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Geological settings of North-West Pacific

The oceanic crust in the North-Western Pacific plate off Japan was formed about 125-140 Ma at the Pacific-Izanagi plate boundary (Nakanishi et al., 1989). The area is covered by pelagic sediments 1.6 km thick and subducts beneath the Japan islands at a speed of 70-90 km/Ma (Seno, 2017). A thicker and wider accretionary prism is observed in the northern Japan Trench (Kodaira et al., 2017). The free-air gravity anomaly locally delineates seamounts and fracture zones (Fig. 2-1A). In the whole area, it displays a negative anomaly associated to the trench and a positive anomaly on the flexural bulge ~150 km seaward. This bulge reflects the bending of the subducting plate and induces horsts and grabens as the plate approaches the trench (Tsuru et al., 2002; Kodaira et al., 2017).

A new magnetic anomaly map on the japan trench area

We compiled both scalar and vector marine magnetic data available in the study area and merged them into a unique scalar magnetic anomaly map (Fig. 2-1B). Many gaps in the scalar anomaly coverage could be filled with the vector data. The resulting map shows two types of anomalies. Near the Japanese Islands, a strong positive anomaly (FIMAB in Fig. 2-1) is caused by the induced magnetization of serpentinite in the fore-arc mantle (Okubo and Matsunaga, 1994; Hyndman and Peacock, 2003; Blakely et al., 2005) and, possibly, of the volcanic arc. On the Pacific plate and subducting slab, alternating positive and negative lineated anomalies are caused by the remanent magnetization of the oceanic crust. Magnetic anomalies M5 to M17 (~124.6-139.7 Ma; Malinverno et al., 2012) are identified between two NNW-SSE-trending fracture zones depicted on the free-air gravity anomaly (Fig. 2-1A), confirming the interpretation of Nakanishi et al., 1989).
In our study area, only anomalies M8r-M10r (129.0-130.8 Ma) are suitable for a detailed analysis as they are linear and only disrupted by a few isolated seamounts. They display a clear seafloor spreading magnetic signal before and after subduction (Fig. 2-1A). Conversely, anomaly M5 is still recognizable but totally subducted, while anomalies M6-M7 are only partially subducted beneath the southern Kuril Trench, roughly parallel to the magnetic anomaly trend. The presence of propagating rifts (Nakanishi, 2011) and the Joban seamount chain affect anomalies M10n1-M15 south of 38°40’N. During this period the (half) spreading rate was 70-80 km/Ma and the studied oceanic crust was formed at a fast spreading center (Nakanishi et al., 1989).

Magnetic structure of the oceanic crust and progressive demagnetization

The amplitude of the slab magnetic anomalies continuously decreases landward and the anomalies disappear beyond 20 km depth below sea level (bsl) (Fig. 2-1C). The increasing depth of the magnetized source preferentially attenuates the shorter anomaly wavelengths of the anomaly, as does demagnetization of the oceanic crust induced by fluids (alteration) and increasing temperature (thermal demagnetization). To separate the effects of increasing depth and demagnetizationthese effects, we compute synthetic magnetic anomalies assuming unchanged magnetic structure of the subducting plate. The observed anomalies decrease faster than the synthetic ones (Fig. 2-2C). Indeed, the RAM decays rapidly, by 20% per km, between 9-12 km bsl, and more slowly, by 2% per km, beyond (Fig. 2-3A).
The observation of two distinct depth intervals with contrasted RAM decay is in good agreement with our knowledge of the magnetic structure model of oceanic crust formed at fast spreading centers, where two magnetic layers are distinguished (Dyment and Arkani-Hamed, 1995). The shallower one, less than 1~1 km thick (Karson, 2002), is made of extrusive basalt which magnetic mineral is titanomagnetite with various Ti content and oxidation state (Curie temperature Tc 100-350°C, Zhou et al., 2001; Gee and Kent, 2007), the deeper one, ~5 km thick, is made of dolerite, gabbro and serpentinized peridotite in which magnetic mineral is magnetite (Tc 580°C) is the dominant magnetic phase. The Natural Remanent Magnetization (NRM) carried by these layers varies: the extrusive layer bears a strong NRM (> 10 A/m) at the ridge axis which decays to ~3 A/m for old oceanic crust. It is the main contributor to the lineated marine magnetic anomalies observed at sea-surface. Conversely, the dolerite and gabbro bear a weaker NRM (~1 to 1.5 A/m) and the serpentinized peridotite variable NRM (0 to 6 A/m) depending the degree of serpentinization (Harrison, 1987). We, therefore, suggest that the sudden decay of RAM between 9-12 km bsl corresponds to the thermal demagnetization of the extrusive basalt layer and the slow decrease beyond 12 km bsl to that of the deeper layers (Fig. 2-3B). Observed magnetic anomalies beyond 18 km bsl are very low and their shape does not match that of the synthetic ones, precluding the calculation of RAM. Their short wavelength content is not consistent with the expected depth of subducting slab, suggesting the presence of shallower sources in the upper plate continental crust.

