Lake Torfadalsvatn: a high resolution record of the North Atlantic ash zone I and the last glacial-interglacial environmental changes in Iceland

High resolution environmental records with a refined chronology are essential to understand, reconstruct and model the climate dynamics of the last glacial-interglacial transition. Sediments from Lake Torfadalsvatn in northern Iceland contain at least four primary volcanic tephras that belong to ash zone I in the North Atlantic deep-sea cores. We chemically define these basaltic/rhyolitic tephras and the high resolution allows us to date them to about 10,800, 10,600, 9300 and 8900 BP. This detailed tephrostratigraphy will act as a refined dating and correlation tool in the North Atlantic region and enable calibration between different absolute chronologies. The pollen stratigraphy of the sediments suggests that by 10,400 ¹⁴C years BP plant colonization of coastal north Iceland had begun. The pollen stratigraphy shows a succession of pioneer plants, from open tundra vegetation towards birch-juniper woodland, which probably also reflects a transition from a cool climate at 10,400 BP to conditions similar to today's sub-polar oceanic climate around 9200 BP. Diatom data largely concur with the climatic information from pollen, indicating gradually increasing productivity in the lake.     

Tectonic analysis of an oceanic transform fault zone based on fault-slip data and earthquake focal mechanisms: the Húsavík–Flatey Fault zone, Iceland

The Húsavík–Flatey Fault (HFF) is an oblique dextral transform fault, part of the Tjörnes Fracture Zone (TFZ), that connects the North Volcanic Zone of Iceland and the Kolbeinsey Ridge. We carry out stress inversion to reconstruct the paleostress fields and present-day stress fields along the Húsavík–Flatey Fault, analysing 2700 brittle tectonic data measured on the field and about 700 earthquake focal mechanisms calculated by the Icelandic Meteorological Office. This allows us to discuss the Latest Cenozoic finite deformations (from the tectonic data) as well as the present-day deformations (from the earthquake mechanisms). In both these cases, different tectonic groups are reconstructed and each of them includes several distinct stress states characterised by normal or strike-slip faulting. The stress states of a same tectonic group are related through stress permutations (σ₁−σ₂ and σ₂−σ₃ permutations as well as σ₁−σ₃ reversals). They do not reflect separate tectonic episodes. The tectonic groups derived from the geological data and the earthquake data have striking similarity and are considered to be related. The obliquity of the Húsavík–Flatey Fault implies geometric accommodation in the transform zone, resulting mainly from a dextral transtension along an ENE–WSW trend. This overall mechanism is subject to slip partitioning into two stress states: a Húsavík–Flatey Fault-perpendicular, NE–SW trending extension and a Húsavík–Flatey Fault-parallel, NW–SE trending extension. These three regimes occur in various local tectonic successions and not as a regional definite succession of tectonic events. The largest magnitude earthquakes reveal a regional stress field tightly related to the transform motion, whereas the lowest magnitude earthquakes depend on the local stress fields. The field data also reveal an early extension trending similar to the spreading vector. The focal mechanism data do not reflect this extension, which occurred earlier in the evolution of the HFF and is interpreted as a stage of structural development dominated by the rifting process.     

Tectonics and metallogeny of the junction zone between the North Asian craton and Pacific tectonic belt

The tectonics and metallogeny of the junction zone between the North Asian craton and Pacific tectonic belt are considered. This zone is characterized by a wide variety of structures superposed on the metamorphic basement, which was formed in the course of a multistage geologic development of the craton from the Precambrian to the Cenozoic. They are related to the craton evolution and its response to the collision and subduction processes in the adjacent orogenic belt, processes in the passive and active continental margins, and plume magmatism. The geological structure of the region includes blocks of metamorphic rocks of the Aldan–Stanovoi shield, Paleoproterozoic volcanogenic troughs, Mesoproterozoic–Neoproterozoic and Early Paleozoic structures of the platform cover, Late Paleozoic volcanic and terrigenous troughs, structures of the Late Mesozoic Okhotsk–Chukotka volcanic belt of the active continental margin, and Late Cretaceous riftogenic structures formed in response to plume magmatism. In total, six metallogenic epochs are recognized in the development of ore mineralization: Archean–Early Paleoproterozoic, Late Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Late Paleozoic, and Late Mesozoic. The minerageny of the junction zone between the craton and Pacific belt is highly diversified, being characterized by distinct evolution in time and space. Each development stage features its own set of mineral resources.     

