NNE-SSW Faults Research Articles

Using Edge Detection Techniques on Aeromagnetic Data to Delineate Structures Controlling Orogenic Gold Deposits in the Sefwi Greenstone Belt, Ghana

ABSTRACT The orogenic-style gold deposits in the Sefwi greenstone belt are associated with quartz veins and occur along shear zones. Edge detection techniques (i.e., analytic signal, vertical, and tilt derivatives) were employed to delineate regional planar fabrics (e.g., shear zones, faults) using aeromagnetic data. The enhancing techniques aided the delineation of NE-SW striking lineaments associated with the Kenyase-Yamfo, Hwidiem, Ketesso, Bibiani, and Afema shear zones in the study area. These shear zones are within the Sefwi belt and at the contact zones between the belt and the adjacent metamorphosed volcano-sedimentary rocks of the Sunyani-Comoé and Kumasi-Afema basins. Low-displacement N-S to NNE-SSW trends which are oblique to the NE-SW shear zones, were also delineated. The local bend and offset along the NE-SW striking faults created by the cross-cutting N-S to NNE-SSW faults serve as favourable trap zones to focus fluids migrating along these shear zones. Fry analysis of known orogenic gold occurrences in the study areas shows that the NE-SW lineaments, with an azimuth of 40°-50°, are predominant in controlling the distribution of deposits. There is also a significant structural control in the N-S to NNE-SSW direction with azimuths 0°-10° and 10°-20°.

Geological Structures Controlling Au/Ba Mineralization from Aeromagnetic Data: Harrat ad Danun Area, Saudi Arabia

Positive and negative magnetic anomalies occupied the total aeromagnetic (TM) map of the Harrat ad Danun area, Saudi Arabia. Reduction to the pole (RTP) maps display the range of magnetic values (−312.4 to 209.4 nT) that vary in shape, size, and magnitude. These anomalies generally follow the NNW–SSE (Red Sea axis trend), NE–SW, and NNE–SSW trends. The NNW-SSE linear negative and positive magnetic anomalies could be brought on by buried faults, shear zones, or subsurface dikes. In the central part, the position of Au and Ba mineralization was connected to this trend. It is concluded that the principal structures are represented by the NNW–SSE, NE–SW, and NNE–SSW tendencies. Based on gridded RTP magnetic data, the 2-D power spectrum was computed and revealed the frequency of the near-surface and deep magnetic components. It is believed that the depths of the shallow and deep magnetic sources are typically 80 m and 570 m, respectively. Additional negative and positive magnetic anomalies with varied amplitudes and frequencies, trending in the NNW–SSE, ENE–WSW, and NE–SW directions, are seen when the high-pass and low-pass maps are closely examined. Many faults in various directions cut into these anomalies. The occurrence of negative linear magnetic anomalies (−36.6 nT to −137.3 nT) at this depth (80 m) is also confirmed by this map. The TDR filter and the Euler deconvolution method were used to identify the horizontal variations in magnetic susceptibility as well as the source position and depth of magnetic sources. The linear clustering rings are thought to be caused by contacts or faults with depths between 1 m to 474 m that are oriented WNW–ESE, NNE–SSW, and NNW–SSE. These faults or contacts are thought to be particularly prominent in the western, eastern, southern, northern, and central zones. The majority of felsic and mafic dikes are found to be connected to subsurface structures, showing that three structural trends—WNW–ESE, NNE–SSW, and NNW–SSE—affect the studied area. This demonstrates that important features and shear zones control the majority of Saudi Arabia’s gold deposits. A negative magnetic anomaly that is centered in the area, trending NNW–SSE and crossing the NNE–SSW fault, is connected to the plotted gold and barite mineralization in the study area. This may imply that these two tendencies are responsible for mineralization. This result raises the possibility of mineralization in the NNW negative magnetic feature located in the western part of the area. The occurrence of gold and barite was significantly impacted by the NNW–SSE and NNE–SSW structural lineaments.

