Zones Of Crustal Weakness Research Articles

Multiple movements recorded in a crustal weakness zone in NE Iberia: The Vallès-Penedès Fault revisited

Structural analysis and Anisotropy of Magnetic Susceptibility are here applied to the fault zone (several hundreds of meters thick) associated with an E-W striking, 20 km-long segment of the Vallès-Penedès Fault (Catalan Coastal Ranges). The results obtained indicate a complex structure, mainly resulting from the imprint of Paleogene compressional events on a fabric inherited from the Variscan orogeny, and its Late-Variscan and Mesozoic extensional history. The prevailing foliation on the fault gouge and microbreccia is oblique (striking NW-SE) to the main E-W trace of the fault. S-C structures affecting the foliation show both sinistral and dextral shear components on NE-SW and NW-SE planes, respectively, consistent with horizontal striations on quartz rods and planar bodies. Pure reverse and normal dip-slip shear components are also inferred from S-C structures at the meso- and micro-scale, indicating a NW to N (reverse), and S-SSE (normal) transport directions. The magnetic foliation averages the orientation of the observed foliation at the outcrop scale and the magnetic lineation shows two maxima. The first maximum has an overall horizontal plunge, parallel to the strike-slip transport direction inferred from S-C structures. The second maximum is parallel to the dip direction of foliation planes and also parallelizes the reverse transport direction and occasionally the intersection lineation between S and C planes, thus revealing the strong heterogeneity and partitioning of deformation. Cataclasis and pressure-solution are the main deformation mechanisms, in agreement with clay composition and low paleotemperatures determined for the Paleogene. The presented results are consistent with Paleogene transpression occurred through strain partitioning between the main thrust separating the Catalan Coastal Range from the Ebro basin (reverse slip) and the Vallès-Penedès fault zone (sinistral strike-slip), with an important influence of dextral shear associated to the secondary, WNW-ESE, segment of the fault (either an oblique ramp or resulting from the re-activation of the Llobregat Fault). A subsequent, Neogene extensional imprint slightly modified the compressional fabrics in the fault zone.

Formation and evolution of the Chain-Kairali Escarpment and the Vishnu Fracture Zone in the Western Indian Ocean

Published models for the plate tectonic evolution of the Western Indian Ocean suggest that the Southern Mascarene Basin opened by oceanic crustal accretion between the continental margins of southwestern India and southeastern Madagascar. However, with the cessation of the Mascarene Basin spreading centre followed by a ridge jump resulting in the opening of the Carlsberg Ridge, almost all the traces of India-Madagascar divergence were carved away from the Indian Plate and attached to the African Plate. According to some recent studies, the Chain-Kairali Escarpment, a prominent feature on the southwestern continental margin of India, is possibly the only trace of India-Madagascar divergence that remained on the Indian Plate. But the exact conjugate correspondence of this feature on the Madagascar side is uncertain. Published plate tectonic reconstructions imply that the Vishnu Fracture Zone on the Indian side and the Mauritius Fracture Zone on the Madagascar side are aligned at chron C22ny (∼49.04 Ma). Based on the near collinearity of gravity anomaly trends, the Chain-Kairali Escarpment appears to be the landward extension of the Vishnu FZ. However, at chron C34ny (∼83.0 Ma), the Chain-Kairali Escarpment was in close proximity to the incipient Mahanoro Fracture Zone. In this study we investigate this incompatibility, using an up-to-date compilation of the tectonic elements from the conjugate regions of India and Madagascar and the latest available rotation parameters that describe India-Madagascar separation through a direct India-Seychelles-Madagascar plate circuit. Our revised plate reconstruction model suggests that the Chain-Kairali Escarpment was formed due to the strike-slip motion between the southeast coast of Madagascar and the then southwest coast of India during the initial stages of India-Madagascar breakup. The migration of the Chain-Kairali Escarpment from the proximity of the Mahanoro FZ and aligning with the Vishnu FZ was the result of several successive events. The first among those events was asymmetric crustal accretion in the Mascarene Basin due to ridge propagation, between chrons C34ny (83.0 Ma) to C33ny (∼73.62 Ma). The Chain-Kairali Escarpment and associated crustal weak zones offshore India appear to have facilitated subsequent initiation of the Mauritius FZ and its conjugate Vishnu FZ during a plate reorganization at about chron C33ny (∼73.62 Ma). The cessation of spreading in the Mascarene Basin and development of full extent of the Carlsberg Ridge, shortly after chron C27ny (60.92 Ma), resulted in the initiation of a long transform fault, coinciding with the Vishnu FZ, which connected the Carlsberg Ridge with the spreading centre of the Madagascar Basin. Therefore, the Vishnu FZ and the Chain-Kairali Escarpment are two independent features created during different episodes of evolution of the Western Indian Ocean and the Chain-Kairali Escarpment is not a landward extension of the Vishnu FZ.