The Skógar Tephra, a Younger Dryas marker in North Iceland

A composite stratigraphical sequence, the Fnjóskadalur Sequence, reveals ten cycles of glacier advances and formation of ice-dammed lakes in Fnjóskadalur in central North Iceland. Chemical analyses of the Skógar Tephra, with its type locality in this valley, have enabled a correlation with Ash zone I in deep sea sediments of the North Atlantic and with the Vedde Ash Bed on land in western Norway, where it is dated to 10,600 BP. The Skógar Tephra is composed of two layers, a basaltic tephra (STP-1) and a rhyolitic tephra (STP-2) erupted almost simultaneously from two different Icelandic volcanoes. The STP-1 tephra originates from the Katla volcano in South Iceland, and the öræfajökull volcano in Southeast Iceland is considered a plausible source of the STP-2 tephra. This new dating of the Skógar Tephra puts the three youngest glacier advances of the Fnjóskadalur Sequence within a 1000 year period between 10,600 and 9650 BP. The redated Late Weichselian glacial history now extracted from the Fnjóskadalur Sequence shows that glaciers in North Iceland were more extended in Younger Dryas and Preboreal times than previously assumed. This fits with the revised deglaciation pattern which has evolved in recent years.     

Tjörnes fracture zone. New and old seismic evidences for the link between the North Iceland rift zone and the Mid-Atlantic ridge

The Tjörnes facture zone (TFZ) connects the EW extension of the Mid-Atlantic ridge north of Iceland to the extension of the North volcanic zone (NVZ) of Iceland. Earthquakes up to magnitude 7 (Ms) can occur in TFZ, volcanic eruptions have been observed and large crustal deformations are expected in similar way as have been observed in the NVZ. Most of the zone is below ocean, which limits the historical information and geological observations. For studying the dynamics of the zone we must rely on interpretation and modelling based on seismic observations, especially on microearthquake observations for the last 10 years. In this paper we demonstrate how microearthquakes can be applied to map the details of the plate boundary, and how this information can be applied to find epicenters and fault planes of large historical earthquakes, also how seismic information can be applied in dynamic modelling and to infer spatial and temporal interplay in activity, and to enhance hazard assessment.     

Tectonic evolution of the Iceland region,North Atlantic

Major hypotheses on the formation of the Iceland region are considered. It is noted that plate- and plume-tectonic genesis is the most substantiated hypothesis for this region. Model estimations of the effect of hot plume on the formation of genetically different oceanic ridges are obtained. Computer calculations are performed for the thermal subsidence rate of aseismic ridges (Ninetyeast and Hawaiian-Emperor) in the asthenosphere of the Indian and Pacific oceans. Comparative analysis of the calculated subsidence rates of these ridges with those in the Iceland region (Reykjanes and Kolbeinsey ridges) is performed. The results suggest that the thermophysical processes of formation of the spreading Reykjanes and Kolbeinsey ridges were similar to those of the aseismic Ninetyeast and Hawaiian-Emperor ridges: the genesis of all these ridges is related to the functioning of a hotspot. Analysis of the heat flux distribution in the Iceland Island and Hawaiian Rise areas is carried out. Analysis and numerical calculations indicate that the genesis of Iceland was initially characterized by the plume-tectonic transformation of a continental rather than oceanic lithosphere. The level of geothermal regime near Iceland was two times higher (100 mW/m²) relative to the Hawaiian Rise area (50 mW/m²) because the average lithosphere thickness of the Reykjanes and Kolbeinsey ridges near the Iceland was approximately two times less (40 km) relative to the thickness of the Pacific Plate (80 km) in the Hawaiian area. The main stages of evolution of the Iceland region are based on geological and geothermal data and numerical thermophysical modeling. The Cenozoic tectonic evolution of the region is considered. Paleogeodynamic reconstructions of the North Atlantic in the hotspot system at 60, 50, and 20 Ma are obtained.     

