A strike‐slip core complex from the Najd fault system,Arabian shield

Metamorphic core complexes are usually thought to be associated with regional crustal extension and crustal thinning, where deep crustal material is exhumed along gently dipping normal shear zones oblique to the regional extension direction. We present a new mechanism whereby metamorphic core complexes can be exhumed along crustal‐scale strike‐slip fault systems that accommodated crustal shortening. The Qazaz metamorphic dome in Saudi Arabia was exhumed along a gently dipping jog in a crustal‐scale vertical strike‐slip fault zone that caused more than 25 km of exhumation of lower crustal rocks by 30 km of lateral motion. Subsequently, the complex was transected by a branch of the strike‐slip fault zone, and the segments were separated by another 30 km of lateral motion. Strike‐slip core complexes like the Qazaz Dome may be common and may have an important local effect on crustal strength.     

The najd fault system revisited; a two-way strike-slip orogen in the saudi arabian shield

The Najd strike-slip fault system extends over the northeastern Arabian Shield in a zone >1200 km in length and >300 km wide. Faults trend NW-SE with strike lengths >500 km but small sinistral displacements of <25 km. Cumulative displacement across the zone is >240 km. Najd faults were active in the late Proterozoic and post-date cratonization of the Shield. Associated secondary structures include grabens, thrust faults, folds and dike swarms. In the southwest of the Najd system, near Zalm, initial faulting was dextral and began earlier than formerly thought. Emplacement of a plutonic complex was controlled by Najd fractures of dextral geometry and displacements. The same fractures were active before and after deposition of a group of volcanosedimentary rocks in grabens orientated consistently with development in a dextral strike-slip regime. Graben deformation was controlled by sinistral motion along the same fractures responsible for graben development and also by younger fractures of sinistral geometry and displacement. Dike swarms in the area are also consistent with early dextral and later sinistral shear of Najd trend. Structures in the Zalm area occur throughout the Najd system and the consistent chronology of older dextral structures dislocated and deformed by younger sinistral faults suggests a reversal in the sense of motion of the Najd system as a whole.     

Orientation studies for gold in the central pediplain of the Saudi Arabian shield

Orientation studies were carried out by the BRGM mission in Saudi Arabia between 1982–1986 to investigate the dispersion of gold in soils and wadi sediments and to define the optimum sample medium around eight prospects in the central Arabian pediplain. A comparison of gold distribution in several sieved fractions with various pedologic horizons shows that the distribution of gold changes abruptly from the coarser sizes near the source to the finest sizes 150 m downstream.The coarse fraction recommended by previous workers is not representative and yields erratic results in the specific environment of the pediplain. Gold is enriched and more homogeneously distributed in the minus 80 μm fraction of the skeletal soils and wadi sediments.Regional geochemical survey can be effective using a sensitive analytical method for gold and the minus 80 μm fraction of the brown gravelly sediment with a minimum density of 2 samples per km². Geochemical halos in the 30–50 ppb range indicate gold mineralization 500 to 1000 m upstream, depending on relief.In a first follow-up stage, continued use of the same size of the upper argillaceous layer with a regular reg sampling grid gives more contrasted and more extensive anomalies than using a coarse material. These anomalies may be slightly offset by the present arid erosion. A second follow-up stage is then recommended at a closer grid and sampling the brown blocky layer below the surficial reg pavement. Again, the use of the finest fraction at this exploration stage has given the best probability of finding a blind gold target.Optical determination of gold on a nonmagnetic fraction of heavy panconcentrate is not recommended, because nugget growth is practically absent in the present arid conditions and flour-sized gold particles are lost in desliming samples.     

