Has the plate boundary shifted from central Hokkaido to the eastern part of the Sea of Japan?

A NS trending Cenozoic fold-and-thrust belt has developed in the western part of the Hidaka Collision Zone (HCZ), central Hokkaido, Japan. A quantitative estimation of the late Cenozoic convergence rate at the front of the Hidaka thrust system is important in revealing the plate tectonic framework around northern Japan. High-resolution seismic reflection profiling across the active fault-related folds was carried out to ascertain the temporal change in the crustal shortening rate. Overlapping ramp anticlines and growth folds within thrust sheets were examined using balanced cross-sections combined with industry seismic and drilling data. The rate of shortening was examined using a 3.5 Ma horizon and late Quaternary horizons at 115 and 41 ka. These horizons show that the convergence rate of the Hidaka thrust system has not decreased during the last 3.5 Ma. This suggests that the plate boundary between the Eurasian (Amurian) and North American (Okhotsk) plates has not jumped from the central part of Hokkaido to the eastern part of the Sea of Japan since 3.5 Ma and that a significant amount of plate convergence is still being absorbed in the Hidaka Collision Zone.     

Must magmatic intrusion in the lower crust produce reflectivity?

The Færoe–Iceland Ridge (FIR) provides a laboratory in which to investigate the reflectivity and velocity structure of thick crust generated above a mantle plume in order to constrain models of underplating and the origins of lower-crustal layering in an environment dominated by young igneous processes. Over 600 km of common midpoint (cmp) data were collected along and across the FIR using a large airgun array with a 240-channel streamer. The interpretation of these data has been integrated with a velocity model of the crust and upper mantle along the FIR obtained from wide-angle seismic arrivals into ocean bottom and land seismometers. Due to the intermediate water depths and the presence of basalt near the water bottom, specialized processing steps were required for the cmp data. A wave equation-based multiple attenuation scheme was applied to the prestack data, which used a forward model of the multiple series to predict and attenuate multiple energy. Array simulations were applied in the shot and receiver domains in order to minimize spatial aliasing and reduce low apparent-velocity noise. Most of the sections over the central (oceanic) portion of the FIR show no pronounced reflectivity, although occasional Moho and/or lower-crustal reflections are observed. We believe that the poor reflectivity results largely from a lack of physical property contrasts rather than being an effect of acquisition or processing, although we also conclude that residual energy from strong multiple reflection remains in the final sections. Amplitude decay and reflection strength vary along the FIR, but there is good signal-to-noise ratio to travel times of at least 9 s (i.e., into the lower crust), implying that the reduced reflectivity beneath the main part of the FIR is not an artifact of signal penetration loss. We conclude that the addition of melt to the lower crust along the trace of the plume apparently did not produce strong physical property contrasts in the lower crust, where little reflectivity is apparent. Perhaps this was because the entire crust was hot at the time of formation. In contrast, igneous intrusion into preexisting continental crust (at the Færoe Islands end of the FIR) and into older igneous crust (at the Iceland end of the FIR) produces significant lower-crustal reflectivity. Strong lower-crustal reflectivity elsewhere beneath the northwestern European continental margins may have a similar intrusive origin.     

On the nature of mantle heterogeneities and discontinuities: evidence from a very dense wide-angle shot record

A seismic survey with a receiver spacing of 50 m provided one of the most densely sampled wide-angle seismic reflection images of the lithosphere. This unique data set, recorded by an 18-km-long spread, reveals that at wide-angles the shallow subcrustal mantle features high amplitude reflectivity which contrasts with a lack of reflectivity at latter travel times. This change in the seismic signature is located at approximately 120–150 km depth, which correlates with the depth estimates of the lithosphere–asthenosphere boundary (LAB) of previous DSS studies. This seismic signature can be simulated by two-layer mantle model. Both layers with similar average velocities differ in their degree of heterogeneity. The shallow heterogeneous layer and the deeper and more homogeneous one correlate with the lithosphere and the asthenosphere, respectively. Studies involving surface outcrops of ultramafic massifs and mantle xenoliths infer that the upper mantle is a heterogeneous mixture of ultramafic rocks (lherzolites, harzburgites, pyroxenites, peridotites, dunites, and small amounts of eclogites). Laboratory measurements of physical properties of these mantle rocks indicate that compositional variations alone can account for the wide-angle reflectivity. A temperature increase would homogenize the mixture, decreasing the seismic reflection properties due to melting processes. It is proposed that this would take place below 120–150 km (1200 °C, the LAB).     

