Rapid tectonic exhumation, detachment faulting and orogenic collapse in the Caledonides of western Ireland

Collision of the oceanic Lough Nafooey Island Arc with the passive margin of Laurentia after 480 Ma in western Ireland resulted in the deformation, magmatism and metamorphism of the Grampian Orogeny, analogous to the modern Taiwan and Miocene New Guinea Orogens. After 470 Ma, the metamorphosed Laurentian margin sediments (Dalradian Supergroup) now exposed in Connemara and North Mayo were cooled rapidly (>35 °C/m.y.) and exhumed to the surface. We propose that this exhumation occurred mainly as a result of an oceanward collapse of the colliding arc southwards, probably aided by subduction rollback, into the new trench formed after subduction polarity reversal following collision. The Achill Beg Fault, in particular, along the southern edge of the North Mayo Dalradian Terrane, separates very low-grade sedimentary rocks of the South Mayo Trough (Lough Nafooey forearc) and accreted sedimentary rocks of the Clew Bay Complex from high-grade Dalradian meta-sedimentary rocks, suggesting that this was a major detachment structure. In northern Connemara, the unconformity between the Dalradian and the Silurian cover probably represents an eroded major detachment surface, with the Renvyle–Bofin Slide as a related but subordinate structure. Blocks of sheared mafic and ultramafic rocks in the Dalradian immediately below this unconformity surface probably represent arc lower crustal and mantle rocks or fragments of a high level ophiolite sheet entrained along the detachment during exhumation.Orogenic collapse was accompanied in the South Mayo Trough by coarse clastic sedimentation derived mostly from the exhuming Dalradian to the north and, to a lesser extent, from the Lough Nafooey Arc to the south. Sediment flow in the South Mayo Trough was dominantly axial, deepening toward the west. Volcanism associated with orogenic collapse (Rosroe and Mweelrea Formations) is variably enriched in high field strength elements, suggesting a heterogeneous enriched mantle wedge under the new post-collisional continental arc.     

Syn-orogenic extension in the Peloritani Alpine Thrust Belt (NE Sicily,Italy): Evidence from the Alì Unit

Structural and petrological analyses on the Alì Unit, in the Peloritani Thrust Belt, document the first evidence for Alpine exhumation associated with syn-orogenic extension in this part of the Calabria-Peloritani Arc. The Alì Unit displays ductile structures occurred during three Alpine deformation phases (D_a1, D_a2, D_a3). D_a1 and D_a3 developed in a contractional context, whereas D_a2 was generated in an extensional regime. The present-day tectonic contact between the Alì Unit and the overlying Mandanici Unit is interpreted as a low-angle extensional detachment responsible for the metamorphic break between the two units. Structural overprinting relationships indicate that the development of D_a2 structures and related tectonic exhumation occurred during syn-convergence extension, and were followed by further nappe stacking in the Peloritani Belt. To cite this article: R. Somma et al., C. R. Geoscience 337 (2005).     

Late- and post-Variscan cooling and exhumation history of the northern Rhenish massif and the southern Ruhr Basin: new constraints from fission-track analysis

Apatite fission-track analyses were carried out on outcrop and core samples from the Rhenish massif and the Carboniferous Ruhr Basin/Germany in order to study the late- and post-Variscan thermal and exhumation history. Apatite fission-track ages range from 291±15 Ma (lower Permian) to 136±7 Ma (lower Cretaceous) and mean track lengths vary between 11.6 m and 13.9 m, mostly displaying unimodal distributions with narrow standard deviations. All apatite fission-track ages are younger than the corresponding sample stratigraphic age, indicating substantial post-depositional annealing of the apatite fission-tracks. This agrees with results from maturity modelling, which indicates 3500–7000 m eroded Devonian and Carboniferous sedimentary cover. Numerical modelling of apatite fission-track data predicts onset of exhumation and cooling not earlier than 320 Ma in the Rhenish massif and 300 Ma in the Ruhr Basin, generally followed by late Carboniferous–Triassic cooling to below 50–60°C. Rapid late Variscan cooling was followed by moderate Mesozoic cooling rates of 0.1–0.2°C/Ma, converting into denudation rates of <1 mm/a (assuming a stable geothermal gradient of 30°C/km). Modelling results also give evidence for some late Triassic and early Jurassic heating and/or burial, which is supported by sedimentary rocks of the same age preserved at the rim of the lower Rhine Basin and in the subsurface of the Central and Northern Ruhr Basin. Cenozoic exhumation and cooling of the Rhenish massif is interpreted as an isostatic response to former erosion and major base-level fall caused by the subsidence in the lower Rhine Basin.     

