࿴Ἠᒣ⬦ᆅᇦ࡟࠾ࡅࡿ㡿ᐙ̿࿴Ἠᖏࡢ

࿴Ἠᒣ⬦ᆅᇦ࡟࠾ࡅࡿ㡿ᐙ̿࿴Ἠᖏࡢ

⇕ࢸࢡࢺࢽࢡࢫ

Thermal tectonics of the Ryoke–Izumi belt in the Izumi Mountains, Southwest Japan

2014 ᖺ 2 ᭶

Ύᐙ ୍㤿

Kazuma SEIKE

࿴Ἠᒣ⬦ᆅᇦ࡟࠾ࡅࡿ㡿ᐙ̿࿴Ἠᖏࡢ

⇕ࢸࢡࢺࢽࢡࢫ

Thermal tectonics of the Ryoke–Izumi belt in the Izumi Mountains, Southwest Japan

2014 ᖺ 2 ᭶

᪩✄⏣኱Ꮫ኱Ꮫ㝔 ๰㐀⌮ᕤᏛ◊✲⛉

ᆅ⌫࣭⎔ቃ㈨※⌮ᕤᏛᑓᨷ ྂ⏕≀Ꮫ◊✲

Ύᐙ ୍㤿

Kazuma SEIKE

Abstract

Thermal maturity of the Izumi Group in the Izumi Mountains, southwestern Japan, was studied by using vitrinite reflectance. The mean random reflectances, ranging from 0.66 to 2.64%, indicate that thermal maturity of the whole region of the Izumi Mountains increases toward the northern margin. Following the result of vitrinite reflectance data, three-dimensional spatial patterns of the thermal structure were estimated by multiple regression analysis of the surface maturity data collected from the western and central parts of the Izumi Mountains, assuming the altitude of each sample reflects vertical maturity gradients. The analysis resulted in a thermal maturity isograd of N78°E, dipping 19° south, and a maximum temperature gradient of 0.400%Rm/km.

The thermal evolution of the Ryoke–Izumi belt in the Late Cretaceous to early Paleogene was examined based on apatite and zircon fission-track (FT) dating of the Sennan Rhyolites, the Takijiri Adamellite, and the Izumi Group in the Izumi Mountains, newly obtained in this study, along with pre-existing geochronological data and the geological structures. The analysis revealed that the geology of the area is subdivided into three blocks on the basis of different cooling histories and east-trending boundary faults: blocks A, B, and C from the south. Block A consists mostly of the Izumi Group, which accumulated on a part of the Sennan Rhyolites and the Takijiri Adamellite at 70.9 ± 3.4 Ma (zircon FT age). Previous studies on K-feldspar K–Ar ages of the Takijiri Adamellite of Block A show that uplift of Block A occurred at ~66 Ma. The higher maturity of the eastern Izumi Mountains must be attributed to the influence of regional uplift in this area. Block B includes most of the Sennan Rhyolites. They were also partly baked by the Takijiri Adamellite and cooled before 73.9 ± 8.2 Ma (zircon FT age). Block C includes the Takijiri Adamellite that was exposed on the north side of the faults after 50.0 ± 6.6 Ma (apatite FT age). The Sennan Rhyolites in Block B were not thermally affected by subsidence of the Izumi sedimentary basin (ISB), indicating that the ISB was formed at 71 Ma by the subsidence of Block A under transtension. The subsequent rapid uplift of the Izumi Group and the Takijiri Adamellite of Block A at 66 Ma was probably caused by reverse faulting under transpression.

To explain the thermal outline of the Izumi sedimentary basin, two hypothesized processes were tested by the computing simulations of thermal models: tectonic block tilting of the Izumi Group around the Median Tectonic Line (MTL) and difference of sedimentation rates between the northern marginal and depocenter of the ISB. Our new model can evaluate influence of sedimentation, subsidence, and uplift rates for thermal stracture and organic maturation. The models show that differences of subsidence and uplift rates can affect thermal maturity and that history of rapid uplift subsequent to rapid subsidence makes thermally low maturation in local regions. Our computer simulations also show that tilting of block was not appropriate for the vitrinite reflectance gradient outline in N–S direction. We conclude that a unique thermal structure of ISB may be formed by rapid subsidence subsequent rapid uplift.

┠ḟ

㸯㸬ࡣࡌࡵ࡟ ... 1

㸰㸬ᆅ㉁ᴫ␎ 㸰㸬㸯 ᆅ㉁༢ඖ ... 4

㸰㸬㸰 ᑼ࢔ࢲ࣓ࣟᒾ ... 4

㸰㸬㸱 Ἠ༡ὶ⣠ᒾ㢮 ... 7

㸰㸬㸲 ࿴Ἠᒙ⩌ ... 10

㸰㸬㸱㸬㸯 ໭⦕┦ ... 10

㸰㸬㸱㸬㸰 ୺㒊┦ ... 13

㸰㸬㸱㸬㸱 ༡㒊┦ ... 13

㸰㸬㸱㸬㸲 จ⅊ᒾᒙ ... 13

㸰㸬㸱㸬㸳 ሁ✚┅ᙧᡂྐࡢ◊✲ ... 15

㸰㸬㸱㸬㸴 ሁ✚ᖺ௦ ... 21

㸱㸬ࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡ ᐃ 㸱㸬㸯 ヨᩱ࡜᪉ἲ ... 23

㸱㸬㸯㸬㸯 ヨ ᩱ ... 23

㸱㸬㸯㸬㸰 Rock-Eval Tmax ... 26

㸱㸬㸯㸬㸱 ࢣࣟࢪ࢙ࣥศ㞳࣭⃰㞟 ... 26

㸱㸬㸯㸬㸲 ᭷ᶵ≀ࡢ㢧ᚤ㙾ほᐹ ... 28

㸱㸬㸯㸬㸳 ࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡ࡢ ᐃࢳࢵࣉࡢసᡂ ... 28

㸱㸬㸯㸬㸴 ࣅࢺࣜࢼ࢖ࢺࡢᖹᆒࣛࣥࢲ࣒཯ᑕ⋡ࡢ ᐃ ... 29

㸱㸬㸯㸬㸵 㕥ᮌ㸦1996㸧ࡢࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡࡜ࡢẚ㍑ࡢࡓࡵࡢ᥮⟬ ... 29

㸱㸬㸯㸬㸶 ྂᆅ ࡬ࡢ᥮⟬ ... 30

㸱㸬㸰 ⤖ ᯝ ... 30

㸱㸬㸰㸬㸯 ࣅࢺࣜࢼ࢖ࢺᖹᆒ཯ᑕ⋡... 30

㸱㸬㸰㸬㸰 ᭷ᶵ≀ほᐹ ... 32

㸱㸬㸰㸬㸱 ྂᆅ  ... 36

㸱㸬㸰㸬㸲 㕥ᮌ㸦1996㸧ࡢRm࡬ࡢ᥮⟬ ... 36

㸱㸬㸰㸬㸳 Rock-Eval Tmax ... 36

㸱㸬㸱 ㆟ ㄽ ... 36

㸱㸬㸱㸬㸯 ᭷ᶵ≀⇍ᡂࡢഴྥ ... 36

㸱㸬㸱㸬㸰 ࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡࡜Rock-Eval Tmaxࡢ┦㛵 ... 42

㸱㸬㸱㸬㸱 ⇍ᡂഴྥつไせᅉࡢ᳨ウ... 42

㸱㸬㸱㸬㸲 ⇕ᒚṔࡢࣔࢹࣝ ... 51

㸱㸬㸲 ࡲ࡜ࡵ ... 56

㸲㸬ᨺᑕᖺ௦ ᐃ 㸲㸬㸯 ヨᩱ࡜᪉ἲ ... 57

㸲㸬㸯㸬㸯 ヨᩱグ㍕ ... 57

㸲㸬㸯㸬㸰 ࣇ࢕ࢵࢩ࣭ࣙࣥࢺࣛࢵࢡᖺ௦ ᐃ ... 59

㸲㸬㸰 ⤖ ᯝ ... 62

㸲㸬㸱 ㆟ ㄽ ... 62

㸲㸬㸱㸬㸯 ࿴Ἠᒣ⬦ᆅᇦࡢ㡿ᐙ̿࿴Ἠᖏࡢࢸࢡࢺࢽࢡࢫ ... 62

㸲㸬㸱㸬㸰 ࿴Ἠᒙ⩌ࡢᮾ㐍࡜㡿ᐙ࣭ᒣ㝧ᖏࡢⅆᡂάືࡢ㛵ಀ ... 78

㸲㸬㸱㸬㸱 ࿘㎶ᆅᇦࡢⰼᓵᒾ㢮ࡢ෭༷ྐ࡜ࡢẚ㍑ ... 81

㸲㸬㸲 ⤖ ㄽ ... 81

㸳㸬⿕⇕ࣔࢹࣜࣥࢢ 㸳㸬㸯 ⇕ᖺ௦࡜ࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡ࢆ⏝࠸ࡓ ᗘᒚṔࡢ᥎ᐃ ... 83

㸳㸬㸰 ᇙἐࡢᩘ್ࣔࢹࣝࢩ࣑࣮ࣗࣞࢩࣙࣥ ... 87

㸳㸬㸰㸬㸯 ཎ⌮ ... 87

㸳㸬㸰㸬㸰 ሁ✚┅࡛ࡢᇙἐຠᯝࡢ᳨ウ ... 94

㸳㸬㸰㸬㸱 ࿴Ἠሁ✚┅ࡢᇙἐࢩ࣑࣮ࣗࣞࢩࣙࣥ ... 98

㸴ࡲ࡜ࡵ ... 105

ㅰ ㎡ ... 107

ᩥ ⊩ ... 108

㸯㸬ࡣࡌࡵ࡟

ⓑள⣖ࡢ࢔ࢪ࢔኱㝣ᮾ⦕㒊࡟ࡣ㸪ࢡࣛ㸫ኴᖹὒࣉ࣮ࣞࢺࡢᩳࡵỿࡳ㎸ࡳ࡟㛵㐃ࡋࡓᶓࡎ

ࢀሁ✚┅ࡀᩘከࡃⓎ㐩ࡋࡓ㸦Okada and Sakai, 1993㸹Choi and Lee, 2011㸧㸬ࡑࡢ୰࡛ࡶ㸪 ᮏ㑥ୖ㒊ⓑள⣔ࡢ࿴Ἠᒙ⩌ࡣ㸪༢୍ࡢሁ✚┅࡟ሁ✚ࡋࡓᆅᒙ࡜ࡋ࡚ᮾ࢔ࢪ࢔᭱኱つᶍࡢ✚

⟬ᒙཌࢆ᭷ࡍࡿ㸬࿴Ἠᒙ⩌ࡀሁ✚ࡋࡓ⊃㛗࡞ሁ✚┅ࡣ࿴Ἠሁ✚┅࡜࿧ࡤࢀࡿ㸦౛࠼ࡤ㸪す

ᮧ, 1984㸧㸬࿴Ἠሁ✚┅ࡣ㸪༡໭⣙ 10 km ᮾす⣙ 300 km ࡟ࢃࡓࡾ୰ኸᵓ㐀⥺㸦Median Tectonic Line㸸MTL㸧࡟ἢࡗ࡚ศᕸࡋ࡚࠾ࡾ㸦Fig. 1㸧㸪MTLࡢᶓࡎࢀ㐠ື࡟క࠸ᙧᡂࡉ

ࢀࡓ ͆඾ᆺⓗ࡞͇ᶓࡎࢀሁ✚┅࡜ࡉࢀࡿ㸦Taira et al., 1983㸧㸬ⓑள⣖௨㝆ࡢᮾ࢔ࢪ࢔⦕㎶

㒊࡟࠾࠸࡚㸪MTLࡣᶓࡎࢀኚ఩ࢆゎᾘࡍࡿᙺ๭ࢆᢸࡗ࡚ࡁࡓ࡜⪃࠼ࡽ࡚࠾ࡾ㸪ᆅయᵓ㐀Ⓨ

㐩ྐࡢ᚟ඖ࡟࠾࠸࡚㔜せ࡞ㄽⅬ࡜࡞ࡗ࡚ࡁࡓ㸦౛࠼ࡤ㸪ᒣ໭࣭኱⸨, 2000㸧㸬࿴Ἠሁ✚┅ࡢ ᙧᡂᶵᵓࡣ㸪ሁ✚ྐ◊✲ࢆ୰ᚰ࡜ࡋ࡚ከᵝ࡞㆟ㄽࡀ⾜ࢃࢀ࡚ࡁࡓࡀ㸦Miyata, 1990㸹ᒣ໭࣭

ఀ⸨, 1999㸹Noda and Toshimitsu, 2009㸧㸪ᚋᮇⓑள⣖ࡢMTLࡢ㐠ືീࢆ᫂ࡽ࠿࡟ࡍࡿୖ

࡛ࡶ㔜せ࡞ࢸ࣮࣐࡛࠶ࡿ㸬

ሁ✚┅ࡢᙧᡂⓎ㐩ྐࡣ㸪ᇶ┙ᒾ࠾ࡼࡧᇙ✚ࡋࡓሁ✚≀ࡢᇙἐྐ࣭⿕⇕ྐ࡟཯ᫎࡉࢀࡿ㸬 ࡍ࡞ࢃࡕ࿴Ἠᒙ⩌ࡢᇙἐ࠾ࡼࡧୖ᪼㐣⛬ࢆ᥎ᐃࡍࡿ୍ࡘࡢ᪉ἲ࡜ࡋ࡚㸪࿴Ἠᒙ⩌ࡢ᭷ᶵ≀

⇍ᡂࡢ᳨ウ࡜ྜࢃࡏࡓ࿴Ἠᒙ⩌࣭ᇶ┙ᒾࡢ⇕ᖺ௦Ꮫⓗ࡞෭༷ྐࡢ᥎ᐃࡀ᭷ຠ࡛࠶ࢁ࠺㸬ሁ

✚┅ࡢ⿕⇕≧ែࡣ㸪ᆅᒙࡢഴື㸪᩿ᒙ㐠ື㸪ሁ✚┅ࡢỿ㝆࡜ሁ✚≀඘ሸ࡟ࡼࡿ⇕ⓗຠᯝ࡞

࡝㸪ሁ✚┅ࡢᙧᡂ㐣⛬ࢆ኱ࡁࡃ཯ᫎࡍࡿࡇ࡜ࡀ㸪ᶆ‽ⓗ࡟⏝࠸ࡽࢀࡿ᭷ᶵ≀⇍ᡂᣦᶆ࡛࠶

ࡿࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡㸦Hood et al., 1975㸧ࢆ⏝࠸࡚᫂ࡽ࠿࡜࡞ࡗ࡚࠸ࡿ㸬౛࠼ࡤ㸪Yamaji and Takahashi㸦1988㸧ࡣ㸪ᮏ㑥⛛∗┅ᆅ᪂➨୕⣔ࡢࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡ࡢኚ໬ഴྥ࡟╔

┠ࡋࡓ⤖ᯝ㸪ሁ✚┅Ⓨ㐩ᵝᘧࡀ∦ⴠࡕሁ✚┅㸦ࣁ࣮ࣇࢢ࣮ࣛ࣋ࣥ㸧࡛࠶ࡿ࡞ࡽࡤࡑࡢኚ໬

ഴྥࡀㄝ࡛᫂ࡁࡿࡇ࡜ࢆ♧ࡋ࡚࠾ࡾࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡ࡀሁ✚┅Ⓨ㐩㐣⛬ࡢุูᣦᶆ࡜ࡋ

