Large-scale silicic alkalic magmatism associated with the Buckhorn Caldera, Trans-Pecos Texas, USA: comparison with Pantelleria, Italy

Three major rhyolite systems in the northeastern Davis and adjacent Barrilla Mountains include lava units that bracketed a large pantelleritic ignimbrite (Gomez Tuff) in rapid eruptions spanning 300,000 years. Extensive silicic lavas formed the shields of the Star Mountain Formation (37.2 Ma-K/Ar; 36.84 Ma ³⁹Ar/⁴⁰Ar), and the Adobe Canyon Formation (37.1 Ma-K/Ar; 36.51-³⁹Ar/⁴⁰Ar). The Gomez Tuff (36.6 Ma-K/Ar; 36.74-³⁹Ar/⁴⁰Ar) blanketed a large region around the 18×24 km diameter Buckhorn caldera, within which it ponded, forming sections up to 500 m thick. Gomez eruption was preceded by pantelleritic rhyolite domes (36.87, 36.91 Ma-³⁹Ar/⁴⁰Ar), some of which blocked movement of Star Mountain lava flows. Following collapse, the Buckhorn caldera was filled by trachyte lava. Adobe Canyon rhyolite lavas then covered much of the region. Star Mountain Formation (~220 km³) is composed of multiple flows ranging from quartz trachyte to mildly peralkalic rhyolite; three major types form a total of at least six major flows in the northeastern Davis Mountains. Adobe Canyon Formation (~125 km³) contains fewer flows, some up to 180 m thick, of chemically homogenous, mildly peralkalic comendite, extending up to 40 km. Gomez Tuff (~220 km³) may represent the largest known pantellerite. It is typically less than 100 m thick in extra-caldera sections, where it shows a pyroclastic base and top, although interiors are commonly rheomorphic, containing flow banding and ramp structures. Most sections contain one cooling unit; two sections contain a smaller, upper cooling unit. Chemically, the tuff is fairly homogeneous, but is more evolved than early pantelleritic domes. Overall, although Davis Mountains silicic units were generated through open system processes, the pantellerites appear to have evolved by processes dominated by extensive fractional crystallization from parental trachytes similar to that erupted in pre- and post-caldera lavas. Comparison with the Pantelleria volcano suggests that the most likely parental magma for the Buckhorn series is transitional basalt, similar to that erupted in minor, younger Basin and Range volcanism after about 24 Ma. Roughly contemporaneous mafic lavas associated with the Buckhorn caldera appear to have assimilated or mixed with crustal melts, and, generally, may not be regarded as mafic precursors of the Buckhorn silicic rocks, They thus form a false Daly Gap as opposed to the true basalt/trachyte Daly gap of Pantelleria. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Flow directions in ash-flow tuffs: a comparison of geological and magnetic susceptibility measurements, Tshirege member (upper Bandelier Tuff), Valles caldera, New Mexico, USA

This study concludes that the elongation axis (K ₁) of the ellipsoid of anisotropic magnetic susceptibility (AMS) is a suitable proxy for flow axis in ashflow tuffs. 153 oriented samples (176 specimens) were studied from 18 sites in the 1.1 Ma Tshirege member of the Bandelier Tuff. These sites are distributed around the Valles caldera at distances of 5–25 km outside of the rim.K ₁ axes correlate well with postulated radial flow axes at 13 sites.K ₁ also agrees with measured geological flow indicators, mainly imbricated larger clasts, at 7 sites. At 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredK ₁ axes, theK ₁ axes correspond well with geological flow indicators, indicating that the divergence of flow from the predicted radial flow pattern is real. Two major topographic buttresses are suggested as the cause of flow divergence for the Tshirege ash flows: the San Pedro buttress northwest of the caldera, and the San Miguel buttress in the southeast. In situK ₁ axes plunge about 7° toward the source at two-thirds of the sites; therefore the plunge ofK ₁ is a plausible in situ indicator for thedirection of flow. Multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash-flow emplacement. These observations raisre the question of how best to represent mean flow directions in ash-flow sheets: by eigenvector methods, by vector-sum methods, or by modes. A method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined. Imbrication, determined in this way at one-third of the sites, dips toward the source, i.e., up-flow. The minimum (K ₃) axis of the AMS ellipsoid correlates with the flow foliation rather than with the larger clast imbrication. The flow axes of ash flows correspond with theK ₁ axes, not with the declination ofK ₃ axes as suggested by some authors. Initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction, which is reversed with a mean ofD=173.5°,I=-38.4°, ₉₅=3.4°N=18. This mean is not different at the 95% confidence level from that of earlier workers. The mean pole, at 098.0°E, 74.8°N,A ₉₅=3.3°,N=18, is about 15° far-sided relative to the expected time-averaged geomagnetic pole, suggesting a history of emplacement too short to adequately average secular variation.     

