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22.2 the CASCADIA SUBDUCTION WEDGE: the ROLE of ACCRETION, UPLIFT, and EROSION—An Essay by Mark T. Brandon1

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22.2 the CASCADIA SUBDUCTION WEDGE: the ROLE of ACCRETION, UPLIFT, and EROSION—An Essay by Mark T. Brandon1 2917-CH22.pdf 11/20/03 5:26 PM Page 566 Brandon, M.T., 2004, The Cascadia subduction wedge: the role of accretion, uplift, and erosion. in In: "Earth Structure, An Introduction to Structural Geology and Tectonics", by B.A. van der Pluijm and S. Marshak, Second Edition, WCB/McGraw Hill Press, p. 566-574 22.2 THE CASCADIA SUBDUCTION WEDGE: THE ROLE OF ACCRETION, UPLIFT, AND EROSION—An essay by Mark T. Brandon1 22.2.1 Introduction 566 22.2.6 The Cascadia Subduction Zone 569 22.2.2 Accretionary Flux 566 22.2.7 Comparison between the Cascadia 22.2.3 Wedges, Taper, and Stability 567 and Alpine Wedges 574 22.2.4 Double-Sided Wedges 567 Additional Reading 574 22.2.5 Subduction Polarity and Pro-Side Accretion 568 22.2.1 Introduction the Mariana system, to as much as 7 km at ocean- continent subduction zones, such as the Makran mar- Given the constant surface area of Earth, there must be gin of southwest Pakistan. There is evidence from a balance between the amount of plate created at modern subduction zones that not all of the incoming spreading centers and the amount consumed at sub- sedimentary section is accreted. The global rate of sed- duction zones and other sites of plate convergence. iment subduction has been estimated in one study to be Today, 80% of this return flow occurs at subduction equivalent to an average thickness of 300–500 m of zones, and the other 20% at continent-continent colli- compacted sedimentary rock along all subduction sion zones and diffuse oceanic convergent zones. Mod- zones combined. This analysis, however, is compli- ern subduction zones have a total length of 51,000 km, cated by deeply subducted sediment that may be and consume plates at an average rate of 62 km/my (or accreted at depth beneath the overriding plate, rather 62 mm/y), with the highest rates (210 km/my, or 210 mm/y) than fully subducted into the mantle. being found along the Tonga Trench in the southwest Pacific. Subduction zones are marked by Benioff-zone earthquakes and active-arc volcanism, which are indi- 22.2.2 Accretionary Flux cations of shearing and bending of the cold subducting An important theme of this essay is that accretion of plate and melting caused by dehydration of the plate. largely sedimentary materials has a strong influence on Seismologists have generated tomographic images that deformation of the overriding plate at subduction show subducting plates penetrating deep into the inte- zones. We are concerned here with the accretionary rior of Earth, locally reaching the core-mantle bound- flux into the subduction zone, which is defined as the ary (see Chapter 14). thickness of accreted materials times the rate of plate Subduction zones are not totally efficient in remov- subduction. Sedimentary rocks are quickly compacted ing the subducting plate. Some fraction of the plate during the subduction process, so we use the equiva- gets left behind as accretionary complexes that accu- lent thickness of fully compacted sedimentary rock mulate at the leading edge of the overriding plate (Fig- when calculating the accretionary flux. This correction ure 22.2.1). In some cases, this accretion might be for compaction ranges from ∼50% for a thin sedimen- episodic, involving the collision of large lithospheric tary section, which would have a high average poros- blocks, called tectonostratigraphic terranes. More ity, to ∼20% for thick sedimentary sections, where the commonly, only the sedimentary cover of the downgo- base of the section is already fully compacted. ing plate is accreted, while the underlying crust and The Makran subduction zone provides an upper mantle lithosphere are fully subducted. The thickness limit for accretionary fluxes at subduction zones. of this sedimentary cover varies considerably, from There, the flux could be as high as 210 km3/my per hundreds of meters at oceanic subduction zones, like kilometer of subduction zone (equal to 7 km sedimen- tary section × 0.8 compaction factor × 37 km/my con- 1Yale University, New Haven, CT. vergence velocity). It is more common to consider the 566 WESTERN HEMISPHERE 2917-CH22.pdf 11/20/03 5:26 PM Page 567 Forearc high Pro-wedge Retro-wedgeow Structural Frontal accretion Subduction lid Su seSediment i en cocover er bdu ction wedgeed thr r zone ust ar zone Subducting p Underplating ro-plate she o- tr Re S Retro-plateretro plate FIGURE 22.2.1 Schematic cross section of a subduction wedge. “S” refers to the S point, the subduction point, where the pro-plate is subducted beneath the retro-plate. accretionary flux from a cross-sectional view. Thus, the convergence velocity. The resulting horizontal the flux is given as km2/my, which represents the shortening allows the wedge to return to its stable cross-sectional area of material added to the upper taper, at which point horizontal shortening stops plate per unit time. because the wedge is now free to slip on it base. Thus, the accretionary flux into the front of the wedge con- trols deformation within the wedge. 