Ding Lin

Blueschist-bearing metamorphic core complexes in the Qiangtang block reveal deep crustal structure of northern Tibet

A 500-km-long belt of metamorphic exposures in the Qiangtang block provides an opportunity to study the internal structure of northern Tibetan crust. Metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist-bearing melange and are bounded by Late Triassic–Early Jurassic, domal, low-angle normal faults. We propose that this melange was underplated to the Qiangtang block and was subsequently exhumed by detachment faulting; both the underplating and the exhumation occurred during early Mesozoic southward subduction of oceanic lithosphere along the Jinsha suture. This model predicts that the deeper crust of much of northern Tibet consists of accretionary melange, in contrast to the continental crystalline crust of southern Tibet, and may account for north-south variations of Cenozoic tectonism in Tibet.

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.

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.

Significant late Neogene east-west extension in northern Tibet

Field mapping in northern Tibet reveals that the normal slip along late Cenozoic northsouth‐trending faults is comparable to that estimated for equivalent structures in southern Tibet. The orientation of fault striations in two north-south‐trending rifts suggests an eastnortheast‐west-northwest direction of extension in northern Tibet, which in turn implies that northeast-striking active faults in northern Tibet have significant left-slip components. Initiation of rifting in northern Tibet postdates the early Oligocene, and possibly occurred after 4 Ma. The broad similarities in the magnitude of slip and the direction of extension for normal faults in both northern and southern Tibet imply that the entire plateau has been extending. This precludes significant eastward extrusion of north Tibet relative to south Tibet and requires a regional boundary condition as the cause of east-west extension for the entire Tibet plateau.

Direct measurement of strain rates in ductile shear zones: A new method based on syntectonic dikes

Received 17 January 2008; revised 31 August 2008; accepted 29 October 2008; published 23 January 2009. [1] We describe a new method to estimate directly ductile strain rates at an outcrop scale from the deformation of dikes emplaced within a shear zone. The method is tested in a well-constrained shear zone: the Ailao Shan–Red River shear zone, for which global strain rates can be calculated from published fault rates. The strain rate was determined by measuring independently the shear strain (g) recorded by the dikes and the age (t) of dikes emplacement. The shear strain was quantified by three different methods that take into account either the stretching of the dikes or their angle variations during deformation or both of them. The values of minimum shear strains range between 0.2 and 9.7 for the less to the most deformed dikes, respectively. The ages of dike emplacement were obtained by Th-Pb sensitive high-resolution ion microprobe (SHRIMP) dating of monazites. We obtained three groups of ages: the younger age is 22.55 ± 0.25 Ma, the intermediate age is 26.81 ± 0.66 Ma, and the oldest ages are 29.89 ± 0.46 Ma and 29.93 ± 0.38 Ma. The geochronological data are in agreement with the structural data, the most deformed dikes being the oldest. The minimum strain rates deduced from these measurements are 3 to 4 � 10 � 14 s � 1 , which is consistent with previous estimates of geological strain rates in ductile shear zones.

Coupled U-Pb dating and Hf isotopic analysis of detrital zircon of modern river sand from the Yalu River (Yarlung Tsangpo) drainage system in southern Tibet: Constraints on the transport processes and evolution of Himalayan rivers

We conducted coupled U-Pb dating and Hf isotope analysis of detrital zircon in modern sand of the Yalu River in southern Tibet. Our work indicates that the presence or absence of distinctive zircon populations in the Yalu main stream depends critically on the geometric configuration of the tributary rivers. The proportion of upper-stream zircon populations in the Yalu River sand decreases systematically in the downstream direction, which is caused mainly by zircon addition from new source areas in the downstream region. In some extreme cases, the upstream zircon signals can completely be lost in the downstream region due to this dilution effect. Analysis of sand modal composition reveals a downstream increase in the proportion of lithic fragments along the Yalu River, from ∼40% to ∼60% over a distance of ∼600 km. This may be attributed to the combined effect of an eastward increase in the topographic relief and an eastward increase in annual precipitation across the Yalu River drainage basin. Quantitative comparison of detrital-zircon ages between the Yalu River sand and Neogene sediments of the eastern Himalayan foreland supports a previous proposal that the Yalu River once flowed directly over the eastern Himalaya, without going around the Himalaya through its eastern syntaxis. The shortcut appears to have been transient, as it is only recorded in specific stratigraphic horizons of foreland sediments. The inferred Yalu River diversion may have been caused by past advances of glaciers or emplacements of giant landslides that temporarily dammed the Yalu River.

Paleocene to Pliocene low-latitude, high-elevation basins of southern Tibet: Implications for tectonic models of India-Asia collision, Cenozoic climate, and geochemical weathering

The elevation history of the Tibetan Plateau promises insight into the mechanisms and dynamics that develop and sustain high topography over tens of millions of years. We present the first nearly continuous Cenozoic elevation history from two sedimentary basins on the southern Tibetan Plateau within the latest Cretaceous to Eocene Gangdese arc. Oxygen-isotope and Δ47 clumped-isotope compositions of nonmarine carbonates allow us to constrain carbonate formation temperature and reconstruct the paleoprecipitation record of the Eocene to Pliocene Oiyug Basin and Paleocene to Eocene Penbo Basin. We exploit the systematic decrease of surface temperature and meteoric water δ18O values with elevation to derive paleoelevation estimates for these basins. Minimally altered and unaltered pedogenic and lacustrine carbonates from the Oiyug Basin yield Δ47, CDES (relative to the carbon dioxide equilibrium scale [CDES]) values of 0.625‰ to 0.755‰, which correspond to temperatures of 1−30 °C using a Δ47 thermometer for low-temperature carbonates. Similarly, the Penbo Basin yielded Δ47, CDES values of 0.701‰ to 0.726‰, corresponding to temperatures of 6−12 °C. The apparent evidence for survival of primary clumped-isotope values in the face of substantial burial and heating is an important result for the field of carbonate clumped-isotope thermometry. Our paleoelevation estimates for the Eocene to Pliocene Oiyug Basin (∼6.5−4.1 km) support previous evidence that high elevations were attained in southern Tibet by at least ca. 30 Ma. Stable-isotope results allow for the possibility of significant topographic subsidence during the Miocene as a result of regional extension. In the Penbo Basin, our paleoelevation estimates for the Paleocene to Eocene Nianbo Formation (4.4 +1.3/−1.7 km) and Eocene Pana Formation (4.1 +1.2/−1.6 km) extend the altitude record of the southern Tibetan Plateau to pre−India-Asia collision. These results support the “Lhasaplano” model of an Andean-type continental margin tectonic system. The rise of the Himalayas and Tibet is often invoked to understand isotopic proxies for global chemical weathering in the Cenozoic and has constrained the debate on the nature of CO2−climate−weathering feedbacks. The nature of the Tibetan paleoelevations from pre- to postcollision, as presented here, indicates that high relief at low latitude prevailed on the Asian margin much earlier than previously thought. Thus, high topography alone at low latitude is not sufficient to account for the Cenozoic weathering proxy record.