Arc Splitting Research Articles

Numerical Modeling of Arc Splitting Process With Ferromagnetic Plate

A 3-D model is developed to simulate an arc splitting process during its motion along arc runners under the effect of the ferromagnetic plate.

The process of arc splitting between metal plates in low voltage arc chutes

The behavior of arcs in low voltage switching devices is affected by several interactions. For studying simple arrangements of arc runners without metallic splitter plates (deion plates) a three-dimensional simulation system has already been developed. It takes into account the plasma fluid dynamics, the current flow within the electrodes and the plasma, and the magnetic field generated by both currents. In order to model the arc splitting process simulations with an insulating barrier were done in a first step. For the simulation of the splitting process between metallic deion-plates the simulation model has to be extended. In experiments including high-speed movies it is shown that this process is a continuous transition of current flow from the still undivided arc to the new arc roots formed on the metal plates. A new simulation model representing the roots by a thin layer of current-dependent resistivity is discussed and simulation results are compared with switching experiments

Arc-trench rollback and forearc accretion: 2. A model template for ophiolites in Albania, Cyprus, and Oman

Abstract Ophiolite assemblages record structural, magmatic, and metamorphic processes that preceded their entrapment in orogenic belts by continental plate collisions. Ophiolite genetic models appealing to ‘oceanic’ or ‘suprasubduction’ provenance are still unable to reconcile several basic problems, including: (1) the association of boninites with oceanic ridge-type structural settings; (2) the diachronous ‘patch-like’ distribution of ophiolites in orogenic belts; (3) disparate ages between and within their mantle and crustal sections; (4) the lack of evidence for ‘obduction’ at modern passive margins. In contrast, the proposal that ophiolite genesis is exclusive to intra-oceanic forearc settings is compelling, given their uniquely shared structural, lithological, and stratigraphic attributes. Forearcs are interpreted to record discrete stages of subduction ‘rollback’ cycles, examples of which begin with subduction nucleation and the formation of boninitic ‘proto-arcs’, followed by arc splitting and concomitant retreat of the evolving arc-forearc complex. Forearc assemblages are likely to resist subduction to become entrapped in orogens, in contrast to denser, recently formed back-arc basin lithosphere, which is reconsumed by subduction following collision of the retreating forearc. As a model for Neo-Tethyan ophiolite genesis, this is predicated on the notion that rollback cycles are driven by ductile asthenosphere mobilized prior to and during collisions of Gondwana fragments with accreting Eurasia. It is also consistent with the apparent correlation of ophiolite ages with collisional events and their conjugate plate kinematic adjustments. Here, we use the slab rollback model as a template for interpreting the structural, magmatic, and metamorphic characteristics of well-studied Tethyan ophiolites, in Albania (Mirdita), Cyprus (Troodos), and Oman (Semail).

A multistage magmatic history for the genesis of the Orford ophiolite (Quebec, Canada): a study of the Mont Chagnon massif

This paper concerns the petrogenesis of the 504 ± 3 Ma Mont Chagnon massif, the southern extension of the Orford ophiolite in the Quebec Appalachians. The evolution of this massif is summarized in three stages marked by different magmatic series. In the Late Cambrian, the onset of southeastern subduction of the Iapetus basin generated an immature oceanic island arc made up of light rare-earth-element-depleted tholeiites, now preserved in the massif as a portion of the intrusive crustal unit, the dyke complex, and part of the lower volcanic unit. A phase of arc splitting, and concomitant partial erosion of the crustal section, was shortly followed by the eruption of rhyolite genetically related to felsic and low-Ti dykes, and trondhjemite. The geochemistry of these magmas bear some similarities with boninitic series. We believe these liquids derived from the partial melting of the Iapetus amphibolitized oceanic crust, with that of its Laurentian-derived sediments and nearby peridotite, either found as a trapped sliver above the subducting slab or as the slab itself. The final stage, preserved in the massif as a part of the intrusive section, the upper volcanic rocks, and the late-stage dykes, represents the back-arc opening. An ocean-island component is involved in the back-arc related petrogenetic processes, producing magmas with compositions intermediate between arc tholeiites and enriched back-arc basin basalts. This is the first report that the Iapetus basin was locally closing as early as Late Cambrian in the southern Quebec area.

