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1.
Understanding the exhumation process of deep-seated material within subduction zones is important in comprehending the tectonic evolution of active margins. The deformation and slip history of superficial nappe pile emplaced upon high-P/T type metamorphic rocks can reveal the intimate relationship between deformation and transitions in paleo-stress that most likely arose from changes in the direction of plate convergence and exhumation of the metamorphic terrane. The Kinshozan–Atokura nappe pile emplaced upon the high-P/T type Sanbagawa (= Sambagawa) metamorphic rocks is the remnant of a pre-existing terrane located between paired metamorphic terranes along the Median Tectonic Line (MTL) of central Japan. Intra- and inter-nappe structures record the state of paleo-stress during metamorphism and exhumation of the Sanbagawa terrane. The following tectonic evolution of the nappes is inferred from a combined structural analysis of the basal fault of the nappes and their internal structures. The relative slip direction along the hanging wall rotated clockwise by 180°, from S to N, in association with a series of major tectonic changes from MTL-normal contraction to MTL-parallel strike-slip and finally MTL-normal extension. This clockwise rotation of the slip direction can be attributed to changes in the plate-induced regional stress state and associated exhumation of the deep-seated Sanbagawa terrane from the Late Cretaceous (Coniacian) to the Middle Miocene. 相似文献
2.
Soft-sediment deformation structures are common on passive continental margins, in trenches at subduction zones, and in strike-slip environments. Rocks from all these tectonic environments are incorporated into orogens, where soft-sediment deformation structures should be common. However, recognizing soft-sediment structures is difficult where superimposed tectonic structures are present. In seeking characteristic features of soft-sediment deformation, it is important to separate questions that relate to physical state (lithified or unlithified) from those that address the overall kinematic style (rooted or gravity driven). One recognizable physical state is liquefaction, which produces sand that has much lower strength than interbedded mud. Hence structures which indicate that mud was stronger than adjacent sand at the time of deformation can be used as indicators of soft-sediment deformation. These include angular fragments of mud surrounded by sand, dykes of sand cutting mud, and most usefully, folded sandstone layers displaying class 3 geometry interbedded with mud layers that show class 1 geometry. All these geometries have the potential to survive overprinting by later superimposed tectonic deformation; when preserved in deformed sedimentary rocks at low metamorphic grade they are indicators of liquefaction of unlithified sediment during deformation. 相似文献
3.
The effects of accretion, erosion and radiogenic heat on the metamorphic evolution of collisional orogens 总被引：4，自引：0，他引：4
Petrological and thermochronological data provide our best record of the thermal structure of deeply eroded orogens, and, in principle, might be used to relate the metamorphic structure of an orogen to its deformational history. In this paper, we present a two-dimensional thermal model of collisional orogens that includes the processes of accretion and erosion to examine the P – T evolution of rocks advected through the orogen. Calculated metamorphic patterns are similar to those observed in the field; metamorphic temperatures, depths and ages generally increase with distance from the toe of the orogen; P – T paths are anti-clockwise, with rocks heating during burial and early stages of unroofing, followed by cooling during late-stage unroofing. The results indicate that peak metamorphic temperatures within the core of a collisional orogen and the distance from the toe of an orogen to the metamorphic core can be related to the relative rates of accretion, erosion and plate convergence. Model orogens displaying high metamorphic temperatures (>600 °C) are associated with low ratios of accretion rate to plate convergence velocity and with high heat flow through the foreland. Model orogens with metamorphic cores far from the toe of the orogen are associated with high ratios of accretion rate to erosion rate. Calculated metamorphic gradients mimic steady-state geotherms, and inverted thermal gradients can be preserved in the metamorphic record, suggesting reconsideration of the concept that the metamorphic record does not closely reflect geothermal gradients within an orogen. 相似文献
4.
Ultrahigh-temperature (UHT) and high pressure (HP) metamorphic rocks generated at different times in Earth history form paired suites within the same tectonic belts in several regions. We evaluate the thermal regimes and fluid circulation patters in different plate tectonic settings and propose a new model involving ridge subduction where the slab window places hot asthenosphere against the base of the overlying plate and provides an ideal setting for the supply of heat and CO2 -rich fluids at relatively shallow domains, which can explain the formation and preservation of UHT metamorphic rocks in the roots of the arc and forearc. The plate geometry below the slab window would inhibit the normal cooling induced by the slab, and a temporary deprivation of slab-derived sediments and hydrous fluids. Our analysis provides a unified model for paired UHT and HP metamorphic orogens and presents a key to the associated thermal and fluid regimes. 相似文献
5.
