Pascale Huyghe

Propagation of the thrust system and erosion in the Lesser Himalaya: Geochemical and sedimentological evidence

Centre de Recherches Pe´trographiques et Ge´ochimiques, BP 20, 54501 Vandoeuvre-les-Nancy,FranceABSTRACTSedimentological and Nd isotope data of two sections of the sub-Himalaya of westernNepal are used as new constraints for understanding the erosion history of the Himalaya.Throughout the deposition of the middle and upper members of the Siwalik Group, theLesser Himalaya contribution to the total detrital input progressively increased from lessthan 20% to 40%. The increasing proportion of Lesser Himalaya sediments started at ca.10–8 Ma and is associated with a coarsening of the maximum grain size at both micro-scopic and macroscopic scales. Thin-skinned tectonics of the Lesser Himalaya thrust sys-tem would have controlled the exhumation of the Lesser Himalaya rocks and would havebegun at 12–10 Ma, taking into account the delay for denudation. Together with otherstudies, these data restrict the onset of movement on the Lesser Himalaya thrust systemto less than 3 m.y. along more than 1750 km. This short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust; thus we suggest instead the simultaneous initiation of several thrusts aheadof the Main Central thrust at ca. 12 Ma. We suggest that a rapid rise of the TibetanPlateau at this time has transformed the Himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state. This regional rising could be the primecause of the increase of sediment influx at ca. 11 Ma around the Himalaya.Keywords: Siwalik formations, neodymium, grain size, erosion, thrust sheets, Himalaya.

Recent movements along the Main Boundary Thrust of the Himalayas: Normal faulting in an over-critical thrust wedge?

Abstract The Main Boundary Thrust (MBT) is one of the major Himalayan thrusts occurring during the Cainozoic, and it is presently incorporated within the Himalayan thrust wedge (Lesser and Outer Himalayas) displaced above the Indian lithosphere. Nonetheless the MBT shows recent normal displacement along most of its length. We suggest that the orientation of the major principal stress within the Himalayan thrust wedge deviates significantly from the horizontal and when this deviation exceeds the dip of the vectors normal to back-tilted thrusts, the normal component of displacement may act along these faults. Steep north-dipping segments of the MBT therefore show a normal component of displacement if a geometrical definition is used, but they are faults in a compressional regime where the major principal stress axis has deviated from the horizontal. Micro-structural data recorded along the Surkhet-Ghorahi segment of the MBT are consistent with a strong deviation of the state of stress. The presence of such peculiar normal faulting along the MBT is used to calibrate the mechanical characteristics of the belt considered as a Coulomb wedge. The following characteristics are suggested: (a) very poor strength contrast between basal decollement and rocks in the wedge body, (b) a high pore fluid pressure ratio (probably close to 0.8–0.9) and a higher fluid pressure ratio (close to 1.0) along the active normal faults if a high internal friction angle (close to the Byerlee value) is considered. The strong deviation in principal stress direction may have recently increased, due to a taper of the Himalayan wedge exceeding the stability boundary and may be controlled by erosion and isostatic uplift rebound of the Himalayan range.

Tectonics, exhumation, and drainage evolution of the eastern Himalaya since 13 Ma from detrital geochemistry and thermochronology, Kameng River Section, Arunachal Pradesh