࡚᭷ຠ࡛࠶ࡿࡇ࡜ࢆ♧၀ࡋ࡚࠸ࡿ㸬໭⡿࢔ࣛࢫ࢝ᕞ࡟࠾ࡅࡿ10,000ᙅࡢ᭷ᶵ≀⇍ᡂᣦᶆࢹ

࣮ࢱࢆ⏝࠸ࡓJohnsson et al.㸦1993㸧ࡢ◊✲࡛ࡣ㸪ከᩘࡢ࣮࣎ࣜࣥࢢࢹ࣮ࢱ࠿ࡽ῝ᗘࡈ࡜

ࡢ஧ḟඖ⇍ᡂ᩿㠃ࢆసᡂࡍࡿࡇ࡜࡛㸪ୖ᪼࡜ᶓࡎࢀ᩿ᒙ㐠ື㸪ᆅᇦⓗ࡞⇕ὶ㔞ࡢ㐪࠸࡞࡝㸪

᭷ᶵ≀⇍ᡂ໙㓄࣭ᕪ␗ࢆࡶࡓࡽࡍᵝࠎ࡞せᅉࡀ༊ูࡋ࡚㆟ㄽࡉࢀ࡚࠸ࡿ㸬Petrunin and

Sobolev㸦2008㸧ࡣ㸪ᩘ್ࣔࢹࣝࢆ⏝࠸࡚㸪ᇶ┙ᒾ࡜ࡍࡿᒾ▼ࡢ ᗘ≧ែ࡟౫Ꮡࡋࡓᘏᛶᗘ

ࡢ㐪࠸ࡀ㸪ࣉࣝ࢔ࣃ࣮ࢺሁ✚┅ࡢỿ㝆ࡢ῝ࡉ࡜ᙧ≧࡟୚࠼ࡿᙳ㡪ࢆぢ✚ࡶࡗ࡚࠸ࡿ㸬࿴Ἠ ሁ✚┅ࡢࡼ࠺࡞ሁ✚┅࡛ࡣ㸪ᆅᒙࡢഴື㸪᩿ᒙࡢⓎ㐩ࡸࡑࢀ࡟ࡼࡿᇶ┙ᒾࡢ㒊ศⓗ࡞ỿ㝆࣭

ୖ᪼ࡢᙳ㡪ࡀ⇕ᖺ௦࠾ࡼࡧ᭷ᶵ≀⇍ᡂ࡟཯ᫎࡋ࡚࠸ࡿ࡜⪃࠼ࡽࢀ㸪࿴Ἠሁ✚┅ࡢỿ㝆࡜ᮾ

㐍ࡢᶵᵓࢆ㆟ㄽ࡛ࡁࡿྍ⬟ᛶࡀ࠶ࡿ㸬ࡇࢀࡲ࡛㸪࿴Ἠᒙ⩌ࡢᇙἐ࣭ୖ᪼࡟ࡼࡿ⿕⇕≧ែࡢ ヲ⣽࡞᳨ウࡸᇶ┙ᒾ㢮ࡢὸᡤ࡛ࡢỿ㝆࣭ୖ᪼࡜࿴Ἠሁ✚┅ࡢⓎ㐩࡜ࡢ᫬㛫ⓗ࣭✵㛫ⓗ㛵ಀ

ࢆලయⓗ࡟᳨ウࡋࡓ౛ࡣ࡞࠸㸬

ࡑࡇ࡛㸪ᮏ◊✲࡛ࡣ㸪ࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡ࢆ⏝࠸ࡓ࿴Ἠᒙ⩌ࡢ⿕⇕≧ែࡢ᳨ウ࠾ࡼࡧ㸪

⇕ᖺ௦Ꮫⓗ࡞ᡭἲ࡟ࡼࡿᇶ┙ᒾࡢỿ㝆࣭ୖ᪼ࢆྵࡵࡓ࿴Ἠᒙ⩌ࡢ⿕⇕ྐࡢ᳨ウࢆ⾜ࡗࡓ㸬 ࡉࡽ࡟㸪ࡇࢀࡽࡢ᳨ウ࡟⿕⇕ࡢᩘ್ࣔࢹࣜࣥࢢࡢ᳨ウࢆຍ࠼࡚㸪࿴Ἠሁ✚┅ࡢᙧᡂⓎ㐩㐣

⛬ࢆ⪃ᐹࡋࡓ㸬࡞࠾㸪୰᪂ୡ࡟࠾ࡅࡿす༡᪥ᮏᘼࡢ᫬ィᅇࡾᅇ㌿ࡲ࡛㸪MTLࡣ໭໭ᮾ㉮ྥ

࡛࠶ࡗࡓ࡜⪃࠼ࡽࢀ࡚࠸ࡿ㸦Otofuji and Matsuda, 1983㸧ࡀ㸪ᮏ◊✲࡛ࡣ⌧ᅾࡢ᪉఩ࢆ⏝

࠸࡚グ㍕࣭㆟ㄽࢆ㐍ࡵࡿ㸬ࡉࡽ࡟㸪ᮏ◊✲࡛⏝࠸࡚࠸ࡿ࿴Ἠሁ✚┅ࡢ໭⦕㸪༡⦕㸪࠾ࡼࡧ ᮾ⦕࡜࠸࠺⾲⌧ࡣ㸪ࡍ࡭࡚⌧ᅾࡢ࿴Ἠᒙ⩌ศᕸ⠊ᅖࡢቃ⏺఩⨨ࢆᇶ࡟ࡋࡓࡶࡢ࡛࠶ࡿࡓࡵ㸪 ሁ✚ᙜ᫬ࡢ࿴Ἠᒙ⩌ศᕸᇦ࡜୍⮴ࡋ࡚࠸࡞࠸ྍ⬟ᛶࡀ࠶ࡿ㸬ࡲࡓ㸪ᮏ✏࡛ᢅ࠺ᨺᑕᖺ௦ࡢ ㄗᕪࡣࡍ࡭࡚2Ȫ㸦Ҹ95㸣ಙ㢗༊㛫㸧࡛♧ࡋ࡚࠸ࡿ㸬

Median Tectonic Line Median Tectonic Line

Upper Cretaceous Izumi Group Upper Cretaceous Onogawa Group Cretaceous-Neogene granitoids

Osaka

Matsuyama

Osaka

Matsuyama

Awaji

Rokko

Awaji

Rokko

Fig. 2

Strike lines

Ryoke San’yo

San’in

134°E 136°E

34°N

36°N

132°E

Fig. 1. Distribution of the Upper Cretaceous Izumi and Onogawa groups along the Median Tectonic Line (the MTL), and distributions of granitic provinces in the Inner Zone of Southwest Japan (modified after Yamakita and Ito (1999), Kamei (2004), and Fujii et al. (2008)).

Kyushu Shikoku

Kii Peninsula

㸰㸬ᆅ㉁ᴫ␎

㸰㸬㸯 ᆅ㉁༢ඖ

࿴Ἠᒙ⩌ࡢ୺ศᕸᇦࡢᮾ㒊࡟఩⨨ࡍࡿ࿴Ἠᒣ⬦ᆅᇦࡢ࿴Ἠᒙ⩌ࡣ㸪㡿ᐙᖏࡢἨ༡ὶ⣠ᒾ 㢮࠾ࡼࡧ㡿ᐙⰼᓵᒾ㢮ࢆᇶ┙ᒾ࡜ࡋ࡚࠾ࡾ㸦Fig. 2㸧㸪ࡇࢀࡽ୕ࡘࡢᆅ㉁༢ඖࡣ㸪໭࠿ࡽ㡿 ᐙⰼᓵᒾ㢮ࡢᑼ࢔ࢲ࣓ࣟᒾ㸪Ἠ༡ὶ⣠ᒾ㢮㸪࿴Ἠᒙ⩌ࡢ㡰࡟MTL࡟ἢࡗ࡚す༡᪥ᮏෆᖏ ࡢ༡⦕㒊࡟ᴫࡡᮾす᪉ྥࡢᖏ≧࡟㓄ิࡋ࡚࠸ࡿ㸦Fig. 3㸧㸬

㸰㸬㸰 ᑼ࢔ࢲ࣓ࣟᒾ

ᑼ࢔ࢲ࣓ࣟᒾ㸦ᒣ⏣ဴ㞝࡯࠿, 1979㸧ࡣ㸪Ἠ༡ὶ⣠ᒾ㢮࠾ࡼࡧ࿴Ἠᒙ⩌໭⦕࡟ἢࡗ࡚ศ ᕸࡍࡿ᪂ᮇ㡿ᐙⰼᓵᒾ㢮࡛࠶ࡿ㸦Fig. 3㸧㸬ᶍᘧᆅࡣἙෆ㛗㔝ᕷ▼ᕝὶᇦࡢᑼ࡛࠶ࡿ㸬ᕷ ཎ࡯࠿㸦1986㸧ࡣ㸪୕ᅜᒣ㸦Fig. 3㸧௨すࡢᑼ࢔ࢲ࣓ࣟᒾࢆ㏆^ࡇᮌᕝ^{ࡂ ࡀ ࢃ}ⰼᓵᒾ࡜࿧ࢇ࡛࠸ࡿ㸬 ᶍᘧᆅ࿘㎶ࡢᑼ࢔ࢲ࣓ࣟᒾࡣ㸪᱈Ⰽࢆ࿊ࡍࡿ࢝ࣜ㛗▼࡜㯮㞼ẕࡀ≉ᚩⓗ࡛㸪࢝ࣜ㛗▼㸪

ᩳ㛗▼࠾ࡼࡧ▼ⱥࡢ๭ྜࡀ࡯ࡰ➼㔞ࡢ࢔ࢲ࣓ࣟᒾ࡟ศ㢮ࡉࢀࡿ㸦ᒣ⏣ဴ㞝࡯࠿, 1979㸧㸬ᶍ ᘧᆅᮾ᪉ࡢኳ^࠶ࡲぢ^ࡳᕝ^ࡀࢃ࿘㎶࡛ᑼ࢔ࢲ࣓ࣟᒾ࡟᥋ࡋ࡚ศᕸࡍࡿኳぢ▼ⱥ㛝⥳ᒾ㸦ᒣ⏣ဴ㞝࡯࠿, 1979㸧ࡣ㸪୺࡟ᩳ㛗▼㸪▼ⱥ㸪㯮㞼ẕ㸪ゅ㛝▼࠿ࡽ࡞ࡾ㸪࢝ࣜ㛗▼ࢆ࡯࡜ࢇ࡝ྵࡲ࡞࠸࡞

࡝㸪ᑼ࢔ࢲ࣓ࣟᒾ࡜ࡣࡸࡸ␗࡞ࡿ≉ᚩࢆᣢࡘࡀ㸪ᑼ࢔ࢲ࣓ࣟᒾ࡜₞⛣㛵ಀ࡟࠶ࡿ࡜ゎ 㔘ࡉࢀ࡚࠸ࡿ㸦ᒣ⏣ဴ㞝࡯࠿, 1979㸧㸬௚᪉㸪ᨻᒸ㸦1982㸧ࡣ㸪ᮾ㒊ࡢ᪂ᮇⰼᓵᒾ㢮࡟ࡘ࠸

࡚ヲ⣽࡟᳨ウࡋࡓ⤖ᯝ㸪ᒣ⏣ဴ㞝࡯࠿㸦1979㸧ࡀᑼ࢔ࢲ࣓ࣟᒾ࡜࿧ࡪᆅ㉁༢ඖࡢ୍㒊࡟

ࡘ࠸࡚ᑠ῝ⰼᓵ㛝⥳ᒾࡢᑼᆺ࡜࠸࠺༊ศࢆ⏝࠸ࡓୖ࡛㸪ᑠ῝ⰼᓵ㛝⥳ᒾ࡜ኳぢ▼ⱥ㛝⥳

ᒾࡢ㛵ಀࡣ㸪₞⛣ⓗ࡛࠶ࡿࡀኳぢ▼ⱥ㛝⥳ᒾࡀⱝᖸඛ⾜ࡋ࡚㈏ධࡋࡓ࡜ゎ㔘ࡋ࡚࠸ࡿ㸬ࡼ

ࡗ࡚㸪ᮏ◊✲࡛ࡣ㸪ࡇࢀࡽࡢ㈏ධᚋࡢ෭༷ྐࢆ᥎ᐃࡍࡿ࡟࠶ࡓࡾ㸪ኳぢ▼ⱥ㛝⥳ᒾ࡜ᑼ

࢔ࢲ࣓ࣟᒾࢆ༊ูࡏࡎ࡟㸪୍ᣓࡋ࡚ᑼ࢔ࢲ࣓ࣟᒾ࡜࿧ࡪࡇ࡜࡜ࡍࡿ㸬

ᑼ࢔ࢲ࣓ࣟᒾ࠿ࡽࡣ㸪௨ୗࡢᨺᑕᖺ௦ࡢሗ࿌ࡀ࠶ࡿ㸬す㒊ᆅᇦ࠿ࡽࡢࢪࣝࢥࣥࣇ࢕ࢵ

ࢩ࣭ࣙࣥࢺࣛࢵࢡ㸦FT㸧ᖺ௦65.7s4.6 Ma㸦KW60㸸ᒣ⏣࡯࠿, 1987㸧࠾ࡼࡧRb–Sr඲ᒾ

࢔࢖ࢯࢡࣟࣥᖺ௦78.3s3.0 Ma㸦Fig. 3ࡢヨᩱY1601-07ࢆ౑⏝㸸Morioka et al., 2000㸧

࡜㸪ᮾ㒊ᆅᇦ࠿ࡽࡢ㯮㞼ẕK–Arᖺ௦97.8s20.4 Ma࠾ࡼࡧ࢝ࣜ㛗▼K–Arᖺ௦72.9s4.6 Ma㸦76122802㸸ᰘ⏣࡯࠿, 1979㸧㸪୺ศᕸᇦᮾ➃㒊࠿ࡽࡢᬑ㏻ゅ㛝▼K–Arᖺ௦86.8s8.6 Ma㸪㯮㞼ẕK–Arᖺ௦80.5s5.0 Ma㸪࢝ࣜ㛗▼K–Arᖺ௦65.8s2.8 Ma࠾ࡼࡧAr–Ar ᖺ௦66.0s2.2 Ma㸦Ng08㸸Shibata et al., 1994㸧㸪࢝ࣜ㛗▼K–Arᖺ௦65.2s4.0 Ma㸦Ng7㸸 ᰘ⏣࡯࠿, 2003㸧㸪ᬑ㏻ゅ㛝▼K–Arᖺ௦78.8s4.8 Ma㸪࢝ࣜ㛗▼K–Arᖺ௦68.3s4.1 Ma

ｖ

ｖ ｖ

ｖ ｖ

ｖ

ｖ

ｖ

ｖｖ

ｖ

ｖ

ｖ ｖ ｖｖ

ｖｖ ｖ

ｖ ｖ

ｖ ｖ

ｖ ｖ ｖ

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ｖ ｖ

ｖ ｖ ｖ

ｖ ｖ ｖ ｖ

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ｖ ｖ ｖｖ ｖ ｖ ｖｖ

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ｖ ｖｖ ｖ ｖ

ｖ ｖ ｖ ｖ

ｖ

ｖ ｖ

ｖ ｖｖ ｖ ｖ ｖ ｖ

ｖ ｖ ｖ ｖ ｖｖ

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ｖ

ｖｖ ｖ ｖ

ｖｖ ｖ ｖ ｖ ｖ ｖ ｖ

ｖ ｖ ｖ ｖ

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ｖ

ｖ ｖ ｖ ｖ

ｖ ｖ

ｖ ｖ ｖ

ｖ ｖ

ｖ

ｖ ｖ ｖ ｖ

ｖ ｖ

ｖ ｖ ｖ

ｖ ｖ ｖ ｖ ｖ ｖ ｖ

ｖ ｖ ｖ

ｖ ｖ

ｖ ｖ ｖ ｖ

ｖ

ｖｖｖ

ｖ ｖ ｖ ｖ ｖ ｖ

ｖ

ăă ăă ăă ă ă

ă ăă ă

Fault / inferred one

NP

M T L

Main facies

Takihata alternation Member Nate Fm.

Hashimoto

Gojo

Izumisano Sennan Huke Kada Wakayama

Azenotani mudstone Member Kasayama conglomerate Mem.

Mutsuo Fm.

