Weakening climatic signal since mid-20th century in European larch tree-ring chronologies at different altitudes from the Adamello-Presanella Massif (Italian Alps)

Tree rings from temperature-limited environments are highly sensitive climate proxies, widely used to reconstruct past climate parameters for periods prior to the availability of instrumental data and to analyse the effect of recent global warming on tree growth. An analysis of the climatic signal in five high-elevation tree-ring width chronologies of European larch (Larix decidua Mill.) from the tops of five different glacial valleys in the Italian Central Alps revealed that they contain a strong summer-temperature signal and that tree-ring growth is especially influenced by June temperatures. However, a moving correlation function analysis revealed a recent loss of the June temperature signal in the tree-ring chronologies. This signal reduction primarily involves the two lowest-altitude chronologies. It is probable that the observed increasing importance of late-summer temperature for tree-ring growth over the past 50 yr is an effect of the lengthening growing season and of the variations in the climate/tree-ring relationship over time. All the chronologies considered, especially those at the highest altitudes, show an increasing negative influence of June precipitation on tree-ring growth. The climatic signal recorded in tree-ring chronologies from the Italian Central Alps varies over time and is also differentially influenced by climatic parameters according to site elevation.     

北极斯瓦尔巴、高亚洲和阿尔卑斯山冰川物质平衡对比研究

基于冰川物质平衡和平衡线高度数据,对北极斯瓦尔巴、高亚洲和阿尔卑斯山的冰川物质平衡变化和平衡线高度空间分布特征进行了对比分析,得出以下结论：（1）阿尔卑斯山冰川年均负物质平衡值最大,为-907 mm;斯瓦尔巴为-431 mm;高亚洲最小,为-264 mm。（2）高亚洲和斯瓦尔巴冰川物质平衡年振幅较小,年际变化较小;阿尔卑斯山冰川物质平衡年振幅较大,年际变化较大。斯瓦尔巴冰川物质平衡趋向正平衡,阿尔卑斯山和高亚洲冰川物质平衡趋向负平衡。（3）斯瓦尔巴内陆的冰川平衡线高度高于沿海地区,高亚洲冰川平衡线高度呈纬向地带性、经向地带性和区域地带性的分布规律,阿尔卑斯山的冰川平衡线高度主要受冰川所处海拔的影响。     

Post-collisional tectonics in the Northern Cottian Alps (Italian Western Alps)

Field mapping and structural analysis have allowed us to characterise the fault geometry and the post-metamorphic tectonics of an area located in the Northern Cottian Alps (inner Western Alps). Two main faulting stages were distinguished here. The first (Oligocene?-Early Miocene) is related to the development of an E–W-striking left-normal shear zone. This shear zone is interpreted as an antithetical of two regional, N–S right-lateral structures: the Col del Lis-Trana Deformation Zone (LTZ) and the Colle delle Finestre Deformation Zone (CFZ). The second faulting stage (post-Early Miocene) is related mainly to the development of N–S normal faults, coeval with the extensional reactivation of the LTZ and the CFZ. We discuss this kinematic evolution in the framework of the geodynamic evolution of the Western Alps.     

Dynamics and mass balance of the 2007 Cima Una rockfall (Eastern Alps, Italy)

On October 12th, 2007 about 40,000 m³ of dolomitic rock detached from the northern wall of the peak known as “Cima Una” (Val Fiscalina, Sesto Dolomites, Bolzano, Italy), and fell 900 m to Fiscalina Valley below. The event generated a dense dust cloud, which traveled up to 4 km from the source area. The failure surface was formed by two near-vertical surfaces, almost perpendicular to each other. The orientation of these surfaces is consistent with two of the main regional tectonic sets. Only a small portion of the fallen material appeared to be preserved as blocks deposited at the base of the rock wall. About a fifth of the fallen mass was deposited on a colluvial cone. The missing mass, estimated to be about 80 %, may be represented by highly fragmented rock in part deposited as sand on the valley floor and in part dispersed as a dense dust cloud generated during the rockfall. There appears to be a deficit of deposited material, which could lead underestimation in the calculation of rock–cliff recession rates. The dynamics of the rockfall, strongly conditioned by the local topography, partially explains the intense rock breakage and the generation of the dust cloud. The rockfall was not caused by an external trigger, such as an earthquake or heavy rainfall; the failure was most likely progressive due to mechanical and physical degradation along highly stressed failure surfaces, possibly promoted by permafrost degradation and freeze and thaw processes.     

