The dynamical interaction between three-dimensional (3-D) buoyant flow and platedriven mantle flo... more The dynamical interaction between three-dimensional (3-D) buoyant flow and platedriven mantle flow beneath a mid-ocean ridge is examined using a combination of laboratory and numerical experiments. T.. u.i•ue •n•-ß . ,• •aou•.L,,•y setup a layer of viscous fluid is heated from below and cooled from above to drive thermal convection. Forced, plate-driven flow is modeled by dragging mylar sheeting in opposite directions across the fluid surface. Uniform viscosity 3-D numerical models have been designed to simulate the laboratory runs, to provide additional information on the flow, and to attempt to isolate the effects of variable viscosity. In one type of experiment we modeled buoyancy on the scale of the partial melting region with a linear heat source beneath the spreading axis. In a second set of experiments the entire base of the tank is heated in order to investigate the interaction between plate-driven flow and larger-scale (upper mantle) convection. The pattern of segment-scale (-100-150 km) convection beneath a spreading center is found to be a strong function of the spreading rate and the Rayleigh number (Ra) of the buoyant flow. Purely two-dimensional (2-D) flow exists only in the case of low Ra (-105) and faster spreading rates (>4-6 cm/yr half rate). Three dimensionality is strongly enhanced during transient pulses of upwelling. Convection on the scale of the upper mantle can contribute significant long-wavelength spatial (-600-1000 km) and temporal (20-40 m.y.) variability in upwelling rates and temperatures at spreading centers and may provide an alternative model for plume-ridge interaction. For a given Ra the upwelling near the spreading axis can essentially be described by three regimes: weakly 3-D at the slow spreading rates, strongly 3-D at slow to intermediate rates, and 2-D at fast spreading rates. The regime boundaries shift toward higher plate velocities with increasing Ra. Comparison of the laboratory and numerical experiments indicates that temperature-dependent viscosity may strengthen the position of focused upwelling centers and cause a sharper transition between 2-D and 3-D upwelling patterns. Taken together, these results suggest that buoyancy-driven, dynamic flow is an important element in the geodynamics of mid-ocean ridges. 16,177
A combination of laboratory and numerical models are used to examine the mantle flow beneath a se... more A combination of laboratory and numerical models are used to examine the mantle flow beneath a segmented ridge generated by the interaction of a linear, buoyant upwelling source with plate-driven flow. In the absence of plate spreading, the linear buoyant source creates a very narrow (across-axis), two-dimensional upwelling pattern. The plate-driven flow consists of a quasi-linear sheet-like upwelling that cuts beneath ridge-transform inside corners and is not centered beneath the spreading segments. When buoyant and plate-driven flows are combined, material rises beneath the inside corners and flows away from the axis asymmetrically. Near the ends of segments, this results in a geometrical misfit between the center of mantle upwelling and the ridge axis. If a similar pattern of mantle flow occurs beneath a segmented mid-ocean ridge, the result will be a thinner crust toward segment ends and possibly a negative correlation between extent of mantle melting and average depth of melting. These results indicate that even with an essentially two-dimensional source, in cases where it is oblique to the actual spreading segments, the upwelling beneath a segmented ridge will appear to be three-dimensional along axis. Since slow spreading ridges are generally more segmented than fast spreading ridges, this effect is likely to be more important at slow spreading ridges.
We present three-dimensional laboratory modeling of the evolution of finite strain and compare th... more We present three-dimensional laboratory modeling of the evolution of finite strain and compare these to shear wave splitting observations in the Northwest U.S. under the High Lava Plains (HLP). We show that relationships between mantle flow and anisotropy are complicated in subduction zones and factors such as initial orientation of the olivine fast-axis, style of subduction, and time evolving flow are important. Due to increased horizontal shear, systems with a component of rollback subduction have simple trench-normal strain alignment within the central region of the backarc mantle wedge while those with more simple longitudinal sinking are often variable and complex. In the HLP, splitting observations are consistent with rollback-driven laboratory results.
