Median Tectonic Zone Research Articles

Development of the Median Tectonic Line-related shear zone, southwest Japan: An analysis of strain localization processes

The development of a mylonite zone along the Median Tectonic Line (MTL), southwest Japan, which developed in the Cretaceous granitic rocks, was examined to understand strain localization during cooling. The mylonites are E-W trending, north-dipping, and represent the hanging wall of the MTL. The mylonite zone includes both protomylonite and mylonite, which were derived from two distinct protoliths: tonalite (protomylonite) and granite (mylonite) and occur at distances of c. 0–300 m and 300–700 m from the MTL, respectively. We used quartz microstructures and crystallographic preferred orientations (CPOs) and two-feldspar thermometry to infer the spatiotemporal development of deformation conditions in the MTL mylonite zone. The mylonitic granite is sub-divided into southern and northern mylonite zones, which exhibit A- and B-type quartz microstructure formed by subgrain rotation and grain boundary migration recrystallization, respectively. In both the mylonitic granite and protomylonitic tonalite, quartz c-axis CPOs primarily display a moderate-temperature Y-maximum pattern, as well as a less common low-temperature type-I crossed-girdle pattern. In the northern zone, quartz deformation initially occurred at moderate temperatures of 435 °C–490 °C, forming mylonitic granite with B-type quartz microstructure. As temperature decreased to 355 °C–435 °C, the strain localized into the southern zone to form A-type quartz microstructure. The protomylonitic tonalite formed under similar temperature conditions. At temperatures below 355 °C, ductile deformation ceased in the majority of the granite and tonalite, with only a narrow zone of ultramylonite up to 50 m wide, deforming adjacent to the MTL at conditions approximating the brittle–ductile transition in quartz (c. 300 °C). Strain localization did not occur within a single shear zone, but in several deforming strands that formed in distinct protoliths, one of which developed into the narrow ultramylonite zone reworked as the brittle MTL, likely controlled by cooling.

The 1596 Keicho Earthquake, a 5‐Day, 300‐km‐Long Sequential Rupture Event in the Median Tectonic Line Fault Zone, Southwestern Japan

AbstractHistorical accounts and geological surveys suggest that a sequential rupture event occurred in 1596 CE in the Median Tectonic Line fault zone (MTLFZ) in southwestern Japan. During this event, known as the 1596 Keicho earthquake, the rupture probably propagated in two or three sequential stages, similar to a series of events that occurred along the North Anatolian fault during 1939–1999. The detailed rupture process of the 1596 earthquake event is unknown, however, because coseismic seismological, geological, and geodetic observation data are not available. To reconstruct the rupture processes, we integrated data from Coulomb stress change analyses, fault geometrical analyses, historical accounts, and rupture history. The results suggest that the rupture initiated on the Kawakami fault on 1 September 1596 and propagated bilaterally, to the Beppu Bay fault system, which ruptured on 4 September, and to the MTLFZ in eastern Shikoku and the Rokko‐Awaji fault zone and Arima‐Takatsuki Tectonic Line in the Kinki area, which ruptured on 5 September. However, there is a small possibility that the rupture initiated at the western end of the MTLFZ and propagated eastward unilaterally. The propagation of ruptures might be sensitively controlled not only by stress perturbation and fault geometry but also by rupture history. Evaluation of fault rupture behavior from various viewpoints can contribute to the mitigation of future seismic disasters in fault zones where detailed coseismic observation data from the past are lacking.

