Central Altyn Tagh Fault Research Articles

Underthrusting of Tarim Lower Crust Beneath the Tibetan Plateau Revealed by Receiver Function Imaging

AbstractThe left‐lateral Altyn Tagh Fault (ATF) system is the northern boundary of the Tibetan Plateau resulted from the India–Eurasia continental collision. How intracontinental deformation across the central ATF responds to the distal collision remains elusive, primarily due to unclear crustal structure. We obtained detailed crustal structure across the central ATF using receiver functions recorded by ∼NW–SE oriented linear dense array. The images reveal the Tarim lower crust is underthrusting beneath the Tibetan Plateau and reaches to a maximum depth of ∼75 km and undergoing partial eclogitization. The two south‐dipping interfaces imaged beneath the Altyn Tagh Range (ATR) represent the thrusting Northern Altyn Fault and its branch fault. Oblique convergent forces extruded upper crustal materials along the thrust faults, creating the pop‐up structure of ATR, supported by low Vp/Vs ratios. Our balanced cross‐section for the Moho suggests intracontinental deformation in the ATR has accelerated since the late Miocene.

GPS Determined Asymmetric Deformation Across Central Altyn Tagh Fault Reveals Rheological Structure of Northern Tibet

AbstractWe establish a continuous GPS transect crossing the central Altyn Tagh fault at 90°E with eight years of observations. GPS velocities along this profile and another one crossing the fault at 86°E suggest a fault slip rate of 12.4 ± 0.7 mm/yr, but with asymmetric straining of adjacent terrain. On the south side, ∼8.2 mm/yr of left‐lateral shear is absorbed across a region ∼210 km from the fault, but only ∼4.2 mm/yr is found on the north side. This estimate of slip rate is ∼30% larger than the consensus estimate of previous models. By treating the deforming regions as elastic plates with different thicknesses overlying a substrata that obeys a linear Maxwell viscoelastic constitutive relationship, we infer a viscosity of ∼5.1 × 1019 Pa s (between 3.5 and 9.1 × 1019 Pa s at 1‐σ) on the south side, beneath northern Tibetan Plateau. This low viscosity, compared to some estimates for the asthenosphere, concurs with the Tibetan Plateau being underlain by a relatively hot and weak lower crust and upper mantle. The effective elastic thickness on the south side is 16.5–20 km, which is significantly smaller than that of the Tarim Basin of >60 km.

Observation-constrained multicycle dynamic models of the Pingding Shan earthquake gate along the Altyn Tagh Fault

Earthquake gates are fault geometric complexities, common in natural fault systems, that conditionally impede earthquake ruptures. This study centers on modeling of multicycle dynamics of the Pingding Shan earthquake gate along the central Altyn Tagh Fault in northwest China. The earthquake gate includes three geometric complexities: a prominent restraining bend, a 4-km wide stepover to the east, and a releasing bend to the west. We use a 2D finite element method to simulate coseismic spontaneous ruptures with interseismic fault stress evolutions computed by an analytic viscoelastic solution. Paleoseismic records and long-term slip-rates are used to constrain the models. We find that fault-geometry-related heterogenous stresses accumulated over earthquake cycles yield complex rupture patterns and help explain earthquake recurrence intervals revealed by paleoseismic records. The three most important contributions to the heterogeneous stresses come from dynamic ruptures passing fault geometric complexities, fault-strike-dependent tectonic loading and relaxation, and stress history from past earthquakes. In the Pingding Shan earthquake gate, the releasing stepover appears to impede ruptures more effectively than the restraining bend. The combined impact of the restraining bend and the releasing stepover makes the Pingding Shan earthquake gate a very effective barrier to 350-km model-spanning ruptures. The best-fit model yields a low recurrence interval of 4.6 kyrs for 350-km-long ruptures, interspersed with more frequent ruptures limited to individual fault segments. A lower static friction tends to reduce the effectiveness of the earthquake gate to impede ruptures. Local fault geometric complexities together with rapid energy release and restrengthening of friction during dynamic ruptures help explain the 0.66 recurrence interval coefficient of variation (COV) recorded at the Copper Mine paleoseismic trench site on the Xorxoli segment. This study provides a method for applying heterogenous initial stresses for single-event dynamic rupture simulations that consider the effect of past earthquakes and fault geometry.