明渠交汇口水流分离区研究

支流汇入主流后,在明渠交汇口下游附近形成分离区,造成过流断面束窄.采用水力学基本理论及试验手段,对等宽明渠交汇口水流分离区的形状及尺寸进行了理论分析和试验研究,提出了分离区收缩系数及能量损失系数的理论表达式并与试验结果进行了比较.研究结果表明,随交汇角或主支流流量相对大小不同,分离区尺寸发生变化;对于给定交汇口几何形状及尺寸,分离区形状基本保持不变,但分离区尺寸随流量比变化并具有较好的相关关系,并随交汇角增大呈有规律增加.     

Holocene vertical deformation outside the active rift zone of north Iceland

In Iceland, the main deformation activity occurs not only in the active rift zone where volcanism and seismicity coexist in fissure swarms but also in the transfer zones (north and south) where the main part of the seismic activity is located. Many studies described extension and vertical displacement associated with rifting and magmatism inside this rift zone. They have classically assumed that no deformation occurs outside the rift zone. However, few studies have already been conducted outside the rift zone. This work focuses on an area west of the rift zone in the Akureyri area, which displays an anomalous topography. We illustrate an ENE tilting of 0.23° since approximately 17,000 years ago using ancient lacustrine shorelines mapped in detail with a differential GPS survey and analysis of SPOT images. This degree of tilting corresponds to a vertical displacement of 160 m, i.e., a deformation rate of 1 cm/year, which is close to values measured in the active zone. This long wavelength deformation is compatible both with (i) the topographic anomaly of the Akureyri area, and with (ii) the very diffuse microseismicity that affects northern Iceland. The origin of this tilting is discussed and the different hypotheses (glacial loading response, rift uplift, transform zone effect, thermal doming, local magmatic intrusion) are reviewed. Our results, structural data and the topography are more consistent with a local magmatic intrusion inducing local doming. This work is the first attempt at quantifying long-term deformation outside the rift zone in Iceland.     

柴达木盆地北缘东段察汗河右行转换挤压剪切带的变形样式及构造意义

转换挤压剪切带普遍发育在汇聚板块边界及碰撞造山带中，对调节造山过程中的增生、碰撞及物质侧向挤出等起重要作用。位于青藏高原东北缘的柴达木盆地北缘构造带（柴北缘构造带）被认为是早古生代原特提斯洋闭合、大陆深俯冲、陆- 陆碰撞和造山后伸展垮塌作用的产物。最近，在柴北缘构造带东段乌兰北部察汗河地区，笔者新厘定出NWW—SEE向展布的右行转换挤压韧性剪切带。本文通过对该剪切带内宏微观构造特征、石英c轴组构、运动学涡度等研究，结合锆石U- Pb年代学数据，来探该讨剪切带的转换挤压样式及对柴达木盆地北缘早古生代造山作用的启示意义。察汗河韧性剪切带内XZ面上发育的宏微观构造以及石英c轴组构共同指示了右行走滑剪切指向，结合糜棱岩的轻微压扁—压扁的应变椭球体形态，共同指示了其具有转换挤压的构造性质；估算的平均运动学涡度限定了其转换挤压变形样式。根据石英颗粒的重结晶机制、c轴组构滑移系以及其开角温度计，确定其变形温度为500～553℃，指示了中地壳变形层次，并结合剪切带内石英动态重结晶颗粒大小，估算其差异应力为28. 5～30. 0 MPa，古应变速率为10-12/s。通过锆石U- Pb定年，获得卷入韧性剪切变形的闪长岩时代为432±3 Ma，结合柴达木盆地北缘已有区域地质资料，推断韧性剪切作用的活动时代为中志留世—早泥盆世（432～396 Ma）。以上资料显示，察汗河韧性剪切带表现出垂直于造山带的水平收缩和平行于造山带的侧向挤出的构造变形样式，伴随着部分由北向南的逆冲分量，形成于柴达木盆地北缘早古生代晚期的碰撞造山阶段，并指示了其造山作用具有斜向汇聚及碰撞特征。     

