Variability in flow and temperatures within mantle subduction zones

Variability in flow and temperatures within mantlesubduction zones

C. KincaidGraduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USA ([email protected])

R. W. GriffithsResearch School of Earth Sciences, Australian National University, Canberra, Australia

[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.

Components: 11,306 words, 14 figures, 1 table.

Keywords: geodynamics; plate slab sinking style; subduction zone evolution.

Index Terms: 8147 Tectonophysics: Planetary interiors (5430, 5724); 8155 Tectonophysics: Plate motions—general; 8450

Volcanology: Planetary volcanism (5480).

Received 15 November 2003; Revised 24 February 2004; Accepted 29 March 2004; Published 9 June 2004.

Kincaid, C., and R. W. Griffiths (2004), Variability in flow and temperatures within mantle subduction zones, Geochem.

Geophys. Geosyst., 5, Q06002, doi:10.1029/2003GC000666.

1. Introduction

[2] Subduction driven processes span a range in

spatial and temporal scales, from the shallow or

early evolution of trenches soon after the initiation

of subduction to the deeper, longer term aspects of

how slabs interact with the 670 km boundary and,

at least in some cases, pass into the lower mantle.

G3G3GeochemistryGeophysics

Geosystems

Published by AGU and the Geochemical Society

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES

GeochemistryGeophysics

Geosystems

Article

Volume 5, Number 6

9 June 2004

Q06002, doi:10.1029/2003GC000666

ISSN: 1525-2027

Copyright 2004 by the American Geophysical Union 1 of 20

Between these extremes lies a set of processes

responsible for thermal and chemical exchange

between Earth’s interior, crust and ocean-atmo-

sphere system. In particular, the consequent chem-

ical recycling of crust, sediment and water through

arc systems ultimately contributes to the growth

and evolution of continental crust. The composi-

tions and fluxes of melts produced in the subduc-

tion factory depend critically on the thermal and

dynamical evolution of the mantle and descending

plate in subduction zones. Here we investigate

three-dimensional patterns in the spatial and tem-

poral evolution of both flow and temperatures in

subduction zones using laboratory experiments,

with a particular focus on the variability of tem-

perature over the surface of a descending slab

segment of finite along-trench dimension. We also

provide more detailed analysis of how different

subduction parameters influence the magnitude of

the vertical component of velocity within the

mantle wedge, which ultimately controls decom-

pression melting within arcs.

[3] Although the influence on arc magmas of

crustal plumbing and the assimilation of crust

during magma ascent remains uncertain, there are

three basic processes leading to magma production

within the mantle wedge beneath arcs. The most

commonly discussed processes are the melting of

the mantle wedge after it has been seeded with

fluids from the underlying slab [Reagan et al.,

1995; Morris et al., 1990; Gill et al., 1993;

Edwards et al., 1993], and the direct melting of

either the slab sediments or the subducted ocean

crust [Marsh, 1979; Yogodzinski et al., 2001;

Drummond and Defant, 1990; Sigmarsson et al.,

1998; Elburg and Foden, 1999; Bryant et al.,

1999]. More recently, arc magma production has

also been attributed to decompression melting

[Sisson and Bronto, 1998; Kincaid and Hall,

2003; Conder et al., 2002], a process most com-

monly associated with ridges.

[4] Slab sinking is commonly discussed in terms of

two modes. One mode is the downward motion of

the slab along a fixed dip trajectory, referred to as

longitudinal or sometimes as downdip sinking (Ud)

(Figure 1). Slabs may also sink with a component

of motion normal to the dip of the slab producing a

translation of the slab through the mantle, a mode

referred to as rollback subduction (Figure 1)

[Elssasser, 1971].

[5] The majority of subduction zone models

have studied large-scale mantle flow in a two-

dimensional (2-D) geometry in response to exter-

nally imposed (kinematic) plate or slabmotion along

a constant dip trajectory. The response of the mantle

to this mode of downdip plate sinking (Figure 1) has

been represented analytically with a corner flow

solution [Tovish et al., 1978]. Numerical models of

downdip subduction have considered coupling be-

tween circulation and thermal evolution of the

mantle [Hsui et al., 1983; Staudigel and King,

1992; Davies and Stevenson, 1992; Furukawa,

1993; Peacock et al., 1994]. The results highlight a

balance between advective heat transport toward the

slab surface by the induced corner flow and conduc-

tive heat exchange between adjacent regions of slab

andmantle as they descend though the uppermantle.

Recent high-resolution models of downdip subduc-

tion predict very efficient advection of hot mantle

upward into the wedge corner and slab surface

temperatures (SSTs) higher than those given by

previous models [van Keken et al., 2002].

2. Rollback Subduction

[6] Observationally based models are increasingly

appealing to three-dimensional (3-D) flow in sub-

duction zones related to rollback motion of the

plate and slab. Seismic anisotropy studies, in which

the fast axes for propagating seismic energy are

assumed to be associated with mantle flow direc-

tions, indicate mantle circulation patterns which are

not consistent with those predicted by 2-D subduc-

tion models. Data from South America [Russo and

Silver, 1996], New Zealand [Marson-Pidgeon et

al., 1999; Matcham et al., 2000; Audoine et al.,

2000] and the Lau Basin [Smith et al., 2001] also

indicate 3-D patterns in mantle flow that are more

complex than can be explained with traditional

corner flow models. Rollback induced circulation

has also been suggested as a mechanism for trans-

porting geochemically distinct mantle from the

ocean side of the slab into the wedge [Pearce et

al., 2001; Turner and Hawkesworth, 1998;Wendt et

GeochemistryGeophysicsGeosystems G3G3

kincaid and griffiths: subduction zones 10.1029/2003GC000666

2 of 20

al., 1997] (Figure 1) and for producing anomalous

melting patterns at slab edges, inferred from arc

volcanoes [Yogodzinski et al., 2001; Gvirtzman and

Nur, 1999].

[7] Modes of rollback sinking involve lateral

translation of the plate and trench through the

mantle and steepening of the subducting litho-

sphere as it sinks. Both styles require mantle to be

Figure 1. (a) Schematic illustrating 3-D aspects of subduction, including different styles of slab sinking and mantlereturn flow. Slab motion along a constant dip is referred to as downdip sinking (UD) and drives a corner flow in thewedge (blue line). Sinking with a component of motion normal to plate dip is rollback (UR), which may involvetranslation (UT) and changes in dip angle (qt). Red lines depict possible return flow paths for rollback-induced motion,around (dashed) or beneath the plate. The coordinate axes are shown, with the position of the slab axis of symmetry(y = 0). (b) Photograph of the subduction apparatus with the tank of glucose syrup and the subducting plate. Thehydraulic components for producing three distinct modes of sinking are labeled.


kincaid and griffiths: subduction zones 10.1029/2003GC000666kincaid and griffiths: subduction zones 10.1029/2003GC000666

3 of 20

displaced from the ocean side to the wedge side

of the system, either beneath the plate tip or

around the plate edge (Figure 1). Return flow in

a horizontal plane driven by a translating plate has

been documented, albeit for higher Reynolds

numbers than are appropriate for the mantle

[Hudson and Dennis, 1985]. Other models have

considered rollback sinking in a 2-D, vertically

oriented plane through the mantle. These show

that flow in the wedge is very different in rollback

cases than cases when slabs sink only with

longitudinal motion [Garfunkel et al., 1986;

Olbertz et al., 1997; Kincaid and Sacks, 1997].

