Ignimbrite Sheets Research Articles

The late Quaternary Diego Hernandez Formation, Tenerife: Volcanology of a complex cycle of voluminous explosive phonolitic eruptions

The Diego Hernandez Formation (DHF; 600–ca. 180 ka) represents the products of the most recent complete cycle of phonolitic explosive volcanism on Tenerife (Canary Islands, Spain). We provide a revised and detailed stratigraphy, new 40Ar/ 39Ar and (U–Th)/He age determinations for major eruptive units, a summary of new chemical data and an overview of the key characteristics of the cycle, including volume estimates, dispersal patterns, eruption styles, phreatomagmatic influences and caldera collapse episodes. The complex stratigraphy of the DHF is divided into 20 named members, each representing a major eruption, as well as numerous unnamed members of limited present-day exposure. The major eruptions are represented by the Fortaleza (370 ka), Roque (347 ka, 3 km 3), Aldea (319 ka, 3 km 3), Fasnia (309 ka, 13 km 3), Poris (268 ka, 3.5 km 3), Arafo (4 km 3), Caleta (223 ka, 3.5 km 3) and Abrigo (between 196 and 171 ka, 20 km 3) Members. The Aldea, Fasnia and Poris Members consist of highly complex successions of plinian fall, surge and flow deposits and several of the eruptions produced widespread and internally complex ignimbrite sheets. Phreatomagmatism occurred most frequently in the opening phase of the eruptions but also recurred repeatedly throughout many of the sequences. Inferred sources of water include a shallow caldera lake and groundwater, and intermittent phreatomagmatic activity was an important influence on eruption style. Another important factor was conduit and vent instability, which frequently loaded the eruption column with dense lithic debris and occasionally triggered column collapse and ignimbrite formation. Most of the major DHF eruptions were triggered by injection of mafic magma into existing phonolitic magma bodies. Two phonolitic magma types were available for eruption during the lifetime of the DHF, but each was dominant at different times. The results presented here support a caldera collapse rather than a landslide model for the origin of the Las Cañadas Caldera, although the evolution of the caldera is evidently more complex and incremental than first thought.

Paleomagnetism and tectonics of the southern Atacama Desert (25–28°S), northern Chile

We report paleomagnetic results for 131 sites from the modern forearc of northern Chile (25°S and 28°S). Remanent magnetization in volcanic and intrusive rocks is mostly primary, while a secondary magnetization is observed in sedimentary rocks. Comparison of locality‐mean directions with expected paleomagnetic directions indicates vertical axis rotations from −7.3° ± 21.6° counterclockwise to 52.7° ± 17° clockwise. Jurassic to Early Cretaceous rocks from the Coastal Cordillera and Cretaceous to Paleocene rocks from the Central Depression show similar magnitude (>30°) clockwise rotations, while more variable rotations occur in Mesozoic to Eocene rocks of the Precordillera. Clockwise rotations in Mesozoic and Paleogene rocks occur in the Chilean Frontal Cordillera south of 27°30′S. Paleomagnetic results in three large Miocene ignimbrite sheets overlying rotated and nonrotated older rocks in the Precordillera and Pre‐Andean Depression which show no relative rotation between sites indicate that most rotations within the study area occurred prior to 18 Ma (early Miocene) and likely during and after the “Incaic” tectonic event, which affected large tracts of the central Andes. The postulated onset of rotations in the north Chilean forearc was contemporaneous with the beginning of horizontal shortening and uplift of the Eastern Cordillera in Bolivia and northwestern Argentina. Rotation of the Chilean forearc, enhancement of the curvature of the central Andes, and the formation of the Bolivian Orocline seem to be, for the most part, closely linked to the evolution of the Eocene‐Oligocene tectonics of the Eastern Cordillera.

Neogene and Quaternary ignimbrites in the area of Arequipa, Southern Peru: Stratigraphical and petrological correlations

