Tectonic implications of Early Miocene OIB magmatism in a near-trench setting: the Outer Zone of SW Japan and the northernmost Ryukyu Islands

Kazuo Kiminami^a, Teruyoshi Imaoka^a, Kazuki Ogura^b, Hiroshi Kawabata^c, Hideo Ishizuka^d & Yasushi Mori^e

^aDepartment of Geosphere Sciences, Faculty of Science, Yamaguchi University, Yamaguchi 753-8512, Japan

^bGraduate School of Integrated Arts and Sciences, Kochi University, Kochi 780-8520, Japan

^cMultidisciplinary Science Cluster, Kochi University, Kochi 780-8520, Japan

^dDepartment of Geology, Kochi University, Kochi 780-8520, Japan

^eKitakyushu Museum of Natural History and Human History, 2-4-1 Higashida, Yahatahigashi-ku, Kitakyushu 805-0071, Japan

Corresponding author. Tel.: +81 83 933 5765; fax +81 83 933 5273
E-mail address: imaoka@yamaguchi-u.ac.jp (T. Imaoka)

This webpage is a summary of: Kiminami, K., Imaoka, T., Ogura, K., Kawabata, H., Ishizuka, H., and Mori, Y., 2017, Tectonic implications of Early Miocene OIB magmatism in a near-trench setting: the Outer Zone of SW Japan and the northernmost Ryukyu Islands: Journal of Asian Earth Sciences. 135, 291-302, doi: org/10.1016/j.jseaes.2016.12.033

1. Introduction

Miocene magmatism in SW Japan has previously been discussed with regard to subduction of the young Philippine Sea Plate (PSP) and opening of the Sea of Japan, which resulted in trench-ward motion of SW Japan. However, the influence of geodynamics on magmatism in the region is still debated (Kimura et al., 2005; 2014). Because of the position of the Neogene-Quaternary arc, Miocene magmatism in the Outer Zone of SW Japan is inferred to have occurred in the fore-arc region (e.g., Shinjoe, 1997). In the Outer Zone of SW Japan and the northernmost Ryukyu Islands, Early Miocene basalt and lamprophyre dikes with alkaline affinities have been reported from Shingu-Otoyo area in northern Shikoku (Uto et al., 1987) and from Tanegashima at the northern end of the Ryukyu Islands (Ogasawara, 1997) (Figure 1).

Figure 1: Map showing the locations of the SW Japan and Ryuku arcs, and associated tectonic and geographical features. Sampling locations are also shown, indicated by red stars. Inset map shows the location and tectonic setting of SW Japan. Depth contours to the upper boundary of the subducting Philippine Sea slab are after Hirose et al. (2008) and Huang et al. (2013). MTL = Median Tectonic Line; YF = Yangsan Fault; TGUF = Tsushima-Goto-Ulleung Fault; IA = Izumo area; MA = Mizunami area; SA = Shitara area; WA = Wajima area.

The occurrence of mafic dikes with alkaline affinities in a near-trench setting is unusual and enigmatic with regard to plate tectonic processes, because the subducting oceanic plate is presumed to have been shallow during the Early Miocene and would act as a barrier to magma rising vertically from great depth. Here, we discuss the timing of the trench-ward motion of SW Japan, its effect on the geometry of the subducting slab, and the tectonic implications of the presence of mafic dikes with alkaline affinities in the near-trench environment during the Early Miocene.

2. Geological background

The SW Japan and Ryukyu arcs are divided by the Median Tectonic Line (MTL) into the Inner and Outer zones. The Outer Zone is underlain mainly by late Paleozoic to Cenozoic accretionary complexes and post-Paleogene igneous rocks. The following tectonostratigraphic units of the accretionary complexes occur from north to south (Figure 2):

Figure 2: Map showing the occurrences of Middle Miocene igneous rocks in the Outer Zone and Setouchi area (compiled from Uto et al., 1997; Kimira et al., 2005, etc.) and sampling locations in the Shingu-Otoyo area and Tanegashima, and the geotectonic divisions of the Outer Zone. The location of the Oligocene magmatic front is after Sakai (1994) and Imaoka et al. (2011). MTL = Median Tectonic Line; BTL = Butsuzo Tectonic Line.

