Zones Of Crustal Weakness Research Articles

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 >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.

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 >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.

Mesoproterozoic bimodal magmatism along the Protogine Zone, S Sweden: three magmatic pulses at 1.56, 1.22 and 1.205 Ga, and regional implications

The Protogine Zone is a major tectonic zone situated at the eastern boundary of the Sveconorwegian orogenic belt in southern Sweden. This structure hosts Mesoproterozoic dolerite dykes and plutonic rocks including ultramafic cumulate, syenite, monzonorite and granite. Pb–Pb zircon evaporation ages are reported for seven samples of rocks along the Protogine Zone, including the Vaggeryd syenite pluton. Two samples of the main syenite facies of the Vaggeryd pluton yield ages of 1220±3 and 1219±3 Ma. Three samples of anorthosite gabbro, aplite and pegmatite, all of which form dykes in the Vaggeryd pluton, yield equivalent ages of 1220±3, 1221±3 and1218±3 Ma. They demonstrate that the different facies recorded in the pluton belong to a single magmatic pulse at 1220 Ma, significantly older than previous estimate of 1204±4 Ma (Jarl 2002). Samples of the Gumlösa-Glimåkra granite in NE Scania and the Åker metabasite, located immediately west of the Vaggeryd syenite, were dated at 1204±3 and 1567±3 Ma, respectively, hence reproducing earlier reported ages for these intrusions determined by conventional TIMS and SIMS U–Pb techniques (1204−15 +16 Ma, Johansson 1990 and 1562±6 Ma, Söderlund et al. 2004a). The 1575–1562 Ma suite of mafic intrusions along the Protogine Zone shows temporal correspondence with rapakivi magmatism in SW Finland and suggests that the Protogine Zone first may have originated as a zone of crustal weakness, which later was reactivated (e.g. at 1224–1215 and ca. 1204 Ma) in an extensional, possibly back-arc, tectonic setting prior to the 1.13–0.95 Ga Grenvillian-Sveconorwegian orogeny.

Mesozoic transtensional basin history of the Eastern Cordillera, Colombian Andes: Inferences from tectonic models

Backstripping analysis and forward modeling of 162 stratigraphic columns and wells of the Eastern Cordillera (EC), Llanos, and Magdalena Valley shows the Mesozoic Colombian Basin is marked by five lithosphere stretching pulses. Three stretching events are suggested during the Triassic–Jurassic, but additional biostratigraphical data are needed to identify them precisely. The spatial distribution of lithosphere stretching values suggests that small, narrow (<150 km), asymmetric graben basins were located on opposite sides of the paleo-Magdalena–La Salina fault system, which probably was active as a master transtensional or strike-slip fault system. Paleomagnetic data suggesting a significant (at least 10°) northward translation of terranes west of the Bucaramanga fault during the Early Jurassic, and the similarity between the early Mesozoic stratigraphy and tectonic setting of the Payandé terrane with the Late Permian transtensional rift of the Eastern Cordillera of Peru and Bolivia indicate that the areas were adjacent in early Mesozoic times. New geochronological, petrological, stratigraphic, and structural research is necessary to test this hypothesis, including additional paleomagnetic investigations to determine the paleolatitudinal position of the Central Cordillera and adjacent tectonic terranes during the Triassic–Jurassic. Two stretching events are suggested for the Cretaceous: Berriasian–Hauterivian (144–127 Ma) and Aptian–Albian (121–102 Ma). During the Early Cretaceous, marine facies accumulated on an extensional basin system. Shallow-marine sedimentation ended at the end of the Cretaceous due to the accretion of oceanic terranes of the Western Cordillera. In Berriasian–Hauterivian subsidence curves, isopach maps and paleomagnetic data imply a (>180 km) wide, asymmetrical, transtensional half-rift basin existed, divided by the Santander Floresta horst or high. The location of small mafic intrusions coincides with areas of thin crust (crustal stretching factors >1.4) and maximum stretching of the subcrustal lithosphere. During the Aptian–early Albian, the basin extended toward the south in the Upper Magdalena Valley. Differences between crustal and subcrustal stretching values suggest some lowermost crustal decoupling between the crust and subcrustal lithosphere or that increased thermal thinning affected the mantle lithosphere. Late Cretaceous subsidence was mainly driven by lithospheric cooling, water loading, and horizontal compressional stresses generated by collision of oceanic terranes in western Colombia. Triassic transtensional basins were narrow and increased in width during the Triassic and Jurassic. Cretaceous transtensional basins were wider than Triassic–Jurassic basins. During the Mesozoic, the strike-slip component gradually decreased at the expense of the increase of the extensional component, as suggested by paleomagnetic data and lithosphere stretching values. During the Berriasian–Hauterivian, the eastern side of the extensional basin may have developed by reactivation of an older Paleozoic rift system associated with the Guaicáramo fault system. The western side probably developed through reactivation of an earlier normal fault system developed during Triassic–Jurassic transtension. Alternatively, the eastern and western margins of the graben may have developed along older strike-slip faults, which were the boundaries of the accretion of terranes west of the Guaicáramo fault during the Late Triassic and Jurassic. The increasing width of the graben system likely was the result of progressive tensional reactivation of preexisting upper crustal weakness zones. Lateral changes in Mesozoic sediment thickness suggest the reverse or thrust faults that now define the eastern and western borders of the EC were originally normal faults with a strike-slip component that inverted during the Cenozoic Andean orogeny. Thus, the Guaicáramo, La Salina, Bitúima, Magdalena, and Boyacá originally were transtensional faults. Their oblique orientation relative to the Mesozoic magmatic arc of the Central Cordillera may be the result of oblique slip extension during the Cretaceous or inherited from the pre-Mesozoic structural grains. However, not all Mesozoic transtensional faults were inverted.

