Characterising deformable substrates of modern and past ice sheets is a key factor in understanding ice mass behaviour and dynamics. This is challenging when field-scale signatures of deformation in soft sedimentary beds are weakly developed or absent. To overcome these difficulties, in this study, we combine micro- and macroscale studies and quantitative analyses to decipher the nature of subglacial sediment deformation under a Pleistocene fast-flowing (streaming) ice sheet. We investigated a complex sedimentary succession at Nieszawa in north-central Poland. A multi-proxy approach involving till micromorphology in thin sections, ripple-cross lamination orientation, till fabric, clast microfabric and the anisotropy of magnetic susceptibility (AMS) data in a closely sampled vertical profile throughout the fluvial sand and till was used to reveal subglacial bed deformation mechanisms and kinematics. This records subglacial deformation of the sedimentary succession at Nieszawa under low basal shear stresses, where simple shear was the dominant bed deformation mechanism. Intergranular advection, decoupling at the ice-bed interface related to high subglacial water pressure, clast ploughing, and basal sliding also took place. The main finding of this study is that AMS reveals the shear strain development and ice flow direction in the structurally and texturally complex deposits under weak coupling conditions at the ice-bed interface beneath a fast flowing/streaming ice sheet, where strain and kinematic indicators other than AMS fabrics are vague or absent.

表征现代和过去冰盖的可变形基底是了解冰块行为和动力学的关键因素。当软沉积床中的野外尺度形变特征不发达或不存在时，这是具有挑战性的。为了克服这些困难，在这项研究中，我们结合了微观和宏观研究以及定量分析，以破译在更新世快速流动（流动）冰盖下冰下沉积物变形的性质。我们调查了波兰中北部Nieszawa的复杂沉积演替。一种多代理方法，涉及到薄切片中的微形态，波纹交叉层压方向，直至织物，沿河流砂层直至井盖的垂直取样剖面中的显微微结构和磁化率（AMS）数据的各向异性被用来揭示冰下层变形机制和运动学。这记录了在低基础剪应力作用下，尼泽瓦沉积相的冰期形变，其中简单剪切是主要的床层变形机制。晶间平流，在冰床界面的脱钩与高冰河下水压，碎屑耕作和基底滑动有关。这项研究的主要发现是，AMS揭示了在快速流动/流动冰盖下方冰床界面处弱耦合条件下，结构和质地复杂沉积物中剪切应变的发展以及冰流向，