Interpretation of a seismic-refraction survey across the Arabian shield in western Saudi Arabia

In February 1978 seismic-refraction profiles were recorded by the U.S. Geological Survey along a 1000 km line across the Arabian Shield in western Saudi Arabia. This report presents a traveltime and relative amplitude study in the form of velocity-depth functions for each individual profile assuming horizontally flat layering. The corresponding cross section of the lithosphere showing lines of equal velocity reaches to a depth of 60–80 km.The crust thickens abruptly from 15 km beneath the Red Sea Rift to about 40 km beneath the Arabian Shield. The upper crust of the western Arabian Shield yields relatively high-velocity material at about 10 km depth underlain by velocity inversions, while the upper crust of the eastern Shield is relatively uniform. The lower crust with a velocity of about 7 km/s is underlain by a transitional crust-mantle boundary. For the lower lithosphere beneath 40 km depth the data indicate the existence of a laterally discontinuous lamellar structure where high-velocity zones are intermixed with zones of lower velocities. Beneath the crust-mantle boundary of the Red Sea rift most probably strong velocity inversions exist. Here, the data do not allow a detailed modelling, velocities as low as 6.0 km/s seem to be encountered between 25 and 44 km depth.     

The Effect of finite strain and deformation history of HALABAN area,eastern Arabian shield,Saudi Arabia

The present study aims to evaluate a relationship between the mineralogy and structural analysis in the Halaban area and to document the tectonic evolution of Halaban and Al Amar faults. The collected samples were taken from deformed granitiods rocks (such as granite, gneisses and tonalite), metasedimentary, metavolcanic, metagabbro and carbonate rocks are trend to NE-SW with low dip angle in the Halaban area. These samples were 8 from granite, 14 metagabbro, 6 metavolcanics, 5 tonalite, 6 metasedimentary, 10 gneisses and 8 carbonate rocks. Our results are described for the different axial ratios of deformed rocks as the following: XZ sections range from 1.10 to 4.60 in the Fry method and range from 1.70 to 2.71 in the Rf/? method. YZ sections range from 1.10 to 3.34 in the Fry method and range from 1.62 to 2.63 in the Rf/Phi method. In addition, XY sections range from 1 to 3.51 in the Fry method and range from 1 to 1.27 in the Rf/? method for deformed granite rocks, metasedimentry rocks, and metagabbro. The stretch axes for measured samples in the X direction axes (S_X) variety from 1.06 to 2.53 in the Fry method and vary from 1.20 to 1.45 in the Rf/? method. The values of the Y direction axes (S_Y) vary from 0.72 to 1.43 in the Fry method, which indicates contraction and extension in this direction and vary from 1.13 to 1.37 in the Rf/? method which indicates extension in this direction. Furthermore, the Z direction axes (SZ) varies from 0.09 to 0.89 in the Fry method and from 0.52 to 0.71 in the Rf/? method. The stretches axes in the Z direction (SZ) show a vertical shortening about 11% to 91% in the Fry method and show vertical shortening about 29% to 48% in the Rf/? method. The studied rock units are generally affected by brittle-ductile shear zones, which are sub-parallel to parallel NW or NNW trend. It assumed that different rock types of have similar deformation behavior. Based on these results, it is concluded that the finite strain is accumulated during the metamorphism after that was started the deformation by thrusting activity. The contacts between the different rock types were deformed during thrusting under semi-brittle to ductile deformation conditions by simple shear. A component of vertical shortening is also involved causing subhorizontal foliation in the Halaban area.     

Deltaic sedimentation in the Lake Hazar pull-apart basin, south-eastern Turkey

Lake Hazar lies within a small pull-apart basin along the East Anatolian Transform Fault in south-eastern Turkey. Deltas are formed where streams debouch into the low-energy lacustrine environment. The facies constituting the deltas include delta plain debris flow, braided stream, and marginal lacustrine deposits; delta front foreset and mouth bar deposits; prodelta and lacustrine deposits. The facies are spatially restricted with sharp transitions. Facies sequences and relationships indicate two distinct styles of deltaic sedimentation. Fan deltas with a tripartite structure characteristic of Gilbert-type deltas comprise the marginal drainage system and form along the basin margins. Mouth bar deltas develop where the axial drainage system of the basin debouches into the lake. The distribution of the two deltaic types is thought to be a function of gradient and controlled by position relative to faults within the basin.     

