Indosinian deformation of the Songpan Garzê Fold Belt, northeast Tibetan Plateau

Deformation of the Songpan Garzê Fold Belt, southwest China, occurred progressively during the Indosinian orogeny (ca. 230-200 Ma). Two phases of south-directed deformation are identified: D1, associated with pervasive thickening of the Songpan Garzi sedimentary pile and detachment of that portion of the fold belt onlapping the Yangtze passive margin; and D2, a localized phase of deformation associated with transpressive shortening of the fold belt's southeast margin. An arcuate form-surface in the southeast fold belt reflects the distribution of D1 and D2 structural phases and the diachronous migration of these phases into the fold belt. This migration accommodated lateral growth and cumulative sinistral transpression within the fold belt's eastern margin. An enigmatic metamorphic complex, the so-called Danba antiform, is constrained to have formed during the early stages of the D2 deformation. This was associated with diapiric upwelling of buoyant migmatised basement and domal distortion of the overlying metasedimentary pile.