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The >1000-km-long Oligocene-Miocene left-lateral Red River shear zone (RRSZ) and metamorphic belt and the Pliocene-active right-lateral Red River fault (RRF), stretching from SE Tibet to the South China Sea, has been cited as one of the primary examples of a lithospheric scale strike-slip fault that has resulted in syn-kinematic metamorphism and partial melting and accommodated several hundred to a thousand kilometers of horizontal motion as a result of the indentation of India into Asia. Alternatively we interpret the metamorphic complexes along the RRSZ as exhumed metamorphic core complexes of older rocks, subsequently affected by Oligocene-Early Miocene leftlateral shear and localized partial melting (leucogranite dykes), Miocene low-angle normal faulting along margins (Range Front faults), and Pliocene active dextral strike-slip faulting (RRF). Along the Ailao Shan (ALS) and Diancang Shan (DCS) ranges in Yunnan, SW China, early amphibolite facies metamorphic rocks were intruded by K-feldspar orthogneisses of Triassic age (Indosinian). LA-ICP-MS U-Pb zircon dating reveals a complex history with zircon cores showing evidence of Indosinian (~239-243 Ma) to Neoproterozoic magmatism. Zircon rims show an Oligocene (~26 Ma) magmatic or metamorphic overprint. Biotite granodiorites and syenites of mantle origin intruded the gneisses during the Oligocene (~35 Ma).Later biotite leucogranites intruded the orthogneisses and migmatite host rocks before a signifi cant phase of tight to isoclinal folding. Ductile, left-lateral strike-slip shear fabrics were superimposed on all lithologies at high temperature (~500-550 °C) for the ALS and lower temperatures (~250-150 °C) after peak metamorphism and after granite intrusion. A few very small biotite (±Grt ± Tur) leucogranite veins and dykes crosscut the ductile strike-slip shear fabrics at Yuanjiang, in the Ailao Shan. Low-angle normal faulting along the margins of the metamorphic massif accommodated fi nal exhumation of the Red River gneisses. Using published U-Th-Pb and 40 Ar/ 39 Ar ages of granites along the shear zone, the age of left-lateral ductile shearing along the RRSZ can be constrained as between the earlier deformed leucogranites (31.9-24.2 Ma) and the later crosscutting dykes (21.7 Ma) with exhumation-related cooling continuing until ~17 Ma.

The Red River shear zone (RRSZ) is a major left-lateral strike-slip shear zone, containing a ductilely deformed metamorphic core bounded by brittle strike-slip and normal faults, which stretches for >1000 km from Tibet through Yunnan and North Vietnam to the South China Sea. The RRSZ exposes four high-grade metamorphic core complexes along its length. Various lithologies from the southernmost core complex, the Day Nui Con Voi (DNCV), North Vietnam, provide new constraints on the tectonic and metamorphic evolution of this region prior to and following the initial India-Asia collision. Analysis of a weakly deformed anatectic paragneiss using P-T pseudosections constructed in the MnO-provides prograde, peak and retrograde metamorphic conditions, and in situ U-Th-Pb geochronology of metamorphic monazite yields texturally controlled age constraints. Tertiary metamorphism and deformation, overprinting earlier Triassic metamorphism associated with the Indosinian orogeny and possible Cretaceous metamorphism, are characterized by peak metamorphic conditions of~805°C and~8.5 kbar between c. 38 and 34 Ma. Exhumation occurred along a steep retrograde P-T path with final melt crystallizing at the solidus at !~5.5 kbar at~790°C. Further exhumation at~640-700°C and~4-5 kbar at c. 31 Ma occurred at subsolidus conditions. U-Pb geochronological analysis of monazite from a strongly deformed pre-kinematic granite dyke from the flank of the DNCV provides further evidence for exhumation at this time. Magmatic grains suggest initial emplacement at 66.0 AE 1.0 Ma prior to the India-Asia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 AE 0.4 Ma. Monazite grains from a cross-cutting post-kinematic dyke within the core of the DNCV antiform provide a minimum age constraint of 25.2 AE 1.4 Ma for the termination of fabric development. A separate and significant episode of monazite growth at c. 83-69 Ma is suggested to be the result of fluid-assisted recrystallization following the emplacement of magmatic units. GeochronologyInsight into the tectonothermal evolution of the DNCV region has been obtained through multisystem geochronology performed on various lithological

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