Graben System Research Articles

Structure, paleogeographic inheritance, and deformation history of the southern Atlas foreland fold and thrust belt of Tunisia

Structural analysis of the southern Tunisian Atlas was carried out using field observation, seismic interpretation, and cross section balancing. It shows a mix of thick‐skinned and thin‐skinned tectonics with lateral variations in regional structural geometry and amounts of shortening controlled by NW‐SE oblique ramps and tear faults. It confirms the role of the Late Triassic–Early Jurassic rifting inheritance in the structuring of the active foreland fold and thrust belt of the southern Tunisian Atlas, in particular in the development of NW‐SE oblique structures such as the Gafsa fault. The Late Triassic–Early Jurassic structural pattern is characterized by a family of first‐order NW‐SE trending normal faults dipping to the east and by second‐order E‐W trending normal faults limiting a complex system of grabens and horsts. These faults have been inverted during two contractional tectonic events. The first event occurred between the middle Turonian and the late Maastrichtian and can be correlated with the onset of the convergence between Africa and Eurasia. The second event corresponding to the principal shortening tectonic event in the southern Atlas started in the Serravalian‐Tortonian and is still active. During the Neogene, the southern Atlas foreland fold and thrust belt propagated on the evaporitic décollement level infilling the Late Triassic–Early Jurassic rift. The major Eocene “Atlas event,” described in hinterland domains and in eastern Tunisia, did not deform significantly the southern Tunisian Atlas, which corresponded in this period to a backbulge broad depozone.

Noble gas isotopes and the chemical composition of geothermal gases from the eastern part of the Büyük Menderes Graben (Turkey)

We present noble gas isotope and gas composition data from eight production wells of the Kizildere geothermal field and from six gas discharging pools from the adjacent Tekke Hamam geothermal field, situated in the eastern segment of the Büyük Menderes Graben system in western Anatolia (Turkey). Gas from both locations is composed mainly of CO 2 with minor contributions of N 2, H 2S, CH 4, O 2, Ar, H 2, C 2+ and He, with higher contents of H 2S, He and C 2+ in Tekke Hamam samples. Air corrected helium isotope ratios reveal a mantle origin for samples from both locations, but 3He/ 4He ratios are lower and more variable at Kizildere (0.96–2.06 R a) than at Tekke Hamam (2.36–2.91 R a, R a is the atmospheric 3He/ 4He ratio of 1.39 × 10 − 6 ). Kizildere gas contains less 4He (0.7–2.2 ppmv) and 3He (~ 2–5 × 10 − 6 ppmv) than gas from Tekke Hamam (2.5–3.5 ppmv and ~ 1 × 10 − 5 ppmv, respectively). CO 2/ 3He ratios of 2–8 × 10 11 at Kizildere and 0.7–1 × 10 11 at Tekke Hamam are higher than average mantle values (~ 2 × 10 9). Within the Kizildere geothermal field, reservoir temperatures and CO 2/ 3He and CH 4/C 2H 6 ratios increase and 3He/ 4He ratios decrease from southwest to northeast. Interpolation of a linear trajectory between 3He/ 4He and temperature from Kizildere delivers temperature of 173 °C–184 °C for the source reservoir of Tekke Hamam, in good agreement with measured reservoir temperatures from well TH-2 at Tekke Hamam (171 °C). The low helium content, the variable helium isotope composition and the higher CO 2/ 3He ratios at Kizildere can be explained by temperature-dependent partial removal of mantle-derived helium, e.g. through vapor separation, followed by addition of radiogenic helium. Kizildere samples derive from the remaining water phase after gas separation, while Tekke Hamam samples may represent the gas phase liberated by the former process. The pristine 3He/ 4He ratio of helium at Kizildere can be calculated as 2.2 R a, close to helium isotope ratios observed at Tekke Hamam. Partial removal of helium may also account for the higher and T-dependent CO 2/ 3He ratios at Kizildere, compared to Tekke Hamam, but such process alone cannot explain the overall high CO 2/ 3He ratios (> 7 × 10 10) and calls for contribution of CO 2 from interaction between geothermal fluids and carbonate reservoir rock. Noble gas abundances indicate mixing with cold meteoric water in the reservoir.

