Back-arc Spreading System Research Articles

An iron isotope perspective on back-arc basin development: Messages from Mariana Trough basalts

The geology and magmatism of the northern Mariana Trough (NMT) represent a type example for the development of a back-arc basin (BAB). It is a mature spreading center in the south and propagating northward via rifting of arc and rear arc lithosphere in response to trench retreat and overriding plate extension in space and time. It is thus essential to fully characterize its tectonomagmatic processes that are potentially of global significance. For this reason, we carried out a comprehensive geochemical study on basaltic samples (glasses) from the NMT axis from 17 to 23°N by analyzing a full suite of major and trace elements, Sr–Nd–Pb–Hf radiogenic isotopes and stable Fe isotopes. The Fe isotopes, in particular, have the potential to offer novel perspectives beyond the aforementioned geochemical variables. From south to north, the δ56Fe of the NMT basalts (NMTB) decrease systematically, varying from a MOR-like value of +0.12‰ to an arc-like value of 0.00‰. Importantly, incompatible element abundances and Sr–Nd–Pb–Hf isotope compositions of the NMTB also show systematic along-latitude variations. Positively correlated with δ56Fe, the abundances of fluid-insoluble elements (e.g., Nb, Ta, Zr, Hf, Ti and heavy rare earth elements) show the south-to-north decrease, suggesting that the NMTB δ56Fe systematics are inherited from the magma source variation. The source variation is most consistent with an asthenosphere source in the south and progressively more depleted harzburgitic lithosphere source of prior melt depletion towards north. On the other hand, there are scattered but significant enrichment trends towards north in terms of Sr–Nd–Pb–Hf radiogenic isotopes and fluid-soluble incompatible elements (e.g., Ba, Rb, Cs, Th, U, K, Pb, Sr and light rare earth elements), which is consistent with south-to-north increase in slab material (e.g., slab fluids, sediments etc.) contribution. These north-to-south magma source systematics defined by the NMTB can be readily understood as a tectonic response to the back-arc basin development with time from initial arc-magmatism dominated by sub-arc lithospheric mantle melting with significant slab material contributions to back-arc rifting and to back-arc spreading center magmatism characterized by increasing asthenospheric mantle melting, decreasing slab material contribution and diminishing sub-arc lithospheric mantle participation. This tectonic-magma source evolution in space and time is manifested by the snapshot of the NMTB magmatism in terms of the geology, petrology and geochemistry.The NMTB δ56Fe values and fluid-insoluble incompatible element abundances are unaffected by slab materials, but controlled by the harzburgitic residues of previous melt extraction depleted in these elements with low δ56Fe at the early arc-magmatism stage in the north and by the asthenosphere of MORB mantle characteristics in the south with well-developed back-arc spreading system. Importantly, these observations affirm the understanding that compositionally depleted and physically buoyant lithosphere is prerequisite as the overriding plate for subduction initiation at its edges.

Reprint of “Magmatism, structure and age of Dove Basin (Antarctica): A key to understanding South Scotia Arc development”

Dove Basin is situated in the south-central Scotia Sea, between Pirie and Bruce banks, and was formed during the development of the Scotia Arc. The basin has a roughly sigmoidal shape, with a prominent NNE–SSW elongated ridge located in its central part, the Dove Ridge, which is considered as an extinct spreading axis. A NE–SW elongated tectonic high, the Dove Seamount, is located in the north-eastern region of the basin, bounded by a normal fault dipping to the southeast. Dredged rocks and geophysical data were collected during the SCAN2004 and SCAN2008 cruises. Dredged samples were recovered from three positive features in the center of the basin, two from Dove Ridge, and one from Dove Seamount. Igneous rocks along the Dove Ridge are mainly tholeiitic basalts, derived from asthenospheric upper mantle within an extensional supra-subduction back-arc tectonic setting, which evolved over time from back-arc basin basalts (BABB) toward Mid Oceanic Ridge Basalts (MORBs). Altered olivine-bearing fine- and medium-grained basaltic rocks were also dredged from Dove Ridge and the seamount, together with minor oceanic island arc basalts and basaltic andesites. The mantle source was affected, up to early Miocene times, by a subducted oceanic slab related to an arc to the east, with Dove Basin forming in a back-arc position. Minor alkaline oceanic island basalts dredged at the seamount might represent a final extensional stage, genetically related with the dying Dove Ridge volcanism or, less probably, to a later, late Miocene–Pliocene extensional stage, producing incipient volcanism deriving from a deeper mantle source.40Ar/39Ar dating of MORB samples dredged from the Dove Ridge provided ages of 20.4±2.6 to 22.8±3.1Ma. These outcrops were later coated by Fe–Mn crusts with Co-Chronometer ages ranging from at least 12.6Ma and probably up to 18Ma. Analysis of magnetic anomaly profiles shows the best fit in the central profile, corresponding to chrons C6B (21.7Ma) to C7 (24.5Ma), although alternative ages may be proposed due to the short length. This interpretation supports the 40Ar/39Ar dating of a late Oligocene to early Miocene age. The spreading was asymmetrical, the asynchronous age of extinction of spreading in the basin being confirmed by the variable character of magnetic anomalies. The western part extended faster than the eastern part, suggesting an eastward location for a westward deepening subduction zone.The age obtained for the Dove Basin is older than the age previously proposed for the Protector Basin located to the west, thus suggesting an opposite polarity for the development of some small oceanic basins of the southern Scotia Sea with respect to others generated by an eastward migrating arc over subducted Atlantic oceanic floor. This apparent contradiction could be explained by considering the Dove Basin as originated from an Oligocene to early Miocene arc–back-arc spreading system, which would contribute to the eastward general development of the Scotia Arc, meanwhile Protector Basin was formed by a middle Miocene extensional phase, not genetically related with the main oceanic spreading along the Dove Basin.

