Passive continental margins are geometrically irregular as a consequence of either triple‐junction evolution or the development of transfer zones in detachment fault systems, whereas active continental margins are smoothly arc‐shaped due to subduction of plates on the Earth's spherical surface. We propose that this basic difference in boundary geometry has played an important role in the latest Paleozoic‐early Mesozoic collision of North and South China. In particular, we suggest that prior to collision, the active southern margin of the North China Block (NCB) was contiguous across the Qilian Shan, Qinling, Dabie Shan, Shandong peninsula of east central China to the Imjingang area of central Korea. The passive northern margin of the South China Block (SCB), in contrast, had a more irregular shape, such that its northeastern segment in northern Jiangsu and eastern Anhui provinces of China extended some 500 km farther north than its western counterparts in northern Sichuan, southern Shaanxi, and northern Hubei provinces. Collision of the NCB and the SCB began by indentation of the northeastern SCB into the eastern NCB in the late Early Permian and lasted until the Late Triassic‐Early Jurassic. The indentation produced the left‐slip Tan‐Lu fault in northeastern China and the right‐slip Honam shear zone in southeastern Korea and caused the northward displacement of the Shandong and the Imjingang metamorphic belts. This model predicts that collision along the Dabie and Qinling metamorphic belt occurred significantly later than along the Shandong belt, which is consistent with radiometric and depositional constraints on the time of collision. The proposed model accounts for the abrupt termination of the Tan‐Lu fault at its south end and the drastic decrease in slip along the Tan‐Lu fault north of the Shandong metamorphic belt. The model also predicts the distribution and ages of metamorphism along the suture and the observed local but intense Triassic deformation (=Indosinian orogeny) in northeastern China and northern Korea, which was previously an enigmatic feature in this region.

Abstract.Structural, sedimentological, magnetostratigraphic, and nøAr/39Ar thermochronological investigations were conducted in the southern Chinese Tian Shan. On the basis of our own mapping and earlier investigations in the area, the Late Cenozoic southern Tian Shan thrust belt may be divided into four segments based on their style of deformation. From west to east, they are (1) Kashi-Aksu imbricate thrust system, (2) the Baicheng-Kuche fold and thrust system, (3) the Korla right-slip transfer system, and (

Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south-central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.

are compatible with the cylindrical volume diffusion at a scale of the crystal diameter (Hames and Bowring, 1994, and references cited therein). Minor radial age gradients may exist in all of the samples; however, we effectively ''averaged out'' such variations by sampling a single size fraction and by using the muscovite total gas ages as the best approximation of the time of cooling through a fixed isotherm. Because of the similar size, composition, and cooling rate, we are confident that all of the muscovites have a very similar closure temperature. Finding the heat source responsible for the age variation in the Presidential Range, that Oliver suggests is due to thermal relaxation of a perturbed geotherm, is problematic. Our muscovite ages are too old to be related to the Mesozoic White Mountain plutonic-volcanic series and too young to be related to the voluminous Carboniferous-Devonian two-mica granitoids in the region. The systematic radial increase in muscovite ages away from the contact with the Sebago batholith (see Eusden and Lux, 1994, Fig. 1) is suggestive of a thermal perturbation caused by the intrusion of this pluton. However, Aleinikoff et al. (1985) determined the age of the Sebago to be ϳ325 Ma. Therefore, our muscovite ages would be too young to be thermally related to this intrusion. Another possibility is that the ''chrontour'' pattern around the Sebago was created by Triassic structural doming of a subhorizontal ''fossil'' isothermal surface.

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