Peizhen Zhang

Continuous deformation of the Tibetan Plateau from global positioning system data

Global positioning system velocities from 553 control points within the Tibetan Plateau and on its margins show that the present-day tectonics in the plateau is best described as deformation of a continuous medium, at least when averaged over distances of .;100 km. Deformation occurs throughout the plateau interior by ESE-WNW extension and slightly slower NNE-SSW shortening. Relative to Eurasia, material within the plateau interior moves roughly eastward with speeds that increase toward the east, and then flows southward around the eastern end of the Himalaya. Crustal thickening on the northeast- ern and eastern margins of the plateau occurs over a zone ;400 km wide and cannot be the result of elastic strain on a single major thrust fault. Shortening there accommodates much of Indias penetration into Eurasia. A description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field.

Geology of the Haiyuan Fault Zone, Ningxia‐Hui Autonomous Region, China, and its relation to the evolution of the Northeastern Margin of the Tibetan Plateau

The Haiyuan area, located along the northeastern margin of the Tibetan Plateau in north central China, provides a laboratory for studying how the plateau has grown in late Cenozoic time. Rocks in the area range from pre-Silurian (Precambrian?) to Recent; the pre-Silurian and Cenozoic rocks form the most extensive outcrops. The pre-Silurian rocks consist of amphibolite- and greenschist-grade metasedimentary and metaigneous rocks unconformably overlain by Silurian and Devonian red beds. All of these rocks are intruded by granodiorite of unknown age. Cenozoic rocks consist of 2.6–3.0 km of Eocene to Miocene red beds that were deposited over an extensive area in this part of China. Pliocene conglomerate contains clasts from all older formations and is interpreted to have been derived from highlands developed during the beginning of Cenozoic deformation in the Haiyuan area. Except for the widespread loess deposits, Quaternary rocks reflect deposition in local tectonic environments. The oldest Cenozoic structures in the Haiyuan area are folds and small thrust faults that generally strike N30°–45°W and involve mostly pre-Quaternary rocks. These structures and all the Quaternary rocks are cut by the Haiyuan left-lateral strike-slip (left-slip) fault zone that generally trends N60°–65°W and is nearly vertical. At the western end of the mapped area a fault zone, which strikes N75°–90°W, forms a left-stepping transfer zone that connects with another segment of the Haiyuan fault zone, which continues N60°–65°W west into Gansu Province. A small basin, the Salt Lake Basin, is marked by active faults in the area of the transfer zone and is interpreted as a pull-apart basin along the left-slip Haiyuan fault zone. At its eastern end the Haiyuan fault zone has an irregular surface trace; east of Luzigou an active fault striking N35°–45°W branches to the south. This southern branch appears to be a younger fault and now accommodates most of the left-slip deformation that formerly occurred on the easternmost part of the Haiyuan fault zone. This younger fault connects through a left-stepping transfer zone to a parallel fault, the Xiaokou fault, that can be traced into the Liupan Shan about 60 km to the southeast. The Laohuyaoxian Basin is interpreted as a very young pull-apart basin in the area of the transfer zone. Matching different geological features across the Haiyuan fault zone yields a total left-slip offset of between 10.5 and 15.5 km, and the best constrained offsets yield 12.9–14.8 km. If left slip began near the end of the Pliocene time or earliest Pleistocene time, it indicates an average slip rate between 5 and 10 mm/yr. Progressively smaller offsets can be determined on progressively younger geological features, but dates for these younger features are too imprecise to constrain slip rates through time. Surface ruptures that formed during the 1920 Haiyuan earthquake (M = 8.7) show mostly left-slip displacement with magnitudes of more than 10 m in some places. Active faulting in the region suggests the Tibetan Plateau may be extending to the northeast in time. In the Haiyuan area, deformation probably began in Pliocene time, compared with a likely earlier initiation to the southwest; thus deformation began about 40–45 m.y. after collision between India and Asia. Formation of the low ranges to the northeast of the Haiyuan area, however, may have developed at different times and deformation may not have propagated regularly to the northeast. A total displacement of 10.5–15.5 km on the Haiyuan fault zone indicates that this fault zone does not accommodate large-scale eastward lateral transfer of continental fragments in the northern Tibetan Plateau.

