A cross section of the eastern Betic Cordillera (SE Spain) according field data and a seismic reflection profile

The BT3 multichannel seismic profile was acquired by the C.G.G. (Compagnie General de Géophysique) in 1977 for hydrocarbon exploration in the eastern Betic Cordillera. REXIMseis Ltd scanned and vectorized a paper copy and then performed post-stack processing, including coherence filtering and deconvolution. The receiver functions of a broad-band seismic station located near the village of Vélez Rubio, at the SE end of the profile, were analysed by Julia et al. [Julia, J., Mancilla, F., Morales, J., 2005. Seismic signature of intracrustal magmatic intrusions in the Eastern Betics (Internal Zone), SE Iberia, Geophysical Research Letters 32, L16304, doi:10.1029/2005GL023274.] to determine the structure of the underlying crust. We have used these Vp data to convert the profile to depth. The profile has a mean SE–NW trend, with a SE-Section 44 km in length followed by a NW-Section 20 km in length. The record includes the first 4 s (twtt), which corresponds to 11 km.Two main areas can be seen in the profile. At the SE-end, a band of high-amplitude discontinuous reflectors dips towards the north. The band is 100 to 200 ms thick, increasing even more northwards. This band reaches the surface at the top of the Maláguide Complex (the upper complex of the Internal Zones). Above these reflectors, an area with chaotic seismic facies and no reflectors corresponds to the outcrops of the olistostromes and turbidites of the Solana Formation, and it is in turn overlain by discontinuous reflectors of the Subbetic rocks.At the NW-end of the profile, a set of high-amplitude continuous reflectors with SE dips point to the location of the Prebetic. Below this section, oblique reflectors of intermediate amplitude indicate the Variscan basement. Over the Prebetic, we have marked the basal thrusts of the Intermediate Units and the Subbetic. Using this seismic data, as well as field observations, we propose a geological cross-section of the upper crust of the eastern Betic Cordillera and a model of the most recent evolution of the orogen. In this model, the Internal Zones and the Subbetic have been welded together from the Middle Burdigalian to the present day and acted as an orogenic wedge that deformed the Intermediate Units and the Prebetic.     

Timing and origin of orogenic remagnetizations in Mississippian carbonates, Sawtooth Range, Montana

Paleomagnetic data and rock magnetic results suggest that a widespread orogenic remagnetization caused a pervasive chemical remanent magnetization (CRM) that resides in magnetite in the Mississippian Madison Group in the Sawtooth Range, Montana, during the Late Cretaceous to Early Tertiary. The CRM is similar to a CRM reported by workers from equivalent units in the southern Canadian Cordillera. The CRM is interpreted to be related to alteration by fluids, and there are two likely fluids: hydrocarbons which migrated into the unit and externally derived radiogenic fluids.     

METAMORPHISM IN THE LESSER HIMALAYAN CRYSTALLINES AND MAIN CENTRAL THRUST ZONE IN THE ARUN VALLEY AND AMA DRIME RANGE (EASTERN HIMALAYA)

METAMORPHISM IN THE LESSER HIMALAYAN CRYSTALLINES AND MAIN CENTRAL THRUST ZONE IN THE ARUN VALLEY AND AMA DRIME RANGE (EASTERN HIMALAYA)1　BrunelM ,KienastJR . tudep啨ｔｒｏ structuraledeschevauchementsductileshimalayenssurlatrans versaledel’Everest Makalu (N啨ｐａｌｏｒｉｅｎｔａｌ) [J].CanadianJ .EarthSciences,1986 ,2 3:1117～ 1137. 2　LombardoB ,RolfoF .TwocontrastingeclogitetypesintheHimalayas :implicationsfortheHimalayanorogeny…     

