Eocene Metamorphism and Anatexis in the Kathmandu Klippe, Central Nepal: Implications for Early Crustal Thickening and Initial Rise of the Himalaya
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Abstract The continental collision between India and Asia has been ongoing since early Eocene time, but the orogenic record is typically dominated by Miocene and younger deformation and metamorphism that largely overprinted earlier Eocene‐Oligocene events. This hinders our understanding of how crustal thickening responds to initial collision and when the Himalayan mountains initially rise. The advancement of spatially precise petrochronology techniques, however, has provided the means to see through the Miocene overprint and enabled the characterization of Eocene metamorphism in different parts of the Himalaya. The current study presents new monazite petrochronology and paired thermobarometry from the Kathmandu klippe in the central Nepalese Himalaya. These data reveal Eocene prograde metamorphism (44‐38 Ma) and partial melting (38‐35 Ma) under peak P‐T conditions of 730 °C–760 °C and up to 10.5 kbar. The migmatites within the Kathmandu klippe is equivalent to the Upper or Uppermost Greater Himalayan Crystallines and should have been exhumed during Eocene‐Oligocene. The new evidence of Eocene metamorphism and anatexis presented herein adds to a growing body of data detailing initial crustal thickening during the early continent collision. The mid‐Eocene crustal thickening event indicates that the Himalayan felsic crust was thickened to a depth of ∼35 km shortly within 10–20 Myr of the initial collision, which was probably responsible for the initial topographic rise of the Himalayan proto‐mountains. Characterizing the effects of this early orogenesis is critical in understanding the Himalayan architecture prior to the better‐preserved Miocene metamorphism and anatexis record and how the orogen may have been preconditioned for the younger stage.Keywords:
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The India-Asia collision is an outstanding smoking gun in the study of continental collision dynamics. How and when the continental collision occurred remains a long-standing controversy. Here we present two new paleomagnetic data sets from rocks deposited on the distal part of the Indian passive margin, which indicate that the Tethyan Himalaya terrane was situated at a paleolatitude of ∼19.4°S at ∼75 Ma and moved rapidly northward to reach a paleolatitude of ∼13.7°N at ∼61 Ma. This implies that the Tethyan Himalaya terrane rifted from India after ∼75 Ma, generating the North India Sea. We document a new two-stage continental collision, first at ∼61 Ma between the Lhasa and Tethyan Himalaya terranes, and subsequently at ∼53-48 Ma between the Tethyan Himalaya terrane and India, diachronously closing the North India Sea from west to east. Our scenario matches the history of India-Asia convergence rates and reconciles multiple lines of geologic evidence for the collision.
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Abstract. Continental collision is a crucial process in plate tectonics, whereas our understanding regarding the tectonic complexities at such convergent plate boundary remains largely unclear in terms of the evolution and the controlling parameters of its lateral heterogeneity. In this study, we conducted a series of two-dimensional numerical experiments to investigate how continental lithospheric thermal structure influences the development of lateral heterogeneity along the continental collision zone. Two end members were achieved: 1) Continuous subduction mode, which prevails when the model has a cold procontinental moho (Tmoho ≤ 450 °C). In this case, a narrow collision orogen develops, and the subducting angle steepens with the increasing retrocontinental Tmoho. 2) Continental subduction with a slab break-off, which generates a relative wide collision orogen, and dominates when the model has a relative hot procontinental Tmoho (≥ 500 °C), especially when the Tmoho ≥ 550 °C. In contrast, Hr is the second-order controlling parameter in varying the continental collision mode, while it prefers to enhance strain localization in the upper part of the continental lithosphere and promote the growth of shear zones there. By comparing the model results with geoscience observations, we suggest that the discrepant evolutionary paths from the continuous subduction underlying the Hindu Kush to the continental subduction after slab break-off beneath eastern Tibet may originate from the inherited lateral inhomogeneity of Indian lithospheric thermal structure. Besides, the high content of crustal radioactive elements may be one of the important factors that controls the formation of large thrust fault zones in the Himalayas.
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Crustal thickening in an oblique continental collision, such as in the South Island of New Zealand, necessarily involves deformation processes in three dimensions (3-D). We have investigated the role played by the strength of the lower crust using simplified 3-D mechanical models. These models show that crustal thickening occurs away from the area of maximum compression, along an axis inclined to the plate boundary (about 10°–20° to the plate boundary in the case of the South Island), and perpendicular to the convergence direction. Furthermore, the specific geometry of the relatively old and strong Australian lithosphere versus the Pacific lithosphere also controls the location of crustal thickening. These conditions could explain the observed mismatch between the locations of maximum elevation and minimum gravity in South Island, NewZealand, as a consequence of decoupled deformation owing to low-viscosity lower crust.
