Wadia Institute of Himalayan Geology

cubic meters of rock and glacier ice collapsed from the steep north face of Ronti Peak. The rock and ice avalanche rapidly transformed into an extraordinarily large and mobile debris flow that transported boulders greater than 20 meters in diameter and scoured the valley walls up to 220 meters above the valley floor. The intersection of the hazard cascade with downvalley infrastructure resulted in a disaster, which highlights key questions about adequate monitoring and sustainable development in the Himalaya as well as other remote, high-mountain environments.

Research Article| June 01, 1987 The closing of Tethys and the tectonics of the Himalaya M. P. SEARLE; M. P. SEARLE 1Department of Geology, The University, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar B. F. WINDLEY; B. F. WINDLEY 1Department of Geology, The University, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar M. P. COWARD; M. P. COWARD 2Department of Geology, Imperial College, Prince Consort Road, London SW7 2BP, United Kingdom Search for other works by this author on: GSW Google Scholar D.J.W. COOPER; D.J.W. COOPER 3Department of Geology, The University, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar A. J. REX; A. J. REX 3Department of Geology, The University, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar D. REX; D. REX 4Department of Earth Sciences, Leeds University, Leeds LS2 9JT, United Kingdom Search for other works by this author on: GSW Google Scholar LI TINGDONG; LI TINGDONG 5Chinese Academy of Geological Sciences, Baiwanzhong, Beijing, China Search for other works by this author on: GSW Google Scholar XIAO XUCHANG; XIAO XUCHANG 5Chinese Academy of Geological Sciences, Baiwanzhong, Beijing, China Search for other works by this author on: GSW Google Scholar M. Q. JAN; M. Q. JAN 6Department of Geology and National Centre of Excellence, Peshawar University, Peshawar, Pakistan Search for other works by this author on: GSW Google Scholar V. C. THAKUR; V. C. THAKUR 7Wadia Institute of Himalayan Geology, Dehra Dun, 248001, India Search for other works by this author on: GSW Google Scholar S. KUMAR S. KUMAR 7Wadia Institute of Himalayan Geology, Dehra Dun, 248001, India Search for other works by this author on: GSW Google Scholar Author and Article Information M. P. SEARLE 1Department of Geology, The University, Leicester LE1 7RH, United Kingdom B. F. WINDLEY 1Department of Geology, The University, Leicester LE1 7RH, United Kingdom M. P. COWARD 2Department of Geology, Imperial College, Prince Consort Road, London SW7 2BP, United Kingdom D.J.W. COOPER 3Department of Geology, The University, Leicester LE1 7RH, United Kingdom A. J. REX 3Department of Geology, The University, Leicester LE1 7RH, United Kingdom D. REX 4Department of Earth Sciences, Leeds University, Leeds LS2 9JT, United Kingdom LI TINGDONG 5Chinese Academy of Geological Sciences, Baiwanzhong, Beijing, China XIAO XUCHANG 5Chinese Academy of Geological Sciences, Baiwanzhong, Beijing, China M. Q. JAN 6Department of Geology and National Centre of Excellence, Peshawar University, Peshawar, Pakistan V. C. THAKUR 7Wadia Institute of Himalayan Geology, Dehra Dun, 248001, India S. KUMAR 7Wadia Institute of Himalayan Geology, Dehra Dun, 248001, India Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1987) 98 (6): 678–701. https://doi.org/10.1130/0016-7606(1987)98<678:TCOTAT>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. P. SEARLE, B. F. WINDLEY, M. P. COWARD, D.J.W. COOPER, A. J. REX, D. REX, LI TINGDONG, XIAO XUCHANG, M. Q. JAN, V. C. THAKUR, S. KUMAR; The closing of Tethys and the tectonics of the Himalaya. GSA Bulletin 1987;; 98 (6): 678–701. doi: https://doi.org/10.1130/0016-7606(1987)98<678:TCOTAT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Recent geological and geophysical data from southern Tibet allow refinement of models for the closing of southern (Neo-) Tethys and formation of the Himalaya. Shelf sediments of the Indian passive continental margin which pass northward into deep-sea Tethyan sediments of the Indus-Tsangpo suture zone were deposited in the Late Cretaceous. An Andean-type margin with a 2,500-km-long Trans-Himalayan (Kohistan-Ladakh-Gangdese) granitoid batholith formed parallel to the southern margin of the Lhasa block, together with extensive andesites, rhyolites, and ignimbrites (Lingzizong Formation). The southern part of the Lhasa block was uplifted, deformed, and eroded between the Cenomanian and the Eocene. In the western Himalaya, the Kohistan island arc became accreted to the northern plate at this time. The northern part of the Lhasa block was affected by Jurassic metamorphism and plutonism associated with the mid-Jurassic closure of the Bangong-Nujiang suture zone to the north.The timing of collision between the two continental plates (ca. 50-40 Ma) marking the closing of Tethys is shown by (1) the change from marine (flysch-like) to continental (molasse-like) sedimentation in the Indus-Tsangpo suture zone, (2) the end of Gangdese I-type granitoid injection, (3) Eocene S-type anatectic granites and migmatites in the Lhasa block, and (4) the start of compressional tectonics in the Tibetan-Tethys and Indus-Tsangpo suture zone (south-facing folds, south-directed thrusts).After the Eocene closure of Tethys, deformation spread southward across the Tibetan-Tethys zone to the High Himalaya. Deep crustal thrusting, Barrovian metamorphism, migmatization, and generation of Oligocene-Miocene leucogranites were accompanied by south-verging recumbent nappes inverting metamorphic isograds and by south-directed intracontinental shear zones associated with the Main Central thrust. Continued convergence in the late Tertiary resulted in large-scale north-directed backthrusting along the Indus-Tsangpo suture zone. More than 500 km shortening is recorded in the foreland thrust zones of the Indian plate, south of the suture, and > 150 km shortening is recorded across the Indian shelf (Zanskar Range) and the Indus suture in Ladakh. There was also large-scale shortening of the Karakoram and Tibetan microplates north of the suture; as much as 1,000 km shortening occurred in Tibet. The more recent deformation, however, involved the spreading of this thickened crust and the lateral motion of the Tibetan block along major approximately east-west–trending strike-slip fault zones. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Gorkha earthquake (magnitude 7.8) on 25 April 2015 and later aftershocks struck South Asia, killing ~9000 people and damaging a large region. Supported by a large campaign of responsive satellite data acquisitions over the earthquake disaster zone, our team undertook a satellite image survey of the earthquakes' induced geohazards in Nepal and China and an assessment of the geomorphic, tectonic, and lithologic controls on quake-induced landslides. Timely analysis and communication aided response and recovery and informed decision-makers. We mapped 4312 coseismic and postseismic landslides. We also surveyed 491 glacier lakes for earthquake damage but found only nine landslide-impacted lakes and no visible satellite evidence of outbursts. Landslide densities correlate with slope, peak ground acceleration, surface downdrop, and specific metamorphic lithologies and large plutonic intrusions.