大陆碰撞造山样式与过程：来自特提斯碰撞造山带的实例 总被引：2，自引：0，他引：2
本文选取特提斯域内比利牛斯、阿尔卑斯、扎格罗斯、喜马拉雅-青藏高原四个地球上最年轻的陆-陆碰撞造山带,对其造山带结构、类型、物质组成、构造岩浆过程等方面进行详细介绍,进而讨论各个造山带的差异性及其缘由,分析碰撞造山普遍性规律。资料分析表明,四个碰撞造山带具有不同的结构和组成。根据板块汇聚方向与造山带边界间的夹角可将造山带分为正向和斜向两种;根据造山带结构可将碰撞带分为对称式和不对称式两种。由此本文将碰撞造山带划分为四种基本式样:正向对称式、正向不对称式、斜向对称式、斜向不对称式,分别以比利牛斯、青藏高原、阿尔卑斯和扎格罗斯碰撞带为代表。综合分析四个造山带碰撞以来的岩浆构造活动,本文发现完整的碰撞过程可以划分为三个阶段,第一阶段主要发生挤压缩短、地壳加厚,高压变质和钙碱性火山岩浆活动;第二阶段以大规模走滑系统发育和高钾钙碱性或钾质火山岩浆作用为特征;第三个阶段挤压应力向碰撞带两侧扩展,同时伴有大型伸展构造系统的发育。在这三阶段演化历程中,比利牛斯只进行到第一阶段,成为幼年夭折的碰撞带;扎格罗斯进行到第二阶段,出现调节挤压应变的走滑系统和钾质超钾质岩浆活动;青藏高原和阿尔卑斯进行到第三个阶段,以发育大型伸展构造和钾质、超钾质岩浆活动为特征,但后者在造山带物质组成和汇聚速率方面显示出比前者更成熟的造山演化程度。因此认为岩石圈组成是碰撞造山带结构的主要控制因素,如果上覆板块具有相对不稳定的岩石圈,会使得碰撞带后陆发育宽广的构造岩浆带,造成造山带呈不对称式结构。 相似文献
6.
7.
Dominique Frizon de Lamotte Jean-Claude Guézou Marie-Anne Albertini 《Geodinamica Acta》2013,26(4):267-281
AbstractThe Cenozoic westward motion of the Betic-Rif internal zone (“Alboran block”) between Iberia and Africa is constrained by paleogeographic considerations and by wrench faulting which affects both sides of the external zones. However, in the Alboran domain itself there was so far no evidence of significant internal deformation related to this westward displacement which was consequently consider as an en bloc” motion. Our work, in Eastern Andalucia, demonstrates that the main tectonic units building up the Betic zone should be regarded as large-scale tectonic sheets with a typical duplex style. The direction of the tectonic transport is to the West. At meso-scale, the major structures exhibit a combination of hindward and foreward dipping imbricates on the respective east and west sides of antiformal stacks or “rigid cores”. On a broader scale, the same geometric framework appears on both east and west sides of the Sierra Nevada window which we interpret as a tectonic culmination on the hangingwall of a Subbetic décollement zone. The development of this tectonics, in retrogressive metamorphic conditions, postdates the ductile deformation of the internal complexes. The morpho-tectonic relationships between the culmination of the metamorphic cores and the Neogene basins give a way to date the westward motion of the “Alboran System of Nappes” of the middle and upper Miocene. 相似文献
8.
