Radial Dike Swarm Research Articles

Effects of crustal-scale mechanical layering on magma chamber failure and magma propagation within the Venusian lithosphere

Understanding the connection between shallow subsurface magmatism and related surface expressions provides first-order insight into the volcanic and tectonic processes that shape a planet's evolution. When assessing the role of flexure, previous investigations assumed homogeneous host rock, but planetary lithospheres typically include crust and mantle material, and the mechanical response of a layered lithosphere subjected to flexure may influence both shallow magma reservoir failure and intrusion propagation. To assess the formation of giant radial dike systems, such as those observed on Venus, we create axisymmetric elastic finite element models of a spherical reservoir centered at the contact between stiff, dense mantle overlain by softer, lighter crust. We analyze magma chamber stability, overpressure at rupture, and resulting intrusion types for three distinct environments: lithostatic, upward flexure, and downward flexure. In the lithostatic case, reservoir failure at the crust-mantle contact favors lateral sill injection. In the flexure cases, we observe that failure location depends upon the crust/lithosphere thickness ratio and, at times, will favor radial dike intrusion. Specifically, upward flexure can promote the formation of giant radiating dike swarms, a scenario consistent with a plume-derived origin. Our results present a mechanical explanation for giant radial dike swarm formation, showing that both the stability of magma chambers on Venus and the type of intrusions that form are influenced by lithospheric layering. Furthermore, where dike swarms occur, our approach provides a powerful new way to constrain local crust/mantle layering characteristics within the lithosphere at the time the swarm was forming.

A new felsic cone-sheet swarm in the Central Atlantic Islands: The cone-sheet swarm of Boa Vista (Cape Verde)

The island of Boa Vista is one of the oceanic islands with the largest amount of felsic rocks reported in the world (50% of the total outcropping igneous rocks of Boa Vista). The felsic rocks are trachytes and phonolites generated within the second main stage of the volcanic activity that lasted about four million years.Three consecutive felsic episodes are distinguished: the first occurred between 14.5Ma and 13.5Ma, the second between 13.6Ma and 12.0Ma, and the third and last between 12.0Ma and 10.4Ma. Their geochemical composition reveals the presence of crystal fractionation processes with participation of feldspars and amphibole as well as different accessory phases involved in each episode.The Boa Vista felsic cone-sheet swarm, the youngest third episode, is formed by several hundreds of peralkaline ne-phonolitic sheets distributed within a circular surface about 11km in radius. The cone-sheets conform a single general pattern characterized by a decreasing dip outward the structure, together with a constant dip of each individual sheet. The average inclination of sheets is around 40° in the sectors next to the centre of the structure whereas it is only about 30° next to its periphery. The magmatic focus of the sheets is located over 3km deep almost coinciding with the convergence centre of a contemporary basic radial dike swarm.

A basic radial dike swarm of Boa Vista (Cape Verde Archipelago); its significance in the evolution of the island

A basic radial dike swarm unrelated to other basic units of Boa Vista (Cape Verde Archipelago) has been localized and characterized in the central sector of the island.According to new radiometric data three main stages in the evolution of Boa Vista are distinguished: the earlier (the Old Volcanic Complex: 17–16Ma) is equivalent to the shield building stage of Hawaii and the later (the Recent Volcanics (8–4Ma) is in some aspects comparable to the post-erosional stage. An important intermediate essentially felsic stage (the Trachytic–Phonolitic Complex: 14.3–12.8Ma) followed the basaltic shield stage. This felsic stage has equivalents in some other oceanic islands as the Canary Islands, specially Gran Canaria, Tenerife, and La Gomera. The central sector of Boa Vista is also occupied by the Felsic Subvolcanic Complex, a unit consisting of phonolitic breccias, syenites and monzonites that represent the Trachytic–Phonolitic Complex hypabyssal roots. The felsic rocks as a whole constitute half of the total amount of igneous rocks on the island making up Boa Vista, the island with the highest percentage of felsic rocks in the Central Atlantic Ocean.More than 200 dikes of the basic radial swarm intruding the Felsic Subvolcanic Complex have been measured. The intensity of the multiple dike injection is sometimes rather high, roughly a dike every 5m. The individual dikes have an observable mean length of about 300m. The composition of these dikes is always foiditic (nephelinites, melilitites, and limburgites), slightly different in composition (more alkaline and richer in incompatible elements) to the other basic units of the island (the Old Volcanic Complex and the Recent Volcanics).The radial dikes converge in an area located NW of the geometrical center of Boa Vista, a zone where the hypothetical center of the Old Volcanic Complex and the Trachytic–Phonolitic Complex edifices must also have been situated. The ages obtained from the dikes (between 14.8 and 11.5Ma) indicate that the radial injections are contemporary with the phonolites and the trachytes of the Trachytic–Phonolitic Complex and yet are part of the intermediate evolutionary stage of Boa Vista.