This study focuses on the correlation of four newly identified rhyolitic ignimbrites located around Arequipa, which were previously described as a single ignimbrite sheet: (1) The Río Chili Ignimbrite (ca. 13.33 Ma) crops out in the Río Chili canyon and consists of a cooling unit of non-welded to partially welded, massive, crystal-rich lapilli tuff. This ignimbrite represents the base of the Neogene ignimbrite succession in the Arequipa area and may be stratigraphically equivalent to the 14–13 Ma Huaylillas Ignimbrites in southern Peru. (2) The second voluminous (∼ 20 km 3) and widespread La Joya Ignimbrite sheet (ca. 4.87 Ma) filled the Arequipa depression. Its probable source is now buried underneath the Chachani volcanic complex. The La Joya Ignimbrite covers both the Arequipa Batholith toward the Río Vitor piedmont and also part of the Altiplano to the East and North of the Chachani complex. (3) The Arequipa Airport Ignimbrite (AAI) sheet (ca. 1.6 Ma, ∼18 km 3), the third and latest infill of the Arequipa basin, consists of a lower white, columnar jointed, massive lapilli tuff, indurated by vapor-phase recrystallization, and is overlain by an upper, pink, lithic-rich, non-welded, massive lapilli-tuff. The source of this ignimbrite is also buried underneath the Chachani complex, as indicated by anisotropy of magnetic susceptibility measurements and component lithology. (4) The Yura non-welded pumice-flow deposits (ca. 1.02 Ma, 1.5 km 3) are restricted to the northern and western flanks of the Chachani complex and may correlate with tuffs of the Capillune Formation on the Altiplano. All ignimbrites are high-K calc-alkaline to alkaline rhyolites, which contain variable amounts of plagioclase, biotite, quartz, sanidine, and Fe–Ti oxides. The modal composition of mineral phases and chemistry of amphiboles and biotites are useful for distinguishing the ignimbrites. Specific geochemical fingerprints, such as Sr, Y, Rb, Ba, Cs, REE, and Sr isotopes were effective discriminator elements. Magmatic differentiation, crustal residence time and contamination, and source melting processes are discussed in order to explain compositional variations. The older La Joya Ignimbrite and Río Chili Ignimbrite reflect initial wetter crustal melts closer to granite composition, whereas with time the younger Arequipa Airport Ignimbrite and Yura Tuffs initial melts are slightly less evolved and dryer due to more extensive crustal melting. The voluminous (> 40 km 3) Neogene ignimbrites of the Arequipa area were produced by three major explosive volcanic pulses in the Central Volcanic Zone of southern Peru, whose genetic relation with tectonic uplift and crustal thickening of the Andean Cordillera are still a matter of debate.

Fluvial responses to volcanism: resedimentation of the 1800a Taupo ignimbrite eruption in the Rangitaiki River catchment, North Island, New Zealand

The potential for the generation of dangerous and damaging lahars and floods in response to the eruption of voluminous pyroclastic debris has become increasingly appreciated in recent years. The style and tempo of this response varies both between eruptions and between individual catchments impacted by a single eruption, so that an understanding of the factors controlling this variation is necessary for precise hazard assessment. The 1800a Taupo eruption from the Taupo Volcanic Centre in the central North Island of New Zealand devastated an area of 20,000 km 2 during eruption of a climactic ignimbrite, impacting the headwaters of all major rivers draining radially from this region. The Rangitaiki River, the subject of this paper, differs from other catchments in that the Taupo ignimbrite buried an essentially flat land surface inherited from a suite of welded ignimbrite sheets erupted between 320–340 and 230 ka. The middle reaches of the catchment are characterised by narrow, steep gorges alternating with low-gradient basins developed in tectonic half-grabens. Initially, remobilisation of pyroclastic material in the headwaters was dominated by hyperconcentrated sheet flows resulting in shallow reworking. In higher gradient areas, reintegration of drainage networks was achieved by incision of deep channels and gullies, assisted by breakouts from ephemeral lakelets developed in ignimbrite-dammed depressions. Braided, and later meandering, streams superseded this pattern as rill and gully systems stabilised and sediment yields fell leading to a decline in drainage density. Gorge reaches acted as efficient conduits for remobilised material while the basins acted as local depocentres for the temporary storage of volcaniclastic sediments, mediating the transfer of pyroclastic debris to the Bay of Plenty coast >100 km to the north. Reworking and resedimentation of pyroclastic debris began immediately after the eruption, peaking early and then rapidly declining so that most geomorphic adjustments occurred within one or two decades of the initial disturbance.

The 7600 ( 14C) year BP Kurile Lake caldera-forming eruption, Kamchatka, Russia: stratigraphy and field relationships

