[1] An extensive series of laboratory experiments is used to investigate the behavior of sheared thermal plumes. The plumes are generated by heating a small circular plate on the base of a cylindrical tank filled with viscous fluid and then sheared by rotating a horizontal lid at the fluid surface. The motion of passive tracers in the plumes is visualized by the release of several dye streams on the hot plate. We systematically examine the dependence of the convective flow on four dimensionless numbers: a velocity ratio, a Rayleigh number, the viscosity ratio, and an aspect ratio. We identify and delineate two transitions in the convective behavior: from a regime where the plume can spread upstream against the shear to a regime where the entire plume is advected downstream, and from a regime of negligible cross-stream circulation to a regime with significant cross-stream circulation and thermal entrainment. Our analysis of the steady profiles of the plumes shows that they initially rise with a constant vertical rise velocity. This rise velocity depends on the buoyancy flux and ambient viscosity but is almost independent of the centerline plume viscosity, which suggests that most of the thermal plume has a viscosity that is much closer to the ambient viscosity than the centerline viscosity. As the plumes approach the lid, they decelerate as the viscous drag on them steadily increases. The lateral spreading of the plumes under the lid is found to be well described by similarity solutions derived for the spreading of compositional plumes on a rigid surface, if the effective viscosity of the thermal plumes is taken to be the ambient viscosity rather than the centerline viscosity. A similar theoretical model is found to roughly predict the upstream spreading of thermal plumes at low shear, but it breaks down at moderate to high shear, where the entire plumes are advected downstream. When our results are applied to the Earth, we find that mantle plumes are mostly divided into only two flow regimes in the upper mantle: plumes under slow moving plates experience upstream flow and negligible cross-stream circulation, while plumes under faster moving plates (including all Pacific plumes) experience significant cross-stream circulation and are advected downstream. We also demonstrate that geochemical heterogeneities in a plume's source region will result in an azimuthally zoned plume and in an asymmetric geographical distribution of geochemical heterogeneities in the erupted hot spot basalts, as is seen in the Hawaiian, Galápagos, Marquesas, and Tahiti/Society island chains. For individual mantle plumes, we determine their diameter and vertical rise velocity as well as the extent of upstream spreading and the rate of lateral spreading under the lithosphere.
This study details the physiological responses of cork oak (Quercus suber L.) to manipulated water inputs. Treatments named as dry, ambient and wet, which received 80, 100 and 120% of the annual precipitation, respectively, were applied to a Mediterranean woodland in southern Portugal. Tree ecophysiology and growth were monitored from 2003 to 2005. The impacts of the water manipulation were primarily observed in tree transpiration, especially during summer drought. Rainfall exclusion reduced the annual stand canopy transpiration by 10% over the 2-year study period, while irrigation increased it by 11%. The accumulated tree transpiration matched precipitation in spring 2004 and 2005 at the stand level, suggesting that cork oak trees rely on precipitation water sources during the peak of the growing season. However, during the summer droughts, groundwater was the main water source for trees. Despite the significant differences in soil water content and tree transpiration, no treatment effects could be detected in leaf water potential and leaf gas exchange, except for a single event after spring irrigations in the very dry year 2005. These irrigations were intentionally delayed to reduce dry spell duration during the peak of tree growing season. They resulted in an acute positive physiological response of trees from the wet treatment one week after the last irrigation event leading to a 32% raise of stem diameter increment the following months. Our results suggest that in a semi-arid environment precipitation changes in spring (amount and timing) have a stronger impact on cork oak physiology and growth than an overall change in the total annual precipitation. The extreme drought of 2005 had a negative impact on tree growth. The annual increment of tree trunk diameter in the ambient and dry treatments was reduced, while it increased for trees from the wet treatment. Water shortage also significantly reduced leaf area. The latter dropped by 10.4% in response to the extreme drought of 2005 in trees from the ambient treatment. The reduction was less pronounced in trees of the wet treatment (−7.6%), and more pronounced in trees of the dry treatment (−14.7%). Cork oak showed high resiliency to inter-annual precipitation variability. The annual accumulated tree transpiration, the minimum midday leaf water potential and the absolute amount of groundwater used Abbreviations: (A), carbon assimilation; (Amax), maximum carbon assimilation; (CV), crown volume; (DBH), diameter at breast height; (DBH inc), trunk diameter increment; (E), sap flux, tree transpiration; (E/DBH), normalized tree transpiration; (ET), potential evapotranspiration; (gs), stomatal conductance; (gsmax), maximum stomatal conductance; (Gt), whole tree specific hydraulic conductance; (LAD), leaf area density; (LAI), leaf area index; (PAR), photosynthetically active radiation; (PCA), projected crown area; (SLA), specific leaf area; (SWC), soil water content; (T), air temperature; (VPD), vapour pressure deficit; («), leaf water potential; (« md), midday leaf wat...
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[1] Patterns of dike swarms around volcanic centers or above mantle plumes are interpreted by a mechanical analysis of regional and dike-induced stresses, in which dike emplacement is controlled and guided by the stress state. Comparisons of dike patterns with patterns of principal-stress trajectories caused by a source and a regional stress system are commonly used to infer paleostresses. However, dike trajectories are determined by a complex interaction between dike-induced stresses and the source/ regional stress system. We present numerical calculations based on a novel boundary-integral formulation, which examines the simultaneous effects of regional stresses, magma pressure, and dike injection on the local stress state around a continuously curving dike. Dike paths are calculated from the condition that dikes propagate by mode I failure. Our results suggest that the magnitude of the regional stresses would be 2 -5 times higher than previous estimates based on principal-stress trajectory analysis.
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