Clastic Dykes Research Articles

Clastic injectites and seismic-induced liquefaction in latest Quaternary travertine deposits (Serre di Rapolano, Italy)

ABSTRACT The liquefaction and consequent injection of clastic dykes (injectites) is often associated with the occurrence of large earthquakes. Injectites are most documented in thick unconsolidated sedimentary sequences and less commonly in rocks as a result of localized clastic mud overpressure due to earthquake-induced shaking. This paper documents near-surface clastic dykes injected into Middle-Late Pleistocene travertine deposits following repeated seismic events. The novelty is that there is a clear relationship between the development of the injectites and the activity of seismogenic faults that cut the entire body of the travertine, deposited at least at 45.5 ± 8.5 ka, and the topographic surface. The clastic slurry, derived from the fluidization of silty-sandy continental deposits at the base of, or intercalated with the travertine body, was in geometric connection with the faults. When this occurred, the over-pressured slurry was injected into the fault zone, filling the fractures and assisting their development by over-pressure hydrofracturing and counteracting the frictional forces on the fault. This evidence, in addition to documenting another case of liquefaction and associated emplacement of clastic dykes, has a twofold significance: i) it records the effects of clastic dykes intruded at very shallow depths, up to the surface, within a capable fault zone; ii) it documents a polyphase injection of clastic material into travertine deposits, never before described, induced by repeated liquefaction phenomena following major seismic events, thus highlighting and reinforcing the importance of travertine for the study of active tectonics and earthquake studies.

Geological conditions and fluid flow history that lead to the development of large clastic dykes in basins: A case study from Kushiro, Japan

AbstractLarge clastic dykes (the Harutori‐Taro and Harutori‐Jiro dykes) and smaller dykes are exposed in the underground Kushiro Coal Mine (KCM), Japan. This study examines these dykes as a case study to investigate the geological conditions and fluid flow history that lead to the development of large clastic dykes in basins. The composition of the dykes indicates the Beppo and/or Harutori formations as their parent unit. Crystallite size distribution (CSD) analysis reveals Ostwald ripening of the kaolinite in the kaolinitised feldspar from the dykes, suggesting stagnant conditions in the parent unit before the dyke was formed. In contrast, smectite CSDs and the high carbonate content of the dykes suggest that large volumes of fluid flowed through the dykes along the established hydraulic gradient, which was triggered by the breaking of the upper seal. The isotopic and chemical compositions of the calcite and aragonite in the dykes, with moderate siderite and rhodochrosite content, indicate the fluid was a warm (>30°C) mixture of freshwater and saltwater, which was transferred from deeper levels of the parent unit towards the crest of an anticline. Immediately after sand injection, the semi‐closed system of the parent unit near the root of the large dyke was transformed into a major flow channel for overpressurised fluids. Subsequently, a large volume of fluid flowed along the vertical conduit (or dyke) over a long period of time (>1 Myr), which removed fluid from a widespread area (i.e., several hundred square kilometres) of the basin. The results show that thin parent units, poor lateral continuity of the upper seal, and spatially heterogeneous overpressurisation do not preclude the formation of large dykes.

Reappraising the seismogenic potential of a low-strain rate region: Active faulting in the eastern Siena Basin (southern Tuscany, Italy)