In a 2-D geometry, rollback drives a return flow

beneath the slab tip (Figure 1) that causes flow

entering the wedge corner to occur along steeper

trajectories. Results indicate this leads to warmer

SSTs [Kincaid and Sacks, 1997] and greater

amounts of decompression melting in the mantle

wedge [Kincaid and Hall, 2003].

[8] Previous dynamical studies have used labora-

tory experiments to model 3-D aspects of roll-

back subduction [Kincaid and Olson, 1987;

Shemenda, 1993; Griffiths et al., 1995; Guillou-

Frottier et al., 1995; Funiciello et al., 2003]. An

advantage of the laboratory models is that they

are not limited by computational grid resolution

on temperature and velocity fields, but instead

limited only by the spatial resolution with which

measurements can be made. The results revealed

a range in interaction styles between negatively

buoyant, viscous slabs and the 670 km interface

[Kincaid and Olson, 1987; Guillou-Frottier et al.,

1995] and a natural time variability between

periods of purely downdip versus rollback sink-

ing [Griffiths et al., 1995]. Laboratory models

have also related patterns in return flow to

seismic anisotropy data [Buttles and Olson,

1998] and shown that longitudinal and rollback

sinking can produce very different wedge circu-

lation and SST patterns [Kincaid and Griffiths,

2003].

[9] Here we report more extensive results on

spatial and temporal variability in circulation and

SST from 3-D laboratory subduction models.

We show that vertical velocities within the wedge

(which will dictate decompression melting effi-

ciency in the mantle) are largest for cases of

rollback sinking with slab steepening, as well as

in experiments with steep, superfast downdip

subduction. Warm wedge material is efficiently

drawn toward the slab surface, creating what has

been termed a ‘‘pinch zone’’ in the apex of the

wedge [e.g., Hsui et al., 1983]. Using appropriate

scaling to the mantle we predict that SSTs are

generally higher than predicted by previous nu-

merical models and are shown to be strongly

dependent on the style of slab sinking. Downdip

sinking produces warmer temperatures along the

slab edge, whereas rollback sinking leads to

warmer temperatures along the slab centerline.

We also find that SSTs vary in time, with larger

values recorded soon after subduction is initiated

and at later times after the pinch zone develops in

the apex of the wedge.

3. Subduction Model

[10] We model the upper 1300 km of the mantle

with glucose syrup held within a 100 cm long �60 cm wide � 40 cm deep transparent acrylic

tank (Figure 1). The subducting slab is repre-

sented with a composite laminate, or Phenolic

sheet, that is 20 cm wide and 2.5 cm thick. The

slab is forced to sink into the glucose along

prescribed trajectories by hydraulic pistons. Two

pistons control downdip (UD) and translational

(UT) plate motion, and a third changes the slab

dip angle with time (qt) (Figure 1b). Piston stroke

rates are controlled with precision flowmeters in

the hydraulic lines. A 2.5 cm thick rigid acrylic

sheet overlies the wedge region of the fluid to

simulate an overriding plate that migrates with

the trench. All plate motions are kinematic, rather

than dynamic. We employ this model simplifica-

tion to allow us to control the relative magnitudes

of downdip versus rollback sinking. This assumes

that the dominant driving force for convective

motion within the upper mantle in subduction

zones is the sinking plate. While the experiments

are kinematic models only, they are designed to

mimic the range in sinking styles seen in previ-

ous experiments with dynamic slabs [Kincaid and

Olson, 1987; Guillou-Frottier et al., 1995;

Griffiths et al., 1995].



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[11] The glucose syrup has a temperature depen-

dent viscosity represented by the exponential law

m ¼ 15 exp 1800= Tþ 93ð Þ � 12:10f g; ð1Þ

where m and T are dynamic viscosity (in Poise) and

temperature (�C), respectively. The maximum

viscosity contrast within the glucose in these

experiments is 10, which is less than that within

the mantle. However, the very large viscosities

within a sinking lithospheric plate are represented

in our model by the elastic plate. The ratio of slab

to ambient fluid viscosity contrast is infinite for the

experiments and likely exceeds 105 for the mantle.

[12] The primary difference between the laboratory

and mantle viscosity structure occurs over a narrow

temperature range in the cool boundary layers

beneath the overriding plate and around the sinking

slab. A comparison between mantle [Kincaid and

Sacks, 1997] and laboratory viscosity laws shows

that over 80–90% of the thermal boundary layer

(TBL) that develops between the ambient wedge

fluid and the surface of the sinking plate viscosity

increases are similar (roughly a factor of 5). Lab-

oratory and mantle viscosity profiles diverge over

the remaining 10% of the TBL, nearest to the

slab surface. Within this portion of the TBL di-

mensionless laboratory viscosity (normalized by

the ambient value) is between 5 and 10. Using a

mantle viscosity law this region of the boundary

layer has a characteristic dimensionless viscosity

>1000, making the mantle fluid in this region part

of the downgoing plate. The weaker temperature

dependence for viscosity in the laboratory models

however, is not expected to significantly influence

the basic variations in flow or temperatures

recorded between the different subduction modes.

In the laboratory models there is no relative mo-

tion, or shear observed within the portion of the

boundary layer where the viscosity laws diverge.

[13] The kinematic subduction models may be

scaled to the mantle through the Peclet number,

Pe ¼ UDD=k; ð2Þ

which represents the ratio of advective heat

transport to conductive transport and connects the

length scales and timescales of flow and heat

conduction. The thermal diffusivity k of the

laboratory fluid and Phenolic plate are both

10�3 cm2 s�1. The corresponding mantle value

is 10�2 cm2 s�1. A relevant length scale, D, is the

thickness of the laboratory slab (2.5 cm) which is

taken to be equivalent to a lithospheric thickness

of 80 km. Equating Pe for the laboratory and the

mantle, slab speeds of 2–11 cm min�1 in the

laboratory correspond to mantle values of 3.5–

19 cm yr�1. The important variables are therefore

the piston stroke rates controlling each of the

modes of slab motion. In all cases the plate

subducts to within 0.5 cm of the base of the tank.

Values for the duration of each experiment are

given in Table 1.

[14] The dimensions of the tank are chosen to

remove the boundaries of the domain as much as

possible from the region of interest. The depth H of

the tank (i.e., the maximum height of sinking)

is 40 cm which scales to a mantle depth of

1300 km. This is roughly a factor of 3 greater

than our depth of interest, which is the upper

400 km of the mantle wedge. In addition, these

models represent whole mantle convection because

there is no barrier to flow associated with the

670 km interface. The width W of the slab, relative

to its thickness (W/D = 8), models a plate segment

of width 650 km, which is intermediate to values for

the Scotia (450 km) and Marianas (1200 km) slabs.