Thermal structure and earthquakes in the shallow subducting oceanic crust

A global compilation over subduction zones suggests that, apart for local effects, heat flow over oceanic lithosphere entering subduction does not significantly deviate from that of normal oceanic lithosphere (Stein, 2003), whereas a more recent study supports higher heat flow over oceanic lithosphere approaching subduction (Harris et al., 2017). Conversely, heat flow over the fore-arc basin is generally low (Stein, 2003; Yamano et al., 2014). Heat flow measurements on the Pacific plate off the Japan Trench range between 50 and 100 mW/m2, significantly higher than the ~50 mW/m2 expected for oceanic lithosphere of this age (Yamano et al., 2014).
Estimating the slab thermal structure is difficult because the hydrothermal circulation in the accretionary prism and overriding continental crust is hard to quantify. Proposed models consider constraints such as the age of oceanic lithosphere, rate of convergence, shear heating and dip of the slab (van Keken et al., 2012; Wada et al., 2015). However, strong uncertainties remain in the shallow subduction zones on the effectiveness of hydrothermal circulation, the permeability of the igneous crust, and its evolution with depth. Consequently, models propose a wide variety of temperature ranges for the shallow subducting slab. For instance, the obtained temperatures are as low as ~100-200 °C from the deformation front to 20 km depth (Hyndman and Peacock, 2003; van Keken et al., 2012; Wada et al., 2015), a consequence of the rapid convergence of old oceanic crust (van Keken et al., 2012). Our study suggests that the Curie temperature of titanomagnetite, within the range 150-350°C, is reached by the extrusive basalt layer at 9-12 km bsl, and the Curie temperature of magnetite, 580°C, is reached by the deeper crustal layers when the slab surface is at 20 km bsl (i.e. at ~22-26 km bsl considering the initial depth of these layers). These temperatures are significantly higher than those predicted by most published models. They agree better with the background thermal gradient of 26.29° ± 0.13°C/km measured on the overriding plate near the trench (Fulton et al., 2013), which predicts 170°C at 10 km bsl (6.5 km below seafloor) and 460°C at 20 km bsl (17.5 km bsf).

Magnetic structure of the subducting plate before entering the japan-kuril trench

