Late-stage tectonic evolution of the Al-Hajar Mountains, Oman: New constraints from Palaeogene sedimentary units and low-temperature thermochronometry

Mountain building in the Al-Hajar Mountains (NE Oman) occurred during two major shortening stages, related to the convergence between Africa-Arabia and Eurasia, separated by nearly 30 Ma of tectonic quiescence. Most of the shortening was accommodated during the Late Cretaceous, when northward subduction of the Neo-Tethys Ocean was followed by the ophiolites obduction on top of the former Mesozoic margin. This shortening event lasted until the latest Santonian - early Campanian. Maastrichtian to Eocene carbonates unconformably overlie the eroded nappes and seal the Cretaceous foredeep. These neo-autochthonous post-nappe sedimentary rocks were deformed, along with the underlying Cretaceous tectonic pile, during the second shortening event, itself including two main exhumation stages. In this study we combine remotely sensed structural data, seismic interpretation, field-based structural investigations and apatite (U-Th)/He (AHe) cooling ages to obtain new insights into the Cenozoic deformation stage. Seismic interpretation indicates the occurrence of a late Eocene flexural basin, later deformed by an Oligocene thrusting event, during which the post-nappe succession and the underlying Cretaceous nappes of the internal foredeep were uplifted. This stage was followed by folding of the post-nappe succession during the Miocene. AHe data from detrital siliciclastic deposits in the frontal area of the mountain chain provide cooling ages spanning from 17.3 to 42 Ma, consistent with available data for the structural culminations of Oman. Our work points out how renewal of flexural subsidence in the foredeep and uplift of the mountain belt were coeval processes, followed by layer-parallel shortening preceding final fold amplification

New age constraints on emplacement of the cevenol granitoids, south french massif central

International audienceDuring the development of the Variscan orogeny, large amounts of granitic melt were produced, giving rise to the intrusion of granitoids at different structural levels. Despite numerous studies, ages available from previous work on the Cevennes granites remain largely imprecise. In order to better constrain the age and emplacement mode of these granites, we have combined U-Pb dating on monazites and zircons and Ar-40/Ar-39 dating on biotites with petrological observations, major element chemical analysis and SEM zircon imaging on five samples from the Aigoual-St Guiral-Liron and Mont Lozere granitic massifs. The results revealed that granitic intrusions and cooling in Southern Cevennes occurred in a short time span at similar to 306 Ma after the main episode of regional metamorphism. Petrological and chemical data suggest that they result from a mixing between mantle-derived basic magmas (lamprophyres) and lower crust acid magmas. At a regional scale the production of these melts occurred at the end of crustal thickening induced by nappe stacking, at the same time as the late anatectic events recorded further north in the Velay dome and the granulite facies metamorphism recorded in metasedimentary granulite enclaves brought up by Tertiary volcanoes of the Velay area (Bournac)

Low-temperature thermochronology in the Peruvian Central Andes: implications for long-term continental denudation, timing of plateau uplift, canyon incision and lithosphere dynamics

In Peru, the western edge of the 4.5 km high Western Cordillera is cut by a>3 km deep canyon. To understand incision by the Cotahuasi-Ocona River and the regional uplift history of this orogenic plateau capped by volcanic rocks, 26 crystalline rock samples were collected for low-temperature thermochronology from vertical profiles parallel and perpendicular to the canyon. Rock cooling histories confirm that most plateau denudation had occurred prior to 24 Ma but plateau incision peaked after c. 14-9 Ma in response to rapid surface uplift. The abrupt occurrence of a rock heating event is also detected during middle Miocene time. This was either a response to the emplacement of low-conductivity, regionally extensive ignimbritic caprock or a response to crustal-scale fluid circulation caused by wet melting of the overriding plate when magmatism resumed c. 24 Ma. The potential for thermochronology to provide information on past geothermal gradients is discussed, showing how it can be used as a proxy for understanding change in subducting slab dynamics, with oscillations in subduction angle having perhaps been the main on-off switch for magmatism in this Cordilleran setting

Low long-term erosion rates in high-energy moutain belts : insights from thermo- and biochronology in the Eastern Pyrenees.