1] Results are presented from field measurements of the spatial and temporal variability of circu... more 1] Results are presented from field measurements of the spatial and temporal variability of circulation and exchange between Narragansett Bay and Rhode Island Sound (RIS) at both tidal and seasonal timescales. Data were collected during 13 acoustic Doppler current profiler (ADCP) surveys between April 1998 and July 1999. Significant horizontal and vertical structure was observed in the instantaneous, tidally driven flow and the residual, nontidal flow. This was especially evident through the deeper East Passage (EP), where persistent inflow was observed along the eastern side, and outflow along the western side of the channel. The residual volume transport of water to RIS is greater through the EP than through the West Passage entrance to the Bay, particularly in summer. Data from the offshore, or coastline-normal survey lines reveal a strong seasonal variability in RIS circulation. A residual (5-15 cm s À1 ) cyclonic coastal current is recorded within RIS during summer, stratified conditions. This residual current, which was not present during the winter surveys, enhances Narragansett Bay-RIS exchange by efficiently advecting the bay outflow to the southwest, away from the entrance. Results also indicate the resupply pathways for deep water to the entrance region vary in summer versus winter. Observed differences in RIS circulation, and related estuary-shelf exchange, have significant implications for physical, chemical, and biological models of Narragansett Bay.
1] Results are presented from field measurements of the spatial and temporal variability of circu... more 1] Results are presented from field measurements of the spatial and temporal variability of circulation and exchange between Narragansett Bay and Rhode Island Sound (RIS) at both tidal and seasonal timescales. Data were collected during 13 acoustic Doppler current profiler (ADCP) surveys between April 1998 and July 1999. Significant horizontal and vertical structure was observed in the instantaneous, tidally driven flow and the residual, nontidal flow. This was especially evident through the deeper East Passage (EP), where persistent inflow was observed along the eastern side, and outflow along the western side of the channel. The residual volume transport of water to RIS is greater through the EP than through the West Passage entrance to the Bay, particularly in summer. Data from the offshore, or coastline-normal survey lines reveal a strong seasonal variability in RIS circulation. A residual (5-15 cm s À1 ) cyclonic coastal current is recorded within RIS during summer, stratified conditions. This residual current, which was not present during the winter surveys, enhances Narragansett Bay-RIS exchange by efficiently advecting the bay outflow to the southwest, away from the entrance. Results also indicate the resupply pathways for deep water to the entrance region vary in summer versus winter. Observed differences in RIS circulation, and related estuary-shelf exchange, have significant implications for physical, chemical, and biological models of Narragansett Bay.
flow must have been convective rather than diffusive in order to preserve the characteristics of ... more flow must have been convective rather than diffusive in order to preserve the characteristics of water input from the Trias. According to the distribution of salinity and other geochemical tracers in the Dogger aquifer, cross-formation flow probably took place via the major Hercynian fault and fracture systems that run through the centre of the Paris basin 4,24 . We conclude that convection through geological accidents is probably the main mode of crossformation mass transfer in basins because diffusive transfer appears to be extremely limited on a basin scale. The detailed role of diffusion on a local scale will need to be assessed in the future by experiments aimed at documenting the migration of highly mobile elements in aquitards. A
1] A series of laboratory experiments is used to model three-dimensional aspects of flow in subdu... more 1] A series of laboratory experiments is used to model three-dimensional aspects of flow in subduction zones, and the consequent temperature variations in the slab and overlying mantle wedge. The effects of longitudinal, rollback and slab-steepening components of motions are considered, along with different thicknesses of the over-riding lithosphere. The results show that the style of plate sinking influences the evolution of subduction zones, both in terms of the speed, orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab. In the simplest case of longitudinal sinking, without rollback motion, velocities in the mantle wedge are 30-40% of the downdip plate speed. Return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds, and there is no mass flux around the edges of a slab segment (of finite width). Rollback subduction leads to flow both around and beneath the sinking slab, with larger velocities in the wedge (up to 150% of the slab speed) and flow focused toward the center of the plate segment. Rollback subduction, in which either the trench migrates or the plate steepens with time, induces shallow and steep wedge return flow trajectories, respectively. The thermal evolution of the plate is strongly influenced by sinking style and rate: highest temperatures are along the edges of the slab for longitudinal sinking, but along the centerline of the slab segment for rollback motion. Slab surface temperatures, when scaled to the mantle, are higher than those given by previous two-dimensional numerical models, and are consistent with recent observational and experimental data on melt compositions.