Northern marginal facies

Southern facies

135°05’ 34°20’

Osaka Bay

Porphyries Sennan Rhyolites

N

Takihata

Kyoshi syncline

Central par

t

Eastern par t

estern par W

t

I-1 I-2

I-3

I-8 I-5

I-7 I-4

I-6

I-10 I-9

TK08 NT03IW07 SH07KK01KK02IW02

SH04 SH01 SH05 SH02 KD02

NT01IW01

KK06

TK07 TK05

TK06 KD01

NT02 KK05

KK04

KK03IW04IW05 IW06KK07

KK08 KK09IW03 KK11 SH09SH08

SH03 SH06

KK10

IW08IW09 IW11

IW10 SH10

This study Suzuki (1996)

Fig. 3 Fig. 2. Geology of the Izumi Mountains and sample localities within the Izumi Group, compiled from Ichikawa et al. (1979), Yamada, N. et al. (1979), Itihara et al. (1986), and Miyata (1990); open circles show the location of samples for organic maturation analysis taken during this study, and solid squares numbered from I-1 to I-10 show the sampling localities of Suzuki (1996) using for vitrinite reflectance.

Samples

Ishi R.Amami R.Ishimi R.Chihaya R. NP

52

4941 54

44 45 506250

34

4062

46

60

41 52

34 37

30 30 23

38

42

4548 54

32 35

263328 41 43

34 48

32 40 4440

40 25

Kashii R. Shinge R. Kin’yuji R.

Kogi R.

Chichioni R. Makio R.

Ishi R. Soba R.

Higashimakio R. Ushitaki R. Kibitani R. Soba R.

Tsuda R. Mide R.

135°25’ 34°25’ N W

W W

FTs1 FTs3

FTsb1 FTa8 76122802 Ng08 Ng7

Y1601-07 KW59

KW60 KW59

76122802 FTs1 FTs3

FTa10

FTsb1 FTa8 KW58 76122802 Ng08 Ng7

Ng10

FTt1 FTa1

TK05

KS04 Mt.Mikuni Mt.Katsuragi

KK06IW03 TK06 TK07 TK08

TK01

TK03

TK04 TK02

TK02 KS03 KS02 KS01-1,2 AZ06 AZ05AZ04AZ03 AZ02 AZ01AZ07 SH11

Mt.Iwawaki Legend

Rock samples (organic maturation analysis) Radiometric dating points of previous studies Izumi Group Porphyries Ryoke metamorphic rocks

Sennan Rhyolites Ogawa Andesite

Amami Quartz-diorite

Takijiri Adamellite conglomerate and sandstone mudstone undivided in lithologyalternat. sandstone, mudstone and conglom. faulttuff layer

Ѝeastern partcentral partЋ Ѝcentral partwestern partЋ

Samples(fission-track dating) fault contactunconformityIzumi Group / basement rock lithology in this study Fig. 3. Geological map in the northern Izumi Mountains, showing sampling locations. Open circles denote sample locations for vitrinite reflectance analysis; grey hexagons denote sample locations for fission-track dating; solid squares denote sample locations for radiometric dating analyzed in previous studies; open stars denote outcrops of the basal unconformity of the Izumi Group overlying the Sennan Rhyolites and Porphyries; and solid stars denote outcrops of fault contacts between the Izumi Group and basement rocks. Three acidic tuff layers within the Izumi Group were used as marker beds (t8, t10, and t11 of Miyata, 1980). The distribution of the Sennan Rhyolites and Porphyries is after Yamada, N. et al. (1979) and Itihara et al. (1986), and the distribution of the Ryoke granitoids in the northern Izumi Mountains, Kii Peninsula is after Yamada, T. et al. (1979), Masaoka (1982), and Itihara et al. (1986). Abbreviations: alternat. = alternations of; conglom. = conglomerate; R. = River.

㸦Ng10㸸ᰘ⏣࡯࠿, 2003㸧࡛࠶ࡿ㸬ࡓࡔࡋ㸪FTᖺ௦࡟㛵ࡋ࡚ࡣ㸪1990ᖺࡢᖺ௦ᶆ‽໬່

࿌㸦Hurford, 1990㸧௨๓ࡢࢹ࣮ࢱ࡛࠶ࡿࡓࡵ㸪⌧ᅾࡣཧ⪃್࡜ࡋ࡚ᢅࢃࡊࡿࢆᚓ࡞࠸㸬ࡲ

ࡓ㸪ヨᩱ76122802ࡢ㯮㞼ẕ࡟ࡣⴭࡋ࠸ኚ㉁ࡀㄆࡵࡽࢀࡿ㸦ᰘ⏣࡯࠿, 1979㸧ࡓࡵ㸪ࡇࡢ㯮

㞼ẕࡢ K–Ar ᖺ௦ࡣ෭༷ྐࡢゎ㔘࡟౑⏝ࡋ࡞࠸㸬ᑼ࢔ࢲ࣓ࣟᒾࡣ㸪௨ୗࡢ⌮⏤࠿ࡽ㸪࿴

Ἠᒙ⩌ሁ✚㛤ጞ௨๓࡟㈏ධࡋࡓ࡜⪃࠼ࡽࢀࡿ㸬ᑼ࢔ࢲ࣓ࣟᒾ࡟ࡼࡗ࡚⇕ኚᡂస⏝ࢆཷࡅ ࡓἨ༡ὶ⣠ᒾ㢮ࡀ㸪㠀ኚᡂࡢ࿴Ἠᒙ⩌࡟ࡼࡗ࡚୙ᩚྜ࡛そࢃࢀ࡚࠸ࡿࡇ࡜㸦ᒣ⏣┤฼࡯࠿, 1979㸧㸪࠾ࡼࡧᑼ࢔ࢲ࣓ࣟᒾ㸦ኳぢ▼ⱥ㛝⥳ᒾ㸧ࡣ࿴Ἠᒙ⩌࡜୙ᩚྜ㛵ಀ࡟࠶ࡿࡇ࡜㸦ᨻ ᒸ㸪1982㸧࠿ࡽ࡛࠶ࡿ㸬

㸰㸬㸱 Ἠ༡ὶ⣠ᒾ㢮

Ἠ༡ὶ⣠ᒾ㢮㸦ᕷཎ࡯࠿, 1986㸧ࡣ㸪࿴Ἠᒣ⬦໭㯄࡟ᗈࡃศᕸࡍࡿ⁐⤖จ⅊ᒾࢆ୺య࡜ࡍ

ࡿᆅ㉁༢ඖ࡛࠶ࡿ㸦Fig. 3㸧㸬ᕷᕝ࣭኱ᶫ㸦1965㸧ࡣ㸪㡿ᐙᖏ༡㒊࡟఩⨨ࡋ࿴Ἠᒙ⩌࡟୙ᩚ

ྜ࡛そࢃࢀࡿⅆᒣᒾ㢮ࡢ࠺ࡕ㸪࿴Ἠᒣ⬦໭㯄࡟ศᕸࡍࡿⅆᒣᒾ㢮ࢆ㸪Ἠ༡ᆅᇦࢆᶍᘧᆅ࡜

ࡋ࡚Ἠ༡㓟ᛶⅆ○ᒾ㢮࡜࿨ྡࡋࡓ㸬୍᪉㸪ᒣ⏣┤฼࡯࠿㸦1979㸧ࡣ㸪Ἠ༡㓟ᛶⅆ○ᒾ㢮࡟

ୗ఩ࡢሁ✚ᒾ㸦ഃ^ࡑࡤᕝ^ࡀࢃ♟ᒾᒙ㸧࠾ࡼࡧᏳᒣᒾ⁐ᒾ㸦ᑠᕝᏳᒣᒾ㸧ࢆຍ࠼࡚㸪Ἠ༡ᒙ⩌࡜ࡋ࡚

෌ᐃ⩏ࡋࡓ㸬ࡑࡢᚋ㸪ᕷཎ࡯࠿㸦1986㸧ࡣ㸪Ἠ༡ᒙ⩌࠿ࡽ᭱ୗ఩ᒙࡢᑠᕝᏳᒣᒾࢆ㝖࠸ࡓ

ഃᕝ♟ᒾᒙ࡜Ἠ༡㓟ᛶⅆ○ᒾ㢮࡟ࡘ࠸࡚Ἠ༡ὶ⣠ᒾ㢮࡜࠸࠺ྡ⛠ࢆ⏝࠸ࡓ㸬Ἠ༡ὶ⣠ᒾ㢮 ࡣ㸪࿴Ἠᒣ⬦໭㯄࡟ᮾす⣙25 km ࡟ࢃࡓࡾ⣽㛗ࡃศᕸࡋ㸦Fig. 1㸧㸪ᴫࡡᮾす㉮ྥ࣭ᛴഴᩳ

ࡢ༡ୖ఩࡛㸪✚⟬ᒙཌࡣ4000 m࡟㐩ࡍࡿ㸦ᕷཎ࡯࠿, 1986㸧㸬࿴Ἠᒣ⬦ࡢす࡟࠶ࡓࡿῐ㊰

ᓥ࡟ࡣ㸪Ἠ༡ὶ⣠ᒾ㢮࡟ᑐẚࡉࢀࡿⅆᒣᒾ㢮ࡀ࿴Ἠᒙ⩌໭⦕࡟ἢࡗ࡚ᑠつᶍ࡟ศᕸࡋ࡚࠾

ࡾ㸪ࡑࢀࡽࢆἨ༡ὶ⣠ᒾ㢮࡜࿧ࡪሙྜࡶ࠶ࡿ㸦౛࠼ࡤ㸪㧗ᶫ࡯࠿, 1992㸧㸬୺࡟ࢹ࢖ࢧ࢖ࢺ࣭

ὶ⣠ᒾ㉁ࡢ⁐⤖จ⅊ᒾ࡜㠀⁐⤖จ⅊ᒾᒙࡢ⧞ࡾ㏉ࡋ࠿ࡽ࡞ࡾ㸪㠀จ⅊㉁࡞ഃᕝ♟ᒾᒙ࠾ࡼ

ࡧ⁐ᒾࢆ୺య࡜ࡍࡿ∵^࠺ࡋᕝ^ࡓࡁࡀࢃὶ⣠ᒾ⁐ᒾࢆྵࡵ࡚㸪17 ᒙ࡟༊ศࡉࢀ࡚࠸ࡿ㸬ࡍ࡞ࢃࡕ㸪ୗ఩

ࡼࡾഃᕝ♟ᒾᒙ㸪W1⁐⤖จ⅊ᒾ㸪∵ᕝὶ⣠ᒾ⁐ᒾ㸪W2⁐⤖จ⅊ᒾ㸪W3⁐⤖จ⅊ᒾ㸪

⏿จ⅊㉁◁ᒾᒙ㸪W4⁐⤖จ⅊ᒾ㸪ዟỈ㛫^{࠾ ࡃ ࡳ ࡎ ࡲ}จ⅊㉁◁ᒾᒙ㸪W5⁐⤖จ⅊ᒾ㸪W6⁐⤖จ⅊ᒾ㸪

⛞ࡁࡧ

㇂ᕝ^ࡓ࡟ࡀࢃจ⅊ᒾᒙ㸪W7⁐⤖จ⅊ᒾ㸪ୗ^ࡋࡶ኱ᮌ^{࠾ ࠾ ࡁ}จ⅊ᒾᒙ㸪W8⁐⤖จ⅊ᒾ㸪W9⁐⤖จ⅊ᒾ㸪ᒸ

୰จ⅊ᒾᒙ㸪࠾ࡼࡧW10⁐⤖จ⅊ᒾ࡛࠶ࡿ㸬Ἠ༡ὶ⣠ᒾ㢮ࡢᄇฟࡣⓑள⣖ࡢⅆᒣ๓⥺࡟࠾

ࡅࡿⅆᡂάືࡢ୍ࡘ࡜ࡉࢀ࡚࠸ࡿ㸦ᒣ⏣, 2005㸧㸬

Ἠ༡ὶ⣠ᒾ㢮ศᕸᇦ࡟ࡣ኱ᑠᵝࠎ࡞つᶍࡢᩬᒾ㢮ࡢᒾᰴࡀㄆࡵࡽࢀࡿ㸦Fig. 3㸧㸬୺࡟▼

ⱥᩬᒾ࠾ࡼࡧⰼᓵᩬᒾ࠿ࡽ࡞ࡿ㸬ᒣ⏣┤฼࡯࠿㸦1979㸧ࡣ㸪ᩬᒾ㢮࡜⁐⤖จ⅊ᒾࡢ༊ูࡀ 㔝እ࡛ᅔ㞴࡞ሙྜࡀ࠶ࡿ࡜ᣦ᦬ࡋ࡚࠾ࡾ㸪ᐇ㝿࡟ᒣ⏣┤฼࡯࠿㸦1979㸧࡛ⰼᓵᩬᒾ㢮࡜ࡉ

ࢀࡓᆅ㉁యࡢゎ㔘ࡀ㸪ᕷཎ࡯࠿㸦1986㸧࡛ࡣ⁐⤖จ⅊ᒾ࡬࡜ኚ᭦ࡉࢀࡓ㒊ศࡶᏑᅾࡍࡿ㸬

ᩬᒾ㢮ࡣ࠸ࡎࢀࡶἨ༡ὶ⣠ᒾ㢮ࡢᄇฟ௨㝆࡟㈏ධࡋࡓ࡜⪃࠼ࡽࢀ࡚࠾ࡾ㸪㟢㢌ほᐹ࠿ࡽ᥎ ᐃࡉࢀࡿ㈏ධ㛵ಀ࠾ࡼࡧᑼ࢔ࢲ࣓ࣟᒾ࡟ࡼࡿ࡜⪃࠼ࡽࢀࡿ⿕⇕ࡢ᭷↓࠿ࡽ㸪Ἠ༡ὶ⣠ᒾ 㢮ሁ✚ᚋ࡛ᑼ࢔ࢲ࣓ࣟᒾࡼࡾྂ࠸࡜⪃࠼ࡽࢀࡿⰼᓵᩬᒾ㢮㸦Ϩ㸧࡜㸪ࡑࢀࡽࢆ㝖࠸ࡓⰼ ᓵᩬᒾ㢮㸦ϩ㸧ࡀ㆑ูࡉࢀ࡚࠸ࡿ㸦ᕷཎ࡯࠿, 1986㸧㸬

Ἠ༡ὶ⣠ᒾ㢮ࡢ໭⦕࡟ࡣ᪂ᮇ㡿ᐙⰼᓵᒾ㢮ࡢᑼ࢔ࢲ࣓ࣟᒾࡀศᕸࡋ㸪୧⪅ࡣ⌧ᅾᗈ࠸

⠊ᅖ࡛᩿ᒙ㛵ಀ࡟࠶ࡿ㸦ᕷཎ࡯࠿, 1986㸧㸬ࡋ࠿ࡋ㸪௨ୗࡢ⌮⏤࡟ࡼࡾᑼ࢔ࢲ࣓ࣟᒾࡀἨ ༡ὶ⣠ᒾ㢮ሁ✚ᚋ࡟㈏ධࡋࡓ࡜⪃࠼ࡽࢀ࡚࠸ࡿ㸬Ἠ༡ὶ⣠ᒾ㢮࡟ࡣᑼ࢔ࢲ࣓ࣟᒾࡢ㈏ධ

᫬ࡢ⇕ኚᡂ࡟ࡼࡿ㯮㞼ẕࡢ⏕ᡂࡀ࠸ࡃࡘ࠿ࡢᆅᇦ࡛ㄆࡵࡽࢀ㸪㈏ධ㛵ಀ࡜Ἠ༡ὶ⣠ᒾ㢮ࡢ

⿕⇕ࢆ♧ࡍ㟢㢌ࡀᏑᅾࡍࡿ㸦ᒣ⏣┤฼࡯࠿㸪1979㸧㸬ࡓࡔࡋ㸪ูࡢぢゎࡶ࠶ࡿ㸬ᕷᕝ࣭኱ᶫ 㸦1965㸧࠾ࡼࡧୖ஭࣭㜿㒊㸦1977㸧ࡣ㸪∵ᕝἢ࠸࡟ศᕸࡍࡿ኱ἑ♟ᒾ㸦ᕷཎ࡯࠿, 1986㸧

