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9 ACTIVE ~GINS: GROUP 3 MASS AND CHEMICAL TRANSFER INTRODUCTION Melting Pot of the Earth: Crustal Formation and Modification at Active Margins Plate tectonic theory depicts the earth as a dynamic and evolving body. Much of the evolution takes place at active margins (convergent plate boundaries), both oceanic and continental, where continental crust is created as oceanic crust is carried back into the earth's interior. The process of subduction creates hazards, such as large earthquakes and explosive volcanic eruptions. Voicanic ash also profoundly affects climate. More benevolent results of subduction are the formation of economically significant ore bodies and geothermal resources. In addition to these socially significant aspects, active margins are of great scientif to interest as regions where the earth's crust. mantle, oceans . and atmosphere interact. Indeed, it now appears that much of earth's crust, atmospheric gases, surface waters, and upper mantle have been subducted and recycled at active margins, perhaps several times. - ~ reasons, active margins are the melting pots of the For these earth. The last decade has seen implementation of new geophysical and geochemical techniques, yielding a rapid improvement in the amount and quality of observational data on subduction systems. For example, seismic tomography now allows us to image the mantle above the subducting slab (the mantle wedge), a region which is central to studies of active margins. ,°Be and U-series disequilibria allow us to constrain the time scale of element recycling through the system. GeochemicaI data for arc volcanic rocks have made possible recognition of global patterns that can be related to variations in input parameters, such as subducted ocean crust or the mantle. Finally, consensus, though not unanimity, that the mantle wedge is the primary source of arc magmatism, and better understanding of the physics of magma formation and extraction are leading to refined models of the complex melting and mixing processes that accompany plate convergence. As a result of these advances, the stage is set for 95
us to pose and answer a new level of questions: it is now realistic to begin to examine the melting pot not as a set of unrelated processes, but as an ordered dissipative system. New field, experimental, and theoretical studies, on land, at sea, and in the laboratory can now be focused on representative active margins e We expect that this integrated approach, with a focus on the arc system, will produce major breakthroughs. Explosive volcanism, metal deposition, and major mass feedback loops accompanying distillation of crust from mantle are all connected through the subduction process. A modern systems approach stresses that these phenomena are all manifestations of a dynamical system whose basic components are illustrated in Figure 1. Oceanic crust enters the active margin system on the downgoing plate (#1~. Mass is transferred out of the downgoing plate (~2) at depths ranging from the trench to the site of magma formation beneath the volcanic arc and backarc basin. Mixing then occurs within the mantle wedge (#3) between slab-derived and wedge-derived components, with the relative effect of the slab contribution apparently decreasing away from the plate boundary. The mantle wedge is thus the melting pot, into which is fed new mantle plus material transferred from subducted Oceanic crust, and from which magmas and fluids are extracted into the crust of the active margin (#4~. This magmatism both creates and modifies the crust. Preexisting crust, in turn, can modify the ascending magma. Ultimately, the magma and fluids appear at the earth's surface ~#5) , where they can be sampled in forearm, volcanic arc, and backarc settings. Critical problems for mass transfer processes operating in each of these numbered regions are discussed below, but the main objective is to use mass flux to understand convergent margins as an integrated dynamical system. ~ ~ in r. VA 8A _~ \~o \~ FIGURE 1 Input from the subducted oceanic crust (#1, SOC) is transferred t#2) and mixed with new mantle in the mantle wedge (#3, MOO), which is the principal source of the continents. Magmas and fluids traverse preexisting crust (~4, C) en route to the surface (#5) where they can be sampled in the forearc (FA), volcanic arc (VA), and backarc (BA). 96
CRITI CAL PROBLEMS Chemica_ Variability of Input from the Subducting Plate The definition of mass fluxes at active margins requires compositional data on the altered basaltic crust and the sedge meets that form the upper part of the subducting plate. The basal tic crust ~ hat is subducted differs from that formed at spreading axes because of compos itional changes caused by seawater. Hign temperature alteration, driven by the forced hydrothermal circulation of seawater near the ridge axis ~ sty ips The basaltic crust of elements, such as K, Rb, and B. However, nydrothermal circulation wanes due to cooling in the newly formed crust as it moves away from the ridge, and all crust older than 20 Ma may have similar hydrothermal circulation signatures. In contrast, the extent of passive low temperature hydrothermal alteration f which enriches the crust in elements, such as U. K, Rb, Cs, and B. varies with the flow patterns and the age of the crust that is subducted. The sediments deposited on the oceanic crust and supplied to subduction