We investigated the active tectonics and earthquake potential of the eastern Siena Basin, a slowly deforming portion of southern Tuscany in the inner Northern Apennines. This region hosts several historical settlements and valuable cultural heritage, but also frequent background seismicity and rare damaging earthquakes in the Mw range 5.0–6.2. We describe in detail an active, capable, and seismogenic fault system that we identified in the eastern Siena Basin, a few kilometers south-east of the city of Siena, thanks to the presence of an active quarry (Cava Capanni) that exploits travertines of Middle Pleistocene-Holocene age. Travertines are unique rock masses that may preserve living evidence of active and seismogenic faulting, thus providing remarkable seismotectonic insight. The active fault system consists of at least two segments rupturing travertines younger than 45 ka, with a cumulative vertical displacement of 111 cm, and an estimated minimum slip rate of 0.02–0.03 mm/y. We maintain that this displacement is the result of at least three coseismic movements accompanied by clastic dykes injected within the fault damage zone due to liquefaction phenomena. The fault system is seen to extend east of the quarry, affecting Pliocene and Mesozoic deposits.The Cava Capanni fault system is evidence of a poorly understood but potentially seismogenic tectonic mechanism of regional extent. Its orientation and kinematics are compatible with the activity of faults that are oriented obliquely or orthogonally to the main chain axis, in contrast with the setting of the axial and outer zone of the Northern Apennines, where extension and compression are accommodated by Apennines-parallel faults.

A glaciotectonic landform in the Shyok valley, Trans-Himalayan Karakoram Range, India

Abstract This study reports and discusses the first case of glaciotectonic landforms in the Shyok valley of the Trans-Himalayan Karakoram Range, Ladakh, where a large decomposed granite megablock (8.2 km2) along with underlying diamicton is thrust over the unconsolidated Quaternary glaciofluvial sediments along a fault gouge zone near the village of Khalsar. The absence of deformation signatures below the fault gouge indicates that the brittle fault acted as a décollement surface under frozen conditions along which the glaciotectonic megablock was translated. The other deformation features include slickensides, ductile shear, thrust propagation fold noses, clastic dykes and rafts of granite and slate within the diamicton sediments. These features indicate a subglacial glaciotectonic nappe origin of the landform. The presence of juxtaposed brittle to ductile deformation fabric, clastic dykes and the superimposition of deformed decomposed granite and diamicton over the undisturbed fluvial sediments indicates a permafrost glacial margin and proglacial environment under sufficient subglacial hydrodynamic conditions for the entrapment and transportation of the glaciotectonic megablock. The deformation fabric consistently shows a southeast orientation, indicating an advancing glacier motion from northwest to southeast. The Siachen Glacier which formerly flowed down the Nubra valley is the most likely cause of the Khalsar glaciotectonic landform.

Early dextral shear motion during the breakup of East Gondwana documented in clastic dykes in the Tethys Himalaya

Much of the geological record of the northern Indian passive continental margin has been lost due to erosion or obscured due to tectonic burial associated with the India-Eurasia collision. Consequently, tectonic models of India's breakup from East Gondwana incorporate limited data from the northern margin of Greater India. We present evidence of the Late Jurassic clastic dykes crosscutting contemporaneous sedimentary sequences in the Tethys Himalaya. We propose that the original orientation of the clastic dykes provides kinematic information about Greater India's breakup from East Gondwana. The clastic dykes are meters to hundreds of meters long and centimeters to meters wide. They taper downwards and crosscut country rocks at high angles. The clastic dykes and their host strata are quartzose sandstones and share similar textures and detrital zircon age populations (dominantly 600–500 Ma and 1000–800 Ma zircon grains). These similarities indicate the clastic dykes and their host rocks share a common source as well as comparable transport and deposition dynamics. Notably, the clastic dykes lack distinct signs of ptygmatic folds, shale/mudrock clasts, surface orientated structures, or bifurcations on dyke margins. Decompaction and structural restoration of the sequences show that the sandstone dykes/fractures and en échelon dyke arrays can be classified into three distinct orientation groups NW (349° − 295°), SW (∼210°) and NE (∼40°). There is no apparent difference in the age or provenance of the clastic material within the different orientation groups. We propose that the clastic dykes signify passively infilled tensile fractures formed within a NW-SE dextral strike-slip megashear system during the Late Jurassic. This system likely developed in a transform zone within Greater India prior to the separation of the Indian Plate from East Gondwana.