[15] The most important aspects of the thermal

setup in the experiments are an upper TBL

representing the lithosphere and an initial slab to

ambient fluid temperature difference (Figure 2).

Experiments began with the slab apparatus being

brought to a temperature of 5�C in a constant

temperature room. The tank containing the glu-

cose syrup at a uniform temperature of 20�C was

kept in another temperature-controlled area. The

tank was then insulated by 7.5 cm foam sheeting

on the sides and base and was wheeled into the

constant temperature room, where the plate and

control apparatus was attached to the top of the

tank. Motion of the slab (subduction) was initiated

when the conductively growing surface TBL

beneath the overriding plate reached the desired

thickness. Experiments were carried out with two

values for the thickness of the TBL (Figure 3),



5 of 20

defined as the depth beneath the overriding plate of

the 15�C isotherm. Figure 3 shows vertical profiles

of temperature and viscosity through the initial

TBL and corresponding rheological boundary

layers (RBL) beneath the overriding plate.

[16] The choice of temperatures for use in the

experiments is arbitrary. Figure 2 illustrates the

model used to relate laboratory and mantle

temperatures. Model temperatures are scaled to

the mantle by assuming the initial temperature

difference (DTL = T1 � To = 15�C) between

the ambient fluid (T1) and plate (To) in the

model corresponds to the estimated potential

temperature difference (DTm = 1050�C) between

the ambient mantle and the surface of the

subducting oceanic plate at a depth of 20 km.

The latter depth is roughly equivalent to the

elastic thickness of the overriding plate. Thus a

1�C difference in the laboratory model corre-

sponds to a 70�C difference in mantle potential

temperature. Profiles of scaled mantle tempera-

ture (Tm) versus depth are generated using the

expression

Tm ¼ T20 þ T*DTm þ gzð Þ; ð3Þ

where z is depth in km, g is adiabatic temperature

gradient (0.5�C km�1) and T20 is an estimate of

mantle SST at 20 km depth (T20 = 350�C). Thequantity T* is a dimensionless potential SST

anomaly evaluated from the model experiments

as

T* ¼ T� Toð Þ=DTL; ð4Þ

where To and T(z, t) are the initial and measured

laboratory SST.

[17] The strength of thermal convection is gov-

erned by the Rayleigh number (Ra) which repre-

Table 1. Parameters for Subduction Experiments and Data on Flow and Temperatures in the Wedgea

Exp. UD, cm min�1 UT, cm min.�1 Dip, deg. TBL, cm F, deg. Uw* Tw, �C W, cm yr�1 Duration, min

4 2 0 49 0.7 1–4� 0.34 16.9� 0.05 2632 2 0 49 0.7 2–4� 0.32 16.7� 0.07 267 2 1 49 0.7 19� 268 4.5 1 49 0.7 18� 11.59 2 2 49 0.7 18.7� 2610 4.5 0 49 0.7 18.5� 11.511 2 4 49 0.7 16.8� 2612 2 0 74 0.7 18� 20.513 4.5 0 74 0.7 18� 914 11 0 49 0.7 0–5� 0.33 17.9� 0.4 4.815 11 0 74 0.7 8–12� 0.38 19.5� 1.5 3.716 2 4 74 0.7 3� 1.6 18.2� 0.3 20.517 2 2 74 0.7 18� 20.519 2 0 qt 0.7 15� 0.75 19.5� 0.8 20.520 2 2 qt 0.7 5–9� 1.2 18.5� 0.7 20.523 2 0 49 1.8 5–6� 0.37 18.6� 0.1 2624 2 0 74 1.8 12–15� 0.42 19.3� 0.4 20.525 11 0 74 1.8 12–15� 0.42 18.7� 2.2 3.726 11 0 49 1.8 6–9� 0.42 18.3� 1.3 4.828 2 2 49 1.8 3–4� 0.88 17� 0.2 2629 2 2 74 1.8 4� 1.7 19.5� 0.5 20.530 2 2 qt 1.8 6–8� 1.4 19.5� 0.8 20.531 2 0 qt 1.8 18–21� 0.85 19.8� 1.2 20.5

aColumns 2–5 list information on the initial/forcing conditions for each experiment. Columns 6–9 list data on representative flow rates and

temperatures in the wedge region of the fluid (see Figures 1 and 2). Ud and UT are the longitudinal and translational plate speeds, TBL refers to thethickness of the thermal boundary layer under the over-riding plate (see Figures 2 and 3), F is the angle of the flow streamlines within the wedgeapex (Figure 2), measured in degrees from the horizontal, Uw* is the dimensionless speed recorded by individual reflectors (defined as U/Ud ormeasured laboratory velocity divided by the longitudinal plate speed), Tw is the maximum temperature measured in the wedge along a vertical linebeginning 5.5 cm from the trench (Figure 2) and W is the vertical velocity within the wedge apex, scaled to mantle values. Values for Ud of 2, 4.5and 11 cm min�1 correspond to mantle values of 3.5, 8 and 19 cm yr�1. Dip angle is given in degrees from horizontal or as parameter qt whichindicates variable dip where slabs steepen from 49� to 74� at qt = 2� min�1. TBL thickness is the depth to the 15�C isotherm at the start of theexperiment, given in cm. The duration (in mins.) of each experiment is given in the last column. To convert from laboratory times to mantle timesmultiply by 2 Ma min�1.



6 of 20

sents the ratio of buoyancy forces to factors resist-

ing convection (thermal diffusivity, viscosity) and

is expressed as

Ra ¼ gaDTLH3

� �=kn: ð5Þ

Here DTL, H, a and n are vertical temperature

difference, fluid depth, expansion coefficient and

kinematic viscosity. Although slab sinking velo-

cities are prescribed in these experiments, we

calculate a laboratory Ra to facilitate comparisons

with mantle values and provide a check on our

choice for laboratory DTL. Values of H = 40 cm,

DTL = 15�C, a = 4.6 � 10�4 �C�1 and n = m/r =

5 � 102 cm2 s�1 in our experiments give Ra 106. Using mantle values in (5), we obtain a

comparable value or Ra assuming an upper

mantle dynamic viscosity in the range m 1019–1020 Poise.

[18] SST was monitored at 1 s intervals using six

rows of thermocouples set into the slab surface

(five thermocouples per row). Distances of ther-

mocouple rows from the slab tip were 1 cm

(row 1), 8.5 cm (row 2), 18 cm (row 3),

23.5 cm (row 4), 31 cm (row 5) and 38.5 cm

(row 6). Rows 1 and 2 sample conditions just

after subduction initiation. The passage of the

larger row numbers record thermal conditions

after the subduction zone has matured. The

temperature versus depth paths for rows 5 and

6 represent nearly steady state conditions. SSTs

recorded for rows 4–6 from similarly positioned

thermocouples (e.g., plate edge) show tempera-

ture changes drop to <2%, when compared at a

reference depth level. Wedge temperature profiles

were obtained by traversing probes through the

fluid (Figure 2) at the end of each experiment.