Abstract Glacier outlines are mapped for the upper Bhagirathi and Saraswati/Alaknanda basins of the Garhwal Himalaya using Corona and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite images acquired in 1968 and 2006, respectively. A subset of glaciers was also mapped using Landsat TM images acquired in 1990. Glacier area decreased from 599.9 ± 15.6 km 2 (1968) to 572.5 ± 18.0 km 2 (2006), a loss of 4.6 ± 2.8%. Glaciers in the Saraswati/Alaknanda basin and upper Bhagirathi basin lost 18.4 ± 9.0 km 2 (5.7 ± 2.7%) and 9.0 ± 7.7 km 2 (3.3 ± 2.8%), respectively, from 1968 to 2006. Garhwal Himalayan glacier retreat rates are lower than previously reported. More recently (1990–2006), recession rates have increased. The number of glaciers in the study region increased from 82 in 1968 to 88 in 2006 due to fragmentation of glaciers. Smaller glaciers (&lt;1 km 2 ) lost 19.4 ± 2.5% (0.51 ± 0.07% a −1 ) of their ice, significantly more than for larger glaciers (&gt;50 km 2 ) which lost 2.8 ± 2.7% (0.074 ± 0.071 % a −1 ). From 1968 to 2006, the debris-covered glacier area increased by 17.8 ± 3.1% (0.46 ± 0.08% a −1 ) in the Saraswati/Alaknanda basin and 11.8 ± 3.0% (0.31 ± 0.08% a −1 ) in the upper Bhagirathi basin. Climate records from Mukhim (∼1900 m a.s.l.) and Bhojbasa (∼3780 m a.s.l.) meteorological stations were used to analyze climate conditions and trends, but the data are too limited to make firm conclusions regarding glacier–climate interactions.