Early Tertiary paleo-thermal effects in Northern England: reconciling results from apatite fission track analysis with geological evidence

The Paleozoic Lake District Block in northwest England has traditionally been thought of as tectonically stable since the Late Paleozoic, receiving only small thicknesses of Late Paleozoic to Mesozoic cover (although some workers have put forward different views). Apatite fission track analysis (AFTA) data from outcrop samples across the region reveal Early Tertiary paleotemperatures around 100 °C, requiring kilometre-scale Late Paleozoic and Mesozoic cover, removed during Tertiary uplift and erosion. With no evidence for elevated basal heat flow in NW England during the Early Tertiary, and no a priori justification for invoking it, earlier studies favoured an explanation involving burial by up to 3 km of overburden removed during Tertiary uplift and erosion. This conclusion was met with scepticism by many workers, and provoked a range of comments and criticisms, with a variety of alternative interpretations put forward, although these are also open to criticism. Results from the West Newton-1 hydrocarbon exploration well on the northern flank of the Lake District gave the first indication of a possibly more realistic interpretation, involving a combination of elevated heat flow and more restricted burial, but some aspects of the interpretation of these data were equivocal. More detailed sampling was therefore undertaken, in order to shed more light on the origin of the elevated Early Tertiary paleotemperatures observed across NW England. New AFTA data in outcrop samples from different elevations around Sca Fell (characterised by the highest elevations in the Lake District with the summit of Scafell Pike at 978 m asl) define an Early Tertiary paleogeothermal gradient of 61 °C/km, and require around 700 m of section removed from the summit during Tertiary uplift and erosion. These results, together with those from the West Newton-1 well, provide strong support for an interpretation involving Early Tertiary paleogeothermal gradients between 50% and 100% higher than present-day values, providing clear evidence of elevated basal heat flow during the Early Tertiary, contrary to earlier assumptions. Combined with amounts of section removed during Tertiary exhumation varying between 0.7 km (from mountain peaks) and 1.5–2 km (from coastal plains and glacial valleys near sea level) over the region, this interpretation finally provides a geologically plausible mechanism for the origin of the observed Early Tertiary paleo-thermal effects in NW England.     

苏鲁高压-超高压变质带折返抬升过程中构造界面和应力场的变化

作为地质历史时期深俯冲作用产物的超高压变质岩,尽管有不同的形成演化历史,但其最终的折返作用无不是在逆冲扩展过程中完成的。因此,逆冲扩展作用及其数值模拟的研究对完整认识苏鲁高压-超高压变质带的折返机制和折返过程具有重要意义。逆冲扩展作用最显观的构造效应是不同层次构造界面和应力场的变化,对于地壳尺度的逆冲扩展作用而言,陆表面和Moho是最重要的活动性构造界面,陆表面的变化导致山体抬升和滞后伸展盆地的形成,出现盆-山相间的构造格局。初步的模拟计算表明,山体的抬升量、滞后伸展盆地的坳陷量和Moho的上拱量与逆冲地块的平移速度和逆冲扩展速度成正相关关系,而且随着时间的推移,山体的抬升速度、滞后伸展盆地的坳陷速度和Moho的上拱速度都有逐渐增大趋势。逆冲扩展过程中,构造应力场的变化总的表现为随着平移速度和逆冲扩展速度的增大和逆冲扩展作用的持续进行,逆冲块体内部由挤压应力状态逐渐向拉张应力状态转化。地块的平移速度是构造强度的一个重要标量,当高压-超高压变质带以仰冲块体为运动载体,沿断裂带向陆壳浅部折返时,构造界面的移动规律基本反映了高压-超高压变质带的折返过程,可见,构造作用的强度和性质应该是制约高压-超高压变质带折返的重要因素之一,在岩石密度差相同的条件下,拉张构造应力场更有利于折返作用的进行。苏鲁高压-超高压变质带的折返是通过多期构造作用完成的,根据数值模拟结果可以推测,每期构造作用都伴随有折返速度由慢到快的变化,在整个折返过程中,构造运动性质和强度的差异导致了折返速度的不均一,总体上,折返速度将随着逆冲地块由挤压向拉张状态的转化和拉张强度（构造作用强度）的不断增强而逐渐增大,最终在以拉张为主导的构造应力场中完成了高压-超高压变质带折返的全过程。     

Episodic,two-stage Neogene extension and short-term intervening compression in Western Turkey: field evidence from the Kiraz Basin and Bozdağ Horst