Paleoseismologic record of earthquakes along the Wuzunxiaoer section of the Altyn Tagh fault and its implication for cascade rupture behavior

The Altyn Tagh fault is one of the few great active strike-slip faults in the world. The recurrence characteristics of paleoearthquakes on this fault are still poorly understood due to the lack of paleoseismic records recorded in high-resolution strata. We document a paleoseismic record in a pull-apart basin along the Wuzunxiaoer section of the central Altyn Tagh fault. The high-resolution strata recorded abundant seismic deformations and their sedimentary responses. Four earthquakes are identified based on event evidence in the form of open fissures, thickened strata, angular unconformities, and folds. The occurrence times of the four events were constrained using radiocarbon dating. Event W1 occurred at AD1220-1773, events W2 and W3 occurred between 407 and 215BC, and event W4 occurred slightly earlier at 1608-1462BC, indicating clustered recurrence characteristics. A comparison of the earthquake records along the Wuzunxiaoer section with other records along the Xorkoli section suggests that both sections ruptured during the most recent event.

Shallow Seismicity Forecast for the India‐Eurasia Collision Zone Based on Geodetic Strain Rates

AbstractGeodetic strain rates from increasing Global Positioning System data provide a promising approach for seismicity forecast. With the strain rate field presented in Zheng et al. (2017, https://doi.org/10.1002/2017JB014465) derived from the most complete and up‐to‐date Global Positioning System data set in the India‐Eurasia collision zone, we forecast the shallow seismicity of this region, and infer that about 11 Mw ≥ 7.5, 36 Mw ≥ 7.0, 109 Mw ≥ 6.5, and 326 Mw ≥ 6.0 earthquakes may occur here every 100 years. We indicate that shallow seismicity forecast may be able to help us distinguish between block and continuum models, and block model cannot well describe the kinematics of the Tibetan Plateau, Tien Shan, West Mongolia, North China, and Myanmar. We suggest that the regions with high forecasted earthquake rates but lack of historical earthquakes are undergoing high seismic risk, such as the west‐central Himalaya (overdue for Mw ≥ 7.5 earthquakes, possibly Mw ≥ 8.0) and the central Altyn Tagh fault (overdue for Mw ≥ 7.0 and Mw ≥ 7.5 earthquakes).

A 6000-year-long paleoseismologic record of earthquakes along the Xorkoli section of the Altyn Tagh fault, China

Long records of paleoearthquakes are essential for understanding earthquake recurrence behavior of active faults and for evaluating regional seismic hazard. However, paleoseismic data on the Altyn Tagh fault (ATF), one of the longest strike-slip fault in Asia, are scarce. We document a long paleoseismic record along the Xorkoli section of the central ATF. Eight and probably nine earthquakes are identified based on event evidence in the form of open fissures, folds, unconformities, and upward fault terminations, with modeled mean (95% confidence) ages of A.D. 1598 (1491–1741) yr (event A), A.D. 797 (676–926) yr (B), B.C. 668 (732–589) yr (C), B.C. 956 (1206–715) yr (D), B.C. 1301 (1369–1235) yr (E), B.C. 2105 (2233–1987) yr (F, probable), B.C. 2663 (2731–2601) yr (G), B.C. 2818 (2878–2742) yr (H), B.C. 3396 (3522–3205) yr (I). The mean recurrence interval is 620 ± 410 yr with a coefficient of variation (COV) of 0.67, indicating that earthquake recurrence is weakly periodic, with individual intervals ranging from as short as 150 yr to as long as 1460 yr. A global compilation of 35 strike-slip paleoseismic sites yields a similar average COV of 0.69. Synthesis of paleoseismic sites from the central ATF indicates that not all earthquakes ruptured to the eastern end of the Xorkoli section, within the Aksay restraining bend. Given that the 420-yr elapsed time since the most recent event, well within a COV of mean interval, a large surface-rupturing earthquake could occur at any time along the central ATF.

