Orientation Of Dykes Research Articles

Breaking the Ring of Fire: How Ridge Collision, Slab Age, and Convergence Rate Narrowed and Terminated the Antarctic Continental Arc

AbstractThe geometry of the Antarctic‐Phoenix Plate system, with the Antarctic Plate forming both the overriding plate and the conjugate to the subducting oceanic plate, allows quantification of slab age and convergence rate back to the Paleocene and direct comparison with the associated magmatic arc. New Ar‐Ar data from Cape Melville (South Shetland Islands, SSI) and collated geochronology shows Antarctic arc magmatism ceased at ∼19 Ma. Since the Cretaceous, the arc front remained ∼100 km from the trench whilst its rear migrated trenchward at 6 km/Myr. South of the SSI, arc magmatism ceased ∼8–5 Myr prior to each ridge‐trench collision, whilst on the SSI (where no collision occurred) the end of arc magmatism predates the end of subduction by ∼16 Myr. Despite the narrowing and successive cessation of the arc, geochemical and dyke orientation data shows the arc remained in a consistently transitional state of compressional continental arc and extensional backarc tectonics. Numerically relating slab age, convergence rate, and slab dip to the Antarctic‐Phoenix Plate system, we conclude that the narrowing of the arc and the cessation of magmatism south of the SSI was primarily in response to the subduction of progressively younger oceanic crust, and secondarily to the decreasing convergence rate. Increased slab dip beneath the SSI migrated the final magmatism offshore. Comparable changes in the geometry and composition are observed on the Andean arc, suggesting slab age and convergence rate may affect magmatic arc geometry and composition in settings currently attributed to slab dip variation.

The Magmatic Rifting of Santos Basin: Aeromagnetic Mapping of Dykes, Terranes and Marginal Structures and the Interplay Between Tectonism and Volcanism

AbstractWe present a new magnetic map integrating continental and oceanic features at the Santos Basin, in order to investigate the connection between onshore and offshore tectonics and magmatism at the early stages of rifting. The magnetic and seismic data evidence the offshore continuity of the Florianópolis and Serra do Mar dyke swarms, while the Ponta Grossa Dyke Swarm is interrupted by the marginal structures. This structural relationship brings to evidence a different timing of formation of the Ponta Grossa Dyke Swarm relative to the early opening of the margin. A very high amplitude magnetic lineament ‐ the Santos Marginal Anomaly (SAMA) reaches up to 1,500 nT and trends NE‐SW, with local E‐W inflections forming the Cananéia and Guaratiba Arcuate anomalies. Our modeling suggests that the SAMA can be associated with dykes and syn‐rift extrusive rocks linked at depth to larger magmatic bodies at the proximal and necking domains. Based on the increasing width and amplitude of the magnetic anomalies oceanwards, we propose a mechanism whereby the igneous accretion (i.e., the amount of magmatism) augmented as rifting evolved from continent to ocean, creating a volcanic rift axis. This rift was segmented by crustal discontinuities that probably nucleated on preexisting rheological contrasts, forming tectonomagmatic segments during the early extensional stages. The onshore‐offshore relationships highlight that where there was an agreement between basement fabric and dyke orientation, inheritance may have influenced their emplacement, although it was not a limiting factor for their formation. The regional geodynamics of the Gondwana opening was the first order control on the emplacements of the dyke swarms, with deviations of the magmatically influenced rift by tectonic inheritance.

Bimodal Holocene flank eruptions at Lanín composite volcano, Southern Volcanic Zone of the Andes: Characterization of their deposits and controls on the vents array

The Pleistocene to Holocene Lanín composite volcano, in the Southern Volcanic Zone of the Andes, has summit and flank eruptions of Holocene age. We present here a characterization of flank vents and their deposits produced during the most recent eruptions of the Lanín volcano. Volcanic facies analysis indicates four unreported Holocene eruptive units sourced from flank vents. The associated deposits have basaltic, basaltic andesitic and trachytic compositions, and were formed mainly by low-volume effusive and mainly Strombolian-like explosive eruptions. Pyroclastic deposits were identified at the NE, E and SW volcano flanks, and correspond to concentrated and diluted pyroclastic density current deposits, in addition to non-welded to welded fall deposits. Basaltic lava flows of pahoehoe morphology were produced in a purely effusive episode. An analysis of the distribution of Lanín's flank vents (dyke's orientation and vent alignments) along fault-kinematic analysis in basement rocks were carried out with the aim of assessing the basement anisotropies that may have controlled the location and orientation of dykes, eruptive fissures and vent alignments. Contrary to expectations, the Liquiñe-Ofqui Fault System does not constitute a first-order control on the orientation of dykes and vents alignments in Lanín. The vents arrangement may be partly influenced by pre-Andean crustal scale fault zones, such as the Mocha-Villarica Fault Zone, and local basement structures. The predominance of flank vents over summit eruptions strongly suggests that the load of the volcanic edifice is a controlling factor.

