第22回名古屋大学年代測定総合研究センターシンポジウム平成21(2009)年度報告

Field observations of the Greater Himalayan Sequence in Eastern Nepal demonstrate a ductile, highly strained package of metamorphic rocks that show extensive evidence of crustal anatexis throughout. These can be distinguished from the Lesser Himalayan sequence below by a distinct reduction in metamorphic grade, an inverted metamorphic sequence and a high strain zone corresponding to the Main Central Thrust. Metamorphic studies are combined with geochronology to demonstrate a protracted period of crustal melting followed by rapid decompression from 18.7 Ma to 15.6 Ma. A metamorphic decompression rate is quantified at c.2mm/yr during this period. This is interpreted to represent exhumation of the Greater Himalayan Sequence by a process of ductile, channelised flow from the mid-crust beneath Tibet. Below a prominent band of kyanite gneiss, previously used to locate the Main Central Thrust, but here mapped within the Greater Himalayan Sequence, partial melting is still exhibited. Here monazites are dated at 10.6 Ma. In the Lesser Himalaya below, allanites record a similar 10.1 Ma event. This implies that following channel flow during the mid-Miocene, the channel widened in the lower-Miocene to incorporate a greater structural thickness. Following these two periods of exhumation and ductile extrusion, separated in time and space, Fission Track studies indicate that much slower, erosion driven exhumation proceeded, at <1 mm/yr. This rate increases slightly in the Pliocene, most likely in response to Northern Hemisphere glaciation; no difference in exhumation is seen across the Greater Himalayan Sequence with respect to the different, earlier, phases of ductile channel flow related exhumation. These results demonstrate the episodic nature of channel flow in the Himalaya and reconcile arguments about the position of the MCT in Eastern Nepal.

Analysis of quartz c-axis fabrics and microstructures from ductily deformed rocks allows for the examination of the kinematics associated with crustal deformation. This thesis expands on the current knowledge of the kinematic evolution of the Himalayas and Scottish Caledonides, by examining samples from the Main Central Thrust (MCT) (Himalayas) and the Sgurr Beag Thrust (SBT) (Scottish Caledonides).  Metamorphic temperatures (Tm) associated above the MCT are inverted; chapter one attempts to test if deformation temperatures (Td) correlate to Tm, indicating that ductile shearing occurred during peak Tm. In the Scottish Caledonides, Td and Tm increase from foreland to hinterland, potentially indicating a right way up thermal structure;  chapter two presents Td and Tm associated with the region around the SBT. Above the MCT, quartz c-axis fabrics yield Td ranging between 500-650 "C, corresponding to the temperatures of dynamic recrystallization for subgrain rotation (SGR) and grain boundary migration (GBM). Up to 1000m above the MCT, Td and Tm are within error of each other, suggesting that shearing occurred during peak Tm; while further away from the MCT  Tm is significantly hotter than Td, suggesting that shearing continued past Tm. Deformation associated with the upper part of the Moine thrust sheet and the SBT yields quartz c-axis fabrics with Td ranging between 395-583 "C, corresponding to the regional dynamic recrystallization. Tm calculations original to this study yield pressure-temperature constraints of 4.8-5.8 kbar and 586-625 "C. Tm is within error of Td, suggesting that deformation and metamorphism were synchronous. / Master of Science

Dynamically recrystallized quartz microstructure and grainsize evolution along mid-crustal shear zones allows for the estimation of tectonic driving stresses and strain rates acting in the mid-crust. Quartz-rich tectonites from three exhumed mid-crustal shear zones, the Main Central Thrust (MCT; Sutlej valley, NW India), South Tibetan Detachment System (STDS; Rongbuk valley, S Tibet), and Moine thrust (NW Scotland), were analyzed. Deformation temperatures estimated from quartz microstructural and petrofabric thermometers indicate steep apparent thermal gradients (80–420 °C/km) across 0.5–2.3 km thick sample transects across each shear zone. Quartz recrystallization microstructures evolve from transitional bulging/sub-grain rotation to dominant grain boundary migration at ~ 200 m structural distance as traced away from each shear zone. Optically measured quartz grainsizes increase from ~ 30 μm nearest the shear zones to 120+ μm at the largest structural distances. First-order Zener space analysis across the Moine nappe suggests strong phyllosilicate control on recrystallized quartz grainsize. Recrystallized quartz grainsize piezometry indicates that differential stress levels sharply decrease away from the shear zones from ~ 35 MPa to 10 MPa at ~ 200 m structural distance. Strain rates estimated with quartz dislocation creep flow laws are tectonically reasonable, between 10⁻¹² – 10⁻¹⁴ s⁻¹. Traced towards each shear zone strain rate estimates first decrease one order of magnitude before rapidly increasing one to two orders of magnitude at structural distances of ~ 200 m. This kinked strain rate profile is likely due to the steep apparent thermal gradients and relatively constant differential stress levels at large structural distances. / Master of Science

L'influence de l'érosion sur la localisation de la déformation dans une chaîne de montagnes est un phénomène souvent envisagé à la suite de modélisations numériques. Or les données géologiques pertinentes en faveur de cette hypothèse sont encore fort peu nombreuses. Aussi, la mise en évidence d'une évolution temporelle et spatiale de la déformation constitue une observable clef pour tester les relations érosion/localisation de la déformation. Nous testons cet effet, sur un objet géologique soumis à des conditions de déplacements aux limites simples et sur un objet géologique soumis à des conditions climatiques et érosives variables latéralement : le flanc sud de l'Himalaya (située au dessus d'un décollement crustal majeur). Elle est soumise à une convergence continue et de valeur constante depuis au moins une dizaine de millions d'années et sa rhéologie est invariante au cours du temps ; elle est en revanche soumise à un gradient climatique d'est en ouest (transversalement par rapport à la direction de convergence), gradient de plus variable au cours du temps. Nous avons mis en oeuvre des techniques de thermochronologie basse température pour consituer une base de données conséquente, que nous avons utilisée dans des modélisations numériques thermo-cinématiques directes et inverses. Nous montrons que 1) au Népal central,le MHT présente une rampe crustale prononcée, et aucun mécanisme de chevauchement en hors-séquence n'est nécessaire pour expliquer les données, 2) la géométrie du MHT varie d'est en ouest, avec une rampe moins prononcée dans l'est de la chaîne, et 3) les variations latérales en terme de mise en place et de cinétique du MFT sont peu importantes.