NNW Trending Research Articles

Quaternary tectonics along oblique deformation zones in the Central Andean retro-wedge between 31°30′S and 35°S

Abstract The distribution of the Quaternary deformation in the outer retro-wedge of the Andes (31°30′–35°S) is controlled by the subduction geometry, the position of the structural front, and the location of oblique pre-Cenozoic mechanical anisotropies. In the Southern Precordillera, Quaternary structures tend to group along the Barreal–Las Peñas deformation zone with a NW to NNW trend. This Cenozoic belt (31°30′–32°40′S) developed on the northern segment of the Triassic Cuyo Basin and broadened laterally during the Quaternary. New radiocarbon ages on dam deposits confirm Holocene tectonic activity on the northwestern edge. The oldest ages of dam deposits are 5810±90 a BP (Cabeceras Creek) and 810±50 a BP (Dolores Creek). Palaeoseismological and seismic data suggest active tectonic growth on the NW and SE extremes. Quaternary tectonics has contributed to modifying the relief along this oblique belt. This contribution is evident from the tectonic uplift of blocks (minimum 90–120 m in the Barreal block), the initial development of intermontane basins (Pampa de los Burros Basin), the Quaternary rejuvenation of tectonic depressions (at least 32–37 m of tectonic subsidence in the Vizcacheras half-graben) and the incipient development of low-relief morphotectonic units by soft-linkage of Quaternary structures.

Integrated hydroacoustic flares and geomechanical characterization reveal potential hydrocarbon leakage pathways in the Perth Basin, Australia

Geoscience Australia (GA) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) conducted a marine survey including monitoring hydroacoustic flares in order to understand the natural leakage pathways in the offshore northern Perth Basin. 186 hydroacoustic contacts were encountered and classified and thirty two were interpreted as possible seeps or expulsion of gases from the subsurface. The contacts were typically distributed above areas of interpreted subsurface faulting. In the survey site Da (15 km2), nine probable seeps and sixteen other contacts were interpreted and are aligned with a fault segment (A2) interpreted on 2D seismic reflection data. The segment A2 is part of a major N–NNW trending fault system intersecting the sedimentary sequence from the near seabed to the Permian units, including the Kockatea Shale source rock located in the oil window. Evaluation of the stress state on the fault segment A2 suggests that it is not critically stressed and therefore not likely prone to reactivation and dilation and vertical leakage under the modelled stress field. We propose that fault segment A2 acts as a baffle delimiting a migration pathway in the wall rock and permitting hydrocarbons generated from the source rock to overcome capillary entry pressures of the overlying marginal seals. The interpreted seeps could therefore be associated with intraformational vertical migration in the wall rock focused by the faults.

The sole of an ophiolite: the Ordovician Bay of Islands Complex, Newfoundland

The early Ordovician ( c . 485 Ma) Bay of Islands Ophiolite Complex was obducted onto the Laurentian rifted margin as the fore-arc of an oceanic arc that collided with the margin during the mid-Ordovician ( c . 470 Ma). The subduction zone was nucleated on an oceanic transform–fracture zone, part of whose remnants occur as the polyphase-deformed and intruded mafic and ultramafic rocks of the Coastal Complex. The ophiolite formed as a suprasubduction-zone fore-arc ophiolite at the spatial and temporal continuation of the ridge normal to the transform–fracture zone–subduction zone system and NE of a trench–trench–ridge triple junction. A two-pyroxene garnet granulite–garnet amphibolite–epidote amphibolite mafic metamorphic sole at the base of the ophiolite was generated, roughly synchronously with the ophiolite, by the metamorphism of mid-ocean ridge basalt mafic rocks in the descending slab at about 10 kbar and quickly attached to the base of the overlying ophiolite during slab flattening, and not by subduction zone extrusion. The metamorphic sole is not a metamorphic aureole at the base of a hot obducting ophiolite. Plate slip vector triangles around the triple junction, before collision and during obduction, are constructed from the orientation of dykes in the sheeted complex and the trends of structures in the high- to lower-temperature parts of the sole and the obducted nappes of oceanic and continental margin rocks beneath the ophiolite. Linear structures in the sole amphibolites trend NNW (the subduction direction); those in the greenschist-facies, obducted, oceanic and continental margin rocks trend WSW (the obduction direction).