Concurrence of load-and-flame structures, balls-and-pillows, clastic injectites and shear deformation bands as indicator of seismicity in mixed siliciclastic-carbonate successions (Finale Ligure Basin, Italy)

The present research explores a diverse suite of soft sediment deformation structures (SSDS) combined with clastic dykes and shear deformation bands distributed throughout a Early Miocene-age mixed carbonate-siliciclastic coastal wedge located on the Ligurian coast (NW Italy) of the Mediterranean region. The studied Aquitanian-Serravallian successions of the Finale Ligure Basin are typified by the presence of lithologically heterogeneous units, internally characterized by abrupt changes of lithofacies distributions and associated interpreted depositional environments. The interface between the terrigenous base and the carbonate wedge in many places features SSDS. Here, we report the discovery of a multiform deformational pattern, ranging from small size (cm-to dm-scale) load-and-flame structures, ball-and-pillows and mini-slumpings, to outsized (several meters in height) chaotic contorted and folded stratigraphic intervals, sand intrusions and diapir- or tree-like water escape structures. Many of the SSDS are related to fast injection of overpressurized fluids after sand liquefaction, and occur in close association with disaggregation and phyllosilicate shear bands, and sand dykelets. The stratigraphic distribution and cyclic repetitions of SSDS-prone intervals unambiguously indicates syn-depositional seismicity, thus suggesting that juxtaposition of SSDS, diapiric injectites, and shear deformation bands can serve as a suitable diagnostic criteria for seismite identification in the field. Since these particular assemblages of deformation structures reflect seismic activity in the area, they provide compelling evidence for previously undocumented tectonic episodes in Langhian-Serravallian times, associated with the latest stages of the Liguro-Provençal Basin opening.

Dinosaur and tree-line invasion of southeastern Australia during Cretaceous greenhouse spikes

The paleosol record of the Early Cretaceous Wonthaggi and Eumeralla formations of Victoria reveals a suite of non-calcareous Histosols and Ultisols, as well as stump casts, indicative of humid temperate ecosystems. At some stratigraphic levels, frigid temperatures are indicated by clastic dykes (ice wedges), load casts of mud (periglacial convolutions), and coal-mantled stone rolls (aapamires). The largest tree stumps and thickest paleosols correspond to known Early Cretaceous greenhouse spikes, as revealed by stomatal index of fossil ginkgo leaves, by black shales of oceanic anoxic events, and both marine and non-marine carbon isotopic anomalies. Temperature and CO2 spikes coincide with appearance of broadleaf deciduous forests, including Ginkgo australis, with dinosaurs, monotremes, placentals, early birds and angiosperms. In contrast, permafrost paleosols represent fern-lycopsid tundra and coniferous taiga vegetation. Early Cretaceous CO2 greenhouse spikes allowed southward migration of tree lines and polar biomes several times during the Cretaceous, which was not continuously a hothouse period. The same greenhouse spikes are recorded by maximal marine transgressions in the Great Artesian Basin of Queensland, and the frigid intervals by dropped pebbles and glendonites in South Australia, New South Wales and Arctic regions.

The Sukil River valley: a natural geological laboratory (case studies from the Ukrainian Carpathians)

Abstract In this chapter we present a brief geological description of the Sukil River valley, which is located in the Ivano-Frankivsk region in the western part of Ukraine, and the interesting geological features found there. Here, different types of rocks are exposed over a small area. The area is rich in various structural features such as joints, small faults, and different types of folds, duplexes and clastic dykes. Two outcrops of folds found in the Cretaceous deposits in the village of Bukivets are unique geosites. In addition, in the city of Bolekhiv, there are the ruins of a saltery. We describe the types of research conducted in this natural laboratory, most of which concerns structural geology. Geological phenomena are studied using both classical approaches and the latest technologies. Electronic compasses were used widely in the study of mesostructures. For the first time, balanced geological sections have been constructed for the territory of the Ukrainian Carpathians both for outcrops and for the entire Sukil River valley. Ground-based laser scanning was used for the detailed mapping of folds in Bukivets. Subsequently, the exposure was studied using unmanned aerial vehicle (UAV) photogrammetry, which was used to construct virtual geological outcrops that can be used widely for educational purposes. The urgency for creating virtual tours became apparent during the COVID pandemic. These models were also used to create the ‘Geo-Carpathians’ project, a Polish–Ukrainian geotourism route through the Carpathian Mountains. These models can be also used to assist in the teaching of the structural geology of the Carpathians, in the geological training of Earth Sciences students, in excursions for different types of Ukrainian and international conferences, for high-school students, and, indeed, for anyone interested in geology.