zones vary significantly in total thickness (150 m to 2500 m) and Ethology (carbonate, pelagic, voicanic~astic, terrig~nous), but then generally have high abundances of many of the incompatible elements (H. C, U. K, Rb, Cs, Ba ,°Be, B. Pb, Sr) that are enriched in magmas erupted at active margins. Therefore, some of these enrichments are thought to reflect a seafloor alteration or sediment contribution to the arc magmas. A first order Question is whether observed regional differences in the chemical composition of arc magmas can be linked to differences in the composition of the subducting plate. For example, do the differences-between the island arc tholeiites of the Bonin arc, the calc-alkaline magmas of the southern Mariana arc, and the alkaline-magmas of the northern Marianas arc reflect contributions from sediments of~different compositions? This issue can be addressed if geochemical reference holes are drilled outboard of the trench as part of transects across arcs, as recommended by COSOD IT. Existing DSDP cores outboard of the Japan, Aleutian, Sunda, and Middle America trenches also should be exploited more fully. Rough mass balance calculations based on isotopic and elemental abundance ratios that are distinctive in sediments indicate that less than 5 percent sediment is required in the hybrid mantle source that melts to yield arc lavas. The sediment contribution does not influence concentrations of the most abundant elements in the source, but it does have a major effect on the budget of incompatible elements, including the carbon and water that play a major rote in explosive volcanism. The effects of modern sediment contributions must be distinguished uniquely 97
from those resulting from the preexisting chemical variation of the mantle and contamination in the crust in order to establish mass flux values. This can be done through improved measurements and interpretations of isotopic tracers which are sensitive to sediment alteration and which have half-lives appropriate to the time scales of plate subduction beneath active margins (e.~., Abe) r and of magma segregation and differentiation (e.g., 30Th, and other actinides) e While the sediment contribution of arc magmas is small by mass, sediment subduction to the depths of magma generation has major consequences. For example, the presence of sediments in the subduction zone may well dictate earthquake locations and mechanisms by determining the degree of mechanical coupling between the downgoing and overlying plates. Additional studies are necessary to provide systematic measurements of all potential tracers of sediments or altered oceanic crust outboard of the trench, a goal not yet achieved. Combination of these sediment studies (part of the input) with investigation of tracers in arc magmas and fluids (the output) will provide a better understanding of the mechanisms of sediment incorporation in arc magmas, a distinction between contributions from sediment and the altered basaltic crust, and a better constraint on the balance of sediment accretion and subduction at active margins. Fundamental advances in understanding of the subduction process await resolution of two paradoxes that have emerged from mass tracer studies. The first is that while the alteration and sediment components are chemically variable at many spatial and temporal scales, the behavior of s~ab-dominated elements in arc layas Is commonly systematic within specific arc segments at 10 -10 year time scales. Efficient homogenization is required. The second is that different tracers of sediment incorporation commonly dogtrot correlate well, presumably ref lecting dif ferent behavior of the elements during transfer from the downgoing plate to the mantle wedge (discusser} below). Mass Transfer Across Plate Boundaries An important aspect of the interaction between plates at convergent margins is the exchange of mass between the subducting and overriding plates. This exchange involves a variety of processes ranging from solid state transfer (e.g., accretion and erosion of the hangingwall crust or mantle) to migration of fluids derived by devolatilization reactions which occur as hydrous and carbonate minerals in the subducting plate are transported to higher pressure and temperature. We focus here on the chemical reactions -occurring in the the uppermost part of the subducting plate because these reactions can transfer the volatile components, HI and C02, into the overlying lithosphere and asthenosphere. Even small amounts of these volatiles can 98
have major effects on (1) theological characteristics; (2) solidus temperatures; (3) stability of minerals; (4) oxygen fugacities-; and (5) concentrations of elements that are highly soluble in volatile-rich fluids. Mineral reactions occur in the subducted plate because the sediments and altered basa~ts that compose its upper part have minerals that are stable only at relatively low pressures and temperatures. However, as the slab is subducted, these minerals are transported to higher pressures and temperatures where they are unstable e Reactions create new mineral assemblages and low-density fluids that can segregate and rise from the downgoing slabe ~ sequence of important reaction and segregation events begins as the slab descends beneath the forearm, and continues during descent through the asthenosphere. Examples of these reactions and segregation processes are the serpentinite diapirs in the