The exhumation history of the central Himalaya is well documented, but lateral variations in exhumation remain poorly constrained. In this study, we identify sediment source areas and examine the late Neogene exhumation history of the eastern Himalaya from the synorogenic sedimentary record of its foreland basin. We present Nd and Hf isotopic data as well as apatite and zircon fission-track analyses from the Miocene–Pliocene Siwalik Group along the recently dated Kameng River section in Arunachal Pradesh, northeastern India. Our isotopic data show that Siwalik Group sediments deposited between 13–7 and <2.6 Ma in Arunachal Pradesh were mainly derived from Higher Himalayan source rocks. In contrast, sediments deposited between ca. 7 and 3 Ma have far less negative eNd and eHf values that require involvement of the Gangdese Batholith and Yarlung suture zone source areas via the Brahmaputra River system. Consequently, these sediments should also record incision of the Namche Barwa massif by this river. Source-area exhumation rates of Himalayan-derived sediments, determined from detrital zircon fission-track data, were on the order of 1.8 km/m.y. in the fastest-exhuming areas. These rates are very similar to those calculated for the central Himalaya and have been relatively constant since ca. 13 Ma. Our results do not support the hypothesis of a major change in exhumation rate linked to either local or regional climate change or to Shillong Plateau uplift during the Miocene, as reported elsewhere. The zircon fission-track data further suggest that exhumation of the Namche Barwa massif between 7 and 3 Ma was much slower than the ∼10 km/m.y. rate recorded in the recent past. Detrital apatite fission-track data indicate deformation of the Siwaliks due to forward propagation of the frontal thrust since around 1 Ma.

The Orinoco turbidite system: Tectonic controls on sea-floor morphology and sedimentation

Because of its location in an active margin context, the sand-rich Orinoco turbidite system is controlled morphologically and tectonically by the compressional structures of the Barbados prism, and as a consequence, the sedimentation system does not exhibit a classic fan geometry. The sea-floor geometry between the slope of the front of the Barbados prism and the slope of the Guyana margin induces the convergence of the turbidite channels toward the abyssal plain at the front of the Barbados accretionary prism. Also, whereas in most passive margins the turbidite systems are commonly organized upstream to downstream as canyon, then channel levee, then lobes, here, because of the control by active tectonics, the sedimentary system is organized as channel levee, then canyons, then channelized lobes. In shallow water, landward of the prism, the system has multiple sources with several distributaries, and progressively downward, the channel courses are more complex with frequent convergences or divergences that are emphasized by the effects of the undulating sea-floor morphologies. Erosional processes are almost absent in the upper part of the turbidite system shallower than 1500 m (4921 ft). Erosion along channels develops mostly between 2000 and 4000 m (6562 and 13,123 ft) of water depth, above the compressional structures of the Barbados prism. Incisions show irregular meandering and sinuous courses in the low-relief segments and less sinuous courses where channels incise the structures. Larger incisions (canyons) are 3 km (1.9 mi) wide and 300 m (984 ft) deep. The occurrence of different phases of successive incisions is responsible for the development of morphologically correlative terraces in both flanks of the canyons. This might be the consequence of two mechanisms: the tectonic activity of the deformation front characterized by progressive uplift and thrusting of recent sediments, and the superimposition of the cycles of the Orinoco turbidite system. Piston-core surveys have demonstrated that turbidite sediments moving through the channel and canyon system and deposited in the abyssal plain are mostly coarse sandy deposits covered by recent pelagic planktonic-rich sedimentation, which indicates that sand deposition slowed down during the postglacial sea level rise.

A comparison of inverted basins of the Southern North Sea and inverted structures of the external Alps

Abstract For several years, the relationships between extensional and contractional features have been studied in detail by numerous authors, and structural rules suggested regardless of the size and the geodynamic context of the inverted structures. Examples from the external part of the Western Alps and its foreland follow these rules (oblique-slip reactivation, buttress effect of normal faults, short-cuts, preserved half-grabens beneath decollement, forced folds, etc.), but are not controlled by the same geodynamic conditions as intra-plate inverted basins. The Jura platform, which is not controlled by extensional tectonics, shows fault reactivation, buttress effects and preserved structures beneath the decollement. Inverted structures are nicely illustrated in the Dauphinois domain of the stretched western Tethyan margin, but the effect of lithospheric weakening inherited from the stretching was reduced at the time of the Neogene shortening, and the maximum burial is induced by nappe loading. Therefore the relative timing of inversion tectonics, maximum burial and maximum temperature are very different in the Bourg-d’Oisans half-graben of the Dauphinois domain than for intra-plate inverted basins of the Southern North Sea, e.g. the Broad Fourteens Basin. Nonetheless, fold structures overlapped by major unconformities developed during early deformation events in restricted areas of the Western Alps. The example of structures in the Devoluy is linked to the inversion of the Vocontian basin, and is followed by a renewal of subsidence prior to the later Alpine shortening. Therefore, the Alps provide geometrical field examples of the interaction between stretching and shortening events, and may give some insight into the geometry of structures on the seismic scale. Intra-plate inverted basins provide examples of folded and faulted structures overlapped by sediments and provide a means of explaining the early shortening events that occurred in the Western Alps which were not related to the alpine evolution.

