A Tale of Two Tibets: Oligo-Miocene Basin Development in the Bangong and Yarlung Suture Zones

Coeval Oligo-Miocene basins along the Bangong and Yarlung suture zones (BSZ and YSZ) in Tibet record drastically different environmental conditions, tectonic processes, and possibly paleoelevations. The Nima and Lunpola basins formed in a contractile tectonic setting during late Oligocene reactivation of the BSZ, and filled with alluvial, fluvial, and lacustrine facies that were strongly influenced by boreal or alpine dry climate and high paleoelevation (>4.6 km). At the same time, the Kailas basin developed along the YSZ approximately 450 km to the west-southwest. The Kailas Formation is >2500 m thick, and rests in buttress unconformity upon ca. 67 Ma silicic volcanic rocks, which are intruded by 55 Ma granite. U-Pb ages from tuffs and detrital zircons in the Kailas Formation indicate deposition at 26-23 Ma, possibly continuing into mid- Miocene time. Unlike the strongly oxidized, evaporitic, and paleosol carbonate-rich deposits of Nima basin, the Kailas basin is dominated by an upward fining succession of alluvial fan to lacustrine deltaic and offshore deposits. Individual lacustrine parasequences exceed 50 m in thickness, and contain laminated black shale that accumulated in profundal settings in very deep lakes. Nearshore facies contain evidence of large storm waves and combined currents, suggesting that Kailas lakes were large and deep. Organic material in Kailas profundal facies consists of amorphous kerogen, fungal spores, and palynomorphs with warm tropical affinities. Provenance data (detrital zircon and modal sandstone point-counts) indicate derivation of sediment almost exclusively from the Kailas magmatic complex and Gangdese magmatic arc. Although the southern flank of the Kailas basin outcrop belt is involved in early Miocene shortening, overall basin fill characteristics suggest accumulation in an extensional tectonic environment. The uppermost part of the Kailas succession shifts abruptly into redbed/marl/paleosol carbonate facies typical of the "high elevation" facies assemblage documented in Nima basin. Oxygen isotope values from paleosol carbonate in these rocks are in the range of --21.5 to --15.4‰ (PDB), suggesting that paleoelevation comparable to Nima basin developed only in the final stages of Kailas basin filling. The absence of paleosol carbonate and abundance of organic matter, woody plant fossils, and deep water lacustrine facies in the bulk of the Kailas section, coupled with evidence for extensional tectonics, suggests potentially much lower elevation in the YSZ during the Oligocene and earliest Miocene. Contrasts between basins in the YSZ and BSZ raise the prospect that mid-Tertiary paleogeography and tectonic processes in the two sutures were profoundly different.