《International Geology Review》2012,54(15):1889-1921
This article describes the geometry and structural architecture of the Viù Deformation Zone (VDZ), a brittle-ductile to brittle structure affecting the metamorphic units of the inner Western Alps, and its role in modifying the pre-existing syn-metamorphic structural setting. The VDZ reactivates and displaces the contact between two different oceanic units, the Lanzo Ultramafic Complex and the Lower Susa–Lanzo Valleys Unit, characterized by polyphase syn-metamorphic deformation. It shows a strike-slip duplex geometry, constituted by N–S reverse-dextral faults linked by NW–SE antithetical sinistral-reverse faults, and represents a contractional step-over zone along a N–S regional dextral-reverse structure, the Col del Lis-Trana Deformation Zone. Formation of these transpressional structures steepened the Lanzo Ultramafic Complex during the last stages of its exhumation. The 3D geometry of the VDZ was strongly controlled by the reactivation of pre-existing structures, such as the buried western edge of the Ivrea body and metamorphic foliations. Brittle reactivation also induced block rotation along the VDZ, causing anomalous kinematic relations between the VDZ-associated faults. This study, hence, shows that in metamorphic orogens the mechanisms generating strike-slip duplexes may be different from those classically provided by the literature, with brittle reactivation and block rotation strongly prevailing on newly formed faults. In such orogens, moreover, rotations induced by transpressional faulting may sometimes be mistaken for steep syn-metamorphic shear zones. Underestimating the effects of later brittle deformation and associated rotations may cause erroneous interpretations of the tectonic evolution of orogens. 相似文献
9.
The Precambrian terranes of southern Peninsular India have been central to discussions on the history of formation and breakup of supercontinents. Of particular interest are the Proterozoic high grade metamorphic orogens at the southern and eastern margins of the Indian shield, skirting the 3.4 Ga Dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics, but also carry imprints of the tectonic framework related to the assembly of the major Neoproterozoic supercontinents – Rodinia and Gondwana. These Proterozoic orogens are described as Southern Granulite Terrane (SGT) in the southern tip and the Eastern Ghats Mobile Belt (EGMB) in the eastern domains of the peninsula. The contiguity of these orogens is broken for a distance of ∼400 km and disappears in the Bay of Bengal. These orogens expose windows of middle to lower crust with well-preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths.Recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems, some of which represent collisional sutures. The SGT and EGMB carry several important features such as fold-thrust tectonics, regional granulite facies metamorphism of up to ultrahigh-temperature conditions in some cases, multiple P–T paths, development of lithospheric shear zones, emplacement of ophiolites, presence of alkaline and anorthositic complexes, development of crustal-scale “flower structures”, transpressional strains, and reactivation tectonics. A heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified. Both EGMB and SGT share a common transpressional deformation history during the latest Neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a subvertical grain conducive to reactivation tectonics. Our synthesis of the spatial distribution, geometry, kinematics and the transpressional strain of the shear zone systems provides insights into the tectono-metamorphic history of the Proterozoic orogens of southern India and their contiguity and complexities. Recent understanding of subduction, accretion and collisional history along these zones together with a long lived transpressional tectonic regime imply that these orogens witnessed identical tectonic regimes at different times in Earth history, although the major and common structural architecture was built during the final assembly of the Gondwana supercontinent. 相似文献
10.
Tertiary geodynamics of Sakhalin (NW Pacific) from anisotropy of magnetic susceptibility fabrics and paleomagnetic data 总被引：1，自引：0，他引：1
Richard Weaver Andrew P Roberts Rachel Flecker David I.M Macdonald 《Tectonophysics》2004,379(1-4):25-42
Sakhalin has been affected by several phases of Cretaceous and Tertiary deformation due to the complex interaction of plates in the northwest Pacific region. A detailed understanding of the strain is important because it will provide constraints on plate-scale processes that control the formation and deformation of marginal sedimentary basins. Anisotropy of magnetic susceptibility (AMS) data were obtained from fine-grained mudstones and siltstones from 22 localities in Sakhalin in order to provide information concerning tectonic strain. AMS data reliably record ancient strain tensor orientations before significant deformation of the sediments occurred. Paleomagnetically determined vertical-axis rotations of crustal rocks allow rotation of the fabrics back to their original orientation. Results from southwest Sakhalin indicate a N035°E-directed net tectonic transport from the mid-Paleocene to the early Miocene, which is consistent with the present-day relative motion between the Okhotsk Sea and Eurasian plates. Reconstruction of early–late Miocene AMS fabrics in east Sakhalin indicates a tectonic transport direction of N040°E. In west Sakhalin, the transport direction appears to have remained relatively consistent from the Oligocene to the late Miocene, but it has a different attitude of N080°E. This suggests local deflection of the stress and strain fields, which was probably associated with opening of the northern Tatar Strait. A northward-directed tectonic transport is observed in Miocene sediments in southeast Sakhalin, mid-Eocene sediments in east Sakhalin, and in Late Cretaceous rocks of west and northern Sakhalin, which may be associated with northwestward motion and subduction of the Pacific Plate in the Tertiary period. The boundaries of the separate regions defined by the AMS data are consistent with present-day plate models and, therefore, provide meaningful constraints on the tectonic evolution of Sakhalin. 相似文献
11.
