Dike Patterns Research Articles

Granitic inselberg erosion controlled by dike swarm array in semiarid Brazil

Numerous inselbergs rise above the regional erosional surface in semiarid northeastern Brazil. The inselbergs are underlain by the Quixadá Pluton and cut by cm-thick granite dike swarms. Five inselbergs with abundant dikes from two different parts of the pluton were selected to investigate the role of dike patterns in the evolution of residual granite topography, including morphological features of hillslopes. The methods included field mapping of landforms, structural measurements of accessible dike arrays, and rock strength determinations using a Schmidt hammer. Unmanned Aerial Vehicle (UAV) photogrammetry was used to build detailed digital surface models of inselbergs and to recognize geometric patterns of dike arrays. In addition, 34 rock samples were collected to determine rock composition and evaluate their resistance to weathering. The results show that >60 % of dikes display a sub-horizontal array composed of low- and medium-angle dikes (up to 45° dip), which tend to increase rock-mass resistance, providing effective barriers against weathering and vertical fracturing, and are associated with the most prominent, steep-sided residual hills. Besides, dike swarm geometry controls the slope outlines of some inselbergs, which commonly conform to dike attitudes. By contrast, steeply-dipping dikes (>60° dip) promote weathering along the dike-host rock interface and are hypothesized to underlie the plains between the inselbergs. At a small scale, dikes control the occurrence and evolution of minor rock landforms such as weathering pans, solution runnels, overhangs, and split boulders. Our findings show that the resistance of inselbergs to erosion is not homogeneous in the granitic pluton but is controlled by local factors such as dike occurrence and dip.

Magma Flow Patterns in Dikes: Observations From Analogue Experiments

AbstractWe conducted analogue experiments to examine flux‐driven and buoyancy‐driven magma ascent, which included a series of isothermal experiments and thermal, solidification‐prone experiments. We measured the internal flow using 2D particle image velocimetry, which indicates that buoyancy has a strong control on the flow pattern of isothermal dikes. Dikes that are not buoyant (likely driven by source pressure) take on a circulating pattern, while buoyant dikes assume an ascending flow pattern. Solidification modifies the flow field so that flow is confined to the dike's upper head region. The lower tail becomes mostly solidified, with a narrow conduit connecting the source to the head. We interpret that this conduit acts as a high velocity point source to the head, promoting a circulating flow pattern, even as the dike becomes buoyant. We then perform particle tracking velocimetry on several particles to illustrate the complexity of their paths. In a circulating flow pattern, particles rise to the top of the dike, descend near the lateral edge, and then are drawn back into the upward flow. In an ascending pattern, particles ascend slightly faster than the propagation velocity, and therefore are pushed to the side as they approach the upper tip. In erupting dikes, particles simply flow to the vent. In the context of crystal growth in magmatic dikes, these results suggest that crystal growth patterns (e.g., normal or oscillatory zoning) can reflect the magma flow pattern, and potentially the driving forces.

Can the geometry of an injection clastic dike swarm indicate a short-term process? Examining the Dead Sea Basin in Israel

Despite the importance of identifying dynamic stress fields, doing so is not straightforward. This is because most geological processes occur slowly and the formation of fractures is generally attributed to long-term processes.Within the seismically active Dead Sea Basin (DSB), radial and concentric injection clastic dikes were emplaced during earthquake events, but a satisfactory explanation for their geometric configuration is lacking. In this study, field observations were used to explore whether the geometry of the injection clastic dike swarm reflects a quasistatic state of stress, such as with a salt diapir, or if it reflects a dynamic short-term stress state associated with the passage of seismic waves. The results show that the radial clastic dikes are not concentrated toward one central point located approximately 3 km east of the Ami'az Plain, that the concentric clastic dikes are arranged by three subsets, and that the spatial density and strain of both fracture sets are focused mainly toward the center of the Ami'az Plain. This evidence does not fit with a quasistatic state-of-stress interpretation of the region that would be associated with a local stress state around a salt diapir. A more plausible explanation is that the geometry of the radial and concentric clastic dike sets was controlled by short-term seismic waves that were probably generated by earthquake events located along an adjacent fault and that were trapped by the sedimentary basin located below the Ami'az Plain itself.The reconstruction of the injection clastic dike pattern may provide an opportunity to more deeply understand the short-term process induced by paleoearthquakes occurring adjacent to sedimentary basins close to the surface.

