GPS survey of the western Tien Shan

This report summarizes the background, field work, data collection and analysis, and future plans associated with a collaborative GPS experiment in the Tien Shan of the former Soviet Union. This project involves the amalgamation of two, separately funded projects, which were proposed separately by PIs Hamburger and Reilinger (NSF number EAR-9115159 and NASA number NAG5-1941) and Molnar and Hager (NSF number EAR9117889 and NASA number NAG5-1947). In addition, the work is being conducted under the auspices of the US-USSR Agreement on Cooperation in the Field of Environmental Protection, with support from the United States Geological Survey

Pn anisotropy beneath the South Island of New Zealand and implications for distributed deformation in continental lithosphere

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 119 (2014): 7745–7767, doi:10.1002/2014JB011233.Pn travel times from regional earthquakes recorded both by stations on New Zealand and by ocean bottom seismographs deployed offshore indicate anisotropy in the uppermost mantle beneath the region. The largest anisotropy of ~8% (±2%, 1σ) lies beneath the deforming part of the South Island to just off its West Coast, a zone roughly 100–200 km wide. The fastest propagation is aligned N60°E (±3°), essentially parallel to the largely strike-slip relative plate motion since 20 Ma, also ~ N60°E. The magnitude of anisotropy decreases abruptly northwest and southeast of this zone, and on the southeast side of the island, the orientation of fastest propagation is between N32°W and N-S. The ~ N60°E orientation of fast propagation is consistent with finite strain within the uppermost part of the mantle lithosphere if the measured 850 km of displacement of the Pacific plate past the Australia plate is spread over a region with a width of 100–200 km. The agreement of this orientation of fast propagation with the orientation or relative plate motion suggests the possibility of but does not require some dynamic recrystallization in rock as cold as 500–800°C, where Peierls creep seems to be the likely deformation mechanism. Such a strain distribution matches deformation of a thin viscous sheet that obeys a constitutive relationship of the form inline image, where inline image is the average strain rate and τ is the operative deviatoric stress, with an average value of n ≈ 3–10. Presumably, the NW-SE fast propagation in the region southeast of the island results from strain that precedes the Cenozoic deformation that has shaped the island.This research was supported by the National Science Foundation under grants EAR-0409564, EAR-0409609, and EAR-0409835.2015-04-2

A multi‐scale study of the dominant catchment characteristics impacting low‐flow metrics

Low flows can impact water use and instream ecology. Therefore, reliable predictions of low-flow metrics are crucial. In this study, we assess which catchment characteristics (climate, topography, geology and landcover) can explain the spatial variability of low-flow metrics at two different scales: the regional scale and the small headwater catchment scale. For the regional-scale analysis, we calculated the mean 7-day annual minimum flow (qmin), the mean of the flow that is exceeded 95% of the year (q95), and the master recession constant (C) for 280 independent gauging stations across the Swiss Plateau and the Swiss Alps for the 2000–2018 period. We assessed the relation between 44 catchment characteristics and the three low-flow metrics based on correlation analysis and a random forest model. Low-flow magnitudes across the Swiss Plateau were positively correlated with the fraction of the area covered by sandstone bedrock or alluvium, and with the area that has a slope between 10° and 30°. Across the Swiss Alps, low-flow magnitudes were positively correlated with the fraction of area with slopes between 30° and 60°, and the area with glacial deposits and debris cover. There was good agreement between observations and predictions by the random forest regression model with the top 11 catchment characteristics for both regions: for 80% of the Swiss Plateau catchments and 60% of the Swiss Alpine catchments, we could predict the three low-flow metrics within an error of 30%. The residuals of the regression model, however, varied across short distances, suggesting that local catchment characteristics affect the variability of low-flow metrics. For the local-scale headwater catchments, we conducted 1-day snapshot field campaigns in 16 catchments during low-flow periods in 2015 and 2016. The measurements in these sub-catchments also showed that areas with sandstone bedrock and a good storage-to-river connectivity had above average low-flow magnitudes. Including knowledge on local catchment characteristics may help to improve regional low-flow predictions, however, not all local catchment characteristics were useful descriptors at larger scales

GPS survey of the western Tien Shan

There were two major developments in 1994 in our collaborative GPS experiment in the Tien Shan of the Former Soviet Union (FSU). Both were motivated by our expectation that we will ultimately obtain better science at lower cost if we involve our colleagues in the FSU more deeply in (1) the collection and (2) the analysis of data. As an experimental test of the concept of having our local collaborators carry out the field work semi-autonomously, we sent 6 MIT receivers to the Tien Shan for a period of 3 months. To enable our collaborators to have the capability for data analysis, we provided computers for two data analysis centers and organized a two-week training session. This report emphasizes the rationale for deeper involvement of FSU scientists, describes the training sessions, discusses the data collection, and presents the results. We also discuss future plans. More detailed discussion of background, general scientific objectives, discussions with collaborators, and results for the campaigns in 1992 and 1993 have been given in previous reports

Application of Global Positioning Measurements to Continental Collision in the Pamir-Tien Shan Region, Central Asia and GPS Survey of the Western Tien Shan