Structural control on volcanoes and magma paths from local- to orogen-scale: The central Andes case

Assessing the parameters that control the location and geometry of magma paths is of paramount importance for the comprehension of volcanic plumbing systems and geo-hazards. We analyse the distribution of 1518 monogenic and polygenic volcanoes of Miocene-Quaternary age of the Central Volcanic Zone of the Andes (Chile-Bolivia-Argentina), and reconstruct the magma paths at 315 edifices by analysing the morphostructural characteristics of craters and cones. Then we compare these data with outcropping dykes, tectonic structures and state of stress. Most magma paths trend N–S, NW–SE, and NE–SW, in decreasing order of frequency. The N–S and NW–SE paths coexist in the northern and southern part of the study area, whereas N–S paths dominate east of the Salar de Atacama. Outcropping dykes show the same trends. The regional Holocene stress state is given by an E–W greatest horizontal principal stress. N–S and NNE–SSW reverse faults and folds affect deposits of 4.8, 3.2 and 1.3Ma BP, especially in the central and southern study areas. A few NW–SE left-lateral strike-slip faults are present in the interior of the volcanic arc, part of which belong to the Calama-Olacapato-El Toro fault. The volcanic chain is also affected by several N–S- and NW–SE-striking normal faults that offset Pliocene and Quaternary deposits. The results indicate different scenarios of magma-tectonic interaction, given by N–S normal and reverse faults and N–S fold hinges that guide volcano emplacement and magma paths. Magma paths are also guided by strike-slip and normal NW–SE faults, especially in the northern part of the study area. Zones with verticalized strata, with bedding striking NE–SW, also acted as preferential magma paths. These data suggest that at convergence zones with continental crust, shallow magma paths can be more sensitive to the presence and geometry of upper crustal weakness zones than to the regional state of stress.

Detection and analysis of morphotectonic features utilizing satellite remote sensing and GIS: An example in SW Jordan

This study investigates the dominant orientations of morphological features and the relationship between these trends and the spatial orientation of tectonic structures in SW Jordan. Landsat 8 and hill-shaded images, constructed from 30m-resolution ASTER-GDEM data, were used for automatically extracting and mapping geological lineaments. The ASTER-GDEM was further utilized to automatically identify and extract drainage network. Morphological features were analyzed by means of azimuth frequency and length density distributions. Tectonic controls on the land surface were evaluated using longitudinal profiles of many westerly flowing streams. The profiles were taken directly across the northerly trending faults within a strong topographic transition between the low-gradient uplands and the deeply incised mountain front on the east side of the Dead Sea Fault Zone. Streams of the area are widely divergent, and show numerous anomalies along their profiles when they transect faults and lineaments. Five types of drainage patterns were identified: dendritic, parallel, rectangular, trellis, and modified dendritic/trellis. Interpretation and analysis of the lineaments indicate the presence of four main lineament populations that trend ~E–W, ~N–S, NE–SW, and NW–SE. Azimuthal distribution analysis of both the measured structures and drainage channels shows similar trends, except for very few differences in the prevailing directions. The similarity in orientation of lineaments, drainage system, and subsurface structural trends highlights the degree of control exerted by underlying structure on the surface geomorphological features. Faults and lineaments serve as a preferential conduit for surface running waters. The extracted lineaments were divided into five populations based on the main age of host rocks outcropping in the study area to obtain information about the temporal evolution of the lineament trends through geologic time. A general consistency in lineament trends over the different lithological units was observed, most probably because repeated reactivation of tectonism along preexisting deep structural discontinuities which are apparently crustal weakness zones. The reactivation along such inherited discontinuities under the present-day stress field is the most probable explanation of the complicated pattern and style of present-day landscape features in SW Jordan.

