Palaeomagnetic evidence for the origin of Madeira

The remanent magnetization of ‘basement’ volcanics from Madeira define three different axes of magnetization, each having a dual-polarity build-up. The suggested oldest of these components, with declination 302 and inclination +4, is assigned to the late Lower Cretaceous and is thought to reflect the age of the early volcanism of the island. Subsequent magnetization overprints seem to have occurred in the Late Cretaceous/Early Tertiary (minor) and in Neogene times, respectively. The latter magnetization, which is strongly developed, was most likely impressed during the extensive volcanism that swept the island in post-Late Miocene. The palaeomagnetic evidence for a Cretaceous origin of Madeira is supported by the finding of Lower-Middle Cretaceous tuff layers in DSDP site 136 which is located only 160 km north of the island. The inferred palaeomagnetic structure of the ‘basement’ rocks of Madeira is similar to that found in the old volcanic complexes of other east central Atlantic islands.     

Sedimentary and tectonic evolution of the Banquan pull-apart basin and implications for late Cenozoic dextral strike-slip movement of the Tanlu Fault Zone

The Banquan Basin is a pull-apart basin with the largest scale and the most prominent structure due to dextral slip of the Tanlu Fault Zone(TLFZ) in late Cenozoic. The depositional history of the basin records the start time and evolution of the right-lateral strike-slip movement of the TLFZ. This paper studies the sedimentary and tectonic evolution of the Banquan Basin by seismic reflection exploration, borehole detection and cosmogenic nuclide chronology. We analyze the coupling relationship b...     

Palaeomagnetic evidence for stationary sources of geomagnetic secular variation

Asymmetrical curvilinear patterns of secular variation with approximately superposed “outward” and “return” trajectories can be attributed to stationary magnetic sources in the core. Some appropriate palaeomagnetic examples are presented here.     

Structural evidence for large-scale top-to-the-north normal displacement along the Median Tectonic Line in southwest Japan

Abstract   The Median Tectonic Line (MTL) is a first-order tectonic boundary that separates the Sanbagawa and Ryoke Metamorphic Belts. Strike-slip movements on the MTL have been well documented by many workers. New field based structural studies in the Sanbagawa Belt close to the MTL reveal a large number of secondary faults and semibrittle shear bands indicating a top-to-the-north normal sense of displacement. The strikes of these shear zones and their spatial distributions suggest that development of these structures is related to movements on the MTL. These results imply that the MTL has a large-scale normal shear component on a regional scale that can help account for the exhumation of the Sanbagawa Belt. Our proposed history of the MTL can also account for changes in the geometry of folds in the Sanbagawa Belt.     

联合InSAR和GPS研究鲜水河断裂带炉霍—道孚段震间运动特征

鲜水河断裂带是位于青藏高原东侧的大型走滑断裂,地震活动性高,其炉霍—道孚段是少数观测到蠕滑现象的段落之一,但目前尚缺乏高分辨率大地测量数据的约束研究.本文综合利用InSAR和GPS观测数据研究炉霍—道孚段的运动特征,提出一种适用于研究震间形变的InSAR和GPS数据融合方法,将InSAR速度场和GPS插值速度场转换至平行断层方向,利用抽样统计方法去除GPS参考框架影响,在数据融合过程中,注重保留远场构造运动特征和近场高分辨率形变信息.建立包括浅层蠕滑、中间闭锁单元和深部滑动的断层运动模型,以融合的形变速度场为约束,利用马尔可夫链蒙特卡罗方法反演炉霍—道孚段断层滑动速率、浅层蠕滑特征和闭锁状态.InSAR速度场剖面分析结果表明：炉霍—道孚段地表蠕滑特征明显,沿走向方向存在差异,炉霍段蠕滑速率明显低于道孚段,表现出从北西至南东速率逐渐增大、减小、再次增大的变化特征,断层左旋蠕滑速率在1.0~5.2 mm·a^-1之间;断层运动模型反演结果表明,炉霍—道孚段蠕滑深度为~9.8 km,浅层蠕滑速率为3.27~4.18 mm·a^-1,断层深部滑动速率为8.12~9.30 mm·a^-1.炉霍段地震复发周期为370~410年,闭锁层厚度~1 km,地震危险性较低.道孚段地震复发周期为59~65年,存在厚度~7.5 km的闭锁区,可积累足够的应变,浅层蠕滑释放的能量为~1.32×10¹⁶ Nm·a^-1,占构造运动加载积累能量的23%~38%,1981年道孚6.9级地震以来,闭锁区近40年积累的地震矩达到3.62×10¹⁷至7.88×10¹⁷ Nm,相当于一次M_W5.6~M_W5.9地震的能量,其地震危险性值得关注.

