U52A-01
The Nicaragua - Costa Rica slab and wedge: imaging deep fluid pathways
The 2004-6 TUCAN broadband experiment (NSF/MARGINS) provides striking images of slab dehydration and wedge structure, through several seismological tools. With advances in receiver-function migration methodology we successfully recover a sharp interface at the top of the downgoing plate, from 40 to 200 km depth, dipping 50-70 deg. beneath Nicaragua. Velocities increase downward across it. Imaging of such a steeply dipping surface is almost unprecedented, as most receiver function migration methods break down for surfaces dipping > 35-45 deg., and relies upon dense coverage far in the backarc to sample the strongly scattered slab signals. The imaged interface lies just below Wadati-Benioff Zone (WBZ) seismicity at depths shallower than 80 km and 5-20 km above the WBZ at greater depths. Possibly, the boundary at shallow depth is the subducting oceanic Moho overlain by comparatively low-velocity metabasalts, whereas beyond 80 km depth the crust has eclogitized so that the strongest velocity discontinuity lies above it, at the base of a fluid-rich wedge. Also, seismicity may step deeper within the subducting plate as the plate heats and dehydrates; the possibilities are not exclusive. Seismic attenuation images show that the 80 km WBZ depth corresponds to the trenchward limit of the hot mantle wedge overlying the subducting plate; rapid heating of the top of the downgoing plate should occur just beyond this depth. Seismic velocity inversions show a vertical column of high Vp/Vs directly beneath the arc also overlying the slab where it is more than 80 km deep. This structure is interpreted as ascending melt, rising from a site of slab dehydration to the volcanic arc. Thus, at 80 km depth there is good evidence that the slab surface heats up rapidly and crust dehydrates to eclogite, triggering fluid ascent toward the volcanic arc.
U52A-02 INVITED
Along-arc Variations in Volatile Cycling in the Nicaragua-Costa Rica Subduction Zone
Recent seismic imaging in the Nicaragua-Costa Rica subduction zone, in combination with geochemical data, has revealed systematic along-arc variations in volatile cycling from the subducting slab, through the mantle wedge, to the melts generated at arc volcanoes. Velocity and attenuation tomography based on P and S phases from local events recorded by the 2004-2006 TUCAN Broadband Seismic Experiment resolve a high velocity, low attenuation subducting slab, a shallow wedge corner with intermediate attenuation, and a slower, more highly attenuating mantle wedge beneath the arc and back-arc. However, velocity and attenuation structures also manifest strong along-arc variations. The subducting slab beneath Nicaragua contains a shallow layer of slow velocities and is more attenuating than the slab beneath Costa Rica, consistent with greater slab hydration (for example, 10-20% mantle serpentinization). Continuing the along-arc trend, attenuation in the Nicaraguan mantle wedge at depths of 60-100 km is significantly higher than in Costa Rica, suggesting that enhanced slab hydration beneath Nicaragua results in a more hydrated wedge. Moving to the arc, olivine melt inclusions indicate that Nicaraguan magmas are more water-rich on average than those erupted in the Costa Rican arc within the region best-imaged by the TUCAN array. Assuming that these wet melts equilibrated with olivine in the mantle wedge, then the inferred decrease in wedge hydration from Nicaragua to Costa Rica can explain the along-arc variation in wedge attenuation. In addition, although bulk magma compositions in the vicinity of the Nicaragua and Costa Rica attenuation profiles are significantly different, they imply a similar temperature of 1265 ± 25°C at a depth comparable to the middle of the wedge (75 km) when primary melt water content is taken into account in calculating olivine-liquid temperatures. A roughly vertical column of high Vp/Vs extends from the slab surface to the arc in Nicaragua, but this feature dies away along the arc to Costa Rica. This anomaly could reflect larger melt fractions in the Nicaraguan wedge, possibly produced by the apparently greater wedge hydration.
U52A-03
Trench-parallel fast axes of seismic anisotropy due to dehydration in subducting slabs
Observations of seismic anisotropy are widely used to constrain models of mantle deformation around subduction zones, and yet cannot distinguish between trench-parallel or trench-normal mantle flow. Inherent in these models is the link between the elastic anisotropy of rocks and the mantle flow regime. In this debate, seismic anisotropy originating within the slab has largely been overlooked. In this contribution, we show that the elastic anisotropy of metamorphosed and deformed rocks in the slab is likely to be very significant (i.e. measurable at the surface) and that dehydration-induced crack damage within the slab will lead to rotation of the seismic fast axes from trench-normal to trench-parallel orientations. We calculate the elastic properties of slab rocks using equilibrium mineral assemblages for the pressure and temperature conditions at a depth of 75 km. The magnitude and orientation of predicted seismic anisotropy for each rock reflects the intrinsic elastic properties of the aligned anisotropic constituent minerals, and locally exceeds 70% for both P wave velocities and shear wave splitting. Integration of the anisotropy over the vertical depth of the slab produces delay times in the range of several seconds. Significantly, our model predicts the rotation of the fast axes of P and S1 waves to be sub-horizontal and parallel to the trench for receiving stations in the forearc. Our model provides an alternative explanation for supra-subduction zone seismic anisotropy that does not require high- stress, high-water conditions or trench-parallel flow in the mantle wedge.