Maximum wedge temperatures (Tw) recorded

along these profiles are listed in Table 1. Circu-

lation patterns were monitored by passing a thin,

1.5 cm thick planar sheet of light horizontally or

vertically through the tank and observing the bright

reflections off micro-bubbles (�0.1 cm diameter)

suspended within the syrup. Digital video images

were recorded over the entire duration of each

Figure 2. Cartoon illustrating the model used to relatelength scales and temperatures in the laboratory model tothose for the mantle. The wedge region of the fluid iscovered by an overriding plate. The temperature of theslab when it enters the fluid is To, or 5�C. Experimentsbegin with a thermal boundary layer beneath the fluidsurface, shown schematically on the right as a plot oftemperature (T) versus depth (z). Here Ts and T1are fluidtemperature beneath the elastic plate (9�C) and theambient fluid temperature (20�C). The correspondingmantle values are 630�C and 1400�C, respectively.Circulation data are obtained from time lapse images ofreflective microbubbles that move passively with thefluid and produce streaks. Streak velocity (Uw) is thestreak length (Ls) divided by the time lapse interval (Dt).As shown in the lower left corner, the steepness of thestreak, measured as the angle f between the streak and thehorizontal, may be converted into a vertical velocity thatis scaled through Pe to a mantle value (W). A maximumtemperature in the wedge (Tw) is recorded along a verticalprobe line (grey line) at the end of each experiment.

Figure 3. (a) Plots of temperature verus depth forcases of thin and thick thermal boundary layers(TBLs). The base of the TBL is defined as the 15�Cisotherm. As described in the methods, the TBL growsconductively within the cold room until the desiredthickness is reached. (b) Corresponding plots ofviscosity variation with depth based on the temperatureprofiles in Figure 3a.



7 of 20

experiment and post-processed using digital Parti-

cle Image Velocimetry to produce time-lapse streak

images of bubble trajectories (Figure 2).

4. Results

4.1. Longitudinal Subduction:Velocity Data

[19] We present results on circulation and tempera-

ture patterns from a set of 23 experiments that

includes cases with downdip and rollback sub-

duction (Table 1). Our experiments begin at the base

of the slip zone between the downgoing slab and the

overlying plate, where the slab and mantle wedge

material become coupled. Figure 4 shows the

time evolution of an experiment with shallow (49�)downdip (longitudinal) subduction from just after

the initiation of subduction through to the point

where the slab tip passed beyond the base of the

upper mantle (20 cm, corresponding to 600 km

in the mantle). As sinking was initiated and the slab

tip entered the syrup, a TBL and associated RBL

grew along the upper surface of the slab. This is

shown by the distortion in the appearance of fine

parallel lines on a diffusing screen resulting from a

temperature-dependent refractive index of the syrup

(Figures 4a–4c). The morphology of the TBL

assumed a characteristic shape that was thin near

the tip of the slab and thickening with distance along

the slab. As the length of slab in the syrup increased,

the pattern of induced circulation in the surrounding

syrup evolved, and the boundary layer was later

pinched toward the slab in the apex of the wedge.

[20] Characteristics of the corner flow for this case

of shallow, longitudinal motion (Exp.32, Table 1)

are shown in Figures 4d–4f. Streak images reveal

an initial transient phase (owing to increasing slab

length) that evolved to near steady state circulation.

Figure 4d shows that at early times there was a re-

circulation cell in the wedge and that this feature

was displaced progressively downward following

the descent of the slab tip. By the last frame shown

the flow in the shallow parts near the slab had

evolved into a typical corner flow pattern, where

fluid moved toward the slab surface along nearly

horizontal trajectories before turning downward

with the slab.

[21] Data on fluid velocities and flow trajectories

within the wedge region of the fluid are summa-

rized in Table 1. We also estimate scaled vertical

velocities in the wedge because these may be used

to gauge the relative importance of decompression

melting processes (velocities were determined by

comparing the lengths of individual streaks to a

reference length scale equivalent to the downdip

speed Ud multiplied by the measurement time

interval, see Figure 2). Velocities in the wedge

for the case of intermediate downdip sinking were

typically less than 40% of Ud, or 0.4Ud. Wedge

flow trajectories were steepest just after subduction

initiation (Figure 4d) and became shallower with

time (approaching 2–4� fromhorizontal; Figure 4f ).

The corresponding vertical velocity in the mantle

for near-steady conditions corresponding to this

case is only 0.07 cm yr�1.

[22] Circulation in the wedge for cases with only

downdip sinking is seen to vary with slab speed, dip

angle and TBL thickness. As expected, wedge

velocities increased with increasing slab speed

(Figure 5), with magnitudes that were again in the

range (0.3–0.4)Ud. Figures 5a–5c show the time

evolution in flow for a case of steep (74�), superfastsubduction (Ud = 11 cm min�1) beneath a thin TBL

(Exp. 15, Table 1). The orientations of particle paths

are steeper (8–12� from the horizontal) and scaled

mantle values for vertical velocity in the wedge

reach 1.5 cm yr�1. The presence of a thicker TBL

beneath the overriding plate produced even steeper

trajectories in the wedge and the largest vertical

velocities recorded in any of these experiments

(Figures 5d–5f, Table 1). As in the case of the

thinner TBL, the maximum upward velocities near

the wedge corner occurred just after initiation of

subduction and decreased with time. Early in this

experiment flow trajectories in the wedge corner

were 26� from horizontal and the largest vertical

velocities were recorded at 2 cm min�1 (mantle

value 4 cm yr�1). As the system approached steady

state, flow trajectories within the wedge corner were

reduced to 12–15� from horizontal, with scaled

vertical mantle velocities of 2 cm yr�1 (Table 1).

[23] The relationships between the orientation of

wedge flow trajectories (e.g., streak angles) and the

different subduction parameters considered here



8 of 20

are summarized in Figure 6. Figure 6a shows the

temporal variability in wedge streak angles for two

end-member cases with only downdip sinking.

Within a given experiment, steeper angles occur

early, just after subduction initiation. Comparisons

of data on flow trajectories from late in the experi-

ments (e.g., steady state) are shown in Figures 6b

and 6c. Streak angles are less sensitive to downdip

Figure 4. Images taken parallel to the strike of the trench showing the time evolution in subduction for casesof downdip sinking (UT = 0) along a fixed, shallow (q = 49�) dip angle. Figures 4a–4c are for a case of fast (UD =4.5 cm min�1) sinking (Exp. 10). These are photographs taken against a diffusing screen marked with fine horizontallines. Distortion of the lines is from thermal gradients in the fluid. Dashed line in Figure 4c is the TBL/RBL above theplate surface. Figures 4d–4f are time lapse streak images oriented though the plate’s centerline. These are for a caseof intermediate sinking (UD = 2 cm min�1; Exp. 32). Streak’s show the evolution of a corner flow. Streak orientationsin the wedge apex (region marked in Figure 4e) are steeper early on (Figure 4d) and shallow with time (Figure 4f;Table 1). The UD scale bar is the streak length for this time lapse window corresponding to the UD velocity. A coloredstreak in d–f that is half this length is made by a reflector moving at half of UD.



