, 86, 627-636.

Zanchetta et al. (2015). Lithosphere, 7, 662-681.

Zanchi A. et al. (2019). Italian Journal of Geosciences, 138, 184-201.

How to cite: Zanchi, A., Locchi, S., and Zanchetta, S.: Early Permian syndepositional tectonics in the Orobic Basin, Southern Alps, Italy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8056, https://doi.org/10.5194/egusphere-egu21-8056, 2021.

11:51–11:53
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EGU21-6843
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ECS
Wanli Gao and Zongxiu Wang

Abstract:The Zongwulong tectonic belt (ZTB) is located between the northern Qaidam tectonic belt and the south Qilian orogenic belt and contains Late Paleozoic and Early- Middle Triassic strata. Structural features and geochronology of Zongwulong ductile shear zone have key implications for the tectonic property of the ZTB. This study integrated field structure, microscopic structure and 40Ar/39Ar laser probe analysis. The shear zone strikes ~NEE-SWW and dips at a high angle, with a NWW-SEE trending and WE stretching lineation, indicating the shear zone as a thrust- slip shear ductile shear. The asymmetric folds, rotating porphyroclast,structural lens and crenulation cleavage can be seen in the field. Mica fish, S − C fabrics, σ type quartz porphyroclastic and quartz wire drawing structure can also be observed under microscope, indicating that the strike- slip- related ductile deformation and mylonitization occurred under low- grade greenschist facies conditions at temperatures of 300° C − 400° C.  The highly deformed
mylonite schist yielded 40Ar/39Ar ages (245.8±1.7)Ma and (238.5±2.6)Ma for muscovite and biotite, respectively, indicating that the shear deformation occurred during the Early- Mid Triassic. Combined with comprehensive analysis of regional geology and petrology, the authors hold that the age of ductile shear deformation represents the time of Triassic orogeny in the ZTB. The oroginic activity was probably related to the oblique collision between the South Qilian block and the Oulongbuluke block after the closure of the northermost Paleo-Tethys ocean.

How to cite: Gao, W. and Wang, Z.: 40Ar/ 39Ar laser dating of Zongwulong ductile shear zone in northeastern Tibetan Plateau :Constrains on the closure time of the northmost Paleo-Tethys ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6843, https://doi.org/10.5194/egusphere-egu21-6843, 2021.

11:53–11:55
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EGU21-10596
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ECS