Configuration of subducting Philippine Sea plate and crustal structure in the central Japan region

A seismic experiment with six explosive sources and 391 seismic stations was conducted in August 2001 in the central Japan region. The crustal velocity structure for the central part of Japan and configuration of the subducting Philippine Sea plate were revealed. A large lateral variation of the thickness of the sedimentary layer was observed, and the P-wave velocity values below the sedimentary layer obtained were 5.3–5.8 km/s. P-wave velocity values for the lower part of upper crust and lower crust were estimated to be 6.0–6.4 and 6.6–6.8 km/s, respectively. The reflected wave from the upper boundary of the subducting Philippine Sea plate was observed on the record sections of several shots. The configuration of the subducting Philippine Sea slab was revealed for depths of 20–35 km. The dip angle of the Philippine Sea plate was estimated to be 26° for a depth range of about 20–26 km. Below this depth, the upper boundary of the subducting Philippine Sea plate is distorted over a depth range of 26–33 km. A large variation of the reflected-wave amplitude with depth along the subducting plate was observed. At a depth of about 20–26 km, the amplitude of the reflected wave is not large, and is explained by the reflected wave at the upper boundary of the subducting oceanic crust. However, the reflected wave from reflection points deeper than 26 km showed a large amplitude that cannot be explained by several reliable velocity models. Some unique seismic structures have to be considered to explain the observed data. Such unique structures will provide important information to know the mechanism of inter-plate earthquakes.     

The crustal structure of the Guayana Shield, Venezuela, from seismic refraction and gravity data

We present results from a seismic refraction experiment on the northern margin of the Guayana Shield performed during June 1998, along nine profiles of up to 320 km length, using the daily blasts of the Cerro Bolívar mines as energy source, as well as from gravimetric measurements. Clear Moho arrivals can be observed on the main E–W profile on the shield, whereas the profiles entering the Oriental Basin to the north are more noisy. The crustal thickness of the shield is unusually high with up to 46 km on the Archean segment in the west and 43 km on the Proterozoic segment in the east. A 20 km thick upper crust with P-wave velocities between 6.0 and 6.3 km/s can be separated from a lower crust with velocities ranging from 6.5 to 7.2 km/s. A lower crustal low velocity zone with a velocity reduction to 6.3 km/s is observed between 25 and 25 km depth. The average crustal velocity is 6.5 km/s. The changes in the Bouguer Anomaly, positive (30 mGal) in the west and negative (−20 mGal) in the east, cannot be explained by the observed seismic crustal features alone. Lateral variations in the crust or in the upper mantle must be responsible for these observations.     

发展中的板块边界:天山-贝加尔活动构造带

亚洲内陆的强地震密集地发生在天山-贝加尔一线,但该处并不存在一条连续的大断裂,学术界对这个问题的认识长期相左。文中分析了这条地震带的时空分布、分区特点、应力状态和活动周期,计算了欧亚大陆的布格重力异常场、均衡重力异常场,反演了上地幔的密度分布和剪切波速分布。发现在这个部位的70~250km的深部有一条北东向的密度、速度陡变带,它是新生代的冷地幔和热地幔的交界带,与浅部构造存在立交关系,对亚洲大陆的现今构造运动和应力场具有重要的控制作用。这个带的地震不同于传统意义上的板缘地震和板内地震,是一种因为深浅构造不同而造成的结构性地震,性质上为大陆内缘地震。文中还就深浅构造的空间立交关系、时间镜像关系进行了讨论,指出在南北地震带和伊朗东侧地震带的立交结构也与上地幔构造有关。天山-贝加尔活动构造带是正在发展中的板块边界,是大陆内部的一个典型构造,北侧为稳定的俄罗斯-西伯利亚次板块,南侧为活动的中国-东南亚次板块。     

