Topographic control of asynchronous glacial advances: A case study from Annapurna, Nepal

Differences in the timing of glacial advances, which are commonly attributed to climatic changes, can be due to variations in valley topography. Cosmogenic 10Be dates from 24 glacial moraine boulders in 5 valleys define two age populations, late-glacial and early Holocene. Moraine ages correlate with paleoglacier valley hypsometries. Moraines in valleys with lower maximum altitudes date to the lateglacial, whereas those in valleys with higher maximum altitudes are early Holocene. Two valleys with similar equilibrium-line altitudes (ELAs), but contrasting ages, are \u3c 5 km apart and share the same aspect, such that spatial differences in climate can be excluded. A glacial mass-balance cellular automata model of these two neighboring valleys predicts that change from a cooler-drier to warmer-wetter climate (as at the Holocene onset) would lead to the glacier in the higher altitude catchment advancing, while the lower one retreats or disappears, even though the ELA only shifted by ~120 m

Himalayan landslide-dam lake record

Abstract About 5400 cal yr BP, a large landslide formed a N400-m-tall dam in the upper Marsyandi River, central Nepal. The resulting lacustrine and deltaic deposits stretched N 7 km upstream, reaching a thickness of 120 m. 14 C dating of 7 wood fragments reveals that the aggradation and subsequent incision occurred remarkably quickly (∼ 500 yr). Reconstructed volumes of lacustrine (∼ 0.16 km 3 ) and deltaic (∼ 0.09 km 3 ) deposits indicate a bedload-to-suspended load ratio of 1:2, considerably higher than the ≤1:10 that is commonly assumed. At the downstream end of the landslide dam, the river incised a new channel through ≥ 70 m of Greater Himalayan gneiss, requiring a minimum bedrock incision rate of 13 mm/ yr over last 5400 yr. The majority of incision presumably occurred over a fraction of this time, suggesting much higher rates. The high bedload ratio from such an energetic mountain river is a particularly significant addition to our knowledge of sediment flux in orogenic environments

Pre- and post-seismic deformation related to the 2015, M_w 7.8 Gorkha earthquake, Nepal

We analyze time series from continuously recording GPS stations in Nepal spanning the pre- and post-seismic period associated to the M_w7.8 Gorkha earthquake which ruptured the Main Himalayan Thrust (MHT) fault on April 25th, 2015. The records show strong seasonal variations due to surface hydrology. After corrections for these variations, the time series covering the pre- and post-seismic periods do not show any detectable transient pre-seismic displacement. By contrast, a transient post-seismic signal is clear. The observed signal shows southward displacements consistent with afterslip on the MHT. Using additional data from stations deployed after the mainshock, we invert the time series for the spatio-temporal evolution of slip on the MHT. This modelling indicates afterslip dominantly downdip of the mainshock rupture. Two other regions show significant afterslip: a more minor zone updip of the rupture, and a region between the mainshock and the largest aftershock ruptures. Afterslip in the first ~ 7 months after the mainshock released a moment of [12.8 ± 0.5] × 10^(19) Nm which represents 17.8 ± 0.8% of the co-seismic moment. The moment released by aftershocks over that period of time is estimated to 2.98 × 10^(19) Nm. Geodetically observed post-seismic deformation after co-seismic offset correction was thus 76.7 ± 1.0% aseismic. The logarithmic time evolution of afterslip is consistent with rate-strengthening frictional sliding. According to this theory, and assuming a long-term loading velocity modulated on the basis of the coupling map of the region and the long term slip rate of 20.2 ± 1.1 mm/yr, afterslip should release about 34.0 ± 1.4% of the co-seismic moment after full relaxation of post-seismic deformation. Afterslip contributed to loading the shallower portion of the MHT which did not rupture in 2015 and stayed locked afterwards. The risk for further large earthquakes in Nepal remains high both updip of the rupture area of the Gorkha earthquake and West of Kathmandu where the MHT has remained locked and where no earthquake larger than M_w7.5 has occurred since 1505

Pre- and post-seismic deformation related to the 2015, M_w 7.8 Gorkha earthquake, Nepal

