Contrasting sediment flux in Val Lumnezia (Graubünden, Eastern Swiss Alps), and implications for landscape development

This paper presents qualitative estimates of sediment discharge from opposite valley flanks in the S–N-oriented Val Lumnezia, eastern Swiss Alps, and relates inferred differences in sediment flux to the litho-tectonic architecture of bedrock. The valley flank on the western side hosts the deep-seated Lumnezia landslide where an area of ca. 30 km² has experienced slip rates of several centimetres per year, potentially resulting in high sediment discharge to the trunk stream (i.e. the Glogn River). High slip rates have resulted in topographic changes that are detectable on aerial photographs and measurable with geodetic tools. In contrast, a network of tributary channels dissects the valley flank on the eastern side. There, an area of approximately 18 km² corresponding to < 30% of the surface has experienced a change in the landscape mainly by rock avalanche and rock fall, and the magnitudes of changes are below the calibration limit of digital photogrammetry. We thus infer lower magnitudes of sediment discharge on the eastern tributaries than on the western valley side, where landsliding has been the predominant erosional process. These differences are interpreted to be controlled by the dip-slope situation of bedrock on the western side that favours down-slope slip of material. Morphometric investigations reveal that the western valley side is characterized by a low topographic roughness because this valley flank has not been dissected by a channel network. It appears that high sediment discharge of the Lumnezia landslide has inhibited the establishment of a stable channel network and has largely controlled the overall evolution of the landscape. This contrasts to the general notion that channelized processes exert the first-order control on landscape evolution and formation of relief and needs to be considered in future studies about landscape architecture, drainage network and sediment discharge.     

Environmental controls on 10Be‐based catchment‐averaged denudation rates along the western margin of the Peruvian Andes

We present ¹⁰Be‐based basin‐averaged denudation rates for the entire western margin of the Peruvian Andes. Denudation rates range from c. 9 mm ka^?1 to 190 mm ka^?1 and are related neither to the subduction of the Nazca plate nor to the current seismicity along the Pacific coast and the occurrence of raised Quaternary marine terraces. Therefore, we exclude a tectonic control on denudation on a millennial time‐scale. Instead, we explain >60% of the observed denudation rates with a model where erosion rates increase either with mean basin slope angles or with mean annual water discharge. These relationships suggest a strong environmental control on denudation.     

Quantification of both normal and right‐lateral late Quaternary activity along the Kongur Shan extensional system,Chinese Pamir

The Kongur Shan Extensional System (KES) is a ~250 km long normal fault system that bounds the Muji–Tashkorgan basin of the Chinese Pamir. It accommodates E–W extension due to the northward indentation of the Pamir salient, and its late Miocene activity has been the focus of tectonic studies. While the KES has a main normal component, ~WNW–ESE‐striking segments have an additional right‐lateral strike‐slip component. Here, we quantify late Quaternary horizontal and vertical slip rates at three locations along the KES, where active faults cut and offset abandoned geomorphic features. We find rates of >3–4 mm a^?1 (horizontal) along the western Muji fault in the north and of ~1.7 mm a^?1 (vertical) and ~1 mm a^?1 (horizontal) along the Kongur Shan fault in the south during the late Pleistocene. These rates are consistent with GPS and late Miocene rates, and imply that E–W extension in the Muji–Tashkorgan basin is faster in the north than in the south.     

