The Digital Terrain Anlysis And Uplift Mechanism Of Qilian Mountains Based On GIS

The uplift process, mechanism and effects of orogen have been a hotspot ingeo-science for a long time. And the relationship between tectonic uplift and erosionis the key to resolve these problems. In the past, there were many studies, whichexplored evidences for the uplift of Tibetan Plateau and Qilian Mountains, in geology,geomorphology, atmospheric science and biology. However, there was still not aconsistent opinion about some problems, such as, whether there was a uniformpeneplanation in Qilian Mountains or not? The main cause of peneplanationdeformation and uplift is tectonic movement or climate change and erosion isostaticeffect? How did the typical coupled mountain with basin geomorphological form beshaped? In the research of the relationship between tectonic uplift and erosion, it wasvery difficult to quantify the amount of uplift directly. However, the uplift of steppedgeomorphic surfaces provided the beginning and ending position of paleo-relief. Byrebuilding paleo-relief, we can calculate the amount of erosion after steppedgeomorphic surfaces deformation and it is also benefit advanced studies for exploringquantitative relationships between uplift and erosion.In rebuilding paleo-relief, it is very important to make sure the stage ofpeneplanation. Whereas the fault have very important influence on partition ofdifferent grade peneplanation, we used some methods such as horizontal curvature,vertical curvature and shaded-relief images, to extract the distribution and propertiesof faults in Qilian Mountains successfully. Then we made overlay analysis on spatialrelationships between faults and different grade peneplanation surfaces. The resultsshowed that there was only 20 percent faluts which cut across different gradepeneplanation surfaces at different altitude and related sediments had multi-periodsand multi-cycled properties. It indicated that those geomorphic surfaces distributing atdifferent altitude in Qilian Mountains were not a uniform planation which wasdisturbed by tectonic movements, but a result of different tectonic movement cycles.The auto-identification and extraction of stepped geomorphic surfaces was alsothe key to rebuild paleo-relief exactly. Studies also indicated that, with scale-up ofstudy area and increase of complexity in geomorphical structure, simple classification methods based on image process were not enough to complete classification ofcomplex geomorphical forms. With introducing slope position index and space-scale,we could resolve this problem well. After statistics, the results suggest that from westto east, from north to south, the elevation of Summit Surface decreased, and itsslope was about 8°. However, the elevation of Main surface decreased fromnorthwest to southeast and its slope was about 4°. Based on these quantitativecharacters, we can rebuild paleo-relief easily.The interpolated calculation based on the result of auto-classification is the laststep in rebuilding paleo-relief. The calculation indicated that surface has been raised600m after the erosion surface broke. The amount of uplift caused by isostatic effectwas 124m which occupied 21% of total amount of uplift. To Main Surface andSummit Surface, the total amounts of uplift were 1550m and 2150m respectively afterthe two surfaces broke, the amounts of uplift caused by isostatic effect were 376m and428m, which occupied 24.3% and 19.9% of total amounts of uplift. Since the crustdepth measured by using artificial earthquakes showed that crust depth correspondedwith elevation of surface, the main reason of the tectonic uplift in Qilian Mountainswas isostatic uplift caused by curst shortening and thickening rather than surfaceerosion.According to the fault strike, we calculated the direction of regional main stressis N32°E, consisting with the direction of compression from southwest to northeastof Indian plate. The character of the arched deformation of Main Surface also showedthat Indian plate continued pushing northwestwards after it collided with Eurasianplate. And it directly caused the surface uplift and the crust shortening and thickeningin Qilian Mountains. It was necessary for the formation of coupled mountain-basingeomorphological form that the discontinuous push blocked by pre-formed transversefaults forming strong compress-strike motion and changed lateral slip motion tovertical strike motion.