The Gramscatho Basin, south Cornwall, UK: Devonian active margin successions

Deep marine deposits of the Gramscatho Basin of south Cornwall reflect two tectonic regimes; Early to Middle Devonian rifting of continental lithosphere with formation of oceanic lithosphere to the south, and Middle Devonian to earliest Carboniferous convergence along its southern margin. Sediments on thinned continental crust to the north and oceanic lithosphere to the south were juxtaposed in the Late Devonian when nappes of deep water flysch and olistostrome were thrust up on to the northern continental margin of the basin. Basin closure was accommodated by forward propagating thrust nappes, accompanied by penecontemporaneous sedimentation. The stratigraphical sequences of major nappes illustrate the progradation of flysch with climactic sedimentation of olistostrome in late Mid- to Late Devonian times. The Lizard Complex, including the Lizard ophiolite, within that nappe stack, constitutes part of one of the GCR sites which are largely in the allochthonous rocks. Many of those sites feature the olistostrome, Roseland Breccia Formation, with its great variety of sedimentary, igneous and metamorphic clasts (up to 1.5 km), and the association of ocean floor basalt and penecontemporaneous acidic volcanics indicative of the coming together of oceanic and continental plates. A site at the top of the parautochthonous continental margin succession displays the erosion products of the youngest nappe as it emerged and advanced across the sediment surface, marking closure of the oceanised Gramscatho Basin and continental collision.     

非开挖铺管中钻头泥包问题的分析与对策

非开挖导向铺管过程中,当钻遏较软岩层或泥页岩层时,经常会遇到钻头被泥包的现象,尤其在扩孔过程中,更为明显。针对这种现象,从三个方面认真了分析钻头泥包的原因,并提出从泥浆方面入手的对策。进行室内推力实验,在武汉江夏区的非开挖施工现场提出配方并加以应用,对提高钻进效率有重要的意义。     

Geometry and kinematics of the Andean thick-skinned thrust systems: Insights from the Chilean Frontal Cordillera (28°–28.5°S), Central Andes

The structure of the Chilean Frontal Cordillera, located over the Central Andes flat-slab subduction segment (27°–28.5°S), is characterized by a thick-skinned deformation, affecting both the pre-rift basement and the Mesozoic and Cenozoic infill of the NNE-SSW Lautaro and Lagunillas Basins, which were developed during the Pangea-Gondwana break-up. The compressive deformation show a complex interaction between Mesozoic rift structures and thrust systems, affecting a suite of Permo-Triassic (258–245 Ma) granitic blocks. We used a combination of geological mapping, new structural data, balanced and restored cross sections and geochronological data to investigate the geometry and kinematics of the Andean thick-skinned thrust systems of the region. The thrust systems include double-vergent thick-skinned thrust faults, basement-cored anticlines and minor thin-skinned thrusts and folds. The presence of Triassic and Jurassic syn-rift successions along the hanging wall and footwall of the basement thrust faults are keys to suggest that the current structural framework of the region should be associated with the shortening of previous Mesozoic half grabens. Based on this interpretation, we propose a deformation mechanism characterized by the tectonic inversion of rift-related faults and the propagation of basement ramps that fold and cut both, the early normal faults and the basement highs. New U–Pb ages obtained from synorogenic deposits (Quebrada Seca and Doña Ana formations) indicate at least three important compressive pulses. A first pulse at ∼80 Ma (Late Cretaceous), a second pulse related to the K-T phase of Andean deformation and, finally, a third pulse that occurred during the lower Miocene.     

Kinematic model for out-of-sequence thrusting: Motion of two ramp-flat faults and the production of upper plate duplex systems

