Arc-continent Collision Zone Research Articles

Zircon U–Pb age and Hf isotope evidence for contrasting origin of bimodal protoliths for ultrahigh‐pressure metamorphic rocks from the Chinese Continental Scientific Drilling project

AbstractA combined study of zircon morphology, U–Pb ages and Hf isotopes as well as whole‐rock major and trace elements was carried out for ultrahigh‐pressure (UHP) eclogite and felsic gneiss from the main hole (MH) of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. The results show contrasting Hf isotope compositions for bimodal UHP metaigneous rocks, pointing to contrasting origins for their protoliths (thus dual‐bimodal compositions). The samples of interest were from two continuous core segments from CCSD MH at depths of 734.21–737.16 m (I) and 929.67–932.86 m (II) respectively. Zircon U–Pb dating for four samples from the two core segments yields two groups of ages at 784 ± 17 and 222 ± 3 Ma, respectively, corresponding to protolith formation during supercontinental rifting and metamorphic growth during continental collision. Although the Triassic UHP metamorphism significantly reset the zircon U–Pb system of UHP rocks, the Hf isotope compositions of igneous zircon can be used to trace their protolith origin. Contrasting types of initial Hf isotope ratios are, respectively, correlated with segments I and II, regardless of their lithochemistry. The first type shows positive ɛHf(t) values of 7.8 ± 3.1 to 6.0 ± 3.0, with young Hf model age of 1.03 and 1.11 Ga. The second type exhibits negative ɛHf(t) values of −6.9 ± 1.6 to −9.1 ± 1.1, with old Hf model ages of 2.11 and 2.25 Ga. It appears that the UHP rocks from the two segments have protoliths of contrasting origin. Consistent results are also obtained from their trace element compositions suggesting that mid‐Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting at the northern margin of the South China Block. Thus, the first type of bimodal magmatism formed by rapid reworking of juvenile crust, whereas the second type of bimodal magmatism was principally generated by rift anatexis of Paleoproterozoic crust. Melting of orogenic lithosphere has potential to bring about bimodal magmatism with contrasting origins. Because arc–continent collision zones are the best place to accumulate both juvenile and ancient crusts, the contrasting types of bimodal magmatism are proposed to occur in an arc–continent collision orogen during the supercontinental rifting, in response to the attempted breakup of the supercontinent Rodinia at c. 780 Ma.

Reworking of juvenile crust: Element and isotope evidence from Neoproterozoic granodiorite in South China

A combined study of geochronology and geochemistry was carried out for Neoproterozoic granodiorites at the southeastern margin of the Yangtze Block in South China. The results demonstrate that sedimentary rocks derived from juvenile crust were melted to form strongly peraluminous granite in an arc-continent collision zone, resulting in continental growth. LA-ICPMS zircon U-Pb dating yields two groups of age at 824 ± 6 and 882 ± 16 Ma, respectively. From CL images, the young zircons are new grains or rims of homogeneous structure with magmatic zoning, whereas most of the old ages occur in cores or as single xenocrysts. The granodiorites show high A/CNK ratios of 1.38–1.89 and strong negative Nb and Ta anomalies as well as high δ 18O values of 11.9–14.0‰ for quartz and 8.1–10.2‰ for zircon, pointing to a supracrustal origin characteristic of S-type granites. On the other hand, they have positive ɛ Hf( t) values of 3.4 ± 1.6 to 5.4 ± 2.6 for zircon, neutral ɛ Nd( t) values of −2.06 to 0.02 and low to medium initial 87Sr/ 86Sr ratios of 0.7033–0.7087 for whole-rock, indicating a magmatic source with significant components of juvenile crust similar to I-type granites. Relatively refractory minerals like zircon, garnet and quartz retain their magmatic O isotope ratios, whereas the other minerals, such as K-feldspar, plagioclase and biotite that show O isotopic disequilibria when paired with quartz, suffered different degrees of post-magmatic alteration at subsolidus temperatures. On the basis of element and isotope geochemistry, we interpret the 882 ± 16 Ma zircons as being inherited from arc-derived igneous rocks, and the 824 ± 6 Ma zircons as having crystallized from an anatectic crustally-derived magma. Arc-continent collision is inferred to have taken place at ca. 900 ± 20 Ma during assembly of the supercontinent Rodinia. Subsequent to weathering of juvenile crust in the back-arc basin, low-maturity sedimentary rocks were deposited from about 880 to 830 Ma between the Yangtze and Cathaysia Blocks. Extensional collapse of the collisional orogen at about 830–820 Ma is suggested as a geodynamic mechanism for burial and melting of the sediments of juvenile crust to produce the S-type granodiorites. As a result, the granodiorites are characterized by the features of both S- and I-type granites, and thus testify to the short-term recycling of juvenile crust with significant contributions to continental growth. The syn- and post-collisional magmatism may be a basic mechanism to result in compositional evolution from arc crust to continental crust.