The dynamical interaction between three-dimensional (3-D) buoyant flow and platedriven mantle flo... more The dynamical interaction between three-dimensional (3-D) buoyant flow and platedriven mantle flow beneath a mid-ocean ridge is examined using a combination of laboratory and numerical experiments. T.. u.i•ue •n•-ß . ,• •aou•.L,,•y setup a layer of viscous fluid is heated from below and cooled from above to drive thermal convection. Forced, plate-driven flow is modeled by dragging mylar sheeting in opposite directions across the fluid surface. Uniform viscosity 3-D numerical models have been designed to simulate the laboratory runs, to provide additional information on the flow, and to attempt to isolate the effects of variable viscosity. In one type of experiment we modeled buoyancy on the scale of the partial melting region with a linear heat source beneath the spreading axis. In a second set of experiments the entire base of the tank is heated in order to investigate the interaction between plate-driven flow and larger-scale (upper mantle) convection. The pattern of segment-scale (-100-150 km) convection beneath a spreading center is found to be a strong function of the spreading rate and the Rayleigh number (Ra) of the buoyant flow. Purely two-dimensional (2-D) flow exists only in the case of low Ra (-105) and faster spreading rates (>4-6 cm/yr half rate). Three dimensionality is strongly enhanced during transient pulses of upwelling. Convection on the scale of the upper mantle can contribute significant long-wavelength spatial (-600-1000 km) and temporal (20-40 m.y.) variability in upwelling rates and temperatures at spreading centers and may provide an alternative model for plume-ridge interaction. For a given Ra the upwelling near the spreading axis can essentially be described by three regimes: weakly 3-D at the slow spreading rates, strongly 3-D at slow to intermediate rates, and 2-D at fast spreading rates. The regime boundaries shift toward higher plate velocities with increasing Ra. Comparison of the laboratory and numerical experiments indicates that temperature-dependent viscosity may strengthen the position of focused upwelling centers and cause a sharper transition between 2-D and 3-D upwelling patterns. Taken together, these results suggest that buoyancy-driven, dynamic flow is an important element in the geodynamics of mid-ocean ridges. 16,177
A combination of laboratory and numerical models are used to examine the mantle flow beneath a se... more A combination of laboratory and numerical models are used to examine the mantle flow beneath a segmented ridge generated by the interaction of a linear, buoyant upwelling source with plate-driven flow. In the absence of plate spreading, the linear buoyant source creates a very narrow (across-axis), two-dimensional upwelling pattern. The plate-driven flow consists of a quasi-linear sheet-like upwelling that cuts beneath ridge-transform inside corners and is not centered beneath the spreading segments. When buoyant and plate-driven flows are combined, material rises beneath the inside corners and flows away from the axis asymmetrically. Near the ends of segments, this results in a geometrical misfit between the center of mantle upwelling and the ridge axis. If a similar pattern of mantle flow occurs beneath a segmented mid-ocean ridge, the result will be a thinner crust toward segment ends and possibly a negative correlation between extent of mantle melting and average depth of melting. These results indicate that even with an essentially two-dimensional source, in cases where it is oblique to the actual spreading segments, the upwelling beneath a segmented ridge will appear to be three-dimensional along axis. Since slow spreading ridges are generally more segmented than fast spreading ridges, this effect is likely to be more important at slow spreading ridges.
We present three-dimensional laboratory modeling of the evolution of finite strain and compare th... more We present three-dimensional laboratory modeling of the evolution of finite strain and compare these to shear wave splitting observations in the Northwest U.S. under the High Lava Plains (HLP). We show that relationships between mantle flow and anisotropy are complicated in subduction zones and factors such as initial orientation of the olivine fast-axis, style of subduction, and time evolving flow are important. Due to increased horizontal shear, systems with a component of rollback subduction have simple trench-normal strain alignment within the central region of the backarc mantle wedge while those with more simple longitudinal sinking are often variable and complex. In the HLP, splitting observations are consistent with rollback-driven laboratory results.
