East-northeast Trending Research Articles

Pre-existing structural control on the recent Holuhraun eruptions along the Bárðarbunga spreading center, Iceland

The active rift zones in Iceland provide unique insight into the geodynamic processes of divergent plate boundaries. The geodynamics of Iceland are studied intensively, particularly, by geophysical methods sensitive to active and/or visible structures such as earthquake seismic and Synthetic Aperture Radar observations or aerial photographs. However, older and less active structures, that may exert a strong control on the presently active geodynamics, are often buried beneath recent volcanic or sedimentary deposits and are—due to their passive mode—overseen by the typical geophysical investigations. Aeromagnetic surveys provide spatial information about subsurface magnetization contrasts relating to both active and inactive structures. However, the aeromagnetic data in Iceland were collected in the 1970-80s and are relevant only to large-scale regional rift studies. With the availability of reliable drones and light-weight atomic scalar sensors, high-quality drone magnetic surveys can provide an unprecedented spatial resolution of both active and passive structures of rift systems as compared to conventional airborne surveys. Here, we present the results of a drone-towed magnetic scalar field and scalar gradiometry study of the north-northeast trending Bárðarbunga spreading center to the north of the Vatnajökull ice cap, Iceland. Our results provide new information about the structural complexity of rift zones with evidence of densely-spaced, conjugate and oblique faults throughout the area. Evidence is shown of a hitherto unknown and prominent east-northeast trending fault structure that coincides with the northern tip of the main eruption edifice of the 1797 and 2014-15 Holuhraun volcanic events. We suggest that this pre-existing structure controlled the locus of vertical magma migration during the two Holuhraun events.

Deposit-scale controls of hypozonal gold mineralization in amphibolite facies metaturbidites: 3D modelling of the Twin Hills deposit, Damara Belt, Namibia

The recently discovered Twin Hills deposit is a new resource in the Pan-African Damara Belt of central Namibia, described by a finely dispersed, low-grade and high tonnage disseminated and vein-style gold mineralization. The underlying assumption of the 3D implicit modelling followed in this study is that the integration of gold grades and ore shells with structural and lithological data from oriented core logging reflects the underlying controls of hydrothermal fluid flow and mineralization at Twin Hills. The deposit comprises distinct clusters of economic-grade gold mineralization, measuring several hundred meters along strike, separated by several kilometer-long intervals of only sporadic and sub-economic gold grades. Mineralization is developed on the subvertical southern limb of a regional-scale syncline hosted in highly-strained compositionally heterogeneous, amphibolite facies metaturbidites. Additionally, the stretch of the main mineralization in the central parts of the ∼12 km long east-northeast-trending mineralized Twin Hills corridor coincides with the change of the fold vergence direction. We suggest that the steep orientation of the wall-rock strata generated high-permeability fluid conduits for focusing regional fluid flow during flexural-slip folding and associated local dextral strike-slip component of D2/3 deformation. Disseminated sulfide mineralization is mostly developed in pelitic units, whereas networks of quartz-sulfide veins and veinlets with subordinate disseminated sulfides are present in rigid psammitic beds in interbedded pelite-psammite horizons. While the wide distribution of finely disseminated sulfides point to the rather pervasive fluid flow along grain-scale permeabilities during recrystallization of mineral fabrics, the formation of fractures and resultant dilatancy in psammitic units created hydraulic gradients and focused fluid flow into the psammite horizons. High-grade ore shoots are controlled by the combination and interplay of two main factors, namely lithological contacts and subtle deflections of bedding of the highly-strained, subvertical metaturbidites. The mineralized corridor of the Twin Hills deposit is largely confined to imbricated and/or tightly folded packages of interbedded pelites-psammites within the otherwise pelite-dominated sequence. It is these units that promoted fracturing, fluid flow and the development of fine-scale vein networks as a result of pronounced strain partitioning between pelitic units (ductile) and interbedded psammites (brittle) during regional deformation. Ore shoots of higher-grade mineralization (>1.5 g/t Au) follow these main lithological contacts, but are laterally confined to subtle (5–15°) clockwise deflections of bedding (dilational jogs) from its regional east-northeast trends. Outside these deflections, gold mineralization is patchy and only sub-economic. The structural and lithological controls of the Twin Hills mineralization closely resemble those of orogenic gold deposits, whereas the fine-grained and dispersed texture of gold and associated sulfide mineralization are more reminiscent of disseminated gold deposits. We suggest that the Twin Hills deposit is an example of a hypozonal orogenic gold deposit formed under peak metamorphic amphibolite conditions.