Imaging sub-caldera structure with local seismicity, Okataina Volcanic Centre, Taupo Volcanic Zone, using double-difference seismic tomography

We examine sub-caldera structure and seismicity in and around the Okataina Volcanic Centre (OVC), Taupo Volcanic Zone, New Zealand, using seismic data collected over 4+ years with a temporary array of broadband and short-period seismometers, supplementing data from the permanent New Zealand seismometer network. We derive a new 3-D image of P-wave seismic velocity for the upper crust in the region, using double-difference seismic tomography and utilising waveform cross-correlations. We subsequently relocate 6989 earthquakes which occurred in the region over the 2010–2021 time period, using the 3D velocity model. The seismicity distribution shows spatial clusters west of Lake Rotomahana, as well as beneath Haroharo and the Makatiti Dome, inside the Okataina caldera. Beneath Makatiti Dome 90% of the events are shallower than 7.7 +/− 0.5 km. Outside of the Okataina caldera event relocations highlight short (∼3–4 km long) streaks of activity in the Ngakuru graben, part of the active Taupo Rift southwest of Okataina caldera. Inside the OVC the relocated seismicity beneath Makatiti appears closely associated with low (∼10%) P-wave velocity anomalies, which we resolve in the ∼5-to-8-km depth range beneath the Okataina caldera, and which are likely related to partial melt and/or fluid-volatile pathways. Moment tensor analyses for two larger-magnitude events (ML4.5 and ML4.9) near Haroharo indicate normal faulting, with NNE-SSW fault strike, but with positive CLVD and positive isotropic components when allowing for a full moment tensor, consistent with a magmatic environment with degassing and/or fluid migration.

Unraveling the emplacement history of a Portuguese post-tectonic Variscan pluton using magnetic fabrics and gravimetry

The Lamas de Olo pluton is a post-tectonic Variscan pluton located in northern Portugal. It is a composite pluton comprising three outcropping biotite granites, similar in mineralogical composition but with different textures and different biotite and magnetite contents: Lamas de Olo, Alto dos Cabeços, and Barragem. Two magnetic fabric techniques, anisotropy of magnetic susceptibility and anisotropy of anhysteretic remanent magnetization, were used to test the coaxiality of magnetite and biotite fabrics. Comparing both magnetic tensors, the fabric of magnetite and paramagnetic minerals was found to be coaxial. The magmas ascended passively through opening fractures and were controlled by the intersection of ENE-WSW, NNW-SSE and NNE-SSW fault systems. The gravimetric study shows that the Lamas de Olo is an asymmetric, wedge-shaped pluton with a thickness of ca. 2.5–3.5 km. The kinematic model points to the presence of a feeder zone following an ENE-WSW direction, with two roots for Lamas de Olo and Alto dos Cabeços granites. Regarding Barragem granite, several conduits were identified along an NNE-SSW fault system, subparallel to the Verín-Régua-Penacova fault. Comparing the shape of three post-tectonic biotite plutons with same age and located along the same regional structure, the Verín-Régua-Penacova fault, we observed that the geometry is mainly conditioned by the nature of the host rocks and characteristics of the crustal structure. Wedge-shaped plutons are intruded into granites, and flat-shaped plutons are hosted in metasediments.

Geological and geophysical investigations of an engineering site characterization for construction purposes in Western Saudi Arabia

Characterizing subsurface site conditions and understanding main geological features are critical components of any safety assessment for any construction, particularly defining subsurface flow paths and weakness (i.e. fracture and fault) zones. Poor definition of flow paths and weakness zones may lead to serious hazards during construction. This study, integrating geophysical and geological investigations, is undertaken to enhance the understanding of both surface and subsurface complexities of an engineering site characterization located in western Saudi Arabia near Red Sea. Two geophysical investigations, Electrical Resistivity Tomography (ERT) and Transient Electromagnetic (TEM), were carried out to characterize the subsurface of the study area. A total of three ERT profiles and sixteen TEM sounding measurements was conducted in the study area. Geological investigation was mainly focused to understanding geological features and bedrock conditions of the two opposite river's abutments at the study site. Geophysical results show NNE-SSW and NNW-SSE fault zones which were confirmed by in-situ geological inspections and by satellite images. The results also reveal the presence of geothermal fluids validated by the observation of hot water flowing in the river along the measurements. This study demonstrates that the integrated geophysical and geological investigations provide means to infer geothermal fluids, locate fractures, and fault zones. This study enabled us to characterize and locate the geological structural framework as well as possible geothermal fluids existence in the study area.

