4:15 PM - 4:30 PM
[SMP25-04] Role of phyllosilicates on slip direction reversals in Himalayan Frontal Fold Thrust Belt
Keywords:brittle-ductile transition zone, opposite shear senses, strain localization, North Almora Thrust, Himalayan Frontal Fold Thrust Belt
Collisional zones such as the Himalayas typically exhibit a sequence of fold-thrust belts with a range of deformation features, from high-temperature plastic domain features towards the central region of the collisional zone to low-temperature brittle deformation features observed in the foreland direction of the thrust. The mechanisms that facilitate the weakening and formation of large thrust faults in collisional zones, and their subsequent propagation into foreland are still not well understood. In the present study, we aim to understand the deformation mechanisms in the Main Central Thrust (MCT) of the Himalayan Frontal Fold-Thrust Belt, which was once an active subduction boundary of the Indian Plate below the Asian Plate. Furthermore, we aim to clarify the mechanism leading to the southward (foreland-ward) propagation of the active subduction boundary in the Himalayan Frontal Fold-thrust Belt.
In the present research, the mylonitised granites of the North Almora Thrust or NAT (equivalent to MCT) are studied at different scale (outcrop to micro-scale) to elucidate the deformation and weakening mechanisms. The study area shows distinct domains of degrees of mylonitisation from the NAT as ultramylonite, mylonite and protomylonite. In addition, sporadic occurrences of phyllosilicate-rich ultramylonite, classified as phyllonite, are observed in the ultramylonite (occurring near NAT). Shear-sense indicators in outcrop structures indicate a general top-to-south shear sense, but a distinct top-to-north shear sense is observed in phyllonite and phyllosilicate-rich zones in ultramylonite.
Microstructural observations indicate grain size reduction by dynamic recrystallization in quartz. Especially the quartz grains show sub-grain rotation and grain boundary migration microstructures indicating predominance of dislocation creep in the quartz grains and deformation temperatures 450–550 ℃. The feldspar has a myrmekite texture and undergoes predominantly brittle deformation with domino-style fractures. The different deformation mechanisms of the mineral phases are attributed to an increase in grain-scale microcracking, fluid intrusion, and dissolution-precipitation mechanisms with subsequent formation of phyllosilicates (predominantly muscovite and biotite) in the ultramylonites. These phyllosilicates play a major role in the weakening and subsequent formation of the North Almora Thrust. In addition, the viscosity contrast between the quartz+plagioclase aggregates and the mica-rich bands after fluid intrusion results in localized zones of differential strain and shear direction, leading to deformation with opposite shear direction. Presence of fluids and ongoing deformation are considered to have led the authigenesis of new fined-grained quartz and phyllosilicates minerals as observed in the pressure shadow zones of coarse feldspar grains (porphyroclasts). The stress build-up and subsequent weakening behavior can be further extended to the thrust propagation mechanism in the Himalayan Frontal Fold-Thrust belt (a continent-continent collision zone), where once a large-scale fault has been formed, the continued subduction of the Indian-plate, after weak plane development, results in foreland-ward stress concentration, that may have led to the south-ward propagation of active large-scale thrusting.
This study highlights the role of fluid intrusion and brittle-ductile deformation mechanisms in controlling the development and movement along large-scale (regional) thrusts in collisional zones.
In the present research, the mylonitised granites of the North Almora Thrust or NAT (equivalent to MCT) are studied at different scale (outcrop to micro-scale) to elucidate the deformation and weakening mechanisms. The study area shows distinct domains of degrees of mylonitisation from the NAT as ultramylonite, mylonite and protomylonite. In addition, sporadic occurrences of phyllosilicate-rich ultramylonite, classified as phyllonite, are observed in the ultramylonite (occurring near NAT). Shear-sense indicators in outcrop structures indicate a general top-to-south shear sense, but a distinct top-to-north shear sense is observed in phyllonite and phyllosilicate-rich zones in ultramylonite.
Microstructural observations indicate grain size reduction by dynamic recrystallization in quartz. Especially the quartz grains show sub-grain rotation and grain boundary migration microstructures indicating predominance of dislocation creep in the quartz grains and deformation temperatures 450–550 ℃. The feldspar has a myrmekite texture and undergoes predominantly brittle deformation with domino-style fractures. The different deformation mechanisms of the mineral phases are attributed to an increase in grain-scale microcracking, fluid intrusion, and dissolution-precipitation mechanisms with subsequent formation of phyllosilicates (predominantly muscovite and biotite) in the ultramylonites. These phyllosilicates play a major role in the weakening and subsequent formation of the North Almora Thrust. In addition, the viscosity contrast between the quartz+plagioclase aggregates and the mica-rich bands after fluid intrusion results in localized zones of differential strain and shear direction, leading to deformation with opposite shear direction. Presence of fluids and ongoing deformation are considered to have led the authigenesis of new fined-grained quartz and phyllosilicates minerals as observed in the pressure shadow zones of coarse feldspar grains (porphyroclasts). The stress build-up and subsequent weakening behavior can be further extended to the thrust propagation mechanism in the Himalayan Frontal Fold-Thrust belt (a continent-continent collision zone), where once a large-scale fault has been formed, the continued subduction of the Indian-plate, after weak plane development, results in foreland-ward stress concentration, that may have led to the south-ward propagation of active large-scale thrusting.
This study highlights the role of fluid intrusion and brittle-ductile deformation mechanisms in controlling the development and movement along large-scale (regional) thrusts in collisional zones.