Electrical resistivity structure across the Late Cenozoic Abaga-Dalinor volcanic field, eastern China, from 3-D magnetotelluric imaging and its tectonic implications

The Abaga-Dalinor volcanic field (ADVF) in Inner Mongolia, China, located to the west of the North-South Gravity Lineament (NSGL), is the largest and less known area of Late Cenozoic intraplate volcanism in Northeast China. Knowledge of the subsurface structure beneath the ADVF will improve our understanding of its evolution process and formation mechanisms. New broadband magnetotelluric (MT) data were collected along a profile of about 300 km that crosses the major basaltic areas exposed at the surface in the ADVF and a series of east-north-east (ENE) trending faults. The electrical resistivity structure of the crust and uppermost mantle across the ADVF was generated by 3-D inversion of full impedance tensor and tipper data. This model reveals a thick high-resistivity layer (>1000 Ωm) in the upper crust that may represent the Precambrian basement and Late Paleozoic-Mesozoic granitic plutons. In contrast, high conductivity anomalies dominate the middle-lower crust. The northward-dipping high-conductivity feature at depths of 20–40 km below sea level under the Chagan Obo fault is suggested to be the involvement of the Early Paleozoic subducted oceanic crust. A remarkably high-conductivity zone (1–20 Ωm) exists in the middle-lower crust beneath the Abaga and Dalinor volcanic areas. It is explained to be a saline fluid zone of ∼0.45%–0.48% that exsolved from the partial melts as magma ascent rapidly. The impermeable upper crust traps these fluids in the middle-lower crust. A moderately high-conductivity anomaly in the uppermost mantle below near the Solonker Suture Zone (SSZ) is attributed to a minimum partial melt of ∼3–4% associated with the localized asthenospheric upwelling, which feeds the Abaga and Dalinor volcanos as a common magma source. Integrating our resistivity model with previous geophysical and geochemical studies, we suggest that intraplate volcanism in the ADVF is mainly controlled by the pre-existing tectonic weak zones. The lithosphere within the SSZ is susceptible to thermal perturbation from the upwelling mantle, providing the preconditions for melt generation. A set of ENE trending faults and an inferred NW-SE trending fault jointly dominated the vertical transport of magma and lateral extension along the faults, which may have influenced the spatial distribution of basalts and volcanic cones at the surface in the ADVF. In addition, we tend to suggest that Late Cenozoic intraplate volcanism in the ADVF could be the result of decompressing melting within the localized small-scale asthenospheric upwelling induced by the edge-driven convection near the NSGL, which is indirectly related to the westward subduction and retreat of the (Paleo-)Pacific plate during the Late Mesozoic to Cenozoic.

Red Sea tectonics unveil one of the largest terrestrial ice streams: New constraints on Late Ordovician ice sheet dynamics

Mega-streamlined landforms on Earth and Mars have been attributed to aeolian, glaciogenic, fluvial, and tectonic processes. Identifying the forces that shaped these landforms is paramount for understanding landscape evolution and constraining paleo-climate models and ice sheet reconstructions. In Arabia, east-northeast trending, kilometer-scale streamlined landforms were interpreted to have been formed by Quaternary aeolian erosion. We provide field and satellite-based evidence for a Late Ordovician glacial origin for these streamlined landforms, which were exhumed during Red Sea–related uplift. Then we use Late Ordovician paleo-topographic data to reconstruct the Late Ordovician ice sheet using identified and previously reported glacial deposits and landforms. Our reconstruction suggests these glacial features are part of a major, topographically controlled, marine-terminating ice stream, with a minimum length of 1000 km extending from the southeast Egypt to northern and central Arabia and possibly more than twice this length if the glaciomarine and iceberg deposits in the present-day western Iran are part of this system. Our results support models that advocate for a single, major, and highly dynamic ice sheet and provide new morphological-based constraints for Late Ordovician climate models.

The Indian Ocean Geoid Low at a plume-slab overpass

The Indian Ocean Geoid Low (IOGL) appears as a prominent feature if the geoid is, as usual, shown with respect to the Earth's reference shape. However, if it is shown relative to hydrostatic equilibrium, i.e. including excess flattening, it appears as merely a regional low on a north-south trending belt of low geoid. For a mantle viscosity structure with an increase of 2–3 orders of magnitude from asthenosphere to lower mantle, which is suitable to explain the long-wavelength geoid, a geoid low can result from both negative density anomalies in the upper mantle and positive anomalies in the lower mantle. Here we propose that the IOGL can be explained due to a linear, approximately north-south-trending high-density anomaly in the lower mantle, which is crossed by a linear, approximately West-Southwest–East-Northeast trending low-density anomaly in the upper mantle. While the former can be explained due to its location in a region of former subduction and inbetween the two Large Low Shear Velocity Provinces (LLSVPs), we propose here that the latter is due to an eastward outflow from the Kenya plume rising above the eastern edge of the African LLSVP. We show that, with realistic assumptions we can approximately match the size, shape and magnitude of the geoid low.