Gravity and magnetic data analysis of block-35, Jiza' basin, Eastern Yemen

The Jiza' basin is located in the eastern part of Yemen, trending generally in the E–W direction. It is filled with Middle Jurassic to recent sediments, which increase in thickness approximately from 3,000 m to more than 9,000 m. In this study, block-35 of this sedimentary basin is selected to detect the major subsurface geological and structural features characterizing this basin and controlling its hydrocarbon potentials. To achieve these goals, the available detailed gravity and magnetic data, scale 1:100,000, were intensively subjected to different kinds of processing and interpretation steps. Also, the available seismic reflection sections and deep wells data were used to confirm the interpretation. The results indicated three average depth levels; 12.5, 2.4, and 0.65 km for the deep, intermediate, and shallow gravity sources and 5.1 and 0.65 km for the deep and shallow magnetic sources. Accordingly, the residual and regional anomaly maps were constructed. These maps revealed a number of high and low structures (horsts and grabens and half grabens), ranging in depth from 0.5 km to less than 4.5 km and trending mainly in the ENE, NW, and NE directions. However, the analytical signal for both gravity and magnetic data also showed locations, dimensions, and approximate depths of the shallow and near surface anomaly sources. The interpretation of the gravity and magnetic anomalies in the area indicated that the NW, NNW, ENE, and NE trends characterize the shallow to deep gravity anomaly sources; however, the NE, NW, and NNE trends characterize the magnetic anomaly sources, mainly the basement. Two-dimensional geologic models were also constructed for three long gravity anomaly profiles that confirmed and tied with the available deep wells data and previously interpreted seismic sections. These models show the basement surface and the overlying sedimentary section as well as the associated faults.

The tectonic crustal stress field and style of faulting along the Pacific North America Plate boundary in Southern California

We invert for the state of stress in the southern California crust using a catalogue of high quality earthquake focal mechanisms (1981–2010). The stress field is best resolved where seismicity rates are high and sufficient data are available to constrain the stress field across most of the region. From the stress field, we determine the maximum horizontal compressive stress (S_Hmax) orientations and the style of faulting across southern California. The trend of S_Hmax exhibits significant regional and local spatial heterogeneities. The regional trend of S_Hmax varies from north along the San Andreas system to NNE to the east in the Eastern California Shear Zone as well as to the west, within the Continental Borderland and the Western Transverse Ranges. The transition zones from one state of stress to the other occur over a distance of only a few kilometres, following a trend from Yucca Valley to Imperial Valley to the east, and the western edge of the Peninsular Ranges to the west. The local scale heterogeneities in the S_Hmax trend include NNW trends along the San Andreas Fault near Cajon Pass, Tejon Pass and the Cucapah Range, as well as NNE trends near the northern San Jacinto Fault and the Wheeler Ridge area. The style of faulting exhibits similar complexity, ranging from predominantly normal faulting in the high Sierra Nevada, to strike-slip faulting along the San Andreas system, to three consecutive bands of thrust faulting in the Wheeler Ridge area and the Western Transverse Ranges. The local variations in the style of faulting include normal faulting at the north end of the San Jacinto Fault and scattered areas of thrust faulting. The regional variations in the SHmax trends are very similar to the pattern of the GPS-measured maximum shortening axes of the surface strain rate tensor field although the strain field tends to be smoother and appears to capture some of the upper-mantle deformation field. The mean trend of S_Hmax departs about approximately 14° to the east from the trend of the maximum shortening directions derived from anisotropy in the upper mantle.

Stress analysis, post-orogenic extension and 3.01 Ga gold mineralisation in the Barberton Greenstone Belt, South Africa

