It is widely believed that the Tibetan plateau is a late Cenozoic feature produced by the Indo-Asian collision. However, because Tibet was the locus of continental accretion and subduction throughout the Mesozoic, crustal thickening during that time may also have contributed to growth of the plateau. This portion of the geologic history was investigated in a traverse through the central Lhasa block, southern Tibet. Together with earlier studies, our mapping and geochronological results show that the Lhasa block underwent little north-south shortening during the Cenozoic. Rather, our mapping shows that ~60% crustal shortening, perhaps due to the collision between the Lhasa and Qiangtang blocks, occurred during the Early Cretaceous. This observation implies that a significant portion of southern Tibet was raised to perhaps 3-4 km elevation prior to the Indo-Asian collision. INTRODUCTIONAlthough it has long been recognized that Tibet was the locus of continental collision and accretion since the early Mesozoic (Allégre et al., 1984;Sengör, 1984;Searle et al., 1987), the style and intensity of deformation produced by each accretional event remain poorly documented (e.g., Tibetan Bureau of Geology and Mineral Resources [TBGMR], 1982). To address this issue, we conducted systematic mapping of the Coqin area in the northcentral part of the Lhasa block ( Fig. 1), during which three thrust systems were documented (Gugu La, Shibaluo, and Emei La) along a 132-km-long north-south traverse (Fig. 2). None of these thrusts appear to have involved metamorphic basement, and thrusts in their hanging walls are cut by plutons that are in turn overlain by essentially flat-lying tuff deposits. Although the ages of sedimentary sequences in the mapped area have been broadly constrained as Paleozoic and Mesozoic by using index fossils (Tibetan Bureau of Geology and Mineral Resources, 1982), the igneous rocks were not previously dated. In this paper we briefly summarize the structural relationships and age constraints for the three thrusts and, in conjunction with allied data, conclude that the southern Tibetan plateau had begun to form during the Early Cretaceous and remained elevated until the Indo-Asian collision began. GUGU LA THRUST SYSTEMThe north-dipping Gugu La thrust places Cretaceous strata (Gugu La sequence) over Cretaceous (?) conglomerate and volcaniclastic rocks (Burial Hill sequence; Fig. 2a). The thrust has a maximum stratigraphic throw in its central part. Fault slickensides indicate a S10°-20°W transport direction.The lower part of the hanging wall consists of ~1-km-thick volcanic breccias and volcaniclastic sandstones. Paleocurrent measurements of crossbeds in the sandstone indicate a north-directed paleoflow. The top of the lower section (~800 m thick) is marked by a laterally extensive limestone layer, which contains abundant Early Cretaceous rudist bivalves (TBGMR, 1982). Above it is an ~500-m-thick sequence of fluvial sandstone that records a change upsection from north-directed to south-directed paleoflow. We inter...

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We used the natural abundance of stable isotopic ratios of hydrogen and oxygen in soil (0.05-3 m depth), plant xylem and precipitation to determine the seasonal changes in sources of soil water uptake by two native encroaching woody species (Pinus ponderosa P. & C. Lawson, Juniperus virginiana L.), and two C(4) grasses (Schizachyrium scoparium (Michx.) Nash, Panicum virgatum L.), in the semiarid Sandhills grasslands of Nebraska. Grass species extracted most of their water from the upper soil profile (0.05-0.5 m). Soil water uptake from below 0.5 m depth increased under drought, but appeared to be minimal in relation to the total water use of these species. The grasses senesced in late August in response to drought conditions. In contrast to grasses, P. ponderosa and J. virginiana trees exhibited significant plasticity in sources of water uptake. In winter, tree species extracted a large fraction of their soil water from below 0.9 m depth. In spring when shallow soil water was available, tree species used water from the upper soil profile (0.05-0.5 m) and relied little on water from below 0.5 m depth. During the growing season (May-August) significant differences between the patterns of tree species water uptake emerged. Pinus ponderosa acquired a large fraction of its water from the 0.05-0.5 and 0.5-0.9 m soil profiles. Compared with P. ponderosa, J. virginiana acquired water from the 0.05-0.5 m profile during the early growing season but the amount extracted from this profile progressively declined between May and August and was mirrored by a progressive increase in the fraction taken up from 0.5-0.9 m depth, showing plasticity in tracking the general increase in soil water content within the 0.5-0.9 m profile, and being less responsive to growing season precipitation events. In September, soil water content declined to its minimum, and both tree species shifted soil water uptake to below 0.9 m. Tree transpiration rates (E) and water potentials (Psi) indicated that deep water sources did not maintain E which sharply declined in September, but played an important role in the recovery of tree Psi. Differences in sources of water uptake among these species and their ecological implications on tree-grass dynamics and soil water in semiarid environments are discussed.

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Abstract. Determining the timing, magnitude, and location of deformation due to the IndoAsian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision. A puzzling result of geological investigations of the Lhasa Block over the past 2 decades has been the apparent lack of significant Tertiary deformation there. Perhaps the most important structural feature of the Lhasa Block is the south directed Gangdese Thrust System, which developed along its southern edge. The thrust system, which separates the Andean-type batholith of southern Asia from rocks of Indian affinity, is obscured at most locations across southeastern Tibet by back thrusts of the younger, north directed Renbu Zedong Thrust System. The best documented site where both thrusts are exposed occurs near Zedong (Zedong Window).

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Figure 1. Location of Hannuoba Basalt within North China craton. A: Setting of study area within North China craton (from Zhao et al., 2001). Small rectangle shows area enlarged in B. B: General geology of area. Most samples were from Damaping (labeled DA in Tables 1 and DR1 [see footnote 1]); sample JSB1 was from Jieshaba. Age of formation of rock units other than Hannuoba Basalt (HB, shading): Q-Quaternary; K-Cretaceous; J-Jurassic; A-Archean. ABSTRACTWe present evidence from zircons entrained within lowercrustal xenoliths in the Cenozoic Hannuoba Basalt of multiple melting events beneath the North China craton in the late Mesozoic. Peak activity was between 180 and 80 Ma, the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks, emplacement of dioritic and lamprophyric dikes, and widespread gold mineralization. The process involved partial loss of mantle lithosphere, accompanied by wholesale rising of asthenospheric mantle beneath eastern China. We correlate these events with lithospheric thinning resulting from the breakup and dispersal of Gondwanaland, accompanied by a major mantle overturn, fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of Asia.