1. The Early Izu-Bonin-Mariana Arc Mark Reagan University of Iowa Osamu Ishizuka Geological Survey of Japan/AIST Barry Hanan, Rosemary Hickey-Vargas, Matthew Heizler, Sherman Bloomer, Patricia Fryer, Jim Hawkins, Teruaki Ishii, Katie Kelley, Jun-Ichi Kimura, Yasuhiko Ohara, Robert Stern, Melinda Woods

2. West Philippine Basin Parece Vela Basin Ogasawara Escarp. ‘04 Shinkai dives Saipan Rota Guam ‘06 Shinkai dives Chichijima Hahajima Kyushu-Palau Ridge Philippine Plate Pacific Plate Izu arc Amami Plateau ca. 116 Ma DSDP 458,459 Mariana Trough West Mariana Ridge Mariana arc Volcano, Bonin arcs ODP 782,786 Shinkai dives Shikoku Basin DSDP 447,448

3. Protoarc Modern arc Second arc Timeline for IBM forearc sites Age in Ma Back-arc spreading West Philippine Basin Parece Vela Basin Mariana Trough First arc Arc minima? Arc volcanism 458, Chi. Saipan, Chi. Ogas, Hahaj Guam Rota 448 Guam (Best 40 Ar/ 39 Ar and K/Ar age dates) Sources: Meijer et al. (1983); Taylor (1992); Lee et al. (1995); Gill et al. (1994); Cosca et al. (1998); Pearce et al. (1999); Deschamps and Lallemand (2002); Straub et al. (2003); Ishizuka et al. (2006); This study S.Mar. forearc Pre-arc

4. Saipan Rota Guam 973 974 975 976 977 On-land and dive locations - Marianas Shinkai 6500 dives 8/27/06-9/5/06

5. 975 974 976 Guam ? ? 1st and 2nd arc: v,i Transitional boninites: v,i MORB-like basalts: v,i Boninites: v Adakitic andesites: v,i Topography of Mariana forearc (from GeoMapApp) 5:1 vertical exaggeration Peridotites, gabbros Trench southeast

6. Representative “Protoarc” compositions

7. site 458 glass NMORB Mariana trench slope Bonin trench slope Debari et al. (1999) PM Normalized Concentrations LREEHREEMREE mostly wedge?mostly slab Forearc basalts - FAB (Ti/V =15-21)

8. LAICPMS incompatible trace element concentrations for consanguineous (?) DSDP site 458 glasses PM Normalized Concentrations BoniniteTholeiite NMORB

9. site 458 (glass) Mariana trench slope (whole rock) Bonin Islands Chichijima (glass) PM Normalized Concentrations 48-49 Ma boninites Melting: harzburgite, shallow, strong slab flux

10. PM Normalized Concentrations Transitional Boninites 44 Ma - Guam Ba/La = 13.7 Ba/La = 28 43 Ma - Ogasawara

12. Representative Mariana arc lava compositions "First arc"

13. 31 Ma Mariana trench slope "First Arc" lavas PM Normalized Concentrations 42 Ma Guam ca. 36 Ma Rota, Saipan 30 Ma Bonin Turbidites Gill et al. (1994)

14. SouthNorth Guam Mar. Trn. Saipan Rota DSDP 458-9 Haha- jima Chichi- jima Ogasa. Ridge ODP 786 Adakitic forearc lavas Boninites Rhyolites Transitional boninites 44 41 42.7 45.1 38.3 36 46.7-45.3 48.2-46.0 44.7 42.9 41.8 43.9 44 >47 MORB-like (FAB) Age in Ma Ages and compositions of protoarc and first arc lavas Ages from this study; Ishizuka et al. (2006); Cosca et al. (1998); Meijer et al. (1983) 49.3 32 ? “Normal” arc lavas ? ? ? After Ishizuka et al. (2006) 31 ?

15. Mesozoic volcanogenic sed. Silicic sediment MORB Arc MORB OIB Total slab flux+ melting

16.  Nd o  Hf o OIB+Pacific volcanogenic sediment Silicic sediment Indian Ocean MORB-OIB+ Philippine basins Pacific MORB Sources: GEOROC; PETDB; Woodhead et al. (2001); Pearce et al. (1999), this study 458, 459, first arc Guam Second arc, modern arc Chichijima 782,786

17. Philippine basins Jurassic volcaniclastic sediment Average Asian loess Cenozoic silicic sediments NHRLIP Sources: Meijer (1976); Hickey-Vargas and Reagan (1987); Hickey-Vargas (1989); Dobson and Tilton (1989); Taylor et al. (1994); Elliott et al. (1997); Hickey-Vargas (1998); Taylor and Nesbitt (1998); Pearce et al. (1999); Debari et al. (1999); Pettke et al. (2000); Sun and Stern, (2001); Woodhead et al. (2001); Ishizuka et al. (2006), this study Mariana FAB Bonin FAB Mariana boninitic Bonin boninitic Mariana first arc Mariana fore-arc andesites

18. Indian-like mantlePacific mantle SWNE Transition West Philippine Basin Pacific Plate (?) Protoarc 49-43 Ma (youngest oldest, 1-d spreading?) Before 50 Ma See Stern and Bloomer (1992), Hall et al. (2003) FAB Boninite Trans. boninites

19. First arc 42-32 Ma Transition 45-42 Ma

20. Conclusions: Volcanic stratigraphy of IBM forearc largely supports Stern and Bloomer model of catastrophic subduction initiation and forearc spreading Oldest subduction related magmas may be FAB Subduction initiation occurred at or near Pacific-Indian mantle domain boundary Late Eocene volcanics record a shift to “normal” arc magma generation processes and therefore “normal” mantle convection 4-7 million years after subduction initiation IB & M arcs have similar histories before opening of PVS basins Not much evidence for loss of forearc area by subduction erosion