发展中的板块边界:天山-贝加尔活动构造带

亚洲内陆的强地震密集地发生在天山-贝加尔一线,但该处并不存在一条连续的大断裂,学术界对这个问题的认识长期相左。文中分析了这条地震带的时空分布、分区特点、应力状态和活动周期,计算了欧亚大陆的布格重力异常场、均衡重力异常场,反演了上地幔的密度分布和剪切波速分布。发现在这个部位的70~250km的深部有一条北东向的密度、速度陡变带,它是新生代的冷地幔和热地幔的交界带,与浅部构造存在立交关系,对亚洲大陆的现今构造运动和应力场具有重要的控制作用。这个带的地震不同于传统意义上的板缘地震和板内地震,是一种因为深浅构造不同而造成的结构性地震,性质上为大陆内缘地震。文中还就深浅构造的空间立交关系、时间镜像关系进行了讨论,指出在南北地震带和伊朗东侧地震带的立交结构也与上地幔构造有关。天山-贝加尔活动构造带是正在发展中的板块边界,是大陆内部的一个典型构造,北侧为稳定的俄罗斯-西伯利亚次板块,南侧为活动的中国-东南亚次板块。     

Deep seismic refraction experiment in northeast Brazil: New constraints for Borborema province evolution

The Borborema Province of northeastern Brazil is a major Proterozoic crustal province that, until now, has never been explored using deep crustal seismic methods. Here are reported the first results obtained from a high-quality seismic refraction/wide-angle reflection profile that has defined the internal seismic velocity structure and thickness of the crust in this region. Almost 400 recording stations were deployed in the Deep Seismic Refraction (DSR) experiment through an NW–SE ca. 900 km linear array and 19 shots were exploded at every 50 km along the line. Data from the 10 southeastern most shots of the seismic profile were processed in this work. The main features and geological structures crossed by the studied portion of the profile belong to the so-called Central Sub-province of the Borborema tectonic province. The crustal model obtained is compatible with a typical structure of extended crust. The model was essentially divided into three layers: upper crust, lower crust, and a half-space represented by the shallower portion of the mantle. The Moho is an irregular interface with depth ranging between 31.7 and 34.5 km, and beneath the Central Sub-province it varies from 31.5 to 33 km depth, where its limits are related to major crustal discontinuities. The distribution of velocities within the crust is heterogeneous, varying vertically from 5.7 to 6.3 km/s in the upper crust and from 6.45 to 6.9 km/s in the lower crust. From the average crustal velocity distribution it is evident that the Central Sub-province has seismic characteristics different from neighboring domains. The crust is relatively thin and crustal thickness variations in the profile are subtle due to stretching that occurred in the Cretaceous, during the fragmentation of Pangaea, opening of the South Atlantic Ocean and separation of South America from Africa.     

Very high strains recorded in mylonites along the Alpine Fault, New Zealand: implications for the deep structure of plate boundary faults

Oblique displacement on the Alpine Fault, which forms the principal structure along the Australian–Pacific plate boundary in South Island, New Zealand, has resulted in exhumation of a kilometre-wide mylonite zone in the hanging wall adjacent to the current brittle fault trace. The mylonites formed under amphibolite facies conditions at depths of ca. 25 km and have been uplifted during the past 5 Ma. A suite of 65–70 Ma pegmatite veins in the hanging wall Alpine schists has been progressively deformed within the mylonite zone and sheared out over a strike length of ca. 100 km. Measurements of the thickness distribution of the pegmatite veins within the non-mylonitised schists and at three localities within the progressively strained mylonites have been used to estimate strain values within the mylonites. The thicknesses approximate a log-normal distribution, with a mean value that is progressively reduced through the protomylonites, mylonites and ultramylonites. By assuming that the thickness distribution currently observed in the schists was the same for the pegmatites within the mylonites before strain, a model of deformation incorporating simple shear and simultaneous pure shear is used to strain the undeformed veins until a fit is obtained with the strained distributions. Shear strains calculated range from 12 to 22 for the protomylonites, 120 to 200 for the mylonites and 180 to 300 for the ultramylonites, corresponding to pure shear values of 1–3 in each case. These values are compatible with the strains predicted if most of the surface displacement on the fault over the past 5 Ma were accommodated within a 1–2-km-wide mylonite zone through the middle and lower crusts. The results suggest that processes such as erosional focussing of deformation and thermal weakening may cause intense strain localisation within the lower crust, with plate boundary deformation restricted to narrow zones rather than becoming increasingly distributed over a widening shear zone with depth.     

