Fluid events and exhumation history of the main central thrust zone Garhwal Himalaya (India)

Sachan, Himanshu K.; Sharma, Rajesh; Sahai, Atul; Gururajan, N. S.

doi:10.1016/s1367-9120(00)00036-5

Cited by 20 publications

(4 citation statements)

References 39 publications

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“…A conductive zone with an inferred well‐connected fluid phase coincides with the maximum in observed micro‐seismicity. Third, abundant fluids are considered essential to generate the observed retrograde metamorphism in rocks believed to have once been at these seismogenic depths [e.g., Sachan et al , 2001].…”

Section: Discussionmentioning

confidence: 99%

Seasonal modulation of seismicity in the Himalaya of Nepal

Bollinger

Perrier

Avouac

et al. 2007

Geophysical Research Letters

124

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[1] For the period 1995 -2000, the Nepal seismic network recorded 37 ± 8% fewer earthquakes in the summer than in the winter; for local magnitudes ML > 2 to ML > 4 the percentage increases from 31% to 63% respectively. We show the probability of observing this by chance is less than 1%. We find that most surface loading phenomena are either too small, or have the wrong polarity to enhance winter seismicity. We consider enhanced Coulomb failure caused by a pore-pressure increase at seismogenic depths as a possible mechanism. For this to enhance winter seismicity, however, we find that fluid diffusion following surface hydraulic loading would need to be associated with a sixmonth phase lag, which we consider to be possible, though unlikely. We favor instead the suppression of summer seismicity caused by stress-loading accompanying monsoon rains in the Ganges and northern India, a mechanism that is discussed in a companion article.

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Section: Discussionmentioning

confidence: 99%

Seasonal modulation of seismicity in the Himalaya of Nepal

Bollinger

Perrier

Avouac

et al. 2007

Geophysical Research Letters

124

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“…Amphibolite facies gneisses in the central Himalaya are pervaded by CO 2 ‐rich fluids (>80 mol.%; Boullier et al. , 1991; Sachan et al. , 2001), especially near the Main Central Thrust (Fig.…”

Section: Discussionmentioning

confidence: 99%

Fluid evolution and thermal structure in the rapidly exhuming gneiss complex of Namche Barwa–Gyala Peri, eastern Himalayan syntaxis

Craw

Koons

Zeitler

et al. 2005

Journal Metamorphic Geology

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High-grade gneisses (amphibolite-granulite facies) of the Namche Barwa and Gyala Peri massifs, in the eastern Himalayan syntaxis, have been unroofed from metamorphic depths in the late Tertiary-Recent. Rapid exhumation (2-5 mm year )1 ) has resulted in a pronounced shallow conductive thermal anomaly beneath the massifs and the intervening Tsangpo gorge. The position of the 300°C isotherm has been estimated from fluid inclusions using CO 2 -H 2 O immiscibility phase equilibria to be between 2.5 and 6.2 km depth below surface. Hence, the near-surface average thermal gradient exceeds 50°C km )1 beneath valleys, although the thermal gradient is relatively lower beneath the high mountains. The original metamorphic fluid in the gneisses was >90% CO 2 . This fluid was displaced by incursion of brines from overlying marine sedimentary rocks that have since been largely removed by erosion. Brines can exceed 60 wt% dissolved salts, and include Ca, Na, K and Fe chlorides. These brines were remobilized during the earliest stages of uplift at >500°C. During exhumation, incursion of abundant topography-driven surface waters resulted in widespread fracture-controlled hydrothermal activity and brine dilution down to the brittle-ductile transition. Boiling water was particularly common at shallow levels (<2.5 km) beneath the Yarlung Tsangpo valley, and numerous hot springs occur at the surface in this valley. Dry steam is not a major feature of the hydrothermal system in the eastern syntaxis (in contrast to the western syntaxis at Nanga Parbat), but some dry steam fluids may have developed locally.

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“…Then, our modelled seismic velocity values reduce between 5 and 15 km depths, suggesting a 10-20% drop in Vp, dVp, Vs and dVs and an increase in Vp/Vs by 10-14%, which could be explained in terms of the presence of meteoric water, aqueous uids and wet-sediments of the Eurasian plate. Several studies [32][33][34] (Boulier et al, 1991;Sachan et al, 2001;Sharma et al, 2008) have shown that the uids in the higher Himalaya and the MCT are dominantly carbonic whereas the uids in the Lesser Himalaya and the MBT are dominantly brines or syntectonic uids (Ojha and Srivastava, 2016). Between 15 and 20 km depth, a slight increase (i.e.…”

Section: Resultsmentioning

confidence: 99%

WITHDRAWN: Seismic Imaging of the Kumaon-Garhwal Himalaya: Insights into the Main Himalayan Thrust and the Crust-Mantle Boundary

Mandal,

Raghavan,

Saha

et al. 2023

Preprint

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The seismic velocity tomography of reveals a high-velocity (5–20% increase in dVp, 4–16% increase in dVs, low Vp/Vs (~ 1.5–1.8)) upper crust between 0 and 20 km depth and a low-velocity (10–20% drop in dVp, 4–12% drop in dVs, high Vp/Vs (~ 1.65-2.0)) lower crust between 20 and 40 km depth. Our modelling detects a low-angle north-dipping low-velocity layer (10–20% drop in Vp and Vs, 10–15% increase in Vp/Vs) at depths of 8–20 km, which may be the Main Himalayan Thrust (MHT). The presence of low-velocity pockets in the lower crust may be indicative of partial melts, while those in the upper and middle crust may be indicative of aqueous or metamorphic fluids. The Common Conversion Point stacking (CCP) of radial P- Receiver Functions (PRFs) detects a double Moho structure below the rupture zone of the 1803 Mw7.8 Garhwal, the 1991 Mw6.8 Uttarkashi, and the 1999 Mw6.5 Chamoli earthquakes, which might be accumulating marked crustal stresses on the MHT. Our modelling also suggests that fluids or high pore-fluid pressure caused the above-mentioned three Mw≥6.5 earthquakes, which occurred in low-velocity zones on the MHT. Our CCP imaging also suggests a north-dipping crust-mantle boundary (with larger positive impedance contrast) at depths of 30–55 km while these CCP images also detect a north-dipping zone with larger negative impedance contrast at depths of 8–20 km, possibly representing the low-velocity MHT. Our joint inversion of radial PRFs and group velocity dispersion data of Rayleigh waves has also mapped an MHT and Moho that dip to the north.

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Fluid events and exhumation history of the main central thrust zone Garhwal Himalaya (India)

Cited by 20 publications

References 39 publications

Seasonal modulation of seismicity in the Himalaya of Nepal

Seasonal modulation of seismicity in the Himalaya of Nepal

Fluid evolution and thermal structure in the rapidly exhuming gneiss complex of Namche Barwa–Gyala Peri, eastern Himalayan syntaxis

WITHDRAWN: Seismic Imaging of the Kumaon-Garhwal Himalaya: Insights into the Main Himalayan Thrust and the Crust-Mantle Boundary

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