HR: 16:45h
AN: T32E-04    [PDF]
TI: Ephemeral Sedimentary Basins and Limits of Lateral Plateau Growth at the Eastern Puna Margin, NW Argentina
AU: * Strecker, M R
EM: strecker@geo.uni-potsdam.de
AF: Universitaet Potsdam, Institut fuer Geowissenschaften Universitaet Potsdam, Potsdam, 14415 Germany
AU: Hilley, G E
EM: hilley@seismo.berkeley.edu
AF: University of California, 377 McCone Hall Department of Earth and Planetary Sciences University of California, Berkeley, CA 94720-4767 United States
AB: Similar to the northeastern margin of the Tibetan Plateau the eastern margin of the Puna Plateau of the southern Central Andes is characterized by laterally growing ranges and intervening intramontane basins in a tectonically active and arid environment. However, in contrast to Tibet, lateral plateau growth along the SE Puna by incorporation of filled sedimentary basins appears to be inhibited. In fact, reverse-fault bounded and sediment filled arid basins straddling this plateau are ephemeral. The relations between deformation, uplift, basin formation, and climate are illustrated well in the Quebrada del Toro at about 24$^{\circ}$ S in the Cordillera Oriental. The basin records the transition from a foreland to a broken foreland setting with range uplift beginning after about 8 Ma. Deformed sediments are overlain by about 1000-m-thick, poorly lithified conglomerates; in the lower 200 m of the section they laterally grade into plant-bearing lake deposits indicating a humid environment similar to the eastern windward slopes of the present basin border. Conglomerate deposition began after 8 Ma following deformation but before deposition of a 4.17 Ma tuff in the lower part of the sequence and continued until 0.98 Ma, when renewed thrusting along the basin margins and within the basin deformed these strata. These deposits were incised to elevations comparable to the present base level, suggesting integration of the Toro basin into the foreland drainage system. However, this event was superseded by a second, important influx of conglomerates, which filled the pre-existing erosional paleo-topography to levels that are up to 700 m above the present trunk stream. Subsequently, these conglomerates and all older units were extensively stripped and incised again to the present local base-level. Incision was episodic as documented by five inset strath terraces that follow the course of the trunk stream. Mapping of the Quaternary deposits and DEM analysis indicate that a minimum of 61.5 km$^{3}$ of material was removed from the basin during the Pleistocene. The average elevation change within the Quebrada del Toro is 158 m, which corresponds to 0.158 mm/yr of exhumation within the ultimate 1 Ma. This exhumation rate appears to be far lower than regional minimum rock uplift rates. Therefore, the basin surface was, and continues to be uplifted, which may be coupled with continuing uplift of the adjacent Puna Plateau. Unlike the margin of the northeastern Tibetan Plateau, where continued thrusting creates intramontane basins that are being filled with sediments that are ultimately incorporated into the plateau realm, the Toro basin is clearly transitory. Identical histories exist in other basins along the Puna edge. In contrast to basins with similar geometries in the arid Puna the basins straddling the eastern plateau margin cannot be sustained as depocenters and closed basins due to a very effective erosional regime of a wetter climate acting on the basin margins. This leads to large rainfall gradients in this area with easterly moisture sources, headward erosion, drainage capture, and ultimately basin exhumation.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1854 Precipitation (3354)
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting