Chris Kincaid

Thermal and dynamical evolution of the upper mantle in subduction zones

We present results from two-dimensional (2-D) numerical experiments on the thermal and dynamical evolution of the subducting slab and of the overlying mantle wedge for a range in subduction parameters. These include subduction rate and the age and rheology of both subducting and overriding plates. Experiments also consider the influence of slab forcing conditions (from purely kinematic to purely dynamic) on the evolution of both the slab and mantle wedge. One goal is to determine how different parameters control thermal evolution of the slab-wedge interface, from just after subduction initiation up through roughly 500–600 km of subduction, where temperatures are approaching steady state. An additional goal is to define optimal conditions for the melting of slab sediments and crust. Results show slab surface temperatures (SSTs) depend strongly on subduction velocity, plate thermal structure, and upper mantle (or wedge) viscosity structure. Fast subduction beneath a thick (>70 km) overriding plate results in the coolest SSTs. Maximum SSTs are recorded as an early transient event for cases of slow subduction ( 100 km) which deflects the zone of maximum shear away from slab-wedge interface.

Experiments on the interaction of thermal convection and compositional layering at the base of the mantle

Accumulation and survival of compositional heterogeneity at the base of the convecting mantle is investigated using laboratory experiments with temperature dependent viscous fluids. Two models of heterogeneity in the D” layer are considered: (1) a reservoir of dense material above the core-mantle boundary and (2) continuous recharge of the D” layer by subduction of cold, negatively buoyant high viscosity slabs of oceanic lithosphere. Survival of a dense reservoir depends on Rp, the ratio of stabilizing compositional buoyancy to destabilizing thermal buoyancy. Rapid mixing occurs for Rp ≤ 1; for Rp ≥ 1, mixing occurs by slow entrainment only. The reservoir model predicts D” layer heterogeneity is concentrated beneath lower mantle upwellings. Differentiation of composite slabs, consisting of dense crustal and light depleted components, occurs in a hot thermal boundary layer. Composite slabs pile up in the basal thermal boundary layer, and upon heating the light component is ejected by Rayleigh-Taylor instabilities. The dense component accumulates in the boundary layer, arrayed in a pattern of interconnected spokes, and is slowly entrained into thermal plumes. According to these experiments the time required for conversion of slabs to plumes in D” is 1–2 Gyr. The correlation between lateral variations in D” layer thickness and lateral variations in lower mantle seismic velocity suggests that D″ may consist of dense material, perhaps former oceanic lithosphere, which has sunk through the lower mantle.

Onset of mid-Cretaceous volcanism by elevation of the 670 km thermal boundary layer

A pulse of ocean crustal production during the mid-Cretaceous begins at the same time that magnetic reversal frequency drops to zero. We suggest that slab penetration into the lower mantle caused a thermal boundary layer normally at 670 km to be rapidly advected upward as the slabs descended through the lower mantle. The onset of near-surface melting due to the rise of this boundary layer coincides with the slabs9 arrival at D″, which drove up the heat flux across the core-mantle boundary and stabilized the magnetic reversal process. The slab penetration event was coincident with the breakup of the Gondwana supercontinent and triggered the subsequent formation of oceanic plateaus in the central Pacific and Indian Ocean basins.

Patterns in seismic anisotropy driven by rollback subduction beneath the High Lava Plains

Received 24 March 2011; revised 25 May 2011; accepted 31 May 2011; published 8 July 2011. [1] 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. Citation: Druken, K. A., M. D. Long, and C. Kincaid (2011), Patterns in seismic anisotropy driven by rollback subduction beneath the High Lava Plains, Geophys. Res. Lett., 38, L13310, doi:10.1029/2011GL047541.

Source and dispersal of jökulhlaup sediments discharged to the sea following the 1996 Vatnajökull eruption

The October 1996 Gjalp eruption beneath Vatnajokull glacier led to one of the largest jokulhlaups (glacial floods) in Iceland in the twentieth century. A catastrophic discharge of meltwater and sediment swept across the Skeidararsandur flood plain to the sea. Tephra from the eruption consists of vesicular sideromelane shards with a basaltic andesite composition (53% SiO 2 , 3% MgO, 0.8% K 2 O). After the flood, sediment samples were collected from the flood plain and off the southeast coast of Iceland, where a major sediment plume had been created by the discharge. Compositions of glass shards from flood-plain and seafloor deposits do not match those of the Gjalp magma. Flood-plain samples consist primarily of blocky to poorly vesicular sideromelane clasts with compositions that are characteristic of Grimsvotn volcanic products (∼50% SiO 2 , 5.5% MgO, 0.4% K 2 O). Marine samples collected near the jokulhlaup outflow into the sea also consist primarily of blocky to poorly vesicular sideromelane clasts with compositions that are, for the most part, similar to products of the Grimsvotn volcanic center. Distal marine samples have more vesicular sideromelane clasts with compositions that are similar to products of the Katla volcanic center (e.g., 48% SiO 2 , 4.5% MgO, 0.8% K 2 O). Significant deposition to the seafloor was apparently limited to an area just offshore of the Skeidararsandur. There is no indication that juvenile volcanic material from the Gjalp eruption was carried by the 1996 jokulhlaup onto the flood plain or into the ocean. Instead, the jokulhlaup carried primarily older volcaniclastic material eroded by the flood.