藏南定结地区早中新世淡色花岗岩的形成机制及其构造动力学意义

定结日玛那穹窿位于高喜马拉雅带中段,由花岗片麻岩、变泥质岩、变基性岩及大量淡色花岗岩等组成,经历了角闪岩-麻粒岩相变质作用。为厘定淡色花岗岩的形成机制以及与高级变质岩的关系,我们对淡色花岗岩和高级变质岩进行了全岩元素和Sr和Nd同位素组成和SHRIMP锆石U-Pb地质年代学测试。全岩元素和Sr-Nd同位素测试结果揭示淡色花岗岩具有以下特征:(1)高SiO₂ (>72%),高Al₂O₃ (>12%)和高A/CNK比值 (>1.0);(2)高Rb,低Sr,高Rb/Sr比值(>1.0);(3)高∑REE和明显的负Eu异常;(4)高Sr同位素初始比值(0.7621~0.8846)和低ε_Nd(t)值(-13.0~-20.2)。淡色花岗岩的高Rb/Sr比值和Sr-Nd同位素系统特征表明其形成机制为主要为白云母脱水部分熔融作用,源区为由花岗片麻岩和变泥质岩组成的混合源区。SHRIMP锆石U-Pb年代学研究揭示出定结地区淡色花岗岩具有21.0±0.7Ma和15.8±0.1Ma 2期年龄,花岗片麻岩的锆石变质增生边年龄为22.2±1.4Ma,与该区的榴辉岩退变质年龄一致。这些数据共同表明,花岗片麻岩和 变泥质岩在22~21Ma发生高级变质和深熔作用,形成早期淡色花岗岩岩浆,在~16Ma进一步深熔,形成晚期淡色花岗岩岩浆。     

Extensional deformation along the southern boundary of the Gyeonggi Massif,South Korea: structural characteristics,age constraints,and tectonic implications

The Permo–Triassic collision of the North and South China blocks caused the development of the Dabie–Sulu Orogen in China and Songrim Orogen in the Korean Peninsula. Extension after this collision is known from the Dabie–Sulu Orogen, but post-orogenic extension is not well defined in the Korean Peninsula. Extensional deformation along the southern boundary of the Gyeonggi Massif in Korea is characterized by top–down-to-the-south ductile shearing and subsequent brittle normal faulting, and was predated by regional metamorphism and north-vergent contractional deformation. Extension occurred between ~220 and 185 Ma based on the ages of pre-extensional regional metamorphism and post-extensional pluton emplacement. ⁴⁰Ar/³⁹Ar dating of syn-extensional muscovite in quartz–mica mylonite yields an age of 187.8 ± 5.6 (2σ) Ma, in agreement with constraints from structural relationships. Together with the extensional deformation identified along the northern boundary of the Gyeonggi Massif (~226 Ma), the extension along the southern boundary is probably related to the exhumation of the massif during late-orogenic or post-orogenic extension associated with the Songrim Orogeny of the Korean Peninsula and forms an important event in the Phanerozoic crustal evolution of East Asia.     

