Tectonic Graben Research Articles

Reconstruction of paleoenvironment during coal deposition in the Neogene Karlovo graben, Bulgaria

Abstract Coal petrologic and mineralogic investigations, along with proximate and Rock-Eval pyrolysis, were performed on samples from a borehole in the center of the Karlovo basin in order to determine the depositional environment of the Karlovo lignite, which formed during the transgressive stage of a tectonic graben. Several thin coaly layers represent the transition from a fluvio-deltaic to a lacustrine environment, but only the uppermost lignite seam, which is about 10-m thick, is considered (sub-) economic. The latter is characterized by medium to high ash yields and low sulfur contents. The coal formed in a rheotrophic, low-lying mire near a freshwater lake with low pH value. The acid environment resulted in low bacterial activity, low gelification, and low sulfur contents. Alumosilicates were altered to kaolinite, another consequence of the acidic peat water. During peat accumulation areas with open water existed in the central basin. Alginite-rich sapropelic sediments with (acidophilic) diatoms accumulated in these anaerobic ponds. Apart from the basin center, xylitic coal indicates a vegetation rich in decay resistant conifers (swamp forest). Relatively dry conditions were established during later peat-forming stages. This is indicated by lower ash yields and increased contents of fusinite and fungal remains. Peat accumulation was terminated by a major flooding event and the establishment of a lake.

Statistical analysis of bed-thickness variation in a Tortonian succession of biocalcarenitic tidal dunes, Amantea Basin, Calabria, southern Italy

The Tortonian biocalcarenitic succession in Amantea Basin was deposited in a ramp setting of a peri-Tyrrhenian shelf embayment, whose northern part was linked with the southern part by a tectonic graben referred to as the Monte Pellegrino strait. The graben was 1.5–2 km wide and 4 km long, had shallowly submerged margins and a south-sloping sublittoral floor dominated by strong, asymmetrical tidal currents. The water depth fluctuated due to episodes of rapid subsidence driven by extensional tectonics. The strait-fill biocalcarenites are up to 120 m thick and consist of planar cross-stratified beds that range in thickness from < 0.2 m to > 20 m and represent 2-D dunes accumulated as a mounded longitudinal complex. Bed thicknesses have been measured in four vertical logs along an outcrop section oblique to the palaeostrait. The stratigraphic pattern of bed-thickness variation has been analyzed by using two versions of the runs test, the Moore–Wallis test, the median-crossing test, the mean squared successive difference test, the Hurst statistic, the Spearman rank-correlation test and the Meacham rank-difference test. The statistical tests indicate that the axial zone and inner flank of the tidal complex show alternating clusters, 5–15 m thick, of thinner and thicker beds, whereas the outer flank and marginal zone show thinning-upward bed packages, 15–25 m thick. An overall thinning-upward trend characterizes the transitional flank zone. The bed packages are considered to form aggradational parasequences, whose varied lateral development is attributed to a differential response of the subtidal system to bathymetric changes. The data set indicates that the bed thicknesses are self-similar, but beds thinner than 250 cm have a different fractal dimension than the thicker beds. The pattern of bed-thickness variation is considered to be a result of internal forcing of a sedimentary system in a state of self-organized criticality, perturbed by bathymetric changes. Fractal property bears important implications for the spatial characteristics of sedimentary succession, allowing bed geometries and volumes to be assessed on the basis of thickness data from isolated logs or drilling cores.

The hydrogeothermal basin of Sofia graben (Bulgaria)

In the Mesozoic basement and Neogene filling of Sofia tectonic graben a non-homogenous system of thermal water reservoirs connected to a united hydrogeothermal basin occurred. The basin contains and reproduces five types of low mineralized thermal waters and four types of mineralized thermal waters with exotic composition. The water temperatures are 30–80(90) °C. The total potential of the basin is assessed at 500 l/s thermal waters and 48 MWt thermal power. The utilization of thermal waters in the area of the Bulgarian capital has its roots and traditions in antiquity, and a promising future. The new town plan of Sofia provides construction of 35 centers for developing a versatile and prosperous hydrothermal business.