The provenance and setting of the Mesoproterozoic Dala Sandstone, western Sweden, and paleogeographic implications for southwestern Fennoscandia

The Mesoproterozoic (Jotnian) Dala Sandstone, western Sweden, is investigated using zircon U-Pb SIMS. The Dala Sandstone was deposited in a gentle sag basin and underwent post-depositional downfaulting and local deformation along the Sveconorwegian Frontal Deformation Zone. The maximum depositional age is lowered to 1.58Ga, lending support to a suggested 1.5Ga maximum age from regional basement age considerations. Associated 1.46Ga volcanism (Öje basalt) indicates that the 1.5–1.4Ga Hallandian-Danopolonian event triggered basin formation; in addition, repeated local magmatism and deformation over several 100m.y. hint at the presence of a crustal weak zone. Provenance data indicate sediment sources in surrounding Svecofennian, Transscandinavian Igneous Belt and Gothian domains; age peaks coinciding with early Gothian phases suggest that older inboard parts of the orogen shielded the Dala region from the younger outboard parts. Decreasing input from the Gothian orogen after 1.46Ga suggests changing regional drainage patterns. Archaean/Paleoproterozoic zircon in pre-Sveconorwegian sediments in southern Norway is proposed to reflect sediment routing from the north, or significant sinistral Sveconorwegian displacement of southern Norway along the Fennoscandian margin. The latter would potentially juxtapose the Dala basin and the Rjukan rift in southern Norway, however, these formed two distinct and separate basins.

La zonificación morfotectónica-volcánica en el análisis morfoestructural del relieve: el caso del municipio de Managua, Nicaragua

The modern Geomorphology has one of the scientific objectives: the morphostructural analysis, which is the set of methods oriented to reveal the structural-tectonic conditions of the earth's crust and its role in the shaping and consolidation of the actual landforms.Having into account the above mentioned, the main objective in this research is to create a Morpho-Tectonics Volcanic Unit (UMVT) of the Municipality of Managua, Nicaragua, which is represented in a topographic Map to a 1:50 000.00 scale, having as a base, the relationship between the geologic structures and the resulting relief shapes originated by the fragmentation of the Nicaraguan Volcanic Chain and the Graben Formation of Managua.The following research methods were applied in order to conduct this study: a) Creation of a digital model of elevation and shapes, b) morphometrics analysis, c) interpretation of air photographs to a 1:40 000,00 scale, d) analysis of the lineal morpho-elements and annular relief in order to complement the failure systems and morpho-lineal, e) a correlative structural analysis with the relief litostratigraphy and its lying conditions and the precision between the tectonic-volcanic units, f) interpretation of the structural patterns of the hydrographic network.The findings obtained from the morphometrics Analysis were: a) the altimeter map that allowed identifying altimetry differences between the main tectonic structures and the volcanic formations of the area. b) The creation of a dissection deep map that identified in a general way, a relationship between the slope and the dissection deep values in the territories: areas with a great number of slopes involves a major dissection and vice versa. c) The slope map which identified that the most important features appreciated, in a general way, were the escarps, which coincide with the mountain sides and erosive deforestation. The minor values are observed in the plains and middle foothills.In according to the geological-geomorphologic conditions, and the specific criteria stated previously, the Municipality of Managua was divided in twelve zones or morphotectonic volcanic units (UMVT), which are mentioned below:UMT I: Cuajachillo. This zone is represented by a low plain; UMTV II. Ticomo Los Martínez: This is a crustal weakness zone; UMTV III. San Judas-Tiscapa: This is a zone represented with middle-high plains with notable internal fissures; UMTV IV. San Juan-Unidad de Propósito: This unit is characterized with middle high plains, with presence of low foothills and with notable disjunctives structures; UMTV V. Barrio Nuevo-Esquipulas: In this zone it is middle and high plains are observed with presence of moderate fissures; UMTV VI. Graben el Aeropuerto: This zone is represented by the presence of the Graben Aeropuerto; UMTV VII. Los Madrigales: Low Foothills Zone, form volcanic origin with few presences of notable structural elements; UMTV VIII. Las Cuchillas: It is geomorphologically classified in pre-mountains with some elliptical features in its configuration; UMTV IX. San Isidro: From the geo-morphological perspective, it is classified as a high foothill, with structural evidences of the recent tectonic activity; UMTV X. Cedro Galan-Monte Tabor: This zone is represented by low foothills with some plain sectors; UMTV XI. Berlin: It is characterized by pre-mountain features; UMTV XII. Costa del Lago: This zone is characterized by a low plain.This classification is a regional characterization and its morphotectonic zoning is constituted from the scientific point of view: as a geo-systemic vision of the structure expression and the geological constitution and the neotectonic activity in the formation, consolidation and differentiation of the tectonic-volcanic relief in Nicaragua, showing the individual characteristics of each of the 12 space units.In this sense, the morphotectonic-volcanic zoning has a significant holistic strength that allows it to understand the space relationship between the composition of the geological substratum, its structures, and tectonics deformities, and at the same time its role in the relief differentiated formation in complex genetics conditions related with the recent and quaternary volcanism. The definition of the twelve units of this morphotectonic volcanic zoning represent an important input for further studies of the seismic dangerous, volcanic and extreme hydro-meteorological in different weakness sceneries and for designing the suitable programs for an adequate management of its risks.These aspects are considerably related with the UMTV, for that reason, its analysis in the weakness context due to flooding and its corresponding affections as well as the economic value of the tangible potential damages provoked by a flooding are clear examples of the relevance presented in this research (Velásquez, 2011).