Erosion, fault initiation and topographic growth of the North Qilian Shan (northern Tibetan Plateau)

New apatite (U-Th)/He from the northeastern margin of the Tibetan Plateau (north Qilian Shan) indicate rapid cooling began at ~10 Ma, which is attributed to the onset of faulting and topographic growth. Preservation of the paleo-PRZ in the hanging wall and growth strata in the footwall allow us to calculate vertical and horizontal fault slip rates averaged over the last 10 Myr of ~0.5 mm/yr and ~1 mm/yr respectively, which are within a factor of two consistent with Holocene slip rates and geodetic data. Low fault slip rates since the initiation of the northern Qilian Shan fault suggest that total horizontal offset did not exceed 10 km. Further, emergence of the northern Qilian Shan occurs during a period of increased aridity in northern Tibet but is associated with only a minor expansion of the northern plateau perimeter, which is well established near collision time. Outgrowth of the northern Qilian Shan at ~10 Ma could be simple propagation of the larger Qilian Shan system, occurring in response to decreased slip rates on the Altyn Tagh fault or as a result of the change in GPE of the central plateau.

Amount and style of Late Cenozoic Deformation in the Liupan Shan Area, Ningxia Autonomous Region, China

The structures of the Liupan Shan area are characterized by numerous active thrust and strike-slip faults that suggest thin-skinned deformation. The structural history of this area can be divided into three phases that probably overlap one another in time and are parts of a single protracted deformation. The oldest Cenozoic deformational phase occurred probably between late Pliocene and early Quaternary time and produced some of the folds and thrust faults in the Liupan Shan and Yueliang Shan. During this phase, deformation was the result of approximately N50°E shortening, and the amount of shortening seems to have been about 1–2 km. The second phase of deformation was dominated by left-lateral strike-slip faulting (left slip) on the N60°W striking Haiyuan fault zone and shortening on north-south trending structures; shortening was associated with a transfer of the left-slip displacement on the Haiyuan fault zone to shortening in areas farther east. Shortening occurred by thrust faulting in the Liupan Shan and Xiaoquan Shan and by folding in the Madong Shan. During this phase the orientation of shortening changed to N60°W. The average amount of shortening on the north-south trending folds in the Madong Shan is about 6.3–7.8 km. Most of the shortening on the Liupan Shan and Xiaoguan Shan thrust faults also occurred during this phase and amounted to a minimum of 4.8–6.3 km and 6.6–7.6 km, respectively, also with an orientation of N60°W. During the third phase of deformation, about 1–1.5 km of late Pleistocene to Recent left slip occurred on the Xiaokou fault, which was transferred into oblique left-slip thrusting in the Liupan Shan. At this time, deformation in the Madong Shan and Xiaoguan Shan ceased or was reduced to a very slow rate. The present, active left-slip on the Haiyuan fault zone is accommodated by shortening in the Liupan Shan area. The total displacement along the Haiyuan fault is essentially the same as the total amount of shortening in the Liupan Shan area. The sequence and interaction of strike-slip and thrust faults in the Liupan Shan area seem to apply to the folds and thrust faults farther north in the southern Ningxia region. Thus the northeastern margin of the Tibetan Plateau appears to grow by shortening oriented northeast and by left slip faulting that transfers material from farther to the west. The total left-slip in the entire southern Ningxia region probably is less than 20–25 km and may be absorbed by shortening within this region. Thus the eastward translation of crustal fragments in the northern part of the Tibetan Plateau may not extend east of southern Ningxia, and if large-scale eastward displacement has occurred, it must lie farther south.

