Major Tectonic Discontinuities Research Articles

Sequence stratigraphy of the paleozoic successions in Ghadames Basin, southern Tunisia: Insights from integrated interpretation of well logs and seismic data.

The Ghadames Basin is considered as huge intracratonic structure, which preserved over 4000 m of Paleozoic sedimentary succession deposited over the Precambrian Basement. The present work consists of an integrated interpretation of the Paleozoic interval in the Ghadames basin of southern Tunisia using well logs and calibrated seismic data. Thus, the adopted sequence stratigraphic approach allowed the identification of a total of 3 s-order sequences, bounded by Taconic, Caledonian, and Hercynian unconformities recognized as major tectonic discontinuities in the study area. The determined second-order sequences have been further subdivided into fourteen third-order sequences with their associated systems tracts, which are limited by sequence boundaries (SB), and presenting transgressive surfaces (TS), and maximum flooding surfaces (mfs). Each sequence is characterized by a unique stacking pattern and distinctive facies, thus reflecting the lithology and the corresponding depositional environment. These sequences are composed of claystone and sandstone facies deposited in varying depositional environments, ranging from a shallow marine setting during the Ordovician period to a deep marine environment during the Early Silurian. A complex deltaic environment was evidenced during the Late Silurian involving to a fluvial environment during the Devonian period. The integration of well logs and seismic data proved to be an efficient approach in the delineation of sequence stratigraphy, revealing in detail the spatial distribution and sequence organizations within the study area. These later are controlled by the relative sea level variations, with eustasy is being the dominant factor. The obtained results are mainly correlatable with the sequence stratigraphy of North African basins at a regional scale and Snedden and Lee chart (2010) at a global scale.

Chemical mass transport during deformation and metamorphism: Insights from the Main Central Thrust and its footwall of Western Arunachal Himalaya, NE India

The Main Central Thrust (MCT) is one of the major tectonic discontinuities stretching about ~2500 km along the length of the Himalaya and it plays important role during the tectono-metamorphic evolution of Himalayan orogen. This contribution aims to find out the element mobility and loss or gain of chemical mass during mylonitization and migmatization of rocks and their relation with deformation and metamorphism within the Main Central Thrust Zone (MCTz) and its immediate footwall along the Bomdila-Dirang-Seppar section of the Western Arunachal Himalaya, India. Based on meso- and microstructural observations, we divided the Dirang Formation in the footwall of the MCT into 3 units: Lower Mylonite Zone (LMZ), Protomylonite Zone (PMZ) and Upper Mylonite Zone (UMZ), with increasing structural distance from Dirang thrust towards NW. Deformation of the Dirang Formation results in enrichment of Na, K and Si with significant gain in mass of ~55% in the UMZ relative to the PMZ. We inferred that shear strain localization during mylonitization in the UMZ induces textural transitions causing chemical alterations at peak metamorphism conditions of 0.62–0.66 GPa and 690°-710 °C. Migmatites of Lower Structural Position (LSP) show enrichment of Si, Na, K, Ca, Mn and P with depletion of Mg and Fe in the leucosome + mafic selvage (LM) relative to the paleosome with ~13% chemical mass gain. On the other hand, migmatites of Upper Structural Position (USP) are characterized by enrichment of Si, Mn and K and depletion of Ca, Mg, Na, P and Fe in the LM relative to the paleosome with ~17% chemical mass gain. Such chemical changes occur during near peak metamorphic conditions of 0.52–0.63 GPa and 680°-720 °C and it is accompanied by the appearance of sillimanite. This study improves our understanding of the chemical modification of rocks in the MCT and its footwall, which were subjected to deformation and metamorphism during its evolution in Himalayan orogen. The emplacement of the GHS along the MCT buried the Dirang Formation to a depth of ~23–25 km. Interaction of fluids, derived from dewatering of the underthrusted Dirang Formation, induced chemical alteration in the UMZ. We interpret that chemical mass transfer in migmatite zones directly influences the metamorphic reactions that occur during deformation and crustal anatexis in intracontinental shear zones, such as the MCT.

