Geology topics

P. Molnar

Publications and source records attributed to P. Molnar.

3 recordsLinked to original sources

USGS2021

Numerical analysis of the effect of subgrid variability in a physically based hydrological model on runoff, soil moisture, and slope stability

In coarse resolution hydrological modeling we face the problem of subgrid variability, the effects of which are difficult to express and are often hidden in the parameterization and calibration. We present a numerical experiment with the physically based hydrological model ParFlow‐CLM with which we quantify the effect of subgrid heterogeneities in headwater catchments within the cell size typically used for regional hydrological applications. We simulate homogeneous domains and domains with subgrid heterogeneities in topography or soil thickness for two climates and soil types. The presence of side slope is the main error source, leading to large underestimation of runoff, and marginally also of evapotranspiration. The spatial distribution of soil saturation in the presence of subgrid variability in topography also leads to underestimation of landslide risk. Soil thickness is the second influential subgrid property, affecting soil moisture distribution and surface runoff formation. Results are consistent for the climates and the soil types considered. The topographic wetness index approach is tested as a way to downscale soil moisture simulations within the domain. Although this method is successful in reproducing some spatial variability and patterns, it fails when the coarse grid mean soil saturation is inaccurate or subgrid topography does not represent subsurface flow paths accurately. We conclude that ignoring subgrid variability in topography and soil thickness in coarse‐scale hydrological models may lead locally to underestimation of runoff and slope instability. Users of such models should be aware of these biases and consider ways to include subgrid effects in coarse‐scale hydrological predictions.

Water Resources Research↗

USGS2011

Magnetostratigraphy of the Neogene Chaka basin and its implications for mountain building processes in the north-eastern Tibetan Plateau

Magnetostratigraphy of sedimentary rock deposited in the Chaka basin (north‐eastern Tibetan Plateau) indicates a late Miocene onset of basin formation and subsequent development of the adjacent Qinghai Nan Shan. Sedimentation in the basin initiated at ∼11 Ma. In the lower part of the basin fill, a coarsening‐upward sequence starting at ∼9 Ma, as well as rapid sedimentation rates, and northward paleocurrents, are consistent with continued growth of the Ela Shan to the south. In the upper section, several lines of evidence suggest that thrust faulting and topographic development of the Qinghai Nan Shan began at ∼6.1 Ma. Paleocurrent indicators, preserved in the basin in the proximal footwall of the Qinghai Nan Shan, show a change from northward to southward flow between 6.5 and 3.8 Ma. At the same location, sediment derived from the Qinghai Nan Shan appears at 6.1 Ma. Finally, the initiation of progressively shallowing dips observed in deformed basin strata and a change to pebbly, fluvial deposits at 6.1 Ma provide a minimum age for the onset of slip on the thrust fault that dips north‐east beneath the Qinghai Nan Shan. We interpret a decrease in sediment accumulation rates since ∼6 Ma to indicate a reduction in Chaka basin accommodation space due to active faulting and folding along the Qinghai Nan Shan and incorporation of the basin into the wedge‐top depozone. Declination anomalies indicate the beginning of counter‐clockwise rotation since 6.1 Ma, which we associate with local deformation, not regional block rotation. The emergence of the Qinghai Nan Shan near the end of the Miocene Epoch partitioned the once contiguous Chaka‐Gonghe and Qinghai basin complex. In a regional framework, our study adds to a growing body of evidence that points to widespread initiation and/or reactivation of fault networks during the late Miocene across the north‐eastern Tibetan Plateau.

Chaka basin↗

USGS1989

Geology of the Ulugh Muztagh area, northern Tibet

Within the Ulugh Muztagh area, north central Tibet, an east-west-trending ophiolitic melange marks a suture that apparently was formed during a late Triassic or slightly younger collision between a continental fragment to the south and the rest of Asia. The southern continental fragment carries a thick sequence of upper Triassic sandstone, but the contact between the sandstone and the ophiolitic melange is covered by a younger redbed sequence of unknown age. A suite of 2-mica, tourmaline-bearing leucogranite plutons and dikes intruded the Triassic sandstone at shallow crustal levels 10.5 to 8.4 Ma. These rocks range from granite to tonalite in composition, are geochemically very similar to slightly older High Himalayan leucogranite and are interpreted to have been derived by the partial melting of crustal material. We interpret this to mean that crustal thickening began in this part of the Tibetan plateau at least by 10.5 Ma. Welded rhyolitic tuff rests on a conglomerate that consists of abundant debris from the Ulugh Muztagh intrusive rocks and has yielded Ar Ar ages of about 4 Ma. The tuffs are geochemically identical to the intrusive rocks suggesting that crustal thickening may have continued to 4 Ma. Crustal thickening probably occurred by distributed crustal shortening similar to shortening now occurring north of Ulugh Muztagh along the northern margin of the Tibetan Plateau.

Earth and Planetary Science Letters↗