THE APPLICATION OF THE TRANSIENT EM METHOD INTO THE GEOTHERMAL FIELD EXPLORATION

In this work we examine the potential of the time-domain electromagnetic method (Transient EM) in the exploration of geothermal field and the study of the geological environment. The method was applied in the geothermal field of Aristino (Alexandroupolis), westwards of the Traianoupolis, where the known hot springs exist. The EM method and more specifically magnetotellurics (MT), it has been applied in the past in Greece by various researchers, but it is the first time the TEM method is applied for geothermal exploration purposes. As it has been shown by previous works, in the wider region of Aristino an important geothermal field is developed, with geothermal fluid temperature of 30°-90° and high content of dissolved salts of 10gr/lt. These factors result to the appearance of highly conductive geoelectrical anomalies which are directly related to the geothermal field. The existence of two drillholes in the region allowed the calibration and the control of the effectiveness of the TEM method. The geophysical survey has delimited the area of local geothermal interest that is formed by hot aquifer at a relatively shallow depth, extended North of Aristino and between the main faults of the region. The two main faults are responsible for the creation of a tectonic graben in the intermediary region and the development of a highly conductive formation which correlates well with the hot aquifer tank. The geothermal fluid circulates through the faulting system having high temperature caused by the contact with magmatic masses. The application of TEM proved the operational advantages of the method in the geothermal field exploration. The relatively limited number of TEM soundings presented here does not allow the export of safe conclusions regarding the geothermal field of the wider region, although the evaluation of results based on the geological and geoelectrical data of the region is extremely encouraging and justifies the use of the TEM method in the systematic study and mapping of geothermal fields.

Water geochemistry of the Seferihisar geothermal area, İzmir, Turkey

The Na–Cl thermal waters of Seferihisar have measured temperatures of 30–153°C, and conductivities of 7400–55200 μS/cm. They issue from Upper Cretaceous to Paleocene Bornova melange rocks, which are made up of sandstone–shale intercalations, conglomerate, mafic submarine volcanics, limestone lenses, serpentinite and limestone bodies, and their complexes, through the intersection of faults. The clay-rich zones of the overlying Neogene terrestrial sediments cap the system. The heat source is the high geothermal gradient caused by graben tectonics of the area. Na and Cl ions mainly dominate the chemistry of thermal waters, thus thermal waters of the Seferihisar area appear to be mixtures of groundwaters and seawater. The seawater contribution in thermal waters varies from 10% to 80%. Assessments from empirical chemical geothermometers, mineral equilibria and silica-mixing model applied on the thermal waters of the Seferihisar geothermal area suggest that the temperatures of the reservoir systems vary between 60 and 180°C. The Seferihisar geothermal area has been physically divided into five main groups and estimated reservoir temperatures provided by the entire methods give the increase as: Doğanbey Burnu, Karakoç spa, Doğanbey, Tuzla and Cumali spa areas in ascending order of reservoir temperature. Hydrogeochemical assessments indicate that thermal waters were mixed with cold waters before and/or after heating at depth in different proportions. The results of mineral equilibrium modeling indicate that the thermal waters of Seferihisar are undersaturated with respect to gypsum and amorphous silica, oversaturated with respect to calcite, dolomite and aragonite, and undersaturated or oversaturated with respect to quartz, chalcedony and anhydrite.

