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Abstract. Deformation at the ends of large intracontinental strike-slip faults that do not simply link other major structures often involves rotations about a vertical axis. We use earthquake slip vectors, surface rupture in earthquakes, and geomorphology to examine the ends of three major strike-slip faults in Mongolia. In these places a simple pattern is seen, consisting of a thrust fault on one side, with a displacement that decreases away from the strike-slip fault, consistent with local rotational deformation. Strike-slip faults that terminate in this way allow the style of faulting to change spatially within a deforming area, for example, from dominantly strike-slip to dominantly dip-slip, while still accommodating the overall deformation required by larger-scale regional motions. Such a change in fault style should also be accompanied by a change in the rotation rate about a vertical axis, which may be detected paleomagnetically. The kind of strike-slip fault termination described here may have consequences for how large strike-slip faults evolve and grow and for the variation in displacement along their length.

S U M M A R YIn this paper, we use observations of earthquake source parameters and gravity to investigate the mechanical properties and the active faulting of the lithosphere in Mongolia. Well-determined earthquake centroid depths, including 10 from inversions of P and SH waveforms that are presented here for the first time, show that the seismogenic thickness (T s ) within Mongolia itself is less than 20 km. However, to both the east, in parts of the Lake Baikal rift system, and the west, adjacent to the Junggar basin and Kazakhstan platform, the seismogenic thickness is considerably greater, and includes essentially the whole crust. From the admittance between the free-air gravity and the topography, and also from profiles across a flexural foreland basin, we determine the effective elastic thickness (T e ) in central Mongolia to be <10 km, though it may be a little greater (<20 km) adjacent to the Gobi-Altay range in the south. Further west, adjacent to the Kazakhstan platform, the same techniques show that T e > 30 km. In both Mongolia and its surroundings, T e is comparable with T s and, where it is well determined, T e < T s . Nowhere do the data require that T e > T s . These data are consistent with the view that the strength of the continental lithosphere resides in its seismogenic part, which in Mongolia is the upper crust, but to both the east and west appears to be the whole crust. The earthquake source parameters also allow us to ask how the active faulting in Mongolia accommodates the velocity field revealed by GPS measurements. It is likely that the entire Mongolian Altay range in the west rotates counter-clockwise relative to stable Asia, and is responsible for the distributed E-W left-lateral shear seen further east in central Mongolia. The admittance observations show no evidence at the present day for convective mantle support, or a 'hotspot', responsible for the elevated region of the Hangay dome in central Mongolia, even though the geochemical data from nodules in late Cenozoic basalts and seismic tomography studies suggest elevated temperatures at shallow depths (<125 km) and probably thinned lithosphere.

Late Pleistocene glaciers around Darhad Basin advanced to near their maximum positions at least three times, twice during the Zyrianka glaciation (at ∼ 17–19 ka and ∼ 35–53 ka), and at least once earlier. The Zyrianka glaciers were smaller than their predecessors, but the equilibrium-line altitude (ELA) difference was < 75 m. End moraines of the Zyrianka glaciers were ∼ 1600 m asl; ELAs were 2100–2400 m asl.14C and luminescence dating of lake sediments confirm the existence of paleolake highstands in Darhad Basin before ∼ 35 ka. Geologic evidence and10Be cosmic-ray exposure dating of drift suggests that at ∼ 17–19 ka the basin was filled at least briefly by a glacier-dammed lake ∼ 140 m deep. However, lake sediments from that time have not yet been recognized in the region. A shallower paleolake briefly occupied the basin at ∼ 11 ka, but between ∼ 11 and 17 ka and after ∼ 10 ka the basin was probably largely dry. The timing of maximum glacier advances in Darhad appears to be approximately synchronous across northern Mongolia, but different from Siberia and western Central Asia, supporting the inference that paleoclimate in Central Asia differed among regions.

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We present remote-sensing and fi eld observations of an ~350-km-long east-west left-lateral strike-slip fault (the South Hangay fault) in the Hangay Mountains of central Mongolia, an area previously believed to be deforming solely by slip on scattered, and randomly oriented, normal faults. The known dip-slip faults are shown to be short segments introduced at bends in the much longer strike-slip fault. Our observations show that the active faulting in the Hangay Mountains is consistent with the regional strain fi eld of Mongolia and does not require, as suggested in other studies, that the faults result from stresses introduced by the locally elevated topography. Our observations help to defi ne the active tectonics of this important part of the India-Eurasia collision. The South Hangay strike-slip fault is a potential source of large-magnitude earthquakes and constitutes a previously unrecognized hazard in this part of Mongolia.

The Altai range (western Mongolia) accommodates NNE-SSW shortening across the northern India-Eurasia collision zone by dextral slip on faults trending NNW-SSE, and anticlockwise, vertical-axis rotations of fault-bounded blocks. However, fault slip-rates and the way in which faulting evolves over time are poorly understood, and form the motivation for this study. We focussed on the HarUs-Nuur fault, a major transpressional fault bounding the eastern margin of the Altai. Three abandoned alluvial fan surfaces, each displaced right-laterally by the fault, were targeted for dating with cosmogenic 10 Be and quartz optically stimulated luminescence (OSL). The first surface (A2) shows an exponential decrease in 10 Be with increasing depth, with a significant inherited compo- * Corresponding author Material from the same sampling pit was dated at ∼19 ka with OSL, but we consider this younger age to be incorrect, possibly due to feldspar contamination or abnormal quartz OSL characteristics. The A2 surface is displaced by 175 m, implying a (maximum) dextral slip-rate of 2.4 ± 0.4 mm yr −1 . A second fan surface (F1) was dated at ∼6 ka with OSL and shows little variation in 10 Be with depth, consistent with this young age. The inherited component is higher than for A2, indicating contrasting levels of inheritance for different periods of fan aggradation. A final surface (F2) shows scattered 10 Be concentrations and lacks material suitable for OSL, so cannot be dated precisely. Using the total vertical displacement across the fault, we place the initiation of movement on the fault at ∼2 Ma, significantly later than the late Oligocene to Miocene (28-5 Ma) onset of shortening in the Altai region. This suggests that deformation in the Altai has widened over time to incorporate new faults at the range margins (such as Har-Us-Nuur), possibly because older faults in the range interior have rotated about vertical axes into orientations that require work to be done against gravity.