Multi-scale, multi-style soft-sediment deformation structures preserved in Cenozoic fluvial deposits, southeastern Brazil: Evidence of impact shaking?

Soft-sediment deformation (SSD) structures preserved in sedimentary successions can record significant seismic events which occurred during or right after deposition. The integrated analysis of sedimentary facies, depositional architecture, and SSD data can determine the relative temporal relationship between deposition and deformational events, and reveal the origin and mechanisms involved in the deformation and the resulting structures. The São Paulo Formation in southeastern Brazil preserves Cenozoic alluvial deposits which characterize most of the geology of the homonymous metropolis area. This succession preserves fluvial channel and floodplain deposits, with associated debris-flow dominated alluvial fan deposits, and preserves abundant SSD structures in a wide range of dimensions and morphologies. These deformation structures include small- and large-scale folds, multi-scale vents of varied geometries, ball-and-pillow structures, and clastic dykes. Here we provide an integrated analysis of sedimentary facies, depositional architecture data, and SSD data from the São Paulo Formation, in order to discuss the deformation mechanisms and plausible triggering mechanisms responsible for the development of such structures. Our interpretation is that SSD structures in the São Paulo Formation were likely triggered by a Cenozoic meteor impact recorded by a crater near the study area.

Deformation of soft sediments and evaporites in a tectonically active basin: Bay St. George sub-basin, Newfoundland, Canada

The Bay St. George sub-basin of SW Newfoundland, part of the larger late Paleozoic Maritimes basin, formed under the influence of strike-slip faulting and the movement of evaporites. New stratigraphic correlations between Newfoundland and other late Paleozoic sub-basins illustrate the effects of both basement and salt movement. Coastal outcrops show complex combinations of synsedimentary, salt-related, and tectonic structures. Map relationships and dramatic thickness contrasts in the Tournaisian Anguille Group indicate that a large, concealed, NE–striking normal growth fault (Ship Cove fault) controlled sedimentation; the exposed Snakes Bight fault originated as a hanging-wall splay. Structures formed during, or soon after deposition include soft-sediment folds, boudins, clastic dykes, and millimetre-scale diapiric bulb structures, formed by overpressuring and liquidization of sediment. These suggest that the sub-basin was tectonically active throughout deposition. Evaporite-related deformation is recorded in the Visean Codroy Group and overlying strata. Comparisons between outcrop and subsurface suggests that significant amounts of evaporite were removed from exposed sections by halokinesis and solution. Complex outcrop relationships indicate salt welds, and suggest that units of the upper Codroy and overlying Barachois groups represent fills of minibasins that subsided into thick evaporites. Field relationships suggest tectonic inversion deposition related to E-W dextral strike slip motion that affected the entire Maritimes basin in the Serpukhovian, producing reverse-sense offsets and contractional folds. Many of the structures in the Bay St. George sub-basin, previously interpreted as post-depositional and purely tectonic, were formed by deformation of unlithified sediment and ductile evaporites during basin development.