Cover sequences at the northern margin of the Antongil Craton, NE Madagascar

The island of Madagascar is a collage of Precambrian, generally high-grade metamorphic basement domains, that are locally overlain by unmetamorphosed sedimentary rocks and poorly understood low-grade metasediments. In the Antalaha area of NE Madagascar, two distinct cover sequences rest on high-grade metamorphic and igneous basement rocks of the Archaean Antongil craton and the Neoproterozoic Bemarivo belt. The older of these two cover sequences, the Andrarona Group, consists of low-grade metasedimentary rocks. The younger sequence, the newly defined Ampohafana Formation, consists of unmetamorphosed sedimentary rocks. The Andrarona Group rests on Neoarchaean granites and monzogranites of the Antongil craton and consists of a basal metagreywacke, thick quartzites and an upper sequence of sericite-chlorite meta-mudstones, meta-sandstones and a volcaniclastic meta-sandstone. The depositional age of the volcaniclastic meta-sandstone is constrained in age by U–Pb laser-ablation ICP-MS analyses of euhedral zircons to 1875 ± 8 Ma (2 σ). Detrital zircons of Archaean and Palaeoproterozoic age represent an input from the Antongil craton and a newly defined Palaeoproterozoic igneous unit, the Masindray tonalite, which underlies the Andrarona Group, and yielded a U–Pb zircon age of 2355 ± 11 Ma (2 σ), thus constraining the maximum age of deposition of the basal part of the Andrarona Group. The Andrarona Group shows a low-grade metamorphic overprint in the area near Antalaha; illite crystallinity values scatter around 0.17°Δ2Θ CuKα, which is within the epizone. The Ampohafana Formation consists of undeformed, polymict conglomerate, cross-bedded sandstone, and red mudstone. An illite crystallinity value of >0.25°Δ2Θ CuKα obtained from the rocks is typical of the diagenetic zone. Occurrences of rhyodacite pebbles in the Ampohafana Formation and the intrusion of a basaltic dyke suggest a deposition in a WSW-ENE-trending graben system during the opening of the Indian Ocean in the Upper Cretaceous, that was characterized by extensive rhyolitic to basaltic magmatism along Madagascar's eastern coast.

Petrology, sedimentology, geochemistry, and absolute age of Triassic volcanosedimentary rocks from the southwest of the West Siberian geosyneclise ( Kurgan Region)

In this study we report new Ar-Ar ages for basalt (261.0 ± 15, 249.4 ± 3.0, 247.0 ± 3.1 Ma) and rhyolite (246.9 ± 2.6 Ma) samples collected from the Triassic volcanosedimentary sequence, which is one of the remarkably complete and structurally complex sections in the southwest of the West Siberian geosyneclise (WSG) (East Kurganskaya-42 (EK-42) and East Kurganskaya-43 (EK-43) wells), largely comprising basalts and conformable layers of rhyolites, acid tuffs, limestones, and silty mudstones. It was shown that basalts, which constitute most of the section, are geochemically similar to medium-K tholeiitic and calc-alkaline basalts in the southern extension of the Koltogory–Urengoi rift (Nikol’skaya-1 well). Rhyodacites and rhyolites from the section of interest belong to the potassic to potassic-sodic series of normal alkalinity. The trace element compositions of the rhyolites and host basalts suggest fractionation from primary magmas during formation of felsic rocks. No low-K sheet-flow basalts have been encountered in the sections penetrated by the three wells (EK-42, EK-43, and Nikol’skaya-1), as opposed to sections in the northern part of the Koltogory–Urengoi rift (TSD-6 well) and the Siberian flood basalt province. Despite their small thickness, the sedimentary units provide evidence for different depositional settings during the evolution of a volcanosedimentary basin. Similar to the Koltogory–Urengoi rift, deposition of the Presnogor’kovsk graben fill indicates intermittent marine conditions with accumulation of hyaloclastites, and fine, calcareous, silt- and clay-rich carbonaceous oozes associated with a background sedimentation, which contain abundant marine fauna, pyrite and rare distal turbidites with glauconites. The geochemical features of these sedimentary rocks were largely defined by a bimodal syndepositional volcanism. During the Early Triassic, a system of rifts and grabens that was developed within the continental plateau over much of the WSG allowed the intermittent ingress of sea-water from the north (in present coordinates) far southwards and southwestwards up to the latitude of Omsk and Kurgan. As a result of extensive erosion of the continental plateau, the packages of low-K sheet-flow basalts were removed from the upper part of the sections of deep wells, which became dominated by the eroded roots of the West Siberian traps (subalkaline and alkaline rift basalts).