Chapter 14 Review of the tectonic controls and sedimentary patterns in late neogene piggyback basins on the barbados ridge complex

Abstract A review of the tectonic control and sedimentary patterns of the late Neogene piggyback basins of the Barbados Ridge Complex is proposed, mainly based on the analysis of seismic reflection profiles and sidescan sonar images. The geometrical relationships between the thrust faults and the syntectonic deposits show that the frontal piggyback basins are controlled by active thrusting. Deformations, with distinct scales, control their evolution: (1) a rapid tilting, strictly localized at the back limb of anticlines, and attributed to migration of active axial surfaces during fault-bend fold propagation; (2) a complex activation of the major thrust system, at a scale of a few kilometers: blind thrusts corresponding to frontal propagation develop seaward whereas motion along thrusts occurs backward. At about 50 km back of the deformation front, abundant muddy material raises up through pre-existing faults and disturbs arcward piggyback basins. The superimposition of the diapiric structures upon deformations linked to tectonic accretion (development of backthrusts and reactivating of forward verging thrusts) and inherited oceanic basement ridges leads to the individualized development of sub-basins bounded by steep topographic features. Most of the sediments of the South Barbados piggyback basins originate from the South American continent and are massively transported to the abyssal plain through canyons. Their course is driven by the main regional structures and their morphology reflects the tectonic activity of the features where they run. Oceanic ridges, by damming and collecting turbidite material control the thickness of sediments added to the complex and then the depth of the decollement and size and filling of the piggyback basins. Tectonics, by generating routes along the faults and excess fluid pressures control the main location and importance of mud diapirs and authigenic deposits. These stiff carbonate crusts preferentially develop on diapiric domes or on the back limb of anticlines up to the edge of supra-prism basins. Clayey diapiric material may be found within the basins where they form important mud flows as well as sliding masses. Tectonic and diapiric structures control gravity reworking processes, whereas structural relieves locally disturb bottom currents and hence control some erosion processes. The development of the piggyback basins of the BRC is closely linked to the evolution of a thrust wedge. The formation of frontal basins is mainly controlled by a forward-verging thrust system that forms a brittle wedge, whereas the development of the arcward basins is mainly controlled by subcretion of deep muds that induces mud diapirism, ductile deformation in the lowermost part of the wedge, a regional gentle slope, and the occurrence of both backward and forward verging thrusts. The evolution of the piggyback basins of the BRC also reflects the north to south changes in width and thickness of the wedge which are mainly related to variations of the increase of the sediment supply. Piggyback basins then evolve from minor depressions filled with very poor sediments in the north to about 10 km long overfilled basins in the south of the complex where abundant sedimentation occurs.

Stable Drainage Pattern and Variable Exhumation in the Western Himalaya since the Middle Miocene

Sedimentary records in peripheral basins of mountain belts record changes in erosion dynamics and drainage-network reorganization, but it is often difficult to discriminate between these different controls. Geochemical provenance data on paleo-Indus deposits from the western Himalayan foreland provide constraints on the possible variation of the position of the drainage divide between the Indus and Ganges river systems. Here we present geochemical (trace element and Hf-Nd isotopic) and thermochronological (detrital zircon fission-track [DZFT]) analyses of modern Indus and Miocene Siwalik sediments from northern Pakistan and compare these with published data on the Indus Fan. Available bedrock isotopic data are used to define three end-member sediment sources (Himalaya, Karakorum, and the Kohistan-Ladakh arc) and to calculate the contribution of each of these sources to the foreland basin and Indus Fan. Our results indicate that since the Miocene the contribution of the Himalayan rivers reaching the Indus in the foreland remained constant, whereas the contributions of sediment sources of the upper Indus catchment changed: those of the Kohistan-Ladakh arc diminished strongly in favor of Karakorum and Himalayan sources. Analysis of the DZFT data from the Miocene foreland basin and sediments of the modern upper Indus reach suggests that the exhumation pattern changed due to an increase in exhumation rate of the Karakorum and Himalayan units of the syntaxis since Miocene times, whereas that in the Kohistan-Ladakh arc remained relatively stable. These results imply that the Indus sediments record changing relative erosion rates in the different source regions rather than widespread drainage rearrangement, as suggested previously.