The lithospheric geodynamics of plate boundary transpression in New Zealand: Initiating and emplacing subduction along the Hikurangi margin, and the tectonic evolution of the Alpine Fault system

In contrast to the normal ‘Wilson cycle’ sequence of subduction leading to continental collision and associated mountain building, the evolution of the New Zealand plate boundary in the Neogene reflects the converse—initially a period of continental convergence that is followed by the emplacement of subduction. Plate reconstructions allow us to place limits on the location and timing of the continental convergence and subduction zones and the migration of the transition between the two plate boundary regimes. Relative plate motions and reconstructions since the Early to Mid-Miocene require significant continental convergence in advance of the emplacement of the southward migrating Hikurangi subduction—a sequence of tectonism seen in the present plate boundary geography of Hikurangi subduction beneath North Island and convergence in the Southern Alps along the Alpine Fault. In contrast to a transition from subduction to continental convergence where the leading edge of the upper plate is relatively thin and deformable, the transition from a continental convergent regime, with its associated crustal and lithospheric thickening, to subduction of oceanic lithosphere requires substantial thinning (removal) of upper plate continental lithosphere to make room for the slab. The simple structure of the Wadati–Benioff zone seen in the present-day geometry of the subducting Pacific plate beneath North Island indicates that this lithospheric adjustment occurs quickly. Associated with this rapid lithospheric thinning is the development of a series of ephemeral basins, younging to the south, that straddle the migrating slab edge. Based on this association between localized vertical tectonics and slab emplacement, the tectonic history of these basins records the effects of lithospheric delamination driven by the southward migrating leading edge of the subducting Pacific slab. Although the New Zealand plate boundary is often described as simply two subduction zones linked by the transpressive Alpine Fault, in actuality the present is merely a snapshot view of an ongoing and complex evolution from convergence to subduction.     

藏南也拉香波早渐新世富钠过铝质淡色花岗岩的成因机制及其构造动力学意义

藏南也拉香波穹隆位于近东西向展布的北喜马拉雅片麻岩穹隆(NHGD)最东端,主要由石榴角闪岩、石榴石云母片麻岩、二云母花岗岩和淡色花岗岩组成.SHRIMP锆石U/Pb定年结果表明也拉淡色花岗岩的结晶年龄为35.3±1.1Ma,明显老于位于该穹隆以西类似的淡色花岗岩(年龄普遍<25Ma).全岩元素和Sr-Nd同位素测试结果揭示：(1)也拉香波淡色花岗岩为过铝质富钠花岗岩;(2)与片麻岩相似,也拉香波淡色花岗岩富集大离子亲石元素(LILE,如K,Sr,Rb和Ba),但亏损Ti,Y,Yb,Sc和Cr;(3)和片麻岩或角闪岩相比,也拉香波淡色花岗岩同时亏损LREE和HREE,但与HREE相比,LREE相对富集;(4)在Sr-Nd同位素系统特征上.淡色花岗岩初始Sr同位素比值与角闪岩的相当,在0.711949～0.719344之间;但远小于片麻岩.而Nd同位素组成在片麻岩和角闪岩之间,在-8.9～-15.0之间.以石榴角闪岩和片麻岩为端元,简单混合计算表明：由石榴角闪岩为主和片麻岩为辅组成的混合源区发生部分熔融作用,各自产生的熔体进行不同程度的混合,可形成类似于也拉香波淡色花岗岩成分的岩浆,其中角闪岩的部分熔融起主要作用.使用Zr在岩浆中的饱和浓度温度计得出岩浆的平均温度为673℃,在此温度下,变泥质片麻岩在高压(～10kbar)条件下的水致部分熔融和角闪岩部分熔融都可形成也拉过铝质富钠淡色花岗岩,但角闪岩的脱水部分熔融起主导作用.在地壳增厚条件下,下地壳角闪岩的部分熔融可能是导致喜玛拉雅造山带从缩短增厚向伸展垮塌转换的主要因素之一.     

