The Chinese Southwestern Tianshan high-to ultra-high pressure low temperature (HP-UHP/LT) metamorphic belt exhibits well-preserved mafic layers, tectonic blocks/slices and boudins of different sizes and lithology embedded within dominant meta-volcanosedimentary rocks. Despite a wealth of previous studies on UHP relicts, P-T paths estimates and age constraints for metamorphism, controversies still exist on P-T-t assessments and regional exhumation patterns (i.e., tectonic mélange versus internally coherent "sub-belt" model). This study focuses on a group of coesite-bearing eclogite samples from a thick (~ 5 meters) layered metabasalt outcrop in order to unravel its detailed tectono-metamorphic evolution through space and time (both prograde, peak and exhumation). Using SIMS zircon U-Pb and oxygen isotope analyses, TIMS Sm-Nd multi-point isochron dating, in situ laser-ICP-MS trace-element analyses, classical thermobarometry and thermodynamic modeling, we link the multistage zircon growth to garnet growth and reconstruct a detailed P-T-time-isotopic evolution history for this UHP tectonic slice: from UHP peak burial ~ 2.95 ± 0.2 GPa, 510 ± 20 ℃ around 318.0 ± 2.3 Ma to HP peak metamorphism ~ 2.45 ± 0.2 GPa, 540 ± 20 ℃ at 316.8 ± 0.8 Ma, then, with eclogite-facies deformation ~ 2.0 ± 0.15 GPa, 525 ± 25 ℃ at 312 ± 2.5 Ma, exhumed to near surface within ca. 303 to ca. 280 Ma. Our P-T-time-isotopic results combined to the compilation of regional radiometric data and P-T estimates notably point to the existence of a short-lived period of rock detachment and exhumation (< 10 Ma, i.e. at ca. 315 ± 5 Ma) with respect to subduction duration. Highlights  Constant δ 18 O of multistage zircon domains suggest a nearly closed metamorphic system.  Multistage zircon ages are tied to metamorphism by test REE partitioning with garnet.  Detailed P-T-t-isotopic history of SW-Tianshan coesite-bearing eclogite is obtained.  Zircon chronology and Sm-Nd isochron constrain peak burial at ca. 315 ± 5 Ma.  Exhaustively compiled regional data outline a short-lived detachment and exhumation.

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The strength of the lithosphere may be constrained qualitatively by field observations on 19 localized vs distributed modes of deformation and by the mineral assemblages formed during 20 deformation. The internal deformation of the Bielsa basement unit of the Pyrenean Axial zone 21 is investigated through structural, microstructural and thermometric data. In this area, 22 shortening is widely distributed as attested by the folded attitude of the interface between the 23 basement and its sedimentary Triassic cover. Shortening is estimated around 1.7 km (13%) 24 from a regional balanced cross-section and should be considered in pre-Pyrenean 25 reconstructions. Shortening probably occurred before strain localization on crustal ramps as 26 suggested by zircon fission-track analysis. Distributed shortening is characterized at small-27 scale by low-temperature mylonites and cataclasites. In thin-section, feldspar originally 28 present in magmatic rocks is partially to totally sericitized. This transformation led to 29 significant weakening of the rock and took place in the 250-350°C temperature range. 30 Sericitization is ubiquitous, even in un-deformed granodiorites. This shows that the 31 weakening effect of sericitization not only occurs in ultra-mylonites, ultra-cataclasites and 32 2 phyllonites but also more generally in the upper crust early during the shortening history, with 33 implications for the shortening style. Estimates of the geothermal gradient suggest that 34 inherited thermicity may also have influenced the style of shortening. We propose that the 35 upper crust was very weak before or at the onset of its shortening due to high-thermal gradient 36 and fluid circulation that induced large-scale sericitization in greenschist facies conditions. 37 This has strong implications on the rheological evolution of the upper crust submitted to 38 metamorphic alteration in the greenschist facies and below.