The 7600 14C-year-old Kurile Lake caldera-forming eruption (KO) in southern Kamchatka, Russia, produced a 7-km-wide caldera now mostly filled by the Kurile Lake. The KO eruption has a conservatively estimated tephra volume of 140–170 km 3 making it the largest Holocene eruption in the Kurile–Kamchatka volcanic arc and ranking it among the Earth's largest Holocene explosive eruptions. The eruptive sequence consists of three main units: (I) initial phreatoplinian deposits; (II) plinian fall deposits, and (III) a voluminous and extensive ignimbrite sheet and accompanying surge beds and co-ignimbrite fallout. The KO fall tephra was dispersed over an area of >3 million km 2, mostly in a northwest direction. It is a valuable stratigraphic marker for southern Kamchatka, the Sea of Okhotsk, and a large part of the Asia mainland, where it has been identified as a ∼6 to 0.1 cm thick layer in terrestrial and lake sediments, 1000–1700 km from source. The ignimbrite, which constitutes a significant volume of the KO deposits, extends to the Sea of Okhotsk and the Pacific Ocean on either side of the peninsula, a distance of over 50 km from source. Fine co-ignimbrite ash was likely formed when the ignimbrite entered the sea and could account for the wide dispersal of the KO fall unit. Individual pumice clasts from the fall and surge deposits range from dacite to rhyolite, whereas pumice and scoria clasts in the ignimbrite range from basaltic andesite to rhyolite. Ignimbrite exposed west and south of the caldera is dominantly rhyolite, whereas north, east and southeast of the caldera it has a strong vertical compositional zonation from rhyolite at the base to basaltic andesite in the middle, and back to rhyolite at the top. Following the KO eruption, Iliinsky volcano formed within the northeastern part of the caldera producing basalt to dacite lavas and pyroclastic rocks compositionally related to the KO erupted products. Other post-caldera features include several extrusive domes, which form islands in Kurile Lake, submerged cinder cones and the huge silicic extrusive massif of Dikii Greben' volcano.

Mio-Pliocene basin development in the eastern part of the Cappadocian Volcanic Province (Central Anatolia, Turkey) and its implications for regional tectonics

The Cappadocian Volcanic Province (CVP) comprises predominantly of a thick succession of volcanogenic rocks and interbedded siliciclastic sediments of Middle Miocene to Recent age in Central Anatolia, Turkey. The conditions of basin development in the eastern part of the CVP have been elucidated by using sedimentological and geomorphological approaches. The prevailing tectonic regime, its extent and causes are also discussed. Sedimentological analysis supported by geomorphological observations revealed a major NE-trending probably normal, border fault and its several synthetics. This tectonic element constitutes the SE margin of the basin and divided the CVP from the Tauride range during Middle Miocene to Pliocene. The basin fill in the study area comprises gravelly alluvial fans near the border fault, while fluvial clastics and lacustrine carbonates dominate towards the centre. Some pyroclastic rocks and lava flows are also made part of the fill. The southeastern basin margin is characterized morphologically by a number of uplifted basement blocks, probably associated with synthetic faults, and some deeply incised canyons in the footwall. These canyons were subsequently filled with a Mid-Pliocene ignimbrite sheet, and represent the sediment supply conduits to the basin. The cessation of filling in the basin was determined by strike-slip faults that uplifted and detached the basin about 2.6 Ma. This date also marks the onset of the neotectonic period in the region. The overall extensional tectonic regime inferred for the eastern CVP appears coeval with events recognised in the southern basins, i.e. Adana and Mut Basins and the eastern Mediterranean. Some physical connections between these basins also have been demonstrated. It is suggested that the CVP and the southern basins were all created during a phase of extension resulting from continued northward subduction of the African plate beneath the Eurasia during the Late Cenozoic.

Bypassing and diachronous deposition from density currents: Evidence from a giant regressive bed form in the Poris ignimbrite, Tenerife, Canary Islands

Research Article| May 01, 2004 Bypassing and diachronous deposition from density currents: Evidence from a giant regressive bed form in the Poris ignimbrite, Tenerife, Canary Islands R.J. Brown; R.J. Brown 1Geology Department, University of Leicester, University Road, Leicester LE1 7RH, UK Search for other works by this author on: GSW Google Scholar M.J. Branney M.J. Branney 1Geology Department, University of Leicester, University Road, Leicester LE1 7RH, UK Search for other works by this author on: GSW Google Scholar Author and Article Information R.J. Brown 1Geology Department, University of Leicester, University Road, Leicester LE1 7RH, UK M.J. Branney 1Geology Department, University of Leicester, University Road, Leicester LE1 7RH, UK Publisher: Geological Society of America Received: 09 Sep 2003 Revision Received: 13 Jan 2004 Accepted: 18 Jan 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2004) 32 (5): 445–448. https://doi.org/10.1130/G20188.1 Article history Received: 09 Sep 2003 Revision Received: 13 Jan 2004 Accepted: 18 Jan 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R.J. Brown, M.J. Branney; Bypassing and diachronous deposition from density currents: Evidence from a giant regressive bed form in the Poris ignimbrite, Tenerife, Canary Islands. Geology 2004;; 32 (5): 445–448. doi: https://doi.org/10.1130/G20188.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Exceptional longitudinal exposure through an ignimbrite sheet on the southern flanks of Las Cañadas volcano, Tenerife, Canary Islands, reveals a giant regressive bed form, >520 m long. It comprises sourceward-dipping diffuse lenses, ∼50 m long, of massive and diffuse-bedded lapilli tuff. Its architectural relationships indicate that at any one moment in time, deposition was restricted to an ∼50-m-long longitudinal depositional zone, bounded both upslope and downslope by extensive bypassing zones. The regressive architecture shows that the zone of deposition gradually migrated sourceward as the current passed. Nonpersistent scours indicate that the current was locally and momentarily erosive, although at many locations there is little evidence for substantial erosion or deposition, and most of the pyroclastic load seems to have entirely bypassed subaerial slopes to enter the Atlantic Ocean, ∼20 km from the source. The ignimbrite is diachronous, and any single vertical section through it represents only a short phase of the current's protracted depositional history. Similarly protracted and complex flow histories elsewhere may be easily overlooked at typical field exposures of ignimbrites, turbidites, and lahar deposits, leading to significantly underestimated flow durations and volumes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Emplacement textures in Late Palaeozoic andesite sills of the Flechtingen-Roßlau Block, north of Magdeburg (Germany)

