Crustal Instability Research Articles

On the Energy Budget of Starquake-induced Repeating Fast Radio Bursts

With a growing sample of fast radio bursts (FRBs), we investigate the energy budget of different power sources within the framework of magnetar starquake triggering mechanism. During a starquake, the energy can be released in any form through strain, magnetic, rotational, and gravitational energies. The strain energy can be converted from three other kinds of energy during starquakes. The following findings are revealed: (1) The crust can store free magnetic energy of ∼1046 erg by existing toroidal fields, sustaining 106 bursts with frequent starquakes occurring due to crustal instability. (2) The strain energy develops as a rigid object spins down, which can be released during a global starquake accompanied by a glitch. However, it takes a long time to accumulate enough strain energy via spindown. (3) The rotational energy of a magnetar with P ≲ 0.1 s can match the energy and luminosity budget of FRBs. (4) The budget of the total gravitational energy is high, but the mechanism and efficiency of converting this energy to radiation deserve further exploration.

Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga

AbstractThe nature of Earth's earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that plate tectonics began in the Hadean, others suggest that the Hadean was characterized by long‐lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace‐element proxies for the tectono‐magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. Here, we present high‐spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1–3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, long‐lived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface‐altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon's parental magmas and flux‐like melting signatures become more prominent. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ∼3.8 and 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling—A possible sign for mobile‐lid tectonics—Occurred in that time period.

Evidence from the Vredefort Granophyre Dikes points to crustal relaxation following basin-size impact cratering

The timescale of the modification stage of basin-sized impact structures is not well understood. Owing to ca. 10 km of erosion since its formation, the Vredefort impact structure, South Africa, is an ideal testing ground for deciphering post-impact modification. Here, we present geophysical and geochemical evidence from the Vredefort Granophyre Dikes, which were derived from the - now eroded - Vredefort impact melt sheet. The dikes have been studied mostly in terms of their composition, while the timing and duration of their emplacement remain controversial. We examined the modern depth extent of five dikes, with three from the inner crystalline core of the central uplift, and two from the boundary between the core and the supracrustal collar of the central uplift, using two-dimensional electrical resistivity tomography. We found that the core dikes terminate near the present erosion surface (i.e., <5 m depth). In contrast, the dikes at the core-collar boundary extend to a depth ≥ 9 m. These observations suggest that the core dikes are exposed near their lowermost terminus. In addition, we obtained bulk geochemical composition of the dikes, finding that the andesitic composition phase is present in the core-collar dikes that is not found in the core dikes. The presence of this phase indicates the episodic emplacement of impact melt into subvertical crater floor fractures.We conclude that the dike formation was protracted and occurred over a time span of at least 104 years. The sequential formation of the Vredefort Granophyre Dikes points to horizontal extension of crust below the impact melt sheet above a kinematic velocity discontinuity, a crustal instability resulting from the dynamic collapse of the transient cavity.

A review of geohazards on the northern continental margin of the South China Sea

Crustal instability to induce the geohazards often results from both the internal tectonic dynamics and external sediment processes. Thus, the detailed geodynamics and kinematics of geohazards are important for understanding the tectonic evolutionary process and evaluating potential geohazards. This paper focuses on the active faults, submarine landslides, earthquakes as well as submarine canyons geomorphology description and kinetics explanation in the northern South China Sea (SCS) continental margin. Firstly, we have estimated the spatial extent, fault-plane geometry and faulting behaviors of the submarine active faults, such as the Littoral Fault Zone, based on cumulative deformation recorded by geophysical seismic reflections and geomorphic markers. We also analyzed the reactivation of preexisting structures, as well as the spatial and temporal distribution and migration of the submarine landslides and canyons. Then, several conceptual models and a comprehensive study of some of the different factors that contributed to the geohazard chain are conducted based on pre-existing theory and available literature. This review study indicates: (i) geohazards of different phases and geohazard chain are generally triggered by both early-phase tectonics-dominated and later-phase sedimentation-dominated processes; (ii) the fault geometric and stress segmentation may have played an important role in controlling on the uneven and asymmetric distribution of earthquakes and submarine canyons of the northern SCS, Especially, the ENE-striking preexisting Littoral Fault Zone is the major seismogenic structure along the Southchina coastline; (iii) the tectono-sedimentary coupling induced landslide and slope failure that has help to shape the modern coastal and slope geomorphology.