U52A-04 INVITED
Common Maximum Depth of Slab-Mantle Wedge Decoupling: Understanding Variations in Fluid Supply and Thermal-petrologic Processes Among Subduction Zones
In subduction zones, aqueous fluid is critical to a diversity of processes such as intraslab earthquakes, mantle-wedge metamorphism, and arc volcanism. Fluid availability at depth depends on the thermal structure of the subducting slab. The age of the slab is the primary factor that controls its thermal structure, but the thermal effect of slab-driven mantle wedge flow is large enough to affect the depths of dehydration and even melting of the slab. Geophysical observations indicate that the shallow part of the forearc mantle wedge does not participate in this flow and thus is decoupled from the subducting slab. We develop 2-D steady-state thermal models for Cascadia, NE Japan, and Kamchatka, where there are sufficient heat flow data to constrain the maximum depth of decoupling. In the models, we use temperature- and stress- dependent mantle rheology, and the age-controlled thermal structure of the slab is advected into the subduction system. Cascadia has a very young (< 10 Ma) and warm slab whereas NE Japan and Kamchatka have a very old (> 100 Ma) and cold slab. These end members show dramatic contrast in their metamorphic, volcanic, and seismological characteristics, but the heat flow data require the subduction interface to be decoupled to a depth of 70-80 km for all the three subduction zones regardless of their slab age. We thus hypothesize that the common depth of decoupling of 70-80 km applies to most, if not all, subduction zones, and we develop models for fourteen other subduction zones to test this hypothesis against available observations. For subduction zones with a very warm slab, such as Cascadia and Nankai, the models predict that slab dehydration peaks in the forearc where the mantle wedge temperature is low enough for serpentinization but too low for melt production. For most other subduction zones, slab dehydration is predicted to peak in the depth range of 80-140 km beneath the high-temperature part of the mantle wedge, promoting arc volcanism. In all models, the mantle-wedge temperatures above where the slab is about 100-km deep reach the melting temperatures estimated by laboratory and geochemical studies. The common maximum depth of decoupling explains multidisciplinary observations, and our modeling results are consistent with conclusions drawn by various research groups through modeling individual subduction zones.
U52A-05
Effects of Rheological Weakening by Fluids and Melts on the Dynamics of Subduction Under an Active Margin: Numerical Modeling
The dynamics of subduction under an active margin is analyzed by using a 2D coupled geochemical- petrological-thermomechanical numerical model of an oceanic-continental subduction process. This model includes spontaneous slab retreat and bending, dehydration of the subducted crust, aqueous fluid transport, partial melting of both crustal and mantle rocks and melt extraction processes resulting in magmatic arc crust growth. The innovation in this model is the consideration of rheological weakening effects by fluids and melts. The numerical experiments revealed that rheological weakening by fluids and melts controls the mode of subduction. By varying the weakening effects, five different regimes can be induced showing the following characteristics: (1) formation of a backarc basin, occurance of plumes that (2) ascend and intrude into the continental crust or (3) extend horizontally beneath the continental plate (underplating) or (4) remain above the subducting plate and (5) neither formation of backarc basins nor occurance of plumes. The transition between the different tectonic regimes of subduction at an active margin is caused by the concurrence of rheological weakening by (1) aqueous fluids percolating from the subducting slab into the mantle wedge and (2) melts propagating from the mantle wedge toward the surface. The aqueous fluids mainly affect the forearc region: strong fluid-related weakening of rocks atop the slab promotes the stacking of sediments and the development of an accretion wedge. In case the material in the subduction channel is weak the coupling of the plates is low facilitating extension in the subduction channel. In contrast, a small weakening effect by fluids results in strong coupling of the plates inducing collision-like subduction and subduction erosion along with lithospheric thickening and sedimentary plumes. Extracted melts rheologically weaken the lithosphere below the arc. Strong rheological weakening by melts together with low coupling of the plates allows for necking of the continental lithosphere and leads to the formation of a backarc basin. In case of sedimentary plumes, weakening of the continental lithosphere by extracted melts generates a weak channel in which the positive-buoyant plumes may ascend. That way silicic intrusions are emplaced in the continental crust. If the continental lithosphere is too strong, the plumes cannot ascend but may extend horizontally leading to underplating.
