Physical modelling of large-scale deformational systems in the South Polar Layered Deposits (Promethei Lingula, Mars): new geological constraints and climatic implications

Deformation systems (DSs) locally affect the South Polar Layered Deposits (SPLDs) along the margins of the Promethei Lingula ice sheet (part of the southern Martian ice-dome). One example is the ‘S₂’ deformation system, characterized by a complex pattern of brittle and brittle–ductile structures related to kilometre-scale shear zones that deform the sequence. Moreover, soft-sediment structures affect one layer located at the base of the S₂. An earlier structural analysis suggested that: (1) two deformation stages (D₁, in which the shear zones developed, and D₂, in which the D₁ structures were reactivated by deep-seated gravitational slope deformation) occurred, driven by gravity; and (2) there are variations in the bulk composition of the SPLD (which is inferred to be mainly composed of water ice plus basaltic dust). This work supports these structural results through thermal and mechanical modelling of the S₂ sequence. Our modelling results suggest that several layers within the S₂ system are probably composed of, or are mixed with, CO₂ ice, and that the development of the observed deformation is inconsistent with present-day physical conditions. Soft-sediment structures probably formed under warmer surface temperatures during the past, with those warmer temperatures favouring or even triggering ice flow/basal sliding of the Promethei Lingula.

变形系统（Deformation systems, DSs）会对普罗米修斯舌状冰盖（Promethei Lingula ice sheet，属于火星南部冰穹的一部分）边缘的南极层状沉积（South Polar Layered Deposits, SPLDs）造成局部改造。其中典型案例为“S₂”变形系统，其特征是发育有与千米级剪切带相关的复杂脆性及脆韧性构造，使地层发生变形。此外，S₂底部的一层地层受到软沉积构造的改造。

此前的结构分析提出两点结论：其一，存在两期变形作用（D₁期形成剪切带，D₂期D₁构造被深部重力斜坡变形作用活化），二者均由重力驱动；其二，南极层状沉积的整体成分存在差异，据推测其主要由水冰与玄武岩尘埃组成。

本研究通过对S₂地层开展热-力学模拟，验证了上述结构分析结果。模拟结果表明，S₂系统内的多个地层可能由二氧化碳冰（CO₂ ice）构成或与之混合，且观测到的变形发育过程与现今物理环境并不相符。软沉积构造大概率形成于过去地表温度更高的时期，彼时的升温环境有利于甚至触发了普罗米修斯舌状冰盖的冰流与基底滑动。