Quartzite terrains, geologic controls, and basin denudation by debris flows: their role in long-term landscape evolution in the central Appalachians     

Robert J. Sas Jr.  L. Scott Eaton 《Landslides》2008,5(1):97-106

A large storm in 1995 that impacted the central Blue Ridge Mountains of Virginia triggered over a thousand debris flows, which deeply incised stream channels in zero-, first-, and second-order basins underlain primarily by granite, metabasalt, and quartzite. This event provided an opportunity to gain insight into geologic controls on debris-flow initiation and rates of basin denudation. Intensive investigations in quartzite terrain indicated that well-developed joints provide for rapid infiltration of rainwater and thus affected debris-flow initiation. Possible mechanisms of slope failure include (1) elevated rates of fracture recharge in steep valleys and/or (2) bedrock bedding planes. Fracture recharge may serve to substantially increase rock pore pressure during high intensity rainfall, especially when coupled with antecedent moisture. The quartzite bedrock tends to break into orthogonal blocks due to jointing. Statistical analysis shows the presence of a minimum of two joint populations that serve as bedrock controls on lateral erosion. Resistant bedding planes, parallel to slope, control vertical erosion below a finer-grained layer of saprolite. The combination of increased recharge and joint orientations in quartzite basins are likely the main factors resulting in the highest measured values of basin denudation from debris flows, relative to other lithologies, in the central Appalachians. Additionally, these joint and bedding planes produce a topographic signature at all scales and, therefore, are an important factor controlling long-term landscape evolution.  相似文献   

Fission-track dating of the south-eastern Bohemian massif (Waldviertel,Austria): thermochronology and long-term erosion     

Ewald?HejlEmail author  Gert?Sekyra  Gertrude?Friedl 《International Journal of Earth Sciences》2003,92(5):677-690

Apatite fission-track thermochronology was applied to the outcropping Variscan basement of the south-eastern Bohemian massif in order to resolve its post-Variscan cooling and denudation history. Nineteen samples could be dated by the grain-population method. The ages range from 235 to 90 Ma (Middle Triassic to Middle Cretaceous). Neglecting some local variations, they show a decrease from the western edge of the Moravian Thaya window towards the west and the south. Fission-track length histograms mainly exhibit broad distributions with arithmetic means between 12.3 and 13.4 µm, and standard deviations between 1.5 and 2.0 µm. Thermochronological model calculations of the measured data demonstrate that the apatite fission-track memory of the samples reaches back until the begin of the Cretaceous, and for some samples even until the Permian. The regional fission-track cooling pattern is not related to the limitations and the internal structure of the Moravian Thaya window, which appears to be an elder—probably Late Carboniferous to Permian—tectonic feature. Also the Vitis fault does not delimit areas of different thermochronological behaviour. Therefore, significant vertical offset on that fault can be excluded for the Cretaceous and Cenozoic. Thermochronological model calculations of fission-track ages and length data are consistent with the assumption of regional sedimentary reburial during the Late Cretaceous. The amount of reburial could have attained 1 km. The Cenozoic bulk denudation was in the order of 1 to 3 km.

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Zusammenfassung Die postvariszische Abkühlungs- und Denudationsgeschichte der südöstlichen Böhmischen Masse wurde mittels Apatit-Spaltspuranalyse untersucht. Neunzehn Proben wurden mit der Kornpopulationsmethode datiert. Die Spaltspuralter liegen zwischen 235 und 90 Ma (mittlere Trias bis mittlere Kreide). Abgesehen von kleinen lokalen Abweichungen, nehmen die Alter von der Westecke des Thayafensters in Richtung Süden und Westen ab. Nahezu alle Histogramme der Spurenlängen zeigen relativ breite Verteilungen mit arithmetischen Mittelwerten zwischen 12,3 und 13,4 µm, und mit Standardabweichungen zwischen 1,5 und 2,0 µm. Thermochronologische Modellberechnungen mit den gemessenen Daten zeigen, daß das Apatit-Spaltspurgedächtnis aller Proben mindestens bis zum Beginn der Kreide, für manche Proben sogar bis ins Perm zurückreicht. Die regionale Verteilung der Spaltspurdaten zeigt keine Übereinstimmung mit den Grenzen und dem inneren Bau des Thayafensters, welches eine offenbar ältere—wahrscheinlich oberkarbonische bis permische—tektonische Struktur darstellt. Auch an der Vitisstörung tritt keine sprunghafte Änderung des thermochronologischen Verhaltens der Gesteine auf. Daher kann ein bedeutender kretazischer oder känozoischer Vertikalversatz an dieser Störung ausgeschlossen werden. Thermochronologische Modellberechnungen der Apatit-Spaltspurdaten unterstützen die Annahme einer regionalen Wiederversenkung infolge oberkretazischer Sedimentbedeckung. Die Mächtigkeit der oberkretazischen Sedimente über dem Grundgebirge könnte bis ca. 1 km betragen haben. Der känozoische Gesamtabtrag lag zwischen 1 und 3 km.

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Long-term denudation rates of actively uplifting hillcrests in the Boso Peninsula, Japan, estimated from depth profiling of in situ-produced cosmogenic Be and Al     

Yuki Matsushi  Sachi Wakasa  Hiroyuki Matsuzaki  Yukinori Matsukura 《Geomorphology》2006,82(3-4):283-294

Depth profiling of in situ-produced cosmogenic ¹⁰Be and ²⁶Al was applied to estimate the rate of long-term denudation on actively uplifting hillcrests made of weakly consolidated sandstone in the Boso Peninsula, Japan. The nuclide concentration in the topmost meter of the hillcrest has reached an equilibrium value owing to surface denudation at 90 mm/kyr. This rate is much smaller than the regional uplift rate, and is consistent with the current isolated residual topography of the hillcrest. By considering the epoch of emergence of the sandstone hillcrests, we estimate the summit denudation rate of the adjacent mudstone hill formation to be 720 ± 110 mm/kyr.  相似文献   

福建武夷山风景名胜区及其附近地区的地貌研究   总被引：2，自引：0，他引：2  

刘尚仁 《热带地理》2004,24(3):216-220,241

武夷山风景名胜区及其附近地区由花岗岩、火山熔岩、变质岩和砂页岩等构成的中山、低山占最大面积.最高海拔约1800m,在海拔350～1200m,有7级剥蚀夷平面,形态有山顶缓坡型、山坡肩膀型、谷底裂点型(占绝大多数).丹霞地貌的顶面坡以圆弧形为主,陡崖坡有罕见的切层洞穴群和最大的晒布岩,崖麓缓坡多由红层组成.河流纵谷常沿着NNE走向较软的中生界砂页岩发育,断层谷断续长数十公里;横谷常沿着NWW的走向节理发育.据6个TL年龄认定河流阶地的沉积年代:第一级阶地Q3 ,第二级阶地Q2或Q2为主,第三级阶地Q2.  相似文献