Effect of tectonic prominence and growth of the Arabian shield on Paleozoic sandstone successions in Saudi Arabia

Paleozoic successions in Saudi Arabia are exposed around and bordering the south, north, and northeastern edge of the Arabian shield. They are represented by the Wajid group in the south and by the Taymah, Tabuk, Qalibah, Huj, and Buraydah groups in the north and northeast. The Wajid group includes Dibsiyah, Sanamah, Khusayyan, and Juwayl formations. The Taymah group includes Siq, Amai’er, Quweira, Saq, and Qasim formations. The Tabuk group includes Zarqa, Sara, and Hawban formations. The Qalibah group includes Baq’a, Qusaiba, and Sharawra formations. The Huj group includes Tawil, Jauf, and Jubah formations. The Buraydah group includes Berwath, Unayzah, and Khuff formations. The Wajid group form one block in the south and the other groups form another block in the north, and they can be correlated. There are similarities between the northern belt which consists of the Cambro-Ordovician formations of the Tayma and Tabuk groups and the southern belt which consists of the Dibsiyah and Sanama formations of the Wajid group. Similarities include sandstone composition, sedimentary environment, paleocurrent directions, unconformities, tectonic events, and influence of Gondwana glaciations. These formations and probably some or all the rest of the Paleozoic formations used to form one block but later separated after erosion caused by gradual tectonic growth, uplift, and prominence of the Arabian shield. During early Paleozoic time, the process started by poststabilization then sedimentation and at a later stage the growth and uplift of the shield occurred gradually. Growth of shields is a fact and it is the only way to explain the exposure of the Wajid sandstone on top of the highest mountain of the shield which exceeds 3,000 m in As Sawdah in Asir area in southwestern Saudi Arabia. The sandstone sediments of these outcrops were deposited on a low lying basin before been raised to this elevation.     

Deformed dike swarms as an implication of transpression deformation in Western Arabian shield,Wadi Fatima,Saudi Arabia

Utilization of satellite images and field observations of dike swarms in pre-Fatima basement show that these dikes are older than the overlaying Fatima Formation. Dikes digitization and orientation analysis on satellite images show that the prevailing trend of the dikes is ENE-WSW. The granitic rocks of pre-Fatima basement and its hosted dikes expose evidences of completely a different deformation regime from the overlaying Fatima Formation. These evidences include shearing, dextral shear indicators, isoclinal folds, deflection and rotation of crystals, mineral elongation, and mylonitic and gneissose textures. Strain analysis results of using Fry method on quartz and feldspar grains support the presence of deformation in these ENE-WSW dikes. These results gave a strain ratio of 2.1:1.3:1, which suggest an amount of 40% stretching in the ENE-WSW direction parallel to the dike walls, and an amount of 30% shortening in the NNW-SSE direction. Mesoscopic and microscopic scale structures confirm the existence of dextral ductile-brittle shearing followed the emplacement of the dikes and before the pure shear deformation that caused the thrusting and folding of Fatima Formation. This ductile-brittle deformation is correlated with the dextral transpression that formed the Fatima Shear Zone (FSZ).     

Wind sedimentation in the Jafurah sand sea, Saudi Arabia

The Jafurah sand sea of the Eastern Province of Saudi Arabia extends along the Arabian Gulf coastline from Kuwait in the north to the Rub Al Khali in the south, a distance of about 800 km. Sand drifts southward to south-eastward from regions of high wind energy in the north to low wind energy in the south. The aeolian landscape is zoned, with areas of deflation, transport and deposition from north to south. Drift rates in the zone of transport, near Abqaiq, range from 2 m³ m-w^-1 yr^-1 on sabkhas, to 29 m³ m-w^-1 yr^-1 on the crests of dunes. Average drift rates of approximately 18 m³ m-w^-1 yr^-1 observed during the study can cause about 1 m of accumulation per 5500 yr in a 100 km zone of deposition downwind, not including the bulk transport represented by the forward advance of dunes. Dune advance ranged from 23 m (2.9 m high dune) to 3 m (23 m high dune) during April-October 1980. The study area consists of dune, interdune, sand sheet and siliciclastic sabkha terrains, each of which is characterized by differing drift rates, and differing rates of erosion or deposition. Sedimentation occurs by lateral movement of dunes and interdunes, and vertical accretion by sand sheets and sabkhas.     

