Landward migration of active folding based on topographic development of folds along the eastern margin of the Japan Sea, northeast Japan

Quaternary International 397 (2016) 563e572

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Landward migration of active folding based on topographic development of folds along the eastern margin of the Japan Sea, northeast Japan Makoto Otsubo*, Ayumu Miyakawa Geological Survey of Japan, AIST, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 25 December 2015

The eastern margin of the Japan Sea, northeast Japan, lies in a strongly compressional area, and contractional deformation in the region is ongoing. We reconstructed the temporal and spatial evolution of contractional deformation across the Tohoku district on the eastern margin of the Japan Sea, by using surface geologic and geomorphologic data. We measured the distance from the fold hinge to the topographic ridge line in antiforms, developed by surface uplift, associated with fold growth under the EeW to WNWeESE compressional stress regime. The topographic development of fold structures, developed in the region since the Pliocene, is consistent with the activity of the folds. When the folding that generates the uplift becomes inactive, the topographic contrasts between hinge and ridge location can be remarkable. Spatial variations in this parameter are consistent with the systematic eastward migration of fold growth in the area. The documentation of fold-related topography thus provides important information for understanding the dynamics of folding along the eastern margin of the Japan Sea. © 2015 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Active fault Earthquake Erosion Fault-related fold Reverse fault Tohoku

1. Introduction The Quaternary tectonics of the Japan island arc are characterized by strong crustal deformation, whose mode and rate are quite different from those of the preceding late Pliocene. The Tohoku district, on the eastern margin of the Japan Sea, lies within a strongly compressive area (Fig. 1) that has been experiencing large, contractional, crustal deformations, since the late Pliocene (Sato and Amano, 1991; Sato, 1994). Fold-and-thrust structures (Sato, 1989) and foldetopographic structures with distributed reverse faults (Okamura et al., 1995; Okamura, 2002; Sato et al., 2002) have developed in response to this contractional deformation. Geodetic surveys in the Tohoku district have detected zones with a high rate of horizontal strain (Sagiya et al., 2000). Within these zones, a number of large, reverse-faulting earthquakes have occurred in the upper crust over the past 10 years (Fig. 1; e.g., Sibson, 2009). When the rates and kinematics of contemporary contractional deformation are similar to those of the long-term regime, the high-strain-rate zones observed at

* Corresponding author. E-mail address: [email protected] (M. Otsubo). http://dx.doi.org/10.1016/j.quaint.2015.11.019 1040-6182/© 2015 Elsevier Ltd and INQUA. All rights reserved.

geological (105
6 years) and geodetic (10 0
2 years) scales should coincide. However, the high horizontal strain-rates recognized at geological and geodetic time-scales are spatially heterogeneous. Rates are consistent in the Niigata region, in the southwestern part of the Tohoku district, but not in the Akita region in the northwestern part of the district (Fig. 1). The accumulation and release of crustal strain in and along a subduction zone are important drivers of active onshore faulting (e.g., Farías et al., 2011). The difference in location between the geologic and geodetic horizontal high-strain-rate zones at the eastern margin of the Japan Sea indicates temporal variations in the continuity of strain accumulation and release. The evidence at time-scales that are intermediate between the geologic and geodetic time-scales is a key to understanding these variations in continuity. However, few data at such time-scales are available for the Tohoku district. We focus here on the erosional and topographic evolution of fold structures developed since the late Pliocene in the Akita and Niigata regions. Topography has long been recognized as an expression of the interplay or coupling between surface processes and tectonic processes (for a brief review, see Merritts and Ellis, 1994). The advantages of topographic analysis are that the surface