Extinction of pull-apart basins

Numerous pull-apart basins are present along the Haiyuan fault zone in northwestern China. Detailed geological mapping of these basins reveals an evolution of faulting and subsidence that leads toward extinction of some pull-apart basins. The most common cause of pull-apart basin extinction appears to be the development of new strike-slip faults along one of the basin-margin normal faults or diagonally across the basin. The strike-slip faults may develop together with normal faults to form a complex zone within the basin. The migration of boundary strike-slip faults toward the basin center can also cause the extinction of pull-apart basins. The extinction of these pull-apart basins suggests that there is probably a tendency for a strike-slip fault zone to straighten itself, and the examples show how this takes place.

Characteristics of amplitude and duration for near fault strong ground motion from the 1999 Chi-Chi, Taiwan Earthquake

A great number of free-field ground motion records are obtained during the 1999 Chi-Chi, Taiwan, earthquake. Records from 130 near fault free-field stations within 55 km to the causative fault surface are used as database, and characteristics of earthquake peak ground acceleration, velocity, displacement and duration are analyzed. According to this study, near fault ground motions are strongly affected by distance to fault, fault rupture directivity, site condition, as well as thrust of hanging wall. Compared with empirical strong ground motion attenuation relations used in China, US and Japan, the PGAs and PGVs recorded in this earthquake are not as large as what we have expected for a large earthquake as magnitude 7.6. However, the largest PGV and PGD worldwide were recorded in this event, which are 292 cm/s and 867 cm, respectively. Caused by nonlinear site effects of soil, peaks and corresponding ratios on E-class site were markedly different from those on other sites. Just as observed in historic earthquakes, fault rupture directivity effects caused significant differences between peaks of ground motion of two horizontal components, but took very slight effects on the duration of ground motion. The significant velocity pulses associated with large PGVs and PGDs, as well as large permanent displacements, which may result from the large thrust of the hanging wall, became the outstanding character of this event. Based on this study, we point out that 3D waveform modeling is needed to understand and predict near fault ground motion of large earthquakes.

Late Quaternary right-lateral slip rates of faults adjacent to the lake Qinghai, northeastern margin of the Tibetan Plateau

By combining terrace riser offsets with terrace ages dated by 14 C, optically stimulated luminescence (OSL), and 10 Be techniques, we determine average slip rates of 1.1 ± 0.3 mm/yr and 1.2 ± 0.4 mm/yr for the Elashan and Riyueshan faults, two north-northwest–trending, right-lateral, strike-slip faults west and east of the lake Qinghai in the northeastern margin of the Tibetan Plateau. These faults are conjugate to the major easterly trending, left-lateral Altyn Tagh, Haiyuan-Qilianshan, and Kunlun faults, and they contribute to the subdivision of the region between the Haiyuan-Qilianshan and Kunlun faults into small blocks tens to ∼100 km in dimension. The relatively low slip rates in this region reflect distributed deformation. The total right-lateral offsets of geological contacts are ∼9–12 km along the Elashan fault and ∼11–12 km for the northern segment of the Riyueshan fault. If long-term slip rates were constant during late Cenozoic time, dates of initiation of dextral movement would be 9 or 10 ± 3 Ma for the two strike-slip faults, concurrent with onsets or acceleration of tectonic deformation in Cenozoic basins nearby. Our study highlights a stage of tectonic deformation in the northeastern margin of the Tibetan Plateau beginning since ca. 8–12 Ma, tens of millions of years after the collision between India and Eurasia began.

