Composite faults in the Swiss Alps formed by the interplay of tectonics, gravitation and postglacial rebound: an integrated field and modelling study
Composite faults in the Swiss Alps formed by the interplay of tectonics, gravitation and postglacial rebound: an integrated field and modelling study
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瑞士阿尔卑斯山由构造、重力和冰后回弹相互作用形成的复合断层：综合现场和建模研究
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发表时间：
2007
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通讯作者：
O. Pfiffner
中科院分区：
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作者：
M. Ustaszewski;A. Hampel;O. Pfiffner
在瑞士阿尔卑斯山的几个山谷的侧翼，在海拔1900米到2400米之间存在保存完好的断层崖，这些断层崖显示出谷侧地块相对于山侧地块的抬升。这些朝山上的断层崖的高度沿断层迹线走向在0.5米到10米以上不等，断层迹线通常与山谷轴线平行。断层崖的形成通常归因于构造运动或重力坡体失稳。在此，我们结合实地数据和数值实验表明，断层崖可能是复合成因的，即构造和重力过程以及冰后期差异抬升可能都对其形成有贡献。由于断层崖与较老的构造断层平行，可能会发生构造位移。然而，由于所研究的山谷缺乏地震活动，构造因素似乎是次要的。由于所研究山谷中的片理和岩性边界的陡倾，重力因素可能很大，这从断层崖不均匀的形态可以看出，这是坡体运动的典型特征。相对于山顶，谷底的冰后期差异抬升为断层崖的形成提供了第三种可能的机制，因为断层崖形成于冰后期，且出现在末次盛冰期被冰填充的山谷的侧翼。有限元实验表明，冰后期的卸载和回弹可引发在陡倾的先存软弱带的滑动，并解释部分所观测到的断层崖高度。根据我们的实地和模拟结果，我们得出结论：朝山上的断层崖的形成主要是由与山谷轴线平行的陡倾片理以及此外山谷中相对于山顶力学性质较弱的岩石所促进的。我们的研究结果意味着，与活动断层相关的地表表现的识别可能会因非构造起源的类似形态结构而受阻。
Along the flanks of several valleys in the Swiss Alps, well-preserved fault scarps occur between 1900 and 2400 m altitude, which reveal uplift of the valley-side block relative to the mountain-side block. The height of these uphill-facing scarps varies between 0.5 m and more than 10 m along strike of the fault traces, which usually trend parallel to the valley axes. The formation of the scarps is generally attributed either to tectonic movements or gravitational slope instabilities. Here we combine field data and numerical experiments to show that the scarps may be of composite origin, i.e. that tectonic and gravitational processes as well as postglacial differential uplift may have contributed to their formation. Tectonic displacement may occur as the fault scarps run parallel to older tectonic faults. The tectonic component seems, however, to be minor as the studied valleys lack seismic activity. A large gravitational component, which is feasible owing to the steep dip of the schistosity and lithologic boundaries in the studied valleys, is indicated by the uneven morphology of the scarps, which is typical of slope movements. Postglacial differential uplift of the valley floor with respect to the summits provides a third feasible mechanism for scarp formation, as the scarps are postglacial in age and occur on the flanks of valleys that were filled with ice during the last glacial maximum. Finite-element experiments show that postglacial unloading and rebound can initiate slip on steeply dipping pre-existing weak zones and explain part of the observed scarp height. From our field and modelling results we conclude that the formation of uphill-facing scarps is primarily promoted by a steeply dipping schistosity striking parallel to the valley axes and, in addition, by mechanically weaker rocks in the valley with respect to the summits. Our findings imply that the identification of surface expressions related to active faults can be hindered by similar morphologic structures of non-tectonic origin.