Bull Earthq Res Inst Univ Tokyo Vol 2-,**2 pp +0- +1- : SK++* + * + Plio-Pleistocene Behavior of the Itoigawa-Shizuoka Tectonic Line in Northern Nagano Prefecture, Central Japan: Paleomagnetism of the Omine-SK++* Tephra + Takeyuki Ueki * + Institute of Geology and Geoinformation, Geological Survey of Japan, AIST Abstract This study aims to reveal Late Pliocene to Early Pleistocene geomorphic and structural evolution of the Omine Belt, North Fossa Magna, central Japan, associated with tectonic movement of the Itoigawa-Shizuoka tectonic line A comparison of the paleomagnetic directions of a distinct wide-spread tephra, which is exposed in di# erent areas, is useful to detect tectonic rotation because it gives instantaneous paleomagnetic directions from proximal to distal areas The Omine-SK++* tephra with an estimated age of + 0/ + 0* Ma is distributed in the Omine Belt as Omine Welded Tu# and in the west Tama Hills of the Kanto Plain as a co-ignimbrite of the Horinouchi, Tu # Progressive alternating field and thermal demagnetization experiments reveal that both tephras have reversed paleomagnetic directions that are almost identical to each other As the west Tama Hills had been in a tectonically stable area since Late Pliocene, there might be no vertical-axis rotation in the Omine Belt Thus, the Omine Belt would not be a pull-apart basin with left-lateral strike-slip faulting between the Otari-Nakayama fault and the active fault system of the Itoigawa-Shizuoka tectonic line Key words: Itoigawa-Shizuoka tectonic line, Omine-SK+** tephra, paleomagnetism, Early Pleistocene, thrust front migration + +32/ ; +321 ; +33* ; +33+ ; +331 ; +332 ;,**+,**0; Okumura,,**+;,**0 / m+*** +/* km,**+; Matsuta et al, + : +33+ ; +33 +332 ;,** +3 ; Okumura,,**+ -* km * km Elouai et al,,**; Sato et al, + 0 m+*** Une,** ab * e-mail: gakusetsu-ueki@aistgojp -*/ 2/01 + + 1 163
+33+ +333 ; +33+ +3 +333 ; Takeuchi,,** Elouai et al,,**; Sato et al,,** ab Ikami et al, +320 ; +321; Fig + Index map of the Itoigawa-Shizuoka tectonic,***;,**+,**1ab; Elouai et line (ISTL), Omine Belt in the north Nagano Prefecture and Tama Hills in the western flank of al,,**; Sato et al,,** ab;,**/ Kanto Plain, central Japan Shaded area indicates the North Fossa Magna Bold line on the ISTL indicates the active fault system AMR, OKH and PHS are the Amurian, Okhotsk and Philippine Sea Plates +333 SK++*,*** SK++*,**0 +332 / +* km +313 +333 ;,***;,**-, +32* +33 +33+, +32- ; +323 ; +33+ +333, 0 -, / thrust front migration: Ikeda, +32-,,**,; Ueki,,**, 0/ km, / +* km,,**, +33+,* * 164
SK++* Fig, Geology of the central to south Omine Belt and surrounding areas, and sampling sites for the Omine Welded Tu# Geologic map is simplified after and Yamada et al ( +323 ) and Kosaka ( +33+ ) + 0 are the sampling sites for the Omine Welded Tu # EMF, KF, ONF and TF are the Eastern Margin fault of the Matsumoto Basin, Kamishiro fault, Otari-Nakayama fault and Terama fault +/ ; +2 ; +32- ; +323 +32-; +323 ; +332 +330 +10 + 1/ Ma,*** +332 -** m +331 SK++*,***,**-,*** SK++* SK++* +0/ +0 Ma +331 ;,***;,**- +0 +- Ma K-Ar FT,,**/ +3-- ; +3-- +2 ; +2 - +33 +33, +33 Kd,/ +3,*** Kd,/,,**,,**,, SK++* 0,, / - + 0 165
Fig - Geology of the west Tama Hills and sampling sites for the Horinouchi, Tu# Geologic map is simplified after Takano ( +33 ) 1 ++ are the sampling sites for the Horinouchi, Tu # Fig Geologic columnar sections of the Oyamada Formation of the Kazusa Group intercalated with cm 1 the Horinouchi, Tu # (HU,) Location of the Site 1++ are shown in Fig - ++, / +* cm +*,* cm,* mm, -, progressive alternating field demagnetization, PAFD progressive thermal demagnetization, PThD 3, - 0 PAFD PThD -,/ cm + + PAFD,/ cm 1 2 +* ++, 1 / 2, + PThD 3,/ cm 1, PAFD PThD +,, 166