It is well known that Middle Miocene felsic to intermediate volcano-plutonic complexes occur in the Outer Zone from central Japan to the Ryukyu Islands. The basaltic dike in the Shingu-Otoyo area yields a well-defined ⁴⁰Ar/³⁶Ar-⁴⁰0K/³⁶Ar isochron age of 17.7 ± 0.5 Ma (Uto et al., 1987) and contains microdiamond-bearing peridotite xenoliths (Mizukami et al., 2008). The Tanegashima lamprophyre gives a hornblende K-Ar age of 18.2 ± 0.9 Ma (Ogasawara, 1997). This Early Miocene mafic magmatism pre-dates the felsic-to-intermediate magmatism in the Outer Zone.

3. Whole-rock chemistry

The Shingu-Otoyo basalts and the Tanegashima lamprophyre show high concentrations of HFSEs, e.g., TiO₂ = 1.9-2.1 wt.% and 2.5-3.4 wt.%, respectively, Nb = 51-60 ppm and 50-55 ppm, respectively, Ta = 3.4- 4.3 ppm and 3.4-3.9 ppm, respectively, and Zr = 168-219 ppm and 209-275 ppm, respectively. Figure 3 shows a recently proposed Nb_N vs. Th_N discrimination diagram (after Saccani, 2015) that enables basalts generated at divergent plate and within-plate settings to be distinguished from basalts generated at convergent plate settings. The Shingu-Otoyo basalts and Tanegashima lamprophyre plot near the average ocean island basalt (OIB) composition (Sun & McDonough, 1989) in the alkaline OIB field, which confirms the OIB nature of these rocks.

Figure 3: Nb_N vs. Th_N diagram (Saccani, 2015) showing data of the Shingu-Otoyo alkaline basalts and Tanegashima lamprophyre. Th and Nb are normalized to the N-MORB composition of Sun & McDonough (1989).

4. Discussion

4.1. Timing of the extensional regime and opening of the western part of the Sea of Japan

On the basis of Miocene paleomagnetic data from SW Japan, Hoshi & Sano (2013) and Hoshi et al. (2015) concluded that clockwise rotation of SW Japan occurred predominantly during the interval 17.5-15.8 Ma. This conclusion is consistent with that obtained from geochronological and paleomagnetic data from Miocene rocks in central SW Japan by Sawada et al. (2013). We adopt the hypothesis of Hoshi et al. (2015) for the timing of clockwise rotation of SW Japan.

Numerous studies (e.g., Kano et al., 2007; Ninomiya et al., 2014; Yoon et al., 2014) concerning geochronology, geochemistry, and tectonic regime in and around the western part of the Sea of Japan have revealed that an extensional regime in the region commenced during the early stage of the Early Miocene, if not before. We presume that the clockwise rotation of SW Japan was preceded by the onset of extensional tectonics in the western part of the Sea of Japan.

The timing of extensional tectonics in the western part of the Sea of Japan is an important factor in determining the nature of the boundary between the Eurasian Plate and the PSP because extensional tectonics with the trench-ward motion of SW Japan would have induced convergence between the both plates. Thus, we suggest that the PSP has been subducted since the Early Miocene.

4.2. Configuration of the subducting PSP and the overriding plate during the Early Miocene

Present-day depths to the top of the subducting slab are ~30-35 km (Hirose et al., 2008; Ito et al., 2009) around the Shingu-Otoyo area, and ~40 km at Tanegashima (Hirose et al., 2008). It is reasonable to assume that these depths are greater than those during the Miocene, because of subsequent accretion. Present-day crustal-scale cross sections illustrate a direct contact between the Shimanto accretionary complex and the subducting slab, with no lower crust and mantle wedge beneath much of the Outer Zone in Shikoku (Igarashi et al., 2011). Thus, it is highly likely that the crust, consisting predominantly of the Shimanto accretionary complex, was coupled with the top of the subducting Philippine Sea slab beneath most of the Outer Zone in the western part of SW Japan and the northernmost Ryukyu Islands during the Early Miocene. Alternatively, if there was a mantle wedge it was very thin and would have been restricted to the northern margin of the Outer Zone.