࡟ࡘ࠸࡚㸪Ἠ༡㓟ᛶⅆ○ᒾ㢮㸦Ἠ༡ὶ⣠ᒾ㢮㸧࡟ᒓࡋ㸪㡿ᐙᖏࡢⰼᓵᒾ㢮ࢆ୙ᩚྜ࡟そࡗ

࡚࠸ࡿ࡜ゎ㔘ࡋࡓ㸬ࡇࢀ࡟ᑐࡋ࡚㸪ᒣ⏣┤฼࡯࠿㸦1979㸧ࡣ㸪ࡇࡢ♟ᒾ࡜㢮ఝࡍࡿ♟ᒾࡀ Ἠ༡ὶ⣠ᒾ㢮࡛௚࡟ㄆࡵࡽࢀ࡞࠸ࡇ࡜ࡸᒾ┦ࡢ≉ᚩ࠿ࡽ㸪ྂ➨୕⣖ࡲࡓࡣ᪂➨୕⣖ࡢሁ✚

≀ࡢྍ⬟ᛶࡀ㧗࠸࡜ࡋ࡚࠸ࡿ㸬ࡲࡓ㸪ᕷཎ࡯࠿㸦1986㸧ࡣ㸪኱ἑ♟ᒾࡀἨ༡ὶ⣠ᒾ㢮ࡢ♟

ࢆ୺࡜ࡋ㸪ᇶ㉁ࡀ㠀จ⅊㉁࡛࿴Ἠᒙ⩌࡯࡝ᅛ⤖ࡋ࡚࠾ࡽࡎ㸪ᮏᆅᇦࡢ໭ഃ࡟᩿⥆ⓗ࡟ศᕸ ࡍࡿ୰᪂⤫㸽ࡢ⏑༡ഛ^{࠿ ࢇ ࡞ ࡧ}ᒙ㸦ྜྷᕝ㸪1973㸧࡜࿴Ἠᒙ⩌ࡢ୰㛫ࡃࡽ࠸ࡢᅛ⤖ᗘ࡛࠶ࡿࡇ࡜࠿ࡽ

ྂ➨୕⣖ࡢἙᕝሁ✚≀࡜ゎ㔘ࡋ࡚࠾ࡾ㸪ᕷᕝ࣭኱ᶫ㸦1965㸧࠾ࡼࡧୖ஭࣭㜿㒊㸦1977㸧ࡢ ゎ㔘ࢆᨭᣢࡋ࡚࠸࡞࠸㸬Ἠ༡ὶ⣠ᒾ㢮ࡢ༡⦕ࡣ㸪࿴Ἠᒙ⩌࡟୙ᩚྜ࡛そࢃࢀࡿ㸪ࡲࡓࡣ࿴

Ἠᒙ⩌࡜᩿ᒙ࡛᥋ࡍࡿ㸬ᩬᒾ㢮ࢆྵࡵࡓἨ༡ὶ⣠ᒾ㢮ࡀ࿴Ἠᒙ⩌࡟┤᥋୙ᩚྜ࡛そࢃࢀࡿ

㟢㢌ࡣ㸪ᮏㄪᰝᆅᇦෆࡢᑡ࡞ࡃ࡜ࡶ16⟠ᡤ࡛☜ㄆࡉࢀࡿ㸦Fig. 3ࡢⓑᫍ༳㸧㸬௚᪉㸪୧ᆅ㉁

༢ඖࡀ᩿ᒙ࡛᥋ࡍࡿ㟢㢌ࡣ㸪10⟠ᡤ࡛☜ㄆࡉࢀࡿ㸦Fig. 3ࡢ㯮ᫍ༳㸧㸬㟢㢌ほᐹ࡛ࡣ㸪ቃ⏺

᩿ᒙࡢᮾす࡬ࡢ㐃⥆ᛶࡣᝏࡃ㸪୙ᩚྜ㛵ಀ࡜᩿ᒙ㛵ಀࡀቃ⏺࡟ἢࡗ࡚⧞ࡾ㏉ࡋ࡚࠸ࡿ㸬ࡲ

ࡓ㸪༡⦕㒊ࡢἨ༡ὶ⣠ᒾ㢮࡟ࡣ㸪ᆅᒙࡢഃ᪉㐃⥆ᛶࡀᝏࡃ㸪࠶ࡿᆅᒙࡢ㉮ྥᘏ㛗ୖࡢୖ఩

ᒙ‽ࡲࡓࡣ┤᥋ⓗ࡞ᒙ఩㛵ಀࡀ୙᫂࡞ᆅᒙࡢ㟢ฟࡀ」ᩘᆅⅬ࡛☜ㄆࡉࢀࡿ㸬ᕷཎ࡯࠿

㸦1986㸧࡟࠾ࡅࡿ㸪ᶔ஭ᕝἢ࠸ࡢୗ኱ᮌจ⅊ᒾᒙ㸪㏆ᮌᕝᨭὶࡢዟỈ㛫จ⅊㉁◁ᒾᒙ➼ࡢ

ࣞࣥࢬ≧ศᕸࡢ࡯࠿࡟㸪ᑠつᶍ࡛ࡣ࠶ࡿࡀࣀụ࿘㎶㸪⛞㇂ᕝᮏὶ࣭ᨭὶ㸪▼ᕝἢ࠸ࡢ࿴

Ἠᒙ⩌ࡢᇶ┙୙ᩚྜ௜㏆࡛ࡶ㸪୙ᩚྜ௜㏆ࡢࡳ࡟࿘㎶࡜␗࡞ࡿᒾ┦ࡢἨ༡ὶ⣠ᒾ㢮ࡀᑠつ ᶍ࡟㟢ฟࡋ࡚࠸ࡿ㸬ࡇࢀࡽ࠿ࡽࡣ㸪୙ᩚྜ௜㏆ࡢᆅᒙࡢ஘ࢀࢆ♧ࡍྍ⬟ᛶࡀ࠶ࡿ㸦ᚋ㏙㸧㸬

Ἠ༡ὶ⣠ᒾ㢮ࡢᨺᑕᖺ௦࡟ࡘ࠸࡚㸪ᒣ⏣࡯࠿㸦1987㸧ࡀ㸪ศᕸᇦす⦕࡛㔠^ࡁࢇ⇃^ࡺ࠺ᑎᕝ^{ࡌ ࡀ ࢃ}ἢ࠸ࡢ

᭱ୖ఩ᒙ‽௜㏆2ᒙ‽㸦Fig. 3ࡢKW58࠾ࡼࡧKW59㸧࡛ࢪࣝࢥࣥFTᖺ௦75.3s5.0 Ma

northern marginal

facies main Facies southern

Facies

Nate Fm.

Kasayama Conglomerate Member Azenotani Mudstone Mem. Takihata Alternation Mem.

Kokawa Formation Iwade Formation

Shindachi Formation

Kada Formation

not exposed

Mutsuo Formation

YYYYYYYYYYYY YYYYYYYYYYYY

YYYYYYYYYYYY YYYYYYYYYYYY YYYYYYYYYYYY YYYYYYYYYYYY

YYYYYYYYYYYY t1 76.5±7.6 Ma (ED1) t4

t7 72.1±8.2 Ma (ED1) t12 72.1±7.2 Ma (ED2) t15 73.1±7.6 Ma (ED1) t27 72.2±7.8 Ma (ED2)

t14

YYYYYYYYYYYY vvvvvvvvvvv vvvvvvvvvvv

t11 t10 t8

The tuff marker bed in Miyata (1980) (Zircon fisson-track ages after Miyata et al., 1993).

F. Formation Member Formation

Abbreviations; ED1: inner surface, ED2: outer surface, F.: Formation, Mem.: Member.

Fig. 4. Lithostratigraphic division of the Izumi Group in the Izumi Mountains, which is modified after the Research Group for the MTL in West Kinki (1981).

The relationship among the three facies; the northern marginal facies, the main facies, and the southern facies are contemporaneous heteropic facies.

Izumi Group

㸦KW59㸧㸪77.4s5.4 Ma㸦KW58㸧ࢆ㸪ᰘ⏣࣭ෆᾏ㸦1992㸧ࡀ㸪ྠ୍ᒾ▼ヨᩱࢆ⏝࠸࡚ࢧ ࢽࢹ࢕ࣥࡢK–Arᖺ௦88.0s5.6 Ma㸦KW59㸧㸪90.4s5.6 Ma㸦KW58㸧ࢆࡑࢀࡒࢀሗ࿌ࡋ ࡓ㸬ࡇࡢ࠺ࡕ㸪FT ᖺ௦ࡣ㸪1990 ᖺ௨๓ࡢࢹ࣮ࢱ࡛࠶ࡿࡓࡵཧ⪃್࡜ࡍࡿ㸬ࡲࡓ㸪∵ᕝ ὶ⣠ᒾ⁐ᒾ࠿ࡽࡣSibata et al. 㸦1978㸧ࡀ඲ᒾࡢK–Arᖺ௦59.4s4.8Maࢆሗ࿌ࡋࡓࡀ㸪 ᄇฟᖺ௦࡛ࡣ࡞ࡃ㸪஧ḟⓗ࡞ኚ㉁స⏝ࡢᙳ㡪࡜⪃࠼ࡽࢀ࡚࠸ࡿ㸦ᒣ⏣┤฼࡯࠿, 1979㸧㸬

㸰㸬㸲 ࿴Ἠᒙ⩌

ୖ㒊ⓑள⣔࿴Ἠᒙ⩌㸦Matsumoto, 1954㸧ࡣ㸪す༡᪥ᮏෆᖏࡢ༡⦕㒊㸪MTLࡢ໭ഃ࡟ἢ ࡗ࡚ᅄᅜ࠿ࡽ⣖ఀ༙ᓥࡲ࡛⣽㛗ࡃᖏ≧࡟ศᕸࡍࡿ㸦Fig. 1㸧㸬ᒾ┦ࡢഃ᪉ኚ໬ࡀⴭࡋ࠸ࡓࡵ

࡟㸪♟ᒾ࣭◁ᒾ࣭Ἶᒾ࠿ࡽ࡞ࡿ㢖⦾࡞ᒾ┦ኚ໬࡜ሢ≧Ἶᒾࢆ≉ᚩ࡜ࡍࡿ໭⦕┦࠾ࡼࡧ༡㒊

┦࡜㸪◁ᒾἾᒾ஫ᒙ࠾ࡼࡧ◁ᒾ♟ᒾ஫ᒙࢆ୺య࡜ࡍࡿ୺㒊┦ࡢ㸱┦࡟኱ูࡉࢀࡿ㸦Figs.2, 4㸧㸬

࿴Ἠᒣ⬦ᆅᇦ࡛ࡣ㸪༡⦕㒊ࢆMTL࡟⏬ࡉࢀ㸪໭⦕㒊ࡣ๓㏙ࡢ㏻ࡾἨ༡ὶ⣠ᒾ㢮࣭ᑼ࢔ࢲ

࣓ࣟᒾࢆ୙ᩚྜ࡟そ࠺࠿㸪ࡑࢀࡽ࡜᩿ᒙ࡛᥋ࡍࡿ㸦Fig. 3㸧㸬኱㒊ศࡢᆅᇦ࡛ࡣᮾす࠿ࡽ༡ す―໭ᮾ㉮ྥ࣭༡ഴᩳ࡛༡ୖ఩ࡢྠᩳᵓ㐀ࢆ࡞ࡍࡀ㸪࿴Ἠᒣ⬦す㒊ᆅᇦ࡟኱つᶍ࡞ྥᩳᵓ㐀 ࡀᏑᅾࡍࡿ㸦Fig. 2㸧㸬ᮏ◊✲࡛౑⏝ࡍࡿ࿴Ἠᒣ⬦ᆅᇦࡢᒙᗎ༊ศࡣ㸪ᇶᮏⓗ࡟㏆␥す㒊MTL

◊✲ࢢ࣮ࣝࣉ㸦1981㸧࡟ᚑࡗࡓ㸦Fig. 4㸧㸬࿴Ἠᒙ⩌໭⦕┦ࡣ඲ᇦࡀභ^ࡴࡘᑿ^࠾ᒙ࡜࿧ࡤࢀࡿ㸦⏣

୰, 1965㸹∾ᮏ࡯࠿, 2004㸧㸬୺㒊┦ࡣ㸪ୗ఩ࡼࡾ㸪ຍኴ^{࠿ ࡔ}ᒙ㸪ಙ^ࡋࢇ㐩^ࡔࡕᒙ㸪ᒾฟ^{࠸ ࢃ ࡛}ᒙ㸪⢊Ἑᒙ࡟༊

ศࡉࢀ࡚࠸ࡿ㸦㏆␥す㒊MTL◊✲ࢢ࣮ࣝࣉ, 1981㸹∾ᮏ࡯࠿, 2004㸧㸬༡㒊┦ࡣྡᡭᒙ࡜࿧

ࡤࢀࡿ㸦㏆␥す㒊MTL◊✲ࢢ࣮ࣝࣉ, 1981㸹∾ᮏ࡯࠿, 2004㸧㸬

㸰㸬㸱㸬㸯 ໭⦕┦භᑿᒙ

࿴Ἠᒣ⬦ᆅᇦ࿴Ἠᒙ⩌ࡢ໭⦕┦භᑿᒙ㸦⏣୰, 1965㸧ࡣ㸪୺࡟㔠⇃ᑎᕝ௨ᮾ࠿ࡽ࿴Ἠᒣ⬦

ᮾ➃ࡲ࡛ࡢ࿴Ἠᒣ⬦໭㯄࡟ศᕸࡍࡿ㸦Fig. 2㸧㸬භᑿᒙࡣࡉࡽ࡟➟ᒣ♟ᒾ㒊ᒙ࠾ࡼࡧ ␏^࠶ࡐࡢ㇂^ࡓ࡟Ἶ ᒾ㒊ᒙ࣭⏿஫ᒙ㒊ᒙ࡟⣽ศࡉࢀࡿ㸦ᕷཎ࡯࠿, 1986㸧㸬୺㒊┦࡜ࡣྠ᫬␗┦ࡢ㛵ಀ࡟࠶ࡿ

ࡓࡵ㸪୧ᆅ㉁యࡢቃ⏺㒊࡛ࡣ᩿⥆ⓗࡲࡓࡣ₞⛣ⓗ࡟୺㒊┦ࡢ◁ᒾἾᒾ஫ᒙ࡬࡜ኚ໬ࡍࡿࡓ

ࡵ㸪ᒾ┦ୖࡢ୺㒊┦ࡀ໭⦕┦୰࡟ᣦ஺㛵ಀ࡟ศᕸࡍࡿሙྜࡀ࠶ࡿ㸦Figs. 3, 5㸧㸬 㸯㸧➟ᒣ♟ᒾ㒊ᒙ

ᑠᯘ㸦1931㸧ࡀ➟ᒣᇶᗏ♟ᒾᒙ࡜ࡋ࡚࿨ྡࡋࡓࡶࡢ࡜ྠ⩏࡛㸪ᶍᘧᆅࡣ኱㜰ᗓἨ༡ᕷ᪂ᐙ^{ࡋ ࢇ ࡆ} ࡢ➟ᒣ௜㏆࡛࠶ࡿ㸬࿴Ἠᒣ⬦୰㒊࡛ࡢ༡໭᪉ྥࡢ✚⟬ᒙཌࡣ⣙10㹼150 㹫࡛࠶ࡿ㸦Fig. 5㸧㸬

ୗ఩ࡢἨ༡ὶ⣠ᒾ㢮࣭ᑼ࢔ࢲ࣓ࣟᒾ࡜ࡣ୙ᩚྜ㛵ಀ㸪ࡲࡓࡣ୍㒊࡛᩿ᒙ㛵ಀ࡛࠶ࡿ㸬ቃ

⏺㟢㢌ࡢ㟢ฟࡀࡼ࠸す㒊ࡢ㟢㢌ほᐹ࡛ࡣ㸪୍⯡࡟࿴Ἠᒙ⩌ࡢᇶᗏ୙ᩚྜ㠃ࡣẚ㍑ⓗᖹᆠ࡛

Azenotani Mudstone Member

Basement

Kasayama Congl. Member

Shindachi Formation Iwade Formation

sandy mudstonemudstone very fine sandstone fine sandstone medium sandstone coarse sandstone sands. with gravel conglomerate Sennan Rhyolites

slump vvvvvvvvvvtuff layerTf

Xnconformitysandstone layer thickness <10 cm 10-30 cm >30 cminterbedded thin sandstone layer bedded mudstone pebbly mudstone

faultLdLegend

alternating sandstone and conglomerate

Classification of the alternating sandstone and mudstone muds. dominant sands. dominant equal amount vvvvvvvvvv vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv vvvvvvvvvvvvvvvvvvvvvvvvvv

vvvvvvvvvvv vvvvvvvvvvv

vvvvvvvvvvv vvvvvvvvvvvv

vvvvvvvvvvvvvvvvvvvvv no exposure

vvvvvvvvvvv vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv

vvvvvvvvvvv no exposure

vvvvvvvvvvv ռ Shikatani

no exposure Tf ձ Azenotani^Ћ

AZ02

vvvvvvvvvvv

TfTfTf

Tf Tf

Tf Tf

Tf Tf

Tf Tf Tf

Tf TfTf Kashii R. Kogi R.Kibitani R. Mide R.

