Occurrence Of Volcanism Research Articles

A classification of gem corundum deposits aimed towards gem exploration

This classification of gem corundum (ruby and sapphire) deposits takes into consideration petrographic data, and the mode of genesis of the deposits. It is aimed at being practical and useful in the field, particularly for prospecting purposes. Deposits are classified into primary and secondary deposits. Primary deposits include igneous and metamorphic deposits. Igneous gem corundum deposits are rare and include sapphire-bearing syenites from Kenya. Emphasis is put on metamorphic deposits, that are subdivided into metamorphic s.s., metasomatic, and anatectic deposits. Many gem corundum deposits result from metasomatic processes. Small-scale metasomatism mostly involves desilication reactions between silico-aluminous rocks (pegmatites, gneisses, etc) and silica-poor rocks (ultramafites, meta-carbonates), and leads to the formation of limited-size deposits. Large-scale (diffuse) metasomatism is usually more difficult to characterize, and therefore is not separated from isochemical metamorphism in this classification. In metamorphic deposits, gem corundum results from transformation of an Al-rich and/or Si-poor protolith. Such deposits include ruby-bearing mafic granulites, ruby-bearing meta-limestones, and ruby/sapphire-bearing gneisses and granulites. An intermediate category includes anatectic deposits. Secondary deposits encompass sedimentary and volcanic (xenoclastic) occurrences. In sedimentary deposits, gem corundum occurs as clasts originating from other lithologies. In volcanic deposits, gem corundums are xenocrysts that have a range of origins. The proposed classification outlines geological environments favorable to the crystallization and distribution of gem corundum, thus facilitating prospecting and mining of this gemstone.

Animated tectonic reconstruction of the Southern Pacific and alkaline volcanism at its convergent margins since Eocene times

An animated reconstruction shows South Pacific plate kinematics, in the reference frame of West Antarctica, between 55 Ma and the present-day. The ocean floor in the region formed due to seafloor spreading between the Antarctic, Pacific, Phoenix and Nazca plates (a plate formed by fragmentation of the Farallon plate early in Oligocene times). The Pacific–Antarctic Ridge remained fairly stable throughout this time, migrating relatively northwestwards, by various mechanisms, behind the rapidly-moving Pacific plate. The Nazca and Phoenix plates also moved quickly, but relatively towards the east or southeast, and were subducted in these directions beneath the South American and Antarctic plates. Segments of spreading centres forming at the trailing edges of the Nazca and Phoenix plates periodically collided with these subduction zones, resulting in the total destruction of the Nazca–Phoenix spreading centre and the partial destruction of the Nazca–Antarctica spreading centre (the Chile Ridge) and Antarctic–Phoenix Ridge, which ceased to operate shortly before its northeasternmost three segments could collide with the Antarctic margin. Following collision of segments of the Chile Ridge, parts of the Antarctic plate underwent subduction at the Chile Trench. After these collisions, slab windows should have formed beneath both the South American and Antarctic convergent margins, and the animation shows occurrences of alkaline volcanism that have been, or can newly be, related to them. Further occurrences of alkali basalts, at the margins of the Powell Basin and, more speculatively, James Ross Island, can be related to the formation of a slab window beneath them following the collision of segments of the South America–Antarctica spreading centre in the northwest Weddell Sea.

Identifying volcanic regimes using Hidden Markov Models

We examine the application of Hidden Markov Models (HMMs) to volcanic occurrences. The parameters in HMMs can be estimated from data by means of the Expectation—Maximization (EM) algorithm. Various formulations permit modelling the activity level of a volcano through onset counts, the intensity of a Markov Modulated Poisson Process (MMPP), or through the intervals between onsets. More elaborate models allow investigation of the relationship between durations and reposes. After fitting the model, the Viterbi algorithm can be used to identify the underlying (hidden) activity level of the volcano most consistent with the observations. The HMM readily provides forecasts of the next event, and is easily simulated. Data of flank eruptions 1600–2006 from Mount Etna are used to illustrate the methodology. We find that the volcano has longish periods of Poissonian behaviour, interspersed with less random periods, and that changes in regime may be more frequent than have previously been identified statistically. The flank eruptions of Mount Etna appear to have a complex time-predictable character, which is compatible with transitions between an open and closed conduit system. The relationship between reposes and durations appears to characterize the cyclic nature of the volcanoes activity.