Flow directions in ash-flow tuffs: a comparison of geological and magnetic susceptibility measurements,Tshirege member (upper Bandelier Tuff), Valles caldera,New Mexico,USA

This study concludes that the elongation axis (K ₁) of the ellipsoid of anisotropic magnetic susceptibility (AMS) is a suitable proxy for flow axis in ashflow tuffs. 153 oriented samples (176 specimens) were studied from 18 sites in the 1.1 Ma Tshirege member of the Bandelier Tuff. These sites are distributed around the Valles caldera at distances of 5–25 km outside of the rim.K ₁ axes correlate well with postulated radial flow axes at 13 sites.K ₁ also agrees with measured geological flow indicators, mainly imbricated larger clasts, at 7 sites. At 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredK ₁ axes, theK ₁ axes correspond well with geological flow indicators, indicating that the divergence of flow from the predicted radial flow pattern is real. Two major topographic buttresses are suggested as the cause of flow divergence for the Tshirege ash flows: the San Pedro buttress northwest of the caldera, and the San Miguel buttress in the southeast. In situK ₁ axes plunge about 7° toward the source at two-thirds of the sites; therefore the plunge ofK ₁ is a plausible in situ indicator for thedirection of flow. Multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash-flow emplacement. These observations raisre the question of how best to represent ‘mean’ flow directions in ash-flow sheets: by eigenvector methods, by vector-sum methods, or by modes. A method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined. Imbrication, determined in this way at one-third of the sites, dips toward the source, i.e., up-flow. The minimum (K ₃) axis of the AMS ellipsoid correlates with the flow foliation rather than with the larger clast imbrication. The flow axes of ash flows correspond with theK ₁ axes, not with the declination ofK ₃ axes as suggested by some authors. Initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction, which is reversed with a mean ofD=173.5°,I=-38.4°, α₉₅=3.4°N=18. This mean is not different at the 95% confidence level from that of earlier workers. The mean pole, at 098.0°E, 74.8°N,A ₉₅=3.3°,N=18, is about 15° far-sided relative to the expected time-averaged geomagnetic pole, suggesting a history of emplacement too short to adequately average secular variation.     

Anisotropy of magnetic susceptibility variations in a single icelandic columnar basalt

Measurements of the anosotropy of magnetic susceptibility in 34 specimens drilled from a single Icelandic columnar basalt segment reveal a preferred, long-axis alignment of magnetic minerals normal to the long axis of the column and well grouped. Maximum elongation of these minerals occurs in regions which crystallized late, as independently indicated by relative samarium concentrations. We propose that the variations in magnetic mineral elongation are controlled by variations in thermal stress in the cooling column, before complete solidification. An upper limit of 75–100 bars is suggested for these stresses.     

Anisotropy of magnetic susceptibility in the Scaglia Rossa pelagic limestone

During folding of the Scaglia Rossa limestone in Umbria, Italy, deformation was mainly accommodated by pressure solution cleavage. Fossils between the cleavage planes appear visibly undeformed, yet the limestone possesses a weak magnetic fabric. The maximum and intermediate principal axes of the magnetic anisotropy ellipsoid define a distinct magnetic foliation plane within which a weak concentration of the maximum axes forms a magnetic lineation. Neither of these features is of sedimentary origin. Results from a slumped outcrop, where bedding and a cleavage induced by overburden compaction have different attitudes, show that the magnetic foliation is caused by the compaction. Comparisons with field-derived structural data suggest that the magnetic lineation was produced tectonically during deformation of the Apennine fold belt.     