Abstract During Late Palaeozoic times, andesite magmas intruded a 100-m thick sequence of Late Carboniferous lacustrine to alluvial siliciclastic rocks, sandwiched between folded Namurian sediments at the base and a thick, partly welded rhyolitic ignimbrite sheet at the top, in the Flechtingen area. An intrusive complex, comprising of two main sills up to 200 m thick and over 20 km in lateral extent, was formed (the Flechtingen Sill Complex, or the ‘lower andesites’, previously interpreted as lava flows). In the supposed feeder area the andesitic magmas locally pierced the ignimbrite seal, forming isolated pipes and domes (the ‘upper andesites’). Thickness variations of the sills suggest ponding of the andesite magma within former depositional troughs and syn-emplacement deformation of the host sequence with the formation of swells, basins and fault-bounded grabens at the top of the sills. Locally, thin ‘failed’ sills are present. In places, sill margins show domains of flattened and aligned vesicles and planar, sharp contacts to the host sediments. However, in many outcrops and drill cores the sill margins consist of variable andesite breccias and peperties. These fragmental rocks reflect auto- to quench-clastic brecciation of chilled andesite magma ( in situ breccias and perlite), variable magma-sediment interactions and later breciation by hydrothermal fluids.

Event-stratigraphy of a caldera-forming ignimbrite eruption on Tenerife: the 273 ka Poris Formation

The 273 ka Poris Formation in the Bandas del Sur Group records a complex, compositionally zoned explosive eruption at Las Canadas caldera on Tenerife, Canary Islands. The eruption produced widespread pyroclastic density currents that devastated much of the SE of Tenerife, and deposited one of the most extensive ignimbrite sheets on the island. The sheet reaches ~ 40-m thick, and includes Plinian pumice fall layers, massive and diffuse-stratified pumiceous ignimbrite, widespread lithic breccias, and co-ignimbrite ashfall deposits. Several facies are fines-rich, and contain ash pellets and accretionary lapilli. Eight brief eruptive phases are represented within its lithostratigraphy. Phase 1 comprised a fluctuating Plinian eruption, in which column height increased and then stabilized with time and dispersed tephra over much of the southeastern part of the island. Phase 2 emplaced three geographically restricted ignimbrite flow-units and associated extensive thin co-ignimbrite ashfall layers, which contain abundant accretionary lapilli from moist co-ignimbrite ash plumes. A brief Plinian phase (Phase 3), again dispersing pumice lapilli over southeastern Tenerife, marked the onset of a large sustained pyroclastic density current (Phase 4), which then waxed (Phase 5), covering increasingly larger areas of the island, as vents widened and/or migrated along opening caldera faults. The climax of the Poris eruption (Phase 6) was marked by widespread emplacement of coarse lithic breccias, thought to record caldera subsidence. This is inferred to have disturbed the magma chamber, causing mingling and eruption of tephriphonolite magma, and it changed the proximal topography diverting the pyroclastic density current(s) down the Guimar valley (Phase 7). Phase 8 involved post-eruption erosion and sedimentary reworking, accompanied by minor down-slope sliding of ignimbrite. This was followed by slope stabilization and pedogenesis. The fines-rich lithofacies with abundant ash pellets and accretionary lapilli record agglomeration of ash in moist ash plumes. They resemble phreatomagmatic deposits, but a phreatomagmatic origin is difficult to establish because shards are of bubble-wall type, and the moisture may have arisen by condensation within ascending thermal co-ignimbrite ash plumes that contained atmospheric moisture enhanced by that derived from the evaporation of seawater where the hot pyroclastic currents crossed the coast. Ash pellets formed in co-ignimbrite ash-clouds and then fell through turbulent pyroclastic density currents where they accreted rims and evolved into accretionary lapilli.