Gold deposits in the Barberton Greenstone Belt (BGB) have formed along a common network of brittle–ductile shear zones that developed during extension and emplacement of felsic porphyry dykes at ∼3015Ma. At a deposit scale, mineralised shears occur in en-echelon arrays, distributed as Riedel, P- and anti-Riedel shears, with envelopes that connect groups of mine workings along NNW, ENE, NE and ESE trends. The dip of the mineralised shear envelopes is generally moderate to steep (50–80°). The existence of the directional alignment of gold deposits at the scale of the greenstone belt has been demonstrated with Fry analysis.Stress inversion for mineralised shear zones from workings near 28 mines using a Bingham axial distribution and an optimized right dihedron approach provide near identical results with σ1 vertical and σ3 horizontal NW. The stress regime is generally radial extensional, or pure extensional. At the scale of the greenstone belt, the mineralised shear envelopes comprise two sets of near-orthogonal trends, positioned symmetrically with respect the measured far field stress: 1. A NW and NE trending pair with the latter accommodating pure normal slip and the former both dextral or sinistral strike-slip; and 2. A NNW and ESE trending pair accommodating dextral transtensional and sinistral transtensional movements respectively.The fact that all mineralised shears developed systematically in response to a similar far field stress suggests that mineralisation formed during a single tectonic stage. 207Pb/206Pb ages of 3017.1±17.8Ma and 3008.8±16Ma from a syn-mineralisation, porphyry dyke near Golden Quarry indicates gold deposition at ∼3015Ma; i.e. ∼85Ma after the craton had stabilised. Older porphyry dykes intruded near Sheba and Consort mines have been dated at ∼3100Ma and ∼3260Ma respectively, and provide a minimum and maximum age constraint on accretionary deformation in the area.We propose that the main phase of gold enrichment in the BGB occurred after tectonic and thermal stabilisation of the crust at a time the craton was acting as a stable terrane. Mineralised brittle–ductile shear zones allowed ingress of deeply sourced fluids during cratonic extension. Mine camps display a cluster spacing of ∼12km, which suggests a regular distribution of controlling shears, possibly as a function of the elastic rigidity of the lithosphere. Rather than linking mineralisation to orogenic processes during greenstone belt accretion, we link gold mineralisation to the opening of fault-bounded intracratonic basins including the Pongola and Witwatersrand Basins, and concomitant igneous activity.

FIA trends along the Precambrian Rocky Mountains: a new approach to timing continental docking

AbstractA similar succession of Foliation Inflection/Intersection Axis (FIAs) trends preserved within porphyroblasts is present in two areas separated by 200 km along the Rocky Mountains. The Precambrian rocks in Central Colorado and Northern New Mexico were affected by deformation and metamorphism from ∼1506 to 1366 Ma. A succession of five FIAs trending W–E, SSW–NNE, NNW–SSE, NW–SE and WSW–ENE is distinguished in Central Colorado and dated at 1506 ± 15 Ma, 1467 ± 23 Ma, 1425 ± 18 Ma, not dated and 1366 ± 20 Ma respectively. To the south in Northern New Mexico, a succession of five FIAs trending SSW–NNE, WNW–ESE, NNW–SSE, NW–SE and WSW–ENE is distinguished and dated at 1482 ± 48 Ma, 1448 ± 12 Ma, 1422 ± 35 Ma, not dated and 1394 ± 22 Ma. The excellent correlation of the sequence of FIA trends and their ages between regions reveals a sixfold‐FIA succession across the region with the first developed FIA set in Central Colorado not present in Northern New Mexico and the third FIA set in the region not present in Central Colorado. Preferential partitioning of W–E trending deformation into the Central Colorado region ∼1506 ± 15 Ma was followed by SSW–NNE trending deformation that affected both regions at 1470 ± 20 Ma. However, preferential partitioning of WNW–ESE trending deformation into Northern New Mexico at 1448 ± 12 Ma left Central Colorado unaffected. Both regions were then affected by the three remaining periods of orogenesis, the first trending NNW–SSE at 1424 ± 15 Ma followed by one trending NW–SE that has not yet been dated, and then one trending WSW–ENE at 1390 ± 19 Ma. This suggests that the Yavapai terrane was tectonized at ∼1506 Ma, prior to amalgamation with the Mazatzal terrane ∼1470 Ma. Subsequent orogenesis was initially partitioned preferentially into the Mazatzal terrane, but the following three periods of tectonism affected both terranes in a similar manner.

The October 20, 2011 Mw 5.1 Talala earthquake in the stable continental region of India