The Pacific plate and associated subducting slab display NNE-SSW magnetic anomalies of Early Cretaceous age (see above). The southern Kuril Trench is roughly parallel to these anomalies, with anomaly M5 (124.58-126.05 Ma) recognizable although totally subducted whereas anomalies M6 and M7 (127.2-128.54 Ma) are only partially subducted. Conversely, the Japan Trench is oblique to anomalies M8 to M11 (128.5-132.67 Ma), which are progressively fading away after passing the Trench to disappear ~100-120 km landward (Okubo et al., 1991). These observations result both from the increasing distance to the magnetized source and to thermal demagnetization of the oceanic magnetic layers (Choe and Dyment, 2019). On the plate prior subduction, the magnetic anomalies are slowly fading away while approaching the trench. To evaluate quantitatively the amplitude variation of these anomalies, we selected data showing clear seafloor spreading magnetic anomalies both in amplitude and wavelength (i.e. 20-40 km), in areas of dense data coverage. Profiles perturbed by local complexities such as seamounts, propagators and fracture zones are discarded.
Investigating the same anomaly reduces possible biases related to paleomagnetic field intensity variation with time. We limited our study to magnetic anomalies M8r to M10Nr (129.0–132.0Ma) off the Japan Trench. We compared the observed magnetic anomaly profiles to synthetic ones computed by assuming that (1) the basement is the top of a 0.5 km-thick magnetized extrusive basalt layer, and (2) the magnetization is the average of all equivalent magnetization profiles inverted from the observed anomaly profiles with the same local geometry. Figure 3-2 presents the example of magnetic anomaly M10 (133.45-133.49Ma), which shows a good fit between the observed and synthetic anomalies East of the outer rise and an increasing misfit between the outer rise and the trench (Fig. 3-2C). We computed the ratio of peak to trough anomaly amplitudes of the observed and synthetic anomalies, hereafter named RAM (Remaining Amount of Magnetization). Figure 3-3A represents the RAM versus the distance to the closest outer rise for all anomalies and, despite some scatter, shows a significant decrease of the RAM toward the trench. We averaged the RAM at 25 km intervals and adopted the corresponding standard deviation as the uncertainty (Fig. 3-3A). The average RAM is constant at 0.93 East of the outer rise and decreases exponentially to 0.7 from the outer rise to the trench (Fig. 3-3A).
The decreasing RAM reflects a loss of magnetization occurring between the outer rise and the trench and can be attributed to two main processes, the alteration and thermal demagnetization of magnetic minerals.

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Decay of magnetization before subduction: alteration as the main process

Two major processes can be inferred to explain the decay of marine magnetic anomalies between the outer rise and the trench. The first one, thermal demagnetization, relates the loss of magnetization to an increase of temperature: the magnetic minerals are heated beyond their Curie temperature and loose their magnetization (Okubo et al., 1991). The second process, alteration, considers that hydrothermal circulation oxidizes the magnetic minerals which transform to less or non-magnetic minerals (Tivey et al., 1993; Dyment et al., 2015).
Moreover, recent heat flow data show values close to the global average (~50Wb/m2) East of the outer rise and scattered, in average higher values between the outer rise and the trench (Yamano et al., 2014; Fig. 3-3B). The higher heat flow observed over the oceanic crust approaching the trench suggests high temperatures that may support thermal demagnetization of titanomagnetite, the magnetic bearer of extrusive basalt that exhibits a strong magnetic intensity but a low Curie temperature (150-350°C; Zhou et al., 2002; Gee and Kent, 2007). However, a recently published thermal structure model (Kawada et al., 2014) concluded that the additional hydrothermal circulation which causes the higher and more scattered heat flow measurements of Yamano et al. (2014) only heats up the oceanic crust at 1.5 km depth by 40°C, insufficient to reach the Curie temperature and significantly demagnetize the plate. We therefore consider that thermal demagnetization only plays a minor role in the observed decay of the marine magnetic anomalies before entering subduction.
Recent active seismic reflection profiles show that the Vp/Vs ratio strongly increases and the high Vp/Vs layer thickens up to 3.5km-deep below seafloor (bsf) from the outer rise to the trench as the hydrothermal activity and subsequent alteration significantly increase (Fujie et al., 2018; Fig. 3-3C). These results and the heat flow measurements (Yamano et al., 2014) support rejuvenation of the hydrothermal circulation within the old oceanic crust. Seawater penetrates the pelagic sediments and the crust along normal faults and fissures opened by the flexure of the oceanic lithosphere at the outer rise as it is observed in most subduction zones (Contreras-Reyes et al., 2008; Shillington et al., 2015; Fujie et al., 2018).
The decay of RAM is therefore most likely caused by the flexure of the lithosphere approaching the trench, the resulting normal faulting and fissuring, the associated rejuvenated hydrothermal circulation, and finally the alteration of the magnetic minerals (Fig. 3-4). It is interesting to note that the decay seems to attenuate from the outer rise to the trench, as does the increasing crustal hydration estimated from Vp/Vs values (Fig. 3-3C). Both the crustal hydration and the magnetic mineral alteration are fast when the faults and fissure open, and tend to saturate with time, when hydrated and altered minerals cover the cracks and their vicinity. For this reason, the RAM decay is much more subdued near the trench (Fig. 3-3A).

