[1] Multichannel seismic (MCS) profiles and bathymetric data from the central Mariana and Izu-Bonin subduction systems image the subducting Pacific Plate from the outer trench slope to beneath serpentinite seamounts on the outer fore arc. Subducting oceanic crust varies along the Mariana margin from 5.3 to 7 km thick and is covered by 0.5-2 km thick sediments and numerous seamounts. Oceanic crustal thickness east of the Izu-Bonin Trench is $6 km. Faulting resulting from flexure of the incoming Pacific Plate begins up to 100 km east of the trench axis, near the 6 km depth contour. The plate is cut by normal faults that reactivate inherited tectonic fabric where that fabric strikes <25°to the trench. Where the strike is >25°, incoming crust breaks along new faults with a trench-parallel strike. The Mariana Trench axis is commonly a graben that accommodates an abrupt change (within <25 km) of plate dip from <4°( commonly 2°) on the incoming plate to >8°beneath the outer fore arc. We infer that the plate fails there rather than simply bends under the applied loads. Along portions of the Mariana margin, subducting seamounts displace the trench axis westward and uplift the toe of the slope. Surprisingly, west of the toe, there is no geophysical evidence of disturbance of the upper plate in response to seamount subduction, nor of significant subduction erosion or sediment underplating. MCS profiles across the base of the Mariana inner trench slope provide evidence for both complete subduction and small-scale accretion of Pacific Plate sediments; however, we found no evidence for long-term sediment accretion. The subducting plate dips 9-12°beneath serpentinite seamounts on the Izu-Bonin and Mariana fore arcs. Along the Mariana margin, the majority of these seamounts are located $50-70 km west of the trench where the mantle wedge is 3-7 km thick between 8-10 km thick fore-arc crust and the top of the subducting plate. The apparent lack of significant deformation of the Mariana fore arc crust by subducting seamounts may be the result of a weak serpentinized mantle wedge and/or progressive fracturing as the subducting plate increases in dip as it passes through the trench graben.

S U M M A R YSerpentinite seamounts, representing some of the first material outputs of the recycling process that takes place in subduction zones, are found on the outer Mariana forearc. Multichannel seismic (MCS) and bathymetric data collected in 2002 image the large-scale structures of five seamounts, as well as the pre-seamount basement geometry and sediment stratigraphy. We present data from three edifices that provide insights into seamount growth and internal deformation processes and allow us to support the interpretation that serpentinite mud volcanoes are formed by the episodic eruption of mud flows from a central region. The presence of thrust faulting at the base of Turquoise and Big Blue Seamounts, along with the low surface slopes (5 • -18 • ) of all the seamounts studied, lead us to infer that these edifices spread laterally and are subject to gravitational deformation as they grow. Numerical simulations using the discrete element method (DEM) were used to model their growth and the origins of features that we see in MCS sections, such as basal thrusts, inward-dipping reflections and mid-flank benches. The DEM simulations successfully reproduced many of the observed features. Simulations employing very low basal and internal friction coefficients (∼0.1 and ∼0.4, respectively) provide the best match to the overall morphology and structures of the serpentinite seamounts. However the simulations do not capture all of the processes involved in seamount growth, such as withdrawal of material from a central conduit leading to summit deflation; compaction, dewatering and degassing of mud flows; mass wasting in the form of sector collapse and growth upon a dipping substrate. A strong reflection beneath the summit of Big Blue, the largest serpentinite seamount on the Mariana forearc, represents the floor of a summit depression that has been partially in-filled by younger muds, supporting the idea that serpentinite seamounts grow by episodic mud volcanism. Boundaries of mud-flow units are visible in bathymetric data and as normal polarity, subhorizontal reflections on seismic profiles. Big Blue Seamount displays complex nesting relationships as it merges with other seamounts to form a large, composite edifice. Flank flows of serpentinite muds on Big Blue and Celestial Seamounts downlap pre-existing forearc substrate. The interface between serpentinite seamounts and the underlying forearc sediments is represented by a reverse polarity reflection beneath Big Blue and Celestial Seamounts, suggesting that the substrate is undercompacted/overpressured and may be a zone of fluid migration. DEM simulations suggest that this boundary represents a distinct décollement along which the seamounts slide laterally. In contrast, Turquoise Seamount grows laterally, not by stable sliding along the top of forearc sediments, but by incorporating them into large basal thrusts.