Magmatism, structure and age of Dove Basin (Antarctica): A key to understanding South Scotia Arc development

Dove Basin is situated in the south-central Scotia Sea, between Pirie and Bruce banks, and was formed during the development of the Scotia Arc. The basin has a roughly sigmoidal shape, with a prominent NNE–SSW elongated ridge located in its central part, the Dove Ridge, which is considered as an extinct spreading axis. A NE–SW elongated tectonic high, the Dove Seamount, is located in the north-eastern region of the basin, bounded by a normal fault dipping to the southeast. Dredged rocks and geophysical data were collected during the SCAN2004 and SCAN2008 cruises. Dredged samples were recovered from three positive features in the center of the basin, two from Dove Ridge, and one from Dove Seamount. Igneous rocks along the Dove Ridge are mainly tholeiitic basalts, derived from asthenospheric upper mantle within an extensional supra-subduction back-arc tectonic setting, which evolved over time from back-arc basin basalts (BABB) toward Mid Oceanic Ridge Basalts (MORBs). Altered olivine-bearing fine- and medium-grained basaltic rocks were also dredged from Dove Ridge and the seamount, together with minor oceanic island arc basalts and basaltic andesites. The mantle source was affected, up to early Miocene times, by a subducted oceanic slab related to an arc to the east, with Dove Basin forming in a back-arc position. Minor alkaline oceanic island basalts dredged at the seamount might represent a final extensional stage, genetically related with the dying Dove Ridge volcanism or, less probably, to a later, late Miocene–Pliocene extensional stage, producing incipient volcanism deriving from a deeper mantle source.40Ar/39Ar dating of MORB samples dredged from the Dove Ridge provided ages of 20.4±2.6 to 22.8±3.1Ma. These outcrops were later coated by Fe–Mn crusts with Co-Chronometer ages ranging from at least 12.6Ma and probably up to 18Ma. Analysis of magnetic anomaly profiles shows the best fit in the central profile, corresponding to chrons C6B (21.7Ma) to C7 (24.5Ma), although alternative ages may be proposed due to the short length. This interpretation supports the 40Ar/39Ar dating of a late Oligocene to early Miocene age. The spreading was asymmetrical, the asynchronous age of extinction of spreading in the basin being confirmed by the variable character of magnetic anomalies. The western part extended faster than the eastern part, suggesting an eastward location for a westward deepening subduction zone.The age obtained for the Dove Basin is older than the age previously proposed for the Protector Basin located to the west, thus suggesting an opposite polarity for the development of some small oceanic basins of the southern Scotia Sea with respect to others generated by an eastward migrating arc over subducted Atlantic oceanic floor. This apparent contradiction could be explained by considering the Dove Basin as originated from an Oligocene to early Miocene arc–back-arc spreading system, which would contribute to the eastward general development of the Scotia Arc, meanwhile Protector Basin was formed by a middle Miocene extensional phase, not genetically related with the main oceanic spreading along the Dove Basin.