Spatio-temporal microseismicity clustering in the Cretan region

Spatio-temporal clustering of microseismicity in the central forearc of the Hellenic Subduction Zone in the area of Crete is investigated. Data for this study were gathered by temporary short period networks which were installed on the islands of Crete and Gavdos between 1996 and 2004. The similarity of waveforms is quantified systematically to identify clusters of microseismicity. Waveform similarities are calculated using an adaptive time window containing both the P- and S-wave onsets. The cluster detection is performed by applying a single linkage approach. Clusters are found in the interplate seismicity as well as in the intraplate seismicity of the continental crust in the region of the transtensional Ptolemy structure. The majority of the clusters are off the southern coast of Crete, in a region of elevated intraplate microseismic activity within the Aegean plate. Clusters in the Gavdos region are located at depths compatible with the plate interface while cluster activity in the region of the Ptolemy trench is distributed along a nearly vertical structure throughout the crust extending down to the plate interface. Most clusters show swarm-like behaviour with seismic activity confined to only a few hours or days, without a dominant earthquake and with a power law distribution of the interevent times. For the largest cluster, precise relocations of the events using travel time differences of P- and S-waves derived from waveform cross correlations reveal migration of the hypocenters. This cluster is located in the region of the Ptolemy trench and migration occurs along the strike of the trench at ∼ 500 m/day. Relocated hypocenters as well as subtle differences in the waveforms suggest an offset between the hypocenters and thus the activation of distinct patches on the rupture surface. The observed microseismicity patterns may be related to fluids being transported along the plate interface and escaping towards the surface in zones of crustal weakness (Ptolemy structure), triggering swarm-like cluster activity along its way.

Microseismic activity in the Hellenic Volcanic Arc, Greece, with emphasis on the seismotectonic setting of the Santorini–Amorgos zone