Sediment compositions in offshore southern Taiwan and their relations to the source rocks in modern arc-continent collision zone

Modal analyses of 31 sand samples collected by piston coring document variations in sediment composition along and across the developing collision zone off southern Taiwan and help constrain sediment transport paths and, by inference, of sediment sources. Overall, sand composition from this region is dominated by lithic-fragment populations, with a QtFL average of Qt 27F 21L 52. Three geographic domains are based on morphotectonics and variations in sand composition: (1) South China Sea and the adjacent slope of the accretionary prism; (2) suture zone south of southern Taiwan and north of the Luzon forearc; and (3) the Luzon forearc basin (North Luzon Trough) and immediate adjacent slopes. Sands from the accretionary prism and the suture zone contain subequal amounts of sedimentary and metamorphic lithic fragments (Ls 51Lv 5Lm 45 and Ls 47Lv 5Lm 48), respectively, whereas sands from the forearc basin are dominated by volcanic lithic fragments (Ls 29Lv 49Lm 22). In addition, compositions of individual sand beds vary dramatically in the forearc basin. This heterogeneity indicates that sediments from different sources have been deposited sequentially, but do not typically mix during transport and deposition. Similar, but less dramatic, within-core variations occur over the accretionary prism. The major sediment input for the accretionary prism is from western Taiwan and is dominated by sedimentary and low-grade metamorphic lithic fragments. There appears to be a second sediment source, however, from southeastern China. This implies that a significant amount of sediment is the input from China to this region. The major component of sands in the suture zone is derived from Taiwan, but medium-grade metamorphic lithic fragments are rather sparse in these sands, considering the extensive and high-relief exposures of metamorphic rocks on the island of Taiwan. The major source of sands in the forearc basin is the active volcanoes of the Batan islands. Episodically, minor components appear to be fed to the forearc basin from the arcward slope of the accretionary prism, presumably by submarine mass wasting. This observation provides support for the hypothesized olistostromal origin for the Lichi Mélange of eastern Taiwan.

Geochemical trends across an arc-continent collision zone: magma sources and slab-wedge transfer processes below the Pantar Strait volcanoes, Indonesia

Four volcanoes in the Pantar Strait, the westernmost part of the extinct sector of the east Sunda arc, show remarkable across-arc variation in elemental abundances (K 2O: 1.2 to 4.3%), trace element ratios (Pb/Ce: 0.4 to 0.18; Ce/Yb: 20 to 55) and isotope ratios ( 143Nd/ 144Nd: 0.51263 to 0.51245; 87Sr/ 86Sr: 0.7053 to 0.7068; 206Pb/ 204Pb: 19.29 to 19.15). Pb isotopes are decoupled from Sr and Nd isotopes, with the frontal volcanoes showing the higher Nd and Pb and lower Sr isotopic ratios. The isotopic and trace element ratios of the volcanic samples are best explained by modification of a MORB-type source (with Indian Ocean island basalt–type Pb isotopic characteristics) by a fluid and a partial melt of subducted continental material (SCM). The frontal volcano contains the highest proportion of the fluid component, with a small contribution of partial melt. The source of the rear-arc volcano is strongly influenced by a partial melt of SCM that had undergone a previous dehydration event, by which it lost most of its fluid-mobile elements such as Pb. The SCM partial melt was in equilibrium with both rutile and garnet, whereas mantle melting took place in the presence of residual mica. The relatively large across-arc increase in incompatible elements can be explained by a combination of increasing addition of SCM partial melt, changing mantle wedge fertility and smaller degrees of partial melting toward the rear of the arc. Comparison with a more westerly across-arc transect shows that the relatively low 143Nd/ 144Nd ratios of the frontal volcano, and the decoupling of Pb from Sr and Nd isotopes are unique to the Pantar Strait volcanoes. This is likely to reflect magma generation in a collisional environment, where the leading edge of the Australian continent, rather than subducted sediment, contributes to the magma source.