Effects of Spur Dyke’s Orientation on Bed Variation in Channel Bend

Spur dykes also known as Groynes are often used to either divert or attract the flow from the main structure to safeguard their life. Those structures may be bridge piers, abutments or any similar hydraulics structures. Spur dykes are also used to save the cutting of banks on concave side of stream. Lots of work have been done in recent past on spur dykes by many investigators in which various hydraulic and geometrical parameters of spur dykes such as discharge, sediment size, flow velocity, shear stress, spur dykes shape, size and submergence etc. are studied in detail. But mostly all the studies were pointed out in straight open channels. Very few studies were done in curved channel and only their similar effects were studied. In present thesis main emphasis is given to study the effect of orientation and location of spur dykes in meandering channel on the bed of downstream side. In the present study experimental work has been carried out in 80° bend and constant discharge (Q = 4.5 l/s) is allowed to pass in channel without spur dyke. It is found that maximum scouring occurs at angular displacement θ = 60° to 80° in the vicinity of outer bank. To minimize this scouring, spur dyke has been installed at angular displacement θ = 20°, 40° & 60° by changing the dyke angle α = 60°, 90° & 120° respectively. It is found that scouring at θ = 60° is reduced by installing spur dyke at angular displacement θ = 40° which is oriented at α = 60° and scouring at θ = 80° is reduced by installing spur dyke at angular displacement θ = 60° which is oriented at α = 60°.

Intrusion fracturing and quartz sand-rich injections in thrust-related fault rocks within the basal shear zone of the Esla Nappe (Cantabrian Zone, NW Iberia)

The Esla Nappe, located in the Cantabrian Zone (Variscan Orogen, NW Iberia), has an estimated 19 km displacement to the NE accommodated in a thin (<2–3 m) shear zone (ENSZ) at least 4 km depth. Fault-rock assemblages record different deformation processes operating in the hangingwall and footwall. While the hangingwall was largely deformed through cataclastic flow localised in a <50 cm band, hangingwall-footwall intervening ultracataclasites and footwall lithologies preserve evidence for an interplay of cataclastic flow and pressure solution. Final hydrofracturing and calcite vein precipitation has contributed to the vertical dilation of the fault rock assemblages.In the late stages of emplacement, the ENSZ was breached through intrusion fracturing and associated clastic sills and dykes injected along re-opened previous discontinuities and anisotropies. Together, they conform an interconnected network of quartz sand-rich lithosomes reaching structural heights of 20 m over the ENSZ. Injections are formed by a mixture of injected quartz grains and host-derived fragments. Dyke orientations suggest that the injection process occurred under large fluid overpressure conditions exceeding lithostatic values in the footwall. Fluid overpressure may have been caused by progressive fluid accumulation beneath a low-permeability ultracataclasite layer at the base of the nappe. Breaching through the shear zone was favoured by a change in the stress regime following the Esla Nappe emplacement.

Field observations and numerical models of a Pleistocene-Holocene feeder dyke swarm associated with a fissure complex to the east of the Tatara-San Pedro-Pellado complex, Southern Volcanic Zone, Chile

