Timing of Late Cretaceous shortening and basin development,Little Hatchet Mountains,southwestern New Mexico,USA – implications for regional Laramide tectonics

Laser ablation‐multi collector‐inductively coupled mass spectrometry U‐Pb geochronology, detailed field mapping and stratigraphic data offer improved insights into the timing and style of Laramide deformation and basin development in the Little Hatchet Mountains, southwestern New Mexico, USA, a key locality in the ‘southern Laramide province.’ The Laramide synorogenic section in the northern Little Hatchet Mountains comprises upper Campanian to Maastrichtian strata consisting of the Ringbone and Skunk Ranch formations, with a preserved maximum thickness of >2400 m, and the correlative Hidalgo Formation with a total thickness >1700 m. The Ringbone Formation and superjacent Skunk Ranch Formation are each generally composed of (1) a basal conglomerate member; (2) a middle member consisting of lacustrine shale, limestone, sandstone, and interbedded ash‐fall tuffs; and (3) an upper sandstone and conglomerate member. Basaltic andesite flows are intercalated with the upper member of the Ringbone Formation and the middle member of the Skunk Ranch Formation. The Hidalgo Formation, which crops out in the northern part of the range, is dominantly composed of basaltic andesite breccias and flows equivalent to those of the Ringbone and Skunk Ranch formations. The Laramide section was deposited in an intermontane basin partitioned across intrabasinal thrust structures, which controlled growth‐stratal development. U‐Pb zircon ages from five tuffs indicate that the age range of the Laramide sedimentary succession is ca. 75–70 Ma. U‐Pb detrital‐zircon age data (n = 356 analyses) from the Ringbone Formation and a Lower Cretaceous unit indicate sediment contribution from uplifted Lower and Upper Cretaceous rocks adjacent to the basin and the contemporary Tarahumara magmatic arc in nearby northern Sonora, Mexico. The new ages, combined with published data, indicate that uplift, basin development, and magmatism in the region proceeded diachronously northeastwards as the subducting Farallon slab flattened under northern Mexico and southern New Mexico from Campanian to Palaeogene time.     

Drainage reorganization and Laramide tectonics in north-central New Mexico and downstream effects in the Gulf of Mexico

The El Rito and Galisteo depocenters in north-central New Mexico archive tectonically-driven Paleogene drainage reorganization, the effects of which influenced sedimentation along the northwestern margin of the Gulf of Mexico. Although separated by ~100 km and lacking depositional chronology for the El Rito Formation, the two aforementioned New Mexican depocenters are commonly considered remnants of a single basin with coeval deposition and shared accommodation mechanism. Detrital zircon U-Pb maximum depositional ages indicate that the El Rito and Galisteo formations are not coeval. Moreover, stratigraphic thickness trends and mapping relationships indicate different accommodation mechanisms for the Galisteo and El Rito depocenters; tectonically-induced subsidence versus infilling of incised topography, respectively. The regional unconformity that bounds the base of both the El Rito and Galisteo formations is a correlative surface induced by local tectonic activity and associated drainage reorganization in the early Eocene, and was diachronously buried by northward onlap of fluvial sediments. Detrital zircon distributions in both depocenters indicate increased recycling of Mesozoic strata above the unconformity, but diverge upsection as topographic prominence of local basement-involved uplifts waned. Sediment capture in these depocenters is coeval with deposition in other externally-drained Laramide basins. Further, it corresponds to a period of low Laramide province-derived sediment input and replacement by Appalachian-sourced sediment along the northwestern margin of the Gulf of Mexico during a basin-wide transgression. This illustrates the potential effect that pockets of sediment storage within the catchment of a transcontinental drainage system can have over the sedimentary record in the receiving marine basin.     