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Fig. 1. Active tectonics of the eastern margin of the Japan Sea. (a) Seismo-tectonic sketch map showing the epicenters of recent reverse-fault ruptures in relation to basin-bounding faults, exposed areas of pre-Neogene basement, actively growing regional antiforms, Quaternary volcanoes, and the volcanic front (modified after Sato, 1994; Sibson, 2009). These features are shown in the context of the subduction plate boundary at the Japan trench and inferred trajectories of maximum horizontal stress (SHmax). N: Niigata-Kobe Tectonic Zone (Sagiya et al., 2000). T: Tohoku Mountain concentrated deformation zone (Hasegawa et al., 2005). NAM: North American Plate. PAC: Pacific Plate. KCTL: KashiwazakieChoshi (Chiba) Tectonic line (Yamashita, 1970). (b) Shortening ratios estimated from geological fold structures in each grid in the Tohoku district (modified after Sato, 1989). Shading indicates areas of thick Neogene sediments (Sato, 1994).

of the crust is easily observed, and topographic observations can be used to constrain parameters that are otherwise difficult to measure, especially when deformation is distributed (Kirby et al., 2003; Boulton and Whittaker, 2009; Barnes et al., 2011). Landforms develop and erosion occurs at an intermediate time-scale, between  and Avouac, the geological and geodetic time-scales (e.g., Lave 2000). Hills and mountains with elevations of less than 1000 m in the Tohoku district of Japan are antiforms (Fig. 2), and have developed by surface uplift associated with fold growth under an EeW to WNWeESE compressional stress regime (e.g., Sato, 1989,

1994). We would expect to find spatial heterogeneity in the elevation of ridges and the distances between the topographic ridges and corresponding fold hinges if fold growth is spatially variable. Based on the degree of fold activity, the balance between erosion and surface uplift controls the topographic development (Fig. 3; Ellis and Densmore, 2006). When tectonic displacement outpaces the rate of erosion, the topographic divide and fold hinge remain close (Fig. 3). Over time, when relief diminishes with respect to rock uplift, and the divide occupies an equilibrium

Fig. 2. Photograph (view to the east) showing the topography (antiforms) generated by folding since the late Pliocene in the southern part of Niigata region. The antiforms have elevations of ~100e700 m. The Echigo Mountains (Mesozoic basement) are more than 1500 m above sea level. PF: Fold structures (antiforms) since the late Pliocene. EM: Echigo Mountains.

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2. Geological background

Fig. 3. Cross-sectional profiles showing topographic development above a blind thrust fault (modified after Ellis and Densmore, 2006). The pattern of rate of tectonic uplift remains constant in each panel. In the topography-building states (I) and (II), rate of tectonic uplift outstrips erosion, whereas in a near-equilibrium phase (III), erosion and rock uplift are in balance. (I) When rate of tectonic uplift exceeds the rate of erosion (i.e., Tectonics > Erosion), the topographic divide (black triangle) is coincident with the fold hinge (white triangle). (II) As fold growth continues, the divide migrates toward the backlimb, creating gentler slopes in the vicinity of the blind thrust. The distance between the topographic divide and the fold hinge remains small during this stage. (III) Over time, the relief becomes subdued and the rate of rock uplift decreases; the divide occupies an equilibrium position between drainage systems with equal concavity (i.e., Tectonics ~ Erosion).

position between drainage systems with equivalent concavity, the topographic differences become large (Fig. 3). In areas of fold topography, many of these same geomorphological observations are also consistent with, and have been used to infer the lateral propagation of a fold (e.g., Keller et al., 1999). We therefore seek to understand the first-order tectonic evolution of the study area by examining the lateral geomorphological development of each fold as the fold width increases. The first-order tectonic evolution of the study area is likely to be recorded by the geomorphological development of fold structures in a transect across the arc. It appears that the across-arc scale is not the main control on the erosion of individual folds or on differences in the erosion of individual folds. In the folded area, the previous researchers' interpretation of drainage divide position further assumes that rock uplift rate (e.g., Shikakura et al., 2012), rock erodibility, and precipitation rate (e.g., Willett, 1999) are spatially uniform and that lateral advection of bedrock is unimportant (e.g., Miller et al., 2007). In this study, we reconstruct the temporal and spatial evolution of contractional deformation across the Tohoku district on the eastern margin of the Japan Sea, by using surface geologic and geomorphologic data in the Akita and Niigata regions. The overall objectives of are to (1) analyze the topographic features of folded rocks, on the basis of fold hinge-line trends and topographic data; (2) characterize styles of fault-related fold development during contractional deformation; and (3) reveal the relationship between geodetic time-scale deformation and geological time-scale deformation.

