Page 279 - Global Tectonics

264 CHAPTER 9

Another first order variation in the nature of sub- base. The amount of subsidence can be measured

duction zones is whether they are accretionary or if drill cores are available from sedimentary sequences
erosive. Historically, oceanic trenches and magmatic in this region. It is then possible to estimate the rate
arcs were considered to be the settings where material of erosion at the base of the forearc crust (Clift &
derived from the continental and oceanic crust is Vannucchi, 2004).
accreted to the margin of the overriding plate in the
form of a wedge of sediments in the forearc region, and

an edifice of igneous material in the magmatic arc. 9.7 ACCRETIONARY
Increasingly however it has been realized that most of
the oceanic crust and pelagic sediments is subducted
into the mantle, and that, in approximately half of the PRISMS
convergent margins, some of the overriding plate is
eroded and subducted. The process by which pelagic
sediments on the downgoing plate are subducted is Where present, an accretionary prism forms on the
known as sediment subduction and the process whereby inner wall of an ocean trench. The internal structure
rock or sediment from the upper plate is subducted is and construction of these features have been deduced

termed subduction erosion. The latter may be derived from seismic refl ection profiles and drilling at active
from the base of the landward slope of the trench or subduction zones, and by the study of ancient subduc-
from the underside of the upper plate. Moreover, the tion complexes now exposed on land.
majority of the material accreted in the magmatic arc Accretionary prisms develop where trench-fi ll turbi-
is thought to be derived from the mantle rather than dites (flysch), and some pelagic sediments, are scraped

subducted crust (Section 9.8). Thus, subduction zones off the descending oceanic plate by the leading edge
have also been characterized as accretionary or erosive of the overriding plate, to which they become accreted.
(Figs 9.1, 9.19). Examples of accretionary margins The Nankai Trough, located south of Japan (Fig. 9.20a),
include the Nankai Trough and Barbados prisms illustrates many of the structural, lithologic and hy-
(Section 9.7) (Saffer & Bekins, 2006); erosive prisms drologic attributes of a large, active accretionary prism
occur offshore of Costa Rica (Morris & Villinger, 2006) with a thick sedimentary section (Moore et al., 2001,
and Chile (Section 10.2.3). 2005). Beneath the prism, the plate boundary is defi ned
On the basis of seismic refl ection profiling data, it by a 20- to 30-m-thick, gently dipping fault or shear

appears that the thickness of sediment on the oceanic zone that separates a deformed sedimentary wedge
plate entering a trench must exceed 400–1000 m for above from a little-deformed section of subducted
sediment to be scraped off and added to the accretion- trench sediment, volcaniclastic rock, and basaltic crust
ary prism. This implies that perhaps 80% of the pelagic below (Fig. 9.20b). This boundary, or décollement, devel-
sediments entering trenches is subducted, and that most ops in a weak sedimentary layer, typically a low perme-
of the sediment accreted in the forearc region is trench ability hemipelagic mud underlying stronger, more
turbidites derived from continental material (von Huene permeable trench turbidites. Above the décollement is
& Scholl, 1991). The accretionary or nonaccretionary a fold and thrust belt composed of listric thrust ramps
nature of a subduction zone will depend in part, there- that rise through the stratigraphic section forming
fore, on the supply of oceanic plate sediments and imbricated arrays. These faults defi ne wedge-shaped
continentally derived clastic material to the trench. lenses that are internally folded and cleaved. At the base
However, the causes of subduction erosion are very of the imbricate series, the décollement slopes down-
poorly understood (von Huene et al., 2004). Typically ward toward the volcanic arc where it becomes pro-
the thickness of trench sediments at accretionary gressively better developed. Away from the arc, it
margins exceeds 1 km (Saffer & Bekins, 2006). Other extends a short distance seaward of the deformation

parameters that correlate with accreting margins are: front, which is marked by the first small proto-thrusts
−1
orthogonal convergence rates of less than 76 mm a and folds located inward from the trench. Farther
and forearc bathymetric slopes of less than 3°. In addi- seaward, the stratigraphic horizon that hosts the décol-
tion to the steeper slope of the forearc region at erosive lement is known as the incipient or proto-décollement
margins, the forearc is characterized by subsidence, zone where the incoming sedimentary section is only
which reflects the thinning of the upper plate along its weakly deformed.