Tilted Blocks Research Articles

Kinematics and geometry of normal faults at the margins of pull-apart basins: An example from the Dead Sea Basin

The parameters controlling the geometrical and kinematical evolution of a normal fault system along the margins of a subsiding pull-apart basin are studied using the case of the Dead Sea Basin. The Western margins of the basin consist of sub-parallel normal faults and basinward tilted blocks that evolved in a layered sedimentary sequence. The faults exhibit narrow domains of fault damage, characterized by mechanical degradation of the surrounding rock materials. The slip surfaces display oblique-normal abrasive striations and waviness. Measurements of normal-to-slip fault surface roughness at more than eight orders of length-scale magnitudes, combined with directly obtained fault dips and slip orientations, suggest that at length-scales higher than ~102 m the dominant geometry of the faults change from generally wavy with multiscale undulations to a characteristic zigzag pattern with two distinct sets. We utilize the numerical, discrete elements, DDA method to explain the structural evolution of the margins. Our results suggest that the sedimentary cover deforms as a coherent system of uniformly sized blocks, exhibiting flexure in the western margins and gravitational collapse eastward into the subsiding basin, predominantly in tilted blocks mode. These conditions explain many of the observed structures in the field, including the extensional belt and the stepped basinward block-tilting of the sedimentary cover. Comparing the wide deformation belt on the western margins of the basin to the much narrower deformation zone on the eastern margins suggests that the existence of a thick sedimentary cover and its initial structure control the width of the margins and the general mode of structural deformation.

3D Model and Structural-Kinematic Evolution of the Pre-Jurassic Crystalline Basement of the Western Georgia

The modern structure of Western Georgia is determined mainly by the meridional (sub-meridional) and latitudinal systems of faults covering different depths of the Earth's crust. The noted faults are often-sided boundaries of the blocks of the crystalline basement of the Earth's crust, creating a picture of its mosaic-block structure. The analysis of the lithofacies and thicknesses of the sedimentary cover developed within their limits, in several cases, indicating their autonomous and inversion nature of development. The comparison of geophysical and drilling data and applying the system analysis method of disjunctive structures made it possible to clarify some issues of the structural-kinematic evolution and morphogenetic of individual blocks and faults of the pre-Jurassic crystalline basement within the limits of the Southern Caucasus. A 3D physical model of the surface of the crystalline basement constructed by us within Western Georgia shows the spatial arrangement and the character of the inversion nature of individual blocks, indicating the manifestations of the Alpine and Late Alpine orogeneses. Analysis of the actual material, geophysical, and geological data for the intra-Caucasian intermountain area allows us to draw the following conclusions: the Georgian Block (a fragment of the Transcaucasian median massif, microplates, and terranes), with a pre-Jurassic crystalline basement exposed in its central part, is divided into the western and eastern subsidence zones, which in turn disintegrate into separate blocks. From the central zone of the uplift of the Georgian Block to the east and west, a gradual "stepwise" subsidence and tilting of the blocks of the crystalline basement is outlined. Similar structures are known in the literature as the so-called tilt blocks.

Morphometric analysis of Chapecó river basin: Searching for vestigial trace of neotectonic on a basaltic landscape at southern Brazil