青藏高原东南部海洋型冰川物质平衡研究进展北大核心CSCD

青藏东南部海洋型冰川具有独特的气候敏感性,普遍呈现加速退缩趋势,这不仅影响区域水资源安全,而且伴生了相应的冰川灾害,是当前青藏高原冰冻圈变化研究的热点区域之一。本文对海洋型冰川物质平衡时空变化特征进行了综述,2000年以来冰川总体处于物质亏损状态,其平均物质平衡介于-0.66~-0.61m w.e.·a^(-1)之间;同时总结了海洋型冰川物质加速变化的驱动因素以及新特征。当前海洋型冰川物质平衡变化研究受观测数据缺乏和模型模拟不确定性等问题限制,尤其现有模型对冰面裂隙增多与扩张、冰崖-冰面湖-表碛相互作用、冰内冰下过程、冰崩、末端冰湖水-冰相互作用等过程的描述过于简化或基本缺失,其机理及影响仍存在较大的不确定性。未来需加强海洋型冰川物质平衡的综合监测,基于多数据和多方法的集成研究提高模型对冰川物质平衡多物理过程的耦合与模拟能力,为开展海洋型冰川物质变化的区域水资源效应和致灾效应研究奠定基础。     

2000—2020年兴都库什东部冰川区物质平衡变化及其影响因素

利用SRTM DEM和ASTER立体像对数据获取的DEM分析了2000—2020年兴都库什东部的冰川物质平衡,并结合CRU TS 4.04气象数据探讨了气温、降水、地形和冰湖对南、北冰川区物质平衡空间差异的影响。结果表明：2000—2020年兴都库什东部冰川区物质平衡为（-0.02±0.04） m w.e.·a^-1,冰川整体呈现微弱的负物质平衡状态。从坡向来看,南坡以正物质平衡冰川居多,北坡以负物质平衡冰川居多。从南、北两个子区域来看,北部冰川区物质平衡为（0.07±0.04） m w.e.·a^-1,南部冰川区物质平衡为（-0.32±0.04） m w.e.·a^-1。北部冰川面积规模大,所处海拔区间高,南部则相反。北部冰川区处于较高的海拔区间且冬季气温较低,导致夏季升温所产生的冰川消融的影响被削弱,冰川物质平衡的分布与降水分布在空间上具有一致性。南部冰川区出现的强烈物质亏损主要是由于夏季气温的急剧升高和冰川处于较低的海拔区间。南、北区域冰前湖和冰面湖面积不断扩大的空间差异性,也在一定程度上加剧了该地区冰川物质平衡的空间差异。     

Glaucophanites and eclogites from Val Chiusella,Sesia-Lanzo zone (Italian Alps)

In the metabasites of Val Chiusella, metamorphic assemblages are present, corresponding to the glaucophane schist facies, i.e. garnet glaucophanites to omphacite-garnet glaucophanites, as well as to the eclogite facies, i.e., glaucophane eclogites, eclogites, and omphacite felses. Both groups of assemblages are divided by the critical reaction 1 zoisite +1 glaucophane 1.2 omphacite+0.8 garnet+0.7 paragonite +1.4 quartz+0.8 H₂O. From textural evidence it is clear that in the investigated area this reaction proceeded to the right according to a prograde metamorphism. Correspondingly, K ^garn-cpx _D(Fe/Mg) values of coexisting garnet-omphacite pairs in the glaucophane schist facies assemblages are higher than in the eclogite facies assemblages and reflect a temperature increase from about 450 ° C to about 550 ° C at minimum water vapour pressures of 12 to 16 kb.     