Rift structures and magmatism focus VMS and gold mineralisation in the Paleoproterozoic Bryah Rift Basin, Australia

• Geophysical interpretation reveals syn -rift structures and magmatic centres. • Magmatic centres control syn -rift VMS and post-rift orogenic gold mineralisation . • Long-lived structures favour high endowment mineralisation. The interaction of structure development and magmatism in rift-settings provides systemic controls on the emplacement of ore deposits both during rifting and subsequently, and a knowledge of these may help to predict better the likely locations of major deposits. The Paleoproterozoic Bryah Rift Basin includes substantial mafic magmatism and deep-crust penetrating structures and possesses syn -rift volcanogenic massive sulphide (VMS) and later gold mineralisation, providing a good opportunity to study interactions between magmatism, structures, and mineralisation. The volcano-sedimentary Bryah Group was deposited in a continental rift developed within the broader Yerrida Basin on the northern margin of the Yilgarn Craton at ca. 2030 Ma. Multi-scale interpretation and forward modelling of gravity and magnetic data were applied to characterise the magmatic and structural patterns of the Bryah Group, enabling a better understanding of its tectono-magmatic development and controls on VMS and gold mineralisation. The interpretation highlights the bounding extensional faults and the inward-deepening structural pattern of the Bryah Rift Basin that reflects the initial rift geometry. Deep-rooted gravity sources show magmatism focused in the southern-central part of the basin, where mafic rocks' thickness can reach more than 10 km. At shallower levels, the rift magmatism extends in three east to east-northeast trending magmatic corridors with an en-echelon arrangement along the rift, oblique to major structures. These magmatic corridors have an intrinsic relationship with the internal structure of the rift. VMS mineralisation associated with mafic magmatism shows a spatial connection with magmatic corridors and major syn -rift faults, whereas VMS mineralisation associated with felsic volcanic rocks is related with off-axis volcanism and regional pre-rift faults. Later orogenic gold mineralisation is also spatially associated with the borders of the magmatic centres and rift faults, despite occurring ~200 Ma later. These associations suggest that primary rift architecture has substantially focused syn -rift VMS mineralisation and also later gold mineralisation, as a consequence of rheological and compositional contrasts.

Gravity, aeromagnetic and electromagnetic study of the gold and pyrite mineralized zones in the Haile Mine area, Kershaw, South Carolina

New high resolution gravity, electromagnetic (EM) and magnetic data are tested for the detection of disseminated sulfides in the area of the Haile Gold Mine, South Carolina. Geophysical interpretations were constrained with densities and mineral concentrations measured for 49,183 samples from 448 drill holes. Positive residual gravity anomalies from spectral analysis correlate with the mineralized ore zone. Similar correlations between positive anomalies and the ore zones are observed in the first vertical derivative of Bouguer anomaly and tilt derivatives of the residual gravity. Euler deconvolution of the gravity field also shows numerous shallow sources in the ore zone. Drill core measurements show that metasediments (Richtex unit), (2.76 g/cm3) and gold bearing samples (2.73 g/cm3) are slightly denser than metavolcanics (Persimmon Fork Formation), (2.69 g/cm3). Correlation coefficients for sample density and mineral percentages are pyrite (0.18) and sericite (0.15), indicating that pyrite is the main mineral increasing sample density. 2D forward gravity models constrained by dense drilling match the predicted depth range of density anomalies from Euler deconvolution. Drilling results confirm a spatial correlation between high densities, high pyrite concentrations, and the mineralized zones. High conductivity anomalies are observed over the Haile ore zone as well as over the metasediments. Core drilling and 2D inversion show that high conductivity anomalies coincide with zones of high pyrite concentrations. The magnetic field is dominated by anomalies produced by granite plutons 3 km north and 5 km west of Haile and northwest-trending Jurassic diabase dikes. East-northeast trending linear anomalies have been sampled and dated as Alleghanian lamprophyre dikes providing the first magnetic map of these intrusions in the southeastern United States.