Horizontal velocities of 26 Global Positioning System (GPS) stations in the northwest Himalayan region provide new constraints on the partitioning of India‐Eurasia convergence and elastic strain accumulation about the locked Main Frontal Thrust (MFT). The northwest‐striking Karakorum fault slips at 11 ± 4 mm/yr and contributes to east‐west extension of southern Tibet and westward motion of the northwest Himalaya towards Nanga Parbat, rather than playing a role in eastward extrusion of Tibet. Crustal shortening across the Himalaya occurs within a zone centered about 100 km north of the Siwalik Foothills and the MFT. Model inversions of the GPS data indicate that the MFT is locked over a width of ∼100 km. Comparison with geologic MFT‐slip‐rate estimates suggests that this zone is building up a slip deficit at a rate of 14 ± 1 mm/yr and will eventually fail in future great earthquakes.

Abstract. Peatlands are a major terrestrial carbon store and a persistent natural carbon sink during the Holocene, but there is considerable uncertainty over the fate of peatland carbon in a changing climate. It is generally assumed that higher temperatures will increase peat decay, causing a positive feedback to climate warming and contributing to the global positive carbon cycle feedback. Here we use a new extensive database of peat profiles across northern high latitudes to examine spatial and temporal patterns of carbon accumulation over the past millennium. Opposite to expectations, our results indicate a small negative carbon cycle feedback from past changes in the long-term accumulation rates of northern peatlands. Total carbon accumulated over the last 1000 yr is linearly related to contemporary growing season length and photosynthetically active radiation, suggesting that variability in net primary productivity is more important than decomposition in determining long-term carbon accumulation. Furthermore, northern peatland carbon sequestration rate declined over the climate transition from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA), probably because of lower LIA temperatures combined with increased cloudiness suppressing net primary productivity. Other factors including changing moisture status, peatland distribution, fire, nitrogen deposition, permafrost thaw and methane emissions will also influence future peatland carbon cycle feedbacks, but our data suggest that the carbon sequestration rate could increase over many areas of northern peatlands in a warmer future.

Nd and Sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences. The potential of the technique is illustrated by this isotopic study of the Main Central thrust system of the Himalaya. Nd isotope data from the Garhwal Himalaya indicate that metasedimentary rocks from the Vaikrita Group (Nd = –14 to –19) correlate closely with those from the High Himalayan Crystalline Series, which constitutes the hanging-wall lithologies of the Main Central thrust. In contrast, metasedimentary rocks from the Munsiari Group (Nd = –23 to –28) show marked similarities to the Lesser Himalayan Series in the footwall of the Main Central thrust. Sr isotopes support the correlations in that the Vaikrita Group shows partial reequilibration at 500 Ma, whereas the Munsiari Group has not undergone Sr isotope homogenization since 1800 Ma. Thus, the Vaikrita thrust that juxtaposes these two formations is recognized as the Main Central thrust in Garhwal Himalaya. The thrust coincides, approximately, with the location of the kyanite isograd, confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the Main Central thrust. &#13;\n&#13;\nAlong the Tons thrust (known locally as the Srinagar thrust) 50 km south of the Main Central thrust, low-grade quartzarenites with Nd-Sr isotope and trace element characteristics typical of Lesser Himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the High Himalayan Crystalline Series. The field relationships most probably result from out-of-sequence thrusting in which Lesser Himalayan Series rocks to the north were emplaced over low-grade equivalents of the High Himalayan Crystalline Series preserved in the external part of the orogen. This study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts.