Early syn-rift igneous dike patterns, northern Kenya Rift (Turkana, Kenya): Implications for local and regional stresses, tectonics, and magma-structure interactions

AbstractFour areas (Loriu, Lojamei, Muranachok-Muruangapoi, Kamutile Hills) of well-developed Miocene-age dikes in the northern Kenya Rift (Turkana, Kenya) have been identified from fieldwork and satellite images; in total, >3500 dikes were mapped. Three areas display NNW-SSE– to N-S–oriented dike swarms, with straight, radial, and concentric patterns in zones <15 km long, and indicate NNW-SSE to N-S regional maximum horizontal principal stress (SHmax) directions in the early to middle Miocene. Individual dikes are typically <2 m wide and tens to hundreds of meters long and have accommodated <2% extension. In places (Loriu, Lojamei, Lokhone high), dikes trend at a high angle to the rift trend, suggesting some local influence (e.g., overpressured magma chamber, cracked lid–style dike intrusions over a sill or laccolith, preexisting fabric in basement) on orientation, in addition to the influence from regional stresses. Only a minor influence by basement fabrics is seen on dike orientation. The early- to middle-Miocene dikes and extrusive activity ended a long phase (up to 25 m.y.) of amagmatic half-graben development in central Kenya and southern Turkana, which lay on the southern edge of the early (Eocene–Oligocene) plume activity. The Miocene dike sets and extension on major border faults in Turkana contrast with larger, more extensive arrays of dikes in evolved systems in the Main Ethiopian Rift that are critical for accommodating crustal extension. By the Pliocene–Holocene, magmatism and intrusion along dikes had become more important for accommodating extension, and the tectonic characteristics began to resemble those of rift basins elsewhere in the eastern branch of the East African Rift.

청송 북동부 유문암질 암맥군의 패턴과 성인

청송 북동부에서 중태산과 갈평지 관입체는 영양소분지에서 각각 가송동층과 도계동층을 관입하여 각각 병반과 암주로 산출된다. 그리고 청송 암맥군은 도계동층을 관입하며 갈평지 암주에 매우 밀접하게 조합되어있다. 이 암맥군은 방사상으로 갈평지에 수렴되고 전체 윤곽이 타원형 분포를 나타낸다. 또한 암맥들은 유상엽리, 구과상, 석정상 구조를 나타내는 유문암맥들이며 하나의 단일 관입상을 나타낸다. 암맥들의 방향성은 관입체에서 응력상태에 의하여 지배되는 것으로 해석할 수 있다. 즉 암맥들은 일반적으로 갈평지 암주의 관입에 의해 퇴적암류의 도움작용 동안에 최고 주응력축에 직각으로 발생하는 방사상 단열에 따르는 수직 인장절리를 따라 주입해 방사상 패턴을 나타낸다. 이 패턴은 본역에서 이 암맥들이 대부분 방사상 절리들 속으로 주입을 충분히 설명해줄 수 있다. Jungtaesan and Galpyeongji intrusions in the northeastern Cheongsong occur as laccolith and stock which intrude Gasongdong Formation and Dogyedong Formation, respectively. Cheongsong dike swarm, intruding the Dogyedong Formation, is closely associated with this stock. The dike swarm is more radial to focus into Galpyeongji and its outline is oval. The dikes of the dike swarm are only rhyolite dikes with flow banded, spherulitic and rare stony structures, and represents a single intrusive phase of magma. It can be interpretated that orientation of the dikes is controlled by stress states. Therefore, the dikes display a radial pattern through occupying vertical joints that have been generally attributed to radial fractures formed during doming of the sedimentary rocks by the intrusion of the Galpyeongji stock. The dike pattern could sufficiently account for dike injections into these joints.