The Talala (Sasangir) area in the Saurashtra region of Gujarat, western India, is experiencing tremors since 2001. The swarm type of earthquake activity in 2001, 2004, and every year from 2007 onward has occurred after the monsoon and lasted 2–3 months each time. In 2007 some 200 shocks (largest Mw 5.0) and in 2011 about 400 shocks down to M1 are well recorded with 1–2 km location error. The focal depths are about 2–10 km and shocks are accompanied by blast-like subterranean sounds. The epicenter (21.09 N 70.45E, focal depth: 5 km from location program, 3 km from MTS) of the October 20, 2011 mainshock occurred about 12-km WNW of Talala town or 8-km SSW of the 2007 M w 5.0 earthquake epicenter. The epicentral trends deciphered from local earthquake data indicate two ENE trends (Narmada trend) for about 50 km length and a conjugate 15-km-long NNW trend (Aravali trend). The focal mechanisms by moment-tensor analysis of full wave forms of two 2007 events of Mw 4.8 and 5.0 and the 2011 event of Mw 5.1 indicate rupture along either of the two trends. The ENE trends follow a gravity low between the gravity highs of Girnar mounts. Seismic reflections also indicate a fault in the area named Girnar Fault. Most of Saurashtra region including the Talala area is covered by Deccan Trap Basalt forming plateaus and conical ridges. There is no major fault within Saurashtra Peninsula though it is believed to have major faults along the boundaries that are non-seismic. The intensity of the October 20, 2011 Talala earthquake is estimated to be 6.5 in MM scale while isoseismals of 6, 5, and 4 and felt distance give Mw 5.1 based on Johnston’s 1994 empirical regressions. The source parameters of the 2011 Talala earthquake are estimated using data from 14 broadband seismograph stations. Estimated seismic moment, moment magnitude, stress drop, corner frequency, and source radius are found to be 1016.6 N-m, 5.1, 1.6 MPa, 1.3 Hz, and 2,300 m, respectively. The b and p values are obtained to be low, being 0.67 and 0.71, respectively. PGA of 35 cm/sec2 is noted and the decay rate of acceleration has been estimated from strong motion data recorded at 5 stations with epicentral distances ranging from 32 to 200 km.

Superposition of Four Generations of Folds and Development of Horse Creek Dome, Black Hills, South Dakota

The study area is a part of the Precambrian basement of the Black Hills that was uplifted during Laramide Orogeny. Early Proterozoic meta-conglomerate and greywacke units occupy the study area and form a crescent fold which is unusual structural pattern, mapped in the Precambrian core of the Black Hills. This study aims to determine how the crescent shape was formed via fieldwork investigation and structural data analysis. Four Precambrian (F1, F2, F3 and F4) folding events affected the study area prior to the uplift. Minor folds indicated that F1 and F4 exhibited isoclinal folding, F2 reflected similar folding and F3 displayed kink band and chevron folding. F2 is associated with axial plane cleavage (S2), trends towards the west, and is associated with regional metamorphism. F3 trends NNW and is associated with spaced cleavage (S3). F4 trends WSW and is associated with spaced crenulation cleavage (S4). The area was structurally divided into five domains. Poles to bedding (S0) and axial plane cleavage (S2) equal area stereographic projections show steeply plunging maxima that have monoclinic symmetry, resulting from superposition of two oblique F2 and F3 folding events. Domain I contains the Horse Creek dome that resulted from superposition of F3 on F2. The meta-conglomerate unit formed a crescent fold, due to superposition of the first two generations of folds. The crescent fold follows the Horse Creek dome, trends and opens westerly, and it is parallel to F4 folds.

The River Mondego terraces at the Figueira da Foz coastal area (western central Portugal): Geomorphological and sedimentological characterization of a terrace staircase affected by differential uplift and glacio-eustasy

A geomorphological and sedimentological characterization of the River Mondego terraces in the Figueira da Foz coastal area, Portugal, is presented. The relief is dominated by a Pliocene a marine sandy unit ~10–15m thick, reaching ~250ma.s.l., that covers a shore platform surface. The River Mondego has incised into the Pliocene relief and its long drainage evolution is recorded within a series of inset river terrace landforms. These river terraces are better preserved on the right-hand (northern) bank, where they form a staircase developed against the uplifted Serra da Boa Viagem structure. A set of five river terraces, represented by sedimentary deposits or erosional straths, are identified, namely: T1 at 128–125m; T2 at 101–90m; T3 at 70–60m; T4 at 29–24m; T5 at 11–10ma.s.l. Some luminescence dating ages were obtained from the river terrace deposits: >390ka from the middle of T4; >170ka from the top of T4; ~120ka from the base of T5; ~109ka from T5 (average age for three samples); and 53±3ka from a colluvium at 35ma.s.l. Some differences in altitude of the uppermost sedimentary unit and of the terraces are here interpreted as resulting from vertical displacements of active faults during the Quaternary. The main tectonic structure is the WNW–ESE trending Quiaios fault, responsible for the regional tilting towards SW. Other probably active faults and tectonic lineaments trend NNW–SSE to N–S, NNE–SSW to NE–SW and WNW–ESE. This study reports a long-term uplift rate of 0.004–0.055m/ka for the last 3.6Ma, but 0.017–0.118m/ka for the last ~1.8Ma (using as references, respectively, the base and the surface of the uppermost sedimentary unit). The facies associations that characterize the older terrace deposits (T1 and T2) consist of poorly sorted fluvial sandy-gravels and silts, but also some colluvium at the top of each terrace. The younger terraces (T3, T4 and T5) show better developed sedimentary structures and less sedimentary matrix; the sedimentary features are indicative of both fluvial and coastal environments (estuary and beach, as nowadays). Under conditions of continuous uplift, the episodes of river down-cutting, valley widening and aggradation in the studied area can be attributed to the rise and fall of sea-level in response to the global Quaternary climatic fluctuations. Considering the number of terraces and the dating obtained, it seems that the control of terrace genesis in this coastal area should be dominated by glacio-eustasy whereby episodic valley incision would have been determined by periods of very low sea-level, probably at ~460–410ka (T3/T4; MIS12), ~200–125ka (T4/T5; MIS6) and 100–14ka (T5/Recent alluvial infill; late MIS5, MIS4, MIS3 and MIS2). Some sandy colluvium deposits on the slopes are probably related with mild-cold and wet climate conditions during the period 60 to 32ka. The aeolian dunes are younger (cold to temperate dry conditions; MIS2 and MIS1).