Anisotropy of magnetic susceptibility studies of depositional processes in the Campanian Ignimbrite,Italy

The late Pleistocene trachytic Campanian Ignimbrite underlies much of the Campanian Plain near Naples, Italy, and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source, the Campi Flegrei caldera. At sites within 15 km of the Campi Flegrei, anisotropy of magnetic susceptibility (AMS) principal directions indicate that, in the absence of significant topography, deposition came from a flow moving in a roughly radial direction. AMS studies of the more distal ignimbrite reveal downhill and/or downvalley flow directions prior to deposition, even where these directions are at high angles to a generally radial transport direction from the vent. On the flanks of Roccamonfina Volcano, flow was directly downhill, as if the source of the ignimbrite was the summit of the volcano. In most localities, the ignimbrite consists of a single massive deposit. In a few localities in the Apennine Mountains, however, the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys. A detailed study of the ignimbrite in the flat Titerno River valley near Massa shows that the AMS fabrics are not due to late-stage creeping during deposition or compaction. Well-defined, but non-parallel AMS fabrics from vertical and lateral sections in the Massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit. Clast compositions and AMS axes at Mondragone indicate that the pyroclastic flow encountered the Monte Massico massif and was partially blocked, so that flow during deposition was toward the Campi Flegrei. Similar AMS data from sites along the edge of the Campanian Plain indicate back-flow off the first ridge of the Apennine Mountains reached at least 5 km from their base. The Campanian Ignimbrite was deposited from a density-stratified pyroclastic flow. The depositional system consisted of the lower, denser portion of the current, and was controlled by topography. The grouping of the AMS axes is interpreted to indicate that deposition occurred under laminar flow conditions.     

Search for strange attractors in magnetic field data: An example from a tertiary volcanic area near Bonn, Germany

Spatial variations of the magnetic field, measured at the Earth's surface, are mainly due to geological inhomogeneities within the crust. The present state of the crust, in turn, reflects its past dynamical processes and evolution. We analyzed the magnetic field intensity along a few but densely sampled profiles in a Tertiary volcanic province near Bonn, Germany. The aim was to find out whether the variation of the magnetic intensity can be related to a deterministic, although chaotic, generating system or to a stochastic one. In the former case, the geodynamical history would depend on a few degrees of freedom, whereas in the latter case, the parameters to take into account were so many that a statistic approach appears to be more suitable. Since strange attractors do not emerge from the analyzed data, we exclude the presence of a chaos generating system. Autocorrelation functions and a rescaled-range analysis, on the other hand, reveal and quantify a certain degree of correlation among successive data points and allow calculation of a range of spectral exponents in log-log diagrams, which are clearly distinct from those of white noise and Brownian motion. Only one of the profiles exhibits similarity to uncorrelated or white noise. In this case, the signal variations do not result from the local geology but from a buried gas pipe-line, aligned subparallel to that profile. If self-organized crustal evolution generally produces signatures that can be described as correlated noise, within a limited bandwidth of spectral exponents, erraneous or disturbed data could eventually be discriminated from records containing purely natural data by applying analysis tools of the dynamic system theory together with autocorrelation tests.     

Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm, Karoo igneous province, northern Botswana

To determine the magma flow direction of the giant, 179 Ma Okavango dyke swarm of northern Botswana, we measured the anisotropy of magnetic susceptibility (AMS) of 23 dykes. Dykes are located in two sections (Shashe and Thune Rivers), which are about 300 km and 400 km from the presumed magma source respectively; the Nuanetsi triple point. We collected samples from the margins of the dykes in order to use the imbrication of magnetic foliation to determine magma flow direction. About half of the magnetic fabric in the dykes is inverse, i.e. with the magnetic foliation perpendicular to the dyke plane. Lateral flow to the west and vertical flow is in evidence in the Shashe section. However, the overall analysis of normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the Shashe section. Across the Thune section, a poorly defined imbricated magnetic foliation also suggests lateral flow to the west.     