Geology of the Skorpion Supergene Zinc Deposit, Southern Namibia

The Skorpion zinc deposit in southwestern Namibia comprises a significant nonsulfide orebody (24.6 Mt at 10.6% Zn) and subordinate amounts of primary base metal sulfide mineralization, which structurally underlies the nonsulfide ores at depth. The host rocks of the blind, nonsulfide orebody and relict sulfide mineralization consist of a mixed volcano-sedimentary sequence of Neoproterozoic age that has formed in an initial continental rift system between the Kalahari cratonic province and the Rio de la Plata cratonic province. The metavolcanic and subvolcanic rocks display a distinct bimodal geochemical composition of within-plate affinity and have been emplaced under an extensional crustal regime. Zircons from a rhyolitic vesicular flow, extruded approximately contemporaneously with the volcanic-hosted sulfide mineralization, have been dated by SHRIMP U-Pb analysis and yield an age of 751.9 ± 5.5 Ma. The basin that hosts the Skorpion deposit has been shaped by horst and (half-)graben tectonics, which also controlled the sedimentary deposition of siliciclastic and carbonate sediments within both shallow and deeper water environments. The rock sequence was affected by a major tectono-metamorphic event the Pan African-Brasiliano orogeny at approximately 550 to 545 Ma. Metamorphism reached uppermost greenschist to lowermost amphibolite facies, and the complex deformational style of the region is the result of intense faulting, folding, and thrusting. The Skorpion orebody is hosted by arkosic metaarenites and subordinately by volcaniclastic metasedimentary rocks. It is irregularly shaped, transgressive to sedimentary layering, and has a relatively flat top, which is covered by a blanket of unmineralized overburden consisting of calcrete, boulder beds and recent sand dunes. At depth, the orebody has a relatively sharp contact with a barren marble unit on the east and south and a more gradational boundary within the siliciclastic host rocks on the west and north. The nonsulfide ore minerals comprise predominantly sauconite (Zn smectite), substantial amounts of hemimorphite and smithsonite, and subordinate amounts of scholzite, tarbuttite, atacamite, hydrozincite, and hetearolite. The ore minerals are mainly euhedral and subhedral crystals and occur as open space fillings in intergranular voids, fractures, and breccias and as replacements of feldspar and mica. The metasedimentary host rocks of the nonsulfide ore contain abundant iron hydroxide pseudomorphs after disseminated sulfides and stringers and after rare semimassive stratiform layers of sulfides. Gossanous, manganese and iron hydroxide-rich, siliciclastic, metasedimentary rocks are common in the western and northwestern vicinity of the nonsulfide orebody. Alteration of the supergene mineralized host rocks includes removal of calcite cement and the breakdown of detrital albite-rich plagioclase and of minor orthoclase. Local alteration phenomena also include silicification and rare secondary barite, both a result of the alteration and breakdown of feldspar. Minor base metal sulfide mineral deposits occur in metarhyolitic flows and felsic hyaloclastic rocks, structurally below the nonsulfide orebody. The primary sulfide minerals comprise pyrite, pyrrhotite, sphalerite, minor chalcopyrite, and galena as well as secondary chalcocite and colloidal sphalerite (brunckite). The volcanic-hosted sulfide ore minerals are deformed and partly recrystallized and thus of premetamorphic and predeformation origin. In contrast, the Skorpion nonsulfide zinc deposit is of postmetamorphic origin, as shown by two hydrous, low-P, low-T ore assemblages. The secondary assemblages occur, at least partly, as delicate euhedral and subhedral crystals in secondary pore space, open fractures, veins, and breccias. The nonsulfide ore contains pseudomorphs from in situ oxidation and replacement of preexisting sulfides and ore and also from the remobilization or precipitation of base metals. The Skorpion nonsulfide zinc deposit is of supergene origin and formed from the oxidation of sulfide protore by circulating meteoric water, as suggested by textural evidence and by initial stable isotope data. The timing of ore formation is still undefined but lies within the large age bracket between 545 Ma (peak metamorphism) and subsequent partial exhumation and the deposition of the nonmineralized overburden of Namib Desert gravels (<21 Ma) and dune sands.