Flood damage reduction by compartmentalization of a dike ring: comparing the effectiveness of three strategies

AbstractCompartmentalization of dike ring areas is currently seen as one of the promising options for managing or reducing the risk of flooding. Recently, several studies on the effectiveness of subdividing dike ring areas by means of compartmentalization dikes were published. These studies show that the total damage after a flood could indeed be smaller, but that the damage could also turn out to be larger on a local level. The available studies on compartmentalization usually consider only one dike pattern in the area concerned. This article develops, assesses and compares three fundamentally different strategies, based on different configurations of the compartmentalization dikes. Not only do we examine the effects of the most probable flood scenarios, we also look at the worst‐case scenario, whereby the water is able to flood into the dike ring area from all sides. The spatial layout of the area codetermines the effectiveness of each strategy. Dike ring area 14, which lies in the west of the Netherlands and that covers some cities, like Amsterdam, Rotterdam and The Hague, was chosen as the area of application. The results show that for this dike ring, the secondary dike strategy is most effective in reducing the number of expected victims under different flood scenarios.

Dike propagation within active central volcanic edifices: constraints from Somma-Vesuvius, Etna and analogue models

Dikes within stratovolcanoes are commonly expected to have radial patterns. However, other patterns may also be found, due to regional stresses, magmatic reservoirs and topographic variations. Here, we investigate dike patterns within volcanic edifices by studying dike and fissure complexes at Somma-Vesuvius and Etna (Italy) using analogue models. At the surface, the dikes and fissures show a radial configuration. At depths of tens to several hundreds of metres, in areas exposed by erosion, tangential and oblique dikes are also present. Analogue models indicate that dikes approaching the flanks of cones, regardless of their initial orientation, reorient to become radial (parallel to the maximum gravitational stress). This re-orientation is a significant process in shallow magma migration and may also control the emplacement of dike-fed fissures reaching the lower slopes of the volcano.

Dike propagation in volcanic edifices: Overview and possible developments

Eruptions are fed by dikes; therefore, better knowledge of dike propagation is necessary to improve our understanding of how magma is transferred and extruded at volcanoes. This study presents an overview of dike patterns and the factors controlling dike propagation within volcanic edifices. Largely based on published data, three main types of dikes (regional, circumferential and radial) are illustrated and discussed. Dike pattern data from 25 volcanic edifices in different settings are compared to derive semi-quantitative relationships between the topography (relief, shape, height, and presence of sector collapses) of the volcano, tectonic setting (presence of a regional stress field), and mean composition (SiO 2 content). The overview demonstrates how dike propagation in a volcano is not a random process; rather, it depends from the following factors (listed in order of importance): the presence of relief, the shape of the edifice and regional tectonic control. We find that taller volcanoes develop longer radial dikes, whose (mainly lateral) propagation is independent of the composition of magma or the aspect ratio of the edifice. Future research, starting from these preliminary evaluations, should be devoted to identifying dike propagation paths and likely locations of vent formation at specific volcanoes, to better aid hazards assessment.

The farthest record of the Central Atlantic Magmatic Province into West Africa craton: Precise 40Ar/ 39Ar dating and geochemistry of Taoudenni basin intrusives (northern Mali)

The Taoudenni basin dike swarm (northern Mali) is one of the densest dike complexes of the Central Atlantic Magmatic Province (CAMP). A combined geometrical (dike directions), geochemical (major and trace elements, Sr and Nd isotopes), and geochronological ( 40Ar/ 39Ar dating) study is presented for the first time on the dikes and sills in this area. The CAMP-related magmatic event is confirmed, revealing a main peak in activity at 198.1 Ma. Dike orientations are clearly correlated with geochemical signatures: the E–W trending dikes have intermediate-Ti concentrations with La/Yb > 8; the N–S or NW–SE trending dikes (and the sills) have low-Ti contents, with La/Yb < 7. Although almost all 40Ar/ 39Ar plateau ages of both dike groups overlap, N–S and intermediate trending dikes could be slightly younger (peak at 197.2 Ma) than the E–W trending dikes (peak at 198.1 Ma). Timing and geochemical evolution on the dikes supported by comparison with contemporaneous lava flows in Morocco suggest that a single sub-continental lithospheric mantle source underwent increasing extents of partial melting (4% during initial stages and 7–15% during later ones) with more pronounced magmatic evolution and crustal contamination in the final stages. Finally, the dike directions are not in agreement with the radial pattern that would be predicted by a plume model for the CAMP. Alternatively, the evolution of the Taoudenni dike pattern may have been controlled by reactivation of older crustal structures and by later massive magma storage at depth in response to thermal incubation beneath the Pangea supercontinent.