The effect of large reservoirs impoundment to the spatial and temporal variations of regional crustal deformation in Hubei Province, China

The total capacity of Three Gorges Reservoir (TGR) and Danjiangkou Reservoir (DJR) is large and has significant seasonal fluctuations, which give rise to crustal instability. In this research, we focus on studying the temporal and spatial variation of crustal deformation in Hubei Province caused by reservoir impoundment of TGR and DJR. The Digital Elevation Model, historical hydrological information, GPS monitoring data and load-induced deformation model are combined to monitor the crustal deformation. The modeled results indicate that in the trapezoidal area between the TGR and DJR, the average vertical deformations at different latitudes have different variation tendencies. The vertical deformation modulus and fluctuation amplitude are larger at the latitude of 33°N/32.5°N from 2003 to 2006 and at the latitude of 31°N/32.5°N from 2008 to 2014, while the latter are much larger than the former. Moreover, from 2008 to 2014, the frequency and the intensity of seismic activities are all enhanced significantly in this region. The modeled results at the GPS sites are consistent with the vertical displacement of GPS monitoring results in trends and the waveform. It can be inferred that the seasonal deformation is elastic. The horizontal deformation components have the same variation trends with that at each GPS monitoring station, which demonstrates that the whole region is moving toward the southeast. The spatial variation of crustal deformation demonstrates that the impoundment of TGR in 2003 causes significant vertical displacements, with the maximum modulus of 32 mm downward located in Xiangjiang River's estuary. When the water storage increases, the maximum value will become larger, and the location will move toward the upstream. Besides, the earthquakes occurred more frequently in the region with maximum deformation modulus.

天然地震形成的一种机制:地壳结构不稳定性

Regional crustal structure instabilities are potentially a significant cause of earthquakes, although current research focuses on other causes such as interactions between tectonic plates and the existence of fluids or faults. The Jiashi area, located at the junction of the Tarim block, the Tianshan orogenic belt, the West Kunlun orogenic belt, and the Pamir syntax, is subject to frequent earthquakes. Wide-angle reflections and refractions and near-vertical seismic reflection profiling data show that the crustal structure of the Jiashi area bears a special instability. The velocity distribution at subsurface depths above ~75 km suggests that the crust is structurally complex and hosts a number of high-and low-velocity blocks. Near-vertical seismic reflection profiles show detailed structural characteristics at a depth of 20 s two-way travel time. Sedimentary cover has well-layered reflections, and there is a stable zone highlighted by Reflectors C and D below the unstable zone of Reflectors A, B and E. A model of the crustal structure instability of the Jiashi area has been established, in which an unstable belt is formed by stacking a combination of high-and low-rigidity blocks that are correspondingly represented by high-and low-velocity blocks. They are hosted in the crust, within the stability belt of Reflectors C and D. The structural instability is aided by regional compression and deep crustal faults that intensify regional seismicity. Crustal structural instabilities also exist in the Japanese archipelago and Erdos Basin.

Natural Disasters and Risk Assessment in Uttarakhand with special reference to Uttarkashi Earthquake.

