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Long records of paleoearthquakes are essential for understanding earthquake recurrence behavior of active faults and for evaluating regional seismic hazard. However, paleoseismic data on the Altyn Tagh fault (ATF), one of the longest strike-slip fault in Asia, are scarce. We document a long paleoseismic record along the Xorkoli section of the central ATF. Eight and probably nine earthquakes are identified based on event evidence in the form of open fissures, folds, unconformities, and upward fault terminations, with modeled mean (95% confidence) ages of A.D. 1598 (1491–1741) yr (event A), A.D. 797 (676–926) yr (B), B.C. 668 (732–589) yr (C), B.C. 956 (1206–715) yr (D), B.C. 1301 (1369–1235) yr (E), B.C. 2105 (2233–1987) yr (F, probable), B.C. 2663 (2731–2601) yr (G), B.C. 2818 (2878–2742) yr (H), B.C. 3396 (3522–3205) yr (I). The mean recurrence interval is 620 ± 410 yr with a coefficient of variation (COV) of 0.67, indicating that earthquake recurrence is weakly periodic, with individual intervals ranging from as short as 150 yr to as long as 1460 yr. A global compilation of 35 strike-slip paleoseismic sites yields a similar average COV of 0.69. Synthesis of paleoseismic sites from the central ATF indicates that not all earthquakes ruptured to the eastern end of the Xorkoli section, within the Aksay restraining bend. Given that the 420-yr elapsed time since the most recent event, well within a COV of mean interval, a large surface-rupturing earthquake could occur at any time along the central ATF. [ABSTRACT FROM AUTHOR]

The southeast margin of the Tibetan plateau is characterized by deeply incised river valleys separated by a perched low relief landscape that gently descends from the high Tibetan plateau towards the southeast. When and how this unique landscape formed is debated. The onset of increased river incision is often interpreted as a proxy for the timing of surface uplift. Here, apatite and zircon (U–Th)/He and apatite fission track thermochronometries are employed to map the spatial and temporal pattern of exhumation in the region. Vertical profiles of granitic rocks were collected near Deqin (∼28.5°N) and Weixi (∼27.5°N). The two transects share a similar exhumation history, with two episodes of relatively fast exhumation (∼100–300 m/Myr) in the Cenozoic: during the Paleocene to Eocene (60–40 Ma) and Miocene to present (20–0 Ma), separated by an intervening period of slow exhumation. A pulse of moderate to high exhumation (70–300 m/Myr) during the mid- to late-Cretaceous (120–80 Ma) is also present in the data. However, the rate and total amount of exhumation near Deqin is larger than at Weixi and is especially pronounced in the interval between 20 Ma to present. We interpret this difference as possibly related to differences in erosion rates between the Lancang (Deqin) and the Jinsha (Weixi) rivers. The Paleocene to Eocene episode of fast exhumation is likely in response to early Cenozoic deformation along tectonic boundary structures, related to the transpressional collision of the Indian plate with this region. Pre-Miocene episodes of fast exhumation corroborate recent paleoaltimetric studies, which show that the southeast margin of the Tibetan plateau was elevated prior to the Oligocene. [ABSTRACT FROM AUTHOR]

Abstract: The evolution of the topography of eastern Tibet reflects the dynamic interaction between processes at Earth's surface and the lower crust. Reconstruction of its spatio-temporal development provides constraints for our understanding of the geodynamics of continental deformation. This study examines the topographic evolution of the southeast margin of the Tibetan Plateau throughout the Cenozoic using stable isotope paleoaltimetry of pedogenic carbonates preserved in Cenozoic basins extending from the northern to the southern portions of the SE margin of the Tibetan Plateau (Yunnan Province, China). Paleoelevations are calculated using a Monte Carlo simulation of the normally distributed uncertainties in each parameter used to calculate elevations from the isotopic value of pedogenic carbonate. Isotope–elevation relationships are determined from a modern sampling transect along the modern Lancang (Mekong) River and an estimated Eocene Rayleigh fractionation model that was randomly resampled. Calculated elevations indicate the SE margin of Tibet was at its present elevation as early as the late Eocene, with elevations of 3.3 km as far south as the northern third of China's Yunnan Province. This result expands the extent of the Eocene Tibetan Plateau more than 1000 km east of that previously known and precludes surface uplift via lower crustal flow in the area of SW Sichuan and Northern Yunnan as a driver for drainage reorganization and an observed middle to late Miocene pulse of rapid exhumation. Elevation estimates from the southern two thirds of the study area parallel to the Red River Fault suggest approximately 1 km of post late Miocene surface uplift, consistent with estimates from previous studies. [Copyright &y& Elsevier]