Tectonic significance of Neoproterozoic magmatism of Nakora area, Malani igneous suite, Western Rajasthan, India

Three magmatic phases are distinguished in the Neoproterozoic Nakora Ring Complex (NRC) of Malani Igneous Suite (MIS), namely (a) Extrusive (b) Intrusive and (c) Dyke phase. Magmatism at NRC initiated with minor amount of (basic) basalt flows and followed by the extensive/voluminous acid (rhyolites-trachytes) flows. The ripple marks are observed at the Dadawari area of NRC in tuffaceous rhyolite flow which suggests the aqueous condition of flows deposition. The emplacement of the magma appears to have been controlled by a well defined NE-SW tectonic lineament and cut by radial pattern of dykes. These NE-SW tectonic lineaments are the linear zones of crustal weakness and high heat flow. The spheroidal and rapakivi structures in the Nakora acid volcanics indicate the relationship between genetic link and magma mixing. Basalt-trachyte-rhyolite association suggests that the large amount of heat is supplied to the crust from the magma chamber before the eruption. The field (elliptical/ring structures), mineralogical and geochemical characteristics of Nakora granites attest an alkaline character in their evolution and consistent with within plate tectonic setting. The emplacement of these granites and associated volcanics is controlled by ring structures, a manifestation of plume activity and cauldron subsidence, an evidence of extensional tectonic environment. NRC granites are the product of partial melting of rocks similar to banded gneiss from Kolar Schist Belt of India. The present investigations suggest that the magmatic suites of NRC rocks are derived from a crustal source and the required heat supplied from a mantle plume.

40Ar/39Ar geochronology, isotope geochemistry (Sr, Nd, Pb), and petrology of alkaline lavas near Yampa, Colorado: Migration of alkaline volcanism and evolution of the northern Rio Grande rift

Volcanic rocks near Yampa, Colorado (USA), represent one of several small late Miocene to Quaternary alkaline volcanic fields along the northeast margin of the Colorado Plateau. Basanite, trachybasalt, and basalt collected from six sites within the Yampa volcanic field were investigated to assess correlations with late Cenozoic extension and Rio Grande rifting. In this paper we report major and trace element rock and mineral compositions and Ar, Sr, Nd, and Pb isotope data for these volcanic rocks. High-precision 40 Ar/ 39 Ar geochronology indicates westward migration of volcanism within the Yampa volcanic field between 6 and 4.5 Ma, and the Sr, Nd, and Pb isotope values are consistent with a primary source in the Proterozoic subcontinental lithospheric mantle. Relict olivine phenocrysts have Mg- and Ni-rich cores, whereas unmelted clinopyroxene cores are Na and Si enriched with finely banded Ca-, Mg-, Al-, and Ti-enriched rims, thus tracing their crystallization history from a lithospheric mantle source region to one in contact with melt prior to eruption. A regional synthesis of Neogene and younger volcanism within the Rio Grande rift corridor, from northern New Mexico to southern Wyoming, supports a systematic overall southwest migration of alkaline volcanism. We interpret this Neogene to Quaternary migration of volcanism toward the northeast margin of the Colorado Plateau to record passage of melt through subvertical zones within the lithosphere weakened by late Cenozoic extension. If the locus of Quaternary alkaline magmatism defines the current location of the Rio Grande rift, it includes the Leucite Hills, Wyoming. We suggest that alkaline volcanism in the incipient northern Rio Grande rift, north of Leadville, Colorado, represents melting of the subcontinental lithospheric mantle in response to transient infiltration of asthenospheric mantle into deep, subvertical zones of dilational crustal weakness developed during late Cenozoic extension that have been migrating toward, and subparallel to, the northeast margin of the Colorado Plateau since the middle Miocene. Quaternary volcanism within this northern Rio Grande rift corridor is evidence that the rift is continuing to evolve.