Postglacial sediment yield to Chilliwack Lake,British Columbia,Canada

Tunnicliffe, J., Church, M. & Enkin, R. J. 2012 (January): Postglacial sediment yield to Chilliwack Lake, British Columbia, Canada. Boreas, Vol. 41, pp. 84–101. 10.1111/j.1502‐3885.2011.00219.x. ISSN 0300‐9483. Seismic records and evidence from sediment cores at Chilliwack Lake provide the basis for a long‐term (postglacial) sediment budget for a 324‐km² Cordilleran catchment. Chilliwack Lake (11.8 km² surface area), situated in the North Cascade Mountains, near Chilliwack, British Columbia, was formed behind a valley‐wide recessional moraine in the final phase of post‐Fraser alpine glaciation. Seismic surveys highlight the postglacial lacustrine record, which is underlain by a thick layer of sediments related to deglacial sedimentation. Sediment cores provide details of grain‐size fining from the delta to the distal lake basin. The cores also show a record of intermittent fire and debris flows. Magnetic measurements of lake sediments provide information on grain size, as well as a dating framework. The total postglacial lake‐floor deposit volume is estimated to be 397 ± 27 × 10⁶ m³. Including estimates of fan and delta deposition, the specific postglacial yield to the lake is calculated to be ～86 ± 13 Mg km² a^?1. The sediment volume in the uppermost (Holocene) lacustrine layer is 128 ± 9 × 10⁶ m³, representing ～41 ± 4 Mg km² a^?1 in the Holocene. Compared with other Cordilleran lakes of similar size, particularly those with glacial cover in the watershed, Chilliwack Lake has experienced relatively modest rates of sediment accumulation. This study provides an important contribution to a growing database of long‐term (postglacial) sediment yield data for major Cordilleran lakes, essential for advancing our understanding of the pace of landscape evolution in formerly glaciated mountainous regions.     

Cretaceous and Cenozoic cooling history of the eastern Qilian Shan,north‐eastern margin of the Tibetan Plateau: evidence from apatite fission‐track analysis

The exhumation history and tectonic evolution of the Qilian Shan at the north‐eastern margin of the Tibetan Plateau has been widely debated. Here, we present apatite fission‐track (AFT) data for 12 Ordovician granodiorite samples along a vertical transect in the eastern Qilian Shan. These thermochronometry data indicate that the eastern Qilian Shan experienced a three‐stage cooling history, including: (i) rapid initial cooling in the late Cretaceous; (ii) a stage of quasi isothermal quiescence from ~ 80 to 24 Ma; and (iii) rapid subsequent cooling beginning in the early Miocene. The inferred cooling rates for the three stages are 6.8 ± 4.9 °C Ma^?1, 0.6 ± 0.2 °C Ma^?1 and 2.7 ± 0.9 °C Ma^?1 respectively (±1 σ). Assuming a geothermal gradient of 25 °C km^?1, the exhumation rates for the three stages are 0.27 ± 0.20 mm a^?1, 0.017 ± 0.007 mm a^?1 and 0.11 ± 0.04 mm a^?1 respectively (±1 σ). We suggest that the late Cretaceous cooling records collision of the Lhasa block with the Eurasian continent and that the Miocene cooling represents uplift/exhumation of the Qilian Shan.     

Mechanism on apparent dip sliding of oblique inclined bedding rockslide at Jiweishan, Chongqing, China

Because of the existence of a front stable rockmass barrier, the failure pattern of an oblique inclined bedding slope is conventionally recognized as a lateral rockfall/topple, and then a transformation into a rockfall accumulation secondary landslide. However, the Jiweishan rockslide, Wulong, Chongqing, which occurred on June 5, 2009, illustrates a new failure pattern of massive rock slope that rockmass rapidly slides along apparent dip, and then transforms into a long runout rock avalanche (fragment flow). This paper analyzes the mechanism of the new failure pattern which is most likely triggered by gravity, karstification, and the processes associated with mining activities. A simulation of the failure processes is shown, using the modeling software, FLAC^3D. The results show that there are five principal conditions for an apparent dip slide associated with an oblique inclined bedding slope are necessary: (1) a block-fracture bedding structure. The rockmass is split into obvious smaller, distinct blocks with several groups of joints, (2) an inclined rockmass barrier. The sliding rockmass (i.e., the rockslide structure before movement) exists along a dip angle and is barricaded by an inclined stable bedrock area, and the subsequent sliding direction is deflected from a true dip angle to an apparent dip angle; (3) apparent dip exiting. The valley and cliff provide a free space for the apparent dip exiting. (4) Driving block sliding, which means the block has a push type of effect on the motion of the rockslide. The “toy bricks” rockmass is characterized by a long-term creeping that induces the shear strength reduction from peak to residual value along the bottom soft layer, and the sliding force is therefore increased. (5) The key block resistance and brittle failure. The pressure on the key block is increased by the driving rockmass and its strength decreases due to karstification, rainfall, and mining. The brittle failure of the karst zone between the key block and the lateral stable bedrock occurs instantaneously and is largely responsible for generating the catastrophic rockslide–rock avalanche. If there was not a pre-existing key block, the failure pattern of such the inclined bedding rockmass could be piecemeal disintegration or small-scale successive rockfall or topple. The recognition of catastrophic failure potential in such inclined bedding slopes requires careful search for not only structures dipping in the direction of movement, but also key block toe-constrained condition.     

