Eric Cowgill

Tectonic history of the Altyn Tagh fault system in northern Tibet inferred from Cenozoic sedimentation

The active left-slip Altyn Tagh fault defines the northern edge of the Tibetan plateau. To determine its deformation history we conducted integrated research on Cenozoic stratigraphic sections in the southern part of the Tarim Basin. Fission-track ages of detrital apatites, existing biostratigraphic data, and magnetostratigraphic analysis were used to establish chronostratigraphy, whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region. Much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults, implying a temporal link between sedimentation and deformation. The results of our studies, together with existing stratigraphic data from the Qaidam Basin and the Hexi Corridor, suggest that crustal thickening in northern Tibet began prior to 46 Ma for the western Kunlun Shan thrust belt, at ca. 49 Ma for the Qimen Tagh and North Qaidam thrust systems bounding the north and south margins of the Qaidam Basin, and prior to ca. 33 Ma for the Nan Shan thrust belt. These ages suggest that deformation front reached northern Tibet only ∼10 ± 5 m.y. after the initial collision of India with Asia at 65–55 Ma. Because the aforementioned thrust systems are either termination structures or branching faults of the Altyn Tagh left-slip system, the Altyn Tagh fault must have been active since ca. 49 Ma. The Altyn Tagh Range between the Tarim Basin and the Altyn Tagh fault has been a long-lived topographic high since at least the early Oligocene or possibly late Eocene. This range has shed sediments into both the Tarim and Qaidam Basins while being offset by the Altyn Tagh fault. Its continuous motion has made the range act as a sliding door, which eventually closed the outlets of westward-flowing drainages in the Qaidam Basin. This process has caused large amounts of Oligocene–Miocene sediments to be trapped in the Qaidam Basin. The estimated total slip of 470 ± 70 km and the initiation age of 49 Ma yield an average slip rate along the Altyn Tagh fault of 9 ± 2 mm/yr, remarkably similar to the rates determined by GPS (Global Positioning System) surveys. This result implies that geologic deformation rates are steady state over millions of years during continental collision.

Reconstruction of the Altyn Tagh fault based on U‐Pb geochronology: Role of back thrusts, mantle sutures, and heterogeneous crustal strength in forming the Tibetan Plateau

[1]xa0Reconstructing deformation along the northwestern margin of the Tibetan Plateau is critical for evaluating the relative importance of microplate versus continuum models of the Indo-Asian collision. Questions regarding this margins evolution are as follows: (1) What is the total offset along the sinistral Altyn Tagh strike-slip system? (2) How has that offset been absorbed in the western Kunlun Shan? (3) Why does the N-S width of the plateau vary along strike? Ion microprobe U-Pb zircon apparent ages of 17 plutons from NW Tibet, together with regional geologic observations, define a discrete, E-W trending boundary between two tectonic belts that has been offset along the Altyn Tagh system by 475 ± 70 km. Kinematic arguments indicate that this offset cannot be the result of north directed thrusting in the western Kunlun Shan. Therefore we propose that south directed faulting in the Tianshuihai thrust belt both offset the tectonic boundary and produced the asymmetry in the plateau. Shortening appears to have been absorbed in the upper crust by thin-skinned thrusting, in the middle/lower crust by east directed ductile flow and/or subduction, and in the mantle by north dipping subduction. Factors controlling the formation of the south directed thrust system appear to be the contrast between the rigid Tarim and the weaker Songpan-Ganzi flysch belt and a mantle suture inherited from late Paleozoic subduction. The evolution of western Tibet leads to a view of continental deformation that integrates elements of the microplate model (e.g., plate-like mantle and crust-mantle decoupling) with aspects of the continuum model (weak crustal flow beneath the plateau).

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.

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.

