Palaeogeographic maps for intervals between the Cretaceous and Late Miocene illustrate the complex evolution of West Java. Basement is of Mesozoic age and in West and Central Java there are ophiolitic and arc rocks accreted to the margin of Sundaland in the Late Cretaceous. The oldest Cenozoic rocks in West Java are Middle Eocene formations in the Ciletuh Bay area that formed in quite different settings. There are volcanogenic turbidites and breccias containing abundant basaltic material that we suggest are deep water deposits, associated with the onset of subduction, formed close to a new arc or in its forearc. Nearby are quartz-rich sandstones deposited predominantly in a shallow marine shelf edge environment interpreted to be derived from basement highs. We assign these rocks to different formations and their present juxtaposition is suggested to be due to thrusting. We interpret there to have been a large southerly prograding delta system in SW Java during the Late Eocene. There is a considerable thickness of quartzrich sandstones, forming an overall shallowing-up sequence, sourced from the north and probably derived from Sundaland. The Oligocene of West Java includes terrestrial quartz-rich sandstones, reefal and foraminiferal limestones and volcanogenic sediments deposited in fluvial to deeper water marine environments. The Early Miocene saw an important phase of explosive arc volcanism in south Java. By the Middle Miocene volcanism had diminished or ceased, allowing carbonates to be deposited on the arc rocks. In the Late Miocene volcanism resumed further to the north resulting in a new phase of volcanogenic turbidite deposition. It is not certain when subduction began beneath West Java and where the arc was situated. Except at Ciletuh the volcanic component of Paleogene * SE Asia Research Group, Royal Holloway University of London sequences is relatively minor. This has suggested that subduction-related volcanism did not commence until the Late Oligocene. However, we suggest that subduction-related volcanism began in the Eocene, but the arc did not become emergent until the end of the Oligocene. Loading by the volcanic arc formed a broadly E-W trending flexural basin to the north of the arc which filled with volcanogenic material from the south and continental clastic debris from the north. The distance between the Paleogene quartz-rich shelf sequences and the volcanic arc has been reduced by Neogene thrusting.

Many rifts develop through multiphase extension; it can be difficult, however, to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier rifting is often deeply buried. Using 2-D and 3-D seismic reflection and borehole data from the northern North Sea, we examine the style, magnitude, and timing of reactivation of a preexisting, Permian-Triassic (Rift Phase 1) fault array during a subsequent period of Middle Jurassic to Early Cretaceous (Rift Phase 2) extension. We show that Rift Phase 2 led to the formation of new N-S striking faults close to the North Viking Graben but did not initially reactivate preexisting Rift Phase 1 structures on the Horda Platform. We suggest that at the beginning of Rift Phase 2, strain was focused in a zone of thermally weakened lithosphere associated with the Middle Jurassic North Sea thermal dome, rather than reactivating extant faults. Diachronous reactivation of the Permian-Triassic fault network eventually occurred, with those faults located closer to the Middle Jurassic to Early Cretaceous rift axis reactivating earlier than those toward the eastern margin. This diachroneity may have been related to flexural down bending as strain became focused within the North Viking Graben, and/or the shifting of the locus of rifting from the North Sea to the proto-North Atlantic. Our study shows that the geometry and evolution of multiphase rifts is not only controlled by the orientation of the underlying fault network but also by the thermal and rheological evolution of the lithosphere and variations in the regional stress field.

Detrital zircon U-Pb geochronology can make an extremely valuable contribution to provenance studies and paleogeographic reconstructions, but the technique cannot distinguish grains with similar ages derived from different sources. Hafnium isotope analysis of zircon crystals combined with U-Pb dating can help make such distinctions. Five Paleogene formations in West Java have U-Pb age populations of 80-50 Ma (Late Cretaceous-Paleogene), 145-74 Ma (Cretaceous), 298-202 Ma (Permian-Triassic), 653-480 Ma (mid-Neoproterozoic-latestCambrian), and 1290-723 Ma (late Mesoproterozoic-early Neoproterozoic). Hf-isotopes have been analyzed for 311 zircons from these formations. Differences in zircon U-Pb age and Hf-isotope populations reflect changing sources with time. Late Cretaceous and Paleogene zircons are interpreted as having been derived from two temporally discrete volcanic arcs in Java and West Sulawesi, respectively. The Java arc was active before microcontinent collision, and the W Sulawesi arc developed later, on newly accreted crust at the SE Sundaland margin. The collision age is estimated to be ca. 80 Ma. U-Pb age and Hf-176/Hf-177(i) characteristics allow a distinction to be made between Cretaceous granitic and volcanic arc sources. Zircons that are older than ca. 80 Ma have a continental Sundaland provenance. Mid-Cretaceous zircons in all upper Eocene and lower Oligocene formations were derived from granites of the Schwaner Mountains of SW Borneo. Permian-Triassic zircons were derived predominantly from granites in the SE Asian Tin Belt. Hf-176/Hf-177(i) ratios permit distinction between Tin Belt granites in the Main Range and Eastern Provinces, and indicate that only the lower Oligocene Cijengkol Formation contains significant input from the Main Range Province, suggesting a partial change in drainage pattern. Older zircon ages are more difficult to interpret but probably record contributions from allochthonous basement and sedimentary rocks that were deposited prior to rifting of continental blocks from Gondwana in the early Mesozoic.

