ENE WSW Trending Faults Research Articles

Origin of the Kyaukmyet Low-Sulfidation Epithermal Gold Prospect, Monywa District, Central Myanmar

The Kyaukmyet prospect is one of the principal epithermal gold prospects in the Monywa District, Central Myanmar; its gold- and base metal-bearing quartz veins contain around 3 g/t gold. Ore minerals are mainly hosted by volcanic and volcaniclastic rocks of the Late Oligocene to Middle Miocene Magyigon Formation. The distribution of magmatic intrusions in the area is controlled by ENE-WSW trending faults; these faults are likely related to ore mineralization. Common ore minerals at the Kyaukmyet prospect include pyrite, sphalerite, galena, chalcopyrite, and electrum. They occur in mineralized crustiform-textured brecciated quartz veins and banded (colloform) and massive quartz veins. Mineralized rock is accompanied by silicification and propylitic and argillic alterations. The alteration mineral assemblages include quartz, adularia, calcite, chlorite, illite/smectite, sericite, and illite. Fluid inclusions in the quartz veins have homogenization temperatures ranging from 148 °C to 304 °C and salinities from 0.35 wt % to 2.75 wt % NaCl equiv. The quartz in the mineralized quartz veins was most likely precipitated at a depth ranges165-256 m below the paleosurface. The precipitation of gold at the Kyaukmyet prospect may have been formed by mixing large amounts of meteoric fluid with small amounts of magmatic fluid. The coexistence of liquid-rich and vapor-rich inclusions and presence of adularia and bladed calcite indicate that fluid boiling is caused the main mechanism of ore formation. The vein textures, ore mineral assemblages, alteration minerals and fluid inclusion data suggest that the Kyaukmyet prospect is a polymetallic low-sulfidation epithermal gold deposit.

Magnetotelluric investigation in the swarm prone intraplate Talala region of Saurashtra, Gujarat, western India

The Talala region of Saurashtra has experienced three moderate seismic events of magnitudes Mw5.0, Mw4.8 (on 6th November 2007), and Mw5.1 (on 20th October 2011) accompanied by swarms. The local earthquake data suggested two epicentral trends (ENE- WSW and NNW-SSE). The focal mechanism of all three earthquakes exhibit strike-slip mechanism with nodal planes sympathetic to these two trends. Based on low gravity values and ambiguous seismic reflections, the ENE-WSW trending fault was inferred and named as Girnar fault. The Magnetotelluric (MT) investigation has been carried out (i) to locate and characterize the Girnar fault and (ii) to ascertain the cause of seismicity in the region. The MT data is acquired along the ~62 km long NW-SE oriented profile, starting from the Sendarda village (NW) to the Hadmatiya village (SE) in the Junagadh district (Saurashtra), India. The regional strike of N19°E is obtained from the MT data. The data is subjected to 2D modeling and two conductors are identified along the profile, the first conductor is located between Ajeb and Aambalgadh village in the northern part of the profile with a resistivity range from 20 to 3500 Ohm.m and the second conductor is ~15 km south of the first conductor near Hirenvel village with a resistivity range from 50 to 3500 Ohm.m. The second conductor continues vertically down to 12–15 km depth and inferred as the Girnar fault. The shallow clustered seismicity (~5 km) near the second conductor advocates that it may be related to stresses induced by intense monsoon on the critically stressed Girnar fault, which has fractured the basement. The basement got fractured either from the local stress accumulation near the structural boundary or from the movement of the fluids to the less permeable crust.