3-D upper crustal tomographic structure across the Vrancea seismic zone, Romania

The VRANCEA99 and VRANCEA2001 seismic refraction experiments are part of a multidisciplinary project to study the Eastern Carpathians in Romania. The objectives of these studies are intended to disclose a more detailed picture of the crustal and upper mantle structures above the seismically active Vrancea region. In this paper we provide additional constraints for the upper crustal structures of the area. The 1999 campaign consisted of a 320-km-long N–S profile and a 70-km-long E–W profile. The intersecting 2001 profile extended in E–W direction from the Hungarian border to the Black Sea. In order to enhance the model resolution, first arrival data from local crustal earthquakes were also included.This configuration allowed for the first time to derive a 3-D velocity model for the upper crust of the Romanian Carpathian Orogen, within a 115×235 km wide region, centred over the Vrancea seismic zone. The 3-D model reveals lateral velocity variations, which were not visible on the in-line interpretations. It allows us to distinguish between foreland platform areas, foreland basins and the Carpathian Orogen. Clear velocity differences between the foreland basins south and southeast of the Eastern Carpathians and the Focsani Basin further north indicate different pre-Miocene sedimentary compositions and geological evolutions of these foreland platforms. The involved Moesian and Scythian platforms are separated by the Trotus Fault system, which is observed as a velocity discontinuity. An upper crustal high-velocity zone, above the northern Vrancea seismic zone, could also be identified. This high-velocity zone is explained by a Middle Pliocene to Pleistocene E–W oriented out-of-sequence thrust of the crystalline basement, below the decollement of the flysch nappes.     

Resolving slip vector azimuths and plate motion along the southern boundary of the South Bismarck Plate,Papua New Guinea

The interaction of the Australian, South Bismarck and Solomon Sea Plates in Papua New Guinea is the source of frequent earthquakes that occur as a result of subduction and arc continent collision. Previous investigators have drawn attention to a discontinuity in the horizontal azimuth of slip vectors along the southern boundary of the South Bismarck Plate, with those to the west of 148°E being systematically rotated 20ndash;30° clockwise compared to those located east of 148°E. This has led to the suggestion that relative motion may be occurring between the Huon Peninsula and New Britain or that more than two plates are acting south of the South Bismarck Plate. Global positioning system (GPS) measurements since 1991 indicate that there is no internal deformation occurring within the South Bismark Plate and that at least two distinct plates are in contact with the southern edge of the South Bismarck Plate. We show from a study of a recent earthquake dataset that the change in slip vector azimuth can be modelled by the interaction of the overriding South Bismarck Plate with the underthrusting Australian and Solomon Sea Plates, consistent with the GPS observations, while maintaining the South Bismarck Plate as a rigid entity. We found that a transition zone exists between 147°E and 148°E where the underlying plate changes from the Australian Plate to the Solomon Sea Plate. There are insufficient data at present to indicate whether or not a third plate, the Woodlark Plate, is also interacting directly with the South Bismarck Plate in this transition zone. Slip vector azimuths were used to estimate an Euler pole (6.74°S, 144.64°E), which describes the relative motion of the South Bismarck and Solomon Sea Plates along the New Britain Trench.     

Crustal structure of the Gujarat region,India: New constraints from the analysis of teleseismic receiver functions

In this study, receiver function analysis is carried out at 32 broadband stations spread all over the Gujarat region, located in the western part of India to image the sedimentary structure and investigate the crustal composition for the entire region. The powerful Genetic Algorithm technique is applied to the receiver functions to derive S-velocity structure beneath each site. A detail image in terms of basement depths and Moho thickness for the entire Gujarat region is obtained for the first time. Gujarat comprises of three distinct regions: Kachchh, Saurashtra and Mainland. In Kachchh region, depth of the basement varies from around 1.5 km in the eastern part to 6 km in the western part and around 2–3 km in the northern part to 4–5 km in the southern part. In the Saurashtra region, there is not much variation in the depth of the basement and is between 3 km and 4 km. In Gujarat mainland part, the basement depth is 5–8 km in the Cambay basin and western edge of Narmada basin. In other parts of the mainland, it is 3–4 km. The depth of Moho beneath each site is obtained using stacking algorithm approach. The Moho is at shallower depth (26–30 km) in the western part of Kachchh region. In the eastern part and epicentral zone of the 2001 Bhuj earthquake, large variation in the Moho depths is noticed (36–46 km). In the Saurashtra region, the crust is more thick in the northern part. It varies from 36–38 km in the southern part to 42–44 km in the northern part. In the mainland region, the crust is more thick (40–44 km) in the northern and southern part and is shallow in Cambay and Narmada basins (32–36 km). The large variations of Poisson’s ratio across Gujarat region may be interpreted as heterogeneity in crustal composition. High values of σ (∼0.30) at many sites in Kachchh and few sites in Saurashtra and Mainland regions may be related to the existence of high-velocity lower crust with a mafic/ultramafic composition and, locally, to the presence of partial melt. The existing tectono-sedimentary models proposed by various researchers were also examined.     