The volcanic arc of the Hellenic subduction zone with its four volcanic centers is of major relevance when evaluating the seismovolcanic hazard for the Aegean region. We present results from a 22-station temporary seismic network (CYCNET) in the central Hellenic Volcanic Arc (HVA). CYCNET recordings allow to analyze the level and spatio-temporal evolution of microseismic activity in this region for the first time. A total of 2175 events recorded between September 2002 and July 2004 are analyzed using statistical methods, cluster analysis and relative relocation techniques. We identify distinct regions with significantly varying spatio-temporal behavior of microseismicity. A large portion of the seismic activity within the upper crust is associated with the presence of islands representing horst structures that were generated during the major Oligocene extensional phase. In contrast, the central part of the Cyclades metamorphic core complex remains aseismic considering our magnitude threshold of 1.8 except one spot where events occur swarm-like and with highly similar waveforms. The highest activity in the study area was identified along the SW–NE striking Santorini–Amorgos zone. Within this zone the submarine Columbo volcano exhibits strong temporal variations of seismic activity on a high background level. This activity is interpreted to be directly linked to the magma reservoir and therein the migration of magma and fluids towards the surface. NE of Columbo where no volcanic activity has yet been reported we observe a similar seismicity pattern with small-scaled activity spots that might represent local pathways of upward migrating fluids or even developing volcanic activity within this zone of crustal weakness. In contrast, the Santorini and Milos volcanic complexes do not show significant temporal variations and low to moderate background activity, respectively. Relating our results to the distribution of historical earthquakes and the GPS-derived horizontal velocity field we conclude that the Santorini–Amorgos zone is presently in the state of right-lateral transtension reflecting a major structural boundary of the volcanic arc subdividing it into a seismically and volcanically quiet western and an active eastern part.

Constraints on the tectonic evolution of the Central European Basin System revealed by seismic reflection profiles from Northern Germany

AbstractA selection of reflection-seismic lines from the southern part of the Central European Basin System and its southern margin were used to establish the spectrum of extensional and compressional deformation structures and to calibrate the timing of these events. The lines are arranged in a N-S and an E-W transect running across the Pompeckj Block and the Lower Saxony Basin in Northern Germany. The structural record provided by the seismic data points to an interplay between far-field horizontal stresses and vertical movements of the basin floor, the latter only weakly correlated with the development of seismic-scale tectonic structures. A detailed seismo-stratigraphic analysis indicates that Late Triassic-Jurassic extension has been the principal control on the structure of the E-W profile, whereas the N-S profile is dominated by compressional structures associated with Late Cretaceous inversion. Overprint effects of extension and inversion tectonics can be classified among a few distinct tectonic events accompanied by movements of the Upper Permian salt: firstly, an episode of subsidence and diapiric rise in the Keuper; secondly, a clear Jurassic-to-Early Cretaceous extension recorded by normal faulting and differential subsidence. The latter was interrupted by a major episode of Late Jurassic uplift. Thirdly, a Late Cretaceous-Early Paleogene basin inversion was associated with approximately N-S compression; and fourthly, recurring extension during the Cenozoic associated with diapiric rise and collapse. The Mesozoic extension is expressed in a number of normal faults that were most active during the Early Cretaceous localised subsidence within the Lower Saxony Basin. The deformation associated with the Late Cretaceous inversion was partly decoupled along the salt. The compressional deformation at the southern margin of the basin was thick-skinned in style, characterized by folding and faulting of the Mesozoic sedimentary fill and pre-Zechstein strata. Further north, towards the centre of the basin, folding and reverse faulting were mostly concentrated above the salt. The tectonic evolution of the investigated area suggests the presence of a zone of crustal weakness along the SW margin of the Central European Basin System which allowed strain localization in response to a favourable oriented stress field.

Different modes of the Late Cretaceous–Early Tertiary inversion in the North German and Polish basins