Western Anatolia (Turkey) is a region of widespread active N-S continental extension that forms the eastern part of the Aegean extensional province. The extension in the region is expressed by two distinct/different structural styles, separated by a short-term gap: (1) rapid exhumation of metamorphic core complexes along presently low-angle ductile-brittle normal faults commenced by the latest Oligocene-Early Miocene period, and; (2) late stretching of crust and, consequent graben evolution along Plio-Quaternary high-angle normal faults, cross-cutting the pre-existing low-angle normal faults. However, current understanding of the processes (tectonic quiescence vs N-S continental compression) operating during the short-time interval is incomplete. This paper therefore reports the results of recent field mapping and structural analysis from the NE of Küçük Menderes Graben—Kiraz Basin—that shed lights on the processes operating during this short-time interval. The data includes the thrusting of metamorphic rocks of the Menderes Massif over the Mio-Pliocene sediments along WNW-ESE-trending high-angle reverse fault and the development of compressional fabrics in the metamorphic rocks of the Menderes Massif. There, the metamorphic rocks display evidence for four distinct phases of deformation: (1) southfacing top-N ductile fabrics developed at relatively high-grade metamorphic conditions, possibly during the Eocene main Menderes metamorphism (amphibolite facies) associated with top-N thrust tectonics (D₁); (2) top-S and top-N ductile gentle-moderatley south-dipping extensional fabrics formed at relatively lower-grade metamorphic (possibly greenschist facies) conditions associated with the exhumation of Menderes Massif along presently low-angle normal fault plane that accompanied the first phase of extension (D₂); (3) moderately north-dipping top-S ductile-brittle fabrics, present configuration of which suggest a thrust-related compression (D₃); and (4) south-facing approximately E-W-trending brittle high-angle normal faults (D₄) that form the youngest structures in the region. It is interpreted that D₄ faults are time equivalent of graben-bounding major high-angle normal faults and they correspond to the second phase of extension in western Anatolia. The presence of thrust-related D₃ compressional fabrics suggests N-S compression during the time interval between the two phases of extension (D₂ and D₄). The results of the present study therefore support the episodic, two-stage extension model in western Anatolia and confirm that a short-time, intervening N-S compression separated the two distinct phases.     

Ductile crust exhumation and extensional detachments in the central Aegean (Cyclades and Evvia Islands).

Abstract

The Aegean continental domain is known to be the site of widespread “back-arc” extension since at least 13 Ma, on the basis of seismotectonic, stratigraphic and fault analysis studies. This extension is documented to overprint structures related to the Mesozoic-Cenozoic Hellenic orogeny. Features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered HP/LT metamorphism across the Aegean. This study presents a structural analysis of the central Aegean area (Cyclades and Evvia Islands), examining in particular the relationship between ductile and brittle deformation, both in the field and on a regional scale. Extension appears to be responsible for most of the ductile deformation within HP rock units that have experienced penetrative greenschist facies and higher grade metamorphic over-printing. On each studied island, progressive extensional deformation has occurred through the development of a major normal-sense detachment zone down to depths of about 18-25 km. Large displacement along the detachment zone accounts for rapid cooling and exhumation of ductile lower crust to form a local metamorphic dome or core complex. Structural and stratigraphic features support a progressive migration of normal faulting away from the dome axis, and a rotation of previously active faults toward low dips, as in kinematic models recently suggested for the development of extensional detachment systems. All the studied domes, except that seen on los Island, show a dominant top-to-the north or north-east sense of shear, while on the southern flank of many of them, an opposite sense of shear is observed, displaying the same progressive evolution from ductile to brittle rock behaviour. This opposite sense of shear is thought not to result from shearing along a major conjugate detachment zone, as in some recent models, but from the accommodation in the ductile crust of upward bending of the brittle upper crust in the footwall of the north-dipping detachment. Available radiometric and stratigraphie data indicate an early minimum age (22-19 Ma) for the onset of extension. The relationship between early metamorphic domes and shallow-dipping detachments, on one hand, and Messinian-Quaternary steep normal faults and grabens, on the other hand, is best explained with the progressive and continuous development of new normal faults away from the domes axes, rather than with a two-stage evolutionary model (core-complex stage, then Basin-and-Range stage) of the type invoked for the North American Cordillera.     