N

Tsuda R.Ĭ ĭ ĮįİıIJ •≔⁝ 

ijĴĵĶ ķ

ո Ohkawa river շ Kibitani river ն Tachibanatani յ Kashii river մ Hiuchi- goetani ճ Inakuraike

0

200 m 100 չ Sobura պ Kamihira

ջ Ohsawa ղ Takinoike

ЋAZ06 ЋKS01-1 ЋFTs1ЋAZ05 ЋAZ04

t10 t8

t11

ЋFTs3 ЋAZ03

ЋKS02ЋKS03

Fig. 5. Columnar sections for the northern margin of the Izumi Group. The three marker beds t8, t10, and t11 are recognized in this sequence. The FT, AZ, and KS sample IDs represent sampling horizons of the fission-track dating and the analyses of vitrinite reflectance of this study. Abbreviations: Congl. = Conglomerate; muds. = mudstone; sands. = sandstone; R. = River.

࠶ࡿ㸬ࡋ࠿ࡋ㸪ቃ⏺㠃ࡢᅗᏛᘏ㛗⥺࡜ᐇ㝿ࡢ࿘㎶ᆅᇦࡢ୙ᩚྜࡢ఩⨨ࡣ୍⮴ࡋ࡞࠸ࡇ࡜ࡀ 㢖⦾࡟⏕ࡌࡿ㸬ࡲࡓ㸪᩿ᒙቃ⏺࡛᥋ࡍࡿ࿴Ἠᒙ⩌࡛ࡣ➟ᒣ♟ᒾ㒊ᒙ┦ᙜᒙࡢ୍㒊ࡲࡓࡣ඲

㒊ࢆḞࡃࡇ࡜ࡀከࡃ㸪᩿ᒙቃ⏺ࡣᗈ⠊ᅖ࡟㐃⥆ࡋ࡞࠸㸬ᒾ┦ࡣ♟ᒾ࠾ࡼࡧ♟㉁◁ᒾ࣭◁ᒾ

ࢆ୺య࡜ࡍࡿ㸬ᇶᗏ㒊ࡣἨ༡ὶ⣠ᒾ㢮ࢆ୙ᩚྜ࡛そ࠺ሢ≧↓ᒙ⌮ࡢᇶᗏ♟ᒾ࡛࠶ࡾ㸪♟ࡣ ளゅ㹼ள෇♟ࡢ୰♟࠾ࡼࡧᕧ♟࡛࠶ࡾ㸪Ὲửࡀᝏ࠸㸬♟✀ࡣ∵ᕝ௨す࡛ࡣἨ༡ὶ⣠ᒾ㢮 ࡀ୺య࡛࠶ࡿࡀ㸪∵ᕝ࠿ࡽᮾ᪉࡛ࡣ㸪ⰼᓵᒾ㢮㸪ࢳ࣮ࣕࢺ㸪Ἠ༡ὶ⣠ᒾ㢮௨እࡢⅆᡂᒾ ࡢ♟ࢆᬑ㏻࡟ྵࡴ㸬ࡑࡢୖ఩࡟㸪♟ᒾ㸪♟㉁◁ᒾ࡜㸪୰⢏◁ᒾ㹼ࢩࣝࢺᒾ࡜ࡢ஫ᒙࡀ⣼㔜 ࡋ㸪ཌࡉᩘcmࡢ♟ᒾࢆᣳᅾࡍࡿῈửࡢⰋ࠸⣽⢏㹼୰⢏◁ᒾᒙࡀⓎ㐩ࡍࡿ㸬᭱ୖ㒊ࡣⅣ㉁≀

ࢆẚ㍑ⓗከࡃᣳࡴῈửࡢࡼ࠸⅊ⓑⰍ⣽⢏◁ᒾࡀⓎ㐩ࡍࡿ㸬ᑠᆺࡢ஧ᯛ㈅㢮ࢆ㝖ࡁ኱ᆺ໬▼

ࡣ࡯࡜ࢇ࡝⏘ฟࡋ࡞࠸㸬 㸰㸧␏㇂Ἶᒾ㒊ᒙ

ᑠᯘ㸦1931㸧ࡀờ^࠶ࡏࣀ^ࡢ㇂^ࡓ࡟㡫ᒾᒙ࡜ࡋ࡚࿨ྡࡋࡓࡶࡢ࡜ྠ⩏࡛㸪ᶍᘧᆅࡣ኱㜰ᗓἨ༡ᕷ᪂ᐙ ࡢᐑᕝἢ࠸㸦ᯘ㐨࠾⳥ᯇ⥺㸧࡛࠶ࡿ㸬࿴Ἠᒣ⬦୰㒊࡛ࡢ༡໭᪉ྥࡢ✚⟬ᒙཌࡣ⣙300㹼800 㹫ࡢ⠊ᅖ࡛࠶ࡿࡀ㸦Fig. 5㸧㸪∵ᕝす᪉ࡢ⏿஫ᒙ㒊ᒙ࡜ࡢ₞⛣㒊࡛ࡣᛴ⃭࡟ῶࡎࡿ㸦Fig.

3㸧㸬ୗ఩ࡢ➟ᒣ♟ᒾ㒊ᒙ࡜ࡣᩚྜ㛵ಀ࡛࠶ࡿ㸬ᮏᒙࡣ◁㉁ࢩࣝࢺᒾ࠾ࡼࡧሢ≧ࢩࣝࢺᒾࢆ

୺య࡜ࡍࡿ㸬ᮏᒙ᭱ୗ㒊ࡣ⅊ⰍࡢἾ㉁⣽⢏◁ᒾࡲࡓࡣ◁㉁ࢩࣝࢺᒾ࠿ࡽ࡞ࡿ㸬ࡇࡢᒙ‽࡟

ࡣ▼⅊㉁ᅋሢࡀㄆࡵࡽࢀ㸪஧ᯛ㈅㢮࣭⭡㊊㢮࡞࡝ࡢ㌾యື≀໬▼ࢆከ⏘ࡍࡿ㸬ࡑࡢୖ఩ࡣ ᮏ㒊ᒙࡢ୺యࢆ࡞ࡍཌ࠸ᬯ⅊Ⰽࡢሢ≧Ἶᒾ࡟₞⛣ࡍࡿ㸬ࡇࡢሢ≧Ἶᒾ࡟ࡣ㸪ࢱ࣐ࢿࢠ≧㢼

໬࠾ࡼࡧഇᒙࡀⓎ㐩ࡍࡿ㸬ࡇࡢᒙ‽࡛ࡣ໬▼ࡢ⏘ฟࡣ⛥࡛࠶ࡿ㸬ᮏ㒊ᒙୖ㒊࡛ࡣ㸪୍㒊࡟

ᬯ⅊Ⰽⴥ⌮ἾᒾࡀⓎ㐩ࡋ㸪◁ᒾⷧᒙࢆ⛥࡟ᣳᅾࡍࡿ㸬୺㒊┦࡬ࡢ₞⛣㒊࡟ࡣཌࡉࡀᩘcmࡢ

◁ᒾⷧᒙࢆ㢖⦾࡟ᣳᅾࡍࡿᒙ⌮ࡢ᫂░࡞ἾᒾࡀⓎ㐩ࡍࡿ㸬 㸱㸧⏿஫ᒙ㒊ᒙ

㏆␥す㒊 MTL ◊✲ࢢ࣮ࣝࣉ㸦1981㸧ࡀ⏿ᒙ࡜ࡋ࡚࿨ྡࡋࡓࡶࡢ࡜ྠ⩏࡛㸪ᶍᘧᆅࡣ Ἑෆ㛗㔝ᕷ▼ᕝἢ࠸ࡢ⏿࡛࠶ࡿ㸬Ἶᒾᒙࢆ㢖⦾࡟ᣳᅾࡍࡿ◁ᒾ♟ᒾ஫ᒙࢆ୺య࡜ࡍࡿ㸬

ୗ఩ࡢ➟ᒣ♟ᒾ㒊ᒙࡢ♟ᒾᒙ࡟⣼㔜ࡍࡿࡀ㸪᫂░࡞౵㣗㠃ࢆకࡗ࡚࠸࡞࠸㸬∵ᕝ࠿ࡽഃ

ᕝ࡟࠿ࡅ࡚ࡣ୧ᆅ㉁యࡢቃ⏺࡟ཌ࠸Ἶᒾᒙࡀᣳᅾࡍࡿࡀ㸪⏿௨ᮾ࡛ࡣ➟ᒣ♟ᒾ㒊ᒙࡢ♟

ᒾ࡟┤᥋⣼㔜ࡍࡿ㸬ࡇࡢἾᒾᒙ࡟ࡘ࠸࡚ࡣ␏㇂Ἶᒾ㒊ᒙ࡜ᒾ┦ࡀ㢮ఝࡋ࡚࠾ࡾ㸪␏㇂Ἶᒾ 㒊ᒙ࡜ࡋ࡚ᢅ࠺࡯࠺ࡀྜ⌮ⓗ࡞ྍ⬟ᛶࡶ࠶ࡿࡀ㸪ᮏ◊✲࡛ࡣᚑ᮶࡝࠾ࡾ⏿஫ᒙ㒊ᒙࡢ୍

㒊࡜ࡋ࡚ᢅ࠺㸬ୗ఩ࡢ➟ᒣ♟ᒾ㒊ᒙ࡜ࡣᩚྜ㛵ಀ࡛࠶ࡿ㸦㏆␥す㒊 MTL ◊✲ࢢ࣮ࣝࣉ, 1981㸧㸬➟ᒣ♟ᒾ㒊ᒙ୰ࡢ♟ᒾ㸪◁ᒾ㸪࠾ࡼࡧἾᒾࡢ஫ᒙ࡜⏿஫ᒙ㒊ᒙࢆẚ㍑ࡍࡿ࡜㸪༢ ᒙࡢഃ᪉࡬ࡢ㐃⥆ᛶࡀࡼ࠸࡞࡝ࡢ㐪࠸ࡀ࠶ࡿ㸬௚᪉㸪୺㒊┦ࡢ◁ᒾ♟ᒾ஫ᒙ࡜ࡣ༊ูࡀᅔ

㞴࡛࠶ࡿ㸬ᮏ◊✲࡛ࡣ㸪⏿஫ᒙ㒊ᒙ࡜୺㒊┦ࡢቃ⏺఩⨨࡟ࡘ࠸࡚㸪㏆␥す㒊MTL◊✲ࢢ

࣮ࣝࣉ㸦1981㸧࠾ࡼࡧ Miyata㸦1990㸧࡟ᚑࡗࡓ㸬♟ࡣ୺࡟ள෇♟ࡢ⣽♟㹼୰♟࡛࠶ࡾ㸪

♟✀ࡣⅆᡂᒾ㢮㸪ࢳ࣮ࣕࢺ㸪ⰼᓵᒾ㢮࡞࡝࡛࠶ࡿ㸬

㸰㸬㸱㸬㸰 ୺㒊┦

࿴Ἠᒙ⩌ࡢ୺యࢆ࡞ࡍ㸬࿴Ἠᒣ⬦ᆅᇦ࡛ࡣຍኴᒙ㸪ಙ㐩ᒙ㸪ᒾฟᒙ㸪⢊Ἑᒙࡢ㸲ᒙ࡟༊

ศࡉࢀࡿ㸦㏆␥す㒊MTL ◊✲ࢢ࣮ࣝࣉ, 1981㸧㸬◁ᒾἾᒾ஫ᒙࢆ୺య࡜ࡋ㸪◁ᒾ࣭Ἶᒾࡢ ẚ⋡ࡣ1㸸100ࢆ㉸࠼ࡿࡼ࠺࡞◁ᒾඃໃ஫ᒙ࠿ࡽཌࡉᩘcmࡢ◁ᒾⷧᒙࢆᣳᅾࡍࡿἾᒾᒙࡲ

࡛ኚ໬ࡍࡿ㸬㟢ฟࡢࡼ࠸᰿᮶⾤㐨ἢ࠸ࡢ᥇▼ሙࡣ㸪ᒾฟᒙୗ㒊ࡢ IZM3㹼4㸦ᚋ㏙㸧ᒙ‽࡟

┦ᙜࡋ㸪ཌࡉ20㹼50 cmࡢ◁ᒾᒙ࡜5㹼10 cmࡢἾᒾᒙࡢ㐃⥆ᛶࡢⰋ࠸◁ᒾ຾ࡕ◁ᒾἾᒾ

஫ᒙࡀほᐹࡉࢀࡿ㸬◁ᒾ♟ᒾ஫ᒙࡀ୺య࡜࡞ࡿᒙ‽ࡀಙ㐩ᒙ࣭ᒾฟᒙࡢୗ㒊࡟☜ㄆࡉࢀࡿ㸬

⢊Ἑᒙࢆ㝖࠸࡚㸪ྛᒙࡣᕧどⓗ࡟ୖ᪉ⷧᒙ໬࡜ୖ᪉⣽⢏໬ࡢഴྥࢆࡶࡘ㸦Miyata, 1980㸹 ᐑ⏣࡯࠿, 1993㸧㸬ࡉࡽ࡟㸪ཌࡉ100㹼500 㹫࣮࢜ࢲ࣮ࡢᑠሁ✚ࢧ࢖ࢡࣝ࡟⣽ศࡍࡿࡇ࡜ࡀ

࡛ࡁ㸪ᕷᕝ࡯࠿㸦1979㸧࡟࠾࠸࡚㸪ಙ㐩ᒙ㸦ୗ㒊ளᒙ⩌㸧࡛ࡣ IZL1㹼9㸪ᒾฟᒙ㸦୰㒊ள ᒙ⩌㸧࡛ࡣIZM1㹼8㸪⢊Ἑᒙ㸦ୖ㒊ளᒙ⩌㸧࡛ࡣIZU1㹼3ࡀ㆑ูࡉࢀ࡚࠸ࡿ㸬ࡲࡓ㸪㏆␥

す㒊MTL◊✲ࢢ࣮ࣝࣉ㸦1981㸧࠾ࡼࡧᐑ⏣࡯࠿㸦1993㸧࡛ࡣ㸪ຍኴᒙࡀK1㹼9㸪ಙ㐩ᒙ

ࡀS1㹼9㸪ᒾฟᒙࡀI1㹼8࡟༊ศࡉࢀ࡚࠸ࡿ㸬

㸰㸬㸱㸬㸱 ༡㒊┦ྡᡭᒙ

༡㒊┦ࡣMTLἢ࠸ࡢ୍㒊ᆅᇦ࡟ศᕸࡋ࡚࠾ࡾ㸪ྡᡭᒙ࡜࿧ࡤࢀࡿ㸦㏆␥す㒊MTL◊✲

ࢢ࣮ࣝࣉ, 1981㸧㸬ᶍᘧᆅࡢྡᡭ࿘㎶࡛ࡣἾᒾࢆ୺య࡜ࡍࡿࡀ㸪⌧ᅾ㢼໬ࡀⴭࡋࡃ㸪㟢ฟ≧

ἣࡶⰾࡋࡃ࡞࠸㸬ᱜụ࿘㎶࡛ࡣ㸪♟ᒾ࣭◁ᒾ࠾ࡼࡧ◁㉁Ἶᒾࡀほᐹࡉࢀࡿ㸬♟✀ࡣࣆࣥࢡ Ⰽࡢ࢝ࣜ㛗▼ࡀ≉ᚩⓗ࡞ⰼᓵᒾ♟ࡀㄆࡵࡽࢀ㸪࡯࠿࡟ࢳ࣮ࣕࢺ㸪ὶ⣠ᒾ㉁ⅆᒣᒾ࡞࡝࡛࠶