Late Pleistocene slip rate of the northern Qilian Shan frontal thrust,western Hexi Corridor,China

We derive a slip rate for a frontal thrust in the western Hexi Corridor along the northern Qilian Shan by combining topographic profiling and ¹⁰Be exposure dating. The active Yumen‐Beidahe thrust fault offsets late Pleistocene alluvial‐fan deposits, and a prominent north‐facing scarp is well preserved. To quantify the slip rate, we surveyed the uplifted terraces and sampled quartz‐rich pebbles on terrace surfaces and river channels to determine surface exposure ages and pre‐depositional inheritance. The minimum vertical slip rate of the fault is 0.73 ± 0.09 mm a^?1. This represents a horizontal shortening rate of 1.26 ± 0.31 mm a^?1 for a fault dip of 30 ± 5°. This estimated slip rate supports the inference made from previous geological and GPS constraints that NNE‐directed shortening across the western Qilian Shan and the Hexi Corridor is distributed on several active faults with a total shortening rate of 4–10 mm a^?1.     

Giant ancient landslide in the Alma water gap (Crimean Mountains,Ukraine): notes to the predisposition,structure, and chronology

Large-scale ancient landslides of the area of more than 5 km² and volume exceeding 200 × 10⁶ m³ are characteristic features of the valleys incised in the northern periphery of the Crimean Mountains (Ukraine). The largely affected area is located in the outermost cuesta range of the Crimean Mountains which consists of rigid Sarmatian limestones overlying weak Middle Miocene and Upper Palaeogene deposits. A giant landslide arose in the Alma water gap as a reflection of several coincident preparatory factors such as suitable bedrock stratification, smectite-rich bedrock exposed to swelling activity, presence of faults parallel to the valley trend, and river capture event which preceded the landslide event. The occurrence of such ancient megaslides is particularly interesting in the area which is characterized by low precipitation (<500 mm/year) and weak contemporary seismicity. It probably reflects a more dynamic environment in humid phases of the Holocene; however, seismic triggering along the Mesozoic suture zone cannot be rejected. Compressional features such as gravitational folds in the central and distal parts of the landslide, which probably correlate with the whole landslide genesis or its significant reactivation, arose, according to the radiocarbon dating, during the Holocene climatic optimum in the Atlantic period. The slope deformation has been relatively quiescent since that time, except minor historic reactivization which took place in the frontal part of the landslide. We suppose that the studied landslide could be classified as a transitional type of slope deformation with some signs of spreading and translational block slides.     

Large earthquake-triggered landslides and mountain belt erosion: The Tsaoling case,Taiwan

The 1999 Chi-Chi earthquake triggered the catastrophic Tsaoling landslide in central Taiwan. We mapped the landslide area and estimated the landslide volume, using high-resolution digital elevation model from airborne LiDAR (Light Detection and Ranging), satellite images, aerial photographs and topographic maps. The comparison between cut and fill volumes, about 0.126 and 0.150 km³, respectively, suggests a volume increase of 19% due to decompaction during landsliding. In April 2002, the cut and fill volumes were about 0.137 and 0.116 km³, respectively. These estimates suggest that 2.5 years after the event, the volume of landslide debris removed by river erosion was nearly 0.045 km³. Such a large value highlights the importance of landslide processes for erosion and long-term denudation in the Taiwan mountain belt. To cite this article: R.-F. Chen et al., C. R. Geoscience 337 (2005).     