Paleostress and outcrop fracture analysis along Himalayan Foothills (Eastern Salt Range), Potwar Plateau, NW Himalaya, Pakistan

The Salt Range (SR) is an ENE-WSW trending fault bend fold that form the range front of the Himalayan fold and thrust belt in Pakistan. The research is carried out in the Eastern Salt Range (ESR) with the objective to determine the paleostress inversion from reduced stress tensors by using fracture data. The surface morphology of the fold is predominantly shaped by Eocene limestone that provide best exposure of joint surfaces that can be used for kinematic and dynamic analysis. We adopted a classical circle inventory method in the field and collected orientation data (dip amount, direction, and strike) from 7 localities (outcrop stations) of the Eocene Sakesar Limestone. Three prominent fracture trends are present in the study area namely, FS-1; (E-W), FS-2; (ENE-WSW) and FS-3; (NNE-SSW). For stress analysis and data processing we used the Tensor Program of Delvaux and Sperner (2003) and calculated seven stress fields by the Right Dihedron Method. The orientations of the principal stress axes (ϭ1, ϭ2, ϭ3) and Stress Ratio (R) depicts ϭ1(Shmax) and ϭ2 are sub-horizontal while ϭ3 is vertical in all stress tensors. The paleostress results show that ϭ1(Shmax) oriented NNE-SSW belonging to a compressive regime. It is suggested that ϭ1 developed more or less perpendicular to the trend of SRT and other relevant structures in the Potwar Plateau.

Mineralogical, fluid inclusion, and stable isotopic study of the Shangshan'gang Au deposit, southeast China: Implications for ore formation and exploration

The Shangshan'gang Au deposit is located in the Cretaceous Dongkeng volcanic basin of northern Fujian Province in southeast China. The deposit is hosted by altered intermediate to silicic volcanic rocks along NE–SW- and ENE–WSW-trending faults. The main types of wall-rock alteration are silicification, sericitization, argillization, and chloritization. Ore mineralization occurred in three stages, which were the base metal, AuAg, and post-ore stages. Electrum and kustelite are the main AuAg minerals. Laser ablation–inductively coupled plasma–mass spectrometry analyses revealed different compositions for the two stages of pyrite, galena, and sphalerite, which reflect changing mineralization conditions. Fluid inclusions revealed an epithermal mineralization environment with low–moderate temperature and salinity. Mixing of ore fluids is the main ore-forming mechanisms which occurred from the early to late stages, as evidenced by the H and O isotopic compositions of the ore-forming fluids. The proportion of meteoric fluid in the ore-forming hydrothermal fluid gradually increased with time. Fluid boiling may have locally occurred in the early stage and promoted ore precipitation. Our results show that the Shangshan'gang Au deposit is an intermediate-sulfidation-type epithermal deposit that gradually changed to a low-sulfidation-type during late mineralization. Given the close temporal and spatial relationship between intermediate-sulfidation-type and porphyry deposits, there is potential for porphyry deposits to exist in the Zhenghe–Jian'ou area.

Historical faulting as the possible cause of earthquake damages in the ancient Roman port city of Ostia

This paper presents an original multidisciplinary (geological-structural-geomorphological and seismological) study aimed at investigating the origin of diffused seismic damages affecting several ancient buildings in the Roman port city of Ostia. We also evaluate the possibility to relate these damages to a previously hypothesized ENE-WSW trending fault, bordering the morphological height upon which the Ostia town was founded. Aimed at this scope, we performed seismic noise measures (by using 14 seismic stations) that show no significantly different response and lack of significant ground motion differential amplifications. The coexistence of (i) no local geological heterogeneities and (ii) low amplification of spectral ratios in the recorded seismic signals seems to exclude that the observed seismic damage may be the consequence of significant site effects. When also the large distance from the strongest Apennine’s seismogenic source areas is considered, the possibility that the observed damage may be the consequence of local events should be considered. We discuss the potentiality of the ENE-WSW trending fault as the source of the observed seismic damages, highlighting the supporting evidence as well as the uncertainties of such interpretation.

Ductile and brittle deformation in Singapore: A record of Mesozoic orogeny and amalgamation in Sundaland, and of post-orogenic faulting