Discussion of the Fault Models of the Northern Marmara Shelf and the Lagoon Küçükçekmece with New Marine Magnetic Data, Turkey

580-km length marine magnetic study was performed at the Northern Marmara Shelf and the Lagoon Kucukcekmece to provide new datasets to understand the submarine structures. The distributions of the relatively complex and differentiated low and high magnetic anomalies indicate that the study area was affected by the multiple tectonic regimes. While the magnetic highs correspond with the folding structures, magnetic low anomalies well match with the lineaments determined from the seismic profiles. Magnetic anomalies support an E-W trending fault, controlling the anticlinorium system and NNE-SSW faults related to the compressional tectonics. These magnetic structures might be associated with Thrace Eskisehir Fault Zone, an older right-lateral strike-slip fault, which was the most dominant tectonic element before the North Anatolian Fault Zone. In addition to three appx, N-S trending faults are defined beneath the Lagoon Kucukcekmece from the total and residual magnetic anomaly grids.

The importance of sequential partial melting and fractional crystallization in the generation of syn-D3 Variscan two-mica granites from the Carrazeda de Ansiães area, northern Portugal

In the Carrazeda de Ansiães region, northern Portugal, a mesozonal granitic suite intruded Precambrian to Ordovician metasedimentary rocks during the late kinematic stages of the Variscan orogeny. In this multiphase granitic complex, consisting of ten granite types, the youngest group of two-mica granites (G7–G10) was emplaced between 318 ± 1 Ma and 316.2 ± 0.7 Ma, as determined by ID-TIMS U–Pb on zircon and monazite. Granite types G7–G9 were affected by the third phase of deformation (D3) before they were completely crystallized, as indicated by their internal NW–SE magmatic foliation concordant with the regional structures. The granite type G10 shows some distinctive textural features, showing a strong brittle deformation, probably due to its preferential emplacement in late NNE-SSW fault zones. Granites G7–G9 have equal or higher amounts of muscovite than biotite and contain surmicaceous enclaves, xenoliths, “schlieren”, and, more rarely, microgranular enclaves. The muscovite-dominant granite G10 does not contain enclaves. These Variscan granites are peraluminous, with ASI ranging between 1.22 and 1.39 and normative corundum of 2.79–4.39%, having the characteristics of S-type granites. In fact, the enrichment in LREE relatively to HREE, the negative Eu anomalies, and similar mean values of (87Sr/86Sr)i, εNdt and δ18O for G7 (0.7156 ± 0.0005; − 8.5; 11.49 ‰) and G8 (0.7155 ± 0.0007; − 8.4; 11.39 ‰) show that these two granite types resulted from sequential partial melting of the same metasedimentary material, where granite G8 would have derived from a higher degree of partial melting than G7. Granites G8–G10 and their minerals show a fractionation trend that is confirmed by modeling of major and trace elements. The subparallel REE patterns and the decreasing REE contents within the differentiation series, the Rb–Sr isochron for G8, G9 and G10 (315.5 ± 5.4 Ma; MSWD = 1.3) and the relatively uniform εNdt and δ18O data suggest that fractional crystallization was the main mechanism, which would have lasted less than 1 Ma. The tin-bearing granites G7 and G10 have ≥ 20 ppm Sn, but the main quartz veins containing cassiterite and wolframite cut granite G10, which contains 31 ppm Sn. Fractional crystallization was responsible for the increase in Sn content in granites from the G8–G10 series and their micas.

Alluvial sedimentation and tectono-stratigraphic evolution in a narrow extensional zigzag basin margin (northern Teruel Basin, Spain)

The northern part of the eastern margin of the extensional Neogene Teruel Basin (central-eastern Spain) consists of a non-linear, zigzag fault zone made of alternating ca. 2 km long, NNW-SSE trending segments and shorter NNE-SSW ones. Good outcrop conditions made possible a comprehensive integrated stratigraphic and structural study, especially focused on coarse clastic sediments deposited along the basin margin. Well-exposed stratal relationships with boundary faults, allowed the analysis of tectonic influence on sedimentation. Synsedimentary deformation includes growth faulting, rollover anticlines, and monoclines and associated onlap stratal terminations, angular unconformities, and other complex growth strata geometries. One of them is the onlap-over-rollover bed arrangement described here for the first time, which reveals the competition between tectonic subsidence and sedimentary supply. Both, the structural inheritance (dense Mesozoic fracture grid) and the dominant, nearly ‘multidirectional’ (σ1 vertical, σ2 ≈ σ3), Pliocene extensional regime with σ3 close to E-W, are considered to have controlled the margin structure and evolution. Tectono-stratigraphic evolution includes: (i) reactivation of inherited NNW-SSE faults and development of W-SW-directed small alluvial fans (SAF) while NNE-SSW segments acted as gentle relay ramp zones; (ii) progressive activation of NNE-SSW faults and development of NW-directed very small alluvial fans (VSAF); during stages i and ii sediments were trapped close to the margin, avoiding widespread progradation; (iii) linking of NNW-SSE and NNE-SSW structural segments, overall basin sinking and widespread alluvial progradation; (iv) fault activity attenuation and alluvial retrogradation. The particular structure and kinematic evolution of this margin controlled alluvial system patterns. Size of alluvial fans, directly set up at the border faults, was conditioned by the narrowness of the margin, small catchment areas, and proximity between faults, which prevented the development of large alluvial fans. The size of the relay zones, only a few hundred meters wide, acted in the same way, avoiding them to act as large sediment transfer areas and large alluvial fans to be established. These features make the Teruel Basin margin different to widely described extensional margins models.