The great tectonic discontinuity of Bahía de Banderas, Mexico

Detachment of the Baja California peninsula from the North American plate altered the crustal region between them, including an extension of the crust between the peninsula and Mexico's mainland. In the SE portion of this region Bahía de Banderas, a 60-km indentation in the littoral of Mexico, is a boundary region across which significant changes in topography, seismicity, seismic attenuation, trench subduction angle, continental platform width, geomagnetic field, and crustal thickness have been documented. Here we identify two additional, major discontinuities: one in gravity and the second in the regional electrical resistivity to 50-km depth, both associated with the region in which the crust changes from extended to un-extended. Under the former, we ratify and extend the presence of the oceanic slab obtained elsewhere with seismological determinations. Plate rollback is also reported in the boundary region. Gravity and electrical resistivity models locate the NW and SE limits of the rolled-back portion of the oceanic slab; within this region, we identify upwelling asthenospheric material. Bahía de Banderas region represents a major tectonic discontinuity, implying the existence of a continental sliver prolonging ~160 km into the San Blas basin north of the bay, the Tres Marías sliver, independent from the Jalisco block, in which crustal fragmentation processes appear to be still in progress. We argue that this sliver is tectonically different from the Jalisco block since at least the late Miocene, when extensional tectonics stretched and thinned its corresponding crust and lithosphere.

Mapping seismic moment and b-value within the continental-collision orogenic-belt region of the Iranian Plateau

In this paper, high-resolution map of the Gutenberg–Richter b-value and seismic moment-release are provided for the Iranian Plateau using the unified and homogeneous part of the seismicity record of the region (January 1995–July 2016). We use these parameters as stressmeters and qualitatively explore their correlations with the GPS velocity field, strain rate, faulting mechanism, attenuation, and structure of the region. Our goal is to reveal the correlations and anomalous patterns that can help to better understand the seismotectonics and the state of present-day crustal stress within the region. A negative correlation between b-value and seismic moment release as well as convergence rates is found. Correlation between geodetic measurements and seismic observations might indicate the existence of a strong mechanical coupling between the basement and the sediment cover across Zagros. High geodetic strain rates east of the Hormuz strait, southern central Alborz, and along the north Tabriz fault correspond to low b-value anomalies at these areas. A strong low b-value anomaly is observed at the major tectonic discontinuity between the Zagros continental collision and the oceanic Makran subduction.

Middle to late Eocene exhumation of the Greater Himalayan Sequence in the Central Himalayas: Progressive accretion from the Indian plate

We investigated a contractional shear zone located in central Nepal, known as Kalopani shear zone. This high-temperature shear zone triggered the early exhumation of the metamorphic core in the Himalayan belt and deeply affected the tectono-metamorphic history of the crystalline rocks soon after the collisional stage. Pseudosection modeling and inverse geothermobarometry reveal that rocks involved in the Kalopani shear zone experienced pressure-temperature conditions between 0.60 and 0.85 GPa and 600 and 660 °C. U-Th-Pb in situ laser ablation−inductively coupled plasma−mass spectrometry and sensitive high-resolution ion microprobe dating on monazite points to retrograde metamorphism related to the Kalopani shear zone starting from ca. 41 to 30 Ma. The kinematics of the Kalopani shear zone and associated erosion and/or tectonics caused the middle-late Eocene exhumation of the Greater Himalayan Sequence in the hanging wall of the Kalopani shear zone at least 9 m.y. before the activities of the middle tectonic-metamorphic discontinuity in the Greater Himalayan Sequence (High Himalayan discontinuity), the Main Central thrust, and the South Tibetan detachment. Structural data, metamorphic conditions, and geochronology from the Kalopani shear zone, compared to those of other major tectonic discontinuities active within the Greater Himalayan Sequence in the Kali Gandaki valley, indicate that shear deformation and exhumation were not synchronous all over the Greater Himalayan Sequence but migrated downward and southward at different lower levels. These processes caused the exhumation of the hanging wall rocks of the activated shear zones. The main consequence is that exhumation has been driven since the middle-late Eocene by an in-sequence shearing mechanism progressively involving new slices of the Indian crust, starting from the metamorphic core of the orogen and later involving the outer portions of the belt. This challenges the common view of exhumation of the Greater Himalayan Sequence mainly driven by the coupled activity of Main Central thrust and South Tibetan detachment between ca. 23 and 17 Ma.

