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The stratigraphic record of volcanic arcs provides insights into their eruptive history, the formation of associated basins, and the character of the deep crust beneath them. Indian Ocean lithosphere was subducted continuously beneath Java from ca. 45 Ma, resulting in formation of a volcanic arc, although volcanic activity was not continuous for all of this period. The lower Cenozoic stratigraphic record on land in East Java provides an excellent opportunity to examine the complete eruptive history of a young, well-preserved volcanic arc from initiation to termination. The Southern Mountains Arc in Java was active from the middle Eocene (ca. 45 Ma) to the early Miocene (ca. 20 Ma), and its activity included signifi cant acidic volcanism that was overlooked in previous studies of the area. In particular, quartz sandstones, previously considered to be terrigenous clastic sedimentary rocks derived from continental crust, are now known to be of volcanic origin. These deposits form part of the fi ll of the Kendeng Basin, a deep fl exural basin that formed in the backarc area, north of the arc. Dating of zircons in the arc rocks indicates that the acidic character of the volcanism can be related to contamination of magmas by a fragment of Archean to Cambrian continental crust that lay beneath the arc. Activity in the Southern Mountains Arc ended in the early Miocene (ca. 20 Ma) with a phase of intense eruptions, including the Semilir event, which distributed ash over a wide area. Following the cessation of the early Cenozoic arc volcanism, there followed a period of volcanic quiescence. Subsequently arc volcanism resumed in the late Miocene (ca. 12-10 Ma) in the modern Sunda Arc, the axis of which lies 50 km north of the older arc.

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.

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