Intracratonic asthenosphere upwelling and lithosphere rejuvenation beneath the Hoggar swell (Algeria): Evidence from HIMU metasomatised lherzolite mantle xenoliths

The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular, anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU Sr–Nd–Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on lherzolite xenoliths and alkaline lavas (including He isotopes, Ra < 9) indicate that there is no requirement for a deep plume anchored in the lower mantle, and that sources in the upper mantle may satisfactorily account for all the geochemical/petrological/geophysical evidence that characterizes the Hoggar swell. Therefore the Hoggar volcanism, as well as other volcanic occurrences in the Saharan belt, are likely to be related to passive asthenospheric mantle uprising and decompression melting linked to tensional stresses in the lithosphere during Cenozoic reactivation and rifting of the Pan–African basement. This can be considered a far-field foreland reaction of the Africa–Europe collisional system since the Eocene.

Alkaline volcanism in the history of the Norwegian-Greenland basin

The opening of the North Atlantic Ocean began in the Late Paleocene and was accompanied by the eruption of submarine and subaerial basalts, which built up submarine plateau and ridges, islands, and volcanoes. The volcanic rocks are dominated by low-K tholeiitic basalts, which associate with almost coeval alkaline rocks (subalkali and alkali basalts and their derivatives, basanites, nephelinites, and others). The oldest alkaline volcanics (58–56 Ma) were formed during the opening of the oceanic rift at its shoulders, in northeastern Greenland and the western Norwegian shelf. It was recently found that 55–53 Ma-old alkali-ultramafic rocks are much more widespread at the eastern coast of Greenland than it was previously thought. The younger occurrences of alkali volcanism with pulses at 30, 10, 5 Ma, and up to the present day were formed on the young oceanic plate and newly formed islands and seamounts. To compare the oceanic and continental volcanism of this region, oceanic volcanics dredged during Cruise 10 of the R/V Akademik Kurchatov were reanalyzed using modern analytical methods (XRF and ICP-MS). This study showed that the oceanic and continental alkaline rocks are significantly different in petrochemical and geochemical characteristics, which is caused by differences in magma generation depths and compositions of the mantle source material. The primary continental alkaline magmas were initially more enriched in incompatible trace elements than oceanic ones. During the shallow-level differentiation of oceanic magmas, trace elements and alkalis could be accumulated in residual melts, but these processes occurred on a minor scale and depended on tectonic conditions.

The Origin of the Moon and Geophysical Consequences*

Summary The classical theory of tidal friction developed by Sir George Darwin has been used by a number of investigators to gain information about the origin of the Moon by tracing its orbital evolution backward in time. The present paper extends tidal theory further by making the dissipation function frequency-dependent, where frequency is defined as the relative angular rate of the satellite and planet. In order to test this modified theory empirically, it is first applied to a study of the origin of Phobos and Deimos, the two satellites of Mars. Contrary to earlier results which maintained their present low eccentricity orbits, it is found that their initial orbits are highly elongated ellipses (suggesting an origin in terms of captured cometary nuclei). When the frequency-dependent theory (even in a two-dimensional version) is applied to the more complicated Earth-Moon system, it leads to a set of dynamical constraints on the origin of the Moon which can be used together with geophysical evidence and evidence of the lunar surface structure. The most likely origin of the Moon is found to be capture as a separate body, rather than its formation in the vicinity of the Earth from smaller planetesimals. Earlier developments of tidal theory had required an initial orbit of very high inclination (90° or even greater) and initial obliquity of the Earth's axis of 90°; all of which would have made capture extremely unlikely and, further, required the dissipation of unreasonable amounts of rotational kinetic energy. Urey had favoured capture of the Moon for geochemical reasons; we now support this hypothesis because of dynamical arguments. Evidence is presented to support the occurrence of intensive heating and volcanism, both in the Earth and in the Moon, approximately four billion years ago. Thus, capture of the Moon may explain the apparently anomalously early development of the Earth's atmosphere and oceans.

