Page 361 - Global Tectonics

344 CHAPTER 10

wide) extensional basins floored by basalt and gabbro tionships suggest that orogenesis and crustal growth in
were created behind one or more island arcs that even- the Lachlan orogen were dominated by magmatism and
tually accreted onto the continental margin (Glen, the recycling of continental detritus during cycles of
2005). Between the volcanic rocks are the accreted parts extension and contraction that lasted from Late Ordovi-
of a huge submarine sediment dispersal system that cian through early Carboniferous times.
developed along the Gondwana margin during the early Cycles of backarc and intra-arc extension, such as
Paleozoic. Diachronous pulses of contractional and those that occurred in the Lachlan orogen, generate
strike-slip deformation followed each extensional cycle, thin, hot lithosphere that may localize deformation
generating upright folds and overprinting cleavages in a during subsequent phases of contraction, collision, and
series of thrust wedges in the upper 15 km of the crust. orogeny (Hyndman et al., 2005). Collins (2002b) illus-
This style of shortening did not lead to the development trated this process in a model of orogenesis involving
of a well-defi ned foreland basin nor a foreland fold and the formation and closure of autochthonous backarc
thrust belt of the type seen in the central Andes (Fig. basins (Section 9.10) above a long-lived subduction
10.5) and the Himalaya (Figs 10.19, 10.20). Instead, it zone. The model begins with a zone of intra-arc exten-
was controlled by the thick (10 km) succession of turbi- sion that evolves in response to the roll back (Section
dites and locally high geothermal gradients. These rela- 9.10) of a subducting slab (Fig. 10.37a). This setting

(a) Interarc rift (Taupo Volcanic Zone) 0 Ma (b) Slab-rollback (crustal extension) 0–20 Myr
Ocean
SL SL plateau
0 0
Oceanic crust
MOHO MOHO Oceanic crust
depth in km 50 Lithosphere depth in km 50 Lithosphere
Slab flux
melting
Asthenosphere Asthenosphere
100 100 Roll back
0 100 0 100
km km
(c) Flat subduction (crustal thickening) 20–30 Myr (d) Slab-rollback (crustal extension) 30–50 Myr
Off-scraped
ocean plateau top
50% remnants Old folded New backarc
SL backarc SL
0 0
Oceanic crust
Oceanic crust
depth in km 50 MOHO Lithosphere depth in km 50 MOHO Lithosphere
Asthenosphere
100 Asthenosphere 100 Asthenosphere
0 100 0 100
km Roll back km
Basalt intra/ Mantle counter New accretionary Old arc or
underplating (granulite flow prism Granulite terrain
facies metamorphism) microcontinent
Decompression Slab flux Old accretionary Backarc basin Lithospheric fragment
melting melting prism
Figure 10.37 Model showing the evolution of the Lachlan Orogen of southeast Australia through accretionary
tectonics involving the creation and destruction of backarc basins above an ocean–continent subduction zone (after
Collins, 2002a, with permission from the Geological Society of America). (a) Intra-arc extension due to the roll-back of a

subducting slab. (b) Backarc basin and remnant arc form. (c) Subduction zone flattens and the upper plate of the
orogen is thrown into compression. Contraction and crustal thickening are focused in the thermally softened backarc.
(d) Extension is re-established and a new arc–backarc system forms.