Magnetic anomalies in the Shikoku Basin: a new interpretation

Tectonophysics, 2001

In order to better understand continental rifting along the eastern margin of the Eurasian plate, we have performed a magnetic inversion to reveal the distribution and characteristics of the belts and basins in the East China Sea and Okinawa Trough. Equivalent magnetizations are obtained by assuming a constant thickness of 5 km of the magnetized layer whose top surface is assigned to the sea¯oor and taking into account the present-day geomagnetic ®eld. Because of absence of magnetic reversals in the study area, a high-magnetization zone can re¯ect a shallow and/or highly magnetized basement. In contrast, a lowmagnetization zone generally corresponds to a rifted basin with thick sediments.

Earth, Planets and Space, 2001

Central Honshu, Japan, is conspicuous for a northward-convex form of zonal geologic structures. To clarify the formation of curvature of the zonal structures in the west of the convex form and further to discuss the tectonic evolution of central Honshu, we carried out a paleomagnetic study of dated (∼15 Ma) dike rocks of the Shitara basin. Samples were collected from 25 basalt dikes trending north-south with a nearly vertical intrusion surface. After stepwise alternating-field and thermal demagnetization, 24 site-mean directions were determined. Three dikes gave anomalous directions, probably resulting from instantaneous recording of a field transition or excursion. The other 21 site-means produced a mean direction, D = 9.7 • , I = 54.5 • , α 95 = 5.2 • , and a paleomagnetic pole at 82.3 • N, 216.6 • E, A 95 = 6.2 •. Although the reversal test is negative at the 5% significance level, the mean direction and pole are time-averaged ones in which the secular variation is averaged out. This was confirmed by studying the angular standard deviation of virtual geomagnetic poles. We conclude that the central Honshu curvature formed when the southwest Japan arc rotated clockwise between 17 and 15 Ma in relation to the opening of the Japan Sea, associated with differential rotation of the eastern part of the arc with respect to the central part. The differential rotation probably resulted from a sinistral shear on the eastern margin of the rotating arc. The formation of the curvature seems not to have borne on the collision of the Izu-Ogasawara arc with Honshu, although the collision probably caused large-scale clockwise rotation of the Kanto Mountains in the east of the northward-convex structure.

Journal of geomagnetism and geoelectricity, 1986

Quaternary International, 2016

Paleomagnetic results are presented for a concentrated deformation zone (CDZ) in a convergent plate boundary region along the eastern Eurasian margin. We sampled late Plioceneeearly Pleistocene tuffs and clastic sediments of the Sasaoka Formation (~750 m thick) in northeastern Honshu, Japan, to test whether the late PlioceneeQuaternary crustal deformation within the CDZ along the eastern margin of the Japan Sea was accompanied by rotation about a vertical axis. Rock magnetic experiments suggest that the principal magnetic carrier is magnetite in the fine tuffs, and magnetic iron sulfide in the fine sandstones. Pre-folding characteristic remanent magnetization was confirmed using a positive bootstrap fold test. We obtained 21 acceptable site-mean characteristic directions that include our preliminary published results, and which cover an interval from ca. 2.7 to 1.7 Ma on the basis of magnetostratigraphic correlations. An updated ageedepth model is given, and this allowed us to make numerical age estimates for key interbedded tuff beds (tephras). A positive fold test also suggests that the Gojome syncline began to develop after 1.7 Ma, which means the folding began long after the initiation of late Pliocene crustal shortening in northeastern Honshu. After 100% unfolding, the overall mean direction (D ¼ 359.7 , I ¼ 54.9 , a 95 ¼ 6.7) is indistinguishable from the geocentric axial dipole field direction, indicating that the Gojome syncline, the most prominent structural element in the study area, developed without vertical-axis rotation. A comparison of our results with Plio-Pleistocene directions reported from other areas reveals no paleomagnetically detectable rotation in or adjacent to the CDZ, except for local rotation near strike-slip faults in central Honshu.

Geophysical Journal International, 2004

A magnetic fabric analysis has been carried out on standard cube samples from one gravity and three multiple cores extracted from the Shiribeshi trough and Okushiri basin in the southern margin of the Japan sea north basin. It is aimed at tracing the flow path of turbidites that are assumed to have deposited in response to the 1993 Hokkaido-Nansei-oki earthquake. Magnetic remanence was used for reorientation to the geographic coordinates. Magnetomineralogical investigations including low-temperature magnetometry, magnetic hysteresis loops and isothermal remanent magnetization (IRM) acquisition experiments indicate that pseudosingle domain to multidomain magnetite is the principal magnetic carrier and is, therefore, capable of providing reliable anisotropy of magnetic susceptibility (AMS) palaeocurrent direction estimates. A well-developed near-horizontal magnetic foliation and minimum susceptibility axes lying close to vertical are recorded at all sites reflecting an original depositional fabric. Clearly defined magnetic lineation was observed at all sites and is considered to reflect the palaeocurrent direction. Down-core changes of susceptibility and key AMS parameters show good correspondence to occurrences of turbidite layers marking the increase of input of influx materials. In agreement with results from recent marine surveys and IZANAGI side-scan sonar images, an NNE transportation trend has been estimated for sediments at sites from the Shiribeshi trough with a possible depositing path initiating from the slope bounding the south and southeastern margin down to the trough floor. Similarly, a SSE palaeocurrent direction has been estimated for sediments from the Okushiri basin with evidence for a relatively strong transporting current flowing through the canyons along the steep slope bounding the north and northeastern margins of the basin. The present results agree with the view that slope failure is the most probable mechanism for the down-slope transport of the sand from the shelves and upper slopes down to floors of basins and troughs in the southern margin of the Japan sea north basin. They further support the ongoing assumption that the 1993 Hokkaido-Nansei-oki and other strong historical earthquakes together with associated tsunamis are the principal triggering forces for the down-slope mass gravitational transport and formation of turbidites in this seismically active area.