Uttarkashi lies in the main Alpine Himalayan belt, one of the most earthquake prone regions of the world. Crustal instability in this belt is ascribed to the movement of the Indian plate towords the Eurasian plate at the rate of about 50mm per year. Besides several local faults, two prominent thrusts tending northwest to southeast, from the conspicuous tectonic features. The basic aim of the present work is taken just one more step to make cities safer before the next disaster strikes. Development of a mitigation plan in more economic and effective in a long run than providing an ad-hoc relief after the disaster. The earthquake caused strong ground shaking over a large area with worst effects suffered in Uttarkashi Bhatwari region. Damage was observed in unreinforced masonry buildings as well as RC frame structures. Good construction performed much better than poor quality construction. The need for RC roof and gable bands in masonry buildings was clearly underlined by the performance of buildings at the ITBP campus at Mahidanda. There was enormous loss due to landslides and collapse of retaining walls. The failure of Gawana bridge needs to be studied. This may trigger revision of the Indian code. The earth is continually evolving and undergoing changes. From our view point at the surface of the Earth, we observe and are affected by both surface processes and the external expression of the activity occurring deep within the earth. This activity may be very slow and take place over millions of years, as in the case of a major earthquake. Much of the large scale earth movement is concentrated along faults or breaks in the earth’s crust e.g. Himalayan belt of Uttarakhand when movement occurs suddenly along a fault; energy is released in the form of an earthquake. Earthquake consists of vertical and horizontal waves like motions of the ground. The horizontal motions cause heavy destructive forces being larger than the vertical waves say 5 to 10 times greater. They may further be along any direction. Aims And Objectives Of Present Work: The basic aim of the present work is taken just one more step to make cities safer before the next disaster strikes. Development of a mitigation plan in more economic and effective in a long run than providing an ad-hoc relief after the disaster. Table :1 Districtwise House types as per MSK Classifcation Name of District No. of Houses Ares of District & houses (Per km) Number of A,B,C type houses with percentage Area under MSK Intensity KM Number of houses in Intensity Area Numbe r a type houses Number B-type houses Number of C type houses Almora 282620 5385 (52.48) A 273530 (96.78%) B 6515 (2.31%) C 2575 (0.91%) VIII 800 VII 1300 VI 3285 41986 68228 177406 40636 66033 166861 968 1573 3974 382

A paleomagnetic study of Permian and Triassic rocks from the Toulon‐Cuers Basin, SE France: Evidence for intra‐Pangea block rotations in the Permian

The identification of a massive shear zone separating Gondwana from Laurasia during late Palaeozoic times is one of the prerequisites for the controversial Pangea B to A transition. Here we present new paleomagnetic data from Permian and Triassic sediments and volcanic rocks from the Toulon‐Cuers basin, SE France, likely to be situated within this intra‐Pangea shear zone. A total of 150 samples from 14 sites were collected in the field; 108 samples yielded reliable paleomagnetic component directions based on stepwise thermal demagnetization up to maximum temperatures of 690°C. After removal of an initial viscous magnetic component from room temperature up to 200°C, a second component of reverse polarity, oriented to the south‐and‐up, was identified in almost all samples of Permian age. The Triassic samples behave similarly, with the notable difference that here, two polarities of magnetization are present. Positive field tests suggest the primary character of this characteristic magnetization. The latitudes of the resulting Early to Mid Permian paleopoles agree well with the corresponding segment of the apparent polar wander path (APWP) for Europe, whereas the longitudes are strung out along a small circle segment, indicating relative rotations between the sampled regions and stable Europe. The Triassic poles, instead, plot close to the Triassic segment of the European APWP and provide an upper time limit for the observed rotations. These results suggest a wrench faulting event associated with intra‐Pangea crustal instability and transformation during the Permian.