1916年8月6日の中央構造線断層帯の地震について

1916年8月6日の中央構造線断層帯の地震について

The Destructive Earthquake on 1916 Aug. 6<sup>th</sup> near Ishizuchi Fault of Median Tectonic Line Active Fault Zone in Ehime Prefecture

The Destructive Earthquake on 1916 Aug. 6<sup>th</sup> near Ishizuchi Fault of Median Tectonic Line Active Fault Zone in Ehime Prefecture

Late quaternary slip rates and vectors on the Median Tectonic Line active fault zone in eastern Shikoku, southwest Japan

The Median Tectonic Line active fault zone (MTLAFZ) extends for about 190 km through Shikoku, southwest Japan. Though the MTLAFZ is the most significant onshore active tectonic feature in southwest Japan, its late Quaternary slip rate has been estimated at only a few locations with reasonable references and ages. Better information on this feature's recent slip rates is critical to understanding the ongoing tectonic processes in the region and evaluating the seismic risk of this fault zone. In this paper, new estimates of the late Quaternary slip rate are reported from the Ikeda and Chichio faults in the central portion of the MTLAFZ. The author mapped late Pleistocene fluvial terrace surfaces and used tephrochronology and radiocarbon dating to constrain the age, and measured offset of terrace risers. The slip vectors of both faults are similar, as derived from piercing points on the bottoms of the terrace risers. The vertical component of displacement is 2–6% of the horizontal component. Long-term slip rates during the late Quaternary were calculated at 7.8–9.1 mm/yr, which is more precise than those in the previous studies and represents the highest slip rate in the MTLAFZ. This rate is also much faster than previously reported shorter-term slip rates of geodetic study. The long-term seismic risk of large earthquakes (>M8) along the Ikeda and Chichio faults in the next 30 years are evaluated to be more than 0.4–1.9% and 0.1–3%, respectively, much higher than a previous estimate of 0–0.3%.

Tectonic landform and geology of the Median Tectonic Line Fault Zone in western Shikoku, southwest Japan

Tectonic landform and geology of the Median Tectonic Line Fault Zone in western Shikoku, southwest Japan

中央構造線活断層帯（四国）における最近の研究成果

中央構造線活断層帯（四国）における最近の研究成果

Evolution of the Median Tectonic Line fault zone, SW Japan, during exhumation

Like many crustal-scale fault zones, the Median Tectonic Line (MTL) fault zone in Japan preserves fault rocks that formed across a broad range of physical conditions. We examined the architecture of the MTL at a large new outcrop in order to understand fault behaviours under different crustal levels. The MTL here strikes almost E–W, dips to the north, and juxtaposes the Sanbagawa metamorphic rocks to the south against the Izumi Group sediments to the north. The fault core consists mainly of Sanbagawa-derived fault gouges. The fault zone can be divided into several structural units, including two slip zones (upper and lower slip zones), where the lower slip zone is more conspicuous. Crosscutting relationships among structures and kinematics indicate that the fault zone records four stages of deformation. Microstructures and powder X-ray diffraction (XRD) analyses indicate that the four stages of deformation occurred under different temperature conditions. The oldest deformation (stage 1) was widely distributed, and had a top-to-the-east (dextral) sense of slip at deep levels of the seismogenic zone. Deformation with the same sense of slip, then became localised in the lower slip zone (stage 2). Subsequently, the slip direction in the lower slip zone changed to top-to-the-west (sinistral-normal) (stage 3). The final stage of deformation (stage 4) involved top-to-the-north normal faulting along the two slip zones within the shallow crust (near the surface). The widely distributed stage 1 damage zone characterises the deeper part of the seismogenic zone, while the sets of localised principal slip zones and branching faults of stage 4 characterise shallow depths. The fault zone architecture described in this paper leads us to suggest that fault zones display different behaviours at different crustal levels.