The Quaternary pyroclastic succession of southeast Tenerife, Canary Islands: explosive eruptions, related caldera subsidence, and sector collapse

A much-revised Quaternary stratigraphy is presented for ignimbrites and pumice fall deposits of the Bandas del Sur, in southern Tenerife. New 40Ar/39Ar data obtained for the Arico, Granadilla, Fasnia, Poris, La Caleta and Abrigo formations are presented, allowing correlation with previously dated offshore marine ashfall layers and volcaniclastic sediments. We also provide a minimum age of 287±7 ka for a major sector collapse event at the Güimar valley. The Bandas del Sur succession includes more than seven widespread ignimbrite sheets that have similar characteristics, including widespread basal Plinian layers, predominantly phonolite composition, ignimbrites with similar extensive geographic distributions, thin condensed veneers with abundant diffuse bedding and complex lateral and vertical grading patterns, lateral gradations into localized massive facies within palaeo-wadis, and widespread lithic breccia layers that probably record caldera-forming eruptions. Each ignimbrite sheet records substantial bypassing of pyroclastic material into the ocean. The succession indicates that Las Cañadas volcano underwent a series of major explosive eruptions, each starting with a Plinian phase followed by emplacement of ignimbrites and thin ash layers, some of co-ignimbrite origin. Several of the ignimbrite sheets are compositionally zoned and contain subordinate mafic pumices and banded pumices indicative of magma mingling immediately prior to eruption. Because passage of each pyroclastic density current was characterized by phases of non-deposition and erosion, the entire course of each eruption is incompletely recorded at any one location, accounting for some previously perceived differences between the units. Because each current passed into the ocean, estimating eruption volumes is virtually impossible. Nevertheless, the consistent widespread distributions and the presence of lithic breccias within most of the ignimbrite sheets suggest that at least seven caldera collapse eruptions are recorded in the Bandas del Sur succession and probably formed a complex, nested collapse structure. Detailed field relationships show that extensive ignimbrite sheets (e.g. the Arico, Poris and La Caleta formations) relate to previously unrecognized caldera collapse events. We envisage that the evolution of the nested Las Cañadas caldera is more complex than previously thought and involved a protracted history of successive ignimbrite-related caldera collapse events, and large sector collapse events, interspersed with edifice-building phases.

Sedimentary characteristics and stratigraphic implications of the Kusandong Tuff, Cretaceous Gyeongsang Basin, Korea

The Cretaceous Gyeongsang Basin, SE Korea, developed on a continental are massif, is filled with 6–9 km thick, nonmarine (alluvial, fluvial, and lacustrine) deposits and subduction-related volcanogenic rocks. The Kusandong Tuff is a thin (2–4 m thick) but laterally extensive (ca. 200 km long) ignimbrite sheet, rhyodacitic in composition and rich in crystals (40–60 vol.%). The tuff has been used as an important key bed for stratigraphic classification and correlation of the basin fill. Overall sedimentary and compositional characteristics of the tuff suggest, however, that the northern and southern parts of the tuff represent different ignimbrite units. The northern part of the tuff (NKT), lying above an erosional contact, consists of a massive division and an overlying stratified division, whereas the southern part (SKT), lying conformably above the substrate, has a basal layered division beneath a massive division. The NKT has distinctively higher crystal (51–61 vol.%) and lithic (7.0–7.8 vol.%) contents compared with the SKT, which contains 35–48 vol.% crystals and 0.2–2.5 vol.% lithic fragments. Major and trace element compositions are also different, suggesting that the NKT and the SKT originated from compositionally different magmas. These results suggest that the use of the Kusandong Tuff as an event stratigraphic unit needs to be reconsidered and the formations adjacent to the Kusandong Tuff need to be redefined, especially in the Milyang subbasin.

