Timing of Collision
Although as one of the most important orogenic belts in China Qinling has been intensively studied (Li et al., 1978; Mattauer et al., 1985; Hsu et al., 1987; Zhang, 1988; Xu et al., 1988; Zhang, et al., 1995, 1996), the timing of the joining of the North and South China blocks has been debated for more than a decade. The controversies obviously stem from different approaches to reconstruction of the integration history. Two contrasting lines of evidence yield two different ages for collision of the North and South China blocks--middle Paleozoic and Late Triassic. The Shangdan suture within the Qinling was regarded in previous studies as the trace along which the North and South China blocks collided. However, there are two sutures within the Qinling: the well-documented Shangdan suture and the newly discovered Mianlue suture. The Late Proterozoic to early Mesozoic evolution of the Qinling involved interactions between the North China block, the North and South Qinling orogens, and the South China block. The middle Paleozoic collision along the Shangdan suture, as constrained by some evidence, accreted only the South Qinling orogen to the southern part (i.e., the North Qinling) of the North China block. Contemporaneous rifting of the South China block and subsequent drifting separated the South Qinling from the South China block during the middle to late Paleozoic. The separation of the South from the North China blocks is supported by other evidences, in particular, geomagnetic data. Evidently it was the Late Triassic collision of the South China block with the South Qinling orogen along the Mianlue suture that led to final integration of the North and South China blocks.
Fig. 11 Simplified tectonic framework of Qinling orogen. SSZ-Shangdan suture zone; MSZ-Mianlue suture zone ( Meng et al., 1999).
1. Evidence for the Middle Paleozoic collision
Evidence favoring collision during the middle Paleozoic comes exclusively from geologic and geochronologic investigations of both the Shangdan suture zone and the North and South Qinling orogen. Mattauer et al. (1985) demonstrated ca. 315 Ma sinistral strike‑slip faulting within the Shangdan suture zone, and deduced that the paleo‑ocean basin (proto‑Tethyan Qinling ocean in Fig. 13A) between the North and South Qinling orogens was closed before Middle Devonian time and that an intracontinental tectonic regime had been established by the Carboniferous, Ms deduction is supported by Paleozoic collision‑ related granitoids along the Shangdan suture zone, which have been dated as 383 ± 8 to 345 ± 11 Ma and are considered to have resulted from the underthrusting of South Qinling continental crust beneath the North Qinling in the Devonian (Zhang et al., 1997). Kroner et al. (1993) also inferred an Early Silurian continental collision and crustal thickening event at the Shangdan suture zone by studying Tongbai granulites in the eastern Qinling orogen. Benthic faunas in the South Qinling orogen were distinct from those of the North China block during the early Paleozoic, but become mixed with faunas of the North China block in the Devonian. Moreover, geochemical studies were recently carried out on fine‑grained deposits in the Paleozoic basins of the northern part of the South Qinling orogen (Gao et al., 1995). The results suggest that the South China block had been accreted to the North China block along the Shangdan suture zone in Silurian‑Devonian time from the fact that the South Qinling orogen began to receive sediments from the North Qinling, the leading edge of the North China block, in the middle Paleozoic.
2. Evidence for the Late Triassic collision
2.1 Paleomagnetism
In contrast, paleomagnetic data, along with geochronologic study of ultrahigh‑pressure metamorphic rocks in the Dabie metamorphic terrane in the eastern part of the Qinling orogen (Fig. 11), do not support the deduction of middle Paleozoic accretion of the South China block to the North China block, but attest to the Late Triassic collision of the blocks. The North China block proved to be separated from the South China block during the Permian, and the separation presumably persisted until the Late Triassic, because the two blocks began to share the same paleomagnetic pole in the Jurassic, indicating that the collision must have taken place prior to the Jurassic. However, all the samples for the paleomagnetic studies on the timing of collision of the North and South China blocks were collected exclusively within the interiors of the blocks. No paleomagnetic data were obtained from the South Qinling area, possibly due to multiphase, intense deformation of the strata there. Paleomagnetic data correlate well with geochronologic study of ultrahigh‑pressure rocks in the Dabie terrane. Coesite‑ and diamond‑bearing eclogites are believed to result from continentcontinent collision; thus, their metamorphic ages can be used to constrain the timing of the collision of the North and South China blocks. A number of ages of these ultrahigh‑pressure rocks have been obtained and range from 232 to 209 Ma (Li et al., 1993; Ames et al., 1993, Okay and Sengor, 1993), suggesting that the North China block collided with the South China block during the Late Triassic.
2.2 Suture Zones
Extensive and more detailed geologic mapping throughout the Qinling mountain region has revealed another suture, called the Mianlue suture zone, at the southern rim of the South Qinling orogen (Zhang et al., 1995. 1996). Thus, there are two Phanerozoic suture zones in the Qinling orogen (Fig. 11), the well‑documented Shangdan suture between the North and South Qinling orogen and the newly discovered Mianlue suture between the South Qinling and the South China block. These two sutures have very different evolutionary histories, and it is thus important to compare their tectonic processes so as to reasonably reconstruct the Qinling tectonic evolution and to put constraints on the timing of the collision of the North and South China blocks.
Fig. 12. Geologic map (A) and cross section (B) of western segment of Mianlue suture zone, showing dismembered ophiolites and associated sedimentary rocks of differing ages. Internal structure is dominated by south‑directed thrusts (Meng et al., 1999). Map location is shown in Fig. 11.