Magnetotelluric soundings across the Taubaté Basin, Southeast Brazil

Thirteen magnetotelluric (MT) soundings were carried out in Neoproterozoic crystalline terrains and Tertiary sediments of the Taubate basin, southeastern Brazil. The soundings were deployed in a cross-strike profile bisecting the basin along one of its thickest sub-basins and extending over mountain plateaus to the southeast and the northwest. Occurrences of numerous alkaline plugs and aligned tectonic grabens in the region are records of intense Mesozoic-Cenozoic tectonic activity. MT analysis techniques were used to evaluate data quality, infer strike direction, and correct for near-surface distortion. As the studied area is located only some tens of kilometers away from the Atlantic Ocean and in one of the most densely populated regions of the country, the data are severely distorted by industrial interference and the coast effect. Because of such effects, the data are modelled using a 2D inversion scheme within periods shorter than 0.1 s for the Taubate Basin sites and shorter than 1–10 s for the off-basin sites, the latter depending on the distance of the site from the coast. The main result observed in the modelling is the identification of a conducting zone below 10 km depths beneath the region. There is poor resolution in the data of structures below this conductor, which is also not imaged beneath the basin. Studies carried out in different tectonic regions of the world have also reported conductive layers at about the same depth but in the studied area it is impossible to reach any conclusion about the total conductance of the layer with the available MT data.

Study of weathering processes developed on old piedmont surfaces in Western Spain: new contributions to the interpretation of the “Raña” profiles

The Hercynian basement of the Iberian Peninsula was uplifted by the Alpine orogeny during the Tertiary. It gave rise to a set of block mountains and tectonic grabens, one of which is the Ciudad Rodrigo Basin. It is located in Western Spain and forms a westward extension of the great Tertiary Duero Basin. The sediments filling this graben are of continental origin, their ages ranging from Palaeogene to Quaternary. Morphologically, the southern part of this basin forms a set of piedmont surfaces (the “Raña” surfaces) appearing above the terrace system of the present rivers. This paper examines the weathering processes developed over these old piedmont surfaces using micromorphological, XR diffraction, scanning electron microscopy (SEM) and microporosity techniques. The Raña sediments are rich in quartzite pebbles and gravel within a clayey matrix. Once deposited, these materials underwent important in situ weathering processes under somewhat hydromorphic conditions, of which hydrolysis, ferrolysis and xerolysis were the most important. All these processes gave rise to: (1) transformation of most of the clasts of shists and slates into a matrix causing the destruction of the original sedimentary structures and a relative concentration of the resistant lithologies (quartzites and quartz); (2) important changes in the clay fraction, leading to a predominance of kaolinite in the upper levels of the profiles, (3) release of elements from primary minerals, Fe being one of the most important, and (4) redistribution of matter, mainly clay and Fe oxyhydroxides, within the profiles. The coexistence of seasonal periods with pF higher than 4.2 repeated over a long time, together with poor internal and external drainage conditions, are the cause of the special features displayed by Raña deposits.

Neotectonic structure of the Lakonikos gulf

Detailed single-channel seismic reflection survey has been carried out in the Lakonikos Gulf, Southern Péloponnèse, aiming to a better understanding of the neotectonic structure of the Lakonikos Basin. Our survey showed that, contrary to the model of a simple N-S asymmetric graben previously considered, a tectonic horst occurs within the tectonic graben of Lakonikos. A subsidence of more than 1000m is produced by the N-S marginal faults, whereas, the N-S faults creating the central horst structure within the Lakonikos graben are high-angle reverse faults, which have uplifted the sea bottom together with the Pleistocene and Holocene sediments by about 100m. Several E-W vertical transcurrent faults, with strike-slip motion deform the N-S structures. The central tectonic horst structure is very recent as the observed deformation of the Upper Pleistocene and Holocene sediments indicated and a transtensional geodynamic regime is suggested.