Magma is transported through the lithosphere as dykes which, during periods of unrest, may feed eruptions at the surface. The propagation path of dykes is influenced by the crustal stress field and can be disturbed by crustal heterogeneities such as contrasting rock units or faults. Moreover, as dykes propagate, they themselves influence the surrounding stress field through processes of stress transfer, crustal deformation and seismic failure. The result is the formation of arrested dykes, as well as contrasting strike and dip angles and dyke segmentation. Here, we study the mechanisms of dyke injection and the role played in modifying the stress field and potential propagation paths of later dyke injections. To do this we combine field data from an eroded and well-exposed shallow feeder dyke swarm with a suite of two-dimensional FEM numerical models. We mapped 35 dyke segments over a ~1 km long dyke swarm exposed ~5 km to the East of Pellado Volcano, in the Tatara-San Pedro-Pellado (TSPP) volcanic complex, Southern Volcanic Zone of the Andes. Detailed mapping of the swarm elucidates two preferential strike orientations, one ~N80°E and the other ~N60°E. Our numerical models simulate both the TSPP volcanic complex and the studied dyke swarm as zones of either magmatic excess pressure or as a rigid inclusion. The crustal segment hosting the volcanic complex and dykes is modelled using an elastic domain subjected to regional compression in select model cases. Model outputs provide the stress and strain fields resulting from the different geometries and applied boundary loads. The model results indicate that individual dyke injections can locally rotate the principal stresses such as to influence the range of orientations over which later dykes will form. The orientation of σ1 at the dyke tip ranges over 60° (±30° either side of the dyke tip) indicating that the strike orientation of later dykes will fall within this range. The effect of adding a bulk regional compression is to locally increase the magnitude of favorably oriented tensile stresses in the bedrock but to reduce the range of σ1 orientations to 40° (±20°). This implies that under a far-field transpressive stress regime, as is common in Andean settings, regional dyke swarms will tend to maintain their strike orientation parallel to the regional bulk stress. These results should be accounted for when studying periods of volcanic unrest in order to discern the location and orientation of potential fissure eruptions in active volcanic areas such as the Southern Volcanic Zone of the Andes.

Chapter 12 Dolerites (1.27–1.25 Ga) and alkaline ultrabasic dykes ( c. 1.14 Ga) related to intracratonic rifting

Abstract Doleritic sills, lopoliths and dykes were emplaced into the Paleoproterozoic craton in central Sweden at 1271–1264, 1259–1256 and c. 1247 Ma, a complex temporal zonation occurring in a WSW–ENE direction. The dolerites are subalkaline to alkaline and show predominantly gabbroic, with a trend towards monzogabbroic and quartz monzodioritic, compositions. Positive ɛ Nd and ɛ Hf values suggest a significant depleted mantle component in the source volume of the parental magmas. Dyke orientations indicate extension, at least locally, in a northwesterly direction, consistent with a magma flow direction determined using the anisotropy of magnetic susceptibility values. Intracratonic rifting linked to the break-up of the supercontinent Columbia, back-arc extension above a subduction boundary in a westwards-retreating mode or a mantle plume tail above a continental hotspot have all been proposed for the tectonic setting. Renewed intracratonic rifting at c. 1.14 Ga in the coastal area in northeasternmost Sweden resulted in the emplacement of alkaline ultrabasic dykes, including carbonatites (beforsites), silico-carbonatites and lamprophyres, in a north–south direction along an older shear belt. The broader tectonic setting of this extensional event is not known.

Emplacement mechanisms of a dyke swarm across the brittle-ductile transition and the geodynamic implications for magma-rich margins

Igneous dykes are the main magma transport pathways through the Earth's crust, and they are considered to contribute to tectonic extension in volcanic rifts. Dykes are typically considered to result from brittle fracturing, even in the ductile crust. A common assumption is that dyke orientation is controlled by tectonic stresses, such that dykes in rifts are expected to be vertical and perpendicular to extension. Here we report on detailed field observations of a spectacularly well-exposed dyke swarm to show that dykes were not systematically emplaced by purely brittle processes and that dyke orientation may differ from the dominant tectonic stress orientations. The dyke complex formed near the brittle-ductile transition during opening of the Iapetus Ocean and is now exposed in the Scandinavian Caledonides. Distinct dyke morphologies related to different emplacement mechanisms has been recognized: 1) Brittle dykes that exhibit straight contacts with the host rock, sharp tips, en-echelon segments with bridges exhibiting angular fragments; 2) Brittle-ductile dykes that exhibit undulating contacts, rounded tips, ductile folding in the host rock and contemporaneous brittle and ductile features; 3) Ductile “dykes” that exhibit rounded shapes and mingling between the soft ductile host rock and the intruding mafic magma. The brittle dykes exhibit two distinct orientations separated by c. 30° that are mutually cross-cutting, suggesting that the dyke swam did not consist of only vertical sheets perpendicular to regional extension, as expected in rifts. We were able to use the well-exposed host rock layers as markers to perform a kinematic restoration to quantify the average strain accommodating the emplacement of the dyke complex: it accommodated for >100% extension, but counter-intuitively it also accommodated for 27% crustal thickening. We infer that the magma influx rate was higher than the tectonic stretching rate, implying that magma was emplaced forcefully, as supported by field observations. Finally, our observations suggest that the fast emplacement of the dyke swarm triggered a rapid shallowing of the brittle-ductile transition, and lead to a considerable weakening of the crust. The interpretations presented here could potentially have large implications for surface topography and seismicity in active rifts and volcanic areas around the world.