Timing of effusive volcanism and collapse events within an oceanic arc island: Basse-Terre, Guadeloupe archipelago (Lesser Antilles Arc)

Timing of effusive volcanism has been investigated at the scale of an active volcanic island within the Lesser Antilles outer arc. Thirty-two new accurate K–Ar ages were obtained throughout the island of Basse-Terre, Guadeloupe, by the use of the K–Ar Cassignol–Gillot technique. Coupled with geomorphological and geochemical data, this study provides a general evolution model for Basse–Terre. Where available, paleomagnetic data are in agreement with the geomagnetic polarity time scale. Time scales of volcanic processes for the three northernmost massifs of the island have been constrained between 2.79 ± 0.04 and 0.435 ± 0.008 Ma. Whereas the Basal Complex lasted only 110 kyr (2.79 ± 0.04 – 2.68 ± 0.04 Ma), the Septentrional and the Axial Chains spread each over about 600 kyr, from 1.81 ± 0.03 to 1.15 ± 0.02 Ma, and from 1.02 ± 0.03 to 0.435 ± 0.008 Ma respectively. This geochronological study demonstrates the temporal North to South migration of volcanism throughout the whole island, at a rate of 18 km/Myr over the last 2.8 Myr, and 25 km/Myr for the last 1.8 Myr. The Basal Complex and Septentrional Chain illustrate the occurrence of fissural volcanism over one third of the island along a NNW–SSE striking axis, whereas the Axial Chain is composed of imbricate composite volcanoes aligned along a NW–SE direction. The southern part of the Axial Chain has been affected by two huge flank-collapses, the first event (640 ka; 5 to 10 km 3) was SW oriented while the second (550 ka; 20 to 25 km 3) was SE oriented. Erosion of the collapse areas reaches 84 km 3/Myr (1250 t/km 2/yr) within the last 650 kyr, and 30 km 3/Myr (5250 t/km 2/yr) during about 200 kyr, before the Sans Toucher volcano grew within the collapse structures. Northern Basse-Terre volcanics are mostly basaltic andesites and andesites, and geochemical characteristics belong to a typical Lesser Antilles Central Islands range, representative of non-primitive magmas. Finally, Basse-Terre volcanic massifs emplacement is sequentially related to both regional active faulting around Guadeloupe archipelago along a main NW-SE striking direction, and the westwards propagation of the Désirade, and then Marie-Galante rift systems.

Late Cretaceous-Early Cenozoic volcanism of Southern Mongolia: A trace of the South Khangai mantle hot spot

The Gobi-Tien Shan volcanic area (in Southern Mongolia) is part of the South Khangai volcanic region (SKVR). The formation of its lava fields was related to three stages of volcanic activity: the Late Cretaceous (88–71 Myr), Paleocene-Early Eocene (62–47 Myr), and Early Oligocene (37–30 Myr). Volcanic occurrences of different age are represented by trachybasalt, trachyandesitobasalt, basanite, and melanephelinite with similar geochemical characteristics, which are also close to the geochemical characteristics of OIB basalt. The isotope composition (Sr, Nd) of the rocks indicates that the magma sources were formed as a result of mixing of a moderately depleted PREMA mantle and an EM-I mantle enriched in neodymium.The patterns of migration of volcanic centers of different ages over the area of interest have been studied. The earliest (Late Cretaceous) volcanic occurrences were concentrated mainly in the south of the area, the Paleocene-Early Eocene eruptions took place at the center of the area, and the Early Oligocene volcanism occurred in the northern area. The observed migration of the volcanic activity centers is related to lithospheric plate motions relative to a localized source of hot mantle (the South Khangai mantle hot spot), which controlled volcanic activity within SKVR. In the lithospheric structure of this region, local asthenospheric high, reaching a depth of ∼50 km, correspond to this hot spot.