ࡿ㸬୺㒊┦࡜ࡣ୺࡟᩿ᒙ㛵ಀ࡜ࡉࢀ࡚࠸ࡿࡀ㸪୺㒊┦ࡢ⢊Ἑᒙ࡜ྠ᫬␗┦ࡢ㛵ಀ࡟࠶ࡿ࡜

ࡉࢀࡿ㸦㏆␥す㒊MTL◊✲ࢢ࣮ࣝࣉ, 1981㸧㸬

㸰㸬㸱㸬㸲 จ⅊ᒾᒙ

࿴Ἠᒣ⬦ᆅᇦࡢ࿴Ἠᒙ⩌࡟ࡣὶ⣠ᒾ㉁ࡢจ⅊ᒾᒙࢆከᩘᣳᅾࡍࡿ㸬ࡑࢀࡽࡣ㘽ᒙ࡜ࡋ࡚

㘽ᒙ␒ྕt1㹼t27㸦Fig. 4࡟୍㒊⾲♧㸧ࡀ㆑ูࡉࢀ࡚࠸ࡿ㸦Miyata, 1980࡞࡝㸧㸬࡞࠾㸪㘽

ᒙt1㹼t7࡟ࡘ࠸࡚ࡣᐑ⏣࡯࠿㸦1993㸧㸪㘽ᒙt5㹼t22࡟ࡘ࠸࡚ࡣMiyata㸦1980㸧㸪㘽ᒙt13 㹼t26࡟ࡘ࠸࡚ࡣ∾ᮏ࡯࠿㸦2004㸧ࡀヲࡋࡃグ㍕ࡋ㸪ྛㄪᰝᆅᇦෆ࡛㏣㊧ࡋ࡚࠸ࡿ㸬࿴Ἠᒣ

⬦ᆅᇦ࿴Ἠᒙ⩌ࡢ㓟ᛶจ⅊ᒾ࡟ࡣ㸪ᖹ⾜ⴥ⌮࣭ࢥ࣮ࣥ࣎ࣜࣗࢺⴥ⌮ࡀࡋࡤࡋࡤㄆࡵࡽࢀࡿ

࡜グࡉࢀ࡚࠾ࡾ㸦⏣୰, 1965㸹㏆␥す㒊MTL◊✲ࢢ࣮ࣝࣉ, 1981㸧㸪ཝᐦ࡟ࡣจ⅊㉁◁ᒾᒙ࣭

จ⅊㉁Ἶᒾᒙ࡜࿧ࡪ࡭ࡁࡶࡢࡀྵࡲࢀ࡚࠸ࡿ㸬ࡋ࠿ࡋ㸪ᗈᇦࡢᒾ┦ᑐẚ࠿ࡽྛᒙ࡜ࡶᇶᮏ

ⓗ࡟୍ᒙ‽࡟Ⓨ㐩ࡍࡿࡇ࡜ࡀᗈࡃ▱ࡽࢀ࡚࠸ࡿࡓࡵ㸦౛࠼ࡤ㸪⏣୰, 1965㸹Miyata, 1980㸧㸪 ᗈᇦⓗ࡟ࡣỈ୰㝆ୗᚋࡢỈᗏ࡛ࡢ୍ḟⓗ࡞⛣ື࡟ࡼࡿࡶࡢࡀ࡯࡜ࢇ࡝࡛࠶ࡿ࡜ゎ㔘ࡋ㸪ᮏ

◊✲࡛ࡶᚑ᮶࡝࠾ࡾจ⅊ᒾ㘽ᒙ࡜⾲グࡋ࡚⏝࠸ࡿ㸬

࿴Ἠᒣ⬦ᆅᇦ୰㒊ࡢ໭⦕㒊࡛ᗈ⠊ᅖ࡟㏣㊧࡛ࡁࡿจ⅊ᒾ㘽ᒙࡣ㸱ᒙ࡛㸪࠸ࡎࢀࡶὶ⣠ᒾ

㉁㸦୍㒊࢞ࣛࢫ㉁㸧࡛࠶ࡿ㸬ྛᒙࡢ≉ᚩ࠾ࡼࡧ఩⨨㛵ಀ࠿ࡽ㸪Miyata㸦1980㸧࡟࠾ࡅࡿt8㸪

t10㸪t11࡟ᑐᛂࡍࡿ࡜⪃࠼ࡽࢀࡿ㸦Figs. 3, 5㸧㸬୍⯡࡟㸪➟ᒣ♟ᒾ㒊ᒙ୰࡛ࡣ50 cm࠿ࡽ

10 m ㏆ࡃࡲ࡛ᒙཌࡀ኱ࡁࡃኚ໬ࡋ㸪␏㇂Ἶᒾ㒊ᒙ୰࡛ࡣഃ᪉࡬ࡢ㐃⥆ᛶࡀⰋዲ࡛ࡣ࡞ࡃ㸪

୺㒊┦࠾ࡼࡧ➟ᒣ♟ᒾ㒊ᒙ࡜␗࡞ࡿ㉮ྥࢆ࡞ࡋ㸪❧ⰼ㇂ࡸᶔ஭ᕝἢ࠸ࡢ㟢㢌࡛ࡣࢫࣛࣥࣉ

ᵓ㐀ࡀ☜ㄆࡉࢀࡿ㸬୺㒊┦୰࡛ࡣ1 m௨ୖ࡜ཌࡃ㐃⥆ᛶࡀࡼ࠸㸬௨ୗ࡟ྛᒙࡢᴫせࢆ♧ࡍ㸬 㸯㸧t8จ⅊ᒾ㘽ᒙ

๤㞳ᛶ࡟ᐩࡴᖹᯈ≧ᒙ⌮ࡢⓎ㐩ࡋࡓ㯤ⓑⰍࡢจ⅊㉁◁ᒾ࡛࠶ࡿ㸬࿴Ἠᒣ⬦໭㯄ࡢᮏㄪᰝ ᆅᇦ㸦Fig. 3㸧࡛ࡣཌࡉࡀ1㹼7 m࡛࠶ࡿ㸬ⴥ⌮ࡀⓎ㐩ࡋ㸪኱㔞ࡢⅣ㉁≀ࢆྵࡴࡇ࡜࠿ࡽ㸪

኱㒊ศࡣሁ✚ᚋ࡟୍ᗘࡣ෌ሁ✚ࡋ࡚࠸ࡿ࡜⪃࠼ࡽࢀࡿ㸬୺࡟⣽⢏◁ᒾ࡛࠶ࡿࡀ㸪ᮏᒙ᭱ୖ

㒊࡛ࡣ⢒⢏㹼ᴟ⢒⢏ࡢ○ᒌ≀ࡀㄆࡵࡽࢀࡿ㸬⢏Ꮚࡢ෇☻ᗘࡣᝏࡃ㸪ᇶ㉁ࡣᑡ࡞࠸㸬▼ⱥ㸪 㛗▼ࢆ୺య࡜ࡋ㸪᭷Ⰽ㖔≀ࡣ⛥࡟㯮㞼ẕ࣭⥳Ἶ▼࠾ࡼࡧࢪࣝࢥࣥࡀྵࡲࢀࡿ㸬ᮏจ⅊ᒾ㘽 ᒙࡣ୺㒊┦ಙ㐩ᒙ᭱ୗ㒊ࡢᒙ‽㸦IZL1㸧࡟ᣳᅾࡍࡿ㸦Miyata, 1980㸧㸬Ἠ༡ᕷ᫛࿴ụୖὶࡢ 㐨㊰ἢ࠸ࡸἨ༡ᕷᐑᕝୖὶ࡞࡝࡛ほᐹ࡛ࡁࡿ㸬Ἠ༡ᕷ࡟࠶ࡿ◁ᕝᅜ㝿ࢦࣝࣇࢡࣛࣈ༡᪉ࡢ 㐨㊰ᕤ஦ࡢ㝿࡟኱つᶍ࡟㟢ฟࡋࡓ㸬

㸰㸧t10จ⅊㘽ᒙ

⅊ⓑⰍ࠿ࡽῐ⥳Ⰽࡢจ⅊㉁◁ᒾᒙ࡛࠶ࡿ㸬࿴Ἠᒣ⬦໭㯄ࡢᮏㄪᰝᆅᇦ㸦Fig. 3㸧࡛ࡣᒙཌ ࡣ0.5㹼10 m࡜ኚ໬ࡍࡿ㸬⢒⢏㹼୰⢏◁࡛㸪▼ⱥ㸪㛗▼ࢆ୺య࡜ࡍࡿ㸬ᯈ≧ࡢᒙ⌮ࡀୗ㒊

࡟Ⓨ㐩ࡍࡿࡇ࡜ࡀ࠶ࡾ㸪ୖ㒊࡯࡝⢏ᗘࡀ⢒ࡃ࡞ࡿ㸬ᮏจ⅊ᒾ㘽ᒙࡣ୺㒊┦ಙ㐩ᒙ୰㒊ࡢᒙ

‽㸦IZL5㸧࡟ᣳᅾࡍࡿ㸦Miyata, 1980㸧㸬Ἠబ㔝ᕷᶔ஭ᕝ࠾ࡼࡧࡑࡢᮾ᪉ࡢ❧ⰼ㇂࡟࠾࠸࡚

ࡣ㸪␏㇂Ἶᒾ㒊ᒙ୰࡛ほᐹ࡛ࡁࡿ㸬 㸱㸧t11จ⅊ᒾ㘽ᒙ

⅊⥳Ⰽ࡛⦓ᐦ࡞จ⅊ᒾᒙ࡛㸪࿴Ἠᒣ⬦໭㯄ࡢᮏㄪᰝᆅᇦ㸦Fig. 3㸧࡛ࡣᒙཌࡣ1㹼10 m࡜ ኚ໬ࡍࡿ㸬⣽⢏จ⅊ᒾ࠾ࡼࡧὶ⣠ᒾ㉁ࡢⅆᒣᒾ∦ࢆ୺య࡜ࡍࡿࡀ㸪ⅆᒣ♟࠿ࡽ⣽⢏ⅆᒣ⅊

ࡲ࡛ࢆྵࡳ⢏ᗘࡢኚ໬ࡀⴭࡋ࠸㸬ᮏจ⅊ᒾ㘽ᒙࡣ୺㒊┦ಙ㐩ᒙ୰㒊ࡢᒙ‽㸦IZL6㸧࡟ᣳᅾ

ࡍࡿ㸦Miyata, 1980㸧㸬࿴Ἠᒣ⬦ᆅᇦ໭⦕㒊࡟࠾࠸࡚᭱ࡶ㐃⥆ⓗ࡟㏣㊧࡛ࡁࡿจ⅊ᒾᒙ࡛㸪 Ἠబ㔝ᕷ✄಴ụ༡᪉࠿ࡽ㈅ሯᕷⷃ^ࡑࡪཎ^ࡽࡲ࡛ほᐹࡉࢀࡿ㸬ࡇࡢ㛫࡟㸪t11ᒙ‽ࡣ㸪୺㒊┦ࡢࢱ࣮

ࣅࢲ࢖ࢺ┦࠿ࡽ໭⦕┦ࡢ␏㇂Ἶᒾ㒊ᒙ࠾ࡼࡧ➟ᒣ♟ᒾ㒊ᒙࡲ࡛ࡢᒾ┦ኚ໬ࡍࡿ㸦Fig. 5㸧㸬

㸰㸬㸱㸬㸳 ሁ✚┅ᙧᡂྐࡢ◊✲

࿴Ἠᒙ⩌ࡢሁ✚ሙ࡛࠶ࡿ࿴Ἠሁ✚┅ࡣ㸪MTLࡢᶓࡎࢀ㐠ື࡟ࡼࡾᙧᡂⓎ㐩ࡋࡓᶓࡎࢀሁ

✚┅࡜⪃࠼ࡽࢀ࡚࠸ࡿ㸦ᕷᕝ࣭ᐑ⏣, 1973㸹ᕷᕝ࡯࠿, 1981㸹Taira et al., 1983㸹Miyata,

1990㸹Tanaka, 1993࡞࡝㸧㸬࿴Ἠᒙ⩌ࡢ௦⾲ⓗ࡞≉ᚩ࡛࠶ࡿ኱つᶍྥᩳᵓ㐀ࡣ㸪᩿ᒙ࡟ࡼ

ࡿᕥᶓࡎࢀኚ఩࡜㏫᪉ྥࡢᮾ࡟ࣉࣛࣥࢪࡋࡓ㠀ᑐ⛠࡞ྥᩳᵓ㐀࡛࠶ࡾ㸪㢮ఝࡍࡿᵓ㐀ࡀ༡

࢝ࣜࣇ࢛ࣝࢽ࢔ࡢRidge Basin㸦Crowell, 1982࡞࡝㸧࡟ࡶㄆࡵࡽࢀࡿ㸬

ᕷᕝ㸦1960㸧ࡣ㸪࿴Ἠᒣ⬦ᆅᇦࡢ࿴Ἠᒙ⩌࡛㸪ᮾ᪉࡟᫬௦ࡀⱝ໬ࡍࡿࡇ࡜ࢆ♧၀ࡋࡓ㸬 ࡑࡢᚋ㸪ᅄᅜす㒊࠿ࡽ㏆␥ᆅ᪉す㒊࡟࠿ࡅ࡚㸪࿴Ἠᒙ⩌ࡢ᫬௦ࡀす࠿ࡽᮾ࡬ⱝࡃ࡞ࡾ㸪ᮾ

࡯࡝ࡼࡾୖ఩ࡢᆅᒙࡀᇶ┙ᒾ㢮ࢆそ࠺ࡇ࡜ࡀᣦ᦬ࡉࢀࡓ㸦ᕷᕝ࣭኱ᶫ, 1965㸹⏣୰, 1965㸹

ᑎᒸ, 1977࡞࡝㸧㸬ᕷཎ࡯࠿㸦1986㸧ࡣ㸪࿴Ἠᒙ⩌ࡢ୺ศᕸᇦ300 km࡟࠾࠸࡚㸪࿴Ἠᒙ⩌

ࡢሁ✚ᮇ㛫ࡣ኱ᆺ໬▼ࡢ᳨ウ࡟ࡼࡗ࡚⣙12 m.y.㸦m.y.㸸ⓒ୓ᖺ㸧࡜᥎ᐃࡉࢀ࡚࠸ࡿࡢ࡛㸪

࿴Ἠᒙ⩌ࡢሁ✚┅୰ᚰࡢ⛣ື㏿ᗘ㸦ᮾ㐍㏿ᗘ㸧ࡣ༢⣧ィ⟬࡛⣙25 km㸭m.y.ࡀぢ✚ࡶࡽࢀࡿ

࡜ࡋ࡚࠸ࡿ㸬௚᪉㸪ᑠ⋢㸦1990㸧ࡣ㸪ᅄᅜす㒊ᆅᇦ࠿ࡽῐ㊰ᓥࡢ࿴Ἠᒙ⩌ࡢྂᆅ☢Ẽᒙᗎ ࡢ᳨ウࢆࡶ࡜࡟㸪C33r㸭C33nቃ⏺࠿ࡽC32n2n㸭C32n1rቃ⏺ࡲ࡛ࡢ8.2 m.y.࡟⛣ືࡋࡓ