True metamorphic isograds or tectonically sliced metamorphic sequence? New high-spatial resolution petrological data for the New Caledonia case study

The metamorphic belt of northern New Caledonia represents one of the best-exposed example of fossil Pacific-type subduction zone and a worldwide reference for high-pressure (HP) metamorphism and metamorphic mineral isograds, marked by a gradual evolution from very low-grade lawsonite-bearing to high-grade epidote-bearing eclogite assemblages. Despite the general agreement on the progressive increase in metamorphism, open debates hinge on the tectonic meaning of such mineral isograds, either interpreted as fossilized continuous metamorphic gradients or as major tectonic discontinuities. We present a new and extensive metamorphic dataset acquired by means of Raman spectroscopy of carbonaceous material and pseudosection modeling. Additional PT estimates were also obtained via traditional thermobarometry. Our dataset indicates the occurrence of two tectonometamorphic domains characterized by distinct patterns and significantly modifies the past tectonic interpretation of the HP terranes. The first domain, rich in metasediments, shows a continuous metamorphic gradient starting at ~300 °C and ~0.8 GPa and reaching the blueschist-eclogite transition at 500–520 °C and ~1.8 GPa and is only locally cut by minor tectonic breaks. The second one, rich in metaophiolites, shows a rather constant metamorphism at 520–550 °C and ~2.4 GPa. The two terranes are separated by a conspicuous pressure gap (0.6 GPa, or ~20 km), but no temperature gap exists. We therefore interpret the metamorphic mineral isograds in the blueschist, metasediment-rich unit as a continuous prograde metamorphic gradient corresponding to ~35 km of accreted material (in a cold subduction zone favoring lawsonite stability) later affected by a factor of two decompressional thinning. Only the epidote isograd, which localized strain as a result of fluid release during decompression, reflects regional reequilibrations. Importantly, no significant tectonic break affects the regional distribution of the classical mineral isograds, and the most significant metamorphic break is best depicted by a lithological contrast (metasediment-rich vs. metamafic/ultramafic-rich domains). Comparable patterns are observed in Tethyan-type orogens like the Western Alps or Corsica. This contribution provides useful insights into the mechanisms of exhumation and stacking of HP terrains and mountain building of both Pacific and Tethyan orogenic belts.

Analysis of the fault pattern in the Lower Lanzo Valley (italian Western Alps): a multi-scale integrated approach

This work combines, at different scales, field mapping, detailed structural analysis, satellite images (Landsat TM and ASTER) and aerial photos interpretation for the study of the fault network affecting the Lower Lanzo Valley (Turin), a key-area of the inner Western Alps bordered by two major post-Oligocene tectonic discontinuities: the Canavese Line and the Col del Lis-Trana Deformation Zone. In the study area, where both the Sesia-Lanzo Zone (Austroalpine Domain) and the Lanzo Ultramafic Complex (Piemonte Zone) crop out, this integrated approach allowed distinguishing an E-W-striking major dextral transcurrent/transpressive fault system. E-W faults, usually sub-parallel, are arranged at times in en-echelon left stepped geometric pattern and linked by minor NE-SW faults. The transcurrent activity of these structures, consistent with a bulk NW-SE shortening direction with a NE-SW extension, occurred under brittle-ductile to brittle structural conditions. Rare mesoscale kinematic data also suggest a transtensive/extensional reactivation for these faults under brittle conditions. Dextral transcurrent/transpressive movements along the E-W fault system analysed in this study could be considered roughly coeval with the dextral activity along the N-S striking Col del Lis-Trana Deformation Zone, consistent with a NE-SW to E-W shortening direction, as their transcurrent activity occurred under similar structural conditions. These different shortening directions may be the result of a complex strain partitioning, in time and space, of the bulk deformation induced, since the Oligocene, in the Western Alps by the NW-SE convergence, coupled with the anticlockwise rotation, of the Adria plate with respect to the European plate. In this kind of scenario the E-W dextral faults affecting the Lower Lanzo Valley accommodated the NW-SE directed convergence whereas the N-S dextral faults, like the Col del Lis-Trana Deformation Zone, adsorbed the rotational component between the two colliding plates. The subsequent normal reactivation of the E-W faults may be related to the extensional regime that affected the inner Western Alps since the Early(?) Miocene, due to the progressive decrease of the convergence between the two plates and the subsequent prevailing of gravitational body forces inside the chain. Seismological data indicate that this stress regime is still ongoing.