Anisotropy of magnetic susceptibility and petrofabric studies in the Garhwal synform, Outer Lesser Himalaya: Evidence of pop-up klippen

Abstract   Geological field, petrographic, and anisotropy of magnetic susceptibility studies help in understanding the evolutionary history of the Garhwal synform that lies in the western Outer Lesser Himalaya. Orientations of the magnetic susceptibility axes reveal large variations at short distances as a result of superimposed deformation, and predominant stress conditions favorable for normal faulting. Rocks forming the outer limbs of the Garhwal Synform are metamorphosed up to the lower greenschist facies. The metamorphic grade increases to chlorite zone in the inner limb and the core is characterized by chlorite–biotite to garnet zones. The different grades of metamorphism are separated by thrusts and the structure is described as a pop-up klippen.     

Anisotropy of magnetic susceptibility of rocks in the Rayleigh Law region: Modelling errors arising from linear fit to non-linear data

The anisotropy of magnetic susceptibility (AMS) within the Rayleigh Law range was investigated theoretically, using mathematical modelling. It was revealed that the orientations of the principal susceptibilities and the shape parameter vary with field so weakly that these variations can be regarded as negligible from the practical point of view. The degree of AMS increases with field according to the degree of anisotropy of the initial susceptibility used and according to the intensity of susceptibility change with field of the mineral considered. The degree of AMS calculated using linear theory is very near to the degree of AMS following from the analysis of AMS within the Rayleigh Law range. If it is desirable to correct the field-dependent degree of AMS, a simple technique is suggested based on measurement of the AMS in two fields. fhrouda@agico.cz     

临夏盆地新近纪郭泥沟剖面磁组构特征及其环境意义

临夏盆地在东亚新生代地层、古气候、古生物研究方面占有举足轻重的地位.本文对盆地东部的郭泥沟剖面进行了详细的岩石磁学和磁组构研究,以揭示从早中新世到早上新世临夏盆地的沉积演化过程.郭泥沟剖面沉积物中的磁性矿物有磁铁矿、磁赤铁矿、赤铁矿和针铁矿,但剩磁载体以磁铁矿和赤铁矿为主.从上庄组和东乡组的褐红色粉砂质粘土到柳树组和何王家组的褐黄色粘土,赤铁矿含量呈现降低的趋势,与沉积物颜色变化一致.郭泥沟剖面沉积物磁组构类型为正常沉积磁组构.结合岩石磁学结果和磁组构参数特征可揭示临夏盆地早中新世-早上新世沉积的演化过程：早中新世上庄组为稳定湖相沉积,古水流方向为NNW,与南北向的大夏河方向一致;中中新世气候发生较明显的干湿波动,形成了东乡组的褐红色湖相粉砂质粘土夹粉砂、砂和青灰色泥灰质粘土条带,古水流方向主要为NNW,沉积过程主要受大夏河控制;中中新世晚期,受青藏高原构造运动影响,沉积相由湖相细粒沉积物转变为虎家梁组河流相砂砾层;同时,盆地的水动力条件也发生改变,晚中新世柳树组湖相沉积过程同时受南北向大夏河和东西向洮河控制,两个方向近垂直的河流共同作用导致柳树组内沉积各向异性度较低,面理和线理均不发育,磁化率最大轴偏角分布比较分散,磁组构确定的古流向为东西向和南北向;早上新世期间,由于受青藏高原隆升影响,沉积了何王家组下部的河流相砂砾层;受构造抬升影响,大夏河重新主导何王家组上部洪泛平原相沉积过程,水动力条件较为单一稳定,古流向主要为N向,与大夏河流向一致.

