[1] High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperaturetime paths inferred from 40 Ar/ 39 Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar 40 Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling (<1°C/m.y.) from Jurassic times until the late Miocene or early Pliocene. The regional extent and consistency of thermal histories during this time period suggest the presence of a stable thermal structure and imply that regional denudation rates were low (<0.1 mm/yr for nominal continental geotherms). Beginning in the late Miocene or early Pliocene, these samples experienced a pronounced cooling event (>30°-50°C/m.y.) coincident with exhumation from inferred depths of $8 -10 km, at denudation rates of 1 -2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period ($3°C/m.y.), suggesting limited exhumation (1 -2 km). However, these samples record a slight increase in cooling rate (from <1 to $3°C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin. We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable Sichuan Basin and thus marks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene ($5 -12 Ma).

Repeated, specific interactions between capsid protein (CP) subunits direct virus capsid assembly and exemplify regulated proteinprotein interactions. The results presented here reveal a striking in vivo switch in CP assembly. Using cryoelectron microscopy, threedimensional image reconstruction, and molecular modeling, we show that brome mosaic virus (BMV) CP can assemble in vivo two remarkably distinct capsids that selectively package BMV-derived RNAs in the absence of BMV RNA replication: a 180-subunit capsid indistinguishable from virions produced in natural infections and a previously unobserved BMV capsid type with 120 subunits arranged as 60 CP dimers. Each such dimer contains two CPs in distinct, nonequivalent environments, in contrast to the quasiequivalent CP environments throughout the 180-subunit capsid. This 120-subunit capsid utilizes most of the CP interactions of the 180-subunit capsid plus nonequivalent CP-CP interactions. Thus, the CP of BMV, and perhaps other viruses, can encode CP-CP interactions that are not apparent from mature virions and may function in assembly or disassembly. Shared structural features suggest that the 120-and 180-subunit capsids share assembly steps and that a common pentamer of CP dimers may be an important assembly intermediate. The ability of a single CP to switch between distinct capsids by means of alternate interactions also implies reduced evolutionary barriers between different capsid structures. The in vivo switch between alternate BMV capsids is controlled by the RNA packaged: a natural BMV genomic RNA was packaged in 180-subunit capsids, whereas an engineered mRNA containing only the BMV CP gene was packaged in 120-subunit capsids. RNA features can thus direct the assembly of a ribonucleoprotein complex between alternate structural pathways. M ost isometric viruses contain capsid proteins (CPs) arranged with quasi-icosahedral symmetry (1). This strategy allows the assembly of capsids containing more than the 60 identical CPs permitted by strict icosahedral symmetry, providing the CP is polymorphic and can adapt to a specific set of alternate, quasi-equivalent environments. Progressively larger capsids with 60T subunits can be formed as the number of quasi-equivalent CP-CP interaction states, T, increases according to T ϭ h 2 ϩ hk ϩ k 2 for integers h and k. Such CP polymorphism offers insight into regulated protein conformational changes, protein-protein interaction, and protein-nucleic acid interaction.By expanding capsid size, quasi-equivalence allows expanding viral genomes. Virus evolution appears to have led to continuing modification of CP polymorphism because many CPs share a similar ␤-barrel core (2) and known capsids range from T ϭ 1 to T ϭ 169 (3, 4). However, little is known about the extent of assembly flexibility possible in a single CP to allow such evolutionary transitions.One group of viruses with well-studied, quasi-icosahedral capsids is the bromovirus genus, a set of positive-strand RNA plant viruses with T ϭ 3 capsids. Brome mosaic virus (...

The Indus Group is a Paleogene, syntectonic sequence from the Indus Suture Zone of the Ladakh Himalaya, India. Overlying several pre-collisional tectonic units, it constrains the timing and nature of India's collision with Eurasia in the western Himalaya. Field and petrographic data now allow Mesozoic-Paleocene deep-water sediments underlying the Indus Group to be assigned to three pre-collisional units: the Jurutze Formation (the forearc basin to the Cretaceous-Paleocene Eurasian active margin), the Khalsi Flysch (a Eurasian forearc sequence recording collapse of the Indian continental margin and ophiolite obduction), and the Lamayuru Group (the Mesozoic passive margin of India). Cobbles of neritic limestone, deepwater radiolarian chert and mafic igneous rocks, derived from the south (i.e. from India), are recognized within the upper Khalsi Flysch and the unconfomably overlying fluvial sandstones of the Chogdo Formation, the base of the Indus Group. The Chogdo Formation is the first unit to overlie all three pre-collisional units and constrains the age of India-Eurasia collision to being no younger than latest Ypresian time (>49 Ma), consistent with marine magnetic data suggesting initial collision in the Arabian Sea region at c. 55 Ma. The cutting of equatorial Tethyan circulation north of India at that time may have been a trigger to the major changes in global palaeoceanography seen at the Paleocene-Eocene boundary. New 4 0~r / 3 9~r , apatite fission-track and illite crystallinity data from the Ladakh Batholith and Indus Group show that the batholith, representing the old active margin of Eurasia, experienced rapid Eocene cooling after collision, but was not significantly reheated when the Indus Group basin was inverted during north-directed Miocene thrusting (23-20 Ma). Subsequent erosion has preferentially removed 5-6 km (c. 200 "C) over much of the exposed Indus Group, but only c. 2 krn from the Ladakh Batholith. Reworking of this material into the Indus fan may complicate efforts to ' interpret palaeo-erosion patterns from the deep-sea sedimentary record.

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