Plio-Quaternary Movement of the East Arabian Block

ABSTRACTNew data presented herein support the previously suggested existence of an East Arabian Tectonic Block in Saudi Arabia. However, the East Arabian Tectonic Block is not exclusively bound by grabens, as was previously thought, but also by strike-slip faults and transpressive folds (in what is named here the Mughrah-Al-Kharj Transpression Zone), which laterally grade into the transtensive Central Arabian Graben System. Evidence for Plio-Quaternary strike-slip motion is presented here for the first time and supports previous contentions that graben formation occurred in Plio-Quaternary time. These new observations allow the development of a tectonic model that succinctly explains the deformation along the margins of the East Arabian Block.With the importance of regional fault structures thus recognized, even some major faults of tectonic dimensions start to emerge in the Eastern Province of Saudi Arabia. For example, radar images from the Space Shuttle Endeavour confirm that the Nisah Fault extends eastward along Wadi As-Sahba, over a total distance of 250 kilometers, which was previously undemonstrable. The Sahba Fault is made visible beneath the Ad-Dahna Sand Sea by virtue of the sand-penetrating capability of the L-band radar. Five to eight kilometers of left-lateral strike separation is visible on either side of Wadi As-Sahba which truncates the southern end of the Ghawar field. The graben structure of the Nisah Fault System had been reported before, but its eastward continuation and any strike separation were not established until now. Furthermore, the Nisah-Sahba Fault System may extend further eastward, beneath alluvial gravels, for an additional 200 kilometers to join a known lineament at the southern end of Qatar. Further extension towards the Gulf into the 600-kilometer long Trans-Arabian Gulf Fault is also suggested here for the first time.Field evidence presented here indicates that a substantial part of the motion of the Central Arabian Graben System took place in the Plio-Quaternary and may continue at present. This has been inferred from a study of the wadi (flood valley) system on the Jubaila and Hanifa outcrops in the vicinity of the graben structures to the south of Riyadh. In fact, an 8-kilometer long section of a mature stream channel, known as Sha’ib al-Miklaf, which was formerly discharging intermittent wadi drift and torrential run-off waters into Wadi Hanifa, has now been cut off from its former catchment area by the graben tectonics. As a result Sha’ib al-Miklaf now lies idle between the active wadis of the Awsat Graben and the Nisah Graben. The occurrence of recent fault motion in the Arabian Platform is further supported by changes in thickness of the Ad-Dahna Sand Sea across the Sahba Fault, as well as by truncation of mega-barchanoids in the Jafurah Sand Sea across the Jaub lineament. Not surprisingly, some modern seismicity in the Eastern Province of Saudi Arabia may be attributed to motion on the Nisah-Sahba Fault System.

Estimates of stress directions by inversion of earthquake fault-plane solutions in sicily

SUMMARY The tectonic stress orientation is estimated in the lithosphere of northern Sicily, the southern Tyrrhenian sea and southern Calabria, and in the Wadati-Benioff zone below the Tyrrhenian, by inversion of fault-plane solutions of earthquakes covering a magnitude range from 2.5 to 7.1. Focal mechanisms of 97 earthquakes are taken from the literature, after a critical evaluation of their data quality. An average misfit of F= 13 indicates that the set of all shallow (<50 km) earthquakes is generated by a heterogeneous stress field. For three subsets, based on regional and magnitude separation, F was small enough (2.8lFs5.9) to support the assumption of a homogeneous stress direction; for an additional subset, with F= 7.4, such a condition is close to being fulfilled even though some heterogeneity appears to be present. The number of earthquakes in these subsets ranged from nine to 22, and the uncertainties of the principal stress directions were generally of the order of 20 at the 90 per cent confidence level. The earthquakes with A425 define a regional stress field with the greatest principal stress, ul, dipping at a shallow angle to the south. In north-eastern Sicily and south-western Calabria the stress field estimated by earthquakes is extensional, with o3 in a direction of WNW, and a near-vertical ol, in agreement with the graben tectonics mapped geologically in this area. In western Sicily the c1 direction is oriented WNW, but this result is judged less reliable than the others, based on the broader confidence limits of the solution and the average misfit of 7.4. The earthquakes in the Wadati-Benioff zone define ol dipping at about 70 to the NW, subparallel to the zone, with o2 horizontal and striking parallel to the zone.