A tentative 2D thermal model of central India across the Narmada-Son Lineament (NSL)

This work deals with 2D thermal modeling in order to delineate the crustal thermal structure of central India along two Deep Seismic Sounding (DSS) profiles, namely Khajuriakalan–Pulgaon and Ujjan–Mahan, traversing the Narmada-Son-Lineament (NSL) in an almost north–south direction. Knowledge of the crustal structure and P-wave velocity distribution up to the Moho, obtained from DSS studies, has been used for the development of the thermal model. Numerical results reveal that the Moho temperature in this region of central India varies between 500 and 580 °C. The estimated heat flow density value is found to vary between 46 and 49 mW/m². The Curie depth varies between 40 and 42 km and is in close agreement with the Curie depth (40±4 km) estimated from the analysis of MAGSAT data. Based on the present work and previous work, it is suggested that the major part of peninsular India consisting of the Wardha–Pranhita Godavari graben/basin, Bastar craton and the adjoining region of the Narmada Son Lineament between profiles I and III towards the north and northwest of the Bastar craton are characterized with a similar mantle heat flow density value equal to 23 mW/m². Variation in surface heat flow density values in these regions are caused by variation in the radioactive heat production and fluid circulation in the upper crustal layer.     

Characteristics of gravity and magnetic fields and deep structural responses in the southern part of the Kyushu-Palau Ridge

The southern part of the Kyushu-Palau Ridge (KPR) is located at the conjunction of the West Philippine Basin, the Parece Vela Basin, the Palau Basin, and the Caroline Basin. This area has extremely complex structures and is critical for the research on the tectonic evolution of marginal seas in the Western Pacific Ocean. However, only few studies have been completed on the southern part, and the geophysical fields and deep structures in this part are not well understood. Given this, this study finely depicts the characteristics of the gravity and magnetic anomalies and extracts information on deep structures in the southern part of the KPR based on the gravity and magnetic data obtained from the 11th expedition of the deep-sea geological survey of the Western Pacific Ocean conducted by the Guangzhou Marine Geological Survey, China Geological Survey using the R/V Haiyangdizhi 6. Furthermore, with the data collected on the water depth, sediment thickness, and multichannel seismic transects as constraints, a 3D density model and Moho depths of the study area were obtained using 3D density inversion. The results are as follows. (1) The gravity and magnetic anomalies in the study area show distinct zoning and segmentation. In detail, the gravity and magnetic anomalies to the south of 11°N of the KPR transition from high-amplitude continuous linear positive anomalies into low-amplitude intermittent linear positive anomalies. In contrast, the gravity and magnetic anomalies to the north of 11°N of the KPR are discontinuous and show alternating positive and negative anomalies. These anomalies can be divided into four sections, of which the separation points correspond well to the locations of deep faults, thus, revealing different field-source attributes and tectonic genesis of the KPR. (2) The Moho depth in the basins in the study area is 6–12 km. The Moho depth in the southern part of KPR show segmentation. Specifically, the depth is 10–12 km to the north of 11°N, 12–14 km from 9.5°N to 11°N, 14–16 km from 8.5°N to 9.5°N, and 16–25 km in the Palau Islands. (3) The KPR is a remnant intra-oceanic arc with the oceanic-crust basement.which shows noticeably discontinuous from north to south in geological structure and is intersected by NEE-trending lithospheric-scale deep faults. With large and deep faults F3 and F1 (the Mindanao fault) as boundaries overall, the southern part of the KPR can be divided into three zones. In detail, the portion to the south of 8.5°N (F3) is a tectonically active zone, the KPR portion between 8.5°N and 11°N is a tectonically active transition zone, and the portion to the north of 11°N is a tectonically inactive zone. (4) The oceanic crust in the KPR is slightly thicker than that in the basins on both sides of the ridge, and it is inferred that the KPR formed from the thickening of the oceanic crust induced by the upwelling of deep magma in the process of rifting of remnant arcs during the Middle Oligocene. In addition, it is inferred that the thick oceanic crust under the Palau Islands is related to the constant upwelling of deep magma induced by the continuous northwestward subduction of the Caroline Plate toward the Palau Trench since the Late Oligocene. This study provides a scientific basis for systematically understanding the crustal attributes, deep structures, and evolution of the KPR.©2021 China Geology Editorial Office.     