Reaction Textures and Metamorphic Evolution of Sapphirine-bearing Granulites from the Gruf Complex, Italian Central Alps

Mineral chemistries and textures are described from a suiteof sapphirine-bearing granulites from the Gruf Complex of theItalian Central Alps. The granulites contain combinations ofgarnet, orthopyroxene, sapphirine, sillimanite, cordierite,biotite, quartz, spinel, corundum, staurolite, plagioclase,K-feldspar, ilmenite and rutile, in assemblages with low (usuallynegative) variance. They are outstanding in that they preservea textural and chemical record of a protracted metamorphic evolution. Reaction textures are common and include: (i) pseudomorphs (e.g.of sillimanite after kyanite); (ii) relatively coarse-grainedmonomineralic reaction rims (e.g. of cordierite between sapphirineand quartz); (iii) fine-grained symplectitic coronas (e.g. oforthopyroxene + sapphirine round garnet); (iv) inclusions, ingarnet cores, of minerals (e.g. staurolite) not found elsewherein the rocks. Detailed microprobe study has revealed large chemical variationswithin each phase. Different textural types of each phase havedifferent compositions, and strong zoning is preserved in garnet(Mg/(Mg + Fe) from 0.30 to 0.61) and coarse sapphirine. Inclusionpopulations in garnet correlate with host composition. The textural and chemical features are interpreted in termsof successive equilibrium assemblages and reactions. Metamorphicconditions operative at each stage in the evolution are calculatedusing published geothermometers and geobarometers as well asthermodynamically calibrated MAS and FASH equilibria. The resultsare used to construct a P—T-time path for the sapphirine-granulites,which can be summarized as follows: (i) Increasing T at high P (>7 kb). Partial melting. (ii) A maximum T of 830 ?C attained at 10 kb. (iii) Almost isothermal decompression, reaching 750 ?C at 5kb, under conditions of low µ_H2O. (iv) Further cooling, and decompression. Localized hydration.Rocks exposed. The P—T-time path is interpreted as the product of a singlemetamorphic cycle (the tertiary ‘Lepontine’ event)and is extrapolated to the Gruf Complex as a whole. When combinedwith published geochronological data, the results indicate anaverage uplift rate in excess of 2 mm/yr for the Gruf Complexbetween 38 and 30 Ma ago. An in situ partial melting origin for the sapphirine-granulitesis favoured. Extraction of an iron-rich granitic liquid froma normal pelitic palaeosome could generate a refractory residuewith the required Mg, Al-rich composition. The change in bulksolid composition during partial melting is thought to accountfor the extraordinarity strong zoning in the garnets.     

On the cause of glacier mass balance variations in the Tian Shan Mountains

Over the period of more than 30 years the mass balance observations have been carried on the several representative glaciers of the Tien Shan. But these data are not sufficient to explain the asynchronous degradation of the glaciation in different parts of the mountains. Special field observations were undertaken to study asynchronous changes in mass balance. The programme used an imitation model of the mass balance monitoring (Dyurgerov 1988). According to the model the annual values of glacier mass balance b_n and equilibrium line altitude ELA_n were replaced by the current values of b_t and ELA_t measured during the ablation season. The imitation model was evaluated on Tuyuksu Glacier in 1987–1989. The results showed that the function b_n(ELA_n) can be replaced by b_t(ELA_t) which is nonlinear and can be approximated by the hypsographic curve of the glacier. After these tests the similar measurements were accomplished in summer of 1989 on the Tuyuksu Glacier, Sary-Tor and Glacier No. 1. It was established that the asynchronous changes in mass balance were primarily due to the increase of summer snow accumulation eastward the Tien Shan. The second reason was the difference in the altitudinal position of the glaciers.     