Synsedimentary deformation structures within the Paleoproterozoic Espanola Formation, Huronian Supergroup, Canada: Implications for basin tectonism and paleodepositional conditions

The Espanola Formation represents part of the lower Huronian Supergroup (2.45–2.2 Ga), which is interpreted to have been deposited in a restricted, fault-bound, intracontinental rift basin during Early Proterozoic episodes of crustal stretching and contemporaneous faulting along the southern margin of the Archean Superior Province. Field examination of Espanola Formation outcrops exposed north of Elliot Lake, Ontario, Canada, revealed widespread synsedimentary deformation structures, which can be broadly grouped into three main categories: (1) forceful-injection structures, (2) slump structures, and (3) soft-sediment deformation structures. Examples of these structures include clastic dykes, large-scale slump structures, intraformational conglomerate, load casts, ball-and-pillow structures, convolute laminae, and dish-and-pillar structures. Most of the reported deformation structures are restricted to discrete stratigraphic horizons, are laterally continuous on an outcrop scale, are abundant and widespread across the studied region and are confined between undisturbed strata of similar lithology. These characteristics strongly indicate an origin by liquefaction and fluidization mechanisms that were triggered by syndepositional rift-related seismic activity, inferred to have been induced by movement along bounding normal faults during subsidence of the lower Huronian basin. Autogenic triggers such as overloading by rapid sedimentation, slump emplacement, inverse density gradient, gravity-driven sediment transport and mobilization, and storm waves may have also locally contributed to the development of the reported soft-sediment deformation structures.

Evolution of Jurassic intertrap deposits in the Parnaíba Basin, northern Brazil: The last sediment-lava interaction linked to the CAMP in West Gondwana

The opening of the Central Atlantic Ocean was accompanied by ascending plume zones that generated large igneous provinces (LIPs), such as the Central Atlantic Magmatic Province (CAMP), with voluminous lava flows and outpourings. One of the best records of the CAMP in the West Gondwana is the Jurassic volcanic-sedimentary succession exposed in the western Parnaíba Basin, northern Brazil. Diagenetic and hydrothermal processes are discussed, focusing on the lava-sediment interaction. The siliciclastic sediments called “intertrap deposits” record a wet desert system characterized by fluvial-aeolian and shallow lake deposits engulfed by lavas of the CAMP at approximately 200–202 Ma. The facies and petrographic studies, based on a drill core and outcrops, led to the interpretation of the palaeoenvironmental and diagenetic evolution of the intertrap deposits. The intertrap deposits are sandwiched by basalts and may be organized in seven sedimentary facies, grouped into two facies associations: 1) ephemeral braided river deposits (FA1), which consist of medium- to coarse-grained pebbly sandstone with poorly sorted angular to sub-rounded grains (basaltic fragments and quartz) and low-angle cross-bedding, even-parallel stratification, and trough cross-stratification, and 2) pond/shallow lake deposits (FA2) composed of tabular beds of sandstone/siltstone rhythmite and laminated mudstone. The main architectural elements of intertrap deposits are 1) channels on volcanic substrate, 2) sand sheets deposited over pre-existent lava topography, 3) tabular beds with laminated rhythmites, and 4) sand-filled fissures/fractures and clastic dykes. The intensification of magmatic processes due to the disruption of Pangaea promoted the formation of extensive volcanic plains, whereas wet desert settings were developed during the quiescent intervals between lava flow episodes. Fluvial channels with subaqueous dunes and sand sheets were incised into the basaltic substrate. The abandoned fluvial plains were affected by flash floods that promoted mechanical infiltration of clays, forming coatings over grains. The heat flow and hydrothermal activity of volcanic rocks increased the devitrification of glassy clasts, releasing silica and precipitating low-temperature authigenic mineral assemblages. The most common pore-filling process was the massive precipitation of early diagenetic-hydrothermal minerals, including smectite, chalcedony, poikilotopic zeolite, megaquartz, and haematite, which partially obliterated the primary porosity and permeability. These exceptional conditions of cementation triggered by basalt eruptions precluded the modification of a primary framework by late stages (post-Jurassic) burial diagenesis. The intertrap sandstones represent the last lava-sediment interaction related to the CAMP recorded in West Gondwana.

