BASINS AFTER BILL: DEPARTURES FROM THE NORM IN OROGENIC HINTERLANDS (Invited Presentation)

Dickinson (1974) provided a template by which basins are understood in terms of flexure or stretching of the lithosphere, accompanied by thermal effects. This presentation highlights orogenic hinterland basins, long excluded from the taxonomy of their more accessible lowland counterparts. Examples in the high Andes and the suture zone between India and Asia provide evidence for complex geodynamic processes in mature orogenic belts. The Miocene (21-9 Ma) Arizaro basin in the central Andean Puna of northern Argentina occupies a circular, 7500 km² region at 3800-4300 m in the hinterland of the Andean thrust belt. Low-T thermochronology and stable isotope paleoaltimetry indicate the basin substrate was exhumed at high elevation during the Eocene. The basin filled with >3.5 km of upward-fining lacustrine, eolian, and fluvial deposits. Initially slow subsidence accelerated to 0.6 km/Myr, and then tapered to near zero by 10 Ma. The basin then shortened internally and inverted. Anomalously thin crust, modeling of the subsidence/deformation history, and a lack of viable alternate mechanisms suggest Arizaro basin formed in response to eclogitization of crust and mantle lithosphere, followed by gravitational removal in the form of a Rayleigh-Taylor instability. Similar Miocene and younger basins are common in the high Andes and may reflect geodynamic interactions among crustal shortening, magmatism, eclogitization, and root foundering in cordilleran hinterlands.

The Oligo-Miocene (ca. 26-18 Ma) Kailas basin in southern Tibet is a conventional-looking rift, filled with alluvial, fluvial and lacustrine deposits in a narrow (<15 km wide) elongate (>1300 km) trough astride the suture zone between the collided Indian and Asian landmasses. Apart from the surprising extensional processes acting in the heart of this ongoing collision, the basin is remarkable for its large deep-water paleolakes, turbidite systems, and coal. Organic geochemical data yield TEX₈₆ of 0.73-0.80 and MBT/CBT of 0.65-0.78, suggesting lake water T of 25°C or higher. Together the data suggest that Kailas basin filled at low elevation and was inverted to very high elevations (>6700 m today) ca. 16 Ma. Indian slab rollback, followed by break-off and then a return to flat-slab subduction can explain the setting and characteristics of Kailas basin.