The Calabrian Stage, Pleistocene highstand in Malta: a new marker for unravelling the Late Neogene and Quaternary history of the islands

Abstract: The Late Neogene and Quaternary sedimentary record within the Maltese islands is very fragmentary and has generally been ignored by sedimentologists. Local stratigraphies for parts of the Quaternary have been erected from vertebrate remains contained in Quaternary cave and fissure deposits of the islands. However, no coherent correlation scheme has been proposed in which to encompass them. The situation is exacerbated by the absence of datable fossils within the majority of Quaternary deposits. Considerable progress in resolving these problems is, however, possible if recourse is made to the post-Messinian (Late Miocene) erosion features evidenced in the topography of the islands. In particular, a hitherto unrecognized marine erosion surface, the San Leonardo Marine Abrasion Surface, is described and dated by reference to the marine deposits (San Leonardo Beds) lying directly above it and similarities to contemporaneous SE Sicily marine deposits. This provides an end-Calabrian Stage (Emilian Sub-Stage) base line upon which to hang fragmentary information on later events that have shaped the topography of the Maltese islands. From this it can be confirmed that a tectonic deformation event spanning latest Messinian to mid- Pliocene times uplifted the islands and reactivated previously incipient graben systems. This deformation episode is shown to have ceased within the islands well before the early Calabrian (before c . 1.6 Ma). Subsequent minor regional uplift and eustacy together with subaerial weathering processes have been the principal agents in shaping the islands. Collectively, these processes have prevented terrestrial animal migrations into the Maltese islands from North Africa since Messinian times. However, the infrequent re-establishment of a marine lowstand isthmus virtually linking Malta with SE Sicily has permitted opportunistic animal migrations into the islands and the presence of these greatly aids in dating the sedimentological events.

Tectonic and stratigraphic evolution of the Cambrian basin of northern Northwest Territories

Abstract The Cambrian basin of the mainland Northwest Territories has been of considerable economic interest since gas was first discovered in Mount Clark Formation sandstones in 1974. A recently completed assessment by the Geological Survey of Canada has estimated that the Cambrian Clastics play contains almost a billion barrels of oil and 10.7 TCF of gas. The play’s reservoirs, seals and source rocks are all of Cambrian age, making the understanding of the basin’s evolution and contents critical to any exploration effort. The basin is a semi-enclosed, epicontinental marine basin, bordered by positive elements and open to a subsiding continental shelf and margin, and represents the eastern portion of a much larger deposystem that developed after the breakup of Laurentia. A multi-disciplinary study of a large regional reflection seismic grid, supported by borehole and potential field data, shows the basin to have evolved in three stages: 1) Early Cambrian pre-rift regional subsidence of the area lying inland of the Mahony and Bulmer arches, and a newly identified high that pre-dated the Mackenzie Trough. It was during this period that the Mount Clark Formation was deposited; 2) A middle Cambrian rift phase during which a graben system, about 50 km wide and over 400 km long, developed in the basin’s core. The grabens are deepest in south Mackenzie Trough where rifting began, and are progressively shallower and younger to the north and northeast. Mount Cap was deposited during this phase and it is expected that the grabens received finer-grained clastics than did the shallower areas on their flanks; 3) A post-rift subsidence phase that began with deposition of the Saline River Formation during the Late Cambrian and continued, with interruptions, until the middle of the Devonian. Throughout Cambrian time, sediment onlapped the basin’s flanks and its internal arches, each layer advancing further than its predecessor. It was not until the latest Cambrian that the internal arches were completely covered by beds of the lower Franklin Mountain Formation. A previously undocumented Cambrian age graben (herein named McConnell Graben) lies in the hanging wall of the unnamed Laramide thrust fault that emerges from the east flank of the McConnell Range. The fault began as a Cambrian normal fault that separated the Bulmer and Mahony arches on the east from McConnell Graben on the west. The present day ‘St. Charles’ range developed in a similar manner. A preliminary exploration risk analysis, based on the predicted distribution of reservoir, seal and source rocks, reveals two attractive areas for exploration. The larger one, centred on Lac des Bois, is about 150 km wide and extends on a NW-SE trend for about 300 km across the Aubry and Good Hope depocentres. The smaller area (less than 50 × 100 km in extent) lies within Mackenzie Plain depocentre.

Crustal structure beneath Beijing and its surrounding regions derived from gravity data

In this paper we use gravity data to study fine crustal structure and seismogenic environment beneath Beijing and its surrounding regions. Multi-scale wavelet analysis method is applied to separating gravity fields. Logarithmic power spectrum method is also used to calculate depth of gravity field source. The results show that the crustal structure is very complicated beneath Beijing and its surrounding areas. The crustal density exhibits laterally inhomogeneous. There are three large scale tectonic zones in North China, i.e., WNW-striking Zhangjiakou-Bohai tectonic zone (ZBTZ), NE-striking Taihang piedmont tectonic zone (TPTZ) and Cangxian tectonic zone (CTZ). ZBTZ and TPTZ intersect with each other beneath Beijing area and both of them cut through the lithosphere. The upper and middle crusts consist of many small-scale faults, uplifts and depressions. In the lower crust, these small-scale tectonic units disappear gradually, and they are replaced by large-scale tectonic units. In surrounding regions of Beijing, ZBTZ intersects with several other NE-striking tectonic units, such as Cangxian uplift, Jizhong depression and Shanxi Graben System (SGS). In west of Taihangshan uplift, gravity anomalies in upper and middle crusts are correlated with geological and topographic features on the surface. Compared with the crust, the structure is comparatively simple in uppermost mantle. Earthquakes mainly occurred in upper and middle crusts, especially in transitional regions between high gravity anomaly and low gravity anomaly. Occurrence of large earthquakes may be related to the upwelling of upper mantle and asthenosphere heat flow materials, such as Sanhe earthquake (MS8.0) and Tangshan earthquake (MS7.8).