Neogene non-marine sedimentation and tectonics in small pull-apart basins of the San Andreas fault system, Sonoma County, California

The San Andreas fault system in northern California forms an 80–90 km wide zone of right-lateral shear. Extensional tectonism within this broad shear zone is indicated by both Neogene silicic volcanic rocks that gradually young in the direction of shear propagation to the north-west and by numerous Neogene faultbounded structural basins filled with thick non-marine sequences. The Little Sulphur Creek basins, three well-exposed 1·5–2 km wide pull apart basins within this shear system, have sedimentation patterns analogous to those of much larger pull-apart basins. They were formed and subsequently deformed by east-west extension and by north-west to south-east-orientated right-slip concurrently with basin filling. Palaeocurrent and maximum-clast size data indicate both lateral sediment transport from fault-bounded basin margins and longitudinal transport down the basin axes. The basins are filled primarily with coarse alluvial-fan and streamflow deposits derived from a surrounding igneous, sedimentary, and metamorphic provenance. Two of the basins contain basin-plain-type lacustrine turbidites that grade laterally into distal alluvial fan, fan-delta, and sublacustrine delta deposits. Talus deposits along the south-west margin of the basins contain megabreccia indicative of active uplift. Structures indicative of dewatering, liquefaction, and slumping suggest penecontemporaneous tectonism.     

Wrench faults,pull-apart basins,and volcanism in central Oregon: A new tectonic model based on image interpretation

A tectonic study of the Newberry Crater region of central Oregon has been based on the interpretation of Landsat Thematic Mapper imagery. Two major faults, the Brothers-Tumalo and Eugene-Denio Faults, pass NW-SE through the region and step to the right at the eastern margin of the Cascades Range. Dextral wrench faulting on these structures during the Tertiary controlled the formation of the La Pine Basin, a pull-apart structure containing Tertiary and Quaternary sediments and volcanics. Tertiary wrench faulting appears to have been associated with rotations of crustal blocks at a plate margin, but was superseded in the Quaternary by extensional faulting of the Basin and Range province. Newberry Crater and other major bimodal volcanic centres in the NW Cordillera (Crater Lake, Medicine Lake, Mt. St. Helens) seem to have a similar tectonic setting in crustal pull-aparts. A relationship between magma type and fault trend at Newberry and Medicine Lake is suggested.     

The Cretaceous volcanic succession around the Songliao Basin,NE China: relationship between volcanism and sedimentation

With volume ratio of 8:1:1.5 amongst acidic, intermediate and basaltic rocks, the Cretaceous volcanics around the Songliao Basin are a series of high‐K or medium‐K, peraluminous or metaluminous, calc‐alkaline rocks, lacking typical basalts and peralkaline members of typical rift‐related types. Their eruption ages range between 133 and 127 Ma, 124 and 122 Ma and 117 and 113 Ma respectively. They are high in total (Rare earth element) REE contents (96.1–326 ppm), enriched in LREE and depleted in HREE (LREE/HREE = 4.6–13.8), with negative Eu and Ce anomalies (Eu/Eu* = 0.04–0.88; Ce/Ce* = 0.60–0.97). They have enriched large‐ion lithophile elements (e.g. K, Ba, Th) and depleted high field strength elements (e.g. Nb, Ti and Y), suggesting a subduction‐related tectonic setting. The volcanic activities migrated from south to north, forming a successively northward‐stepping volcanic series and showing a feature significantly different from the overlying sedimentary sequence striking northeast. Thus, an overlap basin model was proposed. Accompanied by opening of the basin, the volcanogenic succession was formed at the block‐faulting stage (131–113 Ma) owing to the closure of the Mongolia–Okhotsk ocean in the Jurassic and early Cretaceous, while the overlying sedimentary sequence was unconformably deposited at the spreading stage (Albian–Maastrichtian) owing to the oblique subduction of the Pacific plate under the Eurasian plate. The volcanic succession constitutes the lower unit of basin filling and is the forerunner of further basin spreading. Copyright © 2002 John Wiley & Sons, Ltd.     