Mariana forearc, and the slab-derived fluid which is inferred to be a component in volcanic arc and backarc lavas. Al though the composition of the components segregated from the slab must change as a function of temperature and pressure r the important reactions controlling compositions of segregated phases are not known. Clearly, a quantitative understanding of these reactions is necessary to determine the composition of components transferred from the subducted plate to the overlying pi ate or asthenosphere. Because of the important effects of even small amounts of volatile components, we are particularly concerned with partitioning of elements into a fluid or silicate melt that segregates from the slab. Our knowledge of element partitioning between minerals and H2O- and CO=rich fluids in the so ab is very limited, and much less is understood than for sin icate melt/mineral partitioning. An attainable ob; active is to determine how el ement part it ion ing varies as the f lu id pha s e in equip ibrium with minerals in the slab and mantle wedge varies f ram HzO-rich to CO=rich to an alkal i-rich sil icate melt . Identification and understanding of the controlling reactions can be achieved by a combination of thermodynamic calculations and experimental studies designed to mimic the subduction zone environment. Another important experimental and theoretical objective is to understand how slab-derived fluids migrate through and interact with the overlying plate or asthenosphere. Key questions are the mechanism of transport and distance of migration of these fluids. This multi-phase flow problem is analogous to those invioving migration of silicate melts discussed below. Mantle Wedge Processes It is increasingly accepted that the mantle wedge is the dominant site of crust/mantle differentiation. The key issues 99
are: (1) the mechanisms and sites of magma generation and segregation; (2) the chemical composition of melt generated in the mantle wedge; (3) the relation between magma type and physical parameters, such as plate coupling, thermal structure, slab dip, distance from trench, and the amount of sediment subduction; and (4) replenishment rates of fertile mantle within the wedge. A maj or obj ective is to develop integrated physical models f or magma generation and mass bans f er in the mantle wedge. For instance, available data indicate that "arc tholeiitic'' suites generally tend to be in areas without large earthquakes flits le plate coupling), whereas calc-alkaline rocks are more comlac~nly associates] with zones of maj or earthquakes. Field and] geochemic data are requires] to evaluate, in various types of arcs ~ the relationships between magma composition, age and dip of the slab, distance of volcanism from the trench, stress regime in the areas of magma genesis, the amount of sediment subducted as estimated from comparisons of sediment influx with the volume of the accreted prism, and the physical state of the downgoing slab (e.g., thickness, extent of fracturing). These should be linked with the results o f geophysical studies including heat f ~ ow and conductivity variations, and spatial variation in seismic velocities and attenuation, which may reveal degrees of melting and determine the rheological characteristics of likely materials in the mantle wedge. The integrated field' geochemical, and geophysical database for selected arc systems can then be used to model the thermal structure, magma generation, and fluid flow within the mantle wedge. Magma generation and segregation processes have been the subject of detailed modelling along active spreading centers, and geochemica~ tests are being developed to evaluate them in areas of continental extension and rifting (see Passive Margins Group 1, page 27~. In comparison, at destructive plate margins, magma generation is affected by the introduction of hydrous fluids, rather than only by adiabatic upwell ing . Furthermore, the amount of refractory clinopyroxene is likely to be variable and the stress regime more complex in the mantle wedge. These differences between the arc and ridge systems are likely to modi fy the processes of magma segregation and element ~ fractionation. To better constrain models for magma generation in the arc environment, more precise data are needed on changes of magma composition and volumes with age during the evolution of different arc systems. Alkalic rocks may be an important link since they occur in both destructive plate margins and within-plate environments. Relatively uncommon magma types berg., Mg-andesites) may provide important impetus for novel ideas. The sources of minor and trace elements in subduction- related magmas are at present subdivided into a mantle component, 100