The Chapecó River Basin (CRB) is located in the west of Santa Catarina State (South Brazil) and has a relief modelled by a large basalt lava outpouring that occurred in the Paraná Sedimentary Basin. The CRB is characterised by two plateau compartments resulting from regional denudation and associated with a river network that was formed after the basaltic lava completed its flow phase in the Lower Cretaceous. Elements of structural geology together with weathering of rocks and its products, such as regolith, have been commonly used to analyse and interpret the landforms in CRB, whose fluvial landscape is shaped by bedrock channels and incision valleys. Nevertheless, there is a lack of detailed studies about such structural factors in terms of type and origin, and to what extent they may be proxies of tectonic tensions of an intra-plate character. In the lack of guide levels and deposits that allow a better interpretation of recent tectonic deformations, in this work we aimed to evaluate tectonic-structural factors involved in the landscape evolution of the CRB through measurement and analysis of morphometric indices derived from the ALOS PALSAR DEM. We analysed the following morphometric parameters for 43 sub-basins: the basin asymmetry (FA), hypsometric integral (HI), elongation, local relief, slopes distribution, Hack index for the two main river channels (Chapecó and Chapecozinho), and concavity index of the main channels of sub-basins. The results obtained from the morphometric analysis of the river network indicate the existence of a geomorphological disequilibrium of its relief. This result was observed in the transition from the medium to the low basin and in the base levels of the Chapecó and Chapecozinho rivers. Such disequilibrium is shown by convex and irregular longitudinal profiles, knickpoints, sub-basins with high values of asymmetry and hypsometric integral (HI), as well as a low correlation between slope and channel gradients for the sub-basins analysed. This context is consistent with the neotectonic hypothesis of a regional uplift that provide energy to the system to reactivate faults, tilt blocks, and deepened the incision of the river network, leading to the creation of new knickpoints and a transient river network. From the results and analysis, we can conclude that the largest knickzones and waterfalls in the study area can be considered markers associated with quaternary tectonic forcings, although many of these geomorphological features may have originated simply by joints and fractures in the rocks.

Rifted margin with localized detachment and polyphase magmatism: A new model of the northern South China Sea

Continental passive margins are commonly classified into volcanic (or magma-rich) and non-volcanic (or magma-poor) types based on the magmatic budget. It is difficult, however, to use either of these models to define the northern margin of the South China Sea (SCS). Based on 2-D/3-D time/depth-migrated seismic data and borehole data from the Pearl River Mouth Basin in the northern SCS, we examine the detachment fault systems and syn-rift magmatism in the Baiyun and Liwan Sags. The detachment fault systems and related tilted blocks of the northern SCS established in the Baiyun and Liwan Sags in previous studies may be mainly constrained to the Baiyun Sag. The Liwan Sag, however, acts as a “sag-type” basin accompanied by magmatic domes. Radiometric age of igneous samples from wells, coupled with multi-horizon onlapping terminations within syn-rift layers, reveal four-phase rift-related magmatism in the northern SCS. The crustal thinning in the Baiyun Sag is controlled by a detachment fault, but in the Liwan Sag, it is mainly controlled by a preferential thinning of the lower crust. High thermal conditions and intense syn-rift magmatism weakened the lower crust, which contributes to the depth-dependent thinning in the Liwan Sag.

Evaluation of DEM based Neotectonics of Sulaiman range and lobe region (SRLR), Balochistan-Pakistan

The Indian plate’s western boundary is extremely tilted towards the direction of convergence between Asia and India signifying an exceptional example of massive scale oblique continent-continent collision. This transform boundary motion is sinistral in nature and Digital Elevation Model (DEM) based geomorphometry permits the investigation of 3-D characteristics of the regional topography. This technique is specifically useful along inter-plate transform margins where the visual interpretation and topographic assessment is unable to observe the trends of the landforms evolution. In this context, the geomorphometry of the regional surface might be influenced largely by climate change and lower rate of active deformation. The geomorphic indices; Isobase Levels (IBLs), Topographic Asymmetry factor (T-factor) and Hack Gradient Index (HGI) have been executed to analyze fluvial drainage networks in connection to neotectonic movements. The sinistral movement along the Chaman Fault system and relatively less deformed Katawaz Block are the major surface dynamics to compute the style of deformation in the Sulaiman Range and Lobe Region (SRLR). Within the SRLR, the Sulaiman Fold and Thrust Belt (SFTB) is thrusting with a rate of 4–15 mm/yr in a North-South (NS) trend between IndoPak-Eurasian and a shortening of 4–6 mm/yr in East-West (EW) context. Seismicity is typically towards NorthWest-SouthEast (NW-SE) accommodating the moment release in the current century between the IndoPak-Eurasian transform plate boundary in the western most of the western Himalayan Syntaxis. The results obtained from the applied geomorphic indices indicate significant uplift in terms of higher IBL and HGI values all along the SRLR (Harnai, Katwaz, Qilla Saifullah, Zhob, Loralai, Jissa Shareef, Sakhi Sarwar and Fort Munro). The T-factor results also display tilt block neotectonics that are in agreement with the results of IBL and HGI.