㊥㞳ࡀ⣙170 km࡛࠶ࡿࡓࡵ㸪ᮾ㐍㏿ᗘࢆ⣙21 km㸭m.y.࡜ぢ✚ࡶࡗ࡚࠸ࡿ㸬

ᕷᕝ࡯࠿㸦1981㸧ࡣ㸪࿴Ἠᒣ⬦ࡢ࿴Ἠᒙ⩌࡟ࡘ࠸࡚㸪௨ୗࡢ≉ᚩ࡟ὀ┠ࡋࡓ㸬

㸦㸯㸧ྛᒙ࡜ࡶሁ✚ࢧ࢖ࢡࣝࡢୗ㒊࡟ࡣ㸪ᖖ࡟⢒⢏ࢱ࣮ࣅࢲ࢖ࢺᆺࡢᒾ┦ࡢศᕸࡀ 10㹼 15kmࡢ㛫㝸࡛ᣑ኱ࡋ࡚࠸ࡿࡢࡀㄆࡵࡽࢀࡿ㸬

㸦㸰㸧ྥᩳ㍈ࡣᮾ࡬ࣉࣛࣥࢪࡋ㸪ྥᩳ㒊࡟࠶ࡿ⢒⢏ࢱ࣮ࣅࢲ࢖ࢺᆺࡢᒾ┦ࡢᆅᒙࡣᮾ࡬ⱝ ࡃ࡞ࡿ㸬

㸦㸱㸧ࢱ࣮ࣅࢲ࢖ࢺࡢᗏ⑞㸦ࣇ࣮ࣝࢺ࢟ࣕࢫࢺࡸࢢ࣮ࣝࣈ࢟ࣕࢫࢺ㸧࠿ࡽ♧ࡉࢀࡿྂὶྥ

ࡣ୍⯡࡟すࡲࡓࡣす༡す᪉ྥ࡛࠶ࡿ㸬

ᙼࡽࡣ㸪ࡇࢀࡽࡢ≉ᚩࢆ㸪MTLࡢᶓࡎࢀ㐠ື࡟ࡼࡾ࿴Ἠሁ✚┅ࡢ᭱኱ỿ㝆㒊ࡀ10㹼15 km ࡢ㛫㝸࡛ẁ㝵ⓗ࡟ᮾ㐍ࡍࡿࣔࢹ࡛ࣝྜ⌮ⓗ࡟ㄝ᫂ࡋࡓ㸦Fig. 6㸧㸬࠶ࡿ᫬Ⅼࡢሁ✚┅୰ᚰࡀ㸪

┦ᑐⓗࡲࡓࡣ⤯ᑐⓗ࡟ୖ᪼ࡍࡿ࡜ྠ᫬࡟㸪ᮾഃࡀ┦ᑐⓗࡲࡓࡣ⤯ᑐⓗ࡟ỿ㝆ࡍࡿࡇ࡜࡟ࡼ

ࡾ㸪ᆅᒙࡣᮾ࡬ഴࡁ㸪ᮾഃ࡟ሁ✚┅୰ᚰࡀ⛣ືࡍࡿ㸬ࡇࡢሁ✚┅୰ᚰ࡟ᮾ࠿ࡽ○ᒌ≀ࡀ㛫 Ḟⓗ࡟౪⤥ࡉࢀ㸪⢒⢏ࢱ࣮ࣅࢲ࢖ࢺᆺࡢᒾ┦ࡀሁ✚ࡍࡿ࡜࠸࠺ࡶࡢ࡛࠶ࡿ㸬ࡑࡢ࿴Ἠሁ✚

э э

э э

э э

Ѝ Ѝ Ѝ

Ѝ

Eastward migration of the Izumi sedimentary basin

Stratigraphic younging Plunge of syncline

A B

A B

Downwelling

Main supply of sediments

: (

Downwelling Upwelling

Main supply of sediments

Upwelling

Fig. 6. Eastward migration model of the Izumi sedimantary basin by

Ichikawa et al. (1981). The depocenter was migrated eastward from

area A to B with relatively or absolutely lifting of area A. This model

can explain the eastward plunge of syncline and the stratigraphic

younging to the east. Modified after Ichikawa et al. (1981).

┅ࡢᮾ㐍ࡢලయⓗࣉࣟࢭࢫ࡜ࡋ࡚㸪ᕷᕝ࡯࠿㸦1981㸧ࡣᮾഃࡀ㛤ᨺᛶᒅ᭤㸦releasing bend㸧

࡛すഃࡀ཰ᩡⓗᒅ᭤㸦restraining bend㸧࡛࠶ࡿpaired bend㸦Harland, 1971㸹Mann, 2007㸧 ࡀ㡰ḟᮾ࡟⛣ືࡋ࡚࠸ࡃ≧ἣࢆ᝿ᐃࡋ㸪༡࡬ࡢᙇࡾฟࡋࡓᕥᶓࡎࢀ᩿ᒙᮾഃ࡛ࡢᶓࡎࢀᘬ ᙇሙ࡛ࡢሁ✚┅ࡢỿ㝆࡜㸪ᪧሁ✚┅୰ᚰ࡛࠶ࡿすഃ࡛ࡢᶓࡎࢀᅽ⦰ሙ࡛ࡢሁ✚┅ࡢୖ᪼ࢆ

⧞ࡾ㏉ࡍࡇ࡜࡟ࡼࡾᮾ㐍ࡀ⏕ࡌࡓྍ⬟ᛶࢆᥦ♧ࡋࡓ㸬

ࡇࢀࢆ⓶ษࡾ࡟㸪MTLࡢᕥᶓࡎࢀ㐠ື࡟క࠺ሁ✚┅ᙧᶵᵓ࡜ࡋ࡚ࡣ㸪ṇ᩿ᒙࡢ㡰ḟᙧᡂ

࡟ࡼࡾ᭱኱ỿ㝆㒊ࡀẁ㝵ⓗ࡟ᮾ㐍ࡍࡿ∦ⴠࡕሁ✚┅ࣔࢹࣝ㸦㧗ᶫ, 1992㸹Tanaka, 1989㸧

࡜㸪ゎᨺᛶᒅ᭤࡟ࡼࡿࣉࣝ࢔ࣃ࣮ࢺሁ✚┅Ⓨ㐩ࣔࢹࣝ㸦Miyata, 1990㸹ᒣ໭࣭ఀ⸨, 1999㸧 ࡢ஧ࡘࡢࣔࢹࣝࡀᥦၐࡉࢀࡓ㸬๓⪅ࡣ㸪MTLࡢỈᖹ᪉ྥ࡟ᑐࡋ࡚㧗ゅᗘ࡛ᩳ஺ࡍࡿすഴᩳ

ࡢṇ᩿ᒙࡢ㡰ḟᙧᡂ࡟క࠺∦ⴠࡕሁ✚┅ࣔࢹ࡛ࣝ࠶ࡿ㸦Fig. 7㸧㸬ࡇࢀࢆᥦၐࡋࡓ Tanaka 㸦1989㸪1993㸧㸪Tanaka and Maejima㸦1995㸧ࡣ୺㒊┦ࡢᾏᗏᡪ≧ᆅሁ✚≀࠾ࡼࡧ໭⦕

┦ࡢࢹࣝࢱሁ✚≀ࡢሁ✚┦ࢆ᳨ウࡋࡓ㸬࿴Ἠᒣ⬦ᆅᇦ୰㒊࠿ࡽす㒊࡟ศᕸࡍࡿ◁ᒾ♟ᒾ஫

ᒙࢆ୺య࡜ࡍࡿ⢒⢏ᒾ┦࡟ࡘ࠸࡚㸪ሁ✚┦ゎᯒࢆ࠾ࡇ࡞࠸ୖ㒊ࢳࣕࢿࣝሁ✚≀㸦UCA㸧࣭ୗ

㒊ࢳࣕࢿࣝሁ✚≀㸦LCA㸧࣭ศὶࢳࣕࢿࣝሁ✚≀㸦DCA㸧࣭ࢩ࣮ࢺ≧ࢱ࣮ࣅࢲ࢖ࢺ㸦STA㸧࣭

ࢩ࣮ࢺ≧ࢱ࣮ࣅࢲ࢖ࢺᮎ➃┦㸦DSA㸧࣭࣮࢜ࣂ࣮ࣂࣥࢡࢱ࣮ࣅࢲ࢖ࢺࢆ㆑ูࡋࡓ㸬ࡉࡽ࡟㸪 ᮾ㒊ࡢ໭⦕┦࡟ࡘ࠸࡚㸪ᡪ≧ᆅሁ✚≀㸦AFA㸧࣭ࢹࣝࢱࢫ࣮ࣟࣉሁ✚≀㸦DEA㸧࣭ࣉࣟࢹࣝ

ࢱሁ✚≀㸦PDA㸧ࢆ㆑ูࡋࡓ㸬ࡇࢀࡽࡢ⤖ᯝ࠿ࡽ㸪ẁ㝵ⓗ࡞ᾏᗏᡪ≧ᆅࡢⓎ㐩ࢆ୰ᚰ࡜ࡋ ࡓሁ✚┅ᙧᡂ㐣⛬ࢆ᚟ඖࡋ㸪ሁ✚┅ࡢᮾ㐍ࡣẁ㝵ⓗ࡛࠶ࡗࡓ࡜ࡋ࡚ṇ᩿ᒙࡢ㡰ḟᙧᡂ࡟క

࠺∦ⴠࡕሁ✚┅࡛࠶ࡗࡓ࡜࠸࠺ࣔࢹࣝࢆ♧ࡋࡓ㸦Fig. 7㸧㸬

ᚋ⪅ࡢMiyata㸦1990㸧ࡣMTLࡢᒅ᭤ࡀᙜ᫬ࡢᕥᶓࡎࢀ㐠ື࡟ࡼࡾ㛤ᨺᛶᒅ᭤࡜ࡋ࡚స

⏝ࡋ㸪ఙ㛗ࢹ࣮ࣗࣉࣞࢵࢡࢫࡢ㡰ḟᙧᡂࢆࡶࡓࡽࡍ㠀ᑐ⛠ࣉࣝ࢔ࣃ࣮ࢺሁ✚┅ࣔࢹ࡛ࣝ࠶

ࡿ㸦Fig. 8㸧㸬Miyata㸦1990㸧ࡣ㸪࿴Ἠᒣ⬦ᆅᇦࡢ࿴Ἠᒙ⩌࡛ࡣྂὶྥࡀす㸦⏣୰㸪1965

࡯࠿㸧࡟ࡶ㛵ࢃࡽࡎ㸪ࢫࣛࣥࣉ〩᭤ࡀ♧ࡍྂᩳ㠃᪉ྥࡣᮾ᪉ྥ࡛࠶ࡿࡇ࡜ࢆ♧ࡋࡓ㸬୺㒊

┦࡟ㄆࡵࡽࢀࡿࢫࣛࣥࣉ〩᭤ࡀ♧ࡍྂᩳ㠃᪉ྥࡣ㸪ሁ✚┅ᮾ㐍࡟࡜ࡶ࡞࠺ྥᩳᙧᡂ᫬ࡢᆅ ᒙഴືࡢ⤖ᯝ࡛࠶ࡿ࡜ࡋ㸪ఙ㛗ࢹ࣮ࣗࣉࣞࢵࢡࢫࡢ㡰ḟⓎ㐩࡟క࠺ሁ✚┅ࡢᮾ㐍㐣⛬ࢆ᚟

ඖࡋࡓ㸬⌧ᅾ㸪஬ᲄᒅ᭤࡟ࡼࡾMTLࡣ㉮ྥ᪉ྥ࠿ࡽᕥ࡟ᒅ᭤ࡋ࡚࠸ࡿࡀ㸪ࡑࡢཎᆺࡣ࿴Ἠ ᒙ⩌ሁ✚᫬࡟ࡣࡍ࡛࡟Ꮡᅾࡋ࡚࠸ࡓ࡜ᒾᮏ࣭ᐑ⏣,㸦1994㸧㸪ᐑ⏣࡯࠿㸦2012㸧࡛ࡣ⪃࠼࡚

࠸ࡿ㸬௚᪉㸪ᒣ໭㺃ఀ⸨㸦1999㸧ࡣࣉࣝ࢔ࣃ࣮ࢺሁ✚┅ࣔࢹࣝࡢᩘ್ゎᯒࣔࢹࣝᐇ㦂ࢆ♧ࡋ

࡚࠸ࡿ㸦Fig. 9㸧㸬ࡇࡢᐇ㦂࡛ࡣ㸪㐃⥆ࡍࡿ኱つᶍ࡞ྥᩳᵓ㐀ࡀ෌⌧ࡉࢀ࡚࠸ࡿࡀ㸪ᐇ㝿ࡢ

࿴Ἠᒙ⩌࡛ㄆࡵࡽࢀࡿ㞜⾜≧࡟㓄ิࡍࡿ〩᭤㍈࡜ࡣ␗࡞ࡿ㸬ᙼࡽࡣ㸪Miyata㸦1990㸧ࡀⓎ

㐩ࢆ♧၀ࡋࡓ஧ḟⓗ࡞ṇ᩿ᒙࡀⓎ⏕ࡋ࡞ࡃ࡚ࡶሁ✚┅୰ᚰࡢ⛣ືࡣྍ⬟࡛࠶ࡾ㸪ሁ✚┅ࡢ

Basement

:(67 ($67

Ѝ

Ѝ

Ѝ Ѝ

Ѝ

Ѝ

ḵ ḵ ḵ

ḵ

ḵ ḵ ḵ

Channel deposits Distributary channel deposits Sheet flow deposits

BASEMENT

E W

sheet flows

distriburary channels

main channeloverspilled deposits

ABANDONED

SYSTEMS ACTIVE SYSTEMS

EASTWARD BASIN EXTENSION

A

B

Fig. 7. Eastward migration model of the Izumi sedimantary basin (ISB)

after Tanaka (1993). A: Depositonal system sketch of the ISB. Active

system stepwisly shifted to the east owing to the eastward extensions of

the basin. B: Interpretative corss section of the stacking pattern of the

Izumi Group. Modified from Tanaka (1993).

1

Fig. 8. Eastward migration model of the Izumi sedimantary basin by Miaya (1990). Pull-apart basin with extensional duplex showing the stepwise migration of the depocenter, which is followed by stepwise normal faulting F1, F2, and F3. Modified from Miyata and Iwamoto (1994).

㸽

㸮 㸯㸮 㸰㸮㹩㹫

Unconformity Fault

hidden one

Upper Cretaceous Izumi Group

Pyloclastic rocks and granites

Ѝ Ѝ

Ѝ

Ѝ Ѝ

Ѝ Ѝ

M T L

Extensional duplex )

)

)

)

Gojo releasing bend

Erosion level

a-1 b-1

a-2 b-2

Erosion level

Erosion level Erosion level

a-1 b-1

a-2 b-2

10km

10km

MTL

MTL MTL

MTL MTL

MTL

Fig. 9. Eastward migration model of the Izumi sedimantary basin by

Yamakita and Ito (1999). Pull-apart basin showing contenious migration of

the depocenter, tilting strata, and eastward plunge of syncline. Modified

from Yamakita and Ito (1999).

ᮾ㐍ࡣẁ㝵ⓗ࡛ࡣ࡞ࡃ㐃⥆ⓗ࡛࠶ࡿ࡜ࡋࡓ㸬

㸰㸬㸱㸬㸴 ሁ✚ᖺ௦

Matsumoto and Morozumi㸦1980㸧ࡣ㸪୺㒊┦ຍኴᒙ࣭ಙ㐩ᒙ┦ᙜᒙ‽ࡢ␏㇂Ἶᒾ㒊 ᒙ࠿ࡽ⏘ฟࡍࡿ࢔ࣥࣔࢼ࢖ࢺ໬▼ࢆ⏝࠸࡚㸪௨ୗࡢࡼ࠺࡟໭ᾏ㐨ࡢ⼎ዀᒙ⩌࡜᫬௦ᑐẚ ࡋ࡚࠸ࡿ㸬ᶔ஭ᕝ௜㏆ࡼࡾ௨すࡢᆅ࡛⏘ฟࡍࡿ࢔ࣥࣔࢼ࢖ࢺ໬▼⩌ࡣ Pachydiscus (Pachydiscus) kobayashii (Shimizu)㸪Canadoceras tanii Matsumoto and Morozumi㸪 Nostoceras sp. aff. hetonaiense Matsumoto㸪Baculites regina Obata and Matsumoto

࡛≉ᚩ࡙ࡅࡽࢀࡿ㸬Matsumoto and Morozumi㸦1980㸧ࡣ࠾ࡑࡽࡃ࡬ࢺࢼ࢖⤫ୖ㒊㝵

ࡢK6b1㸦Fig. 10㸧࡟ᑐẚࡉࢀࡿ࡜ࡋ࡚࠸ࡿ㸬ᶔ஭ᕝ௜㏆ࡼࡾᮾ᪉࡛⏘ฟࡍࡿ࢔ࣥࣔࢼ

࢖ ࢺ ໬ ▼⩌ 㞟 ࡣ 㸪Pachydiscus (Pachydiscus) sp. aff. flexuosus Matsumoto㸪P.

