The Tibetan Plateau as a Passive Wide-Rift System

摘要：

The extensional grabens in the Tethyan Himalayas and the Lhasa Block form a widespread system of roughly parallel seven major north-south trending grabens with similar spacing (125–195 km), length (300–500 km), and width (∼20 km). The huge size of the area covered by the grabens makes the Tibetan graben array one of the largest rift systems in the world. The review summarizes geological constraints on the time of extension in the Tibetan grabens, geodetic observations on lithosphere deformation, geophysical constraints on crustal and lithosphere thickness, as well as seismic, electrical resistivity and thermal models on the lithosphere heterogeneity in Tibet. These observations combined with numerical and analogue modeling of lithosphere extension show that the evolution of the Tibetan graben array follows general trends for formation of periodic systems of normal faults and grabens during lithosphere extension, if certain conditions are met.

Critical pre-requisites for formation of a parallel graben array are the presence of a strong brittle upper crust above a very thick rheologically weak lower crustal layer and a long-lasting, low-rate extension. The presence of an exceptionally thick (70–80 km) crust with a 40–50 km thick ductile lower crust played a critical role in the Tibetan grabens formation. The graben spacing is controlled by mechanical thickness of the brittle layer, which also controls maximum displacement along the grabens, as well as by viscosity of the ductile lower crustal channel and the total area subject to long, low-rate extension. A comparison of the lithosphere structure below the Tibetan graben array with numerical and analogue modeling of lithosphere extension, as well as with geological observations of extensional systems from other continental locations allows for constraining the lithosphere viscosity structure in Tibet, estimated as ca 1020 Pa s.

The growth of the overthickened crust during the India-Eurasia hard collision contributed to the isostatic raise of the high plateau and created favorable conditions for formation of the system of extensional grabens caused by gravitational spreading of ductile material. Vertical stresses produced during the lithosphere extension created undulating seismic velocity and electrical resistivity anomalies at the lower crustal depths, and lithosphere deformation produced periodic geochemical anomalies with a similar spacing. The 250–350 km spacing of the alternating seismic velocity anomalies at 30–70 km mid-lower crustal depth below the Tibetan graben array closely matches the pattern of the alternating electrical resistivity anomalies at the same depths, but does not match the much smaller graben spacing at the surface in agreement with theoretical models.

In line with earlier publications, this review argues that the Tibetan grabens have been formed as a wide rift system, without leading role of the upper mantle lithosphere. The relict nature of the grabens, which were largely active between 18 Ma and 8 Ma despite the on-going Indian plate subduction, supports passive origin of their formation with overthickened crust, stratified rheology and low-rate extension as critical pre-requisites. In the Qiangtang Block with a thinner crust and hotter geotherms the conditions favorable for formation of a parallel graben system have not been met, and lithosphere extension produced only faults, but not grabens.

基本信息