9 of 20

plate rate than to RBL thickness, dip angle and

style of slab sinking (Figure 6b). The greatest

angles are seen for the rollback cases with slab

steepening. However, Figure 6c shows that the

scaled vertical velocities within the wedge are

strongly related to downdip sinking rate.

4.2. Longitudinal Subduction:Temperature Data

[24] A goal of this work is to characterize how

different modes of plate sinking, along with other

more commonly recognized subduction parame-

ters, modify the thermal evolution of the subduct-

ing lithosphere. We first report the observed

temporal patterns in SST. Results suggest that

SST beneath arcs may vary as the system evolves.

The time variability in SST recorded at a fixed

depth of 4 cm (corresponding to 130 km in the

mantle) is shown in Figure 7. Data are plotted for

thermocouple rows 1 and 2, which are nearer the

tip of the plate and rows 4 and 5 which pass

through this depth horizon after more of the

plate has been subducted through the system. A

Figure 5. Series of time-lapse streak images for cases of purely downdip sinking (UT = 0) along a steep (q = 74�)dip at a super-fast rate (UD = 11 cm min�1). Figures 5a–5c show the time evolution in corner flow for sinkingbeneath a thin TBL (Exp. 15). Figures 5d–5f are for sinking beneath a thick TBL (Exp. 25). The average angles ofthe streaks in the wedge apex (e.g., Figure 4) relative to the horizontal are listed in Table 1. The UD scale bar (e.g.,Figure 4) for Figures 5a–5c is shown in Figure 5a and for Figures 5d–5f is shown in Figure 5d.



10 of 20

consistent temporal pattern in SST is that relatively

high values are recorded with the passage of the

plate tip, followed by a decrease in SST with the

passage of thermocouple row 2. After this early

transient, the system matures and relatively high

SSTs are recorded with the passage of thermocou-

ple rows 4 and 5. This temporal variation in SST

was more pronounced at plate edges (variations of

approximately 1�C) than along the centerline of the

plate (variations of 0.5�C).

[25] Larger variations in SSTwere seen in response

to varying key subduction parameters. The depen-

dence of SST on sinking speed (Ud) is shown in

Figure 8 for two dip angles and two TBL thick-

nesses. Temperatures were generally higher for

cases of slower subduction speed and shallower

dip angles. SST was smaller (at both the slab edge

and centerline) for larger plate speeds, a result that

is consistent with previous 2-D numerical models

[Kincaid and Sacks, 1997]. The spread in SST for

the range in Ud values employed was 2–3�C. Thethicker TBL tended to slightly increase the depen-

dence of SST on slab speed and narrow the spread

in SST values recorded for shallow versus steep

dip cases.

[26] In order to facilitate comparisons between the

subduction models and observational constraints,

the laboratory temperatures are scaled to mantle

values using equation (3). Figure 9 shows plots of

SST versus depth for thermocouple row 5 which

subducts through the system once it has matured

and reached steady state. The warmest SST versus

depth paths are recorded along the plate edge in

cases of shallow (49�) longitudinal subduction

Figure 6. Plots summarizing data on flow trajectorieswithin the mantle wedge for the range in subductionparameters explored in these experiments. (a) Plots offlow angle in the wedge apex (e.g., indicated inFigure 4e) for end-member cases with purely downdipsinking (Red circle = steep dip, thick TBL, UD = 2 cmmin�1; blue oval = shallow dip, thin TBL, UD = 11 cmmin�1). Steeper trajectories are recorded just aftersubduction initiation and decrease with time. Steeperdips, faster sinking and a thicker TBL contribute tosteeper flow trajectories. (b) Summary plot showingsteady state flow trajectories (recorded as angles fromthe horizontal) in the wedge apex for the range inexperiments (red(blue), steep(shallow) dip; thin(thick)ovals, thin(thick) TBL; orange rectangle, qt cases; greenrectangle, UT cases). TBL thickness and slab dip have alarger impact on angle than UD. (c) Similar to Figure 6b,but for the vertical component of velocity in the wedgeapex, scaled to the mantle.

Figure 7. Plots of temperature recorded along theplate’s upper surface as it passes through the 4 cm depthhorizon. Values are plotted relative to the distance of thethermocouple from the plate’s leading edge and soprovide a measure of the time evolution of the system.Data are for cases with only longitudinal sinking.



11 of 20

beneath a thin TBL. Values exceed the wet basaltic

solidus [Holloway and Burnham, 1972] for each of

the subduction rates. SST versus depth paths for

the intermediate and fast subduction rates are also

consistent with recent evidence from experimental

petrology suggesting very high slab temperatures

[Johnson and Plank, 1999; Hermann and Green,

2001]. The runs with a thicker initial TBL predict

cooler paths that remain below the wet basalt

solidus for intermediate and superfast sinking rates.

Temperatures along the slab centerline (Figure 9b)

are generally lower than those at the slab edge

[Kincaid and Griffiths, 2003]. However, the scaled

centerline SSTs for intermediate and fast sinking

rates beneath a thin TBL are also consistent with

the Johnson and Plank [1999] data.

[27] Experiments with steeper (74�) downdip sub-

duction produced generally cooler SST paths, over

a similar range in slab speeds (Figure 10). Only the

case of intermediate speed beneath a thin TBL

produced temperatures that were sufficiently high

to match the recent petrology data. SSTs recorded

at the slab edge again were consistently warmer

then the centerline values. An interesting result is

that SST was higher when the TBL was thicker in

runs with steep, slow downdip motion. This pattern

was not seen in the runs with 49� dip.

4.3. Rollback Subduction:Circulation Patterns

[28] The experiments confirm that both circulation

and slab temperature patterns are modified by the

Figure 8. Plots of SST for cases of downdip sinkingrecorded by row 5 thermocouples located at the (a) edgeversus (b) centerline of the plate when they reach adepth of 4 cm (130 km). Row 5 thermocouples arelocated 31 cm from the plate’s tip. Values are plottedagainst UD and represent nearly steady state conditions.Results are shown for cases of thin and thick TBLs andshallow and steep dip angles.