Singapore bedrock geology is dominated by late Permian to Triassic arc magmatism and a genetically related, essentially Middle to Upper Triassic, marine to fluvial volcano-sedimentary inner forearc succession. These Mesozoic strata are deformed into a pattern of NE-translated ductile–brittle deformation structures during the latest Triassic to earliest Jurassic collision and amalgamation of the Sibumasu continental block with the southern part of the Sukhothai Arc. The subduction-related magmatic complex represented in Singapore by the granitic to gabbroic plutons of the Bukit Timah Centre likely acted as a backstop to thrusting at this time. Collisional tectonics drove progressive shortening and steepened earlier-formed inclined asymmetrical folds, culminating in the regional-scale development of a non-coaxial, NE-vergent and NE-facing, fold and thrust system. In Singapore, the Murai Thrust and Pasir Laba Thrust are identified as major elements of this system; both are associated with SW-dipping thrust-imbricate duplex slices. Two distinct early Cretaceous (Berriasian and Barremian) sedimentary successions overstep these collisional tectonic structures. An array of mostly NE–SW and ENE–WSW trending faults and fractures acts as an important control on bedrock unit distribution across Singapore and are most likely generated by Cenomanian dextral shear stress. That stress locally reactivated faults initiated during orogeny, or even earlier. Knowledge of the geotechnical impact of these structural features is critical to both future development and ongoing management of the subsurface in Singapore.

W and Li-Cs-Ta geochemical signatures in I-type granites – A case study from the Vosges Mountains, NE France

In light of the recent ‘lithium boom’, a screening study for Sn-W and Li-Cs-Ta deposits was carried out and located an early 1980s Bureau de Recherches Géologiques et Minières (BRGM) geochemical survey with W anomalies of >50 ppm in the Sainte-Marie-aux-Mines area, NE France. These anomalous areas, along with historic mining of polymetallic veins in the area, led to the present investigation of the regional potential for Variscan vein-hosted Sn-W-Li-Cs-Ta and Mesozoic-Cenozoic Cu-Pb-Zn-As-Ag-Ni veins in the central Vosges Mountains.A total of 144 stream sediment samples were collected in the central Vosges Mountains covering an area of >200 km2. Multi-element pXRF and ICP-MS assays revealed previously undescribed W-Li-Cs-Ta geochemical anomalies related to major ENE-WSW trending fault zones and splay faults in the I-type Central Vosges Mg-K (‘CVMg-K’) granite. Field mapping evidence, previously published age data along with strong geochemical fractionation trends obtained from K/Rb, Zr/Hf and Nb/Ta ratios, imply that the CVMg-K granites were intruded by younger crustal melts derived from crustal anatexis of metasedimentary and metaigneous source rocks, that potentially exploited pre-existing fault control, and subsequently fractionated and enriched incompatible elements of economic interest.This study demonstrates that Sn-W-Li-Cs-Ta geochemical anomalies can occur in I-type granites, if later orogenic processes leading to crustal anatexis resulted in the production of peraluminous melts, strong magmatic fractionation and emplacement of mineralization along regional fault zones. Therefore, the presence of I-type granites in Variscan and other geological terrains should encourage regional Sn-W-Li-Cs-Ta exploration.