     

Anisotropy of magnetic susceptibility data bearing on the transport direction of mid-tertiary regional ignimbrites,Candelaria Hills area,West-Central Nevada

In west-central Nevada, the Oligocene Candelaria pyroclastic sequence reaches a local thickness of up to 1.3 km, in what has been referred to as the Candelaria trough, but more generally the accumulation of ash-flow tuffs and related volcanic rocks is less than 300 m thick. Complete to near complete outcrops are scattered over about 3200 km² in the Candelaria Hills and surrounding ranges of the Southern Walker Lane structural zone. Three regionally extensive compound cooling units within the overall sequence (25.8 Ma Metallic City, 24.1 Ma Belleville, and 23.7 Ma Candelaria Junction Tuffs) have distinguishing characteristics and are the focus of study. At 106 sites, anisotropy of magnetic susceptibility (AMS) data provide an estimate of transport direction of each tuff. Inferred transport directions based on the AMS data are corrected for a modest clockwise, yet variable magnitude, vertical axis rotation that affected these rocks in late Miocene to Pliocene time, as revealed by paleomagnetic studies. The AMS data show a somewhat orderly pattern of magnetic fabrics that we interpret to define unique transport directions for the Metallic City and Candelaria Junction Tuffs. The low susceptibility and degree of anisotropy of the Belleville Tuff limits our interpretation from this pyroclastic deposit. The Metallic City and Candelaria Junction Tuffs typically show gentle, south–southeast and southeast dipping magnetic fabric imbrication, respectively, and very gently plunging magnetic lineations. These AMS fabric elements indicate the tuffs were transported to the north–northwest and northwest, respectively. The AMS fabric data from the Metallic City and Candelaria Junction Tuffs suggest relatively unrestricted flow during emplacement. Evidence across the 3,200 km² area to support more regionally controlled channelized flow into and/or flow along the east northeast–west southwest axis of the Candelaria trough is lacking. The ignimbrites clearly filled a topographic depression inferred to have formed concurrent with early, localized Basin and Range extension during pyroclastic emplacement, but based on the uniformity of AMS fabric data, we infer that the depression quickly filled and did not hinder flow across the region. Unrecognized eruptive centers for the three ignimbrites may lie buried beneath Neogene basin fill sediments south–southeast of the Candelaria Hills or concealed below younger deposits farther southeast into the Palmetto Mountains. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Revised and prepared for publication in the Bulletin of Volcanology.     

Variation of the magnetic properties in a basaltic dyke with concentric cooling zones

The effects of the variation of magnetic grain size on the magnetic properties of rocks have been studied throughout a reversely magnetized basaltic dyke with concentric cooling zones.Except in a few tachylites in which the magnetic mineral is a Ti-rich titanomagnetite, in the bulk of the dyke the magnetization is carried by almost pure magnetite grains. Although the percentage p of these magnetic oxides varies slightly, the large changes in the various magnetic parameters observed across the dyke are essentially attributable to large variations in the grain size of the magnetic particles.From the outer scoria region, where the magnetic grains are a mixture of single-domain (SD) and superparamagnetic (SP) grains, to the tachylite zone with finely crystallized basaltic glass containing interacting elongated SD particles, one observes an increase of both the ratio of the saturation remanent magnetization and the saturation induced magnetization J_rs/J_is, the bulk coercive force H_c, the median destructive field MDF, the intensity of the remanent magnetization J_r, and the Koenigsberger ratio Q. In the tachylites these parameters reach unusually high values, for subaerial basalts:

$\text{〈}\text{J}{}_{\mathrm{rs}}\text{J}{}_{\mathrm{is}}\text{〉 = 0.3, 〈H}{}_{\mathrm{c}}\text{〉 = 460 O}{}_{\mathrm{e}}\text{, 〈MDF〉 = 620 O}{}_{\mathrm{e}}\text{r.m.s., 〈J}{}_{\mathrm{r}}\text{〉 = 2.7 · 10}{}^{\mathrm{?2}}\text{e.m.u. cm}{}^{\mathrm{?3}}\text{〈Q〉 = 24}$

These parameters decrease in the basalt toward the centre of the dyke where pseudo-single-domain (pseudo-SD) particles coexist together with multidomain (MD) grains. The susceptibility remains approximately constant from the inner basalt to the tachylite, but increases in the scoria up to values 10 times higher owing to the presence of SP particles. The magnetic viscosity increases also drastically toward the margin of the dyke due to an increase of the fraction of the SD particles just above the superparamagnetic threshold.     