从洋-陆俯冲到陆-陆碰撞:回眸与展望 总被引：2，自引：0，他引：2
大陆造山带的经典含义是指由于大陆地壳岩石在板块俯冲-碰撞的巨大挤压应力下,遭受强烈变形、变质和熔融作用,地壳发生大规模缩短、加厚和隆升而形成的地带。分布在大陆边缘和内部的造山带,经历从洋壳扩张、洋-陆俯冲到陆-陆碰撞的造山过程,形成"俯冲增生型"、"陆陆碰撞型"和远离板块边界的"陆内型"造山带。造山带类型的分析是识别地球上造山带机制的钥匙。本文在阐述经典造山带分类的基础上,根据造山带的几何学、热历史、构造样式等特征,讨论了弧形造山带、特殊几何学造山带和走滑造山带的结构、运动学和动力学,以及从洋-陆俯冲到陆-陆碰撞造山在时空上的转化和演化。在回顾造山带研究的基础上,突出在板块汇聚边界的大洋和大陆俯冲带研究的重大进展,提出俯冲带和地幔柱提供了穿越地球层圈物质和能量交换的通道,它们的结合研究是探索全球单层壳-幔大循环假说与板块驱动力的新方向,是统领造山带研究的大思路。对于大陆动力学研究的一些前瞻性问题的思考,强调了造山过程的热扩散模式和变形-变质-深熔-成矿作用的自组织行为,以及地壳熔融在造山中的重要性;强调了流变学在大陆造山带形成和演化中的基础地位,并认为这是造山带研究中亟待解决的问题。作者认为板块水平运动是致使地壳挤压缩短和加厚、形成造山带的主要驱动力;而在板块离散边界(包括大洋中脊)垂直上升流所形成"地貌"上的山链,被称为"伸展造山带",不应属于经典"造山带"的范畴。 相似文献
12.
Tim H.Bell 《地学前缘》1999,(3):4
过去还无人指出过板块相对运动的方向与缓倾斜叶理、逆断层和断层上的线状指示物有直接关系,这是因为缓倾斜构造上的运动方向只和变厚了的造山地层的重力塌陷有关,它们和俯冲板块传递给仰冲板块的推力没有关系。缓倾斜叶理上的运动方向的线状指示物和斑状变晶中的叶理弯曲或叶理交切轴（FIA）并无直接关系,这是因为FIA的指向受缓倾斜叶理和斑状变晶边缘上产生的、近乎垂直的叶理之间的交切面控制。在班状变晶边缘上形成的、近乎垂直的叶理在基质中的方位可能在较大范围内变动,因为它们会在稍早期间形成的叶理再活化作用影响下发生转动或遭到破坏。斑状变晶边缘上近乎垂直的叶理,与形成于早期或晚期的缓倾斜叶理的交线,在后期的生长中被圈闭在班状变晶里,此交线规定出了FIA的方位,而与叶理上的运动方向无关。从美国佛蒙特州阿巴拉契亚山脉采集的FIA资料指出,在125km×35km的一片地区内,在该地岩层所发生的多次变形中,从未曾使早期形成的FIA组的方位发生变动。这种情况要求：后来的每一代褶皱都是由于渐进的。总体不均匀缩短作用造成的。这种情况表明：FIA保存着原始的运动方向,此方向未因以后的变形而转动。非洲板块与欧洲板块的相对运动方向和由阿尔卑斯期变质岩中叶理交切轴（FIAs）所指示 相似文献
13.
