Southwest Tibet Research Articles

Crustal ductile flow and its contribution to tectonic stress in Southwest China

Knowledge of tectonic crustal stress in highland region can provide important insights into relative plate motion, mountain building and genesis of larger crustal earthquakes underneath tectonically active regions. Here we focus on the highland regions in Southwest (SW) China, including southeast Tibet and Sichuan and Yunnan provinces. To better understand the correlation between crustal stress and structural heterogeneities, three-dimensional (3-D) velocity (Vp, Vs) structures in the crust are imaged using a large number of P- and Pn-wave, and S- and Sn-wave arrival times simultaneously. A slow-velocity layer at depths of 16–26 km was clearly imaged in most regions of SW China, suggesting the presence of a ductile layer that may be associated with partial melting and/or aqueous fluids in the crust. The azimuths of the maximum compressional stresses estimated using 147 high-quality stress data revealed a characteristic regional stress field along the fault belts in the study region. The trajectories of the stress axes along the southeastern Tibetan plateau vary from NE–SW in southwest Tibet to E–W in southeast Tibet. Our study indicates that, apart from the influence from the collision between the Indian and Eurasian plates, the present-day crustal stress field along the tectonic zones in SW China is significantly affected by ductile deformation in the crust, suggesting that crustal structural heterogeneity plays an important role in the tectonic stress.

Late Miocene topographic inversion in southwest Tibet based on integrated paleoelevation reconstructions and structural history

Investigations of the deformation history of the Himalayan orogen support interpretations of rapid and striking changes in the landscape of the Tibetan Himalaya and High Himalaya. We examine this issue by integrating oxygen isotope-based paleoelevation reconstructions of the Zada basin in southwestern Tibet with information on the structural evolution of the High and Tibetan Himalaya between 79°E and 84°30′E. δ 18O psw values were calculated from δ 18O cc values from pristine fluvial Miocene gastropod shells. Analyses comparing the most negative δ 18O sw and reconstructed δ 18O psw values to Δδ 18O sw versus elevation relationships based on both thermodynamic models and an empirical data set suggest a decrease in the mean watershed elevation of 1 to 1.5 km since the Late Miocene. Geologic mapping and structural data from crustal scale fault systems in the Zada region and regions to its east indicate a phase of arc-normal shortening and vertical thickening since the Middle Miocene, followed by ongoing arc-parallel extension and vertical thinning. These results suggest that regions in this part of the orogen transitioned from undergoing arc-normal shortening to arc-parallel extension in the Late Miocene, and that arc-parallel extensional structures root deeply within the Himalayan thrust wedge. When combined with data on the distribution, age, and provenance of sedimentary basins, our geologic mapping, structural data, and paleoelevation results suggest that this transition from shortening to extension was accompanied by a topographic inversion from mountains to basins in < 4 m.y. These observations can be explained by a foreland propagating fault system that accommodates outward radial expansion of the Himalayan orogen.

Magnetostratigraphy of the Zanda basin in southwest Tibet Plateau and its tectonic implications

The Zanda basin is one of the very important basins at the north slope of the Himalaya Range. Thus the study of the basin strata will provide critical information about the tectonic evolution of the Himalayan Orogenic Belt. 268 oriented block samples were collected in the 750-m-thick sections of the Zanda basin. The characteristic remanent magnetization (ChRM) was isolated that decays linearly to the origin between 500°C and 690°C for most studied samples. An age range of 9.5–2.6 Ma was estimated from the correlation between our observed polarity column and the Geomagnetic Polarity Time Scale (GPTS). The age of the Zanda basin does not support the models that the South Tibetan Detach system (STDS) is one of the basin controlling faults. Given the sedimentological features in the basin and the tectonic features at the north edge of the basin, the Zanda basin was a half graben that was possibly controlled by the Karakorum fault on the northeast.

Geometry, kinematics, and landscape characteristics of an active transtension zone, Karakoram fault system, Southwest Tibet

We investigate the style of active transtensional deformation and characteristics of the landscape along the southern segment of the Karakoram fault system through field mapping, structural analysis, and examination of digital topography (SRTM, ASTER), multi-spectral (ASTER, Landsat 7+), and panchromatic (Corona photography) imagery. Our data suggests that wrench-dominated transtension is occurring within a 90-km-wide zone; simple shear within the zone is accommodated by right-lateral R and P shears as well as left-lateral R′ shears, vertical shortening is accommodated by ∼north-striking extensional shear zones and horizontal shortening is accommodated by ∼east–west-trending transtension-related corrugations. Drainage divides strike at high angles and subparallel to the transtension zone. Those that strike at high angles to the zone can be explained by movement along north-striking extensional fault systems, while the geometry of drainage divides subparallel to the zone are consistent with uplift of corrugations as well as rapid uplift of mountain ranges by normal-right-slip faulting.