Unveiling the origin of radial grabens on Alba Patera volcano by finite element modelling

The Tharsis region is an 8000-km-wide structural dome that incorporates a concentration of the main volcanic and tectonic activity on the Planet Mars. The area of structural doming is characterised by giant radial graben-dike systems. Nested on a set of these giant dikes to the northern side of Tharsis, is Alba Patera, one of the largest volcanoes in the planetary system. The regional dikes there are in arcuate arrangement and imply an E–W to NW–SE regional extension at Alba Patera. To assess the influence of regional and local tectonics, we studied the dike orientations on the volcano with Viking mosaic data and simulated plausible stress fields with finite element modelling. We found that the influence of a NW–SE regional extension was strong near the volcano centre but decreased rapidly in importance towards the northern pole, i.e., far from the Tharsis centre. By combining this regional stress with a broad uplift that is due to a buoyancy zone of about 1400 km in lateral extent and centred under Alba Patera, we reproduced the radial pattern of dike swarms that diverge from the Tharsis trend. Regional tectonics may have dominated the early stages of dike injection. During the evolution of Alba Patera, however, local updoming controlled the dike pattern, supporting the idea of a hotspot under Alba Patera. The well-expressed dike geometry and characteristics of Alba Patera provide an ideal example for comparative study with analogue hotspots on Earth where plate tectonics and active erosion may complicate the reconstruction of volcanic and tectonic history and the understanding of involved geodynamic processes.

Late Magmatism of the Galway Granite Batholith: II. Composite Dolerite–Rhyolite Dikes

Composite dolerite-rhyolite dikes traverse the Galway Granite batholith and its adjacent envelope. The dikes pertain to the Teach D6ite suite and were previously considered to be of Carboniferous age. New and extended examination of field relationships supports recent radiometric dating for an intrusive period that overlapped with the final consolidation of the Galway batholith. Regional crustal extension produced a complex pattern of fissuring, controlled by various preexisting structures, which permitted ascent of mantle-derived melts into and around the Galway batholith. Ponding of mafic magma at an intermediate level facilitated crustal partial melting and the generation of high-silica, high-alumina rhyolitic melts. The two contrasting magmas then rose into common or proximate dike fissures, rhyolitic injection immediately following that of dolerite. Magma storage in stratified chambers occasionally resulted in the development of a hybrid magma layer, but in all cases minor mingling and mixing beween dolerite and rhyolite magma continued up into the dikes. Rhyolite geochemistry precludes a genetic relationship with the Galway granitoids, despite a few instances where granitic material was entrained into rhyolitic magma. Introduction and setting The 400Ma Galway Granite batholith was emplaced into 470Ma island-arc orthogneisses in the Connemara sector of the Caledonides. This emplacement was followed by the intrusion of two hypabyssal suites: earlier microphyric ('porphyry') dacite dikes (Kinahan 1869; Mohr 2003) and a later complex nexus of dolerite dikes, the Teach D6ite (TD) suite (Mitchell and Mohr 1987; Fig. 1). The numerous and widespread dacite dikes have consistently been considered the youngest igneous rocks pertaining to the Galway batholith (Wager 1932; Wright 1961; Harvey 1967; Coats and Wilson 1971; Senior 1973; Leake 1974). However, new work summarised here suggests that the subsequent TD dikes were a final manifestation of the magmatic episode responsible for the batholith. The regional pattern of TD dikes comprises three major linear trends (Fig. 1). In the central part of the Galway batholith and its northern envelope, the NNE-trending Seanabhain system of dikes is intimately associated with the Shanawon Fault that separates the central and western blocks of the batholith (Feely and Madden 1988; Mohr 1993; Callaghan 1999). The na hUillinni dike system, 3.5km west of the Seanabhan system and parallel to it, projects much farther NNE into the orthogneiss envelope (Fig. 1). Secondly, a grid of ENE-trending dikes Irish Journal of Earth Sciences 22 (2004), 15-32. © Royal Irish Academy 15 This content downloaded from 207.46.13.174 on Sun, 10 Jul 2016 05:04:50 UTC All use subject to http://about.jstor.org/terms 16 Irish Journal of Earth Sciences (2004)