A Neoproterozoic continental rift succession: The volcano-sedimentary Koivib Mountains deposits of Namibia

Neoproterozoic volcanic and sedimentary rocks compose the 750m thick Koivib Mountains succession, which forms part of the Rosh Pinah Formation of southwest Namibia. Detailed sedimentary facies analysis and physical volcanology enabled determination of four lithofacies and component twelve facies that record subaqueous deposition in a tectonically controlled shallow sea. The mafic volcanic lithofacies is composed of mafic: (1) flow, (2) volcaniclastic, and (3) intrusive facies characteristic of seamount building by effusive volcanic processes. The felsic volcanic lithofacies containing felsic: (4) flow and (5) volcaniclastic facies displays features characteristic of proximal to distal and marginal regions of subaqueous dome–flow complexes. The siliciclastic lithofacies composed of (6) shale, (7) thin- to medium-bedded sandstone, (8) thick-bedded sandstone, and (9) sedimentary breccia facies is consistent with a submarine fan environment in which subaqueous density current and turbidity flow processes were predominant. The silicilastic–carbonate lithofacies, which contains (10) massive bedded carbonate, (11) laminated to cross-bedded calcarenite, and (12) graded bedded calcarenite, formed mainly through erosion of primary carbonate and transport of material from a platform to a slope setting. The overall stratigraphy, combined with geochemical results indicating bimodal volcanism in a within-plate tectonic setting, signifies that the Koivib Mountains succession developed as part of a continental rift sequence associated with Neoproterozoic break-up of the Rodinia supercontinent. The structural geology of the Koivib Mountains revealed two generations of folds, including (i) NW trending tight, isoclinal folds (F1) and (ii) NNW trending open folds (F2). The second deformation event refolded F1 folds, producing periclinal structures. The deformation features are consistent with SE transpression associated with the continental collision that occurred during closure of the Adamastor Ocean and formation of Gondwana.

Diverse, discrete, mantle-derived batches of basalt erupted along a short normal fault zone: The Poison Lake chain, southernmost Cascades

The Poison Lake chain consists of small, monogenetic, calc-alkaline basaltic volcanoes located east of the Cascade arc axis, 30 km ENE of Lassen Peak in northeastern California. This chain consists of 39 distinguishable units in a 14-km-long and 2-km-wide zone trending NNW, parallel to nearby Quaternary normal faults. The 39 units fall into nine coherent groups based on stratigraphy, field characteristics, petrography, and major-element compositions. Petrographic differences among groups are expressed by different amounts and proportions of phenocrysts. MgO-SiO 2 , K 2 O-SiO 2 , and TiO 2 -SiO 2 variation diagrams illustrate clear differences in compatible and incompatible elements among the groups. Variation of K 2 O/TiO 2 and K 2 O/P 2 O 5 with MgO indicates that most of the basalts of the Poison Lake chain cannot be related by crystal fractionation at different pressures and that compositions have not been affected significantly by incorporation of low-degree silicic crustal melt or interaction with sialic crust. Limited trace-element and whole-rock isotopic data also suggest little if any incorporation of upper-crustal material, and that compositional variation among groups primarily reflects source compositional differences. Precise 40 Ar/ 39 Ar determinations show that the lavas were erupted between 100 and 110 ka. The migration of paleomagnetic remanent directions over 30° suggests that the entire Poison Lake chain could represent three short-lived episodes of volcanism within a period as brief as 500 yr. The diverse geologic, petrographic, chemical, paleomagnetic, and age data indicate that each of the nine groups represents a small, discrete magma batch generated in the mantle and stored briefly in the lower crust. A NNW normal fault zone provided episodic conduits that allowed rapid ascent of these batches to the surface, where they erupted as distinct volcanic groups, each aligned along a segment of the Poison Lake chain. Compositional diversity of these primitive magmas argues against widespread, long-lived ponding of uniform basalt magma at the base of the crust in this region and against interaction with a zone of melting, assimilation, storage, and homogenization (MASH) in the lower crust.

