Origins of chemical diversity of back-arc basin basalts: A segment-scale study of the Eastern Lau Spreading Center

The northeastern Lau Basin is one of the fastest opening and magmatically most active back-arc regions on Earth. Although the current pattern of plate boundaries and motions in this complex mosaic of microplates is fairly well understood, the structure and evolution of the back-arc crust are not. We present refraction seismic, multichannel seismic and gravity data from a 300 km long east-west oriented transect crossing the Niuafo&#8217;ou Microplate (back-arc), the Fonualei Rift and Spreading Centre (FRSC) and the Tofua Volcanic Arc at 17&#176;20&#8217;S. Our P wave tomography model shows strong lateral variations in the thickness and velocity-depth distribution of the crust. The thinnest crust is present in the Fonualei Rift and Spreading Center, suggesting active seafloor spreading there. In the much thicker crust of the volcanic arc we identify a region of anomalously low velocities, indicative of partial melts. Surprisingly, the melt reservoir is located at ~17 km distance to the volcanic front, supporting the hypothesis that melts are deviated from the volcanic arc towards the FRSC in sub-crustal domains. We identify two distinct regions in the back-arc crust, representing different opening phases of the northeastern Lau Basin. During initial extension, likely dominated by rifting, crust of generally lower upper-crustal velocities formed. During an advanced opening phase, likely dominated by seafloor spreading, crust of higher upper-crustal velocities formed and is now up to 11 km thick. This thickening is the result of magmatic underplating, which is supported by elevated upper mantle temperatures in this region.

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The tectonic and volcanic evolution of the Mangatolu Triple Junction

10.5194/egusphere-egu2020-19897 ◽

2020 ◽

Author(s):

Rebecca Mensing ◽

Margaret Stewart ◽

Mark Hannington ◽

Alan Baxter ◽

Dorothee Mertmann

Keyword(s):

Triple Junction ◽

Hydrothermal Systems ◽

Spreading Center ◽

Valuable Insight ◽

Triple Junctions ◽

Lau Basin ◽

The North ◽

Spreading Centers ◽

Spreading Rates ◽

Back Arc

The Mangatolu Triple Junction (MTJ) is an intraoceanic back-arc spreading center that is host to at least 3 distinct hydrothermal systems. It is located in the NE Lau Basin, which opened due to rollback of the Pacific plate along the Tonga-Kermadec trench. At the MTJ, three spreading centers meet in a ridge-ridge-ridge (RRR)-type triple junction separating the Tonga plate in the east, the Niuafo&#8217;ou microplate in the southwest, and an unnamed microplate in the north. The MTJ is directly linked to the formation and evolution of the Northeast Lau microplate mosaic, as plate fragmentation inevitably results in the formation of triple junctions, but it remains unclear whether the spreading centers are the drivers of plate fragmentation or a consequence of stress relocation related to microplate rotation. Detailed investigation of the geology and structural setting of the MTJ therefore provides valuable insight into the development in the northeast Lau Basin. Here we present the first comprehensive 1:200,000 geological map of the MTJ, based on a compilation of marine geophysical data (hydroacoustics, magnetics, and gravity) derived from 7 research cruises that have investigated the region between 2004 and 2018. Analysis of the mapped geological formations at the MTJ shows the importance of relict arc crust originating from the Tofua Arc in the architecture of the triple junction, which includes three stages of back-arc crust development and extensive off-axis volcanism. The spreading centers along each arm of the MTJ exploit pre-existing crustal weaknesses, interpreted to have formed during initial Lau Basin opening. A reconstruction of the basin opening, based on the mapped features and published spreading rates, revealed that initiation of the MTJ commenced approximately 180,000 years ago, consistent with the very recent and ongoing dynamic evolution of the NE Lau Basin and emerging microplate mosaic. Intersecting fabrics indicate sequential evolution of the 3 arms of the triple junction, with extension along the northeast arm dominant in the early history and more recent extension along the southern and western arms. The results of this study contribute to our growing understanding of the tectonic framework of the northeast Lau Basin and the role of triple junctions in microplate formation.

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Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

Progress in Earth and Planetary Science ◽

10.1186/s40645-021-00454-3 ◽

2021 ◽

Vol 8 (1) ◽

Author(s):

Norikatsu Akizawa ◽

Yasuhiko Ohara ◽

Kyoko Okino ◽

Osamu Ishizuka ◽

Hiroyuki Yamashita ◽

...

Keyword(s):

Isotope Composition ◽

Spreading Center ◽

Chemical Compositions ◽

Olivine Gabbro ◽

Pb Isotope ◽

Shikoku Basin ◽

Source Mantle ◽

Evolutional Process ◽

Back Arc

AbstractThis paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

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Major and trace element chemistry and D/H, 18O/16O, 87Sr/86Sr and 143Nd/144Nd ratios of rocks from the spreading center of the Okinawa Trough, a marginal back-arc basin.