“The Lapponicum Sea”: Matter, Sense, and Affect in the Botanical Exploration of Southwest China and Tibet

In his own mission to versify the planet, or at least an exemplary portion of it, Captain Francis Kingdon-Ward (1885–1958) returned repeatedly, even obsessively, to certain places on the alpine plateaus of Northwest Yunnan and Southeast Tibet, where the vegetation, refracted through the Aleph of what I shall argue was a deliberate, laborious, and disciplined optical practice, became liquid, a sea:

Structural evolution and sequence of thrusting in the Tethyan fold-thrust belt and Indus-Yalu suture zone, southwest Tibet

Regional mapping of a north-south traverse from the India-Nepal-China border junction to Mount Kailas in southwest Tibet—combined with previously published geochronologic and stratigraphic data—is the basis for an incremental restoration of the Tethyan fold-thrust belt and deformation along the Indus-Yalu suture zone. From north to south, the major structural features are (1) the Indus-Yalu suture zone, composed of five south-dipping thrust faults involving rocks interpreted to represent parts of the former Indian passive margin and Asian active margin, (2) the Tethyan fold-thrust belt, composed of a dominantly north-dipping system of imbricate thrust faults involving Precambrian through Upper Cretaceous strata, and (3) the Kiogar-Jungbwa thrust sheet. A linelength cross-section reconstruction indicates a minimum of 176 km of north-south horizontal shortening partitioned by the Tethyan fold-thrust belt (112 km) and IndusYalu suture zone (64 km). Sequential restoration of the cross section shows that the locus of shortening prior to the late Oligocene occurred significantly south (possibly .60 km) of the Indus-Yalu suture zone within the Tethyan fold-thrust belt and that a significant amount of unsubducted oceanic lithosphere was present south of the suture in southwest Tibet until that time. An implication of this result is that postcollision (Oligocene/Miocene) high-K, calcalkaline magmatism may be explained by melting due to active subduction of oceanic crust beneath the Kailas magmatic complex until the late Oligocene. A regional profile across the Tibetan-Himalayan orogen from

Structural evolution of the Gurla Mandhata detachment system, southwest Tibet: Implications for the eastward extent of the Karakoram fault system

Field mapping and geochronologic and thermobarometric analyses of the Gurla Mandhata area, in southwest Tibet, reveal major middle to late Miocene, east-west extension along a normal-fault system, termed the Gurla Mandhata detachment system. The maximum fault slip occurs along a pair of low-angle normal faults that have caused significant tectonic denudation of the Tethyan Sedimentary Sequence, resulting in juxtaposition of weakly metamorphosed Paleozoic rocks and Tertiary sedimentary rocks in the hanging wall over amphibolite-facies mylonitic schist, marble, gneisses, and variably deformed leucogranite bodies in the footwall. The footwall of the detachment fault system records a late Miocene intrusive event, in part contemporaneous with top-to-the-west ductile normal shearing. The consistency of the mean shear direction within the mylonitic footwall rocks and its correlation with structurally higher brittle normal faults suggest that they represent an evolving low-angle normal-fault system. 4 0 Ar/ 3 9 Ar data from muscovite and biotite from the footwall rocks indicate that it cooled below 400 °C by ca. 9 Ma. Consideration of the original depth and dip angle of the detachment fault prior to exhumation of the footwall yields total slip estimates between 66 and 35 km across the Gurla Mandhata detachment system. The slip estimates and timing constraints on the Gurla Mandhata detachment system are comparable to those estimated on the right-slip Karakoram fault system, to which it is interpreted to be kinematically linked. Moreover, the mean shear-sense direction on both the Karakoram fault and the Gurla Mandhata detachment system overlap along the intersection line between the mean orientations of the faults, which further supports a kinematic association. If valid, this interpretation extends previous results that the Karakoram fault extends to mid-crustal depths.

Southward propagation of the Karakoram fault system, southwest Tibet: Timing and magnitude of slip

Southward propagation of the Karakoram fault system, southwest Tibet: Timing and magnitude of slip

Southward propagation of the Karakoram fault system, southwest Tibet: Timing and magnitude of slip

The net slip on the southern portion of the Karakoram fault system in southwest Tibet is estimated by restoring a piercing line defined by two key surfaces in the South Kailas thrust system, a regional counter thrust along the Indus-Yalu suture. Assuming that the thrust system is planar across the Karakoram fault, we calculate 66 ± 5.5 km of normal right slip. Documentation of the South Kailas thrust active at 13 Ma implies that the Karakoram fault in southwest Tibet did not initiate until after the cessation of motion on the thrust. However, field investigations of the central portion of the Karakoram fault system document the fault to have been active at 17 Ma and to have accumulated a maximum of 150 km of right slip. We suggest that these along-strike variations in the magnitude of slip and timing constraints are best explained by southward propagation of the Karakoram fault system. This is inconsistent with major right-lateral slip on the fault system, which was used in support of extrusion models for Tibet.

Tertiary deformation history of southeastern and southwestern Tibet during the Indo-Asian collision

Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south- central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.