Several selected seismic lines are used to show and compare the modes of Late-Cretaceous–Early Tertiary inversion within the North German and Polish basins. These seismic data illustrate an important difference in the allocation of major zones of basement (thick-skinned) deformation and maximum uplift within both basins. The most important inversion-related uplift of the Polish Basin was localised in its axial part, the Mid-Polish Trough, whereas the basement in the axial part of the North German Basin remained virtually flat. The latter was uplifted along the SW and to a smaller degree the NE margins of the North German Basin, presently defined by the Elbe Fault System and the Grimmen High, respectively. The different location of the basement inversion and uplift within the North German and Polish basins is interpreted to reflect the position of major zones of crustal weakness represented by the WNW-ESE trending Elbe Fault System and by the NW-SE striking Teisseyre-Tornquist Zone, the latter underlying the Mid-Polish Trough. Therefore, the inversion of the Polish and North German basins demonstrates the significance of an inherited basement structure regardless of its relationship to the position of the basin axis. The inversion of the Mid-Polish Trough was connected with the reactivation of normal basement fault zones responsible for its Permo-Mesozoic subsidence. These faults zones, inverted as reverse faults, facilitated the uplift of the Mid-Polish Trough in the order of 1–3 km. In contrast, inversion of the North German Basin rarely re-used structures active during its subsidence. Basement inversion and uplift, in the range of 3–4 km, was focused at the Elbe Fault System which has remained quiescent in the Triassic and Jurassic but reproduced the direction of an earlier Variscan structural grain. In contrast, N-S oriented Mesozoic grabens and troughs in the central part of the North German Basin avoided significant inversion as they were oriented parallel to the direction of the inferred Late Cretaceous–Early Tertiary compression. The comparison of the North German and Polish basins shows that inversion structures can follow an earlier subsidence pattern only under a favourable orientation of the stress field. A thick Zechstein salt layer in the central parts of the North German Basin and the Mid-Polish Trough caused mechanical decoupling between the sub-salt basement and the supra-salt sedimentary cover. Resultant thin-skinned inversion was manifested by the formation of various structures developed entirely in the supra-salt Mesozoic–Cenozoic succession. The Zechstein salt provided a mechanical buffer accommodating compressional stress and responding to the inversion through salt mobilisation and redistribution. Only in parts of the NGB and MPT characterised by either thin or missing Zechstein evaporites, thick-skinned inversion directly controlled inversion-related deformations of the sedimentary cover. Inversion of the Permo-Mesozoic fill within the Mid-Polish Trough was achieved by a regional elevation above uplifted basement blocks. Conversely, in the North German Basin, horizontal stress must have been transferred into the salt cover across the basin from its SW margin towards the basins centre. This must be the case since compressional deformations are concentrated mostly above the salt and no significant inversion-related basement faults are seismically detected apart from the basin margins. This strain decoupling in the interior of the North German Basin was enhanced by the presence of the Elbe Fault System which allowed strain localization in the basin floor due to its orientation perpendicular to the inferred Late Cretaceous–Early Tertiary far-field compression.

Plateau‐style accumulation of deformation: Southern Altiplano

Employing surface mapping of syntectonic sediments, interpretation of industry reflection‐seismic profiles, gravity data, and isotopic age dating, we reconstruct the tectonic evolution of the southern Altiplano (∼20–22°S) between the cordilleras defining its margins. The southern Altiplano crust was deformed between the late Oligocene and the late Miocene with two main shortening stages in the Oligocene (33–27 Ma) and middle/late Miocene (19–8 Ma) that succeeded Eocene onset of shortening at the protoplateau margins. Shortening rates in the southern Altiplano ranged between 1 and 4.7 mm/yr with maximum rates in the late Miocene. Summing rates for the southern Altiplano and the Eastern Cordillera, we observe an increase from Eocene times to the late Oligocene to some 8 mm/yr, followed by fluctuation around this value during the Miocene prior to shutoff of deformation at 7–8 Ma and transfer of active shortening to the sub‐Andean fold and thrust belt. Shortening inverted early Tertiary graben and half graben systems and was partitioned in three fault systems in the western, central, and eastern Altiplano, respectively. The east vergent fault systems of the western and central Altiplano were synchronously active with the west vergent Altiplano west flank fault system. From these data and from section balancing, we infer a kinematically linked western Altiplano thrust belt that accumulated a minimum of 65 km shortening. Evolution of this belt contrasts with the Eastern Cordillera, which reached peak shortening rates (8 mm/yr) in between the above two stages. Hence local shortening rates fluctuated across the plateau superimposed on a general trend of increasing bulk rate with no trend of lateral propagation. This observation is repeated at the shorter length and time scales of individual growth structures that show evidence for periods of enhanced local rates at a timescale of 1–3 Myr. We interpret this irregular pattern of deformation to reflect a plateau‐style of shortening related to a self‐organized state of a weak crust in the central South American back arc with a fault network that fluctuated around the critical state of mechanical failure. Tuning of this state may have occurred by changes in plate kinematics, during the Paleogene, initially reactivating crustal weak zones and by thermal weakening of the crust with active magmatism mainly in the Neogene stage.