Quantifying the impact of former glaciation on catchment‐wide denudation rates derived from cosmogenic 10Be

Glacial denudation can significantly perturb terrestrial cosmogenic nuclide depth profiles and, if this is not corrected for, derived apparent denudation rates will overestimate the actual denudation rates. Here we determine how much ¹⁰Be‐derived denudation rates – calculated under the assumption of steady state – deviate from actual denudation rates as a function of three parameters: (1) the total amount of glacial denudation, (2) the post‐glacial denudation rate and (3) the time elapsed since deglaciation. We provide correction lines for the full parameter space explored (glacial denudation: 0.01–100 m; post‐glacial denudation rate: 1–1000 mm/ka; deglaciation: 1–100 ka before present), to evaluate and, if necessary, correct denudation rates for the impact of glacial denudation. Applied to ¹⁰Be‐derived catchment‐averaged denudation rates for formerly glaciated catchments in the Black Forest, Germany, we find that uncorrected denudation rates overestimate actual rates by up to a factor of three.     

High‐resolution reconstruction of a coastal barrier system: impact of Holocene sea‐level change

This study presents a detailed reconstruction of the sedimentary effects of Holocene sea‐level rise on a modern coastal barrier system. Increasing concern over the evolution of coastal barrier systems due to future accelerated rates of sea‐level rise calls for a better understanding of coastal barrier response to sea‐level changes. The complex evolution and sequence stratigraphic framework of the investigated coastal barrier system is reconstructed using facies analysis, high‐resolution optically stimulated luminescence and radiocarbon dating. During the formation of the coastal barrier system starting 8 to 7 ka rapid relative sea‐level rise outpaced sediment accumulation. Not before rates of relative sea‐level rise had decreased to ca 2 mm yr^?1 did sediment accumulation outpace sea‐level rise. From ca 5·5 ka, rates of regionally averaged sediment accumulation increased to 4·3 mm yr^?1 and the back‐barrier basin was filled in. This increase in sediment accumulation resulted from retreat of the barrier island and probably also due to formation of a tidal inlet close to the study area. Continued transgression and shoreface retreat created a distinct hiatus and wave ravinement surface in the seaward part of the coastal barrier system before the barrier shoreline stabilized between 5·0 ka and 4·5 ka. Back‐barrier shoreline erosion due to sediment starvation in the back‐barrier basin was pronounced from 4·5 to 2·5 ka but, in the last 2·5 kyr, barrier sedimentation has kept up with and outpaced sea‐level. In the last 0·4 kyr the coastal barrier system has been prograding episodically. Sediment accumulation shows considerable variation, with periods of rapid sediment deposition and periods of non‐deposition or erosion resulting in a highly punctuated sediment record. The study demonstrates how core‐based facies interpretations supported by a high‐resolution chronology and a well‐documented sea‐level history allow identification of depositional environments, erosion surfaces and hiatuses within a very homogeneous stratigraphy, and allow a detailed temporal reconstruction of a coastal barrier system in relation to sea‐level rise and sediment supply.     

Quartzite terrains, geologic controls, and basin denudation by debris flows: their role in long-term landscape evolution in the central Appalachians

A large storm in 1995 that impacted the central Blue Ridge Mountains of Virginia triggered over a thousand debris flows, which deeply incised stream channels in zero-, first-, and second-order basins underlain primarily by granite, metabasalt, and quartzite. This event provided an opportunity to gain insight into geologic controls on debris-flow initiation and rates of basin denudation. Intensive investigations in quartzite terrain indicated that well-developed joints provide for rapid infiltration of rainwater and thus affected debris-flow initiation. Possible mechanisms of slope failure include (1) elevated rates of fracture recharge in steep valleys and/or (2) bedrock bedding planes. Fracture recharge may serve to substantially increase rock pore pressure during high intensity rainfall, especially when coupled with antecedent moisture. The quartzite bedrock tends to break into orthogonal blocks due to jointing. Statistical analysis shows the presence of a minimum of two joint populations that serve as bedrock controls on lateral erosion. Resistant bedding planes, parallel to slope, control vertical erosion below a finer-grained layer of saprolite. The combination of increased recharge and joint orientations in quartzite basins are likely the main factors resulting in the highest measured values of basin denudation from debris flows, relative to other lithologies, in the central Appalachians. Additionally, these joint and bedding planes produce a topographic signature at all scales and, therefore, are an important factor controlling long-term landscape evolution.     

