The Himalaya is commonly regarded as a cylindrical belt from west to east due to the impressive lateral continuity of the main litho-tectonic units and faults/shear zones, which is a peculiar feature of this mountain range. The exhumation of the metamorphic core of the belt, the Greater Himalayan Sequence (GHS), was favored by two regional scale opposite-kinematics ductile to brittle shear zones: the contractional Main Central Thrust zone (MCTz) at the bottom and the normal-sense South Tibetan Detachment System (STDS) at the top of the GHS itself. In this thesis, I investigated the structural and geochronological evolution of the STDS and the MCTz, with a particular focus on the latter, in two transects in the Indian Western Himalaya: the Alaknanda – Dhauli Ganga Valleys and the Bhagirathi – Gangotri Valley in the Garhwal region. To this aim, I used a multidisciplinary approach, which combines microstructural, chemical and geochronological studies, as well as a kinematic estimate. As micas are ubiquitous in strongly deformed shear zones, the 40Ar/39Ar geochronological method on biotite and muscovite has commonly been employed to constrain the ages of mylonitization. The 40Ar/39Ar step-heating approach, the most useful for the present study, is a key procedure to unravel petrological and chemical complexities because of the recognition of different ages due to different steps of Ar release characterized by different Cl/K and Ca/K ratios. I applied this method, combined with a new methodological approach first developed in detail during this Ph.D. thesis work, to rocks coming from the bounding shear zones of the GHS. This new procedure, named Ar Differential Release Plot (DRP), that allows to clearly identify the influence of the co-existence of phyllosilicates on the trend of Ar release during the heating steps, allowing to select the optimal steps corresponding to the degassing of micas sensu stricto, which leads to more reliable age determinations in such metamorphic polydeformed rocks. The results of 40Ar/39Ar method support the diachroneity of the MCTz and the STDS in the studied areas, suggesting that the models of exhumation of the GHS should account for their lack of contemporaneity. Moreover, a new three-dimensional approach based on the X-ray micro Computed Tomography (microCT) has been developed and applied for the study of the kinematic vorticity using the stable porphyroclasts method. A close examination of literature data shows that our study approach in the study of kinematic vorticity using the microCT has never been applied before. This method has been applied to MCTz rocks from the Bhagirathi valley and the results suggest that the exhumation of the GHS has been affected by a shift of deformation within the MCTz from simple to pure shear dominated flow, coherent with literature data all along the belt. In summary, combining meso‐ and micro‐structural studies as well as chemical analyses and 40Ar/39Ar geochronology, my thesis results points out how MCTz and STDS are not coeval in the studied structural transects and that the MCTz shows a shift in deformation regime during its evolution.

L’Himalaya è classicamente considerata una catena orogenica strutturalmente cilindrica per l’impressionante continuità laterale, da ovest ad est, delle principali unità lito-tettoniche e zone di taglio, caratteristica peculiare di questa catena collisionale. L’esumazione del cuore metamorfico della catena, il Greater Himalayan Sequence (GHS), è favorita dall’attività di due zone di taglio regionali a cinematica opposta: la Main Central Thrust zone (MCTz) a cinematica compressiva e il South Tibetan Detachment System (STDS) a cinematica normale, rispettivamente alla base e al tetto del GHS stesso. In questa tesi ho studiato l’evoluzione strutturale e geocronologica del STDS e della MCTz, con particolare focus su quest’ultima, in due transetti nell’Himalaya indiana occidentale: le valli dell’Alaknanda – Dhauli Ganga e la valle del Bhagirathi – Gangotri nella regione del Garhwal. A questo scopo, ho adottato un approccio multidisciplinare che combina studi microstrutturali, chimici e geocronologici e stime di vorticità cinematica. Poiché le miche sono ubiquitarie nelle zone di taglio, il metodo geocronologico 40Ar/39Ar su biotitie e muscovite è stato ampiamente utilizzato in passato e viene utilizzato tutt’oggi per vincolare l’età della deformazione per taglio. Il metodo 40Ar/39Ar step-heating, il più adatto per questo tipo di studi, è un metodo chiave per risolvere complessità petrologiche e chimiche grazie al riconoscimento di età differenti dovute a un differente rilascio dell’Ar caratterizzato da diversi rapporti Cl/K e Ca/K. Ho applicato questo metodo, combinato con una nuova procedura, l’Ar Differential Release Plot (DRP), che permette di identificare chiaramente l’influenza della coesistenza di fillosilicati nel trend di rilascio dell’Ar durante gli step di riscaldamento, permettendo di selezionare gli step ottimali che corrispondono al degassamento delle miche in senso stretto. Questa nuova procedura permette di determinare l’età in modo molto più accurato in rocce polideformate come quelle studiate. I risultati ottenuti con il metodo 40Ar/39Ar supportano la diacronicità della MCTz e del STDS nelle aree studiate, suggerendo che i modelli di esumazione del GHS dovrebbero tener conto della mancanza di contemporaneità tra queste due strutture. Inoltre, è stato sviluppato un nuovo approccio tridimensionale basato sulla X-ray micro Computed Tomography (microCT) e applicato per lo studio della vorticità cinematica usando il metodo dei porfiroclasti stabili. Un esame dei dati di letteratura ha dimostrato che il nostro approccio nello studio di vorticità usando il metodo dei porfiroclasti stabili non era mai stato applicato prima. Questo metodo è stato applicato a rocce provenienti dalla MCTz nella valle del Bhagirathi e i risultati suggeriscono che l’esumazione del GHS sia stata guidata da una variazione di deformazione all’interno della MCTz da un deformazione dominata da taglio semplice a cui è seguita una deformazione dominata da taglio puro, coerentemente con i dati di letteratura lungo la catena. Riassumendo, combinando studi meso e microstrutturali con analisi chimiche e geocronologia 40Ar/39Ar, i risultati della mia tesi indicano che la MCTz e il STDS non sono coevi nei transetti studiati e che la MCTz mostra una variazione di regime deformativo durante la sua evoluzione.

