The Siwalik formations of northern Pakistan consist of deposits of ancient rivers that existed throughout the early Miocene through the late Pliocene. The formations are highly fossiliferous with a diverse array of terrestrial and freshwater vertebrates, which in combination with exceptional lateral exposure and good chronostratigraphic control allows a more detailed and tem porally resolved study of the sediments and faunas than is typical in terrestrial deposits. Conse quently the Siwaliks provide an opportunity to document temporal differences in species richness, turnover, and ecological structure in a terrestrial setting, and to investigate how such differences are related to changes in the fluvial system, vegetation, and climate. Here we focus on the interval between 10.7 and 5.7 Ma, a time of significant local tectonic and global climatic change. It is also the interval with the best temporal calibration of Siwalik faunas and most comprehensive data on species occurrences. A methodological focus of this paper is on controlling sampling biases that confound biological and ecological signals. Such biases include uneven sampling through time, differential preservation of larger animals and more durable skeletal elements, errors in age-dating imposed by uncertainties in correlation and paleomagnetic timescale calibrations, and uneven taxonomic treatment across groups. We attempt to control for them primarily by using a relative-abun dance model to estimate limits for the first and last appearances from the occurrence data. This model also incorporates uncertainties in age estimates. Because of sampling limitations inherent in the terrestrial fossil record, our 100-Kyr temporal resolution may approach the finest possible level of resolution for studies of vertebrate faunal changes over periods of millions of years. Approximately 40,000 specimens from surface and screenwash collections made at 555 localities form the basis of our study. Sixty percent of the localities have maximum and minimum age esti mates differing by 100 Kyr or less, 82% by 200 Kyr or less. The fossils represent 115 mammalian species or lineages of ten orders: Insectivora, Scandentia, Primates, Tubulidentata, Proboscidea, Pholidota, Lagomorpha, Perissodactyla, Artiodactyla, and Rodentia. Important taxa omitted from this study include Carnivora, Elephantoidea, and Rhinocerotidae. Because different collecting methods were used for large and small species, they are treated separately in analyses. Small spe cies include insectivores, tree shrews, rodents, lagomorphs, and small primates. They generally weigh less than 5 kg. The sediments of the study interval were deposited by coexisting fluvial systems, with the larger emergent Nagri system being displaced between 10.1 and 9.0 Ma by an interfan Dhok Pathan sys tem. In comparison to Nagri floodplains, Dhok Pathan floodplains were less well drained, with smaller rivers having more seasonally variable flow and more frequent avulsions. Paleosol se quences indicate reorganization of topography and dra...
This study uses stable isotope variation within individual Mio-Pliocene paleosols to investigate subkilometer-scale phytogeography of late Miocene vegetation change in southeast Asia between ca. 8.1 and 5 Ma, a time interval that coincides with dramatic global vegetation change. We examine trends through time in the distribution of low-latitude grasses (C 4 plants) and forest (C 3 plants) on Indo-Gangetic fl oodplains using carbon (δ 13 C) and oxygen isotopic (δ 18 O) values in buried soil carbonates in Siwalik Series sediments exposed in the Rohtas Anticline, northcentral Pakistan. Revised, high-resolution magnetostratigraphy and a new 40 Ar/ 39 Ar date provide improved age control for the 2020 m Rohtas section. Carbon isotope results capture lateral variability of C 3 versus C 4 plants at fi ve stratigraphic levels, R11 (8.0 Ma), R15 (6.74-6.78 Ma), R23 (5.78 Ma), R29 (4.8-4.9 Ma), and upper boundary tuff (UBT; 2.4 Ma), using detailed sampling of paleosols traceable laterally over hundreds of meters. Paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fl uvial sediments: channel fi ll, crevasse-splay, and fl oodplain environments. δ 13 C results show that near the beginning (8.0 Ma) and after (4.0 Ma) the period of major ecological change, vegetation was homogeneously C 3 or C 4 , respectively, regardless of paleo-landscape position. In the intervening period, there is a wide range of values overall, with C 4 grasses fi rst invading the drier portions of the system (fl oodplain surfaces) and C 3 plants persisting in moister settings, such as topographically lower channel swales. Although abrupt on a geologic timescale, changes in abundance of C 4 plants are modest (~2% per 100,000 yr) compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the Quaternary. Earlier research documented a sharply defi ned C 3 to C 4 transition in Pakistan between 8.1 and 5.0 Ma, based on vertical sampling, but this higher-resolution study reveals a more gradual transition between 8.0 and 4.5 Ma in which C 3 and C 4 plants occupied different subenvironments of the Siwalik alluvial plain.δ 18 O values as well as δ 13 C values of soil carbonate increase up section at Rohtas, similar to isotope trends in other paleosol records from the region. Spatially, however, there is no correlation between δ 13 C and δ 18 O values at most stratigraphic levels. This implies that the changes in soil hydrology brought about by the shift from forest to grassland (i.e., an increase in average soil evaporation) did not produce the shift through time in δ 18 O values. We interpret the trend toward heavier soil carbonate δ 18 O values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 Ma.
22p.International audienceThe 8 October 2005 Kashmir earthquake ruptured an out-of-sequence Himalayan thrust known as the Balakot-Bagh thrust. The earthquake's hypocenter was located at a depth of 15 km on the ramp close to a possible ramp/flat transition. In the weeks following the earthquake a GPS network was installed to measure postseismic displacement. The initial measurements in November 2005 were followed by other campaigns in January and August 2006, in March and December 2007, and in August 2008 and 2009. Two hypotheses were tested: post-seismic displacements controlled by viscous relaxation of the lower crust or by afterslip along a flat north of the ramp affected by the main shock. A single Newtonian viscosity for the different periods cannot be determined by numerical simulations of viscous relaxation, which may indicate that the viscosity of the lower crust is non-Newtonian or that viscous relaxation does not control postseismic displacements. Numerical simulations using dislocations in a uniform elastic half-space indicate afterslip north of the ramp of the earthquake along a flat connected to the ramp. Slip along the northwestern portion of the flat accrued to about 285 mm between November 2005 and August 2006, while slip along the southeastern portion accrued to 130 mm over the same time period. Residual misfit of the observed and predicted displacements clearly indicated that afterslip is a better explanation for the observations than the hypothesis of viscous relaxation. The time evolution of the afterslip was found to be consistent with that predicted from rate-strengthening frictional sliding
The 18-14 Ma Kamlial Formation Himalayan foreland basin sedimentary rocks in the Chinji Village region, Potwar Plateau, Pakistan, are characterized by: (1) lithofacies indicative of deposition by a large river; (2) a dominant magmatic arc provenance completely unlike the 'recycled orogen' foreland basin deposits stratigraphically below, above, or coeval with these rocks; and (3) subordinate contribution from a rapidly exhuming source, interpreted as either the Nanga Parbat Haramosh Massif or the southern margin of the Asian crust. The start of Kamlial Formation deposition at this locality at 18 Ma marks a major break with the older Murree Formation rocks, which were deposited by rivers draining predominantly the Himalayan thrust stack south of the arc. We interpret this change as the result of diversion of the paleo-Indus River to its present position, which crosses the Kohistan arc and Himalayas and debouches into the foreland. If the rapidly exhuming subordinate source region were the Nanga Parbat Haramosh Massif, then initiation of its uplift would have resulted in significant arc
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