Arccontinent Collision

Orogenic belts that result from the collision between an island arc and a continent typically are smaller than those that form by continent-continent collision (Dewey & Bird, 1970). Arc-continent collision also tends to be relatively short-lived because it usually represents an intermediate step during the closure of a contracting ocean basin. Active examples of this type of orogen occur in Taiwan (Huang C.-Y. et al., 2000, 2006), Papua New Guinea (Wallace et al., 2004), and the Timor-Banda arc region north of Australia (Audley-Charles, 2004). These belts provide important information on the mechanisms by which continents grow, including by the accretion of terranes (Section 10.6.3).

The sequence of events that occurs during arc-continent collision begins as the island arc approaches a continent by the consumption of an intervening ocean. Collision begins when the continental margin is driven below the inner wall of the trench. At this point the positive buoyancy of continental lithosphere slows the rate of underthrusting and may lock the trench. If the continental margin is irregular or lies at an angle with respect to the island arc, the timing of arc-continent collision may vary along the strike of the orogen. Once collision begins, the forearc region and accretionary wedge are uplifted and deformed as thrust faults carry slices of flysch and oceanic crust onto the continental plate. If the two plates continue to converge, a new trench may develop on the oceanward (or backarc) side of the island arc.

The Timor-Banda arc region provides an example of an arc-continent collision in its early stages of development. Prior to 3 Ma, oceanic lithosphere of the Indo-

Australian plate subducted northward beneath the Eurasian plate at the Java Trench (Fig. 10.28a). This subduction created the Banda volcanic arc and a north dipping Benioff zone that extends to depths of at least 700 km. Between 3 Ma and 2 Ma, subduction brought Australian continental lithosphere in contact with the Banda forearc, part of which was thrust southward over the colliding Australian continental margin and is now well exposed on Timor (Harris et al., 2000; Hall, 2002). 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 sequences and the structurally lower unsubducted cover sequences of the Australian continental margin. The 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 rifted continental slope and shelf (Audley-Charles, 2004). Within the adjacent volcanic arc north of Timor, volcanism has stopped on the islands of Alor, Wetar, and Romang. West of the tectonic collision zone volca-nism is still occurring on the islands of Flores, Sumbawa and Lombok, north of the triangular Savu-Wetar forearc basin (Fig. 10.28a).

In eastern Indonesia, east of the Australian-Timor collision zone, seismicity patterns provide evidence of the past northward subduction of Indian oceanic lithosphere beneath the Banda Sea (Milsom, 2001). Figure 10.28a shows the inferred position of the former Banda trench, which represents the eastward continuation of the Java trench before it was obliterated by its collision with Australian continental lithosphere. The distribution of earthquake hypocenters beneath the Wetar Strait and Banda arc marks the location of the descending continental lithosphere to below depths of 300 km (Engdahl et al., 1998). Earthquake records suggest that the upper and lower plates of the subduction zone in the Timor region are now locked (McCaffrey, 1996; Kreemer et al., 2000). North of the Banda arc, Silver et al. (1983) discovered two north-directed thrust faults (the Wetar and Flores thrusts) that appear to represent the precursors of a new subduction zone that is forming in response to the collision (Fig. 10.28a,b).

An example of an oblique arc-continent collision occurs in Taiwan and its offshore regions. This belt is especially interesting because an oblique angle of convergence between the Luzon arc and the Eurasia continental margin has resulted in a progressive young-ing of the collision zone from north to south (Fig.

Tectonic Collision Timor

A Interpreted position of former Banda Trench now below Timor and the Roti-Savu Ridge

Limit of Australian continental lithosphere

A Interpreted position of former Banda Trench now below Timor and the Roti-Savu Ridge

Active plate boundary thrust faults Strike-slip faults

Southern limit of forearc accretionary sequences km

Limit of Australian continental lithosphere

Tectonic Collision Timor

0 20 40 60 80 100 km

Figure 10.28 (a) Tectonic map and (b) interpretive cross-section of the Australia-Banda arc collision zone (images provided by M. Audley-Charles and R. Hall and modified from Audley-Charles, 2004, with permission from Elsevier). Triangles are active volcanoes. Hypocenters for events below 75km depth show the north-dipping Australian lithospheric slab and are based on the data set of Engdahl et al. (1998). Geologic section incorporates data from Hughes et al. (1996), Richardson & Blundell (1996), Harris et al. (2000), Hall & Wilson (2000), and Hall (2002). Australian continental shelf edge is marked by the 200 m contour. Dotted pattern in (a) illustrates that Australian continental lithosphere underlies the islands of Savu, Roti, Timor, Moa, Sermata and Babar. Shelf edge at 5 Ma is estimated from Deep Sea Drilling Program drilling site 262 (black circle).

