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.

No 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.

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.