Two morphologic settings in the northwestern Argentine prone to giant mountain‐front collapse—deeply incised narrow valleys and steep range fronts bordering broad piedmonts—were analyzed through detailed investigations of fossil landslides and related fluvio‐lacustrine sediments. Nine different rhyodactic tephra layers were defined by geochemical fingerprinting of glass, morphology of pumice, stratigraphic relationships, and mineralogy. The age of three tephra could be determined either directly by 40Ar/39Ar dating or relatively by 14C dating of associated sediments: Paranilla Ash ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $$723\pm 89$$ \end{document} ka), Quebrada del Tonco Ash (∼30 ka), and Alemanía Ash (∼3.7 ka). These units permit correlation of several spatially separate landslide deposits. Landslide deposits in narrow valleys were generated in the late Pleistocene between 40 and 25 ka and in the Holocene since ca. 5 ka and correspond to periods characterized by increased humidity in subtropical South America. In contrast, the age of large landslides in piedmont regions is significantly greater but more difficult to define by tephrochronology. However, selected deposits from this second environment have cosmogenic nuclide exposure ages of 140–400 ka. Because of the large distance of the collapsed mountain fronts from eroding streams and because of important Quaternary displacement along the mountain‐bounding faults, we suggest that strong, low‐frequency seismic activity is the most likely trigger mechanism for most of the landslides in this environment.

中文翻译：

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阿根廷西北部大型滑坡时间分布的年代学约束

通过对化石滑坡和相关的河湖沉积物的详细调查，分析了阿根廷西北部容易发生巨大山前崩塌的两种形态环境——深切的狭窄山谷和与广阔山麓接壤的陡峭山脉前沿。通过玻璃的地球化学指纹、浮石的形态、地层关系和矿物学定义了九个不同的流纹火山灰层。三个火山灰的年龄可以通过 40Ar/39Ar 测年直接确定，也可以通过相关沉积物的 14C 测年相对确定：Paranilla Ash ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{ bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2, OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty } \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $$723\pm 89$$ \end{document} ka)、Quebrada del Tonco Ash (∼30 ka) 和 Alemanía Ash （～3.7 ka）。这些单元允许关联几个空间上独立的滑坡沉积物。狭窄山谷中的滑坡沉积物产生于 40 至 25 ka 之间的晚更新世和自约 25 年以来的全新世。5 ka 对应于以南美洲亚热带湿度增加为特征的时期。相比之下，山前地区大型滑坡的年龄要大得多，但更难以通过地质年代学来定义。然而，来自第二个环境的选定矿床的宇宙成因核素暴露年龄为 140-400 ka。由于坍塌的山峰与侵蚀溪流之间的距离很远，而且由于沿山边界的断层发生了重要的第四纪位移，我们认为强烈的低频地震活动是该环境中大多数滑坡的最可能触发机制.