Lithospheric structure and deformation in SE Tibet revealed by ambient noise and earthquake surface wave tomography: Recent advances and perspectives

High-resolution lithospheric structure is essential for understanding the tectonic evolution and deformation patterns of the southeastern Tibetan plateau. This is now possible due to recent advances in ambient noise and earthquake surface wave tomography, and great improvements in data coverage from dense portable array stations deployed in SE Tibet. In this review paper, I first give a brief overview of the tomographic methods from ambient noise and earthquake surface waves, and then summarize the major findings about the lithospheric structure and deformation in SE Tibet revealed by ambient noise and earthquake surface wave tomography as well as by other seismic and geophysical observations. These findings mainly include the 3-D distribution of mechanically weak zones in the mid-lower crust, lateral and vertical variations in radial and azimuthal anisotropy, possible interplay of some fault zones with crustal weak zones, and importance of strike-slip faulting on upper crustal deformation. These results suggest that integration of block extrusion in the more rigid upper-middle crust and channel flow in the more ductile mid-lower crust will be more compatible with the current geophysical observations. Finally I discuss some future perspective researches in SE Tibet, including array-based tomography, joint inversion using multiple seismic data, and integration of geodynamic modeling and seismic observations.

Effect of depleted continental lithosphere counterflow and inherited crustal weakness on rifting of the continental lithosphere: General results

Some non‐volcanic rifted continental margins differ from the predictions of uniform lithospheric extension in two significant ways, namely: 1) large tracts of continental mantle lithosphere are exposed at the seafloor or are inferred to have been emplaced beneath the outer regions of the margin; 2) rifting evolves from distributed extension with multiple rifts to terminal focused rifting. These characteristics are explained by depth‐dependent extension. We investigate this type of deformation using 2D upper‐mantle‐scale dynamical finite element models of rifting that include: 1) lithospheric counterflow, the gravitationally driven lateral flow of depleted lower lithosphere during rifting, as a mechanism for underplating and exhuming continental mantle, and; 2) inherited crustal weak zones that cause initially distributed syn‐rift extension. Depleted lower lithosphere (compositionally less dense than sublithospheric mantle by 50–80 kg m−3, with a moderately ‘dry’ olivine rheology) efficiently flows laterally thereby exhuming zones of continental mantle lithosphere that are respectively up to 90 and 140 km wide. Inherited crustal weak zones that are offset from the main central rift axis can be reactivated as offset rift basins. Extension in these basins depends on their proximity to the central rift and the ease with which crust decouples from mantle: narrow (<50 km) basins when crust does not decouple, and longer‐lived wider basins for moderate decoupling. For weak crust that readily decouples during depth‐dependent extension, the offset rift basins may remain active throughout rifting, giving rise to highly allochthonous crustal terranes that are translated toward and over the central rift axis.

Crustal features of the northeastern South China Sea: insights from seismic and magnetic interpretations

We interpret seven two-dimensional deep-penetration and long-offset multi-channel seismic profiles in the northernmost South China Sea area, which were collected by R/V Marcus G. Langseth during the TAIwan GEodynamics Research (TAIGER) project in 2009. To constrain the crustal characteristics, magnetic inversion and forward magnetic modeling were also performed. The seismic results clearly show tilted faulting blocks in the upper crust and most of the fault plane connects downward to a quasi-horizontal detachment as its bottom in the south of the Luzon-Ryukyu transform plate boundary. North of the plate boundary, a small-scale failed rifted basin (minimum 5 km in crustal thickness) with negative magnetization probably indicates an extended continental origin. Significant lower crustal material (LCM) was imaged under a crustal fracture area which indicated a continent and ocean transition origin. The thickest LCM (up to 6.5 km) is located at magnetic isochron C15 that is probably caused by the magma supply composite of a Miocene syn-rift volcanic event and Pliocene Dongsha volcanic activity for submarine volcanoes and sills in the surrounding area. The LCM also caused Miocene crustal blocks to be uplifted reversely as 17 km crustal thickness especially in the area of magnetic isochron C15 and C16. In addition, the wide fault blocks and LCM co-existed on the magnetic striped area (i.e. C15–C17) in the south of the Luzon-Ryukyu transform plate boundary. Magnetic forward modeling suggests that the whole thick crustal thickness (>12 km thick) needs to be magnetized in striped way as oceanic crust. However, the result also shows that the misfit between observed and synthetic magnetic anomaly is about 40 nT, north of isochron C16. The interval velocity derived from pre-stack time migration suggests that the crust is composed of basaltic intrusive upper crust and lower crustal material. The crustal nature should refer to a transition between continent and ocean. Thus, the magnetic reversals may be produced in two possible ways: basaltic magma injected along the crustal weak zone across magnetic reversal epoch and because some undiscovered ancient piece of oceanic crust existed. The crustal structure discrimination still needs to be confirmed by future studies.

