The Greater Himalayan Sequence (GHS) is the main metamorphic unit of the Himalayan belt, stretching for ~ 2400 km, bounded to the South by the Main Central Thrust (MCT) and to the North by the South Tibetan Detachment (STD) whose contemporanous activity controlled its exhumation between 23 and 17 Ma (Godin et al., 2006). Several shear zones and/or faults have been recognized within the GHS, usually regarded as out of sequence thrusts. Recent multitechniques investigations in the GHS in Central-Eastern Himalaya allowed the Authors to identify two different levels of tectonic and metamorphic discontinuities, above the MCT, both characterized by a top-to-the SW sense of shear (Carosi et al., 2010; Montomoli et al., 2013; Montomoli et al., 2014, Iaccarino et al, 2015): a higher (Kalopani and Tiyar shear zones) and a lower tectono-metamorphic discontinuity (Higher Himalayan Discontinuity: Montomoli et al., 2014). U-Th-Pb in situ monazite ages provide ages of initiation at ~ 36 Ma of the upper tectonic and metamorphic discontinuity and at 26-24 Ma for the lower one, continuing up to ~ 17 Ma. Data on the P and T evolution testify that these shear zones affected the tectono-metamorphic evolution of the belt and different P and T conditions have been recorded in the hanging-wall and footwall of the discontinuities. The GHS is consequently divided into three tectonic sub-units and peak metamorphism were reached in different times in each sub-unit. When the mid unit underwent prograde meamorphism the upper one underwent exhumation by activation of the upper discontinuity. When mid unit was exhumed by activation of the lower discontinuity the lower one underwent prograde metamorphism. All the GHS underwent exhumation during the activation of the MCT. The actual proposed models of exhumation of the GHS, based exclusively on the MCT and STD activities and considering the GHS as a single tectonic unit, are not able to explain the occurrence of the Higher Himalayan Discontinuity and other older in-sequence shear zones. Any model of the tectonic and metamorphic evolution of the GHS should account for the occurrence of the tectonic and metamorphic discontinuities within the GHS and its consequences on the metamorphic path. The channel flow model proposed by Beaumont et al. (2001) implies a coherent exhumation of the GHS due to the focused monsoonal erosion acting on the southern flank of the Himalayas. However, channel flow model is not consistent with the occurrence of the several tectono-metamorphic discontinuites within the GHS, indicating a sequential exhumation of different GHS sub-units (Montomoli et al., 2013, 2014; Iaccarino et al., 2015). We have numerically simulated the interaction between surface and exhumation processes in post-subduction collisional orogens. Two kinds of collisional orogens are obtained: asymmetric and symmetric ones. Asymmetric collisional orogens feature the occurrence of a channel flow-like behaviour, high crustal temperatures and exhumation of partially molten rocks, while symmetric orogens features lower crustal temperatures and different patterns of exhumation. The asymmetric models have been chosen to test the influence of surface processes. By taking a fixed value of focused erosion rate, the timing of the focused erosion and sedimentation rate are independently varied. The results give several insights: 1) focused erosion is not a necessary condition for the exhumation of the partially molten rocks; 2) timing of the focused erosion controls the dynamics of the channel flow and, consequently, the exhumation processes; 3) the sedimentation rate may induce first order modifications on the evolution of the orogeny; 4) sequential exhumation of high grade metamorphic sub-units is achieved when the focused erosion is activated at the final stage of the collision.
Tectonic and metamorphic discontinuities in the Freater Himalayan Sequence in Central Himalaya: insights from natural examples and numerical modelling
MONTOMOLI, CHIARA;IACCARINO, SALVATORE;
2015-01-01
Abstract
The Greater Himalayan Sequence (GHS) is the main metamorphic unit of the Himalayan belt, stretching for ~ 2400 km, bounded to the South by the Main Central Thrust (MCT) and to the North by the South Tibetan Detachment (STD) whose contemporanous activity controlled its exhumation between 23 and 17 Ma (Godin et al., 2006). Several shear zones and/or faults have been recognized within the GHS, usually regarded as out of sequence thrusts. Recent multitechniques investigations in the GHS in Central-Eastern Himalaya allowed the Authors to identify two different levels of tectonic and metamorphic discontinuities, above the MCT, both characterized by a top-to-the SW sense of shear (Carosi et al., 2010; Montomoli et al., 2013; Montomoli et al., 2014, Iaccarino et al, 2015): a higher (Kalopani and Tiyar shear zones) and a lower tectono-metamorphic discontinuity (Higher Himalayan Discontinuity: Montomoli et al., 2014). U-Th-Pb in situ monazite ages provide ages of initiation at ~ 36 Ma of the upper tectonic and metamorphic discontinuity and at 26-24 Ma for the lower one, continuing up to ~ 17 Ma. Data on the P and T evolution testify that these shear zones affected the tectono-metamorphic evolution of the belt and different P and T conditions have been recorded in the hanging-wall and footwall of the discontinuities. The GHS is consequently divided into three tectonic sub-units and peak metamorphism were reached in different times in each sub-unit. When the mid unit underwent prograde meamorphism the upper one underwent exhumation by activation of the upper discontinuity. When mid unit was exhumed by activation of the lower discontinuity the lower one underwent prograde metamorphism. All the GHS underwent exhumation during the activation of the MCT. The actual proposed models of exhumation of the GHS, based exclusively on the MCT and STD activities and considering the GHS as a single tectonic unit, are not able to explain the occurrence of the Higher Himalayan Discontinuity and other older in-sequence shear zones. Any model of the tectonic and metamorphic evolution of the GHS should account for the occurrence of the tectonic and metamorphic discontinuities within the GHS and its consequences on the metamorphic path. The channel flow model proposed by Beaumont et al. (2001) implies a coherent exhumation of the GHS due to the focused monsoonal erosion acting on the southern flank of the Himalayas. However, channel flow model is not consistent with the occurrence of the several tectono-metamorphic discontinuites within the GHS, indicating a sequential exhumation of different GHS sub-units (Montomoli et al., 2013, 2014; Iaccarino et al., 2015). We have numerically simulated the interaction between surface and exhumation processes in post-subduction collisional orogens. Two kinds of collisional orogens are obtained: asymmetric and symmetric ones. Asymmetric collisional orogens feature the occurrence of a channel flow-like behaviour, high crustal temperatures and exhumation of partially molten rocks, while symmetric orogens features lower crustal temperatures and different patterns of exhumation. The asymmetric models have been chosen to test the influence of surface processes. By taking a fixed value of focused erosion rate, the timing of the focused erosion and sedimentation rate are independently varied. The results give several insights: 1) focused erosion is not a necessary condition for the exhumation of the partially molten rocks; 2) timing of the focused erosion controls the dynamics of the channel flow and, consequently, the exhumation processes; 3) the sedimentation rate may induce first order modifications on the evolution of the orogeny; 4) sequential exhumation of high grade metamorphic sub-units is achieved when the focused erosion is activated at the final stage of the collision.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
