PALEOEARTHQUAKES ALONG THE ALTUN ACTIVE FAULT AND ITS SEGMENTATION
阿尔金活断层的古地震与分段

The Altun active fault is a large strike-slip fault with manifest dislocations.The newly activities of the fault are mainly along a principal sliding plane. This principal sliding plane extends from Xorkol on west, through Kuyak, Mandalike,Qingshuiquan, Akatutage, Horkor, Annanba, Aksay and Subei eastward to Kuantanshan, the whole length being about 1600km. Along this principal sliding plane, a series of surface ruptures, deformations and paleoearthquake traces can be found.The existence of numerous paleoearthquake traces proved that the Altun fault has been a very active fault in Holocene. The most active segments were KuyakAqiang, Tula-Huangtuquan, and Uzunxiaol-Aksay.Many clear paleoseismic deformation belts are preserved along the Altun fault.The so called paleoseismic deformation belt refers to the areas where residual deformations caused by one or more paleoseismic events are concentrated and preserved. Twelve paleoseismic belts have been identified, and they are further divided into 28 sub-segments in terms of differences of deformation and other paleoseismic traces.As a whole, the Altun active fault is quite continuous with a flat wave-shaped trace. she secondary faults are obviously of en echelon arrangement, predominantlyin the form of right-step. The segmentation of the Altun fault is of hierarchy, i.e.,a large segment can be divided into subsegments at several levels. Three-level segmentation is suitable for the fault.Segments at the largest level are determined on the basis of active tectonicunits and fault activity. The entire Altun fault is made up of four large segment&;arranging in the form of right-step. Among the four large segments there existthree large barriers: the Tula dual stepover, the Akatuta stepover and the Yulekenstepover. The distribution of paleoseismic ruptures also indicates the existence ofthe four segments because paleoearthquakes at different segments took place in different time intervals.Segmentation .at the second level is based on the rupture coverage of the largest earthquake. The Altun fault is located in the highly compressional belt betweenthe Tibet Plateau and the Tarim Basin, along which there are a lot of lensing com-pressional structural blocks. New fault ruptures have been developed in the arc margins of these compressional blocks. The Altun active fault was just formed by linking of these arc ruptures, and so show a wavelike extending as a whole. Now.at least eleven arc structural segments of the kind can be identified. They are varying in length and arrange in the form of right step, spaced out by some stopovers or interruptions. The convex parts of the arc structural segments point to theNEE direction because of the northward compression of the Tibet Plateau. The segmentation of the Altun fault is controlled by the arc structural segments: an arcstructural is right a persistent rupture segment.Each arc segment is often composed of several en echelon arranged secondaryfaults, which constitute th