Resistivity characterization of the Arjuno-Welirang volcanic geothermal system (Indonesia) through 3-D Magnetotelluric inverse modeling

The Arjuno-Welirang geothermal area is located in East Java, Indonesia. In the Arjuno-Welirang geothermal area, the Arjuno, Welirang and Anjasmoro volcanic complex is dominated by quaternary volcanic rocks and a complex geological structure. Several impressive surface thermal manifestations are found in the study area, including a high temperature solfataric-fumarole (about 137 °C) on the top of Mt. Welirang and also bicarbonate hot springs with a temperature range of 39–55 °C in the northern and western parts of the study area. The occurrences of the surface manifestations are probably controlled by the NW-SE and NNE-SSW fault structures. The occurrence of the high temperature solfataric-fumarole with high magmatic gas contents indicates that the Arjuno-Welirang geothermal area can be classified as a volcanic geothermal system. To investigate the subsurface condition of the geothermal system, Magnetotelluric (MT) survey was conducted within 34 MT stations distributed in the northern and western parts of the study area. Considering the complexity of the subsurface volcanic rocks and the geological structures of the study area, one and two-dimensional MT data inversion approaches could not be applied properly. Accordingly, a 3-D inversion of the MT data will be the most representative approach in such conditions in order to investigate the reservoir structure of the volcanic geothermal system. The 3-D inversion was then applied to all MT data with a frequency range of 100–0.01 Hz. Generally, the quality of the MT data varies from good to excellent. A clear subsurface resistivity structure revealed by the 3-D inversion showed a good correlation with geological and geochemical evidences. Overall, the resistivity structure consists of a conductive (1–10 ohm m) altered rock at the upper part overlying a reservoir zone with a slightly higher resistivity range (20–60 ohm m). The lower part of the resistivity structure showed the highest resistivity value (>80 ohm m), indicating a hot rock region. The main upflow is represented by an updome structure of the base of the conductive layer (BOC) beneath the Mt Welirang summit. It is supported by the occurrence of solfataric-fumaroles on the summit of Mt Welirang. Moreover, the outflow zones of the Arjuno-Welirang geothermal system are indicated by a thickening of the low resistivity layer toward the northern and western parts of Mt. Welirang supported with the occurrence of bicarbonate hotsprings. The existence of magmatic gas components (i.e. H2S and SO2) detected in the fumarole, gives an early warning regarding the possible existence of acidic fluid. Further exploration drilling should consider this issue. A conceptual model is then developed to visualize the geothermal system of the Arjuno-Welirang prospect area by integrating the geological and geochemical data.

Nests of red wood ants (Formica rufa-group) are positively associated with tectonic faults: a double-blind test.

Ecological studies often are subjected to unintentional biases, suggesting that improved research designs for hypothesis testing should be used. Double-blind ecological studies are rare but necessary to minimize sampling biases and omission errors, and improve the reliability of research. We used a double-blind design to evaluate associations between nests of red wood ants (Formica rufa, RWA) and the distribution of tectonic faults. We randomly sampled two regions in western Denmark to map the spatial distribution of RWA nests. We then calculated nest proximity to the nearest active tectonic faults. Red wood ant nests were eight times more likely to be found within 60 m of known tectonic faults than were random points in the same region but without nests. This pattern paralleled the directionality of the fault system, with NNE–SSW faults having the strongest associations with RWA nests. The nest locations were collected without knowledge of the spatial distribution of active faults thus we are confident that the results are neither biased nor artefactual. This example highlights the benefits of double-blind designs in reducing sampling biases, testing controversial hypotheses, and increasing the reliability of the conclusions of research.