Statistical analyses of modern subduction zones, analogue experiments, and numerical simulations indicate that trench-ward motion of the overriding plate leads to shallowing of the angle of subduction (e.g., Heuret et al., 2007). It is well known that the present-day PSP is being underthrust beneath Shikoku at a low angle (Hirose et al., 2008; Huang et al., 2013). In contrast, the subduction angle of the PSP beneath the northernmost Ryukyu arc is higher than that beneath Shikoku. Some authors have proposed that the slab beneath the northernmost Ryukyu arc steepened to its modern geometry after 4 Ma (e.g., Watanabe, 2005) suggesting shallower subduction during the Miocene. We envisage that the Early Miocene trench-ward motion of SW Japan produced a shallowly dipping slab.

4.3. OIB-type magmatism related to “petit spots” or a slab tear in the near-trench environment

The Early Miocene configuration of the subducting and overriding plates allows us to attribute the mafic magmatism with OIB signatures to the upwelling of asthenospheric mantle from beneath the subducting slab, through pathways such as fractures or tears. The cause of the fractures or tears remains unknown, but we propose the following two plausible scenarios. First, the shallowing of the subducted PSP would have caused the slab to flex concavely, as shown in Figure 4a. Concave-upward bending of the subducting plate may have led to brittle fracturing parallel to the hinge line in the flexed region. This could have led to subsequent upwelling of asthenospheric mantle through the fractures, in a similar manner to that illustrated by Hirano et al. (2006).

Figure 4: Schematic diagrams showing two possible scenarios (a and b) that could explain the alkaline mafic magmatism in the Outer Zone of the SW Japan and northernmost Ryukyu arcs. In scenario a, shallowing of the subducted PSP causes the slab to flex in a concave-upwards manner, which leads to brittle fracturing parallel to the hinge line and subsequent upwelling of asthenospheric mantle through the fractures. Scenario b involves segmentation of the down-going Philippine Sea slab through slab tearing, induced by differential motion within the slab, and subsequent upwelling of asthenospheric mantle through the tears. Vertical arrows in figure b show the differential motion of the subducting slab.

The second scenario involves vertical tears of the subducting slab. Some authors have suggested segmentation of the down-going PSP through slab tearing or fracturing beneath SW Japan (e.g., Huang et al., 2013; Cao et al., 2014). In general, slab tearing induces the upwelling of asthenospheric mantle from beneath/behind the slab. We suggest that vertical tearing of the subducting Philippine Sea slab, induced by one or more of the mechanisms, along with brittle fracturing caused by flexure of the slab, played an important role in the formation of OIB-type alkaline magmas during the Early Miocene (Figure 4b).

Vertical tears and fractures in an oceanic plate or subducting slab produce coeval magmatic rocks with OIB signatures. In Figure 3, Nb_N and Th_N values of the Early Miocene mafic dikes from the Outer Zone are compared with those of off-ridge alkaline basalts in the Shikoku Basin (Hickey-Vargas, 1998), alkaline basalts from “petit spot” volcanoes (Hirano et al., 2006), vertical-tear-related alkaline basalts from the Southern Tyrrhenian Basin (Trua et al., 2003) and the Garibaldi Volcanic Belt in the northern Cascade Arc (Mulllen & Weis, 2015). Most lavas originating from off-ridge magmatism in the Pacific Ocean show N-MORB or E-MORB geochemical signatures (e.g., Geshi et al., 2007). The Early Miocene basalts and lamprophyre fall in the same field as alkaline basalts from “petit spot” volcanoes. Tear-related alkaline lavas from the Southern Tyrrhenian Basin have higher Th_N values for a given Nb_N value compared with the samples from the alkaline Shingu-Otoyo basalts and the Tanegashima lamprophyre. This is in agreement with the involvement of a subduction-related component in the genesis of alkaline lavas from the Southern Tyrrhenian Basin, as noted by Trua et al. (2003). Tear-related volcanic rocks from the Garibaldi Volcanic Belt fall largely in the AB field, and display somewhat lower enrichments of OIB-type components than the samples from the Early Miocene mafic dikes in the Outer Zone. Thus, we favor the idea that the Early Miocene OIB magmatism in the near-trench environment of the SW Japan and northernmost Ryukyu arcs occurred in response to “petit spot” or slab-tear-induced magmatism.

5. Conclusions

The Early Miocene OIB-type magmatism in the near-trench environment of the SW Japan and northernmost Ryukyu arcs is most plausibly explained by the upwelling of asthenospheric material from beneath the subducting slab, which migrated through fractures and/or tears in the slab. The fractures or tears may have formed via two mechanisms:

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