0 20 40 60 80 100 km

Figure 10.28 (a) Tectonic map and (b) interpretive cross-section of the Australia-Banda arc collision zone (images provided by M. Audley-Charles and R. Hall and modified from Audley-Charles, 2004, with permission from Elsevier). Triangles are active volcanoes. Hypocenters for events below 75km depth show the north-dipping Australian lithospheric slab and are based on the data set of Engdahl et al. (1998). Geologic section incorporates data from Hughes et al. (1996), Richardson & Blundell (1996), Harris et al. (2000), Hall & Wilson (2000), and Hall (2002). Australian continental shelf edge is marked by the 200 m contour. Dotted pattern in (a) illustrates that Australian continental lithosphere underlies the islands of Savu, Roti, Timor, Moa, Sermata and Babar. Shelf edge at 5 Ma is estimated from Deep Sea Drilling Program drilling site 262 (black circle).

10.29). This geometry has allowed geoscientists to use spatial variations in the patterns of deformation, uplift, and sedimentation to piece together the progressive evolution of an oblique collision. C.-Y. Huang et al. (2000, 2006) used this approach to propose four stages of arc-continent collision beginning with intra-oceanic subduction and evolving through initial and advanced stages before the arc and forearc collapse and subside.

Off southern Taiwan, near latitude 21°N (Fig. 10.29), subduction of South China Sea oceanic lithosphere beneath the Philippine Sea plate results in volca-nism and has formed an accretionary prism and forearc basin (Figs 10.29, 10.30a). The Hengchun Ridge/ Kaoping slope and North Luzon Trough represent these two tectonic elements, respectively. Farther north, near 22°N, the North Luzon Trough narrows at the expense of an expanding accretionary prism (Fig. 10.29). In this latter region arc-continent collision began about 5 Ma and resulted in the formation of a suture between the arc and prism. The suture records both convergent and sinistral strike-slip motion (Malavieille et al., 2002) and separates two zones of contrasting structural vergence. To the east, forearc sequences have been thrust eastward toward the arc, forming the Huatung Ridge (Fig. 10.29). To the west, forearc material on the Asian continental slope and South China Sea basin is carried westward within a growing accretionary prism. On the Hengchun Peninsula, Miocene slates and turbidites of the prism have been uplifted and exposed. These and other observations suggest that the initial stage of oblique arc-continent collision involves the following processes (Fig. 10.30b):

1 uplift and erosion of the accretionary prism and the continued deposition of forearc basin sequences;

2 waning arc volcanism and the build-up of fringing reefs on inactive volcanic islands;

3 arc subsidence, strike-slip faulting, and the development of intraarc pull-apart basins;

4 suturing, clockwise rotation, and shortening of forearc sequences to form a syn-collisional fold and thrust belt.

North of the Huatung Ridge, near 23°N, arc-continent collision has reached an advanced stage (Huang et al., 2006). Here, collision since the Plio-Pleistocene has resulted in the west-directed thrusting and accretion of the Luzon arc and forearc sequences onto the accretion-ary wedge and Asian continent (Fig. 10.30c). These events have led to the uplift and exhumation of the underthrust Eurasian continental crust in the Coastal Range of eastern Taiwan. The last stage in the collision/accretion process is recorded north of about 24°N where the collapse and subsidence of the accreted arc and forearc has occurred over the last one or two million years (Fig. 10.30d), possibly as a result of the northward subduction of the northernmost Coastal Range at the Ryukyu Trench (Fig. 10.29). C.-Y. Huang et al. (2000) postulated that the Longitudinal Valley-Chingshui faults mark the collapsed trace of the arc where it approaches the subduction zone. This sequence of events suggests that orogens formed by arc-continent collision can progress rapidly through the initial stage of collision to an advanced stage and even collapse of the arc and forearc in only a few million years.

10.6 TERRANE ACCRETION AND CONTINENTAL GROWTH

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