Tertiary Shortening along the Eastern Portion of the North Qaidam Thrust System

The Qaidam Basin lies within the northeastern portion of the Tibetan Plateau and is bounded on its northern and southern margins by high mountains built by active, basin directed fold and thrust belts. The development of these thrust belts and Qaidam Basin is essential in understanding the Cenozoic evolution and growth of the northern Tibetan Plateau. However, timing of initiation and total magnitude of shortening along these thrust belts remain poorly constrained. Geologic mapping along the eastern portion of the north dipping North Qaidam thrust system has revealed significant Tertiary shortening concentrated along three primary structures. From SW to NE they are: 1) NW trending km scale folds of Tertiary and Mesozoic sedimentary rocks along the northern margin of the Qaidam Basin, 2) The SW directed Shiashiteng thrust fault which places Paleozoic volcanic and sedimentary rocks over folded Tertiary and Mesozoic rocks, and 3) The S-SW directed Ho Lao thrust fault, 15 km to the northeast of the Shiashiteng fault, which places Proterozoic gneisses over heavily deformed Triassic sedimentary rocks. Minimum constraints on the magnitude of shortening along each of these structures are as follows: Palinspastic reconstruction of folded Tertiary and Mesozoic sediments within the fold belt yields ~9 km of shortening. Field data and interpretation of satellite photos indicate the hanging wall of the Shiashiteng fault has overridden the NW portion of a large anticline. Projection of the anticline beneath the hanging wall yields a minimum of 9 km of SW directed thrusting along the fault. The Ho Lao fault has had a minimum of 4 km of southeastward displacement based on balanced cross sections across the fault, although the total amount of shortening is probably much greater. These constraints yield a minimum of 22 km of S-SW directed shortening during the Tertiary along the North Qaidam thrust system. Initiation of shortening along the North Qaidam thrust system is interpreted to have occurred during the Eocene based on coarse conglomerates and sandstones of Eocene age at the base of the Tertiary section with S-SW directed paleocurrent indicators. This earlier phase of deformation is interpreted to have been accommodated along the Ho Lao thrust fault and other thrusts along strike to the east. Deformation then migrated southward to the Shiashiteng fault. At least 18 km of SW directed shortening has occurred along these structures within the past 5 m.y. based on the presence of Early Pliocene conglomerates and sandstones in both the folds along the northern Qaidam and the footwall of the Shiashiteng fault.