Zircon inheritance in an igneous rock suite from the southern Adamello batholith (Italian Alps)

Zircons from a suite of basic to acidic calcalkaline igneous rocks from the southern Adamello batholith (S AB), Southern Alps, N Italy, display complex U–Pb isotopic patterns which are mainly due to the presence of variable amounts of isotopically heterogeneous, inherited radiogenic Pb, and to minor postmagmatic loss of Pb. Inherited Pb is mainly composed of 1) a 1100 Ma Pb component located in zircons devoid of visible cores and 2) a 450 Ma component associated with conspicuous bubble-rich turbid cores. In concordia representation the data points conform to lower intercept ages of 40 Ma. A linear fit of three samples devoid of visible cores from the granodiorite defines an intrusion age of 39.3 Ma. U–Pb systematics of zircon (in particular U content) and crystal morphology are clearly related. Zircons of type G1, which form relatively late in the zircon crystallization sequence, consistently show the highest U contents in each zircon population. These late zircons, however, are not devoid of inherited radiogenic lead. In a population from a granodiorite, cores are randomly distributed throughout the morphological spectrum. The presence of old inherited zircon components in all investigated samples furnishes proof for involvement of crustal material in the genesis of the S AB rocks. Samples characterized by crustal _Nd and _Sr values usually show enhanced zircon inheritance. Inheritance varies with differentiation and reaches a maximum for intermediate to acidic members; these compositions show the highest Zr saturation temperatures calculated for the rock spectrum studied. Textural relations between zircon and major phases indicate that the magmas of the leucocratic rocks were saturated with Zr at an early stage of crystallization. On the other hand, Zr solubilities and textural relations consistently show, that melts of basic to intermediate rocks were not saturated with Zr. Extension of the Zr solubility model to mineral/melt mixtures of tonalitic bulk composition demonstrates that Zr solubility in the residual melt is drastically reduced by crystallization of plagioclase and amphibole. Survival of xenocrystic zircons in the mafic to intermediate rocks of the S AB can best be explained in terms of dissolution kinetics. Since temperature and H₂O content of these Zr-undersaturated melts were favourable for relatively rapid zircon dissolution, inherited zircons (in particular trace-element rich unstable cores) cannot have been exposed to such conditions over extended time periods. Therefore, the tonalitic or more basic magmas of the S AB cannot have been derived from crustal sources by slow processes such as burial metamorphism.     

Dynamics of an active rock glacier (Ötztal Alps, Austria)

The rock glacier Innere Ölgrube, located in a small side valley of the Kauner Valley (Ötztal Alps, Austria), consists of two separate, tongue-shaped rock glaciers lying next to each other. Investigations indicate that both rock glaciers contain a core of massive ice. During winter, the temperature at the base of the snow cover (BTS) is significantly lower at the active rock glacier than on permafrost-free ground adjacent to the rock glacier. Discharge is characterized by strong seasonal and diurnal variations, and is strongly controlled by the local weather conditions. Water temperature of the rock glacier springs remains constantly low, mostly below 1°C during the whole melt season. The morphology of the rock glaciers and the presence of meltwater lakes in their rooting zones as well as the high surface flow velocities of >1 m/yr point to a glacial origin. The northern rock glacier, which is bounded by lateral moraines, evolved from the debris-covered tongue of a small glacier of the Little Ice Age with its last highstand around A.D. 1850. Due to the global warming in the following decades, the upper parts of the steep and debris-free ice glacier melted, whereas the debris-covered glacier tongue transformed into an active rock glacier. Due to this evolution and due to the downslope movement, the northern rock glacier, although still active, at present is cut off from its ice and debris supply. The southern rock glacier has developed approximately during the same period from a debris-covered cirque glacier at the foot of the Wannetspitze massif.     

Geochemistry of micas from the Finero spinel-lherzolite,Italian Alps

The Finero lherzolite is distinct amongst the tectonically emplaced slices of mantle in the Ivrea Zone (Italian Alps) for its abundant coarse phlogopite. An average composition (SiO₂ 39.9, TiO₂ 0.97, Al₂O₃ 16.0, Cr₂O₃ 1.16, FeO 2.73, MgO 24.5, NiO 0.16, BaO 0.31, Na₂O 0.58, K₂O 8.7, Rb₂O 0.056, Cl 0.03, F 0.10 wt.%) is similar in Fe, Cr, Ni, Ba and F/Cl to primary-textured micas from coarse garnet-lherzolite xenoliths from S. Africa, but is higher in Ti, Na, Rb, and Al, and lower in halogens. The distinct values of Ti and Fe for five specimens of Finero peridotites demonstrate local spatial variation. The overall ranges of TiO₂ (0.5–1.7) and FeO (2.3–3.6) fall within the range for secondary-textured micas in peridotite xenoliths from S. Africa. The Finero micas are lower in both K/Rb and K/Ba than the primary and secondary micas from S. Africa, and their mean values of K/Rb (110–220) and K/Ba (15–39) are lower than for almost all bulk rocks, but fit well with the ranges of 109–180 and 12–49 for the high-K lavas of the Roman region.Although all evidence is indicative rather than conclusive, the chemical properties of the Finero micas are consistent with introduction of an alkaline phase into peridotite during or before emplacement of the Finero complex from the upper mantle into the crust, and the coarse, partly-deformed textures can be explained by incomplete metamorphic equilibration during prolonged deformation. The alkaline phase is tentatively attributed to the uppermost mantle.     