Kinematic models developed here suggest a bewildering array of structural styles can be generated during out-of-sequence thrusting. Many of these structures would be difficult to distinguish from a normally stacked thrust sequence and the process can produce younger-on-older faults that could easily be misinterpreted as normal faults. This paper considers a small subset of this problem within a large model space by considering structures that develop along a pair of ramp-flat faults that are moving simultaneously, or sequentially. Motion on the lower ramp warps the structurally higher fault due to fault-bend folding and when the fault ruptures through the warp it transfers a horse to the upper hanging wall. Continuity of the process generates what is referred to here as an “upper plate duplex” to distinguish the structure from a conventional duplex. Kinematic parameters are developed for two models within this general problem: 1) a system with a fixed ramp in the lower thrust, overridden by an upper thrust; and 2) a double-duplex system where a conventional duplex develops along the lower fault at the same time as an upper plate duplex is formed along the upper fault. The theory is tested with forward models using 2D Move software and these tests indicate different families of structural styles form in association with relative scaling of ramp systems, slip-ratio between faults, and aspect ratios of horse blocks formed in the upper-plate duplex. A first-order result of the analysis is that an upper plate duplex can be virtually indistinguishable from a conventional duplex unless the trailing branch lines of the horses are exposed or imaged; a condition seldom met in natural exposures. Restoration of an upper-plate duplex produces counterintuitive fault geometry in the restored state, and thus, restorations of upper plate duplexes that erroneously assume a conventional duplex model would produce restored states that are seriously in error. In addition, in most of the models some fault segments place younger rocks on older rocks which could be easily misinterpreted as normal fault systems. In some models younger-on-older juxtapositions are significant and if scaled to crustal scale would produce core-complex style structures that would be difficult to recognize as contractional features. Collectively, these observations imply that many areas where simultaneous contraction and extension are inferred may be entirely contractional with younger-on-older relationships generated by out-of-sequence thrust systems. Examples where this process may have occurred are in southwestern North America and the Moine thrust system and future studies should evaluate these systems in light of these models. Distinguishing upper plate duplex from conventional duplex is potentially important in economic evaluations of thrust systems because fluid migration paths would be very different in the two alternatives. The process may also be important in seismogenic mechanisms, particularly in subduction megathrusts, because faults warping faults could produce fault irregularities that would form transient asperities along the fault.     

西藏阿里地区冈底斯花岗岩体的热年代学特征及其构造意义

冈底斯花岗岩带是沿雅鲁藏布江北侧近东西向展布的一条长约2 500 km、宽100～300 km的巨型岩浆岩带。在阿里冈底斯山主峰岗仁波齐峰附近,冈底斯花岗岩体受到多期断裂活动的影响,特别受喜马拉雅大反向断裂和喀喇昆仑断裂活动的改造,造成断裂和岩体的关系出现很多复杂的情况,一些研究者把在北阿伊拉日居山分布的32～25 Ma的花岗岩作为喀喇昆仑断裂活动引起的同构造花岗岩,并把此年龄段归结为断层活动年龄,从而引起了极大的争论。本文的锆石U-Pb年龄指示了岗仁波齐峰地区的冈底斯花岗岩是由110 Ma、60 Ma和50 Ma的3期花岗岩组成,而韧性剪切带内的锆石年龄与附近未变形岩石内的锆石年龄一致,表明锆石的形态并未受到断裂活动的影响。韧性剪切带内云母的氩氩年龄为12 Ma左右,而周围未变形花岗岩的云母氩氩年龄在60～50 Ma左右,由此表明喀喇昆仑断裂在岗仁波齐峰地区是12 Ma开始活动的。由于研究区内韧性剪切带中的变形花岗岩并没有记录32～25 Ma这期热事件,由此排除了断裂在狮泉河-门士一线是32～25 Ma开始活动的可能性。     

Strain partitioning between the Suruga Trough and the Zenisu Thrust: Neogene to present tectonic evolution in the onland and offshore Tokai district, Japan

The Tokai district in central Japan is located close to the convergent boundary between the Philippine Sea and Eurasian plates, and has experienced not only repeated large interplate earthquakes but also intense aseismic movement. In this paper, the spatial and temporal tectonic evolution of the Tokai district, particularly around the Omaezaki area, is discussed to assess whether the district has been and will be active or inactive. According to a geological survey, the horizontal crustal shortening strain can imply the hypothetical tectonic model that the area has been getting less active and the strain rate since the Neogene can be calculated as 12% and 2×10⁻⁶%/year, respectively. The present interseismic horizontal crustal strain and strain rate around the Omaezaki area are approximately 4×10⁻⁷% and 4×10⁻⁹%/year. By comparing these rates, the decrease since Neogene can imply the hypothetical tectonic model that the area has been getting less active influenced by the strain partitioning between the Suruga Trough and the Zenisu Thrust.