Normal faulting in central Tibet since at least 13.5 Myr ago

Abstract

Tectonic models for the evolution of the Tibetan plateau interpret observed east-west thinning of the upper crust to be the result of either increased potential energy of elevated crust or geodynamic processes that may be unrelated to plateau formation. A key piece of information needed to evaluate these models is the timing of deformation within the plateau. The onset of normal faulting has been estimated to have commenced in southern Tibet between about 14 Myr ago and about 8 Myr ago and, in central Tibet, about 4 Myr ago. Here, however, we report a minimum age of approximately 13.5 Myr for the onset of graben formation in central Tibet, based on mineralization ages determined with Rb-Sr and 40Ar-39Ar data that post-date a major graben-bounding normal fault. These data, along with evidence for prolonged activity of normal faulting in this and other Tibetan grabens, support models that relate normal faulting to processes occurring beneath the plateau. Thinning of the upper crust is most plausibly the result of potential-energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath Tibet. This is supported by recent geophysical and geological data, which indicate that spatial heterogeneity exists in both the Tibetan crust and lithospheric mantle.

Crustal–lithospheric structure and continental extrusion of Tibet

“…When taken with small total offsets (Taylor et al 2003), these relatively high slip rates imply that these structures have not been active for the entire period of the Indo-Asia collision, and possibly have been active only in the last 2-3 Ma (Taylor & Peltzer 2006). This is in contradiction to geochronological constraints of the onset of east-west extension at c. 8 Ma (Harrison et al 1995) or earlier at c. 13.5 Ma for normal faulting (Blisniuk et al 2001), or the initiation of slip on the Jiali Fault at between 18 and 12 Ma (Lee et al 2003), which suggests that movement on the central Tibetan faults may have accelerated through time. Meade (2007) did not attempt to resolve each of the conjugate faults on the plateau with the 17-microplate model, particularly in western Tibet where an arbitrary boundary was modelled as taking up 15 mm a À1 of right-lateral slip.…”

“…Since then, dating of over 50 volcanic samples across the plateau has shown that shoshonitic volcanism has occurred sporadically from the time of collision (50 Ma) to today (Chung et al 2005, suggesting that no sudden delamination occurred at c. 8 Ma. North-south aligned normal faults in Tibet were initially assumed to be Pliocene-Pleistocene (Armijo et al 1988) and c. 13.5 Ma in age (Blisniuk et al 2001), but east-west extension is now known to have occurred across the entire plateau from c. 47 Ma to the present day based on dating of adakitic and ultrapotassic dykes . Early interpretations of a change from north-south compression to east-west extension in Tibet (e.g.…”

Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts

“…They are distributed mainly in the southern and central Tibetan Plateau [Molnar and Tapponnier, 1978;Armijo et al, 1986Armijo et al, , 1989Rothery and Drury, 1984], where topography reaches high altitudes. Field observations indicate that the magnitude of this extension is small (<1%) [Armijo et al, 1986] and rifting in southern and central Tibet started about 13.5-14 Ma ago [Yin et al, 1994;Coleman and Hodges, 1995;Harrison et al, 1995;Blisniuk et al, 2001].…”

Extensional Seismogenic Stress and Tectonic Movement on the Central Region of the Tibetan Plateau

“…The weight of a high plateau and the buoyancy acting on the bottom of the plateau crust from the mantle may create an additional vertical compressive stress which can result in the relaxation of the topography and corresponding thinning of the crust together with the normal faulting activity. The altitude of the Tibetan plateau is so high with respect to its surroundings that active normal faulting may also be caused by gravitational collapse toward the plateau margins [25,32,33] A large number of thrust and strike-slip fault events occur in the front of the Himalayan Mountains. Many strikeslip fault events occur in the Karakorum and Altyn mountains.…”

“…The characteristics and mechanism of extensional motions at the high altitudes of the Tibetan plateau have been analyzed [25][26][27][28][29]. The activities of normal faulting earthquakes concentrated at high altitudes were investigated based on distribution characteristics of stress field, GPS data, as well as geothermal data in this paper.…”