Arrayed-boring survey near the eastern end of the Kawakami fault of the Median Tectonic Line Active Fault Zone in central Shikoku, southwest Japan

地形情報を基に推定された断層線の地質学的な証拠を得るため，西条市横黒地点において4本（北からBr.A～D）の群列ボーリング調査を実施した．Br.A～Br.Cコアは4つのユニットで構成されるが，Br.Dには最下位ユニットが観察されない．それぞれの対比層準は南側低下の撓曲構造を示唆しており，対比層準の高度差はBr.BとBr.D間で累積性を示す．さらに，Br.Dの孔内水位が他の孔と大きく異なることから，Br.CとBr.D間に南側低下の断層の存在が示唆される．また，K-Ahの降下層準がBr.CとBr.D間およびBr.BとBr.D間でそれぞれ2.24 mおよび4.03 mの高度差がある．このことから，少なくとも約7300年前以降では，本地点の川上断層の縦ずれ方向の変位速度は0.3 mm/yr（Br.BとBr.D間の撓曲構造を考慮した場合0.55 mm/yr）程度である．

中央構造線活断層帯・重信断層の 歴史時代の断層活動

中央構造線活断層帯・重信断層の 歴史時代の断層活動

Late Holocene faulting history of the Kawakami fault of the Median Tectonic Line Fault Zone in west Shikoku, southwest Japan

Late Holocene faulting history of the Kawakami fault of the Median Tectonic Line Fault Zone in west Shikoku, southwest Japan

Restoration of fault slips revealed by aerial photogrammetry along the Chichio fault of Median Tectonic Line Active Fault Zone in the Eastern Shikoku, Southwest Japan

Restoration of fault slips revealed by aerial photogrammetry along the Chichio fault of Median Tectonic Line Active Fault Zone in the Eastern Shikoku, Southwest Japan

Internal structure of the Median Tectonic Line fault zone, SW Japan, revealed by borehole analysis

The internal structure of the Median Tectonic Line (MTL) fault zone and the processes that prevailed at depths are described based on an analysis of a borehole. The fault plane which defines the boundary between the Ryoke- and Sanbagawa-derived rocks dips at 56° to the north. Immediately beneath the boundary, approximately 40m thick fractured rocks form the major strand of the MTL fault zone. The hanging wall above the boundary comprises variably deformed Ryoke granitoids, including several mylonite zones and cataclasite zones. The fault zone has evolved through a series of faulting events under temperatures ranging from 400 to 200°C. The mineral assemblages of the mylonites and cataclasites immediately above the boundary indicate that these fault rocks were formed at temperatures of about 300°C. These mylonites and cataclasites represent, therefore, fault rocks that formed immediately below and above the brittle–plastic transition, respectively. Development of dissolution seams in these cataclasites suggests that the cataclasite has low strength. The presence of pseudotachylytes in the cataclasite indicates the occurrence of seismicity immediately above the brittle–plastic transition. On the other hand, the very fine grain size of recrystallised quartz in the mylonites indicates high differential stress immediately below the brittle–plastic transition. It is therefore likely that the differential stresses immediately below the brittle–plastic transition are much higher than those immediately above the transition. Formation of laumontite in the major strand of the MTL fault zone occurred at temperatures of around 200°C. The central slip zone of the major strand is about 30-cm thick, and is surrounded by thick gouge zones. This situation is favourable for thermal pressurisation during earthquake slips.

Geomorpholgy, geology and subsurface structure of the Median Tectonic Line active fault zone in the central part of Shikoku

中央構造線活断層帯は日本列島で最長の活断層であり，変位地形の規模も大きく明瞭である．断層露頭も見事である．1995年兵庫県南部地震（M7.3）の発生以降も，多くの調査機関や大学（研究者）が各種の活断層調査を実施してきた．とくに，ボーリング・反射法地震探査・トレンチ掘削調査などが各所で行われ，中央構造線活断層帯の性質・活動履歴・地下構造などもかなり詳しく判明してきた．こうした成果に基づいて，地震調査委員会（2003）から長期評価も公表された．四国中央部における中央構造線活断層帯の代表的な活断層地形や断層露頭などを見学するとともに，地下構造調査の成果も紹介し，活断層に関する総合的な考察を行い，残された課題や問題点などについて検討する．