Fold-and-thrust belts are prominent structures that occur at the front of compressional orogens. To unravel the tectonic and metamorphic evolution of such complexes, kinematic investigations, quantitative microstructural analysis and geothermometry (calcite–graphite, calcite–dolomite) were performed on carbonate mylonites from thrust faults of the Helvetic nappe stack in Central Switzerland. Paleo-isotherms of peak temperature conditions and cooling stages (fission track) of the nappe pile were reconstructed in a vertical section and linked with the microstructural and kinematic evolution. Mylonitic microstructures suggest that under metamorphic conditions close to peak temperature, strain was highly localized within thrust faults where deformation temperatures spatially continuously increased in both directions, from N to S within each nappe and from top–down in the nappe stack, covering a temperature range of 180–380 °C. Due to the higher metamorphic conditions, thrusting of the lowermost nappe, the Doldenhorn nappe, was accompanied by a much more pronounced nappe internal ductile deformation of carbonaceous rock types than was the case for the overlying Wildhorn- and Gellihorn nappes. Ongoing thrusting brought the Doldenhorn nappe closer to the surface. The associated cooling resulted in a freezing in of the paleo-isotherms of peak metamorphic conditions. Contemporaneous shearing localized in the basal thrust, initially still in the ductile deformation regime and finally as brittle faulting and cataclasis inducing ultimately an inverse metamorphic zonation. With ongoing exhumation and the formation of the Helvetic antiformal nappe stack, a bending of large-scale tectonic structures (thrusts, folds), peak temperature isotherms and cooling isotherms occurred. While this local bending can directly be attributed to active deformation underneath the section investigated up to times of 2–3 ma, a more homogeneous uplift of the entire region is suggested for the very late and still active exhumation stage. 相似文献
14.
《International Geology Review》2012,54(5):375-387
We use updated rotations within the Pacific-Antarctica-Africa-North America plate circuit to calculate Pacific-North America plate reconstructions for times since chron 13 (33 Ma). The direction of motion of the Pacific plate relative to stable North America was fairly steady between chrons 13 and 4, and then changed and moved in a more northerly direction from chron 4 to the present (8 Ma to the present). No Pliocene changes in Pacific-North America plate motion are resolvable in these data, suggesting that Pliocene changes in deformation style along the boundary were not driven by changes in plate motion. However, the chron 4 change in Pacific-North America plate motion appears to correlate very closely to a change in direction of extension documented between the Sierra Nevada and the Colorado Plateau. Our best solution for the displacement with respect to stable North America of a point on the Pacific plate that is now near the Mendocino triple junction is that from 30 to 12 Ma the point was displaced along an azimuth of ～N60°W at rate of ～33 mm/yr; from 12 Ma to about 8 Ma the azimuth of displacement was about the same as previously, but the rate was faster (～52 mm/yr); and since 8 Ma the point was displaced along an azimuth of N37°W at a rate of ～52 mm/yr. We compare plate-circuit reconstructions of the edge of the Pacific plate to continental deformation reconstructions of North American tectonic elements across the Basin and Range province and elsewhere in order to evaluate the relationship of this deformation to the plate motions. The oceanic displacements correspond remarkably well to the continental reconstructions where deformations of the latter have been quantified along a path across the Colorado Plateau and central California. They also supply strong constraints for the deformation budgets of regions to the north and south, in Cascadia and northern Mexico, respectively. We examine slab-window formation and evolution in a detailed re-analysis of the spreading geometry of the post-Farallon microplates, from 28 to 19 Ma. Development of the slab window seems linked to early Miocene volcanism and deformation in the Mojave Desert, although detailed correlations await clarification of early Miocene reconstructions of the Tehachapi Mountains. We then trace the post-20 Ma motion of the Mendocino slab window edge beneath the Sierran-Great Valley block and find that it drifted steadily north, then stalled just north of Sutter Buttes at ～4 Ma. 相似文献
15.
Investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets. To illustrate the problems in documenting flow vectors we present integrated structural-metamorphic datasets from two transpressional systems; the Kaoko Belt in Namibia and the Kalinjala Shear Zone in South Australia. These orogens experienced widely differing metamorphic responses to transpressional deformation. Integration of kinematic and metamorphic datasets from the Kaoko Belt indicate shallow up-plunging extrusion trajectories in the orogen core, and show that the maximum stretching direction pattern matches the inferred flow vectors. High-grade domains (800–840 °C and 7.0–8.0 kb) in the orogen core developed low-angle upward-verging maximum stretching direction trajectories, whereas a low-grade domain (575–600 °C and 5.0–5.5 kb) in the orogen core has downward-verging lineation trajectories. The barometric differential between these high-grade and low-grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high-grade domains that were extruded obliquely, for the amount of lateral shear estimated for the orogen core. The Kalinjala Shear Zone in South Australia contrasts strongly with the Kaoko Belt. In this example, the high-grade and high-strain shear zone core of the orogen, experienced high-T/high-P metamorphism with low thermal gradients of 21–26 °C/km and steep decompressive P–T paths. The lower-grade external domains experienced lower-T/lower-P metamorphism with high thermal gradients of 35–37 °C/km. Sub-horizontal maximum stretching directions do not match the vertical extrusional flow in the high-grade core that is indicated by the metamorphic data. This comparison shows that in general and on a gross scale, maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis. In some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow. Consequently, information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record. These two datasets must be used in concert to obtain realistic constraints on first-order material flow trajectories at orogenic scales. The horizontal component of flow is typically best recorded by structural fabrics (maximum stretching direction and sense of shear), whereas the vertical component is typically best recorded by metamorphic information, such as P–T paths, temperature over depth ratio (G) and metamorphic field gradients (i.e. ΔT, ΔP and ΔG) across the orogen. 相似文献
16.
华北克拉通的形成以及早期板块构造 总被引：21，自引：0，他引：21
地球上最早的地壳岩石是高钠的花岗质(TTG)岩石,但是否有更老的洋壳存在过、以及陆壳是怎样形成的,涉及到地球动力学几乎所有的问题。其中板块构造是在什么时候开始的,就是个延续了数十年热度不减的前沿科学问题。流行的说法是板块构造始于新元古代,也有一些学者认为在新太古代就已经开始,或者认为自从地球上有了水的记录,就开始有板块构造。在众多的判别板块构造的标志中,蛇绿岩残片和古老的高压变质岩无疑是两个最具影响力的问题。前者可以确定有远古的古老洋壳存在过并成为缝合带中的残片,后者可以指示曾有地表的岩石单元被俯冲到深部,是俯冲、消减与碰撞的岩石学证据。本文在讨论和比较了太古宙绿岩带与蛇绿岩,以及早前寒武纪高温高压(HTHP)麻粒岩/高温—超高温(HT-UHT)麻粒岩与造山带高压变质带之后,认为尚不能作为板块构造的证据。本文还对华北的新太古代末的稳定大陆形成以及古元古代活动带的裂谷-俯冲-碰撞进行了论述。提出华北克拉通在新太古代末的绿岩带-高级区格局可能标志着热体制下有限的横向活动构造,微陆块被火山-沉积岩系焊接,随后发生变质作用和花岗岩化,完成稳定大陆的克拉通化过程。其构造机制可能是适度规模且多发的地幔柱构造控制下小尺度的横向构造运动的机制。华北克拉通的古元古代活动带有与绿岩带-高级区不同的构造样式,表壳岩带状分布,经受了强烈的变形以及中级变质作用,伴随花岗岩的侵入,虽然没有蛇绿岩和高压变质带,但已表现出板块构造的雏形特征。 相似文献
17.
R. L. Romer 《地学学报》2001,13(4):258-263
The isotopic composition of lead available for incorporation during metamorphic reactions is heterogeneous, depends on the reaction history of the metamorphic rock, and is commonly not accessible for measurement as the precursor minerals have been consumed during the growth of the metamorphic phases. The initial lead composition has a significant effect on the age of low-238 U/204 Pb metamorphic phases (e.g. garnet, rutile, titanite, staurolite, vesuvianite and ilmenite). Using a distinct value (e.g. leached K-feldspar Pb, model Pb) rather than a geologically reasonably constrained range may result in apparently precise, yet inaccurate ages. Since age data from metamorphic minerals are widely used to unravel the P–T–t–d evolution of orogens, inaccurate ages result in: (1) incorrect timing (duration) of P–T loops and associated with it the heat budget and mass transfer in orogens; (2) arbitrary rates (based on the age difference between core and rim) for mineral growth, P–T evolution and deformation; and (3) apparent sequences of isotopic closure for the U–Pb system of contrasting minerals. 相似文献
18.
