The Early Cretaceous Hohhot metamorphic core complex(mcc) of the Daqing Shan(Mtns.) of central Inner Mongolia is among the best exposed and most spectacular of the spatially isolated mcc’s that developed within the northern edge of the North China “craton”.All of these mcc’s were formed within the basement of a Late Paleozoic Andean-style arc and across older Mesozoic fold-and-thrust belts of variable age and tectonic vergence.The master Hohhot detachment fault roots southwards within the southern margin of the Daqing Shan for an along-strike distance of at least 120 km.Its geometry in the range to the north is complicated by interference patterns between(1) primary,large-scale NW-SE-trend-ing convex and concave fault corrugations and(2) secondary ENE-WSW-trending antiforms and synforms that folded the detachment in its late kinematic history.As in the Whipple Mtns.of California, the Hohhot master detachment is not of the Wernicke(1981) simple rooted type:instead,it was spawned from a mid-crustal shear zone,the top of which is preserved as a mylonitic front within Carboniferous metasedimentary rocks in its exhumed lower plate.~(40)Ar—~(39)Ar dating of siliceous volcanic rocks in basal sections of now isolated supradetachment basins suggest that crustal extension began at ca.127 Ma, although lower-plate mylonitic rocks were not exposed to erosion until after ca.119 Ma.Essentially synchronous cooling of hornblende,biotite.and muscovite in footwall mylonitic gneisses indicates very rapid exhumation and at ca.122—120 Ma.Contrary to several recent reports,the master detachment clearly cuts across and dismembers older,north-directed thrust sheets of the Daqing Shan foreland fold-and-thrust belt.Folded and thrust-faulted basalts within its foredeep strata are as young as 132.6±2.4 Ma,thus defining within 5—6 Ma the regional tectonic transition between crustal contraction and profound crustal extension. The Early Cretaceous Hohhot metamorphic core complex (mcc) of the Daqing Shan (Mtns.) Of central Inner Mongolia is among the best exposed and most spectacular of the spatially isolated mcc’s that developed within the northern edge of the North China “craton” . All of these mcc’s were formed within the basement of a Late Paleozoic Andean-style arc and across older Mesozoic fold-and-thrust belts of variable age and tectonic vergence. The master Hohhot detachment fault roots southwards within the southern margin of the Daqing Shan for an along-strike distance of at least 120 km. Its geometry in the range to the north is complicated by interference patterns between (1) primary, large-scale NW-SE-trend-convex convex and concave fault corrugations and (2) secondary ENE-WSW-trending antiforms and synforms that folded the detachment in its late kinematic history. As in the Whipple Mtns. of California, the Hohhot master detachment is not of the Wernicke (1981) simple rooted type: instead, it was spawned froma mid-crustal shear zone, the top of which is preserved as a mylonitic front within Carboniferous metasedimentary rocks in its exhumed lower plate. ~ (40) Ar- ~ (39) Ar dating of siliceous volcanic rocks in basal sections of now isolated supradetachment basins suggest that crustal extension began at ca.127 Ma, although lower-plate mylonitic rocks were not exposed to erosion until after ca.119 Ma.Essentially synchronous cooling of hornblende, biotite.and muscovite in footwall mylonitic gneisses very rapid exhumation and at ca.122-120 Ma.Contrary to several recent reports, the master detachment clearly cuts across and dismembers older, north-directed thrust sheets of the Daqing Shan foreland fold-and-thrust belt. Fully and thrust-faulted basalts within its foredeep strata are as young as 132.6 ± 2.4 Ma, thus defining within 5-6 Ma the regional tectonic transition between crustal contraction and profound crustal extension.