http://www.zetaware.com/public/McKenzie_1978.pdf

Some remarks on the development of sedimentary basins

We summarize evidence for Quaternary and active faulting collected in the field during three Sino-French expeditions to southeastern Tibet (1980–1982). Detailed mapping of Quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the Himalayas in the last 2 ± 0.5 m.y. The maximum horizontal principal stress in south Tibet appears to be only the intermediate principal stress σ2, σ1 being vertical. South of the “chord” joining the eastern and western syntaxes of the Himalayan arc, extensional strains are principally localized within seven regularly spaced rift zones, three of which have been studied in some detail. The extension direction is determined to be N96° E ± 7° mainly from statistical averaging of strikes of newly formed normal faults. Throw rates on normal faults are evaluated for different time spans (2 ± 0.5 m.y., 60 ± 40 kyr, and 10 ± 2 kyr B.P.), using structural and topographic reliefs, as well as synglacial and postglacial vertical offsets. The rate of Quaternary extension is about 1% m.y.−1 along an 1100-km-long ESE traverse across south Tibet. This corresponds to a “spreading” rate of 1 ± 0.6 cm/yr. This rate and the divergent horizontal projections of slip vectors of earthquakes along the Himalayan front constrain the rate at which rigid India underthrusts southern Tibet to be 2 ± 1 cm/yr. Although most of the normal faults appear to be independent of, and nearly orthogonal to, the ≃E-W Mesozoic-Tertiary tectonic fabric, the Yadong-Gulu rift appears to be guided for over 130 km by the older, oblique (≃NE-SW) Nyainqentanglha range and fault zones along it. This reactivated zone is the most prominent left-lateral strike-slip fault system in SE Tibet. Excepting this zone, and the vicinity of the SE extremity of the Karakorum fault, Quaternary strike-slip faulting is rare in south Tibet, i.e., south of the chord between the syntaxes of the Himalayan arc. North of the chord, the tectonic style is different. There minor conjugate strike-slip faulting is widespread and appears to control Quaternary normal faulting, which is more diffuse and subdued than in south Tibet. Along the chord, the presence of a major zone of active right-lateral, en echelon strike-slip faults (Karakorum-Jiali fault zone) probably reflects the greater facility of eastward extrusion in north central Tibet, in response to the northward push of India. The eastern Himalayan syntaxis may be an obstacle to such extrusion movements south of the chord.

Quaternary extension in southern Tibet: Field observations and tectonic implications

SUMMARY
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 decollement 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.

/pdf/partitioning-of-crustal-slip-between-linked-active-faults-in-24550899gq.pdf

Partitioning of crustal slip between linked, active faults in the eastern Qilian Shan, and evidence for a major seismic gap, the ‘Tianzhu gap’, on the western Haiyuan Fault, Gansu (China)

Faulting exerts an important control upon drainage development in active extensional basins and thus helps determine the architecture of the sedimentary infill to a synrift basin. Examples of the interaction between faulting and drainage from the western United States and central Greece may be grouped into a relatively small number of classes based upon the structural position of a drainage catchment: footwall, hangingwall, fault offset and axial. Our
examples illustrate the diversity of erosional effects that might arise because of variations in the spacing, orientation and segmentation of faults and their interactions. Where basement lithology is similar, footwall catchments are generally smaller, shorter and steeper than those of the hangingwall. Footwall-sourced alluvial fans and fan deltas are generally smaller in area than those sourced from similar lithologies in the hangingwall. Wide fault offsets often give rise to large drainage catchments in the footwall. The development of axial drainage depends upon the breaching of transverse bedrock ridges by headward stream erosion or by lake overflow. Once breaching has occurred the direction of axial streamflow is controlled by the potential developed between basins of contrasting widths. Fault migration and propagation leads to the uplift, erosion and resedimentation of the sedimentary infill to formerly active
basins, leading to the cutting of footwall unconformities. The outward sediment flux from structurally controlled catchments is modulated in an important way by lithology and runoff. The greatest contrasts in basement lithology arise when fault migration and propagation have occurred, such that the sedimentary fill to previously active basins is uplifted, incised and eroded by the establishment of large new drainage systems in the footwalls of younger faults. Drainage patterns in areas.where faults interact can shed light on the relative timing of activity and therefore the occurrence of fault migration and propagation. Facies and palaeocurrent trends in ancient grabens may only be correctly interpreted when observations are made on a length scale of 10-20 km, comparable to that of the largest fault segments.

The interaction between normal faulting and drainage in active extensional basins, with examples from the western United States and central Greece

The diffusion model of degradation of topographic features is a promising means by which vertical offsets on Holocene faults might be dated. In order to calibrate the method, we have examined present-day profiles of wave-cut shoreline scarps of late Pleistocene lakes Bonneville and Lahontan. It may be assumed that these scarps were initially at least as steep as the angle of repose. Offsets range from 1 to 12 m, and present slope angles range from 9° to 29°. A parameter called apparent diffusion age, defined as half the mean square horizontal extent of the slope function of each profile, is plotted as a function of scarp offset. The points show a clear trend of apparent age increasing nearly linearly with offset. If linear diffusion held and scarps were initially vertical, apparent diffusion age would be the same for all the shoreline profiles. The increasing trend can only partly be explained by nonvertical initial scarp slope, and therefore the rate of transport of material downslope must increase significantly faster than a linear law in the range of slopes spanned by the data. The transport law must become linear at small slope to reduce scatter between profiles with varying ambient slopes. The transport law adopted for the purpose of dating is k0(1 + 5s2), where s is local slope. The transport coefficient k0 is correlated inversely with fan slope, suggesting that there is a dependence on the particle size distribution. A table is included that allows easy application of the model to scarps with simple initial shape.

Fitting degradation of shoreline scarps by a nonlinear diffusion model

The age of fault scarps in unconsolidated deposits is commonly estimated by several criteria such as the extent of dissection, amount of rounding of the crest of the scarp, and the slope of the face of the scarp. To provide a more quantitative basis for evaluating the ages of fault scarps in western Utah, we have measured numerous profiles across late Quaternary fault scarps and studied the relationship between scarp height, slope angle, and age. Well-defined curves delineated by the data suggest that for a given age, (1) the slope angle of the scarp is proportional to the logarithm of the scarp height and (2) the slope angle decreases with estimated age for scarps of a given height. Owing to the lack of well-determined ages for the scarps studied, age assignments are only approximate; the available information provides a means of ranking scarps according to relative geomorphic age within a general age framework for scarps between several thousand and several hundred thousand years old.

Estimation of fault-scarp ages from a scarp-height slope-angle relationship

Late Cenozoic fault patterns and stress fields in the Great Basin and westward displacement of the Sierra Nevada block: Comment and reply

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Estimation of fault-scarp ages from a scarp-height slope-angle relationship

Late Cenozoic fault patterns and stress fields in the Great Basin and westward displacement of the Sierra Nevada block: Comment and reply