Inversion tectonics during continental rifting: The Turkana Cenozoic rifted zone, northern Kenya

Remote sensing data and revised seismic reflection profiles provide new insights about the origin of inverted deformation within Miocene‐Recent basins of the Turkana rift (northern Kenya) in the eastern branch of the East African rift system. Contractional structures are dominated by weakly inverted sets of fault blocks within &lt;3.7 Myr old synrift series. Most of reverse extensional faults involve components of oblique‐slip, whereas associated hanging wall folds are characterized by large wavelength upright folding. The area of basin inversion is restricted to a 40 × 100 km elongated zone overlying a first‐order N140°E trending fault zone in the basement, referred to as the N'Doto transverse fault zone (NTFZ). In the proposed kinematic model, inversion tectonics is assigned to permutation of principal stress axes (σ1/σ2) in addition to the clockwise rotation of extension (from nearly N90°E to N130°E) during Pliocene. The transition from pure extension (Miocene) to a wrench faulting regime (Pliocene) first results in the development of T‐type fault networks within a dextrally reactivated shear zone (NTFZ). Inversion tectonics occurred later (&lt;3.7 Ma) in response to a still rotated (∼20°) shortening axis (σ1) oriented N40°E that caused the oblique compression of earlier (NS to N20°E) extensional structures within the NTFZ. The origin of basin inversion and strain concentration in the Turkana rift is thus directly linked to a crustal weakness zone, transverse to the rift axis, and involving steep prerift anisotropies.

Crustal memory and basin evolution in the Central European Basin System—new insights from a 3D structural model

The Central European Basin System (CEBS) is composed of a series of subbasins, the largest of which are (1) the Norwegian–Danish Basin (2), the North German Basin extending westward into the southern North Sea and (3) the Polish Basin. A 3D structural model of the CEBS is presented, which integrates the thickness of the crust below the Permian and five layers representing the Permian–Cenozoic sediments. Structural interpretations derived from the 3D model and from backstripping are discussed with respect to published seismic data. The analysis of structural relationships across the CEBS suggests that basin evolution was controlled to a large degree by the presence of major zones of crustal weakness. The NW–SE-striking Tornquist Zone, the Ringkøbing-Fyn High (RFH) and the Elbe Fault System (EFS) provided the borders for the large Permo–Mesozoic basins, which developed along axes parallel to these fault systems. The Tornquist Zone, as the most prominent of these zones, limited the area affected by Permian–Cenozoic subsidence to the north. Movements along the Tornquist Zone, the margins of the Ringkøbing-Fyn High and the Elbe Fault System could have influenced basin initiation. Thermal destabilization of the crust between the major NW–SE-striking fault systems, however, was a second factor controlling the initiation and subsidence in the Permo–Mesozoic basins. In the Triassic, a change of the regional stress field caused the formation of large grabens (Central Graben, Horn Graben, Glückstadt Graben) perpendicular to the Tornquist Zone, the Ringkøbing-Fyn High and the Elbe Fault System. The resulting subsidence pattern can be explained by a superposition of declining thermal subsidence and regional extension. This led to a dissection of the Ringkøbing-Fyn High, resulting in offsets of the older NW–SE elements by the younger N–S elements. In the Late Cretaceous, the NW–SE elements were reactivated during compression, the direction of which was such that it did not favour inversion of N–S elements. A distinct change in subsidence controlling factors led to a shift of the main depocentre to the central North Sea in the Cenozoic. In this last phase, N–S-striking structures in the North Sea and NW–SE-striking structures in The Netherlands are reactivated as subsidence areas which are in line with the direction of present maximum compression. The Moho topography below the CEBS varies over a wide range. Below the N–S-trending Cenozoic depocentre in the North Sea, the crust is only 20 km thick compared to about 30 km below the largest part of the CEBS. The crust is up to 40 km thick below the Ringkøbing-Fyn High and up to 45 km along the Teisseyre–Tornquist Zone. Crustal thickness gradients are present across the Tornquist Zone and across the borders of the Ringkøbing-Fyn High but not across the Elbe Fault System. The N–S-striking structural elements are generally underlain by a thinner crust than the other parts of the CEBS. The main fault systems in the Permian to Cenozoic sediment fill of the CEBS are located above zones in the deeper crust across which a change in geophysical properties as P-wave velocities or gravimetric response is observed. This indicates that these structures served as templates in the crustal memory and that the prerift configuration of the continental crust is a major controlling factor for the subsequent basin evolution.

