Crustal shortening on the margins of the Tien Shan, Xinjiang, China

We present the results of mapping selected cross-sections across the margin s of the Chinese Tien Shan, an intracontinental mountain belt that formed i n response to the India-Eurasia collision. This belt contains significant l ateral variation in topography, structure, and stratigraphy at all scales, and our estimated rates of shortening also reveal a distribution of shorten ing that varies laterally. At the largest scale, it consists of two major h igh mountain ranges in the west that merge eastward into a complex, single high mountain belt with several distinct ranges, then separates farther eas tward into several low mountain ranges in the south and a single narrow hig h mountain range in the north. Active fold-and-thrust belts along parts of the north and south flanks of the Tien Shan involve only Mesozoic and Cenoz oic sedimentary cover, which varies in both stratigraphy and structure from east to west. The southern fold-and-thrust belt decreases in width and com plexity from west to east and ends before reaching Korla. The northern belt begins near the longitude where the southern belt ends, and increases in w idth and complexity from west to east. Within these two fold-and-thrust bel ts are both E-W and N-S variations in stratigraphy at the scale of the fold -and-thrust belts and across individual structures. All these variations ma ke it very difficult to generalize either structure or stratigraphy within the Tien Shan or within local areas. Four maps and cross-sections, two across each of the northern and southern fold-and-thrust belts, imply different magnitudes of shortening. In the eas tern part of the northern belt, a cross-section along the southern part of the Hutubi River yields shortening of 6.2 km, and a section to the north ac ross the Tugulu anticline yields shortening of 5.5 km. The two parts of the cross-section cannot be added because the Tugulu anticline lies 20 km west of the Hutubi River, and diminishes greatly in amplitude toward the Hutubi River. In the western part of the northern belt, cross-sections require 4. 6 to 5.0 km of shortening at Tuositai and 2.12 to 2.35 km across the Dushan zi anticline. The Tuositai structure lies south of the Dushanzi anticline, but shortening in these two areas also cannot be summed, because they seem to be separated by a N-trending strike-slip fault. In the western part of t he southern fold-and-thrust belt, an incomplete cross-section along the Kal asu River suggests shortening of 12.1 to 14.1 km. If the estimated shorteni ng of 6 to 7 km in the Qiulitage anticline, which we did not map, is added, the total shortening in this cross-section would be similar to 18 to 21 km . To the east, a complete cross-section at Boston Tokar yielded shortening of 10.3 to 13.0 km. Calculating long-term shortening rates from these four cross-sections is di fficult, because the time of initiation of deformation is poorly known. In the Kalasu River area of the southern belt, there is evidence that limited shortening of 2 to 4 km occurred in the early Miocene, if major thickness c hanges in deposition of conglomerate unit 3b are interpreted to be growth s trata. Geological evidence suggests that most of the shortening began in bo th belts after the beginning of the deposition of the thick conglomerate un it drown as lower Quaternary on Chinese geological maps. Strata within the middle part of these conglomerates were deposited during the growth of the folds. Presence of Equus near the base of similar conglomerates indicates a Quaternary age, hut the fossil localities are far from most of our cross-s ections, and the contemporaneity of the rocks remains in question. The begi nning of conglomerate deposition may he controlled by climate change, and i f so, the beginning of conglomerate deposition may be generally contemporan eous throughout the region at similar to 2.5 Ma. Deformation began at some time after the onset of conglomerate deposition, but this time is not web c onstrained. Thus we have calculated shortening rates for 2.5, 1.6, and 1.0 Ma that should bracket maximum and minimum slip rates. These calculations y ield the following ranges in the northern fold-and-thrust belt: southern Hu tubi River=2.5 to 6.2 mm/yr: Tugulu anticline = 2.1 to 5.5 mm/yr; Tuositai anticline = 1.8-2.0 to 4.6-5.0 mm/yr: and Dushanzi anticline = 0.8 to 2.1-2 .4 mm/yr; and in the southern fold-and-thrust belt: Kalasu River = 4.6-5.6 (including the Qiulitage anticline = 7.2-8.4) to 12.1-14.1 (including Qiuli tage anticline = 18-21) mm/yr; and at Boston Tokar = 4.1-5.2 to 10.3-13.1 m m/yr. If 2 to 4 km of shortening occurred in the Kalasu River section durin g early Miocene time, the long-term rates for Quaternary time are 3.2-4.8 ( including Qiulitage anticline = 5.6-7.6) to 8.1-12.1 (including Qiulitage a nticline = 14-19) mm/yr. Calculation of the shortening rate across the entire width of the Tien Shal l is difficult because of the rapid lateral variations in structure ana bec ause of active deformation within the range, which we have not studied. The cross-sections at Boston Tokar in the south and Tuositai in the north lie along the same longitude. Adding the shortening rates in these areas would yield a minimum range (using 2.5 Ma as the initiation time) of 5.7 to 7.2 m m/yr. If deformation began at 1.6 or 1.0 Ma, the range of shortening rates would be 10-11.2 mm/yr to 14.9-18.1 mm/yr, respectively. Because the first indication of structural growth with the mapped areas occurs above the base of the conglomerates at the top of the stratigraphic succession, a minimum shortening rate greater than 5.7 to 7.2 mm/yr is more likely. Both the marginal fold-and-thrust belts have a thin-skinned geometry with t he decollement at similar to 6 to 10 km and within Mesozoic and Cenozoic se dimentary rocks. Toward the interior of the range the decollement must pass into the Paleozoic basement rocks and steepen beneath the Ranks of the ran ge. The structural style is similar to that in the Laramide Rocky Mountains and the California Transverse Ranges. The highest parts of the Tien Shan a re adjacent to areas of active shortening. Such a relation might suggest th at the major uplift of the Tien Shan is very young, mostly latest Cenozoic or Quaternary in age. The shortening across the Tien Shan is inhomogeneous and spatially distributed.