which is variably enriched or depleted in ways anal ogous to that seen in MORB (~nid-oceanic ridge basalts) and OTB (oceanic isl and basal tag, ant] an additional component that is a feature of subduction zones. The ~ atter is in some way red ated to the subduction process, whereas the former reflects the preexisting composition of the mantle wedge. In this motley, the mantle wedge varies significantly from place to place, being highly depleted beneath convergent margins ~ hat yield island arc tholeiite suites, and relatively enriched beneath continental arcs . This concept needs to be properly eval utter by detailed transverses across magmatic arcs in oceanic and continental areas, characterized by different age provinces in the overriding p7 ate. Crustal Processes The tangible outputs of the mantle wedge are the magmas that cross the crust-mantle boundary and form new crust. To investigate the mantle wedge, we need to identify the composition of these magmas and to determine their compositional variation in space and time. The identification is complicated by modification of the magmas as they fractionate and interact with preexisting crust. The intracrustal processes are tightly coupled with the nature and mechanical state of the preexisting crust. In both continental and oceanic arcs, intracrustal differentiation results in the redistribution of crustal components, leading to a generally more silicic and hydrous upper crust and a more mafic and less hydrous lower crust. In addition, to evolve-mature continental crust from transient oceanic arc crust requires a second stage of refinement and crustal thickening, thought to occur during arc-continent collision. Often this collision exposes crust that was originally at mid- or even lower-crustal levels. These intracrustal processes are of economic and social significance as they lead to the-emplacement of hydrothermal and magmatic ore deposits. They are important in the thermal evolution of petroleum in overlying sedimentary basins, and they determine the eruptive histories of voicanos. The sequence of processes by which magma interact with the preexisting crust is as enigmatic as the analogous processes operating in accretionary prisms. Obviously, the evolution of arc magmas is coupled with the composition, structure, and state of stress of the preexisting crust, but this coupling is only understood in the broadest sense. Generally' the simplest crustal environments are thought to be oceanic arcs with steeply dipping slabs (i.e., Tonged, while the most complex are continental arcs over shallowly dipping slabs (i.e., Andes). In oceanic arcs, the preexisting crust consists of products of previous arc magmatic events and the basement upon which the arc 101
was built. This basement is thought to consist of oceanic or backarc crust which may be dismembered. In continental arcs, the preexisting crust is older continental crust that has a complex previous history. In extensional regimes, arc magmas may pass with little fractionation to shallow level magma chambers which periodically feed voicanos. In contractional regimes r arc magmas may pond at depth leading to a higher intrusive-extrusive ratio, greater heating of the preexisting crust, and subsequent melting anti mixing with that crust. Heating anc} melting of the crust influence ductile and brittle behavior and have a profound but poorly understood influence on deformation of the crust. The middle to lower oceanic and continental crusts are inaccessible, but processes occurring there can be partially deciphered by studying magmas that reached the surface after having traversed these regions. As a sequence of fossils records evolution, the geochemistry and mineralogy of a series of magmas are a catalogue of interaction of these magmas with the preexisting crust. The tools to decipher this history are available, but they are just beginning to be used to their fu1 lest potential in understanding the evolution of continental margins from a mass balance viewpoint. Integrated field, geochemical, and geophysical studies are needed to determine how new arc magmas refine old crust and influence mechanisms and patterns of crustal deformation. An understanding of the relation between deformation and magmatism is essential as deformation affects how differentiation processes proceed, and it provides a continual mechanism of recycling fusible upper crust back into the hot deforming lower crust. The destiny of--mantle-derived magmas in-the crust -has broad impl ications for continental crustal formation. Most studies agree that new crust is primarily aclded at convergent margins or by underplaying in extensional regions and that this newly added crust is largely of basaltic composition. Ponding of these basalts-at-the crust-mantle boundary-probably-results in fractionation of mafic-minerals, forming new-mantle. The fate of the new mantle is poorly understood. Furthermore, these ideas lead to a fundamental question: how can addition of basalt result in a bulk crust which, by most estimates, has an andesitic composition? ~ There are two possible resolutions to this paradox e First, early (Archean) crustal formation processes were-distinct, and melts extracted from the mantle at that time were more silicic. In this case, recent crustal additions at lower rates lead to a more mafic crust through time. Alternatively, the mafic lower crust formed during compositional stratification at active margins are actually recycled into the mantle through a poorly understood process of crustal delamination. This process could be triggered by thickening and cooling of hot lower crust leading to garnet production and an increase in crustal density to the ~2