Tectonolineament zones of east-north east trending and selected problems of mediterranean tectonics

Tectonolineament zones of east-north east trending and selected problems of mediterranean tectonics

ARCHAEAN GOLD MINERALISATION DURING POST-OROGENIC EXTENSION IN THE NEW CONSORT GOLD MINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA

The area around New Consort Gold Mine (NCGM) was complexly deformed during at least five distinct events named D1NC to D5NC. The D1NC to D3NC events involve progressive shearing and folding linked to the 3.25 to 3.23 Ga accretionary history of the Barberton greenstone belt (D1NC and D2NC), and to the subsequent ~3.1 Ga emplacement and doming of surrounding batholiths (D3NC). During D3NC the area experienced intense strain partitioning with the development of a network of shear zones that envelop structural domains characterised by locally unique deformation histories. Around NCGM, D3NC events involved early shearing (D3aNC) along the Consort Bar followed by two episodes of 600 m scale folding (D3bNC and D3cNC), resulting in a complex fold interference pattern. Pegmatites intruded during D3NC. The critical structures controlling gold mineralisation are D4NC extensional, 10 to 100 m scale, brittle-ductile shear fractures associated with kink bands and crenulation folds that formed concomitantly with gold mineralisation after all the D3NC structures had fully developed, marking a clearly separate event. This network of fractures is distributed along 100 to 200 m wide west-northwesterly, north-northwesterly and east-northeasterly trending corridors. The distribution of high-grade ore zones was controlled by the intersection orientation of D4NC fractures and suitable host lithologies, mainly the silicified hinge zones of D1NC folds and the siliceous Consort Bar. Because these host lithologies were complexly folded in D1NC-D3bNC-D3cNC fold interference patterns, the 3-D distribution of ore zones is highly discontinuous and complex. D5NC structures represent late reverse faults of unknown age, well developed in the nearby Albion Mine. Kinematic analyses and stress inversion using Bingham tensor solutions and a reduced moment tensorn method, of mineralised D4NC fractures along old gold workings at NCGM; consistently indicate a vertical σ1, and a horizontal northwesterly to southeasterly directed σ3, within an extensional stress regime. Results for the nearby Clutha, Albion and Woodstock mines are similar. This study suggests that gold mineralisation in the NCGM area can be linked to an extensional event that may have developed separately from the accretionary events shaping the craton, and may have coincided with deposition of the Dominion Group.

The 20 January 2007 Odaesan, Korea, Earthquake Sequence: Reactivation of a Buried Strike-Slip Fault?

The M w 4.6 Odaesan, Korea, earthquake of 20 January 2007 (11 hr 56 min) had nine locatable foreshocks in the previous 67 hours and nine aftershocks during the 11 hours after the main event. We obtained accurate locations of this sequence by employing the double-difference earthquake relocation method with differential travel-time measurements using waveform cross-correlation. We found that 19 accurately relocated foreshocks, mainshock, and aftershocks aligned along a 1.2 km long lineation with an azimuth of west-northwest–east-southeast (115°), which coincides with a nodal plane of the mainshock focal mechanism that strikes 114°. Regional waveform modeling of the mainshock constrained a focal depth of 11±1 km and the focal mechanism is a pure strike-slip faulting along the vertical nodal planes with a horizontal P axis (plunge=1°) trending east-northeast (69°). The pulse rise time of the mainshock source time function estimated from an empirical Green’s function analysis ranges from 0.1 to 0.16 sec, which yields a source radius of ∼0.6 km and is consistent with the spatial distribution of the foreshock and aftershock sequences. The stress drop of ![Graphic][1] is obtained for the mainshock using a radius of 0.6 km for a circular rupture, and seismic moment ![Graphic][2] . The 20 January 2007, Odaesan earthquake may be the first significant earthquake in southern Korea to date where the fault plane is constrained by accurate locations of the foreshock and aftershock sequences. Its source mechanism, vertical strike-slip fault at depth, suggests that the event represents reactivation of a buried high-angle fault in the Precambrian basement by the contemporary east–east-northeast trending regional horizontal compressive stress. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Proterozoic deformation in the northwest of the Archean Yilgarn Craton, Western Australia

The Narryer Terrane within the northwestern Yilgarn Craton contains the oldest crust in Australia. The Jack Hills greenstone belt is located within the southern part of the Narryer Terrane, and structures cutting it and surrounding rocks have been dated using the 40Ar/ 39Ar technique. The results show that east-trending, dextral, transpressive shearing was related to the 1830–1780 Ma Capricorn Orogeny, followed by further deformation and/or cooling between c. 1760 and 1740 Ma. These results confirm that major deformation has affected the northwestern part of the Yilgarn Craton in an intracratonic setting during the Proterozoic. Proterozoic structures have been interpreted to extend south beyond the Narryer Terrane into the northern part of the Youanmi Terrane (Murchison Domain), and include the Yalgar Fault, previously interpreted as the boundary between the Narryer and Youanmi Terranes. Terrane amalgamation pre-dated the emplacement of c. 2660 Ma granites in both terranes, and the current expression of the Yalgar Fault must represent a younger, reworked, post-amalgamation structure, possibly controlled by the tectonic boundary. However, new aeromagnetic and gravity imagery does not show the eastern part of the Yalgar Fault as a major structure. Its signature is more akin to a series of east- to east-northeast trending faults that are interpreted to be Proterozoic in age. This suggests that this part of the Yalgar Fault may not be a terrane boundary, and is possibly no older than Proterozoic. The 40Ar/ 39Ar dating also shows a younger, less intense deformation and/or cooling event at c. 1172 Ma.