FEATURES CONTROLLING RADIOELEMENTS DISTRIBUTION IN AL AGLAB REGION, NORTH EASTERN DESERT, EGYPT

This work is an approach to integrate multi-discipline set of data including geological and remote sensingas well as airborne, ground radiometry and aeromagnetic geophysical data aiming to establish the controllingfeatures of the radioelement potential distribution within Al Aglab area. The investigated area of Al Aglabregion is located within the Northern Eastern Desert litho-structural domain where the outcropping rock typescomprise: metavolcanics (oldest), quartz diorites, Dokhan volcanics, Hammamat sediments and gabbrogranophyreassociations intruded by monzogranite, syenogranite and post granitic dykes (youngest). Thegranitic rocks display sharp contact with the older pre-existing rock. The radioactive anomalies lay at theintersections between NNW-SSE to N-S and NE-SW to ENE-WSW trending structure lineaments (joints,faults and shear zones) within the syenogranites pluton of G. Al Aglab and along the NE-SW to ENE-WSWtrending felsitic dykes, jasperoid veines and pegmatite bodies.Landsat Thematic Mapper (TM) remote sensing map in great concordance with airborn gamma rayspectrometric data delineate most of the surface geology over the compiled geological map. The extractedstructural lineaments are coherent to those inferred from the analytical signal magnetic maps in completeharmony with the NNW-SSE to N-S and NE-SW to ENE-WSW trending fault systems obtained from theground geological and radiometric investigation. Moreover, different types of alterations such as hematitization,Kaolinitization, chloritization and silicification are recorded within the synogranite, where their distributionis mainly controlled by the intersections between the NNW-SSE to N-S and NE-SW to ENE-WSW trendingstructural lineaments.The studied structural lineaments are arranged according to three main trend clusters around NNW-SSEto N-S, NE-SW to ENE-WSW and NW-SE to WNW-ESE directions representing the main factor controllingthe emplacement of the different granitic intrusions in Al Aglab area. Moreover, most of these lineaments, asobtained from field measurements, show strike-slip movement criteria within an extensional context. All therecorded mineralized occurrences are related to extensional events that create the necessary space either formineral entrapment and/or the percolation of the hydrothermal carrying solutions along these structural trendsespecially at their intersections.Spectrometric data (K, U and Th) shows a very high consistency between the radioelement distributionsand the differentiated outlines of different rock types. Results from integrated field studies, Landsat ThematicMapper (TM) remote sensing data and geophysical data analyses provide important constraints on the tectonicevolution of Al Aglab area and importantly, demonstrate the strength of combining remote sensing data withgeophysical data and geological observations in resolving the surface geometry of major fault systems andtheir impact on the radioelement potential distribution within Al Aglab area.

Relationship between geological structures and groundwater flow and groundwater salinity in Al Jaaw Plain, United Arab Emirates; mapping and analysis by means of remote sensing and GIS

Geological structures can be of great influence groundwater movement and accumulation in the surface and subsurface, and should therefore be taken into consideration in studies related to groundwater contamination impact. This study attempts to investigate the influence of geological structures on groundwater flow and groundwater salinity in Al Jaaw Plain, United Arab Emirates. A set of thematic maps derived from digital elevation model (DEM), LANDSAT, and Spaceborn Imagine Radar-C/X-Band Synthetic Aperture Radar were enhanced by applying Soble filter with 10 % threshold and equalization enhancement to reveal and map geological structures crosscut the entire region. Drainage pattern was derived from DEM automatically using D8 algorithm. The algorithm determines in which neighboring pixel any water in a central pixel will flow naturally. The trends of geological structures and drainage pattern extracted from remote sensing data were correlated with the spatial variation of hydraulic head, thickness aquifer, and groundwater salinity in the region. The results of the study reveal that the wadi courses, thickness of the aquifer, and topography are structural controlled by NNW–SSE, NE–SW, and ENE–WSW trending fault zones, significantly influencing the groundwater flow and groundwater contamination in Al Jaaw Plain.

Morphotectonic segmentation and spatial variability of neotectonic activity along the Narmada–Son Fault, Western India: Remote sensing and GIS analysis

Morphotectonic analysis of landscape using remote sensing and GIS is an effective way of deducing the pattern and spatial variation of neotectonic activity along poorly investigated active faults. In this paper, we evaluate the neotectonic activity along a part of the seismically active but poorly understood Narmada–Son Fault (NSF), a more than 1000km long ENE–WSW trending fault transecting through the central part of the Indian plate. The NSF in the study area is geomorphologically expressed as an ENE–WSW trending line of north facing scarps that delimit the rugged mountainous topography of the uplands to the south and the alluvial basin to the north which prominently slope away from the scarps. The scarps are developed in the south dipping basaltic flows belonging to the Deccan Trap Formation of the late Cretaceous–Eocene and continue westward into Tertiary rocks and further as a paleobank of the Narmada River. Field studies have revealed that the NSF in the study area is divisible into four morphotectonic segments (I to IV), which is attributed to the Tilakwada, Karjan, Madhumati and Rajpardi Faults cutting across the NSF. We characterize the neotectonic activity of the segments through DEM analysis, using geomorphic indices which include the mountain front sinuosity, hypsometric curves, hypsometric integral, asymmetry factor, stream length–gradient index and channel sinuosity. Interpretations of morphometric data are well supported by field data. Both the entire length of the NSF and the cross faults are found to be neotectonically active. We demonstrate that segment II has undergone the highest intensity of neotectonic activity followed by segments III, I and IV. This is corroborated by the highest elevation of scarps and the alluvial deposits, the deepest incision (~40m depth) and steep northward slope of the alluvial plain in segment II. We infer that the NSF is characterized by differential uplift in recent past which agrees with the high angle reverse faulting with oblique-slip movements along the cross faults.