四国の中央構造線断層帯の最新活動に伴う横ずれ変位量分布

Recent paleoseismological studies revealed that the entire length of the right-lateral Median Tectonic Line active fault zone in Shikoku ruptured after the 16th century A.D. Based on aerial photograph interpretation and field observations, we identified geomorphic and artificial features offset along the fault zone that record the amount of right-lateral slip associated with the most recent and previous earthquakes. The amount of offset during the most recent earthquake varies considerably among the active faults that comprise the fault zone: about 7m along the Chichio and Ikeda faults in eastern Shikoku and 2-3m along the Shigenobu and Iyo faults in western Shikoku. In general, surface slip associated with the latest event is greater than 5m between the Zunden and Okamura faults, and decreases gradually to the east and west. There is a correlation between horizontal slip rates of the individual faults and surface slip during the most recent event. This is consistent with the paleoseismic observation that the recurrence intervals of the individual faults do not vary considerably along the fault zone and are in a range of 1000-2000 years. Based on the surface offsets and segmentation model by Goto and Nakata (2000b), we are able to calculate moment magnitudes for scenario earthquakes from the fault zone. Separate ruptures of the western Shikoku, Okamura, and eastern and east-central Shikoku segments would produce earthquakes with magnitudes of Mw7.3-7.4, Mw7.2-7.4, and Mw7.7-7.8, respectively. These ruptures would be as large as or greater than the largest recorded historical earthquake, the 1891 Mw7.4 Nobi earthquake.

Earthquake slip weakening and asperities explained by thermal pressurization

An earthquake occurs when a fault weakens during the early portion of its slip at a faster rate than the release of tectonic stress driving the fault motion. This slip weakening occurs over a critical distance, D(c). Understanding the controls on D(c) in nature is severely limited, however, because the physical mechanism of weakening is unconstrained. Conventional friction experiments, typically conducted at slow slip rates and small displacements, have obtained D(c) values that are orders of magnitude lower than values estimated from modelling seismological data for natural earthquakes. Here we present data on fluid transport properties of slip zone rocks and on the slip zone width in the centre of the Median Tectonic Line fault zone, Japan. We show that the discrepancy between laboratory and seismological results can be resolved if thermal pressurization of the pore fluid is the slip-weakening mechanism. Our analysis indicates that a planar fault segment with an impermeable and narrow slip zone will become very unstable during slip and is likely to be the site of a seismic asperity.

Active fault Research on the Median Tectonic Line Active Fault Zone

Active fault Research on the Median Tectonic Line Active Fault Zone

Provenance analysis using conglomerate clast lithologies: a case study from the Pahau terrane of New Zealand

The field of provenance analysis has undergone a revolution with the development of single-crystal isotope dating techniques using mainly silt to sand sized single minerals. This study focuses on coarser-grained rocks, notably conglomerates, which involve much shorter transport distances and therefore may be used to trace proximal sources. The Torlesse terranes—part of the New Zealand Eastern Province—are accretionary complexes that comprise an enormous volume of quartzofeldspathic sandstones and mudstones with subsidiary conglomerates plus minor oceanic assemblages. Two terranes are recognised in the South Island, the Permian to Late Triassic Rakaia terrane and the Late Jurassic to Early Cretaceous Pahau terrane. Geochronology, geochemistry and Sr–Nd isotopes of igneous clasts from Pahau terrane conglomerates identify the Median Tectonic Zone (s.l.) as a major contributor of detritus to the Pahau depositional basin. SHRIMP U–Pb zircon ages of igneous clasts range from 128 to 123 Ma and from 147 to 135 Ma. Calc-alkaline clasts of the latter range are indistinguishable in age, chemical composition, and petrogenesis from the calc-alkaline granitoids of the Darran Suite, whereas alkaline clasts correlate with the Electric Granite, both parts of the Median Tectonic Zone. An Early Jurassic calc-alkaline rhyolite clast from Kekerengu (188±3 Ma) correlates with the Bounty Island Granite, Campbell Plateau. The petrography and geochemistry of Pahau terrane sandstone clasts indicate the recycling of Permian to early Late Triassic older Rakaia terrane rocks. This is further supported by the detrital zircon age data. Igneous conglomerate clasts are capable of placing exceptionally tight constraints on Pahau provenance and on its Mesozoic tectonic setting within the Southwest Pacific margin of Gondwana. They provide the best evidence yet that the Pahau terrane is locally derived.