Tectonic and magmatic evolution of the Bismarck Sea, Papua New Guinea; review and new synthesis

The Bismarck Sea, located north of Papua New Guinea and just south of the equator, formed during the final stages of a long, complex geological development of the Melanesian Borderland, which resulted from Cenozoic convergence between the Australian and Pacific-Caroline Plates and the opening of back-arc basins. The sea, which straddles two oppositely facing trenches, the inactive Manus trench and the active New Britain trench, covers two basins, the New Guinea Basin to the west and the Manus Basin to the east. These basins are separated by the shallow Willaumez-Manus Rise, trending roughly from WNW to ESE. The origin of these major structural units and their relationship with the present-day zone of major seismicity along the Bismarck Sea Seismic Lineation remains unclear. A detailed examination of geophysical and geochemical data, combined with geologic and geodetic information from surrounding regions, attests that the Bismarck Sea went through some rather unusual events of back-arc development beginning in the middle Pliocene. Around 3.5 Ma, the northern tip of New Guinea came into contact with the Finisterre-Huon Range. The event triggered a back-arc opening that eventually divided the Bismarck seafloor into North and South Bismarck Plates. The rapid opening also caused an upwelling of anomalously hot upper mantle beneath the Bismarck Sea. A large volcanic outflow during this period may have contributed to the formation of the rise west of Manus Island. Both the North and South Bismarck Plates interacted with surrounding plates, and small and large changes may have occurred throughout their history. A sudden shift in local plate motion, perhaps caused by locking such as plate collision, may have caused the overlying lithosphere to decouple from the mantle upwelling. The Willaumez-Manus Rise was probably created under such circumstances when a large volume of rising magma leaked out along a strike-slip plate boundary linking the spreading centers in the New Guinea and Manus basins. An alternative view is that the Willaumez-Manus Rise represents a magmatically robust segment of a single spreading axis that linked the two basins. The further docking of Finisterre-Huon Range with the New Guinea Highlands allowed the South Bismarck Plate to open faster toward the east in the Manus Basin and eventually to jump to the present-day Bismarck Sea Seismic Lineation. The Willaumez Transform was initiated by this more recent plate-boundary reorganization. Seafloor spreading commenced in the Manus Spreading Center <0.78 Ma, and the overall pattern suggests that rifting in the Bismarck Sea is propagating eastwards in the Southeastern Rift and westwards in the Western Spreading Ridges. One feature that distinguishes the Bismarck Sea from most other recently opened back-arc spreading systems in the western Pacific, except for the northern North Fiji and Lau basins, is the anomalously large distance (more than 400 km) between the arc and the spreading axis. This reflects the complex, combined geodynamic processes in this region, i.e., arc-continent collision, mantle upwelling, and rapid back-arc opening specific to this region. Central Bismarck Sea lavas are also unusual as they contain chemical components characteristic of the lower mantle. This may reflect proximity to the Melanesian Borderland, long a site of major subduction, and therefore of upper mantle enrichment by subduction-derived components, compared to other spreading systems.

Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges

P. A. Rona, C. W. Devey, J. Dyment , and B. Murton , editors. Pp. 440. 2010. American Geophysical Union, Geophysical Monograph Series, vol. 188. ISBN 978-0-87590-478-8. Price $129. This book arrived in my mailbox about one month ago, and I immediately began using it for my own research. I referred to it frequently while preparing several review papers. I was so immersed in the book that I completely forgot how I came to own it. Then I received a gentle reminder from the editor that I was overdue with the review. The fact that I had forgotten how the book ended up on my desk, because I was too busy reading it, is probably a better endorsement than any review I can provide here. This is a book for specialists in sea-floor hydrothermal systems. It is part of a superb collection of volumes in the AGU Geophysical Monograph Series that track the history of research in this field. The series has its beginnings in the 1995 introductory volume, Seafloor Hydrothermal Systems (Monograph 91), which was followed by Faulting and Magmatism at Mid-Ocean Ridges (Monograph 106), The Subseafloor Biosphere at Mid-Ocean Ridges (Monograph 144), Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans (Monograph 148), Back-Arc Spreading Systems (Monograph 166), and Magma to Microbe: Modeling Hydrothermal Processes at Ocean Spreading Centers (Monograph 178). Together with a number of similar volumes published by the Geological Society of London, the Geophysical Monographs represent our most important encyclopedia of submarine hydrothermal systems, covering the development of thought since the discovery of the first black smokers on the East Pacific Rise. With these volumes in your library, you can be assured of having at your fingertips the most complete record of knowledge in the field, written as compelling reviews that provide both the historical context of the research and its relevance for future exploration. …

δ37Cl systematics of a backarc spreading system: The Lau Basin

Determinations of δ 37 Cl in glasses and melt inclusions from backarc basin basalts of the Lau Basin (southern Pacific Ocean) constrain the effects of mantle source, subduction, and shallow assimilation. In sections of the Lau Basin not heavily affected by subduction input (central and eastern Lau spreading centers), Cl derived from unfractionated seawater via assimilation in shallow crustal environments is the predominant source of “excess Cl,” and moderates δ 37 Cl to near zero values. In contrast, low-Cl glasses from the Mangatolu triple junction have distinctly negative δ 37 Cl, compatible with recent estimates for δ 37 Cl of the depleted mantle source. Along the Valu Fa Ridge, subduction input manifests as δ 37 Cl that is variable but dispersed toward more negative values in both lavas and melt inclusions. These values are compatible with a second source of lighter Cl within a slab-derived flux derived from altered ocean crust, sediments, and/or serpentinized mantle wedge.