青藏高原东缘峨眉山新生代加速抬升剥蚀作用

本文通过峨眉山基底卷入构造带低温热年代学（磷灰石和锆石裂变径迹、锆石（U-Th）/He）研究,结合典型构造-热结构特征诠释峨眉山晚中-新生代冲断扩展变形与热年代学耦合性.峨眉山磷灰石裂变径迹（AFT）和锆石（U-Th）/He（ZHe）年龄值分别为4~30 Ma和16~118 Ma.ZHe年龄与海拔高程关系揭示出ZHe系统抬升剥蚀残存的部分滞留带（PRZ）.低温热年代学年龄与峨眉山构造分带性具有明显相关性特征：万年寺逆断层上盘基底卷入构造带AFT年龄普遍小于10 Ma,万年寺逆断层下盘扩展变形带AFT年龄普遍大于10 Ma;且空间上AFT年龄与断裂带具有明显相关性,它揭示出峨眉山扩展变形带中新世晚期以来断层冲断缩短构造活动.低温热年代学热史模拟揭示峨眉山构造带晚白垩世以来的多阶段性加速抬升剥蚀过程,基底卷入构造带岩石隆升幅度大约达到7~8 km,渐新世以来抬升剥蚀速率达0.2~0.4 mm·a^-1,其新生代多阶段性构造隆升动力学与青藏高原多板块间碰撞过程及其始新世大规模物质东向扩展过程密切相关.

     

碎屑矿物热年代学研究进展

碎屑矿物热年代学是以沉积物中未遭受热重置的碎屑矿物颗粒为研究对象,将造山带抬升、剥露和盆地沉积联系起来的年代学方法。在物源分析、源区构造热历史、古地形恢复和约束地层年龄等方面有广泛的应用。在概述其基本原理的基础上,从造山带剥露历史分析、物源分析中多种技术的联合运用和现代汇水区侵蚀速率计算3个方面介绍了这一方法近年来的研究进展。     

Structural development of the Tso Morari ultra-high pressure nappe of the Ladakh Himalaya

A continental subduction-related and multistage exhumation process for the Tso Morari ultra-high pressure nappe is proposed. The model is constrained by published thermo-barometry and age data, combined with new geological and tectonic maps. Additionally, observations on the structural and metamorphic evolution of the Tso Morari area and the North Himalayan nappes are presented. The northern margin of the Indian continental crust was subducted to a depth of > 90 km below Asia after continental collision some 55 Ma ago. The underthrusting was accompanied by the detachment and accretion of Late Proterozoic to Early Eocene sediments, creating the North Himalayan accretionary wedge, in front of the active Asian margin and the 103–50 Ma Ladakh arc batholith. The basic dikes in the Ordovician Tso Morari granite were transformed to eclogites with crystallization of coesite, some 53 Ma ago at a depth of > 90 km (> 27 kbar) and temperatures of 500 to 600 °C. The detachment and extrusion of the low density Tso Morari nappe, composed of 70% of the Tso Morari granite and 30% of graywackes with some eclogitic dikes, occurred by ductile pure and simple shear deformation. It was pushed by buoyancy forces and by squeezing between the underthrusted Indian lithosphere and the Asian mantle wedge. The extruding Tso Morari nappe reached a depth of 35 km at the base of the North Himalayan accretionary wedge some 48 Ma ago. There the whole nappe stack recrystallized under amphibolite facies conditions of a Barrovian regional metamorphism with a metamorphic field gradient of 20 °C/km. An intense schistosity with a W–E oriented stretching lineation L₁ and top-to-the E shear criteria and crystallization of oriented sillimanite needles after kyanite, testify to the Tso Morari nappe extrusion and pressure drop. The whole nappe stack, comprising from the base to top the Tso Morari, Tetraogal, Karzok and Mata–Nyimaling-Tsarap nappes, was overprinted by new schistosities with a first N-directed and a second NE-directed stretching lineation L₂ and L₃ reaching the base of the North Himalayan accretionary wedge. They are characterized by top-to-the S and SW shear criteria. This structural overprint was related to an early N- and a younger NE-directed underthrusting of the Indian plate below Asia that was accompanied by anticlockwise rotation of India. The warping of the Tso Morari dome started already some 48 Ma ago with the formation of an extruding nappe at depth. The Tso Morari dome reached a depth of 15 km about 40 Ma ago in the eastern Kiagar La region and 30 Ma ago in the western Nuruchan region. The extrusion rate was of about 3 cm/yr between 53 and 48 Ma, followed by an uplift rate of 1.2 mm/yr between 48 and 30 Ma and of only 0.5 mm/yr after 30 Ma. Geomorphology observations show that the Tso Morari dome is still affected by faults, open regional dome, and basin and pull-apart structures, in a zone of active dextral transpression parallel to the Indus Suture zone.