Electrical structures of the source area of the 1999 Chi-Chi, Taiwan, earthquake: Spatial correlation between crustal conductors and aftershocks

Wide-band magnetotelluric (MT) data were collected on an east–west profile, approximately perpendicular to the local strike of the Chelungpu thrust, through the hypocentral area of the Chi-Chi earthquake for imaging the seismogenic structure. MT data were then inverted for two-dimensional resistivity models plus best-fitting static shift parameters using a nonlinear conjugate gradient algorithm that minimizes the sum of the normalized data misfits and the smoothness of the model. As shown in the inverted 2D resistivity models presented in this paper, an electrical conductor beside the hypocenter of the Chi-Chi earthquake indicates that deep-crustal fluids may participate in the rupture process of the Chi-Chi earthquake. A striking spatial correlation between the crustal conductor and occurrence of aftershocks beneath the Chelungpu fault suggests a postseismic pore pressure adjustment ongoing after the mainshock. Additionally, the hypocenter exhibits an electrical resistive zone, consistent very well with a predicted compact zone from a crustal deformation and transient fluid flow modeling.     

Double-sided onshore–offshore seismic imaging of a plate boundary: “super-gathers” across South Island, New Zealand

Onshore–offshore seismic refraction profiling allows for the determination of crustal and mantle structures in the transition between continental and oceanic environments. Islands and narrow landmasses have the unique geometry of allowing for double-sided onshore–offshore experiments that favor the construction of composite “super-gathers” using the acquisition of onshore–offshore and ocean-bottom seismometer receiver gathers, land explosion shot gathers, and near-vertical incidence multichannel seismic (MCS) profiling. A number of sites at plate boundaries are amenable to the application of double-sided onshore–offshore imaging, including the Indo-Australian/Pacific transform boundary on South Island, New Zealand. By comparing the ratio of island width to mantle refraction (Pn) “maximum” crossover distance, using nondimensional distances, we provide an indicator of raypath “coverage” for crustal illumination. Islands or narrow land masses whose widths are less than twice their maximum crossover distance are candidates for double-sided onshore–offshore experiments. The SIGHT (South Island GeopHysical invesTigation) experiment in New Zealand is located where the width of South Island is sufficiently narrow with respect to its crustal thickness that a double-sided onshore–offshore experiment allows for complete crustal imaging of the associated plate boundary.     

Triassic paleomagnetism from the Western Pyrenees revisited: implications for the Iberian–Eurasian Mesozoic plate boundary