(2020). Geochronology and kinematics of crustal scale shear zones in the Himalayan collisional belt. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2020).

Geochronology and kinematics of crustal scale shear zones in the Himalayan collisional belt

MONTEMAGNI, CHIARA
2020

Abstract

The Himalaya is commonly regarded as a cylindrical belt from west to east due to the impressive lateral continuity of the main litho-tectonic units and faults/shear zones, which is a peculiar feature of this mountain range. The exhumation of the metamorphic core of the belt, the Greater Himalayan Sequence (GHS), was favored by two regional scale opposite-kinematics ductile to brittle shear zones: the contractional Main Central Thrust zone (MCTz) at the bottom and the normal-sense South Tibetan Detachment System (STDS) at the top of the GHS itself. In this thesis, I investigated the structural and geochronological evolution of the STDS and the MCTz, with a particular focus on the latter, in two transects in the Indian Western Himalaya: the Alaknanda – Dhauli Ganga Valleys and the Bhagirathi – Gangotri Valley in the Garhwal region. To this aim, I used a multidisciplinary approach, which combines microstructural, chemical and geochronological studies, as well as a kinematic estimate. As micas are ubiquitous in strongly deformed shear zones, the 40Ar/39Ar geochronological method on biotite and muscovite has commonly been employed to constrain the ages of mylonitization. The 40Ar/39Ar step-heating approach, the most useful for the present study, is a key procedure to unravel petrological and chemical complexities because of the recognition of different ages due to different steps of Ar release characterized by different Cl/K and Ca/K ratios. I applied this method, combined with a new methodological approach first developed in detail during this Ph.D. thesis work, to rocks coming from the bounding shear zones of the GHS. This new procedure, named Ar Differential Release Plot (DRP), that allows to clearly identify the influence of the co-existence of phyllosilicates on the trend of Ar release during the heating steps, allowing to select the optimal steps corresponding to the degassing of micas sensu stricto, which leads to more reliable age determinations in such metamorphic polydeformed rocks. The results of 40Ar/39Ar method support the diachroneity of the MCTz and the STDS in the studied areas, suggesting that the models of exhumation of the GHS should account for their lack of contemporaneity. Moreover, a new three-dimensional approach based on the X-ray micro Computed Tomography (microCT) has been developed and applied for the study of the kinematic vorticity using the stable porphyroclasts method. A close examination of literature data shows that our study approach in the study of kinematic vorticity using the microCT has never been applied before. This method has been applied to MCTz rocks from the Bhagirathi valley and the results suggest that the exhumation of the GHS has been affected by a shift of deformation within the MCTz from simple to pure shear dominated flow, coherent with literature data all along the belt. In summary, combining meso‐ and micro‐structural studies as well as chemical analyses and 40Ar/39Ar geochronology, my thesis results points out how MCTz and STDS are not coeval in the studied structural transects and that the MCTz shows a shift in deformation regime during its evolution.
VILLA, IGOR MARIA
MONTOMOLI, CHIARA
Himalaya; zone di taglio; metodo 40Ar/39Ar; vorticità; MCTz, STDS
Himalaya; shear zones; 40Ar/39Ar method; kinematic vorticity; MCTz, STDS
GEO/08 - GEOCHIMICA E VULCANOLOGIA
English
10-mar-2020
SCIENZE CHIMICHE, GEOLOGICHE E AMBIENTALI
32
2018/2019
open
(2020). Geochronology and kinematics of crustal scale shear zones in the Himalayan collisional belt. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2020).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/269277
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