Neotectonics of The Netherlands: a review

Earthquakes and vertical land movements inferred from geodetic levelling results demonstrate that the Netherlands is situated on a tectonically active part of the Earth's surface. Tectonic subsidence analyses of the sedimentary records in the basins and the history of tectonic uplift inferred from Meuse river terraces show that the current tectonic activity is part of a deformation phase which began in the late Early Miocene, which we take as the start of the neotectonic period. The neotectonic faulting mode is normal-slip. This is in accordance with the present-day orientation of the maximum horizontal stress, and the vertical orientation of the maximum stress. However, the neotectonic fault zones are reactivated Variscan or older wrench faults. These faults have been reactivated repeatedly during the Mesozoic and Cenozoic in normal and wrenching modes, and therefore represent fundamental crustal weakness zones. As a result, the surficial neotectonic fault pattern resembles the inherited wrenching fault pattern, although the faulting mode is normal-slip. This is illustrated for the area where normal faults have displaced the Meuse fluvial terrace system in the southeastern part of the Netherlands. The neotectonic vertical motions in and around the Roer Valley Rift System have affected the courses of the Rhine and Meuse rivers from Pliocene times until the present. In addition, a fluvial terrace staircase developed along the Meuse river in response to the uplift of the Ardennes-Rhenish Massif and its foreland, for example near Maastricht. Furthermore, small fault scarps are present along segments of the bounding fault zones of the Roer Valley Graben: the Peel Boundary Fault Zone and the Feldbiss Fault Zone. In other parts of the Netherlands the neotectonic vertical motions have no direct expression in the morphology. Examples of possible indirect control on the morphology in these areas occur in glacial landscapes, the Zuiderzee Basin and the coastal zone.

Seismic hazard impact of the Lower Tagus Valley Fault Zone (SW Iberia)

The seismic hazard of SW Iberia is composed of two contributions: offshore, large to very large events on the plate boundary between Africa and Eurasia such as the Lisbon earthquake of 1755 or the Gorringe Bank earthquake of 1969; and onshore, moderate to strong intraplate earthquakes on inherited crustal fractures. One of these zones of crustal weakness is the Lower Tagus Valley (LTV) fault zone, which displays the highest level of seismic hazard in Western Iberia. In this paper we review the active tectonics and seismicity of the LTV, integrating previous geophysical data with recent results of paleoseismological investigations, and discuss its impact on the seismic hazard of SW Iberia. We conclude that the seismic zonation for hazard assessment currently in force in the building code is biased towards the scenario of distant offshore rupture, and does not take adequately into account the LTV seismic source.

Triassic and Jurassic transtension along part of the Sorgenfrei–Tornquist Zone in the Danish Kattegat

In the Kattegat area, Denmark, the Sorgenfrei–Tornquist Zone, an old crustal weakness zone, was repeatedly reactivated during Triassic, Jurassic and Early Cretaceous times with dextral transtensional movements along the major boundary faults. These tectonic events were minor compared to the tectonic events of the Late Carboniferous – Early Permian and the Late Cretaceous – Early Tertiary, although a dynamic structural and stratigraphic analysis indicates that the Sorgenfrei–Tornquist Zone was active compared to the surrounding areas. At the end of the Palaeozoic, the area was a peneplain. Regional Triassic subsidence caused onlap towards the north-east, where the youngest Triassic sediments overlie Precambrian crystalline basement. During the Early Triassic, several of the major Early Permian faults were reactivated, probably with dextral strike-slip along the Børglum Fault. Jurassic – Early Cretaceous subsidence was restricted primarily to the area between the two main faults in the Sorgenfrei–Tornquist Zone, the Grenå–Helsingborg Fault and the Børglum Fault. This restriction of basin development indicates a change in the regional stress field at the Triassic–Jurassic transition. Middle Jurassic and Late Jurassic – Early Cretaceous subsidence followed the Early Jurassic pattern with local subsidence in the Sorgenfrei–Tornquist Zone, but now even more restricted to within the zone. The subsidence showed a decrease in the Middle Jurassic, and increased again during Late Jurassic – Early Cretaceous times. Small faults were generated internally in the Sorgenfrei–Tornquist Zone during the Mesozoic with a pattern that indicates a broad transfer of strike-slip/oblique-slip motion from the Grenå–Helsingborg Fault to the Børglum Fault.