Structural evolution from Paleogene extension to Latest Miocene-Recent arc-continent collision offshore Taiwan: comparison with on land geology

Marine structures observed in seismic profiles across the NE South China Sea–Taiwan Strait–Luzon Arc region show that they are comparable to those observed onshore Taiwan. The Hanjiang Depression and Dongsha Uplift off the SE China coast extend to the Penghu Basin and Penghu Platform, respectively, in the Taiwan Strait, while the Tainan Basin in the south of Penghu Platform is connected to the Chaoshan Depression south of the Dongsha Uplift. The Hanjiang Depression and the Penghu Basin are composed of Paleogene syn-rift sequences on the Mesozoic basement and are unconformably overlain by Neogene post-rift strata. Similar successions are found underneath the coastal plain or have been thrust up as a fold-and-thrust belt bordering the Peikang Basement High onshore central Taiwan.East of the Manila Trench, off southwestern Taiwan, parts of the Tainan Basin are thrust into the submarine syn-collision accretionary wedge (the fold-and-thrust Kaoping Slope) in the initial arc-continent collision zone. They have also been exposed in the fold-and-thrust Western Foothills south of the Peikang Basement High in southern Taiwan during an advanced stage of arc-continent collision since the Late Pliocene. Further east, the pre-collision accretionary wedge (the Hengchun Ridge) shoals northward to the Hengchun Peninsula, with Late Miocene turbidites unconformably overlain by Plio–Pleistocene shallow-marine slope basin sequences. In the arc domain, east of the arc-prism boundary fault, the western forearc sequences in the North Luzon Trough are deformed. Two normal-faulted intra-arc basins occur on the eastern part of the Lanhsu Island in the Luzon Arc. The scenario of the deformed forearc sequences and the development of the two intra-arc basins in the initial arc-continent collision zone are analogous to the Lichi Mélange–Taiyuan Basin and the intra-arc basins in the southern Coastal Range, eastern Taiwan.Additionally, the lateral change in the marine structure from the passive Asian continental margin to the Manila subduction system is comparable to the temporal evolution from Paleogene normal faulting, through Neogene subduction, to Latest Miocene-Recent arc-continent collision observed onshore Taiwan.

Processes of forearc and accretionary complex formation during arc-continent collision in the southern Ural Mountains

The southern Ural Mountains of Russia contain a well-preserved, well-exposed Paleozoic accretionary wedge and forearc that can be readily compared to those in active arc-continent collision zones. The early convergent history in the southern Ural Mountains is marked by the generation of boninite-bearing arc tholeiites in the Magnitogorsk forearc, followed by arc tholeiite to calc-alkaline volcanism. With the entry of the East European craton continental crust in the subduction zone, volcanism waned and stopped, and high-pressure metamorphism of its leading edge took place. The arrival of the full thickness of the continental crust at the subduction zone is marked by increased sedimentation in the forearc basin and deposition of arc-derived volcaniclastic turbidites across the subducting slab. These, together with offscraped continental material, the exhumed high-pressure rocks, and a lherzolite massif, formed an accretionary wedge. A broad melange zone containing ultramafic fragments separates the forearc basement from the accretionary wedge, and marks the damage zone that developed along the backstop region. Shallow-water carbonates deposited unconformably on top of the mildly deformed arc record the end of the collision and the collapse of the arc.