Integrated remote sensing data utilization for investigating structural and tectonic history of the Ghadames Basin, Libya

This study was initiated to constrain the geological structure of the Ghadames Basin in northwest Libya. Detailed analysis was based on digital integration of surface data, including SRTM DEM, ETM+ and geologic maps with subsurface data, including well logs and potential field data. Integrated analysis of remotely sensed data of the SRTM and ETM+ were utilized to identify geologic lineaments in the area. Ground-based verification of the remote sensing data was achieved with field work. Interpretation and analysis of the lineaments indicate that the Ghadames Basin is controlled by four main fault systems that trend WNW, NNW, NW and ENE. Well logs and potential field data were used to delineate a detailed picture of the subsurface structure. The potential field data reveal two NE and NNE trending sedimentary basins. The depth of the basement inside the main basin ranges from 2 to 6km. A two-dimensional (2-D) schematic model shows that the basin gradually deepens towards the southwest. The applied data integration gave new insight into the tectonic and structure patterns of the Ghadames Basin and the adjacent areas of northwest Libya.

Seismic reflection constraints on upper crustal structures in the volcanic-covered central Nechako basin, British Columbia1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

In 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic-covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8–12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that ∼1700 and ∼2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin.

Seismological Aspects of the Abou Dabbab Region, Eastern Desert, Egypt

Located about 24 km from the Red Sea, the Abou Dabbab region is characterized by significant microseismic activity. The seismic history of this region is marked by two moderate magnitude earthquakes, the 12 November 1955 Mb 6.1 and the 2 June 1984 Mb 5.1. The 1955 Mb 6.1 earthquake was the largest recorded event to occur within Abou Dabbab. Woodward Clyde Consultants (1982) computed the first-motion focal mechanism for the 1955 event. The mechanism has a strike-slip faulting solution with a normal dip-slip component. The T -axis has a NNW trend. The two nodal planes that were determined for this event give NNW to NW and ENE to ESE strike directions. Earthquakes in the Abou Dabbab region have shown a tendency to cluster in space and time (Morgan et al. 1981). This pattern suggests that the activity seems to have been mainly controlled by local sources ( e.g. , magmatic intrusion) triggered in turn by regional tectonics (Badawy et al. 2008). The heat flow value in Abou Dabbab is twice the average heat flow value in the Eastern Desert of Egypt (Morgan et al. 1985). El Hady (1993) reported from the distribution of earthquake's focal depths and rheological studies that the brittle-ductile transition in the Abou Dabbab area occurs at a relatively shallow depth range (∼9–1. km). The existence of the brittle-ductile transition in this region can be attributed to shallow asthenospheric intrusion, which causes an increase in the temperature. Recordings of microearthquakes in Abou Dabbab from 1 August to 20 August 2004 from 11 portable instruments show that this region is distinguished by intense spatial clustering (Figure 1A). The error estimated in both horizontal distance and depth is less than 0.3 km (Hussein et al. 2008). The cross-section perpendicular to the activity reflects two main spatial clusters with a …

Surface geophysical investigations of landslide at the Wiri area in southeastern Korea

A geophysical survey was undertaken at Wiri area of the Andong in southeastern Korea to delineate subsurface structure and to detect the fault zone, which affected the 1997 mountain–hill subsidence and subsequent road heaving initiated by the intense rainfall. Electrical resistivity methods of dipole–dipole array profiling and Schlumberger array sounding and seismic methods of refraction and reflection profiling were used to map a clay zone, which was regarded as the major factor for the landslide. The clay zone was identified in electrical resistivity and seismic sections as having low electrical resistivity (<100 Ωm) and low seismic velocity (<400 m/s), respectively. The clay zone detected by using geophysical methods is well correlated with its distribution from the trench and drill-core data. The results of the electrical and seismic surveys showed that slope subsidence was associated with the sliding of saturated clay along a fault plane trending NNW–SSE and dipping 10°–20° SW. However, the road heaving was caused by the slope movement of the saturated clay along a sub-vertical NNE-trending fault.