Sequence Stratigraphy and Graben Tectonics: Fennoscandian Border Zone and Central Graben, NW Europe: ABSTRACT

Sequence Stratigraphy and Graben Tectonics: Fennoscandian Border Zone and Central Graben, NW Europe: ABSTRACT

Climatic control on quaternary fluvial sedimentation in the kleszczów graben, central Poland

The Pleistocene sequence of the Kleszczów Graben, central Poland, comprises seven fluvial series, ranging in age from Pre-Tiglian to Holocene. These sequences are investigated in terms of graben tectonics that existed up to the Middle Pleistocene and climate change ranging from periglacial to cool temperate. Warm temperate (interglacial) fluvial deposits are virtually absent. There is no evidence for tectonic control of fluvial sedimentation, although this is the factor that has controlled sediment preservation, deformation and erosion. In contrast, sedimentation has been influenced greatly by climatic conditions and climatic change, mainly as a response to changes in vegetation cover. Similar responses in the form of typical sedimentary successions are observed in fluvial series of different ages. This paper outlines the sequences of erosion and sedimentation and discusses their relation to the regional history of climatic changes, the geomorphic positions of river valleys and the positions of advancing ice-sheets.

Evolution of backarc rifting: Mariana Trough, 20°–24°N

The Mariana Trough is an actively opening backarc basin in the western Pacific. The trough formed by extension which longitudinally split an earlier arc massif creating a crescent‐shaped basin between the remnant West Mariana Ridge and eastern active volcanoes of the Mariana Arc. Opening increases southward from the Volcano Islands near 24°N where the two arcs join. At 18°N in the central Mariana Trough the basin is widest and may be opening primarily by seafloor spreading. We present a synthesis of closely spaced shipboard gravity, magnetic, and bathymetry measurements from the northern basin (20°–24°N), an area that undergoes a significant progressive southward increase in extension. We have identified three stages in the evolution of rifting in this area: (1) asymmetric rifting from 24°N to 22°15′N where faulting and magmatism have migrated laterally to remain near the active volcanic arc side of the basin, (2) a “localization” of rifting from 22°15′N to ∼21°N where the primary zone of rifting separates from the active volcanic arc, and (3) a further concentration of rifting from 21°N to 20°N leading to the formation of deep tectonic grabens near the center of the basin. This last stage may be a precursor to (or incipient) seafloor spreading. We describe a new type of magnetic lineation resolved in a three‐dimensional seafloor magnetization inversion of the trough between 20° and 24°N. The detailed character of these lineations, their association with tectonic structures, and other geophysical observations indicate that they are not seafloor spreading lineations but rather result from magnetic intrusions and volcanism emplaced within preexisting, less magnetic rifted arc crust. The development of these well‐defined magnetization bands indicates that the zone of magmatism remains relatively narrow at any one time throughout the opening of the trough, although tectonic deformation appears to be more widely distributed. In addition, the geometry of the magnetization bands with respect to the remnant arc border faults indicates that rifting propagated rapidly northward.