The relation between differential stress, driving pressure ratio, tensile strength and reservoir failure in a magmatic system – Towards eruption forecasting

In this study a relation is established between differential stress (σd), driving magma pressure ratio (R′) and tensile strength (T) of a rock mass in a volcanic system. It is shown that in σd vs. R′ space, the T curve follows equation of rectangular hyperbola and for dyking or volcanic eruption to occur, this T curve must be exceeded. It is proposed that this theoretical relation can be exploited to understand magma reservoir failure and for volcanic eruption forecasting. Applicability of this concept is demonstrated on Santorini volcanic system (Greece) for which considerable background information is available from previous studies. Published dyke orientation and aspect ratio data from Santorini yield σd and R′ values of 8.04 MPa and 0.22 respectively. Considering a weak quality host rock mass in Santorini, tensile strength curve for T = 1.5 MPa is plotted in σd vs R′ space. Considering magma reservoir volume and rock physical property estimates to be valid, and assuming σd and absolute stress to be constant, it is argued that change in reservoir dynamics would be controlled by variations in volume of magma released from the deeper magma reservoir to shallow reservoir (Vr) in Santorini. Values of R′ and Vr for the events of non-dyking in Santorini are also calculated. Minimum R′ and Vr required to induce dyking and eruption are estimated and it is concluded that the proposed theory can be generally applied for eruption forecasting.

Pegmatite dyke emplacement and the state of stress during cratonization – An example from the Dharwar Craton (South India)

In this study we measured orientations of 797 pegmatite dykes intruding the ∼2.5 Ga Koppal Pluton (Dharwar Craton, South India) and use the data to evaluate the state of stress prevalent during dyke emplacement. We infer that the dykes were emplaced synchronously with late-D3 regional deformation that was controlled by NW-SE directed shortening. Dykes within the Koppal Pluton (KP) have a restricted range of orientations, which is indicative of fluid pressure (Pf) < σ2. Paleostress analysis using dykes yields orientation of σ2 to be steepest (with respect to other principal stress axes), while σ1 is shallow, thus implying oblique-slip during dyke emplacement, but with a strong strike-slip component. Maximum orientation of dykes intruding KP is NNW-SSE, which is oblique to the NNE-SSW oriented magnetic fabric that is known to have developed in the KP during earlier stages of D3. Statistical analysis of dyke thickness data from KP implies accumulation of melt in multiple pockets was important during the pegmatite emplacement process during late-D3 deformation. A kinematic model involving this aspect as well as deformation partitioning during late-D3 along anisotropy developed during early-D3 (magnetic fabric) is envisaged. This explains emplacement of pegmatite dykes having a NNW-SSE strike. Cratonization of Dharwar Craton is known to have occurred at ca 2.5 Ga, which is also the established age of KP. Therefore, the present work on pegmatite dykes provides information about the processes and paleostress prevalent during this cratonization.

Extraction of high-resolution structural orientations from digital data: A Bayesian approach

Measurement of structure orientations is a key part of structural geology. Digital outcrop methods provide a unique opportunity to collect such measurements in unprecedented numbers, and are becoming widely applied. However, orientation estimates produced by plane fitting can be highly uncertain, especially when observed data are approximately collinear or the structures of interest comprise differently oriented segments. Here we present a Bayesian approach to plane fitting that can use data extracted from digital outcrop models to constrain the orientation of structures and the associated uncertainty. We also describe a moving-window search algorithm that exploits this Bayesian formulation to estimate local structure orientations for segmented structures. These methods are validated on synthetic datasets for which both the structure orientation and associated uncertainty is known. Finally, we implement the method in the point cloud analysis package CloudCompare and use it to estimate the orientation and thickness of dykes exposed in cliffs on the island of La Palma (Spain). The results highlight the potential of this method to generate structural data at unprecedented spatial resolution, while simultaneously characterising the associated uncertainties.