Multiple growth of garnet,sillimanite/kyanite and monazite during amphibolite facies metamorphism: implications for the P–T–t and tectonic evolution of the western Altai Range,Mongolia

Four amphibolite facies pelitic gneisses from the western Mongolian Altai Range exhibit multistage aluminosilicate formation and various chemical‐zoning patterns in garnet. Two of them contain kyanite in the matrix and sillimanite inclusions in garnet, and the others have kyanite inclusions in garnet with sillimanite or kyanite in the matrix. The Ca‐zoning patterns of the garnet are different in each rock type. U–Th–Pb monazite geochronology revealed that all rock units experienced a c. 360 Ma event, and three of them were also affected by a c. 260 Ma event. The variations in the microstructures and garnet‐zoning profiles are caused by the differences in the (i) whole‐rock chemistry, (ii) pressure conditions during garnet growth at c. 360 Ma and (iii) equilibrium temperatures at c. 260 Ma. The garnet with sillimanite inclusions records an increase in pressure at low‐P (~5.2–7.2 kbar) and moderate temperature conditions (~620–660 °C) at c. 360 Ma. The garnet with kyanite inclusions in the other rock types was also formed during an increase in pressure but at higher pressure conditions (~7.0–8.9 kbar at ~600–640 °C). The detrital zircon provenance of all the rock types is similar and is consistent with that from the sedimentary rocks in the Altai Range, suggesting that the provenance of all the rock types was a surrounding accretionary wedge. One possible scenario for the different thermal gradient is Devonian ridge subduction beneath the Altai Range, as proposed by several researchers. The subducting ridge could have supplied heat to the accretionary wedge and elevated the geotherm at c. 360 Ma. The differences in the thermal gradients that resulted in varying prograde P–T paths might be due to variations in the thermal regimes in the upper plate that were generated by the subducting ridge. The c. 260 Ma event is characterized by a relatively high‐T/P gradient (~25 °C km^?1) and may be due to collision‐related granitic activity and re‐equilibrium at middle crustal depths, which caused the variations in the aluminosilicates in the matrix between the rock units.     

Effects of geological and tectonic characteristics on the earthquake-triggered Daguangbao landslide,China

The Daguangbao landslide is the largest co-seismic landslide triggered by the Wenchuan earthquake (Ms 8.0) occurred on 12 May 2008. The landslide, which is 4.6 km long and 3.7 km wide, involves a volume of approximately 1.2 × 10⁹ m³. An exposed slip surface, situated at the southern flank of its source area, was observed with a length of 1.8 km along the main sliding direction and an area of 0.3 km². To study the geological and tectonic characteristics of the source area and their contributions to the landslide formation during the earthquake, detailed geological investigations were firstly conducted. And it is reached that the landslide occurred on the northwestern limb of the Dashuizha anticline with its scarp showing several geological structures, including joint sets, local faults, and folds. These tectonic-related structures potentially influenced the failure of the landslide. Secondly, further investigations were focused on the inclined planar sliding surface using 12 exploratory trenches, nine boreholes, a tunnel, borehole sonic data, and micro-images. These data reveal that the rock mass along the sliding surface was the fragmented rock of a bedding fault. A pulverized zone was observed on the sliding surface, which was the zone of shear localization during the landslide. This suggests that the shear failure of the Daguangbao landslide developed within the bedding fault. The rapid failure of the landslide was associated with the degradation of the rock mass strength of the bedding fault both before and during the 2008 Wenchuan earthquake. With this study, we propose that a pre-existing large discontinuity within a slope may be the basis for initiating a large landslide during earthquake.     