Figure 9. Plots of SST versus depth for row 5 thermo-couples for cases of shallow longitudinal subduction(UT = 0). These plots are scaled so as to allowcomparisons between lab. results and recent experi-mental constraints and previous numerical models.Laboratory temperatures have been scaled to mantlevalues using equation (3). The evolution in SST versusdepth as recorded by (a) edge and (b) centerlinethermocouples is shown for a range in UD values (solid,slower; dashed, faster) and TBL thicknesses (thick TBL,blue lines; thin TBL, red lines). Estimated SST fromrecent experimental constraints are shown as a redellipse [Johnson and Plank, 1999] and a red circle[Hermann and Green, 2001]. The range in steady stateSSTs from cases of Kincaid and Sacks [1997] with onlylongitudinal sinking are represented in the bar (top,mantle value of UD = 1.3 cm yr�1; base, UD = 10 cmyr�1). For comparison, estimates of the wet (WP) anddry (DP) peridotite solidus [Takahashi, 1990] are shownand the wet basaltic solidus (WB) [Holloway andBurnham, 1972]. In shallow (steep) dip cases row5 sinks to a scaled depth of 510 km (320 km). Plots aretruncated to show results from the upper 300 km.



12 of 20

rollback component of motion (as first reported

in Kincaid and Griffiths [2003]). The most basic

observation is that rollback sinking drives a mass

flux around the tip of the slab, a motion that is

not apparent in the case of simple longitudinal

subduction. Figures 11a–11c shows the evolution

of the wedge circulation in response to downdip

motion and slab translation (with a fixed dip

angle). In Figure 11a the slab is forced with only

downdip motion and flow in the wedge is

sluggish. However, when rollback is initiated

(Figure 11b) velocities in the wedge increase

immediately. At this stage, with the slab tip still

at shallow levels, return flow to the wedge is

predominantly driven beneath the leading edge of

the plate. As the slab tip moves deeper into the

system (Figure 11c) the return flow becomes

dominated by motion around the edges of the

plate. Streak patterns for this style of rollback

sinking show that shallow mantle wedge trajec-

tories evolve to orientations that are 3�–4�from horizontal, regardless of dip angle, TBL

thickness or rollback speed (Table 1).

[29] In previous dynamic subduction models,

where plates sink because they are relatively dense,

slab dip tended to increase with time [Kincaid and

Olson, 1987; Griffiths et al., 1995; Kincaid and

Sacks, 1997]. Hence we show in Figures 11d–11f

the time evolution of flow patterns in the wedge for

a case of slab steepening (Exp. 31 in Table 1). Here

the position of the trench was fixed (UT = 0) and

the change in dip effectively caused the rollback

component of slab motion to increase with distance

down the plate. Distinct circulation regimes are

identified within the wedge. Flow trajectories within

the shallow wedge (depths < 10 cm) were steep

(15–21� from horizontal). Velocities in this region

were (0.75–0.85)Ud, which was greater than in

purely downdip cases but smaller than in rollback

runs with UT 6¼ 0 (e.g., Figures 11a–11c). Scaled

vertical velocity in the apex of the wedge reached

1.2 cm yr�1. Circulation patterns in the deeper

parts of the wedge were similar to those observed

in runs with translation of both slab and overriding

plate (Figures 11a–11c).

4.4. Rollback Subduction: Temperatures

[30] Slab temperatures in cases of longitudinal

subduction were dramatically different from those

with rollback sinking [Kincaid and Griffiths,

2003]. In cases with rollback, the large scale corner

flow, driven by Ud, and the return flow around the

slab combine to produce a shear flow over the slab

surface. The shear involved enhanced advection

toward the centerline of the slab (1.5UD) relative to

flow around the edges of the slab (<0.4UD),

causing SST to increase dramatically in the center

of the slab over cases without rollback.

[31] Predicted temperature-depth paths for the plate

surface in rollback cases (scaled to mantle temper-

atures; Figure 12) show that SST was consistently

higher along the slab centerline relative to the slab

edge. An overriding plate with a thicker initial

TBL reduced SST and further enhanced the lateral

temperature difference between the center and edge

of the slab. The scaled center-to-edge difference

varied between 50�C for the thin TBL case

(Exp. 20) to >100�C for the thick TBL case

(Exp. 30). The hottest SST path recorded in these

experiments - from the centerline thermocouple in

Figure 10. Similar to Figure 9, for cases with onlylongitudinal sinking but along steeper, 74� dip trajec-tories. SST paths are generally cooler than thoserecorded in the 49� dip cases.



13 of 20

an experiment with relatively slow downdip

motion and translation of a shallow dipping slab

(Exp. 7) - is also shown in Figure 12. In that case

scaled values for SST reached 950�C in the depth

range of 120–130 km. The wedge temperature,

too, for this case was among the highest values

recorded in our experiments (Table 1).

5. Discussion

5.1. Slab Surface Temperatures

[32] Estimates of mass transport and chemical

recycling through arcs require information on tem-

peratures within the subducting lithosphere and the

mantle wedge. In particular, estimates of SST

versus depth are needed for discerning between

models in which slabs melt and those in which the

slab only provides fluids to initiate melting in

overlying mantle. Here we summarize a number

of factors influencing SSTs, in a relative sense, and

then discuss model results in the light of recent

arguments suggesting slabs may be hotter than

previously thought [Johnson and Plank, 1999;

Hermann and Green, 2001].

[33] Key factors governing the thermal evolution

of the plate include subduction rate, the thermal

structure of the incoming plate and the vigor and

geometry of flow in the wedge [Peacock, 2003]

Figure 11. Streak images showing flow patterns in the wedge for rollback subduction cases. Figures 11a–11c arefor a case of intermediate downdip sinking (UD = 2 cm min�1) and fast translation (UT = 4 cm min�1) of a plate thatremains at q = 74� dip (Exp. 16). Here Figure 11a is taken prior to the start of rollback. A transition from slow to fastwedge flow is recorded from Figure 11a to Figure 11b, as rollback starts. Figures 11d–11f are for a case in which dipangle increases, with no plate/trench translation (Exp. 31). These are the steepest angles for streaks recorded in thewedge (Table 1).



14 of 20

(Figure 13). These factors are modeled in the

experiments. However, two factors not included

are plate age and shear heating at the slab

surface, which are thought to influence the SST

at the point where the plate and mantle first

couple. Models with shear heating applied as a

constant heat source show large variations

(500�C) in incoming SST [Peacock, 1992].

More recent models in which shear heating is

explicitly calculated, suggest heat production may

be more limited (100�C). The age of the

incoming plate may be responsible for larger

swings in SST. Models show ranges of 200–

300�C for cases with incoming ages between

25–100 Ma [Kincaid and Sacks, 1997; van

Keken et al., 2002]. In applying our experimental

results we neglect these factors and choose a

‘‘standard’’ initial slab temperature of 350�C in

equation (3).

