Regolith thickness instability and the formation of tors in arid environments
Regolith thickness instability and the formation of tors in arid environments
复制标题
干燥环境中风化层厚度的不稳定性和托尔的形成
DOI：
10.1029/2005jf000405
复制
发表时间：
2006
影响因子：
--
通讯作者：
P. Haff
中科院分区：
文献类型：
--
作者：
M. Strudley;A. Murray;P. Haff
[1] 我们提出的模型结果表明，物理侵蚀-基岩风化反馈是导致孤立基岩突起（tors/inselbergs）形成的原因，这些突起通常会破坏均匀基底岩性的光滑山墙。由于降雨和沉积物输送条件的波动以及取决于风化层厚度的基岩风化机制，可能会出现托尔和更大、节理更紧密且形态复杂的暴露物（岛山）。干旱、花岗岩环境中的水文地球化学考虑和现场观测表明，风化速率与风化层厚度之间的关系在有限的覆盖层厚度下表现出最大值。我们将这种简单的侵蚀风化反馈封装在一个模拟干旱/半干旱景观演化的数值模型中，该模型产生了由托尔打断的低坡度山墙。托尔在有效湿度较高的时期形成，导致局部基准面切口和山墙上的风化层变薄，引起地幔表面以超过裸露基岩风化速率的速度下降的转变。这种情况有利于风化层厚度小于阈值的区域中树突的出现和生长。随后气候或当地基准面的变化使沉积物表面下降率恢复低于裸露基岩风化率，将导致托尔高度逐渐降低，并最终消失。因此，根据该模型，干旱环境中的托尔代表了与气候或当地运输条件波动相关的可能瞬态特征，而不是源自差异性地下风化和风化层剥离的古代景观的重写。
[1] We present model results suggesting that a physical erosion–bedrock weathering feedback is responsible for the development of isolated bedrock knobs (tors/inselbergs) that often punctuate otherwise smooth pediments of homogeneous basement lithology. Tors and larger, more heavily jointed and morphologically complex exposures, inselbergs, may arise as a consequence of fluctuations in rainfall and sediment transport conditions combined with a bedrock weathering mechanism that depends on regolith thickness. Hydrogeochemical considerations and field observations in arid, granitic environments suggest that the relationship between weathering rate and regolith thickness exhibits a maximum for a finite thickness of cover. We have encapsulated this simple erosion-weathering feedback in a numerical model simulating arid/semiarid landscape evolution that produces low-sloping pediments punctuated by tors. Tors form during periods of higher effective moisture, resulting in local base level incision and regolith thinning on pediments, invoking a transition in which mantled surfaces lower at rates exceeding the bare bedrock weathering rate. This condition favors the emergence and growth of tors in areas covered by regolith thickness less than a threshold value. Subsequent shifts in climate or local base level that restore sediment surface lowering rates less than the bare bedrock weathering rate will lead to a progressive decrease in tor height and, ultimately, their disappearance. Thus, according to this model, tors in arid environments represent possibly transient features related to fluctuations in climate or local transport conditions rather than palimpsests of an ancient landscape derived from differential subsurface weathering followed by regolith stripping.