SK++* PAFD PThD nent magnetization, VRM PAFD, G /,* mt 2* mt 2* mt, / 10* R PAFD * +* mt VRM,/ mt +* 2* mt / mt ++,2 + 0 2 /* PThD Magnetic Measurements MMTD-+2 PThD 0/*, MAD *-,, + 0,* +** /** /* PThD +/* /** 0/* -* * + VRM / + A -* PAFD, PThD, 0 Zijderveld, +301 +** Kirschvink, +32* VRM 0 B, + / maximum angular deviation, +/*,** /3* MAD +/ / + A, 0 3, PAFD PThD ** ** PAFD /0* / 0 A 3,, 0,* VRM PThD +* + PAFD PAFD, PThD MAD *3 - + + 0 +- 3 +/3 0,+ - /0 3 McFadden and MacElhinny +322 +1* 3,** - -- 0 PThD PAFD 0* 1 + 0 + PThD 3 a : Fisher, +- k: Fisher, +- +, /,* - -*-* 222-0 ++ 1,, 2, -*, +++ / +,,, PAFD 1 2 +* ++ : 1 2 +* ++ PAFD, PThD PAFD /,/ mt PThD / 1 2 ++ + 0 +,/ +*,133 + / 1 A+ VRM PAFD -2mT * 1 --/ mt /* 2* mt / + A 2* mt + 0 B+ Brunhes 1 VRM Chron Viscous rema- - 1 2 ++ /-13 167
Fig / Progressive alternating field and thermal demagnetization (PAFD and PThD) results for representative samples of the Omine-SK+** tephra plotted on orthogonal vector diagrams Solid and open circles are the projections in the geographic coordinates of remanence vectors on the horizontal, and north-south vertical planes, respectively Unit of magnetization intensity is Am 168
SK++* Fig 0 Equal-area projections of site-mean remanent magnetization directions of the Omine-SK +** tephra + Site-mean (open circles) and overall mean (open rectangles) remanent magnetization directions of the Omine Welded Tu # Remanent magnetization directions are projected in the upper hemisphere Gray ovals correspond to the confidence cones of the overall mean remanent magnetization direction mdec and minc are overall mean declination and inclination a, k, N are radius of confidence cone ( ), Fisher s ( +-) precision parameter, number of sampling sites +0 are the sampling sites for the Omine Welded Tu #, Site-mean (open circles) and overall mean (open rectangles) remanent magnetization directions of the Horinouchi, Tu # 1, 2, +*, ++ are the sampling sites for the Horinouchi, Tu # - Site-mean remanent magnetization direction (open rectangle) at Loc 3, determined by combination of stable remanent directions and best-fit demagnetization circles following the method of McFadden McElhinny ( +322) mdec and minc are declination and inclination of site-mean remanent magnetization direction Nd and Nc are numbers of stable remanent directions and best-fit demagnetization circles A+ -J+ and A, -J, are the specimens for PAFD and PThD Four open circles are the stable remanent magnetization directions of specimens projected in the upper hemisphere Dashed and solid lines are parts of best-fit demagnetization circles projected in the lower and upper hemispheres, applied for principal component analyses by progressive demagnetization experiments The arrows indicate trend of remanent magnetization direction during progressive demagnetization experiments Gray oval corresponds to the confidence cone of the site-mean remanent magnetization direction +*,33, / 1 A, 1 2 +* ++ +/* ** /0* MAD * 31 + 0,* +** +/* PThD +** -/* 0,* -** /-* /0* 002 VRM /3* -/* 169
Table + Site-mean paleomagnetic data for the Omine Welded Tu# and the Horinouchi, Tu # Omine WT and HU, are the Omine Welded Tu# and Horinouchi, Tu # Latitude and longitude of each locality are shown as the World Geodetic System +32 S and D are strike and dip of sedimentary rocks immediately beneath the Omine Welded Tu # Nd and Nc are the numbers of stable remanent magnetization components and best-fit great circles to calculate the site-mean remanent magnetization directions D and I are declination and inclination of in situ site-mean remanent magnetization directions Dc and Ic are declination and inclination of tilt-corrected site-mean remanent magnetization directions MAD is averaged maximum angular deviation of stable magnetization components a and k are radius of confidence cone ( ) and Fisher s ( +-) precision parameter l and f are latitude and longitude of virtual geomagnetic pole -** /-* 0,* VRM VRM / 3 0,, +* ++ B,,** -** /+ 1 + /3* 0,* 0,* VRM MAD,2 0,3 + +* + + /3* 0,* +** 1 2 +* ++ +0,* +22 1 3-0-- + 0, a k / + ++ 2-3, +1/ * + MAD +* 0 + * + 3 : 3 PThD +* +/*,/* PAFD PAFD +/*,/*,** ** / 3 B+ +/ 0- / mt 2* mt 3 VRM 0,* 2 // VRM 2* 3 VRM mt VRM PThD 3 PThD +** +/* PAFD PThD +** +/*,** -** +- +0 2 02 0 170