Shear-wave splitting in the Mariana trough—a relation between back-arc spreading and mantle flow?

A shear-wave splitting analysis was performed in the Mariana trench on data recorded by an array of semi broad-band Ocean Bottom Seismometers (OBS), with the aim of studying the Mariana back-arc spreading system and clarifying the nature of the back-arc process. We look for possible mechanisms relating anisotropy and mantle flow. Direct sampling of ray paths through the slab suggests that anisotropy in the slab is relatively small compared with that of the mantle wedge. Fast polarizations at most OBS are consistent with the direction of relative motion between the subducting Pacific and overriding Philippine plates. Exceptions are seismic stations located on the ridge spreading axis which show fast polarizations perpendicular to the ridge curvature. From simple synthetic modeling we estimate an upper limit for the depth of the layer of mantle flow due to the spreading ridge, but as the data show, its influence wanes quickly with distance from the axis. These results suggest that the “sea-anchor” or “extrusion” models are the most favorable scenarios of back-arc opening among other geodynamic models.

Upper mantle structure of the southwest Pacific from regional waveform inversion

We determine the upper mantle seismological structure of the southwest Pacific by inverting complete regional seismograms recorded by the Southwest Pacific Seismic Experiment, a deployment of 12 broadband seismographs in the Fiji, Tonga, and Niue islands. We first present a method to invert entire seismic waveforms for a laterally homogeneous velocity structure. Partial derivatives are computed using an efficient reflectivity code for differential seismograms [Randall, 1994], and a conjugate gradient method is used to perform the nonlinear inversion. Band pass‐filtered (10–55 mHz) vertical and radial component seismograms, extending from the P arrival to the surface waves, are used to constrain the structure. Waveforms are selected to provide regional distance (400–1500 km) propagation paths predominantly within one of the tectonic regions of the southwest Pacific. Source depths ranging from 10 to 240 km allow the deeper regions of the model to be resolved with relatively short path lengths. Results show extremely large lateral variability in the upper mantle structure of the southwest Pacific. The inactive South Fiji Basin shows upper mantle velocities much lower than those previously observed for the Pacific lithosphere of a similar age. The North Fiji and Lau Basins, containing active back arc spreading systems, show upper mantle shear velocities as slow as 3.8 km s−1. The exceptionally low seismic velocities in the active back arc regions may represent the slowest upper mantle velocities in the world. The velocity difference between the active back arc basins and the old Pacific lithosphere immediately east of the Tonga Trench is maximum at 30–90 km depth, with a variation of ∼18%, indicating partial melting in this depth range. Velocity heterogeneity of &gt;4% extends to a depth of 170 km. Velocity structure below 220 km depth is poorly resolved, but there is some evidence of a 2% difference extending to depths of 350 km.

The geochemistry of tholeiitic and alkalic plutonic suites within the Northland ophiolite, northern New Zealand; magmatism in a back arc basin

The Northland ophiolite of New Zealand consists mainly of tholeiitic basalts with subordinate alkalic volcanic rocks. The alkalic rocks occur together with, but are generally younger than the tholeiitis. Both the tholeiitic and alkalic suites include minor plutonic intrusives which range from gabbro to plagiogranite. Variations within the tholeiitic plutonic suite can be modelled in terms of low pressure crystal fractionation. The evolution of the alkalic suite is more problematic, because each massif appears to be derived from a slightly different parental magma. New trace element chemistry presented in this study shows that the tholeiitic rocks formed from a mantle source that was compositionally modified by supra-subduction zone processes but that the alkalic suite has a different chemical signature. Nevertheless both suites of rocks are interpreted as having formed in a back arc spreading system. In this environment, the addition of fertile mantle to the supra-subduction wedge, facilitated by subduction roll-back, provided a source for the alkalic rocks with mixed geochemical characteristics.