A flexural model for the Paradox Basin: implications for the tectonics of the Ancestral Rocky Mountains

The Paradox Basin is a large (190 km × 265 km) asymmetric basin that developed along the southwestern flank of the basement‐involved Uncompahgre uplift in Utah and Colorado, USA during the Pennsylvanian–Permian Ancestral Rocky Mountain (ARM) orogenic event. Previously interpreted as a pull‐apart basin, the Paradox Basin more closely resembles intraforeland flexural basins such as those that developed between the basement‐cored uplifts of the Late Cretaceous–Eocene Laramide orogeny in the western interior USA. The shape, subsidence history, facies architecture, and structural relationships of the Uncompahgre–Paradox system are exemplary of typical ‘immobile’ foreland basin systems. Along the southwest‐vergent Uncompahgre thrust, ～5 km of coarse‐grained syntectonic Desmoinesian–Wolfcampian (mid‐Pennsylvanian to early Permian; ～310–260 Ma) sediments were shed from the Uncompahgre uplift by alluvial fans and reworked by aeolian‐modified fluvial megafan deposystems in the proximal Paradox Basin. The coeval rise of an uplift‐parallel barrier ～200 km southwest of the Uncompahgre front restricted reflux from the open ocean south and west of the basin, and promoted deposition of thick evaporite‐shale and biohermal carbonate facies in the medial and distal submarine parts of the basin, respectively. Nearshore carbonate shoal and terrestrial siliciclastic deposystems overtopped the basin during the late stages of subsidence during the Missourian through Wolfcampian (～300–260 Ma) as sediment flux outpaced the rate of generation of accommodation space. Reconstruction of an end‐Permian two‐dimensional basin profile from seismic, borehole, and outcrop data depicts the relationship of these deposystems to the differential accommodation space generated by Pennsylvanian–Permian subsidence, highlighting the similarities between the Paradox basin‐fill and that of other ancient and modern foreland basins. Flexural modeling of the restored basin profile indicates that the Paradox Basin can be described by flexural loading of a fully broken continental crust by a model Uncompahgre uplift and accompanying synorogenic sediments. Other thrust‐bounded basins of the ARM have similar basin profiles and facies architectures to those of the Paradox Basin, suggesting that many ARM basins may share a flexural geodynamic mechanism. Therefore, plate tectonic models that attempt to explain the development of ARM uplifts need to incorporate a mechanism for the widespread generation of flexural basins.     

Refining stratigraphy and tectonic history using detrital zircon maximum depositional age: an example from the Cerro Fortaleza Formation,Austral Basin,southern Patagonia

The north–south trending, Late Cretaceous to modern Magallanes–Austral foreland basin of southernmost Patagonia lacks a unified, radiometric, age‐controlled stratigraphic framework. By simplifying the sedimentary fill of the basin to deep‐marine, shallow‐marine and terrestrial deposits, and combining 13 new U‐Pb detrital zircon maximum depositional ages (DZ MDAs) with published DZ MDAs and U‐Pb ash ages, we provide the first attempt at a unified, longitudinal stratigraphic framework constrained by radiometric age controls. We divide the foreland basin history into two phases, including (1) an initial Late Cretaceous shoaling upward phase and (2) a Cenozoic phase that overlies a Palaeogene unconformity. New DZ samples from the shallow‐marine La Anita Formation, the terrestrial Cerro Fortaleza Formation and several previously unrecognized Cenozoic units provide necessary radiometric age controls for the end of the Late Cretaceous foreland phase and the magnitude of the Palaeogene unconformity in the Austral sector of the basin. These samples show that the La Anita and Cerro Fortaleza Formations have Campanian DZ MDAs, and that overlying Cenozoic strata have Eocene to Miocene DZ MDAs. By filling this data gap, we are able to provide a first attempt at constructing a basinwide, age‐controlled stratigraphic framework for the Magallanes–Austral foreland basin. Results show southward progradation of shallow marine and terrestrial environments from the Santonian through the Maastrichtian, as well as a northward increase in the magnitude of the Palaeogene unconformity. Furthermore, our new age data significantly impact the chronology of fossil flora and dinosaur faunas in Patagonia.     

Late-glacial vegetation and environment on the eastern slope foothills of the Rocky Mountains, Alberta, Canada

Stratigraphic pollen analysis done on sediment cores from two sites in the upper North Saskatchewan drainage basin of the eastern slopes foothills of the Rocky Mountains in west central Alberta, Canada combined with sedimentological data provide a local vegetational and environmental history. Radiocarbon AMS dates provide a chronology back to 17960 BP. Reconstruction and interpretation of the local pollen zones includes reevaluation of steppe and grassland as analogs for full- and late-glacial vegetation. Regional vegetation from c. 17960 to 16 100 BP is interpreted as an extremely cold semi-arid Artemisia steppe, the vegetation c. 16 100 to 11 900 BP as an Artemisia-Betula shrubland, and the vegetation c. 11 900–10 200 BP as a Picea woodland, in an environment characterized by consistently arid and windy conditions. This reconstruction emphasizes the significance of aridity, as opposed to simply low temperatures, as the critical factor in determining the late Quaternary vegetation of Alberta.This is the 18th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