Inversion-related features along the southeastern margin of the North German Basin (Elbe Fault System)

A seismostratigraphic approach constrained by well data is used for the interpretation of the deformation style along the central Elbe Fault System (EFS) within the sedimentary succession. Structural analysis allows to qualify, to quantify, and to date tectonic events. The stratigraphic interpretation is complicated by the mobilized Upper Permian Zechstein salt and by erosional events. A first-order quantification of the inversion-related uplift is estimated from vertical fault offsets that reach up to 4 km. The main uplift occurred during the Maastrichtian/Paleocene. Amounts of erosion inferred from comparing the strata thickness on top of the Flechtingen High with the surrounding basinal areas range from 3 to 4 km. The data indicate a changing deformation style: Thick-skinned deformation with southwest-dipping thrusts that vertically offset the pre-Permian basement is observed along the Flechtingen High in the central part of the EFS. Thin-skinned deformation occurs in the North German Basin where salt detaches the post-Permian cover from the barely faulted basement. It is concluded that during the Late Cretaceous/Early Tertiary inversion, the EFS responded to regional compression with uplift and formation of an internal high, the Flechtingen High. A stress-sensitive crustal weak zone beneath the EFS could be the reason for the repeated strain localization in the area.

Eurekan transpressive deformation in the Wandel Hav Mobile Belt (northeast Greenland)

Besides the ∼W‐E trending Kap Cannon Thrust Zone and Harder Fjord Fault Zone, the 300 km long and NW‐SE striking Wandel Hav Mobile Belt represents the third major fault zone in north Greenland. Structural analyses in several areas of this belt suggest that compressive deformation is characterized by transpressive dextral strike‐slip kinematics. This is demonstrated by a combination of folding around ∼W‐E axes, approximately north and south directed reverse faulting, and dextral strike‐slip displacements along NW‐SE trending fault lines. The initial formation of the long, linear faults is interpreted to be related to deformational events in Late Paleozoic, Triassic‐Jurassic, and Late Cretaceous times. Due to the lack of structural evidence, we assume that deformations led to the generation of extension faults which probably were reactivated during dextral strike‐slip tectonism. It is suggested that dextral transpressive deformation was coeval with ∼N‐S compression at the Harder Fjord Fault Zone and Cap Cannon Thrust Zone and took place during Eocene (Eurekan) times. Dextral strike‐slip tectonism in the Wandel Hav Mobile Belt was the result of ∼N‐S compression due to a general northward movement of the Greenland plate. As a zone of crustal weakness, the belt can be interpreted as the onshore equivalent of the main transcurrent fault zone (De Geer Fault) which caused the intracontinental dextral slip of Svalbard (Barents Shelf) relative to north Greenland during Early Tertiary (Eurekan) times and prior to their separation. In this configuration, it represents one part of the belt of Eurekan deformation which extends from the Canadian Arctic islands over north Greenland to the Barents Shelf where compressive deformation is recorded in the West Spitsbergen Fold‐and‐Thrust Belt.

A magnetotelluric study of the Damara Belt in Namibia: 1. Regional scale conductivity anomalies

The Namibian margin is dominated by the late Proterozoic to early Cambrian fold belts of the Damara Orogen, which wrap around and separate the Congo and Kalahari Cratons. This mosaic of relatively ‘soft’ fold belts and ‘hard’ cratons apparently controlled the path for the opening of the South Atlantic in the early Cretaceous. The continents split along the coast-parallel fold belts of the Damara Orogen while the inland fold belt (Damara Belt) was effected by extension and widespread igneous intrusion but never developed to the rift stage. This paper is concerned with the interpretation of magnetotelluric (MT) data along a 200 km NW–SE profile across the Damara Belt in NW Namibia. The regional, two-dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones: (i) a generally very resistive upper crust which is typical for the granites and metasediments of the Damara Belt, (ii) two subvertical conductors in upper to mid-crustal levels which correlate with major tectonic zone boundaries and (iii) a highly conductive middle to lower crust in the southern part of the profile. The geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust. We interpret the high electrical conductivity in terms of graphite (or other forms of mineralization) enrichment along the shear planes. This zone of crustal weakness may have originated in Pre-Damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the Cretaceous rifting and magmatism associated with the opening of the South Atlantic.