The Akato Tagh bend along the Altyn Tagh fault, northwest Tibet 1: Smoothing by vertical-axis rotation and the effect of topographic stresses on bend-flanking faults

To better understand the mechanics of restraining double bends and the strike-slip faults in which they occur, we investigated the relationship between topography and bedrock structure within the Akato Tagh, the largest restraining double bend along the active, left-slip Altyn Tagh fault. The bend comprises a ~90-km long, east-west striking central fault segment fl anked by two N70°E-striking sections that parallel the regional strike of the Altyn Tagh system. The three segments form two inside corners in the southwest and northeast sectors of the uplift where they link. We fi nd that both the topography and bedrock structure of the Akato Tagh restraining bend are strongly asymmetric. The highest and widest parts of the uplift are focused into two topographic nodes, one in each inside corner of the bend. Structural mapping of the western half of the bend suggests the southwest node coincides with a region of anomalously high, bend-perpendicular shortening. We also fi nd that partitioned, transpressional deformation within the Akato Tagh borderlands is absorbed by bend-parallel strike-slip faulting and bend-perpendicular folding, unlike the thrusting reported from many other double bends. Synthesis of these results leads to three implications of general signifi cance. First, we show that focusing of bend-perpendicular shortening into the two inside corners of a restraining double bend may cause both the bend and the fault to undergo vertical-axis rotation, thereby reducing the bend angle and smoothing the trace during progressive deformation. Such vertical-axis rotation may help explain why fault trace complexity is inversely related to total displacement along strike-slip faults. Second, we calculate four independent age estimates for the Akato Tagh bend, all of which are much younger than the Altyn Tagh system. We use these estimates in a companion study to postulate that the Altyn Tagh and similarly multi-stranded strike-slip systems may evolve by net strain hardening. Third, comparison of the Akato Tagh with other restraining double bends highlights systematic differences in the style of borderland faulting and we speculate that these variations result from different states of stress adjacent to the bends. Strike-slip dominated bends such as the Akato Tagh may form where σ H = σ 1 , σ v = σ 2 , and σ h = σ 3 whereas thrust-dominated bends like the Santa Cruz bend along the San Andreas fault form when σ H = σ 1 , σ h = σ 2 , σ v = σ 3 . This hypothesis predicts that the style of faulting along a restraining double bend can evolve during progressive deformation, and we show that either weakening of borderland faults or growth of restraining bend topography can convert thrust-dominated bends into strike-slip dominated uplifts such as the Akato Tagh.

Range-front fault scarps of the Sierra El Mayor, Baja California: Formed above an active low-angle normal fault?

The Canada David detachment, a west-dipping low-angle normal fault, juxtaposes Pliocene and Pleistocene(?) sedimentary units over sheared bedrock in the Sierra El Mayor, Baja California. Along the west-central range front, the detachment is overlapped by Quaternary alluvial fans that are, in turn, cut by fault scarps up to 7 m high. Map relationships suggest that scarp-forming faults may sole into the detachment at very shallow depth (<200‐300 m). Palinspastic restoration of a topographic profile across 15 north-striking scarps suggests that the ratio of net heave to net throw along scarp-forming faults is ~2, consistent with them soling into a low-angle (~30°) fault. These 15 scarps may have all formed in one earthquake, with rupture propagation from depth to the near-surface aided by clay gouge along the detachment. The detachment is abandoned in the footwall east of the scarps.

Two phases of Mesozoic north-south extension in the eastern Altyn Tagh range, northern Tibetan Plateau

[1]xa0The >300-km long, east striking Lapeiquan fault lies in the eastern Altyn Tagh range along the northern margin of the Tibetan Plateau and was interpreted as a north dipping thrust in early studies. However, our mapping shows that the fault is a south dipping normal fault juxtaposing Archean-Proterozoic gneisses beneath an early Paleozoic volcanic and sedimentary sequence. Its dip angle varies from <30° to ∼60°. The central fault segment is expressed as a 30–50 m thick ductile shear zone with well-developed mylonitic fabrics and stretching mineral lineations, where the eastern and western segments are characterized by cataclastic deformation. Kinematic indicators such as asymmetric boudinage, asymmetric folds, and minor brittle and ductile faults within the fault zone consistently indicate a top-south normal-slip sense of shear. The age of the Lapeiquan fault is constrained by two types of information. First, a sequence of Early-Middle Jurassic sediments is locally present in the hanging wall of the Lapeiquan fault. The clasts of the Jurassic strata, particularly the stromatolite-bearing, cherty limestone and purple quartzite, can be correlated uniquely with those in the footwall of the fault. We interpret that the Early-Middle Jurassic strata were deposited in an extensional basin related to motion along the Lapeiquan fault. Second, 40Ar/39Ar thermochronologic analyses indicate two prominent cooling events in the Lapeiquan footwall. The older event occurred in the latest Triassic-earliest Jurassic between ∼220 and 187 Ma, while the younger event occurred in the latest Early Cretaceous at ∼100 Ma. Because the 220–187 Ma cooling ages are widespread in the Lapeiquan footwall, we suggest it to represent the main stage of faulting. We interpreted the younger phase of fault motion at ∼100 Ma to have been related to fault reactivation. The deformation was aided by motion along the south dipping Qiashikan normal fault that merges with the eastern Lapeiqaun fault. From the regional tectonic setting, it appears that Mesozoic extension in northern Tibet to have occurred in a back arc setting during northward subduction of the Tethyan oceanic plate. The findings of Mesozoic extensional structures in northern Tibet suggest that compressive stress induced by collision of the Qiangtang and Lhasa terranes with Asia was not transmitted beyond northern Tibet. This in turn implies that the popularly inferred contractional setting for Mesozoic evolution of the Tian Shan north of Tibet needs a reevaluation based on a combination of both structural and sedimentological observations.