[34] Numerical 2-D models have already shown

that rollback sinking influences patterns of wedge

flow [Garfunkel et al., 1986], temperature and melt

production [Kincaid and Sacks, 1997; Kincaid and

Hall, 2003]. Velocities in the wedge increase and

trajectories steepen as a result of rollback, leading

to a reduced heat loss from wedge material to the

Figure 12. Similar series of plots for row 5 thermo-couples as in Figures 9 and 10, but for cases of rollbacksinking. SST paths are compared for cases with UD =2 cm min�1 and UT = 1.2 cmmin�1, and plate steepeningat qt = 2� min�1. The difference is between Exp. 20 (thinTBL, red lines) and Exp. 30 (thick TBL, blue lines). Thehottest SST path recorded in these experiments is shownfor the centerline in Exp. 7 (red line), which is fixeddip (49�) rollback beneath a thin TBL (UD = 2 cm min�1,UT = 1 cm min�1). The red ovals are the same as inFigure 9 and the red rectangle represents an SST valuefrom Kincaid and Sacks [1997] for a case of rollbacksinking. Data for SST versus depth recorded at the slabcenterline are shown with solid lines. The dashed linesrepresent data values for the plate edge.

Figure 13. Cartoons which illustrate how the SSTachieved by a depth of 130 km in arcs is controlled bythe depth at which mantle wedge and slab parcelsbecome coupled. (a) As the plate sinks, mantle wedgematerial (red squares) is drawn up into the wedge apex(shaded triangular region) and toward the slab surface(blue squares). Red wedge parcels come in contact withand couple to blue slab surface parcels at point I. Fromthis point down to the 130 km depth horizon, thecoupled red-blue parcels move together and exchangeheat via conduction and a TBL grows above the slabsurface (Figures 4a–4c). Heating length (Lk) and time(tk) scales may be defined (orange line and equation inFigure 13a). A fast UD results in a shorter tk and lowerSST for the plate as it passes the 130 km depth horizon.Increasing or decreasing Lk will also effect tk and SST at130 km. Shallowing dip angle increases Lk. InFigure 13b we illustrate how processes within theextreme wedge apex and the depth of slab-wedgecoupling may influence Lk. If the apex is erodedcoupling occurs at II, Lk is large and SSTs are increased.If a stagnant, high viscosity plug grows in the apex,outlined by dashed blue line, the depth of coupling maybe delayed to III, shortening Lk and causing SSTs to bereduced at 130 km depth.



15 of 20

overriding plate as material approaches the sinking

slab. The laboratory results underscore the impor-

tance of subduction style on the evolution of the

system and reveal a strong three-dimensionality in

the mantle’s response to rollback. For example,

SST along slab edges is predicted to be high

relative to the slab’s center in cases with only

downdip motion, and to vary only slightly when

rollback sinking is initiated. A more dramatic

change is that after rollback begins flow velocities

toward the slab centerline immediately increase by

a factor of 3–5 and SSTs rapidly increase by 100–

200�C in this region of the plate.

[35] Another fundamental process governing SST

involves the establishment of a TBL above the slab

near the apex of the wedge. The experiments show

that the boundary layer thickness depends on the

temperature of the incoming wedge material and the

nature of the coupling between the plate and wedge

(Figure 13). The advective heat transport com-

presses isotherms against the slab while thermal

diffusion acts to broaden the temperature gradient.

In early numerical models [Hsui et al., 1983] the

temperature gradient in this pinch zone was ulti-

mately limited by grid resolution, which limited the

SST, whereas results from recent numerical models

[van Keken et al., 2002] and from the present experi-

ments give larger SST values (Figures 9 and 10).

[36] Once the slab surface material becomes cou-

pled to parcels of the mantle wedge and move

downward together (Figure 13), thermal diffusion

becomes the dominant process of heat transfer

normal to the slab. The time that it takes joined

parcels of slab and mantle wedge to reach the zone

of magma generation beneath the arc (assumed here

to be 130 km) becomes an important parameter.

This heating time (tk) is controlled by the downdip

plate speed (Ud) and the distance (Lk) between the

coupling point and the 130 km depth horizon, as

illustrated in Figure 13. This length scale varies

with dip angle and the depth of coupling. Increases

in Lk and therefore tk due to shallower dip angles or

shallower coupling points allow for more diffusive

heat exchange and higher SSTs.

[37] The depth level for coupling between the

wedge material and the slab is highly model

dependent. In our laboratory models, and most

numerical models, sinking is prescribed as a kine-

matic forcing condition and slab-wedge coupling

occurs at relatively shallow levels [Bodri and

Bodri, 1978; van Keken et al., 2002]. Models in

which plates sink dynamically tend to require the

use of slip nodes [Kincaid and Sacks, 1997] or a

form of stress dependent rheology [Christensen and

Yuen, 1984; King and Hager, 1990; Schmeling,

1989] to produce plate-like subduction. These

techniques enable simulated slabs to sink into the

mantle without adhering to the overriding plate.

[38] The combination of slip nodes and downdip

sinking in dynamic models facilitates the growth of

a cool, stagnant, high viscosity region within the

apex of the wedge. This delays the coupling

between slab and wedge, shortens tk and produces

cooler SST (Figure 13). Conversely, erosion of the

viscous region within the wedge apex results in

larger Lk and tk and higher SST. The importance of

this effect is seen in 2-D numerical models where a

rollback-induced increase in Lk results in a 200–

300�C rise in SST at 160 km depth [Kincaid and

Sacks, 1997, Figure 13]. It is consistent therefore,

that in the present laboratory models the highest

SSTs were found when a tight pinch zone

developed at the slab surface (for a combination

of rollback motion and relatively shallow plate-

wedge coupling).

[39] The two scenarios for the wedge apex depicted

in Figure 13b should be distinguishable with geo-

physical observables such as seismic quality factor

(Q) and heat flow. In the case of deep coupling we

expect a high Q and reduced heat flow between the

volcanic front and the trench [Watanabe et al.,

1977]. For shallower plate-wedge coupling the

apex region is more effectively replenished with

warm mantle material moving toward the slab.

Hence Q should be low [Umino and Hasegawa,

1984], heat flow in the region between the arc and

the trench should be relatively high, and decom-

pression melting should be enhanced.

5.2. Case for Hot Slabs

[40] Recent field evidence argues for slab melting

as the source of adakite magmas in a number of arc

systems [Abratis and Worner, 2001; Gvirtzman



16 of 20

and Nur, 1999; Yogodzinski et al., 2001] and for

melting of the slab sediments [Elburg and Foden,

1999]. Melting experiments involving 10Be and Th

systematics indicate that SST reaches nearly

800�C at pressures of 2–3 GPa [Johnson and

Plank, 1999]. Results from high pressure experi-

ments on melting in phengite in the absence of

fluid have also been used to argue for a SST of

950�C at depths of 130 km [Hermann and Green,

2001].

[41] Without more detailed information constrain-

ing the volumes and spatial/temporal patterns for

slab-derived melts relative to other magma types

in arc systems, questions remain as to how hot

the subducted slabs may become, and whether

slabs are able to melt. The majority of previous

thermal models predict slab temperatures too low

for melting of slab material other than sediment,

and sediment melting for only restricted cases of

slow subduction of a young plate [Kincaid and

Sacks, 1997]. The high resolution models of van

Keken et al. [2002] generate significantly larger

SST over a wider range in subduction parame-

ters. From the laboratory results we also suggest

that high SST, capable of melting the plate, may

be possible over a wider range of plate speeds

and ages and that high SST zones will vary in

space and time depending on relative strengths of

longitudinal and rollback sinking (Figures 9, 10,

and 12).