Source, characterictics and some geological features of the thermal waters of Hidirbeyli Region, Aydin, Turkey

The metamorphic units belonging to Menderes Massif occur in the area as basement rocks. These rocks are predominantly formed during Paleozoic. At the top of these rock units, there are some Miocene and Paleocene aged sedimentary rocks, which are covered by Quaternary aged alluviums. The Menderes block (often described as Menderes Massif was elevated due to compressional tectonic forces from south and north side of the block. On the other hand, there have been some active en echelon faults on both sides of the massif. These fault system gave rise to the occurrence of several horst and graben system in the area. The HA±dA±rbeyli geothermal area is found at the western side of Menderes graben, between Bozkoy and Omerbeyli thermal areas. All these thermal spots were developed on the East-west oriented fault belt. At the Germencik-Omerbeyli geothermal area, there have been 9 research-drilling holes made by the MTA (Mineral Research and Exploration General Directorate of Turkey) and these resulted in high temperature and discharge water. As a result of these findings, the investigations were concentrated to the HA±dA±rbeyli area, in where two drilling holes (HB-1 and HB-2) were made to find geothermal steam and waters (if any) in the area. The drillings revealed that there has been some steam plus water. The produced steam from these wells are colorless, odorless and some cloudy. The steam and waters of HB-1 have 5197.47 mg/l minerals, and it has been classified as boric-silisic sodium-bicarbonate waters. On the other hand, the steam and waters of HB-2 can be classified as boric-silisic-cloric sodium-bicarbonate waters with 4745.4 mg/l mineral components. HB-1 and HB-2 are the first deep-wells in the region and the steam and waters of these wells can be used for electricity generation and heating of nearby settlements with 142oC and 120.5oC average water temperatures; the sum of discharge of these two wells is 274 l/s. Key words: Menderes graben, source of thermal water, hydro-geochemistry, HA±dA±rbeyli, Turkey.

The Iapetan rifted margin of southern Laurentia

The Iapetan rifted margin of southern Laurentia includes the northeast-striking Blue Ridge, Ouachita, and Marathon rifts, which are offset by the northwest-striking Alabama-Oklahoma and Texas transform faults, framing the Alabama and Texas promontories and the Ouachita and Marathon embayments of the continental margin. Interpretations of the original trace, structural style, and age of the rifted margin rest on identification of synrift rocks and structures, as well as continental-shelf and off-shelf sedimentary deposits on the passive margin. Both late Paleozoic Ouachita-Appalachian allochthons and post-orogenic Atlantic-Gulf passive-margin deposits cover the Iapetan rift margin, necessitating the use of data from deep wells and geophysical surveys along with geologic maps of the exposed Ouachita-Appalachian thrust belts to characterize the synrift and post-rift rocks and structures. The continental margin and passive-margin shelf strata are primarily in the footwall of the Ouachita allochthon; however, some Ouachita thrust faults displaced shelf-margin basement and cover. Appalachian thrust faults imbricate synrift fill of the intracratonic Birmingham graben and the passive-margin shelf. Palinspastic restoration of thrust-belt structures uses balanced cross sections to locate the original trace of the Iapetan margin. Thickness and subsidence history of the passive-margin successions, as well as a general lack of preserved synrift deposits, indicate an upper-plate structure along the Blue Ridge rift on the Alabama promontory and along the Ouachita rift on the Texas promontory. The upper plate on the Texas promontory is conjugate to a lower-plate rift structure on the Argentine Precordillera. Although data are limited, the evolution of the passive margin along the Marathon rift in the Marathon embayment suggests a lower-plate structure. Geophysical modeling supports a steep continental margin along the Alabama-Oklahoma transform, and a similar structure can be inferred for the Texas transform. The Blue Ridge rift north of the Alabama promontory is dated by synrift volcanic rocks as young as 564 Ma, and passive-margin transgression beginning in earliest Cambrian is documented along the Alabama promontory and farther north. The age of the Ouachita rift is documented by the 530–539 Ma synrift volcanics of the transform-parallel intracratonic Southern Oklahoma fault system, by Early Cambrian synrift sediment along the conjugate rift margin in the Argentine Precordillera, and by late synrift graben-fill of Early to early Late Cambrian age in the rift-parallel intracratonic Mississippi Valley and Birmingham graben systems, as well as by subsidence history of the passive margin on the Texas promontory. The diachroniety of rifting reflects an inboard shift from the Blue Ridge rift to the Ouachita rift along the Alabama-Oklahoma transform and rifting of the Argentine Precordillera from the Ouachita embayment.