A deep-seated slow movement controlled by structural setting in marly formations of Central Italy

The site investigation of low-gradient slopes composed by marly rocks usually focuses on shallow slides in weathered mantling material as it is assumed that the underlying bedrock has higher strength, but deeper investigations may reveal larger, active, deep-seated movements. A typical example of this is found in Montemartano (Perugia, Central Italy). Here aerial photo interpretation and field observations indicate that active movements involve the shallower portion of the slope, formed by a very old and large landslide body extending over an area of about 0.5 km². Borehole core logging and probe inclinometer monitoring reveal that the area corresponding to the deep-seated landslide is moving at a maximum rate of 70 mm/year down to a maximum depth of 40 m. A comparison of inclinometer and piezometer data indicates that the movement seasonally reactivates even when rainfall and piezometer levels are below average values and suggests that structural setting of the whole slope influences both groundwater flow and movement kinematics. This hypothesis is reinforced by seepage analyses and stability analyses yielding a mobilized shear strength close to residual strength of the clayey interbeds of the marly limestone formations. This implies that instability occurs along bedding over a large part of the slide. The importance of these phenomena in land management policy is discussed and the critical aspects of their investigation and monitoring are addressed. The reconstruction of landslide geometry/stratigraphy and geotechnical characterization of the materials is closely considered, particularly as these are complicated by the limited representativeness of field and laboratory investigations in this type of material.     

Constraining Holocene 10Be production rates in Greenland

The absence of a production rate calibration experiment on Greenland has limited the ability to link ¹⁰Be exposure dating chronologies of ice‐margin change to independent records of rapid climate change. We use radiocarbon age control on Holocene glacial features near Jakobshavn Isbræ, western Greenland, to investigate ¹⁰Be production rates. The radiocarbon chronology is inconsistent with the ¹⁰Be age calculations based on the current globally averaged ¹⁰Be production rate calibration data set, but is consistent with the ¹⁰Be production rate calibration data set from north‐eastern North America, which includes a calibration site nearby on north‐eastern Baffin Island. Based on the best‐dated feature available from the Jakobshavn Isbræ forefield, we derive a ¹⁰Be production rate value of 3.98 ± 0.24 atoms g a^?1, using the ‘St’ scaling scheme, which overlaps with recently published reference ¹⁰Be production rates. We suggest that these ¹⁰Be production rate data, or the very similar data from north‐eastern North America, are used on Greenland. Copyright © 2011 John Wiley & Sons, Ltd.     

10Be exposure age constraints on the Late Weichselian ice‐sheet geometry and dynamics in inter‐ice‐stream areas,western Svalbard

The Late Weichselian ice sheet of western Svalbard was characterized by ice streams and inter‐ice‐stream areas. To reconstruct its geometry and dynamics we investigated the glacial geology of two areas on the island of Prins Karls Forland and the Mitrahalvøya peninsula. Cosmogenic ¹⁰Be surface exposure dating of glacial erratics and bedrock was used to constrain past ice thickness, providing minimum estimates in both areas. Contrary to previous studies, we found that Prins Karls Forland experienced a westward ice flux from Spitsbergen. Ice thickness reached >470 m a.s.l., and warm‐based conditions occurred periodically. Local deglaciation took place between 16 and 13 ka. At Mitrahalvøya, glacier ice draining the Krossfjorden basin reached >300 m a.s.l., and local deglaciation occurred at c. 13 ka. We propose the following succession of events for the last deglaciation. After the maximum glacier extent, ice streams in the cross‐shelf troughs and fjords retreated, tributary ice streams formed in Forlandsundet and Krossfjorden, and, finally, local ice caps were isolated over both Prins Karls Forland and Mitrahalvøya and their adjacent shelves.     