Calculation of dike trajectories from volcanic centers

Patterns of dike swarms around volcanic centers or above mantle plumes are interpreted by a mechanical analysis of regional and dike‐induced stresses, in which dike emplacement is controlled and guided by the stress state. Comparisons of dike patterns with patterns of principal‐stress trajectories caused by a source and a regional stress system are commonly used to infer paleostresses. However, dike trajectories are determined by a complex interaction between dike‐induced stresses and the source/regional stress system. We present numerical calculations based on a novel boundary‐integral formulation, which examines the simultaneous effects of regional stresses, magma pressure, and dike injection on the local stress state around a continuously curving dike. Dike paths are calculated from the condition that dikes propagate by mode I failure. Our results suggest that the magnitude of the regional stresses would be 2–5 times higher than previous estimates based on principal‐stress trajectory analysis.

A model for submarine rhyolite dome growth: Ponza Island (central Italy)

The Late Pliocene rhyolitic submarine volcanic rocks of Ponza island (Italy) can be interpreted as the subaqueous equivalent of subaerial dome complexes in terms of geometry and structure. Three coalescing domes of about 1km radius and aligned along a NE-trending regional fracture have been identified. The main difference between subaqueous and subaerial lava domes is that in a subaqueous environment, lava is likely to undergo pervasive hyaloclastic brecciation, so that domes are mainly composed of variously brecciated, in situ to clast-rotated hyaloclastite rather than coherent lava. We suggest that the shape and rheologic behaviour through time of submarine domes are closely controlled by the development and thickness of the particulate hyaloclastic carapace, which assumes the role of the solid crust of domes in subaerial environment. The thickness of the hyaloclastic carapace at Ponza is greater than 150m and emplaced during several different pulses (or eruptions). In the earliest pulses, lava is directly extruded on the seafloor and produces hyaloclastite, the degree of brecciation of which decreases inward to the coherent flow-banded rhyolite lava of the feeder dike. Once the hyaloclastic carapace is formed, further pulses of magma, combined with increase in height of the dome result in a local stress pattern characterised by a vertical σ1⪢σ2=σ3, producing concentric and radial fractures and normal faults. The newly rising magma, shielded by the hyaloclastic carapace, can intrude along these fault and fracture systems and invade previously emplaced but still water-saturated hyaloclastite. This produces the characteristic pattern of dikes observed at Ponza as a series of concentric dikes that are progressively less inclined outward with respect to the dome centre. These late stage dikes in turn produce hyaloclastite at their margins, but generally less fragmented than the embedding hyaloclastite, probably because the ascending magma is better shielded from direct contact with sea water. Periodic gravity collapses of the dome maintain the equilibrium between height and radius of the dome, as suggested at Ponza by the presence of mass flow deposits and the development of small topographic basins on top of the domes, probably related to debris removal. Some of these topographic depressions host subaqueous pyroclastic deposits, suggesting that, if due to vertical growth of the dome, water depth above the dome becomes shallower, hydromagmatic explosions may occur.

Tectonic analysis of the Husavik‐Flatey Fault (northern Iceland) and mechanisms of an oceanic transform zone, the Tjörnes Fracture Zone.