The Khao Yai Fault on the southern margin of the Khorat Plateau, and the pattern of faulting in Southeast Thailand

A study of Google Earth images has revealed a hitherto-unrecorded gently curved lineament within the southern marginal zone of the Khorat Plateau in eastern Thailand. The lineament, confirmed by digital elevation model (DEM) images, is at least 130 km long and coincides with a dip reversal of the Mesozoic Khorat Group. It is interpreted here as a fault, named the Khao Yai Fault, and it has characteristics which make it unusual within the Khorat Plateau. The fault forms the northern boundary of a belt of several ENE–WSW trending fault splays which are thought to link with the Mae Ping Fault further south; this is interpreted as a left-stepping, sinistral strike-slip duplex about 50 km wide and 150 km long. Apatite fission track data indicate that exhumation began during the earliest Palaeogene. The Khao Yai Fault is considered in its regional context which includes the Cardamomes Mountains of Cambodia, the offshore Phuquoc-Kampot Basin, and the Khao Thalai Red-beds outlier of the Khorat Group in Southeast Thailand. The latter is interpreted as a down-faulted sliver of the Khorat Group in the Tha Mai Fault belt which is thought, in turn, to be a splay of Thailand's other major regional fault, the Three Pagodas Fault. Carboniferous, Permian and Triassic shallow-marine rocks with unusual faunas occur in a limited NNW–SSE trending zone to the west and NNW of the Tha Mai Fault and it is suggested that wrench movement on the fault played a part in the emplacement of these rocks.

Analogue and geophysical modelling of the Garmsar Salt Nappe, Iran: constraints on the evolution of the Alborz Mountains

SUMMARY The Alborz Mountains are forming a ∼100-km-wide east–west trending orogenic belt that stretches 2000 m across northern Iran south of the Caspian Sea. The Alborz Mountains consist of salt-bearing Neogene sediments, which are folded and cut by faults. Global positioning system studies indicate N–S directed shortening across the Alborz range, which is accommodated by right and left-lateral strike-slip along ESE–WNW and ENE–WSW trending faults, respectively. A 20 km × 10 km × 03 km sheet of salt extruded over the central plateau of Iran arising at the front of the advancing Alborz Mountains. The extruded salt forms the Eyvanekey plateau between Eyvanekey and Garmsar, which is now known, as the Garmsar Salt Nappe. To get more insights in the evolution of the Garmsar Salt Nappe, analogue modelling has been carried out using PDMS as salt analogue and sand as analogue for the brittle overburden. The structures produced consist of folds and thrusts, which were formed while the salt analogue PDMS was rising up. The modelling results are compatible with our interpretation that the deformation front of the Alborz Mountains advanced SSW when overriding a salt sequence in the Garmsar area. Depth estimations using the gravity and magnetic fields suggest that the salt in the Garmsar Salt Nappe extruded from a depth less than 2000 m.

Structural and tectonic interpretation of geophysical data along the Eastern Continental Margin of India with special reference to the deep water petroliferous basins

The study area encompasses the Eastern Continental Margin of India (ECMI) and the adjoining deep water areas of Bay of Bengal. The region has evolved through multiple phases of tectonic activity and fed by abundant supply of sediments brought by prominent river systems of the Indian shield. Detailed analysis of total field magnetic and satellite-derived gravity data along with multi channel seismic reflection sections is carried out to decipher major tectonic features, basement structure, and the results have been interpreted in terms of basin configuration and play types for different deep water basins along the ECMI. Interpretation of various image enhanced gravity and magnetic anomaly maps suggest that in general, the ENE–WSW trending faults dominate the structural configuration at the margin. These maps also exhibit a clear density transition from the region of attenuated continental crust/proto oceanic crust to oceanic crust based on which the Continent Ocean Boundary (COB) has been demarcated along the margin. Basement depths estimated from magnetic data indicate that the values range from 1 to 12 km below sea level and deepen towards the Bengal Fan in the north and reveal horst–graben features related to rifting. A comparison of basement depths derived from seismic data indicates that in general, the basement trends and depths are comparable in Cauvery and Krishna–Godavari basins, whereas, in the Mahanadi basin, basement structure over the 85°E ridge is clearly revealed in seismic data. Further, eight multichannel seismic sections across different basins of the margin presented here reveal fault pattern, rift geometries and depositional trends related to canyon fills and channel–levee systems and provide a basic framework for future petroleum in this under explored frontier.