Magnetic interpretation of north Gebel El Shallul area, central Eastern Desert, Egypt

The present work utilizes the aeromagnetic data supported by geology and remote sensing satellite data, to delineate surface and subsurface structural elements in north Gebel El Shallul area. Geologically, the area is covered by Precambrian basement rocks to the East (metasediments, metavolcanics, Hammamat sediments and younger granites) and Phanerozoic sediments to the West (Nubian Sandstones and Qouseir clastics). The Landsat image of the area gave additional detailed litholgic information which is very useful in identifying and discriminating the different lithologic rocks exposed in the area. Analysis of the structural lineaments of the Landsat image and the compiled geological map show that most of the well-developed structural lineaments have NW, NNW and ENE trends. The aeromagnetic data were analyzed and processed by several advanced techniques; reduction to the pole, regional-residual separation, second vertical derivative (SVD), analytical signal, Euler deconvolution and shadowgrams. The application of local power spectrum on the reduced to North Pole (RTP) aeromagnetic data indicated that the average depths to the near–surface and deep-seated causative magnetic bodies were found to attain 0.5 and 1.8 km, respectively. Therefore, the filtering of the aeromagnetic data at the two assigned interfaces was conducted to assist the discrimination of the residual (near-surface) and regional (deep–seated) magnetic anomalies. The prepared aeromagnetic maps have been interpreted qualitatively and quantitatively to deduce the structural elements which were used to construct a basement tectonic map for the area. The used analysis techniques helped to reveal many structural elements such as faults of different orders, uplifted blocks (antiforms or horsts) and subsided blocks (basins or synforms). It has been found that the NNW pronounced linear magnetic anomalies, normally and reversely magnetized, are associated with deep-seated diabasic dykes. The interpretation of the basement tectonic map of the area indicated the presence of a set of unexposed subsurface basic dykes running generally in NNW–SSE direction controlling the courses of major wadis in the study area. This set of NNW–SSE basic dykes are dissected by a NE–SW fault system. These two sets of fault systems were found to be matched well with that obtained from the Landsat image and geological map. It was found that the pronounced NNW direction is a predominant structural trend controlling the structural framework of the area.

Kinematic analysis of a transtensional fault system: The Atuel depocenter of the Neuquén basin, southern Central Andes, Argentina

Abstract The Atuel depocenter of the Neuquen basin originated as an Upper Triassic to Lower Jurassic rift system, later inverted during the Andean contractional deformation. In order to study the extensional architecture and the kinematic evolution of this depocenter, we collected a large amount of field and sub-surface data, consisting of slip data from outcrop-scale normal faults, thickness and facies distribution within the synrift deposits, and structural data from angular and progressive unconformities. The Atuel depocenter has a NNW trend, showing a bimodal distribution of NNW and WNW major faults (first and second order faults). On the other hand, from kinematic indicators measured on outcrop-scale faults (third and fourth order faults), we found a mean NE internal extension direction, which is oblique to the general trend of the sub-basin. Taking these particular characteristics into account, we interpreted the Atuel depocenter as an oblique rift system. We evaluated two mechanisms in order to explain the development of this transtensional system: 1) reactivation of upper-crustal NNW-oriented Paleozoic shear zones, and 2) oblique stretching of a previous NNW-oriented lithospheric weakness zone.

New U–Pb and 40Ar/ 39Ar ages from the northern margin of the Barberton greenstone belt, South Africa: Implications for the formation of Mesoarchaean gold deposits