• Probability density functions for repeated strike-slip earthquakes are analyzed. • Low variation in slip (CoV) is required to distinguish multiple earthquakes. • Only a couple of earlier historical rupture data have CoVs low enough to find >2 events. • Peaks in probability density functions are not always individual events. Coseismic slip distributions of repeated earthquakes are important data used to forecast magnitude and recurrence characteristics of future earthquakes. Many studies infer coseismic slip of past individual paleoearthquakes from frequency histograms of geomorphic offsets. Such histograms, often expressed as a COPD (cumulative offset probability distribution), commonly show multiple peaks with similar displacements, which are interpreted to be incremental slip associated with successive paleoearthquakes. However, assumptions in linking COPD peaks to individual earthquakes have not been fully explored. Here, we combine statistical modeling with global historical coseismic surface rupture data to evaluate the conditions required for the approach. The results show that only coseismic slip with low variability allow more than one event identification; for example, even low CoVs (coefficient of offset variation) of 0.25 and 0.35 permit identification of only 2 to 3 events. In contrast, the mean CoV value for all historical ruptures is 0.58 to 0.66. Furthermore, interpreting the first COPD peak as a single-event slip distribution could result in artificially low variation and flatter distribution of offset than observed in historical earthquakes. This study cautions that robust COPD interpretation on large strike-slip faults requires low offset variability, a sufficient number of offset measurements, and additional constraints like historical coseismic slip data or paleoseismic event sequence data. [ABSTRACT FROM AUTHOR]

River incision into a widespread, upland low-relief landscape, and related patterns of exhumation recorded by low-temperature thermochronology, together underpin geodynamic interpretations for crustal thickening and uplift of the eastern Tibetan Plateau. We report results from a suite of 11 (U–Th–Sm)/He cooling-age samples. Eight samples comprise a 1.2 km relief section collected from elevations up to 4800 m in the Jiulong Shan, an elevated, rugged region located in the hinterland of the Yalong-Longmen Shan Thrust Belt, and surrounded on three sides by upland low-relief landscape surfaces. Zircon and apatite cooling ages record two episodes of rapid exhumation in the early Oligocene and late Miocene, that were separated by a period of stability from ∼30 to 15 Ma. The first episode is consistent with a similar pulse evident from the Longmen Shan. The second episode is ongoing, and when integrated with adjacent cooling-age data sets, shows that doming of the Jiulong Shan has resulted in 2 to 4 km of differential exhumation of the plateau interior. We show from a compilation of glacial landform-mapping that the elevation of the plateau surface closely tracks global last glacial maximum equilibrium line altitude. We hypothesize that smoothing of highlands by efficient glacial and periglacial erosion, coupled with potential river captures and conveyance of sediments via external drainage, can yield an apparently continuous low-relief plateau landscape formed diachronously at high elevation. [ABSTRACT FROM AUTHOR]

Abstract: The timing of uplift of the Tibetan Plateau remains controversial, with estimates varying from Eocene to more recent than Pliocene. In particular, the paleoaltimetry of the Qiangtang terrane, on the central Tibetan Plateau, is completely unknown. Here, we present new stable isotope results of fluvial/lacustrine carbonate cement, pedogenic carbonate and marl from the Kangtuo and Suonahu formations deposited between ∼51 and ∼28Ma in the Heihuling–Bamaoqiongzong area of the northern Qiangtang terrane. The lithofacies associations indicate that the Kangtuo formation was deposited in alluvial fan and fluvial floodplain environments, and the Suonahu formation was deposited in near shore lacustrine, playa-lake and channelized fluvial environments. Carbon and oxygen isotope values, coupled with the sedimentary facies interpretations, point to evaporation and low respiration rates in the Eocene–Oligocene paleosols, suggesting an arid climate in the high Qiangtang area at the time. The δ 18Opsw values of paleo-surface water reconstructed from the lowest (i.e. the least evaporated) δ 18Oc (PDB) values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin. The paleoelevation of the northern Qiangtang terrane is reconstructed as above 5000m by at least the middle Oligocene (28Ma), similar to the present elevation in this area. The aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high Lhasa terrane established by the middle Oligocene blocked the northward transport of tropical moisture. Maintenance of high elevation (>5000m) from at least the Oligocene to the present suggests that the Qiangtang crust was already thickened by that time, and that underthrusting of India beneath Asia since then has continued to provide additional material while at the same time driving Asian lower crust eastward by crustal flow. [Copyright &y& Elsevier]