Seismological aspects of the Cairo earthquake, 12th October 1992

On 12 October 1992 a significant earthquake (MB = 5.8) occurred southwest of Cairo in the vicinity of the Dahshour region. For the mainshock, an average moment release M 0 = 9.70E17 n-m; a fault length = 13.8 km; an average displacement = 0.22 m; a stress drop = 0.760 MPa and maximum spectral magnitude = 5.78 were obtained. Spectral magnitude calculations were used in this analysis. The distribution of the well-recorded aftershocks over 15 months using a temporary seismic network installed immediately after the mainshock shows a zone of concentrated activity. Three composite focal mechanism solutions are constructed, using P wave polarity data for 30 earthquakes. These solutions have a mechanism, involving normal and right lateral strike slip motion along E-W to ENE-WSW trending fault plane. The P-wave polarity data of the individual 30 earthquakes are inverted to determine the stress tensor. The stress field estimated is extensional with s3 in the direction of NNE. The stress pattern determined from the inversion is in good agreement with the one estimated from the three composite fault plane solutions, while the neotectonic situation in Northern Egypt supports ENE-WSW extensional movement. However, the region of study has suffered both extensional tectonic activity of the Northern Red Sea rift zone and the compressional tectonic activity along the Hellenic arc. The inconsistency of the principal tension directions may belong to the interaction between the extensional and compressioal tectonics.

Late Pleistocene tectonic-geomorphological development within a passive margin — The Cariatá trough, northeastern Brazil

Studies on the geomorphological evolution of the South American passive margin have been based on the pediplanation model, which predicts that its morphology is a response to regional uniform uplift and concomitant development of erosion surfaces. We combined remote sensing, geological mapping, lithostratigraphic and facies analyses, and luminescence dating in the Cariatá trough, northeastern Brazil, in order to determine how brittle tectonics and climate influenced colluviation and the shaping of local landforms in the Quaternary. Our work indicates that Cariatá is an asymmetrical trough ∼ 40 km long, ∼ 25 km wide, 250–550 m deep, and delimited by ENE–WSW-trending faults to the north and south. We recognized an ENE–WSW-oriented compression related to a strike-slip faulting regime, which corresponds to the present-day stress field in the region. This faulting event led to the deposition of colluvial fans, shed from adjacent uplifted crustal blocks, in a tectonically controlled depression. The colluvial succession is ∼ 45 m thick and presents two facies assemblages that filled the southern and, in particular, the northern borders of the trough: non-cohesive debrisflow and mudflow deposits. Optically stimulated luminescence dates of the sedimentary infill yielded ages at 224–128 ka and 45–28 ka, dominated by debrisflow and mudflow deposits, respectively. These ages may be over-estimated due to poor bleaching of colluvium, but they and our field data suggest that the margins of the trough were tectonically uplifted and eroded twice in the Late Pleistocene. The spasmodic colluvial accretion reflects the occurrence of high-magnitude, low-recurrence episodes probably associated with climate shifts in a semi-arid hillslope system. It follows that the present-day low-lying piedmont in which the Cariatá trough occurs is a juxtaposition of surfaces of various ages. This trough may have counterparts across the region. These conclusions do not corroborate the application of the cyclical pediplanation hypothesis in the area.