Since the pioneering studies of Van der Voo [Tectonophysics 7 (1969) 5] and Van der Voo and Boessenkool [J. Geophys. Res. 78 (1973) 5118], paleomagnetism of Permo-Triassic redbeds and volcanics from the Western Pyrenees has furnished important contributions for delineating the Mesozoic boundary between the Iberian and Eurasian plates. In this paper, we present a new paleomagnetic study focussed on Triassic red beds (23 sites) of the Paleozoic Basque Massifs (PBM). The aim of this study is to complement previous studies done in those massifs to better constrain the complex kinematics of the Western Pyrenees. Two stable magnetic components have been isolated: (1) a dual polarity, pre-folding magnetisation carried by specular hematite; and (2) a secondary, normal polarity component also carried by hematite. Our data confirm both the origin and the rotation pattern of the primary remanence described in previous works. Nevertheless, field tests performed on the secondary component do not confirm the earlier interpretations by Schott and Peres [Tectonophysics 156 (1988) 75] as they indicate a synfolding nature of the remagnetisation instead of a post-folding origin. We consider that the secondary component is better explained if a Cretaceous age is considered. The presence of such remagnetisation in the western Pyrenees strengthens the widespread occurrence of similar remagnetisation events reported in northern Iberia in connection with the extensional tectonic events that occurred during Cretaceous times. A comparison of the rotations recorded by the Triassic component and by the remagnetisation indicate that the Paleozoic units underwent variable tectonic rotations before the remagnetisation was acquired, most likely in connection with the counterclockwise rotation of Iberia with respect to Eurasia. These results favour that the Mesozoic plate boundary between the Iberian and Eurasian plates was a wide domain of distributed deformation and therefore contradict previous interpretations claiming for a discrete plate boundary.     

准噶尔盆地郝家沟剖面三叠纪—侏罗纪之交孢粉组合与古气候

三叠纪—侏罗纪之交（Triassic-Jurassic boundary,TJB）发生了三叠纪末的生物大灭绝（end-Triassic mass extinction,ETE）,海洋和陆地生态系统都发生了不同程度的崩溃。而在陆地生态系统中,植物群首当其冲,但是该时段的植物群变化的详细数据还相对缺乏。本文在新疆准噶尔盆地南缘郝家沟剖面TJB地层系统采样研究,来揭示该过程的植物群响应。根据孢粉属种组成及含量变化划分了3个孢粉组合,对古气候进行了重建。结果表明,准噶尔盆地南缘TJB的气候演变经历了湿热—湿冷—干热的转换过程。同时对ETE和TJB两个重要界线及灭绝发生的期次关系进行了讨论,由孢粉数据结合前人植物大化石、有机碳同位素地层曲线、生物标志化合物和Hg/TOC全球对比的结果,指示ETE和TJB可能分别位于郝家沟组43层底附近和八道湾组49层底附近;并推测三叠纪末陆地植被的灭绝在高纬度的准噶尔盆地记录到3个期次。在ETE与TJB之间地层中发现了与全球多处记录到的孢子含量激增一致的现象,推测该时期陆地生态系统的波动具有全球性。     

Multibeam bathymetry and sidescan imaging of the Rivera Transform–Moctezuma Spreading Segment junction, northern East Pacific Rise: New constraints on Rivera–Pacific relative plate motion

To better understand the recent motion of the Pacific plate relative to the Rivera plate and to better define the limitations of the existing Rivera–Pacific plate motion models for accurately predicting this motion, total-field magnetic data, multibeam bathymetric data and sidescan sonar images were collected during the BART and FAMEX campaigns of the N/O L'Atalante conducted in April and May 2002 in the area surrounding the Moctezuma Spreading Segment of the East Pacific Rise, located offshore of Manzanillo, Mexico, at 106°16′W, between 17.8°N and 18.5°N. Among the main results are: (1) the principle transform displacement zone of the Rivera Transform is narrow and well defined east of 107o15′W and these azimuths should be used preferentially when deriving new plate motion models, and (2) spreading rates along the Moctezuma Spreading Segment should not be used in plate motion studies as either seafloor spreading has been accommodated at more than one location since the initiation of seafloor spreading in the area of the Moctezuma Spreading Segment, or this spreading center is not a Rivera–Pacific plate boundary as has been previously assumed. Comparison of observed transform azimuths with those predicted by the best-fit poles of six previous models of Rivera–Pacific relative motion indicate that, in the study area, a significant systematic bias is present in the predictions of Rivera–Pacific motion. Although the exact source of this bias remains unclear, this bias indicates the need to derive a new Rivera–Pacific relative plate motion model.