Basaltic Volcaniclastics from the Challenger Deep Forearc

University of Rhode Island
[email protected]

Graduate School of Oceanography Faculty Publications

Graduate School of Oceanography

12-24-2014
Basaltic Volcaniclastics from the Challenger Deep Forearc Segment, Mariana Convergent Margin: Implications for Tectonics and Magmatism of the Southernmost Izu–Bonin–Mariana Arc
Robert J. Stern
Minghua Ren
Katherine A. Kelley University of Rhode Island, [email protected]
Yasuhiko Ohara
FFoelrlnoawntdhoisManadrtainddeiztional works at: https://digitalcommons.uri.edu/gsofacpubs The University of Rhode Island Faculty have made this article openly available.
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Citation/Publisher Attribution Stern, Robert J.; Ren, Minghua; Kelley, Katherine A.; Ohara, Yasuhiko; Martinez, Fernando; Bloomer, Sherman H. (2014). "Basaltic volcaniclastics from the Challenger Deep forearc segment, Mariana convergent margin: Implications for tectonics and magmatism of the southernmost Izu- Bonin- Mariana arc." Island Arc. 23(4): 368-382. Available at: http://onlinelibrary.wiley.com/doi/10.1111/iar.12088/abstract

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Authors Robert J. Stern, Minghua Ren, Katherine A. Kelley, Yasuhiko Ohara, Fernando Martinez, and Sherman H. Bloomer
This article is available at [email protected]: https://digitalcommons.uri.edu/gsofacpubs/72

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Basaltic volcaniclastics from the Challenger Deep forearc segment, Mariana convergent margin: Implications for tectonics and magmatism of the southernmost IBM arc

Journal: Island Arc

Manuscript ID: Draft

Manuscript Type: Research Article

Date Submitted by the Author: n/a

Complete List of Authors:

Stern, Robert; U Texas at Dallas, Geosciences Ren, Minghua; U Nevada Las Vegas, Geoscience Kelley, Katherine; University of Rhode Island, GSO Ohara, Yasuhiko; Hydrographic and Oceanographic Department of Japan, Martinez, Fernando; University of Hawaii, Hawaii Institute of Geophysics and Planetology Bloomer, Sherman; Oregon State U, Geosciences

Key words: Subduction, Mariana Arc, basalt, Challenger Deep

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Basaltic volcaniclastics from the Challenger Deep forearc segment, Mariana convergent margin: Implications for tectonics and magmatism of the southernmost IBM arc

Robert J. Stern1*, Minghua Ren2, Katherine A. Kelley3, Yasuhiko Ohara4, Fernando Martinez5, Sherman H. Bloomer6
1Geosciences Dept., U. Texas at Dallas, 800 W. Campbell Road, Richardson TX 75080 USA 2 Dept. Geosciences, University of Nevada, Las Vegas, Las Vegas, NV 89154-4010, USA 3 Graduate School of Oceanography, University of Rhode Island, Narragansett Bay Campus, Narragansett, Rhode Island, USA 4 Japan Agency for Marine-Earth Science and Technology, Natsushima, Yokosuka and Hydrographic and Oceanographic Department of Japan, Koto-ku, Tokyo, Japan 5 Hawai‘i Institute of Geophysics and Planetology, SOEST, University of Hawai’i at Manoa, Honolulu, Hawaii, USA 5 Geosciences Department, Oregon State University, 128 Kidder Hall, Corvallis, Oregon 97331, USA