Toward understanding the relationship between strain accumulation and strain release in the context of the mechanics of the earthquake and mountain building process and quantifying the seismic hazard associated with the globes largest continental thrust system, we describe the late Quaternary expression and paleoseismic evidence for great surface rupture earthquakes at six sites along the Himalayan Frontal Thrust (HFT) system of India. Our observations span a distance of ∼250 km along strike of the HFT. Uplifted and truncated fluvial terrace deposits resulting from the Holocene displacements on the HFT are preserved along canyons of the Ghaggar, Markanda, Shajahanpur, and Kosi Rivers. Dividing the elevation of the bedrock straths at each site by their ages yields estimates of the vertical uplift rate of ∼4–6 mm/yr, which when assumed to be the result of slip on an underlying thrust dipping at ∼20°–45° are equivalent to fault slip rates of ∼6–18 mm/yr or shortening rates of ∼4–16 mm/yr. Trench exposures reveal the HFT to fold and break late Holocene surface sediments near the cities and villages of Chandigarh, Kala Amb, Rampur Ganda, Lal Dhang, and Ramnagar. Radiocarbon ages of samples obtained from the displaced sediments indicate surface rupture at each site took place after ∼A.D. 1200 and before ∼A.D. 1700. Uncertainties attendant to the radiocarbon dating currently do not allow an unambiguous definition of the capping bound on the age of the displacement at each site and hence whether or not the displacements at all sites were contemporaneous. Trench exposures and vertical separations measured across scarps at Rampur Ganda, Lal Dhang, and Ramnagar are interpreted to indicate single‐event displacements of ∼11–38 m. Dividing the observed single‐event vertical components of displacement by the estimated longer‐term uplift rates indicates ∼1330–3250 or more years should be required to accumulate the slip sufficient to produce similar sized displacements. Surface rupture appears to not have occurred during the historical 1905 Kangra ( M w = 7.7), 1934 Bihar‐Nepal ( M w = 8.1), and 1950 Assam ( M w = 8.4) earthquakes, which also occurred along the Himalayan front. Yet we observe clear evidence of fault scarps and displacements in young alluvium and progressive and continued offset of fluvial terrace deposits along the HFT. We suggest on this basis and the size and possible synchroneity of displacements recorded in the trenches that there exists the potential for earthquakes larger than recorded in the historical record and with the potential to rupture lengths of the HFT greater than the ∼250 km we have studied.

Abstract Speleothem proxy records from northeastern (NE) India reflect seasonal changes in Indian summer monsoon strength as well as moisture source and transport paths. We have analyzed a new speleothem record from Mawmluh Cave, Meghalaya, India, in order to better understand these processes. The data show a strong wet phase 33,500–32,500 years B.P. followed by a weak/dry phase from 26,000 to 23,500 years B.P. and a very weak phase from 17,000 to 15,000 years B.P. The record suggests abrupt increase in strength during the Bølling‐Allerød and early Holocene periods and pronounced weakening during the Heinrich and Younger Dryas cold events. We infer that these changes in monsoon strength are driven by changes in temperature gradients which drive changes in winds and moisture transport into northeast India.

Detrital zircon samples from Cambrian and Lower to Middle Ordovician strata were taken across and along the strike of the Himalaya from Pakistan to Bhutan (~2000 km). By sampling rocks from one time interval for nearly the entire length of an orogen, and