Structure of the Central Altyn Tagh Fault revealed by magnetotelluric data: New insights into the structure of the northern margin of the India–Asia collision

The Altyn Tagh Fault (ATF) is a left-lateral, strike-slip fault that forms the northern margin of the Tibetan Plateau and plays a significant role in accommodating the convergence between the colliding Indian and Eurasian plates. As a part of the fourth phase of the INDEPTH project, magnetotelluric (MT) data were collected across the central segment of the ATF to determine the lithospheric-scale structure of the fault system. Dimensionality analyses demonstrated that the MT data can be interpreted using two-dimensional approaches, but some localized 3-D effects are seen. Consequently, both 2-D and 3-D inversions were carried out, and a joint interpretation was made on the basis of these two types of models. Inversion models revealed two major conductors beneath the Qaidam Basin (QB) and Altyn Tagh Range (ATR), respectively. The conductive region beneath the QB was interpreted as a ductile layer in the lower crust to upper mantle that might represent flow beneath the western margin of the QB, whereas the large scale south-dipping conductor beneath the ATR is interpreted as a region with high fluid content formed by metamorphism associated with the oblique underthrusting of the Tarim Block beneath the northern Tibetan Plateau. These fluids migrate upwards through the fault system and have formed serpentinized zones in the crust. Combining these interpretations, a structural model compatible with diverse geophysical observations is proposed, in which we suggest the competing end-member rigid block model and continuum model are reconcilable with the continuum model locally dominant for the study region, as evidenced by a thickened crust.

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 ∼90.5°E 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 (86.4°E) and Akato Tagh (∼88°E) sites. This rate is ∼10 mm/yr less than the upper rate determined near Tura at ∼87°E, 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 ∼90°E 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.

Faulted terrace risers place new constraints on the late Quaternary slip rate for the central Altyn Tagh fault, northwest Tibet

The active, left-lateral Altyn Tagh fault defines the northwestern margin of the Tibetan Plateau in western China. To clarify late Quaternary temporal and spatial variations in slip rate along the central portion of this fault system (85°–90°E), we have more than doubled the number of dated offset markers along the central Altyn Tagh fault. In particular, we determined offset-age relations for seven left-laterally faulted terrace risers at three sites (Kelutelage, Yukuang, and Keke Qiapu) spanning a 140-km-long fault reach by integrating surficial geologic mapping, topographic surveys (total station and tripod–light detection and ranging [T-LiDAR]), and geochronology (radiocarbon dating of organic samples, 230 Th/U dating of pedogenic carbonate coatings on buried clasts, and terrestrial cosmogenic radionuclide exposure age dating applied to quartz-rich gravels). At Kelutelage, which is the westernmost site (37.72°N, 86.67°E), two faulted terrace risers are offset 58 ± 3 m and 48 ± 4 m, and formed at 6.2–6.1 ka and 5.9–3.7 ka, respectively. At the Yukuang site (38.00°N, 87.87°E), four faulted terrace risers are offset 92 ± 12 m, 68 ± 6 m, 55 ± 13 m, and 59 ± 9 m and formed at 24.2–9.5 ka, 6.4–5.0 ka, 5.1–3.9 ka, and 24.2–6.4 ka, respectively. At the easternmost site, Keke Qiapu (38.08°N, 88.12°E), a faulted terrace riser is offset 33 ± 6 m and has an age of 17.1–2.2 ka. The displacement-age relationships derived from these markers can be satisfied by an approximately uniform slip rate of 8– 12 mm/yr. However, additional analysis is required to test how much temporal variability in slip rate is permitted by this data set.

Paleoseismology of the Xorxol Segment of the Central Altyn Tagh Fault, Xinjiang, China

Although the Altyn Tagh Fault (ATF) is thought to play a key role in accommodating India-Eurasian convergence, little is known about its earthquake history. Studies of this strike-slip fault are important for interpretation of the role of faulting versus distributed deformation in the accommodation of the India- Eurasia collision. In addition, the > 1200 km long fault represents one of the most important and exemplary intracontinental strike-slip faults in the world. We mapped fault trace geometry and interpreted paleoseismic trench exposures to characterize the seismogenic behavior of the ATF. We identified 2 geometric segment boundaries in a 270 km long reach of the central ATF. These boundaries define the westernmost Wuzhunxiao, the Central Pingding, and the easternmost Xorxol (also written as Suekuli or Suo erkuli) segments. In this paper, we present the results from the Camel paleoseismic site along the Xorxol Segment at 91.759°E, 38.919°N. There evidence for the last two earthquakes is clear and 14C dates from layers exposed in the excavation bracket their ages. The most recent earthquake occurred between 1456 and 1775 cal A.D. and the penultimate event was between 60 and 980 cal A.D. Combining the Camel interpretations with our published results for the central ATF, we conclude that multiple earthquakes with shorter rupture lengths (?? 50 km) rather than complete rupture of the Xorxol Segment better explain the paleoseismic data. We found 2-3 earthquakes in the last 2-3 kyr. When coupled with typical amounts of slip per event (5-10 m), the recurrence times are tentatively consistent with 1-2 cm/yr slip rates. This result favors models that consider the broader distribution of collisional deformation, rather than those with northward motion of India into Asia absorbed along a few faults bounding rigid blocks.