East Avalonia,the third partner in the Caledonian collisions: evidence from deep seismic reflection data

Further evidence for the existence of the terrane East Avalonia (Cadomia) in north-west Europe, its boundaries and its role in the Caledonian collisional processes comes from studies of deep seismic reflection data at sea and on land. Various sutures are found in the north-east and the north-west, and a generally poor reflectivity dominates in the major part. Further details of reflectivity patterns support the idea that a huge terrane which split from the northern rim of Gondwana moved northward and collided with the merging plates Laurentia-Baltica. Its docking features are analysed.     

Automatic extraction of lineament information from satellite images using digital elevation data

Using conventional visual interpretation in lineament analysis presents two main problems. The first is subjectivity, introduced because of the bias of various interpreters. The second problem is that lineaments detected from satellite images are constrained by the direction of the illumination source. Since lineament identification mainly involves recognition of diagnostic morphological features, the use of digital elevation can contribute significant information about these features. Further, in generating images using digital elevation data, the direction of illumination can easily be controlled. Thus, the use of digital elevation data offers the possibility of revealing features not apparent in regular satellite images.We discuss a sequential line detection method for extraction of linear features from digital elevation data. In this method, raw elevation data is used for generating shaded relief images using the Lambertian reflection model, wherein the illumination direction is controlled by the user. The Directional Segment Detection Algorithm (DSDA) is used for detecting linear topographic features in user-defined trends. Locational information about these linear features is stored in the computer as coordinate pairs amenable to editing and subsequent analysis. Finally, three-dimensional terrain models are generated by combining the digital elevation data and satellite images. The experiments were carried out using digital elevation data of southwest Japan and Landsat MSS data.     

用双椭圆方法确定反射点位置

在反射波地震勘探中使用动校正确定反射点位置用到很多假设,如勘探深度要远远大于炮检距;假定倾斜反射界面的反射点与水平反射界面反射点都位于炮检距的中点;近似认为倾斜反射界面的动校正量等于水平反射界面的动校正量;假定反射面倾角固定且较小等,上述假设一定会造成勘探误差。由于反射点位置和反射面倾角未知,所以理论上无法唯一地确定反射点位置。如果反射波传播的介质的波速一定,从炮点发出的地震波,经反射点后,在接收点被接收,其可能的反射点是椭圆的一部分,但还不能唯一确定反射点;再取炮点和另外一个接收点,其可能的反射点是另外一个椭圆的一部分。如果假定反射面是平面,可以是水平面,也可以是有固定倾角的倾斜平面,该平面在地震波射线平面内是一条直线,该直线一定是两椭圆的公切线。把两椭圆方程和切线方程联立,就可以求解出公切点位置,公切点位置就是反射点位置,这就是用双椭圆确定反射点位置的方法。通过建立模型对勘探方法进行了检验,证实了用双椭圆方法确定反射点位置的有效性。双椭圆方法有一个重要的副产品,就是在确定反射面位置的同时计算出反射面的视倾角。     

中亚造山带东段浅表构造速度结构——深地震反射剖面初至波层析成像的揭露

为揭示中亚造山带浅表结构,对地壳演化与深部过程提供浅部精准约束,利用横过中亚造山带东段（奈曼旗—东乌珠穆沁旗）长达400 km的深地震反射剖面共2 186炮的初至波走时数据,运用初至波层析成像方法获得了自地表向下约3 km厚度的浅表速度结构精细模型。通过模型计算了沉积厚度变化与基岩起伏特征,并在贺根山和西拉木伦缝合带附近获得了呈低速特征的弧前沉积盆地规模与沉积厚度变化特征;在此基础上,综合速度模型与深地震反射剖面的强振幅反射信息,建立了符合剖面南北两侧的古亚洲洋双向俯冲并与中部的残存微陆块发生拼合的构造模型。结果表明：研究区的沉积厚度在0.3~3.0 km范围内变化,区内存在多期岩浆活动及活动构造,林西地区隐伏连续分布的高速结构多为造山花岗岩所导致;古亚洲洋消亡过程在经数亿年演变后仍能在大陆边缘的浅表构造中有迹可循。