Marine Geology, 2002

carried out in collaboration a 120channel reflection seismic survey using an airgun array with total 4200 cubic inch capacity around the eastern Nankai trough in 1996. Data obtained by R/V Hakuho, University of Tokyo were primarily processed by JAMSTEC and were shared by the above three main bodies of this program to complete geoscientific interpretation. The fault configuration around the eastern Nankai trough is very complicated due to the collision of the Izu Peninsula against central Japan. The major remarkable faults around the eastern Nankai area are the Zenisu, the Kodaiba and the Tokai thrusts. The main objective of this paper is to clarify these fault configurations. The Zenisu thrust dips gently northward from the southern slope of the Zenisu ridge, and reaches the oceanic Moho. The location of the Tokai thrust may jump at the Tenryu canyon, and appears to reach the top of the subducting oceanic plate. The Kodaiba fault penetrates a 5.5^5.6-km/s layer, which is the crust of the central Japan forearc. A large fault in the trough axis cuts the entire oceanic crust and has a flower structure, which indicates that it has strike-slip component. Its strike is NW^SE, parallel to the plate motion and perpendicular to the strike of the Zenisu ridge. The top of the subducting oceanic crust of the Philippine Sea plate was defined beneath the continental slope of the central Japan forearc. The angle of subduction becomes abruptly steeper 50 km north of the deformation front. We also observed at the southern roof of the Zenis thrust a drape fold structure which is proof of subduction. We suggest a subduction on the southern side of the Zenisu ridge, strongly deformed oceanic crust at the trough axis and some major faults in the forearc region reaching subducting oceanic crust.

EGU General Assembly 2010, held 2-7 May, 2010 …, 2010

Earth, Planets and Space, 2007

We examined short-term geomagnetic changes related with the 2003 Tokachi-oki earthquake (M 8.0) and the 2004 Kushiro-oki earthquake (M 7.1) in Hokkaido, Japan. However, we could not find the precursory and coseismic signals above several nT at a magnetic station whose epicentral distances were about 120 km and 50 km, respectively. Model calculations showed that co-seismic piezomagnetic fields did not amount to 1 nT at the station in both cases when we assumed the relevant fault parameters, in-situ Curie temperature depth, subsurface magnetic structure and stress sensitivity of rocks. Therefore, it may be reasonable that we could not detect the piezomagnetic signals at the station. We also made model calculations to forecast the piezomagnetic amplitudes caused by M 7.9 and M 8.5 earthquakes which have been expected to occur along the southern Kurile trench in the future. The model calculations reveal the piezomagnetic fields up to about −4 nT and −7 nT are expected in the eastern part of the Hokkaido island for the M 7.9 and M 8.5 earthquakes respectively, encouraging magnetic observations hereafter.

[1] Analyses of normal faults in the Kumano forearc basin of the Nankai Trough reveal multiple normal fault populations in a region generally thought to be under compression. Most faults have offsets of less than 20 m and dips of 60–70 and show no growth structures, indicating that the faults were active for short periods of time. The oldest generation of faults is older than ~0.9 Ma and strikes ~50–60. The next oldest faults strike ~160–170 , are older than 0.44 Ma, and are related to local uplift along the western edge of the region. The youngest faults cut the seafloor; shallow faults near the SE margin of the basin curve from ~100 in the middle of the survey area to ~145 at the SE corner of the area. The pattern of the two youngest fault populations is consistent with the regional stress pattern (maximum horizontal stress subparallel to the trench). Orientations of older fault populations are caused by uplift of the underlying accretionary prism, implying that the forearc basin region is not as stable as previously thought. Reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2%, concentrated near the seaward edge of the basin. The active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge. Components: 7,415 words, 13 figures, 1 table. (2013), Analysis of normal fault populations in the Kumano Forearc Basin, Nankai Trough, Japan: 1. Multiple orientations and generations of faults from 3-D coherency mapping,

Journal of geomagnetism and geoelectricity, 1982

Total intensity data acquired by aeromagnetic surveys were decomposed into vector components, following a procedure proposed by LOURENCO and MORRISON (1973). It has been revealed that the Fossa Magna is a magnetic region. Intense magnetic anomalies distribute inside the region, and no conspicuous anomalies outside. In the north Fossa Magna, a narrow belt of positive anomaly is formed with the vertical component and the total intensity. In the south, on the other hand, the positive anomaly spreads widely covering the whole area. The boundaries of the Fossa Magna correspond to the boundaries of the magnetic region. Across the Itoigawa-Shizuoka tectonic line, the west boundary of the Fossa Magna, a clear contrast is seen with the vertical component. A conspicuous positive anomaly zone of the east component runs north-south along the tectonic line. The southeast boundary of the Fossa Magna is clearly recognized in the vertical component. This boundary causes negative anomalies both in the north and the east components. Besides these large scale features, there are anomalies of small scale with strong intensity. They are either of volcanic or intrusive origin, such as Mt. Fuji and Tanzawa Mountains. The magnetic belt in the north Fossa Magna is probably related to the Miocene formation called the Central Uplift Zone (Chuo-Ryuki-tai), which contains large mass of intrusive rocks and volcanics. Positive Bouguer anomaly along this belt suggests that high density materials are responsible for producing the magnetic belt. In the south, on the other hand, the Bouguer anomaly is negative. Presumably the magnetic anomaly is caused by thick marine sediments of Miocene time with smaller density, which distribute widely in the south Fossa Magna. The thickness of the causative body has been estimated to be about 10km for the magnetization of 3×10-3emu/cm3.