Macroseismic study of 20th May 2007 Sikkim earthquake — Its seismotectonic implications for the region

An earthquake of magnitude 5.0 with its epicenter in the West Sikkim district occurred on the evening of 20th May 2007. The macroseismic study revealed an ENE-WSW isoseismal pattern indicating a trend conforming to that of the major tectonic discontinuities in this part of the Himalaya.

On the origin of El Chichón volcano and subduction of Tehuantepec Ridge: A geodynamical perspective

The origin of El Chichón volcano is poorly understood, and we attempt in this study to demonstrate that the Tehuantepec Ridge (TR), a major tectonic discontinuity on the Cocos plate, plays a key role in determining the location of the volcano by enhancing the slab dehydration budget beneath it. Using marine magnetic anomalies we show that the upper mantle beneath TR undergoes strong serpentinization, carrying significant amounts of water into subduction. Another key aspect of the magnetic anomaly over southern Mexico is a long-wavelength (∼ 150 km) high amplitude (∼ 500 nT) magnetic anomaly located between the trench and the coast. Using a 2D joint magnetic-gravity forward model, constrained by the subduction P– T structure, slab geometry and seismicity, we find a highly magnetic and low-density source located at 40–80 km depth that we interpret as a partially serpentinized mantle wedge formed by fluids expelled from the subducting Cocos plate. Using phase diagrams for sediments, basalt and peridotite, and the thermal structure of the subduction zone beneath El Chichón we find that ∼ 40% of sediments and basalt dehydrate at depths corresponding with the location of the serpentinized mantle wedge, whereas the serpentinized root beneath TR strongly dehydrates (∼90%) at depths of 180-200 km comparable with the slab depths beneath El Chichón (200-220 km). We conclude that this strong deserpentinization pulse of mantle lithosphere beneath TR at great depths is responsible for the unusual location, singularity and, probably, the geochemically distinct signature (adakitic-like) of El Chichón volcano.

Petrology and Geochemistry of Submarine Volcanism in the Sicily Channel Rift

Submarine magmatism in the Sicily Channel Rift began in the early Pliocene and lasted until almost 200 yr ago. We present here petrological and geochemical data on volcanic rocks dredged from Graham and Nameless banks and Pantelleria seamounts in the Pelagian sector of the Sicily Channel Rift. Petrological evidence suggests that the ascent of magmas to the surface was relatively rapid, probably through channels superimposed over the major tectonic discontinuities of the Rift. Major and trace element data indicate an ocean island basalt affinity for Graham and Nameless bank alkaline lavas and a depleted tholeiitic signature for one Pantelleria seamount, which had a shallower mantle source. Sr‐Nd‐Pb isotopic compositions suggest a heterogeneous mantle source involving both focus zone (FOZO) and high‐μ (HIMU) components. Lead isotopic compositions are slightly less radiogenic in the tholeiitic basalt and more radiogenic (closer to HIMU) in the alkaline lavas. We hypothesize that a decompressional melting mechanism affected the mantle beneath the Sicily Channel at different levels. Our data fit in a regional scenario characterized by the presence of HIMU or FOZO components in alkaline and calc‐alkaline volcanics all around Sicily. The explanation for the common isotopic composition could lie in a (fossil) plume head that variably contributed to magmatogenesis in response to variable lithospheric stretching.