Shallow structure and active tectonism of an offshore continental back-arc spreading system: the Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ), the predominant active volcano-tectonic structure within the New Zealand sub-continent, marks the zone of Quaternary back-arc spreading and calc-alkaline volcanism associated with the convergence of the Pacific and Australian plates at the Hikurangi plate-boundary margin, North Island, New Zealand. New bathymetric, shallow seismic reflection, magnetic anomaly, and GLORIA data presented here accurately define for the first time the extent, structure and tectonism of the poorly known offshore portion of the TVZ. These data show that nearly half of the entire ca. 40 km wide TVZ lies offshore, extending northeast for some 150 km seaward to 36°45′S. The southeastern and northwestern boundaries of the offshore TVZ are significant tectonic and bathymetric features, which comprise inward facing normal fault zone (the White Island and Tauranga Fault Zones, respectively). Three, essentially parallel, tectono-morphological units are identified along the length of the offshore TVZ. They comprise (from southeast to northwest) a frontal non-volcanic graben, a volcanic ridge, and a volcanic “back-arc” graben. The spatial position of these units with respect to the Hikurangi subduction margin is not a simple “arc” and “back-arc” relationship. Tectonism within the offshore TVZ, although pervasive, increases from northwest to southeast, so that the area of highest extension and subsidence of the non-volcanic graben occurs in front of the volcanic ridge. Geophysical data also suggest the possible tectonic ablation of blocks of the forearc hinterland into the offshore TVZ along its southeastern margin. The southern Havre Trough and northern TVZ are offset sinistrally by 45–50 km. They overlap by 40 km, and differ in orientation by ca. 7°. Dextral displacement between these two extensional regimes is accomodated by a series of dextrally oblique and en-échelon “bookshelf” faults. The orientation of extensional structures within the offshore TVZ is also dextrally oblique, by 10°–15° to the regional 030° trend of the bounding faults, and agree with axes of maximum extension determined elsewhere from geodetic and seismological data.

Submarine volcanism in the Sumisu Rift, Izu-Bonin arc: submersible and deep-tow camera results

Multi-vent volcanic extrusions in the Sumisu back-arc rift have formed individual cones in the outer rift and short segment en echelon ridges in the inner rift. Alternating or coincident phases of constructional volcanism and mass wasting have produced a terrain consisting mainly of large basalt pillow lavas, lesser amounts of fresh-looking lobate, ropy, and sheet flows along the ridge crests, and volcanic debris ubiquitous throughout the rift. A rhyolite volcano and inactive hydrothermal chimneys occur at the step in the largest en echelon ridge. At present, constructional volcanism predominates over tectonism on the features studies. The development of en echelon basaltic ridges is greater in other western Pacific back-arc rifts with more extension. Continued extension in Sumisu Rift may cause a shift in the concentration of volcanism from the cross-rift trend to an along-axis orientation, eventually leading to the coalescing of the inner rift volcanic ridges into a back-arc spreading system.

南太平洋の背弧海盆における海底拡大系の微細構造 ビスマルク海およびラウ海盆での海底地震観測研究

To identify the detailed nature of the extensional opening system of two active back-arc basins in south Pacific, i.e. the Bismarck Sea and Lau Basin, we deployed the local seismic array of newly developed pop-up type Ocean-Bottom-Seismometers (OBS's). Using P- and S-wave arrival time data, hypocenters of more than two hundred small earthquakes were located with high resolution in both of the two back-arc basins.In the western survey area in the Bismarck Sea, a linear N65°W zone of OBS-located earthquakes defines the major transform fault narrower than 3km. In the eastern survey area, P-wave first motions of the events along stepping WNW-ESE seismic segments suggest the left-lateral strike-slip type focal mechanisms with N20°W T-axes. Therefore, it is inferred that small spreading ridges of en echelon type exist connecting the stepping WNW-ESE seismic segments.In the Lau Basin, an NNW linear trend of OBS-located seismic events has identified, which is continuous to the NW-SE seismic zone along the Peggy Ridge. The seismic trend is composed of several short seismic segments, suggesting a left-stepping en echelon crustal opening system with the NW-SE spreading.Thus we identified the fine-scaled structure of the back-arc spreading systems in the basins. Our result provides useful information of the location of hydro-thermal activity associated with the obliquely extensional spreading.

Microearthquakes along the back-arc spreading system in the eastern Bismarck Sea

OBS’s were deployed for 26 to 29 days in the eastern Bismarck Sea to investigate the back-arc spreading. Hypocenters of 186 shallow earthquakes were determined using P- and S-waves from at least five stations. In the western survey area, a transform fault zone is marked by a linear micro-earthquake activity striking N65°W and less than 5 km wide. The predominant type of their focal mechanisms is strike-slip. In the eastern area, several intermittent zones of micro-earthquakes and their strike-slip type focal mechanisms suggest the location of short-length transform faults separating en-echelon spreading ridges.