Low Quaternary slip rate reconciles geodetic and geologic rates along the Altyn Tagh fault, northwestern Tibet

For more than two decades the slip rate along the active, left-slip Altyn Tagh fault of northwestern Tibet has been disputed, with millennial rates reported to be as much as three times faster than those determined geodetically. This problem is significant because the total offset, plate-boundary length, and age of the Altyn Tagh fault make it the most important single structure accommodating India-Asia convergence north of the Himalayas. Here we show that the central Altyn Tagh fault slipped at only 14–9 mm/a over the past 4–6 ka by tightly bracketing the age of a displaced fluvial terrace riser at Yuemake (88.51°E, 38.19°N). This result contradicts previous latest Quaternary rates and is consistent with those derived from geodetic, paleoseismic, and geologic measurements, and thus resolves the long-standing dispute over the latest Quaternary slip rate along the longest active strike-slip fault in Tibet.

Riser diachroneity, lateral erosion, and uncertainty in rates of strike‐slip faulting: A case study from Tuzidun along the Altyn Tagh Fault, NW China

At late Quaternary timescales, offset fluvial terrace risers are among the most common landforms used to determine rates of strike‐slip faulting. Although diachroneity in the age of riser segments on opposite sides of the fault has been noted previously, an unexplored source of uncertainty associated with deriving slip rates from these markers centers on quantifying the size of the displacement uncertainty such diachroneity introduces. To evaluate the impact of riser diachroneity, we investigated the Tuzidun site (37.73°N, 86.72°E) along the Cherchen He reach of the active central Altyn Tagh Fault. The east bank of the channel is flanked by a left‐laterally offset terrace riser. While the measured offset is 54 ± 3 m, geochronologic measurements and analysis of riser topography indicate that the downstream riser segment formed between 6.0 ± 0.8 ka and 5.7 ± 0.4 ka, while the upstream riser segment may have been laterally refreshed as recently as 0.5 ± 0.2 ka. A valley wall on the west bank of the channel places a maximum limit of 38 ± 6 m on the amount of possible lateral erosion of the upstream riser. This bound, in turn, limits the total offset since formation of the downstream riser to range from 54 ± 3 to 89 ± 7 m. Together, these observations bracket the millennial Altyn Tagh Fault slip rate to range from 9.0 ± 1.3 to 15.5 ± 1.7 mm a−1. More generally, this investigation shows that the observed riser displacement does not necessarily correlate with the age of either riser segment (downstream or upstream of the fault) in cases where one segment is displaced while the other is subjected to lateral erosion. If this diachroneity goes undetected, erroneous slip rate measurements are likely to result.

Impact of riser reconstructions on estimation of secular variation in rates of strike–slip faulting: Revisiting the Cherchen River site along the Altyn Tagh Fault, NW China