Abstract. A large number of Himalayan glacier catchments are under the influence of humid climate with snowfall in winter (November–April) and south-west monsoon in summer (June–September) dominating the regional hydrology. Such catchments are defined as "Himalayan catchment", where the glacier meltwater contributes to the river flow during the period of annual high flows produced by the monsoon. The winter snow dominated Alpine catchments of the Kashmir and Karakoram region and cold-arid regions of the Ladakh mountain range are the other major glacio-hydrological regimes identified in the region. Factors influencing the river flow variations in a "Himalayan catchment" were studied in a micro-scale glacier catchment in the Garhwal Himalaya, covering an area of 77.8 km2. Three hydrometric stations were established at different altitudes along the Din Gad stream and discharge was monitored during the summer ablation period from 1998 to 2004, with an exception in 2002. These data have been analysed along with winter/summer precipitation, temperature and mass balance data of the Dokriani glacier to study the role of glacier and precipitation in determining runoff variations along the stream continuum from the glacier snout to 2360 m a.s.l. The study shows that the inter-annual runoff variation in a "Himalayan catchment" is linked with precipitation rather than mass balance changes of the glacier. This study also indicates that the warming induced an initial increase of glacier runoff and subsequent decline as suggested by the IPCC (2007) is restricted to the glacier degradation-derived component in a precipitation dominant Himalayan catchment and cannot be translated as river flow response. The preliminary assessment suggests that the "Himalayan catchment" could experience higher river flows and positive glacier mass balance regime together in association with strong monsoon. The important role of glaciers in this precipitation dominant system is to augment stream runoff during the years of low summer discharge. This paper intends to highlight the importance of creating credible knowledge on the Himalayan cryospheric processes to develop a more representative global view on river flow response to cryospheric changes and locally sustainable water resources management strategies.

While deformation at the Earth's surface primarily occurs along tectonic plate boundaries, major earthquakes have shaken regions deep within continental interiors. Three of the largest (M &gt; 7.5) historic intraplate earthquakes occurred within the Indian subcontinent, suggesting the possibility of significant intraplate deformation. We consider surface velocities determined from new GPS data collected at 29 continuous GPS stations and 41 survey‐mode GPS stations in India between 1995 and 2007 to find a north‐south shortening rate of 0.3 ± 0.05 nanostrain yr −1 , which may be accommodated by 2 ± 1 mm/yr of more localized convergence across central India. Southward motions at 4–7 mm/yr of sites on the Shillong plateau in northeast India reflect rapid shortening and high earthquake hazard associated with active thrust faults bounding the plateau. The width and magnitude of the elastic strain accumulation field across the Himalaya varies little from ∼76°–90° longitude, but the strain is more broadly distributed and convergence rates are higher along the eastern ∼200 km of the range.

Abstract Heterogeneous glacier mass loss has occurred across High Mountain Asia on a multi-decadal timescale. Contrasting climatic settings influence glacier behaviour at the regional scale, but high intra-regional variability in mass loss rates points to factors capable of amplifying glacier recession in addition to climatic change along the Himalaya. Here we examine the influence of surface debris cover and glacial lakes on glacier mass loss across the Himalaya since the 1970s. We find no substantial difference in the mass loss of debris-covered and clean-ice glaciers over our study period, but substantially more negative (−0.13 to −0.29 m w.e.a −1 ) mass balances for lake-terminating glaciers, in comparison to land-terminating glaciers, with the largest differences occurring after 2000. Despite representing a minor portion of the total glacier population (~10%), the recession of lake-terminating glaciers accounted for up to 32% of mass loss in different sub-regions. The continued expansion of established glacial lakes, and the preconditioning of land-terminating glaciers for new lake development increases the likelihood of enhanced ice mass loss from the region in coming decades; a scenario not currently considered in regional ice mass loss projections.

Abstract Age-constrained pollen data and magnetic susceptibility of an alpine peat profile from the Garhwal Higher Himalaya display a continuous record of climate and monsoon trends for the past 7800 yr. About 7800 cal yr B.P., dominance of evergreen oak ( Quercus semecarpifolia ), alder ( Alnus ), and grasses in the pollen record reflect a cold, wet climate with moderate monsoon precipitation. From 7800 to 5000 cal yr B.P., vegetation was progressively dominated by conifers, indicating ameliorated climate with a stronger monsoon. A warm, humid climate, with highest monsoon intensity, from 6000–4500 cal yr B.P. represents the mid-Holocene climatic optimum. Between 4000 and 3500 cal yr B.P., the abundance of conifers sharply decreased, with the greatest increase in evergreen oak. This trend suggests progressive cooling, with a decrease in the monsoon to its minimum about 3500 cal yr B.P. Two relatively minor cold/dry events at ca. 3000 and 2000 cal yr B.P. marked step-wise strengthening of the monsoon until ca. 1000 cal yr B.P. After a cold/dry episode that culminated ca. 800 cal yr B.P., the monsoon again strengthened and continued until today. A sharp decrease in temperature and rainfall at 4000–3500 cal yr B.P. represents the weakest monsoon event of the Holocene record. This cold/dry event correlates with proxy data from other localities of the Indian subcontinent, Arabian Sea, and western Tibet.

