Page 280 - Global Tectonics

SUBDUCTION ZONES 265

(a) Accretionary forearc

V fluid vents
Accretionary
prism Magmatic front
Trench axis Trench slope Trench slope break Thick forearc basin Active

Trench basin arc
fill

Arc massif
Downgoing oceanic crust
Primary partial melts
20
Subduction channel
10
Lithosphere
km 10 30 50
(b) Non-accretionary forearc

Magmatic front
Exposed basement Serpentinite Active
Seamount mud volcanoes Thin forearc basin arc

Empty trench

Primary partial melts
Subducted seamount Downgoing oceanic crust
20
10

km 10 30 50 Lithosphere

Partially serpentinized mantle Basaltic forearc crust Undeformed sediments

Lithospheric mantle Gabbroic forearc crust Deformed sediments

Figure 9.19 Diagrams contrasting the characteristic features of (a) accretionary, and (b) nonaccretionary, convergent
margins (redrawn from Stern, 2002, by permission of the American Geophysical Union. Copyright © 2002 American

Geophysical Union). V, fluid vents.

Seismic reflection data and the ages of deformed the arc by the addition of new wedges to the toe of the
sediments suggest that the youngest faults in accretion- prism. This process, called frontal accretion, causes older
ary prisms occur at the deformation front and generally thrusts to become more steeply dipping with time and
become older away from the trench (Moore et al., 2001, is responsible for the lateral growth of the prism. Lateral
2005) (Fig. 9.20b). As shortening occurs, old thrust growth requires that the most intense deformation
wedges gradually move upwards and are rotated toward occurs at the oceanward base of the sedimentary pile,