[1] We present new multichannel seismic profiles and bathymetric data from the central Marianas that image the West Mariana Ridge (WMR) remnant arc, both margins of the Mariana Trough back-arc basin, the modern arc, and Eocene frontal-arc high. These data reveal structure and stratigraphy related to three periods of arc volcanism and two periods of arc rifting. We interpret the boundary between accreted backarc basin and rifted arc crust along the Mariana Trough and support these findings with drilling results and recent seismic refraction and gravity studies. We show that with the exception of a few volcanoes behind the volcanic front that straddle the boundary between crustal types, the modern Mariana Arc is built entirely on rifted arc crust between 14 and 19°N. Our data indicate that there is more accreted back-arc seafloor to the west of the Mariana Trough spreading axis than to the east, confirming previous evidence for an asymmetric basin. The rifted margin of the WMR remnant arc forms a stepped pattern along the western boundary of the Mariana Trough, between 15°30 0 and 19°N. In this region, linear volcanic cross chains behind the WMR are aligned with the trend of Mariana Trough spreading segments, and the WMR ridges extend into the back-arc basin along the same strike. These ridges are magmatic accommodation zones which, to the north along the Izu-Bonin Arc, punctuate tectonic extension. For the WMR we hypothesize that rift basins are more commonly the sites where spreading segment offsets nucleate, whereas magmatic centers of spreading segments are sites where magmatism continues from arc volcanism, through rifting to back-arc spreading. The Mariana Trough is opening nonrigidly and is characterized by two predominant abyssal hill trends, NNW-SSE in the north and N-S in the south. Between the only two basin-crossing fracture zones at 15.5 and 17.5°, N-S axes propagated north at the expense of NNW axes.

[1] New seismic data collected in the 14.5°-18.5°N Mariana segment of the Izu-Bonin-Mariana island arc system image six seismic stratigraphic sequences that can be mapped throughout the inner forearc. These sediments were most likely deposited from 35 Ma to the present. The oldest stratigraphic Units 1, 2, and 3 are syn-rift volcaniclastic deposits. Unit 4 deposits accumulated during a period of mild structural inversion, which resulted in several isolated reverse-faulted anticlines within the forearc sedimentary basin. A late period of extensional deformation began near the end of Unit 5 deposition and continued through Unit 6 sedimentation to the present. Seismic lines show that the basement of the forearc is composed of large rotated fault blocks and half grabens with NE, NW, and NNE trends. Fault offset calculations show that basement faults with dips between 45°and 50°account for only 4% total extension in the forearc. South of 16.3°N, normal growth faults initiated during basement extension offset the frontal arc high from a deep forearc basin. From correlations with the known geologic history, we hypothesize that extension during deposition of Units 1 through 3 corresponds to rifting of the Eo-Oligocene Arc, between 35 Ma and 29 Ma, older deposits being too thin to be seismically resolvable. Localized compression during Unit 4 accumulation occurred some time after Eo-Oligocene rifting in the early Miocene. Late-stage normal faulting near the end of deposition of Unit 5 and throughout Unit 6 accumulation may be associated with the opening of the Mariana Trough backarc basin from 8 Ma years to the present. There is a higher density of these later faults in the inner forearc between 15.5°and 17°N and in the outer forearc between 14°N and 18°N. Recent extension is at least partially accommodated by reactivation of older basement faults with the same NE, NW, and NNE-trends. Stratigraphic relationships indicate that the inner forearc south of 16.3°N has differentially subsided and tilted trenchward, possibly as a result of a recent change in subducting slab geometry or subducted relief under the forearc.

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