Artemis, Venus: The largest tectonomagmatic feature in the solar system?

New geologic mapping reveals that Artemis, a unique 2400-km-diameter feature on Venus, is much larger than previously recognized, including a wide outer trough (>5000 km diameter), a radial dike swarm (12,000 km diameter), and a concentric wrinkle ridge suite (13,000 km diameter). Artemis's evolution included formation of its interior and chasma, accompanied by lateral propagation of radial dikes. The escape of dike magma to the surface formed local cover deposits that buried parts of the remaining radial fracture suite. Cover deposits are cut, in turn, by wrinkle ridges that likely formed by coupling of convective mantle flow with the lithosphere. The outer trough formed late relative to radial fractures, cover deposits, and wrinkle ridges. We suggest that Artemis represents the magmatic signature of a deep mantle plume acting on relatively thin lithosphere. As such, it appears to represent the largest tectonomagmatic feature in the solar system. The newly recognized vast extent of Artemis holds implications for the formation of giant radial dike swarms, wrinkle ridge formation, terrestrial planet mantle-lithosphere coupling, and Venus's surface and geodynamic evolution.

Evidence for intrusive activity on Mercury from the first MESSENGER flyby

Images from MESSENGER's first flyby of Mercury have shown convincing evidence for surface volcanism. Here we report on evidence in the new data for several features that are characterized by fractures and graben — rare features on a planet dominated by contractional deformation — that may be linked to intrusive activity. These features include: (1) A floor-fractured crater, interpreted to have been the site of laccolith-like sill intrusions; the feature is similar to some floor-fractured craters on the Moon and shows evidence for individual fractured dome-like uplifts on the floor. (2) A concentric complex of graben, observed inside the peak ring on the floor of the ~ 250-km-diameter Raditladi basin and associated with dark plains and possibly embayed by them; the feature may represent an unusual type of floor-fracturing associated with deeper intrusions and related ring dikes or cone sheets, or the graben may instead be the product of non-magmatic uplift of the basin floor. (3) A large radial graben swarm, Pantheon Fossae, located near the center of the Caloris basin, thus far unique on Mercury, and characterized by hundreds of individual graben segments ranging from ~ 5 km to ~ 110 km in length. In the nexus, graben crosscut one another and produce a local polygonal pattern; others curve away from the center as the nexus is approached. Two scales of graben length are observed; the radius of the dense radially symmetric plexus of graben is ~ 175 km, and a few graben extend to greater radial distances to the north and southwest out to distances that intersect with a ring of generally concentric graben around the outer basin floor. Two width scales of graben are observed; a large graben about 8 km wide emerges from the nexus and extends for ~ 100 km; most graben are less than half this width. Some graben walls appear cuspate, with convex-outward wall segments that resemble crater chain segments. One crater chain with distinctive raised rims parallels nearby graben. Locally, some graben appear in en echelon patterns, and smaller graben sometimes show cross-cutting (superposition) relationships. Abundant impact craters, the most prominent being Apollodorus, and secondary crater clusters and chains are superposed on the graben system; there is little evidence that craters greater than 5 km in diameter have been cut by a graben. This relation implies that the graben swarm formed soon after the emplacement of the Caloris floor plains. These graben are interpreted to be the surface expression of a radial dike swarm emanating from a subsurface magma reservoir. Similar features, in which the dikes contribute to a near-surface stress field that favors radial graben, are known on the Earth, Venus, and Mars. The location of Pantheon Fossae in the center of the Caloris basin suggests that formation of the radial graben structure is linked to basin evolution.

Dike-swarms, key to the reconstruction of major volcanic edifices: The basic dikes of La Gomera (Canary Islands)

The detailed study of the age and distribution of the basic dikes of La Gomera (Canary Islands) has allowed us to reconstruct the evolution of its main volcanic edifice. The dikes of the oldest unit (the Basal Complex) have a N70°–80° dominant direction at regional scale. On the other hand, the dikes of the post Basal Complex subaerial units are the result of local stress fields. The oldest swarm is composed of sills dated at 10.2–9.3 Ma. Four younger radial dike swarms have been identified (S1, S2, S3 and S4) with ages of 9.1–8.4 Ma; 8.2–6.7 Ma; 5.5–4.4 Ma and 5.3–4.0 Ma respectively. The reconstruction of the magmatic focus location using these swarms shows a migration southwards with an average speed of 1.6 mm/year. This temporal sequence of parallel swarms, sills and radial swarms is a pattern frequently repeated during the building of large insular volcanic edifices in the Canary Islands as well as in other archipelagos.