Lower Cretaceous clastic dykes in southern Tibet: Characteristics and palaeogeographic significance

A clastic dyke is a stratigraphically vertical, wall‐like body of clastic material that fills open fissures across strata, and may provide important information on regional depositional processes and tectonic activity. Clastic dyke swarms composed of greyish to brown, fine‐ to medium‐grained lithic‐rich volcaniclastic sandstone are widely exposed in the lower Aptian shales in the Gucuo and Wölong areas of the Tethyan Himalaya in southern Tibet, where they are distributed along a series of normal faults. The length and width of clastic dykes range from 5 to 50 m and from 20 to 150 cm, respectively. The U–Pb age spectra of zircon grains contained in clastic dykes are similar to those in Lower Cretaceous sandstones exposed in the studied areas (i.e., Wölong volcaniclastics) and display a youngest peak at 124.3± 2 Ma. Petrographic analyses show that the underlying strata and dykes contain more volcanic lithic fragments and less feldspar grains than the overlying strata, indicating that the dykes were derived from underlying strata and injected upward along faults. Their formation may be explained by a fault‐induced clastic‐dyke model: normal faults provided conduits for clastic material injected from the underlying unlithified sandstones which were undergoing refluidization owing to over‐pressure. We infer that the liquefaction of underlying strata was induced by earthquakes associated with the extensive magmatic activities and regional extension that affected the Indian passive margin of Neotethys during the Early Cretaceous.

Typical Soft–Sediment Deformation Structures Induced by Freeze/Thaw Cycles: A Case Study of Quaternary Alluvial Deposits in the Northern Qiangtang Basin, Tibetan Plateau

AbstractWith the objective of establishing a distinction between deformation structures caused by freeze/thaw cycles and those resulting from seismic activity, we studied three well–exposed alluvial deposits in a section at Dogai Coring, northern Qiangtang Basin, Tibetan Plateau. Deformation is present in the form of plastic structures (diapirs, folds and clastic dykes), brittle structures (micro–faults) and cryogenic wedges. These soft–sediment deformation features (except the micro–faults) are mainly characterized by meter–scale, non–interlayered, low–speed and low–pressure displacements within soft sediments, most commonly in the form of plastic deformation. Taking into account the geographic setting, lithology and deformation features, we interpret these soft–sediment deformation features as the products of freeze/thaw cycles, rather than of earthquake–induced shock waves, thus reflecting regional temperature changes and fluctuations of hydrothermal conditions in the uppermost sediments. The micro–faults (close to linear hot springs) are ascribed to regional fault activity; however, we were unable to identify the nature of the micro–faults, perhaps due to disturbance by subsequent freeze/thaw cycles. This study may serve as a guide to recognizing the differences between deformation structures attributed to freeze/thaw cycles and seismic processes.

Enigmatic earthquake-generated large-scale clastic dyke in the Biberach area (SW Germany)

We discovered and describe a large-scale (exposed dimensions ~17 m in lateral and ~15 m in vertical extension, and up to 10 cm in width) clastic dyke in the Middle Miocene (between ~17.1 and ~14.5 Ma) sandy deposits (‘Fluviatile Untere Serie’) of the North Alpine Foreland Basin, close to Biberach an der Riß in SW Germany. The dyke appears to be of sedimentary–tectonic origin. It formed by liquefaction and injection of clastic material that occurred simultaneously with fracture opening and propagation in sandy deposits, presumably as a result of strong seismic shaking triggered by an intense earthquake. We, therefore, interpret the dyke as a seismite and a useful palaeoseismic indicator. The clastic dyke cuts through a layer of distal Ries ejecta (‘Brockhorizont’) and, thus, postdates the Ries impact. The age of the Ries event, in combination with other stratigraphic constraints, defines a relatively narrow time frame of ~14.8 to 14.5 Ma for the formation of the clastic dyke. The source of the strong earthquake responsible for dyke formation might be seismicity related to the Alpine orogeny and tectonism, seismic activity in one of the volcanic fields of the Paleogene to Quaternary European Volcanic Province, or seismicity induced by a Miocene impact event in southern Germany. We suggest the relatively strong earthquake in question may have been triggered by a large phreatomagmatic explosion in the Urach-Kirchheim volcanic field or, perhaps more likely, the Steinheim impact event on the eastern Swabian Alb plateau.