Palaeomagnetism and magnetic properties of the Cappadocian ignimbrite succession, central Turkey and Neogene tectonics of the Anatolian collage

The Cappadocian ignimbrite succession of central-southern Anatolia comprises at least nine major and two minor calc-alkaline rhyolitic sheets emplaced at 1–2-Ma intervals between 11.2 and 1.1 Ma. It records the last phase of Neotethyan subduction during final emplacement of the Tauride orogen in southern Turkey. This study reports magnetostratigraphy and describes associated rock magnetic properties. Remanence resides in Ti-poor titanomagnetites. Haematisation is locally produced by post-emplacement oxidation but does not contribute significantly to the palaeomagnetic signature although secondary processes within the ignimbrite sheets have produced composite isothermal remanent magnetisation spectra and variable intensities of magnetisation. Weak anisotropy of magnetic susceptibility describes tensors with maximum axes close to bedding and minimum axes perpendicular to this plane. Directions of k max with weak imbrication mostly suggest flow away from centres of eruption located by gravity and remote sensing. Older ignimbrites (Upper and Lower Göreme, Akdag-Zelve) from the Çardak Centre are all of normal polarity. Later ignimbrites, partly erupted from the Derinkuyu Centre, comprise the Sarımaden (R), Cemilköy (R), Tahar (R), Kızılkaya (R), Incesu (N) and Valibaba-Sofular (R) ignimbrites. The overall (reversed) group mean is D/ I=174/−51° ( N=10 units, R=9.84, α 95=6.6°, k=55) and all magnetisation directions from the Upper Göreme (9.0 Ma) onwards are rotated anticlockwise with respect to Eurasian and African palaeofields. This sense of rotation characterises most of central Anatolia and averages 9±5° in this sector. The rotation rate from 8 to 1 Ma BP was ∼1.25°/Ma but it appears to have accelerated during the latter part of the Quaternary to about an order higher than rates determined from GPS. Rotation has resulted from extrusion of fault blocks during tectonic escape of the Anatolian collage to the southwest and followed crustal thickening as the Afro-Arabia Plate has continued to impinge differentially into Eurasia. Cenozoic magnetic inclinations are systematically shallower across Anatolia than inclinations expected from the geocentric dipole model and the apparent polar wander of bordering major plates. Only part of this difference (∼400 km) can be accommodated by northward movement and crustal thickening in Central Anatolia since mid-Miocene times. Shallow inclinations observed in the Aegean region extend into Turkey; they are observed in young volcanics and appear to reflect a regional geomagnetic anomaly. The pattern of neotectonic declinations across Anatolia shows rotations that are strongly anticlockwise rotated in the east near the Arabian pincer but diminish towards the west to become zero or slightly clockwise at the western extremity of the collage. Rotations also seem to become generally younger towards the south. Crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the Tauride Arc and, by inference, the Cyprus Arc.

Chapter 6 Ignimbrite architecture: constraints on current dynamics

Abstract This chapter explores how ignimbrite architectures and lithofacies associations can be used to infer how flow-boundary zones of pyroclastic density currents vary with time, downcurrent, laterally and with topography. The approach hopefully will be developed further so deposits can be used more precisely to constrain the dynamics of pyroclastic density currents. The initial analysis of a complex ignimbrite sheet should be purely descriptive. An ignimbrite sheet may be divided into 'divisions'. A division is a basic architectural unit, and may comprise a layer, lens or any specified part of the deposit that has some common characteristics and/or bounding surface(s). A division may be characterized by a lithofacies or a group of lithofacies. Conceptualizing architecture in a time-geometry framework To determine how a pyroclastic density current evolved in time and space requires correlation of the internal lithofacies and divisions of an ignimbrite within a time-geometry framework. The deposition of layers, lithofacies and compositional zones in ignimbrites (Fig. 6.1) may be diachronous (see below). It is useful, therefore, notionally to subdivide an ignimbrite sheet with time surfaces, which form time-lines in cross-section. One type of time-surface is a depositional isochron, or depochron (new term), which joins pyroclasts deposited at the same instant in time (Fig. 6.2). A depochron represents an instantaneous aggradation surface. Ignimbrite lithofacies along a depochron record the depositional processes at the flow boundary of an entire pyroclastic density (A) Gradational chemical zonation within massive lapilli-tuff (mLT). The gradual colour variation with height reflects a gradually increasing abundance of dark andesite pumice clasts, reflecting temporal changes

Chapter 7 Overview, key implications and future research

Overview and key implications The vast extent of many ignimbrites shows that eruptions have occurred on almost unimaginable scales, well beyond any modern human experience. Evidence is emerging that plumes derived from large pyroclastic currents have impacted climate and biota on a global scale, whilst certain types of ignimbrites (e.g. extensive rheomorphic ignimbrites) indicate particularly awesome styles of eruption and emplacement that are regionally devastating and which we do not fully comprehend. Such unimaginable eruptions are bound to occur again. If we are to interpret such catastrophic events correctly, and possibly even anticipate the impact of future occurrences, it is essential that ignimbrite sheet architectures are studied further in order to understand the mechanisms, rates and durations of the fundamental processes. Of particular importance in risk mitigation will be the understanding of early stages of such devastating eruptions. The new approaches and descriptive schemes presented in this Memoir are intended to stimulate and facilitate such future work. Below, we summarize the fourteen main points and key implications of our analysis of pyroclastic density current deposition. Many of the ideas are also directly applicable to the interpretation of other types of sediment gravity flow deposits, such as those from turbidity currents, debris flows and lahars. (1) We have reviewed significant recent advances in the understanding of particle transport, segregation and deposition. Valuable insights derive from laboratory experiments and computer models, from analogous processes in nature and in industry, and from observations of volcanic eruptions and their deposits. (2) Clasts in pyroclastic density currents are supported