The Tohoku district is a part of the northeast Japan arc, and is located along the convergent boundary between the North American plate and the Pacific plate (Fig. 1). The compressional deformation in the northeast Japan arc is attributed to the subduction of the Pacific Plate. The geological record in the Tohoku district indicates that the region has been undergoing active compressional inversion since the late Pliocene (~3.5 Ma) (Sato, 1994; Okamura et al., 1995). Extensional back-arc rifting in the early to middle Miocene (22e14 Ma), associated with the opening of the Japan Sea, led to the formation of normal faults that trend NeS to NEeSW and that bound deep sedimentary basins. Some of these basins still contain less than 6 km of Neogene sediments and volcanics (Yamaji, 1990; Sato and Amano, 1991). The blind fault structures under the thick sedimentary layers have been reactivated as reverse faults in the present compressional stress regime, and accommodate much of the ongoing shortening across the arc (Sato, 1994). The present rate of crustal deformation in the Japan arc is constrained by geodetic and seismological data. The present-day regional stress in the Tohoku district is an EeW compressional stress, characterized by reverse-faulting (e.g., Terakawa and Matsu'ura, 2010). The NiigataeKobe tectonic zone (NKTZ) is a zone of high strain rate in the Niigata region of central Japan (Fig. 1; Sagiya et al., 2000), characterized by folds developed since the Pliocene (Fig. 1). In the central and northern Tohoku districts, there is also a high-strain-rate zone on both sides of the volcanic front (Fig. 1; Sagiya et al., 2000; Hasegawa et al., 2005). Across the region, fold structures are widely developed in the sedimentary basins between the volcanic front and the eastern coast of the Japan Sea. The Akita and Niigata regions (Fig. 4) are typical areas of postPliocene fold-and-thrust structures in the Tohoku district. The fold axes in these regions are oriented NeS to NEeSW (Sato, 1989, 1994; Okamura et al., 1995; Sato et al., 2002). There are longer folds, with axial lengths of 10e20 km and wavelengths of 3e5 km, and smaller folds, with axial lengths of 2e3 km and wavelengths of 2e3 km (Suzuki et al., 1971). The smaller folds are generally kink folds with sub-horizontal hinge lines and interlimb angles of 90 e120 , whereas most of the larger folds are box folds (Suzuki et al., 1971). The smaller folds are the dominant fold structures in the study area. Most of the folds are cylindrical folds and asymmetric and verge westward or eastward (Fig. 4c). The hinge lines are only slightly offset from the crests relative to the wavelengths of the folds (e.g., Imamura and Iwata, 2004; Sato et al., 2004; Sato and Kato, 2005, 2010). Some of the folds on the eastern margin of the Japan Sea are currently active (Ota and Suzuki, 1979). Awata and Kakimi (1985) and Kishi and Miyawaki (1996) reported that fold development in this region is migrating from west (back-arc) to east (volcanic front), which is consistent with the history of active faulting in the area (e.g., Doke et al., 2012). The average amount of horizontal shortening at geological time-scales in the area is estimated to be 10%e15% (Fig. 1b; Sato, 1989). The hills and mountains in the study area reach elevations of ~100e700 m and the topography clearly reflects fold development (Fig. 2). Areas of topographic collapse and landsliding are common. The topography is gentle in areas where erosion is dominated by slope failure, but relatively steep in areas with a greater abundance of consolidated sandstone and alternating sandstone and mudstone beds. The local base level is largely uniform throughout the study area except for a folded area near a Quaternary volcano.