Early Permian strike-slip basin formation and felsic volcanism in the Manning Group,southern New England Orogen,eastern Australia

The Manning Group is characterised by rapidly filled strike-slip basins that developed during the early Permian along the Peel--Manning Fault System in the southern New England Orogen. Typically, the Manning Group has been difficult to date owing to the lack of fossiliferous units or igneous rocks. Thus, the timing of transition from an accretionary convergent margin in the late Carboniferous to dominantly strike-slip tectonic regimes that involved development and emplacement of the Great Serpentinite Belt (Weraerai terrane) is not well constrained. One exception are rhyolites of the Ramleh Volcanics that were erupted into the Echo Hills Formation. These developed along the dextral Monkey Creek Fault splay east of the Peel--Manning Fault System. Zircons extracted from the Ramleh Volcanics yield a U–Pb (SHRIMP) age of 295.6?±?4.6?Ma that constrains the minimum age of deposition in this basin to earliest Permian. Whole-rock geochemistry indicates these are peraluminous felsic melts enriched in LREE and incompatible elements with strong depletions in U, Nb, Sr and Ti. These are similar in age and composition to the nearby S-type Bundarra and Hillgrove plutonic supersuites. We suggest that extensive movement along the east-dipping Peel--Manning Fault System was responsible, not only for strike-slip basin development at the surface (Manning Group), but was also the locus for crustal melting that was responsible for generating S-type felsic melts that utilised hanging-wall fault splays as conduits to the surface or to coalesce in the crust as batholiths exclusively to the east of the Peel--Manning Fault System.     

The role of aeromagnetic data analysis (using 3D Euler deconvolution) in delineating active subsurface structures in the west central Arabian shield and the central Red Sea,Saudi Arabia

In this paper, we present a case study of structural mapping by applying the 3D Euler method to the high-resolution aeromagnetic data that was collected in the west central Arabian Shield region and the coastal region of the central Red Sea in Saudi Arabia. We show the 3D Euler deconvolution algorithm and apply it to magnetic potential field data from the west Central Arabian Shield and the Central Red Sea. The solution obtained with 3D Euler deconvolution gives better-focused depth estimates, which are closer to the real position of sources; the results presented here can be used to constrain depth to active crustal structures (volcanisms) for the study area. The results indicated that the area was affected by sets of fault systems, which primarily trended in the NNW–SSE, NW–SE, EW, and NE–SW directions. Moreover, estimated Euler solution map from aeromagnetic data delineated also the boundaries of shallow, small, and confined magnetic bodies on the offshore section of the study area. These nearly exposed basement intrusions are most likely related to the Red Sea Rift and may be associated with structures higher up in the sedimentary section. These volcanic bodies are extended to the continental part (onshore) of the west central Arabian Shield, particularly beneath both sides of the Ad Damm fault zone. This extension verifies that the fault was largely contemporaneous with a major period during the extension of the Red Sea Basin. Moreover, according to the distribution of circular magmatic-source bodies (circular-shaped ring dikes) that resulted from this study, we can state that the clustering of most earthquakes along this fault may most likely be attributed to the active mantle upwelling (volcanic earthquakes), which are ultimately related to volcanic processes. Furthermore, the oceanic crustal structures near and in the Red Sea offshore regions were also estimated and discussed according to the ophiolite occurrences and further opening of the Red Sea. Our results are largely comparable with studies of previous crustal sections, which were performed along the Red Sea Rift and the Arabian Shield. As a result, the areas above these anomalies are highly recommended for further geothermal study. This example illustrates that high-resolution aeromagnetic surveys can greatly help delineating the subsurface active structures in the west central Arabian Shield and the middle coastal region of the Red Sea of Saudi Arabia.