We analyze time series from continuously recording GPS stations in Nepal spanning the pre- and post-seismic period associated to the M_w7.8 Gorkha earthquake which ruptured the Main Himalayan Thrust (MHT) fault on April 25th, 2015. The records show strong seasonal variations due to surface hydrology. After corrections for these variations, the time series covering the pre- and post-seismic periods do not show any detectable transient pre-seismic displacement. By contrast, a transient post-seismic signal is clear. The observed signal shows southward displacements consistent with afterslip on the MHT. Using additional data from stations deployed after the mainshock, we invert the time series for the spatio-temporal evolution of slip on the MHT. This modelling indicates afterslip dominantly downdip of the mainshock rupture. Two other regions show significant afterslip: a more minor zone updip of the rupture, and a region between the mainshock and the largest aftershock ruptures. Afterslip in the first ~ 7 months after the mainshock released a moment of [12.8 ± 0.5] × 10^(19) Nm which represents 17.8 ± 0.8% of the co-seismic moment. The moment released by aftershocks over that period of time is estimated to 2.98 × 10^(19) Nm. Geodetically observed post-seismic deformation after co-seismic offset correction was thus 76.7 ± 1.0% aseismic. The logarithmic time evolution of afterslip is consistent with rate-strengthening frictional sliding. According to this theory, and assuming a long-term loading velocity modulated on the basis of the coupling map of the region and the long term slip rate of 20.2 ± 1.1 mm/yr, afterslip should release about 34.0 ± 1.4% of the co-seismic moment after full relaxation of post-seismic deformation. Afterslip contributed to loading the shallower portion of the MHT which did not rupture in 2015 and stayed locked afterwards. The risk for further large earthquakes in Nepal remains high both updip of the rupture area of the Gorkha earthquake and West of Kathmandu where the MHT has remained locked and where no earthquake larger than M_w7.5 has occurred since 1505

New GPS station network and elastic half-space modeling of Nepalese Himalayan tectonics and earthquake hazard

Considerable controversy still exists over if and how climate and orogenic evolution may be coupled. A prime example of this debate centers on the Himalayan front where profound gradients in both precipitation and topography occur in a similar location – roughly coincident with the Main Central Thrust Zone (MCT) (Fig. 1). Some researchers (e. g., Hodges 2006; Wobus et al. 2005) suggest that the high precipitation rates drive high erosion rates and thus out-of-sequence thrusting and channel flow in this region. Others think the evidence points to a steeper sub-surface ramp causing the topographic rise and thus capturing higher precipi tation rates (e.g., Bollinger et al. 2006; Robinson et al. 2006). In one scenario climate is a driver of orogenic development, in the other it is a passive responder. Some field work suggests that out-of-sequence thrusting is occurring (Mukul et al. 2007). However, a more detailed understanding of modern ground motion will help to determine if out-of-sequence thrusting is indeed occurring. To this end, a new permanent GPS network is being established in the Nepalese Himalaya, which provides greater northeast-southwest transect density of stations than the newly established Caltech network (Bollinger et al. 2006) and more continuous coverage than the campaign GPS (Bilham et al. 1997) of the 1990’s. Six stations were established in June 2008 and an additional 12-15 are planned as funding is secured. Work is also underway to use an elastic half-space model to predict expected surface deformation under different active fault scenarios. Model results will be compared to the GPS results. The project is a joint venture between Central Washington University, USA and Tribhuvan University, Nepal. The influx of permanent GPS stations into Nepal will help better determine if Indian and Nepalese ground motion truly differs (Jade et al. 2007). In addition to helping determine likely tectonic models for the Nepalese Himalaya and give insights into climate-vs-tectonic drivers, this project should help us better understand earthquake hazards in Nepal. For instance, whether only one near-surface fault (Main Frontal Thrust–MFT) is active versus several active faults has considerable impact on earthquake-associated hazards

Anomalous cosmogenic ^3He production and elevation scaling in the high Himalaya

The production rate of cosmogenic ^3He in apatite, zircon, kyanite and garnet was obtained by cross-calibration against ^(10)Be in co-existing quartz in glacial moraine boulders from the Nepalese Himalaya. The boulders have ^(10)Be ages between 6 and 16 kyr and span elevations from 3200 to 4800 m. In all of these minerals ^3He correlates with ^(10)Be and is dominantly cosmogenic in origin. After modest correction for non-cosmogenic components, ^3He/^(10)Be systematics imply apparent sea-level high-latitude (SLHL) apparent production rates for ^3He of 226 atoms g^(-1) yr^(-1) in zircon, 254 atoms g^(-1) yr^(-1) in apatite, 177 atoms g^(-1) yr^(-1) in kyanite, and 153 atoms g^(-1) yr^(-1) in gamet. These production rates are unexpectedly high compared with rates measured elsewhere in the world, and also compared with proposed element-specific production rates. For apatite and zircon, the data are sufficient to conclude that the ^3He/^(10)Be ratio increases with elevation. If this reflects different altitudinal scaling between production rates for the two isotopes then the SLHL production rates estimated by our approach are overestimates. We consider several hypotheses to explain these observations, including production of ^3He via thermal neutron capture on ^6Li, altitudinal variations in the energy spectrum of cosmic-ray neutrons, and the effects of snow cover. Because all of these effects are small, we conclude that the altitudinal variations in production rates of cosmogenic ^3He and ^(10)Be are distinct from each other at least at this location over the last last ~10 kyr kyr. This conclusion calls into question commonly adopted geographic scaling laws for at least some cosmogenic nuclides. If confirmed, this distinction may provide a mechanism by which to obtain paleoelevation estimates