Geochemical and mineralogical analysis of Gruithuisen region on Moon using M3 and DIVINER images

Spectral information from the Moon Mineralogy Mapper (M3) onboard Chandrayaan-1 and DIVINER Lunar Radiometer onboard LRO have been used for geochemical and mineralogical characterisation of the Gruithuisen region on Moon along with morphometrical information from LOLA Digital elevation model. The apparent reflectance of M3 on global mode is used for (1) spectral characterisation (2) estimating the abundance of Ti and Fe using Lucey's method and (3) discriminating non-mare region from mare regions by means of Minimum Noise Fraction (MNF) transform and Integrated Band Depth (IBD) parameters. Christensen frequency (CF) value derived from DIVINER data is used to delineate the silica saturated lithology from the undersaturated rocks as well as to delineate their spatial spread. Low values of FeO, TiO2, and IBD indicate non-mare nature of the domes and highland material, also supplemented by CF values. The highland rocks represent signatures of sodic plagioclase, the end result of plagioclase crystallisation from Lunar Magma Ocean. Compositional variations are observed among the domes. NW dome has highest silica concentration than the other two domes and in turn higher viscosity. It is most likely that the three domes tapped residual liquid from different locations of the residual magma chamber which is in constant mixing. The extrusion is probably a localised phenomenon, where urKREEP welled out along the zone of crustal weakness formed by Imbrium Impact. It is likely that δ dome has extruded over a larger time span than other two features.

Neoproterozoic extension in the Greater Dharwar Craton: a reevaluation of the “Betsimisaraka suture” in MadagascarThis article is one of a series of papers published in this Special Issue on the theme ofGeochronologyin honour of Tom Krogh.

The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U–Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil–Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo–Mesoarchean (3.32–3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55–2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the “Betsimisaraka suture” is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro–Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560–520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo–Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times.

Identification of Quaternary scarps in southwest and central west Western Australia using DEM-based hill shading: application to seismic hazard assessment and neotectonics*

An examination of regionally extensive hill-shaded Shuttle Radar Topography Mission (SRTM) 90 m resolution and Department of Land Information (DLI) 10 m resolution digital elevation data, and discussions with government and industry geologists familiar with the local geology, has resulted in the identification of 38 previously unrecognized linear topographic scarps in the southwest and central west of Western Australia. I contend that most of these relate to Quaternary surface-rupturing earthquakes. If validated, this more than doubles the number of Quaternary fault scarps known from this area, bringing the total to 60. The newly recognized scarps average between 25 and 50 km in length and from ∼1.5 to 20 m in height. The geometric, recurrence and spatial attributes of these features make it possible to propose a model describing the causative seismicity. The model contends that uniform contractional strain in the ductile lithosphere manifests as localized, transient and recurrent brittle deformation in zones of pre-existing crustal weakness in the upper lithosphere. The data presented allow for ready calculation of the maximum probable magnitude earthquake for the southwest of Western Australia, and identify ‘earthquake-prone’ regions of interest to seismic hazard assessors.

Magnetic anomalies across Bastar craton and Pranhita-Godavari basin in south of central India

Aeromagnetic anomalies over Bastar craton and Pranhita-Godavari (P-G) basin in the south of central India could be attributed to NW-SE striking mafic intrusives in both the areas at variable depths. Such intrusions can be explained considering the collision of the Bastar and Dharwar cratons by the end of the Archaean and the development of tensile regimes that followed in the Paleoproterozoic, facilitating intrusions of mafic dykes into the continental crust. The P-G basin area, being a zone of crustal weakness along the contact of the Bastar and Dharwar cratons, also experienced extensional tectonics. The inferred remanent magnetization of these dykes dips upwards and it is such that the dykes are oriented towards the east of the magnetic north at the time of their formation compared to their present NW-SE strike. Assuming that there was no imprint of magnetization of a later date, it is concluded that the Indian plate was located in the southern hemisphere, either independently or as part of a supercontinent, for some span of time during Paleoproterozoic and was involved in complex path of movement and rotation subsequently. The paper presents a case study of the utility of aeromagnetic anomalies in qualitatively deducing the palaeopositions of the landmasses from the interpreted remanent magnetism of buried intrusive bodies.