Jurassic to Early Cretaceous basin configuration(s) in the Fingerdjupet Subbasin, SW Barents Sea

The Fingerdjupet Subbasin in the southwestern Barents Sea sits in a key tectonic location between deep rifts in the west and more stable platform areas in the east. Its evolution is characterized by extensional reactivation of N-S and NNE-SSW faults with an older history of Late Permian and likely Carboniferous activity superimposed on Caledonian fabrics. Reactivations in the listric NNE-SSW Terningen Fault Complex accommodated a semi-regional rollover structure where the Fingerdjupet Subbasin developed in the hangingwall. In parallel, the Randi Fault Set developed from outer-arc extension and collapse of the rollover anticline.N-S to NNE-SSW faults and the presence of other fault trends indicate changes in the stress regime relating to tectonic activity in the North Atlantic and Arctic regions. A latest Triassic to Middle Jurassic extensional faulting event with E-W striking faults is linked to activity in the Hammerfest Basin. Cessation of extensional tectonics before the Late Jurassic in the Fingerdjupet Subbasin, however, suggests rifting became localized to the Hammerfest Basin. The Late Jurassic was a period of tectonic quiescence in the Fingerdjupet Subbasin before latest Jurassic to Hauterivian extensional faulting, which reactivated N-S and NNE-SSW faults. Barremian SE-prograding clinoforms filled the relief generated during this event before reaching the Bjarmeland Platform. High-angle NW-prograding clinoforms on the western Bjarmeland Platform are linked to Early Barremian uplift of the Loppa High. The Terningen Fault Complex and Randi Fault Set were again reactivated in the Aptian along with other major fault complexes in the SW Barents Sea, leading to subaerial exposure of local highs. This activity ceased by early Albian. Post-upper Albian strata were removed by late Cenozoic uplift and erosion, but later tectonic activity has both reactivated E-W and N-S/NNE-SSW faults and also established a NW-SE trend.

Size distribution and roundness of clasts within pseudotachylytes of the Gangavalli Shear Zone, Salem, Tamil Nadu: An insight into its origin and tectonic significance

Gangavalli (Brittle) Shear Zone (Fault) near Attur, Tamil Nadu exposes nearly 50 km long and 1–3 km wide NNE–SSW trending linear belt of cataclasites and pseudotachylyte produced on charnockites of the Southern Granulite Terrane. Pseudotachylytes, as well as the country rock, bear the evidence of conjugate strike slip shearing along NNE–SSW and NW–SE directions, suggesting an N–S compression. The Gangavalli Shear Zone represents the NNE–SSW fault of the conjugate system along which a right lateral shear has produced seismic slip motion giving rise to cataclasites and pseudotachylytes. Pseudotachylytes occur as veins of varying width extending from hairline fracture fills to tens of meters in length. They carry quartz as well as feldspar clasts with sizes of few mm in diameter; the clast sizes show a modified Power law distribution with finer ones (<1000 \({\upmu }\)m\(^{2})\) deviating from linearity. The shape of the clasts shows a high degree of roundness (>0.4) due to thermal decrepitation. In a large instance, devitrification has occurred producing albitic microlites that suggest the temperature of the pseudotachylyte melt was >1000\(^{\circ }\hbox {C}\). Thus, pseudotachylyte veins act as a proxy to understand the genetic process involved in the evolution of the shear zone and its tectonic settings.

The dolerite dyke swarm of Mongo, Guéra Massif (Chad, Central Africa): Geological setting, petrography and geochemistry

AbstractDolerite dykes are widespread in the Mongo area within the granitic Guéra Massif (Chad, Central Africa). Dykes are several hundred metres to several kilometres long, a metre to decametre thick, and vertical, crosscutting the Pan-African granitic basement rocks. They are controlled by major Pan-African NNE-SSW, NE-SW and ENE-WSW faults. Rocks constituting the dykes exhibit typical doleritic textures (

Site - building resonance response in a complex geological setting: Ground motions recorded in the centre of Paleohora Basin and at a rock fractured outcrop site close to the basin edge (SW Crete, Greece)

