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Plate tectonic models indicate subduction of the Kula-Farallon spreading ridge, and thus imply formation of the Kula-Farallon slab window, beneath western North America from Late Cretaceous to middle Eocene time. Seafloor magnetic anomalies, however, provide few, constraints on the configuration of the subducted ridge and the location of the slab window. We use geochemical data from Eocene igneous rocks to provide an estimate of the slab window's position at 50 Ma. Contouring of the trace element ratios [K.sub.2]O/Si[O.sub.2], Rb/Zr, and Ta/Ce for lavas of the Eocene Challis-Kamloops volcanic belt demonstrates a southward trend toward enriched geochemical character. This trend is consistent with a Pacific Northwest position for the slab window; this is supported by the presence of adakites (slab melts) near the Canada-United States border, and by ca. 51 Ma forearc intrusions and volcanic rocks on Vancouver Island that are regarded as products of ridge subduction. A new reconstruction of the slab window's geometry over time indicates that its chemically defined position is kinematically viable. Keywords: Challis-Kamloops, Tertiary, geochemistry, ridge subduction, British Columbia, adakite.

ABSTRACT The drill hole near Digby in southwestern Nova Scotia is the deepest on-shore hole in the Canadian Appalachians, and penetrated to a depth of about 1.5 km. The site [...]

Isotopic and geochemical data indicate that intrusions in the eastern Creignish Hills of central Cape Breton Island, Canada represent the roots of arcs active at approximately 540-585 Ma and approximately 440 Ma. Times of intrusion are closely dated by (1) a nearly concordant U-Pb zircon age of 553 +/- 2 Ma in diorites of the Creignish Hills pluton; (2) a lower intercept U-Pb zircon age of 540 +/- 3 Ma that is within analytical error of (super 40)Ar/(super 39)Ar hornblende plateau isotope-correlation ages of 545 and 550 +/- 7 Ma in the River Denys diorite; and (3) an upper intercept U-Pb zircon age of 586 +/- 2 Ma in the Melford granitic stock. On the other hand, approximately 441-455 Ma (super 40)Ar/(super 39)Ar muscovite plateau ages in the host rock adjacent to the Skye Mountain granite provide the best estimate of the time of intrusion, and are consistent with the presence of granitic dykes cutting the Skye Mountain gabbro-diorite previously dated at 438 +/- 2 Ma. All the intrusions are calc-alkaline; the Skye Mountain granite is peraluminous. Trace element abundances and Nb and Ti depletions of the intrusive rocks are characteristic of subduction-related rocks. The approximately 540-585 Ma intrusions form part of an extensive belt running across central Cape Breton Island, and represent the youngest Neoproterozoic arc magmas in this part of Avalonia. Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic rocks, which bracket the transition from convergence to extension between approximately 540 and 505/520 Ma. This transition varies along the Avalon arc: 590 Ma in southern New England, 560-538 Ma in southern New Brunswick, and 570 Ma in eastern Newfoundland. The bi-directional diachronism in this transition is attributed to northwestward subduction of two mid-ocean ridges bordering an oceanic plate, and the migration of two ridge-trench-transform triple points. Following complete subduction of the ridges, remnant mantle upwelling along the subducted ridges produced uplift, gravitational collapse and the high-temperature/low-pressure metamorphism in the arc in both southern New Brunswick and central Cape Breton Island. The approximately 440 Ma arc magmatism in the Creignish Hills extends through the Cape Breton Highlands and into southern Newfoundland, and has recently been attributed to northwesterly subduction along the northern margin of the Rheic Ocean.

Central Cape Breton Island in Nova Scotia, Canada, is host to -700-630 Ma felsic and associated mafic volcanic rocks that are relatively rare in other parts of the Avalon Composite Terrane, occurring elsewhere only in the Stirling Block of southern Cape Breton Island and in parts of eastern Newfoundland. The mafic rocks of central Cape Breton Island are typically intraplate tholeiitic basalts generated by melting of a garnet-bearing mantle source. They lack a continental trace element and [[Epsilon].sub.Nd] imprint although they were emplaced on continental crust; they resemble oceanic island basalts. Contemporaneous volcanism in the Stirling Block is calc-alkaline and formed in a volcanic arc setting. In the absence of evidence for an intervening trench complex or suture, it may be inferred that the central Cape Breton tholeiites formed in a back-arc setting relative to the Stirling Block. This rifting may represent the initial stages of separation of an Avalonian arc from western Gondwana. The arc rifted further between ~630-610 Ma when the younger Antigonish-Cobequid back-arc basin formed. Subsequently, the extensional arc became convergent, telescoping the back-arc basin. Northwestward migration of calc-alkaline arc magmatism may be related to shallowing of the associated Benioff zone through time.

