Br. Hacker et al., Exhumation of ultrahigh-pressure continental crust in east central China: Late Triassic-Early Jurassic tectonic unroofing, J GEO R-SOL, 105(B6), 2000, pp. 13339-13364
The largest tract of ultrahigh-pressure rocks, the Dabie-Hong'an area of Ch
ina, was exhumed from 125 km depth by a combination of normal-sense shear f
rom beneath the hanging wall Sine-Korean craton, southeastward thrusting on
to the footwall Yangtze craton, and orogen-parallel eastward extrusion. Pri
or to exhumation the UHP slab extended into the mantle a downdip distance o
f 125-200 km at its eastern end, whereas it was subducted perhaps only 20-3
0 km at its far western end similar to 200 km away. Structural reconstructi
ons imply that the slab was >10 km thick. U/Pb zircon and Ar-40/Ar-39 geoch
ronology indicate that exhumation up to crustal depths occurred diachronous
ly 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 bound
ary is represented by the Lower Yangtze fold-thrust belt. NW-trending stret
ching lineations, NE-vergent, WNW-ESE trending < a > folds, dominant top-NW
shear, and conjugate, but overall, asymmetric, shear band fabrics, documen
t that exhumation was accomplished by updip and orogen-parallel extrusion a
ccompanied by layer-parallel thinning. The orientation and shape of the fol
ds, and a change from SE to SW flow directions, imply that the slab rotated
clockwise about a western pivot during exhumation; this rotation was likel
y caused by the eastward increasing depth of subduction mentioned above, co
mbined with a possible marginal basin and a weak eastern plate boundary. Ex
humation of the slab produced considerable shortening in the Lower Yangtze
fold-thrust belt, perhaps producing the foreland orocline.