The relative importance of plate-driven and buoyancy- driven flow at mid-ocean ridges

The dynamical interaction between three-dimensional (3-D) buoyant flow and plate-driven mantle flow beneath a mid-ocean ridge is examined using a combination of laboratory and numerical experiments. In a unique laboratory setup a layer of strongly temperature-dependent 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.

The Dynamics of Water Exchange Between Narragansett Bay and Rhode Island Sound

The Narragansett Bay estuary represents an important natural and economic resource. Narragansett Bay has a long history of scientific study, whichmakes it ideal for understanding the interplay between anthropogenic impacts, estuarine science and management. Over the past 40 years, numerous studies have focused on the biological and chemical processes within Narragansett Bay (e.g., Hicks, 1959; Kremer and Nixon, 1978; Pilson, 1985; Keller, 1988; Bender et al., 1989; Hinga et al., 1989; Nixon, 1997; Granger and Buckley, 1999; Keller et al., 1999; Brush, 2002; Oviatt et al., 2002; Prell et al., 2004; Bergondo et al., 2005). Regions of the bay have been instrumented for long-term monitoring projects (e.g., NB-PORTS sites; Rhode Island Department of Environmental Management buoys) and are sampled by monthly surveys with towed instruments. Observations suggest that large-scale, climate-induced changes (Hawk, 1998) have occurred and may be linked to modifications in the bay’s ecosystem (Hawk, 1998; Oviatt et al., 2002; Sullivan and Van Keuren, 2003). Anthropogenic and natural stresses are also apparent in the increasing number and severity of low oxygen events in more developed regions of the bay (Saarman et al., 2002; Bergondo, 2004; Bergondo et al., 2005; Deacutis et al., 2006; Chapters 11 and 12). Large-scale engineering projects will also have an impact; such as recent dredging of the Providence River shipping channel and a stormwater holding facility planned for Providence to reduce storm-water discharge and ultimately nutrient flux. Estuarine circulation lies at the heart of multidisciplinary approaches to coastal management. Ecosystem-based models require information on the mixing, flushing and transport of water between sub-regions of the estuary. Moreover, bio-chemical processes, such as those controlling oxygen levels within Narragansett Bay, are also influenced by how efficiently water is

Mesoscale convective system surface pressure anomalies responsible for meteotsunamis along the U.S. East Coast on June 13th, 2013

Two destructive high-frequency sea level oscillation events occurred on June 13th, 2013 along the U.S. East Coast. Seafloor processes can be dismissed as the sources, as no concurrent offshore earthquakes or landslides were detected. Here, we present evidence that these tsunami-like events were generated by atmospheric mesoscale convective systems (MCSs) propagating from inland to offshore. The USArray Transportable Array inland and NOAA tide gauges along the coast recorded the pressure anomalies associated with the MCSs. Once offshore, the pressure anomalies generated shallow water waves, which were amplified by the resonance between the water column and atmospheric forcing. Analysis of the tidal data reveals that these waves reflected off the continental shelf break and reached the coast, where bathymetry and coastal geometry contributed to their hazard potential. This study demonstrates that monitoring MCS pressure anomalies in the interior of the U.S. provides important observations for early warnings of MCS-generated tsunamis.

Combined laboratory and numerical studies of the interaction between buoyant and plate-driven upwelling beneath segmented spreading centers

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.

Water Quality Modeling in the Rio Chone Estuary

Abstract Water quality within the Rio Chone estuary, a seasonally inverse, tropical estuary, in Ecuador was characterized by modeling the distribution of biochemical oxygen demand (BOD) and dissolved inorganic nitrogen (DIN) within the water column. These two variables are modeled using modified advection-diffusion equations within a two-dimensional, laterally-averaged hydrodynamic model. The model includes sources of salt, BOD and DIN from shrimp mariculture ponds in the region surrounding the estuary. The model was successful in simulating seasonal concentrations in DIN and BOD over a range in source concentrations. Seasonal BOD measurements along the length of the estuary were coincident with dissolved oxygen concentrations in the estuary (high BOD generally corresponding to low dissolved oxygen). Results suggest that the citing of shrimp ponds near the head of the estuary should be avoided in order maintain estuarine water quality and to maximize production.