GEOCHEMICAL JOURNAL ◽

10.2343/geochemj.25.121 ◽

1991 ◽

Vol 25 (2) ◽

pp. 121-136 ◽

Cited By ~ 47

Author(s):

Hiroji Honma ◽

Minoru Kusakabe ◽

Hiroo Kagami ◽

Shigeru Iizumi ◽

Hitoshi Sakai ◽

...

Keyword(s):

Trace Element ◽

Okinawa Trough ◽

Spreading Center ◽

Trace Element Chemistry ◽

Element Chemistry ◽

The Okinawa Trough ◽

Back Arc

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A highly diverse silicic end member of a bimodal magma suite formed in an active continental back arc, Okinawa Trough

10.5194/egusphere-egu21-6713 ◽

2021 ◽

Author(s):

Arran Murch ◽

Kenichiro Tani ◽

Takashi Sano ◽

Shigekazu Yoneda

Keyword(s):

Magma Mixing ◽

Okinawa Trough ◽

Fractional Crystallisation ◽

Chemical Diversity ◽

Crustal Assimilation ◽

Arc Volcanism ◽

The North ◽

The Okinawa Trough ◽

Formation Conditions ◽

Back Arc

The Okinawa Trough (OT) is an incipient continental back-arc basin that extends from Kyushu in the north to Taiwan in the south. The Okinawa Trough can be split in to three segments, the Northern (NOT), Middle (MOT), and Southern (SOT) with active back-arc volcanism restricted to volcanic centres located in en-echelon grabens the MOT and SOT. Previous studies have shown magmatism in the OT is bimodal (basaltic to rhyolitic), with at least two types of silicic melts inferred to form through pure fractional crystallisation from basalt and by fractional crystallisation along with minor crustal assimilation (Shinjo and Kato, 2000).Here we present petrological descriptions, along with major, trace element and Sr&#8211;Nd isotopic data for 75 silicic end member samples recovered as both lava and pumice, collected during the R/V Sonne HYDROMIN1 and 2 cruises in 1988 and 1990, respectively. Samples were dredged from various seafloor knolls and ridges located in the Io and Iheya grabens and from Izena Hole in the MOT, and from a single volcanic ridge in the Yaeyama graben and a single isolated knoll in the SOT.Results show a chemically highly diverse silicic end member magmas, with at least four identifiable groups based on differences in the degree of enrichment of incompatible elements (LREE, K, Rb, Ba, etc.). Each group contains at least one dense lava sample suggesting the chemical diversity is a primary feature of magmatism in the Okinawa Trough rather than a result of the floating in of pumiceous material from various locations.Using petrological descriptions and the chemistry of samples along with MELTS modelling we plan to calculate magma formation conditions and identify any evidence of magma mixing or crustal assimilation. In doing so we hope to provide a model to explain the diversity of silicic magma chemistry in the MOT and SOT.&#160;Shinjo, R., and Kato, Y. (2000). Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin. Lithos 54, 117&#8211;137. doi:10.1016/S0024-4937(00)00034-7.

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A new species of alvinocaridid shrimp Rimicaris Williams & Rona, 1986 (Decapoda: Caridea) from hydrothermal vents on the Mariana Back Arc Spreading Center, northwestern Pacific

Journal of Crustacean Biology ◽

10.1093/jcbiol/ruz046 ◽

2019 ◽

Vol 39 (5) ◽

pp. 640-650 ◽

Cited By ~ 3

Author(s):

Tomoyuki Komai ◽

Thomas Giguère

Keyword(s):

New Species ◽

Hydrothermal Vents ◽

Coi Gene ◽

Spreading Center ◽

Northwestern Pacific ◽

Distal Margin ◽

Mitochondrial Coi ◽

Mitochondrial Coi Gene ◽

Back Arc ◽

A New Species

AbstractA new species of the alvinocaridid shrimp genus RimicarisWilliams & Rona, 1986, R. falkoraen. sp., is described and illustrated based on material from deep-sea hydrothermal vents (3,630–3,912 m deep) on the Mariana Back Arc Spreading Centre, northwestern Pacific, representing the tenth described species of the genus. The new species is morphologically most similar to R. paulexa (Martin & Shank, 2005), but the presence of numerous short setae scattered on the carapace surface, the relatively long antennular stylocerite usually reaching the distal margin of article 2 of the antennular peduncle and the spiniform posteromesial projection of the uropodal protopod distinguish the new species from all congeners. Genetic analysis using the barcoding region of the mitochondrial COI gene supports the recognition of the species as new.

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