Specifications of thermal waters and their classification on the base of neural network method: Examples from Simav geothermal area, Western Turkey

Western Turkey is one of the best known geothermal fields in Turkey. There are numerous geothermal energy plants (for example KA±zA±ldere, Nazilli, HA±dA±rbeyli, Balcova, Tuzla) in Aegean region, Western Turkey. Simav geothermal field is located within the Aegean Graben System in Western Anatolia. Rock units in the study area are mainly the formation of Menderes Massive. The Simav geothermal waters were divided into four types: (1) Eynal, (2) Citgol, (3) geothermal water and (4) cold water. In this study, we aimed to introduce a method for classifying waters in the study area using some parameters such as temperature, pH, electrical conductivity and major ions by means of Artificial Neural Network (ANN) method. The data at hand obtained from wells indicate that the drilled water can be used for drinking and irrigation, the data also reveals that the ground water flows towards the desiccated lake. The cold water analysis gave high CO3+HCO3, Ca, Mg ion values, and low NH4, NO3, Fe, NO2, Al and Mn ion values. On the other hand, the hot water analysis indicate that a cation trend of Na+K>Ca>Mg and an anion trend of HCO3+CO3>SO4>Cl. While preparing the training data set in ANN method, for input, T (°C), EC (mS), pH, Na (mg/l), K (mg/l), Ca (mg/l), Mg (mg/l), CO3 (mg/l), HCO3 (mg/l), Cl (mg/l) and SO4 (mg/l) values of 50 water samples from the study area were used. Four output values were used. In each output value, the known water was represented by 1 and others by 0. A test data set of 15 samples in which the T, EC, pH, Na, K, Ca, Mg, CO3, HCO3, Cl and SO4 values are known but their group are unknown was prepared. And these input values were run in ANN model in order to see how the waters were grouped. The advantages of artificial neural networks can be exploited to solve this problem. The most common ANN architecture is Multilayered Perceptrons, which was used in this study. For this solution, the first artificial neural network model using Extended Delta-Bar-Delta (EDBD) algorithm has been successfully implemented. Mean Square Error result of these model obtained by EDBD algorithm is 1.3x10–3. These results show that the group in which the waters in the study area fall can be determined with high accuracy by using some parameters of water such as the ion content of water. Key words: Geothermal waters, chemistry, hydrology, Artificial Neural Network (ANN), Simav, Turkey.

Some geological and hydrogeochemical characteristics of geothermal fields of Turkey

Presently, active hydrothermal areas of Turkey can be divided into three major provinces which correlate particular volcano-tectonic features. These fields are essentially found at western, central and eastern part of the country. There has been no particular geothermal energy fields in the southern part of Turkey since this region is mostly covered by ~2 km thick carbonate sequences and has suffered little recent rifting. The western Turkish geothermal provinces, where horst and graben systems are well developed, have the hottest fluids among others. In this region, normal faults penetrate into the crust and thus the meteoric waters are heated to greater temperatures than in the east where a compressional tectonic regime dominates. In the western Turkish geothermal fields, the fluids have a distinctly higher pH (>7) values than those in the east. This resulted that the Si content of the western Turkish hydrothermal fluids is considerably higher than the others. The geothermal field at Tuzla, situated on the Aegean shore in NW Turkey, has ionic component twice that of conventional seawater. Because of this, the area provides a good opportunity to investigate the interactions between seawater and hot dry rock. Key words: Geothermal fields, seawater, hot dry rock.

Fault tectonics, neotectonic stresses, and hidden uranium mineralization in the area adjacent to the Strel’tsovka Caldera

The scheme of recent fault tectonics and neotectonic stresses of the area adjacent to the Strel’tsovka Caldera in the southeastern Transbaikal region has been compiled for the first time on the basis of structural and geomorphic study. The faults were ranked by criteria of slip direction stability along separate segments and their expression in topography. Neotectonic stresses of corresponding ranks were ascertained as well. The heterogeneity of neotectonic stress field is related to the mosaic development of compression, extension, and the three-axial stress state. Comparison of morphogenetic features of recent and older faults shows the different character of the deformation mechanism and orientation of tectonic displacements. It has been established that the Strel’tsovka Caldera and its northwestern segment, in particular, developed as an echeloned system of pull-apart grabens, but the caldera itself is situated in a recent rise, whereas the northwestern segment is located in a neotectonic depression corresponding to the Dry Urulyungui Basin filled with volcanic and sedimentary rocks. Such a structure markedly expands the outlook for discovery of hidden uranium mineralization in the studied area. The elaborated scheme of neotectonic faults and stresses reflects the postore geodynamic setting and completes the reconstruction of geodynamic conditions pertaining to the periods of preore preparation and ore-forming tectonomagmatic reactivation.