Ophiolites in ocean–continent transitions: From the Steinmann Trinity to sea-floor spreading

Before the theory of plate tectonics took hold, there was no coherent model for ocean–continent transitions that included both extant continental margins and fragmentary ancient examples preserved in orogenic belts. Indeed, during the early 1900, two strands of thought developed, one relying on the antiquity and permanence of continents and oceans, advocated by the mainstream of the scientific community and one following mobilist concepts derived from Wegener's hypothesis (1915) of continental drift. As an illustration of the prevailing North-American view, the different composition and thickness of continental and oceanic crust and the resulting isostatic response showed “how improbable it would be to suppose that a continent could founder or go to oceanic depth or that ocean floor at ± 3000 fathoms could ever have been a stable land area since the birth of the oceans” [H.H. Hess, Trans. R. Soc. London, A 222 (1954) 341–348]. Because of the perceived permanence of oceans and continents, mountain chains were thought to originate from narrow, elongated, unstable belts, the geosynclines, circling the continents or following “zones of crustal weakness” within them, from which geanticlines and finally mountain belts would develop. This teleological concept, whereby a geosyncline would inevitably evolve into a mountain chain, dominated geological interpretations of orogenic belts for several decades in the mid-twentieth century. However, the concept of permanence of oceans and continents and the concept of the geosyncline had already met with the critiques of Suess and others. As early as 1905, Steinmann considered the association of peridotite, “diabase” (basalt/dolerite) and radiolarite (a typical ocean–continent transition assemblage), present in the Alps and Apennines, as characteristic of the deep-ocean floor and Bailey (1936) placed Steinmann's interpretation into the context of continental drift and orogeny. Indeed, in both authors’ writings, the concept of ophiolites as ocean crust is apparent. Between 1920 and 1930, the stage was thus potentially set for modern mobilist concepts that were, however, to prove attractive to only a small circle of Alpine and peri-Gondwanian geologists. After the Second World War, the 1950s saw the rapid progress of the geophysical and geological exploration of oceans and continental margins that provided the data for a reevaluation of the geosynclinal concept. Actualistic models now equated the former preorogenic miogeosyncline of Stille (1940) and Kay (1951) with passive continental margins [C.L. Drake, M. Ewing, G.H. Sutton, Continental margin and geosynclines: the east coast of North America, north of Cape Hatteras, in: L. Ahrens, et al. (Eds.), Physics and Chemistry of the Earth 3, Pergamon Press, London, 1959, pp. 110–189], the (American version of the) eugeosyncline and its igneous rocks with “collapsing continental rises” [R.S. Dietz, J. Geol. 71 (1963) 314–333] and the ophiolites, the Steinmann Trinity, of the (European) eugeosyncline with fragments of oceanic lithosphere [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599–620]. The concept of sea-floor spreading [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599–620; H.H. Hess, Mid-oceanic ridges and tectonics of the sea-floor, in: W.F. Whittard, R. Bradshaw (Eds), Submarine Geology and Geophysics, Colston Papers 17, Butterworths, London, 1965, pp. 317–333] finally eliminated the weaknesses in Wegener's hypothesis and, with the coming of the “annus mirabilis” of 1968, the concept of the geosyncline could be laid to rest. Ocean–continent transitions of modern oceans, as revealed by seismology and deep-sea drilling, could now be compared with the remnants of their ancient counterparts preserved in the Alps and elsewhere.

Swarms in Andaman Sea, India — a seismotectonic analysis

The seismotectonic characteristics of 1983–1984, 1993 and 2005 swarms in Andaman Sea are analysed. These swarms are characterised by their typical pulsating nature, oval shaped geometry and higher b values. The migration path of the swarms from north to south along the Andaman Spreading Ridge is documented. While the first two swarms are located along existing mapped rift segments, the 2005 swarm appears to have generated a new rift basin along 8°N. The analysis and supporting evidences suggest that these swarms were generated by intruding magmatic dyke along the weak zones in the crust, followed by rifting, spreading and collapse of rift walls. CMT solutions for 2005 swarm activity indicate that intrusion of magmatic dyke in the crustal weak zone is documented by earthquakes showing strike slip solution. Subsequent events with normal fault mechanism corroborate the rift formation, collapse and its spreading.

