Temporal And Spatial Variation In The Cenozoic Deformation Patterns Of The Sanjiang Orogenic Belt

Locating in southeastern Tibetan Plateau, east of the Eastern Himalaya Syntax, the Sanjiang orogenic belt recorded a complicated Cenozoic deformation as responding to the India-Eurasia continent-continent collision. Our field observations, zircon U/Pb dating, and muscovite 40Ar/39 Ar analytical results revealed the structural pattern and deformational history of the Xuelong Shan metamorphic complex as below: folds of gneissosity or schistosity and associated shear belts which are parallel to the axial plane of the folds. The folds and shear zones formed during 28-26 Ma. The structural pattern and deformational history are distinctive in contrasting with those of the Diancang Shan and Ailao Shan shear zones to the south. The temporal and spatial variation in structural pattern may provide a positive constrain on the India-Eurasia continent-continent collision. A growing accumulation of detail structural and geothermochronological data makes it possible to reveal the variation. Synthesizing available data, we find that the Baoshan and Tengchong blocks were firstly(50-45Ma) deformed displaying fold-and-reversal fault pattern; this structural pattern gradually expanded northeastward into the Lanping-Simao block, causing a change in basin-pattern within the block and two ductile shear zones along its western and eastern boundaries, respectively. The western(34Ma) shear zone has an earlier starting time than its eastern(<30Ma) counterpart. Both shear zones have similar deformational-history: pure-shear(compression), simple shear(lateral sliding), and/then pure-shear(uplifting) again. The starting times of southern segments of both shear zones are slightly earlier than their northern ones. The ductile-deformed lower crustal rocks along the shear zones uplifted heterogeneously as well likely due to a complicated block rotation according to the paleomagnetic data. It is worthy pointing out that the northernmost tips of all shear zones mainly record compressional deformation at the latest time. Such a Cenozoic strain pattern suggests that, during the continent-continent collision, the Sanjiang orogenic belt deformed mainly by pervasive shortening and heterogeneous block rotation, while as the scale of southeastward extrusion of block is limited.