The accretionary history of southern South America from the latest Proterozoic to the Late Palaeozoic: some palaeomagnetic constraints

Abstract It is now accepted that southern South America was formed from several terranes of diverse origin and evolution. However, a detailed history of the accretionary processes has not been unravelled yet. Palaeomagnetism can play an important role in such an endeavour. Palaeomagnetic constraints on the tectonic evolution of this region in the Proterozoic and Palaeozoic are reviewed and discussed. Data from the Rio de la Plata craton suggest that this block was already attached to most major Gondwana blocks by the end of the Proterozoic and may have formed a single continental mass with Congo-Sao Francisco, West Nile and Arabia throughout most of the Vendian. A large ocean separating these cratons from Amazonia and West Africa, prior to Gondwana assembly, is supported by available palaeomagnetic data. To the west of the Rio de la Plata craton is the Pampia terrane. Despite lack of palaeomagnetic data, geological evidence supports a model of Early Cambrian collision between these blocks. An Early Ordovician magmatic arc, the Famatina-Eastern Puna belt, which had developed on the western margin of the already accreted Pampia terrane, shows a systematic pattern of large clockwise rotation that has been interpreted as representative of the whole terrane. The favoured tectonic model portrays a continental magmatic arc with a back-arc basin to the east that was closed when the terrane rotated. There is little doubt of a Laurentian origin for the Cuyania (Precordillera) terrane, given the amount and diversity of evidence, including palaeomagnetism. The tectonic mechanism for accretion and its timing are still controversial. New palaeomagnetic data from Late Ordovician rocks of Cuyania support the ‘Laurentian plateau’ hypothesis, which suggests that Cuyania was still linked to Laurentia well into the Ordovician. Nevertheless, these new data do not rule out the more generally favoured ‘microcontinent model’. To the west of Cuyania is the Chilenia terrane, separated by a belt of ophiolites of Late Ordovician age. Very little is known about this terrane, although some U-Pb ages and Nd model ages point to a Laurentian origin for its basement. Lack of palaeomagnetic data precludes determining its kinematic evolution. The Arequipa-Antofalla block may actually be a composite terrane. Palaeomagnetic data obtained so far come exclusively from the southern Antofalla block. Recently acquired data in the western Puna of Argentina confirm the originally proposed distribution of Early Palaeozoic palaeomagnetic poles, which, despite several uncertainties, delineate a pattern of significant counterclockwise rotations with a possible anomaly in palaeolatitude for the late Cambrian. The data suggest a major tectonic discontinuity between the Eastern and Western Puna of Argentina in the Early Palaeozoic. Four palaeomagnetic poles of Devonian to Permian age from the North Patagonian Massif are consistent in position and age with the Gondwana apparent polar wander path, suggesting that both continental masses have not experienced major relative displacement since the Devonian. The data do not, however, rule out a restricted separation of Patagonia orthogonal to its northern boundary in the Early or Middle Palaeozoic and subsequent collision in the Late Palaeozoic.

Clustering of Earthquake Events in the Himalaya - Its Relevance to Regional Tectonic Set-up

The earthquake events of Himalaya of magnitude ≥5.0 from the time window 1905–2000 are statistically analysed. The inter-event time between earthquakes shows Hurst phenomena of temporal clustering which are spatially located in five distinct domains along the Himalayan fold-thrust belt. Out of these, two domains, one around Uttaranchal-Nepal border and the other around Nepal-Sikkim border reveal maximum number of temporal clusters and thus considered as seismically most potential zones of the Himalaya. Both these zones are located at the interface of the orthogonally disposed major tectonic discontinuities of the Peninsular Shield and Himalayan fold-thrust belt. Such zones are geologically most favourable locales for strain accumulation during later-tectonic movement. Statistical analysis points towards a probability of recurrence of seismic events in near future in these two zones. However, validity of such statistical results can be ascertained by detailed geological and geophysical modelling of the terrain.

