Relict Back-Arc Basins: Principles of Recognition and Possible New Examples from China

Abstract

The Junggar, Tarim, and Qaidam Basins are commonly considered to be cratonic blocks surrounded by orogenic belts and have thus been called intermontane basins. The author proposes that those basins be compared to the Black Sea and Caspian Basins, and suggest that Junggar was formed during Carboniferous Period, and Tarim and Qaidam during Permian period, as back-are basins behind volcanic arcs on the southern active margin of Paleozoic Asia. The relatively thin crust and the presence of very large positive magnetic anomalies under the Mesozoic sediments of those basins indicate that their deepest depressions are floored, at least in part, by oceanic rocks. The oldest sediments in those basin are very likely marine shales. After arc-continent collisions during Late Paleozoic and Triassic period, the basins became partially enclosed, and the euxinic sediments in those partially restricted basins could well be the source beds of the crude oils found recently in major oil fields of those basins. Junggar, Tarim, and Qaidam became inland basins with continental sedimentation after their communications to open sea were severed by the rising mountain chains. Isostatic basin subsidence permitted the accumulation of thick Mesozoic and Paleogene sediments before tectonic rejuvenation along Neogene faults.