Geohistory Analysis of the Tabas Block (Abdoughi-Parvadeh Basins) as Seen from the Late Triassic through Early Cretaceous Subsidence Curves

The study area is situated in the Middle part of the Tabas Block. It contains outcrops of rocks that formed along longitudinal faults in Early Cimmerian orogenic phase. The basin subsided along these faults from the Late Triassic to Early Cretaceous, which include two sedimentary cycles. A sedimentary cycle, related to Upper Triassic to Bajocian is known as Shemshak group. Another sedimentary cycle, related to Bathonian to Upper Jurassic, is known as MAGU group. The Cretaceous rock units include red coarse-grained sandstone, at the base changing upward into red gypsifferous marl, rudist limestone and conglomerate, which deposited in a shallow marine environment. Subsidence analysis is applied to geologically disconnected objects in a manner that departs from its traditional use in basin analysis. However, as it introduces quantified data on the behavior of the crust in response to tectonics, it was shown to be an efficient tool in sorting out the major events amongst various local evidences for crustal instability. Based on the subsidence curves plotted for tectonic domain of the Tabas Block , the major obtained results include: - subsidence curves shown a relatively high subsidence rate in the Upper Triassic to the Middle Cretaceous, that coupled with rifting environment in this time; - this rifting was rapidly ceased in the Late Cretaceous to form Aulacogen( Failed Rift) and caused to generate oceanic crust and overriding of Sabzevar-Nain-Baft Ophiolites in this time; - subsidence rate in this part of Tabas Block was increasing towards the west and the north.

Cumberland batholith, Trans-Hudson Orogen, Canada: Petrogenesis and implications for Paleoproterozoic crustal and orogenic processes

Large volume, plutonic belts, such as the ∼ 221,000 km 2, ca. 1.865–1.845 Ga Cumberland batholith (CB) of the Trans-Hudson Orogen in Canada, are major components of Paleoproterozoic orogenic belts. In many cases, they have been interpreted as continental arc batholiths. The petrogenesis and tectonic context of the CB and implications for crustal growth and recycling are interpreted herein based on a 900 km geochemical-isotopic (Nd–O) transect across it and into granitoid plutons within bounding Archean cratons in central and southern Baffin Island. The mainly granulite grade CB, emplaced over an age span of between 14 and 24 Ma, consists mainly of high-K to shoshonitic monzogranite and granodiorite, but also includes low- and medium-K granitoid rocks. Metaluminous to slightly peraluminous compositions and δ 18O (VSMOW) values (+ 6 to + 10‰) indicate derivation from infracrustal (I-type) sources. ε Nd 1.85 Ga signatures (− 12 to − 2) of both mafic and felsic units suggest a dominance of evolved sources. Isotopic signatures in the interior of the CB (− 2 to − 7) are more radiogenic than those within Archean domains in central (− 8 to − 15) and southern (− 5 to − 19) Baffin Island. The isotopic transect is interpreted as ‘imaging’ an accreted microcontinental block (Meta Incognita) and bounding Archean cratons. The CB includes granites of arc, within-plate (A-type) and post-collisional affinity and volumetrically minor mafic rocks with both arc and non-arc features. (La/Yb) CN and Sr/Y values range from < 1 to 225 and < 1 to 611, respectively. In these respects, some CB granitoid rocks resemble Paleozoic adakitic granites, interpreted as partial melts of greatly thickened crust within post-collisional settings, such as Tibet. Thus, the CB likely encompasses various non-consanguineous magmatic suites generated at deep- to mid-crustal depths. Although CB granitoid rocks undoubtedly had important crustal sources, it is hard to assess the relative contribution of mantle-derived magmas. The CB is best interpreted as a post-accretion batholith resulting from large-scale lithospheric mantle delamination followed by the upwelling of hot asthenospheric mantle leading to voluminous crustal partial melting. Contributors to crustal instability which may have facilitated such delamination included: (a) a collage of recently assembled small cratons underlain by hot, weak lithosphere with mantle-depth structural breaks within this segment of the Trans-Hudson Orogen; (b) the gabbro-eclogite phase transformation, and (c) a greatly thickened crustal section (> 60 km), as evidenced by adakitic granites.