Tectonic evolution of accretionary prism in the arc-continent collision terrane of Taiwan

The thick sedimentary and meta-sedimentary rocks west of the Eocene-Paleozoic metamorphic basement of Taiwan represent an accretionary prism developed between the Eurasian continent and the Philippine Sea plate. The accretionary prism consists of a subduction wedge in the east and a collision prism in the west. The deep-marine subduction wedge developed during the eastward subduction of the South China Sea oceanic crust since the Early Miocene. In the Central Range, a regional unconformity with mylonite structure occurred between the Miocene deep-marine slates-turbidites and the Paleozoic-Eocene metamorphic basement. The unconformity marks the tectonostratigraphic break between the overlying subduction wedge and the underlying underthrust Eurasian continent. The subduction wedge extends from the western Central Range southwards through the Hengchun Peninsula to the offshore Hengchun Ridge. Subduction of the South China Sea oceanic crust further led to the oblique arc-continent collision starting about 6.5 Ma in northern Taiwan. During the collision, the shallow-marine passive margin and foreland sequences were progressively incorporated to the collision prism by a series of west-vergent thrusts in the Hsüehshan Range and the Western Foothills. The Kaoping Slope west of the subduction wedge of the Hengchun Ridge represents the modern collision prism in the active arc-continent collision zone. The collision prism is juxtaposed against the subduction wedge to the east along the Lishan-Laonung-Hengchun fault, which extends offshore to the 30-km-wide fault zone between the Kaoping Slope and the Hengchun Ridge. Before the onset of the arc-continent collision, the Lishan-Laonung-Hengchun fault developed along the northern part of the proto-Manila trench and acted as the thrust front located to the west of the subduction wedge. At present, the thrust front has migrated southwestward to the west of the collision prism. The arc-continent collision propagated southwards and yielded the time-transgressive deformations from north to south to result a south tapering configuration of Taiwan. This paper is the first to recognize the consistent occurrence of the subduction wedge and collision prism onshore and offshore Taiwan. This allows reconstruction of the tectonic evolution of the accretionary prism during the subduction and collision tectonics of Taiwan.

Evolution of Pliocene to Pleistocene sedimentary environments in an arc-continent collision zone: evidence from the analyses of lithofacies and ichnofacies in the southwestern foothills of Taiwan

Lithofacies and ichnofacies analyses reveal an excellent example of gradually subsiding and rapid infilling and uplifting of the sedimentary basin in the southwestern foothills of Taiwan, along the strike of the active Plio-Pleistocene orogeny. Ten well-exposed sections, roughly evenly distributed along a southwestern transect, were studied to construct the three-dimensional distribution of lithofacies and ichnofacies. Basin evolution is discussed in terms of temporal and spatial facies variations. In early Pliocene time, the shallow marine facies of the northern part of the study area were deposited on the passive Eurasian continental shelf. The sedimentary basin was deeper in the south, with a lower slope and/or basin floor setting. The ongoing arc-continent collision caused flexural bending of the crust, inducing gradual subsidence of the basin during Pliocene and early Pleistocene time. Around 0.98 Ma, the onset of more rapid orogenic movements accelerated the basin subsidence, as indicated by rapid deepening of the facies northward along the basin axis, as well as by the widespread development of mass-gravity transportation deposits and channel cutting structures. However, the basin was soon infilled and uplifted as part of the orogen within 0.8 my. Basin subsidence is much greater than previously indicated.

Evolution of Pliocene to Pleistocene sedimentary environments in an arc-continent collision zone: Evidence from the analyses of lithofacies and ichnofacies in the southwestern foothills of Taiwan

Abstract Lithofacies and ichnofacies analyses reveal an excellent example of gradually subsiding and rapid infilling and uplifting of the sedimentary basin in the southwestern foothills of Taiwan, along the strike of the active Plio-Pleistocene orogeny. Ten well-exposed sections, roughly evenly distributed along a southwestern transect, were studied to construct the three-dimensional distribution of lithofacies and ichnofacies. Basin evolution is discussed in terms of temporal and spatial facies variations. In early Pliocene time, the shallow marine facies of the northern part of the study area were deposited on the passive Eurasian continental shelf. The sedimentary basin was deeper in the south, with a lower slope and/or basin floor setting. The ongoing arc-continent collision caused flexural bending of the crust, inducing gradual subsidence of the basin during Pliocene and early Pleistocene time. Around 0.98 Ma, the onset of more rapid orogenic movements accelerated the basin subsidence, as indicated by rapid deepening of the facies northward along the basin axis, as well as by the widespread development of mass-gravity transportation deposits and channel cutting structures. However, the basin was soon infilled and uplifted as part of the orogen within 0.8 my. Basin subsidence is much greater than previously indicated.