Soft-sediment deformation: deep-water slope deposits of a back-arc basin (middle Eocene-Oligocene Kırkgeçit Formation, Elazığ Basin), Eastern Turkey

The Kirkgecit Formation (middle Eocene-Oligocene) is exposed in an approximately E-W direction around the city of Elazig, eastern Turkey. This unit represents marly and sandy deep-marine sediments deposited in a slope setting with channels and contain soft-sediment deformation structures (SSDSs). These SSDSs include slump folds, chaotic strata, load casts, flame structures, convolute laminations, clastic dykes, water-escape structures and syn-sedimentary faults. The main underlying deformation processes are liquefaction, fluidization and gravity-induced loading. Some of the SSDS could be interpreted as seismites, which originated due to earthquakes that also triggered high-density mass flows. This interpretation is consistent with the active tectonics in the Elazig back-arc basin. The SSDS in the study area are compared with SSDS in other regions, as well as with SSDS in other marine deposits. In this way, findings obtained from the study area are compared with SSDS at different locations in the marine Kirkgecit Formation around Elazig and with similar structures developed in marine settings worldwide, such as SE Crete and NW Africa.

A Late Glacial surface rupturing earthquake at the Peel Boundary fault zone, Roer Valley Rift System, the Netherlands

Paleoseismological trenching studies constrain recurrence times and magnitudes of faulting events and earthquakes on active faults. In a trench along the central part of the Peel Boundary fault zone (PBFZ), southeastern Netherlands, evidence was found for such a large faulting event that occurred around 14 ka. The event caused a fault scarp in unconsolidated sediments of ∼1 m height. A colluvial wedge was formed next to the scarp. A second faulting event offsets this colluvial wedge by 0.2–0.1 m. This event can be tentatively dated at ∼13 ka. During or immediately after the second event, a large clastic dyke intruded along the fault plane. The dyke is not faulted, but its emplacement did cause some minor thrust faulting around the injection.The sudden character of the main faulting event, the brittle deformation style of loam layers, the lack of growth faulting in the colluvial wedge, the clastic dykes and the flame structures demonstrate that the main faulting event was a surface rupturing earthquake. Based on the scarp height, the estimated moment magnitude is about 6.8 ± 0.3. Similar observations in a previous trench site suggest that the length of the surface rupture was at least 32 km. The earthquake took place during the Weichselian (Würmian) Late Glacial. This timing corresponds to the start of the glacio-isostatic forebulge collapse in the Netherlands. Glacio-isostatic movements have been invoked before to explain earthquake events in the Roer Valley Rift System in which the PBFZ is situated, and in northern Germany and Denmark. If these earthquakes can indeed be attributed to a collapsing forebulge, their ages should show a decrease in the direction of ice-sheet retreat. This might indeed be the case, as the ages decrease from 14 ka and 13 ka in this trench via 13–16 ka at the Osning Thrust Zone, NW Germany, to 12–14 ka in northern Denmark.

Sedimentary environment and diapirism of the Pleistocene–Holocene deposits of the Zirku salt plug, southern Arabian Gulf

The Zirku salt plug is one of eleven emergent salt plugs in the southern Arabian Gulf. Its diapiric core consists of the Late Neoproterozoic–Early Cambrian ‘Hormuz Series’, which are unconformably overlain by ‘post-Hormuz’ calcarenite and breccia. The lithological and sedimentological characteristics of the calcarenite and breccia indicate that they were deposited in a beach and adjacent intertidal flats. It is suggested on stratigraphic grounds and the consideration of previous works that the ‘post-Hormuz’ deposits may be considered Pleistocene–Holocene in age. Diapirism affected the Pleistocene–Holocene deposits and produced mini-diapirs, intrusion and piercement structures, extruded gypsum flows, clastic dykes and diapiric uplift shown by elevated marine terraces and tidal notches. The elevated terraces and notches record sea-level high stands, uplift rates and sea-level fluctuations during the late Quaternary. The calculated rate of uplift of the Zirku salt plug during the period between 128 ka and 11 ka, is nearly constant at 1.05 mm/yr. Periods of transgressions and regressions were inferred from the late Quaternary sea-level curves. In the field, transgression is marked by onlap and the filling up of wadi channels, and regression by producing a new lower base level of wadis and the occurrence of gypcrete interbedded with calcarenite.