Modern erosion rates in the High Himalayas of Nepal

Current theories regarding the connections and feedbacks between surface and tectonic processes are predicated on the assumption that higher rainfall causes more rapid erosion. To test this assumption in a tectonically active landscape, a network of 10 river monitoring stations was established in the High Himalayas of central Nepal across a steep rainfall gradient. Suspended sediment flux was calculated from sampled suspended sediment concentrations and discharge rating curves. Accounting for solute and bedload contributions, the suspended sediment fluxes were used to calculate watershed-scale erosion rates that were then compared to monsoon precipitation and specific discharge. We find that, in individual watersheds, annual erosion rates increase with runoff. In addition, our data suggest average erosion rate increases with discharge and precipitation across the entire field site such that the wetter southern watersheds are eroding faster than the drier northern watersheds. The spatially non-uniform contemporary erosion rates documented here are at odds with other studies that have found spatially uniform long-term rates (105–106 yr) across the pronounced rainfall gradient observed in the region. The discrepancy between the modern rates measured here and the long-term rates may be reconciled by considering the high erosional efficiency of glaciers. The northern catchments were much more extensively glacierized during the Pleistocene, and therefore, they likely experienced erosion rates that were significantly higher than the modern rates. We propose that, in the northern watersheds, the high rates of erosion during periods of glaciation compensate for the low rates during interglacials to produce a time-averaged rate comparable to the landslide-dominated southern catchments

Bedload-to-suspended load ratio and rapid bedrock incision from Himalayan landslide-dam lake record

About 5400 cal yr BP, a large landslide formed a \u3e 400-m-tall dam in the upper Marsyandi River, central Nepal. The resulting lacustrine and deltaic deposits stretched \u3e 7 km upstream, reaching a thickness of 120 m. 14C dating of 7 wood fragments reveals that the aggradation and subsequent incision occurred remarkably quickly (∼ 500 yr). Reconstructed volumes of lacustrine (∼ 0.16 km3) and deltaic (∼ 0.09 km3) deposits indicate a bedload-to-suspended load ratio of 1:2, considerably higher than the ≤ 1:10 that is commonly assumed. At the downstream end of the landslide dam, the river incised a new channel through ≥ 70 m of Greater Himalayan gneiss, requiring a minimum bedrock incision rate of 13 mm/yr over last 5400 yr. The majority of incision presumably occurred over a fraction of this time, suggesting much higher rates. The high bedload ratio from such an energetic mountain river is a particularly significant addition to our knowledge of sediment flux in orogenic environments

Applying geodesy and modeling to test the role of climate controlled erosion in shaping Himalayan morphology and evolution

The Himalaya-Tibet system is the archetype of continent-continent collision, but the role of climate in modulating orogenesis is a relatively new paradigm that has not been well tested with field-based deformation measurements. Phenomenal monsoon precipitation (\u3e3 m/year) falls along the Himalayan front, and the resulting erosion is thought by some to promote out -of-sequence thrusting or even channel flow within the High Himalaya, leading to the observed, profoundly steep morphology. Others attribute High Himalayan morphology to a more traditional paradigm of a steeper underlying décollement ramp. The two paradigms predict different patterns of current deformation, but both at rates readily measurable with global positioning system (GPS). In this paper we review the current impasse which researchers from both sides of the debate have reached using methods of structural mapping, morphological analysis, spirit-leveling, seismicity, thermochronometry, cosmogenically-determined erosions rates, and thermokinetic modeling and propose that the addition of continuous geodetic measurements of surface deformation combined with elastic half-space modeling could help resolve the issue. To this end we deployed a network of 6 permanent GPS stations in the Nepal Himalaya in summer 2008 and have plans to expand to it to 16 stations. Preliminary model results demonstrate that within a couple years differences between the two paradigms should be discernable