BIOEROSION OF THE PLIO-PLEISTOCENE TRANSGRESSION OF SOUTHERN ITALY

During the Pliocene and Lower Pleistocene, the structural unit of the Murge was largely submerged by a marine transgression. As the sea advanced, the limestone basement surface was invaded by bioeroding organisms whose borings are well preserved through burial by the calcareous sediments of the Calcarenite di Gravina formation. The tilted—block topography of the submerging Murge unit produced different coastal environments ranging from protected gentle slopes to exposed vertical clifflines. Details of the bioerosion were examined at four localities that cover a range of those environments. The 35 ichnotaxa recognized (excluding microborings) are treated taxonomically where necessary, and their relative distributions and occurrences are studied. Three new ichnospecies are erected: Gastrochaenolites cor, Maeandropolydora barocca and M. crassa. Four recurring assemblages are recognized: Caulostrepsis/ Maeandropolydora assemblage, G. cor assemblage, G. torpedo/ Entobian assemblage and a High Diversity Entobian—Dominated assemblage. All four indicate shallow marine environments, their relative distributions being influenced by environmental factors such as the attitude of the substrate surface and the hydrodynamic energy level.

Structure of massively dilatant faults in Iceland: lessons learned from high-resolution unmanned aerial vehicle data

Abstract. Normal faults in basalts develop massive dilatancy in the upper few hundred meters below the Earth's surface with corresponding interactions with groundwater and lava flow. These massively dilatant faults (MDFs) are widespread in Iceland and the East African Rift, but the details of their geometry are not well documented, despite their importance for fluid flow in the subsurface, geohazard assessment and geothermal energy. We present a large set of digital elevation models (DEMs) of the surface geometries of MDFs with 5–15 cm resolution, acquired along the Icelandic rift zone using unmanned aerial vehicles (UAVs). Our data present a representative set of outcrops of MDFs in Iceland, formed in basaltic sequences linked to the mid-ocean ridge. UAVs provide a much higher resolution than aerial/satellite imagery and a much better overview than ground-based fieldwork, bridging the gap between outcrop-scale observations and remote sensing. We acquired photosets of overlapping images along about 20 km of MDFs and processed these using photogrammetry to create high-resolution DEMs and orthorectified images. We use this dataset to map the faults and their damage zones to measure length, opening width and vertical offset of the faults and identify surface tilt in the damage zones. Ground truthing of the data was done by field observations. Mapped vertical offsets show typical trends of normal fault growth by segment coalescence. However, opening widths in map view show variations at much higher frequency, caused by segmentation, collapsed relays and tilted blocks. These effects commonly cause a higher-than-expected ratio of vertical offset and opening width for a steep normal fault at depth. Based on field observations and the relationships of opening width and vertical offset, we define three endmember morphologies of MDFs: (i) dilatant faults with opening width and vertical offset, (ii) tilted blocks (TBs) and (iii) opening-mode (mode I) fissures. Field observation of normal faults without visible opening invariably shows that these have an opening filled with recent sediment. TB-dominated normal faults tend to have the largest ratio of opening width and vertical offset. Fissures have opening widths up to 15 m with throw below a 2 m threshold. Plotting opening width versus vertical offset shows that there is a continuous transition between the endmembers. We conclude that for these endmembers, the ratio between opening width and vertical offset R can be reliably used to predict fault structures at depth. However, fractures associated with MDFs belong to one larger continuum and, consequently, where different endmembers coexist, a clear identification of structures solely via the determination of R is impossible.