PGE exploration targets to the west of the Pilanesberg, South Africa

The main approach to PGE target selection west of the Pilanesberg was to use the geological models for the setting of PGE mineralisation (together with their associated geophysical and geochemical signatures) in the southwestern lobe of the Bushveld Complex where abundant information is available. A small gravity anomaly to the west of the Pilanesberg could represent stratigraphic thickening of the Bushveld stratigraphy or a local magma feeder zone. If the Merensky Reef is present in this area then it could be expected to be of the narrow, pegmatoidal, potholed (including contact type) and high-grade variety, as in the southwestern and northwestern lobes of the Bushveld Complex. High precision, airborne magnetic and radiometric surveys over the area, carried out by the Council for Geoscience, demarcate Upper Zone stratigraphy and a range of dykes, faults and probable scattered bodies of iron-rich ultramafic pegmatites (IRUPS). Of particular significance are a set of northeasterly trending dykes and/or faults of the Vlakfontein swarm, which could host Ni (PGE) sulphide bodies of the type already known from the area. Anomalously low radiometric signatures were used to demarcate areas of unequivocal mafic/ultramafic rock outcrops or sub-outcrops. Higher radiometric signatures reflect Transvaal floor rocks while extremely high signatures occur over the Pilanesberg alkalic rocks. In terms of soil geochemistry, chromium is a good indicator of the Lower and Lower Critical zones of the Bushveld Complex. The transition from a high to a low chromium signature in the Rustenburg area is broadly coincident with the base of the Upper Critical Zone, which contains the important UG2 and Merensky Reef platiniferous layers. The same high to low chrome transition is seen to the west of the Pilanesberg and is considered to be an important PGE target indicator. Anomalously high nickel values occur over the Lower Zone rocks to the west of the Pilanesberg, although in the Rustenburg sector the most pronounced nickel geochemical anomalies are shown to be related to the impact of mining including tailings dams, smelters etc. The expansive nickel anomaly west of the Pilanesberg probably reflects the greater abundance of altered (including birbiritized) olivine-bearing rocks (harzburgites) and the presence of nickeliferous pipes. The transition area from a high-nickel (>220ppm) to a low-nickel signature, as in the case of chromium, defines a good PGE target area. Vanadium and titanium as expected show an anomalous build-up in the Upper Zone rocks of the Bushveld Complex. V also shows anomalous values over the upper chromitite zone and over IRUPS. The zone east of the vanadium anomalies to the west of the Pilanesberg therefore emerges as one with PGE potential. By integrating the interpretations of the above data sets, six main PGE target areas emerge to the west of the Pilanesberg. The most important targets include a well-defined zone of probable Critical Zone rocks as well as east-northeasterly trending linear targets along the Vlakfontein and related fault/dyke sets. Possible fault-, basal- and offset-style mineralisations are identified in the northwestern part of the area.

Revisiting the “Yanbian Terrane”: Implications for Neoproterozoic tectonic evolution of the western Yangtze Block, South China

There is a consensus that the nature and evolution of the widespread Neoproterozoic igneous rocks along the western margin of the Yangtze Block are important for understanding the tectonic evolution of South China, but the petrogenesis and tectonic affiliations of these rocks are still highly debated. The Yanbian region in the southwestern Yangtze Block preserves Neoproterozoic igneous packages that provide a rare opportunity for unraveling the regional tectonomagmatic evolution. SHRIMP U–Pb zircon data show two major episodes of mafic–ultramafic magmatism in the region. The elder one is represented by a basaltic suite within the Yanbian Group that formed at ca. 920–900(?) Ma. Geochemical and Nd isotopic data suggest these basalts were derived from a highly depleted asthenospheric mantle in a continental back-arc basin, rather than being part of an ophiolite suite formed in an oceanic setting as previously suggested. A major fold and thrust event took place during the final stage of the Sibao orogenesis at ca. 900 Ma, forming the east-northeasterly trending structures within the Yanbian Group rocks (and regional pre-900 Ma rocks as well). The younger magmatic episode is represented by the non-deformed mafic–ultramafic intrusions dated at 820–800 Ma. Mineral compositions and whole-rock geochemistry suggest that they are affinitive with rift-related, rather than arc-related, magmatism, with some pre-existing arc components being assimilated during their genesis. These mafic–ultramafic intrusions belong to the ca. 830–740 Ma anorogenic igneous provinces along the north-south trending Kangdian Rift in the western Yangtze Block. The igneous rocks in the Yanbian region had thus recorded an important tectonic transition from a back-arc setting during the Sibao Orogeny at ≥900 Ma to a continental rift setting after ca. 825 Ma in the southwestern Yangtze Block. This tectonic transition most likely corresponded to the amalgamation and breakup of the Rodinia supercontinent. We see neither geological nor geochemical evidence for the existence of a mid-Neoproterozoic (830–740 Ma) magmatic arc or an ophiolitic complex in the Yanbian region as suggested by previous studies.