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-μ Volcanism in Africa

The East African Rift System is important to understanding plume-initiated rifting as manifest in the geochemistry of mafic lavas erupted along the rift throughout its evolution. We present new data from high-MgO Tertiary lavas from Turkana, northern Kenya, to investigate regional melt source components, to identify the depths and degrees of melting, and to characterize spatially and temporally the chemical structure of the underlying mantle. The Turkana area is a region of high lithospheric extension that sits between two topographic uplifts thought to be surface expressions of one or more upwelling mantle plumes. Thinning of local crust is believed to be accompanied by widespread removal of the mantle lithosphere, causing the asthenosphere to be in close contact with the overlying crust. New geochemical data on basanites, picrites and basalts (MgO >7 wt %) tightly constrain the primary melt source regions of Tertiary volcanism. Initial isotopic signatures (143Nd/144Nd = 0·51267–0·51283, 87Sr/86Sr = 0·7031–0·7036) and trace element abundances (Ce/Pb ∼30, La/Nb = 0·6–0·8 and Ba/Nb = 3–10) in these lavas are consistent with derivation from sub-lithospheric sources. Basalts and picrites erupted between ∼23 and 20 Ma have Sr–Nd–Pb–He isotopic characteristics indicative of high-μ influence, record high depths and degrees of partial melting, and are associated with rift propagation to the north and south. Accordingly, these lavas sample a source region that is geochemically distinct from that reflected both in Oligocene Ethiopian flood basalts and in the modern Afar region. The geochemical data support numerical and theoretical models as well as tomographic results providing for a complex thermal structure in the mantle beneath East Africa and are interpreted to reflect isotopically distinct plume heads beneath Tanzania and Afar that are derived from the chemically heterogeneous South African superplume.

Distribution of Early Tertiary volcanic rocks in south Sumatra and west Java

Abstract The evolution of volcanism in Sumatra and Java during Tertiary and Quaternary time can be divided into three phases: (i) lava flows of the Early Tertiary event (43–33 Ma) consisting of island arc tholeiites; followed by (ii) eruption of tholeiitic pillow basalt at the beginning of the Late Tertiary (11 Ma); and succeeded by (iii) medium‐K calc‐alkaline magmatism in the Pliocene and Quaternary. The present available field data on the occurrence of Paleogene volcanic rocks and subsurface data in south Sumatra and northern west Java indicate a much larger area of distribution of the volcanic rocks than previously recognized. Because the eastward continuation of the northern west Java volcanic rocks had not been found, early investigators were inclined to assume that they continued to south Kalimantan. In contrast, the early Tertiary volcanic rocks that occupy the south coast of Java can be traced further east as far as Flores. The occurrence of Paleogene volcanics in south Sumatra and northern west Java can be interpreted as a Paleogene volcanic arc that was presumably related to the late Cretaceous–Paleogene trench parallel to Sumatra and west Java due to subduction of the Indian Plate toward the northeast (Meratus trend).

Effect of lithospheric root on decompression melting in plume-lithosphere interaction models

Global distribution of large igneous provinces (LIPs) shows that volcanic events associated with diapiric plume heads have often occurred near the margins of cratonic lithosphere. Previous numerical models of plume–lithosphere interaction and decompression melting assumed the lithosphere as a rigid lid of mostly uniform thickness. Here we present a 3-D numerical model of a starting mantle plume, consisting of a diapiric head and a trailing conduit, which interacts with a moving lithospheric plate of non-uniform thickness. We study in particular the effect of a lithospheric root on plume flow and partial melting. The plume is deflected away from the root, preferentially in the upstream direction. The presence of a root facilitates more than three-fold enhancement in the melting rate by restricting the horizontal spreading of plume material and the diffusion of thermal anomaly in comparison to the model having no root. The large melting rate is sustained for longer duration in the presence of depletion buoyancy of the residue. Our model does not require an excessively high plume temperature or compositionally heterogeneous plume head and reproduces the characteristics of melting rates and melt volumes seen in flood basalt volcanic provinces and the trailing hotspot tracks. The model also supports occurrence of volcanism of different ages at a given location as a result of initial melting in the upstream region and its subsequent passage over the plume conduit due to plate motion. Inferences for the occurrence of such late-stage volcanism are discussed for the Deccan Volcanic Province. Cenozoic volcanism of eastern Australia is also discussed as an example of deflection of a plume due to the presence of thick lithosphere.

A satellite image approach to the study of lineaments, circular structures and regional geology in the Golcuk Crater district and its environs (Isparta, SW Turkey)

Important volcanic occurrences such as a sub-volcanic stock, caldera, volcano cones, circular, semicircular and elliptical morphologic structures have been identified in the Golcuk Crater Lake district and surrounding area, SW Turkey. Our satellite image analysis shows that the volcanic activity occurred during different volcanic phases. Maps of lineaments and geology of the region were prepared using colors, drainage patterns, textural features, vegetation cover and morphological forms as interpretation parameters. In addition, the major fracture directions of the region were detected from lineament analysis as trending NE, N, and NW. The relationship of Pliocene Golcuk volcanism with tectonism was defined and these data are interpreted. The satellite image reveals that the volcanic structures and the Golcuk Crater generally occur at the intersections of different lineaments, where the number of lineaments increases. Lineament analysis on Landsat satellite images is a useful tool for detecting volcanic regions, alteration and mineralization zones.