An Unusual Enstatite-Forsterite Basalt from Kolbeinsey Island, North of Iceland

This is the first account of volcanic rocks erupted from a northernextension of the MidAtlantic Ridge to a locality between Icelandand Jan Mayen. The islet of Kolbeinsey (67° 08' N., 18°36' W.) is being rapidly eroded and now measures 52x36 m, andreaches only to 7.5m above sea level. Two identical specimensof vesicular basalt were collected by the Icelandic Coastguardvessel Aegir in 1962, and have now been chemically analysed.The mineral compositions were determined by electron microprobeanalysis. The rock carries micro-phenocrysts of highly magnesianolivine (Fo_{98 8}) and of plagioclase (An₈₅ to An₃₀). Brown andlemon-yellow grains of augite (Wo₄₂ En₄₅ Fs₁₃) are accompaniedby pale-yellow, euhedral to subhedral, groundmass orthopyroxenesof highly magnesian composition (Wo₄ En₈₈ Fs₈) and with a calciumcontent higher than found previously in enstatites. The chemistryof the basalt, also, is unusual in showing Fe₂O₃ = 8.98 percent and FeO = 0.38 per cent. The high state of oxidation isconsidered in relation to the production of forsterite and enstatitefrom basalt magma. Historic submarine volcanism in the regionis discussed in relation to bathymetric and geophysical data.     

A prediction of minimum age for the Weichselian maximum glaciation in North Iceland

The Weichselian glaciation in Norht lceland is locally divided into three main stages:(1) The maximum stage, when North Iceland was ice-covered northwards to the island of Grimsey; (2) the ice-lake stage, when a series of ice-dammed lakes were formed in Fijóskadalur; and (3) the Langhöll Stadial, ¹⁴C age about 10,000 B.P., an advance restricted to the valleys on both sides of Eyjafjördur, after the final emptying of the youngest lake in Fnjöskadalur, By combining changes in strandine gradients with time, an age of about 20,700 B.P. for the oldest ice-dammed lake is predicted. As this a ge is greater than the assumed age, 18,000 B.P., of the maximum extent of the Weichselian glaciation, it is unlikely that the maximum occurred at that time. Possibly, the maximum extent of the Weichselian glaciation in North lceland took place concurrenly with some of the early s tadials that have been identified in Arctic Canada, in East Greenland and on Svalbard.     

Strain analysis of a shear zone in a granodiorite

A ductile shear zone in a late Precambrian granodiorite, from the Rouergue (southwest part of the French Massif Central) has been studied.A single episode of deformation is responsible for the formation of a foliation and a well-defined lineation which are localized into an elongated zone, a few decimeters wide.The strain features can be attributed to a simple-shear mechanism (Ramsay and Graham, 1970), so that the main parameters of the deformation are defined.At stages of increasing deformation, the quartz isotropic sub-fabric of the undeformed host rock is progressively transformed into an anisotropic fabric composed of a single oblique girdle while the subgrain size progressively decrease and the dislocation density remains constant. It is suggested that the gliding planes of quartz are the basal plane (0001) and a predominant prismatic plane 101̄0 the slip directions may be a for both glide-planes.The results obtained in this investigation provide a basis for a high voltage electron microscope (H.V.E.M.) study which shows that the fabrics development may be related to dislocation processes. The difference of strain rates in the host rock and in the shear zone is calculated from the dislocation microstructures.     

Immature and mature transform zones near a hot spot: The South Iceland Seismic Zone and the Tjörnes Fracture Zone (Iceland)

We describe and compare the two transform zones that connect the Icelandic rift segments and the mid-Atlantic Ridge close to the Icelandic hot spot, in terms of geometry of faulting and stress fields. The E–W trending South Iceland Seismic Zone is a diffuse shear zone with a Riedel fault pattern including N0°–N20°E trending right-lateral and N60°–N70°E trending left-lateral faults. The dominant stress field in this zone is characterised by NW–SE extension, in general agreement with left-lateral transform motion. The Tjörnes Fracture Zone includes three major lineaments at different stages of development. The most developed, the Húsavík–Flatey Fault, presents a relatively simple geometry with a major fault that trends ESE–WNW. The stress pattern is however complex, with two dominant directions of extension, E–W and NE–SW on average. Both these extensions are compatible with the right-lateral transform motion and reveal different behaviours in terms of coupling. Transform motion has unambiguous fault expression along a mature zone, a situation close to that of the Tjörnes Fracture Zone. In contrast, transform motion along the immature South Iceland Seismic Zone is expressed through a more complicate structural pattern. At the early stage of the transform process, relatively simple stress patterns prevail, with a single dominant stress field, whereas, when the transform zone is mature, moderate and low coupling situations may alternate, as a function of volcanic–tectonic crises and induce changes in stress orientation.     