Variations in Late Cenozoic–Recent strike-slip and oblique-extensional geometries, within Indochina: The influence of pre-existing fabrics

From Yunnan to Northern Thailand, Late Cenozoic–Recent faults strike predominantly NNE–SSW, N–S to NNE–SSW and NE–SW to ENE–WSW. Associated sedimentary basins are aligned NE–SW to N–S. The regional fault patterns are commonly interpreted as strike-slip dominated deformation throughout the area. Releasing bend and en echelon stepping patterns on faults bounding sedimentary basins indicate sinistral displacement on NE–SW to ENE–WSW trending faults. Yet, in the escape tectonics model left lateral displacement on the NE–SW to ENE–WSW faults is thought to have occurred late in the Miocene, whilst earlier motion was dextral. However, in Yunnan the NE–SW Shmax direction required for dextral motion on the N–S Sagaing, Nanting and Gaoligong fault zones is consistent with sinistral motion on ENE–WSW striking faults, which is still their sense of motion today. In Northern Thailand the dextral–sinistral switch model during the Miocene is not tenable because the Fang basin is of Late Oligocene–Pliocene age, and requires similar age sinistral motion on the ENE–WSW Mae Chan fault in order to have opened. In an alternative model, Northern Thailand is interpreted to have evolved predominantly by oblique extension. The Golden Triangle area marks a transition from transtensional deformation in the north to oblique extension in the south. The activation of pre-existing fabrics strongly affects both strike-slip and extensional faults and has given rise to the similar extensional and strike-slip fault patterns. Multiple episodes of basin inversion in Northern Thailand during the Miocene require short-term changes in stress pattern. To produce the inferred changes in stress pattern it is suggested that stresses radiating out from the Himalayan syntaxis exert a strong influence, but were not the only important forces acting on the region.

‘Singwang strike-slip duplex’ around the Pohang Basin, SE Korea: its structural evolution and role in opening and fill of the Miocene basin

In order to elucidate the structural characteristics of the Yangsan Fault in southeastern Korea the Ganggu-Angang area has been studied in terms of fault branching and fault linkages. According to the results of this study, eight faults (e.g., the Winmal, Docheon, Yugye, Gojusan, Naengsu, Malgol, Ogeum and Moa faults) branch from the Yangsan Fault, and divide it into at least nine segments. The strike separations of these faults suggest that they are much longer than their presently exposed lengths and that the majority of them are linked to the Jangsa Fault as connecting faults. This distribution of faults comprises the ‘Sing-wang strike-slip duplex’ with the geometry of an extensional strikeslip duplex or pull-apart basin. Block faulting took place along the connecting faults and related transfer faults, resulting in the deposition of coarse grained sediments in fan-delta systems. The shape of basalt masses around the northern tip of Yangsan Fault reveals that this fault had a sinistral motion, possibly in relation to the Pliocene (?) rotation of the Pohang-Ulsan Block. At a recent stage, the Yangsan Fault was displaced by WNW-ESE, E-W or ENE-WSW trending faults. Quaternary reactivation occurred along the Yangsan Fault but not along the Jangsa Fault.

Triassic stratigraphy and structural evolution of a rift basin: the Eç Çour basin, High atlas of Marrakech, Morocco

Stratigraphic and structural analysis of the Triassic sedimentary basins of the High Atlas of Marrakech, in particular of the Eç Çour basin, indicates that the Proterozoic–Paleozoic block (“Massif Ancien”) formed a paleogeographic high during this period, with the basement block acting as an east–west trending horst, separating two large half-grabens to the north and to the south. The marginal half-grabens, which were filled by red beds essentially of continental origin, were most likely linked via transverse pathways. In the Eç Çour basin, the Triassic sedimentary deposits are subdivided into three formations of which the first two are continental and the third shows marine influences. Structurally, this area is characterized by two main (ENE–WSW and NNE–SSW) fault directions. The ENE–WSW trending faults, which include the major South Atlas fault, have an extended history of displacement during pre-Triassic Hercynian to Late Hercynian and during Triassic times. In contrast, the NNE–SSW-striking faults appear to have only a syndepositional Triassic history with, rarely, a late Hercynian heritage. This structural pattern is the result of a pure extensional regime during which synsedimentary normal faulting caused the assymmetry of the basin. This was reflected by high subsidence rates in the NE that progressively decrease to the NW towards the rising Proterozoic–Paleozoic massif of the High Atlas. During the tertiary Alpine–Atlasic orogeny, the ENE–WSW-striking normal faults were inverted.