Slip rates are heavily contested for many of the first-order strike–slip faults within the Indo–Asian collision zone. Rates determined geodetically are generally lower than those reported using reconstructions of offset landforms, and it is unclear if this discrepancy reflects true secular variation in slip history, systematic errors in interpretation, or both. Here I examine the methods used to derive slip rates from reconstructions of displaced fluvial risers, and show that such rates are subject to potentially important uncertainties that have largely been overlooked. Review of published data demonstrates that the slip rate can vary by a factor of 1.2 to 5 for the same site, depending on whether the reconstruction uses the age of the upper or lower terrace for the initiation of riser offset. To reduce this epistemic uncertainty, I have developed six geomorphic indices that can be used to identify the most accurate end-member reconstruction: (1) comparison of riser offset with inset channel width; (2) similarity of riser and tread displacements; (3) morphological analysis of the scarp profile; (4) riser deflection in plan view; (5) diachroneity of terrace abandonment; and (6) whether riser crests or bases yield the slip vector. Without the use of these geomorphic observations the epistemic uncertainty associated with current methods for determining slip rate from offset fluvial risers is likely to be so large that there is no resolvable discrepancy between the GPS and geologic rates within the Indo–Asian collision zone. Application of three of these indices to new field mapping of the Cherchen He (River) Site (∼ 86.4°E) along the central Altyn Tagh Fault yields a revised slip rate of 9.4 ± 2.3 mm/yr, equivalent within error to the rates previously derived from GPS and paleoseismic studies. Although reinterpretation of published data from the Sulamu Tagh site (∼ 87.4°) is consistent with this revised rate, additional work is needed to reconcile this result with some of the rates reported near Aksay (∼ 94°E).

Cenozoic Left-Slip Motion along the Central Altyn Tagh Fault as Inferred from the Sedimentary Record

Several Cenozoic sedimentary basins are present along the central segment of the Cenozoic Altyn Tagh fault (ATF) that marks the northern boundary of the Tibetan Plateau. Field investigations reveal that basin sedimentation and subsequent deformation are controlled by left-slip motion along the Cenozoic ATF. In order to better understand the temporal and spatial interactions between the Altyn Tagh fault and the development of adjacent basins, we divided Cenozoic sedimentary sequences into three subunits based on lithologic variation and the presence of unconformities. From our own observations and regional correlations, we suggest that slip along the central ATF began in the Early Oligocene. Reconstruction of sedimentary relationships among the basin, slip along the fault, and offset topography suggests that the ATF experienced four stages of Cenozoic leftslip motion. The offset of a Late Miocene sequence and its possible correlation across the Altyn Tagh fault suggests 80-100 km left slip. This yields an average slip rate of 10-12.5 mm/yr assuming that the sequence was deposited at ~8 Ma.

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

To better constrain the ongoing rates of deformation in northern Tibet, the ages of fluvial and glacial geomorphic markers left‐laterally displaced by the Altyn Tagh Fault have been determined by radiocarbon and 10Be‐26Al cosmic ray exposure dating. Two sites were investigated: Cherchen He and Sulamu Tagh, both near Tura (∼37.6°N, 86.6°E). The sites are geomorphologically distinct with Cherchen He dominated by fluvial processes and the Sulamu Tagh by glacial action. Nine offsets ranging from 166 to 3660 m with ages between 6 and 113 ka yield an average slip rate of 26.9 ± 6.9 mm/yr. Landscape evolution appears to have been modulated by climate change and is temporally consistent with the δ18O 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.

Initiation and Long-Term Slip History of the Altyn Tagh Fault

New Tertiary piercing points along the eastern and central Altyn Tagh fault, the northern boundary of the Tibetan Plateau, allow construction of the first well-defined time-displacement curve for the fault. Displacement-history analysis indicates: (1) late Oligocene-earliest Miocene inception of the Altyn Tagh fault; (2) 375 ± 25 km of total left-lateral slip on the eastern and central segment of the Altyn Tagh fault; and (3) an average long-term Cenozoic slip rate of approximately 12-16 mm/year. These results demonstrate that Himalayan deformation propagated well into the interior of Asia by early Miocene time and that a significant amount of India-Asia convergence was accommodated by sinistral slip on the Altyn Tagh fault.

Late Cenozoic sedimentary process and its response to the slip history of the central Altyn Tagh fault, NW China

The ENE-striking Altyn Tagh fault (ATF), extending along the northern edge of the Ti-betan Plateau, is one of the major important strike-slip faults, and has been known as one of the key areas to debate the eastward extrusion and crustral shortening models of the Tibetan Plateau during and after India-Asia collision. This paper mainly presents new evidence of Late Cenozoic sedimentary process to reconstruct the slip history of the ATF during the Late Cenozoic. Field measurements and laboratory analyses of the sedimentary characteristics in the Late Cenozoic basins in the central Altyn Tagh fault suggest that Late Cenozoic sedimentary sequence should be divided into three units according to facies changes. The paleo-topography reconstruction shows that the sedimentarion in these basins was tightly related with the fault, indicating that the ATF has experienced at least three stages of strike slipping in the Late Cenozoic. New geological data from the Late Cenozoic sedimentary basins and the formation of the present Suo’erkuli basin provide evidence for the displacement of the fault. The result shows that the 80–100 km left-lateral strike-slip displacement of the fault has been accumulated in the Late Cenozoic.