Vertical and lateral propagation of radial dikes inferred from the flow-direction analysis of the radial dike swarm in Komochi Volcano, Central Japan

Dikes with lateral and outward intrusion directions (named L-type dike) and dikes with vertical and upward intrusion directions (V-type dike) are recognized in a radial dike swarm of Komochi Volcano, Japan, by the combined analysis of the preferred orientations of deformed vesicles and anisotropy of magnetic susceptibility (AMS). Intrusion directions of magmas were examined for 41 dike outcrops. Among them, 19 dikes were classified as L-type dike and 15 dikes were as V-type. The L-type dikes distribute over the area of the dike swarm radiating from the central conduit, which is named Daikokuiwa stock, and the V-type dikes occur mainly in the peripheral portion of the volcano. The L-type dikes are characterized with evolved whole-rock compositions similar to those of rocks consisting of the central conduit, whereas the V-type dikes have less-fractionated composition and are enrich in mafic phenocryst as compared to the L-type dikes. The outward intrusion directions of the L-type dikes and their petrological similarity to the rock of the central conduit indicate that these dikes intruded from the shallow part of the central conduit where the magmas underwent fractional crystallization and degassing. The petrological characteristics of the V-type dikes suggest that the less-fractionated magma intruded directly from a magma chamber, in which mafic phenocryst crystals accumulated. The larger dike thickness and higher magnetic foliation of the V-type dike as compared to the L-type dike indicate higher magmatic overpressure, which was possibly result of the direct connection to the pressurized magma chamber and vertical-growth of the dikes with buoyant magma. Solidification of the central conduit is favorable for the accumulation of internal excess pressure in the magma chamber to break the wall rock. Fissure eruptions independent of the central conduit have occurred in many volcanoes and these fissure eruptions might be fed by the dikes directly propagated from the deeper magma chamber.

Mantle plumes from top to bottom

Hotspots include midplate features like Hawaii and on-axis features like Iceland. Mantle plumes are a well-posed hypothesis for their formation. Starting plume heads provide an explanation of brief episodes of flood basalts, mafic intrusions, and radial dike swarms. Yet the essence of the hypothesis hides deep in the mantle. Tests independent of surface geology and geochemistry to date have been at best tantalizing. It is productive to bare the current ignorance, rather than to dump the plume hypothesis. One finds potentially fruitful lines of inquiry using simple dynamics and observations. Ancient lithospheric xenoliths may reveal heating by plumes and subsequent thermal equilibration in the past. The effect at the base of the chemical layer is modest 50–100 K for transient heating by plume heads. Thinning of nonbuoyant platform lithosphere is readily observed but not directly attributable to plumes. The plume history in Antarctica is ill constrained because of poor geological exposure. This locality provides a worst case on what is known about surface evidence of hotspots. Direct detection of plume tail conduits in the mid-mantle is now at the edge of seismic resolution. Seismology does not provide adequate resolution of the deep mantle. We do not know the extent of a chemically dense dregs layer or whether superplume regions are cooler or hotter than an adiabat in equilibrium with the asthenosphere. Overall, mid-mantle seismology is most likely to give definitive results as plume conduits are the guts of the dynamic hypothesis. Finding them would bring unresolved deep and shallow processes into place.

Speculations on the nature and cause of mantle heterogeneity

Hotspots and hotspot tracks are on, or start on, preexisting lithospheric features such as fracture zones, transform faults, continental sutures, ridges and former plate boundaries. Volcanism is often associated with these features and with regions of lithospheric extension, thinning, and preexisting thin spots. The lithosphere clearly controls the location of volcanism. The nature of the volcanism and the presence of ‘melting anomalies’ or ‘hotspots’, however, reflect the intrinsic chemical and lithologic heterogeneity of the upper mantle. Melting anomalies—shallow regions of ridges, volcanic chains, flood basalts, radial dike swarms—and continental breakup are frequently attributed to the impingement of deep mantle thermal plumes on the base of the lithosphere. The heat required for volcanism in the plume hypothesis is from the core. Alternatively, mantle fertility and melting point, ponding and focusing, and edge effects, i.e., plate tectonic and near-surface phenomena, may control the volumes and rates of magmatism. The heat required is from the mantle, mainly from internal heating and conduction into recycled fragments. The magnitude of magmatism appears to reflect the fertility, not the absolute temperature, of the asthenosphere. I attribute the chemical heterogeneity of the upper mantle to subduction of young plates, aseismic ridges and seamount chains, and to delamination of the lower continental crust. These heterogeneities eventually warm up past the melting point of eclogite and become buoyant low-velocity diapirs that undergo further adiabatic decompression melting as they encounter thin or spreading regions of the lithosphere. The heat required for the melting of cold subducted and delaminated material is extracted from the essentially infinite heat reservoir of the mantle, not the core. Melting in the upper mantle does not requires the instability of a deep thermal boundary layer or high absolute temperatures. Melts from recycled oceanic crust, and seamounts—and possibly even plateaus—pond beneath the lithosphere, particularly beneath basins and suture zones, with locally thin, weak or young lithosphere. The characteristic scale lengths—150 to 600 km—of variations in bathymetry and magma chemistry, and the variable productivity of volcanic chains, may reflect compositional heterogeneity of the asthenosphere, not the scales of mantle convection or the spacing of hot plumes. High-frequency seismic waves, scattering, coda studies and deep reflection profiles are needed to detect the kind of chemical heterogeneity and small-scale layering predicted from the recycling hypothesis.