U-series, SrNdPb isotope and trace-element systematics across an active island arc-continent collision zone: Implications for element transfer at the slab-wedge interface

We present U-series, SrNdz.sbnd;Pb isotope and trace-element results of a regional study of geochemical systematics across an island arc-continent collision zone in the East Sunda Arc of Indonesia. Samples from four active volcanoes exhibit a striking compositional range from low-K tholeiitic to ultrapotassic, but all are characterised by high 87 Sr 86 Sr (0.7053–0.7067), radiogenic lead isotope ratios ( 206 Pb 204 Pb = 18.99–19.15), low ( 230 Th) ( 232 Thz) (0.66–0.85), and low 143 Nd 144 Nd (0.51255–0.51272), except for high 143 Nd 144 Nd (>0.51286) at the volcanic front. Low ( 230 Th) ( 232 Th) ratios are also found in terrigenous sediments in front of the arc, which, in combination with Srz.sbnd;Ndz.sbnd;Pb isotopic constraints, indicates that subducted continental material contributes to magma sources in this arc sector. The volcanoes close to the trench show a large excess of 238U over . 230Th (up to 80%) and of 226Ra over 230Th (up to 800%). In addition, they are enriched in elements thought to be mobile in hydrous fluids during slab-wedge transfer, such as Ba, Pb, and Sr. In contrast, Uz.sbnd;Thz.sbnd;Ra systematics are close to equilibrium in the volcanoes behind the front. Abundance patterns of incompatible trace elements in these rocks are similar to those of the terrigenous sediments, so that, in comparison with the arc-front lavas, they possess low Ba/La, Ba/Th, La/Th, Pb/Ce, and Zr/Nb. Higher concentration levels and less interelement fractionation form conspicuous differences with the front volcanics. Our combined isotopic and trace element data are consistent with three-component mixing whereby a slab-derived hydrous fluid and a siliceous melt are both added to the sub-arc mantle source. The hydrous fluid largely controls the input in the shallow part of the subduction zone, whereas the siliceous melt dominates the flux at deeper levels. Sedimentary material is considered to be the primary source of both. The large U-Th-Ra disequilibria at the front suggests that element transfer is a currently active process associated with present-day subduction of continental material.

Origin of the Sumba forearc basement

An interpretation of acoustic basement structures in the eastern Sunda/western Banda forearc region suggests structural continuity between Sumba and Northern Timor. The area between Sumba and Timor is delineated as a zone of continuous structures, without major strike-slip offsets. These structures trend east-west in the west but gradually change towards the east into NE-SW trending structures. A major NE-SW trending discontinuity west of Sumba between 117° 30′ and 118° 30′E marks the transition between an intraoceanic volcanic arc system in the west and a volcanic arc-continent collision zone in the east. The extent of the Sumba basement suggests either a common (Late Jurassic) rift/drift history for both Sumba and Northern Timor or an (Early Miocene) magmatic welding of two continental fragments of different origin, resulting in structural continuity between two microplates. The first explanation implies that Sumba originated from the north-west Australian continental margin south of Tethys, from which it drifted away in the Late Jurassic. The second hypothesis, which is favoured, proposes a northern provenance of Sumba. The island would then originate from the south-east Asian continent, north of Tethys, from which it was detached in the Eocene.

Global Positioning System Measurements of Crustal Deformation in the Taiwan Arc-Continent Collision Zone

Global Positioning System Measurements of Crustal Deformation in the Taiwan Arc-Continent Collision Zone

Gamilaroi Terrane: A Devonian rifted intra-oceanic island-arc assemblage, NSW, Australia.

