Page 347 - Global Tectonics

330 CHAPTER 10

the Eurasian side of the suture. This process Australian plate subducted northward beneath the
predicts that channel crust south of the Eurasian plate at the Java Trench (Fig. 10.28a). This
Indus–Zangbo suture will show Indian subduction created the Banda volcanic arc and a north
crustal affinities, whereas channel material dipping Benioff zone that extends to depths of at least

north of the suture will have Eurasian crustal 700 km. Between 3 Ma and 2 Ma, subduction brought

affinities in a manner consistent with Australian continental lithosphere in contact with the
geologic observations (Section 10.4.4). Other Banda forearc, part of which was thrust southward over
similar models predict the formation of the colliding Australian continental margin and is now
gneiss domes similar to those observed in the well exposed on Timor (Harris et al., 2000; Hall, 2002).
Greater Himalaya. The downgoing Australian continental slope choked
the subduction zone and created a fold and thrust belt
(Fig. 10.28b) that has deformed both the forearc
10.5 ARC–CONTINENT sequences and the structurally lower unsubducted cover
sequences of the Australian continental margin. The
COLLISION Australian sequences include pre-rift Late Jurassic to
Permian sedimentary rocks of a Gondwana cratonic
basin, and younger post-rift Late Jurassic to Pliocene
continental margin deposits that accumulated on the
Orogenic belts that result from the collision between rifted continental slope and shelf (Audley-Charles,
an island arc and a continent typically are smaller 2004). Within the adjacent volcanic arc north of Timor,
than those that form by continent–continent collision volcanism has stopped on the islands of Alor, Wetar,
(Dewey & Bird, 1970). Arc–continent collision also and Romang. West of the tectonic collision zone volca-
tends to be relatively short-lived because it usually nism is still occurring on the islands of Flores, Sumbawa
represents an intermediate step during the closure of and Lombok, north of the triangular Savu-Wetar forearc
a contracting ocean basin. Active examples of this basin (Fig. 10.28a).
type of orogen occur in Taiwan (Huang C.-Y. et al., In eastern Indonesia, east of the Australian–Timor
2000, 2006), Papua New Guinea (Wallace et al., 2004), collision zone, seismicity patterns provide evidence of
and the Timor–Banda arc region north of Australia the past northward subduction of Indian oceanic
(Audley-Charles, 2004). These belts provide important lithosphere beneath the Banda Sea (Milsom, 2001).
information on the mechanisms by which continents Figure 10.28a shows the inferred position of the former
grow, including by the accretion of terranes (Section Banda trench, which represents the eastward continu-
10.6.3). ation of the Java trench before it was obliterated by
The sequence of events that occurs during arc–con- its collision with Australian continental lithosphere.
tinent collision begins as the island arc approaches a The distribution of earthquake hypocenters beneath
continent by the consumption of an intervening ocean. the Wetar Strait and Banda arc marks the location
Collision begins when the continental margin is driven of the descending continental lithosphere to below
below the inner wall of the trench. At this point the depths of 300 km (Engdahl et al., 1998). Earthquake
positive buoyancy of continental lithosphere slows the records suggest that the upper and lower plates of
rate of underthrusting and may lock the trench. If the the subduction zone in the Timor region are now
continental margin is irregular or lies at an angle with locked (McCaffrey, 1996; Kreemer et al., 2000). North
respect to the island arc, the timing of arc–continent of the Banda arc, Silver et al. (1983) discovered two
collision may vary along the strike of the orogen. Once north-directed thrust faults (the Wetar and Flores
collision begins, the forearc region and accretionary thrusts) that appear to represent the precursors of a
wedge are uplifted and deformed as thrust faults carry new subduction zone that is forming in response to
slices of flysch and oceanic crust onto the continental the collision (Fig. 10.28a,b).

plate. If the two plates continue to converge, a new An example of an oblique arc–continent collision
trench may develop on the oceanward (or backarc) side occurs in Taiwan and its offshore regions. This belt
of the island arc. is especially interesting because an oblique angle of
The Timor–Banda arc region provides an example convergence between the Luzon arc and the Eurasia
of an arc–continent collision in its early stages of devel- continental margin has resulted in a progressive young-
opment. Prior to 3 Ma, oceanic lithosphere of the Indo- ing of the collision zone from north to south (Fig.