Triggering processes of deep-seated gravitational slope deformation (DSGSD) in an un-glaciated area of the Cavargna Valley (Central Southern Alps) during the Middle Holocene

Triggering mechanisms and causative processes of deep-seated gravitational slope deformations (DSGSD) in Alpine and high mountain areas include, among others, post-glacial de-buttressing, earthquake-induced ground shaking, or co-seismic surface faulting. Distinguishing between climatic or tectonically driven factors is challenging since faults and fracture systems can play both an active and/or passive role in the process initiation. We applied an integrated approach, including morpho-structural analysis, geologic field survey, a paleoseismological approach applied to trenching, radiocarbon dating and detailed sedimentological analysis, to a DSGSD located in the Cavargna Valley (N Italy), an area that was never occupied by extensive ice tongues in the Upper Pleistocene and Holocene. We were able to document at least two phases of movement from the Middle Holocene onwards and related triggering factors. Thanks to the comparison with dated lacustrine turbidites in the Alpine Lakes, we argue a seismic triggering for the landslide onset, with a possible source located in southern Switzerland, close to the Insubric Line. The later evolution of the landslide was instead driven by climatic predisposing conditions (humid and cold), instead, corresponding to a phase of enhanced slope instability.

     

Contrasting thermomechanical evolutions in the Southalpine metamorphic basement of the Orobic Alps (Central Alps, Italy)

In the Southern Alps a progressive metamorphic zonation, with an increase in the geothermal gradient from NE to SW, has been widely proposed. However, recent investigations have shown that the greenschist metamorphic imprint of the low-grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions. On the other hand, in the medium-grade zone, a later low-pressure, high-temperature (LPHT) metamorphic event has also been proposed. In an attempt to resolve these different interpretations, new petrological and partly new structural data have been obtained for two sectors of the Orobic Alps, traditionally attributed to different metamorphic zones. Thermobarometric determinations, supported by microstructural analysis, indicate the following different pressure-retrograde paths in each sector: (1) in the Val Vedello basement (VVB) rocks, a first metamorphic imprint characterized by P = 7–9 kbar and T = 570–610°C was followed by a greenschist retrogression ( P ≤ 4 kbar and T ≤ 500° C); (2) in the Lario basement (LB) rocks, the first detectable metamorphic stage, characterized by mineral assemblages indicating P = 7–9 kbar and T = 550–630° C, was followed by a LPHT event, synkinematic with F2 extensional deformation. A greenschist retrogression marks the final uplift of these rocks.
Reinterpretation of the available geochronological data indicates a diachronism for the two thermomechanical evolutions. In the light of these data, we interpret the retrograde P–T–t path of the VVB rocks as a pre-Permian post-thickening uplift and the retrograde P–T–t evolution of the LB rocks as a Permo-Mesozoic uplift related to the extensional tectonic regime of the Tethyan rifting.     

Kinematics of the northern part of the Simplon line (Central Alps)

Structural studies were carried out along the northern part of the Simplon line in order to constrain the kinematic history of the line during Tertiary times. The ductile strain accumulated within both basement units and Mesozoic cover reveals the upper units sheared towards the west-southwest. The Simplon line can be described as a major shear zone along which the main movement occurred perpendicular to the line. This shearing developed during retrograde metamorphic conditions and postdates earlier deformations associated with the high temperature Lepontine episode. Finally, it was followed by the formation of the Berisal syncline (back-folding episode) which generated some brittle movements towards the south, localized at the contact between the lower and upper Pennine units (i.e. on the Simplon line).     