As demonstrated here, the Shangdan suture zone records a middle Paleozoic collisional event, resulting from the closing of the paleo‑ocean basin between the North and South Qinling orogens. Recent studies, however, show that the North and South Qinling collision along the Shangdan suture zone was a prolonged and complicated, three-stage process (Zhang et al., 1995). The middle Paleozoic collision represented only the initial‑stage collision and presumably mainly occurred in the eastern segment of the suture because the collision‑related structures (Mattauer et al., 1985) and granulites (Kroner et al., 1993) were primarily observed in the east and rarely found in the west. The subsequent two stages, lasting from the late Paleozoic to Late Triassic, are characterized by the closing of some remnant basins within the suture zone and by the westward migration of the suturing (Yin and Nie, 1993). In particular, the late stage (Late Triassic) was coincident with the collision of the South Qinling orogen and the South China block along the Mianlue suture zone and was accompanied by widespread intrusion of collision‑type granites (Zhang et al., 1995).
The Mianlue suture zone is along the southern rim of the South Qinling orogen and is manifested as a zone of ophiolites composed of ultramafic rocks, gabbros, metabasalts, and radiolarian cherts (Fig. 12). The metabasalts show no Eu anomaly and yield average (La/Yb)N, (Ce/Yb)N, and dEu values of 0.51, 0.54, and 0.99, respectively. The rare earth element (REE) distribution displays depletion in the light REEs, showing characteristics of typical normal (nonplume) mid‑oceanic ridge basalts (Lai et al., 1996). The dismembered Mianlue ophiolites are now mixed with Devonian‑Permian metasedimentary rocks, but chronology of the ultramafic and volcanic rocks is still poorly constrained. The metavolcanic rocks give metamorphic ages of 242 ± 2 to 221 ± 13 Ma, agreeing well with the ages of collision‑type granites dated as 206 ± 2 to 220± 2 Ma. Nevertheless, radiolarians in the cherts in association with the ultrarmafic and mafic volcanic rocks give an age of Carboniferous, suggesting that the ocean basin probably came into existence during the Carboniferous. This deduction is further supported by the occurrence of an inferred breakup unconformity, usually regarded to coincide with the transition from rifting to drifting, between the Devonian and the Carboniferous‑Permian strata on the southern side of the Mianlue suture zone (Meng et al., 1996). These facts prove that the Mianlue suture zone was the product of the closure of a paleo‑ocean between the South Qinling orogen and the South China block in Late Triassic time.
3. Tectonic Development
The Qinling orogen tectonic evolution can be envisaged as a prolonged and complex interaction between the North China block, the South China block, and the South Qinling orogen. The Shangdan suture zone is the trace of closure of the proto‑Tethyan Qinling ocean (Fig. 13A), once separating the South Qinling (northern margin of the South China block) from the North Qinling (southern margin of the North China block) orogens between Late Proterozoic and early Paleozoic time (Zhang, 1988). The separation is evidenced by the complete differences in stratigraphy and sedimentation between the North and South China blocks since the Late Proterozoic. For example, the late Sinian (800‑‑600 Ma) platform carbonates that are widespread over the South China block and the South Qinling orogen contrast strikingly with the equivalent, less‑developed elastic rocks of the North China block. Subduction of the proto‑Tethyan ocean beginning in the Ordovician resulted in the development of an arc‑trench system and a backarc basin in the North Qinling orogen, which then evolved into the southern active continental margin of the North China block (Fig. 13B). The South Qinling orogen, however, was concomitantly in an extensional setting and served as the northern passive continental margin of the South China block. Rift sedimentation and alkalic magmatism were active along the southern edge of the South Qinling orogen from Ordovician to Silurian time (Huang et al., 1992), implying that a rift system had developed there.
Fig. 13. Integrating processes of the North and South China blocks through Qinling orogen Q; S = south, N = north. Note simultaneous subduction and collision at northern edge and rifting and drifting at southern edge of South Qinling orogen through Paleozoic. South Qinling orogen had been accreted to North Qinling orogen at end of Middle Devonian, but simultaneously drifted apart from South China block. Separation had been maintained by paleo‑Tethyan Qinling ocean until Late Triassic, when South China block was finally integrated with North China block through collision along Qinling orogen. NCB = North China block; SCB = South China block; SSZ = Shangdan Suture Zone; MSZ = Mianlue Suture Zone (Meng et al., 1999).
Coeval with the Devonian collision at the Shangdan suture zone, the southern edge of the South Qinling orogen started to be gradually rifted away from the South China block (Fig. 13C), and an ocean basin developed there during the Carboniferous and Permian (Fig. 13D). As a result, the South Qinling orogen was separated from the South China block during the Carboniferous to Early Triassic by the paleo‑Tethyan Qinling ocean (Fig. 13D), which might have been part of the northern paleo‑Tethyan realm and thus linked to the contemporaneous South Kunlun paleo‑ocean to the west. Convincing evidence for the separation is the contrasting stratigraphic development of the South Qinling orogen and the South China block beginning in the Devonian. Subduction of the paleo‑Tethyan Qinling ocean is assumed to have started in the Early Triassic (Fig. 13E) and resulted in the development of island‑arc calc‑alkalic volcanic rocks (Lai et al., 1996). As constrained by chronology of the metabasalts and collision‑type grantoids, collision between the South Qinling orogen and the South China block apparently took place in the Late Triassic (Fig. 13F). Concurrent collision‑related ‑granite intrusions are common along the Shangdan suture zone and might be genetically attributed to intracontinental collision and crustal thickening due to strong northward movement of the South China block in the Late Triassic.
