LATE CENOZOIC TRANSPRESSIONAL BASIN DEVELOPMENT WITHIN A MESOZOIC CONTINENTAL RIFT PROVINCE, GOBI ALTAI, MONGOLIA

In the Gobi Altai region of southern Mongolia, transpressional basins are actively forming in an intraplate, intracontinental setting. Individual basin evolution is directly coupled to Late Cenozoic development of adjacent mountain ranges but an earlier Jurassic-Cretaceous basin history affected the entire region. Key questions are: 1) how does the modern basin architecture reflect the underlying inherited Mesozoic basin system and, 2) has Cenozoic transpressional deformation involved reactivation of Mesozoic normal faults?

Mesozoic sediments are exposed along range margins and in topographically inverted belts within range interiors. Uninverted basin-bounding normal faults occur adjacent to the Ih Bogd range and syn-sedimentary extension during Mesozoic alluvial and lacustrine deposition is suggested by thickness distributions around larger faults (offsets >50m) and thickness changes across smaller-scale faults (offsets <5m). South of Nemegt Uul, a Late Cretaceous lacustrine-deltaic succession also contains small normal faults (offsets <50cm). Along the southern Nemegt range front, a south-directed Cenozoic thrust fault places Palaeozoic rocks above overturned Mesozoic sediments, but does not appear to reactivate any pre-existing structure. The most convincing examples of normal fault reactivation are where basinal thrust faults near mountain fronts are directed back towards the front. Therefore, the SE Mongolian Mesozoic rift system extended into the Gobi Altai region, but reactivation of normal faults played a limited role in Cenozoic uplift and basin development.

The largest Cenozoic fan systems are linked to drainages that penetrate deep into the range, have a complex architecture at the range front and incise Mesozoic sediments beyond the front. The 3-D architecture of the youngest basin fill successions is largely unrelated to the geometry of the older basins although Mesozoic facies distributions and palaeoflow indicators suggest the present ranges were palaeohighs. The basin history around Nemegt Uul and Ih Bogd is dominated by Mesozoic extension, but Late Cenozoic transpressional uplift has led to complex overprinting relationships along the active range fronts where alluvial fan complexes exist in various stages of aggradation, incision, erosion, and re-sedimentation.