Large near-surface block rotations at normal faults of the Iceland rift: Evolution of tectonic caves and dilatancy

Abstract Surface ramps in normal fault zones of the Iceland plate boundary have been described in many studies, but their structure and evolution are not well understood. We show that surface ramps are manifestations of large tilted blocks (TBs) formed in dip relays of normal faults. Based on existing modeling studies, we propose three classes of TBs defined by kinematics and location of the hinge of the TB. TBs are considered a member of the family of fault relay structures that form near the surface, commonly, but not exclusively, in columnar basalts with orthotropic strength. We present high-resolution aerial vehicle–based observations of a representative set of normal faults in Iceland and compare these with geometric models we derived from modeling studies. We predict extensive tectonic cave (fluid conduit) systems under the TB, which interact with magma and groundwater flow. The general fault structure is dominated by large, subvertical open fractures reactivating cooling joints that are locally filled by basalt rubble. We propose the existence of a hybrid failure zone at larger depths before the effective vertical stress is sufficient to initiate shear fractures in intact basalt.

Depositional architecture and structural evolution of a region immediately inboard of the locus of continental breakup (Liwan Sub-basin, South China Sea)

AbstractNew 3-D seismic data and regional 2-D seismic profiles from the northern South China Sea, the most extensive data set imaging a distal rifted margin in the world, are used to characterize a region located immediately inboard of the locus of Cenozoic continental breakup. The interpreted data set images a ∼6-km-thick continental crust in which the Moho and the base of syn-rift sediment are observed as clear, well-resolved seismic reflections. This extremely thinned continental crust was offset at its base by a complex detachment fault system from which oceanward-dipping listric faults propagated vertically to bound six separate tilted blocks, in a style akin to tectonic rafts. The seismic reflection data allowed us to investigate the thickness of syn- and post-rift strata above tilt blocks, revealing that the early-middle Eocene syn-rift topography was gradually blanketed in the late Eocene (ca. 38 Ma). After 33 Ma (earliest Oligocene), the main depocenter on the margin migrated to the south of the Liwan Sub-basin, i.e., oceanwards, as recorded by the thickening of strata within a breakup sequence. This work is important as it demonstrates how closely structures and sedimentation within the Liwan Sub-basin were controlled by a basal, rift-related detachment system, which is imaged in detail by 3-D seismic data for the first time on a rifted continental margin. Continental breakup was marked by a shift in the locus of subsidence (and crustal stretching) toward ocean crust, within a time period spanning ∼16 m.y. We extrapolate our findings from the South China Sea to the development of asymmetric passive margins across the world.

Breaking up is hard to do: Global cartography and topography of Pluto's mid-sized icy Moon Charon from New Horizons

The 2015 New Horizons flyby through the Pluto system produced the first high-resolution topographic maps of Pluto and Charon, the most distant objects so mapped. Global integrated mosaics of the illuminated surface of Pluto's large icy moon Charon have been produced using both framing camera and line scan camera data (including four-color images at up to 1.47 km pixel scales), showing the best resolution data at all areas of the surface. Digital elevation models (DEMs) with vertical precisions of up to ∼0.1 km were constructed for ∼40% of Charon using stereo imagery. Local radii estimates for the surface were also determined from the cartographic control network solution for the LORRI framing camera data, which validate the stereo solutions. Charon is moderately cratered, the largest of which is ∼250-km across and ∼6 km deep. Charon has a topographic range over the observed hemisphere from lowest to highest of ∼19 km, the largest topographic amplitude of any mid-sized icy body (including Ceres) other than Iapetus. Unlike Saturn's icy moons whose topographic signature is dominated by global relaxation of topography and subsequent impact cratering, large-scale tectonics and regional resurfacing dominate Charon's topography. Most of Charon's encounter hemisphere north of the equator (Oz Terra) is broken into large polygonal blocks by a network of wide troughs with typically 3–6 km relief; the deepest of these occur near the illuminated pole and are up to 13 km deep with respect to the global mean radius, the deepest known surfaces on Charon. The edge of this terrain is defined by large tilted blocks sloping ∼5° or so, the crests of which rise to 5 or 6 km above Charon mean, the highest known points on Charon. The southern resurfaced plains, Vulcan Planitia, consist of rolling plains, locally fractured and pitted, that are depressed ∼1 km below the mean elevation of the disrupted northern terrains of Oz Terra that comprise much of the northern hemisphere (but ∼2–2.5 km below the surfaces of the blocks themselves). These plains roll downward gently to the south with a topographic range of ∼5 km. The outer margins of Vulcan Planitia along the boundary with Oz Terra form a 2-3-km-deep trough, suggesting viscous flow along the outer margins. Isolated massifs 2–4 km high, also flanked by annular moats, lie within the planitia itself. The plains may be formed from volcanic resurfacing of cryogenic fluids, but the tilted blocks along the outer margins and the isolated and tilted massifs within Vulcan Planitia also suggest that much of Charon has been broken into large blocks, some of which have been rotated and some of which have foundered into Charon's upper “mantle”, now exposed as Vulcan Planitia, a history that may be most similar to the disrupted terrains of Ariel.

