Integration of natural data within a numerical model of ablative subduction: a possible interpretation for the Alpine dynamics of the Austroalpine crust

A numerical modelling approach is used to validate the physical and geological reliability of the ablative subduction mechanism during Alpine convergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the Alpine belt: the Austroalpine Domain. The model predictions and the natural data for the Austroalpine of the Western Alps agree very well in terms of P–T peak conditions, relative chronology of peak and exhumation events, P–T–t paths, thermal gradients and the tectonic evolution of the continental rocks. These findings suggest that a pre‐collisional evolution of this domain, with the burial of the continental rocks (induced by ablative subduction of the overriding Adria plate) and their exhumation (driven by an upwelling flow generated in a hydrated mantle wedge) could be a valid mechanism that reproduces the actual tectono‐metamorphic configuration of this part of the Alps. There is less agreement between the model predictions and the natural data for the Austroalpine of the Central‐Eastern Alps. Based on the natural data available in the literature, a critical discussion of the other proposed mechanisms is presented, and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results; these factors include variations in the continental and/or oceanic thickness, variation of the subduction rate and/or slab dip, the initial thermal state of the passive margin, the occurrence of continental collision and an oblique convergence.     

Debris flow and slide deposits at the top of the Internal Liguride ophiolitic sequence, Northern Apennines, Italy: A record of frontal tectonic erosion in a fossil accretionary wedge

Abstract In the Northern Apennines, the Internal Liguride units are characterized by an ophiolite sequence that represents the stratigraphic base of a late Jurassic–early Paleocene sedimentary cover. The Bocco Shale represents the youngest deposit recognized in the sedimentary cover of the ophiolite and can be subdivided into two different groups of deep sea sediments. The first group is represented by slide, debris flow and high density turbidity current-derived deposits, whereas the second group consists of thin-bedded turbidites. Facies analysis and provenance studies indicate, for the former group, small and scarcely evoluted flows that rework an oceanic lithosphere and its sedimentary cover. We interpret the Bocco Shale as an ancient example of a deposit related to the frontal tectonic erosion of the accretionary wedge slope. The frontal tectonic erosion resulted in a large removal of materials, from the accretionary wedge front, that was reworked as debris flows and slide deposits sedimented on the lower plate above the trench deposits. The frontal tectonic erosion was probably connected with subduction of oceanic crust characterized by positive topographic relief. This interpretation can be also applied for the origin of analogous deposits of Western Alps and Corsica.     

褶皱—冲断构造的动力学研究现状和力学分析初探

综述了褶皱—冲断构造的几何特征,造山带的岩石物理性质以及临界锥体力学分析研究现状,认为运用临界锥体力学原理可以很好地解释褶皱—冲断构造发育和发展动力学机制。     

浅变质岩在示踪大别—苏鲁造山带大陆板块俯冲与折返过程中的意义

中国大别-苏鲁造山带为大陆板块俯冲形成的碰撞造山带,该带北缘和内部产有原岩时代为新元古代-晚古生代的浅变质岩。这些浅变质岩对应于扬子板块北缘前寒武变质基底和扬子板块北缘古生代大陆架沉积物,形成过程于印支期扬子板块向北俯冲过程中的刮削作用密切相关,与大洋板块俯冲过程中刮削形成的加积楔具有类似的动力学过程。对大别-苏鲁造山带浅变质岩的深入研究,不仅有助于揭示大陆板块俯冲过程中高压-超高压岩石形成与折返过程,而且确定了扬子板块与华北板块之间的缝合线位置位于大别造山带北淮阳带的北部和苏鲁造山带的五莲-蓬莱群的北侧。     

Ar/Ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a Mesozoic accretionary wedge, Otago Schist, New Zealand

Structural thickening of the Torlesse accretionary wedge via juxtaposition of arc-derived greywackes (Caples Terrane) and quartzo-feldspathic greywackes (Torlesse Terrane) at 120 Ma formed a belt of schist (Otago Schist) with distinct mica fabrics defining (i) schistosity, (ii) transposition layering and (iii) crenulation cleavage. Thirty-five ⁴⁰Ar/³⁹Ar step-heating experiments on these micas and whole rock micaceous fabrics from the Otago Schist have shown that the main metamorphism and deformation occurred between 160 and 140 Ma (recorded in the low grade flanks) through 120 Ma (shear zone deformation). This was followed either by very gradual cooling or no cooling until about 110 Ma, with some form of extensional (tectonic) exhumation and cooling of the high-grade metamorphic core between 109 and 100 Ma. Major shear zones separating the low-grade and high-grade parts of the schist define regions of separate and distinct apparent age groupings that underwent different thermo-tectonic histories. Apparent ages on the low-grade north flank (hanging wall to the Hyde-Macraes and Rise and Shine Shear Zones) range from 145 to 159 Ma (n=8), whereas on the low-grade south flank (hanging wall to the Remarkables Shear Zone or Caples Terrane) range from 144 to 156 Ma (n=5). Most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline metamorphic mica and lesser detrital mica. Several of the hanging wall samples with ages of 144–147 Ma show no evidence for detrital contamination in thin section or in the form of the age spectra. Apparent ages from the high-grade metamorphic core (garnet–biotite–albite zone) range from 131 to 106 Ma (n=13) with a strong grouping 113–109 Ma (n=7) in the immediate footwall to the major Remarkables Shear Zone. Most of the age spectra from within the core of the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial retention interval for white mica (430–330 °C). Samples with apparent ages of about 110–109 Ma tend to give concordant plateaux suggesting more rapid cooling. The youngest and most disturbed age spectra come from within the ‘Alpine chlorite overprint’ zone where samples with strong development of crenulation cleavage gave ages 85–107 and 101 Ma, due to partial resetting during retrogression. The bounding Remarkables Shear zone shows resetting effects due to dynamic recrystallization with apparent ages of 127–122 Ma, whereas overprinting shear zones within the core of the schist show apparent ages of 112–109 and 106 Ma. These data when linked with extensional exhumation of high-grade rocks in other parts of New Zealand indicate that the East Gondwana margin underwent significant extension in the 110–90 Ma period.     

