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Abstract. The Woodlark Basin in the western Pacific forms a continuous system of active continental rifting evolving to well-developed seafloor spreading. Thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin. Magnetic isochrons indicate a single Euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins. In agreement with rigid plate tectonic models, continental rifting initiated geologically synchronously (at --6 Ma) along the length of the protomargins within a deforming plate boundary zone. Strain localization and seafloor spreading, however, developed in a time transgressive fashion from east to west within this zone of deformation. Spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the Solomon and Coral Seas. The transition to spreading occurred after a rather uniform degree of continental extension: 200+_40 km. Both early and late stage rifting involved highand low-angle normal faults. We identify distinct styles in the transition from rifting to spreading which we refer to as nucleation, propagation, and stalling. These breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures. Continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating. The initial spreading segments achieved much of their length at nucleation (within rift basins separated along strike by accommodation zones), with subsequent lengthening by spreading propagation into rifting continental crust. This early propagation, and the subsequent development of transform faults between initially nontransform spreading segment offsets, produced rift and spreading segmentation boundaries that are not simply correlated. The spreading centers nucleated approximately orthogonal in strike to the opening direction but, as the protomargins were oblique to this direction, nucleation jumps occurred in order to maintain the new spreading centers within the protomargins. Thus stepwise spreading nucleation in order to remain within a rheologically weak zone, rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis, is the dominant form of the rifting-to-spreading transition in the Woodlark Basin.

This paper focuses on the processes of arc rifting in the context of the volcanic, structural, and sedimentologic evolution of the Izu-Bonin-Mariana arc-trench system. Middle and late Eocene supra-subduction zone magmatism formed a vast terrane of boninites and island arc tholeiites (>300 × 3000 km) that is unlike active arc systems but is similar to many ophiolites. A modern-style volcanic arc developed by the Oligocene, along which intense tholeiitic and calc-alkaline volcanism continued until 29 (Mariana) and 27 Ma (Izu-Bonin). The Eocene-Oligocene arc massif was stretched during protracted Oligocene rifting, creating sags and half graben in the forearc and backarc. Minima in arc volcanism Many arc segments go through a cycle of (1) frequent volcanism before and during rifting; (2) reduced and/or less disseminated volcanism during latest rifting and early backarc spreading, as new frontal arc volcanoes are being constructed and growing to sea level; and (3) increasingly vigorous volcanism during middle and late stage backarc spreading, and until the next rift cycle begins. Even within periods of intense volcanism, 100-km-long arc segments may be quiescent for periods of up to 400 k.y. The frontal arc volcanoes, because of their thicker crust and higher heat flow, create a linear zone of lithospheric weakness that controls the location of arc rifting. Differences in plate boundary forces at the ends, more than in the middle, of volcanic arcs may significantly influence their proclivity to rift.

[1] Abstract: We propose that across-arc differences in the geochemistry of Izu-Bonin arc magmas are controlled by the addition of fertile-slab fluids to depleted mantle at the volcanic front, and residual-slab fluids to fertile mantle in the back arc without slab melting or contemporaneous back arc spreading. The arc consists of a volcanic front, an extensional zone, and seamount chains (the Western Seamounts) that trend into the Shikoku Basin. Each province produces a distinct suite of arc-like volcanic rocks that have relative Nb depletions and high ratios of fluid-mobile elements to high field strength elements. The volcanic front has the lowest concentrations of incompatible elements and the strongest relative enrichments of fluid-mobile elements (high U/Nb, Ba/Nb, Pb/Zr, Th/Nb, 206 Pb/ 204 Pb, e Nd , and 87 Sr/ 86 Sr). A fluid derived from both sediment and altered oceanic crust explains most of the slabrelated characteristics of the volcanic front. The Western Seamounts and some of the extensional zone rocks have lower e Nd , 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, Ba/Th, and U/Th; moderate Ba/Nb and U/Nb; and similar or higher Th/Nb and Th/Nd. Although the lower e Nd and higher Th/Nd tempt a sediment melt explanation, a lack of correlation between the strongest sediment proxies, such as e Nd , Th/Nb, and Ce/ Ce*, precludes sediment melts. The subduction component for the Western Seamounts is probably a fluid dehydrated from a residual slab that was depleted in fluid-mobile elements beneath (as well as trenchward of) the volcanic front. This depleted fluid is added to elementally and isotopically more enriched mantle beneath the Western Seamounts.

The Corinth Rift, central Greece, enables analysis of early rift development as it is young (<5 Ma) and highly active and its full history is recorded at high resolution by sedimentary systems. A complete compilation of marine geophysical data, complemented by onshore data, is used to develop a high-resolution chronostratigraphy and detailed fault history for the offshore Corinth Rift, integrating interpretations and reconciling previous discrepancies. Rift migration and localization of deformation have been significant within the rift since inception. Over the last circa 2 Myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift, but this transition did not occur synchronously along strike. Isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620-340 ka, characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at~30 m/kyr. Since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin, demonstrated by lateral growth of discrete depocenters at~40 m/kyr. A single central depocenter formed by circa 130 ka, indicating full fault linkage. These results indicate that rift localization is progressive (not instantaneous) and can be synchronous once a rift border fault system is established. This study illustrates that development processes within young rifts occur at 100 kyr timescales, including rapid changes in rift symmetry and growth and linkage of major rift faults.

The Mariana, east Scotia, Lau, and Manus back-arc basins (BABs) have spreading rates that vary from slow ( 6 50 mm/yr) to fast ( s 100 mm/yr) and extension axes located from 10 to 400 km behind their island arcs. Axial lava compositions from these BABs indicate melting of mid-ocean ridge basalt (MORB)-like sources in proportion to the amount added of previously depleted, water-rich, arc-like components. The arc-like end-members are characterized by low Na, Ti and Fe, and by high H 2 O and Ba/La; the MORB-like end-members have the opposite traits. Comparisons between basins show that the least hydrous compositions follow global MORB systematics and an inverse correlation between Na8 and Fe8. This is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures (Lau/Manus hotter than Mariana/Scotia). This interpretation accords with numerical model predictions that faster subduction-induced advection will maintain a hotter mantle wedge. The primary compositional trends within each BAB (a positive correlation between Fe8, Na8 and Ti8, and their inverse correlation with H 2 O(8) and Ba/La) are controlled by variations in water content, melt extraction, and enrichments imposed by slab and mantle wedge processes. Systematic axial depth (as a proxy for crustal production) variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate. Hydrous fluxing enhances decompression melting, allowing depleted mantle sources just behind the island arc to melt extensively, producing shallow spreading axes. Flow of enriched mantle components around the ends of slabs may augment this process in transform-bounded back-arcs such as the east Scotia Basin. The re-circulation (by mantle wedge corner flow) to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the East Lau Spreading Center, Manus Southern Rifts, and Mariana Trough and the very depleted lavas of east Scotia segments E8/E9. The crust becomes mid-ocean ridge (MOR)-like where the spreading axes, further away from the island arc and subducted slab, entrain dominantly fertile mantle. ß

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