Tectono‐sedimentary evolution of the Plio‐Pleistocene Corinth rift, Greece

AbstractThe onshore central Corinth rift contains a syn‐rift succession >3 km thick deposited in 5–15 km‐wide tilt blocks, all now inactive, uplifted and deeply incised. This part of the rift records upward deepening from fluviatile to lake‐margin conditions and finally to sub‐lacustrine turbidite channel and lobe complexes, and deep‐water lacustrine conditions (Lake Corinth) were established over most of the rift by 3.6 Ma. This succession represents the first of two phases of rift development – Rift 1 from 5.0–3.6 to 2.2–1.8 Ma and Rift 2 from 2.2–1.8 Ma to present. Rift 1 developed as a 30 km‐wide zone of distributed normal faulting. The lake was fed by four major N‐ to NE‐flowing antecedent drainages along the southern rift flank. These sourced an axial fluvial system, Gilbert fan deltas and deep lacustrine turbidite channel and lobe complexes. The onset of Rift 2 and abandonment of Rift 1 involved a 30 km northward shift in the locus of rifting. In the west, giant Gilbert deltas built into a deepening lake depocentre in the hanging wall of the newly developing southern border fault system. Footwall and regional uplift progressively destroyed Lake Corinth in the central and eastern parts of the rift, producing a staircase of deltaic and, following drainage reversal, shallow marine terraces descending from >1000 m to present‐day sea level. The growth, linkage and death of normal faults during the two phases of rifting are interpreted to reflect self‐organization and strain localization along co‐linear border faults. In the west, interaction with the Patras rift occurred along the major Patras dextral strike‐slip fault. This led to enhanced migration of fault activity, uplift and incision of some early Rift 2 fan deltas, and opening of the Rion Straits at ca. 400–600 ka. The landscape and stratigraphic evolution of the rift was strongly influenced by regional palaeotopographic variations and local antecedent drainage, both inherited from the Hellenide fold and thrust belt.