SK++* + 0- a k 01 -*/ + / / +, + 0, +2+ + /,,, 1 ++, +1- / /3 * a k 3 + 1, + 0,, + 0-1 / /*/ 1 ++ 0 3 3 + + ++,**, * // + /0 *, km 2* a, +323, SK++* thermoremanent 0 magnetization, TRM, detrital remanent magnetization, DRM a k 3 3 1, 0 +, SK++*,,, DRM depositional, detrital remanent magnetization, ddrm,,**,, thrust front migration,**,; Ueki,,**, /,,**, +33+ +333 ; +33+ +3 +333 ; Takeuchi,,**,: +323 -,*** m +33+ +3 +333 171
Takeuchi,** +331 +0 -/ - +2 -+ 33 +32- / + 3- +323 / + +++ +33+ -1 Kirschvink, JL, +32*, The least-squares line and plane and the analysis of palaeomagnetic data Geophys Jour Roy astron Soc, 0,, 0331+2 +/ 0+ -* 1,**0 A ++/,*2,,* +32* +/ -+ 0 +33+,0 1/ +* +333,+ /23 / +313 + no +0 +03 +2, +331 : +*0 -, -3 2,**- --0,**+ Elouai, D, Sato, H, Hirata, N, Kawasaki, S, Takeshita, T,,* /31* Kato, N and Takeda, T,,**, Deep seismic reflection Matsuta, N, Ikeda, Y and Sato, H,,**, The slip rate along profiling across the Northern Fossa Magna : The ERI the northern Itoigawa-Shizuoka tectonic line active +331 and the JNOC +330 seismic lines, active faults and fault system, central Japan Earth Planets Space, /0, geological structures Earth Planets Space, /0, +--+ +--2 +-,- +--* Fisher, R, +-, Dispersion on a sphereproc Roy Soc London, Ser A,,+1,, -*/ +2,**0 0 +2+ +3 +2 0 -+,0 +*/ +,* Ikami, A, Yoshii, T, Kubota, S, Sasaki, Y, Hasemi, A, Moriya, T, Miyamachi, H, Matsu ura, RS and Wada, K, +320, A seismic-refraction profile in and around Nagano Prefecture, central Japan Jour Phys Earth, -, /1 1 Ikeda, Y, +32-, Thrust front migration and its mechanism:,**1a evolution of intraplate thrust systems Bull Dep Geogr Univ Tokyo, +/, +,/ +/3 2,,/ -/,**,,0*,**1b 172
SK++* 2, -1,0 +,+ +-0 McFadden, PL and McElhinny, MW, +322, The combined +321 analysis of remagnetization circles and direct observa- no +1 3, 3- tions in paleomagnetism Earth Planet Sci Lett, 21, +0+ +1, +3,**, K-Ar,**/ /- -3,/ +3-- 0-1- * ** *+ +33 +3 +** 01/ 03+,**, +* 2-32 no -, ++ ++/ +332 +33+ +* +2 +32 +** /* //+ +330 +3 +2 3, 30 +333 /*,3,,+ /2- /22,*** Takeuchi, A,,**, Basement-involved tectonics in North Fossa Magna, central Japan: The significance of the +*0 /+ 03 northern Itoigawa-Shizuoka Tectonic Line Earth Planets +33* Space, /0, +,0+ +,03 2 +/,+ +333 +333,*** A 2+ 0 +11 +20,**, Okumura, K,,**+, Paleoseismology of the Itoigawa- Shizuoka tectonic line in central Japan Jour Seismol, /, ++ -+ +/ +3-- +33 * 1332*+ Ueki, T,,**,, History of fault activity based on the paleo- 0,/ -2 geography and paleo-geologic-structure since Pliocene: Examples of the northeastern Shikoku Island, southwest +332 Japan, and the Omine area of the Northern Fossa Magna region, central Japan PhD thesis of Tokyo Met- /* -/ /+ ropolitan University, ++0p Sato, H, Iwasaki, T, Kawasaki, S, Ikeda, Y, Matsuta, N,,**/, Takeda, T, Hirata N and Kawanaka, T,,** a, Formation and shortening deformation of a back-arc rift basin ++ 0,-0,/ revealed by deep seismic profiling, central Japan Tectonophysics, Une, H, +32/, On the tectonic landforms in the northwestern -22, 1 /2 part of the Nagano Prefecture, central Japan Bull Sato, H, Iwasaki, T, Ikeda, Y, Takeda, T, Matsuta, N, Imai, Geogr Surv Inst,,3, 22 +*+ T, Kurashimo, E, Hirata, N, Sakai, S, Elouai, D, Kawanaka, +323 T, Kawasaki, S, Abe, S, Kozawa, T, Ikawa, T,,* + Arai, Y and Kato, N,,** b, Seismological and geologi- +321 cal characterization of the crust in the southern part of * 2331 northern Fossa Magna, central Japan Earth Planets Zijderveld, JDA, +301, AC demagnetization of rocks : Space, /0, +,/- +,/3 Analysis of results,in Methods in Palaeomagnetism edited +333 by Collinson, D, Creer, K and Runcorn, S, Elsevier, /-,1/,3* pp,/0,20 Received May +/,,**2 Accepted September,3,,**2,**0 173