Synchronous N-S and E-W extension at the Tibet-to-Himalaya transition in NW Bhutan

Despite ~50 Myr of continuous continent-continent collision, contractional structures in the Himalayan-Tibetan orogen are today limited to the northern and southern margins of the system, while extension dominates much of the interior. On the Tibetan Plateau, Cenozoic E-W extension has been accommodated by strike-slip faults and extensional grabens, while N-S extension at the Tibet-to-Himalaya transition has been accommodated by the South Tibetan fault system (STFS). The genetic relationship between N-S and E-W extension is disputed, although age constraints indicate temporal overlap of at least 7 Myr. In NW Bhutan the two intersect where the STFS basal detachment is cut by the Yadong cross structure (YCS), an extensional half graben that provides a rare opportunity to constrain relative timings. We report U-Pb zircon dates from four STFS footwall leucogranites consistent with episodic magmatism during the middle-late Miocene and in situ U(-Th)-Pb monazite and xenotime dates from three metasedimentary rocks ranging from late Oligocene to middle Miocene. We suggest that amphibolite facies footwall metamorphism was ongoing at the time the basal STFS detachment initiated as a ductile structure in the middle-late Miocene. Late-stage granitic intrusions may reflect footwall melting during extensional exhumation along the STFS, but post-metamorphic and post-intrusion fabrics suggest that most displacement occurred after emplacement of the youngest granites. Some of the oldest YCS-related fabrics are found in a deformed 14 Ma leucogranite, implying middle Miocene ductile deformation. This observation, along with evidence for subsequent brittle YCS deformation, suggests that N-S and E-W extensional structures in the area had protracted and overlapping deformation histories

Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U-th)/He thermochronology

High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperature-time paths inferred from 40Ar/39Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar40Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling (<1°C/m.y.) from Jurassic times until the late Miocene or early Pliocene. The regional extent and consistency of thermal histories during this time period suggest the presence of a stable thermal structure and imply that regional denudation rates were low (<0.1 mm/yr for nominal continental geotherms). Beginning in the late Miocene or early Pliocene, these samples experienced a pronounced cooling event (>30°-50°C/m.y.) coincident with exhumation from inferred depths of ~8-10 km, at denudation rates of 1-2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period (~3°C/m.y.), suggesting limited exhumation (1-2 km). However, these samples record a slight increase in cooling rate (from <1 to ~3°C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin. We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable Sichuan Basin and thus marks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene (~5-12 Ma)

Rochechouart 2017-Drilling Campaign: First Results

No abstract available

Evolution métamorphique du dôme de Kangmar (Sud-Est-Xizang, Tibet): Implications pour les zones internes himalayennes

International audienc

40Ar/39Ar geochronology of flood basalts from the Kerguelen Archipelago, southern Indian Ocean: Implications for Cenozoic eruption rates of the Kerguelen plume

The 6500 km2 Kerguelen Archipelago formed on the nothern Kerguelen Plateau (NKP) (4 X 105 km2) which is a shallow submarine plateau belonging to the Kerguelen large igneous province in the southern Indian Ocean. Flood basalts make up 85% of the archipelago and are interpreted as the most recent volcanism ( < 40 Ma) from the Kerguelen hotspot which has erupted basalt for the last 115 million years. Based on 40AR/39Ar incremental heating of acid-leached groundmass separates, we report isochron ages ranging from 29.26 ± 0.87 Ma to 24.53 ± 0.29 Ma for 15 basalts from five stratigraphic sections from diverse regions of the archipelago. The oldest dated basalt from the archipelago (~ 29 Ma) is much younger than the ~ 40 Ma age of conjunction between the hotspot and the Southeast Indian Ridge. Basalt eruption seems to have ceased shortly after ~ 24 Ma although small volume, highly evolved lavas and plutons continued to form in the archipelago. The basalt age data suggest an average lava accumulation rate of ~ 1.6±0.9 km/my during the Oligocene. The archipelago's volumetric eruption rate (0.009 km3/yr) is lower than estimates made for the Cretaceous Kerguelen Plateau (1.7 km3/yr) and the Ninetyeast Ridge hotspot track (0.18 km3/yr), suggesting that the late Cenozoic extrusive activity of the Kerguelen plume is waning. Cenozoic volcanism attributed to the Kerguelen plume occurs over a diffuse area with Quaternary eruptions at Heard and McDonald Islands and within the Kerguelen Archipelago. The decreasing eruption rate and areally diffuse volcanism may be explained by the thick lithosphere of the Cretaceous Kerguelen Plateau overriding and insulating the plume. However, if the undated NKP, which underlies the archipelago, formed during the Cenozoic, then, the crustal production rate of the plume from 40 Ma to the present (~ 0.25 km3/yr) would be similar to the crustal production rate (0.23 km3/my) previously estimated for the formation of the Ninetyeast Ridge (~ 82-38 Ma).SCOPUS: ar.jinfo:eu-repo/semantics/publishe