Abstract Glaciers in the Karakoram exhibit irregular behavior. Terminus fluctuations of individual glaciers lack consistency and, unlike other parts of the Himalaya, total ice mass remained stable or slightly increased since the 1970s. These seeming anomalies are addressed through a comprehensive mapping of surge-type glaciers and surge-related impacts, based on satellite images (Landsat and ASTER), ground observations, and archival material since the 1840s. Some 221 surge-type and surge-like glaciers are identified in six main classes. Their basins cover 7,734 ± 271 km 2 or ~43% of the total Karakoram glacierised area. Active phases range from some months to over 15 years. Surge intervals are identified for 27 glaciers with two or more surges, including 9 not previously reported. Mini-surges and kinematic waves are documented and surface diagnostic features indicative of surging. Surge cycle timing, intervals and mass transfers are unique to each glacier and largely out-of-phase with climate. A broad class of surge-modified ice introduces indirect and post-surge effects that further complicate tracking of climate responses. Mass balance in surge-type and surge-modified glaciers differs from conventional, climate-sensitive profiles. New approaches are required to account for such differing responses of individual glaciers, and effectively project the fate of Karakoram ice during a warming climate.

The 26 December 2004 Sumatra earthquake produced static offsets at continuously operating GPS stations at distances of up to 4500 kilometers from the epicenter. We used these displacements to model the earthquake and include consideration of the Earth's shape and depth-varying rigidity. The results imply that the average slip was >5 meters along the full length of the rupture, including the approximately 650-kilometer-long Andaman segment. Comparison of the source derived from the far-field static offsets with seismically derived estimates suggests that 25 to 35% of the total moment release occurred at periods greater than 1 hour. Taking into consideration the strong dip dependence of moment estimates, the magnitude of the earthquake did not exceed Mw = 9.2.

Sr and Nd concentrations and isotope compositions in sediment of the Ganga River, from Gangotri to Rajmahal, and its tributaries have been measured to determine provenance and the spatial variability in physical erosion among the Ganga subbasins. Sr and Nd in silicates range from 37 to 138 and from 10 to 36 μ g/g, with 87 Sr/ 86 Sr and ɛ Nd of 0.7474–0.8428 and −25.5 to −15.5, respectively. The results suggest that &gt;65% of Ganga mainstream sediments are derived from the Higher Himalayan Crystallines highlighting intense physical erosion in this region. The 87 Sr/ 86 Sr values of sediments in the Gangetic plain show nearly identical trends during two seasons, with a sharp and significant decrease at Barauni downstream of Gandak confluence. This brings out the major impact of the sediment contribution from the Gandak to the Ganga mainstream. Model calculation suggests that about half of the Ganga sediment at Rajmahal is sourced from the Gandak. The erosion rates in the Himalayan subbasins of the Ganga range between 0.5 and 6 mm/a (where a is years), with the Gandak having the highest erosion rate. High relief and intense precipitation over the headwater basins of the Gandak appear to drive the rapid and focused erosion of this basin. The results of this study and those in literature suggest that the eastern syntaxis (Brahmaputra), the western syntaxis (Indus), and the Gandak have much higher physical erosion rates than the other Himalayan basins. Focused erosion in the hot spots of these river basins contributes significantly to the global riverine sediment budget and influence regional tectonics.

The lower Lesser Himalayan sequence marks the northern extremity of the exposed Indian plate, and is generally interpreted as a passive margin. Five lines of evidence, however, collectively suggest a continental arc setting: (1) igneous intrusions and volcanic rocks occur at this stratigraphic level across the length of the Himalaya, (2) ages of intrusive and metavolcanic (?) rocks cluster at 1780–1880 Ma but also indicate a long-lived igneous process, (3) detrital zircon ages in clastic rocks cluster at 1800–1900 Ma, with a unimodal age distribution in some rocks, (4) the mineralogy and chemistry of metasedimentary rocks differ from typical shales and suggest a volcanogenic source, (5) trace-element chemistries of orthogneisses and metabasalts are more consistent with either an arc or a collisional setting. Intercalation of volcanic rocks with clastic sediments and a general absence of Proterozoic metamorphic ages do not support a collisional origin. An arc model further underscores the profound unconformity separating lower-upper Lesser Himalayan rocks, indicating that a Paleoproterozoic arc may have formed the stratigraphic base of the northern Indian margin. This, in turn, may indicate disposition of the Indian plate adjacent to North America in the ca. 1800 Ma supercontinent Columbia. Felsic orthogneisses ("Ulleri") likely represent shallow intrusions, not Indian basement.