Synclinal-horst basins: examples from the southern Rio Grande rift and southern transition zone of southwestern New Mexico, USA

In areas of broadly distributed extensional strain, the back‐tilted edges of a wider than normal horst block may create a synclinal‐horst basin. Three Neogene synclinal‐horst basins are described from the southern Rio Grande rift and southern Transition Zone of southwestern New Mexico, USA. The late Miocene–Quaternary Uvas Valley basin developed between two fault blocks that dip 6–8° toward one another. Containing a maximum of 200 m of sediment, the Uvas Valley basin has a nearly symmetrical distribution of sediment thickness and appears to have been hydrologically closed throughout its history. The Miocene Gila Wilderness synclinal‐horst basin is bordered on three sides by gently tilted (10°, 15°, 20°) fault blocks. Despite evidence of an axial drainage that may have exited the northern edge of the basin, 200–300 m of sediment accumulated in the basin, probably as a result of high sediment yields from the large, high‐relief catchments. The Jornada del Muerto synclinal‐horst basin is positioned between the east‐tilted Caballo and west‐tilted San Andres fault blocks. Despite uplift and probable tilting of the adjacent fault blocks in the latest Oligocene and Miocene time, sediment was transported off the horst and deposited in an adjacent basin to the south. Sediment only began to accumulate in the Jornada del Muerto basin in Pliocene and Quaternary time, when an east‐dipping normal fault along the axis of the syncline created a small half graben. Overall, synclinal‐horst basins are rare, because horsts wide enough to develop broad synclines are uncommon in extensional terrains. Synclinal‐horst basins may be most common along the margins of extensional terrains, where thicker, colder crust results in wider fault spacing.     

Seismic evidence for the Pleistocene depositional changes in Lake Hovsgol, Mongolia, and implications for the age model and the sediment grain size record of KDP-01 drill core

This paper seeks to arrive at a consistent interpretation of (1) the age model, (2) the grain size record, and (3) seismic reflection data from Lake Hovsgol (a.k.a Khubsugul or Hövsgöl), Mongolia, reported by Fedotov et al. (2007, earlier by Fedotov et al. 2002, 2004). In their most recent contribution, the grain size record of the KDP-01 drill core is interpreted as a climatic signal while little consideration is given to lake-level changes and hence to basin-wide changes in depositional setting evident from seismic profiles; also, a nearly linear age model is at odds with the seismic evidence for a major angular unconformity in the sediment strata. The lack of regional seismic stratigraphic analysis has thus led to an improbable interpretation of the Lake Hovsgol sediment grain size record and ultimately to an improbable scenario of Mongolian glaciation history. Using the available seismic profiles, here we show that the drill core penetrated several transgressive/regressive sedimentary sequences and a major angular unconformity. Therefore, the drilled sediment section cannot represent continuous sediment accumulation and the Brunhes age model across the unconformity cannot be nearly linear; the time interval representing a hiatus remains to be determined. The assumed nearly linear age/depth relationship in the upper 23 m above the angular unconformity is also an unlikely relationship, given the evidence of repeated changes in lake level, and hence in the depositional setting and sedimentation rates. We further propose a qualitative reference model for changes in the Lake Hovsgol depositional setting (presented as a step-by-step animation – see supplementary material) based on manually ‘backstripping and rebuilding’ the seismic pattern. We argue that this model provides a useful template of the likely sediment facies changes in the deep axial part of the Hovsgol basin: our crude model in fact captures the major depositional trends in the KDP-01 drill core section located some 10 km NW along the seismic line. We contend that changes in the depositional setting provide the first-order control on sediment grain size in the Hovsgol record. Our study provides important new constraints on the nature of sedimentary proxy records in Lake Hovsgol and on their interpretation as a record of Mongolian glaciation history.     