Late Palaeozoic to Triassic evolution of the Turan and Scythian platforms: The pre-history of the Palaeo-Tethyan closure

A number of en échelon-arranged, southwest-facing arc fragments of Palaeozoic to Jurassic ages, sandwiched between two fairly straight east-northeast trending boundaries, constitute the basement of the Scythian and the Turan platforms located between the Laurasian and Tethyside units. They have until now largely escaped detection owing to extensive Jurassic and younger cover and the inaccessibility of the subsurface data to the international geological community. These units are separated from one another by linear/gently-curved faults of great length and steep dip. Those that are exposed show evidence of strike-slip motion. The arc units originally constituted parts of a single “Silk Road Arc” located somewhere south of the present-day central Asia for much of the Palaeozoic, although by the late Carboniferous they had been united into a continental margin arc south of the Tarim basin and equivalent units to the west and east. They were stacked into their present places in northern Afghanistan, Turkmenistan, Caucasus and the northern Black Sea by large-scale, right-lateral strike-slip coastwise transport along arc-slicing and arc-shaving strike-slip faults in the Triassic and medial Jurassic simultaneously with the subductive elimination of Palaeo-Tethys. This gigantic dextral zone (“the Silk Road transpression”) was a trans-Eurasian structure and was active simultaneously with another, similar system, the Gornostaev keirogen and greatly distorted Eurasia. The late Palaeozoic to Jurassic internal deformation of the Dniepr–Donets aulacogen was also a part of the dextral strain in southern Europe. When the emplacement of the Scythian and Turan units was completed, the elimination of Palaeo-Tethys had also ended and Neo-Tethyan arcs were constructed atop their ruins, mostly across their southern parts. The western end of the great dextral zone that emplaced the Turan and Scythian units horsetails just east of north Dobrudja and a small component goes along the Tornquist–Teisseyre lineament.

PETROGRAFIA E MICROESTRUTURAS DE TECTONITOS DA ZONA DE CISALHAMENTO RIBEIRA, VALE DO RIBEIRA, SP E PR: IMPLICAÇÕES NO ZONEAMENTO METAMÓRFICO E ESTRUTURAL DO SUPERGRUPO AÇUNGUI E UNIDADES CORRELATAS

The Ribeira Shear Zone, in the homonymous valley, Southeastern Brazil, is a dextral transcurrent, east-northeast trending structure that cuts metasedimentary and metabasics rocks of the Acungui Supergroup and granitic plutons. Petrographic and microstructural data indicate a long deformational history of the shear zone that is characterized by mylonites formed under amphibolite (500°-600°C) to greenschist (300°-400°C) facies conditions, followed by formation of breccias. A clearly spatial distribution of the mylonites of different metamorphic degrees indicate that the Ribeira Shear Zone had a fundamental role in the metamorphic and structural zoning of part of the Acungui Supergroup and correlated units. A vertical displacement about 3,5-4,4 km is inferred between the neighboring tectonic blocks.

PETROGRAFIA E MICROESTRUTURAS DE TECTONITOS DA ZONA DE CISALHAMENTO RIBEIRA, VALE DO RIBEIRA, SP E PR: IMPLICAÇÕES NO ZONEAMENTO METAMÓRFICO E ESTRUTURAL DO SUPERGRUPO AÇUNGUI E UNIDADES CORRELATAS

The Ribeira Shear Zone, in the homonymous valley, Southeastern Brazil, is a dextral transcurrent, east-northeast trending structure that cuts metasedimentary and metabasics rocks of the Açungui Supergroup and granitic plutons. Petrographic and microstructural data indicate a long deformational history of the shear zone that is characterized by mylonites formed under amphibolite (500°-600°C) to greenschist (300°-400°C) facies conditions, followed by formation of breccias. A clearly spatial distribution of the mylonites of different metamorphic degrees indicate that the Ribeira Shear Zone had a fundamental role in the metamorphic and structural zoning of part of the Açungui Supergroup and correlated units. A vertical displacement about 3,5-4,4 km is inferred between the neighboring tectonic blocks.