Abstract The Gamilaroi terrane comprises part of the New England orogen of eastern Australia and contains Devonian strata preserving evidence of their development in a variety of arc-related environments. Radiolarian studies provide new and more precise constraints on stratigraphy and together with geochemistry allow development of a new model for evolution of the terrane. A diverse suite of volcaniclastic sedimentary rocks intercalated with regionally extensive extrusive and high level intrusive meta-felsic igneous rocks of low-K and calc-alkaline volcanic affinity is present. These rocks constitute the oldest in-situ volcanic rocks in the Gamilaroi terrane and are similar to those found in modern intra-oceanic volcanic island-arc settings. Basaltic intrusions and lavas occur at higher stratigraphic levels. Major and trace element compositions, together with relatively flat chondrite-normalized REE distributions, resemble those from rocks formed in modern intra-oceanic island-arc rift zones. The close spatial association of these rock types together with an absence of continent-derived detritus is consistent with interpretation of development of the Gamilaroi terrane in an intra-oceanic island arc which experienced local rifting. This interpretation is a departure from previous models for the New England orogen, which interpret the Gamilaroi terrane as a forearc basin that developed above a long-lived (Cambrian to Permian), west-dipping subduction zone fringing the continental margin of Gondwana. By analogy with modern arc-continent collision zones in which crust between colliding continents and island arcs is preferentially subducted beneath the approaching arc, we suggest that oceanic crust intervening between Gamilaroi terrane and Gondwana was subducted eastwards under the western margin of the Gamilaroi terrane arc. During a latest Devonian collision event the Gamilaroi terrane was obducted onto the Gondwana margin. This resulted in subduction flip and subsequent development of an east-facing continental margin arc system on top of the Gamilaroi terrane.

Sm?Nd dating of multiple garnet growth events in an arc-continent collision zone, northwestern U.S. Cordillera

Integrated petrologic and Sm−Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P−T studies indicate a single stage of metamorphism with final equilibration at ∼600°C and 8–9 kbar. The Sm−Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A40Ar/39Ar age for this hornblende indicates cooling through ∼525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm−Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent “ages” of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (∼128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at ∼128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.

Marine geology in the arc-continent collision zone off southeastern Taiwan: Implications for late neogene evolution of the coastal range

Marine geology in the region between Taiwan and Luzon is characterized by a convergent tectonics in the south and an oblique arc-continent collision in the north. In the collision zone six geologic provinces are recognized. They are the Manila Trench, the accretionary prism of the Hengchun Ridge, the sutural basin of the Southern Longitudinal Trough, the subduction mélange of the Huatung Ridge, the forearc basins of the Taitung and the North Luzon Troughs, and the volcanic arc of the North Luzon Ridge. The oceanic crust of the South China Sea is subducting eastward beneath the Philippine Sea plate along the Manila Trench south of 20°N. Further to the north, the Trench has changed progressively its axis toward the west from the subduction mélanges off southeastern Taiwan at 3 Ma to the Hengchun peninsula of southern Taiwan at 2-1 Ma, and then to the present-day location off southwestern Taiwan. Because of the continuing collision and the westward shifting of the Manila Trench, sediments deposited in the region are passively incorporated into the accretionary prism and deformed to the Hengchun Ridge. After the collision the Southern Longitudinal Trough, consisting of a 2 km thick of orogenic sediments, represents a sutural basin superposed on the plate boundary between the accretionary prism of the Hengchun Ridge (Asian continent) and the subduction mélange of the Huatung Ridge (Philippine Sea plate). The Huatung Ridge is a bathymetric high connected to the southern Coastal Range and is characteristic of low, negative free-air gravity and magnetic anomalies. The Ridge contains deeply weathered sedimentary blocks with abraded deep-water late Pliocene foraminifera and identical clay mineral compositions as those in the onshore Lichi Mélange. This implies that at least a portion of the Huatung Ridge is composed of mélange materials and represents a former subduction zone extending from the Lichi Mélange of the southern Coastal Range. The North Luzon Trough and the Taitung Trough are the constructed foreac basins with 2 km of orogenic sediments unconformably overlying the volcanic islands and the subduction melange. The Taitung Trough is southward connected to the North Luzon Trough through a bathymetric passage, which was formed by narrowing the North Luzon Trough as a consequence of clockwise rotation of the Lutao volcanic island during the late Pleistocene. A detailed study of the four Neogene tectonostratigraphic units in the Coastal Range shows that the overall history of the formation of the Range can be related to the present-day geologic provinces and the on-going collision processes in the region off southeastern Taiwan. The Range had evolved from a southward accretion of the Shuilien-Loho remnant forearc basin and the Chimei volcanic pair at 2 Ma, followed by the Chengkung-Taiyuan remnant forearc basin and the Chengkuangao volcanic island pair at 1 Ma, and the Taitung Trough and the Lutao volcanic island pair presently in process. By analogy, the northern part of the North Luzon Trough and the Lanhsu volcanic island will be accreted to the Coastal Range in the next million years. The paired forearc basin-volcanic island accretion model proposed in this study may thus have implications for mountain ranges that evolved from similar tectonics of oblique arc-continent collision.