Axial–transverse fluvial interactions in half-graben: Plio-Pleistocene Palomas Basin, southern Rio Grande Rift, New Mexico, USA

Accurate magnetostratigraphic dating of Plio-Pleistocene alluvium in the Palomas half-graben permits correlation of transverse and axial deposits, thus enabling analysis of the movement of alluvial facies belts in time and space for the first time. Northern areas show evidence for basinward progradation of footwall-sourced Matuyama-age alluvial fan deposits over axial channel belt deposits of the ancestral Rio Grande, despite both deposits having similar deposition rates. This gradual ‘forced’ westward migration of the axial belt was in opposition to ongoing eastward growth of hangingwall-sourced fans and tectonic tilt imposed by the bounding Caballo normal fault. Fan growth was coincident with a recently proposed gradual climatic shift that may have increased sediment flux out of transverse catchments. It is also possible that continuing tectonic footwall uplift and divided retreat caused catchment areas to increase, contributing to these trends. Southern areas of the Palomas half-graben feature late Gilbert/early Gauss deposits indicative of rapid westwards progradation of large low-gradient, footwall-sourced fans over axial deposits. This ‘forced’ migration of the ancestral Rio Grande may have occurred due to footwall catchment and fan growth consequent upon initiation and growth of the Red Hills Fault. Subsequent eastward movement of the axial channel belt in late Gauss and Matuyama times overwhelmed these large fans. We attribute this to continued tilting on the Red Hills Fault and to development of the Jornada Fault to the south-east, the axial river belt avulsing north and eastwards through a developing Red Hills/Jornada crossover transfer zone. We conclude generally that facies architecture of axial and transverse elements in half-graben must reflect both climatic influences and the effects of fault development. Careful field mapping, accurate dating and palaeoclimatic studies are all necessary to determine the relative importance of these controls. Although adequate as broad guides, previous purely ‘fixist’ tectonosedimentary models allow for no fault growth, decay or climatic modulation of facies trends and are thus generally inadequate to explain important aspects rift basin stratigraphy.     

The Palliser Rockslide, Canadian Rocky Mountains: Characterization and modeling of a stepped failure surface

This paper presents the results of an investigation of the prehistoric Palliser Rockslide, Rocky Mountains, Canada. Conventional aerial photograph interpretation and field mapping are complemented by terrestrial digital photogrammetry. These techniques allow quantification of the rockslide debris volume and reconstruction of the pre-slide topography. It has been estimated that the volume of rock involved in the most recent large rockslide is 8 Mm³. Terrestrial digital photogrammetry is used in the characterization of the failure surface morphology, which is subdivided into four types of step-path geometry comprising both pre-existing discontinuities and intact rock fractures. Incorporation of these data into various rock slope stability numerical modeling methods highlights a complex failure mechanism, which includes sliding along a large scale curved failure surface, intact rock bridge fracturing and lateral confinement. A preliminary quantification of the contribution of intact rock bridges to the shear strength of the failure surface is presented in terms of the apparent cohesion, apparent tensile strength and cumulative length of the intact rock segments.     

The Middle Jurassic Camaraca Formation, Arica, Chile: palaeomagnetism, K—Ar age dating and tectonic implications

Summary. Palaeomagnetic and K–Ar whole rock age studies are reported from samples of the Camaraca Formation exposed near Arica (18.6° S, 70.3° W), Chile. The Camaraca Formation is composed of andesites and interbedded marine shales which yield a fauna placing the formation in the Bajocian–Callovian stages of the Jurassic. Our sampling sites span a strati-graphic thickness of 2 km and yield a reversal stratigraphy of: N–R–N. The K–Ar isochron method, applied to least altered samples from the formation, gives a sharply defined age of 157 Myr which is in agreement with the palaeontologically assigned age of the formation. Normal and reversed directions of remanent magnetization, isolated by of and thermal de-magnetization methods, are statistically antiparallel. The pole position, computed from these directions of magnetization, is at 010° E, 70%0 S ( A ₉₅= 6.0°). This pole position, when compared with the well-studied Chon Alice Formation of Argentina, suggests that the sampling region has under-gone a 28°± 28° counter-clockwise rotation about a local vertical axis. The large uncertainty (between 0° and 56°) in this estimate is due to the large scatter in the South American reference data. When compared with African Jurassic palaeopoles, with allowance made for the opening of the South Atlantic, a counter-clockwise rotation of 44°± 14° is indicated.     