     

The palaeoposition of Madagascar: Palaeomagnetic evidence from the Isalo group

Palaeomagnetic results are reported from the continental facies of the Triassic-Jurassic Isalo Group of Madagascar. Stability of the magnetic remanence was tested using the alternating field and progressive thermal demagnetization techniques. Results from 8 sites, 6 located in northwestern Madagascar and 2 from the southwestern region, yield a palaeomagnetic pole at 74.2°S, 97.1°E (N = 8, A₉₅ = 6.3°). Three models previously proposed to describe the drift history of Madagascar relative to Africa are considered and the relevant geological and geophysical information is reviewed. The palaeomagnetic data are only consistent with the pre-drift model which places Madagascar off the east coast of Africa adjacent to Kenya and Tanzania. This is also the continental drift fit favoured on geological grounds.     

Palaeomagnetic evidence from the Ordovician and Silurian of northwest Peninsular Malaysia

The Ordovician and Silurian Setul Limestone of the Langkawi Islands, northwest Peninsular Malaysia, has a mean magnetic vector ofD = 338°,I = 62° after cleaning and correction for tilt. This is equivalent to a palaeolatitude of 43°, and a palaeomagnetic pole at 46°N, 76°E. The Silurian part of the Setul limestone also shows a similar direction. The Ordovician results are equivalent to a palaeolatitude of 43°, N or S. Recent reconstructions, based on palaeontology, place Indochina and China in the northern hemisphere in the Ordovician; if this is correct, a palaeolatitide of 43° for Langkawi would imply that Malaya-Indochina was the most northerly continental fragment at that time.     

Palaeomagnetism of late Precambrian dolerite dykes from Varanger peninsula,north Norway

Palaeomagnetic investigations have been carried out on 12 dykes of Late Precambrian age from the Varanger peninsula, north Norway. The dykes are separated into two groups, the Kongsfjord dykes and the Båtsfjord dykes. In the Kongsfjord dykes, titanomagnetite is almost entirely erased, as a result of an extreme degree of alteration. Pyrrhotite is the dominating magnetic mineral, and only three stable specimen directions can be defined. In the Båtsfjord dykes, however, the most important magnetic constituent is nearly pure magnetite, and a two-axis magnetization structure is revealed. The directions of the major component conform to a Fisherian distribution, and are assumed to represent the relative Late Precambrian field. Superimposed on this magnetization is a minor component which is assumed to be of Caledonian origin, probably Ordovician. This latter remanence is in accordance with other Middle Palaeozoic results obtained in Western Europe. The upper age limit of the Late Precambrian field is discussed, and it is proposed that the polar shift from the Late Precambrian position to the main Palaeozoic group may have occurred as late as Middle Ordovician.     

The penultimate deglaciation: High-resolution paleoceanographic evidence from a north–south transect along the eastern Nordic Seas

The penultimate termination has been studied with focus on oceanographic changes in the eastern Nordic Seas and the influence of these changes on the surrounding ice sheets and vice versa. Repeatedly, major changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) occurred during the studied interval. Times of strong overturning and increased heat transport northwards were of importance in triggering one minor and two major disintegration events. The two major disintegration events were separated by a deglacial pause, characterized by a strong AMOC. The same variability is seen throughout the eastern Nordic Seas, from the Faeroe–Shetland Channel in the south to the Fram Strait in the north. Some of the oceanographic changes occurring during the penultimate termination are comparable with changes seen through the last deglaciation. Reduced winter moisture flux and increased summer melting due to the present insolation forcing further amplified the rate of ice sheet disintegration. Calculated sea-level change through TII shows a mean change of 121 ± 4 m, 41 ± 16 m in the first step and 80 ± 13 m in the last step.     

Ca. 13 Ma strike-slip deformation in coastal Sonora from a large-scale,en-echelon,brittle-ductile,dextral shear indicator: implications for the evolution of the California rift