Definitions of Terms used

accumulative . A downstream increase in a current parameter, such as velocity, concentration or capacity (see p. 2). for example, accumulative velocity is downstream spatial acceleration. Term coined by Kneller & Branney (1995). agglomerate . Welded or non-welded coarse-grained deposit formed mainly of juvenile pyroclasts (typically spatter, scoria, pumice, or obsidian ‘bombs’ or lapilli) whose shapes and/or external surface textures indicate that they were partly fluidal during transport. Agglomerates can be deposited by pyroclastic density currents (p.61), by pyroclastic fall or ballistically. Those within ignimbrite sheets commonly contain subordinate lithic blocks, pumice lapilli and pumiceous ash, and grade into pumiceous ignimbrite and/or massive lithic breccia (p. 57). In powder technology, loosley bonded clusters or clumps of fine cohesive particles are referred to as ‘agglomerates’ agglomeration . Adherence and clustering together of ash particles to form agglomerations: i.e. loose or firm clumps, pellets and/or accretionary lapilli. Fine ash may agglomerate within an eruption plume or in a pyroclastic density current initially due to electrostatic effects or (when moist) surface tension effects, and the agglomerations may be rapidly cemented by precipitation of slats as water evaporates (p. 83). agglutination . Very rapid, syn-depositional welding of hot, fluidal pyroclasts prior to burial by aggrading ignimbrite (p. 84). Agglutination resultsin a welded tuff in which former clast outlines are still visible (cf. coalescene in which they are not). amalgamation . Inferred seamless joining of deposits from successive discrete depositional events to produce a single bed with ni trace of a hiatus (p. 95).

Facies in an Upper Permian volcanic succession, Emmaville Volcanics, Deepwater, northeastern New South Wales

The Upper Permian Emmaville Volcanics at Deepwater, northeastern New South Wales, consist of a diverse succession of calc‐alkaline silicic‐intermediate ignimbrites, volcaniclastics and minor lavas. This 2.5 km‐thick sequence underlies and outcrops extensively along the northern margin of the Dundee Rhyodacite Outlier at Dundee. Detailed mapping and facies analysis have revealed eight locally mappable units namely; Magistrate Volcanic Member (rhyolitic ignimbrites), Wollundi Mudstone Member, Dellwood Ignimbrite Member, Marrawarra Rhyolite Member, Top‐Crossing Sandstone Member, Arranmor Ignimbrite Member, Yarramundi Andesite Member (lava, breccia) and Welcome Volcanic Member (rhyolitic ignimbrites). All volcanic units are contained in two fault‐bounded blocks of different lithology and structure. The volcanic succession ranges in composition from andesite to high‐silica rhyolite (58.6–78% SiO2). Chemical characteristics include enrichment in K2O (>3.5%), Al2O3 and large‐ion lithophile elements (LILE: Rb, K and light rare‐earth elements (LREE)), and depletion in high field strength elements (HFSE: Ti, Nb and Zr). These geochemical attributes reflect a continental subduction‐related signature. The facies architecture indicates that the principal volcanic features of the Late Permian palaeogeography in northeastern New South Wales was a topographically subdued depression flanked by low‐angle ignimbrite sheets with rhyolitic‐intermediate volcanic centres rising gently from the sloping terrain. The succession demonstrates that during the Late Permian andesitic volcanism was present, although localised. A modern analogue for the setting of the Emmaville Volcanics is the Quaternary Taupo Volcanic Zone (New Zealand).

