PROVENANCE ANALYSIS OF THE XORKOL BASIN DEDUCES SEQUENTIALLY YOUNGING TERTIARY PIERCING POINTS ALONG THE ALTYN TAGH FAULT

The ENE-trending, left-slip Altyn Tagh fault (ATF) defines the northern boundary of the Tibet Plateau, and is commonly perceived to have been playing an important role in accommodating India/Eurasia convergence in the Cenozoic. In order to define the fault’s slip history, a basin analysis has been performed in the Xorkol basin, a small Tertiary strike-slip basin bounded by the eastern strand of the ATF to the east.

Two detailed stratigraphic sections measured in the Xorkol basin are adjacent to the ATF. Both sections include an Oligocene formation (layer 1 to 20), a Lower Miocene formation (layer 21 to 58) and an overlying Miocene formation (layer 59 to 61). Paleocurrent data (n=325) from layer 1 to 61 indicate that paleo-flow direction was from the southern side of the ATF. Clast counts (n=1376) in the field and 73 clast samples illustrate four different types of clast assemblages. Four sequentially younging Tertiary piercing points with sequentially decreasing offsets were deduced by matching these assemblages with the basement terranes on the southern side of the ATF (Table 1).

These piercing points indicate that (1) there is 370±30 km Cenozoic offset near the eastern end of the fault; (2) at least 200 km and possibly more than 300 km of offset was accomplished in the Early Miocene and the Oligocene; and (3) the average Pliocene and Quaternary slip rate along the eastern strand of the ATF is at most 8±4 mm/year.

Table 1. Clast assemblages in the basin, source terranes on the southern side of the ATF and offsets indicated by sediment-source matches

Layer(age)	Clast assemblage in Xorkol basin	Source terrane	Offset
1-28 (Olig.-earliest Mio.)	metadacite, dolomitic limestone, micrite, dolomite, metasandstone, slate, and phyllite	Cambrian strata of North Qilian	370±30 km
32-45 (Early Miocene)	monzogranite, granodiorite, diorite, biotite gneiss, schist, phyllite, and marble	Danjinshankou	200±40 km
46-54 (Early Miocene)	alkali feldspar granite, syenogranite, granodiorite, biotite gneiss, schist, phyllite, and marble	40 km SW of Danjinshankou	160±30 km
61 (Miocene)	syenogranite, monzogranite, granodiorite, plagiogranite, gneissic amphibolite, schist, phyllite, and marble	Oboliang Bei Shan of NW Qaidam	40±20 km