Palaeogeographic maps for intervals between the Cretaceous and Late Miocene illustrate the complex evolution of West Java. Basement is of Mesozoic age and in West and Central Java there are ophiolitic and arc rocks accreted to the margin of Sundaland in the Late Cretaceous. The oldest Cenozoic rocks in West Java are Middle Eocene formations in the Ciletuh Bay area that formed in quite different settings. There are volcanogenic turbidites and breccias containing abundant basaltic material that we suggest are deep water deposits, associated with the onset of subduction, formed close to a new arc or in its forearc. Nearby are quartz-rich sandstones deposited predominantly in a shallow marine shelf edge environment interpreted to be derived from basement highs. We assign these rocks to different formations and their present juxtaposition is suggested to be due to thrusting. We interpret there to have been a large southerly prograding delta system in SW Java during the Late Eocene. There is a considerable thickness of quartz- rich sandstones, forming an overall shallowing-up sequence, sourced from the north and probably derived from Sundaland. The Oligocene of West Java includes terrestrial quartz-rich sandstones, reefal and foraminiferal limestones and volcanogenic sediments deposited in fluvial to deeper water marine environments. The Early Miocene saw an important phase of explosive arc volcanism in south Java. By the Middle Miocene volcanism had diminished or ceased, allowing carbonates to be deposited on the arc rocks. In the Late Miocene volcanism resumed further to the north resulting in a new phase of volcanogenic turbidite deposition.

Continental SE Asia is the site of an extensive Cretaceous-Paleocene regional unconformity that extends from Indochina to Java, covering an area of c. 5 600 000 km(2). The unconformity has previously been related to microcontinental collision at the Java margin that halted subduction of Tethyan oceanic lithosphere in the Late Cretaceous. However, given the disparity in size between the accreted continental fragments and area of the unconformity, together with lack of evidence for requisite crustal shortening and thickening, the unconformity is unlikely to have resulted from collisional tectonics alone. Instead, mapping of the spatial extent of the mid-Late Cretaceous subduction zone and the Cretaceous-Paleocene unconformity suggests that the unconformity could be a consequence of subduction-driven mantle processes. Cessation of subduction, descent of a northward dipping slab into the mantle, and consequent uplift and denudation of a sediment-filled Late Jurassic and Early Cretaceous dynamic topographic low help explain the extent and timing of the unconformity. Sediments started to accumulate above the unconformity from the Middle Eocene when subduction recommenced beneath Sundaland.

Paleogene sedimentary rocks in southwest Java record detrital contributions from different sources with different ages. The Ciletuh, Ciemas, Bayah, Cijengkol and Cikalong Formations have depositional ages of Middle Eocene to Early Oligocene. Samples from the Ciletuh Formation are deep marine volcanogenic sandstones whereas samples from the other formations are terrestrial and marginal marine quartz-rich sandstones. All contain abundant zircons from which U-Pb ages have been obtained by Laser Ablation ICPMS dating. Zircons yield a wide range of ages that span the Phanerozoic and Proterozoic, with rare Archean grains. Common age clusters include Late Cretaceous-Paleogene (40–80 Ma), Cretaceous (70– 130 Ma), Permian-Triassic (190–270 Ma) and Late Neoproterozoic-Cambrian (480–590 Ma), although not all clusters are present in all samples. The zircon age spectra, combined with field observations, palaeocurrent measurements and light mineral analyses, are used to identify possible sediment sources. The zircon ages are interpreted to indicate a Cretaceous and Early Paleogene volcanic source for the Ciletuh Formation and Sundaland sources for all other formations. Cretaceous zircons are present in all younger (Upper Eocene and Oligocene) quartzose sedimentary rocks. The Middle Eocene volcanogenic Ciletuh Formation contains abundant Cretaceous and Paleogene zircons but few zircons of greater age. In contrast, Cretaceous zircons are almost entirely absent from the Middle Eocene Ciemas Formation. Permian–Triassic zircons in the Ciemas Formation and all other quartz-rich sandstones are interpreted to be derived from granites of this age in the Malay Peninsula and the Indonesian Tin Islands. These zircons are interpreted to be derived from Cretaceous granites that are distributed across the Sunda Shelf, and in