PRELIMINARY GEOCHEMICAL DATA ON VOLCANIC ROCKS FROM THE OLT NAPPE (TRANSYLVANIAN NAPPES, ROMANIA): IMPLICATION FOR THEIR ORIGIN IN THE TRANSYLVANIDES

The Transylvanian nappes (Rarau, Haghimas and Olt nappes - Fig. 1a) are interpreted as remnants of the Main Tethyan Suture in the Carpathians. According to some authors (Sandulescu and Russo-Sandulescu, 1981; Sandulescu, 1984; Russo-Sandulescu et al., 1983; Sandulescu in Ichim and Sandulescu, 1997), they consist of various types of sedimentary and Mesozoic magmatic rocks related to the ophiolites of Transylvanides (Fig. 1a), which in turn represent a Tethyan oceanic crust associated to the Vardar Zone (Bortolotti et al., 2002). The nature of volcanics from the Olt nappe (ON) is still poorly constrained. Cioflica et al. (1965) reported the occurrence of sub-alkaline and alkaline volcanics interpreted as evolved magmatic products of a tholeiitic ophiolitic series. Nonetheless the alkaline chemistry of volcanics is not consistent with the interpretation of these rocks as ophiolitic remnants linked with the Transylvanides ophiolites (Sandulescu, 1984; Sandulescu in Ichim and Sandulescu, 1997), since no alkaline rocks are found in these ophiolites. The aim of this paper is thus to present preliminary geochemical data on volcanic rocks from the ON in order to assess their possible genetic relationships with the Transylvanides ophiolites.

Core Description and a Preliminarily Sedimentology Study of Site 1202D, Leg 195, in the Southern Okinawa Trough

ODP Site 1202 of Leg 195 was designed primarily for a high-resolution study of the paleoceanography of the Kuroshio Current in the southern Okinawa Trough off NE Taiwan. Four holes were drilled in which Hole 1202D is described in detail in this study for an assessment of core quality for paleoceanography study and understanding of sedimentological features, especially turbidite sedimentation and the sediment provenances during the Late Quaternary in the southern Okinawa Trough. Pelagic mud with insignificant silt or sand layers is observed from the core top down to 133 m (mbsf; Marine Isotope Stages 1-3), but the silt-sand layer ratio (SLR: total thickness of silt and sand layers/1.5 m of core) increases gradually from a value of ＜10% between 133 and 167 m to values ＞50% between 223 and 279 m, followed by decreases to values ＜10% between 310 and 337 m and to ＜3% between 337 and 407 m (Stage 4). These silt-sand layers were most likely derived by fine-grained turbidite gravity flows, which were very active during Stages 3 and 4. Slate fragments, quartz grains, mica flakes and volcanic detritus are the major components in the coarse fraction of wash residues (＞250μm). Slate fragments are commonly found in fine-grained turbidite dominant intervals (160-280 m), while mica flakes can be observed in the muds throughout the core. The major detrital components were derived primarily from the Miocene slate belt of the pre-collision accretionary prism of the Central Range in northern Taiwan. The occurrence of volcanics could represent submarine volcanic activity in the active-opening Okinawa Trough back-arc basin off NE Taiwan. Shallow-marine fossils including benthic foraminifers, echinoids, bryozoans and mollusks are also found in the fine-grained turbidite dominant intervals. These fossil assemblages could have been deposited in the shallow shelf and then transported to the depositional site along with voluminous terrigenous materials derived from Taiwan, via submarine channels or by slope failures due to frequent earthquakes induced by plate convergence/collision and extension in the southwestern Okinawa Trough off NE Taiwan. It is concluded that the top 133 m of the core is better suited for paleoceanographic reconstruction.

