J. van der Woerd

Holocene left-slip rate determined by cosmogenic surface dating on the Xidatan segment of the Kunlun fault (Qinghai, China)

Cosmogenic dating, using in situ {sup 26}Al and {sup 10}Be in quartz pebbles from alluvial terrace surfaces, constrains the late Holocene slip rate on the Xidatan segment of the Kunlun fault in northeastern Tibet. Two terrace risers offset by 24 {+-} 3 and 33 {+-} 4 m, having respective ages of 1799 {+-} 388 and 2914 {+-} 471 yr, imply a slip rate of 12.1 {+-} 2.6 mm/yr. The full range of ages obtained ({le}22.8 k.y., most of them between 6.7 and 1.4 k.y.) confirm that terrace deposition and incision, hence landform evolution, are modulated by post-glacial climate change. Coupled with minimum offsets of 9--12 m, this slip rate implies that great earthquakes (M {approximately}8) with a recurrence time of 800--1000 yr, rupture the Kunlun fault near 94 E.

Rapid slip along the central Altyn Tagh Fault: Morphochronologic evidence from Cherchen He and Sulamu Tagh

consistent with the d 18 O record from the Guliya ice cap in the West Kunlun; the features of interest were all formed by glacial and fluvial processes subsequent to marine isotope stage 5e, with the youngest features having formed during the early Holocene Optimum. This ‘‘near-field,’’ morphochronological slip rate is averaged over many earthquake cycles and is hence little affected by interseismic strain. It is kinematically consistent with other, somewhat lower, geomorphic slip rate measurements to the east. The average rate, and lower bounds obtained from alternate interpretational models, 18.4 mm/yr, cannot be reconciled with the most rece geodetic measurements (� 7 mm/yr), suggesting that interseismic strain and interactions with adjacent faults may lead to disparate geologic and geodetic rate estimates. This late Pleistocene-Holocene, morphochronologic rate would imply that, at this longitude, the Altyn Tagh Fault, on the north edge of Tibet, might absorb almost as much of India’s convergence relative to Siberia as the Himalayan Main Frontal Thrust does on the southern edge of the plateau. INDEX TERMS: 1035 Geochemistry: Geochronology; 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 8102 Tectonophysics: Continental contractional orogenic belts; 8107 Tectonophysics: Continental neotectonics; KEYWORDS: slip rates, cosmogenic dating, Indo-Asian collision

Uniform slip‐rate along the Kunlun Fault: Implications for seismic behaviour and large‐scale tectonics

A long-term slip-rate is derived from concordant 10 Be, 26 Al and 14 C dating of cumulativeoffse ts along much of the length of the Kunlun Fault. Values at 6 sites indi- cateuniform slip (11 .5 ± 2.0 mm/yr) since ∼40 kyr BP. This relatively high slip rate corresponds to a first-order discon- tinuity in the Asian crustal velocity field. M∼ 8a nd M∼7.5 earthquakes on 2 segments of the fault recur with charac- teristic slip (∼10 ± 2ma nd 4.4 ± 0. 4m ) ev e ry∼850 and ∼420 yrs, respectively.

Estimating the return times of great Himalayan earthquakes in eastern Nepal : evidence from the Patu and Bardibas strands of the main frontal thrust

The return times of large Himalayan earthquakes are poorly constrained. Despite historical devastation of cities along the mountain range, definitive links between events and specific segments of the Main Frontal Thrust (MFT) are not established, and paleoseismological records have not documented the occurrence of several similar events at the same location. In east central Nepal, however, recently discovered primary surface ruptures of that megathrust in the A.D. 1255 and 1934 earthquakes are associated with flights of tectonically uplifted terraces. We present here a refined, longer slip history of the MFT’s two overlapping strands (Patu and Bardibas Thrusts) in that region, based on updated geomorphic/neotectonic mapping of active faulting, two 1.3 km long shallow seismic profiles, and logging of two river-cut cliffs, three paleoseismological trenches, and several pits, with constraints from 74 detrital charcoals and 14 cosmogenic nuclide ages. The amount of hanging wall uplift on the Patu thrust since 3650 ± 450 years requires three more events than the two aforementioned. The uplift rate (8.5 ± 1.5mm/yr), thrust dip (25° ± 5°N), and apparent characteristic behavior imply 12–17.5m of slip per event. On the Bardibas thrust, discrete pulses of colluvial deposition resulting from the coseismic growth of a flexural fold scarp suggest the occurrence of six or seven paleo-earthquakes in the last 4500 ± 50 years. The coeval rupture of both strands during great Himalayan earthquakes implies that in eastern Nepal, the late Holocene return times of such earthquakes probably ranged between 750 ± 140 and 870 ± 350 years.

Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)

The western part of the Tanghe Nan Shan range southwest of Subei (western Gansu, China) is presently growing on thrust ramps splaying from the left-lateral Altyn Tagh Fault. Late Cenozoic thrusting has folded and sliced Oligocene-Miocence red beds into an imbricate wedge, capped by warped and uplifted Quaternary terraces that form a 2- to 5-km-wide ledge, north of the steeply faceted range front. Seismic scarps 1.5 to 4.5 m high cut young fans along the outer thrusts. Carbon 14 dating of organic remains collected on strath terraces constrains the chronology of deposition and incision by the streams. Most of the fans and terraces in the southern part of the Subei basin appear to have been emplaced after the last glacial maximum, particularly during the early Holocene optimum (9–5 ka). Measurements of the shapes of the warped terraces constrain minimum and maximum throws on the outer thrusts. The minimum vertical throw of 34 ± 2 m of a surface dated at 8411 ± 530 years B.P. at one site provides a minimum vertical uplift rate of 4.1±0.5 mm/yr. The maximum possible uplift (115±15 m) of the oldest terrace surface, whose probable age is 15 to 18 ka, places an upper bound on the uplift rate of 7±2 mm/yr. The thrust geometry at depth and the cumulative shortening (10–20 km) deduced from balancing sections logged across the imbricate thrust wedge are consistent with a shortening rate of about 5 mm/yr and an onset of thrusting at about 4±2 Ma. Such a shortening rate implies a significant northward decrease in slip rate along the Altyn Tagh Fault. The recent growth of the western Qilian mountain ranges thus appears to be intimately coupled with sinistral motion on the Altyn Tagh Fault and the extrusion of Tibet.

Incision rate of the Yellow River in Northeastern Tibet constrained by (10)Be and (26)Al cosmogenic isotope dating of fluvial terraces: implications for catchment evolution and plateau building

Abstract Unlike other large rivers flowing out of Tibet, the Yellow River escapes from the plateau towards the NE crossing no less than five NW–SE striking, actively growing ranges and intervening basins. Thick Plio-Quaternary deposits and fluvial terraces testify to a phase of aggradation and sediment infill up to the average surface elevation (3200–3250 m a.s.l.) of the Gonghe, Guide and Qinghai Lake basins. A set of seven main terraces across the Gonghe Basin suggests progressive down-cutting of the Yellow River carving the 500 m deep Longyang gorge at the basin exit. 10Be and 26Al concentrations in quartz of surface and sub-surface samples of four terraces constrain the timing of incision by determining the burial age of the deposit and the exposure age of its surface. Modelling the depth dependence of the 10Be concentration and the 26Al/10Be ratio allows us to constrain the onset of the ongoing phase of incision to 120–250 ka. These ages suggest long-term incision rates between 2–6 mm a−1. Together with the present morphology of the Yellow River terraces across the Gonghe basin and the Longyang gorge, our results imply rapid river catchment evolution and interaction between river dynamics, tectonic and climate in northeastern Tibet.

The Pingding segment of the Altyn Tagh Fault (91 °E): Holocene slip-rate determination from cosmogenic radionuclide dating of offset fluvial terraces

Morphochronologic slip-rates on the Altyn Tagh Fault (ATF) along the southern front of the Pingding Shan at 90.5E are determined by cosmogenic radionuclide (CRN) dating of seven offset terraces at two sites. The terraces are defined based upon morphology, elevation and dating, together with fieldwork and high-resolution satellite analysis. The majority of the CRN model ages fall within narrow ranges (<2 ka) on the four main terraces (T1, T2, T3 and T3′), and allow a detailed terrace chronology. Bounds on the terrace ages and offsets of 5 independent terraces yield consistent slip-rate estimates. The long-term slip-rate of 13.9+/-1.1 mm/yr is defined at the 95% confidence level, as the joint rate probability distribution of the rate derived from each independent terrace. It falls within the bounds of all the rates defined on the central Altyn Tagh Fault between the Cherchen He (86.4E) and Akato Tagh (88E) sites. This rate is 10 mm/yr less than the upper rate determined near Tura at 87E, in keeping with the inference of an eastward decreasing rate due to progressive loss of slip to thrusts branching off the fault southwards but it is greater than the 9+/-4 mm/yr rate determined at 90E by GPS surveys and other geodetic short-term rates defined elsewhere along the ATF. Whether such disparate rates will ultimately be reconciled by a better understanding of fault mechanics, resolved transient deformations during the seismic cycle or by more accurate measurements made with either approach remains an important issue.