Semi-automatic extraction and mapping of dyke swarms based on multi-resolution remote sensing images: Applied to the dykes in the Kuluketage region in the northeastern Tarim Block

The geometry and spatial distribution of mafic dyke swarms provide information about geodynamics. However, mapping large-scale dyke swarms manually is a time-consuming and expensive process. Here we present an efficient method to map dyke swarms semi-automatically by using multi-resolution remote sensing images. This method is applied in the Kuluketage region in the northeastern Tarim Block, northwest China where mafic dykes of multiple trends are widely exposed and with the dominant NW trend being of probable Neoproterozoic age. The tracing of dykes, including their orientation, length, thickness, density, and distribution is done by integrating 30-m multispectral Landsat 8 and 2-m panchromatic CORONA KH-4B imagery. The Gramm-Schmidt Spectral Sharpening approach is used to generate color composite and fusion images with 2-m resolution. The Canny edge detector and Hough transform are used to detect dyke edges. Combining the detection of dyke edges with ArcGIS™, the dyke orientation, length, density, thickness, and exposed area are measured. The accuracy of the semi-automatic method exceeds 75% in comparison with manual mapping of the dykes in the Kuluketage region.

Effect of volcanic dykes on coastal groundwater flow and saltwater intrusion: A field‐scale multiphysics approach and parameter evaluation

AbstractVolcanic dykes are common discrete heterogeneities in aquifers; however, there is a lack of field examples of, and methodologies for, comprehensive in situ characterization of their properties with respect to groundwater flow and solute transport. We have applied an integrated multiphysics approach to quantify the effect of dolerite dykes on saltwater intrusion in a coastal sandstone aquifer. The approach involved ground geophysical imaging (passive magnetics and electrical resistivity tomography), well hydraulic testing, and tidal propagation analysis, which provided constraints on the geometry of the dyke network, the subsurface saltwater distribution, and the sandstone hydrodynamic properties and connectivity. A three‐dimensional variable‐density groundwater model coupled with a resistivity model was further calibrated using groundwater and geophysical observations. A good agreement of model simulations with tide‐induced head fluctuations, geophysically derived pore water salinities, and measured apparent resistivities was obtained when dykes' hydraulic conductivity, storativity, and effective porosity are respectively about 3, 1, and 1 orders of magnitude lower than the host aquifer. The presence of the dykes results in barrier‐like alterations of groundwater flow and saltwater intrusion. Preferential flow paths occur parallel to observed dyke orientations. Freshwater inflows from upland recharge areas concentrate on the land‐facing side of the dykes and saltwater penetration is higher on their sea‐facing side. This has major implications for managing groundwater resources in dyke‐intruded aquifers, including in coastal and island regions and provides wider insights on preferential pathways of groundwater flow and transport in highly heterogeneous aquifer systems.

Structure and tectonic setting of the SE Sardinia mafic dyke swarm. Insights for the stress state during magma emplacement in the upper crust

The SE Sardinia mafic dyke swarm intruded in Permian times into the Late Variscan granitoids of the Sàrrabus massif. In order to characterize the structure of the dyke swarm and to assess the stress state and tectonic regime at the time of dyking, we have performed a multi-methodological study which encompasses paleostress analysis using the orientation distribution of dykes, estimation of crustal extension from dyke thickness/spacing ratios and true dilation direction from the matching geometry of offsets. The mean orientation of the dyke swarm is ∼N150°, with secondary main trends orientated ∼N77° and ∼N10°. The fracture network (mostly joints) exhibits multiple orientations grouped in four or more sets, being the orientation and density patterns in granitoid and dykes significantly different. Some of the pre-existing joints were exploited by the dykes, while others were reactivated as faults during and after dyke intrusion. The paleostress analysis yielded σ1 sub-vertical, indicating that an extensional tectonic regime predominated in the area during the emplacement of the dyke swarm. σ3 was sub-horizontal, sub-parallel to the mean NE–SW dilation direction. The inferred relative stress magnitudes are characterized by a σ1 much larger than the other two principal stresses. In this context, intrusion of the dyke swarm occurred under low magma pressure conditions and preferentially into a pre-existing joint nerwork during a regional extensional event. The SE Sardinian dyke swarm has been compared with other contemporary mafic dykes of the western Mediterranean region, by attempting to restore the effects of the Neogene and eo-Alpine rotational events. The reconstruction gives a complex crustal-scale pattern in terms of dyke orientation, a fact that could be partly attributed to differential rotations associated with localized strike-slip movements that might have operated coevally with regional extension or transtension during Permian times.