Evolution of Jurassic–Cretaceous magmatism in the Khambin volcanotectonic complex ( western Transbaikalia)

The Khambin volcanotectonic complex is a horst framing the Late Cretaceous Lake Gusinoe basin in the northwest. This complex is due to the intracontinental rift conditions which existed in western Transbaikalia in the Late Mesozoic. They gave rise to a system of subparallel grabens and horsts in present-day topography. The magmatic evolution of this complex spans from 159 to 117 Ma and is divided into three stages. The first stage (159–156 Ma) witnessed the formation of thick (up to 1500 m) volcanic masses of trachybasalts, basaltic trachyandesites, trachytes, trachydacites, trachyrhyolites, and pantellerites. The next two stages were the formation of isolated ancient volcanoes (127–124 Ma) composed of trachybasalts, basaltic trachyandesites, phonotephrites, tephriphonolites, and alkali trachytes and the formation of the Murtoi (Lake Gusinoe) essexite dike (122–117 Ma). The main trends for igneous associations from early to late stages are reduced magmatism and reduced rock diversity because of the decreasing portion of felsic volcanic rocks. Mafic rocks show an increase in total alkalinity, the content of incompatible elements (Th, U, K, Rb, Pb, Nb, Ta, Zr, Hf), total REE content, and the LREE/HREE ratio. The Sr–Nd isotopic composition of these rocks remained nearly constant and corresponds to that of OIB-EMII mantle sources. Compositional variations are attributed to a time-dependent decrease in the degree of partial melting of a similar magma source.

Morpho-Sedimentary evidence of the Huoshan Fault’s late Cenozoic right-lateral movement in the Linfen Graben, Shanxi Graben System, North China

The Huoshan Fault, being of NEN strike, is one of the most important faults in the Shanxi Graben System of North China; it is the location of the 1303 A.D. Hongtong earthquake (MS = 8.0). The late Pleistocene and Holocene offset of some gullies that cross the fault and some fault scratches have proved its right lateral movement during the late Quaternary; however, until now, geological evidence to support the movement in the Neogene and early Quaternary was scarce. Our work provides further crucial evidence that supports both its movement in the late Cenozoic and the total right-lateral displacement since the Pliocene. The difference in the outcrop heights of the Pliocene sediment along the fault, the difference in the geomorphological development along the fault, the inconsistency in the lithological composition of the Pliocene proluvial gravels with the rock types within the catchments of the current upper stream, and the offset of the Pliocene alluvial gravels all completely indicate that the fault has always moved right-laterally since the Pliocene. Additionally, this evidence indicates that the accumulative displacement is up to 12.5 km. Based on the horizontal and vertical displacement of the fault since the Pliocene, the time-averaged horizontal slip rate of the fault is estimated to be about 3.5 mm/a, while the ratio of the horizontal to vertical offsets is about 3.8; these data are roughly close to the results that were acquired from the Holocene and the present movement of the fault. This similarity in the tectonic movement parameters may imply that the intensity as well as the configuration of the regional stress field has remained constant, and that no significant changes have taken place since the Pliocene.

Fault linkage and graben stepovers in the Canyonlands (Utah) and the North Sea Viking Graben, with implications for hydrocarbon migration and accumulation

Segmented graben systems develop stepovers that have important implications in the exploration of oil and gas in extensional tectonic basins. We have compared and modeled a representative stepover between grabens in Canyonlands, Utah, and the North Sea Viking Graben and, despite their different structural settings, found striking similarities that pertain to other graben systems. In both cases, the stepovers represent relatively high parts within the graben systems that are likely to be among the first to be filled with hydrocarbons generated in deeper parts of the grabens. Furthermore, the relay ramps and smaller fault offsets in stepovers ease hydrocarbon migration and allow stepovers to act as preferred migration routes from deep graben kitchens to structurally higher traps in the basin. Graben stepovers and their related structures should be paid special attention during exploration because they may represent hydrocarbon accumulations complementary to larger traps along the graben flanks. These observations explain the location of the Kvitebjorn, Valemon, and Huldra fields in a stepover structure of the Viking Graben and encourage increased focus on similar graben stepovers in the Viking Graben and other graben systems.