The Rancho Nuevo semi-circular structure is a geomorphological structure defined by drainage patterns in coastal Sonora, about 160 km NW of Hermosillo. The structure is about 15 by 30 km, and it is cored by felsic to intermediate plutons (granodiorite, monzogranite, quartz-porphyry) covered by Miocene volcanic rocks. This work is focused on the deformation of the intrusives which cover most of study area.The plutons are a co-magmatic suite dated between 71 ± 1.1 and 67.9 ± 1.0 Ma (U-Pb zircon, LA-ICPMS). The most voluminous unit is a granodiorite characterized by conspicuous sigmoidal fractures at the scale of high resolution satellite images, along which rhyolite dikes were emplaced about 13.2 Ma. Magnetic fabric (AMS) and paleomagnetic data were collected from 27 sites in the granodiorite. Magnetic fabrics are weak but well developed, and are characterized by steep foliation planes with strikes that follow the sigmoidal fracture pattern and suggest NE-SW to NW-SE flattening after emplacement. The characteristic magnetization is of dual polarity, but it is dominantly reverse consistent with emplacement during chron C31r. The prevalent magnetization is southwest and moderately steep negative (ten sites), a discordant direction rotated clockwise about 41 ° ± 11 with respect to the expected Late Cretaceous reference direction, also indicating gentle southward tilt. There is, however, paleomagnetic evidence suggesting that the structure did not rotate as a rigid body, but it deformed internally instead. These data are interpreted to indicate that the Rancho Nuevo semicircular structure is a large-scale, dextral, brittle-ductile shear indicator. The age of the dikes and the fact that they are covered discordantly by rocks assigned to the tuff of San Felipe indicate that northwest, srike-slip, motion of Baja California peninsula (and thus the Pacific plate relative to North America) was accommodated by faults in coastal Sonora about 13 Ma ago.     

Paleocene large-scale normal faulting along the Median Tectonic Line, western Shikoku, Japan

Abstract   The Median Tectonic Line (MTL) in southwest Japan, a major east–west-trending arc-parallel fault, has been defined as the boundary fault between the Cretaceous Sambagawa metamorphic rocks and Ryoke granitic and metamorphic rocks, which are unconformably covered by the Upper Cretaceous Izumi Group. The juxtaposition by faulting occurred after the deposition of the Izumi Group. Based on detailed fieldwork and previous studies, the authors reconstruct the kinematic history along the MTL during the Paleogene period, which has not been fully understood before. It is noted that although the strata of the Izumi Group along the MTL dip gently, east–west-trending north-vergent folds with the wavelength of ∼300 m commonly develop up to 2 km north from the MTL. Along the MTL, a disturbed zone of the Izumi Group up to 400 m thick, defined by the development of boudinage structures with the transverse boudin axis dipping nearly parallel to the MTL, occurs. Furthermore, east–west-trending north-vergent folds with the wavelength of 1–5 m develop within the distance up to 60 m from the MTL. The disturbed zone with the map-scale north-vergent folds along the MTL, strongly suggests that they formed due to normal faulting with a top-to-the-north sense along the MTL. Considering that the normal faulting is associated with the final exhumation of the Sambagawa metamorphic rocks, and its juxtaposition against the Izumi Group at depth, this perhaps occurred before the denudation of the Sambagawa metamorphic rocks indicated by the deposition of the Lower Eocene Hiwada-toge Formation. Dynamic equilibrium between crustal thickening at depth (underplating) and extension at shallow level is a plausible explanation for the normal faulting because the arc-normal extension suggests gravity as the driving force.     

New evidence of tectonic uplift and transform of movement style along Ailao Shan-Red River shear zone

Using cooling curves of K-feldspars obtained by specific⁴⁰Ar/³⁹Ar step heating procedure and multiple diffusion domain modeling (MDD model), together with results of dating hornblende, biotite, muscovite, and apatite, this research relates the cooling history of 3 areas, from Ejia in NWto Jinping in SE of Ailao Shan-Red River shear zone, and near Hekou (distance about 390 km). The results indicate that the slip distance of Ailao Shan-Red River shear zone is more than 350 km (at least based on the study done within China). And between the end of the left-lateral slip movement and the beginning of the right-lateral slip movement, the uplift of shear zone from SE to NW was homogeneously extended.     

New sonar evidence for recent catastrophic collapses of the north flank of Tenerife, Canary Islands

Previous sonar surveys show that the north flank of Tenerife has been subject to at least four major landslides during the past 1 Ma. The youngest, Icod, affected the region to the north of the Teide-Pico Viejo complex, the world's third highest oceanic volcano. Recently, we obtained the first detailed acoustic images of Icod using a deep-tow side-scan sonar. The images suggest that Tenerife's north flank has experienced at least two types of flow deposit in the recent past. The older flow deposit, Icod I, is characterised by a 15- to 20-km-wide, >65-km-long, chaotic debris avalanche deposit which includes several very large blocks. We believe the deposit to be ~170 ka, and that it represents the mass-wasting products of the Cañadas edifice, remnants of which are now found in the Las Cañadas caldera wall. The younger flow deposit, Icod II, associated with a shute in its proximal part, appears to have produced a less chaotic deposit in its distal part which clearly preserves flow structures such as latitudinal boulder ridges and longitudinal shear structures. The sonar images cannot determine how much younger Icod II is than Icod I, although it is likely that they are a consequence of the same lateral collapse event. There is evidence from the shute area for erosional scour and sediment deposition since the Icod landslide. If this is correct, then it suggests that mass wasting is an ongoing process that has already started to modify the Teide-Pico Viejo complex itself.     

