Swarm Impact Of Tibet

Abstract:

Abstract

An unusual finding of ultrahigh-pressure (UHP) nitrides like cubic boron nitride, osbornite in association with micro-diamond with Os, Ir inclusion, coesite and stishovite, boron carbide along with other complex alloys, have been reported in the past from Luobusa ophiolite of southern Tibet. These minerals were found at shallow depths in the placer deposits of the YarlungZangbo region(YZR). For the formation of these UHP minerals, pressure in the range of 4-10Gpa and temperature in the range of 700 ⁰ c to 1300 ⁰ c is required. Thus these minerals have been extruded from the deep upper mantle, or they have an impact origin. Dobrzhinetskaya [1] and Wen-JiBai [2] have favored the mantle origin because they did not find any evidence of impact. In the event of mantle origin, its hard to justify perfect preservation of coesite crystals, low oxygen fugacity (fO2 ), and it also faces the challenges concerning the source of nitrogen and boron in the deep mantle. Hence it is critical to re-examine the possibility of an impact. In the event of placer deposits, source craters can generally be traced many kilometers away from the findings. Hence for placer deposits of YZR, it is critical to consider a series of lakes in the region (82˚30'E, 29˚N and 90˚30'E, 33˚N). These lakes are connected to the YarlungZangbo river by multiple streams and small rivers. These lakes are unusually closely aligned and have a steep slope facing circular features. These lakes are interconnected by network of fractures and faults, hinting towards a series of impact craters. These lakes have diameter from 1 Km to 65 Km and are linearly aligned. The unusual shapes of these lakes, and spread over such a large area is possibly an outcome of a swarm impact by a comet at an extremely low impact angle. The low density coupled with low angle of impact will result in to the formation of “large flat-floored oblique craters”. Muong Nong and Indochinites reported from various parts of Asia, strongly support the oblique impact of comet on Indochina border. Our study suggests that the UHP nitrides, micro diamonds, stishovite, boron carbide and other UHP minerals reported from YZR are the result of oblique impact by comet in the Tibetan region between the latitudes of 82˚30'Eand 90˚30'E and 29˚N and 33˚N.

1)Dobrzhinetskaya(2009) PNAS 106, 19233-19238

2)Fang (2007) Geology 35, 875-878.