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lithospheric slab breakoff and topographic regrowth of the Eastern Pyrenees : linking surface morphology with subcrustal processes through thermochronology

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A fission-track and (U-Th)/He thermochronometric study of the northern margin of the South China Sea: an example of a complex passive margin

Zircon fission track (ZFT), apatite fission track (AFT) and (U–Th)/He thermochronometric data are used to reconstruct the Cenozoic exhumation history of the South China continental margin. A south to north sample transect from coast to continental interior yielded ZFT ages between 116.6 ± 4.7 Ma and 87.3 ± 4.0, indicating that by the Late Cretaceous samples were at depths of 5–6 km in the upper crust. Apatite FT ages range between 60.9 ± 3.6 and 37.3 ± 2.3 Ma with mean track lengths between 13.26 ± 0.16 µm and 13.95 ± 0.19 µm whilst AHe ages are marginally younger 47.5 ± 1.9–15.3 ± 0.5 Ma. These results show the sampled rocks resided in the top 1–1.5 km of the crust for most of the Cenozoic. Thermal history modeling of the combined FT and (U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. 1) Initial phase of rapid cooling that youngs to the north, 2) a period of relative (but not perfect) thermal stasis at ~ 70–60 °C which increases in duration from the south to the north; 3) final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin

Timing, slip rate, displacement and cooling history of the Mykonos detachment footwall, Cyclades, Greece, and implications for the opening of the Aegean sea basin

We constrain the slip and cooling history of the Mykonos detachment footwall using thermochronometry. A U–Pb zircon age of 13.5 ± 0.3 Ma dates intrusion of the Mykonos monzogranite. 40Ar/39Ar hornblende and biotite ages from the monzogranite are 12.7 ± 0.6 Ma and 10.9 ± 0.6 Ma, whereas zircon and apatite fission-track ages range from 13 ± 0.8 Ma to 10.7 ± 0.8 Ma and 12.5 ± 2.2 Ma to 10.5 ± 1.8 Ma. (U–Th)/He ages range from 13.6 ± 0.6 Ma to 9.0 ± 0.7 Ma for zircon and 11.1 ± 0.5 Ma to 8.9 ± 0.4 Ma for apatite. The ages in part overlap within 2{sigma} errors and together with the long apatite fission-track lengths (>14 µm) support rapid cooling at rates >100 °C Ma–1. The low-temperature thermochronometric ages decrease east-northeastwards in the direction of hanging-wall transport on the Mykonos detachment. Age–distance relationships show that the Mykonos detachment slipped at an average rate of 6.0 +9.2/–2.4 km Ma–1 causing c. 30 km of offset and c. 12 km of exhumation. This result indicates that Miocene low-angle normal faulting was not important for the exhumation of the Cycladic blueschist unit. The opening of the Aegean Sea basin in the Miocene was controlled by a few large-magnitude low-angle normal faults

Low long-term erosion rates in high-energy mountain belts: insights from thermo- and biochronology in the eastern Pyrenees

We relate the high elevation of flat topography in the Eastern Pyrenees to the resurrection of a mountain belt which prior to ~ 12 Ma was a low-relief landscape, or peneplain, bevelling eroded stumps of the Pyrenean compressional orogen. New apatite fission-track and (U–Th)/He data together demonstrate the consistently young age (< 30 Ma) of mappable remnants of the peneplain irrespective of their current elevations. An independent biochronology of micromammalian assemblages collected from fissures in limestone pavements is also presented and confirms the conclusion that all the relict land surfaces have survived as islands of low erosion in this high-energy mountain environment because of recent and perhaps rapid post-deformation surface uplift. Imaging of lithospheric structure beneath the Pyrenees based on geophysical modelling suggests that topography in the Eastern Pyrenees is anomalously elevated for crustal thicknesses of only 25–35 km, and that the elevation is ascribable to thinning by thermal erosion of the subcrustal lithosphere. Uplift occurred after 12 Ma as a result of lithospheric thinning, and was accompanied by volcanism and crustal extension. It outpaced opportunities for denudation to erode the uplifting peneplain entirely, even at its currently observed maximum elevations of 2.4–2.9 km in the crest zone. Overall, this study defines an example of nonequilibrium topography in a region of past plate convergence where post-orogenic uplift has been causing transient landscape response to thermally-driven thinning of the lithosphere. It shows that compression and mountain building do not always occur together: there can be compression without mountain building and mountain building without compression

Exhumation controlled by transcurrent tectonics: the Argentera-Mercantour massif (SW Alps)

New apatite fission track (AFT) and (U-Th)/He ages from the Argentera–Mercantour External Crystalline Massif of the SW Alps document an exhumation history along transcurrent dextral faults. Significant AFT age differences across the strikes of the main faults have been obtained from the NE (12.9 Ma) to the SW (5.2 Ma) and are linked to vertical uplifts along these right-lateral transpressional structures. Such segmentation is not observed in AHe ages. AHe ages have preserved younger ages (4–5 Ma) on the eastern hangingwall side of the High Durance extensional system. Such an exhumation history is interpreted to result from a transition of transpressional to transtensional regimes at 8–5 Ma during continuing Adria–Europe convergence. These data show that recent extensional deformation in the SW Alps can be related to the development of a transtensional domain in the Ubaye–Embrunais depression linked to ongoing strike-slip deformation along the Argentera–Mercantour