Seismic site characterization is an important parameter in earthquake hazard assessment for risk mitigation studies, which are essential in engineering design practices and urban planning providing useful information to governmental or private civil protection authorities. Recent advances from site effect studies envisage for the future the importance of incorporating microzonation as a tool for determining resonance prone buildings especially for the most earthquake hazardous municipalities. This study aims to assess the frequencies of vibration of selected masonry and reinforced concrete buildings and to examine potential soil - building resonance in a complex geological setting of the small scale Paleohora Basin (southwest Crete). Ambient noise survey was performed in masonry and reinforced concrete buildings, on soil foundation and on soil at several distances from the base of the selected buildings. The selected public and private (masonry and RC) building constructions are located in the centre of the Basin which is characterized by heterogeneities induced by large scale E-W fault and at rock fractured (of complex orientation and opening of fractures and joints) outcrop site close to the margins of the Basin in the southeast bordered by an NNE-SSW fault. The spatial horizontal to vertical spectral ratio (HVSR) technique using ambient noise reveals that soil-building resonance phenomena could be inferred in the small scale alluvial Paleohora Basin.Two amplified peaks in the centre of the Basin and one amplified peak at the rock fractured outcrop site close to the Basin margins are observed from the data sets. In the centre of the Basin, the amplified peak at medium frequency (3.85–7.11 Hz) is related to the near subsurface irregularities locally induced by E-W faulting, while the low frequency (0.75 Hz) is related to the overlain Quaternary deposits. At the rock fractured outcrop site close to the margins of the Basin the one amplified peak at medium frequencies (2.5 Hz) is related to the fractured limestone outcrop. Weak motion earthquake data following the main earthquake event that occurred near to the southwest coast of Paleohora are recorded in the centre of the Basin (at the schoolyard) and on rock fractured outcrop close to the margins of the Basin. The HVSR of the weak motion earthquake data present in the center of the Basin two amplified peaks at low (0.75 Hz) and medium frequencies (6.56 Hz) and at the rock fractured outcrop site close to the Basin edge one amplified peak at medium frequencies (2.58 Hz). This study conducted in the complex small scale and dense populated Basin, highlights the necessity of incorporating the determination of prone resonance buildings into urban planning for risk mitigation studies, specifically in this earthquake hazardous municipality.

Late Variscan deformation in the Iberian Peninsula; a late feature in the Laurentia–Gondwana dextral collision

The Late Variscan deformation event in Iberia is characterized by an intraplate deformation regime induced by the dextral oblique collision between Laurentia and Gondwana. This episode in Iberia is characterized by NNE–SSW brittle to brittle–ductile strike-slip faults, which are considered by the classic works as sinistral strike-slip faults. However, the absence of Mesozoic formations constraining the age of this sinistral kinematics led some authors to consider it as the result of Alpine reworking. Structural studies in Almograve and Ponta Ruiva sectors (SW Portugal) show that NNE–SSW faults have a sinistral kinematics and are occasionally associated with E–W dextral shears. Moreover, this kinematics is related to the late deformation episodes of Variscan orogeny. In Almograve sector, the Late Variscan structures are characterized by NNE–SSW sinistral kink bands, spatially associated with E–W dextral faults. These structures are contemporaneous and affect the previously deformed Carboniferous units. The Ponta Ruiva Sector constrains the age of deformation because the E–W dextral shears affect the Late Carboniferous (late Moscovian) units, but not the overlying Triassic series. The new data show that the NNE–SSW and the E–W faults are dynamically associated and result from the Late Variscan deformation episode. The NNE–SSW sinistral faults could be considered as second-order domino structures related to first-order E–W dextral shears, linked to Laurentia–Gondwana collision during Late Carboniferous–Permian times.

A Neotectonic based Geomorphic Analysis using Remote Sensing Data to Delineate Potential Areas of Hydrocarbon Exploration: Cachar Area, Assam

Abstract: The Assam Arakan fold thrust belt has highly deformed folded units of Tertiary sediments bounded by eastward dipping thrust slices with a convexity towards west. In the Tripura-Cachar region, this folded belt is characterized by the occurrence of wide synclines and narrow anticlines that hosts a number of hydrocarbon producing fields. In the Cachar area of Assam, most of these fields occur in the culmination of anticlinal structures. Other wells drilled in analogous structural settings are found to be dry. In this paper a neotectonic based geomorphic analysis is carried out to delineate a fault network and geomorphic highs in Cachar area as expressions of sub-surface structures which had subsequently been validated by available geophysical data. Of these geomorphic highs, those that are in the synclinal areas are believed to represent subtle sub-surface structural highs. Synclinal structures associated with NNE-SSW faults might be considered interesting for hydrocarbon exploration and are subsequently categorized following their degree of confidence for exploration of hydrocarbon. Additionally, a genetic model of the structures in the region is also proposed.