Structure and deformational character of strike-slip fault zones

Strike-slip fault zones observed either in the field or in model experiments generally consist of several subparallel faults which make these zones complicated in geometry and kinematics. The geometry of a strike-slip fault or shear zone is dependent on arrangement (pinnate or en echelon), on step (left step or right step), and on the rank )smaller faults within larger faults) of the subparallel fault. The relations and interactions of these three factors create a variety of dynamic circumstances and tectonic settings within the strike-slip fault zones. These include pull-aparts in the release area between subparallel faults, push-ups in the jogs where the subparallel faults overlap, and pivotal movements, or rotation, of single faults along the whole fault zone. Each kind of tectonic setting is in itself characteristic, each setting consists of many subtypes, which are controlled chiefly by the geometric parameters of the subparallel faults. One of the most important phenomena revealed in the field work is two different kinds of evolution of strike-slip fault zones: one is the evolution of a zone with a tensile component, which is related to the growth of rock bridges, and the other, of one with a compressional component, which develops by the destruction of rock bridges. In this paper we discuss, on the basis of recent research on four strike-slip fault zones in China, the essential characteristics of strike-slip faults and the possible causes of the observed structural phenomena. Attention is focussed on the deformation, development, and distribution of horizontal displacements within strike-slip fault zones.

Paleoseismology of the northern piedmont of Tianshan Mountains, northwestern China

The northern piedmont of the Tianshan Mountains consists of three rows of Cenozoic EW-striking fold and reverse fault zones, with en echelon right-lateral steps. The southernmost row involves sediments as young as lower Pleistocene, but there is no evidence of activity along this row during the last 30,000 years. The central row is composed of three linear anticlines (Houerguos, Manas, and Tugulu) and associated reverse faults. The northernmost row includes the Dushanzi, Halaande, and Anjihai anticlines and their associated reverse faults. Abundant fault scarps and folds of late Pleistocene to Holocene river terraces across the anticlines within the central and the northernmost rows indicate recent folding and reverse faulting. We divide the northern piedmont into the Dushanzi and Manas fold and reverse fault zone. In the Dushanzi zone, we excavated 15 trenches across scarps controlled by reverse faults and back thrusts. By comparing 11 trench logs among the 15 trenches, we identify three paleoearthquakes since 13,000 years B.P. The first event occurred between 11,300 and 13,300 years B.P., and the second and the third events occurred 6300–8400 and 3000–5000 years B.P., respectively. Considering the uncertainties of the data, the average recurrence interval for large earthquakes in the Dushanzi zone is about 4000 years. A large earthquake along this zone is expected in the near future because the elapsed time since the last surface-rupturing event is already 3000–4000 years. Three large trenches and several small trenches excavated across the fault scarps along the Manas fold and reverse fault zone reveal four events. The first, second, and third events occurred at 18,000–13,000 years B.P., 11,300–10,500 years B.P., and 6900–3600 years B.P., respectively. The fourth, the latest one, is the 1906 M = 7.7 Manas earthquake. Field investigation suggests that the 1906 Manas earthquake occurred along a blind thrust fault. This earthquake formed three discontinuous zones of fresh surface ruptures, the longest of which is only 8 km long along the eastern segment of the Tugulu reverse fault, and was associated with a zone of uplift 130 km long related to the Manas earthquake. The average recurrence interval along this zone is probably 5000–6000 years. Therefore it is unlikely that a large earthquake will occur along this zone in the near future because the Manas earthquake occurred only 89 years ago.

Rupture terminations and size of segment boundaries from historical earthquake ruptures in the Basin and Range Province

Abstract The fault-segmentation method is commonly used to estimate the potential earthquake size. Segment boundaries play an important role in arresting earthquake ruptures from event to event. In the Basin and Range Province, earthquake rupture terminations are commonly associated with structural discontinuities, but not all-structural discontinuities have the capability to terminate an earthquake rupture. The size of structural discontinuities with respect to the rupture length and displacement may play an important role in controlling rupture termination. Studies of the geometric pattern of seven well-documented historical earthquake ruptures in the Basin and Range Province reveal three important relationships between rupture termination and the size of structural discontinuities. Firstly, terminations of normal faulting earthquakes are often associated with structural discontinuities, at least for the five historical ruptures studied in this paper. Secondly, the sizes of structural discontinuities at the end of earthquake rupture zones are generally the largest among the structural discontinuities within the rupture zone. Thirdly, there appears to be a trend that larger earthquake ruptures are stopped by larger structural discontinuities. The size of structural discontinuities that stopped the earthquake seems to scale with the length and displacement of earthquake surface rupture.