Provenance of the Murihiku Terrane, New Zealand: evidence from the Jurassic conglomerates and sandstones in Southland

Sedimentary petrological, geochemical, and geochronological studies were carried out to reveal the provenance of the Murihiku Terrane, Southland, New Zealand. The Murihiku Terrane comprises Late Permian to Early Cretaceous volcaniclastic sediments that were deposited at the fore-arc or back-arc basin of the eastern Gondwana margin. The strata in the Waikawa district, Southland, are Middle or early Late Jurassic and comprise sandstones interbedded with conglomerates and mudstones. The sediments can be petrologically classified into two groups: one dominated by intermediate-mafic volcanic detritus (type-A), and the other more quartz bearing (15–20% in QFL), dominated by felsic volcanic and plutonic detritus (type-B). The chemistry of detrital clinopyroxene in type-A sediments is compatible with their derivation from island arc volcanics. Whole-rock major- and trace-element analyses suggest that granitoid and gabbroid clasts associated with type-B sediments are also of volcanic arc origin. Geochemical characteristics of the plutonic clasts can be compared closely with the Median Batholith (Median Tectonic Zone). Whole-rock K–Ar ages of the clasts (156±8 and 177±9 Ma for gabbroid clasts and 156±8 and 212±11 Ma for alkali feldspar granite clasts) are consistent with the Median Batholith magma cooling. Small quantities of grandite and pyrope-bearing garnet grains in type-B sediments might be derived from metamorphosed intermediate-mafic volcanic rocks to amphibolite facies. Granitic cataclasite and hornfels are also compatible with derivation from the Median Batholith that was accreted to the Brook Street Terrane.

New Zealand's Geological Foundations

New Zealand is a fragment of Gondwana that, before Late Cretaceous sea floor spreading, was contiguous with Australia and Antarctica. Only about 10% of the area of continental crust in the wider New Zealand region (Zealandia) is emergent above sea level as the North and South Islands. No Precambrian cratonic core is exposed in onland New Zealand. The Cambrian to Early Cretaceous basement can be described in terms of nine major volcano-sedimentary terranes, three composite regional batholiths, and three regional metamorphic-tectonic belts that overprint the terranes and batholiths. The terranes (from west to east) are: Buller, Takaka, Brook Street, Murihiku, Maitai, Caples, Bay of Islands (part of former Waipapa), Rakaia (older Torlesse) and Pahau (younger Torlesse). The western terranes are intruded by three composite batholith (>100 km 2) sized belts of plutons: Karamea-Paparoa, Hohonu and Median, as well as by numerous smaller plutons. Median Batholith (including the Median Tectonic Zone) is a recently-recognised Cordilleran batholith that represents the site of subduction-related magmatism from ca. 375–110 Ma. Parts of the terranes and batholiths are variably metamorphosed and deformed: Devonian and Cretaceous amphibolite-granulite facies gneisses are present in Buller, Takaka, Median and Karamea-Paparoa units; Jurassic-Cretaceous subgreenschist-amphibolite facies Haast Schist overprints the Caples, Bay of Islands and Rakaia Terranes; Cretaceous subgreenschist facies Esk Head and Whakatane Mélanges bound the Pahau Terrane. In the South Island, small areas (<5 km 2 total) of Devonian, Permian, Triassic and Jurassic Gondwana sequences have been identified. In the North Island a widespread Late Jurassic overlap sequence, Waipa Supergroup (part of former Waipapa Terrane), has recently been proposed.