Late Paleogene rifting along the Malay Peninsula thickened crust

Sedimentary basins often develop above internal zones of former orogenic belts. We hereafter consider the Malay Peninsula (Western Sunda) as a crustal high separating two regions of stretched continental crust; the Andaman/Malacca basins in the western side and the Thai/Malay basins in the east. Several stages of rifting have been documented thanks to extensive geophysical exploration. However, little is known on the correlation between offshore rifted basins and the onshore continental core. In this paper, we explore through mapping and seismic data, how these structures reactivate pre-existing Mesozoic basement heterogeneities.The continental core appears to be relatively undeformed after the Triassic Indosinian orogeny. The thick crustal mega-horst is bounded by complex shear zones (Ranong, Klong Marui and Main Range Batholith Fault Zones) initiated during the Late Cretaceous/Early Paleogene during a thick-skin transpressional deformation and later reactivated in the Late Paleogene. The extension is localized on the sides of this crustal backbone along a strip where earlier Late Cretaceous deformation is well expressed. To the west, the continental shelf is underlain by three major crustal steps which correspond to wide crustal-scale tilted blocks bounded by deep rooted counter regional normal faults (Mergui Basin). To the east, some pronounced rift systems are also present, with large tilted blocks (Western Thai, Songkhla and Chumphon basins) which may reflect large crustal boudins. In the central domain, the extension is limited to isolated narrow N-S half grabens developed on a thick continental crust, controlled by shallow rooted normal faults, which develop often at the contact between granitoids and the host-rocks. The outer limits of the areas affected by the crustal boudinage mark the boundary towards the large and deeper Andaman basin in the west and the Malay and Pattani basins in the east.At a regional scale, the rifted basins resemble N-S en-echelon structures along large NW-SE shear bands. The rifting is accommodated by large low angle normal faults (LANF) running along crustal morphostructures such as broad folds and Mesozoic batholiths. The deep Andaman, Malay and Pattani basins seem to sit on weaker crust inherited from Gondwana-derived continental blocks (Burma, Sibumasu, and Indochina). The set of narrow elongated basins in the core of the Region (Khien Sa, Krabi, and Malacca basins) suffered from a relatively lesser extension.

Crustal-scale block tilting during Andean trench-parallel extension: Structural and geo-thermochronological insights

Despite a long history of plate convergence at the western margin of the South American plate that has been ongoing since at least the Early Paleozoic, the southern Peruvian fore-arc displays little to no evidence of shortening. In the light of this observation, we assess the deformation history of the southern Peruvian fore-arc and its geodynamic implications. To accomplish this, we present a new structural and geo-thermochronological data set (zircon U-Pb, mica 40Ar/39Ar, apatite and zircon fission-track and zircon (U-Th)/He analyses) for samples collected along a 400 km long transect parallel to the trench. Our results show that the Mesoproterozoic gneissic basement was mainly at temperatures ≤350°C since the Neoproterozoic and was later intruded by Jurassic volcanic arc plutons. Along the coast, a peculiar apatite fission-track age pattern, coupled with field observations and a synthesis of available geological maps, allows us to identify crustal-scale tilted blocks that span the coastal Peruvian fore-arc. These blocks, bounded by normal faults that are orthogonal to the trench, suggest post-60 Ma trench-parallel extension that potentially accommodated oroclinal bending in this region. Block tilting is consistent with the observed and previously described switch in the location of sedimentary sources in the fore-arc basin. Our data set allows us to estimate the cumulative slip on these faults to be less than 2 km and questions the large amount of trench-parallel extension suggested to have accommodated this bending.

Chapala half-graben structure inferred. A magnetometric study

Many geologic and tectonic-structural studies refer to the Chapala lake structure in general terms as a graben (or more generally as rift). However, no formal study has addressed its structure. The Chapala Lake is located in a major topographic depression surrounded by broad middle to late Miocene volcanic-capped plateaus. We conducted a magnetic survey comprising five S-N lines crossing the Chapala Lake. One profile comprises a land extension (southwards along roads). Magnetic anomalies obtained are mainly smooth and large scale features associated to the sedimentary infill, local and high amplitude anomalies are due to shallow basaltic and andesitic tilted blocks, and highest amplitudes due to the outcropped basaltic flows of the Los Alacranes and Mezcala islands. The models obtained point to a large structural high located at the central depression portion that separates two structural lows. The structural lows correspond to half-graben type structures. Through the faults delimiting it were emplaced the basaltic flows of the Mezcala and Los Alacranes islands. The presence of half-graben type structures indicates that the crust has been subject to an extensional tectonic.

