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The distribution and magnitude of forces driving lithospheric deformation in the India‐Eurasia collision zone have been debated over many decades. Here we test a two‐dimensional (2‐D) Thin Viscous Shell approach that has been adapted to explicitly account for displacement on major faults and investigate the impact of lateral variations in depth‐averaged lithospheric strength. We present a suite of dynamic models to explain the key features from new high‐resolution Sentinel‐1 Interferometric Synthetic Aperture Radar as well as Global Navigation Satellite System velocities. Comparisons between calculated and geodetically observed velocity and strain rate fields indicate: (a) internal buoyancy forces from Gravitational Potential Energy acting on a relatively weak region of highest topography (>2,000 m) contribute to dilatation of the high plateau and contraction on the margins; (b) a weak central Tibetan Plateau (∼1021 Pa s compared to far‐field depth‐averaged effective viscosity of at least 1022–1023 Pa s) is required to explain the observed long‐wavelength eastward velocity variation; (c) localized displacement on fault systems enables strain concentration and clockwise rotation around the Eastern Himalayan Syntaxis. We discuss the tectonic implications for rheology of the lithosphere, distribution of geodetic strain, and partitioning of active faulting and seismicity. Plain Language Summary: The collision of the Indian Plate with Eurasia has created the Tibetan Plateau, one of the largest deforming regions in the continents. The mode of deformation has been a focus for heated debate and has inspired two contrasting tectonic models: (a) The deformation is localized on major faults separating "blocks" or (b) the strain is distributed throughout a "continuum." We approximate the India‐Eurasia collision by treating the continent as a thin viscous shell with regional variations in strength, explicitly accounting for displacements on selected major faults. We present a suite of models to explain the key features of new geodetic measurements from satellites. The best‐fit model involves a weak Tibetan Plateau, a particularly weaker central plateau, and four strong regions outside the plateau, and allows displacements on major faults. This represents the deformation field of the India‐Eurasia collision zone as a combination of continuous distributed deformation and focused strain on major faults. Key Points: A suite of faulted viscous shell models testing key parameters explain new observations from geodesy for the India‐Asia collisionThe India‐Asia collision is explained by the balance between buoyancy and boundary forces, slip on faults, and internal strength variationsCentral Tibetan Plateau has a vertically‐averaged effective viscosity of ∼1021 Pa s, 1–2 orders lower than the surrounding area [ABSTRACT FROM AUTHOR]

Dynamic subsidence and uplift of plates are often explained by the vertical motion of density anomalies in the mantle. Such models predict surface vertical motion rates of less than 100 m Myr−1 at long‐wavelengths with a timespan of tens of Myr. However, during periods of relative sea‐level stability, some of the phases of vertical motion on stable portions of plates have occurred at rates greater than 100 m Myr−1 during episodes that may last only a few Myr. Here, we show that vertical surface motions, with rates greater than 100 m Myr−1 and durations less than a few Myr, can be explained by changes in basal shear stress caused by variation in horizontal motion of a viscous plate relative to the asthenosphere. We apply our physical model to the short‐lived mid‐Eocene immersion of the southern margin of Australia. [ABSTRACT FROM AUTHOR]

Crustal deformation along the present-day northern margin of the Tibetan Plateau has occurred since mid to late Eocene time, soon after India collided with Eurasia. Assuming that on these distance and time scales the lithosphere can be approximated using a thin viscous sheet, we show that far-field lithospheric deformation caused by an indenting boundary is expected to start shortly after collision begins. When a strong region analogous to cratonic lithosphere like that of the Tarim Basin is included in the calculations, shear and vertical strain rates concentrate along the southern edge of the strong region, resulting in enhanced deformation there. In calculations scaled to the dimensions of India and the Tibetan Plateau, shear and vertical strain rates on the northern edge of the plateau increase soon after collision begins. Crustal thickening accelerates later in the calculations. We propose that the apparent increased exhumation (and perhaps surface uplift) around 10 Ma may signal the arrival of the thickening front. Citation: Dayem, K. E., P. Molnar, M. K. Clark, and G. A. Houseman (2009), Far-field lithospheric deformation in Tibet during continental collision, Tectonics, 28, TC6005, doi: 10.1029/2008TC002344.