Spatio-temporal variability in debris-flow activity: a tree-ring study at Geisstriftbach (Swiss Alps) extending back to AD 1736

Past debris-flow activity on the forested cone of the Geisstriftbach torrent (St. Niklaus, Valais, Swiss Alps) was assessed from growth disturbances in old conifer trees, providing a much improved record of past events. The study of 633 tree-ring sequences sampled from 252 European larch (Larix decidua Mill.), Norway spruce (Picea abies (L.) Karst.) and Silver birch (Betula pendula Roth.) trees allowed reconstruction of 53 debris-flow events since AD 1736. The spatial analysis of trees affected during particular events on the geomorphic map allowed for a spatial representation of individual events and a reconstruction of four flow patterns. Based on our results and Siegfried maps, we believe that before the formation of a dogleg near the cone apex in the late 1890s, debris flows preferentially used the channels located in the west-southwestern part of the Geisstriftbach cone. This study contributes to our understanding of debris-flow processes on cones and provides an example of how dendrogeomorphic techniques may help in the reconstruction and understanding of debris flows in Alpine areas.     

Metasomatism of continental crust during subduction: the UHP whiteschists from the Southern Dora-Maira Massif (Italian Western Alps)

The ultrahigh‐pressure pyrope whiteschists from the Brossasco‐Isasca Unit of the Southern Dora‐Maira Massif represent metasomatic rocks originated at the expense of post‐Variscan granitoids by the influx of fluids along shear zones. In this study, geochemical, petrological and fluid‐inclusion data, correlated with different generations of pyrope‐rich garnet (from medium, to very‐coarse‐grained in size) allow constraints to be placed on the relative timing of metasomatism and sources of the metasomatic fluid. Geochemical investigations reveal that whiteschists are strongly enriched in Mg and depleted in Na, K, Ca and LILE (Cs, Pb, Rb, Sr, Ba) with respect to the metagranite. Three generations of pyrope, with different composition and mineral inclusions, have been distinguished: (i) the prograde Prp I, which constitutes the large core of megablasts and the small core of porphyroblasts; (ii) the peak Prp II, which constitutes the inner rim of megablasts and porphyroblasts and the core of small neoblasts; and (iii) the early retrograde Prp III, which locally constitutes an outer rim. Two generations of fluid inclusions have been recognized: (i) primary fluid inclusions in prograde kyanite that represent a NaCl‐MgCl₂‐rich brine (6–28 wt% NaCl_eq with Si and Al as other dissolved cations) trapped during growth of Prp I (type‐I fluid); (ii) primary multiphase‐solid inclusions in Prp II that are remnants of an alumino‐silicate aqueous solution, containing Mg, Fe, alkalies, Ca and subordinate P, Cl, S, CO₃^2‐, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), U and Th (type‐II fluid), at the peak pressure stage. We propose a model that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists and that could also explain the genesis of other Mg‐rich, alkali‐poor schists of the Alps. During Alpine metamorphism, the post‐Variscan metagranite of the Brossasco‐Isasca Unit experienced a prograde metamorphism at HP conditions (stage A: ～1.6 GPa and ≤ 600 °C), as indicated by the growth of an almandine‐rich garnet in some xenoliths. At stage B (1.7–2.1 GPa and 560–590 °C), the influx of external fluids, originated from antigorite breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies and Ca in the orthogneiss toward a whiteschist composition. During stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), the metasomatic fluid influx coupled with internal dehydration reactions involving Mg‐chlorite promoted the growth of Prp I in the presence of the type‐I MgCl₂‐brine. At the metamorphic peak (stage D: 4.0–4.3 GPa and 730 °C), Prp II growth occurred in the presence of a type–II alumino‐silicate aqueous solution, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligible. This fluid, showing enrichment in LILE and depletion in HFSE, could represent a metasomatic agent for the supra‐subduction mantle wedge.