北秦岭二郎坪岩群两侧韧性剪切带的构造变形及其意义

北秦岭二郎坪岩群南、北两侧分别被朱夏韧性剪切带和瓦乔韧性剪切带与秦岭岩群和宽坪岩群分开,这两条韧性剪切带对二郎坪弧后盆地的演化起着十分重要的作用。本文对这两条剪切带进行了详细的几何学、运动学和⁴⁰Ar-³⁹Ar年代学研究。几何学和运动学分析结果指示瓦乔剪切带具有由北向南逆冲剪切的运动学特征,而朱夏剪切带早期具有由南向北逆冲的运动学特征,而后期又发生右行走滑活动。对瓦乔剪切带和朱夏剪切带内的糜棱岩中白云母进行了⁴⁰Ar-³⁹Ar法定年,结果指示瓦乔剪切带逆冲活动发生在387±1.7 Ma,朱夏韧性剪切早期逆冲发生在晚古生代,后期右行走滑的年龄为146±2.8 Ma。综合两条剪切带的构造变形特征和年代学数据,结合前人的地质资料,本文认为二郎坪弧后盆地曾沿着瓦乔剪切带和朱夏剪切带发生双向式俯冲。     

The Little Ice Age glacier maximum in Iceland and the North Atlantic Oscillation: evidence from Lambatungnajökull, southeast Iceland

This article examines the link between late Holocene fluctuations of Lambatungnajökull, an outlet glacier of the Vatnajökull ice cap in Iceland, and variations in climate. Geomorphological evidence is used to reconstruct the pattern of glacier fluctuations, while lichenometry and tephrostratigraphy are used to date glacial landforms deposited over the past ˜400 years. Moraines dated using two different lichenometric techniques indicate that the most extensive period of glacier expansion occurred shortly before c . AD 1795, probably during the 1780s. Recession over the last 200 years was punctuated by re-advances in the 1810s, 1850s, 1870s, 1890s and c . 1920, 1930 and 1965. Lambatungnajökull receded more rapidly in the 1930s and 1940s than at any other time during the last 200 years. The rate and style of glacier retreat since 1930 compare well with other similar-sized, non-surging, glaciers in southeast Iceland, suggesting that the terminus fluctuations are climatically driven. Furthermore, the pattern of glacier fluctuations over the 20th century broadly reflects the temperature oscillations recorded at nearby meteorological stations. Much of the climatic variation experienced in southern Iceland, and the glacier fluctuations that result, can be explained by secular changes in the North Atlantic Oscillation (NAO) Advances of Lambatungnajökull generally occur during prolonged periods of negative NAO index. The main implication of this work relates to the exact timing of the Little Ice Age in the Northeast Atlantic. Mounting evidence now suggests that the period between AD 1750 and 1800, rather than the late 19th century, represented the culmination of the Little Ice Age in Iceland.     

The tephrochronology of Iceland and the North Atlantic region during the Middle and Late Quaternary: a review

The tephrochronology of Iceland and the North Atlantic region is reviewed in order to construct a unified framework for the last 400 kyr BP. Nearly all of the tephra layers described are also characterised geochemically. A number of new tephra layers are analysed for the first time for their geochemical signature and a number of pre‐Holocene tephra layers have been given an informal denotation. The tephrostratigraphy of Ash Zone II is highlighted. Where possible the rhyolitic tephra layers found outside Iceland have been correlated to known Icelandic tephra layers or to the volcanic source area. The application of tephra fallout in various depositional environments is described and discussed. Copyright © 2000 John Wiley & Sons, Ltd.