Traveltime tomography of a dense wide-angle profile across Orphan Basin

We used first arrivals and Moho reflections from the 500-km-long Orphan Basin Wide-Angle Velocity Experiment (OBWAVE) profile with 3–5-km instrument spacing to construct a traveltime tomography section and to delineate the Moho discontinuity across the Orphan Basin. The Orphan Basin is a failed rift located offshore Newfoundland, Canada, showing thinned continental crust over an unusually wide region. We observed (1) a zone of extreme crustal thinning ([Formula: see text]-thick crust) with no evidence for mantle serpentinization, (2) basement morphology exhibiting tilted blocks linked to the crustal thinning, and (3) a thicker central crustal segment that is probably related to prerift structural inheritance. Comparison with the adjacent Jeanne d’Arc Basin to the southeast suggested the presence of a decoupling zone between the two basins accommodating the difference in extension rates. There was a good correlation between the tomographic velocities and the reflection structure derived from a coincident seismic reflection profile except in an area in which the reflection seismic data suggested the presence of a deep sedimentary basin. The velocity model computed in this work indicated that this area consists of prerift basement rather than Jurassic or older sediments. Tomographic models computed by varying the density of the recording instrument array gave insight into the relationship among the target size, the instrument spacing needed to resolve it, and the velocity model uncertainty. These results may help guide the design of future wide-angle reflection and refraction surveys across rifted structures.

Skewed orientation groups in scatter plots of earthquake fault plane solutions: Implications for extensional geometry at oceanic spreading centers

AbstractSystematic analysis of earthquake focal solutions derived from centroid moment tensors shows well‐defined orientation groups in scatterplots of fault plane normals and associated slip line vectors. Consideration of the geometry implied by these orientation groups can allow resolution of the ambiguity inherent in the choice as to which of the two conjugate fault plane solutions should apply, and in many cases, the same classification can be applied to the entire orientation group. Examining scatter plots of data from normal fault earthquakes on spreading ridges typically shows orthogonal relations but there are also many cases where there is a skew with respect to the great circles defined by faults on adjacent transform faults. This can be explained by finite rock strength in the adjacent transforms, requiring resolved shear stress to allow movement, thus requiring rotation of the trajectories of the deviatoric stress axes: anticlockwise for right‐lateral transforms and clockwise for left‐lateral transforms. This asymmetry also requires formation of tilt block geometries reminiscent of Basin and Range style continental extension.

First microbialites associated to organic-rich facies of the Oceanic Anoxic Event 2 (Northern Tunisia, Cenomanian–Turonian transition)

From northern Tunisia, small-scale well-preserved microbialites, contemporaneous to the global oceanic anoxic event 2 (OAE 2) are first reported on the southern Tethyan Margin. These microbialites are encased within the pelagic organic-rich black shales of the Bahloul Formation (Cenomanian–Turonian transition). Biostratigraphic, petrographic, and geochemical investigations carried out to constrain their biogenicity and genesis character led to consider them as thrombolites and stromatolites occurring in lenticular bioherms/biostromes and columnar bodies co-relatable to the global ‘filament event’ of the authors, close to the base of the Watinoceras ammonite zone. Abundant clotted micrite, cyanobacterial filaments, and algal tissues point to the key process of microbial carbonate precipitation and to a major role played by microbes in the stabilisation and subsequent lithification, which in turn favoured the preservation of the original structure of the microbialites. These microbially induced carbonate formations are considered as favoured by chemosynthetic fauna of bivalve molluscs and lithistid sponges which were able to host symbiotic microbial communities. The latter contributed to the precipitation of authigenic calcite and non-carbonate minerals (e.g. pyrite) fuelled by microbial activity under sulphate-reducing conditions. The carbonate body onset is considered to be initiated by seafloor instability due to syndepositional fault acting that induced the appraisal of uplifted tilted blocks within oxygenated waters but near the anoxic water masses. Generated depressions allowed the development of chemosynthetic-based communities. Deep faults related to Triassic salt domes acted as conducts for hydrocarbon and salt brine expulsion to the seafloor and the microbialite growth was enhanced by an abrupt uprising sea level under warmer conditions.