The effect of early Alpine thrusting in late-stage extensional tectonics: Evidence from the Kulidzhik nappe and the Pelevun extensional allochthon in the Rhodope Massif, Bulgaria

In the northeastern Rhodope Massif, the Kulidzhik nappe exposes a unique juxtaposition of a high-grade basement allochthon onto a low-grade Mesozoic unit, and the counterpart Pelevun extensional allochthon belonging to the same unit. The Kulidzhik nappe tectonostratigraphy comprises structurally upward: (i) a lower unit consisting of high-grade basement orthogneisses; (ii) a low-grade greenschist-phyllite unit consisting of Jurassic extrusive rocks and metasedimentary rocks; (iii) the nappe allochthon built by the lower high-grade basement unit orthogneisses; and (iv) Eocene sedimentary rocks and Oligocene volcanic cover rocks. The Pelevun extensional allochthon is heterogeneous, and consists of Mesozoic low-grade unit marbles and greenschists and the upper high-grade basement unit. We have combined structure and kinematics, with lithological information and 40Аr/ 39Аr geochronology to constrain the tectonic evolution and regional significance of the Kulidzhik nappe and the Pelevun extensional allochthon. Mineral chemistry reveals igneous phases of the granitic protolith of the allochthonous orthogneisses with textures related to ductile deformation and metamorphism higher than 500 °C. Their trace element patterns are indistinguishable from the high-grade basement orthogneisses in the eastern Rhodope. Mineral chemistry of the metamorphic assemblage in the underlying greenschists is consistent with medium-grade greenschist-facies metamorphism at temperatures well below 450 °C, whose geochemistry defines transitional MORB to IAT affinities with a strong arc imprint. The greenschist's composition is extremely similar to the composition of a supra-subduction zone Jurassic arc extrusive suite that occurs in the low-grade unit of the eastern Rhodope. The structural elements in all of the tectono-metamorphic units and the nappe surface indicate top-to-the NNE tectonic transport. Deformation evolved from ductile to brittle conditions coevally with a progressive decrease from lower amphibolite to weak greenschist-facies metamorphism towards the structural top. Two klippen gave plateau 40Аr/ 39Аr mica ages of 154.23 ± 0.66 Ma and 156.70 ± 0.81 Ma, respectively, which reflect cooling following the greenschist-facies event below ∼ 350 °C, during shallow crustal level thrust emplacement and exhumation of the Kulidzhik allochthon. The hanging-wall Pelevun extensional allochthon preserves an internal NNE-directed ductile tectonic transport trend, but is underlain by a SSW-directed ductile–brittle extensional detachment and has experienced NE-SW brittle extension on high-angle normal faults. An amphibole 40Аr/ 39Аr inverse isochron age of 156.58 ± 0.60 Ma constrains a Late Jurassic upper greenschist-facies tectono-metamorphic event, whereas a white mica plateau age of 39.66 ± 0.47 Ma provides evidence for Middle Eocene cooling and exhumation of the Pelevun extensional allochthon in the hanging wall of the detachment. We relate the Kulidzhik nappe to Late Jurassic crustal deformation during arc-continental margin collision that involved NNE-directed nappe staking and metamorphism of continental margin basement and island arc units. The nappe shares a tectono-metamorphic history with the nappes of the adjacent Strandzha Massif, implying a region-wide early Alpine orogenic system. Our results reveal a record of early Alpine thrust tectonics and show the significance of crustal accretion-related assembly for the tectonic evolution of the Rhodope Massif. Both the Late Jurassic thrusting event and the subsequent Cretaceous thrusting event thickened the Rhodope crust creating crustal instability, which influenced Tertiary crustal extension.

