OCR Text |
Show recent volcanic activity. However, the fact that the elastic plate thickness estimates are so similar is rather unexpected. The Lahontan high shoreline also exhibits a regional tilt signal. Its magnitude and direction are very similar to those seen on the Bonneville and Provo shorelines in the eastern Great Basin. At roughly the same time as Bonneville and Lahontan were reaching their highest levels, the Altiplano region in the Central Andes of Bolivia, Peru and Chile contained a lake of very similar size and character. At its highest level Lake Minchin had a surface area of 60,300 km2 in a drainage basin of 201,200 km2, which it shared with Lake Titicaca ( 9,500 km2). At present, lake Titicaca is spilling to the south at an elevation of 3810 m, and the late Pleistocene level was only about 5 m higher. Lake Minchin reached an elevation of 3780 m. The highest shoreline of Lake Minchin records about 30 m of isostatic deflection and a similar magnitude of regional tilting. The Minchin basin is extremely flat floored, and the lake had attained roughly 50% of its maximum surface area when it was only 10 m deep. It was also even more successful than Lake Bonneville in accreting most of the neighboring lakes at a relatively low level. In springs surrounding the Salar de Ascotan, which was the most distal sub- basin of the Minchin system, there is a distinctive population of fish ( orestia ascotanensis) which appears to have diverged from species present in Lakes Minchin and Titicaca. The sill which separates the Ascotan lake from the main Minchin lake is high ( 3770 m) and thus the Ascotan connection to the main lake was brief. |