Comment and Reply on "Postcollision extension in arc-continent collision zones, eastern Indonesia"

Comment and Reply on "Postcollision extension in arc-continent collision zones, eastern Indonesia"

Postcollision extension in arc-continent collision zones, eastern Indonesia

Postcollisional extension superimposed on arc-continent collision complexes is a common feature in the orogenic belts of eastern Indonesia. In the southern Banda arc, the arc-continent collision event is essentially a Pliocene and younger feature, but already an element of extension is being superimposed on the compressional structures of the collision complex. It is thus likely that the extension commences within a very short period (<5 m.y.) after the initiation of collision-related compressional deformation. A similar history of fold-belt development immediately followed by extension can be inferred for the Lengguru fold belt and the adjacent Wandamen-Wondiwoi terranes of Irian Jaya and for the Gulf of Bone in Sulawesi. It is suggested that the extension results from decoupling of the subducting oceanic lithosphere from the unsubductable continental lithosphere and that the superimposition of extension is a virtually unavoidable consequence of the arc-continent collision.

Interpretation of 40Ar/ 39Ar and K/Ar dating evidence from the Aileu Formation, East Timor, Indonesia

The Aileu Formation, East Timor, is a zoned metamorphic complex exposed in a modern arc—continent collision zone. During collision, extensive retrogression occurred. K/Ar and 40Ar/ 39Ar age measurements on hornblendes are interpreted as indicating that the retrograde metamorphism, and hence collision, occurred at ∼ 8 Ma ago, Late Miocene. Mica K/Ar ages from two samples reflect cooling of the terrain to < 300°C by 5.5 Ma ago. Relict Mg-rich hornblendes lost most of their pre-existing radiogenic Ar, and Fe-rich hornblendes were completely reset, during the retrogressive phase at middle amphibolite facies conditions. At lower grades of retrogressive metamorphism, the Mg-rich hornblendes retained appreciable radiogenic Ar, whereas Fe-rich hornblendes lost most of their accumulated radiogenic Ar even at upper greenschist facies. Thus, under these conditions, chemical composition is an important factor controlling Ar loss from hornblende, although recrystallization also plays a significant role.

Deformation and metamorphism of the Aileu Formation, north coast, East Timor and its tectonic significance

The large block of metamorphic rocks along the north coast of East Timor is of special interest as it occurs at the boundary between continental and oceanic crust in an island arc-continent collision zone. A detailed study of the structure and metamorphic history of 400 km 2 of this formation showed it has a complex history of penetrative deformation but the structure is coherent. Pelites, psammites and limestones interlayered with dolerites and amphibolites have been metamorphosed in a medium pressure environment. They now form a metamorphic province zoned from low greenschist facies in the southwest to upper amphibolite facies in the east. The earliest recognised deformation phase predated the metamorphism and produced a widespread layer—parallel schistosity but no recognisable folds. The second deformation phase post-dated the metamorphic maximum and micropetrological evidence indicates a gradual cooling during this event. This deformation produced tight folds with an axial plane schistosity and transposed the earlier structures. The progressively weaker third and fourth phases developed crenulation cleavages and related folds, under greenschist facies conditions. Open, fifth phase, macroscopic folds were probably synchronous with strike slip faulting parallel to the north coast. Later dip slip faulting juxtaposed the Aileu Formation with Permian and Mesozoic sediments of very low metamorphic grade. Reconnaissance K/Ar radiometric dating using hornblende and biotite showed the prograde metamorphic maximum occurred before 11 Ma ago and implies that the second, and strongest, deformation phase occurred in the late Miocene. This young age establishes the relationship of the deformation events to the collision between Australia and the Inner Banda Arc. The proposed models for the structure of Timor must be modified to fit the deformation history of the Aileu Formation. If Timor is essentially autochthonous, the Aileu Formation was probably deposited in a Palaeozoic graben and the metamorphic maximum may have occurred in the Jurassic. The overthrusting models must be modified in the light of the close correlation in time between penetrative deformation and emplacement of the proposed thrust sheets. The analogy proposed between Timor and ‘normal’ convergent margins is not supported but it may be possible to draw analogies with the Molucca Sea.