Structural setting of late Svecofennian granites and pegmatites in Uusimaa Belt, SW Finland: Age constraints and implications for crustal evolution

Complex late Svecofennian deformation patterns and voluminous granitic magmatism at ∼1.85–1.80 Ga characterise the Palaeoproterozoic Uusimaa Belt in SW Finland. Detailed structural analysis and ion microprobe U–Pb zircon geochronology were used to (i) evaluate the evidence for crustal extension that was assumed to pre-date the predominant contractional deformation, and (ii) integrate tectonic evolution with granitic magmatism. The early Svecofennian structures (D 1 and D 2) are only locally preserved in specific parts of the Uusimaa Belt, while the late Svecofennian structures occur throughout the area. Coaxially deformed granite dykes and weak S 3 foliations in granite most likely indicate that crustal extension (D 3) took place concurrent with the initial stages of crustal melting during ∼1835–1825 Ma. Subsequent evolution involved continued granitic magmatism and a shift to contraction at ∼1820 Ma. Contraction started under horizontal ∼N–S bulk compression and resulted in intense upright as well as N–NW vergent asymmetric folds (D 4). Continued deformation with bulk compression shifting towards E–W at <1820 Ma caused strain localisation into sub-vertical transpressive shear zones with east-side-up shear sense (D 5). Another expression of sub-vertical stretching during D 5 is the generation of sheath and doubly plunging F 5 folds. The volume of granite magmatism decreased from D 3 to D 5. Also the mode of granite emplacement shifted from large flat-lying sheets during D 3 towards thinner migmatising sheets and veins during D 4, while D 5 involved intrusion of cross-cutting pegmatite dykes along sub-vertical crustal weakness zones.

Mesozoic–Cenozoic exhumation events in the eastern North Sea Basin: a multi‐disciplinary study based on palaeothermal, palaeoburial, stratigraphic and seismic data

ABSTRACTFour Mesozoic–Cenozoic palaeothermal episodes related to deeper burial and subsequent exhumation and one reflecting climate change during the Eocene have been identified in a study of new apatite fission‐track analysis (AFTA®) and vitrinite reflectance data in eight Danish wells. The study combined thermal‐history reconstruction with exhumation studies based on palaeoburial data (sonic velocities) and stratigraphic and seismic data. Mid‐Jurassic exhumation (ca. 175 Ma) was caused by regional doming of the North Sea area, broadly contemporaneous with deep exhumation in Scandinavia. A palaeogeothermal gradient of 45 °C km−1 at that time may be related to a mantle plume rising before rifting in the North Sea. Mid‐Cretaceous exhumation affecting the Sorgenfrei–Tornquist Zone is probably related to late Albian tectonic movements (ca. 100 Ma). The Sole Pit axis in the southern North Sea experienced similar inversion and this suggests a plate‐scale response along crustal weakness zones across NW Europe. Mid‐Cenozoic exhumation affected the eastern North Sea Basin and the onset of this event correlates with a latest Oligocene unconformity (ca. 24 Ma), which indicates a major Scandinavian uplift phase. The deeper burial that caused the late Oligocene thermal event recognized in the AFTA data reflect progradation of lower Oligocene wedges derived from the uplifting Scandinavian landmass. The onset of Scandinavian uplift is represented by an earliest Oligocene unconformity (ca. 33 Ma). Late Neogene exhumation affected the eastern (and western) North Sea Basin including Scandinavia. The sedimentation pattern in the central North Sea Basin shows that this phase began in the early Pliocene (ca. 4 Ma), in good agreement with the AFTA data. These three phases of Cenozoic uplift of Scandinavia also affected the NE Atlantic margin, whereas an intra‐Miocene unconformity (ca. 15 Ma) on the NE Atlantic margin reflects tectonic movements of only minor amplitude in that area. The study demonstrates that only by considering episodic exhumation as an inherent aspect of the sedimentary record can the tectonic evolution be accurately reconstructed.

Tectonic boundaries, crustal weakness zones and plume-subcontinental lithospheric mantle interactions in the Serra do Mar dyke swarm, SE Brazil

Tholeiitic basalts of Early Cretaceous age (ca.132 Ma) integrate the Serra do Mar Dyke Swarm in Southeastern Brazil. The dolerites are included in a transitional tholeiitic series that can be divided in a low-TiO 2 and high-TiO 2 suites on a geochemical basis. Parental compositions have La/Yb N and La/Nb N &gt;1 pointing to a predominantly contribution from the subcontinental lithospheric mantle in the generation of the two suites. However, the occurrence of a high-TiO 2 dolerite in the swarm with La/Nb N =0.3 and La/Yb N =9.9 ratios can indicate a contribution of a fertile sublithospheric mantle in the generation of the Serra do Mar Dyke Swarm likely to be represented by the Tristan da Cunha plume. This paper proposes that the Tristan da Cunha plume may have melted locally in places where the Gondwanaland lithosphere had undergone greater thinning due to the reactivation of crustal weakness zones nearby major terrane boundaries amalgamated in Proterozoic times resulting in the generation of restricted volumes of plume-derived tholeiitic basalts.