Structure and Facies Development of the Dutch/North German Rotliegende Basin

The apparent east-west extension of the southern Rotliegende basin, stretching from southern England via the Netherlands and north Germany to Poland, developed from several subbasins running in a northwest-southeast direction. The orientation of the subbasins and the graben systems have largely been caused by a regional stress field, which existed in the Late Paleozic of northern central Europe. The maximum extension was in an east-west direction. The graben systems of northern Germany and the southern part of the North Sea are running roughly north-south and are connected via a parallel set of wrench faults. The subbasin with the largest Rotliegende thickness lies in the German part of the North Sea. It subsided in the region where the rift axis of the north-south-running north German graben system experienced left lateral displacments by northwest-southeast-running wrench faults. The active graben zone extended into the Horn-Bamle-Oslo graben. The initial Dutch subbasin was connected with the early central graben and merged with the north German subbasin in the course of the progressive sedimentation of the basin. In contrast to the north German subbasin, where the initial sedimentation was mainly determined by the north-south-directed graben tectonics, intensive northwest-southeast-directed step faults developed in the Dutch subbasin. Themore » initial subbasins were arranged in an en echelon pattern and merged during the main subsidence of the basin. The origin of the subbasins is linked to the Stephanian basins. Their development continued while several climate changes occurred up to the early Mesozoic. The development of the intracontinental sedimentation from the small initial subbasin to the widespread southern Rotliegende basin can therefore be divided into three main stages: initial stage-tectonics more effective than climate cycles, main stage-equal effect of tectonics and climate cycles, and late stage-climate cycles more effective than tectonics.« less

Hydrothermal vent distribution along the East Pacific Rise crest (9°09′–54′N) and its relationship to magmatic and tectonic processes on fast-spreading mid-ocean ridges

Using the near-bottom ARGO imaging system, we visually and acoustically surveyed the narrow ( < 200 m wide) axial zone of the fast-spreading East Pacific Rise (EPR) along 83 km of its length (9°09′–54′N), discovered the Venture Hydrothermal Fields, and systematically mapped the distribution of hundreds of hydrothermal features relative to other fine-scale volcanic and tectonic features of the ridge crest. The survey encompasses most of a 2nd order ridge segment and includes at least ten 4th order (5–15 km) segments defined by bends or small lateral offsets of the ridge crest or axis (Devals). 4th order segmentation of the ridge crest is clearly expressed in the high-resolution ARGO data by the fine-scale behavior of the ridge axis and by changes in the characteristics of the axial zone (axial lava age, extent of fissuring, axial morphology and structure, etc.) across segment boundaries. The distribution and along-strike variability of hydrothermal features corresponds closely to the morphotectonic/structural segmentation of the ridge. On the 2nd order scale, we find that high T hydrothermal activity correlates with: (1) shallowing of the axial magma chamber (AMC) reflector to depths < 1.7 km beneath the ridge axis; and, (2) with the presence of a well-developed axial summit caldera (ASC). Previous work refers to this feature as an axial summit graben (ASG); however, the extent of volcanic collapse along the ASG revealed by the ARGO survey adds to evidence that on fast-spreading ridges it is an elongate volcanic caldera rather than a tectonic graben, and supports the introduction of “axial summit caldera” as a more accurate descriptor. All but 1 of the 45 active high T vent features identified with ARGO are located within 20 m of the margins of the ASC. Despite the significant extent of our track coverage outside the ASC, no important signs of venting were seen beyond the axial zone. On the 4th order scale, the abundance and distribution of hydrothermal features changes across 4th order segment boundaries. We find that high T vents are most abundant where: (1) the ASC is very narrow (40–70 m), (2) the AMC reflector is most shallow ( < 1.55 km beneath the axial zone), and (3) the axial lavas are youngest and least fissured. To explain the observed distribution of vent activity, a two-layer model of ridge crest hydrothermal flow is proposed in which 3-D circulation at lower T in the volcanic section is superimposed on top of axis-parallel high T circulation through the sheeted dike complex. In the model, circulation parallel to the ridge axis is segmented at the 4th order scale by variations in thermal structure and crustal permeability which are directly associated with the spacing of recent dike intrusions along strike and with cracking down into the sheeted dikes, especially along the margins of the ASC. Based on ratios between numbers of active high T vents and inactive sulfide deposits along particular 4th order segments, and on corresponding volcanic and tectonic characteristics of these segments, we suggest that the individual 4th order segments are in different phases of a volcanic-hydrothermal-tectonic cycle that begins with fissure eruptions, soon followed by magma drainback/drainage and accompanying gravitational collapse, possible development of an ASC, and onset of hydrothermal activity. The hydrothermal activity may wax and continue for up to several hundred years where an ASC is present. The latest phase in the cycle is extensive tectonic fissuring, widening of the ASC by mass wasting along its margins, and waning of hydrothermal activity. In the ARGO area, where full spreading rates are 11 cm/yr, the entire cycle takes less than ∼ 1000 years, and the tectonic phase does not develop where the time interval between eruptions is significantly less than 1000 years.