The Mwγ 9.0 2004 December 26 Sumatra-Andaman and Mw= 8.7 2005 March 28 Nias earthquakes, which collectively ruptured approximately 1800 km of the Andaman and Sunda subduction zones, are expected to be followed by vigorous viscoelastic relaxation involving both the upper and lower mantle. Because of these large spatial dimensions it is desirable to fully account for gravitational coupling effects in the relaxation process. We present a stable method of computing relaxation of a spherically-stratified, compressible and self-gravitating viscoelastic Earth following an impulsive moment release event. The solution is cast in terms of a spherical harmonic expansion of viscoelastic normal modes. For simple layered viscoelastic models, which include a low-viscosity oceanic asthenosphere, we predict substantial post-seismic effects over a region several 100s of km wide surrounding the eastern Indian Ocean. We compare observed GPS time-series from ten regional sites (mostly in Thailand and Indonesia), beginning in 2004 December, with synthetic time-series that include the coseismic and post-seismic effects of the 2004 December 26 and 2005 March 28 earthquakes. A viscosity structure involving a biviscous (Burgers body) rheology in the asthenosphere explains the pattern and amplitude of post-seismic offsets remarkably well.

The ∼2500 km long Himalayan arc has experienced three large to great earthquakes of M w 7.8 to 8.4 during the past century, but none produced surface rupture. Paleoseismic studies have been conducted during the last decade to begin understanding the timing, size, rupture extent, return period, and mechanics of the faulting associated with the occurrence of large surface rupturing earthquakes along the ∼2500 km long Himalayan Frontal Thrust (HFT) system of India and Nepal. The previous studies have been limited to about nine sites along the western two‐thirds of the HFT extending through northwest India and along the southern border of Nepal. We present here the results of paleoseismic investigations at three additional sites further to the northeast along the HFT within the Indian states of West Bengal and Assam. The three sites reside between the meizoseismal areas of the 1934 Bihar‐Nepal and 1950 Assam earthquakes. The two westernmost of the sites, near the village of Chalsa and near the Nameri Tiger Preserve, show that offsets during the last surface rupture event were at minimum of about 14 m and 12 m, respectively. Limits on the ages of surface rupture at Chalsa (site A) and Nameri (site B), though broad, allow the possibility that the two sites record the same great historical rupture reported in Nepal around A.D. 1100. The correlation between the two sites is supported by the observation that the large displacements as recorded at Chalsa and Nameri would most likely be associated with rupture lengths of hundreds of kilometers or more and are on the same order as reported for a surface rupture earthquake reported in Nepal around A.D. 1100. Assuming the offsets observed at Chalsa and Nameri occurred synchronously with reported offsets in Nepal, the rupture length of the event would approach 700 to 800 km. The easternmost site is located within Harmutty Tea Estate (site C) at the edges of the 1950 Assam earthquake meizoseismal area. Here the most recent event offset is relatively much smaller (&lt;2.5 m), and radiocarbon dating shows it to have occurred after A.D. 1100 (after about A.D. 1270). The location of the site near the edge of the meizoseismal region of the 1950 Assam earthquake and the relatively lesser offset allows speculation that the displacement records the 1950 M w 8.4 Assam earthquake. Scatter in radiocarbon ages on detrital charcoal has not resulted in a firm bracket on the timing of events observed in the trenches. Nonetheless, the observations collected here, when taken together, suggest that the largest of thrust earthquakes along the Himalayan arc have rupture lengths and displacements of similar scale to the largest that have occurred historically along the world's subduction zones.