U52A-06
KALMAR – A Russian-German Collaborative Project in the Kurile-Kamchatka and Aleutian Marginal Sea - Island Arc Systems
KALMAR is a Russian-German collaborative project that investigates the triple junction of the Kurile- Kamchatka and Aleutian Island Arc system. This system is seismically and volcanically the most active subduction zone on Earth with an ongoing eruption record in the Kamchatka volcanic front since the Pleistocene. The amount and explosivity of the eruptions point towards a significant influence of fluids in the subducted crust and upper mantle on the composition of the magmas and the volcanological evolution. The significant release of climate-relevant magmatic gases is related to the most active volcanoes associated with the Kamchatka-Aleutian-Triple Junction. In order to better understand the processes that control the subduction, KALMAR investigates the geodynamic-volcanic-magmatic and the oceanographic-climatic evolution of the Kurile-Kamchatka-Aleutean Arc system in five closely coupled subprojects. The subprojects address the processes involved in a wide range of geophysical, tectonic, volcanological and petrological processes as well as paleo-oceanographic and climate research from land and marine sites. The different scientific approaches will enable us to better understand the input and output into the subduction factory and their effects on the global climate. Here we present an overview of the work done within KALMAR; specific research focuses within the project are presented in various sessions of the AGU 2008 Fall Meeting.
U52A-07
Millennial Variations of Magma and Volatile Fluxes Inferred From Time-Series Study of Klyuchevskoy Volcano, Kamchatka
We report preliminary results of a time-series study aimed at deciphering the evolution of Klyuchevskoy volcano, the most productive arc volcano on Earth. About 100 individual cinder layers representing continuous volcanic activity of Klyuchevskoy volcano since 9500 years BP to the present were collected from tephra sections ~15 km northeast of the volcano summit. Regional 14C-dated marker tephras were identified in these sections and allowed us to bracket the ages of individual tephra samples within ~100 years. Geochemical data on volcanic glasses, minerals and bulk cinders demonstrate that the extent of Klyuchevskoy magma fractionation and, possibly, assimilation varied cyclically through time with a period of ~3000 years during the Holocene. The least evolved magmas erupted at ~9000, 6000 and 3000 years BP and the most evolved at ~7500, 4000 and 1000 BP. Variations in the accumulation rate of pyroclastic deposits on the volcano's slope suggest that the volcanic productivity varied by a factor of two (between 60 and 120 million tons per year) and peaked during periods when magma fractionation decreased with time. Data on melt inclusions in primitive olivines suggest a relatively constant composition of parental Klyuchevskoy melts, at least with respect to volatiles. As a result of variable degrees of differentiation, the amount of volatiles released from fractionating Klyuchevskoy magmas also varied significantly through time and appears to be relatively low at present. We interpret the temporal changes in magma composition and volcanic productivity to reflect millennial-scale variations of the mantle magma flux that controls the extent of magma fractionation and, possibly, assimilation in continuously recharging and evolving magma reservoir beneath Klyuchevskoy volcano. Extrapolating from these results, we speculate that magma and volatile fluxes from the sub-arc mantle to the crust are variable on a time scale of a few thousand years, perhaps, due to pulsing fluid release from the subducting plate that governs the rate of fluid-fluxed mantle melting.
U52A-08
Subduction of Seawater-Derived Noble Gases and Halogens: Evidence from Wedge Mantle Peridotite
Subduction volcanism is generally considered to form a 'subduction barrier' that efficiently recycles volatile components contained in subducted slabs back to the Earth's surface (Staudacher and Allegre, 1988). Nevertheless, subduction of sediment and seawater-dominated pore fluids to the deep mantle has recently been proposed to account for the convecting mantle heavy noble gas (Ar, Kr and Xe) non-radiogenic elemental abundance and isotopic pattern (Holland and Ballentine, 2006). To verify whether and how subduction fluids preserve a seawater signature, we have determined noble gas and halogen compositions of the Higashi-akaishi peridotite body in the Sanbagawa metamorphic belt, southwest Japan, in which former water-rich inclusions exhumed from depths greater than 100 km are contained as serpentine dominated micro-inclusions (Mizukami et al., 2004). The striking similarities of the observed noble gas and halogen compositions with marine pore fluids challenge a popular concept, in which the water flux into the wedge mantle is only by hydrous minerals in altered oceanic crust and sediment (e.g., Schmidt and Poli, 1998). Subduction and closed system retention of unbound marine pore fluid to at least 100 km depth is required. The subducted halogen and noble gas elemental ratios are clearly distinct from those of arc volcanic gases. This implies that the Higashi-akaishi peridotite body has frozen in and preserved an inferred but previously unseen part of the volatile recycling process. Return of these volatiles to the atmosphere via arc volcanism requires the addition of a mantle component and fractionation during degassing. A small proportion preserved in the downgoing slab can explain the heavy noble gases observed in the convecting mantle. References: Holland G. and Ballentine C. J., Nature, 441, 186-191 (2006). Mizukami T. et al., Nature, 427, 432-436 (2004). Schmidt M.W. and Poli S., Earth Planet. Sci. Lett., 163, 361-379 (1998). Staudacher, T. and Allegre C.J., Earth Planet. Sci. Lett., 89, 173-183 (1988).