Quartz Rheology and Short-time-scale Crustal Instabilities

We present numerical results of thermal-mechanical feedback in crustal quartz rheology and contrast this behavior to the vastly different character of an olivine mantle. In the numerical experiments quartz is found to have a very strong tendency for short-time-scale instabilities, while our numerical experiments show that olivine has a decisive tendency for a stable thermally lubricated slip. At the same time, olivine can also go through a transitional period of creep bursts, which are physically caused by multiple interacting ductile faults at various length and time scales which collocate quickly into a major shear zone. Since olivine has this strong propensity to self organize in a large apparently stable fault system, it lacks the dynamics of interacting ductile faults evident in other minerals. Quartz behaves totally different and keeps its jerky slip behavior for prolonged deformation. An example is shown here in which a 30×50 km piece of a wet quarzitic crust is extended for about 2 Ma. The associated total displacement field clearly shows the unstable slipping events, which have a characteristic time frame of one to several years. In contrast, olivine is very stable and has a much longer time scale for thermal, instability of 100 kyrs.

Late Pleistocene to Holocene record of changing uplift rates in southern Calabria and northeastern Sicily (southern Italy, Central Mediterranean Sea)

A combination of published and new radiometric dates on uplifted Holocene fossil beaches from northeastern Sicily and southern Calabria (southern Italy) is compared with the altitude of the inner margin of the Last Interglacial (LIg) (Late Pleistocene, ∼124 ka) and older marine terraces in order to gain a regional-scale outline of uplift rates and their temporal changes in a region which is one of the fastest uplifting sectors of the Central Mediterranean Sea. Late Holocene radiocarbon dates from Ioppolo (southern Calabria) and Ganzirri (northeast Sicily), two newly discovered sites are here presented for the first time. The Holocene uplift rates are highest at St. Alessio and Taormina in eastern Sicily (2.4 mm/y) and at Scilla in southwestern Calabria (2.1 mm/y), two sites located across the Messina Straits and which separate the island of Sicily from mainland Italy. Uplift rates decrease towards the south and north from this centre of uplift. Late Holocene uplift rates show an apparent increase of between 64 and 124% when compared with the longer-term uplift rates calculated from the LIg highstand terraces. Furthermore, we discovered that the locations of fastest Late Pleistocene and Late Holocene uplift rates spatially coincide. To what extent the Holocene increase in uplift rates results from incomplete elastic strain release along the major extensional faults which frame the seismotectonic of the area, or indicate a true change in regional tectonic processes, is not resolved. Nonetheless, the heterogeneity of uplift, with a well-defined centre that crosses the Messina Straits, and its persistence at different time-scales indicates a tight connection between wider regional processes and fault-related displacement in controlling crustal instability in this area.

Post-Variscan (end Carboniferous-Early Permian) basin evolution in Western and Central Europe

Abstract The Variscan orogeny, resulting from the collision of Laurussia with Gondwana to form the supercontinent of Pangaea, was followed by a period of crustal instability and re-equilibration throughout Western and Central Europe. An extensive and significant phase of Permo-Carboniferous magmatism led to the extrusion of thick volcanic successions across the region (e.g. NE German Basin, NW part of the Polish Basin, Oslo Rift, northern Spain). Coeval transtensional activity led to the formation of more than 70 rift basins across the region. The various basins differ in terms of their form and infill according to their position relative to the Variscan orogen (i.e. internide or externide location) and to the controls that acted on basin development (e.g. basement structure configuration). This paper provides an overview of a variety of basin types, to more fully explore the controls upon the tectonomagmatic-sedimentary evolution of these important basins.

Variations of gravity anomaly roughness in Chugoku district, Japan: Relationship with distributions of topographic lineaments

Tectonic movement may disturb lateral continuities of crustal structures at weak zones. These lateral discontinuities of density structure cause undulations of the gravity anomaly field whose complexity can be an indicator of past crustal instability. On the other hand, topographic lineaments are formed along weak crustal zones. We would expect therefore that gravity anomaly roughness bears some relation to the distribution of surface lineaments. We develop a statistical method by employing the Bouguer anomaly standard deviation as an index of gravity anomaly roughness in terms of which we interpret the spatial distribution of topographic lineaments in Chugoku district, Japan. Locations of the lineaments, which could be diagnostic of the heterogeneity of the previously damaged crust, tend to coincide with those of high roughness areas of gravity anomaly fields.