Distribution of faults and extensional strain in fractured carbonates of the North Malta Graben

Faults in carbonate rocks occur over a wide range of scales, some of which lie below the resolution of seismic reflection surveys. For this reason, our knowledge of the spatial and size distributions of subseismic faults is very limited. In this study, we address this issue by directly measuring the positions and displacements of extensional faults along a total of 56 km (35 mi) of outcrop and map-derived cross sections from the pelagic carbonates of the Maltese Islands. The data cover close to four orders of displacement magnitude and provide insight into the organization of the fault system associated with the North Malta Graben system. Virtually all the brittle extension in the Maltese Islands is accommodated by faults, with displacements ranging from centimeters to greater than 100 m (328 ft). This extension is not homogeneously distributed across the islands but is localized in a few kilometer-wide higher strain zones, which are separated by virtually undeformed lower strain regions. At the centimeter scale, both regions have an average fault density of about 101 m1 (spacing 10 m [33 ft]). In the lower strain areas, few of these faults attain displacements of greater than 10 m (33 ft) and would be below the resolution of most commercial seismic surveys. In the higher strain regions, however, faults with displacements of greater than10 m (33 ft) occur at spacings of about 1 km (0.6 mi). This difference is reflected in the scaling exponents of D = 0.84 (higher strain) and D = 1.26 (lower strain), with larger faults dominating the deformation when D is greater than 1. Higher strain zones are more prominent in the North Malta Graben and produce an overall extension of approximately 3.3%, whereas they occur only locally in the surrounding horst regions where the extension is about 1.8%. These results indicate that high-quality seismic reflection surveys across the Malta platform might miss between 40 and 45% of the total extension and that estimates of subseismic faulting need to consider differences in the scaling of the higher and lower strain zones.

Pattern and timing of late Cenozoic rapid exhumation and uplift of the Helan Mountain, China

The Helan Mountain, an intraplate deformation belt in the North China Craton, is located in the northern portion of the China North-South seismic belt, and at the northwestern margin of the Ordos Block. The Cenozoic deformation history of the Helan Mountain is characterized by extension along the eastern Helan Mountain fault (EHSF), resulting in the exhumation and uplift of the Helan Mountain, relative to the rifting of the adjacent Yinchuan Basin. Here we present new apatite fission track (AFT) data from several transects adjacent to the EHSF in the central and northern Helan Mountain. AFT ages from the northern Helan Mountain (Dawukou and Zhengyiguan transects) range from 10 Ma to 89 Ma, whereas AFT ages from the southern Helan Mountain (Suyukou transect) are greater than 71 Ma. The AFT data analysis reveals initiation of rapid uplift and exhumation of the Helan Mountain at 10–12 Ma. Additionally, a plot of the AFT ages versus their mean track length shows a distinctive “boomerang” pattern indicating that the Helan Mountain experienced a discrete phase of accelerated exhumation beginning at 10–12 Ma. Spatially, AFT samples systematically increase in age away from the EHSF and are consistent with late Cenozoic exhumation that was slow in the southwestern Helan Mountain and rapid in the northeastern Helan Mountain, as well more rapid adjacent to the EHSF and slower away from the EHSF. Obviously, the spatial distribution of late Cenozoic exhumation indicates that normal faulting of the EHSF is related to southwestward tilting and rapid exhumation of the Helan Mountain beginning at 10–12 Ma. The uplift and exhumation of the Helan Mountain was a response to the intensive extension of the northwestern margin of the Ordos Block in the late Cenozoic; this occurred under a regional extensional stress field oriented NW-SE along the Yinchuan-Jilantai-Hetao and the Weihe-Shanxi graben systems adjacent to the Ordos Block.

Active tectonics of the Alaotra–Ankay Graben System, Madagascar: Possible extension of Somalian–African diffusive plate boundary?