Upper-mantle anisotropy beneath the Altai–Sayan region of central Asia

The Altai–Sayan region of central Asia is actively deforming despite its being located several thousands of kilometers away from the collisional front between India and Eurasia. To add constraints to the role of the upper mantle in this type of tectonic environment, we analyzed intermediate-period, analogue records of SKS and SKKS to determine the state of mantle anisotropy in this region. We find that for all stations located in actively deforming areas, the fast direction of anisotropy correlates well with indicators of Cenozoic to the present-day deformation, and correlates with older structures only when they happen to align with younger ones. The general east-northeast trend of the fast direction of anisotropy in active regions also aligns with the principle extensional strain axis determined from earthquake moment tensors. At one station (NVS), located outside of any actively deforming region, the N–S fast direction may correspond to the long-wavelength strain field of the mantle beneath much of Siberia. These correlations, along with the size of the delay times (1–2 s), suggest that the anisotropy in the upper mantle beneath the Altai–Sayan is responding to recent tectonic activity.

Timing of Interplay between Hydrothermal and Surface Fluids in the Navan Zn + Pb Orebody, Ireland: Evidence from Metal Distribution Trends, Mineral Textures, and 34S Analyses

Models of genesis for the Navan orebody are of two distinct types. An early hypothesis that mesothermal (though nonmagmatic) deposition of ore began when a supernatant seawater brine still had access to the host sediments (during the early to mid-Mississippian), has been challenged by recent suggestions favoring a later (mid- to late Mississippian to Pennsylvanian) mineralization derived from cooler fluids traversing either the Mississippian/Pennsylvanian basin or the underlying basement from the south. These models, characterized here as Irish-type and Mississippi Valley-type respectively, are interrogated as to their particular predictions with regard to metal distributions and sulfur isotope patterns associated with various fault geometries. The basal 5 lens of the Navan Zn + Pb deposit contains ~70 percent of the known tonnage of the ~90 Mt orebody and, thus, is the focus of this examination. Lead distribution patterns suggest that migration of metal-bearing fluids was principally directed up early to mid-Mississippian, near vertical north-northeast, northeast, and east-northeast minor normal faults. These faults predate or are coeval with the major extensional, partly listric, east-northeast faults which now control the general disposition of the deposit. Only where these major east-northeast faults cross putative deep-seated northeast (Caledonoid) and northwest structures are they associated with lead enrichments. A systematic δ 34 S survey in the 5 lens across five minor north-northeast– through to east-northeast–trending faults associated with distinct lead enrichments, and one east-northeast–trending, partly listric, major extensional fault adjacent to that trend, revealed positive δ 34 S values (1–18‰) for galena, sphalerite, and marcasite sampled within 3 m of all the faults on the profile. Sulfides with positive δ 34 S values associated with the deep-seated, metal-bearing fluid generating the Navan deposit have been highlighted by previous workers (Anderson et al., 1998). The evidence reported here strongly suggests that the metal-bearing fluids rose through all the fractures. Conversely, negative δ 34 S values (–1 to –26‰) were returned in galena and sphalerite sampled 3 m or more from these faults. These negative values indicate that locally derived bacteriogenic sulfide, reduced from seawater sulfate, dominated away from these faults. Pyrite δ 34 S values suggest a background level of –29 ± 3.0 per mil across the profile. However, pyrite δ 34 S values as low as –34 ± 2.7 per mil were recorded in one sample collected from within 1 m of a fault. Thus, fluids containing highly fractionated, bacteriogenic sulfide also gravitated into these faults on at least one occasion. There is also evidence suggesting that the metal-bearing solutions periodically displaced the locally derived bacteriogenic sulfide-bearing fluid in and near the faults. Mineral sulfide petrography is used to contextualize the sampling and to give a qualitative indication of the degree of chemical disequilibrium of the system. Mineral textures demonstrating comminution and dissolution are revealed by this study that, when coupled with evidence of isotopic overprinting, force the conclusion that mineralizing fluids first invaded the host lithologies during mid-Lower Mississippian times, coincident with active faulting. There is no evidence of reactivation of the minor fault sets encountered in the study area during post-Chadian tectonism, though the major, partly listric, east-northeast extensional faults were reactivated at that time. Ore deposition was effected by bacteriogenic sulfide (reduced from Mississippian seawater sulfate) reacting with rising, metal-bearing mesothermal fluids. Thus any model invoking an onset of mineralization later than the mid-Lower Mississippian does not stand up to this scrutiny.