Integrating WorldView-2 imagery and terrestrial LiDAR point clouds to extract dyke swarm geometry: Implications for magma emplacement mechanisms

Dyke geometries are useful indicators of the palaeostress field during magma emplacement. In this paper, we present a multi-scale extraction method of dyke geometries by integrating WorldView-2 (WV2) imagery and terrestrial light detection and ranging (LiDAR) data. Color composite and fusion WV2 images with 0.5-m resolution were generated by using the Gramm–Schmidt Spectral Sharpening approach, which facilitates the discrimination of dyke swarms and provides the ability to measure the orientation, exposed length, and thickness of dykes in sub-horizontal topographic exposures. A terrestrial laser scanning survey was performed on a sub-vertical exposure of dykes to obtain LiDAR data with point spacing of ~0.02m at 30m. The LiDAR data were transformed to images for extracting dyke margins based on image segmentation, then the dyke attitudes, thicknesses, and irregularity of dyke margins were measured according to the points on dyke margins. This method was applied at Sijiao Island, Zhejiang, China where late Cretaceous mafic dyke swarms are widespread. The results show that integrating WV2 imagery and terrestrial LiDAR improves the accuracy, efficiency, and objectivity in determining dyke geometries in two and three dimensions. The ENE striking dykes are dominant, and intruded the host rock (mainly granite) with sub-vertical dips. Based on the aspect ratios of the dykes, the magmatic overpressure was estimated to be less than 11.5MPa, corresponding to a magma chamber within 6.6km in the lithosphere.

Eocene–Oligocene stratigraphy and structural history of the Karaburun area, southwestern Black Sea coast, Turkey: transition from extension to compression

AbstractThe stratigraphic succession exposed in the Karaburun area (southern Black Sea coast, NW Turkey) records multiple changes in depositional and tectonic settings during Cenozoic times. It starts with the Middle–Upper Eocene Soğucak Formation of reef limestone that across a normal fault, omitting the lower part of the Lower Oligocene Ceylan Formation (deep-marine shale unit), abuts the upper part of the Ceylan Formation that is made up of two facies: (1) shallow-marine sandstone and (2) shallow-marine limestone units containing horizons of submarine slumps. Both facies are unconformably overlain by the fluvial Upper Miocene Çukurçeşme Formation. The tectonic record includes: (1) latest Eocene – Early Oligocene NE–SW extension, (2) Early Oligocene NE–SW shortening and (3) Late Miocene NW–NE extension. The earliest normal faults cutting the Soğucak and the lower part of the Ceylan formations are associated with clastic dykes injected into the deep-marine shale. These structures suggest a disruption of the Eocene carbonate platform and are also known in the neighbouring Thrace Basin. The following NE–SW shortening created the NE-vergent Karaburun Thrust that is synchronous with the shallowing and inversion of the Ceylan Basin. Rotation of the stress field is recorded by changes in clastic dyke orientation and their deformation. Compression caused multiple westerly directed submarines slides from uplifts in easterly located regions. This event is not recorded in the Thrace Basin. Finally, the Miocene tectonic activity formed NW- and NE-striking normal faults. The outlined tectonic history includes Early Oligocene extensional and compressional episodes recorded in the southern margin of the Black Sea that had hitherto not been known.