Structural Controls on the Geochemistry and Output of the Wells in the Olkaria Geothermal Field of the Kenyan Rift Valley

The Olkaria geothermal field is located in the Kenya Rift valley, about 120 km from Nairobi. Geothermal activity is widespread in this rift with 14 major geothermal prospects being identified. Structures in the Greater Olkaria volcanic complex include: the ring structure, the Ol’Njorowa gorge, the ENE-WSW Olkaria fault and N-S, NNE-SSW, NW-SE and WNW-ESE trending faults. The faults are more prominent in the East, Northeast and West Olkaria fields but are scarce in the Olkaria Domes area, possibly due to the thick pyroclastics cover. The NW-SE and WNW- ESE faults are thought to be the oldest and are associated with the development of the rift. The most prominent of these faults is the Gorge Farm fault, which bounds the geothermal fields in the northeastern part and extends to the Olkaria Domes area. The most recent structures are the N-S and the NNE-SSW faults. The geochemistry and output of the wells cut by these faults have a distinct characteristic that is the N-S, NW-SE and WNW-ESE faults are characterized by wells that have high Cl contents, temperatures and are good producers whereas the NE-SW faults, the Ring Structure and the Ol’Njorowa gorge appear to carry cool dilute waters with less chloride concentration and thus low performing wells. Though the impacts of these faults are apparent, there exists a gap in knowledge on how wide is the impact of these faults on the chemistry and performance of the wells. This paper therefore seeks to bridge this gap by analysis of the chemical trends of both old wells and newly drilled ones to evaluate the impacts of individual faults and then using buffering technique of ArcGis estimate how far and wide the influence of the faults is. The data was obtained after the sampling and analysis of discharge fluids of wells located on six profiles along the structures cutting through the field. Steam samples were collected with a stainless steel Webre separator connected between the wellhead and an atmospheric silencer on the discharging wells whereas the analysis was done in house in the KenGen geochemistry laboratory. The results indicates that Olkaria field has three categories of faults that control fluid flow that is the NW-SE trending faults that bring in high temperature and Cl rich waters, and the NE-SW trending Olkaria fracture tend to carry cool temperature waters that have led to decline in enthalpies of the wells it cuts through. The faults within the Ol Njorowa gorge act to carry cool, less mineralized water. Though initially, these effects were thought to be in shallow depths, an indication in OW-901 which is a deeper at 2200 m compared to 1600 m of OW-23 well that proves otherwise. This is, however, to be proved later as much deeper wells have been sited.

Role of Lithology and Subsurface structures detected by potential field data in controlling the radioactive mineral accumulation at Natash area, Eastern Desert, Egypt

Wadi Natash area is located in the southern part of the Eastern desert of Egypt. It has a great importance for containing accumulations from the radioactive minerals of Uranium, Thorium and Potassium. An integrated potential study was carried out on the study area with the aim of locating depths to causative bodies with sufficient magnetic susceptibility that may represent magmatic intrusions with relation to the radioactivity location and delineate the subsurface structures affecting the area. Both magnetic and Bouguer data as well as radiometric data were interpreted rapidly for source positions and depths using Euler deconvolution, Werner deconvolution and 3D modeling techniques. The results deduced from the trend analyses show that the major fault trend affecting the area have NNW-SSE (Red Sea–Gulf of Suez trend) direction intersected by the less predominant NNE-SSW(The Gulf of Aqaba–Dead Sea trend) and WNW-ESE (Najd Fault System) fault trends. The causative bodies were imaged at depths ranging from 0.3km to about 1.5km. The depths along the interpreted profiles display discontinuities in potential field markers due to presence of the NNW-SSE fault trends act as pass channels for the hydrothermal solutions.It can be stated that the radioactive mineral accumulations were caused by the hydrothermal solutions rich with radioactive minerals as a result of intruding Natash volcanic to the granitic rocks. The Qouseir clastics and the Nudian sand stone were affected by these solutions and show a positive response for the radioactive minerals.

Deformation of the Top Basement Unconformity west of the Alpine Fault (South Island, New Zealand): seismotectonic implications

Geometry of the Top Basement Unconformity (TBU) west of the Alpine Fault has been reconstructed through a set of cross-sections linking surface and subsurface geology. Onshore, the TBU shows tectonic relief of several kilometres between antiformal pop-ups and synformal depressions in contrast with a smoother topography offshore. This geometry arises from reverse slip on sets of north–south to NNE–SSW faults, mostly dipping 50–66° both west and east, that control folding of the TBU and overlying cover sequence. Some of these faults are inherited Upper Cretaceous–Palaeogene normal faults that displaced the TBU during the extensional phases and were later reactivated as reverse faults under compression, whereas others appear to be newly propagated Neogene reverse faults. The faults that deform the TBU have vertical displacements of 3–5 km and lengths of >150 km, and have the potential of being reactivated in the present stress field. Currently active faults comprise a set of blind reverse faults that propagate upsection from pre-existing extensional fault fabric in the basement, imposing a short-wavelength undulation on the TBU.