point that the lower crust is actually denser than the underlying mantle. Delamination of the lower crust and mantle lithosphere could then occur, possibly associated with terrane accretion and continental collision. The delaminated lower crust could be stored in the lithosphere until subsequent melting results in a return to the crust or recycling into the asthenosphere. A similar process could have operated in the Archean. The paradox of the disparity between the composition of material added to the crust and the bulk composition of the crust points to our lack of research on crustal formation processes involving active margins. Chemical Variability of the Output: Geochemistry of Magmas and Fluids at Active Margins Two phenomena, active volcanism and hydrothermal venting, are the dramatic outputs of the active margins system. In addition to being important as hazards and sources of economic mineralization , they also sample the current output along and across arc systems e By a detailed sampling program combined with a judicious choice of geochemical tracers, the red ative role of the various input parameters can be constrained. Older volcanic rocks and ve in mineral i z at ions make it pass ibI e to study pa st outputs as well, so that time sequences can be developed. New observational tools developed during the past decade have invigorated study of both phenomena. As a result, globally consistent patterns in magma composition have emerged. These, in turn, seem to be related to variations in the kind and style of input and tectonically controlled mass transfer processes. A systematic analytical study of volcanic rocks-and fluids should be carried out in representative forearc-voicanic arc-backarc systems in order to develop mass flux models, especially for elements and isotopes that trace different input components. The sampling and analytical program should emphasize components that are transferred across the plate boundary rather than those that are recycled near the earth's surface. However, distinguishing between fluxes from the underthrust plate, mantle wedge, and sites of intracontinental differentiation is an important objective. Forearc fluids from accretionary prisms and serpentinite diapirs, magmatic rocks from the full width of the volcanic arc, and volcanic rocks from backarcs should all be sampled along a representative length of the arc. Metal deposits in all localities should be included. To gain historical perspective about output diversity, volcanic rocks and mineralized veins related to the entire last volcanic cycle (i.e., episodic pulse of activity) of the arc system should be dated and analyzed. In order to emphasize subcrustal components in volcanic rocks, thorough igneous geochemical characterization is 103
necessary, including multi-element' multi-isotope systematic study of suites that are well-defined with respect to stratigraphy-and mineralogy. Special attention should be given to isotopic tracers with decay rates appropriate to the time scale of orogenic processes. Episodicity of Volcanism: Reflection of a Nonlinear System Explosive volcanism poses significant threats to inhabitants of active margins, as the 1980 eruption of Mount Saint Helens demonstrated powerfully. This explosibi~ity is a direct consequence of the melting pot. Seawater enters the system as the result of first being added to oceanic crust, then lost from it as the result of dehydration reactions during subduction. Some of the water that enters the mantle wedge participates in magma genesis, is further concentrated by intracrustal magmatic differentiation, and finally causes explosive eruption if it separates rapidly from large magma volumes. As a result, the rates and episodicity of magma formation, transfer, and dif ferentiation all af feet volcanic behavior. The episodic release of magmas into anc] onto the crust of magmatic arcs reflects nonlinear behavior within the crust and mantle. The episodes occur on several time scales' and a fundamental goal is to associate episodes with magma generation processes within dissipative structures of the upper crust, lower crust, and upper mantle. On the shortest time scales (101 to 103 yr) are the repeat intervals between eruptions from established volcanic centers. The prediction of eruptions has an empirical (historical) basis, and the quasi-periodic eruptive history encourages the investigation of mechanisms involving upper crustal magma chamber dynamics, magmatic "triggering" by quasi-periodic release of magmas from greater depth, or nonlinear buoyant rise. The characteristics of eruptions clearly depend on volatile release. Determination of the volatile content of the magmas, and of the rocks surrounding the magma body, are incompletely explored approaches for investigating and predicting explosive potential. Because a volcanic center may erupt thousands of times during its lifetime, the documentation and explanation of steady state behavior should yield important clues about-volcano dynamics. The waning of activity, culminating in extinction, is the fate of arc vo~canos We have some idea of a volcano's life expectancy (10 to 10~ yr) and also an expectation that, over the scale of severe, million years, a new volcano is born for every one that dies. The longevity of volcanism may trace the stability of dissipative structures--convective cells, diapirs, Rayleigh-Taylor instabilities--in the underlying mantle. Our ideas about flow of peridotite and basaltic melt in the mantle 104
