Glacial advances constrained by 10Be exposure dating of bedrock landslides, Kyrgyz Tien Shan

Numerous large landslide deposits occur in the Tien Shan, a tectonically active intraplate orogen in Central Asia. Yet their significance in Quaternary landscape evolution and natural hazard assessment remains unresolved due to the lack of "absolute" age constraints. Here we present the first 10Be exposure ages for three prominent (N107 m3) bedrock landslides that blocked major rivers and formed lakes, two of which subsequently breached, in the northern Kyrgyz Tien Shan. Three 10Be ages reveal that one landslide in the Alamyedin River occurred at 11–15 ka, which is consistent with two 14C ages of gastropod shells from reworked loess capping the landslide. One large landslide in Aksu River is among the oldest documented in semi-arid continental interiors, with a 10Be age of 63–67 ka. The Ukok River landslide deposit(s) yielded variable 10Be ages, which may result from multiple landslides, and inheritance of 10Be. Two 10Be ages of 8.2 and 5.9 ka suggest that one major landslide occurred in the early to mid-Holocene, followed by at least one other event between 1.5 and 0.4 ka. Judging from the regional glacial chronology, all three landslides have occurred between major regional glacial advances. Whereas Alamyedin and Ukok can be considered as postglacial in this context, Aksu is of interglacial age. None of the landslide deposits show traces of glacial erosion, hence their locations and 10Be ages mark maximum extents and minimum ages of glacial advances, respectively. Using toe to-headwall altitude ratios of 0.4–0.5, we reconstruct minimum equilibrium-line altitudes that exceed previous estimates by as much as 400 m along the moister northern fringe of the Tien Shan. Our data show that deposits from large landslides can provide valuable spatio temporal constraints for glacial advances in landscapes where moraines and glacial deposits have low preservation potential

Evidence of a plate-wide tectonic pressure pulse provided by extensometric monitoring in the Balkan Mountains (Bulgaria)

The EU-TecNet monitoring network uses customized three-dimensional extensometers to record transient deformations across individual faults. This paper presents the first results from two newly established monitoring points in the Balkan Mountains in Bulgaria. The data from Saeva Dupka, recorded across an EEN-WWS striking fault, show sinistral strike-slip along the fault and subsidence of the southern block. Much of the subsidence occurred around the time of the distal MW = 5.6 Pernik Earthquake. An important transient deformation event, which began in autumn 2012, was reflected by significant compression and following extension, across the monitored fault. The data from Bacho Kiro, recorded across a NE–SW striking fault, show sinistral strike-slip along the fault and subsidence of the north-western block. The same important deformation event was reflected by changes in the strike-slip, dip-slip, and horizontal opening/closing trends. These results have been compared to data from other monitoring points in the Western Carpathians, External Dinarides, and Tian Shan. Many of the sites show evidence of simultaneous displacement anomalies and this observation is interpreted as a reflection of the plate-wide propagation of a tectonic pressure pulse towards the end of 2012

Strong Central Asian seasonality from Eocene oysters indicates earlymonsoons and aridification

International audienceClimate models suggest that the onset of Asian monsoons and aridification have been governed by Tibetan plateauuplift, global climate changes and the retreat to the west of the vast epicontinental Proto-Paratethys sea duringthe warm Eocene greenhouse period (55-34 million years ago). However, the role of the Proto-Paratethys seaon climate remains to be quantified by accurate and precise reconstructions. By applying a novel intra-annualgeochemical multi-proxy methodology on Eocene oyster shells of the Proto-Paratethys sea and comparing resultsto climate simulations and sedimentology analyses, we show that the Central Asian region was generally arid witha high seasonal contrast characterized by hot and arid summers and wetter winters.Hotter and more arid summers despite the presence of the Proto-Paratethys may be explained by warmerEocene global conditions with a strong anticyclonic Hadley cell descending at Central Asian latitudes and astronger Foehn effect from the emerging Tibetan Plateau to the south. This implies that the shallow sea did nothave a strong dampening thermal effect on the monsoonal circulation in contrast to previous circulation modelsresults but in agreement with recent evidence for Eocene summer monsoons.Enhanced winter precipitations, relative to modern, is linked to a westerly moisture source coming fromthe Proto-Paratethys sea at that time. Additional bulk sediment stable isotope data from marine limestones andpedogenic carbonates suggest a gradual decrease in this westerly moisture source, which is in line with the retreatof the Proto-Paratethys followed by the Oligo-Miocene orogeny of the Central Asian ranges (Tian Shan andPamir) shielding the westerlies

Pliocene-Pleistocene initiation, style, and sequencing of deformation in the central Tien Shan

In response to the Indo-Asian collision, deformation of the Tien Shan initiated at ∼25 Ma along the northwestern margin of the Tarim Basin. 300 km north, the Kyrgyz Range began deforming ∼15 Ma later. Although multiple intervening structures across the Tien Shan are currently active, the sequencing of initial deformation across the orogen's entire width remains poorly known. To determine whether deformation migrated sequentially northward or developed less predictably, we documented deformation patterns within the Naryn Basin in south-central Kyrgyzstan. Detailed mapping and a published balanced cross section across the Naryn Basin suggest that deep-seated, relatively steeply dipping thrust faults have disrupted the basin during late Cenozoic deformation. Dating of deformed fluvial terraces with ages between ∼10 and 250 ka constrains the rate of deformation across relatively young structures in the Tien Shan interior. Based on geodetic surveys of dated terraces, local rates of relative rock uplift span from 0.3 to 3.5 mm/yr. Folding rates and patterns are temporally persistent at a given site. Moreover, they mimic modern geodetic rates measured from interferometric synthetic aperture radar. Extrapolating these rates into the past suggests that structures within the interior of the Naryn Basin formed in the last 1 Myr, whereas the ranges surrounding the basin initiated at least 1-4 Myr earlier. Hence, within the Naryn Basin itself, deformation has migrated from margins to interior. Similarly, these new chronologies indicate that at least some deformation in the interior of the Tien Shan initiated millions of years later than along either orogenic margin. ©2014. American Geophysical Union. All Rights Reserved