Correlation of Byrd and Selborne Groups, with implications for the Byrd Glacier discontinuity, central Transantarctic Mountains, Antarctica

South of Byrd Glacier, central Transantarctic Mountains, Antarctica, Lower Cambrian Shackleton Limestone (Byrd Group) is divided into four informal members. The transition from Shackleton carbonates to clastics of Starshot Formation is conformable, with alternating units of limestone and argillite, and a thick succession of pillow basalt. Douglas Conglomerate is interbedded with and overlies Starshot Formation, and overlies folded Shackleton Limestone unconformably. Lower amphibolite‐grade Selborne Group is composed of Madison Marble and Contortion Schist. The contact between the two formations is interbedded marble and schist, and includes metabasalt. We correlate Byrd Group with Selborne Group, Shackleton Limestone with Madison Marble, and Starshot Formation and Douglas Conglomerate with Contortion Schist. These correlations and the lack of equivalent lithologies to the north of Byrd Glacier imply that Byrd Glacier is the northern boundary of the Byrd depositional basin, and reinforces the notion that it marks a major tectonic discontinuity crossing the Ross Orogen.

A discussion on papers “Nature of a major tectonic discontinuity in the Svecofennian province of northern Sweden” by Rutland et al. (PR 112, 211–237, 2001) and “Age of deformation episodes in the Palaeoproterozoic domain of northern Sweden, and evidence for a pre-1.9 Ga crustal layer” by Rutland et al. (PR 112, 239–259, 2001)

A discussion on papers “Nature of a major tectonic discontinuity in the Svecofennian province of northern Sweden” by Rutland et al. (PR 112, 211-237, 2001) and “age of deformation episodes in the Palaeoproterozoic domain of northern Sweden, and evidence for a pre-1.9 Ga crustal layer” by Rutland et al. (PR 112, 239-259, 2001).

Nature of a major tectonic discontinuity in the Svecofennian province of northern Sweden

The volcanic rocks of the Skellefte ore district (ca. 1.89–1.87 Ga), like other volcanic complexes of similar age in the Svecofennian orogenic province, have been regarded as juvenile arc material developed over a contemporaneous subduction zone, and subsequently accreted to the continent, during a period of very rapid crustal growth. A modelled geoelectric anomaly, dipping NE beneath the district from the southern province, has provided support for the subduction/ collision hypothesis. We have used recently acquired geological and geophysical evidence to suggest an alternative to this model. Regional aeromagnetic and gravity data indicate that the Skellefte district is the site of a major tectonic discontinuity. The province to the north of this discontinuity provides evidence of an Archaean basement to within 100 km of Skellefteå. The Skellefte Group, on the southern margin of this province, displays a narrow belt of strong upright folding, trending W to WNW. We correlate this folding with the second major deformation episode (D 2), of similar trend, in the metamorphic complex of the southern province (here informally called the Robertsfors Group). The family of D 2 structures, on both sides of the province boundary, defines the 80 km wide Skellefte Zone. D 2 in the Robertsfors Group consists of discrete ductile shear zones and associated upright folds. These zones are consistently steep, display southside up kinematics, and are superposed on earlier NE trending folding (D 1) and migmatisation in the Robertsfors Group. We infer from the style of D 2 that it could not be responsible for the accretion of an exotic volcanic arc to the continent. Upper crustal thickening was probably largely achieved during D 1. We conclude from the style and orientation of the D 1 structures in the Robertsfors Group, their absence from the Skellefte Group, and their relation to granitoid intrusions, that D 1 pre-dates deposition of the Skellefte Group. We infer that the Robertsfors Group forms part of the previously unknown basement of the Skellefte Group. The latter is therefore regarded as a supracrustal sequence, deposited in a rift setting, rather than a juvenile volcanic arc. It is also concluded that the basement stratigraphic package, which also occurs to the southwest of the Skellefte ore district, and has variable internal structures, is responsible for the modelled geoelectric anomaly. We therefore interpret the tectonic discontinuity as the boundary between pre- and post-1.9 Ga rock groups, both of which have been intruded by post-1.9 Ga granitoids and affected by the localised upright D 2 (post-1.87 Ga) deformation in the Skellefte Zone. The volcanism of the Skellefte ore district and the Ni bearing mafic intrusions of the southern province are attributed to an episode of extension, between D 1 and D 2, which was probably related to a contemporaneous active margin lying west of the present exposure of Svecofennian rocks. D 1 in the Robertsfors Group is attributed to the closure of an extensive marginal basin, possibly involving subduction, prior to Skellefte Group deposition (i.e. before 1.9 Ga). Geochronological data from the Robertsfors Group on the ages of D 1 and D 2, and the regional significance of pre-1.9 Ga metamorphic complexes, are discussed in a companion paper.