22.2.3 Wedges, Taper, and Stability Erosion will cause similar feedback, as it tends to The accreted material tends to accumulate in front of reduce wedge taper, which causes horizontal shorten- and beneath the leading edge of the overriding plate, ing and a return to a stable taper. Erosion has a strong forming a wedge-shaped body that grows with time influence on the evolution of subaerial convergent (Figure 22.2.1). A useful analogy is the way that wedges, such as the Himalaya or Alps, but subduction snow piles up in front of a moving plow, forming a wedges are commonly submerged below sea level tapered wedge that moves with the plow (represent- where erosion is not as active. Nonetheless, erosion ing the overriding plate). The wedge entrains snow can be locally important when a subduction wedge from the roadbed, which causes the wedge to grow becomes emergent, which is common during the later (see Chapter 18). evolution of wedges at continental subduction zones An important discovery of the 1980s was that accre- (e.g., Figure 22.2.1). Erosion acts to drive deformation tionary wedges tend to maintain a self-similar form as and also serves to limit the growth of the wedge. This they grow. This behavior is expected for a wedge made is illustrated by the examples that follow, with particu- of frictional materials, in that the wedge taper must be lar emphasis on the Cascadia margin of western North blunt enough to overcome the frictional resistance to America. slip on the subduction thrust that underlies the wedge. A wedge that is able to slip along its base is called a stable wedge, in that the wedge does not deform or 22.2.4 Double-Sided Wedges change shape as it rides above the subduction thrust. An important development in wedge theory occurred The taper geometry of the wedge introduces an impor- during the early 1990s, with the recognition that most tant feedback. Frontal accretion tends to drive the convergent orogens consist of two wedges, arranged wedge into a substable taper, where the wedge taper is back-to-back. This idea was first proposed for colli- now too slender to overcome friction on the subduction sional orogens, like the Alps, but it was also recog- thrust. The wedge becomes locked to the subducting nized as applicable for wedges that form at subduction plate, and thus must deform internally to account for zones. 22.2 THE CASCADIA SUBDUCTION WEDGE 567 2917-CH22.pdf 11/20/03 5:26 PM Page 568 Pro-wedge Retro-wedge S Mylar = Pro-plate Flat backstop = Retro-plate FIGURE 22.2.2 Sketch of sandbox experiment producing a Subduction double-sided wedge. Mylar spool of mylar "plate" The basic problem with the snowplow analogy was ing that the size of an actively deforming wedge is that it was hard to identify where the strong plow- defined by the volume of accreted sediment. The con- blade, or backstop as it is commonly called, was cept of a double-sided wedge provides a different view, located within the overriding plate. Many authors used in that the active wedge involves both accreted sedi- relative strength to define backstops within the crust. ments and upper-plate rocks. This result stems from the However, in many cases, the backstop region could be fact that the width of the wedge is determined by the shown to be deforming along with the accreted mate- motion of the underlying mantle plates, and not by any rial in front of it. This contradicted the basic idea of a specific distribution of strength in the crust. Using backstop, which is supposed to be a rigid part of the terms like subduction wedge or convergent wedge overriding plate. when describing double-sided wedges helps to distin- The solution to the backstop problem was inspired guish this model from that of a small wedge of accreted by sandbox experiments (Figure 22.2.2) in which a sediment bounded by a strong crustal backstop. mylar sheet serves as the subducting plate and a “fixed” flat-lying plate serves as the overriding plate. This arrangement of “plates” was covered with contin- 22.2.5 Subduction Polarity uous layers of sand, representing the deformable crust above the plates. A motor was then used to draw the and Pro-Side Accretion mylar downward through a slot in the base of the sand- The term facing is used to describe the polarity of sub- box, thus simulating the motion of the subducting duction. For instance, a west-facing subduction zone plate.
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