Geological framework of the Ebro continental margin and surrounding areas

The eastern and southeastern margins of Iberia were affected during Neogene time by a rifting tectonic process superimposed on the Alpine structures. The Valencia Trough, situated off the northeastern coast of Iberia, has been defined as a rift system that began its activity in late Oligocene-Early Miocene time. The opening of the Valencia Trough produced a series of tectonic grabens created by fractures which strike parallel to the coastline. As a consequence of the rifting, subsidence occurred and depositional sequences of Miocene age were subsequently deposited over the Triassic and Mesozoic basement. Some of the Mesozoic faults were reactivated and affected the Tertiary deposits. The depositional history was controlled mainly by extensional faulting during the late Paleocene-early Miocene, and by rapid subsidence. During the late Pliocene a second extensional period occurred in the Iberian Chain resulting in the present configuration of the continental margin in the study area. The Pliocene-Quaternary evolution of the margin was largely controlled by fluctuation of the Ebro River sediment supply and sea-level oscillations.

Surface faulting caused by the Kalamata (Greece) earthquakes (13.9.86)

A great number of neotectonic faults traverse the Messinian gulf and its geological prolongation on the continent towards the north, which forms the Kalamata-Kyparissia tectonic graben. On the 13th September 1986, the whole Messinia area was shaken by an earthquake ( M = 6.2); it caused much damage, mainly in the city of Kalamata. Catastrophes were limited to a relatively small area, in the form of a triangle. The striken area is a second-order tectonic graben (called the Dimiova-Perivolakia graben) which is developed on the eastern margin of the greater Kalamata-Kyparissia tectonic graben. Alpine and post-Alpine (Neogene and Quaternary) formations outcrop in the graben. The recent earthquakes caused small fractures (4–5 m long) and small-scale reactivation of neotectonic faults. Both have been observed in almost all formations. The seismic fractures form a zone or zones. They consist of small parallel fractures arranged en échelon, which allows us to conclude that in some places the fracture zones are of right-lateral and in others of left-lateral character. This, in fact, coincides with the kinematics of the main neotectonic fault-zones of the graben. This, in conjunction with other morphotectonic observations, suggests that the deformation of the area is connected with coupling or even torsion phenomena.

Geodetically controlled taphrogenesis in back-arc environments: three examples from Central and Northern Greece

Repeated leveling data collected in the vicinity of three tectonic grabens in the Hellenic back-arc area reveal a tendency to continuation of taphrogenesis processes that started in the Quaternary or earlier. These processes are associated with earthquakes of short or long recurrence intervals: Corinthiakos—tens of years, Atalandi—hundreds of years. Although in most cases the geodetic displacement signal is small, I suggest that it reflects real motions, because first, the measurement specifications are very strict, and second, geologic and other data confirm geodetically derived displacements.

Geological and geophysical investigations at Sierra del Medio massif, Argentina

Geological investigations were performed at Sierra del Medio (Cushamen County, Chubut Province in Central Patagonia), a mountainous massif of about 25 km by 8 km of migmatic origin, which emerges from a depressed tectonic trench or graben called Pampa de Gastre. The most ancient rocks belong to biotitic and amphibolic schist that passed almost entirely to tonalitoid migmatites with a second process producing granitic rocks. Boreholes were drilled on the basis of conclusions from Landsat satellite imagery and aerial photographic sets, followed by field work on geological, petrographic, geophysical and hydrogeological features at surface, structural interpretation supported by geostatistical computations.