Hot orogens, tectonic switching, and creation of continental crust

Many granulite terrains were too hot to have formed during continental collision. Rather, along with many high-grade metamorphic terrains that typify continental crust, most formed in accretionary orogens during tectonic switching, when prolonged lithospheric extension was interrupted by intermittent, transient contraction. Based on modern and ancient examples, tectonic switching occurs when slab retreat induces upper plate extension, causing arc splitting, formation of microcontinent slivers, and backarc basins; then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction (or slab flip) and crustal thickening. During slab retreat, basaltic magmas produced from decompressed asthenosphere advect into the extending orogen, causing granulite facies metamorphism and granite generation, but subsequent thickening during flat subduction cools the region. Thickening is focused in the thermally softened backarc region and, if sediment filled, a hot, short-lived (~10 m.y.), narrow (50– 100 km) orogenic belt forms. Such thickening is often misleadingly ascribed to arc or microcontinent collision. Once slab-retreat mode is reestablished, lithospheric extension recommences and a new arc-backarc system forms, generally outboard. Arrival of another plateau will reverse the procedure, and another short-lived, hot orogen will form within the orogenic system. Cycles of tectonic switching efficiently produce continental crust.

Arc restrikes yielding back-commutations in the contact gap of low voltage interrupters

Arc commutation from the arc chute back into the contact gap of a low voltage circuit-breaker caused by a high arc voltage pulse due to arc splitting was investigated in a model interrupter with the aid of a synthetic test method. In the experiment a well defined time after the arc left the contacts a high voltage ramp was applied across the contact gap. The criterion considered was the breakdown value of the voltage. Recovery time (in the range of 500 μs to 2 ms), geometry and materials of contacts (Cu, Ag/C, Ag/Ni, Ag/MeO) and walls (nongassing and gassing) were varied. Increasing recovery time increased the breakdown voltage significantly due to the decrease of the temperature in the contact area. The minimum breakdown value corresponded to the instantaneous reignition voltage. An increase of the wall distance caused an insignificant increase of the breakdown voltage due to the decrease of field distortion. Gassing wall materials caused slightly higher breakdown voltages than nongassing walls. Increasing contact gap from 2 mm to 4 mm increased the breakdown voltage. No influence was observed at lower (1.5 mm) and at higher contact gaps up to 7 mm. The influence of the contact material was generally little; Cu contacts yielded insubstantially lower breakdown voltages than the other contact materials investigated.

Auroral-arc splitting by intrusion of a new convection channel

Abstract. During a run of the Common Programme Three of the EISCAT radar the splitting of an auroral arc was observed by high time-resolution, ground-based cameras when the UHF radar beam was close to the arc. The evening eastward electrojet situation with a large-scale northward ionospheric electric field was disturbed by the intrusion of a convection channel with southward electric field from the east. The interaction of the new convection channel with the auroral arc caused changes in arc brightness and arc splitting, i.e. the creation of a new arc parallel to the pre-existing auroral arc. The event is described as one possibility for the creation of parallel arcs during slightly disturbed magnetic conditions far from the Harang discontinuity.

Lithostratigraphy of volcanic and sedimentary sequences in central Livingston Island, South Shetland Islands

Livingston Island contains several, distinctive sedimentary and volcanic sequences, which document the history and evolution of an important part of the South Shetland Islands magmatic arc. The turbiditic, late Palaeozoic–early Mesozoic Miers Bluff Formation (MBF) is divided into the Johnsons Dock and Napier Peak members, which may represent sedimentation in upper and lower mid-fan settings, respectively, prior to pre-late Jurassic polyphase deformation (dominated by open folding). The Moores Peak breccias are formed largely of coarse clasts reworked from the MBF. The breccias may be part of the MBF, a separate unit, or part of the Mount Bowles Formation. The structural position is similar to the terrigenous Lower Jurassic Botany Bay Group in the northern Antarctic Peninsula, but the precise stratigraphical relationships and age are unknown. The (?) Cretaceous Mount Bowles Formation is largely volcanic. Detritus in the volcaniclastic rocks was formed mainly during phreatomagmatic eruptions and redeposited by debris flows (lahars), whereas rare sandstone interbeds are arkosic and reflect a local provenance rooted in the MBF. The Pleistocene–Recent Inott Point Formation is dominated by multiple, basaltic tuff cone relicts in which distinctive vent and flank sequences are recognized. The geographical distribution of the Edinburgh Hill Formation is closely associated with faults, which may have been reactivated as dip-slip structures during Late Cenozoic extension (arc splitting).