Gold mineralization in the Mesoarchaean New Consort gold mine in the Barberton greenstone belt, South Africa, is situated in the immediate hanging wall of an exhumed, composite granitoid gneiss dome known as the Stentor Pluton. The granitoid-greenstone contact is characterized by a contact-parallel high-strain zone that separates the mid-crustal gneisses from the greenschist facies greenstone belt. In contrast to the lower greenschist facies deposits in the central parts of the greenstone belt, the mineralized shear zones at New Consort are hosted by distinctly higher grade metamorphic rocks, ranging from upper greenschist to upper amphibolite facies conditions. A distinct change of ore and alteration assemblages with increasing metamorphic grade points to a temporal and spatial relationship between gold mineralization and the juxtaposition of the Stentor gneiss dome against the overlying greenstone belt. Examination of the structural and petrologic evolution of the mineralized high-strain zone, combined with new U–Pb and Ar–Ar data and previously published ages, indicates a multistage tectono-metamorphic evolution. Early high-T, medium-P metamorphism (ca. 600–700 °C and 5 ± 1 kbar) in the structurally deeper parts of the mine is interpreted to be related to an early period of orogen-parallel (NE–SW) extension and associated detachment shearing during the main (D 2) accretionary event in the centre of the greenstone belt, between ca. 3260 and 3225 Ma. Recent work has shown that these early high-T fabrics were reactivated at ca. 3105 Ma during the subsequent D 3 tectonism and associated emplacement of large potassic granite batholiths. The gold mineralization was coeval with NE–SW extensional exhumation and solid-state doming of the Stentor gneiss dome during the subsequent D 4 deformation. This late-stage deformation and the gold mineralization postdate the emplacement of the D 3-related potassic granites by more than 50 million years, as indicated by (1) a U–Pb age of 3046 ± 27 Ma for a retrograde titanite generation in a D 2-related amphibolite (sample 113), and (2) a 3027 ± 7.5 Ma age for hydrothermal titanite from the so-called footwall lens mineralization, which temperature has been estimated at 590 ± 40 °C. This high-T event is also recorded by a “corrected” 3040 ± 4 Ma hornblende Ar–Ar age from the same sample, suggesting that cooling of the Stentor gneiss dome shortly followed the exhumation. A considerably younger Ar–Ar age (2966 ± 4 Ma) has been obtained for an amphibolite from the contact-parallel high-strain zone, pointing to non-uniform cooling following the mineralization. The gold mineralization was largely contemporaneous with the emplacement of NNW trending, syn-kinematic granitic pegmatites and strain localization along the pegmatite swarm at ca. 3030–3040 Ma. The mineralization is also considerably younger than the gold mineralization in the greenschist facies gold deposits in the Barberton greenstone belt, which has previously been dated at ca. 3080 Ma. In conjunction with previously published ages, the data indicate the Au mineralization in the Barberton greenstone belt to have occurred during, at least, two mineralization episodes at ca. 3080 and 3040 Ma. The main phase of Au mineralization was introduced during the end of an either protracted or episodic structural and thermal evolution of the granitoid-greenstone terrain that may have lasted some 200 Ma.

Seismicity and fluid geochemistry at Lassen Volcanic National Park, California: Evidence for two circulation cells in the hydrothermal system

Seismic analysis and geochemical interpretations provide evidence that two separate hydrothermal cells circulate within the greater Lassen hydrothermal system. One cell originates south to SW of Lassen Peak and within the Brokeoff Volcano depression where it forms a reservoir of hot fluid (235–270 °C) that boils to feed steam to the high-temperature fumarolic areas, and has a plume of degassed reservoir liquid that flows southward to emerge at Growler and Morgan Hot Springs. The second cell originates SSE to SE of Lassen Peak and flows southeastward along inferred faults of the Walker Lane belt (WLB) where it forms a reservoir of hot fluid (220–240 °C) that boils beneath Devils Kitchen and Boiling Springs Lake, and has an outflow plume of degassed liquid that boils again beneath Terminal Geyser. Three distinct seismogenic zones (identified as the West, Middle, and East seismic clusters) occur at shallow depths (< 6 km) in Lassen Volcanic National Park, SW to SSE of Lassen Peak and adjacent to areas of high-temperature (≤ 161 °C) fumarolic activity (Sulphur Works, Pilot Pinnacle, Little Hot Springs Valley, and Bumpass Hell) and an area of cold, weak gas emissions (Cold Boiling Lake). The three zones are located within the inferred Rockland caldera in response to interactions between deeply circulating meteoric water and hot brittle rock that overlies residual magma associated with the Lassen Volcanic Center. Earthquake focal mechanisms and stress inversions indicate primarily N–S oriented normal faulting and E–W extension, with some oblique faulting and right lateral shear in the East cluster. The different focal mechanisms as well as spatial and temporal earthquake patterns for the East cluster indicate a greater influence by regional tectonics and inferred faults within the WLB. A fourth, deeper (5–10 km) seismogenic zone (the Devils Kitchen seismic cluster) occurs SE of the East cluster and trends NNW from Sifford Mountain toward the Devils Kitchen thermal area where fumarolic temperatures are ≤ 123 °C. Lassen fumaroles discharge geothermal gases that indicate mixing between a N 2-rich, arc-type component and gases derived from air-saturated meteoric recharge water. Most gases have relatively weak isotopic indicators of upper mantle or volcanic components, except for gas from Sulphur Works where δ 13C–CO 2, δ 34S–H 2S, and δ 15N–N 2 values indicate a contribution from the mantle and a subducted sediment source in an arc volcanic setting.