Sedimentation patterns support seismogenic low-angle normal faulting, southeastern California and western Arizona

Research Article| September 01, 1999 Sedimentation patterns support seismogenic low-angle normal faulting, southeastern California and western Arizona Julia M. G. Miller; Julia M. G. Miller 1Department of Geology, Vanderbilt University, Nashville, Tennessee 37235 Search for other works by this author on: GSW Google Scholar Barbara E. John Barbara E. John 2Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82070-3006 Search for other works by this author on: GSW Google Scholar Author and Article Information Julia M. G. Miller 1Department of Geology, Vanderbilt University, Nashville, Tennessee 37235 Barbara E. John 2Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82070-3006 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1999) 111 (9): 1350–1370. https://doi.org/10.1130/0016-7606(1999)111<1350:SPSSLA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Julia M. G. Miller, Barbara E. John; Sedimentation patterns support seismogenic low-angle normal faulting, southeastern California and western Arizona. GSA Bulletin 1999;; 111 (9): 1350–1370. doi: https://doi.org/10.1130/0016-7606(1999)111<1350:SPSSLA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Miocene synextensional strata flanking the Chemehuevi and southern Sacramento Mountains in southeastern California and western Arizona indicate deposition during seismically active low-angle normal faulting. These data complement existing structural, isotopic, and fission-track studies, which show that the regionally developed Chemehuevi-Sacramento detachment-fault system was initiated and allowed movement of the hanging wall over more than 18 km within the seismogenic regime at moderate to low angles of dip (≤ 30°). Individual faults within the detachment-fault system are corrugated parallel to the east-northeast transport direction, resulting in broad mullion structures (50–550 m amplitude and 1.5–10 km wavelength). Slip occurred along the rooted fault system between ca. 23 and 12 Ma; the average slip rate was ∼7–8 mm/yr during peak tectonic extension from ca. 19 to 15 Ma.Tertiary strata preserved in tilted hanging-wall blocks are between 2 and 3 km thick. Mafic and intermediate volcanic rocks (ca. 23 to 18.5 Ma) at the base of the section constitute 40% to 50% of most basin fills. Alluvial-fan conglomerate, sedimentary breccia, and megabreccia (somewhat >15.5 to somewhat <13.9 Ma) dominate the unconformably overlying sedimentary succession. Decreasing dip angles in progressively younger strata through this volcanic-sedimentary succession, and angular unconformities between units, form corroborative evidence with geochronologic data and show that these strata accumulated during extension. Sedimentary facies and clast types indicate proximal deposition in small basins distributed along east-northeast–trending regions that parallel and overlie synforms in the underlying corrugated Chemehuevi-Sacramento detachment fault. The composition, thickness, and distribution of these volcanic and sedimentary strata support their accumulation near a gently dipping normal fault. Clast types show an inverted stratigraphy recording erosion to progressively deeper structural levels in the source region. Emplacements of thick (<750 m) megabreccias (rock-avalanche deposits), derived from both the hanging wall and the footwall, were likely triggered by earthquakes. Tilted and displaced conglomerate and megabreccia (younger than ca. 15 Ma) contain footwall clasts and indicate breaching of the detachment fault, erosion of the footwall, and late movement on the gently dipping Chemehuevi-Sacramento detachment fault at the Earth's surface.These data show that the gentle dip and corrugated shape of the Chemehuevi-Sacramento detachment-fault system, when it was seismically active and allowed movement at and near the Earth's surface, controlled the location and fill of basins during progressive extension. Stratigraphic, structural, and thermal data therefore corroborate and challenge the assumption that low-angle normal faults are aseismic. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lineaments and fracturing in the Neogene rocks of the Almazán Basin, northern Spain

The Almazán Basin (northern Spain) is a Tertiary basin, 90×45 km wide, located in the western part of the Iberian Chain. It is filled with folded Palaeogene (2500–3000 m thick) and horizontal Neogene (500 m) beds. From the study of a satellite image, more than 2300 lineaments have been identified in the Neogene horizontal rocks. Their absolute orientation maximum is 060–070, and relative maxima are 100–110 and 120–140. Some spatial variations in the regional trends can be observed: (1) Throughout the central part of the basin, the east–northeast trending lines are the most representative. (2) Near the eastern border of the Basin the northwest–southeast set (parallel to the trend of thrust and folds in the older rocks) is dominant. (3) In the northwestern part of the basin the northwest–southeast and WNW–ESE sets coexist with the main east–northeast set. The maximum density of lineaments is found along a WNW–ESE line that corresponds with the axis of maximum thickness for the Tertiary units. A detailed field study of fracturing at the outcrop scale (more than 2300 fractures measured) was carried out. The orientation distribution of lineaments coincides with the orientation of left-lateral faults at the outcrop scale, and differences are greater when considering all kinds of fractures in different sectors. The origin of fracturing at different scales in the Almazán Basin can be related to the Neogene stress field and the influence of previous structures: (1) The northwest–southeast set of fractures formed as a result of reactivation of older faults, inherited from the late Variscan fracturing. (2) The main ENE–WSW set of fractures corresponds to neoformed left-lateral joints and faults under the northeast–southwest Neogene compression, the dextral conjugate set of joints within this system (NNE–SSW) being much less developed.