wedge are primitive at present, and the locations of voicanos, and their life expectancy, combined with possible e secular change in the composition of primitive magmas, is one way to constrain physical mode] s of flow in the mantle wedge. For example, if heterogeneities exist in the downgoing plate, such as chemical singularities clue to subduction of an oceanic ridge, then the duration of the same chemical s ingularities in the voicanos may give the-best available information about the convective pattern and the efficiency of mass transfer through the mantle wedge. Because the fundamental control of volcano location appears to be in the source region, the migration of volcanic loci along and across the strike of any arc should be used to monitor changes in the physical state of the mantle wedge. On shill longer time scales of 5 to 25 Ma, fundamental changes in arc geometry occur. Migration of the vo~ canic front, splitting of the arc to form a backarc spreading center, and the cessation of volcanism al together all occur on this time scale. So also cio episodes of crustal deformation. There are indications of coupling between these phenomena: some volcanic arcs may be relatively quiescent during episodes of spreading in adjacent backarc basins. Large changes in volcanic production rate--magmatic episodes--also occur in the 5- to 25-Ma time scale. These changes certainly are related to changes in the mantle under the arc. However, they appear to be regional and perhaps global in scale, and thus may be difficult to explain by some singularity in the subducted plate itself. Nonlinearity on this time scale may be a fundamental property of mass extraction from the mantle wedge. As a result of this range in time scales of pulsed output' we must integrate over at least one magmatic episode of regional significance in order to address the range of pertinent problems. Of more practical importance, the accumulation of economic resources that depend on hydrothermal circulation and on thermal state Leggy, both base metals and petroleum) may be formed and destroyed at very infrequent times over the history of a convergent margin. NEEDED STUDIES Field, analytical, experimental, and theoretical studies are needed, all focused on specific representative arc systems. From the standpoint of diversity in volcanic output, these systems should include one from each of the following categories. (1) One should be dominated by highly depleted magmas in which the signatures of recycled materials would remain the clearest. Examples include the Tonga, Kermadec, Bonin, New Britain, and Scotia arcs. Generally, these are characterized by weak plate coupling and backa-rc spreading. (2) The other extreme should be a continental margin where isotopically distinctive 105
subcontinental mantle potentially is involved, but where deeper components are not masked by crusta1-1evel processes. Examples include the South Cascades, South Andes, Central America, and Japan. (3) The third should be dominated by more fertile but still young mantle. Examples include the Aleutians, Marianas, Antilles, and Sunda arcs. Generally, these are characterized by geochemical and geophysical parameters intermediate between those of the first two. The field program should include extensive sampling of volcanic and phonic rocks, crustal and mantle xenoliths, and fluids, both along and across the strike of the arc system, both on ~ and and at sea, as necessary . It shout ~ include geophys ical studies such as 1 ocal seismic experiments, including ocean bottom seismometers, deployed to image the mantle wedge from sl ab to crust and from the trench to the axis of backarc volcanism. Surveys of stress orientation, heat flow, and conductivity are needed. Studies should determine the location of the plate boundary independent of epicenter loci (e.g. ~ where are the earthquakes relative to the plate boundary, in the depth range O to 200 km?); the attenuation and heat flow gradients between the volcanic and aseismic fronts; the length scale of attenuating regions; contrasts between the volcanic arc and backarc; and differences in these respects between arcs. The analytical program must provide coordinated and thorough multi-element, multi-isotope studies of both the inputs and outputs at the three types of arcs. Special emphasis should be placed on tracers, which may distinguish input components or mass transfer processes' and which constrain time scales of mass transfer through the subduction system, Voicanic rocks, fluids, and veins must be studied in sufficient detail to identify chemical features acquired within the crust,and dated with sufficient accuracy to relate them to the local geodynamic history e The experimental program should also include as geochemical objectives the partitioning of elements between a range of residual minerals and H~-CO2 fluids plus alkali-rich silicate melts, under the P-T-fO2 conditions of the subducting plate and mantle wedge. Also, determining the character of primitive basaltic magma within the mantle remains a difficult but important experimental objective. Geophysical experiments are needed concerning multi-phase flow of both fluids and silicate melts in the mantle wedge environment. The final goal is the development of quantitative models of the arc system. The models must explain the spatial and temporal pattern of output, and be consistent with what is learned about the input parameters, the chemical and physical processes that transfer material into the wedge, the physical behavior of the wedge, and the effects of intracrustal differentiation. Decades 106
of study of the processes relevant to arc subsystems have prepared us for this larger objective; that is, to define the relationships between the various subsystems and thereby to achieve an integrated view of the whole arc system. 107