(Neodesmoceras) cf. gracilis Matsumoto㸪Gaudryceras izumiense Matsumoto and Morozumi࡞࡝࡛≉ᚩ࡙ࡅࡽࢀࡿ㸬Matsumoto and Morozumi㸦1980㸧ࡣࡇࢀࢆ࡬ࢺ ࢼ࢖⤫ୖ㒊㝵ࡢK6b2㸦Fig. 10㸧࡟ᑐẚ࡛ࡁࡿ࡜ࡋ㸪࣐࣮ࢫࢺࣜࣄࢳ࢔ࣥ㝵㸦72.1㹼66.0 Ma㸸Ogg and Hinnov, 2012㸧࡛࠶ࡿ࡜ࡋࡓ㸬Morozumi㸦1985㸧ࡣ㸪Matsumoto and Morozumi㸦1980㸧ࡢ෌᳨ウ⤖ᯝ࠿ࡽ㸪࿴Ἠᒣ⬦ᆅᇦࡢ࿴Ἠᒙ⩌ࢆ㸪Nostoceras hetonaiense ᖏ࠿ࡽPachydiscus (Pachydiscus) sp.aff. compressus ᖏ࡟ᑐẚࡋ㸪࿴Ἠᒣ⬦ᆅᇦࡢ඲ ᇦࡀ࣐࣮ࢫࢺࣜࣄࢳ࢔ࣥ㝵࡛࠶ࡿ࡜ゎ㔘ࡋࡓ㸦Fig. 10㸧㸬ᶫᮏ࡯࠿㸦2001㸧ࡣ㸪ᨺᩓ⹸໬▼

ࢆ⏝࠸࡚᫬௦ᑐẚࢆ⾜ࡗ࡚࠸ࡿ㸬ᙼࡽࡣ㸪࿴ḷᒣ┴࿴ḷᒣᕷ῝ᒣࡢຍኴᒙ㸪኱㜰ᗓ㈅ሯ ᕷ ⷃ ཎ ࡢ ᒾ ฟ ᒙ ࠾ ࡼ ࡧ ࿴ ḷ ᒣ ┴ ᶫ ᮏ ᕷ ቃ ཎ ࡢ ⢊ Ἑ ᒙ ࡢ ࠺ ࡕ ࡢ ィ 5 ⟠ ᡤ ࠿ ࡽ Amphipyndax tylotus _䠄Foreman䠅࡞࡝ࢆྵࡴ࡯ࡰྠᵝࡢ໬▼⩌ࢆᚓࡓࡀ㸪ࡇࢀࡽࢆᅄ

୓༑ᖏ࡛ࡢᨺᩓ⹸໬▼ᖏ㸦▼⏣࣭ᶫᮏ, 1998㸧ࡢAtᖏ࡟ᑐẚࡋ㸪ࡑࡢ᫬௦ࡣ๓ᮇ࣐࣮

ࢫࢺࣜࣄࢳ࢔ࣥᮇ࡛࠶ࡿ࡜ࡋࡓ㸬ᨺᑕᖺ௦࡜ࡋ࡚ࡣ୺㒊┦ࡢจ⅊ᒾ㘽ᒙ࡛ࢪࣝࢥࣥFTᖺ ௦ࡀ5ᒙ‽࡛ ᐃࡉࢀ࡚࠸ࡿࡀ㸦Fig. 4㸹ᐑ⏣࡯࠿, 1993㸧㸪ࡇࢀࡽ࡟ࡣᖺ௦ᶆ‽່࿌௨๓ࡢ ᖺ௦್ࡸヲ⣽ࡀ♧ࡉࢀ࡚࠸࡞࠸ ᐃ⤖ᯝࡀྵࡲࢀࡿ㸬ࡑࡢ࠺ࡕࡢ3ヨᩱ㸦Fig. 11࡟᥇㞟ሙ ᡤࢆࣉࣟࢵࢺ㸧࡟ࡘ࠸࡚ࡣ㸪 ᐃࢆᘬࡁཷࡅࡓி㒔ࣇ࢕ࢵࢩ࣭ࣙࣥࢺࣛࢵࢡ࡟↷఍ྍ⬟࡛㸪 ࢮ࣮ࢱ㍑ṇࡀ⾜ࢃࢀ࡚࠸ࡿ㸬ࡑࢀࡽࡢᖺ௦࡟ࡘ࠸࡚ࡣୗグࡢ㏻ࡾ࡛࠶ࡿ㸬t1 จ⅊ᒾ㸦ヨᩱ

ྡIZ1㸧ࡣ77.1s7.2 Ma㸪t7จ⅊ᒾ㸦ヨᩱྡIZ2㸧ࡣ72.6s8.0 Ma㸪t15จ⅊ᒾ㸦ヨᩱྡ

IZ4㸧ࡣ73.3s7.4 Ma࡛࠶ࡿ㸬ࡼࡗ࡚㸪࿴Ἠᒣ⬦す⦕㒊ࡢຍኴᒙ᭱ୗ㒊௜㏆࡛ᚓࡽࢀࡓ77.1

Maࢆ㝖ࡅࡤᒙ‽࡟㛵ಀ࡞ࡃ඲ᇦ࡛ᴫࡡ72㹼73 Ma㸦s8.0 Ma㸧࡛࠶ࡾ㸪ㄗᕪ⠊ᅖࢆྵࡵ

࡚ᮏᆅᇦࡢ໬▼ᒙᗎࡢゎ㔘㸦Morozumi, 1985㸧࡜኱ࡁ࡞▩┪ࡣ࡞࠸㸬

Fig. 10. Bisotratigraphic correlations of the Izumi Group (Morozumi, 1985).

?

?

? Shikoku

Awaji Island Izumi Mountains

K6b2

K6b1

K6a3

K6a2 Range of the Izumi Group Zones of the Izumi G. Subdivison

in Hokkaido Stage

Sphenoceramus schmidti Metaplacenticeras subtilistriatum Baculites kotanii Didymoceras sp.

Didymoceras awajiense Pravitoceras sigmoidale Pachydiscus awajiensis Nostoceras hetonaiense Pachydiscus

aff. subcompressus

Maastrichtian Campanian

Lower Upper

㸱㸬ࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡ ᐃ

㸱㸬㸯 ヨᩱ࡜᪉ἲ 㸱㸬㸯㸬㸯 ヨ ᩱ

ࣅࢺࣜࢼ࢖ࢺ཯ᑕ⋡ ᐃヨᩱ࡜ࡋ࡚㸪࿴Ἠᒣ⬦ᆅᇦࡢ໭⦕┦࠿ࡽ19ヨᩱ㸪୺㒊┦࠿ࡽ35 ヨᩱ㸪༡㒊┦࠿ࡽ3ヨᩱ㸪ྜィ57ヨᩱࢆ⏝࠸ࡓ㸦Figs. 2, 3, 11㸧㸬໭⦕┦ࡢヨᩱࡢෆヂࡣ㸪

➟ᒣ♟ᒾ㒊ᒙ࠿ࡽ4ヨᩱ㸦KS01-1, KS01-2, KS02㹼04㸧㸪␏㇂Ἶᒾ㒊ᒙ࠿ࡽ7ヨᩱ㸦AZ01 㹼AZ07㸧㸪⏿஫ᒙ㒊ᒙ࠿ࡽ 9 ヨᩱ㸦TK01㹼TK08㸧࡛࠶ࡿ㸬୺㒊┦ࡢヨᩱࡢෆヂࡣ㸪ୗ

఩ࡼࡾຍኴᒙ࠿ࡽ2ヨᩱ㸦KD01, KD02㸧㸪ಙ㐩ᒙ࠿ࡽ10ヨᩱ㸦SH01㹼SH10㸧㸪ᒾฟᒙ࠿

ࡽ11ヨᩱ㸦IW01㹼IW11㸧㸪࠾ࡼࡧ⢊Ἑᒙ࠿ࡽ11ヨᩱ㸦KK01㹼KK11㸧࡛࠶ࡿ㸬༡㒊┦࡟

ࡘ࠸࡚ࡣྡᡭᒙࡢ 3ヨᩱ㸦NT01㹼NT03㸧࡛࠶ࡿ㸬ᮏ✏࡛ࡣ౽ᐅୖ࿴Ἠᒣ⬦ᆅᇦࡢ࿴Ἠᒙ

⩌ࢆす㒊㸪୰㒊㸪ᮾ㒊ࡢ3ᆅᇦ࡟༊ศࡋࡓ㸦Fig. 2㸧㸬ࡇࢀࡽࡢ༊ศࡣ࿴Ἠᒙ⩌໭⦕㒊ࡢ≉

ᚩࢆᇶ‽࡜ࡋ࡚࠾ࡾ㸪໭⦕㒊୙ᩚྜ௜㏆ࡢ㟢㢌ࡀḞዴࡍࡿ⠊ᅖࢆ࿴Ἠᒣ⬦ᆅᇦす㒊㸪໭⦕

㒊࠿ࡽ༡⦕㒊ࡲ࡛ࡀ㟢ฟࡋ㸪༡໭ᖜࡀ࡯ࡰ୍ᐃࡢ⠊ᅖࢆ࿴Ἠᒣ⬦ᆅᇦ୰㒊㸪໭⦕㒊୙ᩚྜ

ࡢ᪉ྥࡀᮾ໭ᮾ―す༡す࠿ࡽᮾ༡ᮾ―す໭す᪉ྥ࡬࡜ኚ໬ࡋ㸪༡໭ᖜࡀᮾ㒊࡯࡝⊃ࡃ࡞ࡿ⠊

ᅖࢆ࿴Ἠᒣ⬦ᆅᇦᮾ㒊࡜࿧ࡪ㸬඲57ヨᩱࡢ࠺ࡕ㸪࿴Ἠᒣ⬦ᆅᇦす㒊࡟11ヨᩱ㸪୰㒊࡟36 ヨᩱ㸪ᮾ㒊࡟10ヨᩱࡀ఩⨨ࡍࡿ㸦Fig. 2㸧㸬ヨᩱࡣ⢒⢏ሁ✚≀ࢆ㑊ࡅ࡚㸪Ἶᒾ㸪◁㉁Ἶᒾ㸪 ᴟ⣽⢏◁ᒾ㸪ྵ♟Ἶᒾ࠿ࡽ᥇㞟ࡋࡓ㸦Table 1㸧㸬໭⦕┦࡛ࡣ➟ᒣ♟ᒾ㒊ᒙ࣭⏿஫ᒙ㒊ᒙ୰

࡟ᣳᅾࡍࡿ◁㉁ࢩࣝࢺᒾࡲࡓࡣྵ♟Ἶᒾࡸᴟ⣽⢏◁ᒾ㸪␏㇂Ἶᒾ㒊ᒙࡢࢩࣝࢺᒾࡸ◁㉁ࢩ

ࣝࢺᒾ㸪୺㒊┦࡛ࡣ◁ᒾἾᒾ஫ᒙࡲࡓࡣཌ࠸Ἶᒾᒙ࠿ࡽᚓࡽࢀࡓἾᒾࡸᴟ⣽⢏◁ᒾࢆ⏝࠸

ࡓ㸬ྛヨᩱࡣྍ⬟࡞㝈ࡾಖᏑ≧ែࡀⰋ࠸㒊ศࢆᒾ▼ࣈࣟࢵࢡ࡜ࡋ࡚᥇㞟ࡋࡓ㸬ᐊෆ࡟࡚ࡑ

ࢀࡽࢆ༑ศ࡟ỈὙᚋ㸪஝⇱ࡉࡏ࡚࠿ࡽ㢼໬㒊ศࢆ㝖ཤࡋ㸪㕲ங㖊࡛◚○ࡋࡓ㸬ࡉࡽ࡟#5 ࠾

ࡼࡧ#3.2ࡢ⠠࡛ศ⏬ࡋ㸪⢏ᚄ4㹼6 mmࡢࢣࣟࢪ࢙ࣥᢳฟ⏝ヨᩱࢆసᡂࡋࡓ㸬

ᮏ◊✲࡛ࡣ㸪ヨᩱ᥇㞟ᆅⅬࢆ㸪⦋ᗘ࣭⤒ᗘ࣭ᶆ㧗࡜࡜ࡶ࡟ᮾす᪉ྥࡢ㊥㞳࡜༡໭᪉ྥࡢ

㊥㞳࣭ሁ✚┅ᖜ࡜࠸࠺ᗙᶆࢆタᐃࡋ࡚౑⏝ࡋ㆟ㄽࡍࡿ㸦Table 1㸧㸬ᮾす᪉ྥࡢᗙᶆཎⅬࡣ㸪 ᯇᒣ㸦ᮾ⤒132°42’㸧࡟タᐃࡋ㸪ᮾすࡢ㊥㞳ࡣ໭⦋35°00’࡟ἢࡗࡓ㊥㞳࡜ࡍࡿ㸬༡໭᪉ྥࡢ ᇶⅬࡣ㸪໭⦕㒊ࡢᇶ┙୙ᩚྜ࣭᩿ᒙቃ⏺࡜༡⦕㒊ࡢ≀㉁ቃ⏺MTL࡟⨨࠸ࡓ㸦Fig. 11㸧㸬໭

⦕࡟ࡘ࠸࡚ࡣᮏ◊✲࡟ࡼࡿ⤫ྜᆅ㉁ᴫ␎ᅗ㸦Fig. 3㸧㸪࠾ࡼࡧ༡⦕࡟ࡘ࠸࡚ࡣMiyata㸦1980㸧

࡜ᒸ⏣࡯࠿㸦1999㸧࠿ࡽసᡂࡋࡓMTLࡢ఩⨨ࡼࡾసᡂࡋࡓቃ⏺⥺㸦Fig. 11㸧ࢆᇶ࡟ࡋ࡚㸪

⦋ᗘ⤒ᗘ࡟ࡼࡾᆅᅗୖ࡟ࣉࣟࢵࢺࡋࡓྛᆅⅬࡲ࡛ࡢ㊥㞳ࢆ⟬ฟࡋࡓ㸬ࡑࡢ୙ᩚྜ࠿ࡽ≀㉁

ቃ⏺MTL࡜ࡢ㛫ࡢ㊥㞳࡟ࡘ࠸࡚ࡣሁ✚┅ᖜ࡜࿧ࡪࡇ࡜࡟ࡍࡿ㸬ࡓࡔࡋ㸪࿴Ἠᒣ⬦ᆅᇦす㒊

0 10 km Fig. 11. Map of sample localites with northern and southern bounradries of the Izumi Group in the Izumi Mountains, using for coordinate mesurement of localites (distance from the northern margin and MTL, distance of basin width etc.). The northern boundary was refered from Figure 2. The southern boundary (MTL) was compiled after Miyata (1980) and Okada et al. (1999). Black filled circles show the location of samples for vitrinite reflectance analysis, and grey filled circles numbered from I-1 to I- 10 show the sampling localities of Suzuki (1996) used for vitrinite reflectance. Black filled squares (IZ1, IZ2, and IZ4) show the localiti es of zircon fission-track dating after Miyata et al. (1993).

I-1 I-2

I-3

I-8 I-5

I-7 I-4

I-6

I-10 I-9

TK08 NT03IW07 SH07

KK01

KK02IW02

SH04 SH01 SH05 SH02 KD02

NT01IW01

KK06

TK07 TK05

KS04TK06 KD01

NT02 KK05

KK04

KK03IW04IW05 IW06KK07

KK08 KK09IW03 KK11 SH09SH08 SH03 SH06

KK10

IW08

IW09IW11IW10 SH10 IZ1 IZ2

IZ4

AZ02 AZ01

AZ07AZ03 AZ04AZ05AZ06 KS01-1,2 KS02KS03 TK01 TK02TK03

TK04

A B C 100 m contour interval