Tectonic Evolution
A variety of models have been proposed for the Qinling tectonic evolution in recent years. Different models are based upon different data and, as a result, many controversies have arisen. In particular, debates are focused upon the timing of the collision of the South and North China blocks. There exist two groups of evidence: one indicates Middle Paleozoic collision, while the other suggests a Late Triassic age (Fig.14). Most of the previous models considered the Shangdan suture as the trace of collision of the North and South China blocks. The discovery of the Mianlue suture sheds new light on the understanding of the Qinling evolution (Meng et al., 1999). We present here a synthesis of the Qinling tectonic evolution.
The Qinling orogen experienced a prolonged continental divergence and convergence between blocks. During the period from Late Neoproterozoic to Early Paleozoic times, the South Qinling was the northern margin of the South China block, and the North Qinling was the southern margin of the North China block, separated by a Proto-Tethyan Qinling Ocean. The North Qinling evolved into an active margin when the Proto-Tethyan Qinling Ocean subducted northward during Ordovician time. Collision of the South and North Qinling took place in Middle Paleozoic along the Shangdan suture. Synchronous with the collision, rifting occurred at the southern rim of the South Qinling and was followed by the opening of the Paleo-Tethyan Qinling Ocean during the Late Paleozoic, resulting in the splitting of the South China block from the South Qinling. Collision of the South Qinling and the South China block came about in the Late Triassic along the Mianlue suture. The Late Triassic collisional orogeny caused extensive fold-and-thrust deformation and granitoid intrusions throughout the Qinling, and led to final amalgamation of the North and South China blocks.
Fig. 14. Existing evidence for timing of collision of the North and South China blocks. (a) Evidence favoring Middle Paleozoic collision. (b) Evidence supporting Late Triassic collision. NCB=North China block; SCB=South China block; SQ=South Qinling; NQ=North Qinling; SS=Shangdan suture; MS=Mianlue suture; UHP=ultra-high-pressure.
1. Late Neoproterozoic¯Early Paleozoic
Prior to Late Neoproterozoic time, the Qinling experienced a tectonic cycle from Paleoproterozoic¯Early Mesoproterozoic intra-continental rifting to Late Mesoproterozoic¯Early Neoproterozoic convergence (Gao et al., 1996). Continental rifting resumed from the Early Sinian time (Late Neoproterozoic). The Early Sinian (800¯700 Ma) rifting brought about breakup of the Qinling continent and opening of the Proto-Tethyan Qinling Ocean, separating the North from South Qinling. Rift-related bimodal magmatism and sedimentation are characteristic of the South Qinling. Volcanites and tillites rest unconformably on the transitional basement and are then overlapped by Late Sinian¯Early Paleozoic marine siliciclastics and platform carbonates (Fig. 15). Rifting became more intense in the southern subzone from the Early Ordovician, as manifested by coeval turbidite sedimentation and alkaline and basic extrusion . The North Qinling underwent a transition from passive to active margins during the Ordovician as a result of northward subduction (present orientation) of the Proto-Tethyan Qinling Ocean ( Zhang, 1988). The Danfeng arc massif evolved along its southern margin , with synchronous development of forearc ( Meng et al., 1997) and backarc basins ( Sun et al., 1996).
Fig. 15. A model showing sedimentation, basin development, and tectonic evolution of the Qinling orogen since Late Neoproterozoic times. See the text for detailed explanation. B=basin; M=margin; SS=Shangdan suture; MS=Mianlue suture; TQO=Tethyan Qinling Ocean.
2. Middle Paleozoic
Initial collision of the North and South Qinling took place in Middle Paleozoic times. Some remnant basins may have survived the collision at embayments due to irregular configurations of leading edges of the approaching margins, as exemplified by continuation of Silurian¯Lower Carboniferous marine sedimentation in some individual areas along the Shangdan suture. The back-arc basin in the North Qinling was closed at the same time, and intruded by collision-related biotite granites.
The collision along the Shangdan suture did not exert a strong influence upon the interior of the South Qinling, since no significant stratigraphic breaks are present in the Lower¯Middle Paleozoic successions in the middle subzone (Fig. 15). Crustal uplifting, however, might affect both the southern subzone of the South Qinling and the northern edge of the South China block since Late Silurian time because there exists a regional parallel unconformity between the Lower¯Middle Silurian and overlying strata. Rifting occurred along the southern rim of the South Qinling during the Devonian and Carboniferous and was accompanied by alkaline extrusions and turbiditic sedimentation.
3. Late Paleozoic¯Middle Triassic
Continued collision along the Shangdan suture closed the remnant basins in Late Paleozoic time and led to deposition of molasses within the North Qinling. Collision-related biotite granite intrusions aged from 345 to 315 Ma also occurred along the Shangdan suture. A foreland basin was developed south of the Shangdan suture in Early Carboniferous time in response to the south-directed thrusting of the North Qinling ( Hu et al., 1993).
Notwithstanding compressive deformation at the Shangdan suture, the southern South Qinling underwent crustal stretching and subsiding, and received marine deposition from Carboniferous to Middle Triassic times (Fig. 15). Following the Devonian rifting at the southern rim of the South Qinling, the Paleo-Tethyan Qinling Ocean was created and gradually split the South China block from the South Qinling. The Paleo-Tethyan Qinling Ocean was connected with contemporaneous southern Kunlun Ocean to the west ( Yang et al., 1996), and together they were a northernmost branch of the Paleo-Tethyan realm ( Fig. 16).
Fig. 16. Diagram showing genetic linkage between the Qinling and adjacent orogens to the west. Note that the Mianlue suture (MS) is connected with the Southern Kunlun suture (SKS), and the South Qinling and the Kunlun orogens together represent a northern branch of the Paleo-Tethyside orogenic system.
4. Late Triassic¯Cenozoic
Collision of the South Qinling and the South China block took place along the Mianlue suture during Late Triassic time and the whole Qinling was affected, as manifested by formation of thrust¯fold systems and extensive granite intrusions (Zhang et al., 1996). A foreland basin was formed at the northern edge of the South China block because of southward thrusting of the South Qinling in Late Triassic¯Middle Jurassic times. South-directed thrust propagation became more pronounced in eastern Qinling in Late Jurassic time, and eventually led to burial of most parts of the Mianlue suture. In contrast, the northward thrusting in the Liuba terrane may give a chance for the western segment of the Mianlue ophiolitic complex to have been preserved. The foreland basin strata were folded in Late Jurassic time and unconformably overlain by Lower Cretaceous beds. Rapid uplifting happened during Late Cretaceous and Cenozoic, and was accompanied by intermontane basin development ( Xue et al., 1996) and strike¯slip faulting (Peltzer et al., 1985)