Genesis and geometry of tilted blocks in the Theban Hills, near Luxor (Upper Egypt)

The desertic Theban hills between the edge of the alluvial plain of the Nile River and the prominent cliffs at the eastern edge of the Theban Plateau consist of imbricated tilted blocks organized in parallel groups representing successive generations of gravitational collapse structures (or slumps). The older (distal) generations correspond to low, rounded hills farther from the Theban cliffs. The youngest (proximal) generation forms higher hills with young relief. The tilted blocks rest along listric faults on the substratum (Tarawan Chalk and Esna Shale Formations) and abut the Theban cliffs. Faults and warping occur at the contacts between proximal and distal tilted blocks. We hypothesize that the emplacements of the tilted blocks were related to major Pleistocene pluvial episodes, each marked by active flow of the Nile River and significant recess of the Theban cliffs. Tectonic thinning and intensive erosion of the Esna Shale Formation were determinant in shaping the Theban landscape.

The crustal structure of the north-eastern Gulf of Aden continental margin: insights from wide-angle seismic data

The wide-angle seismic (WAS) and gravity data of the Encens survey allow us to determine the deep crustal structure of the north-eastern Gulf of Aden non-volcanic passive margin. The Gulf of Aden is a young oceanic basin that began to open at least 17.6 Ma ago. Its current geometry shows first- and second-order segmentation: our study focusses on the Ashawq–Salalah second-order segment, between Alula–Fartak and Socotra–Hadbeen fracture zones. Modelling of the WAS and gravity data (three profiles across and three along the margin) gives insights into the first- and second-order structures. (1) Continental thinning is abrupt (15–20 km thinning across 50–100 km distance). It is accommodated by several tilted blocks. (2) The ocean–continent transition (OCT) is narrow (15 km wide). The velocity modelling provides indications on its geometry: oceanic-type upper-crust (4.5 km s−1) and continental-type lower crust (>6.5 km s−1). (3) The thickness of the oceanic crust decreases from West (10 km) to the East (5.5 km). This pattern is probably linked to a variation of magma supply along the nascent slow-spreading ridge axis. (4) A 5 km thick intermediate velocity body (7.6 to 7.8 km s−1) exists at the crust-mantle interface below the thinned margin, the OCT and the oceanic crust. We interpret it as an underplated mafic body, or partly intruded mafic material emplaced during a ‘post-rift’ event, according to the presence of a young volcano evidenced by heat-flow measurement (Encens-Flux survey) and multichannel seismic reflection (Encens survey). We propose that the non-volcanic passive margin is affected by post-rift volcanism suggesting that post-rift melting anomalies may influence the late evolution of non-volcanic passive margins.

Reactivations of boundary faults within a buried ancient rift system by ductile creeping of weak shear zones in the overpressured lower crust: The 2004 mid-Niigata Prefecture Earthquake

We elucidated fine-scale heterogeneities of the seismogenic structure associated with the 2004 mid-Niigata Prefecture Earthquake (thrust fault), Japan, by deploying a dense portable seismic array in the southwestern edge of the source region to observe the aftershocks. A velocity model inverted from double-difference tomographic analysis with first arrival times shows that most aftershocks were aligned between sedimentary strata in the hanging wall and the basement in the footwall. The basement is characterized by clear step-like and tilted block structures that gradually deepen toward the west. The domino-tilted block structures of the basement reveal evidence of a Miocene rift system buried beneath the thick sedimentary sequence. The aftershocks appear to be aligned roughly along pre-existing boundaries of the step-like array of tilted blocks. In addition, we used the Natural Earthquake Reflection Profile (NERP) method to image reflective zones in the lower crust, defining a set of relatively strong reflectors. Low seismic velocity anomalies combined with high electrical conductivity in the lower crust suggest that a plausible explanation for the reflective zones is the presence of reservoirs of overpressured crustal fluids. Given these considerations, we propose that pre-existing boundary faults within the buried ancient rift system were reactivated through ductile creeping of weak shear zones in the overpressured lower crust.