Late Holocene earthquake history of the central Altyn Tagh fault, China

The Altyn Tagh fault accommodates sinistral motion between the Tibetan Plateau and the Tarim block within the India-Eurasia collision zone. We used well-preserved evidence for surface-rupturing earthquakes to reconstruct the earthquake history for the central Altyn Tagh fault. We identified three geometric fault segments bounded by left steps and a bend. Geomorphic offsets indicate that the most recent event had maximum surface displacement of ;5.5 m in the west (38.58N, 90.08E), ;7 m in the central part of our study area, and ;4 m in the east (38.88N, 91.58E). The 14 C dates and trench logs of disrupted sediments indicate that these offsets occurred either in a single earthquake with a surface- rupture length .240 km dated as 680 6 108 yr B.P. or as two events. If there were two events, the westernmost recent event occurred 518 6 268 yr ago, whereas the eastern event occurred 650 6 80 yr ago and had a surface rupture length .155 km. We find two events in the past 0.8-2.2 k.y. in the west and two or three events in the east, yielding recurrence intervals of 0.7 6 0.4 k.y. and 1.1 6 0.3 k.y., respectively. These recurrence rates for major earthquakes are lower than expected if the long-term fault slip rate is .20 mm/yr. Explanations for the discrepancy include an overdue major earthquake, or accelerated deformation elsewhere in the India-Eurasia orogen.

Magnitude of post-Middle Jurassic (Bajocian) displacement on the central Altyn Tagh fault system, northwest China

The Altyn Tagh fault system, a key structural feature in the tectonic collage of central Asia, is an active left-lateral strike-slip fault system. However, its age of initiation, kinematic history, and total magnitude of displacement are not well known. Middle Jurassic nonmarine sedimentary rocks that crop out along the central segment of the Altyn Tagh fault were deposited in a contiguous Tarim-Qaidam foreland-style basin that was geodynamically linked to Mesozoic contraction in the Tian Shan, Bei Shan, Qilian Shan, and Kunlun Shan. This study documents the best evidence of a piercing point to date, consisting of an offset lacustrine shoreline demarcated by Aalenian-Bajocian open lacustrine strata to the west and fluvial and alluvial strata of comparable age to the east. Detrital compositions, palynology, paleocurrents, and lithostratigraphy support correlation of these sections. Restoration of this Jurassic facies boundary indicates 400 ± 60 km of post-Bajocian left-lateral separation on the Altyn Tagh fault. Consistent with this result, restoration of the Jurassic shoreline aligns felsic plutons across the western segment of the Altyn Tagh fault system that may have been left-laterally offset by 360 km. These two pairs of offset features suggest that the current best estimate for net magnitude of post-Bajocian left-lateral separation on the Altyn Tagh fault is about 360 km.

Cenozoic vertical-axis rotation of the Altyn Tagh fault system

Paleomagnetic declination data were collected from Cenozoic strata of the southern rim of the Tarim basin to address whether the Altyn Tagh fault has undergone significant rotation during the Indian-Asian collision. Results from the eastern and central Altyn Tagh fault zone suggest that it has undergone no significant rotation since the Oligocene. This implies that the boundary between the Tarim and Tibet has remained relatively stationary during most of the Cenozoic. In contrast, declination data from the western Kunlun Shan on the eastern limb of the Pamir orocline, where the Karakax segment of the Altyn Tagh fault system terminates, suggest that the range has undergone clockwise rotations of between 19.3° ± 8.6° and 27.8° ± 5.8°. Such rotation is in mirror image with the documented counterclockwise rotation of 20°–50° in the western Pamir orocline and implies relatively small displacements on the Karakorum fault. Our results also suggest that the Karakax fault may have formed as an accommodation zone between the western Kunlun Shan and the Karakorum Mountains.