A marked contrast in strength (or viscosity) within a continuously deforming zone can lead to concentration of shear strain in the weaker material adjacent to the boundary between them, but localization comparable to the width of the Altyn Tagh shear zone requires an additional weakening process. During numerical experiments on a thin viscous sheet indented by a rigid object, a shear zone develops adjacent to a strong region mimicking the Tarim Basin, when the boundary between the weak and strong regions is oblique to the orientation of convergence. The width of this shear zone narrows with increased strain, and for comparable penetration by the indenter, the strain is more concentrated for larger values of n, the exponent that relates strain rate to a power of stress, and for smaller values of the Argand number Ar, a measure of buoyancy-induced stress relative to viscous stress. Increasing concentration of shear occurs as the indentation develops without weakening because of a change in material properties. Additional localization develops with the inclusion of strain-dependent weakening associated with, for instance, a temperature increase due to shear heating. For such localization to scale to the width of the Altyn Tagh fault zone of Tibet, the initial temperature near the Moho must be relatively low (~600[degrees]C), and a large value of n (~10) is required. This suggests that deformation there is described by a high-strength flow law, such as that proposed by Evans and Goetze (1979), in which the lithosphere would deform approximately plastically. doi: 10.1029/2008TC002264.

The separation of zones of apparent downwelling flow at the ends of the Sierra Nevada suggests a relatively large wavelength (~500 km) of unstable growth, but Rayleigh-Taylor instabilities for plausible rheological structures with a fixed top boundary condition require much shorter wavelengths (

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2007.01252.x Byline: HEATHER L. REYNOLDS (*[dagger]), GARY G. MITTELBACH ([dagger]), TARA L. DARCY-HALL (*), GREGORY R. HOUSEMAN ([double dagger]), KATHERINE L. GROSS ([dagger]) Keywords: clonal plants; coexistence; grasslands; resource heterogeneity; soil nutrients; species diversity; species sorting Abstract: Summary Coexistence theory predicts that greater heterogeneity of resources or other fitness-constraining environmental factors will promote species diversity, yet this classic mechanism of coexistence has rarely been tested in manipulative field experiments. Here we present results from the fourth year of a long-term experiment designed to test the heterogeneity-diversity hypothesis in a low productivity grassland where we directly manipulated the spatial heterogeneity of soil nutrients. In addition to unfertilized controls, we created heterogeneous and homogeneous nutrient enrichment treatments by applying fertilizer to field plots in a patchy or uniform manner. Fertilization treatments were crossed with seed addition and cover reduction treatments, to allow examination of the effects of recruitment limitation and competition from established vegetation on diversity responses to nutrient heterogeneity. In contrast to predictions, we found that spatially heterogeneous soil nutrient supply did not promote increased species richness or greater species sorting relative to a uniform supply of soil nutrients. Instead, both nutrient enrichment treatments depressed plant species richness (especially at small spatial scales) and, depending on cover reduction and on spatial location in the field, increased the abundance of two spreading clonal species. Our results do not support the heterogeneity-diversity hypothesis, and suggest that the response of clonal species to nutrient enrichment may constrain how plant diversity responds to resource heterogeneity. Future tests of the heterogeneity-diversity hypothesis should explicitly consider the scale of resource heterogeneity and foraging capability of the plant species in the community. Author Affiliation: (*)Department of Biology, Jordan Hall 142, Indiana University, Bloomington, IN 47405-7005, USA, and ([dagger])W.K. Kellogg Biological Station, Michigan State University, Hickory Corners, MI 49060-9516, USA ([double dagger])Department of Ecology and Evolution, 2041 Haworth Hall, University of Kansas, Lawrence, KS66045, USA Article History: Received 17 August 2006; revision accepted 21 March 2007 Handling Editor: Michael Hutchings Article note: Correspondence: Heather L. Reynolds (tel. +1 812 855 0792; fax +1 812 855 6705; e-mail hlreynol@indiana.edu).

