Exhumation of ultrahigh-pressure continental crust in east central China: Late Triassic-Early Jurassic tectonic unroofing

The largest tract of ultrahigh-pressure rocks, the Dabie-Hong'an area of China, was exhumed from 125 km depth by a combination of normal-sense shear from beneath the hanging wall Sine-Korean craton, southeastward thrusting onto the footwall Yangtze craton, and orogen-parallel eastward extrusion. Prior to exhumation the UHP slab extended into the mantle a downdip distance of 125-200 km at its eastern end, whereas it was subducted perhaps only 20-30 km at its far western end similar to 200 km away. Structural reconstructions imply that the slab was >10 km thick. U/Pb zircon and 40Ar/39Ar geochronology indicate that exhumation up to crustal depths occurred diachronously between 240 and similar to 225-210 Ma, reflecting a vertical exhumation rate of >2 mm/yr. The upper boundary of the slab is the Huwan shear zone, a normal-sense detachment that reactivated the plate suture. The lower boundary is represented by the Lower Yangtze fold-thrust belt. NW-trending stretching lineations, NE-vergent, WNW-ESE trending < a > folds, dominant top-NW shear, and conjugate, but overall, asymmetric, shear band fabrics, document that exhumation was accomplished by updip and orogen-parallel extrusion accompanied by layer-parallel thinning. The orientation and shape of the folds, and a change from SE to SW flow directions, imply that the slab rotated clockwise about a western pivot during exhumation; this rotation was likely caused by the eastward increasing depth of subduction mentioned above, combined with a possible marginal basin and a weak eastern plate boundary. Exhumation of the slab produced considerable shortening in the Lower Yangtze fold-thrust belt, perhaps producing the foreland orocline.