Late Cretaceous reactivation of the Norumbega fault zone, Maine: Evidence from apatite fission-track ages

Apatite fission-track ages from opposite sides of the Norumbega fault zone in southern Maine reveal a significant Early Cretaceous time-temperature discontinuity across the structure. Ages west of the fault zone range from 113 to 89 Ma, while those to the east are significantly older, ranging from 159 to 140 Ma. Modeling of track length distributions reveals that at 120 Ma, an ∼55 °C difference in temperature existed between opposite sides of the fault zone currently exposed at the surface. This discontinuity is best explained by an episode of significant (to 2 km of net vertical separation) east-side-down vertical movement along the Norumbega fault zone in Late Cretaceous time. The modeling suggests contemporaneous cooling and that final movement along the fault zone occurred ca. 80 Ma. These results, when combined with earlier structural and thermochronological studies, suggest that faults mapped within the Norumbega fault zone were intermittently active from Middle Devonian to Late Cretaceous time, and that the structure is a fundamental zone of crustal weakness. The prolonged period of activity likely reflects stresses associated with late Paleozoic dextral transgression, early Mesozoic rifting associated with the initial opening of the North Atlantic Ocean basin, and processes associated with the late Mesozoic uplift and exhumation of the northern Appalachian orogen.

The Elbe Fault System in North Central Europe—a basement controlled zone of crustal weakness

The Elbe Fault System (EFS) is a WNW-striking zone extending from the southeastern North Sea to southwestern Poland along the present southern margin of the North German Basin and the northern margin of the Sudetes Mountains. Although details are still under debate, geological and geophysical data reveal that upper crustal deformation along the Elbe Fault System has taken place repeatedly since Late Carboniferous times with changing kinematic activity in response to variation in the stress regime. In Late Carboniferous to early Permian times, the Elbe Fault System was part of a post-Variscan wrench fault system and acted as the southern boundary fault during the formation of the Permian Basins along the Trans-European Suture Zone (sensu [Geol. Mag. 134 (5) (1997) 585]). The Teisseyre–Tornquist Zone (TTZ) most probably provided the northern counterpart in a pull-apart scenario at that time. Further strain localisation took place during late Mesozoic transtension, when local shear within the Elbe Fault System caused subsidence and basin formation along and parallel to the fault system. The most intense deformation took place along the system during late Cretaceous–early Cenozoic time, when the Elbe Fault System responded to regional compression with up to 4 km of uplift and formation of internal flexural highs. Compressional deformation continued during early Cenozoic time and actually may be ongoing. The upper crust of the Elbe Fault System, which itself reacted in a more or less ductile fashion, is underlain by a lower crust characterised by low P-wave velocities, low densities and a weak rheology. Structural, seismic and gravimetric data as well as rheology models support the assumption that a weak, stress-sensitive zone in the lower crust is the reason for the high mobility of the area and repeated strain localisation along the Elbe Fault System.

Tectonics and paleogeography of the Marajó Basin, northern Brazil

The Marajó Basin area presents geologic and geomorphologic features chiefly due to the Mesozoic extension and post-Miocene neotectonics. The extension event with an Early and a Late Cretaceous phases originated four sub-basins that constitutes the Marajó Basin, with a thick continental clastic sequence showing marine influence. NW and NNW normal faults and NE and ENE strike-slip faults controlled the basin geometry. The extension, related to the Equatorial Atlantic opening, propagated into the continent along crustal weakness zones of the Precambrian Tumucumaque, Amapá and Araguaia orogenic belts. The neotectonic event is a strike-slip regime which developed transtensional basins filled in by Upper Tertiary shallow marine (Pirabas Formation) and transitional sequences (Barreiras Group), followed by Quaternary fluvial deposits and transitional sequences derived from the Amazon and Tocantins rivers and the Marajoara estuary. The current landscape has a typical estuarine morphology. The coast morphology presents sea-cliffs on transitional Upper Tertiary sequences, while inwards dominate hills sustained by Mid-Pleistocene lateritic crust, with a flat erosive surface at 70 m. In the eastern Marajó Island several generations of paleochannels associated with fluvial-estuarine sequences are recognized, while a fluvial-marine plain is widespread on its western side.