A re‐evaluation of eclogite facies metamorphism in SW Japan: proposal for an eclogite nappe

Known eclogite occurrences in the Sanbagawa metamorphic belt of SW Japan are dominantly in metagabbro bodies which have complex polyphase metamorphic histories. These bodies are generally described as tectonic blocks and their relationship to the Sanbagawa metamorphism is unclear. New findings of foliated eclogite in the Seba and Kotsu areas show that eclogite facies metamorphism is much more widespread than generally thought. Evidence that the foliated eclogite units originated as lavas or sediments implies that these units can be treated as a high‐grade part of the subduction‐related Sanbagawa metamorphism. Although separated by an along‐strike distance of 80 km, the Seba and Kotsu eclogites have very similar garnet and omphacite compositions, suggesting that they were formed under similar metamorphic conditions. However, differences in the associated retrograde assemblages (epidote–amphibolite in the Seba unit and epidote–blueschist in the Kotsu unit) suggest contrasting P–T paths. In both units, the eclogite rocks occupy the highest structural level of the Sanbagawa belt and overlie rocks metamorphosed at lower pressure. The lower boundary to the eclogite units is therefore a major tectonic discontinuity locally decorated with lenses of exotic material. These features can help trace the boundary into other areas. The previously known outcrops of eclogite show enough similarities with the newly found areas to suggest that all the eclogite facies rocks in the Sanbagawa belt constitute a single nappe that lies at the highest structural levels of the orogen.

Structural history and tectonic evolution of the boundary between the Wilson and Bowers terranes, Lanterman Range, northern Victoria Land, Antarctica

The boundary between the Wilson and Bowers terranes is one of the major tectonic discontinuities of northern Victoria Land (Antarctica) and is referred to as the Lanterman Fault Zone by most authors. Fieldwork carried out in this area indicates that the structural evolution of this boundary is polyphase and four main tectonic phases can be recognized: (1) west over east thrusting of Ross age (i.e. around 500 Ma), coeval with amphibolite and amphibolite–greenschist facies transition metamorphism; (2) sinistral strike-slip shearing, coeval with greenschist facies metamorphism; this event may belong to the Ross Orogeny or may represent evidence of the Borchgrevink Orogeny (i.e. around 360 Ma); (3) large-wavelength folding of late-Ross or Borchgrevink age; (4) Cenozoic brittle tectonics, expressed by small to km-scale structures, with dextral strike-slip displacement. Evidence for this structural evolution is not confined to a single tectonic lineament, but occurs throughout the area close to the boundary between the Wilson and Bowers terranes. It follows that the docking of the Wilson and Bowers terranes was complex and polyphase and it is not possible to define a single tectonic setting for the Lanterman Fault Zone, since it was the site of contrasting tectonic regimes at different times and at different structural levels.