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Abstract Dehydration and anatexis of ultrahigh‐pressure ( UHP ) metamorphic rocks during continental collision are two key processes that have great bearing on the physicochemical properties of deeply subducted continental crust at mantle depths. Determining the time and P–T conditions at which such events take place is needed to understand subduction‐zone tectonism. A combined petrological and zirconological study of UHP metagranite from the Sulu orogen reveals differential behaviours of dehydration and anatexis between two samples from the same UHP slice. The zircon mantle domains in one sample record eclogite facies dehydration metamorphism at 236 ± 5 Ma during subduction, exhibiting low REE contents, steep MREE – HREE patterns without negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 651–750 °C and inclusions of quartz, apatite and jadeite. A second mantle domain records high‐ T anatexis at 223 ± 3 Ma during exhumation, showing high REE contents, steeper MREE – HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, high temperatures of 698–879 °C and multiphase solid inclusions of albite + muscovite + quartz. In contrast, in a second sample, one zircon mantle domain records limited hydration anatexis at 237 ± 3 Ma during subduction, exhibiting high REE contents, steep MREE – HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, medium temperatures of 601–717 °C and multiphase solid inclusions of albite + muscovite + hydrohalite. A second mantle domain in this sample records a low‐ T dehydration metamorphism throughout the whole continental collision in the Triassic, showing low REE contents, steep MREE – HREE patterns with weakly negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 524–669 °C and anhydrite + gas inclusions. Garnet, phengite and allanite/epidote in these two samples also exhibit different variations in texture and major‐trace element compositions, in accordance with the zircon records. The distinct P–T–t paths for these two samples suggest separate processes of dehydration and anatexis, which are ascribed to the different geothermal gradients at different positions inside the same crustal slice during continental subduction‐zone metamorphism. Therefore, the subducting continental crust underwent variable extents of dehydration and anatexis in response to the change in subduction‐zone P–T conditions.
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Abstract. Continental collision is a crucial process in plate tectonics, whereas our understanding regarding the tectonic complexities at such convergent plate boundary remains largely unclear in terms of the evolution and the controlling parameters of its lateral heterogeneity. In this study, we conducted a series of two-dimensional numerical experiments to investigate how continental lithospheric thermal structure influences the development of lateral heterogeneity along the continental collision zone. Two end members were achieved: 1) Continuous subduction mode, which prevails when the model has a cold procontinental moho (Tmoho ≤ 450 °C). In this case, a narrow collision orogen develops, and the subducting angle steepens with the increasing retrocontinental Tmoho. 2) Continental subduction with a slab break-off, which generates a relative wide collision orogen, and dominates when the model has a relative hot procontinental Tmoho (≥ 500 °C), especially when the Tmoho ≥ 550 °C. In contrast, Hr is the second-order controlling parameter in varying the continental collision mode, while it prefers to enhance strain localization in the upper part of the continental lithosphere and promote the growth of shear zones there. By comparing the model results with geoscience observations, we suggest that the discrepant evolutionary paths from the continuous subduction underlying the Hindu Kush to the continental subduction after slab break-off beneath eastern Tibet may originate from the inherited lateral inhomogeneity of Indian lithospheric thermal structure. Besides, the high content of crustal radioactive elements may be one of the important factors that controls the formation of large thrust fault zones in the Himalayas.
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Abstract. Continental collision is a crucial process in plate tectonics. However, in terms of the evolution and the controlling parameters of its lateral heterogeneity, our understanding of the tectonic complexities at such a convergent plate boundary remains largely unclear. In this study, we conducted a series of two-dimensional numerical experiments to investigate how continental lithospheric thermal structure influences the development of lateral heterogeneity along the continental collision zone. The following two end-members were achieved. First, continuous subduction mode, which prevails when the model has a cold procontinental Moho temperature (≤450 ∘C). In this case, a narrow collision orogen develops, and the subducting angle steepens with the increasing retrocontinental Moho temperature. Second, continental subduction with a slab break-off, which generates a relative wide collision orogen and dominates when the model has a relatively hot procontinental Moho temperature (≥500 ∘C), especially when the Moho temperature ≥ 550 ∘C. Radioactive heat production is the second-order controlling parameter in varying the continental collision mode, while it prefers to enhance strain localization in the upper part of the continental lithosphere and promote the growth of shear zones there. By comparing the model results with geological observations, we suggest that the discrepant evolutionary paths from the continuous subduction underlying the Hindu Kush to the continental subduction after slab break-off beneath eastern Tibet may originate from the inherited lateral inhomogeneity of the Indian lithospheric thermal structure. Besides, the high content of crustal radioactive elements may be one of the most important factors that controls the formation of large thrust fault zones in the Himalayas.
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Abstract. Continental collision is a crucial process in plate tectonics, whereas our understanding regarding the tectonic complexities at such convergent plate boundary remains largely unclear in terms of the evolution and the controlling parameters of its lateral heterogeneity. In this study, we conducted a series of two-dimensional numerical experiments to investigate how continental lithospheric thermal structure influences the development of lateral heterogeneity along the continental collision zone. Two end members were achieved: 1) Continuous subduction mode, which prevails when the model has a cold procontinental moho (Tmoho ≤ 450 °C). In this case, a narrow collision orogen develops, and the subducting angle steepens with the increasing retrocontinental Tmoho. 2) Continental subduction with a slab break-off, which generates a relative wide collision orogen, and dominates when the model has a relative hot procontinental Tmoho (≥ 500 °C), especially when the Tmoho ≥ 550 °C. In contrast, Hr is the second-order controlling parameter in varying the continental collision mode, while it prefers to enhance strain localization in the upper part of the continental lithosphere and promote the growth of shear zones there. By comparing the model results with geoscience observations, we suggest that the discrepant evolutionary paths from the continuous subduction underlying the Hindu Kush to the continental subduction after slab break-off beneath eastern Tibet may originate from the inherited lateral inhomogeneity of Indian lithospheric thermal structure. Besides, the high content of crustal radioactive elements may be one of the important factors that controls the formation of large thrust fault zones in the Himalayas.
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