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Fig. 4. Geological and topographical maps of the (a) Akita and (b) Niigata regions (after Osawa and Suda, 1980a,b; Takeuchi et al., 1994; Takeuchi et al., 2007; Takahashi et al., 2010). (c) Geologic cross section between I and II in the Niigata region (Niigata Prefecture, 2000). NN11 and SN01 are box folds (Suzuki et al., 1971). Red lines show the hinge lines of late PlioceneeRecent folds. Blue lines show the mountain ridge lines in the folds. Triangles indicate Quaternary volcanoes. KCTL: KashiwazakieChoshi (Chiba) Tectonic line (Yamashita, 1970). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3. Measurements of the distance between the hinge and ridge lines of folds Quantifying relief is an important step toward characterizing fold morphology. In this study, we make the following assumptions: (1) bedrock uplift occurs mainly in the axis (hinge line) of the anticline under a compressional stress regime; (2) during fold development the uplift axis is fixed and the uplift rate remains constant; and (3) the spatial distribution of the physical properties of folded bedrock is largely uniform throughout the study area. The uplift rate within the active fold zone that we studied is ~1 mm/y, whereas the uplift rate of the non-active fold zone is ~0.1 mm/y (Komatsubara, 1993; Kim, 2004). Landsliding and channel flow (flowing water) are the main agents of erosion in the study area (e.g., Komatsubara et al., 2014), and drive erosion at rates of 0.1e0.3 mm/y (Fujiwara et al., 2005). Because the mountains formed by the folding in this area do not exceed an elevation of 1000 m, we consider that the spatial variations in surface gradient among the folds are small, which means that the spatial heterogeneity of the erosion potential is small enough to be insignificant. Therefore, given the above geological and geomorphological

environment, we consider that the hingeeridge distances are a suitable indicator of fold activity because the distances increase when the uplift rate is less than the erosion rate. A small fold that is in topographic equilibrium could have a shorter hingeeridge distance than a large fold that is far from topographic equilibrium. Therefore, it is necessary to account for this effect by normalizing the data using a non-dimensional metric, such as the hingeeridge distance divided by the total width of the fold. In this way it is possible to compare the data from one fold with another. We calculated the normalized horizontal distance (Dnh), which is expected to vary as a function of fold growth. Suppose that we have a number N of folds. The (i) (i) ith Dnh (i ¼ 1, 2, N) is given by the equation D(i) where nh ¼ d /W d(i) is the ith hingeeridge distance and W(i) is the width of the ith fold. We measured the distance between the fold hinge lines (red lines in Fig. 4) and the mountain ridge lines (blue lines in Fig. 4) for folds along the eastern margin of the Japan Sea. We targeted 44 folds that were activated since the late Pliocene, comprising 12 folds in the Akita region (numbered AK01e12) and 32 folds in the Niigata region (numbered NN01e19 [northern]

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and SN01e13 [southern]). The Niigata region is divided into northern and southern areas by the KashiwazakieChoshi (Chiba) Tectonic line (Fig. 4; Yamashita, 1970). Using 1:200,000 geological maps (Osawa and Suda, 1980a,b; Takeuchi et al., 1994; Takeuchi et al., 2007; Takahashi et al., 2010) and a 50 m grid meshed digital elevation model (DEM) published by the Geospatial Information Authority of Japan, we measured the horizontal distances between the fold hinge lines and the ridge lines.

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region, the hingeeridge distance and Dnh for 32 folds varied from 56 m to 1896 m and 0.04 to 0.47, respectively (Table 1). The histograms of the Dnh that we calculated in the Akita and Niigata regions are shown in Fig. 6 (c, d), plotted at 0.05 intervals. In both regions, the Dnh of the folds are concentrated in the range 0.00e0.30. Fig. 7 shows that Dnh decreases toward the east in the Akita region (Fig. 7a), whereas folds with low Dnh are distributed over much of the Niigata region (Fig. 7b).