Anisotropy of magnetic remanence: A brief review of mineralogical sources,physical origins,and geological applications,and comparison with susceptibility anisotropy

The magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low-field magnetic susceptibility (AMS). However, alternative methods based on remanent magnetization (measured in the absence of a magnetic field) rather than induced magnetization (measured in the applied field) have distinct advantages for certain geological applications. This is particularly true for; (1) adjunct studies in paleomagnetism, in order to assess the fidelity with which a natural remanence records the paleofield orientation; (2) studies of weakly magnetic or weakly deformed rocks, for which susceptibility anisotropy is very difficult to measure precisely; and (3) quantitative applications such as strain estimation. The fundamental differences between susceptibility and remanence (and their respective anisotropies) are due to several factors: (1) susceptibility arises from all of the minerals present in a sample, whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals; (2) ferromagnetic minerals are generally more anisotropic than para- and diamagnetic minerals; (3) for ferromagnetic minerals, remanence is inevitably more anisotropic than susceptibility; and (4) a number of common minerals, including single-domain magnetites, possess an inverse anisotropy of susceptibility, i.e., they tend to have minimum susceptibility parallel to the long axis of an individual particle; remanence is immune to this phenomenon. As a consequence of all these factors, remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample.Contribution number 9102 of the Institute for Rock Magnetism, University of Minnesota.     

Rhyolitic ignimbrites in the Rogerson Graben,southern Snake River Plain volcanic province: volcanic stratigraphy,eruption history and basin evolution

The 80 km long NNE-trending Rogerson Graben on the southern margin of the central Snake River Plain, Idaho, USA, hosts a rhyolitic pyroclastic succession, 200 m thick, that records a period of successive, late-Miocene, large-volume explosive eruptions from the Yellowstone–Snake River Plain volcanic province, and contemporaneous extension. The succession, here termed the Rogerson Formation, comprises seven members (defined herein) and records at least eight large explosive eruptions with numerous repose periods. Five high-grade and extremely high-grade ignimbrites are intercalated with three non-welded ignimbrites and two volcaniclastic deposits, with numerous repose periods (palaeosols) throughout. Two of the ignimbrites are dominantly rheomorphic and lava-like but contain subordinate non-welded pyroclastic layers. The ignimbrites are typical Snake River Plain high-silica rhyolites, with anhydrous crystal assemblages and high inferred magmatic temperatures (≤ 1,025°C). We tentatively infer that the Jackpot and Rabbit Springs Members may have been emplaced from the Bruneau–Jarbidge eruptive centre on the basis of: (1) flow lineation trends, (2) crystal assemblage, and (3) radiometric age. We infer that the overlying Brown’s View, Grey’s Landing, and Sand Springs Members may have been emplaced from the Twin Falls eruptive centre on the basis of: (1) kinematic indicators (from the east), and (2) crystal assemblage. Furthermore, we have established the contemporaneous evolution of the Rogerson Graben from the emplacement of the Jackpot Member onwards, and infer that it is similar to younger half-graben along the southern margin of the Snake River Plain, formed by local reactivation of Basin and Range structures by the northeastwardly migration of the Yellowstone hot-spot. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The dyke swarm of Mount Calanna (Etna, Italy): an example of the uppermost portion of a volcanic plumbing system