Abstract: Palaeogene sedimentary rocks exposed in West Java were derived from local volcanic sources and central Sundaland, the continental core of SE Asia. Detrital zircons from seven sandstone samples contain U–Pb age populations with ages of 50–80 Ma, 74–145 Ma, 202–298 Ma, 480–653 Ma and 723–1290 Ma. Late Cretaceous and Palaeogene zircons in Middle Eocene forearc sandstones are interpreted as derived from two spatially and temporally discrete volcanic arcs located in Java and Sulawesi respectively. In contrast, all other populations have a Sundaland provenance. Most Permian–Triassic zircons were derived from granites of this age in the SE Asian Tin Belt. Mid-Cretaceous zircons in all Upper Eocene and Lower Oligocene formations were derived from the Schwaner Mountains of SW Borneo. The differences in zircon populations reflect changing Sundaland sources with time. In the Middle Eocene, sediment was derived mainly from the Tin Belt. From the Late Eocene onwards a Borneo source became more important. Older zircon ages are from SE Asia basement that once formed part of Gondwana. Zircons also record the timing of microcontinental collision at the Java margin (c. 80 Ma) that halted Cretaceous subduction and probably resulted in the elevation of large parts of continental SE Asia. Supplementary material: Methods, sample locality list, tables for U–Pb zircon age measurements, point count data and heavy mineral assemblages, sandstone petrographic descriptions and palaeocurrent data are available at www.geolsoc.org.uk/SUP18489.

The Malay Peninsula lies on two continental blocks, Sibumasu and East Malaya, which are intruded by granitoids in two provinces: the Main Range and Eastern. Previous models propose that Permian–Triassic granitoids are subduction-related and syn-to post-collisional. We present 752 U–Pb analyses that were carried out on zircons from river sands in the Malay Peninsula; of these, 243 grains were selected for Hf-isotope analyses. Our data suggest a more complex Sibumasu–East Malaya collision history. 176Hf/177Hfi ratios reveal that Permian–Triassic zircons were sourced from three magmatic suites: (a) Permian crustally-derived granitoids, (b) Early-Middle Triassic granitoids with mixed mantle–crust sources, and (c) Late Triassic crustally-derived granitoids. This suggests three Permian–Triassic episodes of magmatism in the Malay Peninsula, two of which occurred in the Eastern Province. Although the exact timing of the Sibumasu–East Malaya collision remains unresolved, current data suggest that it occurred before the Late Triassic, probably in Late Permian–Early Triassic. Our data also indicate that Sibumasu and East Malaya basements are chronologically heterogeneous, but predominantly of Proterozoic age. Some basement may be Neoarchaean but there is no evidence for basement older than 2.8Ga. Finally, we show that Hf-isotope signatures of Triassic zircons can be used as provenance indicators.

(1) SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK, (2) Fault Dynamics Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK, (3) Vening Meinesz Research School of Geodynamics, Faculty of Earth Sciences, Universiteit Utrecht, Budapestlaan 4, 3584CD Utrecht, The Netherlands

ABSTRACT Java is part of a volcanic island arc situated in the Indonesian archipelago at the southern margin of the Eurasian Plate. Sundaland continental crust, accreted to Eurasia by the Early Mesozoic, now underlies the shallow seas to the north of Java where there has been considerable petroleum exploration. Java has an apparently simple structure in which the east–west physiographic zones identified by van Bemmelen broadly correspond to structural zones. In the north there is the margin of the Sunda Shelf and, in southern Java, there are Cenozoic volcanic arc rocks produced by spatially and temporally discrete episodes of subduction-related volcanism. Between the Sunda Shelf and the volcanic rocks are Cenozoic depocentres of different ages containing sedimentary and volcanic material derived from north and south. This simplicity is complicated by structures inherited from the oldest period of subduction identified beneath Java, in the Cretaceous, by extension related to development of the volcanic arcs, by extension related to development of the Makassar Straits, by late Cenozoic contraction, and by cross-arc extensional faults which are active today. Based on field observations in different parts of Java, we suggest that major thrusting in southern Java has been overlooked. The thrusting has displaced some of the Early Cenozoic volcanic arc rocks northwards by 50 km or more. We suggest Java can be separated into three distinct structural sectors that broadly correspond to the regions of West, Central and East Java. Central Java displays the deepest structural levels of a series of north-directed thrusts, and Cretaceous basement is exposed; the overthrust volcanic arc has been largely removed by erosion. In West and East Java the overthrust volcanic arc is still preserved. In West Java the arc is now thrust onto the shelf sequences that formed on the Sundaland continental margin. In East Java the volcanic arc is thrust onto a thick volcanic/sedimentary sequence formed north of the arc in a flexural basin due largely to volcanic arc loading. All the components required for a petroleum system are present. This hypothesis is yet to be tested by seismic studies and drilling, but, if correct, there may be unexplored petroleum systems in south Java that are worth investigating.