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.11.001 Byline: Trudi Hoogenboom, Gregory A. Houseman Keywords: Venus Abstract: In this study we explore the idea that coronae have formed on Venus as a result of gravitational (Rayleigh-Taylor) instability of the lithosphere. The lithosphere is represented by a system of stratified homogeneous viscous layers (low-density crust over high density mantle, over lower density layer beneath the lithosphere). A small harmonic perturbation imposed on the base of the lithosphere is observed to result in gravitational instability under the constraint of assumed axisymmetry. Topography develops with time under the influence of dynamic stress associated with downwelling or upwelling, and spatially variable crustal thickening or thinning. Topography may therefore be elevated or depressed above a mantle downwelling, but the computed gravity anomaly is always negative above a mantle downwelling in a homogeneous asthenosphere. The ratio of peak gravity to topography anomaly depends primarily on the ratio of crust to lithospheric viscosity. Average observed ratios are well resolved for two groups of coronae ([approximately equal to]40 mgalakm.sup.-1), consistent with models in which the crust is perhaps 5 times stronger than the lithosphere. Group 3a (rim surrounding elevated central region) coronae are inferred to arise from a central upwelling model, whereas Group 8 (depression) coronae are inferred to arise from central downwelling. Observed average coronae radii are consistent with a lithospheric thickness of only 50 km. An upper low-density crustal layer is 10-20 km thick, as inferred from the amplitude of gravity and topography anomalies. Author Affiliation: School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK Article History: Received 4 October 2004; Revised 4 August 2005

Bird populations in grasslands have experienced declines coinciding with loss and fragmentation of prairies. The United States Department of Agriculture (USDA)‐administered Conservation Reserve Program (CRP) is the most extensive grassland restoration program in North America and it has especially benefitted grassland birds. Grazing by domestic cattle has been restricted in CRP during avian nesting seasons despite the potential improvements in structuring habitat for a greater diversity of grassland bird species. Potential negative consequences of grazing in CRP grasslands include trampling of nests by cattle, reductions in nest concealment from predators, and attraction of brood‐parasitic brown‐headed cowbirds (Molothrus ater). We designed an experiment to test for effects of cattle grazing in CRP fields during the nesting season on nest survival and brood parasitism of 5 bird species that commonly nest in CRP grasslands: mourning dove (Zenaida macroura), grasshopper sparrow (Ammodramus savannarum), dickcissel (Spiza americana), and eastern (Sturnella magna) and western (S. neglecta) meadowlarks. Grazing was implemented during summers 2017 and 2018 on 17 of 36 fields followed by a year of rest on all fields in 2019. Of the 879 nests on grazed fields, only 4 were likely trampled by cattle (vs. 54% of all nests estimated as failing because of depredation). Experimental grazing (grazed vs. ungrazed fields) had variable effects on nest survival and cowbird parasitism among the bird species analyzed. Negative effects of grazing on daily nest survival of dickcissel and meadowlarks were apparent, at least in some years. We found no direct effects of grazing on nest survival of mourning dove or grasshopper sparrow. Probability and intensity (cowbird offspring/nest) of cowbird parasitism in dickcissel nests was higher on grazed versus ungrazed sites but only in conservation practice (CP) CP2 (vs. CP25 fields). Parasitism probability of grasshopper sparrow nests by cowbirds was higher on grazed fields in the 2 years after introduction of cattle in 2017. Greater vegetative concealment around nest sites was associated with reduced cowbird parasitism of meadowlark and grasshopper sparrow nests and higher nest survival for grasshopper sparrows. Reductions in vegetative height caused by longer‐term or high‐intensity grazing might therefore have negative consequences for some grassland birds by increasing nest site visibility and exposure to cowbird parasitism. Our results indicate that cattle grazing in CRP fields during the nesting season might have some negative effects on reproductive success of some grassland bird species, at least in the short term; however, the potential improvements of structuring habitat to accommodate more grassland bird species and increasing landowner participation in the CRP are considerable. [ABSTRACT FROM AUTHOR]

The geodynamics of the Tarim Basin and the Tian Shan located in central Asia was investigated using a series of numerical experiments that draws together the area's lithospheric strength parameters. The approximation of of the deformation field in the India-Asia collision and the crustal thickness distributions were shown by the use of thin viscous sheet model representation. Results suggests that a strong Tarim Basin transfers strain to the Tian Shan which causes significant crustal thickening in the region.