The inversion of ∼ 1000 fault slip data collected in the Flateyjarskagi Peninsula allows reconstruction of four main tectonic regimes. These include normal and strike‐slip faulting modes and are related to the general behavior of the Husavik‐Flatey Fault (HFF), a major structure of the Tjörnes Fracture Zone connecting the Kolbeinsey Ridge and the North Icelandic Rift. The two most important regimes (E‐W and NE‐SW extensions), consistent with the right‐lateral motion along the Husavik‐Flatey Fault, constitute the main tectonic group. The two others (NW‐SE and N‐S extensions), forming the subordinate tectonic group, are incompatible and result from drastic stress permutations. The relationships between these stress regimes imply not only σ1/σ2 and σ2/σ3, stress permutations but also σ1/σ3 reversals. A critical review of other data available, such as lava bedding, dike, and major fault attitudes, allows us to complete the structural pattern of the Flateyjarskagi peninsula and to highlight the mechanism of the transform zone. The complex pattern of dikes and faults in the northern part of Flateyjarskagi can be explained by the superposition of several processes: (1) a transform‐perpendicular extension (E‐W to ESE‐WNW trends), (2) a simple shear (NNE‐SSW to NE‐SW trends), and (3) a stress perturbation due to the transform motion (NW‐SE trends). An important factor controlling the transform mechanism is the variation of coupling along the HFF. The obliquity between the direction of transform motion and the trend of extension for the two main regimes may vary between 20° and 90°, reflecting repeated changes of the coefficient of friction along the HFF. Such change from very low mechanical coupling (weak fault) to moderate friction may occur very rapidly since it takes place several times in a few years, as shown by focal mechanisms of earthquake analysis.

The Tana basin, Ethiopia: intra-plateau uplift, rifting and subsidence

The Tana basin is situated on the northwestern plateau of Ethiopia, west of the Afar depression. The basin is perched on a topographic high. New data from digital elevation modelling and satellite imagery analysis confirm the basin's location at the junction of three grabens: the Dengel Ber (buried), Gondar (exposed by erosion) and Debre Tabor (reactivated). This structural complex was notably active during the build-up of the mid-Tertiary flood basalt pile, into which the Tana basin is impressed. Fault reactivation occurred in the Late Miocene–Quaternary, accompanied locally by predominantly basaltic volcanism. Fault-slip indicators are consistent with crustal subsidence centred on the present morphologic basin. Concentric and radial dike patterns in the Tana region indicate that diking and basin formation were contemporary. Tana rifting and magmatism occurred above the inferred western side of the Afar mantle plume-head.

Mechanical modeling of circumferential and radial dike intrusion on Galapagos volcanoes

A distinctive and unusual pattern of eruptive fissures is observed on the active volcanoes of the Galapagos islands, reflecting circumferential dike intrusion near the calderas and radial dike intrusion on the volcano flanks. Elastic finite-element models were used to investigate how a stress field could be produced and maintained to promote both circumferential and radial dike emplacement. Modeling results show that magma reservoirs of Galapagos volcanoes are probably diapiric, because this shape promotes both circumferential and radial intrusions, but magma pressure alone cannot create the observed pattern of dikes. Loading by volcano growth and magma reservoir pressure could produce a stress field of suitable orientation but insufficient magnitude. The intrusion of circumferential dikes could alter the stress field in a way that promotes future radial diking, and vice versa. Faulting or slumping within the calderas or on the volcano flanks in response to repeated intrusions could also create a stress field conducive to continued intrusion.

Dike emplacement on Venus and on Earth

Many long linear troughs are visible in synthetic aperture radar images of the surface of Venus. Their geometry closely resembles that of dike swarms on Earth. Such troughs will be formed if dikes fail to reach the surface, or if the magma in them is partially drained. Dikes that form closed ellipses in plan view can be produced by intrusions near a depression, and intersecting dikes can result if the stress field changes with time. Because of the absence of erosion and sedimentation, all these effects are better displayed on Venus than they are on Earth. The magnitude of the regional stress on Venus can be estimated from the dike patterns to be about 3 MPa, or similar to that of Earth. Dikes consisting of en echelon segments whose total length is 1500 km or more occur on both Earth and Venus. On Earth their widths are generally greater than 30 m and are sometimes as great as 250 m, and they are likely to be similar on Venus. The melt velocity during their emplacement is predicted to be a few meters per second, and the calculated rate of heat loss is sufficiently slow that they can remain molten as they propagate for distances of more than 1000 km. The same calculations apply to sills, though less is known about their geometry. The results therefore suggest that magma intrusion in thin sheets extending over 1000 km or more could produce regional crustal thickening and uplift on both Earth and Venus.