Evolution of an accretionary complex along the north arm of the Island of Sulawesi, Indonesia

Abstract Seismic reflections across the accretionary prism of the North Sulawesi provide excellent images of the various structural domains landward of the frontal thrust. The structural domain in the accretionary prism area of the North Sulawesi Trench can be divided into four zones: (i) trench area; (ii) Zone A; (iii) Zone B; and (iv) Zone C. Zone A is an active imbrication zone where a decollement is well imaged. Zone B is dominated by out‐of‐sequence thrusts and small slope basins. Zone C is structurally high in the forearc basin, overlain by a thick sedimentary sequence. The subducted and accreted sedimentary packages are separated by the decollement. Topography of the oceanic basement is rough, both in the basin and beneath the wedge. The accretionary prism along the North Sulawesi Trench grew because of the collision between eastern Sulawesi and the Bangai–Sula microcontinent along the Sorong Fault in the middle Miocene. This collision produced a large rotation of the north arm of Sulawesi Island. Rotation and northward movement of the north arm of Sulawesi may have resulted in southward subduction and development of the accretionary wedge along North Sulawesi. Lateral variations are wider in the western areas relative to the eastern areas. This is due to greater convergence rates in the western area: 5 km/My for the west and 1.5 km/My for the east. An accretionary prism model indicates that the initiation of growth of the accretionary prism in the North Sulawesi Trench occurred approximately 5 Ma. A comparison between the North Sulawesi accretionary prism and the Nankai accretionary prism of Japan reveals similar internal structures, suggesting similar mechanical processes and structural evolution.     

Early Tertiary rupture of the Pacific plate: 1700 km of dextral offset along the Emperor trough-Line Islands lineament

Between 67 and ～40 Ma ago a northwest-southeast-trending fracture system over 8000 km long split the Pacific plate and accumulated at least 1700 km of dextral offset between the east and west portions. This system, here named the Emperor fracture zone (EFZ) system, consisted of several segments, one along the present trace of the Emperor trough and another along the Line Islands, joined by short spreading ridges. The EFZ terminated at its northern end against the Kula-Pacific ridge, and at its southern end in a ridge-transform system, called the Emperor spreading system, which extended to the west, north of Australia.The finite angular velocity vector describing the relative motion between the East and West Pacific plates is ～0.6°/Ma about a pole at 36°N, 70°W. This vector, added to the known Early Tertiary motion of the Pacific plate with respect to the global hotspot reference frame, accounts in large part for the NNW trend of the Emperor seamount chain relative to the WNW Hawaiian trend, without violation of the integrity of the Antarctic plate. The Meiji-Emperor and Emperor-Hawaiian bends date, respectively, the initiation (～67 Ma ago) and cessation (～40 Ma ago) of seafloor spreading on the Emperor spreading system.The postulated Early Tertiary relative motion along the EFZ between the East and West Pacific plates explains (1) the present misalignment of the two sets of magnetic bights of the Pacific, (2) the abrupt truncation of eastern Pacific bathymetric lineaments against the Emperor trough and Line Islands, (3) the contrast in paleolatitude between the eastern and western Pacific as indicated by paleomagnetic and sedimentologic studies, and (4) the anomalous gravity signature of the central Hawaiian ridge that indicates that the ridge loaded thin hot lithosphere.     