Palaeozoic volcanism in the Avalon Terrane of northern Nova Scotia occurred during three time intervals: Cambrian-early Ordovician, late Ordovician-early Silurian and middle-late Devonian. In the Meguma Terrane of southern Nova Scotia, Palaeozoic volcanism is limited to the middle Ordovician. Geochemical data show that most of these volcanic rocks are bimodal, within-plate suites. Initial [[Epsilon].sub.Nd] signatures range from +5.4 to -1.9 in the rhyolites and +6.8 to +2.7 in the basalts, a difference attributable to the absence or presence, respectively, of a significant crustal component. The data and regional tectonic settings of the Avalon and Meguma terranes suggest that the volcanism was generated in three different within-plate settings: (1) Cambrian-early Ordovician volcanism related to thermal decay of late Proterozoic arc magmatism during transtensional deformation; (2) middle Ordovician-early Silurian volcanism during sinistral telescoping between Laurentia and Gondwana where extensional bends in the Appalachians produced rifting; and (3) Devonian volcanism resulting from lithospheric delamination during dextral transpression and telescoping. In each setting, active faults served as conduits for the magmas. Nd isotopic data indicate that the source of the Palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern Nova Scotia and did not substantially change with time. This crustal source appears to have separated from the mantle during the Proterozoic, a conclusion consistent with the hypothesis that the Palaeozoic rocks in Nova Scotia were deposited upon a late Proterozoic oceanic-cratonic volcanic arc terrane. The Nd data, when combined with published faunal, palaeomagnetic and U-Pb isotopic data, suggest that the Avalon Terrane was peripheral to Gondwana off northwestern South America during Neoproterozoic and early Palaeozoic times.

The Bolton Igneous Group rocks in the Southern Quebec Appalachians appear to be tectonic slices in an accretionary prism, with the rocks generated in the Iapetus Ocean's ocean ridge environment. The rocks are pre-Taconian basaltic pillow lavas with compositions which are similar to transitional mid-ocean ridge basalts. The Dunnage zone's mafic volcanic rocks can be separated into three groups by the TiO2 content, consisting of the high-TiO2, medium TiO2 and low TiO2 groups.

Geochemical analysis of Late-Silurian Early-Devonian interbedded volcanic rocks in the northwestern mainland Appalachians suggests that volcanic rocks are biomodal lavas that erupt in a continental intraplate tectonic setting. Volcanism start coincides with a switch from sinistral to dextral transpression alons the orogen. Orogen produces oblique rifting in the Quebec Reentrant and the rift zone undergoes a clockwise rotation during progressive deformation. This rifting generates thinning of crust, upwelling of the asthenosphere, melting and magmatism.

The Pioneer Formation of southwest British Columbia, Canada, is loaded with triassic pillow basalts. The triassic pillow basalts contain argillites at the base and elastic sediments at the top due to the partial melting of parental rocks. The superimposition of mantle plume and oceanic plate accounts for the restructured bridge river area strata.

Multidimensional modeling of phenomena and processes occurring during the expansion of the laser-produced plasma for different irradiation conditions related to both the laser beam parameters and the target constructions is a very complex issue, especially when modeling requires consideration of kinetic processes associated with the development of various types of microscopic instability. Multidimensional PIC codes create such a possibility, but their use is limited to modeling phenomena even in a very narrow timescale due to the limited computational capabilities of current supercomputers. For this reason, the paper attempts to interpret the results of the spontaneous magnetic field (SMF) measurements obtained during the PALS (Prague Asterix Laser System) experiment [Pisarczyk et al., AIP Adv. 10, 115201 (2020); Pisarczyk et al., Phys. Plasmas 22, 102706 (2015)] based on the 2D magneto-hydrodynamic (MHD) model [Jach et al., Computer Modeling of Dynamic Interaction of Bodies by Free Point Method (PWN, Warsaw, 2011)]. The MHD equations were used with included arbitrary (i) current of hot electrons treating it as an additional external current and (ii) ion-sound instability responsible for the increase in anomalous resistance in areas with high temperature and low-density plasma. The spatial distribution of magnetic fields and current density obtained from 2D modeling are in acceptable agreement with the experimental results [Pisarczyk et al., Plasma Phys. Controlled Fusion 62, 115020 (2020); Zaraś-Szydłowska et al., AIP Adv. 10, 115201 (2020); Pisarczyk et al., Phys. Plasmas 22, 102706 (2015)]. The inclusion of temporal changes in anomalous resistance in modeling allowed us to explain the persistence of high SMF amplitude at the level of several megagauss after the laser pulse ended due to the effect of magnetic field freezing. [ABSTRACT FROM AUTHOR]