Tectono-sedimentary evolution of the peripheral basins of the Alboran Sea in the arc of Gibraltar during the latest Messinian-Pliocene

In the peripheral basins of the Alboran Sea, five stratigraphic units (latest Messinian-Pliocene) separated by discontinuities and representing transgressive–regressive cycles have been recognized. The first unit (LM) is latest Messinian in age and precisely characterizes the Lago-Mare event at the end of the Messinian Salinity Crisis, i.e. just before the opening of the Strait of Gibraltar at the beginning of the Pliocene. The three following units (Pl-1, Pl-2 and Pl-3) are Zanclean in age, whereas the last one (Pl-4) is Piacenzian. These four Pliocene units consist of alluvial, deltaic, and littoral deposits in the marginal areas, changing to open marine deposits with planktonic components in the basinal areas, although their extension varies in each basin. Regionally, these units do not necessarily stack in a single stratigraphic succession because of tectonics that controlled their hosting basins. Thus, the LM and Pl-1 units occur only in the Malaga and Estepona-Marbella basins, revealing that the onset of the sedimentation after the Messinian evaporitic stage and the Pliocene transgression was not a single and synchronous event in the western Alboran Sea. Moreover, the Pl-3 and Pl-4 units do not appear in all basins, so that the subsequent continentalization process of these Alboran peripheral areas during the Pliocene was also diachronous.The sedimentary evolution of the peripheral basins was controlled mainly by tectonics. During the latest Messinian-early Pliocene, the sedimentation took place in a context marked by a NNW–SSE compression and ENE–WSW perpendicular tension. The onset of the sedimentation (LM and Pl-1 units) could be linked to preexisting E–W faults that mark part of the borders of the Malaga basin and the Estepona-Marbella sector. During the deposition of the Pl-2 unit, the movements of E–W, NW–SE, and NE–SW normal faults determined a continuous subsidence in several basins, resulting in the accumulation of thick clastic marine sequences (i.e. Malaga, Vélez-Málaga, and Nerja basins in Spain and Tirinesse basin in Morocco). Tectonic activity during the early Zanclean leads to a new paleogeographic configuration of the Alboran peripheral areas. The main features are: (i) continentalization of the Nerja sector in the Betics, Tetouan, and Oued Laou-Tirinesse sectors in the Rif; (ii) on the contrary, a period of intense subsidence started in the coastal sectors between Torremolinos and Manilva, allowing the development of the Pl-3 unit directly on the substratum; and (iii) the Malaga, Vélez-Málaga, and Malalyine basins maintained the marine regime, so their sedimentary infilling recorded the Pl-2–Pl-3 unconformity. Nevertheless, these last basins emerged shortly afterwards, before the end of the early Zanclean (FO of Globorotalia puncticulata), probably in relation to the beginning of the sea-level fall which characterizes the upper part of the TB 3.4 cycle by Haq et al. (1987).During the late Zanclean, sedimentation occurred only in the Betic basins, where NNE–SSW faults – conjugated with NW–SE faults – induced a major subsidence, permitting better development of the Pl-3 unit. On the contrary, NW–SE faults in the sector between Malaga and Nerja, and NE–SW faults in the Tirinesse basin, became practically inactive. Before the end of the Zanclean, the subsidence ceased also in the westernmost Betic basins, thus causing emersion, firstly in the sector between Torremolinos and Manilva and, slightly later, in the San Roque-Algeciras sector. Thus, the whole geodynamic activity conditioned the time–space evolution of the northern edge of the Alboran Sea, which was emerging throughout the Zanclean, successively from the E to the W. A similar E to W continentalization trend can be suggested for the Rifian Pliocene sectors when taking into account the Oued Laou-Tirinesse basins that emerged before the Malalyine one.Moreover, the unit boundaries do not coincide with those of the familiar Exxon coastal aggradational curve, but rather with the local or regional tectonic activity. Consequently, the correlation of the unit boundaries with those of the Pliocene deposits of the eastern Betic basins remains difficult. According to the biostratigraphical data, the Pl-1, Pl-2, and Pl-3 units correspond to the Pliocene-I by Montenat (1990), while the Pl-4 unit may be equivalent to the Pliocene-II.