A new multidisciplinary study, combining geology, petrography, and geochemistry, on the rocks of the isolated hill of Mount Calanna (Mount Etna, Italy) has provided evidence for the existence of a dyke swarm, formed by more than 200 dykes distributed over an area of ~0.7 km², with an intensity of intrusion up to 40%. All bodies are deeply altered, and the geological and mesostructural surveying of 132 dykes revealed that they intruded in E–W direction, with an average dip of 60°. The faults affecting the outcrop have in general an E–W strike and dip of ~55°: these have all normal motion and have been interpreted as coeval with the dykes. This interpretation contrasts with the previous hypothesis that considered Mount Calanna as a thrust resulting from compressive deformation resulting from the gravitational spreading of the volcanic edifice. Mount Calanna is here interpreted as the uppermost portion of a vertically extensive magmatic plexus that fed the eruptive activity of one (or more) eruptive center/s sited in the Valle del Bove area. Measurements of the apparent densities on 23 dykes and host rock samples give an average value of 2,420 kg/m³ for the entire complex, ~15% lower than the density expected for hawaiitic magma, placing an important constraint on the geophysical identification of similar structures. Considering that Mount Etna is not an old eroded edifice but an active and growing volcano, the exposure of this subvolcanic structure can be regarded as exceptional. Its geometry and physical characteristics can be thus regarded as an interesting example of the present-day shallow plumbing system of Mount Etna as well as of other basaltic volcanoes.     

Applying multi‐parameter runoff elasticity to assess water availability in a changing climate: an example from Texas,USA

Adaptation and mitigation efforts related to global trends in climate and water scarcity must often be implemented at the local, single‐catchment scale. A key requirement is understanding the impact of local climate and watershed characteristics coupled with these regional trends. For surface water, determination of multi‐parameter runoff elasticities is a promising tool for achieving such understanding, as explored here for two surface‐water dependent basins in Texas. The first basin is the water supply for Dallas‐Ft. Worth (DFW), and exhibits relatively high precipitation elasticity (proportional change in runoff to change in precipitation) ε_P = 2.64, and temperature elasticity ε_T = ? 0.41. Standard precipitation–temperature elasticity diagrams exhibit unusual concave contours of runoff change, indicating influence of additional parameters, which can be isolated using multi‐parameter approaches. The most influential local parameter in DFW is unexpected reduced runoff fraction in cooler wetter years. Those years exhibit increased summer (JJA) precipitation fraction, but predominant cracking soils in DFW minimize JJA runoff, yielding negative . A comparative basin near Houston shows positive , reflecting the local impact of tropical cyclones and lesser abundance of cracking soils. Both basins exhibit positive elasticity to 1‐year previous precipitation (e.g. DFW ε_{P ? 1} = 1.24), reflecting the influence of soil moisture storage. Only DFW exhibits negative elasticity to 2‐year previous precipitation (ε_{P ? 2} = ? 0.65), reflecting multi‐year influence of vegetation growth and increased evapotranspiration. Using these elasticities, analysis of historical multi‐decadal climate departures for DFW indicates the 80% decrease in runoff during the 1950–1957 drought of record was primarily caused by reduced precipitation. Runoff 56% above‐normal during an unprecedented 1986–1998 wet period was primarily caused by increased precipitation. Since 2000, despite precipitation slightly above normal, runoff has decreased 20%, primarily in response to ～ 1^°C warming. Future precipitation droughts superimposed on this new drier normal are likely to be much more severe than historical experience would indicate. Copyright © 2014 John Wiley & Sons, Ltd.     

柴达木盆地北缘中新生代地层的磁组构特征及其沉积-构造学意义

柴达木盆地沉积地层记载着青藏高原东北部的构造演化信息.对该盆地路乐河地区上中生界-新生界地层系统采样,获得千余块定向岩心样品.岩石磁学研究表明样品中的磁性矿物主要为赤铁矿和磁铁矿;磁组构研究表明为初始沉积磁组构特征.磁组构特征指示了自中侏罗统大煤沟组（J₂d）至早中新统下油砂山组（N₂¹y）7个地层单位沉积时期,古水流方向共经历了4次阶段性的变化,表明柴达木块体相应地发生了4次旋转.在中-晚侏罗世块体逆时针旋转约22°;至早白垩世,块体又顺时针旋转约65°;在65.5~32 Ma期间块体旋转方向再次改变,逆时针旋转约63°;到32~13 Ma阶段块体又发生约50°的顺时针旋转.柴达木块体的旋转及其方向的转换,可能与其南的羌塘块体、拉萨块体和印度板块阶段性北向碰撞挤压紧密相关.拉张环境与挤压环境的多次转换可能与中特提斯的关闭、新特提斯的张开和闭合、高原快速隆升后其边部松弛相联系.