Experiments on rift zone evolution in unstable volcanic edifices

Large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses. Flank deformation is often caused by slip along a décollement within or underneath the volcanic edifice. We studied how such a stressed volcanic flank may bias the rift zone development. The influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence; here our analog experiments provide new insights. We injected colored water into gelatin cones and found systematic orientations of hydro-fractures (dikes) propagating through the cones. At the base of the cone, diverse friction conditions were simulated. By variation of the basal creep conditions we modeled radial dike swarms, collinear rift zones and three-armed rift systems. It is illustrated that a single outward-creeping flank is sufficient to modify the entire rift architecture of a volcano. The experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcano’s architecture.

The felsic dikes of La Gomera (Canary Islands): identification of cone sheet and radial dike swarms

On the northern part of La Gomera there exists a great abundance of trachytic–phonolitic dikes showing a broad diversity in dip and strike. Several methods have been applied in order to separate these dikes in different sets, localise the area from where they derive, and reconstruct the geometry of the swarms. The oldest dikes correspond to a radial swarm dated at 8 Ma. The felsic activity migrated then southwestwards and a second radial swarm and a cone sheet complex were developed between 7.5 and 6.4 Ma ago. The cone sheet complex is 10 km in diameter and shared its centre with that of the second radial structure. The cone sheets exhibit an outward decrease of dip angle whilst every individual sheet maintains a constant inclination. This geometry reflects the existence of an ancient single dome-shaped shallow magma chamber situated some 1650 m below present sea level. The eastern radial swarm represents a felsic episode that could mark the ending of the Lower Old Basalts, the earlier subaerial activity of La Gomera. The two other dike swarms represent a younger episode coeval with the Upper Old Basalts.

Lateral flow and ponding of starting plume material

Many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle. Flood basalts, radial dike swarms, and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere. The buoyant plume head material ponds at the base of the lithosphere and then flows laterally. Relief on the base of the lithosphere acts as an upside‐down drainage pattern with enclosed catchments for this flow. That is, plume material preferentially collects beneath regions where the lithosphere is locally thin, such as thermally subsiding sedimentary basins. An example is uplift associated with the Iceland starting plume which inverted the Mesozoic Irish Sea basin. Additional relief at the base of the lithosphere is associated with rifting and seafloor spreading. In the case of the northern North Sea, material from the Iceland starting plume ponded beneath a basin with locally thin lithosphere. Rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading. Rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head. An example is the volcanic passive margin off Nova Scotia which is 2000 km from the center of the Bahama starting plume head. Numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 Pa s.

The Eocene Big Timber stock, south-central Montana: Development of extensive compositional variation in an arc-related intrusion by side-wall crystallization and cumulate glomerocryst remixing

