THE ALJIN-BEISHAN GEOWEDGE AND ITS TECTONIC SIGNIFICANCE

This paper proposes a new tectonic concept—"geowedge".The"wedge"structure refers to a wcdegeshaped geologic block that is sandwiched by two strikeslip faults converging in one direction and diverging in the opposite and has migrated an mass. The Aljin-Beishan geowedge is distributed in an ENE-WSW direction, with the southern part being pointed and the northern part wide. Its original position was located north of the Tarim basin and the Qaidam basin with the geowedge tip lying in the vicinity of Annanba. Through a slip over a long distance in the Cenozoic, the tip of the geowedge migrated to the northern margin of the Kunlun Mountains with-a migrated distance of about 800km. According to the existence of the Aljin-Beishan geowedge and geological processes, the author puts forwards a new recognition on the tectonics of northwestern China: before the insertion and migration of the geowedge, the eastern and western Kunlun fold belts had not been displaced but had been connected and had very harmonious strikes, forminga tectonic system;Tarim and Qaidam had been a unified platform, which was divided into two after the insertion of the geowedge:the Tianshan fold system and Qilian fold system had been a unified tectonic system, which was later separated, and the connecting part between them migrated southwards to form the present Aljin tectonic belt; the Sino-Korean extended westwards as far as the Junggar block and the Dunhuang fault uplift is the western part of the Alxa platform uplift that was cut off from the latter; the Beishan fold belt is the southmigrating part of the Outer Junggar and Alaty-Khingan and Mongolia fold system. Thus it is considered that northwestern China has a regular tectonic framework. The paper puts forwards a tectonic framework of "three geosynclines and two platforms",i.e. in the north there occurs the Outer Junggar and Altay-Khingan and Mongolia fold system, which is a part of the Central Asian-Mongolian arcuate geosynclinal fold region, and southwards there occur successvely the SinoKorean platform （which extends westwards to the Junggar block）, Tianshan-Qilian fold system, Tarim-Qaidam platform and Kunlun fold system. They are arranged parallelly and the geosyncline and platform appear alternately.