     

significance of high-resolution loess stratification based on grain size and magnetic susceptibility analysis to paleo-earthquake study: a case study of dongyugou loess section,at hongtong,shanxi province

As an important technology to paleoseismologic research, trenching has been used to identify paleo-earthquakes recorded in strata, combined with dating technology. However, there have been some bigger uncertainties and limitations. For instance, subtle strata in loess sediment cannot be interpreted only by naked-eye, which seriously affects identifying paleo-earthquake horizon and time. Therefore, how to improve the accuracy and reduce the uncertainty of paleo-earthquake identification is the important problem we are currently facing. Dongyugou loess section, located in the northeastern corner of Linfen Basin, Shanxi Province, cuts across the Huoshan piedmont fault. The section exposes not only the well-developed loess sequence, but also several obvious faulting events. Thus, this loess section is a better site to make a high resolution study to improve the accuracy and reduce the uncertainty of paleo-earthquake identification. Based on the high-resolution grain size and magnetic susceptibility analysis, and associated with visual interpretation by naked-eye, we made a high-resolution stratification of Dongyugou loess section, including high-resolution thickness of each stratum and its upper and bottom boundaries. Based on the high-resolution stratification and their comparison between two fault walls, we identified three earthquake events, which occurred after formation of u5-7, u4 and u2, corresponding to their stratification depth of 7.1m, 4.7m and 2.9m in hanging wall. Based on results of OSL dating and average sedimentation rate of hanging wall, we estimated that the three events occurred around 45.8ka(between (48.1±1.5)～(43.2±2.5)ka), 32.8ka(between (35.0±2.4)～(30.6±1.3)ka) and 23.3ka(between (26.4±0.8)～(20.9±0.7)ka). According to the thickness difference of three loess-paleosol sedimentary cycles between two fault walls, we calculated the coseismic vertical displacements of the three events as 0.5m, 0.4 and 1.3m, respectively. Compared with other segments of the Huoshan piedmont fault zone, we found the southernmost segment is the weakest, with longer recurrence interval of about 11ka and lower vertical slip rate of 0.048mm/a. The high-accuracy grain size and magnetic susceptibility analysis offers an effective method for reducing the uncertainties of the paleo-earthquake research in loess area.     

Causes and mobility of large volcanic landslides: application to Tenerife, Canary Islands

Giant volcanic landslides are one of the most hazardous geological processes due to their volume and velocity. Since the 1980 eruption and associated debris avalanche of Mount St. Helens hundreds of similar events have been recognised worldwide both on continental volcanoes and volcanic oceanic islands. However, the causes and mobility of these enormous mass movements remain unresolved. Tenerife exhibits three voluminous subaerial valleys and a wide offshore apron of landslide debris produced by recurrent flank failures with ages ranging from Upper Pliocene to Middle Pleistocene. We have selected the La Orotava landslide for analysis of its causes and mobility using a variety of simple numerical models. First, the causes of the landslide have been evaluated using Limit Equilibrium Method and 2D Finite Difference techniques. Conventional parameters including hydrostatic pore pressure and material strength properties, together with three external processes, dike intrusion, caldera collapse and seismicity, have been incorporated into the stability models. The results indicate that each of the external mechanism studied is capable of initiating slope failures. However, we propose that a combination of these processes may be the most probable cause for giant volcanic landslides. Second, we have analysed the runout distance of the landslide using a simple model treating both the subaerial and submarine parts of the sliding path. The effect of the friction coefficient, drag forces and hydroplaning has been incorporated into the model. The results indicate that hydroplaning particularly can significantly increase the mobility of the landslide, which may reach runout distances greater than 70 km. The models presented are not considered definite and have mainly a conceptual purpose. However, they provide a physical basis from which to better interpret these complex geologic phenomena and should be taken into account in the prediction of future events and the assessment of landslide related hazards.