Long offset tensor apparent resistivity surveys of the Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ) is the centre of rhyolite volcanism in New Zealand and is characterised by numerous calderas and active geothermal systems. Three long-offset multiple-source (tensor) bipole–dipole surveys, which together span the width of the TVZ, have been used to determine the resistivity structure down to depths of 5–8 km. These data have been interpreted using the constraints provided by extensive shallow (<1 km) resistivity mapping made using the Schlumberger electrode array. The deep resistivity structure defines three distinct zones within the TVZ, each with boundaries approximately parallel to the eastern margin. The eastern zone is nearly 20 km wide and contains highly conductive rocks (<10 Ωm) that extend from about 2 km to at least 3.5 km depth. The conductive nature of these rocks is believed to reflect clay-rich volcaniclastics which have infilled a series of overlapping and coalescing calderas. About 70% of the geothermal discharge of the TVZ (>4000 MW) occurs through this eastern zone. Along the centre of the TVZ lies the Taupo Fault Belt in which resistivities are an order of magnitude higher (>200 Ωm to depths of at least 7 km) than in the eastern zone. The Taupo Fault Belt also has low heat-flux, high seismicity and is undergoing rapid tectonic extension. A third zone along the western side of the TVZ contains conductive rocks below 500 m depth. Drilling shows that the high conductivities occur in thick ignimbrite sheets with ages greater than 1 Ma. These low resistivities are caused by clay minerals produced by low-temperature alteration.

Systematics of xenocrystic contamination: preservation of discrete feldspar populations at McCullough Pass Caldera revealed by 40Ar/ 39Ar dating

Single crystal 40Ar/ 39Ar dating of K-feldspars from silicic volcanic rocks containing xenocrysts often yields a spectrum of ages slightly older than those of juvenile sanidine phenocrysts. In contrast, feldspars from thin, low-volume units of the Tertiary (14 Ma) McCullough Pass Tuff define discrete age populations at ∼14 Ma, ∼15 Ma, and ∼1.3 Ga, reflecting the time of eruption, xenocrysts from an older ignimbrite exposed in the caldera wall, and Proterozoic basement K-feldspars, respectively. Conductive cooling and diffusion modelling suggests preservation of such discrete populations is likely only when xenocrystic material is incorporated into the magma very near or at the surface, or is engulfed in thin, rapidly cooled pyroclastic flows during emplacement. Incorporation of xenocrysts into the subvolcanic magma chamber, into thick rhyolite domes or lava flows, or into large, welded ignimbrite sheets will result in partial or total resetting of the K/Ar isotopic system. Similarly, petrographic evidence such as exsolution lamellae may be homogenized under these conditions but not in thin ignimbrites. Extremely low diffusion rates for disordering of the Al–Si tetrahedral siting of basement feldspars suggests that they will retain their ordered structural state given rhyolitic magma temperatures. Thus, even when petrographic and K/Ar isotopic evidence for xenocrystic contamination is obscured, it may be preserved in the form of Al–Si ordering.

The Polychnitos ignimbrite of Lesvos island

Miocene volcanism on Lesvos was particularly explosive giving rise to two extensive pyroclastic formations, the Sigri pyroclastics to the west and the Polychnitos ignimbrite to the east of the island. The Polychnitos ignimbrite at 17.2±0.5 Ma (Borsi et al.1972) is part of the shoshonitic succession on Lesbos which ranges in composition from basalt to rhyolite and is both underlain and overlain by calcalkaline volcanic rocks (Pe-Piper and Piper 1993) resting on a late Paleozoic metamorphic basement which has acted as an impediment to the free flow of the ignimbrite. The Polychnitos ignimbrite consists of eight lithological units, six of which are presumed to be facies of the same ignimbrite sheet ("PK", "PU", "MGF I, II, III", "Z"). Ignimbrite deposition at elevated temperatures is advocated by its columnar jointing, eutaxitic texture, gas escape structures and glassy zones of intense welding. The typical mineral assemblage of all Polychnitos ignimbrite units consists of plagioclase, Kfeldspar and biotite. It displays phenocryst microtextures indicative of magma mixing. Magma mixing is corroborated of glasses of two discrete compositions. Lithic clast measurements indicate a northeasterly trending fissure vent passing from the northeastern corner of the Kalloni Gulf.

Volcanic successions and the role of destructional events in the Western Mátra Mountains, Hungary: implications for the volcanic structures / Successions volcanologiques et événements tectoniques dans les Monts Mátra occidentaux : implications pour les structures volcaniques

Abstract The original volcanic edifice of the western part of the mid-Miocene Matra Mountains, North Hungary, may have been a group of small- scale andesitic centres built upon an ignimbrite sheet of the Middle Rhyolite Tuff of the Pannonian Basin. A previously suggested large caldera is not supported by the available volcanological, structural-geological, and geomorphological data. Late Miocene to Pleistocene tectonic movements resulted in the asymmetrical uplift of the mountains, broken up by NW-SE and NE-SW-trending faults. As a response, long-term tectonic displacements as well as catastrophic debris avalanches and lahars contributed to the rearrangement of volcanic successions and orographie elements probably in the latest Miocene to Pliocene.