Resistivity structure of a seismic gap along the Atotsugawa Fault, Japan

Seismicity along the Atotsugawa Fault, located in central Japan, shows a clear heterogeneity. The central segment of the fault with low-seismicity is recognized as a seismic gap, although a lot of micro-earthquakes occur along this fault. In order to elucidate the cause of the heterogeneity in seismicity, the electrical resistivity structure was investigated around the Atotsugawa Fault by using the magnetotelluric (MT) method. The regional geoelectrical strikes are approximately parallel to the fault in a low-frequency range. We constructed two-dimensional resistivity models across the fault using TM-mode MT responses to minimize three-dimensional effects on the modeling process. A smooth inversion algorithm was used, and the static-shifts on the apparent resistivity were corrected in the inversion process.A shallow, low resistivity zone along the fault is found from the surface to a depth of 1-2 km in the best-fit model across the high-seismicity segment of the fault. On the other hand, the corresponding low resistivity zone along the low-seismicity segment is limited to a shallower depth less than 1 km. The low resistivity zone along the Atotsugawa Fault is possibly due to fluid in the fracture zone; the segment with higher levels of seismicity may have higher fluid content in the fault zone compared with the lower seismicity segment. On a view of the crustal structure, a lateral resistivity variation in a depth range of 3-12 km is found below the fault trace in the high-seismicity segment, while a resistive layer of wide extent is found at a depth of about 5 km below the fault trace in the low-seismicity segment. The resistive layer is explained by less fluid condition and possibly characterized as high rigidity. Differences in the resistivity structures between low and high-seismicity segments of the fault suggest that the seismic gap in the central part of the Atotsugawa Fault may be interpreted as a locked segment. Thus, MT is an effective method in evaluating a cause and future activity of seismic gaps along active faults.The lower crust appears as a conductive zone beneath the low-seismicity segment, less conductive beneath the high-seismicity segment. Fluid is inferred as a preferable cause of the conductive zone in this study. It is suggested that the conductive lower crust beneath the low-seismicity segment is recognized where fluid is trapped by an impermeable layer in the upper crust. On the other hand, fluid in the lower crust may upwell to the surface along the high-seismicity segment of the fault.     

Earthquake Triggering along the Xianshuihe Fault Zone of Western Sichuan,China

Western Sichuan is among the most seismically active regions in southwestern China and is characterized by frequent strong (M 6.5) earthquakes, mainly along the Xianshuihe fault zone. Historical and instrumental seismicity show a temporal pattern of active periods separated by inactive ones, while in space a remarkable epicenter migration has been observed. During the last active period starting in 1893, the sinistral strike–slip Xianshuihe fault of 350 km total length, was entirely broken with the epicenters of successive strong earthquakes migrating along its strike. This pattern is investigated by resolving changes of Coulomb failure function (CFF) since 1893 and hence the evolution of the stress field in the area during the last 110 years. Coulomb stress changes were calculated assuming that earthquakes can be modeled as static dislocations in an elastic halfspace, and taking into account both the coseismic slip in strong (M 6.5) earthquakes and the slow tectonic stress buildup associated with major fault segments. The stress change calculations were performed for faults of strike, dip, and rake appropriate to the strong events. We evaluate whether these stress changes brought a given strong earthquake closer to, or sent it farther from, failure. It was found that all strong earthquakes, and moreover, the majority of smaller events for which reliable fault plane solutions are available, have occurred on stress–enhanced fault segments providing a convincing case in which Coulomb stress modeling gives insight into the temporal and spatial manifestation of seismic activity. We extend the stress calculations to the year 2025 and provide an assessment for future seismic hazard by identifying the fault segments that are possible sites of future strong earthquakes.     

希腊Ionian海Kefallinia转换断裂带的地震复发行为

采用历史地震资料和统计方法研究了希腊Ionian海Kefallinia转换断裂带的两条断层(Kdallinia和Lefkada)的整体强地震复发行为.研究发现,Kefallinia断层的地震复发行为表现出丛集性特征,存在时间可预测复发行为;Letkada断层的地震复发行为具有近随机性特征.不存在时间可预测及滑动可预测的复发行为.造成其地震复发行为不同的原因可能有:(a)沿Kefallinia断裂带走向,由静力触发引起的断层分段间的相互作用;(b)Lefkada断层的地震复发行为受北部和东部的断层系影响.另外,活跃期内地震复发间隔分布的分散性较大.可用韦伯(Weibull)分布近似描述;而平静期持续时间分布的分散性不够,这样的分布不便进行拟合.     

Recurrence Behaviors of Earthquakes along the Kefallinia Transform Fault, Ionian Sea, Greece

We examined the whole strong earthquake recurrence behaviors of two fault zones along the Kefallinia Transform, Ionian Sea, Greece, using seismological data and statistical methods. Our data include 29 events with %M%>5^5 for the period 1636～2003. We found different recurrence behaviors for the Kefallinia Fault Zone (clustering and time-predictable recurrence behaviors) and the Lefkada Fault Zone (near random and non-slip-predictable or non-time-predictable recurrence nature). The different modes may be attributed to: (a) segment interaction along-strike (Kefallinia) by static triggering and (b) the influence of fault systems to the north and east on the recurrence on Lefkada. Within the active periods, earthquake recurrence intervals are distributed in a more dispersed fashion, and can be fitted well by a Weibull distribution. In contrast, the distribution of the quiet periods is relatively less dispersed and difficult to describe by suitable probability functions.