* Corresponding author [email protected], Tel. +001-972-883-2442 Fax +001-972883-2537
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ABSTRACT Convergent margin igneous activity is generally limited to 100-200 km from the trench except where spreading ridges are subducted or in association with SubductionTransform Edge Propagators (STEP faults). The southernmost Mariana forearc, facing the Challenger Deep, subducts Mesozoic seafloor and is not in a STEP fault setting but includes at least one site where tholeiitic basalts recently erupted close to the trench, the SE Mariana Forearc Rift (SEMFR). Here we present evidence of young basaltic volcanism from another site ~100 km west of SEMFR. Shinkai 6500 diving during YK1308 (Dive 1363) recovered volcaniclastics from ~5.5 to 6km deep in the inner wall of the Mariana Trench, ~50 km NE of the Challenger Deep. Glassy fragments are tholeiitic basalts similar to MORB except for much higher contents of magmatic water (~2% H2O vs. <0.2% H2O in MORB) and spikes in trace element diagrams at Rb-Cs-Ba, K, Pb, and Sr. Dive 1363 basalt glasses are similar to basalts from SEMFR erupted near the trench and to basalts of the Mariana Trough backarc basin. Basalt fragments and palagonitized matrix dominate the three samples we studied, but small xenocrysts and xenoliths derived from mantle peridotite and Neogene volcanics are also present, probably torn from the vent walls. Dive 1363 hyaloclastites erupted at 3-6 km water depth accompanied by vigorous degassing of volatiles, most likely CO2. These results provide further evidence that the forearc adjacent to the Challenger Deep has been invaded by asthenospheric mantle and is unusually weak. The weakness of the overriding plate may result in weak coupling between the subducting Pacific plate and overriding Mariana plate and this may be partly responsible for the great depth of the Challenger Deep.

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Keywords: Subduction, Mariana Arc, basalt, Challenger Deep

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INTRODUCTION The southernmost part of the ~1500 km-long Mariana arc system - where the Mariana Trench bends sharply west (Fig. 1a) - is a region that is tectonically active and poorly understood. Here, the Pacific Plate subducts almost orthogonally beneath the easternmost Philippine Sea (Mariana Plate) at about 30 mm/year (Bird 2003). Because the plate boundary trends E-W here, it cuts across the southern part of the Mariana Trough, an actively spreading back-arc basin (BAB; Fig. 1a). This combination of strong convergence and extension is associated with the deepest point on Earth’s solid surface, the Challenger Deep. It also causes the adjacent part of the Mariana Trough just north of the Trench to be seismically and magmatically active and to deform rapidly and complexly. We know from GPS studies that the southernmost Marianas (Fig. 1b) is the most rapidly deforming part of the 3500 km long Izu-Bonin-Mariana (IBM) arc system (Kato et al. 2003), but we are only beginning to understand how deformation and magmatism are distributed over this deeply submerged region. Tectonics of the southernmost Marianas have a strong influence on the Mariana Trough BAB. Fryer (1995) first noted that the Mariana Trough had a different expression south of ~14°N and concluded that this reflected a different tectonic style in this region relative to that farther north. For most of its ~1200 km length, the Mariana Trough opens slowly E-W along a ridge system with slow-spreading axial valley morphology producing a variable but somewhat thin (3.5-4 km) volcanic crust estimated from gravity and bathymetry values (Kitada et al, 2006). South of ~14°N the spreading center bends increasingly westward and develops a fast-spreading axial-high morphology, although actual spreading rates are not likely high (Martinez et al., 2000). Gravity data suggest

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somewhat thicker crust in this area (6.7 km) (Kitada et al., 2006). The complexly deformed region – which we call the Southernmost Mariana Trough-Trench Complex (SMTTC) – is delimited on the east by the West Santa Rosa Bank Fault (WSRBF) Fryer et al., (2003). WSRBF can be traced as a 5 km scarp south of 13°N, diminishing in relief northwards until it is replaced by a northeast-trending fault scarp south of Tracey Seamount (Fig. 1b). The northern limit of the region affected by SMTTC tectonics lies ~13°30’N, about where the Mariana volcanic front loses definition southward. This is also about where the BAB spreading center changes from an axial rift in the north into the inflated Malaguana–Gadao Ridge (MGR; Fig. 1b), which is underlain by the only known magma chamber in the Mariana Trough BAB (Becker et al. 2010). The Southern Mariana Forearc Ridge (Fig. 1B) separates the Mariana Trough to the north from the trench and Challenger Deep to the south; where this ridge has been sampled, it is composed of Miocene arc volcanics (Ohara et al., in preparation). More evidence that the SMTTC is unusually active comes from how the locus of arc volcanism is disrupted in this region. For ~3000 km north from Tracey seamount (~13°40’N) all the way to Japan, discrete and well-developed volcanoes of the IBM active arc define a pronounced string of stratovolcanoes that is separated from the trench by a broad (~150-200 km wide) forearc. As is characteristic for other magmatic arcs, IBM arc volcanoes are typically found ~100-150 km above the subducted Pacific Plate. Such a line of discrete, long-lived volcanoes is typical for mature convergent margins and is known as the ‘magmatic front’ (Matsuda & Uyeda, 1971). The magmatic front marks where fluids and sediment melts released from the subducting plate trigger melting of convecting asthenosphere, and arc volcanoes build up over time where these melts rise to