Geochemistry of the Cambrian volcanic-hosted massive sulfide-rich Mount Read Volcanics, Tasmania, and some tectonic implications

The Cambrian Mount Read Volcanics belt in western Tasmania hosts five major gold-rich massive sulfide deposits and remains the focus of intense exploration activity and geologic research. A geochemical study of least altered lavas and shallow intrusions, with emphasis on basaltic to andesitic compositions, was carried out to determine the primary magmatic affinities and tectonic setting of eruption of the Mount Read Volcanics, to aid in internal correlations within the belt, and to provide starting-material estimates for mineralization-related alteration studies.Three calc-alkaline suites (one extending to shoshonitic compositions) and two tholeitiic suites have been distinguished within the Mount Read belt. Suite I is voluminous and includes the Eastern sequence, Central Volcanic Complex, Tyndall Group, the intrusive quartz-feld-spar porphyries and granitoids, and the andesitic lavas of the Que-Hellyer footwall sequence. Suite I andesites are transitional, medium to high K calc-alkaline rocks ((La/Yb) N = 5-12, avg 8.1) and are the least light REE-enriched calc-alkaline lavas in the Mount Read Volcanics.Suite II comprises intrusive and extrusive, often hornblende-porphyritic andesites and dacites mainly from the upper part of the southern Central Volcanic Complex. They are more P 2 O 5 and light REE enriched ((La/Yb) N = 10-26, avg 16.7) than suite I and have high K calc-alkaline affinities.Suite III includes basaltic and andesitic lavas from the Que-Hellyer hanging-wall sequence in the northern part of the belt, the Lynch Creek basalts near Queenstown, and intrusive basalts in the Howards Plains area northwest of Queenstown. Suite III rocks show a striking compositional range, from low TiO 2 (0.4-0.5%), low P 2 O 5 (<0.1%) basalts with (La/Yb) N values from 8 to 12 and almost flat heavy REE patterns, to low TiO 2 (0.4-0.8%) but strongly to exceptionally P 2 O 5 and light REE-enriched basalts with up to 350 times chondritic La and (La/Yb) N values up to 34. The latter are regarded as shoshonites.Tholeiitic pillow basalts of the Henty fault wedge, and the closely related dikes of the Henty dike swarm, constitute suite IV: these have TiO 2 contents from 1.0 to 1.6 percent, very low Nb contents (<3 ppm), and only slightly light REE-enriched REE patterns ((La/Yb) N = 1.4-3.4). They are considered to be most similar to suprasubduction zone basalts erupted during the early phase of arc splitting and back-arc basin development and indicate a period of tension and rifting of the Mount Read belt in post-Central Volcanic Complex but pre-Tyndall Group time.Suite V is constituted by the Miners Ridge basalts and includes low TiO 2 (<0.7%) basalts with strong light REE depletion ((La/Yb) N <0.5) and higher TiO 2 (to 2.6%) lavas with only slight light REE depletion ((La/Yb) N = 0.9). Suite V basalts are tentatively correlated with rift tholeiite sequences associated with the Crimson Creek Formation, and therefore, are regarded as part of the pre-Mount Read Volcanics basement.The three calc-alkaline suites occur in a general stratigraphic order (with suites I and II being more or less coeval near Queenstown); they indicate a magmatic evolution from transitional medium to high K rocks through high K rocks, to strongly enriched shoshonitic rocks. Closest modern analogues are postcollisional volcanics in regions of arc-continent collision (e.g., East Papua and the Roman province).The occurrence of boninite-derived chromites (probably derived from the allochthonous mafic-ultramafic complexes outcropping just west of the Mount Read belt) in two sedimentary units within the Mount Read Volcanics sequence, below suite III basalts, supports a postcollisional origin for at least the major part of the Mount Read Volcanics. Other similar postcollisional sequences, particularly if formed in a submarine setting, must be considered highly prospective for VHMS deposits.