In this report, we summarize what we have accomplished with five years of funding from NASA under its DOSE program, and with a comparable level of funding from NSF. We describe the development of a GPS network in the Tien Shan of Kyrgyzstan and Kazakhstan of the former Soviet Union, the analysis of data, and the main results. This discussion presents the state of the current network, which has grown significantly since the termination of our DOSE grants, with continued support both from NSF through its continental dynamics program and from NASA's SENH program. Although grants from NASA's DOSE program did not support this growth not directly, it did so indirectly by building the infrastructure that has enabled further expansion in an area where otherwise there would be only a small GPS presence. We note how the network has grown over time, but the emphasis of this discussion is on the quantity and quality of measurements that we have made

Upper mantle seismic anisotropy at a strike-slip boundary: South Island, New Zealand

New shear wave splitting measurements made from stations onshore and offshore the South Island of New Zealand show a zone of anisotropy 100–200 km wide. Measurements in central South Island and up to approximately 100 km offshore from the west coast yield orientations of the fast quasi-shear wave nearly parallel to relative plate motion, with increased obliquity to this orientation observed farther from shore. On the eastern side of the island, fast orientations rotate counterclockwise to become nearly perpendicular to the orientation of relative plate motion approximately 200 km off the east coast. Uniform delay times between the fast and slow quasi-shear waves of nearly 2.0 s onshore continue to stations approximately 100 km off the west coast, after which they decrease to ~1 s at 200 km. Stations more than ~300 km from the west coast show little to no splitting. East coast stations have delay times around 1 s. Simple strain fields calculated from a thin viscous sheet model (representing distributed lithospheric deformation) with strain rates decreasing exponentially to both the northwest and southeast with e-folding dimensions of 25–35 km (approximately 75% of the deformation within a zone 100–140 km wide) match orientations and amounts of observed splitting. A model of deformation localized in the lithosphere and then spreading out in the asthenosphere also yields predictions consistent with observed splitting if, at depths of 100–130 km below the lithosphere, typical grain sizes are ~ 6–7 mm.New Zealand. Ministry of Research, Science, and TechnologyNational Science Foundation (U.S.). Continental Dynamics Program (Grant EAR-0409564)National Science Foundation (U.S.). Continental Dynamics Program (Grant EAR-0409609)National Science Foundation (U.S.). Continental Dynamics Program (Grant EAR-0409835

Phi Mesons from a Hadronic Fireball

Production of $\phi$ mesons is considered in the course of heavy-ion collisions at SPS energies. We investigate the possible difference in momentum distributions of $\phi$ mesons measured via their leptonic ($\mu^+\mu^-$) and hadronic ($K^+K^-$) decays. Rescattering of secondary kaons in the dense hadron gas together with the influence of in-medium kaon potential can lead to a relative decrease of a $\phi$ yield observed in the hadronic channel. We analyze how the in-medium modifications of meson properties affect apparent - reconstructed momentum distributions of $\phi$ mesons. Quantitative results are presented for central Pb+Pb collisions at $E_{beam}=158 GeV/A$.Comment: style Revtex4,9 pages, 5 figures. submitted to Phys. Rev.

Thermochronology of the modern Indus River bedload: New insight into the controls on the marine stratigraphic record

The Indus River is the only major drainage in the western Himalaya and delivers a long geological record of continental erosion to the Arabian Sea, which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains. In this study we collected thermochronologic data from river sediment collected near the modern delta. U-Pb ages of zircons spanning 3 Gyr show that only ∼5% of the eroding crust has been generated since India-Asia collision. The Greater Himalaya are the major source of zircons, with additional contributions from the Karakoram and Lesser Himalaya. The 39Ar/40Ar dating of muscovites gives ages that cluster between 10 and 25 Ma, differing from those recorded in the Bengal Fan. Biotite ages are generally younger, ranging 0–15 Ma. Modern average exhumation rates are estimated at ∼0.6 km/m.y. or less, and have slowed progressively since the early Miocene (∼20 Ma), although fission track (FT) dating of apatites may indicate a recent moderate acceleration in rates since the Pliocene (∼1.0 km/m.y.) driven by climate change. The 39Ar/40Ar and FT techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean. Localized regions of tectonically driven, very rapid exhumation (e.g., Nanga Parbat, S. Karakoram metamorphic domes) do not dominate the erosional record

Pleistocene drainage reorganization driven by the isostatic response to deep incision into the northeastern Tibetan Plateau

Pleistocene drainage basin integration led to progressive excavation of Tertiary-Quaternary sedimentary basins along the Yellow River in the northeastern Tibetan Plateau. Cosmogenic burial dating of ancestral river deposits and basin fill from two key watershed divides confirms a fluvial connection between basins at 0.5–1.2 Ma, prior to excavation by the Yellow River. Preservation of the relict depositional surface that represents the maximum height of basin fill allows reconstruction of the volume of eroded material across a broad region. We quantify the isostatic response to this erosional unloading using a two-dimensional (not one-dimensional) flexural model. Calculated maximum vertical displacements for different effective elastic thicknesses vary from ~160 m to ~260 m near the Pleistocene spillway from the Qinghai paleo-lake. We suggest that the isostatic response to fluvial excavation along the Yellow River defeated local tributaries, isolated Lake Qinghai, and led to the development of an internally drained basin in the past 0.5–1.2 Ma