Travel time of accreted igneous assemblages in western Pacific orogenic belts and their associated sedimentary rocks

Accreted igneous assemblages in orogenic belts maybe divided into three types depending on whether they derive from seamounts, ocean ridges or subduction-related ophiolites. Seamount type basalts are associated with shallow water sediments—mostly reefoidal limestones. Ocean ridge type basalts are generally overlain by pelagic cherts. Subduction-related ophiolitic eruptives, often underlain by gabbroic and ultramafic rocks, are associated with hemipelagic mudstones. The age of such diverse eruptive lithologic assemblages reflects the time taken for them to have traveled from their locus of generation to their place of accretion at a continental margin. This relationship has been established for each type of accretionary complex, examples being taken mostly from Japan and the western Pacific rim in order to represent evolutionary processes at a typical active plate margin. In general, the seamount types are older, ridge types are of intermediate age, and the ophiolitic types are by far the youngest, usually close to zero age. Seamount type basalts are accreted by shallower scraping of the seamount's sediment apron together with fragments of seamount basalt, ridge type, by peeling due to permeability contrast, and the ophiolitic types by deeper scraping as a consequence of an inflected temperature gradient. Accordingly, it is concluded that the ophiolitic rocks are generated close to the trench and may be accreted as a result of ridge subduction.     

The Misis–Andırın Complex: a Mid-Tertiary melange related to late-stage subduction of the Southern Neotethys in S Turkey

The Mid-Tertiary (Mid-Eocene to earliest Miocene) Misis–Andırın Complex documents tectonic-sedimentary processes affecting the northerly, active margin of the South Tethys (Neotethys) in the easternmost Mediterranean region. Each of three orogenic segments, Misis (in the SW), Andırın (central) and Engizek (in the NE) represent parts of an originally continuous active continental margin. A structurally lower Volcanic-Sedimentary Unit includes Late Cretaceous arc-related extrusives and their Lower Tertiary pelagic cover. This unit is interpreted as an Early Tertiary remnant of the Mesozoic South Tethys. The overlying melange unit is dominated by tectonically brecciated blocks (>100 m across) of Mesozoic neritic limestone that were derived from the Tauride carbonate platform to the north, together with accreted ophiolitic material. The melange matrix comprises polymict debris flows, high- to low-density turbidites and minor hemipelagic sediments.The Misis–Andırın Complex is interpreted as an accretionary prism related to the latest stages of northward subduction of the South Tethys and diachronous continental collision of the Tauride (Eurasian) and Arabian (African) plates during Mid-Eocene to earliest Miocene time. Slivers of Upper Cretaceous oceanic crust and its Early Tertiary pelagic cover were accreted, while blocks of Mesozoic platform carbonates slid from the overriding plate. Tectonic mixing and sedimentary recycling took place within a trench. Subduction culminated in large-scale collapse of the overriding (northern) margin and foundering of vast blocks of neritic carbonate into the trench. A possible cause was rapid roll back of dense downgoing Mesozoic oceanic crust, such that the accretionary wedge taper was extended leading to gravity collapse. Melange formation was terminated by underthrusting of the Arabian plate from the south during earliest Miocene time.Collision was diachronous. In the east (Engizek Range and SE Anatolia) collision generated a Lower Miocene flexural basin infilled with turbidites and a flexural bulge to the south. Miocene turbiditic sediments also covered the former accretionary prism. Further west (Misis Range) the easternmost Mediterranean remained in a pre-collisional setting with northward underthrusting (incipient subduction) along the Cyprus arc. The Lower Miocene basins to the north (Misis and Adana) indicate an extensional (to transtensional) setting. The NE–SW linking segment (Andırın) probably originated as a Mesozoic palaeogeographic offset of the Tauride margin. This was reactivated by strike-slip (and transtension) during Later Tertiary diachronous collision. Related to on-going plate convergence the former accretionary wedge (upper plate) was thrust over the Lower Miocene turbiditic basins in Mid–Late Miocene time. The Plio-Quaternary was dominated by left-lateral strike-slip along the East Anatolian transform fault and also along fault strands cutting the Misis–Andırın Complex.     

南安集海-南玛纳斯背斜的构造分析及其横向转换研究

南安集海背斜和南玛纳斯背斜属于准噶尔南缘山前中西段第一排构造带,构造变形异常强烈,呈典型的堆跺式双重构造.通过应用断层相关褶皱的构造几何分析方法,用计算机对南安集海-南玛纳斯背斜带内的基干地震剖面进行了构造解析,南玛纳斯背斜是在霍-玛-吐断裂上叠加一堆跺式双重构造组成,向西南玛纳斯背斜上的堆跺式双重构造逐渐消失,同时霍-玛-吐断裂位移量逐渐转换成反向逆冲断层,形成南安集海背斜的复合型构造三角楔.通过构造解析,弄清了它们在三维空间中的过渡转换关系,对后继的南缘山前构造带的勘探开发具有参考意义.