An integrated magnetization model of the subducting plate

This study and our previous work (Choe and Dyment, 2019) offer an integrated view on the magnetic structure of the Pacific Plate before and after entering subduction in the Japan-Kuril Trench. Here we show that, after passing the outer rise, the magnetic anomaly amplitude decreases by roughly 20% due to the alteration of magnetic minerals induced by rejuvenated hydrothermal circulation. From our previous study, half of the remaining 80% is further erased between 9-12 km depth of the slab surface below sea level (bsl) due to the thermal demagnetization of the extrusive basalt layer above the Curie temperature of titanomagnetites (150-350°C). The last 40% are finally removed between 12-20 km depth of the slab surface bsl by thermal demagnetization of the deeper crust above the Curie temperature of magnetite (580°C; Gee and Kent, 2007).
Heat flow, Vp/Vs structure, and magnetization decay concur in supporting rejuvenated hydrothermal circulation and the subsequent alteration as an important process on the plate before subduction. As a consequence, the crust entering subduction is already very altered, and it is therefore unlikely that the drop of magnetization observed between 9 and 12 km may be due to any vigorous hydrothermalism and alteration. Thermal demagnetization appears as the only process able to generate such a magnetization drop. A major difference between the oceanic crust before and after subduction is, the heat mined by hydrothermal circulation before subduction is released to the ocean as the hydrothermal system is open; the thermal structure is only marginally affected, as noted by Kawada et al. (2014). Conversely, after subduction, the hydrothermal system is closed. The seawater trapped in the thick aquifer of Kawada et al. (2014) continues to mine the heat from depth but cannot escape due to the overlying accretionary prism, pelagic sediments and the possibly impermeable decollement surface, resulting in the same fluid convecting and heating the oceanic crust by thermal blanketing (Granot and Dyment, 2019). As a result, the heat flow measured on the seafloor is low (Kawada et al., 2014; Yamano et al., 2014).

Table of contents :

1.1 Motivation
1.2 Subduction zones
1.3 Research framework
2.1 Abstract
2.2 Introduction
2.3 Geological settings of North-West Pacific
2.4 Data and methods
2.5 A new magnetic anomaly map on the japan trench area
2.6 Magnetic structure of the oceanic crust and progressive demagnetization
2.7 Thermal structure and earthquakes in the shallow subducting oceanic crust
2.8 Acknowledgement
2.9 Supplementary materials
2.9.1 Crossover correction for scalar and vector magnetic anomalies
2.9.1 Uncertainties in the slab geometry
2.9.1 Effect of a dipping slab on the inclination and declination of magnetization, consequences on the amplitude of magnetic anomalies
3.1 Abstract
3.2 Introduction
3.3 Data and methods
3.4 Magnetic structure of the subducting plate before entering the japan-kuril trench
3.5 Decay of magnetization before subduction: alteration as the main process
3.6 An integrated magnetization model of the subducting plate
3.7 Acknowledgement
4.1 Abstract
4.2 Introduction
4.3 Data and methods
4.3.1 Selection of study area
4.3.2 Magnetic data collection and processing
4.3.3 Basement geometry data and processing
4.3.4 Remaining amount of magnetization
4.4 Results
4.4.1 Magnetization loss in old oceanic lithosphere
4.4.2 Magnetization loss in young oceanic lithosphere
4.5 Discussion
4.5.1 Comparison betwee the two categories Before subduction After subduction
4.5.2 Discussion for the other subduction zones
4.6 Acknowledgement
4.7 Supplementary materials
4.7.1 Japan-Kuril
4.7.2 West Aleutian
4.7.3 Central Aleutian
4.7.3 South Alaska
4.7.5 Cascadia
5.1 New high-resolution magnetic anomaly map
5.2 Decaying magnetic signals in subduction zones
5.3 Perspectives
A1.1 Introduction
A1.2 Proton precession magnetometer
A1.3 Shipboard three-component magnetometer
A2.1 Introduction
A2.2 Misfit analysis


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