The Eocene Big Timber stock in the Crazy Mountains of south-central Montana is an elliptical, 8 by 13 km, compositionally and texturally diverse composite intrusion with a well-developed radial dike swarm. A sharp intrusive contact separates its two phases: the core of the intrusion is fine-grained quartz monzodiorite, and the volumetrically dominant remainder is composed of medium-grained diorite and gabbro. Differentiation-related major oxide variation within the stock is extensive and spatially nonsystematic. However, abundances of most trace elements were not strongly influenced by differentiation; late zircon and apatite fractionation caused moderate heavy and slight light rare earth element abundance depletions, respectively. Mineral compositions and assemblages indicate crystallization between ≈950 and 700 °C at a pressure of ≈0.8 kbar (3 km). Mixing models indicate that fractionation of varying amounts of plagioclase, orthopyroxene, clinopyroxene, magnesio-hastingsite, hornblende, biotite, titanite, apatite, and magnetite (the stock9s principal constituents, with quartz and potassium feldspar) and remixing of these minerals and residual liquids controlled compositional evolution in the reservoir. Crystals apparently nucleated at the reservoir wall while residual silicate liquid was displaced inward and remixed. Some crystals were plucked from the solidification front, as indicated by glomerocrysts present throughout the stock, and also remixed with residual liquid. Solidification of the reservoir represented by the stock involved heat loss to enclosing wall rock, side-wall crystallization, and subsequent, variably effective, crystal-liquid remixing. This process is an important variant of conventionally invoked models pertaining to solidification of intrusions and explains extensive, relatively nonsystematic compositional variation. The genesis of compositional evolution in other intrusions characterized by extensive, spatially nonsystematic variation may result from the important process documented herein. Compositional and geologic relationships are consistent with magma genesis related to subduction and magmatic-arc processes inboard from the western edge of the early Cenozoic North American plate. Arc magmatism in south-central Montana during Eocene time is consistent with models pertaining to early Cenozoic southward sweep and westward retreat of magmatism. Magmatism represented by the Big Timber stock provides significant new support for steepening subduction, westward retreat of the subduction hinge line, and development of an asthenospheric mantle wedge that fueled renewed magmatism beneath the western edge of the North American continent following early Cenozoic shallow subduction.

子持火山の地質と活動年代

Komochi volcano (1,296 m), located at the southernmost part of NE Japan arc, is a Quaternary composite volcano with a diameter of approximately 6 km and a volume of approximately 10 km3. The basement is composed of the Kirigakubo Formation, a member of the Miocene rhyolitic tuff formation, and Iwamoto volcanoes, dacite lava domes (ca. 6 Ma). The rocks of the Komochi volcano consist of low-K andesites with small amounts of basaltic andesite and dacite. All rocks of the Komochi volcano belong to the hyperthenic rock series. The volcanic activity of the Komochi volcano can be divided into following three stages: Ayado stage (c.a. 1.6 Ma), Early Komochi (0.9 Ma-) and Late Komochi (0.6-0.2 Ma) volcano stages which is subdivided into the two kinds of activities: stratovolcano-forming and lava dome-forming. At any stage, a large number of andesitic dikes intruded, which comprise radial dike swarm in the circumference of the Daikokuiwa neck. The Komochi volcano has a life span of about 1.4 m.y. and calculated production rate of 0.14-0.25 km3/104 yr., which is very small compared to lager volcanoes around Komochi volcano, for example Myoko and Hakone volcanoes, and even smaller than similar volcanoes in the Shin-etsu Highland area.

Changing stresses during emplacement of the radial dike swarm at Spanish Peaks, Colorado

A two-dimensional elastic model was used to analyze the pattern produced by the mapped extent of over 600 km of dikes in the Spanish Peaks, Colorado, radial dike swarm. Optimal regional stress fields were found for dikes of four different lithologies. The direction of maximum principal horizontal regional stress during intrusion of syenite and syenodiorite dikes was N80°E. This was also the direction of minimum principal horizontal regional stress during intrusion of granite and granodiorite dikes.

The geology of Pepin Island and part of the adjacent mainland

The oldest rocks of the Pepin Island Igneous Complex are the Echinus Granite, Platform Gneiss, and Fall Conglomerate of the Pepin Group. These rocks are intruded by a plug and radial dike swarm of ...

Patterns and Origin of Radial Dike Swarms Associated with West Spanish Peak and Dike Mountain, South-Central Colorado

West Spanish Peak and Dike Mountain in south-central Colorado are stocks which cut Tertiary sedimentary rocks near the axis of the La Veta syncline, the structural trough of the Raton basin. Associated with these stocks are radial dike swarms. The outline of the West Spanish Peak dike swarm is elliptical. The Dike Mountain swarm is more radial, and its outline is oval. Both systems are elongated normal to the axis of the La Veta syncline. The dikes of the West Spanish Peak swarm are of diverse rock types and represent several separate magmatic phases. Those of the Dike Mountain swarm are facies of syenodiorite and probably represent a single phase of magmatic invasion. The dikes occupy vertical joints that have been generally attributed to radial fissuring during doming of the sedimentary rocks by the emplacement of the stocks. Structural studies, however, indicate that several systems of shear and tension joints resulted from intermittent orogenic stresses of varying direction and magnitude during folding of the syncline before invasion of the magmas, and the writer suggests that selective intrusion into this joint complex accounts for the dike patterns.