Late cenozoic reconfiguration of low order fluvial channels under conditions of crustal instability (NE of Asia) /[ Reconfiguration des systèmes fluviatiles d'ordre inférieur en contexte d'instabilité crustale au Cénozoïque récent (Nord-Est de l'Asie).

Low order nver channels are highly responsive to tectonic movements which cause local intra-valley restructuring, channel overdeepening, and, finally, reconfiguration of nver basins Lacking a tolerance to changes of tectonic conditions, which is typical of large river valleys, they can be viewed as an indicator of changes in regional structure and geodynamics. These features are especially pronounced in areas of crustal instability, such as a system of ridges and rift valleys on the north-east of Sakha (Yakutia Republic), extending along the boundary between North-American and Eurasian plates.

On the Stratigraphy of the Palaeoproterozoic Deweras and Lomagundi Groups (Magondi Supergroup) in Zimbabwe

The Magondi Belt at the northwest rim of the Archaean Zimbabwe Craton extends over a length of 250 km from the Munyati River area in the south to the Shamrocke Mine area in the north. It is composed of the rocks of the Palaeoproterozoic Magondi Supergroup, which is subdivided into the Deweras, Lomagundi and Piriwiri Groups. The predominantly continental sediments and basalts of the Deweras Group were deposited in a rift-related basin margin environment marked by crustal instability. The depositional environment of the overlying Mcheka and Nyagari Formations of the Lomagundi Group is more stable and reflects shelf margin conditions. The Deweras sediments and basic volcanic rocks occurring in the southern part of the belt show a good stratigraphic correlation with those of the Silverside Mine area located between the middle and the northern part of the belt. A correlation of the Silverside Deweras rocks with the successions of the northern part of the belt (Mhangura and Doma West areas) becomes more difficult due to facies change, the presence of two impersistent amphibolite sills in different stratigraphic positions of the Deweras Group, and the distinct transgressive character of the marine Lomagundi rocks. Based on field relations, deformation and similar chemical composition it is suggested that the amphibolite sills of the Doma West area are equivalents of the Deweras metabasalts occurring in the Silverside area and in the southern part of the belt. The amphibolite sills were folded together with the enclosing Deweras sediments during the Magondi Orogeny that occurred between 2000 and 1800 Ma. Refolding of the Deweras rocks took place around 530 Ma during the Pan-African Orogeny. In the northernmost part of the belt (Shamrocke and Makuti-East areas), deformed amphibolites are also found in the Lomagundi Group. Using the immobile trace elements Zr/TiO 2 versus SiO 2 and Nb/Y versus SiO 2 of whole rock geochemical data the Deweras metabasalts of the Silverside area were classified as subalkaline basalts. The plot coincides with that of Deweras basic lavas of the Munyati, Mupfure and Alaska areas (southern part of the belt) and amphibolite sills occurring in the Deweras Group of the Doma West area (northern part of the belt). The metabasalts of the Silverside, Munyati, Mupfure and Alaska areas are characterised by a similar trend of their primitive-mantle normalised element patterns.

Topography of the Moho undulations in France from gravity data: their age and origin

