Change from Late Tertiary compression to Quaternary extension in southern Tibet during the India‐Asia Collision

Abstract

Field studies in Southern Tibet show that subsequent to folding resulting from the India‐Asia collision (50–40 Ma), reverse faults and backthrusts formed as a result of NNE‐SSW and NNW‐SSE trending compressions. They affect conglomerates of Oligocene‐Miocene age and are probably contemporaneous with shearing on the Himalayan Main Central Thrust and folding of Paleogene basins along the northern border of Southern Tibet (25–16 Ma). Rift systems postdate compressional structures. They were active during the Pleistocene and probably during the Pliocene (5–2 Ma). Change from a compressional to an extensional regime in Tibet probably occurred during the Late Miocene (10–5 Ma). This change is tentatively correlated with uplift inferred by Chinese workers from Paleogene to present‐day variations of the Tibetan vegetation. Recent and active faulting in the High Plateau results from a WNW‐ESE striking extension; this is in agreement with Tibetan focal mechanisms. The Himalayan Lowlands are subjected to a NNE‐SSW trending compression as demonstrated by superficial deformations and seismicity. Therefore the horizontal maximum compressive stress trajectory is roughly parallel to India‐Asia convergence, σ Hmax being σ 1 in the Lowlands and σ 2 in the High Plateau. This change results from the increase of the vertical principal stress value due to the weight of the elevated mass, σ zz is σ 3 in the Lowlands and σ 1 in the High Plateau. Thus the third principal stress axis is σ 2 in the Lowlands and σ 3 in the High Plateau, allowing extension to occur in the WNW‐ESE direction. Extensional tectonics are significantly different in Southern and Northern Tibet. Southern Tibet being constrained between the Himalayan syntaxes, the E‐W extensional principal deviatoric stress value σ 3 is of rather small magnitude, and the N‐S trending σ 2 value is extensional, as demonstrated by analysis of Recent faults. Normal components prevail along all the azimuths; in the field this favors formation of rift systems of large axial extension. On the contrary, north of the Himalayan syntaxes, Northern Tibet “flows” easily eastward. The E–W extensional principal deviatoric stress value σ 3 is of higher magnitude, and the N‐S stress σ 2, is compressional, as shown by inversion of focal mechanism data. This favors strike‐slip components on faults which deviate from the N‐S direction, as expressed in the field by shorter N‐S trending graben linked with normal strike‐slip faults. These faults permit some N‐S shortening in Northern Tibet, without thickening, in response to India‐Asia collision. In high plateaus which are subjected to lateral compression such as the Andean Altiplano and Tibet, the extensional direction is basically controlled by the convergence direction, being roughly orthogonal to the latter.

Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau

“…Thus reactivation of these structures probably began prior to the transition from N-S compression to E-W extension of the Tibetan Plateau between 5 and 10 Ma (Mercier et al, 1987;Pan and Kidd, 1992). Uplift of the Tibetan Plateau itself remains a topic of some debate (Molnar, 1989;Harrison et al, 1992;Burbank et al, 1993;England, 1993;Turner et al, 1993), as timing constraints are indirectly inferred from palaeobotanical studies, thermochronological data or lithospheric modelling.…”

Evolution of North Himalayan gneiss domes: structural and metamorphic studies in Mabja Dome, southern Tibet

“…Le Fort, 1975;Searle, 1983;Ratschbacher et al, 1994;Quidelleur et al, 1997;Searle et al, 1997;Yin et al, 1994Yin et al, , 1999 and extensional structures (e.g. Molnar and Tapponnier, 1975;Armijo et al, 1986;Mercier et al, 1987;Ratschbacher et al, 1994) in a variety of orientations.…”

Oligocene-Miocene middle crustal flow in southern Tibet: geochronology of Mabja Dome

“…Le Fort 1975;Searle 1983;Ratschbacher et al 1994;Yin et al 1994Yin et al , 1999Quidelleur et al 1997;Searle et al 1997a;Godin et al 1999) and extensional structures (e.g. Molnar & Tapponnier 1975;Armijo et al 1986;Mercier et al 1987;Burchfiel et al 1992;Ratschbacher et al 1994) in a variety of orientations.…”