Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, andthermochronologic constraints

Structural, thermobarometric, and thermochronologic investigations of the K angmar Dome, southern Tibet, suggest that both extensional and contractiona l deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimen tary rocks; metamorphic grade dies out up section and is defined by a serie s of concentric kyanite-in, staurolite-in, garnet-in, and chloritoid-in iso grads. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in app roximately E-W trending tight to isoclinal folds of bedding with an associa ted moderately to steeply north dipping axial planar foliation, S1. The sec ond event, D2, resulted in a high strain mylonitic foliation, S2, which def ines the domal structure, and an associated approximately N-S trending stre tching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome t o top N shear on the north dipping flank. The central part of the dome exhi bits either opposing shear sense indicators or symmetric fabrics. Microtext ural relations indicate that peak metamorphism occurred post-D1 and pre- to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of similar to 445 degrees C and 370 MPa in garnet zone rocks, i ncreasing to 625 degrees C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a singl e structural horizon across the dome and the apparent gradient in pressure is similar to 20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica Ar- 40/Ar-39 thermochronology yields 15.24 +/- 0.05 to 10.94 +/- 0.30 Ma coolin g ages that increase with depth and young northward within a single structu ral horizon across the dome. Diffusion modeling of potassium feldspar Ar-40 /Ar-39 spectra yield rapid cooling rates (similar to 10-30 degrees C/Myr) b etween similar to 11.5 and 10 Ma and apatite fission track ages range from 7.9 +/- 3.0 to 4.1 +/- 1.9 Ma, with a mean age of similar to 5.5 Ma. Both d ata sets show symmetric cooling across the dome between similar to 11 and 5 .5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica Ar-40/Ar-39 isochrons are domed, whereas potassium feldspar Ar-40/ Ar-39 and apatite fission track isochrons are not, suggesting that doming o ccurred at similar to 11 Ma. Our data db not support simple, end-member met amorphic core complex-type extension, diapirism, or duplex models for gneis s dome formation. Rather, we suggest that the formation of extensional fabr ics occurred within a zone of coaxial strain in the root zone of the Southe rn Tibetan Detachment System (STDS), implying that normal slip along the ST DS and extensional fabrics within the Kangmar Dome were the result of gravi tational collapse of overthickened crust. Subsequent doming during the midd le Miocene is attributed to thrusting upward and southward over a north dip ping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in t he Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tib et.