Plate collision and paleostress trajectories in a fold-thrust belt: The foothills of Taiwan 总被引：1，自引：0，他引：1
Field analyses of compressional faulting and folding in the Foothills of western Taiwan enable us to reconstruct paleostress trajectories over a large area and to establish the relative chronology of tectonic events. Two main compressional events have contributed to the present structure of the fold-thrust belt. Stratigraphic data show that these events are Plio-Pliocene in age. Older normal faulting indicates NNW-SSE extension across the Chinesse passive continental margin during the Neogene. The two main compressional events of the Taiwan collision correspond to similar fan-shaped distributions of maximum compressive stress trajectories, with a counterclockwise shift of 30°–50° between the two events. Using the relationship between recent stress trajectories and the direction of recent plate motion as a guide, we reconstruct the direction of plate convergence for the older event. We suspect that the relative motion Philippine Sea plate-Eurasia has rotated counterclockwise of at least 35°–45° in Taiwan during collision. This conclusion is in agreement with independent plate tectonic reconstructions. Several problems provide objectives to further tectonic and paleomagnetic studies, including the duration and diachronism of compressional events as well as possible clockwise rotation of northernmost Taiwan. 相似文献
19.
Victor A. Ramos 《Geological Journal》2010,45(1):2-25
The analyses of the main parameters controlling the present Chile‐type and Marianas‐type tectonic settings developed along the eastern Pacific region show four different tectonic regimes: (1) a nearly neutral regime in the Oregon subduction zone; (2) major extensional regimes as the Nicaragua subduction zone developed in continental crust; (3) a Marianas setting in the Sandwich subduction zone with ocean floored back‐arc basin with a unique west‐dipping subduction zone and (4) the classic and dominant Chile‐type under compression. The magmatic, structural and sedimentary behaviours of these four settings are discussed to understand the past tectonic regimes in the Mesozoic Andes based on their present geological and tectonic characteristics. The evaluation of the different parameters that governed the past and present tectonic regimes indicates that absolute motion of the upper plate relative to the hotspot frame and the consequent trench roll‐back velocity are the first order parameters that control the deformation. Locally, the influences of the trench fill, linked to the dominant climate in the forearc, and the age of the subducted oceanic crust, have secondary roles. Ridge collisions of seismic and seismic oceanic ridges as well as fracture zone collisions have also a local outcome, and may produce an increase in coupling that reinforces compressional deformation. Local strain variations in the past and present Andes are not related with changes in the relative convergence rate, which is less important than the absolute motion relative to the Pacific hotspot frame, or changes in the thermal state of the upper plate. Changes in the slab dip, mainly those linked to steepening subduction zones, produce significant variations in the thermal state, that are important to generate extreme deformation in the foreland. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
20.
S. WALLIS 《Journal of Metamorphic Geology》1998,16(1):83-95
Tectonic processes that have been proposed to explain the transport to the surface of regional metamorphic belts can be broadly divided into two types. (i) Corner-flow within a convergent margin bounded by two essentially rigid plates associated with extension at shallow levels. This type of model assumes deformation is distributed throughout the margin and that any discontinuities are of secondary importance. (ii) Expulsion or extrusion of coherent metamorphic nappes. In this second idea, tectonic discontinuities are fundamental in the transport to the surface of metamorphic rocks. The wealth of geological data available from a variety of studies in the Sanbagawa metamorphic belt, southwest Japan makes it well-suited for studying the relative importance of continuous vs. discontinuous deformation in the process of exhumation. In the Sanbagawa belt a sudden decrease in metamorphic pressure going down section of several kilobars suggests the presence of a major tectonic contact separating two major regional nappes: an overlying higher-pressure Besshi nappe and an underlying lower-pressure Oboke nappe. Major tectonic discontinuities have also been proposed within the Besshi nappe, however, indicators of metamorphic temperature, the results of radiometric age dating, and microstructural studies all suggest that post-metamorphic discontinuities are minor and that this nappe formed and remained as an essentially coherent unit. Lithological associations and petrological studies suggest the following positions for the two nappes. The Besshi nappe formed deep within the former accretionary wedge, adjacent to the overlying mantle wedge, and with a dip of roughly 30 °C. In contrast, the Oboke nappe formed at moderate depths within the accretionary wedge, was distant from the mantle wedge, and was roughly horizontal. Penetrative deformation that post-dates the peak of metamorphism has affected nearly all of the Sanbagawa belt and has played an important role in its exhumation. However, the presence of a broad coherent Besshi nappe overlying the lower-pressure Oboke nappe suggests that some process such as buoyancy-driven extrusion was also important in the exhumation process and in forming the structure of the Sanbagawa metamorphic belt. 相似文献