The tectonics and paleo-drainage of the easternmost Himalaya (Arunachal Pradesh, India) recorded in the Siwalik rocks of the foreland basin

The Siwalik sedimentary rocks of the Himalayan foreland basin preserve a record of Himalayan orogenesis, paleo-drainage evolution, and erosion. This study focuses on the still poorly studied easternmost Himalaya Siwalik record located directly downstream of the Namche Barwa syntaxis. We use luminescence, palaeomagnetism, magnetostratigraphy, and apatite fission-track dating to constrain the depositional ages of three Siwalik sequences: the Sibo outcrop (Upper Siwalik sediments at ca. 200–800 ka), the Remi section (Middle and Upper Siwalik rocks at >0.8–<8.8±2.4 Ma), and the Siang section (Middle Siwalik rocks at <9.3±1.5 to <13.5±1.5 Ma). Cretaceous-Paleogene detrital zircon and apatite U-Pb ages, characteristic of the Transhimalayan Gangdese Batholiths that crop out northwest of the syntaxis, are present throughout the Sibo, Remi, and Siang successions, confirming the existence of a Yarlung-Brahmaputra connection since at least the Late Miocene. A ca. 500 Ma zircon population increases up section, most strikingly sometime between 3.6 to 6.6 Ma, at the expense of Transhimalayan grains. We consider the ca. 500 Ma population to be derived from the Tethyan or Greater Himalaya, and we interpret the up-section increase to reflect progressive exhumation of the Namche Barwa syntaxis. Early Cretaceous zircon and apatite U-Pb ages are rare in the Sibo, Remi, and Siang successions, but abundant in modern Siang River sediments. Zircons of this age range are characteristic of the Transhimalayan Bomi-Chayu batholiths, which crop out east of the syntaxis and are eroded by the Parlung River, a modern tributary of the Siang River. We interpret the difference in relative abundance of Early Cretaceous zircons between the modern and ancient sediments to reflect capture of the Parlung by the Siang after 800 ka.

Weathering in the Himalaya, an East-West Comparison: Indications from Major Elements and Clay Mineralogy

Studying past weathering regimes is important for a better understanding of the influence of climate on weathering, erosion, and runoff. The Himalayan foreland basin contains a record of tectonics and paleoclimate since Miocene times. Spanning the entire mountain range, the Mio-Pliocene detrital Siwalik Group allows studies to directly compare the western and eastern Himalaya within similar sedimentary settings. In this study, we use major elements and clay mineralogy to reconstruct the weathering regime along strike since Miocene times. We studied previously dated Dharamsala (pre-Siwalik) and Siwalik sections in the western (Joginder Nagar, Jawalamukhi, and Haripur Kolar) and eastern (Kameng) Himalaya in order to constrain variations in weathering regimes along strike. The compilation of the three sections in the west makes for one of the longest continuous sedimentary records in the Himalaya, spanning over 20 My. The K/Al ratio is used as a reliable weathering proxy and shows a trend toward more intense weathering over time in both the west and the east, but with sediments in the western Himalaya generally more weathered than those in the east, despite higher precipitation in the east. Clay minerals and major elements indicate similar lateral variations in weathering. More intense weathering in the west is linked to a more seasonal climate, permitting weathering of sediments during the dry season, whereas higher runoff in the east leads to more rapid erosion and sediment transport, inhibiting extensive weathering.