1] Results are presented from field measurements of the spatial and temporal variability of circu... more 1] Results are presented from field measurements of the spatial and temporal variability of circulation and exchange between Narragansett Bay and Rhode Island Sound (RIS) at both tidal and seasonal timescales. Data were collected during 13 acoustic Doppler current profiler (ADCP) surveys between April 1998 and July 1999. Significant horizontal and vertical structure was observed in the instantaneous, tidally driven flow and the residual, nontidal flow. This was especially evident through the deeper East Passage (EP), where persistent inflow was observed along the eastern side, and outflow along the western side of the channel. The residual volume transport of water to RIS is greater through the EP than through the West Passage entrance to the Bay, particularly in summer. Data from the offshore, or coastline-normal survey lines reveal a strong seasonal variability in RIS circulation. A residual (5-15 cm s À1 ) cyclonic coastal current is recorded within RIS during summer, stratified conditions. This residual current, which was not present during the winter surveys, enhances Narragansett Bay-RIS exchange by efficiently advecting the bay outflow to the southwest, away from the entrance. Results also indicate the resupply pathways for deep water to the entrance region vary in summer versus winter. Observed differences in RIS circulation, and related estuary-shelf exchange, have significant implications for physical, chemical, and biological models of Narragansett Bay.
1] Results are presented from field measurements of the spatial and temporal variability of circu... more 1] Results are presented from field measurements of the spatial and temporal variability of circulation and exchange between Narragansett Bay and Rhode Island Sound (RIS) at both tidal and seasonal timescales. Data were collected during 13 acoustic Doppler current profiler (ADCP) surveys between April 1998 and July 1999. Significant horizontal and vertical structure was observed in the instantaneous, tidally driven flow and the residual, nontidal flow. This was especially evident through the deeper East Passage (EP), where persistent inflow was observed along the eastern side, and outflow along the western side of the channel. The residual volume transport of water to RIS is greater through the EP than through the West Passage entrance to the Bay, particularly in summer. Data from the offshore, or coastline-normal survey lines reveal a strong seasonal variability in RIS circulation. A residual (5-15 cm s À1 ) cyclonic coastal current is recorded within RIS during summer, stratified conditions. This residual current, which was not present during the winter surveys, enhances Narragansett Bay-RIS exchange by efficiently advecting the bay outflow to the southwest, away from the entrance. Results also indicate the resupply pathways for deep water to the entrance region vary in summer versus winter. Observed differences in RIS circulation, and related estuary-shelf exchange, have significant implications for physical, chemical, and biological models of Narragansett Bay.
flow must have been convective rather than diffusive in order to preserve the characteristics of ... more flow must have been convective rather than diffusive in order to preserve the characteristics of water input from the Trias. According to the distribution of salinity and other geochemical tracers in the Dogger aquifer, cross-formation flow probably took place via the major Hercynian fault and fracture systems that run through the centre of the Paris basin 4,24 . We conclude that convection through geological accidents is probably the main mode of crossformation mass transfer in basins because diffusive transfer appears to be extremely limited on a basin scale. The detailed role of diffusion on a local scale will need to be assessed in the future by experiments aimed at documenting the migration of highly mobile elements in aquitards. A
1] A series of laboratory experiments is used to model three-dimensional aspects of flow in subdu... more 1] A series of laboratory experiments is used to model three-dimensional aspects of flow in subduction zones, and the consequent temperature variations in the slab and overlying mantle wedge. The effects of longitudinal, rollback and slab-steepening components of motions are considered, along with different thicknesses of the over-riding lithosphere. The results show that the style of plate sinking influences the evolution of subduction zones, both in terms of the speed, orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab. In the simplest case of longitudinal sinking, without rollback motion, velocities in the mantle wedge are 30-40% of the downdip plate speed. Return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds, and there is no mass flux around the edges of a slab segment (of finite width). Rollback subduction leads to flow both around and beneath the sinking slab, with larger velocities in the wedge (up to 150% of the slab speed) and flow focused toward the center of the plate segment. Rollback subduction, in which either the trench migrates or the plate steepens with time, induces shallow and steep wedge return flow trajectories, respectively. The thermal evolution of the plate is strongly influenced by sinking style and rate: highest temperatures are along the edges of the slab for longitudinal sinking, but along the centerline of the slab segment for rollback motion. Slab surface temperatures, when scaled to the mantle, are higher than those given by previous two-dimensional numerical models, and are consistent with recent observational and experimental data on melt compositions.
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