The Lake Alaotra–Ankay rift valley of Central Madagascar forms a NE–SW oriented depression filled with Neogene to recent sediments and is part of a more regional post-Miocene graben system that strikes N–S across much of the central part of the island. The region is characterized by a number of small earthquakes, active hot springs, steep fault-scarp bounded valleys, and several levels of terraces. Young, deeply incised topography related to intense tropical weathering, is largely concentrated near active fault systems. The origin and evolution of this extensional structure and its morphological expressions, however, have not been clearly documented. Mountain front sinuosity values indicate that the eastern side of the rift valley is tectonically more active than the western side, and that rapid erosion under subtropical conditions is causing rapid weathering of fault scarps. The active river network within the Alaotra–Ankay rift shows clear signs of surface re-adjustment to recent tectonic activity. The drainage network is asymmetric, with most channels on the east sides of valleys and controlled by regional eastward tilting. First-order rivers flow parallel to the basin axis. Zones of intense lavaka (erosional gully) formation are correlated with active fault systems and seismicity. Extension in the Ankay–Alaotra rift is oriented roughly the same as in the morphologically similar East African rift system, located only 500 km to the west. The eastern edge of the African continent is moving somewhat independently from the rest of Africa and the portion of the African plate east of the East African rift is regarded as a separate plate, the Somali plate. However, few plate configurations have clearly defined the southern extension of the Africa-Somali plate boundary to where it must join the Southwest Indian Ocean ridge, a necessary requirement of any true microplate bounded by plate boundaries. Instead, the southeast part of the African plate, including the Somali microplate, seems to be dominated by a number of individual rift systems, overall defining a broad diffusive extensional plate boundary. We suggest that this diffusive plate boundary includes one segment that extends off the coast of Africa through the Comores, then cuts through northern Madagascar and extends down the active Ankay–Alaotra rift to the fault-block dominated southeastern coast of Madagascar, then extends southeastward to the Southwest Indian Ocean Ridge. Thus, the southeastern part of the African plate is considerably more fragmented by plume-related uplift and diffuse extensional boundaries than previously envisaged.

Application of the geomechanical facies approach and comparison of exploration and evaluation methods used at Soultz-sous-Forêts (France) and Spa Urach (Germany) geothermal sites

Earth-reservoirs are increasingly exploited today with the extraction of resources, such as heat and hydrocarbons, and the large-scale emplacement of waste, such as CO2 sequestration. The characterization, site investigation, predictive modeling and long-term monitoring are dependent on the processes being investigated and modeled. In most cases complex coupled processes have to be addressed in a geologically complex rock mass system. In this paper we present a conceptual holistic framework known as geomechanical facies linking all the scales of investigation, characterization and reservoir development methods. We demonstrate this concept on the work undertaken during the design and development of the enhanced geothermal systems (EGS) systems at the forefront of European Hot-Dry-Rock (HDR) technology, Soultz-sous-Forets (France) and Spa Urach (Germany). Soultz-sous-Forets is situated within granitic rocks and an active tectonic graben system in the central part of the Rhine Graben. It presents conditions of lithology, temperature, stress, hydraulics and geochemistry that are very different from those at Spa Urach, located in a very dense gneiss formation in the South German crystalline complex. Spa Urach exhibits more elastic behavior and is set tectonically within an almost inactive strike-slip stress field described in more detail in Sects. “Drill core analysis” and “Hydraulic stimulation at Spa Urach”. This paper compares the exploration and field development methods used at these two sites against the back drop of the geomechanical facies concept. Issues addressed include the key parameters for flow and heat transport properties, coupled hydro-mechanical process identification, the success of the HDR reservoir as a heat exchanger and exploration techniques applicable to the different facies. Identification of the key geomechanical facies gives an indication as to which technologies will prove more efficient in the application of HDR technology. The results of this study will hopefully help in developing heat recovery schemes for the long-term economical operation of future HDR plants and EGS as well as assist in the understanding of engineered geosystems.

Quaternary deformation around the Palo Negro area, Pampa Norte, Argentina

Abstract The Pampa Norte region is a great plain characterized by low slopes and accumulation of hundreds of meters of thick loose sediments. A high morphostructure denominated San Guillermo block stands out in the central plain, the Tostado–Selva scarp forming its western boundary. It is located in an intraplate setting characterized by low tectonic activity. However recent uplift can be inferred by means of terrain analysis and the sedimentology of the Palo Negro Fm. Pond deposits (Palo Negro Fm.) observed in the scarp suggest topographic inversion during the Late Quaternary in the Palo Negro area. The morphology indicates that the deformation was widely distributed, forming a gentle (ca. 5 m amplitude and 13 km wavelength) asymmetric fold. Low sinuosity lineaments located in the base of the scarp, coincident with knick points in the topographic profile, can be interpreted as the projection of tip-lines by high-angle fault-propagation. This geometry is compatible with reverse kinematics on blind faults. A deformation style with reactivation of pre-existing faults is consistent with structural observations. Seismic reflectors suggest the presence of Cretaceous high-angle normal faults, and the orientation of lineaments is similar to the orientation of the graben systems and transverse accommodation zones originated during the opening of the South Atlantic Ocean. One OSL dating of 67.4 ± 5.1 kyr B.P (from Palo Negro Fm. supposed as deposited on a flat plain floor) and a height difference of 9.5 m measured in the Tostado–Selva scarp account for an averaged uplift rate of 0.14 mm/year.