Characteristic features of the North Anatolian Fault in the eastern Marmara region and its tectonic evolution

Shallow seismic profiles are used to image the tectonic and stratigraphic setting in Izmit Bay, eastern Marmara Sea. Four seismic units were detected in the sediments of Izmit Bay, which carry the effects of the tectonic uplift and global sea-level variations. The area appears to be a negative flower structure controlled by the northern branch of the North Anatolian Fault. In the late Pliocene the North Anatolian Fault reached Marmara Sea as a master fault. Small-scale faults were evolved around the master fault under a regime of dextral shear. The northward bending of the master fault gave rise to en-echelon faults, opening the sub-basins in Izmit Bay as releasing bends. The secondary faults which developed as low-angle oblique to the master fault are also the products of dextral shear. To the south, short dextral ENE–WSW trending faults are interpreted as ‘P’-shears, while to the north, dextral WNW–ESE trending faults represents ‘R’-shears. These faults absorbed the deformation at the western end of the ruptured zone during the Izmit Earthquake (August 17, 1999), causing the migration of failure stress westward.

First palaeomagnetic results from the Cycladic Massif, Greece, and their implications for Miocene extension directions and tectonic models in the Aegean

The Aegean region is characterised by major fault-bound blocks with markedly contrasting structural trends. Recent models account for these variations either through differential block rotation or through a process of radial extension. The Cycladic Massif in the back-arc region to the Hellenic trench represents a critical area where apparently divergent extension directions are observed. This contribution provides the first palaeomagnetic results from this area. Magnetic remanence and AMS data are presented for Middle Miocene granodiorites on the islands of Mykonos and Naxos. These rocks are exposed in the footwalls of major extensional detachments within the metamorphic core complexes that characterise the Cycladic Massif. A clear correlation between the orientation of an L-S fabric developed within the intrusions and the magnetic fabric data is observed, demonstrating the presence of a ‘normal’ magnetic fabric of extensional origin. The mean extension directions inferred from the orientations of the maximum axes of the magnetic susceptibility ellipsoids are markedly divergent at 064° (Mykonos) and 351° (Naxos). Northeasterly in situ magnetic remanences are observed on Mykonos, whereas those on Naxos are directed to the NNW. Neither direction corresponds to the appropriate palaeomagnetic reference direction for the Miocene. Standard palaeomagnetic tilt corrections cannot be applied at these sites because of the lack of palaeohorizontal markers. Deviations of in situ remanences away from the reference direction are potentially due to rotation around any one of an infinite number of net tectonic rotation axes. Each of these may be arbitrarily decomposed into components of tilting and vertical axis rotation. Geologically realistic solutions which restore the in situ magnetisations back to the reference direction may be found by consideration of regional structural trends, assuming that tilting is controlled by the major block-bounding faults. These solutions indicate significant components of differential vertical axis rotations across the Cyclades. Mykonos is bound by NW-SE trending regional faults and is shown to have rotated clockwise by ca. 22°. In contrast, Naxos is bound by ENE-WSW trending structures and has rotated anticlockwise by ca. 33°. The apparent divergence of extension directions inferred from both the magnetic anisotropy and structural data is resolved by correcting the mean AMS data for subsequent rotation using the rotation solutions derived from the magnetic remanence data. The data imply that the primary Middle Miocene extension direction in the Cycladic Massif was nearly uniform along a NNE-SSW azimuth. We consider that the uniform extension direction and opposing senses of block rotation determined for this portion of the Cycladic massif are most consistent with a recent model describing deformation within the Aegean in terms of a system of broken, fault-bounded slats. Within this scenario, Mykonos occupies a position on a clockwise rotating slat which continues northwest, at least as far as Evvia. A major ENE-WSW trending fault between the islands, previously identified on shallow seismic sections, separates this domain from an anticlockwise rotating slat which includes Naxos.