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[1] In easternmost Bhutan the South Tibetan detachment (STD) is a ductile shear zone that juxtaposes the Radi (or Sakteng) klippe of the Tethyan Sedimentary Series from underlying high-grade Greater Himalayan rocks. In situ LA-ICPMS U-Th-Pb analysis of metamorphic monazite from the immediate footwall and hanging wall of the STD within the shear zone at the base of the klippe, constrains north vergent normal shear to between 25 and 20 Ma. Coeval thrusting on the Main Central Thrust during this time supports a phase of channel flow-viscous wedge model activity, lasting only ∼3 Ma. Geochronologic data from the eastern Himalaya indicate alternating mechanisms for extrusion of the metamorphic core of the orogen from the Late Oligocene through to the Late Miocene, switching from channel flow-viscous wedge behavior to critical taper-frictional wedge behavior, each phase lasting approximately only 2 to 5 Ma. The tectonic evolution of the eastern Himalaya is comparable to central and western Himalayan tectonics during the Early Miocene, but during the Middle Miocene metamorphism and magmatism in the eastern Himalaya migrated toward the orogenic hinterland, a process not widely documented elsewhere in the Himalaya, thus highlighting the need for an orogenic model in three spatial dimensions. Citation: Chambers, J., R. Parrish, T. Argles, N. Harris, and M. Horstwood (2011), A short-duration pulse of ductile normal shear on the outer South Tibetan detachment in Bhutan: Alternating channel flow and critical taper mechanics of the eastern Himalaya, Tectonics, 30, TC2005,

Key litho-tectonic units in the Himalaya e.g. the Greater Himalayan Sequence, and the major faults that bound them e.g. the Main Central Thrust, can be traced continuously along the 2400 km strike of the orogen (Fig. 1). Understanding the metamorphic evolution of such units together with the recognition of the nature, location and timing of the principle structural elements of an orogenic belt is fundamental both to defining the tectonic architecture of that belt and to understanding the mechanical behaviour of continental crust during its deformation. The Greater and Lesser Himalayan Sequences are two key litho-tectonic units in the Himalaya. Using provenance studies based on detrital zircon ages (DeCelles et al. 2000;

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Magmatic stoping, i.e., the formation, transfer into, and movement through magma of older plutonic and metamorphic host-rock xenoliths, was widespread in the Mesozoic Sierra Nevada batholith (California, United States). However, the prevailing view that stoped blocks form by rapid thermal shattering and collapse into chambers may not be the dominant process of block formation and displacement into chambers in the Sierra Nevada. In detailed studies in and around the Tuolumne Batholith and Jackass Lakes pluton, we found evidence for the following history of block formation in slightly older, fairly isotropic plutonic host rocks: (1) low stress sites developed, leading to planar zones of increased porosity; (2) focused porous fl ow of fi rst felsic melts followed by intermediate melts led to growth of magma fi ngers, which in turn led to increased porosity and loss of host-rock cohesion; and (3) connection of mag matic fi ngers resulted in the formation of dike-like channels in which fl ow facilitated removal of all host-rock material in these planar zones. Once formed, blocks were initially displaced by repeated magma injections along these channels, often resulting in uni directional growth in these zones creating local magmatic sheeted complexes along block margins. Free block rotation occurred when suffi cient nonlayered magma surrounded the host block; in some cases, segments of former sheeted zones remain attached to rotated blocks.In anisotropic metamorphic host rocks, focused porous fl ow may have locally played a role, but the dominant processes during initial block formation were cracking, parallel and at high angles to anisotropy, and intrusion of magma by channel fl ow. Subsequent initial block displacement and eventual rotation are identical to those in the nearly iso tropic host rock. The driving forces for the development of low-stress sites, cracking, dilation, and magma fl ow remain uncertain, but likely refl ect the interplay between regional stress, magma buoyancy stresses, thermal gradients, and host-rock properties, and not simply rapid heating and thermal expansion cracking. Thus a number of processes may drive block formation, some of which are rapid (thermal shattering, roof collapse) whereas others occur over longer durations (incremental magma pulsing and formation of sheeted complexes, regional deformation).