ALBERT

Notes: We have studied the Cenozoic and active tectonics of the north-eastern rim of Tibet west of the Yellow River (Gansu, China) where the western Haiyuan Fault enters the eastern Qilian Shan, a high mountainous region, which was the site of the 1927 May 23, M= 8-8.3, Gulang earthquake. Fieldwork, combined with analysis of aerial photographs and satellite images, reveals consistent cumulative left-lateral offsets of postglacial geomorphic features along the fault, but no recent rupture. West of the Tianzhu pull-apart basin, the levelling of offset-terrace risers implies Holocene horizontal and vertical slip rates on the steeply south-dipping, N110E-striking fault of 11 ± 4 and 1.3 ± 0.3 mm yr-1, respectively. The presence of subordinate, mostly normal, throws due to local changes in fault strike, and kinematic compatibility at the SW corner of the Tianzhu basin, constrains the azimuth of the fault-slip vector to be N110-115E. On the less prominent, N85-100E-striking Gulang Fault, which splays eastwards from the Haiyuan Fault near 102.2°E, less detailed observations suggest that the average Holocene left-slip rate is 4.3 ± 2.1 mm yr-1 with a minor component of ˜˜N-directed thrusting, with no recent seismic break either. East of ˜˜103°E, coeval slip on both faults thus appears to account for as much as 15 ± 6 mm yr-1 of left-lateral movement between NE Tibet and the southern edge of the Ala Shan Platform, in a N105 ± 6E direction. West of ˜˜103°E structural and geomorphic evidence implies that ˜˜NNE-directed shortening of that edge across the rising, north-eastern Qilian mountain ranges occurs at a rate of 4 ± 2 mm yr-1, by movement on right-stepping thrusts that root on a 10-20°S-dipping décollement that probably branches off the Haiyuan Fault at a depth of ˜˜25 km. The existence of fresh surface breaks with metre-high free faces on a N-dipping, hanging-wall normal fault south of the easternmost, Dongqingding thrust segment, and of half-metre-high pressure ridges on that segment, indicates that the 1927 Gulang earthquake ruptured that complex thrust system. The ˜˜4 mm yr-1 shortening rate is consistent with the inference that the thrusts formed and move as a result of orthogonal slip partitioning in a large restraining bend of the Haiyuan Fault.Based on a retrodeformable structural section, we estimate the cumulative shortening on the Qilian Shan thrusts, north of the Haiyuan Fault, to be at least 25 km. The finite displacements and current slip rates on either the thrusts or the left-lateral faults imply that Cenozoic deformation started in the Late Miocene, with slip partitioning during much of the Plio-Quaternary. Assuming coeval slip at the present rates on the Haiyuan and Gulang Faults in the last 8 Ma would bring the cumulative left-lateral displacement between NE Tibet and the Ala Shan Platform to about 120 km, consistent with the 95 ± 15 km offset of the Yellow River across the Haiyuan Fault, but many times the offset (˜˜16 km) inferred on one rccent strand of that fault east of the river. Relative to the SE Gobi Desert, NE Tibet thus appears to have moved by a fair amount in the Late Cenozoic and is still moving fast. While some of this motion probably contributes to displace (towards the ESE) and rotate (CCW) the south-west edge of the Ordos block, much of it appears to be transmitted to the South China block, which leads, with the additional contribution of other large left-slip faults to the south and despite thrusting in the Lungmen Shan, to the extrusion (towards the ESE-SE) of that block relative to the Gobi, hcncc to north-eastern Asia.The ˜˜260 km long western Haiyuan Fault links two faults that ruptured about 70 years ago during two great earthquakes only seven years apart. Despite spectacular evidence of Holocene movement, it bears no trace of a large earthquake in the past eight centuries, either in the field or in the historical record. Given its relatively high slip rate, it should therefore be singled out as one of the most critical sites for impending great earthquakes (at least M ≥ 7.5, probably M ≥ 8) in the region. That such a seismic gap, called here the ‘Tianzhu gap', lies only ˜˜100 km north of Lanzhou and Xining, largest population centres of west-central China, makes instrumental monitoring of that fault particularly urgent. That the M ˜˜ 8, Gulang earthquake ruptured a complex thrust surface under high mountains in a restraining bend of the Haiyuan strike-slip fault suggests that the occurrence of comparable earthquakes in other areas with similar fault geometry, such as south of the big bend of the San Andreas Fault in California, should not be ruled out.

Notes: We present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active Aegean region. We focus first on the 130 km long Corinth Rift, an asymmetric graben where a flight of terraces of marine origin are uplifted. We show that the edges of the terraces lie in the footwall of the normal fault bounding the Corinth Rift and correspond to sea-level highstands of laic Pleistocene age. Using a detailed analysis of aerial and SPOT imagery supported by field observations, we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault. The elevation of the terraces' inner edges was estimated at 172 sites with an error of ±5m. This data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level. To date the deformation, we correlate the Corinth terraces with late Pleistocene oxygen-isotope stages of high sea-level stands and with global sea-level fluctuations. Using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary-element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault. We show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000. All the terraces are fitted for fault slip increasing in proportion to terrace age, and the component of regional uplift is found to be less than 0.3 mm yr−1. The best fits give a slip rate of 11±3 mm yr−1 on the main rift-bounding fault over the last 350 kyr. Other geological and morphologic information allows us to estimate the total age of the main fault (∼1 Ma) and to examine the mechanical evolution of the Corinth Rift. The minimum observed sediment thickness in the Gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6–7 mm yr−1 (40 per cent less than estimated with modelling). Even this slip rate is nearly 10 times higher than for comparable features in most of the Aegean and elsewhere in the world.At a larger scale, the spacing and asymmetry of the rift systems in the Aegean suggest strain localization in the upper mantle, with slow extension starting 15 Myr ago or earlier. The more recent (1 Myr), rapid phase of rifting in Corinth partly reactivated this earlier phase of extension. The younger faulting in Corinth appears to result from its present location in the inhomogeneous stress field (process zone) of the south-westward propagating tip of the southern branch of the North Anatolian Fault. We extend these relations to propose a mechanical model for the Late Cenozoic evolution of the Aegean. As the Arabia/Europe collision progressed in eastern Turkey it caused Anatolia to move to the west and the North Anatolian Fault to propagate into the Aegean, where the early slow extension started to be modified about 5 Ma ago. The process of propagation dramatically increased the activity of some but not all of the earlier rifts. The model we present is compatible with tectonic observations, as well as with the seismicity, the palaeomagnetic rotations and the displacement field now observed with GPS and SLR.

Notes: Field work carried out in Gansu province and complemented with analysis of SPOT panchromatic scenes allows us to characterize the deformations along the eastern segment of the Altyn Tagh fault and to place bounds on its Holocene left slip rate. East of 96°E, the long-term, left-lateral offset of stream channels, alluvial fans, and terrace edges is about 50 m. These offsets are most probably of Holocene age (12 ± 2 ka) and imply that the corresponding derived slip rate is 4 ± 2 mm yr−1. This observation is consistent with a north-eastward along-strike decreasing slip rate on the Altyn fault due to partitioning of slip on multiple, more easterly trending splays.