Kinematics and Vorticity in Kangmar Dome, Southern Tibet: Testing Patterns of Mid-crustal Ductile Deformation During the Himalayan Orogeny

Tom Wagner
September 2009

Abstract

The channel flow hypothesis states that mid-crustal rocks in southern Tibet flowed like a fluid on geologic timescales southward toward the Himalayan front. Channel flow is driven by a low-viscosity mid-crust, a horizontal pressure gradient between Tibet and India, and surface denudation along the southern flank of the high Himalaya. Flow is predicted to be top-north simple shear at the top of the channel grading into pure shear at the center and top-south simple shear at the base. To test this hypothesis, I completed detailed analyses of the kinematics, deformation temperatures, and mean vorticity (W_m) of D2 deformation, which is characterized by north-south horizontal extension and vertical thinning, that is preserved in mid-crustal (chloritoid- to kyanite-grade) metapelites overlying an orthogneiss in the core of Kangmar Dome, southern Tibet. Mylonitic D2 fabrics define an ~2260–3300 m wide zone of deformation at depth, extending from the garnet-in isograd to ~1000 m below the metapelite/orthogneiss contact; the deeper parts of the orthogneiss appear undeformed. Quartz and feldspar textures and mineral assemblages in equilibrium indicate that D2 deformation temperatures ranged from ~300 to 450°C in chloritoid-zone rocks to ≥600°C in kyanite-zone rocks, the same as peak metamorphic temperatures. Kinematics at these temperatures were characterized by a near equal mix of top-north and top-south shear in garnet-zone rocks, and dominantly top-north shear and top-south shear in staurolite- to kyanite-zone rocks on the north and south flanks of the dome, respectively. D2 vorticity values record an increase in pure shear component with depth from ~45% pure shear in chloritoid-zone rocks, to ~61% in garnet-zone rocks, to ~64% in staurolite- to kyanite-zone rocks, likely the result of an increase in lithostatic load with structural depth. The distribution of shear sense and vorticity estimates suggests that D2 deformation across Kangmar was characterized by a bulk pure shear component (vertical thinning and north-south horizontal extension) below the garnet-in isograd. Shear bands, which cross-cut the D2 mylonitic foliation, are characterized by dominantly top-north shear on the north flank of the dome and a mix of top-north and top-south shear on the south flank, and formed at ~300–400°C, overlapping with ⁴⁰Ar/³⁹Ar mica closure temperatures. These relations suggest that the shear bands formed during a lower temperature phase of deformation that postdates the mylonitic D2 high-temperature phase. The ⁴⁰Ar/³⁹Ar mica ages, which are middle-Miocene (Lee et al., 2000), provide age constraints for this phase of deformation. Finally, the metapelite/orthogneiss contact is characterized locally by breccia and fault gouge (Lee et al., 2000). The ductile deformation patterns documented in Kangmar show that high strain, high-temperature D2 ductile deformation was the consequence of a bulk pure shear deformation in the ductile root zone of the Southern Tibetan Detachment System. This root zone was subsequently cut by the Southern Tibetan Detachment System fault, which is now exposed at the contact between the orthogneiss and overlying metapelites. A later shear band phase of deformation occurred during exhumation of these mid-crustal now residing within the hanging wall of the southward vergent Gyirong-Kangmar Thrust fault. My interpretations imply that channel flow is not preserved within the exposed mid-crustal rocks of Kangmar, but may be present at deeper structural levels.

Update: Tom's thesis was published in 2010 in Tectonics [download PDF file].