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

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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.