Is the North Altyn fault part of a strike-slip duplex along the Altyn Tagh fault system?

Although the Altyn Tagh fault system has played an important role in the Indo-Asian collision, its geometry and tectonic evolution remain poorly known. Between 86° and 92°E, this system is at least 100 km wide and is bounded to the north and south by the North Altyn and Altyn Tagh faults, respectively. Mapping along the Jianglisai reach of the North Altyn fault indicates that Miocene(?) to Pliocene(?) motion was predominantly left to left-reverse slip, with transport vectors trending N45°–60°E. Map relationships suggest that total offset on the fault is >120 km. These results are inconsistent with previous models of the Altyn Tagh fault system in which oblique convergence along the northern margin of the Tibetan Plateau is partitioned into thrusting on the North Altyn fault and left slip on the Altyn Tagh fault. An alternative hypothesis is that the North Altyn fault is the northern boundary of a transpressional strike-slip duplex within which the structurally elevated Altyn Mountains were created. Our model suggests that transpressional deformation may be restricted to this strike-slip duplex and need not characterize the entire margin.

The Akato Tagh bend along the Altyn Tagh fault, northwest Tibet 2: Active deformation and the importance of transpression and strain hardening within the Altyn Tagh system

We investigated active deformation within the Akato Tagh restraining double bend to determine the age of the active Altyn Tagh fault relative to the Altyn Tagh system and thereby evaluate the extent to which this system evolved by net strain-hardening or softening. Active structures were mapped based on their geomorphology and disruption of Quaternary(?) deposits/surfaces. The style of active faulting is strongly correlated with fault strike: 065°-070° striking segments show pure left-slip whereas faults with more northward or eastward strikes are transtensional or transpressional, respectively. Our mapping further suggests that the 065° to 070°-striking western and eastern segments of the Akato Tagh bend are characterized by pure strike-slip motion, with partitioned transpression along the ∼090°-striking central segment of the double bend. It remains unclear how this active, bend-perpendicular shortening is absorbed. In conjunction with previous work, the present study fails to support the commonly held idea that Tarim-Tibet motion is strongly oblique to the Altyn Tagh system. Estimates for the age of the Akato Tagh bend derived in a companion study suggest the bend is only a few million years old. The current principal trace is probably similarly young because formation of the bend by recent deformation of an old trace should result in transpression along the western and eastern segments, contrary to the pure left-slip shown here. The Altyn Tagh system comprises multiple fault strands in a zone ∼100 km wide across strike. Because the main trace appears to be much younger than the system in which it is embedded, we speculate that this system evolved by the sequential formation and death of short-lived fault strands. In particular, we suggest that geometrically complex strike-slip fault systems such as the Altyn Tagh may form via system strain hardening, where this net response reflects a dynamic competition between hardening and softening processes that are active simultaneously within the fault zone. Hardening mechanisms may include growth of restraining bend topography or material hardening of phyllosilicate-rich gouge, whereas softening processes might include R-P shear linkage or reduction in bend angle by vertical axis rotation. Our analysis suggests that net strain hardening of a fault system can produce continental deformation that is spatially localized over the 1-5 m.y. during which an individual strand is active, but distributed over the 10-100 m.y. corresponding to the life-span of the whole fault system and the collision zone in which it is contained. Thus, time scale is critically important in determining whether or not continental deformation is spatially distributed or localized.