We reveal the subsurface bedrock topography and sedimentary succession of one of the deepest glacially-formed basins in the Eastern Alps: the Lienz Basin in the Upper Drau Valley (Tyrol), by means of seismic reflection. A dense source-receiver spacing, supplied by autonomous receivers, and a prestack depth-migration processing scheme were essential to distinguish the various deposits in fine detail, such as slumping, fan delta deposits, and a modified monocline on the basin flank. These details support our interpretation of the seismic stratigraphy that consists of, e.g., subglacial till of last glacial maximum (LGM) age and possibly older, laminated basin fines, and gravel/coarse sand. The maximum depth of the basin is 622 m, at the junction of two major basement faults that are not clearly visible in the seismic reflections. We regard the overdeepening in this longitudinal valley as the result of glacier confluence during the LGM. Subglacial meltwaters utilized the higher erodibility of faulted rocks, as indicated by channel structures. The adverse slope (2.6%) along the valley axis exceeds the gradient ice-surface slope (0.4–0.5%) during the LGM by more than fivefold. We thus suggest this feature is a product of a pre-LGM phase, since adverse slopes greater than ~ 1.2 times the ice surface slope promote the freezing of water in subglacial channels and prevent efficient water flushing of sediments. Integrating other studies allows us to estimate the local overdeepening of the Lienz Basin and that of the whole Upper Drau Valley to be 146 m and 530 m, respectively. At the beginning of lacustrine sedimentation, we estimate the paleo-water depth to be at least 216 m.

中文翻译：

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借助反射地震揭示冰川超深谷的形状和地层：利恩茨盆地（奥地利）

我们通过地震反射揭示了东阿尔卑斯山最深的冰川形成盆地之一的地下基岩形貌和沉积演替：上德劳谷（蒂罗尔州）的利恩茨盆地。自主接收器提供的密集的接收器间距以及叠前深度迁移处理方案对于区分各种沉积物（例如下沉，扇状三角洲沉积物和盆地侧面的改性单斜线）非常重要。这些细节支持了我们对地震地层的解释，例如，由下冰川期到最后一个冰川最大年龄（LGM）以及可能更老的叠层盆地细屑和砾石/粗砂组成。在地震反射中看不到的两个主要地下断层的交界处，盆地的最大深度为622 m。我们认为该纵向山谷的过度加深是LGM期间冰川汇合的结果。如河道结构所示，冰川下的融化水利用了较高的断层岩石易蚀性。在LGM期间，沿谷轴的不利斜率（2.6％）超过冰面梯度斜率（0.4–0.5％）的五倍以上。因此，我们认为此特征是LGM前期的产物，因为大于冰面坡度的〜1.2倍的不利坡度会促进冰下河道中的水冻结，并阻止有效的冲刷沉积物。综合其他研究，我们可以估计利恩茨盆地和整个上德劳河谷的局部超深分别为146 m和530 m。在湖泊沉积开始时，我们估计古水深度至少为216 m。