[42] It is important to note that while the spatial

and temporal patterns in SST measured in the

experiments are robust, the absolute, scaled man-

tle values are highly model dependent. We have

derived mantle pressure-SST paths (Figures 9,

10, and 12) using the model in Figure 2 and

equation (3). However, varying the depth of

coupling (taken here to be thickness of the elastic

portion of the overriding plate) or modifying our

choices for T20 or T1 in equation (3) will change

these paths.

5.3. Implications for DecompressionMelting

[43] Rocks in arc systems reflect magma genera-

tion by decompression melting in the mantle

wedge. These include typical low H2O, MORB-

type tholeiite basalts [Sisson and Bronto, 1998] and

more exotic rocks such as boninites or high Mg

andesites. The latter are thought to result from

decompression melting of a highly refractory

source that has been enriched in incompatible

elements by slab-derived fluids [Crawford et al.,

1981]. The volume of decompression melting

produced in the wedge depends on the temperature

and vertical velocity of the wedge material, in

addition to volatile contents.

[44] Previous 2-D models have suggested that a

slab-steepening mode of rollback [Kincaid and

Sacks, 1997] or the relative motion between the

trench and overriding plate associated with back

arc spreading [Kincaid and Hall, 2003], favor

steeper flow trajectories into the wedge apex,

enhancing decompression melting within the

wedge [Conder et al., 2002; Kincaid and Hall,

2003]. The laboratory models suggest that the

velocity field in the wedge is more complicated

than predicted by 2-D models. Although rela-

tively steep wedge trajectories are associated

with the early stages of rollback sinking, more

mature rollback sinking tends to draw material

into the wedge, from around the slab edge,

along low angle flowlines. For purely longitu-

dinal sinking, decompression melting is most

likely immediately after initiation of subduction

(Figure 6a). At later times the vertical component

of velocity in the wedge is gradually reduced.

Melting-favorable velocities are also seen over

longer periods of time for cases of superfast

longitudinal subduction, particularly when slabs

sink beneath thicker overriding plates (Figures 6b

and 6c).

[45] Crawford et al. [1981] point out that the

appearance of boninites in the Mariana region is

coincident with the cessation of arc magmatism

and the start of back arc spreading. Our labora-

tory models suggest that a pulse of enhanced

vertical motion in the wedge may be expected

when plates begin sinking with a component of

slab steepening. Such a rollback-induced pulse of

melting might produce either MORB-type, low

H2O, tholeiite basalts or boninite magmas,

depending on the degree to which the mantle



17 of 20

wedge has been seeded with slab derived fluids.

Similarly, either initiation of subduction or a

sudden increase in sinking rate, are predicted

to produce pulses of enhanced decompression

melting.

5.4. Temporal Patterns in SubductionZone Temperatures

[46] Our results reveal interesting temporal variabil-

ity in SST (Figure 7). For cases with only downdip

sinking, values recorded along the slab surface as it

passes though a designated depth level of 4 cm (or

130 km in the mantle) appear to cycle between

periods of relatively high and low SST values.

These stages are associated with variations in TBL

thickness against the slab (Figures 4 and 14). As the

slab passes through the system, a thin TBL is

established first near the tip and thickens with

distance along the slab, and hence thickens with

time at a fixed depth position. From the reference

frame of the moving plate this pattern in TBL

thickening may be described as a Blasius problem

of flow of warm material over a planar cool plate.

Thermocouple row 1 in our models is located near

the tip and records relatively high SST. Thermo-

couple row 2 is further up the slab, where the thicker

TBL reduces heat flux into the plate (Figure 14).

However, unlike a Blasius problem, the wedge flow

has a component normal to the plate and pinches the

TBL in the wedge apex, causing SSTat that depth to

rise again. This result is different from numerical

models, in which SST at a given depth decreases

monotonically as more slab passes though the

system [Kincaid and Sacks, 1997]. This simple

schematic model does not apply to cases with

rollback.

[47] There is evidence for similar periods of ele-

vated SST in subduction zones [Elburg and Foden,

1999; Bryant et al., 1999]. Chemical variations in

the rocks from Sangihe Arc indicate periods of

greater concentrations of fluid mobile elements

arising from the slab (Sr, Ba, U) and other periods

of higher concentrations of the less mobile ele-

ments (Nb, Zr, Ti) [Elburg and Foden, 1999].

These changes can be interpreted as variations

between periods of hydrous fluxing from the slab

and periods of direct melting of the slab, reflecting

periods of cooler and hotter SST, respectively.

Future experiments will consider in more detail

what factors influence the time variability in SST.

6. Conclusions

[48] We report results from kinematic laboratory

experiments that suggest there may be significant

differences in mantle circulation and temperature

structure in response to subduction with and with-

out a rollback component. In cases of longitudinal

sinking without rollback motion, flow in the mantle

wedge is 30–40% of the downdip plate speed.

Flow trajectories tend to be nearly horizontal

except in cases of superfast sinking beneath a thick

thermal boundary layer beneath the over-riding

plate. In purely longitudinal subduction there is

no mass flux around the slab. Rollback subduction

drives flow around and beneath the sinking slab,

velocities within the mantle wedge are larger and

Figure 14. Cartoon models illustrating temporalevolution of subduction zones from (a) just aftersubduction initiation to (b) a point where the systemhas matured. For discussion purposes the location ofrow numbers from the laboratory experiments areshown (in white). The TBL is thin above row 1 and isthick above row 2. The TBL thins again in the wedgeapex for the passage of row 4. Heat flux (qT) is inverselyrelated to TBL thickness. The pattern in TBL thicknessdepicted in Figure 14b is apparent in Figures 4a–4c.



18 of 20

are focused toward the centerline of a finite-width

slab segment. The velocities can exceed 150% of

the downdip sinking speed of the slab. Vertical

velocities within the wedge also increase dramati-

cally when plates steepen as they sink. Rollback

motion with trench migration, on the other hand,

leads to small vertical velocities in the wedge. In

fully dynamic, 3-D subduction models the plate is

not rigid and is distorted by the lateral return flow

[Kincaid and Olson, 1987], which may result in

less vigorous circulation around the edges of the

plate.

[49] The experimental results suggest that the sur-

face temperature of the sinking plate varies with

position across the slab segment, in a trench-

parallel direction, and that this variation is sensitive

to the style of slab sinking. Downdip sinking

favors slab melting along the edges of the slab,

while rollback sinking favors slab melting along

the slab centerline. We infer from these results that

zones of active slab melting in the mantle may

alternate between the center of the segment and the

edges of the segment, if the system oscillates

between periods of downdip and rollback subduc-

tion modes. Both longitudinal and rollback cases

are able to produce slab surface temperatures

(scaled to the mantle) that are consistent with

recent observational and experimental constraints

calling for very hot subducting slabs.

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Variability in flow and temperatures within mantle subduction zones

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