Subvolcanic structure of the central dike swarm associated with the ring complexes in the Shitara district, central Japan

The Subvolcanic structure of the central dike swarm associated with the Miocene Otoge ring complex and the Shitara igneous complex, central Japan, has been reconstructed. The central dike swarm was supplied from several aligned magma reservoirs. Flow lineations observed at the margin of the dikes converge towards a region that is regarded as a magma reservoir about 1–2 km below present sea level. The minimum diameter of the magma reservoir corresponds to the width of the central dike swarm, estimated to be about 3–4 km. The inferred magma reservoir of the Otoge ring complex, may have a zoned structure, as suggested by the flow lineations of dikes and the arrangement of cone sheets. Felsic magma occupied the upper part, about 1–2 km below present sea level, and basic magma the lower part, deeper than 2 km. The centre of the Shitara igneous complex is interpreted to be composed of several other shallow magma reservoirs. The distribution pattern in plan view of the central dike swarm is summarized from the frequency of dikes (defined by the number of dikes per kilometre in the direction normal to the trend of the dike swarm) and the variations of the different properties of individual dikes along the dike swarm. It has a plane of symmetry normal to the dike swarm above the magma reservoir. The patterns critical to a general understanding of the dike formation are: 1. A region of low dike frequency is present above the magma reservoir and a radial dike pattern occurs around the magma reservoir. 2. From both sides of the magma reservoir, the axes of high dike frequency extend symmetrically along the central zone of the dike swarm. 3. The number as well as the individual and total thickness of felsic dikes increases towards the magma reservoir. 4. The number of basic dikes increases towards both sides of the magma reservoir, while the individual thicknesses of basic dikes increase with distance from the magma reservoir.

Premineralization radial dikes of tourmalinized fluidization breccia, Julcani District, Peru

Dikes of fiuidization breccia composed largely of fragments of dacite lava are exposed over an area of about 3 km 2 in the central part of the Julcani volcanic center, central Peru. The dikes, which predate later quartz-alunite-pyrite and argillic alteration stages and economic enargite-tetrahedrite mineralization, exhibit a crude but definite radial pattern centered on the zone of later W-Au mineralization. Fragments of Paleozoic rock in dikes in outer areas of the pattern roughly constrain the location of an inferred early central vent structure 1.0 to 1.5 > or = km in diameter now buried beneath the dacite domes that host the breccia dikes.The microfragmental matrix of the dikes has been subject to strong quartz-tourmaline-pyrite alteration that did not affect dacite wall rock or larger dacite fragments within the dikes. The alteration was probably produced by fluids closely related to those that fluidized the dike material during emplacement.Fluid inclusions containing cubes of halite + or - sylvite are present in some quartz fragments within the breccia dikes and in phenocrystic quartz in dacite lava adjacent to the dikes. Estimated salinities require minimum trapping temperatures of about 260 degrees to 450 degrees C and show that fluid pressures during dike emplacement at times were appreciably greater than hydrostatic pressures. A few inclusions contain relatively large grains provisionally identified as hematite, anhydrite and/or gypsum, and carbonate, indicating the existence of solutions rich in these components.The high boron concentrations and highly saline fluid inclusions in the breccia dikes, the radial dike pattern, the high f (sub O 2 ) and f (sub S 2 ) of the magmas, and the general magmatic-hydrothermal history all suggest that the breccia dikes at Julcani were produced by the explosive release of magmatic fluids from a pluton crystallizing beneath the central part of the district.Areally restricted elongate zones of shattered dacite postdate the fiuidization breccia dikes. Tourmaline is absent and only minor particle rotation, rounding, and transport are evident. These zones probably resulted from the locally explosive formation of steam by the intrusion of late dikes into dacite lavas saturated with ground water.

Spreading structure of the Troodos ophiolite, Cyprus

Orientations of dikes within the sheeted dike complex of the Troodos ophiolite reveal primary spreading structure produced at a complex ridge/transform intersection. Three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes. Faults flatten at depth into a detachment within the upper parts of the plutonic complex. Large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids. We suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north-trending (present coordinates), slow-spreading ridge crest. A simple model for ridge migration indicates eastward jumps of 8–13 km; changes in ridge orientation are suggested by changes in trends of dikes and graben axes. Comparison of the pattern of dikes near the Arakapas fault zone with the structure of active ridge/transform intersections suggests that the fault is a right-offset (sinistral) transform, in contrast to earlier models in which a ridge to the west of the exposed Troodos complex was proposed.