Palaeoseismic structures in Quaternary sediments of Hamburg (NW Germany), earthquake evidence during the younger Weichselian and Holocene

Investigations at a construction site in Hamburg (NW Germany) exposed palaeoseismic structures from moderate to strong earthquakes. Based on the large size of large blowout clastic dykes being up to 2.0 m wide and 2.5 m high, as well as the occurrence of infill structures, erratics/rafts of up to 9 kg in weight, seismites s.s. and decimetre-scale folds or seismoslumps, the magnitude of the earthquakes could be in the order of up to M ≥ 6. This is significantly higher than previously assumed for prehistorical seismic events in NW Germany. The structures are assigned to earthquakes possibly related to NW–SE-oriented faults that are evident in Lidar and SAR surface interpretations. Organic material from blowout-related infill bowls, assumed to be synchronously formed collapse depressions, are analysed for 14C ages. The dates reveal ages between 31,500 and 1200 14C cal a BP, indicating five prehistoric earthquakes. Accordingly, three of these earthquakes occurred before and after the main Weichselian glaciation phase (ca. 31,350, 26,850 and 18,980 14C cal a BP), and two occurred during the Subatlantic (ca. 4900 and 1200 14C cal a BP). The Weichselian events indicate seismic activity in the ice marginal zone during or following interstadials/Dansgaard–Oeschger events, as well as suggesting that advancing ice sheets foster earthquakes.

Mineralogical features of the clastic dykes of the Eastern Carpathians Skybova zone

We determined several areas with outcrops of clastic dikes which occur in the rocks of the Menilite suite of the Upper Paleogene period in the so-called Skybova zone [Ukr. Ски́ бова зо́на – the largest tectonic zone within the Carpathian folded structure. The word “skyba” derived from Polish, and is used in relation to nappe – Translator`s Note ] of the Eastern Carpathians. The objective of this article is to reveal the peculiarities of bedding, mineralogical composition and structural-texture peculiarities of the clastic dikes of the Sukyl, Stryi and Skhidnytsia river basins. During our research, we used the method of field structural-geological surveys, traditional method of laboratory analysis of mineralogical-petrographic composition of rocks in thin sections. As a result, we studied the conditions of bedding of clastic dikes, mineral composition and structural-texture peculiarities. We determined that the dikes are represented by aleuro-sandstone and aleurolite with quartz-carbonate cement. Aleurolite most often represents pre-selvage parts of dikes. Mineral grains are mostly formed by quartz of different degrees of roundness. In the selvages of the studied dikes, we observed a decrease in the sizes of mineral grains, enrichment of these parts of dikes by organic compound, increase in the content of carbonate minerals in the rock cement and numerous microdeformations of mineral grains. Also we determined an insignificant content of ore minerals in some studied plots. Additional analysis conducted for a polished sample which characterizes the vertical section of dike in the area of the river Sukyl allowed us to determine the structural signs in its selvages, indicating injection character of dikes` upward introduction to the bearing rocks. We studied the microdeformation of dikes` mineral grains, which are represented by veinles, microfaults and microshifts withclear mixing of their fragments. The obtained results indicate the formation of clastic dikes of the Skybova zone of the Eastern Carpathians in conditions of compression, when relatively flexible material of selvages of the dikes, represented by carbonates and organic compound, contributed to the introduction of the latter to the layer of flysch through the system of tectonic faults of north-west stretch.