Oligocene-Miocene middle crustal flow in southern Tibet: Geochronology of Mabja Dome

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Department or Administrative Unit

Geological Sciences

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New U-Pb zircon, monazite, 40Ar/39Ar, and apatite fission track ages provide constraints on the timing of formation and exhumation of the Mabja Dome, southern Tibet, shed light on how this gneiss dome formed, and provide important clues on the tectonic evolution of middle crustal rocks in southern Tibet. Zircons from a deformed leucocratic dyke swarm yield a U- Pb age of 23.1+ 0.8 Ma, providing the first age constraint on the timing of middle crustal ductile horizontal extension in the North Himalayan gneiss domes. Zircons and monazite from a post-tectonic two-mica granite yield ages of 14.2+ 0.2 Ma and 14.5+ 0.1, respectively, indi- cating that vertical thinning and subhorizontal stretching had ceased by the middle Miocene. Mica 40 Ar/39Ar ages from schists and orthogneisses increase structurally down-section from 12.85+ 0.13 Ma to 17.0+ 0.19 Ma and then decrease at the deepest structural levels to 13.29+ 0.09 Ma. Micas from the leucocratic dyke swarm and post-tectonic two-mica granites yield similar 40Ar/39Ar cooling ages of 13.48+ 0.13 to 12.84+ 0.08 Ma. The low-temperature steps of potassium feldspar 40Ar/39Ar spectra yield ages of c. 11.0- 12.5 Ma and apatite fission track analyses indicate the dome uniformly cooled below c. 1158C at 9.5+ 0.6 Ma. Based on these data, calculated average cooling rates across the dome range from c .4 0 -608C/million years in schist and orthogneiss and following emplacement of the leucocratic dyke swarm, to c. 3508C/million years following emplacement of the two-mica granites. The mylonitic foliation, peak metamorphic isograds, and mica 40Ar/39Ar chrontours are domed, whereas the low-tempera- ture step potassium feldspar 40Ar/39Ar and apatite fission track chrontours are not, suggesting that doming occurred between 13.0 and 12.5 Ma and at temperatures between 370 and 2008C. Our new ages, along with field, structural and metamorphic data, indicate that the domal geometry observed at Mabja developed by middle-Miocene southward-directed thrust faulting upward and southward along a north-dipping ramp above cold Tethyan sediments. The structural, metamorphic and geo- chronologic histories documented at Mabja Dome are similar to those of Kangmar Dome, suggesting a common mode of occurrence of these events throughout southern Tibet. Vertical thinning and horizontal stretching, metamorphism, generation of migmatites, and emplacement of leucogranites in the domes of southern Tibet are synchronous with similar events in the Greater Himalayan Sequence that underlie the high Himalaya. These relations are consistent with previously proposed models for a ductile middle-crustal channel bounded above by the South Tibetan detachment system and below by the Main Central thrust in the High Himalaya that extended northward beneath southern Tibet. The North Himalayan gneiss domes lie within the Tethys Himalaya south of the Indus- Tsangpo Suture Zone (ITSZ) and north of the South Tibetan detachment system (STDS) and crop out within the axis of the North Himalayan antiform.