Exhuming the Sanbagawa metamorphic belt: the importance of tectonic discontinuities

Tectonic processes that have been proposed to explain the transport to the surface of regional metamorphic belts can be broadly divided into two types. (i) Corner‐flow within a convergent margin bounded by two essentially rigid plates associated with extension at shallow levels. This type of model assumes deformation is distributed throughout the margin and that any discontinuities are of secondary importance. (ii) Expulsion or extrusion of coherent metamorphic nappes. In this second idea, tectonic discontinuities are fundamental in the transport to the surface of metamorphic rocks. The wealth of geological data available from a variety of studies in the Sanbagawa metamorphic belt, southwest Japan makes it well‐suited for studying the relative importance of continuous vs. discontinuous deformation in the process of exhumation. In the Sanbagawa belt a sudden decrease in metamorphic pressure going down section of several kilobars suggests the presence of a major tectonic contact separating two major regional nappes: an overlying higher‐pressure Besshi nappe and an underlying lower‐pressure Oboke nappe. Major tectonic discontinuities have also been proposed within the Besshi nappe, however, indicators of metamorphic temperature, the results of radiometric age dating, and microstructural studies all suggest that post‐metamorphic discontinuities are minor and that this nappe formed and remained as an essentially coherent unit. Lithological associations and petrological studies suggest the following positions for the two nappes. The Besshi nappe formed deep within the former accretionary wedge, adjacent to the overlying mantle wedge, and with a dip of roughly 30 °C. In contrast, the Oboke nappe formed at moderate depths within the accretionary wedge, was distant from the mantle wedge, and was roughly horizontal. Penetrative deformation that post‐dates the peak of metamorphism has affected nearly all of the Sanbagawa belt and has played an important role in its exhumation. However, the presence of a broad coherent Besshi nappe overlying the lower‐pressure Oboke nappe suggests that some process such as buoyancy‐driven extrusion was also important in the exhumation process and in forming the structure of the Sanbagawa metamorphic belt.

A stable and 40Ar/39Ar isotope study of a major thrust in the Helvetic nappes (Swiss Alps): Evidence for fluid flow and constraints on nappe kinematics

Stable isotope and 40 Ar/ 39 Ar measurements were made on samples associated with a major tectonic discontinuity in the Helvetic Alps, the basal thrust of the Diablerets nappe (external zone of the Alpine Belt) in order to determine both the importance of fluids in this thrust zone and the timing of thrusting. A systematic decrease in the δ 18 O values (up to 6‰) of calcite, quartz, and white mica exists within a 10- to 70-m-wide zone over a distance of 37 km along the thrust, and they become more pronounced toward the root of the nappe. A similar decrease in the δ 13 C values of calcite is observed only in the deepest sections (up to 3‰). The δD SMOW (SMOW = standard mean ocean water) values of white mica are −54‰ ± 8‰ ( n = 22) and are independent of the distance from the thrust. These variations are interpreted to reflect syntectonic solution reprecipitation during fluid passage along the thrust. The calculated δ 18 O and δD values (versus SMOW) for the fluid in equilibrium with the analyzed minerals is 12‰ to 16‰ and −30‰ to +5‰, respectively, for assumed temperatures of 250 to 450 °C. The isotopic and structural data are consistent with fluids derived from the deep-seated roots of the Helvetic nappes where large volumes of Mesozoic sediments were metamorphosed to the amphibolite facies. It is suggested that connate and metamorphic waters, overpressured by rapid tectonic burial in a subductive system escaped by upward infiltration along moderately dipping pathways until they reached the main shear zone at the base of the moving pile, where they were channeled toward the surface. This model also explains the mechanism by which large amounts of fluid were removed from the Mesozoic sediments during Alpine metamorphism. White mica 40 Ar/ 39 Ar ages vary from 27 Ma far from the Diablerets thrust to 15 Ma along the thrust. An older component is observed in micas far from the thrust, interpreted as a detrital signature, and indicates that regional metamorphic temperatures were less than about 350 °C. The plateau and near plateau ages nearest the thrust are consistent with either neocrystallization of white mica or argon loss by recrystallization during thrusting, which may have been enhanced in the zones of highest fluid flow. The 15 Ma 40 Ar/ 39 Ar age plateau measured on white mica sampled exactly on the thrust surface dates the end of both fluid flow and tectonic transport.

Fluid transport in lineaments

Fluid transport in lineaments