新疆天山南坡表土花粉的初步研究

横亘于亚洲中部的天山山系是一个完整的自然地理单元,山地植被和土壤具有垂直分布明显和区域性差异大的特点,故对花粉的传播、保存、搬运与沉积具有重大影响.根据对西北干旱地区天山中段南坡一条从海拔4 285 m的天山一号冰川到海拔1 200 m的和静段的样带所采集的38个表土花粉样品的孢粉组合图武、植被样方调查资料以及排序结果...     

Magnetostratigraphy of the Qiliting section (SE China) and its implication for geochronology of the red soil sequences in southern China

The red soil sequences in southern and southeastern China provide important terrestrial archives of palaeoclimate and palaeoenvironment and are important sources of Palaeolithic settlements and, therefore, early human occupation. Here we present detailed palaeomagnetic investigation coupled with mineral magnetic and petrographic analyses of a red soil sequence at Qiliting, southeastern China. The sediments contain both strongly magnetic minerals (such as maghemite and magnetite) and weakly magnetic minerals (such as haematite and possibly goethite). Maghemite and haematite were identified as the main carriers of the natural remnant magnetisations. Only haematite dominates the characteristic remnant magnetisations. Palaeomagnetic findings suggest that the Qiliting sedimentary sequence recorded the early Brunhes Chron and the late Matuyama Chron, including the Jaramillo Subchron. Our finding, coupled with previously published magnetostratigraphy, establishes that the red soil sequences in southern China began to accumulate no later than the Jaramillo age (i.e. ∼1.0 Ma). It is further suggested that the capabilities of recording magnetic polarity stratigraphy in Chinese red soil sequences are variable between areas because of different climate regimes.     

Snowball morphology and SEM analysis of pedogenic gypsum, southern New Mexico, U.S.A.

Late Quaternary soils in southern New Mexico support previous work that suggested pedogenic gypsum accumulates in successive stages as a function of time, similar to pedogenic carbonate. However, gypsum also occurs as small (0·5–1 mm), powdery spheres that we term snowball morphology. The snowball morphology represents one of the initial stages of pedogenic gypsum development. Scanning electron microscope analyses indicate that soil microorganisms and organic material may play a crucial role in the development of this morphology. However, no trends were found in the crystal habits of gypsum suggesting the snowball morphology forms in a dynamic environment.     

Compound and hybrid clinothems of the last lowstand Mid-Adriatic Deep: Processes,depositional environments,controls and implications for stratigraphic analysis of prograding systems

Clinoforms with a range of scales are essential elements of prograding continental margins. Different types of clinoforms develop during margin growth, depending on combined changes in relative sea level, sediment supply and oceanographic processes. In studies of continental margin stratigraphy, trajectories of clinoform ‘rollover’ points are often used as proxies for relative sea-level variation and as predictors of the character of deposits beyond the shelf-break. The analysis of clinoform dynamics and rollover trajectory often suffers from the low resolution of geophysical data, the small scale of outcrops with respect to the dimensions of clinoform packages and low chronostratigraphic resolution. Here, through high-resolution seismic reflection data and sediment cores, we show how compound clinoforms were the most common architectural style of margin progradation of the late Pleistocene lowstand in the Adriatic Sea. During compound clinoform development, the shoreline was located landward of the shelf-break. It comprised a wave-dominated delta to the west and a barrier and back-barrier depositional system in the central and eastern area. Storm-enhanced hyperpycnal flows were responsible for the deposition of a sandy lobe in the river mouth, whereas a heterolithic succession formed elsewhere on the shelf. The storm-enhanced hyperpycnal flows built an apron of sand on the slope that interrupted an otherwise homogeneous progradational mudbelt. Locally, the late lowstand compound clinoforms have a flat to falling shelf-break trajectory. However, the main phase of shelf-break bypass and basin deposition coincides with a younger steeply rising shelf-break trajectory. We interpret divergence from standard models, linking shelf-break trajectory to deep-sea sand deposition, as resulting from a great efficiency of oceanographic processes in reworking sediment in the shelf, and from a high sediment supply. The slope foresets had a large progradational attitude during the late lowstand sea-level rise, showing that oceanographic processes can inhibit coastal systems to reach the shelf-edge. In general, our study suggests that where the shoreline does not coincide with the shelf-break, trajectory analysis can lead to inaccurate reconstruction of the depositional history of a margin.     