Table 1 Geometry and distance from the volcanic front of the folds treated in this study. W: fold width. Average D: Average value of hingeeridge horizontal distance. Average Dnh: Average normalized hingeeridge distance. V.F.: Volcanic front. Fold ID

Strike of fold hinge

W of folds (m)

Average D (m)

Average Dnh

AK01 AK02 AK03 AK04 AK05 AK06 AK07 AK08 AK09 AK10 AK11 AK12

NNEeSSW NeS NeS NeS NeS NNEeSSW NeS NEeSW NNEeSSW NNEeSSW NNEeSSW NNEeSSW

4536 3470 3925 3271 1118 4733 1845 3626 2698 3667 4480 2951

489 852 694 445 175 450 135 1104 440 269 471 664

(±2) (±4) (±3) (±4) (±5) (±4) (±2) (±8) (±6) (±6) (±2) (±2)

0.11 0.25 0.18 0.14 0.16 0.10 0.07 0.30 0.16 0.07 0.11 0.22

(±0.03) (±0.04) (±0.04) (±0.05) (±0.06) (±0.05) (±0.03) (±0.09) (±0.06) (±0.07) (±0.02) (±0.02)

76 88 81 79 84 83 84 91 90 84 88 94

NN01 NN02 NN03 NN04 NN05 NN06 NN07 NN08 NN09 NN10 NN11 NN12 NN13 NN14 NN15 NN16 NN17 NN18 NN19

NNEeSSW NNEeSSW NEeSW NNEeSSW NNEeSSW NEeSW NEeSW NNEeSSW NEeSW NEeSW NEeSW NEeSW NEeSW NNEeSSW NNEeSSW NEeSW NEeSW NNEeSSW NEeSW

4224 4590 3133 2943 4232 1571 2435 2464 2274 4048 3926 2257 2713 6086 3955 2338 2160 2668 4783

1073 1048 248 398 999 313 802 586 653 1896 476 379 702 508 240 354 341 100 324

(±5) (±4) (±5) (±3) (±4) (±10) (±7) (±7) (±12) (±4) (±8) (±3) (±4) (±3) (±4) (±2) (±2) (±2) (±1)

0.25 0.23 0.08 0.14 0.24 0.20 0.33 0.24 0.29 0.47 0.12 0.17 0.26 0.08 0.06 0.15 0.16 0.04 0.07

(±0.04) (±0.03) (±0.04) (±0.03) (±0.03) (±0.08) (±0.07) (±0.07) (±0.11) (±0.04) (±0.08) (±0.03) (±0.04) (±0.03) (±0.04) (±0.03) (±0.03) (±0.02) (±0.01)

114 111 106 106 124 118 105 104 108 113 104 104 109 94 98 89 89 86 95

SN01 SN02 SN03 SN04 SN05 SN06 SN07 SN08 SN09 SN10 SN11 SN12 SN13

NNEeSSW NNEeSSW NNEeSSW NEeSW NEeSW NNEeSSW NEeSW NEeSW NEeSW NEeSW NEeSW NEeSW NEeSW

6225 5648 3178 5129 4995 4914 3939 7212 1144 2504 1395 2877 1543

416 232 1262 884 374 529 190 779 56 96 82 387 92

(±6) (±3) (±6) (±2) (±7) (±4) (±2) (±2) (±2) (±2) (±2) (±5) (±3)

0.07 0.04 0.40 0.17 0.07 0.11 0.05 0.11 0.05 0.04 0.06 0.13 0.06

(±0.07) (±0.04) (±0.06) (±0.03) (±0.08) (±0.05) (±0.02) (±0.02) (±0.02) (±0.03) (±0.03) (±0.07) (±0.05)

81 90 97 85 80 77 84 78 74 77 75 73 70

The landsliding occurred in this study area causes that some polarities switch along strike (e.g., SN01, SN02, SN10, SN12 and SN 13; Takeuchi et al., 1994, 2007; Takahashi et al., 2010). Measurements were taken at ~50 m intervals along the fold, from the northern to the southern ends of the hinge line (Fig. 5), and the average and standard deviation (1s) were calculated for each fold. The analysis of Dnh for the Akita and Niigata regions is presented in Figs. 6 and 7, and Table 1. In the Akita region, the hingeeridge distance and Dnh for 12 folds varied from 135 m to 1104 m and 0.07 to 0.30, respectively (Table 1). In the Niigata

Distance from V.F. (km)

For each fold, we obtained the relationship between Dnh and the distance from the volcanic front in the Tohoku district (Fig. 8). The volcanic front is a suitable criteria for comparison of the folds between districts, because it is oriented parallel to the subduction trench and has remained at a constant distance from the trench since the late Pliocene (Yoshida et al., 1995). The results show that Dnh values are smaller at sites located closer to the volcanic front (Fig. 8). Because the orientation of the volcanic front changes from NEeSW to ENEeWSW in the southern part of the Niigata region (Fig. 1), this region is closer to the volcanic front than the northern region (Fig. 8b).