Discussion and Reply: Cenozoic volcanism, tectonism and stream derangement in the Snowy Mountains and northern Monaro of New South Wales

The article discusses the occurrence of Cenozoic volcanism, tectonism and stream derangement in the Snowy Mountains and northern Monaro of New South Wales. The article states that the uplift in the Tumut and Tumbarumba catchments had been largely completed prior to the outpouring of basalt in Early Miocene times. The authors attribute the general northerly fall in the elevation of the Early Miocene surface on the plateau in the Tumut catchment to a northerly, post-basaltic tilting. The southernmost remnant of the Tumbarumba basalt lies at an elevation of only -400 metre, which is 1200 metre below the elevation of basalt in the upper part of the Tumut catchment on the plateau to the east.

Paleoclimate implications of high latitude precession-scale mineralogic fluctuations during early Oligocene Antarctic glaciation: the Great Australian Bight record

Sediments from ODP Site 1128 in the Great Australian Bight record isotopic and mineralogic variations corresponding to orbital parameters and regional climate change during the early Oligocene climate transition and Oi1 glacial event. Bulk carbonate stable isotope analyses reveal prominent positive oxygen and carbon isotope shifts related to the inferred major increase in glaciation at approximately 33.6 to 33.48 Ma. The oxygen isotope excursion corresponds to a prolonged period of low eccentricity, suggesting ice-sheet growth during low seasonality conditions. The clay mineralogy is dominated by smectite throughout. The exclusive occurrence of highly crystalline smectite from 33.6 to 33.5 Ma suggests the occurrence of explosive volcanism that correlates with the positive oxygen isotope shift. The dominance of mixed-layer smectite from 33.5 to 33.4 Ma and an increase in illite following 33.4 Ma indicates a transition from cool, wet conditions to cool, dry conditions over Australia during the Oi1 glaciation. Clay mineralogy and carbonate percentages reveal precession-scale oscillations during the Oi1 event. Kaolinite varies inversely with smectite and percent carbonate. Variations in precipitation and runoff, and wind velocities during southern hemisphere summer perihelion and high eccentricity intervals may account for the precession-scale oscillations.

Statistical analysis of tropical explosive volcanism occurrences over the last 6 centuries

Explosive volcanic eruptions are very violent events, each with clearly individual characteristics. Instead of looking at specific eruptions, we focus here on the long term record of tropical explosive volcanism. To assess the distribution of volcanic occurrences over several centuries we employ sulfate measurements from polar ice cores. From these measurements, we compile a binary time series of sulfate deposition events that left a signature at both poles. Because of the binary nature of this problem, a series of logistic statistical models are compared to best represent the data. These models demonstrate the existence of a strong multi‐decadal variation in the occurrence of sulfur rich tropical eruptions. More precisely, a combination of a trend and a cycle explains best the evolution of eruption history. To corroborate this result with a record independent of the ice cores, we confirm these long‐term variations with the most recent Volcanic Explosivity Index (VEI).

The occurrence of Middle-Miocene volcanism at Mount Hermon, northern Israel

The occurrence of Middle-Miocene volcanism at Mount Hermon, northern Israel

Conodont biostratigraphy of the Lower to Middle Devonian Deserters Formation (new), Road River Group, northeastern British Columbia

A new Lower to Middle Devonian basinal unit of the Road River Group, herein formally named the Deserters Formation, contains argillaceous, crinoidal limestone and black shale deposited in a linear sub-basin of the Ospika Embayment, southern Kechika Trough. The abrupt lateral facies changes in the region, facies thickness, and occurrence of volcanics indicate a period of extensional tectonism. A total of 53 (4–5 kg each) samples from the Deserters Formation yielded 7766 conodont elements assigned to 14 genera representing 33 species. The formation ranges from the Lochkovian (eleanorae Zone, or lower part of the delta Zone of the Cordilleran Region) to Eifelian (australis Zone). The temporal constraints established by conodont biostratigraphy allow correlation to the Grizzly Bear Formation, a regionally restricted Lower to Middle Devonian unit in the Selwyn Basin to the north.

Time-clustering analysis of volcanic occurrence sequences

We investigate the time dynamics of worldwide volcanic activity in order to find the presence of time correlation structures, by applying the Fano factor (FF) method. The analysis, performed on 35 time-occurrence volcanic eruptive sequences with volcanic explosivity index (VEI≥0) has revealed fractal behaviors in the most of the data sets considered, with fractal exponent α ranging from ∼0.2 to 0.9.