Thermal and mechanical behaviour of the orogenic middle crust during the syn‐ to late‐orogenic evolution of the Variscan root zone,Bohemian Massif

We combine structural observations, petrological data and ⁴⁰Ar–³⁹Ar ages for a stack of amphibolite facies metasedimentary units that rims high‐P (HP) granulite facies felsic bodies exposed in the southern Bohemian Massif. The partly migmatitic Varied and Monotonous units, and the underlying Kaplice unit, show a continuity of structures that are also observed in the adjacent Blanský les HP granulite body. They all exhibit an earlier NE?SW striking and steeply NW‐dipping foliation (S3), which is transposed into a moderately NW‐dipping foliation (S4). In both the Varied and Monotonous units, the S3 and S4 foliations are characterized by a Sil–Bt–Pl–Kfs–Qtz–Ilm±Grt assemblage, with occurrences of post‐D4 andalusite, cordierite and muscovite. In the Monotonous unit, minute inclusions of garnet, kyanite, sillimanite and biotite are additionally found in plagioclase from a probable leucosome parallel to S3. The Kaplice unit shows rare staurolite and kyanite relicts, a Sil–Ms–Bt–Pl–Qtz±Grt assemblage associated with S3, retrogressed garnet?staurolite aggregates during the development of S4, and post‐D4 andalusite, cordierite and secondary muscovite. Mineral equilibria modelling for representative samples indicates that the Varied unit records conditions higher than ~7 kbar at 725 °C during the transition from S3 to S4, followed by a P?T decrease from ~5.5 kbar/750 °C to ~4.5 kbar/700 °C. The Monotonous unit shows evidence of partial melting in the S3 fabric at P?T above ~8 kbar at 740–830 °C and a subsequent P?T decrease to 4.5–5 kbar/700 °C. The Kaplice unit preserves an initial medium‐P prograde path associated with the development of S3 reaching peak P?T of ~6.5 kbar/640 °C. The subsequent retrograde path records 4.5 kbar/660 °C during the development of S4. ⁴⁰Ar–³⁹Ar geochronology shows that amphibole and biotite ages cluster at c. 340 Ma close to the HP granulite, whereas adjacent metasedimentary rocks preserve c. 340 Ma amphibole ages, but biotite and muscovite ages range between c. 318 and c. 300 Ma. The P?T conditions associated with S3 imply an overturned section of the orogenic middle crust. The shared structural evolution indicates that all mid‐crustal units are involved in the large‐scale folding cored by HP granulites. The retrograde P–T paths associated with S4 are interpreted as a result of a ductile thinning of the orogenic crust at a mid‐crustal level. The ⁴⁰Ar–³⁹Ar ages overlap with U–Pb zircon ages in and around the HP granulite bodies, suggesting a short duration for the ductile thinning event. The post‐ductile thinning late‐orogenic emplacement of the South Bohemian plutonic complex is responsible for a re‐heating of the stacked units, reopening of argon system in mica and a tilting of the S4 foliation to its present‐day orientation.     

Thermal interaction of middle and upper crust during gneiss dome formation: example from the Montagne Noire (French Massif Central)

This paper aims to decipher the thermal evolution of the Montagne Noire Axial Zone (MNAZ, southern French Massif Central) gneiss core and its metasedimentary cover through determination of P–T paths and temperature gradients. Migmatitic gneiss from the core of the dome record a clockwise evolution culminating at 725 ± 25 °C and 0.8 ± 0.1 GPa with partial melting, followed by a decompression path with only minor cooling to 690 ± 25° C and 0.4 ± 0.1 GPa. Field structural analyses as well as detailed petrological observations indicate that the cover sequence experienced LP‐HT metamorphism. Apparent thermal gradients within the cover were determined with garnet–biotite thermometry and Raman Spectroscopy on Carbonaceous Matter. High‐temperature apparent gradients (e.g. ～530 °C km^?1 along one transect) are explained by late brittle–ductile extensional shearing evidenced by phyllonites that post‐date peak metamorphism. In areas where normal faults are less abundant and closely spaced, gradients of ～20 to 50 °C km^?1 are calculated. These gradients can be accounted for by a combination of dome emplacement and ductile shearing (collapse of isotherms), without additional heat input. Finally, the thermal evolution of the MNAZ is typical for many gneiss domes worldwide as well as with other LP‐HT terranes in the Variscides.