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Fig. 5. Close up of the fold numbered NN15 in the Niigata region. (a) Method used to measure the horizontal distance between the fold hinge line (red line) and the mountain ridge line (blue line) for the NN15. Distances were measured at ~50 m intervals along each fold hinge line, from north to south. d: hingeeridge distance. W: width of fold. (b) Cross section (AeB) for the NN15. The downward-pointing black and white triangles show the topographic divide (ridge line) and the fold hinge, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4. Discussion and conclusions The distances between the hinge and ridge lines, as measured along the eastern margin of the Japan Sea, do not exceed 2000 m (Fig. 6). We interpret the distances as fold-scale events because the fold wavelengths are over 2000 m in the study area. Hence, the distances reflect the relative importance of folding and erosion. The study area has many folds with small Dnh (Fig. 6) but few folds with large Dnh (Fig. 6). We used Dnh so that the distance between the hinge and ridge lines is not dependent on the width of the folds. When fold-related uplift becomes inactive, the topographic contrasts can be remarkable (Fig. 3; Ellis and Densmore, 2006), leading to increased Dnh. Hence, the topographic behavior of folds is therefore a useful indicator of the activity of the fold. Because most of the present folds have small Dnh (Fig. 6), we suggest that they are actively growing. The topographic divides migrated toward the foreland region in ~16% of the folds examined (Fig. 8). The results for these folds may underestimate the horizontal distance between the ridge and hinge lines in the case of a non-active fold, given that the migration rate of the divide toward the foreland is generally lower than that of the divide toward the backlimb. In the study area, most of the present folds are convex kink shapes, though some folds (NN11 and SN01) are box folds (Suzuki et al., 1971).

Because the Dnh value for both fold shapes is the same within error (Fig. 8b), we infer that the Dnh value does not primarily depend on the fold geometry. We considered the relationship between the across-arc migration of fold growth and the spatial variation in Dnh. The across-arc variations in Dnh measured in this study relate to the distance from the volcanic front in the Tohoku district (Fig. 8), and the maximum rate at which Dnh decreases towards the volcanic front is not significantly different in either region (Fig. 8). As presented above, the topographic characteristics of folds in the region are consistent with the degree of activity of the folds, and indicate that the landward migration of fold growth is generally constant. On the other hand, high Dnh values were obtained for the western part of the Niigata region (Fig. 4b). The large Dnh of NN10 (Fig. 4b) may be due to strong coastal erosion on one side of the anticline, and the large Dnh of NN5 (Fig. 4b) may reflect a structurally controlled landform related to erosion-resistant volcanic rocks along the ridge line. Hence, our results indicate that the gap between geologic and geodetic high-strain-rate regions in the northern Tohoku district, including the Akita region, is a result of gradual migration of the locus of crustal deformation, from west to east. The spatiotemporal distribution of volcanism shows a recent (5e0 Ma) migration of volcanism from the back-arc region to the volcanic front (Honda and Yoshida, 2005), which may be correlated with dynamic movement in the mantle wedge (Yoshida et al., 2013). The distribution of geodetic high-strain-rate regions is broadly coincident with the area of high heat flow in the Tohoku district (Hasegawa et al., 2005; Townend and Zoback, 2006). The high crustal temperatures might be the key control on upper plate strength (Townend and Zoback, 2006). Therefore, we suggest that the gap between geological and geodetic high-strain-rate regions reflects the migration of contractional deformation, which followed the landward migration of the geothermal spatial heterogeneity. The spatial variations in Dnh that we estimated in this study provide important information about folding dynamics along the eastern margin of the Japan Sea. In this study area, an unconformity that developed during folding (Kishi and Miyawaki, 1996) indicates that the fold growth occurred over a time-scale of ~500 ky. At 103
4 y time-scales, geomorphic analysis of erosional  and Avouac, 2000). Hence, features is a useful method (e.g., Lave understanding the erosional characteristics and evolution of mountains is a key to filling the temporal gap between geodetic time-scales (10 0
2 years; e.g., Allmendinger et al., 2009) and geological time-scales (105
6 years; e.g., Robinson and McQuarrie, 2012). The values of Dnh in the Niigata region range widely. The northern and southern Niigata regions have smaller Dnh/DVF values (where Dnh/DVF is the ratio of Dnh to distance from the volcanic front (DVF) for each fold) than the Akita region (Fig. 8b), indicating that folds in the western part of the northern and southern Niigata regions have small Dnh values. Previous studies reported that folding commenced prior to ~3e2 Ma (e.g., Kishi and Miyawaki, 1996). These folds, which might therefore be expected to have large Dnh, are located in the geodetic highstrain-rate zone (Figs. 1 and 8b; Sagiya et al., 2000). When uplift ceases, the height of a mountain range decreases exponentially with time due to erosion, dependent on an erosion time constant (Ahnert, 1970; Pinet and Souriau, 1988; Ikeda, 1990). Observations of sediment yields have implied that the erosion time constants for the mountain ranges in the Tohoku district are of the order of 2.4 ± 1.3 Ma (Ikeda, 1990). However, these mountain ranges, which have evidence for unroofing of up to ~400 m or more, still preserve prominent topographic relief