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the surface. South of Tracey seamount, the line of arc volcanoes is poorly defined (Fig. 1B, Stern et al., 2013), despite the fact that the southern Mariana Trough is underlain by a subducted slab that can be traced to 150 km depth (Gvirtzman & Stern, 2004). Poor definition of the magmatic arc SW of 13°40’N reflects tectonic instability in the SMTTC. Multiple sites of extension frequently divert the supply of arc magmas, not allowing magma supply to focus beneath discrete volcanoes so that these can grow to become large stratovolcanoes, as is seen for the arc to the north (Stern et al., 2013). Complex deformation in the SMTTC reflects three interacting causes: 1) subduction of the Pacific plate, which induces asthenospheric convection at the same time that it supplies magma and fluids to the overlying mantle, causing flux melting; 2) BAB opening, which keeps lithosphere thin and causes decompression melting; and 3) rapid rollback of a narrow, short slab, which adds trench-normal extensional stresses to the overriding plate. Gvirtzman and Stern (2004) concluded that the plate-coupling zone along the Challenger Deep forearc segment was unusually narrow, only 50 km wide compared to ~150 km wide beneath the forearc farther north. The unusually narrow plate coupling zone allows convecting asthenosphere to penetrate closer to the trench than is found for other forearcs, and this allows asthenosphere to be fluxed by shallow, slabderived hydrous fluids and melt (Ribeiro et al., 2013a, b). The result is an unusually weak forearc that is volcanically active much closer to the trench than normally occurs. It is not easy to identify where igneous activity occurs in this complexly deforming region. The Southeast Mariana forearc rift (SEMFR) marks one such region of forearc igneous activity, floored by 2.7–3.7 Ma low-K tholeiitic basalts (Ribeiro et al., 2013a, b). SEMFR lavas were produced by partial melting of a BAB-like mantle source,

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metasomatized by sediment melt and aqueous fluids released from dehydration of the subducted oceanic crust. SEMFR melts were probably generated when the Mariana Trough backarc basin (BAB) first began to open in this region. But where else does igneous activity occur in this enigmatic and complexly-deforming region, and how close to the trench does igneous activity occur? In this contribution we present new evidence that MORB-like basalts recently erupted very close to the trench, ~100 km west of SEMFR.

SAMPLE COLLECTION Regional multibeam bathymetry in the area was obtained by US Law of the Sea mapping project (Armstrong, 2011) as well as on R/V Yokosuka (Fig.2 A). In Dec. 2011 to Jan. 2012 R/V Thomas G Thompson obtained two swaths of deep-towed (~500 m altitude) IMI-30 sidescan sonar imagery over the area (Martinez et al., 2012, Fig. 2B). Samples were collected during dive #1363 of the manned submersible Shinkai 6500 on Sept. 10, 2013 as part of JAMSTEC research cruise of R/V Yokosuka (YK1308). The dive site was located ~ 11°38’N, 143°E, ~ 30 km north of the trench axis, ~7.5 km west of the Shinkai Seep Field (Ohara et al., 2012), and ~60 km ENE of the Challenger Deep (Fig. 2). The dive traversed north up the inner wall of the Mariana Trench from 6094 m to 5584 mbsl and was intended to search for additional forearc seeps and communities. Previous studies suggest that the Moho is exposed at ~5500 mbsl near the study area, so we expected to recover peridotites. 18 samples were collected during this dive, consisting of peridotites and moderately lithified volcaniclastic sediments (hyaloclastities), composed of sand-sized, reddish-brown matrix with pieces of basaltic

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