The effect of syndepositional deformation within the Upper Permian Capitan Platform on the speleogenesis and geomorphology of the Guadalupe Mountains, New Mexico, USA

The Guadalupe Mountains in New Mexico and Texas are home to more than 300 caves. Caves have been formed within the Upper Permian Capitan carbonate platform and are oriented along two structural trends, one of which is parallel to the platform margin and the other of which is roughly perpendicular to it. Our recent studies of the Capitan Platform have identified syndepositional faults associated with growth monoclines and synclines in Slaughter Canyon, New Mexico, and these are also parallel to the platform margin. In this study, we demonstrate that syndepositional faults and folds are also present in Rattlesnake and Walnut Canyons, as much as 19 km along strike, and that they have exerted control on karstification of the Guadalupe Mountains from the Upper Permian until present.Three distinctive episodes of karst formation have been recognised in outcrops on the basis of karst-filling deposits and crosscutting relationships. The syndepositional “Phase 1 karst” was formed along syndepositional faults and fractures and is filled by platform-derived sediments. The burial “Phase 2 karst” is filled by post-Permian siliciclastics and is limited to the youngest syndepositional faults and fractures that penetrate the platform in the proximity of its terminal margin. Connectivity of these youngest faults and fractures to the platform top and the overlying stratigraphy is inferred to have controlled the distribution of the Phase 2 karst. The “Phase 3 karst” includes the present cave systems, which were mainly formed by sulphuric acid produced by mixing of fossil and fresh underground waters in conjunction with the uplift of the Guadalupe Mountains in the Late Tertiary, and have since been modified by vadose karst processes. The Phase 3 karst caves are not solely developed along syndepositional faults and fractures as the earlier karst palaeocaverns are, but also follow another, uplift-related, structural trend.Syndepositional folds, faults, and fractures in the Capitan Platform have influenced the shaping of the modern surface geomorphology of the Guadalupe Mountains by controlling drainage and, hence, erosion. Trellis drainage parallel to the platform margin is developed where syndepositional folds, faults, and fractures occur. The morphology of the trellis drainage varies systematically across the range in response to the character of the deformation structures and karst features along which the drainage channels have developed.     

Magnetostratigraphy of the Neogene Chaka basin and its implications for mountain building processes in the north‐eastern Tibetan Plateau

Magnetostratigraphy of sedimentary rock deposited in the Chaka basin (north‐eastern Tibetan Plateau) indicates a late Miocene onset of basin formation and subsequent development of the adjacent Qinghai Nan Shan. Sedimentation in the basin initiated at ～11 Ma. In the lower part of the basin fill, a coarsening‐upward sequence starting at ～9 Ma, as well as rapid sedimentation rates, and northward paleocurrents, are consistent with continued growth of the Ela Shan to the south. In the upper section, several lines of evidence suggest that thrust faulting and topographic development of the Qinghai Nan Shan began at ～6.1 Ma. Paleocurrent indicators, preserved in the basin in the proximal footwall of the Qinghai Nan Shan, show a change from northward to southward flow between 6.5 and 3.8 Ma. At the same location, sediment derived from the Qinghai Nan Shan appears at 6.1 Ma. Finally, the initiation of progressively shallowing dips observed in deformed basin strata and a change to pebbly, fluvial deposits at 6.1 Ma provide a minimum age for the onset of slip on the thrust fault that dips north‐east beneath the Qinghai Nan Shan. We interpret a decrease in sediment accumulation rates since ～6 Ma to indicate a reduction in Chaka basin accommodation space due to active faulting and folding along the Qinghai Nan Shan and incorporation of the basin into the wedge‐top depozone. Declination anomalies indicate the beginning of counter‐clockwise rotation since 6.1 Ma, which we associate with local deformation, not regional block rotation. The emergence of the Qinghai Nan Shan near the end of the Miocene Epoch partitioned the once contiguous Chaka‐Gonghe and Qinghai basin complex. In a regional framework, our study adds to a growing body of evidence that points to widespread initiation and/or reactivation of fault networks during the late Miocene across the north‐eastern Tibetan Plateau.