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Fig. 6. Histogram showing the frequency distribution of hingeeridge distances in the (a) Akita and (b) Niigata regions, and of the normalized horizontal distance (Dnh) in the (c) Akita and (d) Niigata regions.

and numerous ~M7 reverse faulting type earthquakes have occurred in this zone (Fig. 1a; e.g., the 2004 Mw 6.6 Mid-Niigata Prefecture earthquake and the 2007 Mw 6.6 Niigataken Chuetsu-oki earthquake). In a fold-and-thrust belt, the thrusts in the hinterland can become reactivated over geological time-scales (Miyakawa et al., 2010; Yamada et al., 2014). Hence, it is also possible that fault-related folding can become reactivated. Fault reactivation and fault-related folding continue in the recent geodetic high-strain-rate zone. During reactivation of faults that underlie fault-related folds, the deformation mechanisms of thick sedimentary layers in the upper crust becomes progressively more fault-driven, rather than fold-driven, as the

fault propagates to the surface (e.g., Twiss and Moores, 1992). In this case, it is expected that hanging-wall uplift will increase the elevation of the folded structures, and the Dnh will vary from large to small (Figs. 8b and 9b). Therefore, we suggest that the folds with unusually small Dnh in the Niigata region are experiencing a transition from fold-driven to fault-driven deformation. In contrast, the Dnh values are large in the Akita region, where fault reactivation is minor, because the high-strain-rate zone in the Akita region is located closer to the volcanic front (Figs. 1, 8a and 9a). There are a few folds with small Dnh in the western part of the Akita region (Fig. 8a). Future work will investigate the quantitative mechanical implications of Dnh for fault-related fold dynamics.

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Fig. 7. Spatial distribution of Dnh, based on the hingeeridge distances in the (a) Akita and (b) Niigata regions. Color of the hinge line indicates the Dnh value (see the color scale in the figure) for each fold. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Plots of Dnh against the distance from the volcanic front for the (a) Akita and (b) Niigata regions of the Tohuku district, illustrating the across-arc spatial variability in the topographic characteristics of folds. Horizontal axes show the distance from each fold to the volcanic front in the Tohoku district. Error bars are 1s. Open circles indicate folds for which the topographic divide migrated toward the foreland.

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Fig. 9. Schematic model of the eastward migration of fold development in the (a) Akita and (b) Niigata regions. The downward-pointing black and white triangles show the topographic divide (ridge line) and the fold hinge, respectively. Faults with thick and thin half-arrows are re-activated faults and faults with low activity, respectively. V.F.: Volcanic front.

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