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 Sino‐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 ∼200 km away. Structural reconstructions imply that the slab was >10 km thick. U/Pb zircon and 40 Ar/ 39 Ar geochronology indicate that exhumation up to crustal depths occurred diachronously between 240 and ∼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.