Dyke intrusion into a pre-existing joint network: The Aiguablava lamprophyre dyke swarm (Catalan Coastal Ranges)

A structural analysis has been performed in the Upper Permian lamprophyric dyke swarm of Aiguablava (Costa Brava, NE Iberia). Emplacement of sub-vertical dykes is related to the presence of a widespread joint network, mostly developed during the cooling and decompression of the late Variscan leucogranitic host rocks. The joint pattern consists on multiple sub-vertical joint sets among which two orthogonal sets at ≈N23° and ≈N113° are predominant. A sub-horizontal set is also prominent. The sub-vertical dykes have a mean N100°–N125° trend, which corresponds to the trend of one of the main joint sets. However, dyke segmentation is noticeable at the Dm- to cm-scale, and this is inferred to be related to dyke propagation and emplacement along the variably oriented pre-existing joints. A mean robust sub-horizontal NNE–SSW net dilation direction was measured from matching dyke jogs, markers in the host rock and analysis of maximum dyke thicknesses, and this is in line with the minimum principal stress axis (3) derived from a three-dimensional paleostress analysis from dyke orientations. The inferred maximum principal stress axis (1) is sub-vertical, indicating that dykes intruded under conditions of tectonic extension. Furthermore, a Mohr construction allowed calculation of the stress ratio φ=0.29 (close to a prolate stress ellipsoid) and a driving pressure ratio R′=0.27, which corresponds to a magmatic pressure almost equal to the intermediate principal stress axis, σ2. It is inferred that many of the pre-existing joint sets were exploited by the magma, the WNW–ESE joint set (normal to σ3) being the most favorable for dyke emplacement. The present study highlights the structural control of a pre-existing fracture network on emplacement of the Aiguablava lamprophyres in the upper crust during late Permian NNE–SSW brittle extension.

Tectonic controls on the Yamanlar volcano and Yuntdağı volcanic region, western Turkey: Implications for an incremental deformation

Over the past ten years, it has been proposed that the western part of the Menderes Massif was strongly structurally-controlled by the İzmir–Balıkesir transfer zone (İBTZ). Yamanlar volcano is a key area for understanding the deformation of Miocene volcanoes in western Turkey because of its progressive extensional tectonics. Structural analysis provides that this volcano has undergone the incremental tectonic controls in western Turkey since Early Miocene. The volcano experienced deformation and erosional processes associated with activity of intense tectonic regime that resulted in the dissection of the southern flank of the volcano mostly by NE–SW-striking oblique and strike-slip faults together with cross-cutting faults during and after Miocene period. The orientation of volcanic domes, dykes and intrusive bodies indicates successive and reactive tectonic phases that caused incremental complex movements of numerous fault blocks during the destruction area of the Yamanlar volcano.Structural data documents that the region around İzmir Bay and Yuntdağı volcanic complex is prevailed by far more than three deformation phases. Cross-faults do not reflect to the NE–SW-trending transtensional deformation thereafter Early–Middle Miocene for just the area of the Yuntdağı volcanic system. The nearly E–W-oriented faults are inferred to be the most likely cause of the destruction of both the Yamanlar and Dumanlıdağ volcanoes coincident with the last stage of the deformation phases the region experienced.This study proposed that the western part of the MMCC was affected by different deformational phases instead of İBTZ resulting in different stage magma ascents: (i) early-Miocene transtensional tectonic; (ii) middle Miocene extensional tectonic related to slab-retreat and (iii) late Miocene tectonic block rotation.

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).

Evaluation of factors influencing transmissivity in fractured hard-rock aquifers of the Limpopo Province

Geologically-complex fractured aquifers underlie large parts of the semi-arid Limpopo Province where some of the greatest groundwater needs in South Africa occur. It is important to identify potentially high-yielding zones that can be targeted for water supply. The study covered 7 distinct groundwater regions within Limpopo Province, together covering about 63 500 km2. Results from over 4 000 pumping-test analyses indicated that geological setting (e.g. aureole of granitoids), proximity and orientation of dykes and lineaments and proximity of surface-water drainages may exert an influence on borehole productivity.Although dykes are poor groundwater targets, drilling dykes composed of dolerite may prove to be more successful. Lineaments striking perpendicular to the current maximum horizontal stress seem to be more favourable targets, which is inconsistent with the predicted regime. Due to the complex geological history, it is difficult to link open discontinuities toa distinct recent or past tectonic event. Regional stress-field data, as in this case, may not account for local, possibly highly significant, stress-field variations. The hydrogeological importance of several factors related to groundwater occurrence, can be used as a working reference for future groundwater-development programmes.