Abstract: Within the Himalayan collisional belt, granotoids occur along two sub-parallel belts, the Northern Himalayan Gneiss Domes (NHGD) and the High Himalayan Crystalline Series (HHCS). In the Yardoi area of NHGD, two-mica granite, a new type granite occurs in the core of the Yardoi gneiss dome (YGD), Dala and Quedang from north to south, and extends at least 50km long. These granites have similar mineral composition, elemental and radiogenic isotope geochemistry, and age of formation. SHRIMP zircon U/Pb dating indicates that the Yardoi and the Quedang two-mica granites formed at 42.6±1.1Ma and 42.8±0.6Ma, respectively, similar to the Dala pluton. These two-mica granites have (1) high SiO2 (>68wt.%), Al2O3 (>15wt.%), and A/CNK(>1.0); (2) relatively high Sr and LREE, but low Y(<10ppm) and Yb (<1ppm); (3) high Sr/Y (>40 and up to 250) and La/Yb (>30); (4) very weak or no Eu anomalies; and (5) as compared with those in the Himalayan leucogranites, low initial Sr (87Sr/86Sr(i)<0.7120) and similarly unradiogenic Nd (εNd(i)=−8.9–−15.0) isotopic compositions. These granites have initial Sr and Nd isotope compositions similar to those in the amphibolites but significantly different from those in the metapelite and granitic gneiss. Two-mica granites from the Yardoi area are of peraluminous granite with relatively high Na/K and Sr/Y ratios. Such features are distinct from those in the younger leucogranites along the HHCS as well as in the NHGD, and require melting of source consisting dominantly of amphibolite at thickened crustal conditions. This is also supported by the presence of amphibolites with similar Sr and Nd isotope compositions, and similar ages of metamorphism. Two-mica granites of similar age also occur in the other NHGD gneiss domes and along the HHCS belt, implying that Mid-Eocene melting of thickened crustal materials was widespread and might be a primary factor that led to the formation of high density materials (e.g. eclogitic rocks) beneath the Tethyan Himalaya. [Copyright &y& Elsevier]

• New high-resolution magnetostratigraphic study constrains the Gonjo Basin from 69 to 41.5 Ma. • Rapid SR and rotation periods suggest pulses of Tibetan crustal shortening at 69-67 Ma and 52-48 Ma. • Tibetan crustal shortening pulses correlate with India-Asia convergence rate. Models that aim to explain the causes of the significant Indian plate motion acceleration around 70 Ma, and the subsequent deceleration around 52 Ma predict different scenarios regarding crustal shortening of the Tibetan Plateau, which can be tested by precisely determining the timing of regional shortening events in Tibet. Here we attempt to determine this timing by presenting a high-resolution magnetostratigraphy of a ∼3.5 km thick sedimentary sequence in the syn-contractional Gonjo Basin, east-central Tibet. We successfully isolated the primary remanence as confirmed by positive fold and reversal tests. Correlation to the geomagnetic polarity time scale reveals a 69–41.5 Ma age for the Gonjo Basin sedimentary succession. Average sedimentation rates indicate two episodes of enhanced sediment accumulation rate at 69–64 Ma and 52–48 Ma, which coincide with periods of vertical axis rotation recorded in the basin fill. This coincidence suggests a tectonic cause, which given regional structures we interpret as shortening pulses. Our results are similar to those from basins elsewhere in southern, central and northern Tibet, suggesting plateau-wide, synchronous shortening pulses at ∼69–64 Ma and ∼52–48 Ma. These pulses are synchronous with major acceleration and deceleration of India-Asia convergence rate, suggesting that both the acceleration and deceleration of India-Asia convergence may be associated with enhanced crustal deformation in Tibet, which we use to evaluate previous dynamic models explaining the Indian plate motion changes and India-Asia collision processes. [ABSTRACT FROM AUTHOR]