Provenance of Cretaceous clastics in the Subhercynian Basin: constraints to exhumation of the Harz Mountains and timing of inversion tectonics in Central Europe

The Harz Mountains and the adjacent Subhercynian Cretaceous Basin figure as the most prominent surface representative for Late Cretaceous inversion structures in Central Europe. Facies, depositional architecture and provenance of the basin fill reflect mechanisms and timing of the exhumation of the Harz. From Hauterivian to Early Santonian there is no evidence for detrital input from the nearby Harz area. Sediments are mature quartzarenites derived from Paleozoic basement rocks and/or recycled Permian to Mesozoic sedimentary rocks. This situation changed drastically in Middle to Late Santonian when freshly exhumed and eroded Mesozoic sedimentary cover rocks of the Harz were delivered into the basin. Feldspar and lithoclasts reflect erosion of Triassic and, in places, Jurassic to Turonian strata. Apatite and garnet in heavy mineral spectra are derived from largely unweathered Lower Triassic Buntsandstein as indicated by apatite and garnet chemistry. In Early Campanian, Paleozoic lithoclasts indicate erosion cutting down into the basement of the Harz. Simultaneous strong decrease of feldspar, garnet and apatite suggest an almost complete removal of the 2–3 km thick Mesozoic cover of the Harz within only 2–4 Myr. This translates into an exhumation rate of approximately 1 mm/a consistent with apatite fission track data from granitoid rocks of the Harz Mountains.     

Evidence for exhumation of a granite intrusion in a regional extensional stress regime based on coupled microstructural and fluid inclusion plane studies – An example from the Velence Mts., Hungary

A new approach, the fluid inclusion plane technique coupled with fluid inclusion microthermometry and field measurements have been applied to demonstrate the exhumation and relative vertical displacements of an allochthonous, Permian granite intrusion (Velence Mts.) which was situated in the Middle Triassic at the western end of the Neotethys rift system. The pressure and the temperature conditions during the Permian fluid flow (>350 °C and ∼2 kbar) in the granite were considerably higher than during the Triassic fluid flow (<250 °C and ∼0.5 kbar), which indicates the exhumation of the granite intrusion in a primarily extensional tectonic stress regime. The fluid inclusion planes with NE–SW and NW–SE strike prove stress field permutation that can be explained by the exhumation of the host granite during the Middle Triassic fluid flow.It is suggested that the exhumation of the Velence Mts. along with the regional fluid flow can be connected to the passive pre-rift phase of the northern Adriatic Block, during the Middle–Late Triassic. The variations in homogenization temperatures of Triassic regional fluid inclusion assemblages between the blocks of the granite, as well as variations in the fluid inclusion plane orientations, suggest a post-Triassic vertical segmentation and relative block rotation of the granite.     

Exhumation history and faulting activity of the southern segment of the Longmen Shan,eastern Tibet

The Longmen Shan (LMS), which constitutes the eastern border of the Tibetan Plateau, is about 400 km in length and characterized by a steep topographic transition from the Sichuan Basin to the plateau. The 2008 M_w7.9 Wenchuan earthquake and 2013 M_w6.6 Lushan earthquake were associated with the central to northern segments and southern segment of the LMS fault belt, respectively. In this paper, zircon and apatite fission track (ZFT and AFT, respectively) dating in combination with previously published low temperature thermochronology studies are used to constrain both the exhumation history and fault activity along the LMS, with a special focus on the southern segment. In the southern segment of the LMS, the ZFT ages in the hanging wall of the Wulong-Yanjing fault 10–14 Ma, increasing to ca. 30 Ma to the northwest of the faults and to 100–200 Ma in the plateau region. The AFT ages are 3–5 Ma at the mountain front and increase to 8–26 Ma in the plateau. We show that these age distributions are controlled by fault geometry. Two stages of rapid exhumation were identified using apatite fission track length modeling and the age distributions in the southern segment of the LMS. The first stage is from ca. 30 Ma and the second stage is from 3–5 Ma to present. In contrast with the middle segment of the LMS, the Cenozoic exhumation rate is higher in the southern segment of the LMS, which may be due to the influence of the collision between the India and Eurasia plates and/or different faulting mechanisms in the different segments.     

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.     

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.     

俯冲洋壳的折返及其相关问题讨论

大洋俯冲带中高压(HP)和超高压(UHP)岩石的折返机制一直以来都是俯冲工厂中最不为人知的问题之一.本文根据搜集全球折返到地表的洋壳榴辉岩基础数据(包括岩石学特征、峰期温压条件和折返P-T轨迹),初步探讨了洋壳榴辉岩的折返机制.根据峰期矿物组合、温压条件和对应的地温梯度,典型大洋俯冲带中的榴辉岩可以分为三类:含柯石英的UHP硬柱石榴辉岩(2.7～ 3.2GPa,470 ～ 610℃,5～7℃/km)、HP硬柱石榴辉岩(1.7～2.6GPa,360～ 620℃,5～8℃/km)和HP绿帘石榴辉岩(1.5 ～2.3 GPa,540 ～ 630℃,7～12℃/km).与大陆俯冲碰撞造山带中的HP-UHP榴辉岩相比,洋壳榴辉岩具有较低的峰期温压条件和较高的低密度含水矿物的含量,但是普遍缺失高密度的蓝晶石.已有的俯冲洋壳的折返模式都基于一个假设:洋壳榴辉岩密度比周围地幔大.因此,洋壳榴辉岩的折返必须借助于低密度的蛇纹岩或者变沉积岩.MORB体系的热力学模拟研究表明,俯冲洋壳的矿物组合、矿物含量和密度主要受低密度含水矿物(如硬柱石、绿泥石、蓝闪石和滑石等)的稳定性控制,并且在同等深度条件下,冷俯冲洋壳的密度低于热俯冲洋壳的密度.经历冷俯冲(～6℃/km)洋壳的密度在＜ 110～ 120km(P ＜3.3 ～ 3.6GPa)的深度仍小于周围地幔,但是经历热俯冲(～ 1O℃/km)洋壳的密度在＞60km(P＞1.8GPa)的深度就已经超过周围地幔.结合高温高压实验资料和地球物理观察数据,我们认为在＞120km的深度,俯冲基性洋壳本身密度大于周围地幔,不存在低密度的地幔楔蛇纹岩(蛇纹石已发生分解),并且大洋板块的俯冲角度突然增大可能阻碍了更深部的低密度变沉积岩的折返.以上这三个方面的原因可能导致现今折返到地表的洋壳榴辉岩和变沉积岩的形成深度普遍小于120km.折返过程中硬柱石脱水分解会导致洋壳密度增大,退变形成的蓝晶石榴辉岩的密度大于周围地幔,无法折返,这可能是全球洋壳榴辉岩中普遍缺失蓝晶石的主要原因.     

