South-central Tibet Research Articles

Surface wave propagation in central Asia: Observations of scattering and multipathing with the Kyrgyzstan broadband array

We studied the propagation of Rayleigh waves at regional distances in central Asia using a combination of array processing techniques and surface wave analysis. We present results from the detailed analysis of three representative events recorded by a 10‐station, broadband network that has been running in the central Asian country of Kyrgyzstan since 1991: an Ms = 5.1 event near Ashkhabad, Turkmenistan; an Ms = 5.8 event in south central Tibet; and the October 7, 1994, nuclear explosion at Lop Nor. We find there is a remarkable difference in the propagation characteristics of surface waves along these three paths. The path from the event in Turkmenistan is simple and is well approximated by propagation through a laterally homogeneous medium. Array processing shows the entire Rayleigh wave train stacks coherently and arrives from an azimuth close to that predicted by a great circle path. Furthermore, estimates of dispersion curves and fundamental mode signals determined for individual stations show little variation across the array. The Tibet and Lop Nor paths are completely different. We find strong evidence for complicated multipathing and scattering effects along both of these paths. We observe a three‐stage pattern in the Tibet case: (1) the early, lowest‐frequency part of the Rayleigh wave packet arrives as a coherent signal from close to the great circle path azimuth; (2) this is overpowered in the period range around 20 s by a strong multipath signal that propagates across the array from a much more southerly azimuth; and (3) periods below 20 s rapidly become incoherent, and the signal does not have a well‐defined direction of propagation. The Lop Nor path shows similar complexity. On this path there is little dispersion for measurable periods greater than 10 s, so the low‐frequency energy arrives in an Airy phase. The Airy phase stacks somewhat coherently (it stacks, but significant power is lost in the best beam), and slowness analysis shows it arrives from an azimuth 14° north of the great circle path azimuth. Like the Tibet event, the direction of propagation of the shorter periods cannot be resolved. These results are consistent with crustal structure in central Asia. The multipathing and scattering from the Tibet and Lop Nor events are not surprising given these waves propagate across highly variable crust and high topography resulting from the collision of India with Eurasia. The relative simplicity of the Turkmenistan event path reinforces this as almost all of that path traverses the relatively undeformed Turan platform of Uzbekistan.

Isotopic Preservation of Himalayan/Tibetan Uplift, Denudation, and Climatic Histories of Two Molasse Deposits

Two distinctive molasse deposits within the Indo-Asian collision zone have been investigated to help understand the post-Oligocene evolution of the Himalaya and southern Tibetan plateau. The Siwalik Group (predominantly fluvial sandstones and siltstones), is widespread throughout the foothills of the Himalaya from Pakistan to eastern India. Paleomagnetic analysis of a measured section in the Bakiya Khola, southeastern Nepal, constrains depositional ages () to between 10.8 and 4.9 Ma. The average accumulation rate during this interval was 0.4-0.5 mm/yr. dating of 126 detrital K-feldspars from nine horizons in the Bakiya Khola section ( to ~2.2 Ma) provide snapshots in time of the cooling ages exposed at the earth's surface in this drainage basin. The minimum age in the spectra from seven of the nine horizons averages only 3 m.y. older than , indicating exceptionally rapid unroofing in the Himalaya throughout that interval. Half of the grains from these nine sections yield minimum ages between 5 and 21 Ma. This latter age corresponds to the onset of a widespread unroofing event throughout the Himalaya and southern Tibet. Carbonates in paleosols exposed along Bakiya Khola yield carbon isotopic values averaging between 11 and 7.0 Ma, and about between 7.4 Ma to the top of the section at about 2 Ma. This shift, previously recognized in Siwalik Group sediments in Pakistan, is interpreted to result from a change from dominantly plants (i.e., trees) to dominantly plants (i.e., grasses). The first appearance of grasses favored by a warm growing season marks a significant ecological shift that may be related to intensification of the Asian Monsoon brought about by uplift of the Tibetan Plateau. In south-central Tibet, the Kailas conglomerate developed a thickness of over 3 km due to uplift and erosion of the Gangdese belt. Age assessment of this unit has been problematic (Late Eocene to Miocene?). K-feldspar results from a cobble from the Kailas conglomerate, presumed to be from the Gangdese volcanic suite, yields an age spectrum characterized by relatively slow cooling (5°C/Ma) between 27-19 Ma followed by very rapid cooling (100°C/Ma) between 19-18 Ma. The feldspar microstructure indicates that it has been heated to >400°C subsequent to eruption. This thermal history is consistent with deep burial in the volcanic pile subsequent to extrusion followed by rapid unroofing beginning at 20 to 19 Ma. These results suggest an Early Miocene upper limit for the depositional age of the Kailas conglomerate, and extends the evidence for a substantially enhanced unroofing at this time in the Gangdese-Himalaya system.

Fractal Approach: Quantification of Seismicity in South Central Tibet

The South Central Tibet (SCT) region is inferred to be an extensional regime since it is predominantly associated with normal faulting with E-W oriented horizontal tension axis representing extension in eastwest direction. In this paper the fractal analysis is carried out using the seismicity data (mb≥3.5) for last 47 years (1963-2009) in SCT to the north of Central Himalaya. The entire study area is bounded by 29o-31o N and 85o-90o E. The study escorted to the recognition of a clustering events in two consecutive fifty events window having low spatial fractal correlation dimension (Dc) value ranging from 0.3638 to 0.5451 during the period between 07.26.1998 to 03.23.2002. Relatively two lower Dc obtained prior to strong event. Spatial-temporal clustering of events during low Dc period actually specifies a highly stressed region, leading to increase of shear strain causing weak zone from where the rupture propagation may ultimately nucleate causing large earthquake. This type of clustering pattern study using the well constrained catalogue data for the SCT region, which has a record of high seismic activity for the entire last two decades, can assist to enhance the understanding and mitigation of earthquake hazard.Keywords: Fractal spatial; correlation dimension (Dc); clustering; seismicityDOI: http://dx.doi.org/10.3126/bibechana.v8i0.5699 BIBECHANA 8 (2012) 116-126

Out-of-sequence deformation and expansion of the Himalayan orogenic wedge: insight from the Changgo culmination, south-central Tibet

DOI = 10.3126/hjs.v5i7.1279 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.84