The complete gravity data set from France and part of the neighboring countries has been analyzed to compute the topography of the Moho undulations. This work is based on an improved filtering technique and an appropriate assumed density contrast between the crust and the upper mantle. Comparison with deep seismic refraction data reveals that this relief map expresses the continuity and geometry of the Moho undulations better than the sparsely distributed seismic refraction data in France. This gravity Moho map, though may not give absolute depths at places, provides a far better correlation with surface geology than the result from other geophysical techniques. Four domains have been recognized: (a) the Alpine domain where all the Moho undulations are concentric with the Alps; (b) the Armorican domain in which all the undulations are north-west/south-east oriented; (c) the Pyrenean domain, in which the undulations are parallel with the Mountain chain; and (d) the Massif Central Domain which does not show clear structural orientation because of the influence of the strong heat flow located at the lower crust/upper mantle interface. Study of the topography and of the superficial structures associated with these undulations reveals that the undulations delineated in the Alpine Domain result from the Tertiary compression which shaped the Alps. The Armorican Domain was first created during the Lower to Middle Cretaceous opening of the Bay of Biscay. It is now slightly affected by the Tertiary to Quaternary closure of this Bay. The Pyrenean Domain was successively shaped by the Lower Cretaceous oblique opening of the Bay of Biscay and by the Upper Cretaceous to Eocene northward displacement of Spain. Comparison between the Moho undulations map and the stress map of France reveals that most of the undulations are perpendicular to the actual shortening directions. This observation suggests that the Mesozoic, Cenozoic and Quaternary stress directions were roughly the same. Massif Central is characterized by the convergence of these three sets of undulations. Its Post-Oligocene uplift was probably the result of the converging stresses recognized in the three surrounding domains. When the Moho undulations and the topography are compared, two types of periodic crustal instabilities can be recognized. One corresponds to the buckling of the crust developed under compression, the other to boudinage which was associated with extension. Both phenomena show a typical wavelength of 200–250 km which is in agreement with the results of the actual physical and numerical modeling currently available.

Gas geochemistry and seismotectonics: a review

Publications on soil and spring gases have been examined regarding their relationships with both tectonic and seismic activities. The main sources, behaviours and uses of species detected in soils and springs are displayed, and their mode of sampling and analysing briefly described. The main patterns of degassing in soils are described and we outline the wide range of geochemical signatures as the result of both permeability and mineralogical contrasts. Because thermomineral waters have been in contact with great volumes of crustal rocks at various depths, spring gases might be more representative of the local environment than soil gases. Moreover, gas signature comparisons show that spring gases are much more enriched with deep gases and slightly contaminated by atmospheric gases. Therefore, they can be considered as better samples for identifying precursors of earthquakes. Environmental perturbations are examined, and it is shown from divergent cases that pressure, temperature, soil moisture or earth tides may generate very high perturbations of the degassing process. Such effects demonstrate that no systematic correction law can be proposed and that removing external contributions from gas concentrations must be performed case by case. This demonstrates therefore the need for the simultaneous measurement of external parameters during gas monitoring. A qualitative examination of about 150 claimed precursors proposed in the literature has been reviewed. As noted by previous authors, anomalies appear at distances sometimes much greater than typical source dimensions, and occur in the field of strain higher than 10 −9, most of them being in the field of strain higher than 10 −8. Taking into account the very high heterogeneity of such a set of data, we can suggest that amplitudes of gas anomalies are independent of both magnitudes and epicentral distances of related earthquakes, suggesting local conditions to control amplitudes. On the contrary, precursory time and duration of anomalies seem to increase both with magnitudes and epicentral distances. Abundant evidence demonstrates the major role of crustal fluids in the earthquake cycle. Many works have outlined the fact that crustal instabilities can appear as the result of low stress/strain perturbations during loading. It has been suggested that motion of fluids may occur at various scales, from microcrack fluid transfer up to changes of hydraulic levels of water tables. The study of subsequent anomalies is expected to supply a tool for earthquake prediction. Following previous authors, we outline the need for further methodological improvements, including the setting up of multiparameter station networks and the simultaneous recording of the main external parameters (atmospheric pressure, water and air temperature, soil moisture) for signal processing.

Earthquake precursors and crustal 'transients'

For the public, the main question that seismologists should ask themselves is, "Can earthquakes be predicted?". Nature's earthquake prediction debate follows this simple line of inquiry, although presented in a slightly more subtle form by Ian Main: "How accurately and reliably can we predict earthquakes, and how far can we go in investigating the degree of predictability that might exist?" This is still, however, a question formulated under social pressure. I argue that this question should be left to one side by scientists to allow progress in a more general and comprehensive framework, by studying the whole set of crustal instabilities — or 'transients' — and not only earthquake precursors.