Ultrahigh-pressure texture inheritance during retrogression: Evidence from magnetofabrics in eclogites and ultramafic rocks (Chinese Continental Scientific Drilling project)

In order to contribute to a better understanding of exhumation related retrogression processes ultrahigh-pressure (UHP) mafic and ultramafic rocks from the Chinese Continental Scientific Drilling (CCSD) in the Maobei eclogite body of the Sulu ultrahigh-pressure metamorphic belt in eastern China were studied for their magnetofabrics. Variably retrogressed eclogites and serpentinized ultramafic rocks were retrieved from the depth interval of 100 to 1000 m of the borehole. A vein network of irregular shaped veins with a retrograde metamorphic assemblage cuts across the eclogite foliation at both low and high angles. SEM imaging of the eclogites documents that magnetite associated with retrograde pargasitic amphiboles developed around shape-preferred garnet. SEM imaging of the serpentinized ultramafic rocks documents that magnetite rims grew around shape-preferred garnet and that magnetite formed within a mesh texture related to serpentinization. Syn-serpentinization magnetite growth increased bulk susceptibilities, but reduced anisotropy. Maximum susceptibility axes from both eclogites and serpentinized garnet-peridotites trend N-S, i.e. parallel with the stretching lineation as defined by olivine and omphacite grains. This implies that the magnetic fabric mimics the UHP fabric and survived retrogression and that the fabric is inherited from the UHP stage. As a consequence, retrogression was not associated with substantial ductile deformation and the mafic-ultramafic Maobei body behaved as a rigid body within a ductile deforming quartzo-feldspathic matrix during exhumation. Internal strain is restricted to brittle fracturing, associated fluid circulation and vein formation facilitating retrograde reactions in the mafic-ultramafic rocks.     

Genesis of garnet peridotites in the Sulu UHP belt: Examples from the Chinese continental scientific drilling project-main hole, PP1 and PP3 drillholes

The main hole (MH), and pre-pilot holes PP1, and PP3 of the Chinese Continental Scientific Drilling Project (CCSD) penetrated three different garnet peridotite bodies in the Sulu ultrahigh pressure (UHP) metamorphic belt, which are 80 m, 120 m, and 430 m thick, respectively. The bodies occur as tectonic blocks hosted in eclogite (MH peridotite) and gneisses (PP1 and PP3 peridotites). The peridotites in the MH are garnet wehrlites, whose protoliths were ultramafic cumulates based on olivine compositions (Fo_79-89) and other geochemical features. Zoned garnet and omphacite (with 4-5 wt.% Na₂O) are typical metamorphic minerals in these rocks, and, along with P-T estimates based on mineral pairs, suggest that the rocks have undergone UHP metamorphism. SHRIMP U-Pb isotope dating of zircon from the garnet wehrlite yielded a Paleozoic protolith age (ca. 346-461 Ma), and a Mesozoic UHP metamorphic age (ca. 220-240 Ma). The peridotites in PP1 consist of interlayered garnet (Grt)-bearing and garnet-free (GF) peridotite. Both types of peridotite have depleted mantle compositions (Mg# = 90-92) and they display transitional geochemical features. The intercalated layers probably reflect variations in partial melting rather than pressure variations during metamorphism, and the garnets may have been formed by exsolution from orthopyroxene during exhumation. These peridotites were probably part of the mantle wedge above the subduction zone that produced the UHP metamorphism and thus belonged to the North China Block before its tectonic emplacement. The exhumation of the subducted Yangtze Block brought these mantle fragments to shallow crustal levels. The ultramafic rocks in PP3 are dominantly dunite with minor garnet dunite. Their high Mg# (92-93) and relatively uniform chemical compositions indicate that they are part of a depleted mantle sequence. The presence of garnet replacing spinel and enclosing pre-metamorphic minerals such as olivine, clinopyroxene and spinel suggests that these rocks have undergone progressive metamorphism. SHRIMP U-Pb isotope dating of zircon from these rocks yielded two age groups: 726 ± 56 Ma for relic magmatic zircon grains and 240 ± 2.7 Ma for the newly formed metamorphic zircon. The older group is similar in age to granitic intrusions within the Dabie-Sulu belt, suggesting that the PP3 garnet peridotite may record the early emplacement of the peridotite into the crust. The younger dates coincide with the age of UHP metamorphism during continent-continent collision between the Yangtze and North China Blocks, suggesting that these peridotites were subducted to depths equivalent to the coesite facies and later exhumed. Thus, the garnet peridotites in the CCSD cores include both ultramafic rocks that existed originally in the subducted plate and rocks from the mantle wedge above the subducted plate, i.e., part of the North China Block.     

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.     

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.